Report 2012_3363_Guideline for Inspection of Decommissioned Offshore Structures

DET NORSKE VERITASTM

REPORT NO./DNV REG NO.: 2012-3363 / 13XE6KG-4 REV 0, 2013-07-03

REPORT

GUIDELINE FOR INSPECTION OF DECOMMISSIONED OFFSHORE

STRUCTURES

NORSK OLJE OG GASS

DET NORSKE VERITAS

Report for Norsk Olje og Gass

Guideline for inspection of decommissioned offshore structures

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DNV Reg. No.: 13XE6KG-4 Revision No.: 0

Date : 2013-07-03 Page ii of iii

Table of Contents Page

1 GENERAL .......................................................................................................................................... 1 1.1 Introduction ............................................................................................................................... 1 1.2 Scope ......................................................................................................................................... 2 1.3 Objective and use ....................................................................................................................... 2 1.4 Structure of document ............................................................................................................... 2

2 TERMINOLOGY AND DEFINITIONS ............................................................................................ 3

3 ABBREVIATIONS ............................................................................................................................. 4

4 AREAS TO BE INSPECTED ............................................................................................................. 5 4.1 Structural strength ...................................................................................................................... 5 4.2 Corrosion ................................................................................................................................... 7 4.3 Inspection Technology .............................................................................................................. 7 4.4 Other .......................................................................................................................................... 8

5 INFORMATION GATHERING ......................................................................................................... 9

6 SELECTION OF AREAS ................................................................................................................. 10 6.1 Basis for selection .................................................................................................................... 10 6.2 Workshop – Selection of inspections ...................................................................................... 11 6.3 Documentation ......................................................................................................................... 11

7 PREPARATIONS AND EXECUTION OF INSPECTIONS ........................................................... 12 7.1 Group A – Structural Strength ................................................................................................. 13

7.1.1 General ............................................................................................................................. 13 7.1.2 A.1 Low fatigue lives and/or reported cracks .................................................................. 13 7.1.3 A.2 Grout connection between leg and pile ..................................................................... 15 7.1.4 A.3 Pile/sleeve connections .............................................................................................. 16 7.1.5 A.4 Piles ........................................................................................................................... 17 7.1.6 A.5 Members with unintended flooding .......................................................................... 18 7.1.7 A.6 Splash zone (fatigue) ................................................................................................. 19 7.1.8 A.7 Mechanical damaged structural members ................................................................. 19 7.1.9 A.8 Grout reinforced structural components .................................................................... 20 7.1.10 A.9 Pile to topside connection/condition of splice connections ....................................... 22 7.1.11 A.10 Ring stiffened joints ................................................................................................ 22 7.1.12 A.11 Single sided welds ................................................................................................... 23 7.1.13 A.12 Closure welds .......................................................................................................... 24 7.1.14 A.13 Conductors, risers and caissons – structural condition ............................................ 25 7.1.15 A.14 Cast joints ................................................................................................................ 25 7.1.16 A.15 Materials and welding ............................................................................................. 26 7.1.17 A.16 Material test of highly loaded parts ......................................................................... 26 7.1.18 A.17 Measurement of residual stresses ............................................................................ 27 7.1.19 A.18 Test of corroded material ........................................................................................ 27

7.2 Group B – Corrosion ............................................................................................................... 28

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7.2.1 General ............................................................................................................................. 28 7.2.2 B.1 CP system .................................................................................................................. 28 7.2.3 B.2 Coating ....................................................................................................................... 30 7.2.4 B.3 Coating repair ............................................................................................................ 31 7.2.5 B.4 Splash zone (corrosion) ............................................................................................. 31 7.2.6 B.5 Riser and riser clamps (corrosion) ............................................................................. 32

7.3 Group C – Inspection Technology ........................................................................................... 33 7.3.1 General ............................................................................................................................. 33 7.3.2 C.1 Details that have inspection findings from operation ................................................ 34 7.3.3 C.2 Areas of difficult/challenging underwater inspection ............................................... 35 7.3.4 C.3 Verification of new inspection techniques ................................................................ 35

7.4 Group D – Other ...................................................................................................................... 36 7.4.1 D.1 Bolts ........................................................................................................................... 36 7.4.2 D.2 Repair clamps ............................................................................................................ 36 7.4.3 D.3 Marine growth ........................................................................................................... 37

8 POST-PROCESSING OF DATA ..................................................................................................... 38 8.1 Group A - Structural strength .................................................................................................. 38

8.1.1 Evaluation of methods for structural calculations ............................................................ 38 8.1.2 Grout connections ............................................................................................................ 38

8.2 Group B – Corrosion ............................................................................................................... 38 8.2.1 CP system ......................................................................................................................... 38

8.3 Group C – Inspection Technology ........................................................................................... 38

9 STORAGE OF RESULTS ................................................................................................................ 39 9.1 Group A - Structural strength .................................................................................................. 39 9.2 Group B – Corrosion ............................................................................................................... 41 9.3 Group C – Inspection Technology ........................................................................................... 43 9.4 Group D – Other ...................................................................................................................... 43

10 REFERENCES .................................................................................................................................. 44

Appendix 1: Example format for workshop

Appendix 2: Checklist for workshop

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1 GENERAL

1.1 Introduction Many offshore structures are approaching the end of their original design life. In the coming years these will either be decommissioned or their operating lifetime will be extended. .

In April 2010 the PSA activities regulation §50 was published, ref. /1/: “When facilities are disposed of, the operator shall carry out studies of the structure’s condition. The results shall be used to assess the safety of similar facilities.”

In the guidance note to §50 it is specified that “the examinations should particularly be carried out with a view towards projected new facilities and use of facilities beyond their original planned lifetime in mind”. PSA has however not specified what type of inspection which shall be performed.

PSA expects that the industry identifies areas on decommissioned structures which should be inspected/tested to provide information of general value for life extension and design of similar facilities. The industry should organize experience transfer and make relevant data available to ensure that all operators learn from these inspections.

This guideline has been financed by NOG and prepared in co-operation with the industry in order to establish a common set of criteria for which areas to inspect on decommissioned structures, when to inspect, what type of inspections that should be performed and how to report the results to ensure that the lessons learned can be incorporated into the inspection and maintenance program on structures which are intended to operate beyond the original design life. Over time the experience and learning from decommissioned structures may also be utilised in design of new structures.

Inspection of decommissioned structures is a unique opportunity to investigate how the structures actually have performed over 20-40 years operation; were the designs adequate or unfavourable? The following is examples of what may be possible to learn and obtain:

Relation between design calculations and real conditions; i.e. how the structure have performed

Increased knowledge related to offshore repair and reinforcement solutions

Confirm condition of non-accessible areas (e.g. piles, bolts/fasteners, clamps)

Verify inspection findings from operation

Sharing information via uniform collection of data

Inspection of structures brought onshore will provide added value and over time;

contribute to increase the confidence to methodologies and criteria used in design

contribute to increase the confidence to modern ‘state-of-the-art’ methodologies and criteria to document safe life extension

help the operators to direct the inspections and maintenance to the most important areas

reveal poor design & repair solutions

provide information of the goodness/capability of the inspection methods used offshore

provide information of how much contingency is built into design codes which can be utilised in life extension

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contribute to reduce the extent of inspections

1.2 Scope The purpose of the guideline is to establish a common approach for planning, execution and documentation of inspection of decommissioned jacket structures.

The guideline is applicable for all structural parts of the jacket structures, including topside structures. Topside process equipment is not covered by this guideline.

1.3 Objective and use The project specific decommissioning plan shall include the extent of inspection that shall be performed on the decommissioned structure. This guideline should be used to decide which areas of the decommissioned jacket structures that shall be inspected and what type of inspections that shall be performed.

This guideline also specifies what type of information that should be stored in relation to these inspections. The intention is to ensure a common reporting of data so that sharing of information for future assessments is possible.

1.4 Structure of document Section 4 lists all candidate areas for inspection which is covered in the guideline. Each area has a dedicated designation/ID which will follow this area for inspection in the guideline.

Section 5 describes and lists the type of information that should be gathered prior to the selection of areas to be inspected.

Section 6 describes how the selection of areas for each structure shall be performed as part of the cessation project.

Section 7 gives details of how the inspections defined in Section 4 shall be performed. This Section is also intended to be used during the selection of areas for inspection.

Section 8 describes how the data should be post-processed.

Section 9 describes how the results should be stored and reported.

Figure 1-1 gives an illustration of the work flow for this guideline.

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Figure 1-1 Illustration of work flow for this guideline.

2 TERMINOLOGY AND DEFINITIONS may indicates a permissible course of action

shall indicates a mandatory requirement

should indicates a preferred course of action

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3 ABBREVIATIONS ALS Accidental Load Limit States

CP Cathodic Protection

DFF Design Fatigue Factor

DFI Design, Fabrication, Installation

ET Eddy Current Testing

FLS Fatigue Limit States

FMD Flooded Member Detection

MPI Magnetic Particle Inspection

MSF Module Support Frame

MT Magnetic Particle Testing

NDE Non Destructive Examination

NDT Non Destructive Testing

NOG Norsk Olje og Gass

POD Probability Of Detection

PSA Petroleum Safety Authority

SCF Stress Concentration Factor

ULS Ultimate Limit States

UT Ultrasonic Testing

VE Visual Examination

VT Visual Testing

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4 AREAS TO BE INSPECTED The candidate areas for inspection have been classified into 4 groups:

Group A: Structural strength

Group B: Corrosion

Group C: Inspection technology

Group D: Other

Each area of inspection has been given a dedicated ID that will follow this area of inspection throughout the document. Each area for inspection has been given an inspection code:

Code 1: Minimum recommended requirement; i.e. should be performed on all structures.

Code 2: Supplementary inspection; up to project to decide if inspection shall be performed based on checklists given in Appendix 2 and engineering judgement.

Code 3: Supplementary testing; laboratory testing which is up to project to decide if should be performed.

Inspection areas with Code 1 and 2 are detailed further in this document. Inspection areas with Code 3 will require development of test procedures and is not covered in more detail in this guideline.

4.1 Structural strength Table 4-1 lists the candidate areas for inspection defined in the Group A “structural strength”.

Table 4-1 Candidate areas for inspection defined in Group A – Structural Strength. ID Code Areas to be inspected Purpose A.1 1 Low calculated fatigue

lives and/or reported cracks

Increase understanding of fatigue phenomena to improve fatigue analysis methods. See Sec. 7.1.2.

A.2 1 Grout connection between leg and pile

Check how grout has behaved. Check that the assumptions made in design are acceptable. See Sec. 7.1.3.

A.3 1 Pile/sleeve connections Check how grout has behaved. Check that the assumptions made in design are acceptable. See Sec. 7.1.4.

A.4 1 Piles Check that piles below mud-line have no damage or cracks to increase the confidence to the analysis. See Sec. 7.1.5.

A.5 1 Members with unintended flooding

Detect cause of flooding. See Sec. 7.1.6.

A.6 2 Splash zone (Fatigue) Inspect highest utilized joints in the horizontal frames in order to validate/calibrate analysis procedure, and in-service inspection methods in order to reduce uncertainties in future assessments. See Sec. 7.1.7.

A.7 2 Mechanically damaged structural members

Obtain data on capacity of damaged structural elements. See Sec. 7.1.8.

A.8 2 Grout reinforced structural Check the quality of grout and degree of filling to gain

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ID Code Areas to be inspected Purpose components experience for selection of grout material and testing

for later grout operations. See Sec. 7.1.9. A.9 2 Pile to topside

connection/condition of splice connections

Verify how these connections have worked. See Sec. 7.1.10.

A.10 2 Ring stiffened joints Check the condition of the inner ring stiffeners with the objective to validate the fatigue analysis procedure for the stiffener itself and the weld between stiffener and chord. See Sec. 7.1.11.

A.11 2 Single sided welds Check the condition of the root area in single sided welds with the objective to validate the fatigue analysis procedure. See Sec. 7.1.12.

A.12 2 Closure welds Welds made under difficult condition in the yard with limited NDT. Possible sites for fatigue cracking in life extension. See Sec. 7.1.13.

A.13 2 Conductors, Risers, Caissons - structural condition

To get an overview of the structural condition of the conductors/risers/caissons. Main focus area is connection point to structure. See Sec. 7.1.14.

A.14 3 Cast joints Confirm quality of cast joints. See Sec. 7.1.15. A.15 3 Materials and welding Knowledge about the steel quality and quality of

welding of steels used in early platforms. See Sec. 7.1.16.

A.16 3 Material test of highly loaded parts

Establish stress/strain curves to assess possible ageing. Perform fatigue testing to establish remaining fatigue life. See Sec. 7.1.17.

A.17 3 Measurement of residual stresses

Improve understanding of the residual stresses in a structure that has served for several years in order to remove possible conservatism in the assessment methods. See Sec. 7.1.18.

A.18 3 Test of corroded material Establish S-N curves for structures with free corrosion. See Sec. 7.1.19.

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4.2 Corrosion Table 4-2 lists candidate areas for inspection defined in the Group B “Corrosion”.

Table 4-2 Candidate areas for inspection defined in Group B – Corrosion. ID Code Areas to be inspected Purpose B.1 1 CP system Knowledge of contingency of the CP-system can be

utilised for lifetime extension of other structures. See Sec. 7.2.2.

B.2 1 Coating Improve knowledge of coating degradation. See Sec. 7.2.3.

B.3 1 Coating repairs Evaluate if repair methods are good/poor. Can be used as input on other existing structures. See Sec. 7.2.4.

B.4 1 Splash zone (corrosion) Establish corrosion rates to be able to document longer life and larger capacity for existing structures. See Sec. 7.2.5.

B.5 2 Riser and riser clamps (corrosion)

Establish data on how risers and riser clamps perform with respect to corrosion in order to improve assessment/inspection methods. See Sec. 7.2.6.

B.6 2 Waterfilled closed compartment

Inspection of corrosion condition in closed compartments. See Sec. 7.2.7.

4.3 Inspection Technology Table 4-3 lists candidate areas for inspection defined in the Group C “Inspection technology”.

Table 4-3 Candidate areas for inspection defined in Group C – Inspection technology. ID Code Areas to be inspected Purpose C.1 1 Details that have

inspection findings from operation

Confirm finding. Cause of defect may be determined. See Sec. 7.3.2.

C.2 2 Areas of difficult/challenging underwater inspection

Validation of underwater inspection capability. See Sec. 7.3.3.

C.3 2 Verification of new inspection techniques

New inspection techniques may be tested offshore and verified with onshore inspection. See Sec. 7.3.4.

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4.4 Other Table 4-4 lists candidate areas for inspection defined in the Group D “Other”.

Table 4-4 Candidate areas for inspection defined in Group D – Other. ID Code Areas to be inspected Purpose D.1 1 Bolts Determine how bolts and other fasteners perform over

time to improve future specifications. See Sec. 7.4.2. D.2 2 Repair clamps Increase understanding of the performance of repair

clamps in order to design efficient clamps in the future and to prolong the life of existing clamps. See Sec. 7.4.4.

D.3 2 Marine growth Validate the recommendations given in NORSOK N-003. See Sec. 7.4.5.

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5 INFORMATION GATHERING Information from the design phase, installation phase and operation phase of the jacket structure is important in order to select the appropriate locations for the inspection and in order to learn from the inspection. This information will determine the usefulness of follow-up analyses and tests after removal.

Table 5-1 to Table 5-3 shows the type of documentation that should be available for the project. In some projects documentation can be a challenge to locate and the table therefore also list some important information that should be gathered as a minimum if the documents cannot be found.

Table 5-1 Information that should be available from design and fabrication of the structure. Documentation Minimum information requested DFI resumes (including modifications)

Design premises, design codes, design lives, materials selection, incidents during fabrication,

CP-design report Design code, CP design zones with number of anodes and anode types, CP design life

Coating specification Type of coating, coating thickness, surface preparation NDT specifications NDT methods, acceptance criteria

Table 5-2 Information that should be available after transportation and installation of the structure. Documentation Minimum information requested As laid survey/inspection report after installation

Condition of jacket structure when installed. Any deviations from original design? Were there any fatigue issues during transportation? Any incidents during temporary phases?

Table 5-3 Information that should be available from operation of the structure. Documentation Minimum information requested Reassessment reports ULS Reassessment reports ALS Reassessment report FLS Inspection reports (ROV, NDT) Inspection history: Reported damages, incidents, what has

been inspected and what has not been inspected, areas of concern, what type of inspection techniques have been used, anode consumption, CP potential readings?

Documentation of reinforcement Reason for reinforcement, type of reinforcement, design premise and design calculations for reinforcement, year of installation.

Repair history What have been repaired, when and how? Non-repaired inspection findings Type of finding and reason for decision not to repair.

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6 SELECTION OF AREAS The selection of areas to inspect shall be discussed and decided for each structure brought ashore. The benefit from the inspections on the decommissioned structures may be acknowledged in other parts of the Company than in the cessation project team, where the main focus typically has been decommissioning at low cost. In the discussion of selection of areas it is therefore crucial to bring in support from resources within technical disciplines; structural strength, inspection and materials. Resources from license partners or external consultants may also be invited.

An inspection plan for decommissioned structures shall be developed either as part of the cessation project or by other parts of the organisation and thus given as input to the cessation project. The selection of areas should comprise the following three steps:

1) Establish basis for selection of areas for inspection

2) Workshop to detail locations and specify type of inspections to be performed

3) Include requirements to inspection in the decommissioning plan

6.1 Basis for selection It is crucial to have an overview of the history of the structure as basis when selecting areas for inspection of the decommissioned structure. It would be beneficial to involve the personnel responsible for the inspection planning and people in the organization responsible for the operation/maintenance of the platform.

A workshop package presenting the history of the structure should be prepared as basis for the selection of areas for inspection on the decommissioned structure. The following information should as a minimum be used as input for the preparation of the workshop package:

DFI resumes

Reports on platform modifications and reinforcements

Reported incidents during operation and installation phases

Platform annual conditions reports (minimum latest revision)

List of repairs and maintenance on structure during lifetime

Inspection results

Reanalysis reports

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6.2 Workshop – Selection of inspections The workshop package, see Section 6.1, should form the basis for this workshop and detailed drawings of the structure shall be available. The workshop should involve personnel within the disciplines material technology, structural technology and inspection technology. The detailed locations and extend of the minimum recommended inspections, details regarding inspection and supplementary inspections should be concluded in the workshop.

Table 4-1 to Table 4-4 show the minimum recommended and supplementary inspections for the structures. Minimum recommended inspections (code 1 – green) should be performed on all structures. Guidance on this is given for each ID code in Section 7. The supplementary inspections (code 2 – yellow) should be discussed in the workshop and the decision should be made based on checklists given in Appendix 2 and conclusions made during the workshop.

Appendix 1 shows example of a format that may be used in the workshop. Appendix 2 presents some checklists/control questions that may be used during the workshop.

6.3 Documentation Requirements to inspection during decommissioning shall be detailed in the decommissioning plan. The decommissioning plan should as a minimum include the following:

Detailed requirement to type and extend of inspection at specific locations of the structure (reference to areas on drawings and tag no.)

All minimum recommended topics for inspection in this guideline (code 1 – green) should be covered or Company should justify why the inspection is not relevant

Requirements to documentation of inspections performed offshore prior to removal of structure and onshore.

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7 PREPARATIONS AND EXECUTION OF INSPECTIONS An inspection plan for decommissioned structures shall be developed either as part of the cessation project or by other parts of the organisation and thus given as input to the cessation project.

This section provides i) motivation for the proposed inspections listed in Section 4, ii) information required prior to inspection, iii) type of inspection needed prior to offshore removal and iv) type of inspection/activity proposed at the demolition yard.

Appendix 2 contains a summary table listing all potential inspections with purpose and motivation, including a check list (control questions) which can be used as a support when planning which areas to inspect and type of examination.

As stated in Section 4, inspection ID’s with Code 1 are minimum recommended inspections.

Inspection ID’s with Code 2 should be discussed in workshop based on the operation history of the jacket in question, and supported by the checklist in Appendix 2. The project decides if inspection/examination of these areas will provide information of general value for design of new structures and/or for life extension purpose.

Inspection ID’s with Code 3 may be extensive and involve laboratory testing. Only motivation for the proposed inspections is given in Section 7.

It is emphasized that a ‘no finding’ at the demolishing yard may be equally important as ‘a finding’ as regards to providing experience feedback for life extension and new designs. Similarly, a structure characterized as ‘robust’ may also be equally important to inspect as a structure with much findings during operation etc.

Information required prior to inspection: describes what type of information that should be available prior to inspection. This information should be available and discussed during the workshop (Section 6.2) in order to specify the exact location for the inspection on each structure. The information is also relevant for the assessment of the results, post-processing and storage of results.

Inspection prior to offshore removal: describes type of inspection that should be performed before the structure is removed offshore. This inspection will be performed by removal contractor for safety reasons and the information from this inspection should be documented and submitted to Company. The results from these inspections should be used in the assessment of the results, post-processing and storage of results. Further, to ensure that the results are not influenced by the removal operation, it may be advantageous to perform some inspections prior to offshore removal.

Inspection on yard: describes the type of inspection that should be performed onshore when the structure has been shipped to the yard where demolishing will take place. The results from these inspections shall be documented, post-processed and stored.

An inspection report shall be prepared from all inspections performed on the decommissioned structures. Section 9 presents the information that should be included in such a report. For some inspections, post processing of data is required prior to the reporting, see Section 8.2. For other inspections, post-processing may be performed after inspection results from several structures have been gathered, see Section 8.

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7.1 Group A – Structural Strength

7.1.1 General Onshore inspection provides a unique opportunity to increase the knowledge on how the jacket structures have behaved during decades of operation in the harsh environment of the North Sea.

A general visual inspection onshore should be done prior to demolishing to inspect for visible anomalies/notabilities in order to detect any major unexpected behaviour. This initial visual inspection will give a better understanding of the systems and areas of particular interest for detailed inspections and assessment of the condition.

Life extension beyond the original planned service life requires new fatigue analysis aiming for more ‘realistic’ fatigue lives. To document required fatigue life there may then be a need to use more refined fatigue analyses both with respect to wave load recipe and hotspot stress range generation.

There is accordingly a need to validate/calibrate these modern fatigue analysis techniques, and closer investigation over time of decommissioned jackets is a means to accomplish this.

Prior to inspection onshore, a fatigue analysis should be carried out using modern analysis techniques. This is not needed if already done as part of a previous reassessment work.

Areas with inspection findings during operation should be investigated onshore.

For jackets with pile clusters, a check of the grouted connections should (as part of the inspection planning process) be done according to the new design formulas given in Annex K of NORSOK N-004. The grouted connections should be examined onshore.

Appendix 2 contains a list of all alternative inspections, including a check list (control questions) which can be used as a support when planning which areas to inspect.

With reference to Table 4-1, the areas with id’s A.1 to A.5 should be inspected (minimum recommended/Code 1) for any jackets brought to shore. These areas are strongly recommendedto inspect as the findings will be of value for all jackets for which life extension is planned as well as of value in connection with design of new structures.

With reference to Table 4-1, the areas with id’s A.6 to A.14 should be inspected (optional/Code 2) based on check lists and the operation history of the jacket in question. The project decides if inspection/examination of these areas will provide information of general value for design of new structures and/or for life extension purpose.

With reference to Table 4-1, the areas with id’s A.15 to A.19 may be inspected (optional/Code 3) based on check lists and the operation history of the jacket in question. These inspections may involve laboratory testing and will require development of test/inspection procedures which is not detailed further in this guideline.

7.1.2 A.1 Low fatigue lives and/or reported cracks The selection of areas to inspect for fatigue needs to be decided by the evaluation team for each structure. The selection criteria depend on the importance of the weld connection for the global structural integrity, complexity of the joint, inspection/repair history, similar joints in operating platforms etc.

There is a need to improve the understanding of the fatigue phenomena to validate and calibrate the fatigue analysis methods, in particular for the purpose of inspection planning and life extension.

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Information required prior to inspection: All weld connections with calculated fatigue utilisation ratio higher than 0.5 (as calculated according to modern analysis techniques and with DFF = 1) should be examined for fatigue cracks. Joints with lower fatigue damage may be more applicable to select in some structures. The final selection of weld connections to inspect should be based on an overall assessment of the specific structure, see Section 6. The fatigue lives shall be calculated using accurate methodology, e.g.:

hydrodynamic coefficients dependent on the relative surface roughness (e), the Reynold’s number (R), and the Keulegan-Carpenter number (K)

hotspot stress ranges according the generalized influence function method.

Mean values in S-N curve.

For further information on fatigue analysis, see /9/.

In addition, the following information should be reviewed prior to workshop and presented at the workshop:

Drawings, design and material specifications, design premises, DFI, design reports.

Inspection history.

The methodology, criteria and calculated fatigue lives from the original design work.

The methodology, criteria and calculated fatigue lives from any reassessment work.

Any joint connections with particular low life shall be emphasized.

Any cracks registered in-service and associated repair work.

Any members reported as flooded from FMD.

Inspection methods.

Inspection prior to offshore removal: The marine contractor needs to conduct a rather comprehensive inspection of the jacket to ensure a safe removal. Damages and cracks (e.g. members which are unintentionally flooded) that may affect the operation are looked for.

The team planning for inspection at the demolition yard needs to cooperate with the marine contractor. The marine contractor will also benefit from the operation history provided by the operator.

The findings from the inspection by the marine contractor should be transferred to the team planning for inspection at the demolition yard.

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Inspection on yard:

Removal of marine growth and coating.

Close visual inspection and Magnetic Particle Testing (MT) should be performed after removal of coating.

Ultrasonic Testing (UT) should be performed to ensure that potential cracks starting in weld root will be detected.

It may be considered to store samples for future references (e.g. qualification of NDT-methods or further investigation of the cause of the crack, remaining fatigue life etc.). It is important that such joint(s) are stored in a non-corrosive environment (i.e. indoor).

Table 9-1 lists the information that should be stored regarding this area of inspection.

7.1.3 A.2 Grout connection between leg and pile There is significant lack of data on the condition of grouted piles after installation, e.g. how well is it grouted, corrosion of steelwork, condition of circumferential welds (any cracking), evidence of effect of pile driving etc.

In the structural reassessment and design work for extreme storms and fatigue it is common practice to assume that the leg section is compact, with respect to stiffness as well as for static strength and estimation of stress concentration factors (SCF).

Recent experience from check of the grout quality between leg and pile indicates that this is not necessarily true. Accordingly, opening of the leg to check the grout quality at some locations along the leg should be done.

As inspections of these areas are impossible offshore, it is particular important to increase the knowledge of the quality of grout and grout filling to (over time) provide increased confidence in analysis and assessment procedures.

Information required prior to inspection:

The following information should be reviewed prior to workshop and presented at the workshop:

Grout specification with type of grout etc.

Reports from testing of the grout (if available)

Reports from grout filling (if available)

The number of and exact location of grout connections to be inspected should be decided in the workshop, see Section 6.2. It may also be considered to take core samples for future examination.

Inspection prior to offshore removal:

The marine contractor should inform the Company in case of any incidents related to piles (e.g. loss of a pile). This may be an indication of poor grout quality.

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Inspection on yard:

Look at the grout exposed at the ends of the leg.

Carefully cut out steel at various positions along the leg, leaving ‘windows’ for inspection and sample collection.

Assess the quality of the grout; i.e. colour, consistency.

It may be considered to store core samples for future references. It is important that such core sample(s) are stored in a dry environment.

Take photos, prepare a report from the inspection and store grout samples.


7.1.4 A.3 Pile/sleeve connections New formulas have been developed to check the capacity of the grouted connections between the pile and pile sleeve with respect to extreme storms and fatigue. Reference is made to Appendix K in NORSOK N-004. Inspection of these connections to investigate how they have behaved is needed as the previous formulas for capacity checks of these connections may be non-conservative.

As inspections of these areas are impossible offshore, it is particular important to verify if these grout connections have functioned as expected.



Structural drawings of the pile/sleeve connections/foundation design.

Design reports for ULS, ALS and FLS analyses of the pile/sleeve connections.

Grout specification with type of grout etc.

Reports from testing of the grout (if available).

Reports from grout filling (if available)

The number of and exact location of pile/sleeve connections to be inspected should be decided in the workshop, see Section 6.2. It may also be considered to take core samples for future examination.

Inspection during offshore removal:

The marine contractor should inform the Company in case of any incidents related to piles (e.g. loss of a pile). This may be an indication of poor grout quality.

Inspection on yard:

See Section 7.1.3 (A.2 grout connection between leg and pile).


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7.1.5 A.4 Piles It is crucial that the piles maintain their integrity in case of life extension.

Accordingly, the part of the pile below mudline should be investigated for any anomalies or cracks in the welds to (over time) provide increased confidence to the analyses for extreme storm and fatigue. The pile will be cut at some distance below mudline and will therefore be available for inspection at the demolition yard.

As inspections of the piles are impossible offshore, it is particular important to increase the knowledge on how piles behave to provide confidence to the analysis procedures. Information from recovered piles is very valuable.



Pile drawings, design and material specifications, design premises, DFI, design reports.

Utilisation of all piles in the area just below mudline for extreme storm loads (ULS and ALS).

Fatigue damage from pile driving and from operation.

Reassessment reports.

Any indications of scouring?

The number of and exact location of pile to be inspected should be decided in the workshop, see Section 6.2. It may also be considered to take samples for future examination.


Company should be informed if the marine contractor observes any anomalies on the pile.

Company should be informed if any damages to the pile are made during cutting of pile.

Inspection on yard:

Visual inspection of the part of the pile below the leg, i.e. the most utilized piles (storm/fatigue).

Removal of coating (if applicable).

Close visual inspection and Magnetic Particle Testing (MT) should be performed of any welds available on the part brought to shore.

Ultrasonic Testing (UT) should be performed to ensure that potential cracks starting in weld root will be detected.


Take photos and prepare a report from the inspection and store segment of pile for further testing.


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7.1.6 A.5 Members with unintended flooding Some braces have been reported as water filled in-service, but the cause of water filling may not have been identified and explained. Inspection onshore may reveal the reason for water filling, which can be of value for similar situations for existing installations. Information required prior to inspection:


Drawings with indication of members which are unintentionally flooded.

Results from in-service inspections of member.

The number of and exact location of braces to be inspected should be decided in the workshop, see Section 6.2.


It is anticipated that the marine contractor performs FMD to identify members which are water filled, and that efforts are made to check the cause of water filling.

Company should be informed if marine contractor identifies any member with unintentional water filling.

Inspection during offshore removal: For FMD finding it should be confirmed that the findings are correct; i.e. that the braces actually are filled.

Inspection on yard:

Visual inspection of the part of the member(s) which have been found unintentionally flooded to check the cause of flooding.

If reason for flooding of members is not found by visual inspection, ET/MT should be performed on the structural members welded connection.

In case of any crack like defects, the coating should l be removed and tested by MT and UT to define crack extension and geometry.

Water filled braces should be inspected internally for wall thickness reduction which may reduce the strength (e.g. due to microbiolocially induced corrosion). If sectioning of braces is inconvenient, video inspection may be used.

Take photos and prepare a report from the inspection.


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7.1.7 A.6 Splash zone (fatigue) These areas may be difficult to inspect without use of special equipment/climbers.

It is crucial for the global integrity that main connections in these areas are intact and function according to expectations, in particular if wave-in-deck needs to be accounted for in the structural reassessments. As inspections of these areas may be difficult and infrequent due to limited access and high expenses, it is particular important to increase the knowledge on how joint connections in these areas resist the fatigue loads.

In case of subsidence and/or higher design waves (or increased water level), structural members in this area will be exposed to additional loads not originally designed for, i.e. local ‘Morison loads’, wave slamming loads, variable buoyancy due to continuous variable submergence, loads due to wave exit etc. For platforms with subsidence inspection in the splash zone, A.6 should be performed on minimum 2 joints.


The following information should be reviewed prior to workshop and presented at the workshop for all platforms with subsidence or other highly utilised splash zones:

Results from inspections in this area.

Analysis reports, e.g. fatigue analyses due to wave slamming and variable buoyancy. For wave slam calculations it is recommended to follow the procedures in Ref./10/.

Drawings indicating the calculated fatigue life in these connections.


It is anticipated that the marine contractor inspects the structural components in the splash zone areas/top part of jacket. This is particular important if the jacket has subsided as the members in these areas then have been exposed to significant variable loads not accounted for in design.

Company should be informed if marine contractor identifies any cracks or damages in these areas.

Inspection on yard:

See Sec. 7.1.2 (A.1 Low fatigue lives).


7.1.8 A.7 Mechanical damaged structural members Mechanical damages due to boat impacts, dropped objects etc. may reduce the strength of a structural component. Mechanical damages may also originate from other sources such as submarines, whales and hydrostatic pressure.

Inspection according to this Section should be performed if mechanical damages on structural members have occurred in the history of the platform. Specific locations for inspection should be selected in the workshop. It may be considered to take samples for future laboratory testing.

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Reports from inspections/repair of damaged members caused by boat impact and dropped objects.

Sketches/Photos/Videos of the damage, and description of damage/dent.


It is anticipated that the marine contractor inspects for any visible damages/dents caused by boat impact or sudden drop.

Company should be informed if marine contractor identifies any previously detected cracks or damages in these areas.

The marine contractor should assess and note any damage during decommissioning to steel tubulars, piles etc., and notify Company.

Inspection on yard:

Visual inspection of the damaged structural component.

Measure the damage (width, depth etc).


It may be considered to store samples for future references (e.g. laboratory testing of remaining capacity and/or full scale testing. It is important that such joint(s) are stored in a non-corrosive environment (i.e. indoor).


7.1.9 A.8 Grout reinforced structural components Grout filling is a common method to i) reinforce a jacket to increase the global ultimate capacity, ii) increase the static and fatigue strength of a joint, iii) increase the member strength of a brace or leg to better resist boat impact and iv) reinforce repair clamps.

As the structural analyses assume good quality of the grout as well as complete filling, it is crucial that the grout works as intended. Several examinations made onshore of grout reinforced braces, legs and clamps have revealed that this is not always fulfilled.

Experience feedback will be of value for future grouting operations and test procedures.

A.8 should be performed if grout filling of structural components has been done on the structure. If several grout filling reinforcements have been performed, as a minimum one structural component should be evaluated. It may be considered to take drill core samples for laboratory testing in the future.

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Drawings showing which members have been grouted.

Information on the purpose of grouting.

Grout specifications/type of grout.

Reports from testing of grout prior to offshore operation (if available).

Reports from the offshore operation (if available). (Any incidents during the grout operations?)


It is anticipated that the marine contractor will identify the grout filled braces with FMD, and that the Company confirms which members are grout filled.

Inspection on yard:

Identify the grout filled members selected for inspection (if any) from the workshop.

Cut loose the parts as specified in the inspection package prepared.

Efforts should be made not to remove the steel end caps until time of inspection. In this way the degree of filling and the condition of the grout close to the interface with the leg (or brace) can be retained as exactly as possible.

To allow for a close visual inspection of the grout, sections of steel should be removed (‘windows’) at the ends of the member, and at the middle of the member.

Do a qualitative evaluation of the grout quality and degree of filling.

It may be considered to store drill core samples for future references (e.g. crush tests to determine the compressive strength). It is important that such joint(s) are stored in a dry environment.


Discuss and decide if testing in a laboratory may provide information of general value for life extension and new design.


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7.1.10 A.9 Pile to topside connection/condition of splice connections Examine damages in the pile to topsides connections. Cracks in the shim plate welds have been observed in the past.

The purpose of this inspection is to confirm design assumptions.

A.9 should be performed if the inspection history shows indications of cracks. It may be considered to take samples for laboratory testing in the future.


The following information should be reviewed prior to workshop and presented at the workshop if the inspection history shows indications of cracks:

Design reports and specifications.

Reports from inspection of the condition of the MSF splices.

Information related to the most loaded splice connection.


It is anticipated that the marine contractor inspects this area, and reports to Company of any findings.

Inspection on yard:

Cleaning of area and visual inspection to register any anomalies.

In case of any crack like defects, the coating should be removed and crack inspected by MT and UT to define crack extension and geometry.




7.1.11 A.10 Ring stiffened joints Ring stiffeners are key components for confirming joint strength and fatigue performance in life extension. The analysis procedures for calculation of fatigue life in the weld to the chord wall and the ring inner edge are old, and rather simplified for the weld connection to the chord wall.

NDT of inner ring stiffeners is very difficult during platform operation.

It will therefore be of value to check the condition of the inner ring stiffeners, and to investigate if cracking at welds between stiffeners and joint has taken place.

Confirmation of the validity/accuracy of the parametric SCF equations used in design of ring stiffened joints is needed.

A.10 should be performed if the structure is equipped with ring stiffened joints. As a minimum, one ring stiffened joint should be examined. It may be considered to take samples for laboratory testing in the future.

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Design specifications, design briefs.

Location of ring stiffeners, type and dimensions.

Purpose of ring stiffeners.

Methodology used in design for capacity check of ring stiffeners (ULS/ALS, FLS).


Any cracks observed by marine contractor in areas with ring stiffeners should be reported to Company.

Inspection on yard:

Open the joints selected for inspection.

Visual inspection for any anomalies. Any cracking at welds between ring stiffener and joint?

In case of any crack like defects, the coating should be removed and tested by MT and UT to define crack extension and geometry.




7.1.12 A.11 Single sided welds Cracks may develop from the root of single sided welds. This is e.g. the case when braces are welded directly to the legs without stubs. Such cracks may be difficult to detect during in-service inspection.

The root area of single-sided welded tubular joints may be more critical with respect to fatigue cracks than the outside region connecting the brace to the chord. It is normally recommended that stubs are provided for tubular joints where high fatigue strength is required, such that welding from the backside can be performed.

Failure from the root has been observed at the saddle position of tubular joints where the brace diameter is equal. Ref. /11/.

A.11 should be performed if the structure is equipped with single sided welds. As a minimum, one single sided weld connection, which during operation has experienced the highest dynamic loads, should be examined. It may be considered to take samples for laboratory testing in the future.

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Design specifications, design briefs, drawings

Identify any single sided welds, e.g. locations with no stubs towards chords.

Reports from inspection of connections with single sided welds.


Any cracks observed by marine contractor in areas with single sided welds should be reported to Company.

Inspection on yard:

See Sec. 7.1.11(A.10 Ring stiffened joints).


7.1.13 A.12 Closure welds Welds made under difficult conditions in the yard, with limited access for NDT, may be possible sites for fatigue cracking in life extension. The weld quality should be checked. It should be checked if any crack is present.

If the structure has closure welds, as a minimum one closure weld connection, which during operation has experienced the highest dynamic loads, should be examined. It may be considered to take samples for laboratory testing in the future.



Overview of closure welds


It is anticipated that the marine contractor reports any cracks in closure welds to Company.

Inspection on yard:

See Sec. 7.1.11 (A.10 Ring stiffend joints).


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7.1.14 A.13 Conductors, risers and caissons – structural condition The conductors, risers and caissons are safety critical items on a platform. The connection points between the structure and the conductor/riser/caisson are crucial and there is limited access to inspection in these areas during operation.

A.13 should be performed if modern analysis show low calculated fatigue life(< 60 years) or if the history indicate that cracking may be found in these areas. As a minimum one connection point should be examined.

For inspection of corrosion conditions on conductors, risers and caissons, see Section 7.2.6 (B.5 Inspection riser and riser clamps).



Overview of risers/conductors/caissons and their condition from inspection reports.

Design documents with basis and criteria for design.

Reassessment reports.

Repair reports.


Condition of risers and riser clamps if possible.

Inspection on yard:

Close visual inspection of the condition of connection point between structure and conductor/riser/caisson; i.e. is there any wear in this area and is there any signs of cracking in the connection to the structure?



7.1.15 A.14 Cast joints Early cast materials have limited performance data.

Check condition of casting, fracture toughness, quality of any repair welding and if there are any associated cracking.


The following information may be reviewed prior to workshop and presented at the workshop:

Reports from fabrication of cast joints.

DFI resume.

Check if defects were detected and left un-repaired during manufacturing. Fracture mechanics analysis done?

Any repair welding done.

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7.1.16 A.15 Materials and welding There may be lack of knowledge of the specification and performance of steels used in early platforms and of the quality of welding. Steel quality and properties are necessary input for life extension.

It should be considered to select some structural components for laboratory testing to check if material and welding quality is according to the design specifications. Material aging may also have taken place during service, which may have reduced the material properties. Such tests may involve:

Tensile tests

Charpy test

Fracture toughness test

Cross section examination to check for welding defects

Micro structure examination

Chemistry

Fatigue tests

Residual stress measurements

See Ref. /5/ for comprehensive similar testing.



Drawings to identify potential joints of different geometry and complexity as candidates for material and weld testing.

Utilisations with respect to extreme storms and fatigue.

Material certificates

Welding procedure qualification records

7.1.17 A.16 Material test of highly loaded parts Laboratory tests may be performed to establish stress/strain curves to assess possible ageing effects. This is of most interest towards parts that have been highly loaded.



Identify structural components which have been highly loaded.

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7.1.18 A.17 Measurement of residual stresses The magnitude of the residual stresses is of importance for failure modes such as buckling,fatigue and unstable fracture. It is uncertain how the residual stress levels change over time in a dynamically loaded structure.

For example, residual stresses have been measured in a pile from an Ekofisk installation and the results from the laboratory results are reported in /8/. The findings have direct relevance for how to assess the fatigue damage accumulation in piles.

It would be of interest to extend the experience database with more measurements of residual stresses in piles. See also A.4 and /8/.

It is a need to improve the understanding of the residual stresses in a structure that has served for several years in order to remove possible conservatism in the assessment methods. Information required prior to inspection:


With ref. to A.4, discuss and decide if a part of a pile shall be checked for residual stresses.

Any other structural components that could be possible candidates for assessment of residual stresses?

7.1.19 A.18 Test of corroded material Static strength of naturally corroded material requires input that should be based on tests. For fatigue it may be of interest to check S-N curves for structures with free corrosion.



Check with inspection reports if any structural components have been exposed to free corrosion.

Decide if some structural components shall be tested in laboratory for assessment of fatigue strength.

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7.2 Group B – Corrosion

7.2.1 General The corrosion control for offshore structures is a combination of Cathodic Protection (CP), coating and corrosion allowance. An understanding of the factors that influences the corrosion rate as well as lack of corrosion is important input for life time extensions.

Cathodic protection with sacrificial anodes is the main corrosion protection of all parts of the submerged structure. Coating is often applied in combination with CP in order to reduce the required amount of anodes. However, several of the oldest structures are not coated below the splash zone. Inspection of the remnants of the anodes and protectiveness of calcareous deposits should give significant information relevant for inspection and maintenance of structures intended to operate beyond service life.

In the splash zone, the CP will only be partly efficient in the lower parts (when submerged). In the splash zone above lowest astronomical tide, corrosion allowance and coating is applied as corrosion protection. Considering the harsh environment in this zone and limited access for inspection during operation, inspection of the condition of coating and estimates of corrosion rates in the splash zone should give significant information relevant for life extension. Identification of critical areas of e.g. crevice corrosion should also be included.

Coating is the corrosion protection in marine atmosphere (above splash zone). In the design phase, materials selection and design of components (to avoid accumulation of water) is also important for corrosion control.

The candidate areas for inspection are given in Section 7.2.2 to 7.2.6 as well as in Table 4-2. The CP system, coating and coating repair evaluation and corrosion in splash zone are minimum recommended inspections and are classified with Code-1 (green). Inspection of risers and riser clamps are defined with code-2 (yellow), and up to the project to decide if these inspections are to be done.

7.2.2 B.1 CP system The CP system should be assessed based on visual inspection of anodes and potential measurements performed during operation.

If the CP design report is available, the subdivision of the CP zones from the design should be followed. Minimum one of the CP zones should be selected for detailed inspection on each structure. If the subdivision for the CP design is not known, the CP system inspection should be divided into the following zones:

Splash zone / Top zone

General / Bulk zone

Near seabed zone

After inspection, recalculations with today’s codes for the actual lifetime of the anodes should be performed and results shall be compared with actual remnants of anodes in order to estimate the contingency of the CP system, see Section 8.2.

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Location and size of anodes (e.g. drawings)

Inspection history (visual inspection and potential mapping)

Areas with increased consumption of anodes and retrofitting (if applicable) should be highlighted

CP design report, if available

The number of and exact location of anodes to be inspected shall be decided in the workshop, see Section 6.2. Minimum one defined CP zone should be selected for detailed inspection. In this zone, the weight of a representative selection of anodes (number to be decided in the workshop) shall be established.

Inspection prior to offshore removal: Prior to offshore removal the number of anodes, locations, estimated remnants from visual inspection and potential measurements for the selected CP zone for further evaluation should be available to the project. The workshop should decide if this information is available from inspection during operation or if some additional measurements are required prior to offshore removal.


If anodes from the selected CP zone(s) are removed offshore, the anodes shall be uniquely marked and brought offshore for inspection.

Inspection on yard:

For the selected zone(s), the number of anodes should be counted and estimates of remaining anode mass shall be performed visually (for comparison with last CVI offshore).

A minimum of 3 representative anodes should be cut down, washed with high pressure washer and weighed from each zone. To select anodes representing the minimum and maximum anode mass left in the zone is beneficial.

Dimension (length, thickness, width) of the anodes that is weighted shall be measured.

It should be evaluated if the anodes is uniformly or localized corroded

The weighted anodes shall be photo documented.


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7.2.3 B.2 Coating Inspection of coating on decommissioned structures is a unique possibility to improve the knowledge of the coating quality. It is a presumption for these inspections that the type of coating is known. Over time these inspections can result in a database showing expected lifetime and condition for different type of coatings. The inspection will evaluate if the coating is good/fair/poor according to ISO 4628. An inspection plan will divide the substructure in to relevant zones and examine them depending on environmental exposure. The relevant zones will be:

Atmospheric zone Splash zone (tide sone) Submerged zone



Type of coating Inspection history (which areas are defined as good/fair/poor from inspection during

operation?) Coating design specification and technical files (if applicable)

The number of and exact location of coated area to be inspected shall be decided in the workshop, see Section 6.2. For each of the zones (submerged, splash zone, marine atmospheric zone) minimum 3 locations should be selected for detailed inspection on yard. To select areas representing both good and fair/poor coating quality is beneficial.


Any inspection results from the selected areas for detailed inspection should be made available.

Inspection on yard:

1. Visual inspection and photo documentation of different zones, evaluating degradation Assessment of degree of blistering, based on ISO 4628-2 Assessment of degree of rust, based on ISO 4628-3 Assessment of degree of cracking, based on ISO 4628-4 Assessment of degree of flaking, based on ISO 4628-5

2. Field measurements of

Dry film thickness of coatings Using magnetic gauge testing Using a paint inspection gauge. (Check thickness of each coat/layer and

total thickness) Pull off adhesion testing using dollies, based on ISO 4624


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7.2.4 B.3 Coating repair The aim for gathering information on coating repair is to evaluate the successfulness of the repair procedure. This information can then be used on other existing structures for selection of repair method. It is a presumption for inspection B.3 that the repair coating type and method is known.



Areas where coating repairs have been performed

Inspection history for coating repair (when was the area repaired, any need for multiple repairs, method for repair?)

Technical documentation and procedures describing the repair.

The number of and exact location of repaired coating to be inspected shall be decided in the workshop, see Section 6.2.

Inspection on yard:

See Section 7.2.3 (C.2 Coating on offshore structures).


7.2.5 B.4 Splash zone (corrosion) The splash zone is designed with corrosion allowance. Thickness measurements in the splash zone above upper astronomical tide should be performed in order to compare with requirements in NORSOK M-001. Information required prior to inspection:


Design corrosion allowance in splash zone

Inspection history from operation (any areas with known reduced wall thickness?)

Type of coating in splash zone?

Amount of subsidence of the structure during lifetime?

The number of and exact location of areas to be inspected shall be decided in the workshop, see Section 6.2. A representative location for measurements of wall thickness in the splash zone above upper astronomical tide (at time of removal of structure) should be selected.

Inspection on yard:

Visual inspection of coating condition and extend and type of corrosion Wall thickness measurement


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7.2.6 B.5 Riser and riser clamps (corrosion) High corrosion rates have been reported in some cases externally on carbon steel risers and riser clamps underneath damaged coating. Detailed inspections in these locations will give increased knowledge of the reason for these damages and the extent.



Location and type of risers and riser clamps

Inspection history (any known areas with wall thickness reduction, any repairs, method for repair?)

Drawing of design of riser and riser clamps, including information of coating (any field joints in the splash zone?)


Inspection on yard:

See section 7.2.5 (B.4 Corrosion in splash zone).

It may be considered to take samples of riser and riser clamp damages to laboratory for closer failure investigation.


7.2.7 B.6 Waterfilled closed compartments Closed compartments (e.g. legs, braces and piles) may be partly or completely water filled. On some structures these closed compartments are filled with chemically treated water (e.g. addition of biocide and/or oxygen scavenger) while on other structures the water may be untreated. Inspection of these closed compartments will confirm if the risk for corrosion is low or if microbiologically induced corrosion needs to be taken into account.



Location of waterfilled and closed compartments

Description of type of water treatment (e.g. untreated seawater, biocide treatment, oxygen scavenger etc.)


Inspection on yard:

The closed compartment should be opened and inspected internally for extent and type of corrosion. Measurements of wall thickness reduction should be performed.


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7.3 Group C – Inspection Technology

7.3.1 General There are several types of underwater in-service inspection techniques. General Visual Inspection (GVI) and Close Visual Inspection (CVI) are typically performed on a yearly basis. Depending on the results from these inspections, potential types of defects and their expected location, more costly and detailed inspections like Magnetic Particle Testing (MT), Eddy Current Testing (ET), and Ultrasonic Testing (UT) is performed. In seldom cases even radiographic testing (RT) might be necessary.

Table 7-1 shows inspection techniques which are used on jacket structures and limitations for each method.

Table 7-1 Inspection techniques. Inspection techniques Equipment/applic

ation Limitations What can be learnt?

Visual Testing (VT); includes also inspection of marine growth and anode consumption

GVI

ROV, Camera. Inspection performed on the entire submerged part of the structure and topside.

Accessibility, Visibility, ROV/camera limitations. Surface methods. Subjective opinions from inspector regarding marine growth, anode consumption.

Verify findings and subjective estimates of e.g. marine growth and anode mass. Does e.g. the marine growth “hide” some findings that should have been further inspected?

CVI

MT (Magnetic Particle testing)

Prods/yoke, colour contrast or fluorescent. Method typically used where it is expected to be findings.

Need to remove coating. Mostly dependent upon diver inspection (working conditions). Surface methods.

Inspection on yard to confirm size, location and geometry of finding.

ET (Eddy Current Testing)

Probes, ROV or diver operated. Preferred NDT method for in-service inspection since coating do not need to be removed

Accessibility. Need to scan very thoroughly in order to achieve complete coverage. Surface methods. Max. 2 mm crack depth can be measured. Can be used on coated surfaces up to 2 mm in thickness.

Compare highly utilised areas inspected with ET in operation, with MT in yard when coating is removed.

UT (Ultrasonic Testing)

UT general

ROV or diver operated

Accessibility. UT possible with smooth coating but difficult with layers of coating and e.g. blistering. Partly filled members (FMD) may not be detected

Compare highly utilised areas inspected with UT in operation, detailed inspection when dismounted.

FMD Leading tool for underwater inspection

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Inspection techniques Equipment/application

Limitations What can be learnt?

RT (Radio-graphic testing)

RT general

ROV or diver operated. Rarely used due to radiation hazard.

Radiation hazard, only used for volumetric detection (not cracks but significant wall thickness reduction).

Inspection on yard to confirm size, location and geometry of finding.

FMD Leading tool for underwater inspection

Partly filled members may not be detected

In-service inspection underwater is more difficult and limited compared to topside, due to cost of ROV spread and/or diver spread. Inspection of areas with limited access like e.g. twelve o’clock position in complex nodes and painted/coated structural parts will result in possible uncertainties which could affect the probability of detection.

Inspection of decommissioned structures is a unique opportunity to verify the capability of offshore inspection methods with detailed measurements performed on yard. The capability of new inspection techniques may also be verified on decommissioned structures.

The location of the inspections performed on decommissioned structures should be based on input from fatigue calculations and results from inspection history on the structure.

7.3.2 C.1 Details that have inspection findings from operation A representative number of known defects (minimum 2) should be inspected prior to offshore removal and on yard to confirm size, location and geometry of findings. For FMD findings reference is given to Section 7.1.6 (A.5).

Information required prior to inspection: All findings from the inspection history of the structure should be listed with defined location (tag no.). The inspection method(s), the year for the first finding, progress and corrective actions should be brought up. Based on this information there should be established a plan for what type of NDT technique that shall be performed prior to offshore removal and on yard.

Inspection prior to offshore removal: Final NDT-inspection of findings should be performed offshore for comparison with results from inspection on yard.

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Inspection on yard:

Close visual inspection prior and after removal of marine growth

Verification of results from NDT performed offshore on inspection findings. The technique(s) to use on yard depends on type of finding and location. As a minimum, the same technique as used for offshore inspection should be performed onshore. Results from both onshore and offshore inspection shall be stored for comparison (e.g. length, width, depth of cracking).

In case of any crack like defects, the coating should be removed and tested by MT and UT to define crack extension and geometry.

It may be considered to store samples of structure parts with findings for future references (e.g. qualification of NDT-methods or further investigation of damage).


7.3.3 C.2 Areas of difficult/challenging underwater inspection Inspection of decommissioned structures is a unique opportunity to verify the capability of offshore inspection methods with detailed measurements performed on yard. Repeated inspection on land and comparison with underwater results can be used to verify the capability of offshore inspection methods.

C.2 should be performed if experimental techniques have been used during operation e.g. due to limited accessibility (e.g. UT guided wave, ET-PET).

Information required prior to inspection and inspection prior to offshore removal and on yard should be performed according to procedure defined in Section 7.3.2 (C.1 Inspection findings).


7.3.4 C.3 Verification of new inspection techniques Examples of new techniques that can be verified on decommissioned offshore structures are High Resolution Imaging (HRI) and use of ET with ROV.

C.3 should be performed if Company has introduced new techniques during the lifetime of the structure or if Company has introduced new techniques on other installations.

Information required prior to inspection: Locations with relevant areas for the new inspection techniques should be selected. If applicable, both locations with and without findings should be chosen.

Inspection prior to offshore removal: Locations with relevant areas for the new inspection technique should be tested with both the traditional method and the new inspection method.

Inspection on yard:

Close visual inspection after removal of marine growth

Verification of NDT performed offshore. If inspection of finding, the results shall compare length, depth of finding offshore vs. onshore for both the new and traditional technique.

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7.4 Group D – Other

7.4.1 D.1 Bolts Fasteners are critical components and the conditions of the bolts are not always possible to establish without dismounting with subsequent inspection.

Information required prior to inspection: It should be established what is the material selection and what type of coating has been applied (if applicable) on the fasteners. It should further be established at what operating conditions the fasteners have experienced (submerged with CP, splash zone or atmospheric zone).

A selection of bolts should be inspected on the decommissioned structures. The amount and exact location of bolts to be inspected shall be decided in the workshop, see Section 6.2. Bolts with findings from inspection history should be discussed in the workshop. As a minimum 3 locations of bolts should be inspected as detailed below and minimum one of these locations shall be in the splash zone.

Inspection on yard:

Close visual inspection of corrosion and coating (if applicable)

Photo documentation

Dismantling of bolts in stainless steel on yard and evaluation of crevice corrosion beneath bolt heads/nuts and threaded area. Dimensional measurement if significant damage is found.


7.4.2 D.2 Repair clamps Repair clamps are often grouted to ensure sufficient strength in severe weather conditions. Repair clamps are crucial structural members that are not inspectable after installation. Inspection of repair clamps on decommissioned structures should thus be performed in order to be able to rely on grouted repair clamps in the future.



Location of repair clamp and reason for installation

Drawing showing design of repair clamp

Installation procedures and logs

Grout specifications/type of grout.

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Inspection on yard:

Visual inspection of external part of repair clamp and surrounding areas

Opening of repair clamp

Do a qualitative evaluation of the grout quality (i.e. colour, consistency) and degree of filling.

Inspect repaired area underneath clamp. Where damage as expected?

Photo documentation


7.4.3 D.3 Marine growth Marine growth may cause increased hydrodynamic actions, increased weight, increased hydrodynamic additional mass and may influence hydrodynamic instability as a result of vortex shedding and possible corrosion effects. Thickness of marine growth is thus an input in calculation of structural actions, and NORSOK N-003 shows a table with thickness of marine growth at different water levels.



Thickness of marine growth from inspection history

The table in NORSOK has been updated based on measurements during operation. However, if the marine growth thickness on the decommissioned structure is known to be significantly different than NORSOK N-003, inspection should be performed. Location for inspection of marine growth shall be decided in the workshop.

Inspection on laying barge:

Visual inspection of type of marine growth (hard/soft and e.g. types like mussel, sea grass, sponges, barnacles etc.)

Thickness of marine growth


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8 POST-PROCESSING OF DATA An inspection report shall be prepared on all inspection performed on the decommissioned structures. Section 9 presents the information that should be included in such a report. For some inspections, post processing of data is required prior to the reporting, see Section 8.2. For other inspections, post-processing may be performed after inspection results from several structures have been gathered, see Section 8.1, 8.3 and 8.4.

8.1 Group A - Structural strength

8.1.1 Evaluation of methods for structural calculations When several structures have been inspected (A.1, A.6, A.8, A.10, A.11, A.12), the fatigue analysis methods should be evaluated.

8.1.2 Grout connections If any of the grouted connections (A.2, A.3, A.8) are found not to have the quality as expected, the reason for this shall be evaluated. Grouting operational logs and procedures should be reviewed in order to find the cause to the poor grout quality. The evaluation should conclude on a recommendation for future grouting.

A discussion should also be made on the validity of the assumptions made in design and reassessment/life extension work.

8.2 Group B – Corrosion

8.2.1 CP system After gathering data on anode consumption for structures an estimation of contingency of the CP system should be performed. This can lead to important input to life time extension projects. The coating breakdown factor is as well a critical parameter and a long term coating break down factor can be estimated for the used coatings.

For the selected CP zone, recalculation of the CP-system shall be performed with today’s code (DNV-RP-B401/NORSOK M-503). Required input parameters for this calculation are:

Steel surface area for selected CP zone

Type of coating (if applicable)

Type of anodes from design (approximately weight and dimensions)

Actual operating lifetime of structure

The results from the actual remaining amount of anode mass and dimensions should be compared with the recalculated CP system. The contingency factor for the CP system should be calculated according to the following equation:

CP contingency = Actual remaining anode mass / Required anode mass with today’s codes

8.3 Group C – Inspection Technology When a database has been populated with a large number of inspection results, probability of detection (POD) can be established.

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9 STORAGE OF RESULTS An inspection report shall be prepared on all inspection performed on the decommissioned structures. Section 9 presents the information that should be included in such a report.

9.1 Group A - Structural strength Table 9-1 to Table 9-6 list the information that should be stored regarding the inspection of findings related to Group A – Structural Strength.

Table 9-1 Results from inspection of joints - A.1, A.6, A.8, A.10, A.11, A.12. Information Installation year Original design code Location Original methodology and criteria for fatigue life calculations Original fatigue life Methodology and criteria for reassessment Calculated fatigue life with new methodology Method/Technique for detection offshore Size of finding offshore (length, width, depth) – if applicable Method/Technique for detection on yard Size of finding onshore (length, width, depth) Expected condition of joint confirmed (yes/no)? Confidence to modern analysis method (yes/no)? Photo documentation

Table 9-2 Results from inspection of grout - A.2, A.3, A.8. Information Installation year Original design code Location Type of grout Filling as expected or problems (yes/no)? Description of type of problems (if applicable) Results from testing of grout prior to installation Incidents related to pile during offshore removal (e.g. loss of pile)? Description of grout inspected on yard Grout conditions as expected (yes/no)? Reason for grout condition not as expected (if applicable) Assumptions made in design and reassessment/life extension valid? Photo documentation

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Table 9-3 Results from inspection of piles - A.4, Information Installation year Original design code Location Description of type of damage/crack (if applicable) Calculated fatigue life from design/reassessment? (driving vs operation) Assumptions made in design and reassessment/life extension valid? Photo documentation

Table 9-4 Results from inspection of members with unintended flooding – A.5. Information Installation year Original design code Location Cause of flooding Photo documentation

Table 9-5 Results from inspection of mechanical damages - A.7, A.9. Information Installation year Original design code Location Description of damage Damage observed during operation? Any remedial measures done during operation? Photo documentation

Table 9-6 Results from inspection of conductors, risers and caissons - A.13. Information Installation year Original design code Location Description of condition Any failures observed during operation ? Any remedial measures done during operation ? Photo documentation

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9.2 Group B – Corrosion Table 9-7 Results from inspection of CP system - B.1. Information Installation year Location for CP zone evaluated Original CP design code Type of anode, including net/gross weight and dimensions Type of coating on structure Anode consumption from last visual inspection of anodes offshore Potential of structure from last inspection offshore Actual operating life of CP system Anode consumption from visual inspection of anodes on yard Location of anodes that was cut down and measured Weight of anodes measured on yard Dimension of anodes measured on yard Anode uniformly or localized corroded Photo documentation CP contingency

Table 9-8 Results from inspection of coating - B.2. Information Installation year Location of inspection (submerged/splash zone/marine atmospheric zone) Type of coating Results from inspection of coating offshore Degree of blistering on yard (ISO 4628-2) Degree of rust on yard (ISO 4628-3) Degree of cracking on yard (ISO 4628-4) Degree of flaking on yard (ISO 4628-5) Actual operating life of coating system Coating thickness Pull off strength

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Table 9-9 Results from inspection of coating repair - B.3. Information Installation year Location of repaired coating Type and method of coating repair Multiple repairs? If yes, how long time between repairs. Results from inspection of coating repair offshore Degree of blistering on yard (ISO 4628-2) Degree of rust on yard (ISO 4628-3) Degree of cracking on yard (ISO 4628-4) Degree of flaking on yard (ISO 4628-5) Actual operating life of coating repair system Repair system acceptable (Yes/No)

Table 9-10 Results from inspection in splash zone (corrosion) - B.4. Information Installation year Design corrosion allowance in splash zone on structure Actual operating life of structure in splash zone inspected (note subsidence may reduce lifetime) Type of coating in splash zone Results from inspection offshore (any known wall thickness reduction, condition of coating) Condition of coating on yard (Poor/Fair/Good) Wall thickness reduction in splash zone above lower astronomical tide

Table 9-11 Results from inspection of riser and riser clamps - B.5. Information Installation year Type of riser material/clamp material Type of coating riser/clamp Actual operating life of riser/clamp Results from inspection offshore (any known wall thickness reduction, condition of coating) Condition of coating on yard (Poor/Fair/Good) Wall thickness reduction

Table 9-12 Results from inspection of waterfilled closed compartment - B.6. Information Installation year Type of closed compartment (leg/brace/pile etc) Type of water treatment Actual operating life Results from inspection (no corrosion, severe corrosion, pitting corrosion) Wall thickness reduction Water treatment acceptable (Yes/No)

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9.3 Group C – Inspection Technology Table 9-13 list the parameters that should be stored regarding the inspection of findings.

Table 9-13 Results from inspection technology - C.1, C2, C.3 Information Method/Technique for detection offshore Method/Technique for detection on yard Type of finding Location of finding Size of finding offshore (length, width, depth) Size of finding on yard (length, width, depth) Confirmation of reason for finding (yes/no) Secondary effects of finding not detected previously (e.g. FMD- condition inside)

9.4 Group D – Other Table 9-14 list the parameters that should be stored regarding the inspection of bolts.

Table 9-14 Results from inspection of bolts - D.1. Information Installation year Type of bolts Coating type Application Environment (submerged, splash zone, atmospheric zone) Temperature Corrosion damage (Yes/No) Description of type of corrosion (crevice, pitting, general) Condition of coating (description) Photo documentation

Table 9-15 Results from inspection of repair clamps - D.2. Information Installation year Location for repair clamp Reason for repair clamp Type of repair clamp Type of grout Installation procedure Degree of filling and grout quality (colour, consistency) after opening of clamp Results from inspection of areas underneath clamp. Photo documentation Were the damage underneath clamp as expected (Yes/No) Was the repair clamp working as intended (Yes/No)

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Table 9-16 Results from inspection of marine growth - D.3. Information Location of marine growth measurement (water depth) Type of marine growth Thickness of marine growth offshore Thickness deviation from NORSOK N-003

10 REFERENCES /1/ Gerhard Ersdal (PSA), ‘PSA View and Expectation of Activities Regulation §50”

October 6th 2011 in Sandnes (JIP Steering committee meeting).

/2/

University of Stavanger, ‘Material testing of decommissioned offshore structures’ A report prepared for the PSA programme on Ageing and Life Extension. 13.12.2007

/3/

Gerhard Ersdal (PSA) and John Sharp (Cranfield University), ‘Priorities for Testing Components/Steelwork from Decommissioned Structures’ June 2007

/4/

Gerhard Ersdal (PSA) and John Sharp (Cranfield University), ‘Proposed Best Practice for Testing Recovered Steelwork/Components from Decommissioned Offshore Platforms’, Draft 1.June 2007.

/5/

Inspection of Frigg jackets DP1, DP2 and QP (2009)

/6/

Inspection of Ekofisk 2/4 R (2010)

/7/

Inspection of Ekofisk 2/4 W (2011)

/8/

Inge Lotsberg et al,“Fatigue Testing and S-N Data for Fatigue Analysis of Piles” OMAE 2008-57250, June 15-20 2008, Estoril, Portugal

/9/

Det Norske Veritas, ‘Appendix A – Fatigue Analysis of Jacket Structures’ Appendix to the report 2011-066 ‘Use of Probabilistic Methods for Planning of Inspection for Fatigue Cracks in Offshore Structures’.

/10/

Ridley, J. A., ‘A Study of Some Aspects of Slamming.’ NMI Report R 158 OT-82113, 1982. Department of Energy, NMI Project 302025.

/11/

Det Norske Veritas, DNV-RP-C203: Fatigue Design of Offshore Steel Structures

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APPENDIX 1

EXAMPLE FORMAT FOR WORKSHOP

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ID Topic Inspection

Category1) Location of inspection (Tag.no, ref. drawing)

Reason for selecting/not selecting optional

inspections

Comments

1 2 Group A - Structural Strength A.1 Low fatigue lives and/or reported

cracks X

A.2 Grout connection between leg and pile

X

A.3 Pile/sleeve connections X A.4 Piles A.5 Members with unintended

flooding X

A.6 Splash zone (fatigue) X A.7 Mechanical damaged structural

members X

A.8 Grout reinforced structural components

X

A.9 Pile to topside connection/condition of splice connections

X

A.10 Ring stiffened joints X A.11 Single sided welds A.12 Closure welds X A.13 Conductors, risers, caissons –

structural condition

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ID Topic Inspection Category1)

Location of inspection (Tag.no, ref. drawing)

Reason for selecting/not selecting optional

Comments

Group B - Corrosion B.1 CP system X B.2 Coating X B.3 Coating repairs X B.4 Splash zone (corrosion) X B.5 Riser and riser clamps (corrosion) X B.6 Waterfilled closed compartments X Group C – Inspection Technology C.1 Details that have inspection

findings from operation X

C.2 Areas of difficult/challenging underwater inspection

X

C.3 Verification of new inspection techniques

X

Group D - Other D.1 Bolts X D.2 Repair clamps X D.3 Marine growth X

1) Code 1 (green): Minimum recommended requirement; i.e. should be performed on all structures. Code 2 (yellow): Supplementary inspection; up to project to decide if inspection should be performed based on checklists given in Section 7 and engineering judgement.

- o0o -

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APPENDIX 2

CHECK LIST FOR WORKSHOP

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ID Code Purpose Comment Check list A.1 1 Low fatigue lives and/or reported

cracks

Improve understanding of the fatigue phenomenon to improve fatigue analysis methods.

We often observe that some joint connections have very low theoretical fatigue lives, while no cracks have been observed in 30-40 years in service. NDT inspections of such joints are of interest. Typical areas: horizontal frames.

Numerous analysis methods exist to predict fatigue life, with large spread in the results. Inspection of joints onshore (under better controlled conditions than offshore) gives us the opportunity to validate modern refined fatigue analysis methodologies. It is crucial to have confidence to these methods in life extension work.

‘Modern’ fatigue analysis carried out ? Fundamental natural period ? Any exceptional low calculated fatigue life (< 10 years) ? Low calculated fatigue life in any major joints? Low calculated fatigue life in conductor frame areas? Low calculated fatigue life in riser connections? Low calculated fatigue life in upper horizontal jacket frame? Operating platforms with similar joint geometries? Platform exposed to any subsidence/fatigue in upper hor. frame? Low fatigue life in the piles from pile driving/waves? Any complex joints for which par. formulae might not be valid? Other joint configurations for which par. formulae might not be valid? Any joint with ring stiffeners? Assess if such joints should be examined. Any areas with (serious) crack development observed in service? Any areas with (serious) crack development observed in-service, and

which have long life from analyses? (cracks may originate from other sources such as poor workmanship and transportation).

Any defects registered during fabrication? Fabrication site ? (sign. fat. damage may have originated from transport). Evaluate if one or more joints should examined closer, either at the

decommissioning yard or send part(s) of the joint to laboratory for detailed testing (material strength, toughness, fatigue behaviour).

A.2 1 Grout connection between leg and pile Document performance of piles; a key element in demonstrating life extension.

Remove uncertainty about joint behaviour for joints on composite members.

There is significant lack of data on the condition of grouted piles after installation, lack of NDE data.

How well grouted, corrosion of steelwork, condition of circumferential welds (any cracking), evidence of effect of pile driving.

The stiffness modelling of the leg members in all jacket analyses assumes a fully composite action between leg and pile. The static and fatigue strength of leg joints assume a fully composite action between leg and pile. Possible areas with lack of shear load capacity. (voids?) Measurements of grout compressive shear strength.

A part of a leg from an installation that has experienced large hydrodynamic loads may be tested in the

Grout specification available? Reports from grout testing available? Reports from grout operation offshore available? (Any incidents?) Piles through the legs for this jacket? Pile groups? Any high utilisations in joints with respect to joint capacity? Any low fatigue life in any of the leg joints or pile sleeve joints? Any incidents related to piles during removal of jacket offshore? Any abnormal platform behaviour registered in-service? Keep samples of grout material for later tests in laboratory?

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ID Code Purpose Comment Check list laboratory to document that this assumption is valid.

A.3 1 Pile/sleeve connections Check how grout has behaved.

New design formulas for ULS and FLS checks of grouted pile/sleeve connections have been developed (2011), and may be verified through close examination onshore. See also comment to A.2

Water depth ? Deck weight ? Any tension in jacket legs ? Any dynamic amplification ? Reanalysed for ULS and FLS according to new design formulas ? Inspected during in-service ? Platform exposed to any extreme storms ? Hs, max ? Any abnormal platform behaviour registered in-service ? Keep samples of grout material for later tests in laboratory?

A.4 1 Piles Check that piles below mud-line have no damage or cracks to increase the confidence to the analyses.

Inspection of the piles is difficult/impossible during in-service. It is crucial that the foundation works properly when planning for life extension.

A part of the pile is available onshore, and should be inspected.

Welds in the pile should be examined.

Any pile highly utilised (ULS/FLS) from design or reassessment analyses ? Scour ? Any inspection made during in-service ? Possibility for tension in the piles ? (deck weight?) Evaluate if a part of the pile should be tested in laboratory.

(material strength, toughness, fatigue behaviour).

A.5 Members with unintended flooding Detect cause of flooding.

Some braces have been reported as water filled in-service, but the cause of water filling may not have been identified and explained. Inspection onshore may reveal the reason for water filling, which can be of value for similar situations for existing installations.

Any member unintentionally flooded in jacket? Cause found?/any cracks detected?

A.6 2 Splash zone (fatigue) Detect sign of fatigue in the horizontal frame in order to calibrate analyse procedures, and in-service inspection methods in order to reduce uncertainties in future assessments.

These areas may be difficult to inspect without use of special expensive equipment/climbers.

It is crucial for the global integrity that main connections in these areas are intact and function according to expectations. As inspections of these areas may be difficult and infrequent due to limited access and high expenses, it is particular important to increase our knowledge on how joint connections in these areas resist the fatigue loads.

In case of subsidence and/or higher design waves (or increased water level), structural members in this area will be exposed to additional loads not originally designed for, i.e. local ‘Morison loads’, wave slamming loads, variable buoyancy due to continuous variable submergence, loads due to wave exit etc.

Platform exposed to subsidence? Any cracks observed in this area in-service? Conductor frame in upper horizontal jacket frame? Fatigue analyses performed in terms of subsidence? Assess if inspection should be done in order to validate/calibrate the recipe

for fatigue damage calculation caused by wave slam and variable buoyancy (e.g. the Ridley method).

Evaluate if one or more joints should examined closer, either at the decommissioning yard or send part(s) of the joint to laboratory for detailed testing (material strength, toughness, fatigue behaviour).

Evaluate if a fracture mechanics analysis should be done to assess the remaining fatigue life under out-of-plane bending.

Inspection/examination of value for life extension?

A.7 2 Mechanical damaged structural members Obtain data on capacity Mechanical damages due to boat impacts, dropped Any known damages caused by a boat impact or a dropped object?

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ID Code Purpose Comment Check list

of damaged structural elements.

objects etc. can reduce the strength of a structural component.

With detailed inspections and mechanical testing the size/shape of the damage and material properties will be determined, which will give valuable data for ultimate capacity analyses.

Full scale capacity tests can be performed in laboratory to validate analytical/numerical capacity analyses, which will give valuable knowledge and increase the confidence to the theoretical analyses.

Any incidents occurred during operation? (any hit by submarine, whales ?) Evaluate if the structural component(s) should be tested in the laboratory?

(size and shape of damage, material properties). Inspection/examination of value for life extension?

A.8 2 Grout reinforced structural components Check the quality of grout and degree of filling to gain experience for selection of grout material and testing for later grout operations.

Grout filling is a common method to i) reinforce a jacket to increase the global ultimate capacity, ii) increase the static or fatigue strength of a joint, iii) increase the member strength of a brace or leg to resist boat impact and iv) reinforce repair clamps.

As the structural analyses assume good quality of the grout as well as complete filling, it is crucial that the grout functions as intended. Several examinations made onshore of grout reinforced braces, legs and clamps have revealed that this is not always fulfilled.

Experience feedback will be of value for future grouting operations.

Any structural component grouted during in-service? (leg, brace, joint, clamp,..)

Why was the structural component grouted? What type of grout was used? What type of testing was done onshore prior to grouting offshore? Specifications available? Any findings in grouted components offshore? Inspection/examination of value for life extension?

A.9 2 Pile to topside connections/condition of splice connections

Check condition. How have these connections worked?

Examine damages in the pile to topsides connections. Cracks in the shim plate welds have been observed in the past.

Any damage observed in these areas during in-service? Previous reassessments revealed high utilisation in these areas? Subsidence/wave-in-deck during in-service? Deck elevated? Inspection/examination of value for life extension?

A.10 2 Ring stiffened joints Check the condition of the inner ring stiffeners, with the objective to verify that we can trust our simplified fatigue checks of the stiffener and the weld between stiffener and joint.

Ring stiffeners are key components for confirming joint strength and fatigue performance in life extension. The analysis procedures for calculation of fatigue life in the weld to the chord wall and the ring inner edge are old, and rather simplified for the weld connection to the chord wall.

NDT of inner ring stiffeners is very difficult during platform operation.

It will therefore be of value to check the condition of the

Any ring stiffeners in the jacket? Purpose of the ring stiffener? (fatigue, transport/launch, static strength..) Fatigue life of the stiffener itself? Basis for the fatigue calculations original? Any Finite Element analysis done in design or in reassessment work? Any crack indications observed in-service? Dimension of ring stiffeners?

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ID Code Purpose Comment Check list inner ring stiffeners, and to investigate if cracking at welds between stiffeners and joint has taken place.

Confirmation of the validity/goodness of the parametric SCF equations used in design of ring stiffened joints is needed.

A.11 2 Single sided welds Check the condition of the root area in single sided welds, with the objective to verify that we can trust our simplified fatigue checks of the root area.

Cracks may develop from the root of single sided welds. This is the case when braces are welded directly to the legs without stubs. Such cracks may be difficult to observe during in-service inspection.

The root area of single-sided welded tubular joints may be more critical with respect to fatigue cracks than the outside region connecting the brace to the chord. It is normally recommended that stubs are provided for tubular joints where high fatigue strength is required, such that welding from the backside can be performed.

Failure from the root has been observed at the saddle position of tubular joints where the brace diameter is equal.

Any single sided welds in the jacket? Any braces welded directly to the leg without stubs? Any anomalies made during inspections offshore? Quality of workmanship? Fatigue analysis procedure used in design?

A.12 2 Closure welds Check welds made under difficult conditions in the yard.

Welds made under difficult conditions in the yard, with limited NDT. Possible sites for fatigue cracking in life extension. Weld quality, any cracking present etc.

Any closure welds in structure? Any anomalies/cracks found in closure welds? Weld quality/workmanship?

A.13 2 Inspection of risers and riser supports – structural condition (ref. also B.5).

To get an overview of how riser and riser supports have performed over time to possible improve design and assessment methods.

The riser is a safety critical item on a platform. A visual inspection of most of the risers and riser connections for cracks and damages is recommended.

Suggest to take a look at some risers and clamps and look at how the bolts and bolt coatings have performed (any brittleness developed, corrosion (general or dissimilar metal) and loss of coatings, fatigue damage). We can also look at how well the clamp lining materials have performed, have they become hard or brittle, have they degraded or decomposed? Examine corrosion (internal and external) and other signs of damages in the risers, esp. the bottom ends.

Confer any reassessment reports on risers made during operation. Any risers/riser clamps with heavy corrosion (see B.5)

A.14 2 Inspection of caissons and caisson supports To get an overview of how caissons and

Damages to caissons and caisson supports have been observed during offshore inspections. Caissons have

Any cracks/damages in caisson supports registered in-service? Any incidents, e.g. caisson loosened and fell down?

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ID Code Purpose Comment Check list caisson supports have performed over time to possible improve design and assessment methods.

even loosened, fell down and damaged members beneath.

Any damages in case of dropped caisson? Ref. A.6.

A.15 3 Cast Joints Confirm quality of cast joints.

Early cast materials had limited performance data. Condition of casting, fracture toughness, quality of any repair welding and any associated cracking.

Any cast nodes in the structure? Any reports from fabrication available? Any cracks during casting? If so, any fracture mechanics analysis done?

A.16 3 Materials and welding Knowledge about the steel quality and quality of welding is needed for life extension.

Lack of knowledge of the specification and performance of steels used in early platforms and of the quality of welding. Steel quality and properties are necessary input for life extension.

Fracture toughness of early steels, weld quality and any cracking evident, residual stresses.

It should be considered to select some structural components for laboratory testing to check if material and welding quality is according to the design specifications. Such tests may involve tensile tests, charpy tests, cross section examination, micro structure examination, chemistry, hysteresis tests, fatigue tests, residual stress measurements.

Life extension of any ‘similar’ platforms with same designer/same fabrication yard?

Consider laboratory test of piles with respect to residual stresses and fatigue.

It should (over time) be built up a database to increase our confidence to the steel and weld quality of old installations planned for life extension. (ref. the extensive tests of joints by Total from the decommissioned Frigg platforms). It should be considered to supplement this information in the database.

A.17 3 Material test of highly loaded parts Establish stress/ strain curves to assess possible ageing.

Laboratory tests: Establish stress/ strain curves to assess possible ageing. It is of most interest toward parts that have been highly loaded.

Any areas which have been particular highly loaded? Test part of pile brought onshore?

A.18 3 Measurement of residual stresses Improve the understanding of the residual stresses in a structure that has served for several years in order to remove possible conservatism in the assessment methods.

The magnitude of the residual stresses is of importance for failure modes such as buckling and fatigue. It is uncertain how the residual stress levels develop over time in a dynamically loaded structure.

For example, residual stresses have been measured in a pile from an Ekofisk installation and the results from the laboratory results are reported in Error! Reference source not found.. The findings have direct relevance for how to assess the fatigue damage accumulation in piles.

It would be of interest to extend the experience database

Consider to test a part of pile. See Ref. Error! Reference source not found. for motivation to measure

residual stresses in pile.

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ID Code Purpose Comment Check list with more measurements of residual stresses in piles.

A.19 3 Test of corroded material Establish mechanical data for corroded material to be able to determine capacity of such structures.

Static strength of naturally corroded material requires input that should be based on tests. For fatigue it may be of interest to check SN-curves for structures with free corrosion.

Group B - Corrosion B.1 1 CP system Knowledge of

contingency of the CP-system can be utilised for lifetime extension of other structures. Confirm if the corrosion protection systems have functioned according to the assumptions. Confirm the design assumptions for anode supports.

Detailed inspection of anodes in order to establish remaining capacity of the CP-system and coating condition, and to compare with the input parameters used in design.

Original CP design code? Any areas with more consumption of anodes? Reason for that? Results from potential measurements? Any indications of limited protection? Define representative area for detailed investigation of CP system. Number, location and type of anodes (weight, dimensions) in this area? Results from visual inspection – consumption of anodes in this area? Are there both results from visual inspection and potential measurements in this area or is there a need for gathering some data before offshore removal? How many anodes shall be cut down, cleaned and measured? Where are they located?

B.2 1 Coating Improve knowledge of coating quality.

It is a presumption that coating system is known in order to learn from the inspection.

What type of coating is applied on the structure in submerged zone, splash zone and marine atmospheric zone? Lifetime of the coating systems installed? Any areas with known damages on coating? Select representative areas for detailed inspection (both poor and good quality is of interest).

B.3 1 Coating repairs Evaluate if repair methods are good/poor. Can be used as input on other existing structures.

It is a presumption that coating repair system and procedure is known in order to learn from the inspection.

Have there been any coating repairs or mainteinances? Location? Need for any multiple repairs on some locations? Repair procedure and type of repair coating system?

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ID Code Purpose Comment Check list B.4 1 Splash zone (corrosion)

Establish corrosion rates to be able to document longer life and larger capacity for existing structures.

Corrosion allowance is accounted for in design. An investigation to check if the extent of corrosion (utilisation of the design corrosion allowance) and coating degradation to establish if the corrosion allowance and the coating degradation taken place can justify a lifetime extension (compare with design calculations).

Thickness measurements can be made more efficient onshore.

What is the design corrosion allowance in the splash zone? What is the coating type and thickness in the splash zone? Any areas with known wall thickness reduction?

B.5 2 Risers and riser clamps (corrosion) Establish data on how risers perform with respect to corrosion in order to improve assessment/inspection methods.

Internal corrosion: Establish loss of wall thickness by external ultrasonic measurements and cut out of pipe sections subjected to extensive corrosion for laboratory examination in order to establish the cause of the corrosion (e.g. CO2-corrosion, MIC etc.).

Riser material and materials selection in riser clamps? Type of riser coating? Any coating underneath riser clamp? Any locations of riser clamps in the splash zone? Any locations of field joint of riser coating in the splash zone? Any known areas with wall thickness reduction underneath riser clamps or riser coating?

B.6 2 Waterfilled closed compartments Establish what type of corrosion mitigation that is required internally

Is there a risk for MIC and what type of corrosion mitigation should be carried out in waterfilled legs, braces, piles etc.

What type of water is filled in the closed compartment? Has there been any treatment of the water (e.g. biocide, O2-scavenger etc.)? Any indication of corrosion?

Group C – Inspection Technology C.1 1 Details that have inspection findings from

operation Confirm finding and determine cause of defect

It will be beneficial to inspect details that have been reported with inspection findings to better determine the cause of the defect. It is also noted that some findings reported offshore are false; no finding found onshore. Confirm size of finding

Where are the findings located? Is any of the finding located in non-predicted areas (should be considered interesting)? What is the size of all the findings? What type of inspection method has been used? Time of first finding? Development of defect? Any areas where findings has been detected and repaired? What type of repair? Any development after repair?

C.2 2 Areas of difficult/challenging underwater inspection

Verification of underwater inspection capability

In-service inspection underwater is difficult and limited because of cost and diver time.

Location of challenging underwater inspection? Has any techniques with limited experience been applied? What type of technique? Any findings? Size and location if finding?

C.3 2 Verification of new inspection techniques

New inspection techniques may be tested offshore and verified with onshore inspection

Examples are: - high quality digital photos and videos. - EC inspections with use of ROV

Any new techniques that should be tested? Location with finding where the technique may be tested? Size and locating of finding? Location with no finding where the technique may be tested?

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ID Code Purpose Comment Check list Group D - Other D.1 1 Bolts

Determine how bolts and other fasteners perform over time to improve future specifications.

Fasteners are critical components, and the condition of the bolts is not always possible to establish without dismounting with subsequent inspection. In order to evaluate the integrity of the bolted connection, the following areas are of interest: - Coating degradation (if coating has been applied). - Corrosion due to water ingress in the e.g. thread

area (dimensional measurements). - Extent of crevice corrosion beneath bolt heads/nuts

(determination of extent of corrosion). - Evaluation of materials selection/combinations and

selection of coating method, if applicable.

This to give an estimate of the life time of bolted connections, durability of coating applied; identify areas with high risk for corrosion; identify areas with high fatigue load etc.

Critical bolts identified with extensive damage (e.g. cracks, galling) may be subjected to a more detailed inspection in the laboratory.

What types of bolts are installed on the structure in the different operating conditions (submerged with CP, in splash zone, marine atmospheric zone)? Materials selection? Type of coating (if applicable)? Any bolts with findings? Any locations where bolts have been replaces (how many times)? Any bolts located in especially highly utilised areas and/or critical components? Any bolts located in unfavourable design?

D.2 2 Repair clamps Increase understanding of the performance of repair clamps in order to design efficient clamps in the future and to prolong the life of existing clamps.

Significant lack of data on the condition of such repairs after installation. Lack of NDE data. Continuing good performance of repairs necessary for life extension.

Overall condition of repair, extent of grouting (if any), bond to steelwork, cracking at welds, if repair neoprene lined condition of liner.

Method of repair clamp relevant for other structures? Location of repair clamp? Reason for installation of clamp? Procedure for installation of clamp? How is the expected condition of the clamp in order to ensure that it fulfilled its intention?

D.3 2 Marine growth

Validate or revise the recommendations given in Norsok standard N-003.

Measurements of marine growth have previously been undertaken from installations brought onshore. Heavily marine growth (hard) has been observed.

If marine growth is removed offshore, may be some measurements can be made prior to removal.

Any areas where thickness of with marine growth is expected to be significant different from NORSOK N-003? Location?

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Documents

Report 2012_3363_Guideline for Inspection of Decommissioned Offshore Structures