DNV RP C103 Column Stabilized Units

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RECOMMENDED PRACTICE

DET NORSKE VERITAS AS

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DNV-RP-C103

Column-Stabilised UnitsAPRIL 2012


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Det Norske Veritas AS April 2012

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FOREWORDDET NORSKE VERITAS (DNV) is an autonomous and independent foundation with the objectives of safeguarding life,

property and the environment, at sea and onshore. DNV undertakes classification, certification, and other verification andconsultancy services relating to quality of ships, offshore units and installations, and onshore industries worldwide, andcarries out research in relation to these functions.

DNV service documents consist of among others the following types of documents: Service Specifications. Procedural requirements.

Standards. Technical requirements. Recommended Practices. Guidance.

The Standards and Recommended Practices are offered within the following areas:A) Qualification, Quality and Safety MethodologyB) Materials TechnologyC) StructuresD) SystemsE) Special FacilitiesF) Pipelines and RisersG) Asset OperationH) Marine Operations

J) Cleaner EnergyO) Subsea Systems


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Recommended Practice DNV-RP-C103, April 2012Changes Page 3

CHANGES

GeneralThis document supersedes DNV-RP-C103, October 2010.

Text affected by the main changes in this edition is highlighted in red colour. However, if the changes involve

a whole chapter, section or sub-section, normally only the title will be in red colour.

Main changes

General Updates regarding wave calculation requirements, reference changes and clarifications in text.

Sec.2 2.2.2 and 2.2.3 Maximum wave height removed and replaced by significant wave height. 2.2.2 Formula for waveheight adjusted to be in function of the hundred year wave- H 100 . 2.3.3 Contingency factor of 1.3 for ULS, when a Morison model is utilised, is adjusted to 1.1 in accordance

with DNV-OS-C103. 2.3.6 Adjusted wave asymmetric factor for certain calculations. Sec.4

4.3 Contingency factor of 1.2 for ULS, when a Morison model is utilised, is adjusted to 1.1 in accordancewith DNV-OS-C103.



Recommended Practice DNV-RP-C103, April 2012Contents Page 4

CONTENTS

1. THE COLUMN-STABILISED UNIT ................................................................................. 61.1 Introduction...........................................................................................................................................61.2 Important concept differences.............................................................................................................6

1.3 Design principles and parameters .......................................................................................................61.3.1 General....................................................................................................................................................................61.3.2 Design conditions ...................................................................................................................................................71.3.3 Limit states .............................................................................................................................................................7

1.4 Abbreviations ........................................................................................................................................7

2. ENVIRONMENTAL CONDITIONS AND LOADS.......................................................... 82.1 Introduction...........................................................................................................................................82.2 Environmental conditions ....................................................................................................................82.2.1 General....................................................................................................................................................................82.2.2 Regular wave parameters........................................................................................................................................ 82.2.3 Irregular wave parameters ......................................................................................................................................92.2.4 Extreme wave data................................................................................................................................................ 10

2.2.5 Wave theory.......................................................................................................................................................... 112.2.6 Long term description of the sea .......................................................................................................................... 112.2.7 Wave energy spreading function .......................................................................................................................... 11

2.3 Wave loads...........................................................................................................................................122.3.1 Worldwide operation ............................................................................................................................................122.3.2 Site specific harsh environment............................................................................................................................ 122.3.3 Benign waters ....................................................................................................................................................... 122.3.4 Morison equation..................................................................................................................................................122.3.5 Linearisation of the drag force.............................................................................................................................. 122.3.6 Asymmetry factor in air gap calculation ............................................................................................................. 12

3. DESIGN LOADS................................................................................................................. 123.1 Introduction.........................................................................................................................................12

3.2 Loads to be applied in global and local models................................................................................133.3 Permanent loads (G)...........................................................................................................................133.4 Variable functional loads (Q).............................................................................................................133.5 Environmental loads (E) ....................................................................................................................133.5.1 General.................................................................................................................................................................. 133.5.2 Current loads......................................................................................................................................................... 133.5.3 Wind loads............................................................................................................................................................ 133.5.4 Pressure height for sea pressure............................................................................................................................ 133.5.5 Reaction forces of anchor lines and thrusters....................................................................................................... 14

3.6 Accidental loads (A)............................................................................................................................143.7 Deformation loads (D) ........................................................................................................................143.8 Tank pressures and sea pressures .....................................................................................................143.8.1 General.................................................................................................................................................................. 143.8.2 Tank pressures, ULS............................................................................................................................................. 153.8.3 Sea pressures, ULS............................................................................................................................................... 163.8.4 Sea pressure at wave trough, ULS........................................................................................................................163.8.5 Sea pressures, ALS heeled condition ................................................................................................................... 173.8.6 Combination of tank and sea pressures ................................................................................................................ 18

4. GLOBAL RESPONSE ANALYSIS................................................................................... 194.1 Introduction.........................................................................................................................................194.2 Stochastic analysis...............................................................................................................................204.3 Design wave analysis...........................................................................................................................204.4 Simplified design wave analysis.........................................................................................................22

4.5 Methods and models ...........................................................................................................................224.6 Characteristic global hydrodynamic responses ...............................................................................234.6.1 General.................................................................................................................................................................. 234.6.2 Split force between pontoons, F S ......................................................................................................................... 234.6.3 Torsion moment, M t ............................................................................................................................................. 24



Recommended Practice DNV-RP-C103, April 2012Contents Page 5

4.6.4 Longitudinal shear force between the pontoons, F L ............................................................................................. 264.6.5 Longitudinal acceleration of deck mass ............................................................................................................... 274.6.6 Transverse acceleration of deck mass ..................................................................................................................284.6.7 Vertical acceleration of deck mass .......................................................................................................................294.6.8 Vertical wave bending moment on the pontoon................................................................................................... 29

4.7 Characteristic global static responses ...............................................................................................304.8 Global structural model .....................................................................................................................314.8.1 Types of global structural model .......................................................................................................................... 314.8.2 Assessment of single model solutions ..................................................................................................................324.8.3 Mass modelling..................................................................................................................................................... 324.8.4 Boundary conditions............................................................................................................................................. 33

5. LOCAL STRUCTURAL ANALYSES AND STRENGTH CRITERIA ........................ 345.1 Introduction.........................................................................................................................................345.2 Local structural analyses....................................................................................................................345.2.1 General.................................................................................................................................................................. 345.2.2 Pontoons ...............................................................................................................................................................355.2.3 Columns................................................................................................................................................................ 355.2.4 Pontoon/column and column/deck intersections .................................................................................................. 365.2.5 Brace to column connection (twin pontoon units)................................................................................................ 365.2.6 Deck structure.......................................................................................................................................................365.2.7 Damaged structure................................................................................................................................................ 385.3 Superimposing responses ...................................................................................................................395.4 Strength criteria ..................................................................................................................................405.4.1 General.................................................................................................................................................................. 405.4.2 Excessive yielding ................................................................................................................................................ 405.4.3 Buckling................................................................................................................................................................ 405.4.4 Brittle fracture.......................................................................................................................................................405.4.5 Fatigue .................................................................................................................................................................. 40

6. MISCELLANEOUS ............................................................................................................ 416.1 Support of mooring equipment .........................................................................................................416.1.1 General.................................................................................................................................................................. 416.1.2 Design loads ......................................................................................................................................................... 416.1.3 Horizontal and vertical design angles for the fairlead supports ........................................................................... 426.2 Wave slamming...................................................................................................................................426.3 Air gap .................................................................................................................................................426.4 Transit condition.................................................................................................................................436.4.1 General.................................................................................................................................................................. 436.4.2 Transit analysis ..................................................................................................................................................... 436.4.3 Wave loads ........................................................................................................................................................... 43



Recommended Practice DNV-RP-C103, April 2012Sec.1. The Column-Stabilised Unit Page 6

1. The Column-Stabilised Unit1.1 IntroductionThis Recommended Practice (RP) presents recommendations for the strength analyses of main structures of column-stabilised units.The design principles, overall requirements, and guidelines for the structural design of column-stabilised unitsare given in the DNV Offshore Standards:

DNV-OS-C101 Design of Offshore Steel Structures,General (LRFD method)

DNV-OS-C103 Structural Design of Column-stabilised Units (LRFD method) DNV-OS-C201 Design of Offshore Units (WSD method).

LRFD is use of the load and resistance factor design method. WSD is use of the working stress design method.Details given in this RP are referring to the LRFD method.The design principles with use of the LRFD method are described in DNV-OS-C101 Sec.2 with different limitstates. A limit state is a condition beyond which a structure or part of a structure exceeds a specified designrequirement. Design by the LRFD method is a design method by which the target component safety level isobtained by applying load and resistance factors to characteristic reference values of loads (load effects) andstructural resistance.The general principles described in this RP may also be applied with use of the WSD method. The design

principles with use of the WSD method are described in DNV-OS-C201 Sec.2, with design conditionsdescribed by different modes of operation (draughts) or phases during the life of the unit. The following designconditions shall normally be considered: installation, operating, survival, transit, accidental, and damaged.

1.2 Important concept differencesThe methods outlined in this RP are mainly developed for the analyses of twin pontoon units and ring pontoonunits. Consequently this should be taken into account when other concepts are considered.Ring pontoon designs normally have one continuous lower hull (pontoons and nodes) supporting 4-8 verticalcolumns. The vertical columns are supporting the upper hull (deck).Twin pontoon designs normally have two lower hulls (pontoons), each supporting 2-4 vertical columns. The4-8 vertical columns are supporting the upper hull (deck). In addition the unit may be strengthened with

diagonal braces supporting the deck and horizontal braces connecting the pontoons or columns.There are basically two ways of keeping the unit in position:

mooring by anchor lines (passive mooring system) dynamic positioning by thrusters (active mooring system).

A combination of these methods may also be utilised.The units are normally designed to serve at least one of the following functions:

production drilling accommodation special services (e.g. diving support vessel, general service, pipe laying vessel, etc.).

Units intended to follow normal inspection requirements according to class requirements, i.e. typically drillingunits with inspection in sheltered waters or dry dock every 4-5 years, shall be designed with the design fatiguelife equal to the service life, minimum 20 years, as given in DNV-OS-C103 Sec.5.Units intended to stay on location for prolonged period, i.e. typically production units without plannedinspections in sheltered water or dry dock, shall also comply with the requirements given in DNV-OS-C103Appendix A. These supplementary requirements for permanently installed units are related to:

site specific environmental criteria inspection and maintenance fatigue.

1.3 Design principles and parameters

1.3.1 GeneralStructures and elements thereof, shall possess ductile resistance unless the specified purpose requiresotherwise.Structural connections are, in general, to be designed with the aim to minimise stress concentrations and reducecomplex stress flow patterns.



Recommended Practice DNV-RP-C103, April 2012Sec.1. The Column-Stabilised Unit Page 7

Structural strength shall be evaluated considering all relevant, realistic load conditions and combinations.Scantlings shall be determined on the basis of criteria that combine, in a rational manner, the effects of relevantglobal and local responses for each individual structural element.

1.3.2 Design conditions

Different modes of operation of a column-stabilised unit are usually characterised in terms of designconditions. Changes in the design conditions of a column-stabilised unit are usually accompanied bysignificant changes in draught, ballast, riser connections, mooring line tension, or distance from an adjacent

platform, etc. Limited variation of some of these parameters may be contained within a specific designcondition, so the definition of design conditions is to some extent an arbitrary choice by the designer, arrangedto cover all relevant combinations in a systematic and convenient way.

A typical set of design conditions is listed in Table 1-1. All relevant design criteria must be checked andsatisfied for each design condition. The design criteria are expressed in terms of limit states.

The designer will normally specify a limited range of environmental conditions for some of the designconditions. These limitations must be clearly documented in the design analysis and in the operational manual.It is the duty of the operator to carefully adhere to these limitations, so that they may also be applied in design.

1.3.3 Limit states

A limit state formulation is used to express a design criterion in a mathematical form. The limit state functiondefines the boundary between fulfilment and contravention of the design criteria. This is usually expressed byan inequality, as in DNV-OS-C101 Sec.2 D201. The design requirement is fulfilled if the inequality is satisfied.The design requirement is contravened if the inequality is not satisfied. The following limit states are includedin the present RP:

Ultimate Limit States (ULS) corresponding to the ultimate resistance for carrying loads

Fatigue Limit States (FLS) related to the possibility of failure due to the effect of cyclic loading

Accidental Limit States (ALS) corresponding to damage to components due to an accidental event or operationalfailure.

Table 1-1 indicates which limit states are usually considered in the various design conditions.

Limiting design criteria for transfer from one mode of operation (draughts) to another mode of operation shall be clearly established and documented. Different modes of operation or phases during the life of a column-stabilised unit may be decisive for the design. For each of the relevant design conditions the design criteria shallinclude relevant consideration of the following items:

intact condition, structural response and strength (ULS, FLS, ALS) damaged condition, structural response and strength (ALS) air gap (ULS) compartmentation and stability requirements (intact (ULS) and damaged (ALS)).

1.4 Abbreviations

Table 1-1 Design conditions and limit states Installation Operating Survival Transit Accidental Damaged

ULS x x x xFLS (x) x (x) (x)ALS x x

ALS Accidental Limit StatesDFF Design Fatigue Factor DNV Det Norske VeritasFLS Fatigue Limit StatesLRFD Load and Resistance Factor DesignRAO Response Amplitude Operator RP Recommended PracticeSCF Stress Concentration Factor ULS Ultimate Limit StatesWSD Working Stress Design



Recommended Practice DNV-RP-C103, April 2012Sec.2. Environmental Conditions and Loads Page 8

2. Environmental Conditions and Loads2.1 IntroductionThe suitability of a column-stabilised unit is dependent on the environmental conditions in the area of theintended operation. A drilling unit may be intended for worldwide operation or operation in a specific region.A production unit may be planned to operate at a specific site. Such a site may be harsh environment or benignwaters.Hence the environmental conditions and environmental loads depend on the area where the unit is intended tooperate. A column-stabilised unit is normally designed for one of the following conditions:

worldwide specific region or site(s).

The environmental conditions with general importance for column-stabilised units are described by a set of parameters for definition of:

waves current wind snow and ice temperature water depth.

The applied environmental conditions should be stated in the design basis/design brief.Typical environmental loads to be considered in the structural design of a column-stabilised unit are:

wave loads, including variable pressure, inertia, wave "run-up", and slamming loads wind loads current loads snow and ice loads.

Due consideration should be made to site specific environmental phenomena such as hurricanes, cyclones etc.Design for worldwide operation shall be based on DNV-RP-C205 with use of the scatter diagram for the NorthAtlantic as given in DNV-RP-C205 App.C. See also 2.2 and DNV-OS-C101 Sec.3 E200, DNV-OS-C103 Sec.4

B100, DNV-OS-C103 Sec.5 B500 and DNV-OS-C103 Appendix B.Design for specific region or site shall be based on specified environmental data for the area(s) the unit shalloperate, see DNV-OS-C101 Sec.3 E and DNV-OS-C103 Appendix A and DNV-OS-C103 Appendix B. The

parameters describing the environmental conditions shall be based on observations from or in the vicinity of the relevant location and on general knowledge about the environmental conditions in the area. DNV-RP-C205may be used as guidance for determination of the environmental loads.

2.2 Environmental conditions

2.2.1 GeneralThe most significant environmental loads for the hulls of column-stabilised units are normally those induced

by waves. In order to establish the characteristic response, the characteristics of waves have to be described indetail. This description may either be based on deterministic design wave methods or on stochastic methodsapplying wave energy spectra.If a design condition is limited to a certain range of environmental conditions, then these limitations are appliedin the evaluation of the environmental loads for that design condition, rather than the actual environmental datafor the site or region. Care should be taken to choose the most unfavourable combination of environmentalconditions from the specified range.The description of waves is related to the method chosen for the response analysis, see Chapters 3 and 4.More details for wave, wind and current conditions are given in DNV-RP-C205.

2.2.2 Regular wave parametersDeterministic methods are used when the sea state is represented by regular waves defined by the parameters:

wave height, H wave period, T.

The reference wave height for a specific location is the 100 year wave, H 100 , defined as the maximum wavewith a return period equal to 100 years.In order to ensure a sufficiently accurate calculation of the maximum response, it may be necessary toinvestigate a range of wave periods.




There is also a limitation of wave steepness. Wave steepness is defined by:

The combinations of wave height and wave period that are considered should imply a value of steepness thatis less than the following limit:

The wave height and period combinations on the steepness limit are given by:

where T is in seconds and H is in metres.The design wave data are represented by the maximum wave height as well as the maximum wave steepness.The wave lengths are selected which are the most critical to the structure or structure part to be investigated.

2.2.3 Irregular wave parametersStochastic analysis methods are used when a representation of the irregular nature of the sea is essential. Aspecific sea state is then described by a wave energy spectrum, which is characterised by the following

parameters:

significant wave height, H s average zero-up-crossing period, T zThe design and analyses for ULS and FLS shall, for worldwide operation, be based the scatter diagrams for the

North Atlantic given in DNV-RP-C205 App.C.An appropriate type of wave spectrum should be used.A Pierson-Moskowitz wave spectrum representing fully developed seas is applicable when the growth of thewaves is not limited by the size of the generation area. Unless the spectrum peak period is close to a major peak in the response transfer function (e.g. a resonance peak) the Pierson-Moskowitz spectrum is assumed to giveacceptable results.The JONSWAP wave spectrum is a peak enhanced Pierson-Moskowitz spectrum and takes into account theimbalance of energy flow in a sea state when the waves are in the process of growing under strong winds; i.e.the seas are not fully developed. This is the case for extreme wave conditions in the North Sea. The JONSWAPwave spectrum is usually applied for ultimate strength analyses of structures operating in harsh environments.Sea states comprising unidirectional wind waves and swell should be represented by recognised double peakedspectra for example the spectrum proposed by Torsethaugen. If wind waves and swell with different meandirections are critical, then due account of such conditions shall be made.For fatigue analyses, where long term effects are essential, the wave scatter diagram is divided into a finitenumber of sea states, each with a certain probability of occurrence.For extreme response analysis, only sea states comprising waves of extreme height or extreme steepness needto be considered.For design purposes the maximum wave height H max corresponding to the 90% percentile in the extreme valuedistribution is used:

where N is the number of waves in the sea state.

S 2 H

g T 2--------------=

S

17--- for T 6s

1

7 0.93H100----------- T2 36( )+

-------------------------------------------- for T 6s>=

H

0.22T 2 for T 6s

T2

4.5 0.6/H 100 T2 36( )+---------------------------------------------------------- for T 6s>

=

Hma x H s 0.5 ln 1 p1 N----

=




The duration of a storm is of the order of a few hours, and the number of waves will normally be of order 10 3.Consequently:

The steepness of a specific sea state is defined by:

The sea steepness need not be taken greater than the 100 year sea steepness for unrestricted service (worldwideoperation), which normally may be taken as:

Then the significant wave height and period combination on the steepness limit are given by:

For units intended for unrestricted service (worldwide operation) a maximum H s = 17.3 m for 100 year return period will cover most locations.

2.2.4 Extreme wave dataThe 100 year return period is used as the basis for structural analyses in the Ultimate Limit States (ULS).In connection with fatigue analysis (FLS) a return period equal to the required fatigue life is used as the basisfor wave load analysis. The required fatigue life is normally equal to the specified service life of the unit;minimum 20 years for newbuildings with planned periodic inspection in sheltered waters, see DNV-OS-C103Sec.5. For modifications etc. of units in service a reduced fatigue life may be accepted, but should normally not

be less than 15 years.In connection with accidental loads or damaged conditions (ALS) a return period not less than one year is takenas the basis for wave load analysis.If the limiting operating criteria are given as maximum regular or irregular (significant) wave heights only, the100 year wave height steepness should still be considered.The maximum wave height corresponding to a specific return period may be obtained from a wave heightexceedance diagram. If wave height exceedance data are plotted in a log/linear diagram, the resulting curve willin many cases be close to a straight line, see Figure 2-1. Such results are obtained for areas with a homogenouswave climate. Other results may be obtained for areas where the climate is characterised by long periods withcalm weather interrupted by heavy storms of short duration.

Hma x 2.12H s

S s2 H s

g T z 2

----------------=

S s

110------ for T z 6s

115------ for T z 12s

Linear interpolation for 6s < T z 12s

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DNV RP C103 Column Stabilized Units