Design-life extension of deepwater fields requires surveillance of critical subsea components By Christian Bucherie & François Migeon : Bureau Veritas / Vincent Alliot: Schlumberger

Abstract:

The first deepwater fields of the early 2000s are approaching the midterm of their design life. Tiebacks of peripheral reservoirs to existing surface facilities may encourage operators to extend the operation of their subsea assets beyond their original design. This was the case in the 1990s in the North Sea, where steel jackets and platform structures had been submitted to extreme environmental load conditions. This paper briefly reviews some aspects of the recertification processes for those fixed-jacket structures. The lessons learned can be used now to develop robust methodologies for recertification of deepwater assets. A surveillance plan is needed for subsea components subjected to fatigue, material aging, and corrosion that will enable prediction of the actual life with an acceptable degree of confidence. Although such a plan relies on the results of an inspection, maintenance, and repair program, it also requires site data collected through dedicated instrumentation. This information enables reassessment of the asset status based on accurate real-load scenarios rather than conservative engineering assumptions. The surveillance plan and associated reengineering process should recommend replacement of certain components and validate the requested extension, which in itself justifies the investment cost.

I. EXPERIENCE OF LIFE EXTENSION OF FIXED OFFSHORE STRUCTURES

Before discussing the life extension of deepwater facilities, we present a review of how the operating lives of offshore structures designed and installed in the 1970s have been extended through reassessment and recertification by Bureau Veritas (BV) (Figure 1).

1970 1980 1990 2000 2010

1994: First recertification

1974: First certification

Service life life extension

Figure 1: Certification timeline

When a request is made for an aging platform to remain in service longer than its design life, a comprehensive reappraisal of the platform conditions is necessary; this must be carried out before any life extension is granted and a certificate of fitness is reissued. The condition reappraisal gives the owner and certifying authority an up-to-date and state-of-the-art picture of the platform characteristics and condition with respect to data assumptions and methods used at the build time. This picture should provide adequate backup for a sound appraisal of the fitness-for-purpose and of the in-service inspections to be performed for the forthcoming extended life period. As a result, this recertification process must be carried out in two stages:

Establish and implement an asset integrity management policy and process for the operating life of the structure.

Reassess the structure integrity through reengineering and in light of its operating history.

A. ASSET INTEGRITY MANAGEMENT FOR MONITORING FIXED AND FLOATING OFFSHORE

STRUCTURES

Generally, the project and offshore operating teams jointly develop the asset integrity management process to meet the corporate policy implemented throughout the organization. To support such an approach, BV has developed services to assist day-to-day structural integrity management (SIM) in accordance with API RP 2 SIM.

Typically for a jacket structure, risk-based criticality rankings of underwater joints are provided to assist the management of the facilities in making appropriate decisions concerning the inspection, maintenance, and repair program. A tool based on API SIM and compatible with certification and other BV tools is provided for reporting of anomalies. The whole process is based on a semi quantitative assessment and can be summarized as shown in Figure 2.

Data Evaluation Strategy Program

Detailed work scopes for inspection activities and offshore execution to obtain quality data

Overall inspection philosophy and strategy and criteria for in-service inspection

Evaluation of structural integrity and fitness for purpose; development of remedial actions

Managed system for archive and retrieval of SIM data and other pertinent records

The data Figure 2: SIM process

Collected data is required to adequately assess the integrity of the structure and validate the life extension; it is managed using dedicated tools. For example, a criticality ranking is allocated to each underwater joint of a fixed jacket based on the standard method of evaluating the likelihood of failure versus its consequences. The structural design is also reviewed to consider design parameter variations including water depth, weight increase due to concretion, soil characteristics, actual environmental loads condition, etc. BV has developed dedicated tools to assist in establishing a reliable inspection, maintenance, and repair program. Software obtained through the BV SIM engine (Figure 3) facilitates the computation of collected data and risk evaluation to produce useful information for managing the program.

General design data As-built

documentation Inspection history Structural analysis

with Finite Element model

SIM Engine

Risk Matrix Inspection Frequency Inspection

CAMPAIGN 2.2 Inspection results assessment

2.1 Inspection planning

1.2 SIM Evaluation

1.1 Data Collection

Figure 3: BV software computation process

BV, as an FPSO/FSO classification society, has also developed dedicated tools for managing asset integrity of floaters (Figure 4):

Figure 4: BV floater asset integrity management tools

B. DESIGN-LIFE-EXTENSION APPRAISAL PROCESS

The appraisal is an extremely critical process; without it, there may be serious financial consequences or unacceptable risk exposure for the facilities. For this reason, a rigorous method must be implemented to correctly assess the asset integrity status and identify the measures to extend the design life of the facilities. Keeping the operations of the field efficient and economically viable, without compromising the safety, must be the main driver of a life extension. BV uses a reappraisal process that includes four major phases (Figure 5):

Phase 1 SCREENING

- Platform inventory - Site conditions

Phase 2 RE-ANALYSIS

- Inplace - Fatigue

Phase 3 SURVEY

PROGRAM

Phase 4 CONCLUSIONS

Figure 5: Reappraisal process

An outline of the activities for each phase is given in the following sections. Similar to the SIM described above, the life cycle management would include, for each “hot spot” in the risk matrix, the acquisition of data, the reanalysis to evaluate effects from data modified since previous acquisition, the impact of results on risk, the actions to take (on the asset and on the inspection program, planning the data acquisition campaign, carrying out the campaign, etc.). Equipment that are particularly complex or that are “black box” because of confidentiality issues require particular FMECA (Failure mode, effects, and criticality analysis) and/or RAM

(Reliability, Availability, Maintenance) analysis aimed at identifying most likely failure modes:

First, give particular attention to quality during design, fabrication, control, test, and installation phases.

Second, determine most useful ways to inspect and/or monitor while in service. Supplier manuals and recommendations are to be strictly followed. Connectors (both mechanical and electrical) need particular attention.

a. Screening and Inventory

General

The SIM put in place by the operator will greatly facilitate this process because it will contain most of the information required to make the assessment. The inventory will consist mainly of data collected from periodical inspection reports, site work reports, initial building files (as-built), and other available documents, complemented as needed by site information such as pictures or data collected from dedicated instruments.

Platform Arrangement

Inventory should cover all aspects affecting environmental and gravity loads and should include

initial design data and fabrication records historical record of addition, change, removal of items, loads, etc. present situation (for topsides, this will generally imply a reassessment of loads).

Platform Conditions

The inventory should give an overview of past in-service inspections and outstanding findings in areas such as marine growth thickness and extent. Records of cleaning operations; scour observations; records of dumping operations, damages, accidents and subsequent repairs should also be included.

Site Conditions

For environmental and soil conditions, the inventory must include initial reports and related design data, and recent data that may be used to update or to complement initial data:

measurement of water depth, in particular the relative level between platform and sea reference level used to revise uncertainties and possible variations

measurements of wave and other environmental data at the platform or a nearby site to improve the accuracy of initial measurement, typically taken when no platform was on site or obtained by extrapolation (if unavailable, employ a specialist)

soil investigations at any adjacent platform built later–where more recent techniques of investigations and analyses may be available.

Figure 6: Platform conditions

Previous Analyses

A summary of initial design analyses and any later analyses is needed, including dates, purpose, type of analysis, methods used, design parameters, and other features of interest.

Future of the Platform

The operator defines the proposed plan for the future of the platform: life extension platform use (functions, manning, etc.) and modifications related to platform

arrangement and loads, including additions (e.g., platform upgrading) or provisions for such, and intended removals (e.g., conductors, boat-landing or other appurtenances no longer used)

maintenance program (e.g., marine growth cleaning criteria).

Results of the Inventory

The results of the inventory must be made available to BV for review at the earliest time possible, together with the operator’s proposed initial plan for the subsequent phases. These should be regularly updated, as relevant, together with the progress of the following phases.

b. Reanalysis

Reanalyses of the platform, as illustrated in Figure 7, should provide an up-to-date picture of the platform strength, which will be used as basis

for the special survey confirm the suitability of the structures for a life extension update data for later inspections during the life-extension period.

Reanalyses should be based on the results of inventory, the plan for platform usage, and the maintenance program so that they fully consider the actual platform arrangement, any modifications found necessary from a first-pass analysis, up-to-date design parameters, and actual structural conditions (nonrepaired damages, etc.).

Analyses normally cover reassessment of the platform air-gap, topsides load distribution on its supports, in-place strength of substructure and of the integrated deck primary framing, capacity of foundations, and fatigue strength of substructure.

PUSHOVER

PLASTIFICATION

GLOBAL & LOCAL STABILITY

JOINT STRENGTH

FAILURE MODES TO BE ACCOUNTED FOR JACKET PUSHOVER

ANALYSIS

FOUNDATION STRENGTH

RECERTIFICATION BASED ON PUSHOVER ANALYSES

Phase 2 REANALYSIS Phase 1

SCREENING - Platform inventory

- Site conditions

In-place analysis Fatigue analysis

Pushover analysis Higher analysis level

CONVENTIONAL CHECKS (Storm and Input data conditions)

MEMBERS CHECK

FOUNDATIONS CHECK

CONNECTIONS CHECK

FOUNDATION CRITERION

CRITERIA FOR SAFETY FACTOR EVALUATION OF

EACH COMPONENT

RECERTIFICATION BASED ON CONVENTIONAL ANALYSES

YIELD CRITERION

PUNCHING CRITERION

BUCKLING CRITERION

Figure 7: Reanalysis process

If needed, conventional analysis may be complemented by more advanced methods (e. g., pushover analysis and fatigue crack propagation) and reliability methods, if properly documented and backed by conventional analysis. Prior to reengineering, operator specifications must provide a clear picture of all design data and parameters in relation to the inventory, and the intended content of reanalysis. The conclusions of the reanalysis should outline the critical (or most critical) features of platform strength and areas on which a special survey should focus, the proposed course of actions to clarify the inadequacies revealed by analysis (if any), and salient aspects for consideration at time of later inspections. For fixed-jacket and offshore deck structures or floaters, reanalysis relies mainly on numerical models that are correctly calibrated. This approach is acceptable because these

models have been checked through extensive use and independent verification. With correct input data, the results are reliable and sufficiently accurate even for fatigue analysis. The analysis will assess the behavior of the structure and may simulate operations under impaired conditions, with some element failure. The operator reviews this type of information before making a final decision on whether the asset has to be replaced, repaired, or upgraded.

c. Special Survey

A special survey inspection should be performed after the reanalysis; the inspection should be guided by the reanalysis results and used to complete the evaluation of platform condition and to correlate its actual condition with the survey results.

Figure 8: Reinspection of platform

The inspection shall include scour, marine growth, cathodic protection, local damage, straightness, flooded members, pipe thickness and corrosion, riser attachment, anode depletion and attachment, close inspection, and nondestructive testing of selected welds at nodes. Consideration is given to

areas with earlier cracks, unresolved indications, or other recorded anomalies areas determined as critical by analysis areas not covered by earlier inspections, as relevant from results of analysis.

An inspection program prepared by the operator will define the technical methods and locations for the various inspections and must also include provision of random inspection in areas to be defined on-the-spot by the attending surveyor. The BV survey normally includes attendance to the major inspection operations, as found necessary by the surveyor, and a review of inspection records and reports. Instrumentation, such as strain gauges, can also be installed at specific member location for a limited time to record stress induced to the structure by environmental and operating loads. The collected data are compared with the numerical model and will increase confidence in the numerical analysis.

d. Conclusions

From the results of the three phases just described, the operator will prepare conclusions concerning the fitness of the platform with respect to a life extension. These will specify relevant platform data (including future provision) for which the life extension is anticipated and a checklist of work to be carried out for that purpose. A new long-term inspection program should be prepared by the operator based on the results of evaluations and above conclusions. If BV approves the operator conclusions, the agreement for a life extension will be confirmed by a new certificate of design, the issue of a certificate of fitness for the next period (5 years maximum), and the approval of the inspection program.

II. SPECIFICS OF DEEPWATER OIL AND GAS ASSETS

A deepwater environment is defined by the existence of floating support structures with compliant dynamic riser structures to transfer fluid, communication, and energy between seabed hardware and surface process facilities. Going deeper makes the offshore operations more expensive and interventions more risky. Offshore components are normally subjected to a strict QA and QC program by manufacturers and installers, as well as to close scrutiny by the purchaser and operator, especially when components lack track records in the deepwater environment. The field component configuration and the marine environment generate high-frequency cycling loads on the structures involved. These necessitate the development of sophisticated and more complex (compared to that for a fixed platform) numerical model to predict the fatigue damage ratio that will be experienced by the structures. Results of fatigue analyses are extremely sensitive to the number of cycles, intensity of the load variation, and quality of the materials or welds involved in their fabrication. To cover some of these uncertainties, design codes recommend using a high safety coefficient (up to 10 for dynamic risers or even more for pipelines transporting fluids with high levels of corrosive content such as H2S or CO2). Because of protection or insulation coating and sometimes a lack of easy access, these structures are often more difficult to inspect than jacket structures. Therefore, the fatigue assessment can no longer rely only on a survey campaign. The conservative approach of using a high safety coefficient is quite satisfactory to cover the design life but inevitably raises the question of how to address life extension. For these reasons, the exact methodology used for the jacket structure cannot be followed for dynamic compliant structures linking the surface facilities to the seabed infrastructure. This is also true for subsea manifolds, PLEMS, piping, connectors, flowlines, risers (rigid or flexible), or umbilicals. For these components, a single failure at any point will most probably result in leakage or a complete lack of tightness or function, hence leading to a

quick halt to operation and loss of production or expensive downtime. In contrast, simple steel systems are efficiently protected by corrosion management (inhibitors, coating, cathodic protection). The risks of structure failure influence the decision of monitoring the behavior of the structures and the performance deterioration of the subsea components to allow efficient preventive maintenance. For fixed-jacket structures it is possible to inspect and detect defects, cracks, etc. The challenge in deepwater fields is to achieve access to monitor for structure deformations, displacements, vibrations, high stresses, and unforeseen loads or stresses before events can occur. It is essential, through careful design studies, to identify the most sensitive elements of a structure and hot spots exposed to risk so that proper monitoring measures can be put into place as part of the design process. Farming-in or relaunching engineering analyses to create the most useful, efficient inspection or monitoring program are an option. Similarly, particular risks of instability (upheaval buckling, lateral buckling, walking, etc.) or failure under pressure and temperature expansion cyclic effects can be analyzed through dedicated commercial simulators. These support decisions on stabilization or particular arrangements to mitigate the effects of such phenomena.

Flexible Risers Systems

Figure 9: Flexible riser system

Prior to start an assessment of life extension fro flexible riser the following information produced at the design stage needs to be collected and reviewed:

identify limit states verify safety factors validate designs

and/or assessed life extension requests. The assessment of life extension for flexible riser is typically based on

provision of initial flexible design dossier review of detailed end-fitting drawings (generally confidential) at manufacturer’s

premises confirmation of the original design’s basis assumptions, in particular the meteocean

data provision of historical data regarding flexible pipe operation: internal pressure,

temperature, fluid composition, and shutdown records reassessment, as necessary, by manufacturer of global configuration for fatigue

damage in steel wire reinforcements and aging of plastic layers, plus independent analyses as needed

replacement of depleted anodes and removal of excessive marine growth provision of survey and monitored data from dedicated instrumentation inspection reports, including video files of inspections by remotely operated

vehicles. This requires the provision of an updated inspection and maintenance program which includes annual visual inspection or annulus test, installation of a cooling water system for the production riser top section, pressure test of riser bore, and pressure test of annulus to confirm no leak of the outer sheath.

III. MONITORING EQUIPMENT TO EXTEND OPERATING LIFE OF SUBSEA ASSETS

A structural integrity assessment for a deepwater asset relies on complex numerical models that consider the dynamic behavior of floating and compliant structures. These models are generally difficult to calibrate without feedback from onsite data, and design engineers generally have to integrate some conservatism to estimate an acceptable level of performance throughout the design life of the structure. Dedicated monitoring instrumentation can either be installed at the project development stage or be retrofitted during the operating life of the asset.

A. DYNAMIC FLEXIBLE RISERS: LIFE-EXTENSION PROCESS

The significant cost of replacement makes the dynamic flexible risers particularly good targets for assessment of life extension. As discussed in the techniques for fixed structures, it is important to analyze critical asset elements and their associated failure modes. Technical systems are then considered for monitoring and recording the conditions of these elements over time. In the case of a flexible system, the submerged pipe structure and top section are particularly vulnerable to corrosion and adverse load conditions.

The flexible riser’s annulus–between the inner carcase and the outer sheath–is subjected to material deterioration resulting from a combination of fatigue and corrosion; the latter is due to seawater (outer sheath rupture) or condensation water or sour products (H2S or CO2). The flexible system’s top section–from the end fitting to the tip of the bend stiffener–is potentially subjected to armour wire breakage resulting from extreme load conditions or fatigue.

a. Armour wire breakage: Monitoring solutions

A nonintrusive sensor has been developed that can be retrofitted on a flexible pipe to detect wire breakages. The system is based on the principle that tensile armour is the main contributor to the axial and torsion stiffness of an unbonded flexible pipe. As a result, broken tensile armour wires generate transient and permanent axial elongation and torsion twist in the pipe structure that can be measured by strain gauges. The system consists of integrating fiber optics inside a composite material designed and shaped as a clamp to fit the geometry of the flexible riser, with Bragg gratings acting as an optical strain gauge. The technology provides strain measurements of pipe deformation along a specific direction, which are then translated into the torsion and elongation values by appropriate postprocessing software (Figure 10).

Figure 10: Armour wire breakage detection system

Full-scale tests have validated the measurement in several configurations. The equipment is particularly suited for retrofit solutions in which monitoring is required after the flexible riser has been designed, fabricated, and installed.

b. Fatigue at Bend stiffener location: Monitoring solutions

A dynamic riser design requires careful assessment of the bending in cases of extreme and fatigue loading. The locations sensitive to the bending are dependent on the riser configuration and environmental loading from external and internal sources. Flexible riser is normally supported with a polyurethane bend stiffener or rigid bellmouth close to the splash zone. Monitoring the bending at this location (highest bending moment) allows removal of the design assumptions and thus the assumptions made to increase the production life of the riser (Figure 11).

Figure 11: Bending moment evolution across bend stiffener

Figure 12 illustrates a system can be either strapped around the bend stiffener or located at the tip of the bend stiffener to measure the bending moment experienced by the bend stiffener and, by computation, calculate the effective bending moment seen by the flexible pipe.

Figure 12: Monitoring system to be placed at the tip of the bend stiffener

c. Flexible riser annulus

A flooded annulus can lead to excessive corrosion and reduce the fatigue life of the armour layers. To assess the integrity of the riser, an automated annulus surveillance system has been developed that combines controlled venting of the annulus gas with pressure, temperature, and flowrate measurements for independent calculations of the diffusion rate of gas and the volume of liquid (i.e., vapor condensation, water ingress) that may have entered the annulus. The data collected are used to reengineer the fatigue damage ratio experienced by the structure throughout its operational life. Data such as the nature and quantity of the gas to which the internal structure was exposed allow reassessment of the impact of the corrosion on the structure and redefinition of the fatigue damage ratio. With this information, the operator specialist can more precisely assess the remaining design life. This system addresses the shortcomings of costly conventional annulus vent–gas monitoring, which does not provide reliable information on gas flow rates or water vapor emissions and is intermittent by nature. The automated system offers a real-time solution that delivers detailed information for integrity assessments, eliminating the need for vacuum tests. Flow measurements and interpretation of the reservoir are applied to the annulus to obtain detailed information about fluid content and connectivity.

Figure 13: Riser annulus condition surveillance integration onboard FPSO

Figure 14: Interpretation of riser surveillance data

d. Distributed sensor

A distributed fiber-optic sensor offer an accurate means to monitor aging or performance drift of an asset, as well as to provide a temperature profile of a pipeline or a riser. Technip and Schlumberger have developed a method for integrating an optical fiber within a flexible structure (Figure 15) at the design stage; this is done by incorporating small tubing inside the

flexible armors. The fiber-optic sensor is then pumped inside the tubing once the flexible riser is fully commissioned (Figure 16). Although the temperature data is primarily used for flow assurance purposes, it delivers information on the status of some parameters that impact the thermal behavior of the structure. Ageing or failure of the inner component of a flexible riser will likely create a change in the way thermal flux travels between the inner pipe and the external sheath. Optional alarms can signal the operational teams aboard the facilities when a

redefined change in temperature in a specific location occurs.

Figure 15: Principle for integrating a fiber-optic sensor inside a flexible riser

Figure 16: Typical temperature profile from a

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IV. REFERENCES

International Standard Organization (ISO) 19902, Petroleum an

Petronas Technical Specification, 1987, Practice for Assessment of the Structural Integrity of Existing Fixed Structure, PTS 20.063, May.

OTC-2010 20980: Subsea sensors for non intrusive monitoring of temperature, pressure and asset integrity by Guillaume Thivend Schlumberger.

Rio Oil and Gas Conference 2010: IBP3466_10 INTEGRITY MANAGEMENT THROUGH DISTRIBUTED TECHNOLOGIES INTEGRATED WITHIN FLEXIBLE RISERS by Layla El Hares Schlumberger, Patrick Lestanc Technip, Hugues Corrignan Technip