ABS Group’s Brian Gibbs and David Hua discuss long-term strategies to extend the service life of aging offshore assets.
Structural integrity is critical to sustaining stable, reliable, and optimal performance.
As more floating structures near the end of their original design lives, asset owners are evaluating ways to rehabilitate and safely extend service life to avoid nonproductive downtime. A number of service providers offer full life cycle asset integrity management (AIM) programs to facilitate early-stage proactive measures to protect the asset. An effective AIM system is critical to preserving the condition and operability of assets while maintaining compliance with regulatory requirements.
ABS Group develops work scopes, manages in-service inspection programs (ISIPs), and acts as a customer representative during facility visits made by the US Coast Guard and classification society. If structural deficiencies are identified, the company serves as technical advisor to provide solutions and obtain regulatory approval for any necessary repairs.
A new time horizon
For FPSOs and other floating production systems, the intended service life typically is 20-25 years. The scantlings of the structure are designed based on this assumption. For an asset to go beyond the originally approved design life, the owner must demonstrate that the asset can be maintained fit-for-service for the new life span. The critical first step in the decision-making process is determining whether it is feasible to extend the asset’s life so it can operate to the new time horizon.
Life extension is an integral part of the asset life cycle.
If the asset, such as a spar or semisubmersible, is conventionally moored, the primary line of investigation and preservation targets the integrity of the hull and mooring systems. There are many components that need to be evaluated during the initial assessment. The evaluation includes the integrity of the primary structure, the extent of corrosion or steel loss, the remaining fatigue life, and in many cases compliance with current design codes as well as the additional loads that have been placed on the structure over time (e.g. process equipment). All of these evaluation criteria are fundamental to future asset integrity and require regulatory approval. In some cases, they also must be approved by a classification society.
Once the condition of the structure is assessed, the means of mitigating identified issues can be developed, with options ranging from enhanced monitoring to structural refit.
To extend the life of a structure, it is necessary to have in-depth knowledge of its condition. Lack of historical operating data is a challenge when assessing an asset’s viability for life extension.
Other challenges include previous damage, deteriorated process equipment, noncompliance with new regulations, a change in flag state, the potential for the asset to operate in more severe environments, weight issues (such as marine growth or additional equipment), and structural capacity. A lack of proper maintenance procedures also can contribute to gradual deterioration.
Life extension readiness
Following a recommended life extension process helps owners better understand the condition and capability of an asset to address any deficiencies. The benefits to operators are straightforward: improved inspection and planned maintenance, more confidence in future performance, compliance with new regulation and classification society rules, and increased return on investment.
The life extension program evaluates life extension readiness. The approach to the program developed and employed by ABS Group consists of assessing the facts and completing a preliminary assessment to reduce study uncertainties.
Evaluating an asset for life extension begins with obtaining and reviewing the baseline information. The purpose of this first phase of work is to estimate the likely level of effort that will be required to extend service life for the desired number of years. The decision is both a technical and economic one: is there sufficient justification to extend the life of the asset based on likely upgrades and repairs required versus future production rates? Thickness gauging records, for example, can provide insight into the rate at which the asset is corroding. Assuming the asset will be in the same geographic location, this corrosion rate can be used to predict the vessel’s condition in the future. In turn, this allows the estimation of the likely tonnage of steel replacement needed to restore the structure to acceptable condition. Overall, the assessment of the facts involves collecting and reviewing class records, weight control records, thickness gauging reports, and inspection reports.
Once the status of the asset is known in terms of its current condition and compliance with current design codes, a more detailed assessment is performed. The more detailed assessment could include global reanalysis of the hull structural model, focusing on the fatigue-sensitive areas and any areas with substantial or excessive corrosion.
The asset undergoes non-destructive examination, during which engineers gather ultrasonic thickness measurements following appropriate classification requirements. The final step in this phase is inspection of underwater structures: appurtenances, moorings and tendon systems, risers, and corrosion protection systems are variously inspected for damage, external corrosion, cracking or other deterioration. From this information, a preliminary assessment is completed to reduce uncertainties.
At this point, the team conducts a corrosion evaluation and carries out a scantling assessment, identifying areas requiring repairs or upgrade and defining solutions for structural strength and structural modifications to fatigue-prone locations. Equipment upgrades and other life extension activities also are identified.
It may be necessary to complete advanced analysis to determine the requirements of upgrades to activities, or repairs or modifications to equipment. Engineering analyses can include global strength analysis, spectral fatigue analysis, and local finite element analysis of critical components or areas. The fatigue and strength sensitive areas are screened, and the fatigue lives for the most sensitive connections are determined.
Using the results of the analyses, the team develops a set of life extension recommendations that can include:
• Identifying areas of the asset where remedial actions are recommended to meet the design fatigue life required for the proposed service.
• Identifying necessary design changes and modifications.
• Determining the value of condition monitoring systems in the ongoing monitoring of system health.
• Revising the existing ISIP.
The final stage of the process consists of issuing a plan for regulatory approval and developing and implementing the corrective action plan.
ABS Group’s life extension program work flow illustrates how life extension risks are managed. A systematic approach to data evaluation, survey, and analysis is adopted leading to the development of mitigation plans and facility upgrades.
Putting the process to work
The focus of a life extension study is to demonstrate that careful reevaluation of the design and informed assessments of expected future damage, including fatigue and corrosion, contribute to successful life extension.
In many cases the studies are carried out at the end of the original design life, but this is not always the case. One study performed by ABS Group evaluated a deepwater non-ship-shaped floating production installation that had been in service for less than half its design life. A routine periodic inspection of the ballast and void tank structure revealed conditions ranging from coating breakdown and blistering to total coating failure and associated corrosion. With life extension in mind, the asset owner addressed these issues by cleaning and eliminating corrosion sources, such as water ingress and humidity and correct treatment of ballast water.
Another example shows how careful and comprehensive measurement, combined with the knowledge of structural behavior, prevented the need for steel renewal in a watertight compartment. A leak in chain locker drains had allowed water to enter the void space below, causing extensive corrosion and pitting of the uncoated bottom plate. The conventional practice would have been to cut out and replace the bottom plate, but this was not practical because of the restrictions on hot work.
To attack the problem, the team implemented a comprehensive gauging program to record the size, depth, and number of pits. Next, the team determined a reduced effective plate thickness using techniques that account for the presence of pits, combined with the gauged plate thickness. From these data, the effective plate thickness was calculated. The analysis determined there was sufficient remaining strength without any repairs.
Another concern was the risk of loss of water-tight integrity should the pits penetrate through the full plate thickness. This issue was addressed by eliminating the source of water ingress and thoroughly drying the void space. Enhanced monitoring was recommended to assure that wet conditions that could lead to corrosion and pitting would be detected at an earlier stage.
In another case, a routine survey of a FPSO revealed that a substantial portion of a horizontal girder in a wing ballast tank was corroded beyond the renewal limit. Again, the conventional approach would be to cut out and replace the defective steel. Hot work was not a major issue in this case, but the practicalities of large-scale renewal of structural steel while the FPSO was on station created major restrictions for the repairs. Gaugings taken during the survey were sufficient to develop a good picture of the extent and location of corrosion.
As part of the team’s evaluation, the girder was divided into zones across its width, and a thickness was assigned to each zone. New section properties were calculated, and as a result, the team determined that the section properties could be augmented by additional stiffening as an alternative to the conventional cutout-and-renew approach. The dimensions of the repair steel were selected to allow the pieces to pass through the tank hatch and to be moved into place without the use of power equipment – only chain falls were required.
These few examples illustrate how in-depth understanding of structures and novel approaches can be applied to asset service life.
Experience leads to solutions
While design life is not an exact calculation and involves uncertainties, assumptions, and conservatism, software tools are becoming more sophisticated, and engineers can better model a structure’s behavior to determine recommended actions.
Owners that want to extend the operational life of offshore assets have access to a number of creative mitigation solutions that can be carried out by experienced teams with the knowledge to rehabilitate aging offshore units. The benefits of these programs are substantial and can minimize the loss of production that would normally result from removing the critical infrastructure from the field for a prolonged repair program.
Brian Gibbs is director of inspection and verification services for ABS Group. He has spent more than 30 years of his career in asset integrity management (AIM). He is experienced in condition assessment, corrosion technology, failure analysis, corrosion prevention, maintenance strategy development, technical due diligence, risk assessment, ISIP maintenance, risk-based and prescriptive verification, and design and implementation of AIM programs.
David Hua is director of engineering for ABS Group. He has designed both commercial and naval vessels. Hua led the development of a Rapid Response Damage Assessment system, conducting engineering analyses, inspections, and risk assessments for the first FPSO in the US Gulf of Mexico, and he has developed ISIPs such as risk-based inspections for spars, semisubmersibles, and TLPs. He has hands-on experience in life extension projects on both ship-shaped and non-ship-shaped facilities.