DeepStar Accomplishments in Continued-Service Realm

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Offshore operators must grapple with ways to keep aging floating facilities on station beyond their original design life. DeepStar is an operator-funded research-and-development joint-industry consortium that includes members of the oil community such as oil and gas companies, vendors, regulators, and academic and research institutes working in multidisciplinary technology areas. The complete paper highlights successes of the consortium in the realm of continuous service.

Mooring-Integrity Management (MIM)

Moorings on floating production systems (FPSs) are safety-critical systems, but recent research indicates that there is an imbalance between the critical nature of mooring systems and the attention that they receive. Although they are meticulously engineered against a considerable array of known factors that could accelerate standard wear and tear, there have been numerous underlying causes of mooring-system failures historically. In addition to the complex technical consequences of a mooring-leg failure, one must consider the significant financial burdens posed by support, replacement-part, and lost-production costs. Because mooring systems are designed for a single-leg-failure scenario, production might be limited by reduced operating conditions while replacement mooring-leg components are procured, and any subsequent or concurrent shutdown period might affect production negatively while the repairs are made. Should the mooring-leg failure occur at a key structural connection of the mooring leg to the floating unit, then the repairs can be compounded further and, in some instances, might require removal of the system from the field.

After this issue was brought to the consortium committee’s attention, the ­riser-integrity-management (RIM) project was initiated, with the primary goal of developing MIM guidelines for permanent mooring systems. These guidelines are intended to serve as the foundation for future work within the industry for risk-based inspection planning for mooring systems.

A more-comprehensive, risk-based design of an MIM program can improve the safety performance of an operating asset significantly, with the added benefit of extending the longevity of its use as practically as possible while in the field. At the same time, application of a risk-based inspection program can be used to reduce costs further by targeting inspection activities to high-risk areas or components, acting as an early-warning system through early detection of changes that precede line failure.

MIM Guidance Document. The guidance document encompasses ­integrity-management procedures for mooring systems of permanent FPSs that are typically used in offshore oil and gas production. This is relevant to moored FPS facilities of varying shape and function, ranging from a moored mobile-offshore-production unit to a floating liquefied-natural-gas vessel. This document draws upon existing standards and guidelines for the inspection and integrity management of moorings, risers, marine hulls, and offshore facilities and presents a framework for the risk-based integrity management of permanent mooring systems. Furthermore, this guidance document also presents risk-based procedures to provide operators with tools to improve inspection planning and scheduling at the mooring-component level.

This guidance is applicable to all types of FPSs that rely on a permanent mooring system for station keeping. It may also be applied to other elements of an offshore production system that rely on a permanently installed mooring, such as catenary anchor leg mooring buoy export terminals.

Case Studies. To provide practicality to the guidance document, two case studies were reviewed within this study, with Case 1 focusing on a Gulf of Mexico submersible and Case 2 focusing on a North Sea floating production, storage, and offloading vessel. The first case study assumes a generic new-build semisubmersible to be installed at a water depth of 2000 m. This case study further assumes that the MIM program is initiated from the beginning and the deliverables are what would be expected during the front-end-engineering-and-design phase of the life cycle. The second case involved a vessel installed at a water depth of 400 m; it was assumed that it had been in operation for 5 years when the operator decided to implement an MIM program. These case studies are detailed in the complete paper.

Floating-Systems-Integrity Management (FSIM) and RIM

For any operator, the ability to continue production safely from an existing facility represents the most economical way to produce oil. During the design phase, facilities are developed with an expected lifespan already in mind. Through means of safe operations, coupled with the experience gained over the course of many years in production, any operator can go through a continuous-service assessment to determine if a realistic scenario exists to operate safely for any length of time beyond the original design life.

The project of interest created a scenario resulting in the development of a standard guideline for continuing service for aging floating infrastructures. This was achieved by conducting an industrywide survey of integrity-management procedures and then reviewing the integrity-management codes for floating systems. The project also successfully completed three case studies that provided an example of application of continued service assessment guidelines to a variety of continuing-service scenarios.

Floating-Systems and Riser Continued Service. The main objective of the project was to define a methodology to assess the feasibility of a floating structure to continue service beyond its original design life. The methodology is to be applicable to all floating structure types and operating regions. It is also to provide the details necessary to demonstrate the feasibility of continued service and to develop an implementation plan for continued service. The guideline is intended to cover all components of a facility, including hull, marine systems, topside structures, and moorings and applies to floating-structure types such as ship shapes, tension-leg platforms, spars, and semisubmersibles. The project was expanded to cover all dynamic risers connected to permanent floating platforms, umbilicals, and riser components.

The proposed assessment methodology is primarily based on risk assessment and the use of risk as the means to determine acceptance for continued service. As the basis for this document, two main tasks were performed with the purpose of understanding the current state of the industry with regard to prolonging the operating life of floating structures. First, a questionnaire was developed and distributed to industry operators to determine current experience with assessing and implementing continued-service projects. This was followed with a review of any existing guidance documents and technical papers related to continued service. The results from these two tasks were drawn upon in the development of this guideline.

FSIM and RIM Guidance Documents. The intention of these efforts was to provide guidance to owners, operators, and engineers in the development, implementation, and delivery of a process to manage the structural integrity of floating structures and risers. The structural-integrity-management (SIM) system’s goal is to provide a formal process for ensuring the integrity of the structure throughout its intended design service life on a fit-for-purpose basis. The advantage of implementing an SIM system is that an operator can benefit significantly from design decisions such as providing sufficient access for inspection and maintenance. Approaches to dealing with SIM will also vary depending upon field life, the type of floating structure, and the sophistication of the regional infrastructure near the floating-structure location. In turn, these factors can influence the philosophical approach to the specifications of an SIM system, varying from emphasis on the use of monitoring equipment to emphasis on the extensive use of inspections. Additionally, design decisions on safety factors, design margins, corrosion protection, redundancy, and system reliability will also influence the SIM strategy. Regardless, the resulting SIM system should aim to maintain the integrity of the structure throughout its designed service life.

Conclusions

Some of the key lessons from the ­integrity-management program include the following:

  • It is important to start such programs early.
  • Decisions related to integrity management during a structure’s later years can have a profound effect on the feasibility of continuing service beyond the design life.
  • It is important to conduct initial review and assessment steps before immediately recommending a detailed analysis.

If continued service is planned for an asset, then appropriate maintenance strategies need to be both pragmatic and proactive to maximize asset life. Finally, it must be remembered that life extension can be extremely costly; continued-service costs are not necessarily linear, and the cost-vs.-benefit analysis needs to highlight longevity vs. the costs of repair and continued maintenance.

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 27840, “Collective Summary of DeepStar Accomplishments Within Continued Service,” by Joseph Gomes, DeepStar, and Greg Kusinski, Chevron, prepared for the 2017 Offshore Technology Conference, Houston, 1–4 May. The paper has not been peer reviewed. Copyright 2017 Offshore Technology Conference. Reproduced by permission.

DeepStar Accomplishments in Continued-Service Realm

01 May 2018

Volume: 70 | Issue: 5

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