USCG Discusses Evaluation of Continued Service for Offshore Facilities
The assessment of service life extension is not unique to oil and gas facilities, and commonalities exist in evaluating the equipment and components in other sectors. For example, the International Space Station was originally designed to last until 2015, but over the past few years its life expectancy was extended to 2020 and then 2024. The US water infrastructure contains millions of miles of buried pipe, out of sight, and well past its service life. There are some locations where wooden water mains, made of planks lashed together with coils of metal, remain in service after more than 150 years. Heart pacemakers are also inaccessible for regular inspection. They are designed so that the battery will “fail” first, which is gradual and can be detected by a doctor. A replacement can be scheduled before a catastrophic electronic component failure occurs.
David Martyn, program manager, Outer Continental Shelf Inspection Activities at the US Coast Guard (USCG) Eight District, gave these examples in a session at the Offshore Technology Conference in Houston on Monday. He said that while there are marked differences in engineering between the examples and offshore facilities, the basic principles are similar to evaluate continued service: assess the current state of the item; compare it to the design standards; and examine the future anticipated operations.
Martyn and Tracy Phillips, chief of the tank vessel and offshore division at the USCG Marine Safety Center, summarized their findings from a review of the records from 44 floating facilities in the US Gulf of Mexico (GOM). The review looked at nonconformance in the following: corrosion (defined as wastage exceeding allowable limits); fatigue (identification of a fracture or crack); and initial defects (structural nonconformance identified after the structure was installed but related to design, fabrication, or material flaws).
Martyn said that when looking at the absolute number of identified structural nonconformities for all facilities during each year in service, it is remarkable that the number remains in the single digits. He noted the importance of the relative scale—each facility contains thousands of individual components that can fail.
He added that while the data represent the structural issues that were serious enough to be reported to or identified by the USCG, the individual facility maintenance and repair records may contain additional structural issues that were proactively corrected, and thus not required to be reported. Given this possibility, the analysis does not estimate the probability of failure, and the small variable population of facilities does not allow a precise statistical analysis.
The nonconformance curve shown in the figure below is similar to the generic bathtub curve that demonstrates that the early life of a mechanical system has a relatively higher probability of failure, which accounts for initial defects in the system. The reliability improves as the defects are exposed and corrected during a break-in period. During the mid-life period, reliability peaks and then begins to decline. Near the end of the system’s service life, the reliability decreases with age as components wear out.
The figure shows that the structural service lives of GOM floating offshore facilities, when measured in absolute numbers of nonconformities, show the same general trend.
Martyn said, “You must take into account the growth and size of offshore platforms when looking at the stats. There are only three facilities near 20 years of service compared to more than 40 with at least 1 year of service, so they show fewer nonconformities. Some structures today are designed for longer service life, and today’s structures benefit from lessons learned from older facilities.”
Martyn concluded that while floating offshore facilities in the GOM are showing an overall trend of decreasing reliability with age, the actual number of serious structural issues related to individual facilities is small. This provides strong evidence to support the argument that many of these facilities will be good candidates for continued service.
Phillips proposed a strategy for operators to help streamline their development of a proposal for facility life extension. Because both the USCG and the US Bureau of Safety and Environmental Enforcement (BSEE) are involved, she said this strategy can stave off delays due to disorganization or misdirection of efforts.
The eight suggested steps are: Schedule an initial conceptual meeting with the USCG and regional BSEE office; present a detailed proposal that includes data, surveys, risk assessment, and engineering analysis; USCG and BSEE approval of proposal; execution of stage 1 (data and surveys); consult USCG and BSEE; execution of stage 2 (risk assessment and engineering analysis); correction and mitigation; and final submission and authorization.
Phillips added that nonstructural assessments, such as human factors, are also important considerations in the life of an asset. “Not only can the equipment harm a human, but a human can harm the equipment.”
07 November 2017
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