A detailed test-based local fatigue assessment of brownfield flexible risers confirms the suitability of continued service beyond original design life for an operator in the Gulf of Mexico.
REGION Gulf of Mexico WATER DEPTH 2000 ft
A spar platform in the Gulf of Mexico was approaching the end of its 20-year design life. The operator wanted to extend production three years beyond its original design life but required engineering verification to confirm that it could continue to operate safely for that duration.
2H and its sister company Clarus were contracted to assess the remaining service life of the six flexible risers – four production and two export, as part of the operator’s continued service assessment programme. This involved revisiting the original design data and analysis, annulus testing to determine integrity and a local fatigue assessment with measured operational and environmental conditions.
Annulus testing of the flexible risers was undertaken to confirm the integrity of the annulus. Using the test results and operational history, we conducted global and local fatigue assessments that demonstrated that the riser system had enough remaining life to continue production for the desired number of years.
We conducted the flexible riser assessment in accordance with API-RP-17B and API-SPEC-17J and improved upon the original analysis by the suppliers. The original analysis included regular wave loading and considered first order motions alone, but with spars, second order pitch motions can be significant. We conducted the global assessment using random sea loads and included the second order motions. We also included VIM fatigue cycles in the local cross section assessment.
The enhanced analysis combined with site-specific metocean data and actual annulus conditions enabled us to give the client a more reasonable estimate of remaining service life than was possible with a design level assessment methodology.
The local assessment was conducted in BFLEX which is the only commercially available API-RP-17B compliant software. We developed a novel algorithm to incorporate global curvature and tension histograms into the BFLEX local cross section model. This transposition method saved a significant amount of time and man-hour cost.
One of the biggest challenges we faced was the lack of documentation due to the age of the project and the multiple handovers of the asset over its lifetime. Additionally, the flexible suppliers were unable to provide critical design inputs such as S-N curve or friction factors.
We reviewed the original documentation and carefully extracted reliable data to develop a design basis document. Where there were gaps in the data, we made conservative engineering approximations based on our past experience and existing public-domain literature. These included conservative assumptions of spar low-frequency pitch/surge and mean offsets from multiple hull studies over the field life. The fatigue prediction was bounded by the use of S-N curves that were conservative for the particular annulus condition determined by tests. We used a VIM response diagram of a similar sized spar instead. We also considered environment directionality to help relieve some of the layers of conservativism stemming from the approximations.
Life Extension Verified
2H’s riser engineering experience and state of the art numerical tools allowed us to execute the project on a demanding schedule in just three months.
Our expertise in detailed riser analysis and use of validated local analysis software helped confirm that sufficient service life remained for all risers for the client’s desired period of extended operations. Evidence was provided to the client to support their continued service application to BSEE and CDA.