Deepwater development in Gulf of Mexico (GoM) started about two decades ago. It is the time to evaluate the global integrity of the offshore production unit and riser system and explore the possibility of life extension. This paper investigates life extension of deepwater risers used for a spar application in GoM.
A spar in GoM has been put into service for the past 17 years since it was installed in 2000. It was expected to extend the riser design life of 20 years by 10 more years. This paper first introduces the configuration of Steel Catenary Risers (SCRs) and Top Tensioned Risers (TTRs) used for the Spar platform. It then discusses the philosophy for life extension of deepwater risers by assessing the integrity of the riser system. Latest technology and monitored data are used for the assessment.
The TTRs and SCRs are modelled using latest analysis programs that were not readily available when the risers were designed. The interaction between the TTR and buoyancy can tensioning system is modelled as a pipe-in-pipe system that considers the function of centralizers with a gap size. The contact interaction between buoyancy can / stem and supporting guides of the hull platform is modelled as non-linear spring elements. The spring elements connect normally to the hull centerline and allow vertical sliding movement of the buoyancy can tension / stem system with friction. The base line inspections of the vessel, SCRs, and TTRs was carried out in the same inspection campaign with the spar hull structure. As-built information, production and operational data was also used for a continuous service assessment. Environmental condition data to the state of the art, including measured spar VIM data, was used in the assessment of VIM fatigue damage to the risers. The investigation was based on the up-to-date analytical tools including latest FEA program ABAQUS and VIV prediction Shear7. It further computes the riser global performance including dynamic strength and fatigue damage with the contributions from wave fatigue, VIV and VIM. This paper finally draws a conclusion for the life extension of the risers for the application in GoM.