Gulfstar - Porches and Pull Tubes - Comparing Two Riser Interface Options

Abstract

Abstract Historically, Spar platforms have utilized both riser porches and riser pull tubes for interfacing steel catenary risers (SCRs) to the floating system. The riser porch requires a stress joint at the hang-off end of the SCR to withstand tension and bending loads at the fixed connection. The pull tube requires special hardware (e.g., a double-tapered stress joint) near the keel of the Spar where platform motions are first transferred to the riser system. Each of these options for the riser interface has advantages and disadvantages. The Gulfstar FPS utilized both pull tubes and a riser porch for riser interfaces. This paper evaluates the experience designing and building the two different types of riser interfaces for the Gulfstar FPS and the experience offshore during installation of the risers. The paper quantifies the structural appurtenances required for each interface in addition to the equipment required for riser installation, which is significantly different for each interface type. The evaluation results in important lessons learned and recommendations for both types of riser interface strategies. The paper also summarizes the keel interface utilized on the Gulfstar FPS. The Gulfstar FPS incorporates a different approach for the keel interface with the pull tube support compared to other recent Spar projects in the Gulf of Mexico. The results demonstrate the compatibility of the riser keel interface with the platform motions. The information and results presented in this paper are applicable to engineers, designers and fabricators when evaluating riser interface options for floating systems, particularly Spar platforms.