Riser VIM Fatigue Design Induced by Deep Draft Semi-Submersible

SBM Atlantia Inc. (SBMA) designs and constructs DeepDraft Semi^® Floating Production Units (FPU). Two of these units are installed in the South-Central Gulf of Mexico at water depths of 8,000-ft and 6,000-ft. Both units are operational. Each of these FPUs is characterized by four square columns interconnected by four rectangular pontoons. Model tests and field measurements have indicated that these types of vessels are susceptible to Vortex Induced Motions (VIM). Typical loop/eddy current conditions at these Gulf of Mexico locations are likely to induce semi FPU VIM response. The paper addresses the impact of the semi-submersible FPU VIM motion on the fatigue design of the Steel Catenary Risers (SCR). Semi VIM induced riser fatigue design is performed using time domain approach and the scatter diagrams for the surface currents. Riser fatigue damage rates consider the effects of both the inline and transverse semi VIM responses. A comprehensive sensitivity study has been carried out, and the results demonstrate that riser VIM fatigue is primarily affected by the combination of column shape, current heading and intensity, the individual riser configuration, mooring line pretention and polyester stiffness. A simplified approach based on quasi-static Stress Transfer Functions (STF) is proposed. The STF is developed based on the VIM motion amplitude without considering the effect of the mean FPU offset induced by the current. The STF allows one to quickly develop a preliminary map identifying the feasible riser configurations. Such map can be used during the Front End Engineering Design (FEED) studies to initiate viable riser configurations. Finally, it is important to note that the preliminary field VIM measurements are significantly less than those observed during the model tests. Therefore, the fatigue life estimates based on the model test observations are considered to be conservative.