Wave loading fatigue performance of Steel Catenary Risers (SCRs) in an Ultra-deepwater application is assessed. Selectedhost vessels are a truss Spar and a Semi-submersible platform located in a Gulf of Mexico environment. Fatigue damage calculations are based on motion time traces calculated from both uncoupled and fully coupled vessel/risers/mooring motion analysis models. Further for the truss Spar buoy platform, assessments of the relative merits of different SCRhang-off strategies are assessed. Guidance on expected SCR fatigue performance for both a Spar buoy platform and a semisubmersible platform at this very challenging water depth is provided. Further guidance is provided on methodologies for derivation of vessel motions. Also, the need for coupled analysis is evaluated.
Ultra deep water has proven successful so far in the Gulf of Mexico, and opportunities remain in this area as well as other deepwater basins around the world. SCRs are economic and proven riser system solutions. SCR solutions have been adopted for many deepwater projects developed with a floating host. Experience of deepwater SCR applications in the Gulf of Mexico (GoM) shows that fatigue is usually one of the most challenging feasibility issues for SCR design. Wave loading fatigue contributes significantly to the total fatigue performance, through wave induced vessel motions. This is particularly true for large diameter SCRs. The SCR wave loading fatigue damage is related to the combined effect of various parameters, such as environmental conditions, fluid density, riser diameter, water depth, host vessel type and its motion behaviour.
This paper discusses fatigue performance of SCRs in ultra deepwater applications. Case studies have been performed for risers in 10,000 feet water depth in a typical GoM environment. A truss Spar buoy and Semi-submersible have been modelled as potential hosts for 16-inch diameter SCRs. These are considered the most likely potential solutions in GoM ultra deepwater locations. An FPSO/FSO host vessel would also pose a solution for ultra deepwater, however, in the GoM such a solution has yet to be adopted. Vessel motions have been derived for the selected host types. Riser fatigue analyses have been performed based on derived vessel motions to evaluate fatigue performance of SCRs from these hosts.
SCR designs are very sensitive to motion characteristics of the host platform. A main design issue for the SCR concept is fatigue.
In ultra-deepwater, the combined mass of the mooring lines, risers and umbilicals takes up a significant proportion of the system total mass. The hydrodynamic damping due to drag force on these slender bodies also has significant effects on resonant response. A coupled analysis is where these inertial and damping effects are included and solved simultaneously with the vessel motion response. This study evaluates vessel motions for both fully coupled, and uncoupled analysis. SCR fatigue performance is evaluated using both sets of motions.
