Steel catenary risers (SCRs) used in conjunction with a turret moored FPSO in deepwater environments present significant design challenges. The large vertical motions at the FPSO turret induce severe riser response. This results in difficulty meeting strength and fatigue design criteria at the Touch Down Point (TDP) and at the riser hang off location. It is typically considered challenging to achieve feasibility for a conventional SCR application on a turret moored FPSO. Previous industry work for an SCR application used with other floating hosts has demonstrated that SCR strength and fatigue response can be improved using heavy and light coatings strategically placed along the riser [1]. An optimization study is performed, based on previous industry work, which demonstrates that a weight optimized configuration can enable the application of an SCR on a turret moored FPSO. The effect of adding different coatings along the length of the SCR is discussed. The position, length, and density of the coating type are varied to determine an optimum configuration for both strength and fatigue response. This paper will also discuss observations which may help explain why weighted sections can improve SCR response at the critical area.
Hybrid risers represent an excellent way to isolate the riser from most of the host vessel motions and thereby limit riser fatigue. A common arrangement features the riser supported by a buoyancy can via a tether chain. The tether chain is a cheap simple way to make the connection while providing flexibility for installation. However, in service the tether is under very high tension, and the chain is not really flexible in the face of small amplitude fatigue loads. The friction effectively “welds” the chain together. Moment and torque input to the system by first order vessel motions and vortex induced vibrations are carried through the chain and induce fatigue loading in the links. Analysis of the chain can be problematic because the determination of the detailed stress in the chain requires a refined FEA model with contact element between the links. From the global sense the analysis may require running hundreds of sea-state realizations in the time domain and the vortex induced vibration (VIV) assessment of thousands of current profiles. In this paper an efficient numerical method is described to rigorously determine fatigue damage at locations throughout the chain.
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