Polyester rope mooring system has been widely accepted as a reliable and cost effective choice for deepwater mobile offshore drilling and floating production units in recent years. Polyester rope is made of a visco-elastic material and its stiffness is affected by mean tension, tension amplitude, loading period, and loading history. This creates complexity and uncertainty in the design of hull, mooring and riser systems.
This paper summarizes the results of a JIP study investigating the effect of polyester rope stiffness on the SCR design of a semi-submersible FPU at two water depths in the Gulf of Mexico (GoM). A two-slope static-dynamic stiffness model proposed in the recently issued ABS “Guidance Notes on The Application of Fiber Rope for Offshore Mooring” is used in the global motion analysis to provide input for the riser analysis. Parametric studies of SCR extreme analysis, wave fatigue analysis, and VIM fatigue analysis have been performed. Based on the results, recommendations on the impact of rope stiffness to the riser design are presented.