The stiffness model of highly extensible polyester mooring lines is studied as part of dynamic analysis of a floating production, storage and offloading unit (FPSO). In more detail, coupled dynamics of FPSO and deepwater mooring system with taut polyester mooring lines is observed. Calculations are done in time domain utilizing nonlinear finite element method (FEM). Newly derived additional stiffness matrices are used to approximate the influence of elongation of mooring line to its geometric stiffness. Presented case study is based on example from the literature with available experimental results.
Polyester mooring lines are commonly used at deep-water locations for a FPSO. Recommended way to describe dynamics of FPSO is to set a coupled model that also contains dynamics of mooring system. Breaking point of a polyester rope can be found at 15% elongation. Such high elongation has great influence on mooring system response. Therefore, high elongation of polyester lines should be included in the coupled model to achieve better prediction of FPSO dynamics. ISSC 2006 Technical Committee 1.2. reports of three papers which deals with coupled dynamics of moored vessel. Tahar & Kim (2003) developed a computer program for hull/mooring/riser coupled dynamic analysis of a tanker based turret - moored FPSO. Garret (2005) performed a fully coupled global analysis of floating production system, including the vessel, the mooring and riser system. Kim et al. (2005) solved simultaneously the vessel and mooring line dynamics. The vessel global motions and mooring tension were tested at the wave basin of Offshore Technology Research Center (OTRC, Texas, USA) for the non-parallel wind-wave-current 100-year hurricane condition in the Gulf of Mexico. Stiffness nonlinearity of polyester mooring cables was studied by Fernandes et al. (1999). They examined acceptance tests performed with actual full scale cables.