A six (6) Degree-Of-Freedom (6-DOF) numerical code, named as COUPLE, has been developed to study nonlinear dynamic interaction between floating structures and their mooring/tendon system. By considering the dynamic interaction, accurate predictions can be made of the motion responses and tension responses of moored offshore structures, such as Spars, Semi-submersibles and Tension Leg Platforms (TLPs) under ocean wave impact, especially in deep water. There are two major modules in COUPLE for computing wave loads applied on a floating structure (hull) and dynamics of its mooring lines/tendons respectively. Wave loads on the hull are predicted based on the second-order diffraction/radiation theory, and/or the Morison equation. Accurate wave kinematics are crucial to the use of the Morison equation, which are predicted using a nonlinear Hybrid Wave Model (HWM). Mooring lines/tendons are modeled based on a slender rod theory and their dynamics are simulated using a finite element method. In this study, the assumption of the slender rod of infinitesimal elongation has been removed to allow for large elongation of the rod. The two modules are coupled by matching the forces and displacements of the hull and its mooring/tendon system at their connections (fairleads or porches) .
Two types of model tests were used to verify COUPLE: single mooring line oscillation model tests and moored structure model tests in a wave basin. Cable dynamic program was examined through the comparison with the model tests of a uniform chain and a chain with inserted spring. The integrated numerical code, COUPLE, was verified against model tests of a mini TLP. In addition, to investigate the effects of dynamic interactions on a hull and its mooring system, numerical simulations of a JIP Spar in three different water depths (from 318 – 1018 m) were made using both quasi-static analysis and COUPLE. In the case of 318 m water depth, the numerical simulation was also examined against the corresponding laboratory measurements.
