Hull/mooring coupled dynamic analysis of a truss spar in time domain

Nonlinear hull/mooring coupled dynamic analysis of a truss spar in waves with collinear steady winds and currents is numerically carried out in the time domain and the results are compared with 1:61-scale experiments as well as uncoupled analyses. The first- and second-order wave forces, added mass and radiation damping, and wave drift damping are calculated from a second-order diffraction/radiation hydrodynamics program. The total wave force time series is generated based on a 2-term Volterra series model. The hull/mooring coupled dynamics are then solved using a time-domain 3D mooring dynamics program based on a generalized-coordinate-based finite element method. The mooring lines are coupled to the platform through generalized springs and dampers. A case study was conducted fur the Marlin truss spar with 9 taut mooring lines in 3240-ft (988-m) water depth. The numerical results show that dynamic effects are very important for the present mooring design. The motion and tension spectra of uncoupled analyses with a linear massless spring or nonlinear massless spring are also compared with those obtained from fully coupled analysis to assess the importance of hull/mooring coupling and mooring-line damping.