Two existing numerical schemes are dynamically linked to compute the nonlinear responses of a slack moored slender body to steep irregular ocean waves. The combined program, known as COUPLE, allows for nonlinear wave forces estimated at the instantaneous position of the body and the dynamic interactions between the mooring system and body. The motions of a slack moored SPAR and tensions in its mooring lines using COUPLE are predicted and compared with the corresponding laboratory measurements and predictions based on the assumption of a static mooring system. The comparisons indicate that COUPLE is reliable and accurate in simulating the dynamic interactions between a mooring system and a slender-body structure. Current laboratory tests simulate a slack mooring system by taut cables and springs. This kind of simulations may not truly model the dynamic effect of mooring system on a moored structure in the real sea. Hence, the numerical simulation based on COUPLE can be a valuable tool to compensate these laboratory tests, especially in deep water.
Slack moored large offshore structures, such as SPARs, usually have very low natural frequencies because of their huge mass and relatively small restoring stiffness. Owing to their low natural frequencies, the responses of SPARs to steep ocean waves in the wave frequency range are expected to be small. Although the nonlinear low-frequency wave forces are small in magnitude, the structure may experience large low-frequency motions, known as slow drift motions, because the exciting frequency is close to the natural frequency. When oscillating near the natural frequency, the magnitude of slow-drift motions is very sensitive to the damping. Many studies revealed that the damping induced by a mooring system could be substantial in reducing the slow-drif motions of the moored structure (Huse 1986; Huse and Matsumoto 1988, 1989; Huse 1991; Webster 1995).