Experimental study and numerical simulation on the slow-drift oscillation of a semi-submersible in irregular waves

This paper presents an experimental investigation on the slow-drift oscillation of a semi-submersible. The soft-moored model was subjected to two sea states for studying the slow-drift response in irregular waves. The effect of wave direction was considered by setting the vessel in head, quartering and beam sea. Tests under a series of regular waves were carried out for measuring mean drift forces. Viscous damping was measured with decay tests in still water. In addition, a numerical simulation of slow-drift oscillation was also performed to complement the experimental work. In the numerical work, quadratic transfer functions were computed by using Pinkster's theory, and Aranha's approximation was employed to calculate drift damping. The comparison between experimental and numerical results showed generally good agreement. Both experimental and numerical work demonstrated that maximum amplitudes of slow-drift oscillation may be larger in modest seas than in rough seas.