A semisubmersible moored in waves experiences a steady offset and two types of motion—a first-order motion at frequencies corresponding to the incident wave frequencies and a slowly oscillating motion near the natural frequency of the semisubmersible/mooring system. An extensive wave tank testing of a semisubmersible model was undertaken in which the motions of the semisubmersible and the loads in the mooring lines were measured. The semisubmersible was tested in the tank in a head sea as well as a beam sea heading in a series of regular waves, regular wave groups and irregular waves. The test results of the steady offset and first-order and slowly oscillating motions are presented for each heading and for each of these wave series as functions of the wave period. The experimental results are correlated with theoretical results based on a 3-D diffraction theory which takes into account the appropriate first and second-order terms. It is found that the nonlinear damping terms are quite important in explaining the behavior of the moored semisubmersible in waves and that the steady drift loads in wave groups can be determined from results based on regular waves.

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