THE DESIGN WAVE CONDITIONS FOR DETERMINING WAVE DRIFT FORCES ON MOORED SHIPS AND SEMI-SUBMERSIBLES

A moored vessel is required to withstand the environmental forces due to a combination of wind, waves and current. Present mooring design rules all specify that environmental condition with 50 or 100 year return period be assumed with the wind, waves and current acting co-linearly. Evidence is presented which shows that the present design rules may not represent the most onerous condition. The dynamic response to the wave drift forces is greatest in short steep waves and these may provide the most severe conditions when combined with a wind at a heading to the waves. It is also suggested that conditions with the wind, waves and current not in line further increase the mooring forces. Recent mooring failures out-of-line environmental conditions should also be examined.