Analysis on Wave-Induced Transient Lateral Vibration of Four-leg Structure Supported by Seabed Layer—Analysis of lateral vibration of an offshore structure with consideration of the effect of seabed layer (4th Report)

摘要

When analyzing the lateral vibration of an offshorestructure supported by seabed layer, the effect of this layer must betaken into consideration. In the third report, a method has beenproposed for reducing a four-leg offshore structure supported byseabed layer to one-leg structure, and its natural frequency oflateral vibration has been analyzed theoretically. Furthermore, thevalidity of this method has been clarified by the model experiment.In this paper, the wave-induced resonance amplitude as well asbending stress in the leg of the offshore structure were analyzedtheoretically and simulated numerically.The main results obtained are as follows:(1) With respect to the off-shore structure considered in thisstudy, the first resonance of lateral vibration could be inducedin it by practical wave-excitations. Once the resonance occurs,the leg's amplitude of the structure will increase almostlinearly as the time elapses.(2) The maximum bending stress caused by the first-mode oflateral vibration always occurs at the top of the leg.(3) As soon as the vibration is fallen into the resonant state, themaximum bending stress in the leg increases almost linearlywith the time, and then reaches the yield strength of the legmaterial in about 50 seconds, causing the failure of the structure.(4) Based upon the above results, the criteria for designingoffshore structures are proposed from the viewpoint ofevading the resonance phenomena caused by surface waves.