The problem discussed concerns the dynamic response of offshore bottom supported structures to the action of periodic deep water waves when yielding is considered in the structure. Results of mathematical studies show that unbounded response can occur after several near-breaking waves have passed a typical offshore structure. Also, the predicted response of a structure, which is considered to, yield, is significantly affected by the number of degrees of freedom considered in the idealization.


Ocean structure engineers have recently been concerned with the design of offshore tower structures for dynamic behavior. Pursuant to the design procedure, knowledge of the dynamic loads and structural behavior is necessary. The research concerned with the loading on these structures has been pertinent and of high quality. It is surprising that very few tudies have been started with the purpose of finding basic structural behavior, viz. deformation patterns or even failure regimes. The basic simulation has not been made for the case where materials in the structure exceed the elastic limit.

The relationship of force on structures to fluid motion due to free gravity waves - considered to be a design load case-has received a great deal of attention (1,2,3,4). Large amounts of experimental data on wave forces have been gathered for shallow water cases where structural motions are small. Unfortunately no data has been gathered for cases where there is structural dynamic wave interaction type response.

Theoretical studies have frequently(2,3,S) sought to characterize wave forces using statistical methods. The following comments extracted from Ref.(2) are pertinent: " ... The statistical techniques ... neglect the time sequence of the phenomena as they assume the random phase. Thus, one entire class of information is often thrown away in the analysis of waves. From an engineering standpoint, it is the groups of several periodic waves, which are almost always the highest waves in the wave system, that are most effective in causing structural damage..." These comments are unfortunate because the statistical method makes a valuable contribution to wave force evaluation by characterizing the spectrum of the water surface elevation(6). Clearly a reconciliation of views between the deterministic wave train and surface spectrum approaches is needed.

Other studies on wave forces were concerned with effects of structural motions on the forces. The governing equation, originally developed by Morison, et. al.(l), has Deen generalized so that relative fluid velocities and accelerations are related to force on the structure (7,8). In certain cases these modifications will significantly alter the predicted results. Experimental verification of these proposed modifications is needed.

Perhaps the most comprehensive representation of structural behavior during application of wave forces was attempted by Edge and Mayer(9). This linear mathematical model was structured to include effects of soil and waves on the tower.

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