DESIGN SEASTATES FOR EXTREME RESPONSE OF JACKUP STRUCTURES

The extreme response of a jackup structure is studied We consider how design seastate histories cart be introduced to reduce the cost of tim e-domain response analysis. We first identify critical wave characteri stics for extreme response prediction. In quasi-static cases, the maxi mum wave crest height, eta(max), is shown to best explain extreme deck sway. For more flexible structures we introduce a new wave characteri stic, S-D, based on response spectral concepts from earthquake enginee ring. Finally, we show how accurate response estimates can require few er time-domain analyses, provided design seastates are pre-selected to ensure that eta(max) or S-D is near its average value. With respect t o standard Monte-Carlo simulation, these design seastates achieve at l east a 50-percent reduction in response variability and hence at least a fourfold savings in needed simulation cost. These results may lend insight, not only into time-domain simulation, but also into more fund amental questions of jackup behavior. They also suggest that, at least in quasi-static cases, still simpler design wave methods based on eta (max) may suffice. We illustrate and evaluate some such design wave me thods here (e.g., the ''new wave'' model and others based on Slepian t heory).