Characterizing the excitation and response of a spar buoy in random seas

Abstract

The spar concept has found many applications in oceanographic science and ocean engineering. These include oceanographic buoys, the FLIP research vessel and the much larger Neptune and Diana spar platforms in the Gulf of Mexico. Based upon these actual designs one can establish the range of wave and platform design parameters to be used in the numerical simulation for this study. Attention in this study is focused upon the characterization of the excitation and the response of spar buoy designs using Slepian modeling techniques. A Slepian model provides conditional estimates of behavior above a specified threshold level. It requires knowledge of the covariance structure of the various underlying processes and its first or second derivatives depending upon the type of model. For a Type I Slepian model, information on the derivatives at the point of threshold crossing and its distribution are needed to complete. On the other hand information on peaks and the distribution of the second derivative at the peaks are needed to develop a Type II Slepian model. Simulated wave data and wave-induced response of a spar buoy are used as the basis for investigating the Slepian models. Estimates of mean behavior after an event are modeled as well as probabilistic behavioral estimates. Earlier researchers have established that for a very few special cases analytical probability distribution functions can be tied to covariance functions. These ideas are examined for practical use and the issue of scaling the analytical results discussed. The practical application of this approach for characterizing the response behavior is discussed by application to oceanographic and larger ocean platform spar buoy problems.