The importance of accurate calculation of the nonlinear extreme wave kinematics in the design of offshore platforms

The purpose of this work is to highlight the importance of the calculation of the nonlinear extreme wave kinematics appropriate for load calculations of the elements of a platform in deep water. Numerical calculations have been undertaken using a fully nonlinear three-dimensional wave model capable of accurately representing a realistic distribution of wave energy in both the frequency and the directional domains (Bateman, Swan, & Taylor 2001, Bateman, Swan, & Taylor 2003). The purpose of these calculations is to provide what is essentially an exact benchmark against which to assess the accuracy of the commonly applied design wave solutions; linear and 2nd order random waves and nonlinear steady wave solutions. The platform, which is used for the analysis, is a compliant tower set-up in deep water. The load calculations were made by applying the well-known Morison’s equation on all the members of the structure. Through the static analysis, the differences of the results are obvious in the case were geometric nonlinearity is taken into account in the analysis. In order to complete these analyses a well-known software for static and dynamic analysis of structures was used (SAP2000). The results have shown that the fact that none of the common design methods describes accurately the particle velocities, lead not only to a misestimate of the applied loads on the structure investigated but also to important deviations from the predicted displacements of the basic elements of the structure.