Wave and Earthquake Response of Offshore Structures: Evaluation of Modal Solutions

Direct numerical integration of the equations of motion is used to evaluate the adequacy of modal solutions, as applied to the analysis and design of large offshore structures under wave and earthquake loadings. Two such solutions are considered: mode acceleration and simple modal superposition. Both these methods are widely used by the offshore industry but recently their adequacy in predicting member forces under wave loadings has been questioned. After reviewing the underlying assumptions and the differences between the two solutions in relation to the two types of loading, results from analyses of a template platform, modeled with 1,428 degrees of freedom, are compared to those obtained by a direct solution in geometric coordinates. It is found that for wave loadings the mode acceleration method gives excellent results, while simple modal superposition can lead to erroneous member forces and stresses even if a large number of modes is used. For earthquake loadings, on the other hand, mode acceleration is not any better than simple modal superposition, which gives answers that may be considered acceptable for design applications.