Validation of an efficient computer model for spectral analysis of offshore platforms

This paper outlines the background to the development of a frequency domain response model, SPEC, to analysis offshore platforms and the validation of model results. Wave induced loads on the structure are considered to be given by the modified Morison equation. The nonlinear drag term in the Morison equation was linearised in a consistent manner resulting in an equivalent linearised damping matrix. The model was validated in number of different ways as detailed. The eigensolver was compared against the results of industry standard packages and also validated against the first five natural frequencies and mode shapes that were estimated for an existing platform based on a structural monitoring study. Computed spectra of displacements were compared with the results from two separate spectral analyses. Firstly, the spectra of displacements for a simple cantilever were computed for a given input spectrum and compared against the results from a semi-analytical method. Secondly. a model was developed of a large three dimensional lattice platform. The platform was then subject to a random storm event and spectra of displacements computed. The platform was then subjected to the same storm using the software analysis package ASAS and spectra of displacements computed.