The application of a spectral response model to fixed offshore structures

This paper outlines the application and validation of a frequency-domain response model, SPEC, developed to perform structural analysis of offshore platforms. 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. Using the theory of linear systems, complex structural receptances were computed by subjecting the structure to gravity sinusoidal waves only; this approach halved the computational time required to compute the receptances. The model was validated in a number of different ways as detailed. The eigensolver was compared against the results of an industry standard package 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 for a large three-dimensional lattice platform. The platform was then subjected 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.