Analysis of the effects of the leg-spacing on spectral response of offshore structures

The authors have applied a computer-based model to perform spectral analysis of jacket platforms. The analysis is carried out in a consistent manner using the finite element method. The proper estimation of the phasing effects of the ocean waves on structural responses is dependent, inter alia, on the accurate calculation of nodal forces. Using the finite element approach the vector of nodal forces can be most accurately computed taking into account the varying distributed loads along the structural members. The model, developed by the first author, calculates cross-receptances of response for all degrees of freedom that enable the phase effects due to the spatial extent of the structure to be properly accounted for. This model includes the phase effects of wave loadings on the platform and it can thus give a more accurate estimation of response spectra which are generally required for design calculations; this is far more accurate than calculating wave loadings on an equivalent cantilever as is often done.