The forces on offshore platforms with slender members are generally estimated using the well known Morison's equation. The direct application of this equation for probabilistic analysis was difficult because of the nonlinear drag term. This article outlines a development for linearising the drag force term in Morison's equation and the incorporation of this into a finite element program developed by the authors. The program was used by the authors to carry out spectral analysis of a jacket platform using mode superposition. The results are compared with the results of an analysis using a well established equivalent linearisation technique developed by Borgman. (C) 1999 Elsevier Science Ltd and Civil-Comp Ltd. All rights reserved.