A quadratic theory for the fatigue life estimation of offshore structures

Abstract It is the purpose of the paper to present a method for prediction of the fatigue life of flexible offshore platforms, which takes into account the non-linearities in the waves and the stochastic nature of the sea. For convenience the equations are carried out for single leg platforms only. A quadratic theory is adopted for describing the wave elevations, the wave potentials, and the short-crestedness of the sea. In order to determine the loads the linearised Morison equation is applied. The deflections of the platform are calculated by superposition of the natural modes assuming the Timoshenko beam theory to be valid. The stress distribution is found, and a statistical method able to handle the non-linearities is applied. Finally, the fatigue damage based on S-N curves is determined. A numerical example considering a single leg platform for the North Sea is given.