Structural Fatigue Assessment of Offshore Platform Considering the Effect of Nonlinear Drag Force

Fatigue assessment for jacket platforms is an indispensable practical issue. Because of the small-scale leg diameter, these structures are often drag dominated and wave-induced force in these structures can be tackled by using either linear or nonlinear form of Spectral Morison Equation. However, it is really complicated and difficult to incorporate nonlinear form of the Morison Equation to acquire the spectral density of the wave force, which is an important step of fatigue estimation. In this paper, in order to estimate the influence of nonlinear effect in wave force, fatigue assessments containing nonlinear effect for the fixed offshore structure are presented. Firstly, shallow-water jacket model locating at a water depth of 20 m is established and involved in calculation. Besides, for the sake of validating the effectiveness of the nonlinear term, the linear and nonlinear form of wave-induced force spectral densities are calculated by the Morison Equation in frequency domain. Secondly, the fatigue life of the jacket platform is assessed in time domain, where time-history of wave force can be obtained by transforming the linear or nonlinear wave force spectral densities from frequency domain to time domain. After the contrast of acquired fatigue life, the comparative results can indicate that the nonlinear drag force contributes a 14% fatigue damage to the total and the influences of the nonlinear term cannot be ignored for the jacket model.