Tendon fatigue analysis of tension leg platform using a new time-domain quasi-dynamic method

Due to the non-Gaussian characteristics of the tendon tension response of tension leg platform (TLP), the common statistical model is not adequate to estimate the fatigue damage accurately in the frequency domain, thus the time domain analysis is required. A new quasi-dynamic method is proposed to calculate the tendon tension response considering various factors. An equivalent analytical tendon model is employed and the post-treatment has been carried out for the reconstruction of the high-frequency stress response based on the vertical motion QTFs of each tendon by HydroSTAR. Forty sets of environmental cases are extracted from a scatter diagram. The rainflow counting method is applied to deal with the stress time series and the fatigue damage can be obtained combined with the relevant S-N curve. The number of cycles of tension for each tendon is discussed, considering the contribution of the first-order wave load, the second-order diff-frequency wave load, and the second-order sum-frequency wave load. The comparison of annual fatigue damage based on the quasi-dynamic method and coupled method by Orcaflex is carried out. The study proves that the quasi-dynamic method is an efficient way to estimate the fatigue life of each tendon in the time domain considering numerous cases of discrete sea states.