Investigation of which sea state yield the dominating contribution to fatigue accumulation in offshore structures

Abstract

Majority of the fatigue damage on offshore structures is generally assumed to be caused by relatively frequently occurring moderate sea states, i.e. sea states with significant wave height in the range of 4m - 8m.On the contrary, Økland [17] claims that the dominating fatigue damage is caused due to higher sever sea states. These two claims in regard to the major cause of fatigue damage are opposite to each other, therefore, this thesis aims to investigate the inter relationship between fatigue damage verses sea state severity. To perform this study, the thesis has identified the Kvitebjør Statoil jacket platform, in which 3-hour duration wave records are available from 1957 to 2013. These short term waves are assumed as a stationary Gaussian process in which the sea surface elevation process is completely described by Pierson-Moskowitz wave spectrum. In this thesis, the analysis is performed using the spectral-based fatigue assessment method, which is frequency domain analysis and attempts to account for the random nature of sea states in a rational manner. The analysis is performed by developing unidirectional transfer function or Best fit RAO, which is generated from 21 Gaussian sea surface processes and their corresponding linear nature response processes using the Fast Fourier Transform method (FFT). Response spectrum for a given sea state is generated using the Best fit RAO and furthermore standard deviation and number of response cycles of this response process are determined assuming as a narrow band response process. The standard deviation is used to determine the scaling parameter of the Rayleigh-distribution, which represents the distribution of stress ranges for short term. The Rayleigh distribution and the number of response cycles of the process are combined to calculate the number of cycles for a constant stress range in a given stress block. The number of cycles to failure corresponding to the stress range in the given stress block are determined from "T" S-N curve. The effect of the accumulated fatigue damage on the structure is observed by analyzing the S-N curve with double and single slope. Finally, the linear damage calculation by "Miner-Palmgren" summation is used to evaluate the accumulated fatigue damage. Furthermore, the Best fit RAO is assumed to deviate at four different frequency ranges and the fatigue damage for each deviated RAO is calculated. The fatigue analysis results have asserted the first claim that fatigue damage in the structure is observed to be caused due to the moderate sea states. 60% to 65% of the accumulated fatigue damage is induced in the structure by the moderate sea states. This is because; the moderate sea states are more than the higher sea states and have relatively higher stress ranges than the lower sea states. On the other hand, when straight S-N curve with slope m = 3 is used, the accumulated fatigue damage are observed to be overestimated, particularly due to the lower sea states, i.e. sea states with significant wave height 2m to 6m.