Prediction of long-term fatigue damage of a hydraulic cylinder of a wave energy converter subjected to internal fluid pressure induced by wave loads

This paper deals with fatigue analysis of the high-pressure hydraulic power take-off (PTO) machinery in a heaving-buoy wave energy converter (WEC) exposed to irregular incident waves, with a focus on the hydraulic cylinder. To estimate the fatigue damage of the relevant critical joints of the cylinder, time-domain simulations are required to obtain the pressure loads inside the hydraulic pump. In this paper, the relevant dynamic responses are calculated by using the previously established nonlinear WEC model. Long-term pressure loads are obtained by combining the short-term time-simulation results and the occurrence probability of each sea state, and two models, Weibull and generalized Gamma distributions, are applied to fit the long-term rainflow-counted pressure cycles. Based on the pressure cycle distribution, the long-term fatigue damage of the cylinder with two different kinds of material, carbon, low alloy steel and nickel–chromium–molybdenum–iron alloy, is obtained by using the SN-Miner Palmgren approach. Two main parameters, cylinder diameter and thickness-diameter ratio, are investigated. Moreover, the parameter combination with which the cylinder has an expected fatigue life and possesses minimum material volume is supplied. Finally, the relative contribution of fatigue damage from different sea states is obtained and the most important wave conditions are identified.