In this study, a numerical model is adopted to investigate the wave-seabed interactions around offshore wind turbine four–pile jacket foundation. Unlike the previous study, irregular wave loading will be considered. The present model is adopted from PORO–FSSI–FOAM solver, which was based on the olaFlow solvers for wave motion, the Biot equation for the porous seabed. The present model will be validated with the recent wave flume test for random wave–induced pore pressures around a mono–pile. The irregular waves generated by diverse spectra, such as the JONSWAP spectrum etc., propagate forward from the inlet boundary. The numerical simulation consequences manifested that the maximum value of the pore pressure and soil liquefaction depth became greater in the irregular wave system, compared with the cases with regular waves. Furthermore, the random distribution of the fluid velocity and the soil response is observed, which is attributed to the irregularity of waves.
The selection of applicable offshore wind turbine foundation forms is one of the key issues in nearshore energy exploitation. Among ranges of foundation types, the jacket foundation supported by the four-cylinder group is playing an important role because of its convenient construction and mechanical stability. Construction of marine structures in ocean environments are normally undertaken by violent sea climate involving wave-alone, wave plus current, wind and earthquake and so on, which may lead to the seafloor region around the body liquefied, followed by the structure itself would likely be damaged. Numerous accidents have been report in the literature in China, Japan and Spain etc (Zhao, 2011; Carlos et al., 2011).
Based on the observations in the laboratory and field measurements, two mechanisms of the wave-induced soil response have been reported in the literature (Sumer and Fredsøe, 2002) depending on the manner that the pore pressure is generated. The first mechanism resulted from the transient or oscillatory excess pore pressure and is accompanied by attenuation of the amplitude and phase lag in the pore pressure changes (Yamamoto et al., 1978). This type of seabed liquefaction is caused by the wave-induced seabed seepage near wave troughs (Jeng, 2012). The second mechanism is termed as the residual pore pressure, which is the build-up of excess pore pressure caused by contraction of the soil under the action of cyclic loading (Seed and Rahman, 1978). In this study, we focus on the oscillatory mechanism and the associated seabed liquefaction.