Numerical investigation of wave-induced loads on an offshore monopile using a viscous and a potential-flow solver

Abstract

Wave-induced loads on an offshore monopile were investigated using a viscous-flow solver and a potential-flow solver. Considered were only two steep regular wave trains, with the same height but different periods. The potential-flow approach relied on a weakly nonlinear time-domain solver (i.e., Green function). The viscous-flow solver used three solution schemes, namely, an Euler scheme that neglected the flow viscosity, a Laminar scheme that numerically integrated the full Navier–Stokes equations without a model accounting for the fluctuating portion of the velocity field, and a Reynolds-averaged Navier–Stokes scheme that modeled turbulent flow via a two-equation eddy-viscosity model. Numerical uncertainties of viscous-flow simulations were quantified, and computed results were validated against model test measurements. Overall, the main characteristics of wave-induced forces and bending moments were well predicted by all adopted numerical schemes. In terms of higher harmonic components, predictions only from the viscous-flow solver compared favorably to the experimental measurements, including the local free surface elevations, higher harmonic forces/moments, and the secondary wave load cycles. The effect of strong nonlinear motion of free surface on the wave-induced total loading was secondary, and the contribution from the associated viscous and turbulent effects was also very limited. Nevertheless, they attribute to the generation of higher harmonic forces/moments as well as the secondary load cycle. In addition, the effects of wave steepness on the global, first- and higher harmonic forces were addressed.