Wave Loads on Monopile Foundations Revisited – New High-Quality Experiments for Validation of a Novel Engineering Model

Nonlinear horizontal wave forces on a vertical cylinder representative of an offshore wind monopile foundation are investigated experimentally and numerically. The studied regime corresponds to long-period waves with minimal wave diffraction around the column, and with wave amplitudes of the same order as the cylinder radius, such that inertial potential-flow forcing dominates the overall hydrodynamic excitation. The high-quality wave flume experiments were conducted at the UWA Coastal and Offshore Research Laboratory and utilised phase-manipulated focused wave groups for clean extraction of the different harmonic content, in both the undisturbed free surface and the loads. The measured data is used for a detailed validation of a novel force prediction model for in-line wave forces on slender surface-piercing vertical cylinders. The so-called T-FNV model is found to correctly represent the dominant nonlinearities and provides efficient yet accurate loading estimates even in steep wave conditions. The unique formulation in terms of only the free surface elevation data makes the model highly applicable to engineering practice due to its ease of application and computation speed.