Extreme wave loads on monopile substructures: precomputed kinematics coupled with the pressure impulse slamming load model

Monopiles are nowadays the preferred substructure type for bottom-fixed offshore wind turbines at shallow to intermediate water depths. At these locations, the large waves that contribute to extreme loads are strongly nonlinear. Therefore they are not easily reproduced via the simple engineering models who are commonly used in the offshore industry.

In the current approach, we develop a design pattern which improves this standard methodology. To retain nonlinearity in the force computations, we have precomputed a number of wave realizations by means of a potential fully-nonlinear code (OceanWave3D), for a wide span of nondimensional water depths and significant wave heights.

The designer can then extract a wave kinematics time series from the precomputed set, scale it by the Froude law, and couple it with a suitable force model to compute loads. To complete the picture, slamming loads are calculated by means of the so-called pressure impulse model, recently developed at DTU.

Rather than computing the time series of the slamming load, the model uses a few parameters, all except one determinable from the incident wave to calculate the pressure impulse. First comparisons with experimental results, obtained in the framework of the DeRisk project, are promising. The force and the wave elevation statistics from the precomputed simulations show a good agreement with the experimental force and wave elevation signals.

Some discrepancies are present, due to an imperfect scaling and to the differences in the physical and numerical domains. The computed loads from the slamming model match the experimental ones quite closely, once the wave celerity extracted by the precomputed kinematics is corrected by an ad-hoc factor.