Analysis of experimental data: The average shape of extreme wave forces on monopile foundations and the NewForce model

Experiments with a stiff pile subjected to extreme wave forces typical of offshore wind farm storm conditions are considered. The exceedance probability curves of the nondimensional force peaks and crest heights are analysed. The average force time history normalised with their peak values are compared across the sea states. It is found that the force shapes show a clear similarity when grouped after the values of the normalised peak force, F/(ρghR2), normalised depth h/(gT2p) and presented in a normalised time scale t/Ta. For the largest force events, slamming can be seen as a distinct 'hat' on top of the smoother underlying force curve. The force shapes are numerically reproduced using a design force model, NewForce, which is introduced here for the first time to both first and second order in wave steepness. For force shapes which are not asymmetric, the NewForce model compares well to the average shapes. For more nonlinear wave shapes, higher order terms has to be considered in order for the NewForce model to be able to predict the expected shapes.