Calculation of slamming wave loads on monopiles using fully nonlinear kinematics and a pressure impulse model

Abstract

The design methods for highly nonlinear wave loads on monopile structures has over the past years been extended with methods based on pre-computed fully nonlinear wave kinematics. Yet, the slamming events of the strong sea states cannot currently be predicted with these methods. We here present a simple recipe for the application of a recent pressure impulse based slamming load model in combination with fully nonlinear wave kinematics and validate the results against lab measurements of uni- and multi-directional storm sea states. The experimental slamming loads are extracted from lab measurements equivalent to 954 full scale hours. Six methods for the extraction of the slamming force are developed and analysed in detail, with a final selection of two for the further analysis. The experimental analysis shows that the frequency of slamming is larger in uni-directional sea states relative to sea states with directional spreading, and with slightly smaller force impulses. The calculated slamming frequencies from the measurements are used in the application of the numerical slamming model. It is shown that the application is straightforward and robust and involves an intuitive selection of the model inputs from the incident wave kinematics. A generally good agreement between the model and measurement distributions of the force impulse is observed. The difference between 3D and 2D slamming impulses, though, is found to be larger in the numerical model. This is traced to the numerical particle velocities in the wave crests. The pressure impulse model is next extended by assuming a predefined generic slamming force time variation and through calibration of the peak slamming force, a generally good agreement between the model and ensemble-averaged measured slamming force time series is obtained, given the uncertainty in the slamming load extraction. It is also observed that the commonly used non-dimensional slamming force peak of 2π is unrealistically large in the irregular slamming waves because of the 3D effects of small curling factors.