Probability of wave slamming and the magnitude of slamming loads on offshore wind turbine foundations

Abstract

Physical model tests were analyzed with the objective to establish a formulae for the probability of wave slamming on offshore wind turbine foundations and the associated slamming force. Furthermore, the effect of wave slamming is analyzed for a monopile wind turbine foundation using a state-of-the-art aeroelastic model. The laboratory measurements were carried out in Deltares’ Atlantic Basin as part of the joint industry project “wave impacts on fixed turbines”, in short JIP-WiFi. Long crested and irregular waves typical for the North Sea were applied and more than 130 design storms and 500 slamming impacts were analyzed. A simple closed form expression for the probability of wave slamming is presented and a reasonable agreement with the laboratory measurements shown. A criterion for when wave slamming should be included in design computations is formulated based on the sea state steepness. It is observed that wave slamming occurs during tests with a flat seabed, and that seabed features, such as sand waves, significantly increase the probability of wave slamming. Furthermore, a simplified slamming load formulation is proposed based on observations from a subset of the laboratory tests. The magnitude of the slamming load is validated against the remaining part of the laboratory measurements and compared with existing slamming load formulations. The new formulation is in better agreement with the underlying measurements than existing formulations and is of a simpler form which makes it easier to apply. The new slamming load formulation is applied in design computations for a realistic wind turbine with the offshore wind farm Gemini as base case. The computations show that wave slamming, at this specific location, is not governing the structural design. However, wave slamming may introduce high accelerations in the transition piece located in the free surface zone where waves are impacting the structure.