Numerical estimation and application of slamming wave impact on monopile structures

Abstract

For the offshore wind industry it is essential to further reduce costs to be competitive to traditional energy resources. One of the options to achieve so is optimizing support structure design. Using XL-monopiles allows for larger turbines, deeper water access and consequently can provide for a more cost effective support structure. Within this market, Energieonderzoek Centrum Nederland (ECN) operates with its in-house developed software package to evaluate turbine responses. To further improve the model, a better insight is needed in breaking wave impacts on structures.
This thesis is focused on developing an enhanced method to evaluate breaking waves on monopile structures to identify the effect of slamming waves on XL-monopiles. Based on wave tank measurements, a method to identify slamming wave impacts is developed and tested. Later, those wave tank measurements are reproduced and the identification method verified.
Wave tank experiments were carried out at the Atlantic Basin at Deltares within the joint industry research project WiFi. For the experiments, two monopile scale models were placed in the wave tank, both equipped with multiple load and pressure sensors. During the tests the monopiles were exposed to series of wave trains, the irregular wave trains with a total of approximately 5000 waves, created a large sample size needed for the experiment. The wave measurements from the tank are analysed numerically to identify breaking waves. In this thesis a slamming wave event definition is proposed as follows:
• Front crest steepness 푆 should reach breaking limit
• The slamming impact of the wave should be more than 4 times the standard deviation of the force time series
The numerical computations are carried out using the potential flow solver OceanWave3D to generate a sea state with comparable characteristics to the wave tank measurements. At first, the generated sea state appeared to lack the necessary wave height. By adjusting the input in the OceanWave3D program, a sea state was found that matches the measurements. Wave energy dissipation is checked throughout the measurements wave tank and compared to dissipation in the numerical model. It was shown that both the wave tank and numerical model show resembling dissipation.
Hydrodynamic loading on the monopile foundation is assessed using the kinematics obtained from the OceanWave3D model. Several methods were combined to come to the total wave loading. First the Morison approach was used to account for the non-slamming part for the wave load. Using OceanWave3D, the force coefficients are calculated applying DNV guidelines. The second part, the slamming part of the wave load, is obtained in multiple phases. First, based on the above mentioned slamming wave criteria, wave steepness and impact forces, the individual waves are evaluated and scored as potential slamming. Secondly, using conservation of momentum and kinematics derived from OceanWave3D, the slam load is calculated. This slam load is determined evaluating the impact velocity of all waves individually and added to the Morison load if identified as slamming. Finally, when the calculated impact loads from the numerical model are compared to the recorded loads in the wave tank measurements, large similarity can be noticed.
After comparison of the developed slam load representation to the DNV method of slam load estimation, the result is a less conservative slam load approximation. This is due to the evaluation of slamming impact velocities per wave, opposed to the assumption of a single impact velocity for the whole sea state. The results of this thesis can further implemented in turbine response evaluation tools by ECN resulting in a more optimized calculation model. It will enable the design of more (cost) efficient XL-monopile structures.