Edge scour around an offshore wind turbine

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Abstract

Wind energy has experienced an enormous growth in the last years and is becoming more and more popular as an alternative for conventional power. Large growth numbers are also expected for the coming years, for onshore as well as for offshore wind energy. Currently, a wind power capacity of almost 4 GW is installed offshore in the European Union, meeting 0.4% of the European electricity need at this moment. More than 137 GW of offshore wind energy is being planned, consented or under construction in the European waters. This total of offshore wind power capacity could provide approximately 14% of the European demand for electricity. With an expected offshore installation of 1 MW per year, 9.9 million tonnes of CO2 emissions can be avoided annually [EWEA (2011b)]. The first offshore wind park in The Netherlands is Offshore Wind park Egmond aan Zee (OWEZ). The 36 wind turbines in this wind park have a capacity of 3 MW, together producing sufficient renewable energy for more than 100,000 households, approximately the size of the city of Eindhoven [NOORDZEEWIND (2011)]. In order to reach the mentioned numbers for the offshore wind energy, some engineering challenges need to be overcome. One of these challenges in offshore wind park construction is the bed protection around the turbine foundations. Surveys of the bed levels around the wind turbine foundations in OWEZ have shown that just beyond the scour protection edge scour develops. This can cause damage to the electricity cables buried in the bed. A damaged electricity cable results in down time of the wind turbine and the wind turbines connected to it further in the string. The required burial depth of the cables is therefore governed by the location and depth of the edge scour hole. Edge scour is not very well understood and therefore difficult to quantify. Improved insight in the development of edge scour is valuable for science and industry. The present exploratory study revealed that more research is needed to compute edge scour development correctly with the software package Delft3D-Flow. However, even if the model results in Delft3D-Flow would be perfect, considerable or even unacceptable computational times need to be overcome. Therefore, a less time consuming method to calculate the edge scour depth would be very valuable. In order to develop such a method for calculating edge scour depth, the following objectives have been studied: 1. Gain insight in the hydrodynamic and morphodynamic processes around the foundation of an offshore wind turbine with bottom protection, focussing on edge scour. 2. Explore the possibilities of applying Delft3D-Flow for modelling the hydrodynamic and morphodynamic processes around the foundation of an offshore wind turbine with bottom protection, focussing on edge scour. 3. Develop a model to predict the depth and rate of edge scour around the foundation of an offshore wind turbine. The research reveals that tidal asymmetry is of major concern in the development of edge scour. Edge scour develops mainly downstream of the wind turbine for the dominant tide. In addition, lee-wake vortices downstream the wind turbine play an important role in the formation of the edge scour holes. The performance of Delft3D-Flow is rather poor in this specific situation. A lack of resemblance between the bed levels computed with Delft3D-Flow and the measured bed levels in OWEZ exists. Most likely this is due to mediocre performance of the two-dimensional model with respect to the hydrodynamics in this specific situation. It is recommended to investigate this in more detail. Due to the lack of detailed measurements of the hydrodynamics, it is difficult to validate the model results. In order to calculate the edge scour depth, the Edge Scour Prediction Model (ESPM) has been developed. This is a model based on mathematical relations of development towards an equilibrium in time and empirical relations for the equilibrium edge scour depth and characteristic timescale. The Delft3D-Flow model is applied to assess the amplification factor as input for the ESPM. The ESPM has proven to reproduce the edge scour depth as function of time in OWEZ reasonably well. In addition, it can be a valuable tool for a first impression of the edge scour depth in new designs at other locations.