Comparison of impulse based substructuring and fully coupled analysis for offshore wind turbines

Abstract

With an ever increasing demand for clean renewable energy, wind energy is a rapidly expanding market. Due to noise and visual pollution as well as limited space on land, offshore wind turbines are getting more popular. One of the main concerns of offshore wind energy are the high costs. The rough offshore environment makes the installation of the turbine challenging and the support structure heavy. While the wind turbine in an off-the-shelf product the support structure has to be tailor made for every wind park because of the unique combination of the water depth, soil conditions, and environmental conditions. Dynamic simulations have to be done to optimize the support structure, making it as cheap as possible while still being able to withstand the environmental forces for 20 years. It is important to bring the costs of offshore wind energy down. One method of doing this is by increasing the accuracy of the dynamic simulation needed to design the support structure of the wind turbine. The support structures and the wind turbine are designed by two separate companies that dont want to share each others design details. In current practice the support structure is simplied and send to the wind turbine designer, who then does the simulations. This simplication can result in signicant errors. An alternative to this is impulse based substructuring. With this method instead of a simplied model of the support structure, its impulse responses or greens functions are shared. With these impulse responses the dynamic response of the support structure can be accurately calculated, without revealing its design. This method is tested with both a monopile and a jacket support structure and is compared to a fully coupled method. Dierent amounts of eigenfrequencies are added to the impulse response to nd the number required for suciently detailed results. Also a method is proposed to compensate for the exibility of the truncated modes. It is shown that this compensation is needed for accurate results when a limited amount of modes are used. In addition to this, the impulse responses are parameterized. Two parameters are used for each support structure; wall thickness and diameter for the monopile and leg thickness and diameter for the jacket. For several combinations of these parameters the impulse responses are calculated after which a polynomial function is tted to the results to be able to estimate the responses for random inputs for the parameters. It is shown that when done properly only little extra error is introduced in the results. Impulse based substructuring is a viable alternative to the current industry practice, giving accurate results while not sharing design details.