Comparison of impulse based substructuring and fully coupled analysis for offshore wind turbines
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With an ever increasing demand for clean renewable energy, wind energy is a rapidlyexpanding 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 offshoreenvironment makes the installation of the turbine challenging and the support structureheavy.While the wind turbine in an off-the-shelf product the support structure has to be tailormade 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 optimizethe support structure, making it as cheap as possible while still being able to withstandthe environmental forces for 20 years. It is important to bring the costs of offshore windenergy down. One method of doing this is by increasing the accuracy of the dynamicsimulation needed to design the support structure of the wind turbine.The support structures and the wind turbine are designed by two separate companiesthat dont want to share each others design details. In current practice the support structureis 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 basedsubstructuring. With this method instead of a simplied model of the support structure,its impulse responses or greens functions are shared. With these impulse responses thedynamic response of the support structure can be accurately calculated, without revealingits design.This method is tested with both a monopile and a jacket support structure and is comparedto a fully coupled method. Dierent amounts of eigenfrequencies are added tothe impulse response to nd the number required for suciently detailed results. Also amethod 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 modesare used.In addition to this, the impulse responses are parameterized. Two parameters are usedfor each support structure; wall thickness and diameter for the monopile and leg thicknessand diameter for the jacket. For several combinations of these parameters the impulseresponses are calculated after which a polynomial function is tted to the results to beable to estimate the responses for random inputs for the parameters. It is shown thatwhen done properly only little extra error is introduced in the results.Impulse based substructuring is a viable alternative to the current industry practice, givingaccurate results while not sharing design details.