Dimensioning sea loads on offshore wind turbines in shallow to intermediate waters

Offshore wind turbines are fully exposed to the elements, and the forces acting on them from the sea are important to their design, behavior, reliability and cost. The DIMSELO project focused on developing knowledge and simulation tools for engineers related to sea loads on offshore wind turbines in shallow and intermediate waters. This allows for more realistic prediction of sea loads, reduction of uncertainties and reduction of the cost of offshore wind energy. This includes both the behavior of the ocean waves itself and the resulting forces on offshore wind turbines. The most exciting innovations from DIMSELO are perhaps two new methods that allows for more realistic computer simulations of waves. The first innovation is a new method that allow engineers to simulate extreme waves more realistically. The second innovation is a new method that for the first time allow engineers to simulate medium large waves realistically. We then wanted to see what the consequences of using different existing and new computer simulation tools were. We put all the different options into IFE's offshore wind turbine simulation tools 3DFloat, so we could do proper apples to apples companions. We then designed three wind turbines to test the different engineering tool on, a 10MW monopile wind turbine in 25m water depth, a 10 MW monopile wind turbine in 35m water depth and a 10MW wind turbine on a jacket substructure in 35m depth. We then proceeded to compare all the different tools engineers could use with respect to the calculated resulting loads. We saw that the different engineering tools could produce significantly different results with respect to for example the estimated life span of an offshore wind turbines. The life time of a turbine can vary up to 15% depending on what methods are used. The project also had other activities. It worked on the development of the next generation of ocean wave models with an even higher level of realism. We developed methods that allowed us to simulate realistic irregular waves and the loads they cause by use of CFD simulations (Computational Fluid Dynamics). We worked on wind loads on offshore wind turbines and showed that the two approved engineering methods for simulating the wind could result in significantly different loads on offshore wind turbines.

The lifetime cost of energy (LCOE) is currently relatively high for offshore wind turbines and very important for the viability of large scale deployment of environmentally friendly offshore wind energy. In some cases the hydrodynamic sea loads drives the design, dimensions and cost of the offshore wind turbines. Our main focus will be on how to with sufficiently accurately model the sea behaviour and the following dimensioning sea loads with engineering tools. There is a significant potential for cos t reductions if larger, but unproven wind turbine rotors were to be used offshore. Enhanced knowledge of their performance with respect to interaction between wind, rotor, structure, sea and dimensioning loads is needed to enable their deployment. This wi ll be our other main area of research. Our focus will be on conditions and offshore wind turbine concepts relevant for North Sea sites with shallow to intermediate water depths.