The Dimensioning Sea Loads (DIMSELO) project: Paper

DIMSELO is a Competence Building for Industry project (KPN), granted by the Norwegian Research Council under the ENERGIX program, which ran from 2013 to 2017. The project’s objective was to quantify the consequences of using advanced sea loads modeling in integrated simulations of offshore wind turbines in shallow to intermediate waters.

During the project, engineering hydrodynamics load and wave kinematics models of increasing complexity and fidelity were chosen from the literature and implemented. The effect of different model combinations on the substructure loads was was tested for three reference turbines: a DTU 10 MW rotor positioned on a monopile at a water depth of h = 25 m, on a second monopile at h = 35 m and on a jacket, also at h = 35 m.

In this paper, the fatigue loads in a production case for the h = 25 m monopile was calculated via three different load models: the well-known Morison model, the Rainey nonlinear force model and the McCamy-Fuchs linear diffraction model. The models were coupled to kinematics coming both from linear irregular waves and second order irregular waves.

The comparison showed that using the McCamy-Fuchs diffraction theory reduced the predicted fatigue damage by 15% with respect to a base case where the Morison load model was used. Nonlinear wave kinematics and nonlinear force models influenced the force calculations but did not alter the total fatigue damage, since the load cases with high wave steepness were less likely to happen.

In parallel to the research on engineering models, detailed wave loads models were also developed during DIMSELO. By means of CFD, it was possible to reproduce experimental time series of wave loads on a cylinder induced by regular and irregular nearly-breaking waves. Also, a spectral solver for the fast resolution of the fully nonlinear diffraction problem was devised at DTU during DIMSELO.

The solver, which showed encouraging results in the preliminary testing, can be coupled to any nonlinear kinematics solver to calculate the force on a cylinder retaining full nonlinearity and diffraction.