The hydrodynamic loads on a monopile is calculated using different hydrodynamic models and CFD.
The experiment of J. Grue and M. Huseby (2002) is reproduced in OpenFoam (left) and Orcaflex (right). Video is running at 25% speed of real experiment.
As part of validation of the wave model in the CFD software OpenFoam we reproduced a model scale experiment from J. Grue and M. Huseby (2002). The wave tank used in the experiments is 24.57 meters long, with water depth of 0.6 meters. Monopile diameter is 0.06 m. Wave height is 0.11 m with period 0.86s. Assuming model scale factor of 100, this means full scale diameter of 6m, wave height of11 m, period of 8.6s and depth of 60m. We used OpenFoam version 10(openfoam.org). This version includes Stokes 5th order wave model among others. We also ran the same setup in Orcaflex with Morison model and potential theory (1st order).
The relative importance of inertial forces, drag forces and diffraction are determined by the Kuelegan-Carpenter number KC, and the relation D/wavelength, whereD is monopile diameter. For this setup the KC number is about 6 and D/wavelength is about 0.05. This is in the regime where inertia forces dominates, but drag should be considered.
The figures below shows a comparison of horizontal force onthe monopile for experimental data, CFD, Morison and potential theory. Relative good agreement with all models are achieved. Notice that Morison force is conservative using added mass coefficient Ca=1.0 and drag coefficient Cd=1.0. CFD achieves the best match, and also captures the secondary load cycle (at t=3.3 and t=4.1). The green curve is a combination of potential theory below water level and Morison only above water level. This approach captures the additional load from instantaneous water level not included in potential theory. We also compare Morison force with and without drag, the contribution from drag is negligible for this case.
The bending moment at monopile bottom is also plotted. Notice that the bending moment is slightly underestimated by Morison and potential theory models with added mass Ca=1.0.
