Comparison of Morison Forces With CFD Modelling for a Surface Piercing Column of a FOWT

An accurate estimation of the low-frequency wave loads on floating wind turbines is important for design of mooring systems. In order to add wave drift viscous effects, to the potential flow loads from radiation/diffraction codes, one approach would be to use the drag term in Morison’s equation integrated to the instantaneous free surface. The aim of this study is to define the drag coefficients depending on distance to free surface and KC (Keulegan-Carpenter) number. A systematic series of CFD simulations of Stokes fifth order waves has been done for a fixed structure resembling the floating wind turbine structure studied in WINDMOOR project. Simulations are run with the CFD code STAR-CCM+. The structure is a tripod composed of three vertical columns joined at the bottom by pontoons and at the top by deck beams. As one of the steps towards the analysis of the whole structure, the sectional CFD forces on one of the columns is studied here. Morison mass and drag coefficients are determined for each cross section of the column from the CFD simulation. Obtained sectional coefficients and total forces are compared with the available experimental data in the literature for a circular cylinder. The possible interaction effects between the columns are investigated. Forces on the sections close to the free surface are addressed and the accuracy of the Morison formula for these sections is discussed.