Steep Wave Loads From Irregular Waves on an Offshore Wind Turbine Foundation: Computation and Experiment

Bo Terp PaulsenDTU Mechanical EngineeringDK-2800 Kgs. LyngbyDenmarkEmail: botp@dtu.dkHenrik BredmoseDTU Wind EnergyDK-2800 Kgs. LyngbyDenmarkEmail: hbre@dtu.dkHarry B. BinghamDTU Mechanical EngineeringDK-2800 Kgs. LyngbyDenmarkEmail: hbb@dtu.dkSigne SchloerDTU Wind EnergyDK-2800 Kgs. LyngbyDenmarkEmail: sigs@dtu.dkABSTRACTTwo-dimensional irregular waves on a sloping bed and theirimpact on a bottom mounted circular cylinder is modeled bythree different numerical methods and the results are validatedagainst laboratory experiments. We here consider the perfor-mance of a linear-, a fully nonlinear potential flow solver and afully nonlinear Navier-Stokes/VOF solver. The validation is car-ried out in terms of both the free surface elevation and the inlineforce. Special attention is paid to the ultimate load in case of asingle wave event and the general ability of the numerical modelsto capture the higher harmonic forcing. The test case is repre-sentative for monopile foundations at intermediate water depths.The potential flow computations are carried out in a two-dimensional vertical plane and the inline force on the cylinderis evaluated by the Morison equation. The Navier-Stokes/VOFcomputations are carried out in three-dimensions and the forceis obtained by spatial pressure integration over the wettet areaof the cylinder. In terms of both the free surface elevation andthe inline force, the linear potential flow model is shown to be oflimited accuracy and large deviations are generally seen whencompared to the experimental measurements. The fully nonlin-ear Navier-Stokes/VOF computations are accurately predictingboth the free surface elevation and the inline force. However, thecomputational cost is high relative to the potential flow solvers.Despite the fact that the nonlinear potential flow model is car-ried out in two-dimensions it is shown to perform just as goodas the three-dimensional Navier-Stokes/VOF solver. This is ob-served for both the free surface elevation and the inline force,where both the ultimate load and the higher harmonic forces areaccurately predicted. This shows that for moderately steep irreg-ular waves a Morison equation combined with a fully nonlineartwo-dimensional potential flow solver can be a good approxima-tion.1 IntroductionFor most offshore engineering cases an accurate determina-tion of hydrodynamic loads is crucial for an economic yet safedesign. Traditionally these loads are estimated from laboratoryexperiments and/or numerical computations. Laboratory experi-ments are often costly and normally restricted to small scale dueto the limited size of the test facilities. In the numerical com-putations one is not restricted by the size of the experimental1 Copyright © 2013 by ASME