A study of wave forces on an offshore platform by direct CFD and Morison equation
Virginia Polytechnic Institute and State University, Blacksburg, Virginia, 24060, USA
Engineering simulation tools for hydrodynamic loads on offshore platforms (e.g. NREL’s HydroDyn and Orcina’s OrcaFlex) use simplified theories and empirical models. The most common is the Morison’s equation which accounts for unsteady drag and inertial forces created by incident waves. Coefficients are required for these models and are typically derived from wave-tank or pulsatile U-Shaped water tunnel experiments.
The first step of this study focuses on the application of Morison’s equations for computing forces on a 3D platform using both computational fluid dynamics computed and empirical coefficients. Wave forces on a simple structure, such as a bottom–mounted monopile, are estimated by a strip–wise application of Morison’s equation on each section. The fluid particles orbit and kinematics at strip–wise depth are readily derived from wave theory, and the total wave force on the supporting structure is computed by performing a simple integration in the vertical direction over each section.
The next step is the presentation of 3D multiphase RANS simulation of the wind-turbine platform in single-harmonic regular waves. Simulation results from full 3D simulation will be compared to the results from Morison’s equation. We are motivated by the challenges of a floating platform which has complex underwater geometry (e.g. tethered semi-submersible). In cases like this, our hypothesis is that Morison’s equation will result in inaccurate prediction of forces, due to the limitations of 2D coefficients of simple geometries, and that 3D multiphase RANS CFD will be required to generate reliable predictions of platform loads and motions.
© Owned by the authors, published by EDP Sciences, 2015
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