Benchmark study of NEMOH vs WAMIT for the OC4-DeepCwind semi-submersible FOWT

This repository presents a benchmark study comparing two potential-flow Boundary Element Method (BEM) panel solvers, NEMOH and WAMIT, for the OC4-DeepCwind semi-submersible platform. The hydrodynamic analysis was conducted using NEMOH and benchmarked against the published WAMIT data for the OC4 test case available from the OpenFAST repository. NEMOH is an open-source BEM code developed to compute wave loads on offshore structures, including added mass, radiation damping, and diffraction forces. Notably, NEMOH is the only open-source solver that offers second-order solutions, which makes it suitable for this study.

The primary motivation for this work is to integrate an open-source BEM solver like NEMOH with OpenFAST for time-domain simulations under combined wave and wind conditions. The WAMIT OC4 hydrodynamic data include first-order results across different wave headings, while the second-order forces are only available for head seas (0 degrees). Therefore, this study aims to provide a complete hydrodynamic solution, including first- and second-order data for a range of wave headings from 0 to 180 degrees, leveraging NEMOH.

The comparison results show good overall agreement between NEMOH and the WAMIT reference data for the 0 wave heading.  The second-order solution followed the overall trends seen in WAMIT but showed greater deviations compared to the first-order data. More details in the attached report. 

NEMOH datasets are attached as WAMIT-format converted using BEMRosetta for 0-180 wave directions. The files needed as inputs for OpenFAST are .1 (frequency-dependent hydrodynamic added-mass and damping matrix from the wave radiation problem). The .3 file contains the frequency- and direction-dependent first-order wave-excitation loads. The .hst file contains the hydrostatic restoring (stiffness) matrix. The 12.d and 12.s second-order difference-frequency and second-order sum-frequency, respectively.