Development and verification of modelling practice for CFD calculations to obtain hydrodynamic damping of offshore vessels

Estimates for hydrodynamic damping on offshore vessels are important input parameters for simulations studies on
vessels operating at sea, e.g. moored vessels in waves, wind and current, workability and operability investigations, Dynamic Position studies, ship-to-ship operations and safety studies of vessels. To accurately predict the motions of vessels this quantity should be determined with confidence. Traditionally, model experiments in water basins using so-called decay or forced oscillation tests are carried out to determine the hydrodynamic damping. With recent advancements in CFD modelling, the offshore industry has started using CFD as an alternative method to compute the hydrodynamic damping of offshore vessels. In order to help adopt CFD as a widely accepted tool, there is a need to develop confidence in CFD predictions. Therefore, a practical Modelling Practice is developed within the Reproducible CFD JIP to obtain the hydrodynamic damping values using CFD simulations. The Modelling Practice describes the geometry modelling, computational mesh, model set-up and post-processing for these type of CFD calculations. This Modelling Practice is verified and validated by three independent verifiers against available model test data.
This paper provides an overview of the calculated CFD results from the verifiers when following the CFD Modelling
Practice. The results are benchmarked against available model test results for a barge-shaped and tanker-shaped FPSO. The objective of the presented work is to obtain the hydrodynamic damping for the surge, sway and roll motions at low KC values and it is a continuation of the results presented in earlier papers OMAE2020-19173 and OMAE2021-62411. By following this Modelling Practice, the CFD predictions for the quadratic damping coefficients as well as the natural periods are consistent between the different verifiers, i.e. within 20% from each other, and within 20% of the model test results. Exceptions to this general statement are found for the surge direction where a large difference is found between the calculated CD values and the measured CD values as well as for the roll direction, where the
CFD results between the various verifiers can show larger differences depending on the numerical settings.
Therefore, we conclude that when following the developed Modelling Practice for CFD simulations, consistent results can be obtained for the hydrodynamic damping of offshore vessels in surge, sway and roll direction, but that additional sensitivity studies still need to be carried out for specific situations.