Urans predictions of drift loads on a semi submersible in steep waves

The last decade, many semi-submersible designs have been tested in MARIN’s wave basins as a result of recently modified classification society guidelines requiring to investigate the air-gap of semi-submersibles in waves and the occurrence of wave impacts on deck. The main objective of these tests is to investigate the motions, air-gap and mooring line loads in extreme irregular seas. In several of these model test projects, it was found that accurately predicting the motions of the platform using numerical simulations is not an easy task. Especially, the measured horizontal and vertical low-frequency resonant motions were regularly found to be much larger than predicted by numerical results based on linear potential flow theory. These low-frequency motions are mainly caused by the wave drift loads, i.e. the mean and low-frequency wave loads resulting from the non-linear behavior of the structure in the waves. As these drift loads can be affected by higher than second order effects, model testing is still the preferred method to quantify the loads and resulting motions in survival sea states. However, model test results are most often not available in the early design stage and modifying the design may not be easy when model test results become available, further on in the project. Therefore, the development of a reliable and fast prediction technique would be valuable to improve the design of moored semi-submersibles. Such a method may be CFD, which has the potential to predict the motions and drift loads applied on semi-submersibles in steep waves, without and with the presence of current. To obtain confidence in the accuracy of the CFD results, a validation of these results is essential in the first place.
In this paper, the motions and loads applied on the Stena Don semi-submersible in head-on waves are calculated using MARIN’s CFD code ReFRESCO. The objective is to assess whether the motions and the mean drift loads can be correctly predicted using CFD for regular waves of different steepness. The horizontal drift loads are focused on in this paper. In order to assess the accuracy of the results and the capability of CFD, a stepwise approach has been followed. First, the wave generation and propagation is investigated without any floater present. Then, the soft-moored semi-submersible is simulated in regular waves, with and without current, and the vessel motions and mean drift loads are analyzed. Finally, the CFD results are compared to results from potential flow based linear diffraction calculations, and to results from the wave basin. This study confirms that CFD is able to calculate the drift loads accurately in various regular waves. Furthermore, it can be observed that linear potential flow theory severely underpredicts the mean drift loads for steep waves with or without current.