On the Validity and Sensitivity of CFD Simulations for a Deterministic Breaking Wave Impact on a Semi Submersible

Abstract

To assess the integrity and safety of structures offshore, prediction of run-up, green water, and impact loads needs to be made during the structure’s design. For predicting these highly non-linear phenomena, most of the offshore industry relies on detailed model testing. In the last couple of years however, CFD simulations have shown more and more promising results in predicting these events, see for instance [1]–[4]. To obtain confidence in the accuracy of CFD simulations in the challenging field of extreme wave impacts, a proper validation of such CFD tools is essential. In this paper two CFD tools are considered for the simulation of a deterministic breaking wave impact on a fixed semi submersible, resulting in flow phenomena like wave run-up, horizontal wave impact and deck impacts. Hereby, one of the CFD tools applies an unstructured gridding approach and implicit free-surface reconstruction, and uses an implicit time integration with a fixed time step. The other CFD tool explicitly reconstructs the free surface on a structured grid and integrates the free surface explicitly in time, using a variable time step. The presented simulations use a compact computational domain with wave absorbing boundary conditions and local grid refinement to reduce CPU time. Besides a typical verification and validation of the results, for one of the CFD tools a sensitivity study is performed in which the influence of small variations in the incoming breaking wave on the overall results is assessed. Such an analysis should provide the industry more insight in the to-be-expected sensitivity (and hence uncertainty) of CFD simulations for these type of applications. Experiments carried out by MARIN are used to validate all the presented simulation results.