In order to gain greater insight into the phenomenon of breaking waves slamming on the legs of large-volume offshore platforms, MARINTEK’s Wave Impact Load Joint Industry Project (JIP) has studied a long-crested breaking-wave impact on a rectangular cylinder with simplified deck structure.
The prediction of extreme wave impact loads on the columns of offshore structures is of major importance for the design and limit-state analysis of column-based offshore platforms such as semisubmersibles and gravity-base structures. To investigate this problem, one of the sub-tasks of MARINTEK’s Wave Impact Loads JIP has focused on an idealised model test setup of a rectangular cylinder in breaking waves.
The first challenge was to improve the wave boundary condition used in the CFD simulation in order to match the exact motion history of the wave-maker with the measured free-surface elevation throughout the computational domain. Among the simulation parameters specifically tuned to capture the physics of this simulation, the numerical diffusion of the wave propagation was significantly diminished by the use of a higher-order time-integration scheme for volume of fluid (VOF).
Figure 1 shows a comparison of the results obtained by CFD against the measurements. Details of the fluid velocity at the wave crest are presented in Figure 2.
The commercial CFD tool Star-CCM+ was used to reproduce the experiments. The model consists of a vertical column with a fragment of a horizontal deck attached. Although the horizontal loads on the cylinder were the focus for these experiments, a simplified horizontal deck structure was attached on top of the column both in order to obtain a more representative structure for fluid-structure interactions, and to facilitate wave-in-deck measurements, which is another important area of interest with regard to design loads.
The wave elevation obtained from the CFD simulation of a long-crested breaking wave and its impact on the cylinder and deck structure was compared to the free-surface elevation measurements.
An almost exact match was obtained between the computed wave profile and the measured wave profile, and spatially averaged slamming pressures look fairly similar to the model test observations.
To see the figures in this article, please open the pdf version of the article in MARINTEK Review No. 2-12
