Sheltering Analysis of Gravity Based Structures in Shallow Water

Abstract

This thesis has addressed the hydrodynamic problem of sheltering effects from gravity based structures by both numerical and experimental methods. GeniE is used for creating panel models of the structures studied. Wadam is used to calculate the frequency-domain normalized wave elevation surrounding the structures, based on linear potential theory. In order to validate and investigate the numerical solutions calculated from Wadam, model tests were conducted in March 2018. Wave periods from 5 to 15 [s] were studied, with wave steepness varying from $H/\lambda$ = 1/100 to 1/30.

Three different model setups were tested, with focus on different aspects of the problem at hand. The first setup focused on the wave field surrounding the structures. In the second setup a revision was made to the original setup by creating a model of a LNG-carrier behind the structures. This model was fixed, and gave an indication on the free-surface elevation in the gap between the vessel and the structures. The third and final model aimed to gain a better understanding of a third-order phenomena occurring due to wave interactions between the incoming and reflected wave fields.

Most of the results obtained from the numerical simulations was validated by the experimental model tests. The largest discrepancies occurred in regards to the wave elevations in the gap between the modules and the vessel, due to viscous effects. In addition, third-order effects occur at wavelengths corresponding to the length of a module, showing an increase in wave elevation on the models with increasing steepness.

The conclusion of this thesis is two-sided. The structures provides a lowered normalized wave elevation on the leeward side for most wave periods and directions studied. However, this was in the case with no vessel present. When a vessel is modeled behind the structure, large free-surface elevations occur in the gap between them. In this case, the interaction between the moored vessel and the structures will be of great importance.