Wave-induced nonlinear effects on a pontoon-type Very Large Floating Structures (VLFS)

Abstract

The wave-induced nonlinear effects on the pontoon-type Very Large Floating Structure (VLFS) is investigated in this thesis. Typical dimensions of the pontoon-type VLFS such as a floating airport are 5000m in length, 2000m in width, while only a few meters in depth. The nonlinear effects studied here mainly mean that the bottom slamming may occur in the front of the structure when the amplitude of relative wave motion at bow is larger than the instantaneous draft of the structure.

A two-dimensional (2D) fully-nonlinear Numerical Wave Tank (NWT) based on a twodimensional Boundary Element Method (BEM) is developed in time domain to investigate the wave-induced nonlinear effects on the pontoon-type VLFS. The robustness of the numerical solver for the nonlinear wave-rigid-structure interaction is verified by implementing convergence analyses and comparing with the analytical solution or available experimental data. Two main types of bottom slamming have been observed in Yoshimoto’s experiment (1997). One type involves an air cavity that occurs near the front edge. The other one occurs without air entrapment. For both types of impact, bottom slamming can occur with a very small angle between the free surface and the bottom. A local analytical solution is then coupled with the BEM solver around the impact area. The occurrence of bottom slamming with an air cavity is mainly examined by varying the parameters of the incident waves and the water depth. The entrapped air cavity deforms and moves under the interaction with the surrounding flow. Eventually it tends to detach from the bottom and collapse into bubbles. The latter aspect is not discussed in this thesis.

Due to the elastic characteristics of the pontoon-type VLFS, the global linear hydroelastic response of the structure is investigated in frequency domain (FD) and its effects on the occurrence of the local bottom slamming are considered in the local nonlinear analysis (LNS) in time domain. This FD+LNS algorithm is a unidirectional coupling, where the information goes from the global to the local analysis and not vice versa. The local hydroelasticity is assumed not significant for the global effect, due to the large difference between local and global natural frequencies.

The pontoon-type VLFS has great horizontal extensions and the sea floor topography can vary notably along the length and width of the structure. Therefore the effects of the variable bottom on the hydrodynamic response of a rigid body and the hydroelastic response of a flexible body, respectively, are examined numerically in this thesis. The hydroelastic analysis of the structure is performed by assuming more general three-dimensional (3D) conditions. Subsequently, the uneven-bottom effects on the occurrence and features of local bottom slamming are also examined either by the 2D BEM solver for a rigid body, or by the FD+LNS solver for a flexible body.