Prediction Of Structural Responses Of Flexible Polymer Hull Undergoing Water Impacts Using Coupled Eulerian-Lagrangian Analysis

Abstract

Slamming, which is when a marine craft hull impacts the water surface at high speed, is one of the critical load cases considered by designers of high-speed, light boats. These loads have the potential to produce both local and global deformation onto a vessel. The structural design of a polyethylene boat hull to resist slamming loads is particularly challenging, due to the low stiffness and strength of the material, which may result in greater deformation compared to more traditional aluminium and fibre-glass boats. The goal of this work was to develop and validate a computationally efficient approach for predicting the structural response of a complete boat hull using Coupled Eulerian-Lagrangian Analysis, implemented in a finite-element solver. This method was applied to a newly designed, 7-meter long polyethylene hull to assess its local and global response to slamming loads. To minimise the computational cost associated with running large Coupled-Eulerian-Lagrangian simulations on the complete hull, the research started with a mesh optimisation study to determine the lowest mesh density required to adequately model the effects of fluid impact onto a boat structure. Preliminary analyses were then conducted on the 7m hull using the optimal mesh to assess the structural strength of the hull in relation to slamming loads. To validate the numerical analysis method, experiments were conducted, where a nominally rigid, 1:10 scale boat model was subjected to constant velocity water impacts in a custom-built slam testing facility. With refinement to the initial numerical model, hydrodynamic forces measured in the experiments were found to correlate closely with the corresponding simulations. Following validation and refinement of the initial numerical model, a more thorough design review of the new 7m hull was conducted. Analyses of the complete hull indicated that maximum stresses on the proposed structure would exceed the yield limit for polyethylene during typical slamming impacts. Although global bending was found to be relatively low, significant bending of individual panels and girders along the length of the vessel was observed. One of the methods investigated to strengthen the hull was increasing the thickness of three key structural members from 12 mm to 20 mm, which reduced maximum stress in the structure by around 40%, while only increasing the material mass of the hull by 4%. Overall, the research demonstrated that with careful model formulation, Coupled Eulerian-Lagrangian simulations can be effectively and efficiently used for the assessment and optimisation of hull structures subjected to water impact.