Numerical investigation of freak wave slamming on a fixed deck structure

Abstract

Wave impact loads on box-shaped structures highly depend on the wave morphology. This paper conducts a numerical study of freak wave impacts on a fixed, box-shaped deck. A numerical wave flume characterized by enhanced momentum conservation is developed, showing satisfactory accuracy and stability in reproducing freak wave impacts. By changing the horizontal locations of the deck, comparative analyses of the kinematics and dynamics on the front, top and bottom walls of the deck are performed. Based on the morphological features of the wavefront and overturning wave tongue, a quantitative approach for classifying the impact types is proposed. Four impact types are identified, including the unaerated impact of a non-breaking wave, the well-developed plunging breaker impacts with air entrapment on the top or front wall, and the broken wave impact. By investigating the characteristics of each impact type, it is found that the wave shapes and impact behaviours vary significantly on the front and top walls but show high similarities on the bottom wall. The well-developed plunging breaker applies the largest wave pressures and forces, especially when air entrapment happens. Significant negative pressures appear on the top and bottom walls, and the sharp right angles on the edges of the front wall play an important role in the generation of such negative pressures. The influences of entrapped air pockets on wave loads highly depend on their locations. In particular, the entrapped air results in large pressures and insignificant air cushioning effects on the front wall. The findings of the present study would advance the knowledge of the breaking wave impact on box-shaped deck structures, especially the behaviours of the air entrapment and the influence on impact loads, which could underpin the design and assessment of coastal and ocean structures with deck platforms.