In head seas, the offshore construction vessel Pioneering Spirit experiences wave amplification in the slot between its bows. Wave run-up results in a water jet at the closed end. To avoid potential damage of structures in the slot and improve operational safety, more insight into this phenomenon is needed. The aim of this research is to investigate under what circumstances water jets occur within the slot of a floating structure in waves and find suitable means for attenuation of this effect. The solution should not interfere with pipelay equipment in the slot and be detachable for when the slot is required for other purposes.

Literature research on wave damping solutions in moonpools, floating breakwaters, and fixed free surface breakwaters indicates that a plate type fixed free surface breakwater offers an elegant option for wave attenuation. However, limited published research exists on the optimal design of such plates and their application for preventing wave run up and water jets on adjacent structures. Therefore, this study evaluates simple designs to gain new insights.

A linear Boundary Element solver is used to find a simplified model of the slot in which large waves occur, in order to save computational time when using a nonlinear solver.
It follows that the assumption that the vessel does not radiate waves can be made, seen as the effect of radiation on the wave elevations in the slot is small. A simplified model of the hull that is found to experience similar linear wave effects has a box-like shape with a slope underneath the hull and oblique planes to recreate the wave amplifying effect of the slot.


The Computational Fluid Dynamics (CFD) solver ComFLOW is used to simulate two different sea states in which water jets occur in the slot of the simplified model. Seen as three-dimensional (3D) simulations require a significant amount of computational time and power and water jets also occur in a two-dimensional (2D) setting, the simulations are performed in 2D.
The wave attenuation mechanisms in terms of reflected, lost and transmitted wave energy are evaluated for each breakwater design as well as the mean net force on the breakwater.
The results show that a plate under an angle of 60 degrees has small wave transmission and reflection, induces large wave energy loss and experiences a small mean net force. Furthermore, this model prevents the most water jets in both sea states compared to other models.
The results also indicate a linear correlation between the transmission coefficients of breakwaters and their average reduction of the net force on the hull corresponding to the water jets. There is no direct link between the prevention of water jets and the transmission past breakwaters, seen as the occurrence of water jets depends on the phase and shape of the wave as well as the incident wave height.

Further optimisation of the geometry of the 60 degree inclined plate and addition of porosity and appendages is recommended to increase energy dissipation and reduce forces on the breakwater. Additionally, future research should explore the relationship between transmission coefficients and the occurrence of water jets and also consider other factors beyond wave height that contribute to jet formation such as phase and the shape of the wave.