Impact of box-type floating breakwater on motion response of hydrodynamically coupled floating platforms downstream

Abstract

Floating breakwaters are applicable in several offshore applications to protect downstream structures from excessive wave loads and to reduce their motion response. This thesis investigates the impact of the leading platform, functionally a breakwater, on the RAOs and response of the platforms behind it in frequency domain. The structures are only hydrodynamically coupled. All models were analysed for head waves only. Variables in model design are investigated using a combination of diffraction software and solving equations of motion in six degrees of freedom for each body. The variables are the gap between the breakwater and the first platform, the width, i.e. side perpendicular to wave direction, of the breakwater and the gap between platforms. Base case dimensions for the platforms and breakwater are chosen based on the natural frequencies and wave transmission coefficients respectively. These choices are made to function well for the wave spectrum at the chosen site. Each case analysed included a breakwater and 10 platforms downstream of it. It was found that he imperfect efficiency of a breakwater means that the first few platforms behind it act as breakwaters too; albeit for much lower wave energies. Increasing the gap between the breakwater and the first platform behind it results in a decrease in RAO of the platforms due to increasing hydrodynamic coupling. The impact on pitch RAOs is greater than the impact on the heave RAOs. Increasing the width of the breakwater results in minimal reduction in RAOs of the platforms behind it at large gap sizes. At small gap sizes, there is an adverse relationship. Hydrodynamic coupling between the platforms can lead to shared natural frequencies within the design frequency range, leading to a large motion response. This can be prevented by changing the gap size and thereby the hydrodynamic coupling and moving the natural frequency outside of the design range. A breakwater and multiple platforms downstream of dimension L=100 m, B=100 m, T=5 m, with a gap of 80 m between the breakwater and platforms and 100 m between the platforms themselves was shown to effectively reduce the motions of the downstream platforms. The first few platforms exhibit higher heave than the other platforms, but similar pitch to the other platforms. Therefore, their use cases in a floating city must be chosen accordingly. The potential negative impacts of hydrodynamic coupling between the platforms means that the platforms must be further apart from each other, resulting in floating cities with a much larger footprint than previously expected. Further research into the motion response for 2D structure layout and different wave directions would be interesting follow-ups to this thesis.