Form Finding for a Submerged Floating Tunnel: The Clever Cross-section for Coastal Crossings

Although the concept of a submerged floating tunnel (SFT) originates from the early 1900s, the cross-section is always assumed to be circular or rectangular. In this research, it is investigated whether this assumption is valid. In order to find the optimal cross-section, the optimization process is split into two targets. The first targets aims for an optimization of material use considering the large hydrostatic water pressure. The second target guarantees a tensile force in the tethers that support the SFT, while keeping the buoyancy weight ratio (BWR) as close to 1.0 as possible. This is beneficial for both the tunnel tube and the tether system. The optimal cross-section considering the optimization of material use is an 'egg-shape' or an oval which deviates slightly from a circle. This is modelled by the form-finding process as executed in the Grasshopper software. This is due to the relative pressure difference between SFT-top and SFT-bottom. The ovalization decreases for increasing depth. The second target results in an ellipsoidal shape, with a flat bottom and convexly shaped top (in case the SFT is anchored to the seabed). This shape reduces the drag and turbulence on the SFT, while a lift force is generated contributing to the tensile force in the tethers. An analogy with an aeroplane wing is made. The magnitude of the lift and drag force is evaluated by solving for the Panel Method in Python. A significant reduction in drag and turbulence becomes visible. Moreover, some significant lift forces are generated by the encountering flow. The wave load must be compensated by the BWR to assure a tensile force in the tethers. All together, two types of solutions are presented. The ideal solution would be an inner (concrete) tube which absorbs the water pressure with a steel exoskeleton to reduce the drag and generate a lift force. The exoskeleton has a flat bottom and a convexly shaped top (when anchored to the seabed) A compromise between the targets is also possible and presented within the research. This compromise also has a flatter bottom and a more convexly shaped top, but still has some appearances of a circle.Civil Engineering | Hydraulic Engineering | Hydraulic Structures and Flood Ris