Analysis and Design of Ship Collision Barriers on a Submerged Floating Tunnel subjected to Large Ship Collisions
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- Institutt for marin teknikk 
In this thesis, the global response of a floating bridge, with a submerged floating tunnel at the midspan, is studied when subjected to high energy ship impacts. The bridge is a concept pro- posed for use when crossing Sognefjorden, and at the crossing site, the fjord is 3,600 meters wide and 1,200 meters deep. The extreme depth of the fjord means conventional anchoring methods does not suffice, and instead, an artificial seabed is used. 35 meters below the sea surface, pipe bundles will be pulled across the fjord, and put under tension. The floating bridge and submerged tunnel will be anchored to this to provide the bridge with horizontal stiffness. Collision barriers protect the submerged floating tunnel against ship collision until the tunnel reaches a depth of 12 meters below the sea surface. The collision barrier is connected to the tunnel by a fail safe, a member designed to fracture at a certain load. Color Magic is the ship used for the impact analyses, with a mass of 37,500 metric tons, including added mass of10%,andamaximumspeedof10 m.Eigenvalue analyses have been conducted for the global bridge model, which is modeled in USFOS. Two different methods of modeling the anchoring were used for the eigenvalue analyses, one with springs and one with pipes. ÅF Engineering has provided values to be used as comparison for the eigenperiods. The model used to obtain the results use pipes for an- choring, but does not have collision barriers attached. For the highest eigenperiods, the US- FOS model without the collision barriers, and with pipe anchoring, corresponds well with ÅF Engineering s result, respectively 183.54 and 184.09 seconds. The eigenperiods with spring anchoring and collision barriers are in the range from 14.84 to 259.93 seconds.A parametric study was done to evaluate the effect of altering impact velocities, impact lo-cations and fail safe strength. The strength of the fail safe was chosen to be 30 MN, and theimpact velocities used are 6 and 10 m . Three different collision sites were chosen, one for seither side of the two collision barriers, and one for collision against the floating bridge. For collisions against the barriers, the tunnel is kept safe for both velocities at both sites. At the end of the barrier, and at the collision site, permanent damage will be done, but it will not be critical for total collapse. Impact against the floating bridge produces two plastic hinges, one at the collision site and one at the transition between bridge and tunnel. There are large deformations of the bridge beam, and fracture of the bridge is a distinct possibility. This collision will lead to a total collapse of the bridge, as it will be unusable until major repairs are done.