Risk assessment of ship allision in extreme fjord crossings

Norwegian authorities have an ambition to develop the E39 road as a continuous Coastal Highway Route between Kristiansand and Trondheim without ferries. The western coast of Norway is characterized by deep and long fjords cutting into the mountain landscape. These fjords are to be crossed, but this is challenging since the fjords are long, and up to 1350 meters deep and 5 km wide at favourable crossing points.

Floating bridges are probably suitable to cross the fjords, but the coast and the fjords are exposed to significant ship traffic. Some of the new bridges along E39 will, when realized, be the world’s largest bridges of their kind, and critical to the future communication in Norway. Hence, the bridges need to be designed such that the risk from severe ship allisions from the ship traffic passing the bridge is within acceptable limits.

The Norwegian rules for bridge engineering demands that the design ship(s) should be assessed in a separate risk analysis, where the design ship size and mass, the ship's speed at collision and the associated accidental actions (impact energy and momentum) are determined such that the risk acceptance criteria for the bridge crossing is fulfilled.

The risk assessment includes both frequency analysis, and impact analysis. Previous ship allision research has not focused on floating bridges. Long and slim constructions, like a 5 km long floating bridge, have other challenges than fixed bridges. This paper reviews and discusses the activities in a risk assessment suitable for floating bridges.

We argue that ship allision risk may be a major contribution to the total risk. The risk assessment should pay more attention to the impact analyses of floating bridges, to understand how the bridge responds to a ship impact, like the distribution of the impact energy absorbed in global deformations and the impact energy to be dissipated through crushing of pontoons or bridge girder.

The flexibility of floating bridge concepts can be beneficial when subjected to a ship allision, and it is important to understand how the bridge responds, both global and local, to an allision.