A Submerged Floating Tube Bridge Concept for the Bjørnafjord Crossing: Alternatives for Station Keeping

A submerged floating tube bridge (SFTB) concept has been developed for the Bjørnafjord crossing. Two designs are currently considered feasible: pontoon stabilized and tension leg stabilized SFTB. Previous studies have found that the vertical motions and accelerations are largest for the pontoon stabilized concept, although both designs are within the requirements. With respect to ship impact, seabed mooring is favorable. No ship impact is likely at the current depth of the bridge, and this is considered a great advantage. Submarine impact on the tethers must be considered, however. Using a failsafe link between the pontoons and the tube itself significantly lowers the stress in the bridge due to ship impact. The link will break at impact energies above a given threshold, and release the pontoon. The bridge is designed to survive loosing one pontoon. It is seen that the bridge survives impact even without the failsafe, thus it is not a critical element, but the stress reduction is beneficial. The cost of the tension leg moored alternative is substantially higher than that of the pontoon stabilized one. Both concepts are considered feasible, but the final decision depends on more accurate knowledge of the soil conditions in the fjord.