Analysis and Design of Floating Bridge over Bjørnefjorden - Floating Bridge Subjected to Large Ship Collisions

In This Master s Thesis, a bridge design for the crossing of Bjørnafjorden have been investigated. The design was developed in a cooperation between COWI, Aas Jakobsen, Johs Holte As and Global Maritime, as a part of The Norwegian Public Roads Administrations (NPRA) project ferry free E39 . The project is still in the early stages, where the feasibility of different bridge designs are being evaluated. The design analyzed in this project is a curved floating bridge, with a cable-stayed section near land in the south end, to allow for ship traffic to pass under the bridge. It is floating freely without moorings, and the curved design help to carry shear forces through membrane stresses. The bridges global strength against ship collisions have been investigated throughout this Thesis. Theoretical background is presented initially, before different analyses are conducted in the software USFOS. An initial suggestion of a bridge model used in the USFOS analyses, were developed by Post. doc Yanyan Sha. During the work with this Master's Thesis, this much time were spent to improve this model, for the analyses to give realistic results, despite the significant number of uncertainties being present in this early stages of the project. An eigenvalue analysis was conducted, and large period deflection modes were observed for horizontal bending of the bridge girder. The maximum eigenvalue was found to be 62.66 seconds. The results of the eigenvalue analysis were compared with the results obtained by COWI et al., through the software Orcaflex, and were found to correspond well. This gave gave confidence for the mode being able to represent thes structural response of the bridge fairly well. No information were obtained for the damping level of the bridge, and to reduce the uncertainties this would introduce for the final results, a separate damping assessment were performed. An analysis, where the pontoon closest to the navigational channel, was exposed to a ship impact with collision energy of 1250 MJ, was run several times with different damping levels. By comparing the results from these analyses, it was found that even for large changes in the level of damping introduced to the model, the change in the final results were only in the order of a few percent. This is most likely because the most critical response occur shortly after impact, and little energy have had the time to be lost to damping. In the main ship collision analysis three different collision scenarios were investigated, and due to uncertainties with respect to the what collision energies the bridge should be designed against, a range of different collision energies were introduced. In Collision scenario 1, the ship hits the short end of the pontoon closest to the navigational channel. The maximum collision energy of 900 MJ, gives a maximum plastic utilization factor of 0.905, occurring third cross-beam to the north of the struck pontoon. Due to difficulties in modeling the pre-tension in the wires of the cable stayed section, false stresses occur due to the permanent loads of the bridge. Because of this, the value of the plastic utilization is not used for the bridge girder of this part of the bridge. Instead, the increase in the plastic utilization is found. For the maximum collision energy, the increase in the plastic utilization is found to have a maximum value of 0.486, in the girder above the connection to the column of the struck pontoon. From this it is concluded that the bridge will survive the impact, as long as the plastic utilization factor due to the permanent loads do not exceed 0.5. From the analyses of collision scenario 2, where the pontoon closest to the navigational channel is struck in the transverse direction, very high plastic utilization factors were observed in the column of the struck pontoon. At the top of the column, in the connection to the bridge girder, the plastic utilization factor was found to be 0.992 for the lowest collision energy investigated of 500 MJ. For the maximum collision energy of 900 MJ, large plastic deformations are observed, and it is thus advised to strengthen the upper part of the pontoon columns. In collision scenario 3, the plastic utilization factors observed, are low, with a maximum value of 0.763 in one of the cross beams. The global strength of the bridge thus seem to be sufficient for a collision like this, into the bridge girder, and no strengthening are advised. The collision energies used for this collision scenario are much lower than for collision scenario one and two, with a maximum value of 450 MJ. This is a result of it being further away from the navigational channel, and thus was found to be less prone to collisions in the analysis performed by the SSP