Analysis and Design of Bjørnafjorden TLP Supported Suspension Bridge Subjected to Large Ship Collisions and Extreme Environmental Loads
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- Institutt for marin teknikk 
The objective of this thesis is to study the global response of a tether anchored floatingsuspension bridge over Bjørnafjorden when it is subjected to ship impacts and extremeenvironmental loads. Bjørnafjorden south of Bergen is one of the fjords, where the ferrycrossing is proposed to be replaced by a bridge. The proposed crossing has a lengthof over 4 000 metres and depths of over 450 metres. Due to the length and depth ofthis fjord it is not possible to use a conventional bridge. Instead the proposed designis to build a three span suspension bridge supported by two TLP floaters. Each of themain spans has a length of 1 380 metres, and the sailing height is 45 metres. One ofthe features of this design is a circular tube at the sea surface. This is connected to thefloater through tethers and is thought to give stability in the installation phase and actas a barrier against ship impacts. The bridge has eigenperiods spanning from 100 to 4seconds, making it behave dynamically for many types of loads. The geometry of the barrier was established based on simplified methods in order to finda combination minimising motions and strain energy. One of the key questions in thisthesis is how this barrier influences the response of the bridge due to ship impacts. Inorder to establish this, several collision analyses were conducted with energies spanningfrom 50 to 1 200 [MJ]. With the vessels both colliding against the barrier and directlyagainst the floater. In the analyses the ship was modelled as a nodal mass given an initialvelocity. It was connected to the bridge through springs. One of these where one wasgiven the force deformation characteristic of the vessel, while the other spring was used toensure that the system did not have any tensile stiffness. One of the key results from theseanalyses was that the barrier increased the global motions, making it disadvantageousfor the global response. Another key question in this thesis was to investigate the response of the bridge whensubjected to extreme environmental loads. The load components used were the 100 yearwind speed, and the second order drift force in the 100 year sea state. A stochasticwind model was used to account for the time and spatial variability of the wind speeds.Since both the wind and wave loads are of a stochastic nature, 30 one hour simulationswere conducted. This made it possible to use extreme value statistics to find the 100year response to environmental loads. The 90 % percentile was used to account for shortterm variability. These analyses showed that environmental loading yields a larger globalresponse than ship collisions, making it governing for design against global response.