Global Analysis of a Floating Bridge
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- Institutt for marin teknikk 
There is an ongoing political debate in Norway concerning how to improve the communication between communities on the western coastline. A proposed solution is to build floating bridges over some of the crossings that today are being connected by ferry traffic. The vast complexity of such constructions makes simplified computer models important in the initial stages of the design process. This thesis will describe the modeling and analysis of the floating bridge concept PontoMar developed by LMG Marin. The main focus will be to establish a simplified model in order to observe the overall behavior of the bridge. The study will focus on displacements of the floaters, deflections in the road beams and global distributions of forces and moments. Computer programs found suitable for the task are Genie, Wadam, Riflex and Simo. Genie is used for creating panel models of the floaters, while Wadam is used to calculate hydrodynamic forces and coefficients based on linear potential theory. The global model is established in Simo and Riflex via the graphical user interface program DeepC. Static and dynamic analyses are carried out using Riflex and Simo coupled. Simo simulates the motions and station keeping behavior of the floaters, while Riflex calculates the responses in the bridge beam and mooring lines. Static analyses are carried out by simulating current and wind loading in a number of directions, while dynamic analyses are carried out as one hour time domain analyses implementing irregular waves based on a 100 year storm condition. The waves propagate in the same mean direction as the current, and a spreading function is included to realistically model the directional distribution of wave energy. Results from both static and dynamic analyses show that the bridge is very sensitive to the load direction. Concave loading results in the smallest floater displacements, and thus smallest road beam responses, which supports the idea that the bridge should be curved with its convex side pointing inwards the fjord. Static results are generally within requirements; the exception is the static floater roll angles, which are unacceptably large. The dynamic results show forces and moments almost 10 times larger than the equivalent static components. The maximum deflection is 4.66 meter, which is over half the freeboard. The conclusion is that wave induced motions are the critical design factor. Alterations in floater design should be conducted in order to reduce roll response. Further the freeboard has to be increased in order to avoid slamming or even water entry. Detailed stress calculations have to be conducted to determine necessary arrangement and properties of the road beams and arches. Eigen frequencies and corresponding mode shapes should be calculated to avoid resonant motions being excited by current or wind. Axial forces in the mooring lines are below the breaking strength; however fatigue- and damaged condition analyses need to be conducted before the mooring system design can be verified.