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Modelling and Analysis of a Floating Bridge

Vika, Eirik Breisnes
Master thesis
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URI
http://hdl.handle.net/11250/2564491
Date
2018
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  • Institutt for marin teknikk [2449]
Abstract
The Norwegian National Public Road Administration is working on a project to

improve the coastal road E39, connecting the cities along the west coast of Norway.

Today, this road has seven ferry crossings which are to be replaced by permanent

connections, to a total expected cost of 340 billion NOK.

Several of these fjords are wider and deeper than what existing designs can

manage, such that new technology needs to be developed. For some of the fjord

crossings, floating bridge concepts have been developed and concluded to be feasible

solutions. For the about 4500 meters wide Bjørnafjorden, south of Bergen, there

are two floating bridge concepts which are in the process of being further assessed.

One of these concepts is a straight bridge, laterally supported by pre-tensioned

mooring lines. This concept was modelled in the software SIMO-RIFLEX, where

a static, eigenvalue and dynamic analysis were performed in order to illustrate

modelling aspects and calculation procedures. Panel models of the pontoon were

modelled in GeniE and imported into HydroD where hydrodynamic analyses were

carried out in Wadam.

From the eigenvalue analysis, a significant limitation was identified in the eigenvalue

calculation codes in SIMO-RIFLEX, as the catenary mooring lines were not

properly accounted for. Therefore, a second model was made where the mooring

system was replaced by a linearised implementation. For this model, the eigen

periods and mode shapes were coinciding well with those obtained by the NPRA.

The first 30 eigen periods were differing with a mean value of 3.9 % when only the

infinite-frequency added mass matrix was considered. By updating the added mass

for a selected set of modes, differences of less than about 2 % were found.

The eigenvalue calculations revealed several modes that can be triggered by

environmental loads. Laterally dominated modes at high periods with negligible

damping, which can be important for the response in slowly varying wind, and

laterally dominated modes close to the peak period for the 100-year wind waves,

were identified. Additionally, modes dominated by pontoon motions along the

bridge girder close to this period were found, possibly important for the dynamic

weak axis bending moments in the high bridge.

From the dynamic analyses in regular waves, response patterns related to the

identified modes were present when the bridge was subjected to waves from different

directions, respectively. The maximum weak axis bending moment in the bridge

girder for the conditions examined was found in the high bridge for a response

pattern related to the mentioned modes dominated by pontoon motions along the

bridge girder. This moment had a magnitude of 9.1E + 05 kNm, where the dynamic

moment only contributed with 15 %. The results from the analyses performed were

therefore seen to give indications on possible room for girder length to girder height

ratio optimisation and should be further assessed based on analyses performed for

the actual environmental conditions in the fjord.
Publisher
NTNU

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