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The paper will look into the hydrodynamic loads and responses on the proposed Submerged Floating Tube Bridge (SFTB) through the Digernessund by the Norwegian Public Roads Administration (Statens vegvesen, NPRA). The aim is to show how different hydrodynamics aspects during the prelimiary design can be simply addressed under the given environmental conditions. Different SFTB systems are introduced as the first step. A simplified method based on modal analysis is introduced and implemented for evaluation of the motions and stress, bending moments along the bridge. Firstly, a 2D Boundary Element Method (BEM) solver is developed and verified, which is further used for solving the hydrodynamics coefficients of different bridge cross sections. The 3D hydrodynamic coefficients of pontoons are solved by the commercial software AQWA. The analysis procedure of the simplified method for the global SFTB responses is presented. The Eigen periods of the Bjørnefjord SFTB is re-calculated by the present model as a first validation of the implementation. The loads and responses of the bridge under given wave conditions are then estimated. The evaluation of the possibility of vortex induced vibrations of the current SFTB design is given.

The European highway E39 ferry-free Project led by the Norwegian Public Roads Administration (Statens vegvesen, NPRA) is in its full momentum on solving the technological challenges to cross the deep (up to 1350 m) and wide (up to around 6 km) Norwegian fjords. Different types of crossing solutions have been explored up to now. The current paper is related to the submerged floating tube bridge (SFTB) solution. But the model itself can be applied to any other structure that may experience loads induced by a passing vessel. The method of Xiang and Faltinsen (2010) for evaluating the calm water interacting loads between two vessels up to the point of collision has been extended to include an SFTB. Equations of loads are extended to include also the vertical load and pitch moment on the body in question. The vertical load induced on the SFTB is investigated for the purpose of identifying the significance of this effect on the design. The investigation includes this effect under different maneuver scenarios of the passing vessels: a single ship passing, two ships advancing in side-by-side positions, encountering or overtaking maneuvers. In addition, parametric studies on the submergence of floating tube bridge, and vessel speeds are studied to get more understanding of this effect. A simple exercise shows that it is worth of further investigation on ship-induced surface waves effects on the structures in vicinity of the ship path. Finally, the evaluated loads are compared with other types of loads from the analysis results of the on-going NPRA SFTB design.

A submerged floating tube bridge (SFTB) concept is one of the three alternatives for the Bjørnafjord crossing in Norway. Two SFTB designs have been developed: one pontoon-stabilized and one tether-stabilized. There are no submerged floating tube bridges yet built and installed. Consequently there is no direct practical experience with assembly and installation of such massive string-like structures. The purpose of this article is to describe possible means of fabrication, assembling, jointing and installation of SFTB elements and the tube-bridge itself. All major project activities have been divided into stages. Further, these stages have been subdivided into factual steps to demonstrate feasibility based on various criteria. The major construction challenge is to assembly fabricated SFTB elements into one 4580 m string in a floating condition and then to tow and install it in the fjord. The focus was to utilize existing technologies and experiences available from offshore industry to the fullest extent, so that conventional tools and methods could be applied to this new application. Possible solutions both for the tether-stabilized and pontoon-stabilized alternatives — with differences in the foundation/tether and pontoon installations and associated SFTB interfaces — have been developed.