Combined SFT and floating bridge

The main reason for combining a floating bridge and a submerged floating tunnel is to provide a ship channel. Four different concepts will be presented in the following.

Y-shaped SFT combined with floating bridge

A concept proposed for the crossing of Sognefjorden is given in fig. 2.13. The con-cept consists of a floating bridge resting on pontoons, connected to two submerged floating tunnels making a Y-shape. The SFTs are connected to the seabed with tethers, providing vertical stiffness. The Y-shape of the submerged part of the bridge provides horizontal anchoring and separation of the submerged floating tunnels towards sepa-rate rock tunnels at the landfalls.[Statens Vegvesen, 2011]

Figure 2.13: Y-shaped concept for a combination of two submerged floating tunnels and a floating bridge.[Statens

Vegvesen, 2017, Statens Vegvesen, 2011]

2.4. SUBMERGED FLOATING TUNNEL 15 Concept for the crossing of Rovdefjord

Figure 2.14: Illustration of Rovdefjord bridge [Snøhetta, 2016b]

Rovdefjord bridge will be an important link between the southern Sunnmøre and the outer parts of Nordfjord, at the west coast of Norway. The first conceptual ideas for a bridge crossing the Rovdefjord were launched in the 1980s. In 2011, the foundation Rovdefjordsambandet AS was established to investigate the possibilities for a ferry-free crossing of Rovdefjord. The companies Reinertsen AS, dr. techn Olav Olsen, Snøhetta and SINTEF have, on behalf of Vanylven Utvikling AS, developed a con-cept for crossing the Rovdefjord with a SFT (fig.2.14). In 2016, the local council Sande kommune, approved the municipal sector plan for the Rovdefjord bridge.

[Sande kommune, 2016] However, the bridge has not been built yet.

Figure 2.15: Illustration of Rovdefjord bridge showing the spiral connection between the SFT and the floating bridge at Saudeholmen. [Snøhetta, 2016b]

The proposed bridge concept has a total length of 3500 m and consists of a rock tunnel, a floating bridge of length 1500 m and a SFT of length 230 m. The floating bridge, which is curved in the horizontal plane, is a steel girder connected to 14 concrete pon-toons. The floating bridge is further connected to the SFT by a spiral culvert at a small

island called Saudeholmen, fig. 2.15. The SFT is curved in the horizontal plane, con-sists of concrete, and has a circular cross-section.[Sande kommune, 2016] The SFT has no intermediate supports and is thus supported only by its buoyancy and the connec-tions at the two ends.

Artificial seabed

Figure 2.16: Illustration of Artificial seabed concept. [Snøhetta, 2016b]

An ongoing research project called Artificial seabed started in 2014 and is a collabora-tion project between the former Reinertsen, Dr. Techn. Olav Olsen, Snøhetta, Sapa, Hy-dro, Deep Ocean Group and Sintef. The project is supported by The Research Council of Norway.[Snøhetta, 2016a] The concept (fig. 2.16) is a mooring system for a combined floating bridge and SFT, which enables a flexible placement of the SFT independent of the water depth on site and sea bottom conditions. The idea is a submerged anchoring system which provides lateral stiffness of a slender bridge through side mooring. The anchoring system consists of two pretensioned bundles of steel pipes across the fjord.

The bundles consist of three steel pipes, which are neutrally buoyant, submerged to about 35 meters and horizontally curved (fig. 2.17). Transverse steel pipes connect the two bundles making the total anchoring system a stiff horizontal frame.[Reiso et al., 2017]

A crossing which has been evaluated for the Artificial seabed concept is the Bjør-nafjord. For this particular crossing, the distance between the bundles at the bridge ends are 800m, while the distance at mid fjord is 80m. Both the floating bridge and the SFT are connected to the Artificial seabed by mooring. Transverse loads are transferred to the submerged anchoring system, and then transferred further to the abutments as axial forces. The concept allows for reduced span due to side mooring, compared to a horizontally curved end-moored concept. Another advantage is that the ship passage can be optimally placed with respect to the ship traffic. However, the concept require large submerged mooring chambers. These chambers can be accessed by land.[Reiso et al., 2017]

2.4. SUBMERGED FLOATING TUNNEL 17

Figure 2.17: Illustration of the artificial seabed, mooring system, pontoons, ship barri-ers and the combined floating bridge and SFT bridge. [Snøhetta, 2016b]

Chapter 3

Developed concept of a SFT for the Digernessund

The Digernes strait is one of the crossings along the route E39, with an an existing suspension bridge called Stordbrua. This bridge is a part of the triangle link connecting Haugaland and Sunnhordaland, and consists of the 677 m long Stordbrua and a 7.8 km submerged tunnel crossing the Bømlafjorden meeting at a small island named Føyno.

Spatial restrictions at Føyno resulted in a road alignment with a gradient which does not meet the requirement for the new E39. The requirement is a maximum gradient of 5%, and two lanes per driving direction. Therefore, alternative modifications of the link have been considered.[Engseth et al., 2016]

Figure 3.1: Illustration of the proposed SFT for the Digernessund. (Illustration:[Engseth et al., 2016])

The submerged tunnel over the Bømlafjorden has a minimum elevation of−260m be-low the free surface, which govern the high gradient between the Stordbrua and the submerged tunnel. A submerged floating tunnel bridge is considered a suitable alter-native to existing bridges requiring high gradients accessing roads, like the crossing of Digernes strait. On behalf of the NPRA, Dr. techn. Olav Olsen has prepared a feasi-bility study for a SFT for this crossing (fig.3.1).[Engseth et al., 2016] This thesis takes advantage of the work done by dr. techn. Olav Olsen in the feasibility study.

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3.1 Functional requirements

Some functional requirements for the SFT are given in tab. 3.1. According to [Vegdirek-toratet, 2015] the crossing shall have a design life of 100 years. The NPRA has advised the limiting values of vertical and horizontal deflections of L/350 and L/200 respec-tively. For horizontal deflections, L represents the total length between the abutments, while for vertical deflections, L represents the total length between vertical supports.

Further, NPRA has advised maximum accelerations as summarized in tab.3.1 to ensure pedestrian comfort.[Engseth et al., 2016] Horizontal vibration 0.3 m/s² Vertical vibration 0.5 m/s²

The bridge girder should ensure water tightness in operation and temporary condi-tions. The design criteria for serviceability limit state is zero crack width, w_k = 0 and membrane compression in the longitudinal direction in the outer fibre. There is also re-strictions with respect to the minimum compression zone height. For the ultimate limit state and accidental limit state, the strains in the PT-cables and reinforcements are lim-ited to the elastic region. Hydrostatic stability should also be ensured in accordance with requirements in the DNV-OS-C301.[Engseth et al., 2016]