The Norwegian Road Public Administration, Norway
ABSTRACT: The aim of the present paper is to provide a description of the ongoing work regarding the feasibility studies that are currently being performed. Different alternatives are being discussed. Environmental loads, constructability of the structure and the risk of ship impact can be listed among the decisive aspects of the design.
1 INTRODUCTION
The Norwegian Public Road Administration (NPRA) has initiated one of the most ambitious and ground-breaking large scaled infrastructure programs whose objective is to connect Kristiansand to Trondheim without ferry crossing.
The crossing of Bjørnafjorden constitutes one of the most challenging parts of the project. Different solutions are currently being investigated. The present paper deals exclusively with the feasibility study that is being conducted regarding a floating solution whose combined constraints in terms of depth of the seabed and total length would be unprecedented.
The NPRA is being assisted by engineering consultancies and research institutions with exten- sive experience with large bridge projects. The purpose of the feasibility study is to provide the Norwegian authorities with the means to reach a conclusion as to the constructability and cost effectiveness of a floating solution.
This paper summarizes the main steps of the analysis that is being performed as well as the difficulties that need to be overcome. The response of the structure to wave loads, the modelling of ship impact loads or the determination of appropriate construction methods can be listed among the central issues that need to be addressed.
2 DESCRIPTION OF THE DIFFERENT ALTERNATIVES 2.1 Concept
The Crossing is envisaged as a Multi-Span Structure resting on floating elements referred to as Pontoons. Some of the Pontoons would be anchored to the seabed by means of mooring lines whose properties and arrangement remain to be determined.
The main objective of the feasibility study is to develop a floating bridge concept for the crossing of Bjørnafjorden that will remain valid regardless of the final position of the navigation channel.
Different alternatives are currently being investigated with regards to the type of high bridge, the position of the navigation channel and the curvature of the bridge. The various alternatives are discussed below.
2.2 Navigation channel located at the South of Bjørnafjorden 2.2.1 Alternative 1A: Curved bridge – wide navigation channel
The navigation channel is assumed to be 400 m wide and is placed outside the south island. The bridge deck consists of two box girders and follows a horizontal curve with a radius of 5000 m.
Figure 1. Wide navigation channel – South.
The passage over the navigation channel would be supported by means of a cable-stayed whose pylon will be placed on the island.
The bridge deck would be fixed in its southern extremity but simply supported by the pylon, allowing the deck to move sideways.
2.2.2 Alternative 2A: Curved bridge – narrow navigation channel
This scenario requires the blasting of the island located at the south of Bjørnafjorden. The width of the navigation channel would then be 200 m. The bridge deck would consist of two box girders placed in a horizontal curve with an assumed radius of 5000 m. The end supports are assumed to be hinged.
The road elevation gradually increases so as to reach the desire clearance. In the considered alternative the passage over the Channel would be supported by a cable-stayed bridge whose pylon would be positioned ashore.
2.2.3 Alternative 3A: Side anchored straight bridge – narrow navigation channel
In this scenario, a side-anchored straight solution is considered. It is assumed that the bridge deck consists of one box girder and that the pylon is positioned ashore. Similarly to Alternative 2A a cable-stayed is envisaged over the navigation channel and its pylon is placed ashore.
2.2.4 Alternative 4A: Side anchored straight bridge – wide navigation channel
Like alternative 3A, the bridge is assumed to be straight and side-anchored. Its deck consists of one box girder. The pylon of the cable-stayed high bridge rests on the island allowing for the navigation channel to be placed outside the island.
The bridge deck is restrained laterally by the pylon, which entails that the latter has to be designed accordingly.
2.3 Navigation channel located in the middle of Bjørnafjorden 2.3.1 Alternative 1B: Curved bridge – network arch
The navigation channel is positioned in the middle of the fjord. The high bridge is assumed to be a network arch and the bridge deck follows a curve similar to Alternative 1A.
The bridge deck consists of two box girders connected with transverse elements maintaining a constant spacing of 75 m. The height of the arch is initially set as 80 m and it supports a span of 400 m. The bridge deck is hinged at both ends allowing exclusively rotations about the vertical axis.
2.3.2 Alternative 2B: Straight bridge – network arch
Alternative 2B differs from Alternative 1B in that the bridge is assumed to be straight and side anchored. The bridge decks consists one unique girder.
The pontoons supporting the network arch will be side-anchored. In addition two pontoons, located at a quarter and three quarters of the total length of the bridge, will be side-anchored.
Figure 2. Navigation channel – middle of the fjord.
2.4 Pontoon Geometry
It has been determined that the displacements in heave of the pontoons shall be restrained below a certain threshold set as Linfl/350, where Linflis defined as the length of the influence line regarding
heave motions of the pontoons. For combined roll and heave from traffic the freeboard change at the edge of the pontoons is limited to 1.0 m. The roll of the pontoon due to the action of 1 year static wind has been restricted to 0.5 degree.
The criteria listed above have been used to determine target values for the stiffness in roll and heave of the pontoons. The final dimensions of the pontoons will be chosen in order to provide the structure with sufficient stiffness.
2.5 Mooring system
As described in the above paragraphs, the straight bridge alternatives have to be side-anchored. The introduction of mooring lines into the system induces an additional risk and a certain number of uncertainties. The number of mooring lines as well as their dimensions remains to be further investigated.
3 WAVE AND WIND LOADS 3.1 Methodology
At the time of writing, the main focus had been placed on the development of a reliable methodology to capture the response of such a complex solution to wave and wind loads.
The present paragraph describes the assumptions and simplifications that have been made. It is possible that the final conclusions alter significantly.
The two main software packages that have been used are Novaframe, developed byAas-Jakobsen (Norway) and Orcaflex, developed by Orcina. The purpose of the different models is to predict the response of the structure under the dynamic environmental loads it is exposed to (static and turbulent wind, current, first and second order waves).
Important simplifications have been made in order to facilitate the comparison and the calibration of the key parameters.
Preliminary evaluations of wind and wave loads are based on the data collected by the weather stations located in the vicinity of the fjord. The most relevant station is located at open sea, outside Bjørnafjorden. Local wind and waves are then evaluated by extrapolation of the available data.
Weather buoys have now been positioned at relevant locations in the fjord and will collect data throughout the entire lifecycle of the project increasing the statistical confidence in the prediction of extreme values.
3.2 Hydrodynamic loads
3.2.1 Characterization of the expected wave loads
In order to characterize the hydrodynamic loads the bridge will undergo, it is required to define the following parameters:
– Wave Spectrum.
– Directional spreading spectrum
– Spatial variation of the above parameters – Current parameters
The chosen wave spectrum could be two-peaked in order to combine the effects of wind and swell components. Second order effects shall as well be accounted for.
3.2.2 Methodology
In the model developed in Orcaflex, the bridge deck is modelled as a continuous beam element, with adequate stiffness and mass properties, rigidly connected to the pontoons. The high bridge is also incorporated. The effect of the mooring system is included in the model by means of simplified horizontal springs. The latter simplification, as mentioned in paragraph 3.1, has been made in order to make it possible to compare the findings obtained in Orcaflex with those obtained in Novaframe. In a later stage of the project, a more detailed analysis the mooring lines will be performed.
The pontoons are modelled as “Vessels” whose hydrodynamic properties are determined from diffraction and potential theory. Another software, WAMIT, is used to analyze the behavior of the pontoons in the presence of waves. It is based on linear and second-order potential theory. The panel method is used to determine the velocity potential as well as the fluid pressure on the outer surface of the pontoons. Sets of Response Amplitude Operators (RAOs) can then be exported from WAMIT into Orcaflex.
3.3 Wind loads
The wind dynamic response of the structure is based on procedures for random variables and stochastic processes. The wind load and the response of the structure can be subdivided into two terms:
– A mean wind load and a mean structural response due to constant (or static) wind velocity taken as the mean wind velocity during the assumed stationary period (usually taken as a 10 minutes period).
– A fluctuating wind load and a fluctuating structural response characterized by the turbulence intensity of the wind and the standard deviation of the response.
The expected extreme structural response rmax during the assumed stationary period can be
expressed as follows:
where µr= mean value of structural response(static wind); σ = Standard deviation of response
(Fluctuating wind load); and kp= peak factor of the fluctuating structural response.
The structural response of the considered structure to the static component of wind can be evaluated independently from its response to dynamic wind. The global response to wind loads can finally be obtained by summation of the static and dynamic components.
As a first approach, the guidelines from NS-EN1991-1-4 have been used to evaluate the effects from wind. The design group is currently trying to elaborate a reliable methodology to describe the distribution of the wind along the bridge.
3.4 Modal analysis
3.4.1 Calibration of the hydrodynamic stiffness of the pontoons
The restoring coefficient corresponding to roll and pitch movements of the pontoon can be evaluated as follows:
where V =Volume of the considered pontoon; zG= Elevation of the centre of gravity;
zB= Elevation of the centre of buoyancy and Awp=Area of the water plane.
Equation (2)can be rewritten:
where zG,i refers to the elevation of the centre of gravity of the different elements forming the
bridge.
For every displacement of the pontoons a new equilibrium needs to be found. It is described by equation (4):
where t refers to the considered time step.
In order to take into account the variation of the buoyancy forces acting on the pontoons, their hydrostatic stiffness is defined as follows in Orcaflex:
where C
44is the modified hydrodynamic stiffness used in Orcaflex from which is deducted the
weight of the elements connected to the pontoons (the pontoon itself corresponds to element i = 1 inequation (5)).
3.4.2 Findings
Initially the bridge was modelled in Novaframe and Orcaflex without high bridge so that the comparison of the eigenmodes obtained in both softwares would be facilitated. The influence of added mass was not accounted for in this preliminary model. The same side-anchoring is assumed in the different models discussed in the present paragraph.
The ten first Eigen periods can be found inTable 1.
Similar eigen forms and eigen periods are obtained in both programs.
In a subsequent stage, the high bridge was incorporated in the model.Table 2gives the eigen periods of the first ten modes.
Table 2shows that the correspondence between the two softwares in terms of eigen forms and eigen periods remains valid after a high bridge has been added to the models.
Finally the added mass generated by the pontoons is added to the models.Table 3shows the influence of added mass.
Table 1. Model without high bridge – no added mass. Mode 1 2 3 4 5 6 7 8 9 10 Eigen Periods [s] 36 35 34.7 32.5 25.7 24.6 20.3 16.8 16.2 15.3 Orcaflex Eigen Periods [s] 36.6 35.2 34.9 32.6 25.6 24.5 21.4 18.3 16.1 15 Novaframe
Table 2. Model with high bridge – no added mass.
Mode 1 2 3 4 5 6 7 8 9 10 Eigen Periods [s] 48.6 35.3 35.1 34.4 26.9 26 17.4 17.2 15.1 14.6 Orcaflex
Eigen Periods [s] 50.1 35.2 34.9 33.7 26.4 25.7 17.4 17.1 15 14.1 Novaframe
Table 3. Model with high bridge – with added mass.
Mode 1 2 3 4 5 6 7 8 9 10 Eigen Periods [s] 57.2 43.7 39.1 39 30.1 29.8 22.2 20.5 20.4 16.7 Orcaflex Eigen Periods [s] 55.7 40.0 39.1 38.3 33.0 30.4 22.4 20.5 18.9 17.5 Novaframe 4 SHIP IMPACT 4.1 Context
A ship impact analysis has been initiated. Its purpose is to provide a basis for the determination of potential ship impact scenarios. Probabilities of impact as well as a levels of impact energy are associated to each of the considered impact scenarios.
Ship collision and the resulting impact energy could prove dimensioning for the floating bridge. The risk assessment regarding ship collisions has been carried out by SSPA Sweden AB. The results from this analysis will play a decisive role in the choice of the final bridge layout.
4.2 Probabilistic approach
The existing ship traffic and past trends were analyzed in order to establish a prognosis of the ship traffic for 2070 in terms of number of ships, size and speed.
The prevailing traffic following a north-south route together with the traffic entering the area of the planned crossing represent a risk of collision with the planned bridge. Traffic prediction will greatly influence the choice of design ship.
An established American calculation model issued by American bridge design guidelines (AASHTO, 2009) was used as a starting point for Monte Carlo simulations. Collision energy was calculated and systematically simulated based on the design ships and validated against the AASHTO methodology.
Figure 3. (a) USFOS model and (b) Response in Orcaflex.
The output of the risk assessment described above can be summarized by the two following aspects:
– Identification of a design ship
– Identification of events whose occurrence probability exceeds a probability criterion of 10−4
per year. These scenarios cannot be ignored and further analysis has to be carried out.
SSPA uses a probabilistic approach based on Monte Carlo simulations to reflect the real traffic situation. The modelled traffic pattern is based on long term real Automatic Identification System (AIS) recordings from the area and predicted route alterations resulting from the construction of the planned bridge. A large number of impact simulations has been performed, covering a large variety of types of failures. The most frequent root cause for collision accidents is human error, Other causes are technical failure (loss of propulsion or failure in steering system) or a combination of technical and human failure.
The risk of collision could be reduced by introducing traffic regulations in terms of speed and category of vessels that would be allowed to enter the fjord area. Energy dissipation mechanisms would also be an option.At this stage, the benefit of risk mitigating measures has not been examined. 4.3 Bridge response
The design group has used simplified models to develop a better understanding of the expected response of the floating bridge under a ship impact scenario.
An assembly of springs has been defined in USFOS where the contribution of the mooring system, the bridge deck as well as the water plane stiffness and viscous effects are accounted for. The ship impact is modelled by means of an impulse load. The response of the structure is then exported into Orcaflex in the form of time series. The loads undergone by the structure can then be studied. It appears from the first models that have been established (Figure 3aandFigure 3b) that the impact energy is dissipated throughout the entire bridge.
5 CONSTRUCTION STAGE
Regardless of the solution the design group will opt for, the construction phase entails a large number of uncertainties. Given the large scale of the structure and its innovative nature, very few projects can be used as references.
The structural capacity of the different bridge elements will have to be investigated during the temporary phases.
Initially the following assumptions are made:
– The pontoons will be produced in Norway at a location yet to be determined.
– Bridge elements of 30 to 40 m will be transported to the construction site and welded. – Lifting vessels equipped with dynamic positioning systems will used to lift the bridge deck
sections to the desired elevation. Installation aids such as bumpers guides can be considered. Temporary mooring lines could constitute a good supplement to installation vessels.
The construction process will have to follow strict weather restrictions throughout the entire process which could result in significant additional costs.
6 CONCLUSIONS
It has not yet been determined whether the construction of a floating bridge was a viable solution for the crossing of Bjørnafjorden. The topics discussed in the present paper represent crucial aspects of the feasibility study.
A floating bridge concept applied to a crossing of this scale represent an opportunity for the parties involved in the project to further develop their knowledge and understanding of the floating bridge technology through close collaboration between the offshore and bridge construction industry.
REFERENCES
Statens Vegvesen. 2011. Håndbok V499, Bruprosjektering. SINTEF. Bridge across Bjørnafjorden Meteocean conditions NORSOK STANDARD N-001. 2010. Rev. 7, Juni.
DET NORSKE VERITAS. 2010. Offshore Standard DNV-OS-E301, Position Mooring, October. DET NORSKE VERITAS. 2010. Offshore Standard DNV-OS-E302, Offshore Mooring Chain, October. DET NORSKE VERITAS. 2010. Offshore Standard DNV-RP-C205, Environmental Conditions and Environ-
mental loads (October).
AASHTO 2010. Guide Specifications and Commentary for Vessel Collision Design of Highway Bridges.
Multi-Span Large Bridges – Pacheco & Magalhães (Eds.) © 2015 Taylor & Francis Group, London, ISBN 978-1-138-02757-2