· 2016-11-11 · / , rev. 2 Revision Date Reason for Issue Prep. by Contr. by Appr. by 01 04.05.16 Issued for client review Proj.team THS SAH 02 31.05.16 Issued for Approval Proj.team

REPORT

BJØRNAFJORD SUBMERGED FLOATING TUBE BRIDGE

K3/K4 TECHNICAL REPORT

/ , rev.

2

Revision Date Reason for Issue Prep. by Contr. by Appr. by

01 04.05.16 Issued for client review Proj.team THS SAH

02 31.05.16 Issued for Approval Proj.team THS SAH

REPORT

Project name:

BJØRNAFJORD SUBMERGED FLOATING TUBE BRIDGE

Document name:

K3/K4 TECHNICAL REPORT Project number : 12149-03

Document number : 12149-OO-R-310

Date : 31.05.2016

Revision : 02

Number of pages : 340

Prepared by : Project Team

Controlled by : Tore H. Søreide

Approved by : Stein Atle Haugerud

K3/K4 TECHNICAL REPORT / 12149-OO-R-310, rev. 02

3 Table of Content

PREFACE ................................................................................... 6

1 EXECUTIVE SUMMARY ..................................................... 7

1.1 Technical solutions .................................................................................... 7

1.2 Major features .......................................................................................... 8

1.3 Challenges ............................................................................................... 9

2 OPPSUMMERING ........................................................... 11

2.1 Beskrivelse av tekniske løsninger ............................................................... 11

2.2 Viktige egenskaper .................................................................................. 12

2.3 Utfordringer ........................................................................................... 13

3 INTRODUCTION ............................................................ 15

3.1 Project context ....................................................................................... 15

3.2 Terms and definitions .............................................................................. 16

4 SFTB TECHNOLOGY ....................................................... 17

4.1 SFTB Simply Explained ............................................................................. 17

4.2 Proven technology ................................................................................... 19

4.3 Water tightness ...................................................................................... 21

5 CONCEPT ROBUSTNESS ................................................. 22

5.1 Definition ............................................................................................... 22

5.2 Sensitivity studies ................................................................................... 22

5.3 Relevant experience ................................................................................ 23

5.4 Ballast operation ..................................................................................... 24

5.5 Concrete tubes capacity ........................................................................... 24

5.6 Tethers ................................................................................................. 27

5.7 Pontoons ............................................................................................... 29

5.8 Design status ......................................................................................... 30

6 DESIGN PREMISES ........................................................ 31

6.1 Design codes .......................................................................................... 31

6.2 Functional requirements ........................................................................... 32

6.3 Bjørnafjorden bathymetry and soil conditions ............................................... 37

6.4 Environmental conditions .......................................................................... 40

7 PROPOSED LAYOUT ....................................................... 44

7.1 General ................................................................................................. 44

7.2 Pontoon variant ...................................................................................... 55

7.3 Tether variant......................................................................................... 65

7.4 Key figures ............................................................................................ 76

8 ARCHITECTURAL INTENT ............................................... 77

8.1 Introduction ........................................................................................... 77

8.2 Pontoons – “Islands in the stream” ............................................................. 77

8.3 The submerged tunnel “The world’s longest gallery” ...................................... 78

9 DESIGN APPROACH ....................................................... 80

9.1 General ................................................................................................. 80

/ , rev.

4 9.2 Design philosophy ................................................................................... 80

9.3 Concept screening and design considerations ............................................... 81

9.4 Response analyses .................................................................................. 83

9.5 Structural design ..................................................................................... 91

10 LOADS ......................................................................... 97

10.1 Introduction ........................................................................................... 97

10.2 Self-weight ............................................................................................ 98

10.3 Deformation loads ................................................................................. 102

10.4 Traffic loads ......................................................................................... 103

10.5 Wave loads .......................................................................................... 103

10.6 Current loads ....................................................................................... 109

10.7 Tidal loads ........................................................................................... 110

10.8 Temperature loads ................................................................................ 111

10.9 Water pressure on Tether SFTB ............................................................... 111

10.10 Earthquake load .................................................................................... 112

10.11 Combination of environmental loads ......................................................... 112

10.12 Accidental loads .................................................................................... 113

10.13 Design combinations .............................................................................. 117

11 GLOBAL RESPONSE ANALYSES ..................................... 121

11.1 Introduction ......................................................................................... 121

11.2 Eigenperiods ........................................................................................ 130

11.3 Global static response ........................................................................... 139

11.4 Global Response ................................................................................... 170

11.5 Sensitivity studies ................................................................................. 194

12 STRUCTURAL DESIGN .................................................. 200

12.1 Introduction ......................................................................................... 200

12.2 Design parameters ................................................................................ 201

12.3 Cross sections ...................................................................................... 206

12.4 Design of concrete tubes with ShellDesign ................................................. 209

12.5 Tether-stabilized SFTB ........................................................................... 218

12.6 Pontoon-stabilized SFTB ......................................................................... 234

12.7 Design of Crossbars ............................................................................... 244

12.8 Design of horizontal bracing .................................................................... 248

12.9 Design of Pontoons ................................................................................ 255

12.10 Submarine impact - Local design of tube wall ............................................. 257

13 SFTB OUTFITTING ....................................................... 268

13.1 Ballast system ...................................................................................... 268

13.2 Drainage system ................................................................................... 274

13.3 Ventilation ........................................................................................... 275

13.4 Electrical installations............................................................................. 276

13.5 Fire protection ...................................................................................... 279

14 CONSTRUCTION.......................................................... 280

14.1 Major fabrication scope .......................................................................... 280

14.2 SFTB elements ...................................................................................... 280

14.3 Tether fabrication .................................................................................. 285


5 14.4 Tether foundations ................................................................................ 285

14.5 Pontoons ............................................................................................. 286

14.6 Caissons .............................................................................................. 287

14.7 Jointing of elements .............................................................................. 287

14.8 Ground works at bridge site .................................................................... 293

14.9 Schedule ............................................................................................. 293

15 INSTALLATION ........................................................... 295

15.1 General ............................................................................................... 295

15.2 SFTB Dock Operations and Element Tow ................................................... 299

15.1 Temporary mooring system and SFTB assembly ......................................... 303

15.2 Installation of tunnel rock plugs and landfalls ............................................. 306

15.3 Installation of rock anchors (for tether alternative only) ............................... 307

15.4 Pre-installation of tethers (for tether alternative only) ................................. 310

15.5 Submergence test of assembled SFTB ....................................................... 312

15.6 SFTB tow and landfall hook-up ................................................................ 315

15.7 Tether hook-up (Tether alternative) ......................................................... 322

15.8 Pontoon installation (Pontoon alternative) ................................................. 324

15.9 Marine operation vessel overview ............................................................. 325

16 OPERATION AND MAINTENANCE ................................... 328

16.1 Objective and scope, delimitation ............................................................. 328

16.2 Operation overview ............................................................................... 329

16.3 Traffic safety ........................................................................................ 330

16.4 Maintenance guidelines .......................................................................... 331

16.5 Design for minimum maintenance ............................................................ 331

16.6 Restricted operation .............................................................................. 332

16.7 Availability ........................................................................................... 332

17 RISKS AND UNCERTAINTIES ........................................ 334

17.1 Risk analyses HAZID and FMECA .............................................................. 334

17.2 Uncertainties ........................................................................................ 335

18 REFERENCES .............................................................. 340

/ , rev.

6 PREFACE This report presents the assessment study for a Submerged Floating Tube Bridge (SFTB)

over the Bjørnafjord. The study is commissioned by the Norwegian Public Road

Administration Region West under contract no. 2014072111 awarded October 2014. Client

representative has been Jorunn Hillestad Sekse.

The three companies of consulting engineers Reinertsen, Dr. techn. Olav Olsen and

Norconsult have entered a partnership for execution of the Submerged Floating Tunnel

Bridge (SFTB) study for the Bjørnafjorden crossing. The objective is to take the engineering

of the SFTB alternative to a level of detail where we can have adequate confidence in the

technical feasibility and construction costs. Thereby, the SFTB can be regarded as a safe,

robust and viable option in the governmental regional plan for E39 Stord-Os.

Reinertsen and Olav Olsen have been involved in most of the floating bridge- and SFTB

studies completed in Norway, and have extensive experience with offshore structures.

Norconsult is the largest multidiscipline consultant in Norway, and is a leading player within

engineering for transportation and communication.

The partnership has been strengthened with selected subcontractors who are all highly

qualified within their respective areas of expertise. Among them we find dr.ing Arne

Nestegård (DNV GL), with 30 years’ experience in research and development of methods for

calculation of wave loads and response. You also find Aker Solutions, a global supplier of

products and services within offshore oil and gas, who has advised the partnership on marine

operations, mooring- and ballasting systems. The following companies have contributed to

the study:

— Reinertsen

— Dr.techn. Olav Olsen

— Norconsult

— DNV GL

— Aker Solutions

— Snøhetta

— Marintek

— Svend Ole Hansen ApS

Finally, we would like to thank the NPRA for the opportunity of working with this state of the

art project, both highly interesting and highly technically challenging.


7 1 EXECUTIVE SUMMARY

1.1 Technical solutions

In the optimization phase for the Submerged Floating Tube Bridge (SFTB) across the

Bjørnafjord the western corridor between Svarvhella and Røtinga is still the crossing corridor,

however as shown in Figure 1-1 the route is moved towards west out of the shelter Flua

area. The background is the cost estimate made during the previous feasibility stage, coming

up with the bottom stabilized solution from Flua to shore as a cost driver.

> Figure 1-1: Western corridor for SFTB alternatives

The span is now 5350m for the pontoon stabilized concept and 5495 for the tethered

solution. The routings at shore approaches are adjusted correspondingly to fit the SFTB

alignment.

As concerns the present concept optimization further experience from offshore solutions has

been incorporated on concrete structures, tether solutions and marine operations,

respectively. The two concepts have been run in parallel through the same set of design

premises, both satisfying current regulation requirements on safety and risk assessment.

The safety format is based on Eurocode as the structure code regime in Norway since year

2010.

The tether-stabilized SFTB is vertically anchored to the seabed by steel tube tethers for

every 200 m. With top bridge at -30 m, the tethered concept allows free surface ship

passage. It also allows submarines to navigate in submerged position over the tubes. By the

submerged solution a major part of the wave excitation is eliminated, giving a calm tube

bridge without operational restrictions even for the 100-year wave condition.

The pontoon-stabilized SFTB is vertically supported by the water plane stiffness of the

pontoons. This allows for free submarine passage under the bridge, but restricted surface

ship traffic. As the pontoons attract loads from surface waves, the present concept shows

more vertical flexibility and motion than tether-stabilized SFTB. However, the motions are

still well below comfort and driving limits also for 100-year return conditions.

/ , rev.

8 The tube cross-section is mainly determined by the size of the standard tunnel profile as

prescribed by the NPRA for the two running bores. Initially, in the first project phase a tunnel

profile T12.5 including two driving lanes and a continuous shoulder (3.0 m) was specified for

the running bores. For the current phase the Norwegian Public Road Authority (NPRA)

altered the tunnel profile to T9.5 (tunnel class E), see Figure 1-2.

> Figure 1-2: Typical layout of cross tube with lay-by arrangement (pontoon variant)

The fabrication in dry dock and floating of each 200 m element prior to coupling, allows for

accurate weight control of the installed SFTB. Weight variation during operation due to

technical equipment, water absorption in concrete, marine growth, asphalt thickness as well

as traffic itself is a part of the design loads, together with buoyancy variation due to

seawater density. These loads are given their safety factors according to Eurocode for the

design control. There is a large volume for water ballast, which may be activated for possible

extra unexpected weight variation over time, if occurring. This is an extra redundancy in

design beyond safety factors and normal practice for bridges.

1.2 Major features

Characteristics for the two SFTB concepts are:

The SFTB submergence eliminates most loads from wind generated sea. The vertical

motions become small and well below the project specific acceptance limits, even for

100 year storm condition. For SLS the acceleration is in the range of 0.05 m/s2.

Horizontally, the arch geometry implies flexibility for thermal expansion. The twin tube

cross section in combination with curved geometry guarantees acceptable slenderness

and resistance regarding environmental loads. Also horizontal motions become small,

and well below the project specific acceptance limits. For SLS the acceleration is around

0.1 m/s2.

As a consequence of small motions, traffic can be maintained during the most extreme

storm conditions. Although no criterion is specified for operational regularity, zero

downtime due to weather is seen as a major advantage for the SFTB.

Two tubes with two driving lanes and one service lane together with a flat vertical

profile gives high tunnel safety. There are escape routes between the two tubes every

200 m, both in the traffic level and bicycle/footpath level.

Water leakage is eliminated by strict water tightness requirements, and the fact that

there is no penetration to sea. The reinforcement strain limit on 2.17 ‰ in tension for

SLS 100-year combination together with requirement on minimum compression zone

over wall thickness are based on offshore design practice.


9 Impact from surface ships on the tubes is not possible due to large clearance above

the tubes. The SFTB is designed to resist ship impact on the pontoons and submarine

impact on the tubes. For the SFTB it is consequently not necessary to introduce speed

limitations on the ship traffic.

The SFTB is almost neutrally buoyant vertically and is stabilized either by tethers to

seabed or by pontoons to the surface. The tethers are pre-stressed to take care of load

variations, the requirement being no loss of tension. For the pontoon solution the

vertical stiffness is the waterline area of the pontoons, and load variations is taken

care of by variation in pontoon draught.

Both concepts are characterized by low structural and hydrodynamic damping. For the

tether stabilized SFTB the design philosophy has thus been to tune the governing

natural periods of the bridge outside the range of excitation period, heave and roll in

particular. The stiffness governs tether cross-section area instead of the ultimate

strength. For the pontoon solution similar tuning is not possible within practical

pontoon dimensions. Benefit is here taken of higher hydrodynamic damping than for

tethered concept.

The low damping makes especially the tether concept sensitive to variation in wave

period. In parallel with ongoing site measurements analytic and numerical simulations

for wind generated sea should form the basis for a conservative period range to go

into further design.

The twin tube cross-section has by tests shown stable behavior under current and

wave action, eliminating the need for extra design remedies to eliminate uncontrolled

motion. Wind tunnel tests underline twin tube as the preferable cross-section rather

than a rectangular box. Independent of doubts among experts on the validity of the

wind tunnel tests, the conclusion hereof is in line with sound engineering judgement.

The SFTB is designed for minimum maintenance with materials that are not sensitive

to deterioration. Structural elements in the splash zone are particularly exposed to

corrosion, and that is the reason for concrete pontoons. The concrete platforms in the

North Sea with sufficient concrete cover and low permeability concrete have

demonstrated excellent durability over 40 years of service.

Seabed properties vary over the span, with soft soil in the mid and deeper part, turning

to harder soil and rock towards the ends. For the tether stabilized concept this makes

extra need for a flexible foundation design, going from conventional gravity anchor

into possible rock drilling solutions. During the present optimization phase, with limited

input on seabed parameters available, the foundation alternatives have been generally

outlined rather than presenting foundation solution for each specific support location.

There is among users a general reservation on road tunnels, also affected by some recent fire

accidents here in Norway. Independent of concept, any subsea solution adds extra arguments.

To change the opinion can only be reached by fair information and is outside the scope of this

study, however during the optimization phase effort has been placed on obtaining high level

safety for all foreseen hazard scenarios as fire, submarine impact and ship collision.

1.3 Challenges

Both SFTB concepts have been designed according to Eurocode with national application

addendum for Norway. The design control concerns the completed bridge in operation with

the final shore connections and moorings established. Construction, tow and installation

phases have not undergone the same level of detailed analysis, though the free floating

bridge is evaluated regarding tug forces, needed waterline area of preliminary towers and

/ , rev.

10 VIV. The large mass distributed along a flexible structure makes the bridge in installation

phase a more complex system than for operation phase, and this is to be given major

attention in upcoming design work.

The shore connections including establishment of fixed support by ballasting the SFTB down

to pre-installed caisson are also part of marine operations that need further outline. The idea

is now that the connection to rock tunnel will be free of global forces that require complex

operations and rock anchoring. A step-by-step evaluation of the shore connection procedure

also needs additional effort.

The seabed foundations represent a heavy element for the tethered SFTB. Due to the lack of

data during the present optimization phase, the effort has been more on foundation concept

evaluation than on detailed calculations. In this respect it is fair to say that even though

solutions are outlined for soft soil and rock conditions there is still a major technical step up

to the design level obtained for the bridge structure.

The environmental loads applied, and waves in special, are considered conservative with

significant wave height 3.0 m. Current velocities seem a bit more uncertain at present stage,

and high current velocities may impact design to avoid dynamic load effect such as VIV. As

concerns accidental scenarios as submarine impact, ship collision and explosions these

depend on risk studies instead of nature, and may be subject to change during upcoming

design process. The coordination with Norwegian Navy to verify future design loads from

submarine impact has up to now been held within a limited forum, and should be subject to

a more formal documentation where a variety of relevant submarine hulls are considered.

The safety format for the present study is based on Eurocode which by its origin was

intended for conventional land based bridges. For the SFTB additional loads apply from

marine growth, water ingress and variation in sea water density. Also, the reference stage is

now the balanced as installed situation which in addition to as built properties also includes

ballast pattern. So both load pattern as well as reference level are different from what is

relevant for a bridge on shore. An evaluation of the applicability of the Eurocode design

format is required before going into the detail design phase.


11 2 OPPSUMMERING

2.1 Beskrivelse av tekniske løsninger

Studiet har konkludert med to alternative løsninger for krysningen over Bjørnafjorden;

Rørbru med stagforankringer til bunnen og rørbru med pongtonger i overflaten. Begge

løsninger ligger i bue i horisontalplanet, og sideveis stivhet oppnås med buevirkning samt

bøyestivhet og innspenninger i begge landfester.

Omfattende erfaring fra offshore virksomheten er benyttet i dette prosjektet, både innen

marine betongkonstruksjoner, strekkstagløsninger og marine operasjoner.

De to alternative bruløsningene er utviklet parallelt og med de samme forutsetninger, og

tilfredsstiller dagens krav til sikkerhet i henhold til Eurokodene.

I dette studiet av rørbru over Bjørnafjorden er den vestre linjen mellom Svarvhella og

Røtingen valgt som hovedkorridor, men nå flyttet lenger vest i forhold til den første fasen av

prosjektet som der det var en bunnforankring på grunnen Flua. Bakgrunnen for endringen er

å unngå kostbare grunnarbeider under vann fra Flua til Røtinga. Et lenger rørbruspenn gir

beskjeden økning i horisontale reaksjonskrefter.

Spennet er nå 5350 m for det pontongforankrede alternativet og 5495 for det

stagforankrede. Den horisontale linjeføringen er tilpasset tilførselsveiene i begge ender av

brua.

> Figure 2-1 Vestre corridor for rørbrualternativer

Strekkstagløsningen er vertikalforankret til sjøbunnen med forankringsgrupper med typisk

avstand 150 meter. Overkant av rørbrua ligger i 30 meters dybde, noe som sikrer at

skipstrafikken kan gå uhindret over brua. Det tillater også ubåter å gå i neddykket posisjon

over brua. Når brua ligger så dypt, vil mye av bølgevirkningen være borte, slik at man får en

bru med relativt små bevegelser selv for en 100 års bølgetilstand.

Pongtongløsningen opplagret på pongtonger i overflaten med typisk avstand 150 meter, der

pongtongenes vannplan gir nødvendig stivhet i vertikal retning. Ved å unngå forankringer til

/ , rev.

12 bunnen, vil det være fri passasje for ubåter under brua. Det vil imidlertid være restriksjoner

for skipstrafikken på overflaten. Pongtongene er både mykere vertikalt enn strekkstagene,

og pådrar seg i tillegg bølgekrefter. Denne løsningen har derfor større vertikalbevegelser enn

strekkstagløsningen, men bevegelsene er likevel godt innenfor komfortkravene.

Det doble rørtverrsnittet, som tilsvarer standardprofilet T9,5 for aktuell ÅDT, gir rom for to

kjørebaner, i tillegg til gang- og sykkelbane samt servicetunnel. Mellom rørene er det

tverrforbindelser for hver 200 meter med plass for rømningsvei mellom de to

kjøreretningene, se figur Figure 2-1.Figure 1-1

Brua bygges i 200 meter lange seksjoner i tørrdokk. Ved å utføre flytetester før

sammenkobling av seksjoner, vil man oppnå en langt mer nøyaktig vektkontroll enn på

ordinære konstruksjoner. Laster fra toleranser, marin begroing, vannabsorpsjon, varierende

utstyrsvekter, varierende asfalttykkelser, trafikk og variasjon i oppdrift grunnet endringer i

vannets tetthet, behandles alle som ordinære laster med lastfaktorer i henhold til Eurokode.

Det er store ballastvolumer i brua, noe som gir muligheter for å ballastere brua dersom man

får varige endringer av laster over tid. Dette gir reservekapasitet utover det som er vanlig

for ordinære bruer.

2.2 Viktige egenskaper

Viktige egenskaper for rørbruene er:

Stor dypgang eliminerer mye av belastningen fra bølger. Vertikalbevegelsene blir

små, og godt innenfor kravene til bruer, selv for en 100 års bølgetilstand.

Akselerasjonen er i størrelsesorden 0,05 m/s2 i bruksgrensetilstand. Dette gir seg

også utslag i reduserte spenninger i brua.

I horisontalretning gir buen brua mulighet for å bevege seg på grunn av

temperaturendringer. Det doble rørtverrsnittet i kombinasjon med bueformen, gir

høy bøyestivhet og kapasitet mot horisontale laster fra bølger og strøm. Det gir også

små bevegelser horisontalt, godt innenfor kravene. Akselerasjonen er i

størrelsesorden 0,1 m/s2 i bruksgrensetilstand.

Som en konsekvens av de små bevegelsene, vil trafikken gjennom brua gå uhindret

selv under ekstreme værforhold.

To adskilte kjørebaner med bred skulder og horisontal vertikalkurvatur, gir en veg

med høy sikkerhet. Det er rømningsveier for hver 200 meter både i nivå med

kjørebanene og i nivå med gang- og sykkelveien.

Vannlekkasjer vil ikke være et problem, da det er svært strenge tetthetskrav til

betongen, samt at det ikke er gjennomføringer av rør eller lignende i veggene mot

sjøen. Armeringstøyningen i strekk er begrenset til 2.17 ‰ i SLS med 100 års


13 returperiode for lastene, som sammen med minimum trykksonehøyde over

veggtykkelsen utgjør vanlig prosjekteringspraksis for offshorekonstruksjoner.

Skipsstøt mot rørene er umulig da brua ligger så dypt. Brua er dimensjonert for og

tåler både ubåtstøt mot selve brua og overflate skipsstøt mot pongtongene.

Rørbruene har nær oppdriftsnøytrale rør stabilisert med enten strekkstag eller

pongtonger. Stagløsningen har strekkstag med en forspenning som sikrer at de ikke

går i slakk i noen lasttilstand. For pongtongløsningen sikrer man tilsvarende at det er

tilstrekkelig fribord på pongtongene.

Begge løsninger kjennetegnes av liten struktur- og hydrodynamisk dempning. Et

viktig prinsipp i studiet har derfor hvert å prøve å styre egenfrekvensene spesielt i

hiv og rull unna frekvensområdet for vindgenererte bølger.

Ved hjelp av vindtunneltester, har man påvist at brua oppfører seg stabilt under

påvirkning av bølger og strøm uten at man må ta i bruk avbøtende tiltak. Testene

viste også at dobbelt rørtverrsnitt var klart å foretrekke fremfor et rektangulært

tverrsnitt.

Den neddykkede rørbrua er utformet for et minimum av vedlikehold med

motstandsdyktige materialer. Konstruksjoner i skvalpesonen er spesielt utsatt for

korrosjon, og følgelig er det valgt pongtonger i betong. Betongplattformene i

Nordsjøen, med god armeringsoverdekning og lav permeabilitet, viser meget god

bestandighet.

Grunnforholdene varierer langs brulengden og dette gir behov for fleksible

forankringsløsninger for strekkstagalternativet. Her er det mulig med konvensjonelle

gravitasjonsankere (senkekasser) eller boret og gyst fjellforankring

En rørbru med bue i horisontalplanet vil være en sikker og trygg bruløsning gjennom hele

den planlagte levetiden. Brua har kapasitet til å motstå alle laster den utsettes for med god

margin. Bevegelser i brua fra bølger, strøm og vind er relativt små, og de vil ikke skape

ubehag for trafikantene eller forårsake hindringer for trafikken, selv under helt ekstreme

værforhold. Den utførte risikoanalysen konkluderte med at brua har tilstrekkelig robusthet til

å motstå alle relevante ulykkes- og uforutsette hendelser.

Studiet konkluderer med to alternative løsninger. Både rørbru med strekkstagforankringer og

rørbru med pongtonger er teknisk gjennomførbare, og kan tas med i betraktningen ved

videre planlegging av krysning av Bjørnafjorden. Dersom grunnforholdene er gode nok, vil vi

anbefale den strekkstagforankrede rørbrua, da denne har større robusthet og er mindre

påvirket av miljølaster som bølger, strøm og tidevann. Den ligger i sin helhet under

vannoverflaten og vil ikke være synlig i landskapet.

2.3 Utfordringer

Begge rørbrualternativer er prosjektert i henhold til krav i Eurokoden med nasjonalt tillegg

for Norge. Dimensjoneringen omhandler hovedsakelig ferdig tilstand med landfester og

forankring på plass. Bygge-, taue- og installasjonsfaser er vurdert, men ikke analysert i

samme detaljgrad. Egne analyser er kjørt for flytefaser for å evaluere kapasitet mot

tauekrefter, nødvendig vannplansareal for midlertidige tårn, VIV etc. Utfordringen her er en

stor masse fordelt over en lang og fleksibel konstruksjon, og dette bør analyseres videre i en

detaljeringsfase.

/ , rev.

14 Ilandføring er planlagt ved å ballastere rørbrua ned i en preinstallert senkekasse som gir en

fast innspenning i ferdig tilstand. Dette gjør overgangen til fjelltunnel fri fra globale

reaksjonskrefter som krever komplekse installasjonsoperasjoner og fjellforankring med høy

kapasitet. Videre stegvis detaljering av senkekassekoblingen er et område som krever

ytterligere arbeid.

Strekkstagforankringen er et viktig element for dette alternativet. Ettersom det underveis i

prosjektet var et mangelfullt grunnlag på grunnforhold er det brukt mer ressurser på å

utvikle flere gjennomførbare løsninger enn å detaljere det foretrukne. Det er derfor

fremdeles et stort gap mellom detaljnivået oppnådd for brukonstruksjonen og detaljnivået

for forankringsløsning.

Miljølastene i prosjekteringsgrunnlaget, og spesielt bølgelastene, synes å være konservative.

Strømhastighetene er noe mer usikre, både til positiv og negativ side, og økte

strømhastigheter kan påvirke design av rørene for å unngå dynamiske lasteffekter som VIV.

Ulykkeslasttilfellene, eksempelvis skipsstøt, ubåtstøt og eksplosjon, baserer seg på en

risikoanalyse, og resultatene av disse vil kunne endres dersom forutsetningene endres. Når

det gjelder ubåtstøt har dette vært behandlet i et begrenset forum, og dette bør

dokumenteres mer formelt hvor flere relevante skrogtyper vurderes.

Formatet for konstruksjonssikkerhet er Eurokode, som opprinnelig er tiltenkt konvensjonelle

bruer på land og ikke marine konstruksjoner. Før detaljprosjektering bør egnetheten til

Eurokode som prosjekteringsstandard evalueres.


15 3 INTRODUCTION

3.1 Project context

The Norwegian Public Roads Administration (NPRA) has been commissioned by the

Norwegian Ministry of Transport and Communications to develop plans for a ferry free

coastal highway E39 between Kristiansand and Trondheim. The 1 100 km long coastal

corridor comprises today 8 ferry connections, most of them are wide and deep fjord

crossings that will require massive investments and longer spanning structures than

previously installed in Norway. For some of these fjords the Submerged Floating Tube Bridge

(SFTB) is regarded as an attractive crossing solution.

Based on the choice of concept evaluation (KVU) E39 Aksdal Bergen, the ministry of

transport and communications has decided that E39 shall cross Bjørnafjorden between

Reksteren and Os.

The NPRA has commenced the work on a

governmental regional plan with

consequence assessment for E39 Stord-Os.

This plan will recommend a route from Stord

to Os, including crossing solution for

Bjørnafjorden, and shall be approved by the

ministry of Local Government and

Modernisation.

NPRA pursues the development of a

permanent link over the Bjørnafjord through

parallel studies comprising both floating and

submerged floating bridge concepts. The

assessment study for the SFTB is carried out

by the design group REINERTSEN – Olav

Olsen – Norconsult et al.

The applicability of the SFTB technology has

recently been proven in a feasibility study for

the 3.7 km wide and 1 300 m deep

Sognefjord. The study takes the SFTB

alternative to a higher level of detail to give

great confidence in the technical feasibility

and construction costs. Thereby, the SFTB

can be regarded as a safe, robust and viable

option in the governmental regional plan for

E39 Stord-Os.

> Figure 3-1: Area included in regional plan E39 Stord-Os

/ , rev.

16 3.2 Terms and definitions

3.2.1 Terminology

Atmospheric zone: The external surfaces

of the unit above the splash zone Bulkhead: Interior watertight diaphragm

or wall for watertight partition of compartments. Caisson: Prefabricated, temporary floating

structure installed on the seabed by ballasting.

Compartment: Enclosed volume bounded

by watertight structural elements consisting of a single cell or an array of communicative cells. Cell: Unit volume in a compartment

bounded by structural elements. Immersed tunnel: Prefabricated fully

submerged tunnel resting on the sea bed. Return period: Average interval of time in

years between exceedances of an event magnitude. Splash zone: The external surfaces that

are periodically in and out of the water. Submerged Floating Tube Bridge

(SFTB): A free spanning, fully submerged

tunnel floating in water partly supported by it’s buoyancy (also termed Archimedes bridge). Submerged zone: The part of the unit

which is below the splash zone.

Free-floating body motions

3.2.2 Notations

H Height

D Diameter

F Force

f Natural frequency

g Acceleration of gravity (9.81 m/s2)

3.2.3 Abbreviations

ALS Accidental Limit State

EQU Loss of equilibrium

FE Finite element

FEM Finite element model

FLS Fatigue Limit State

GEO Failure of ground

GM Metacentre Height

HAZID Hazard Identification

HAT Highest Astronomical Tide

LPT Linearized Potential Theory

LAT Lowest Astronomical Tide

MSL Mean Sea Level

SFTB Submerged Floating Tube Bridge

NOS Numbers

NPRA Norwegian Public Roads

Administration

NTP National Transportation Plan

OD Outer Diameter

ROV Remotly Operated Vehicle

SLS Serviceability Limit State

STR Failure of structure

ULS Ultimate Limit State

VIV Vortex-induced vibrations

TLP Tension Leg Platform

RP Return Period


17 4 SFTB TECHNOLOGY

4.1 SFTB Simply Explained

The SFTB differs from the conventional immersed tunnels as a free floating structure in

water at some depth below sea surface. The vertical forces are in a SFTB mainly carried by

buoyancy of the bridge structure. The SFTB’s may be classified into two categories, based on

the type of vertical support:

Tether stabilized SFTB

Pontoon stabilized SFTB

An explanation of each concept is further given.

4.1.1 Tether stabilized SFTB

A tether stabilized SFTB is stabilized by tension legs. Hence the SFTB has to have positive

net buoyancy in order to maintain tension in the tethers for all relevant load cases. The net

buoyancy can be distributed along the length of the SFTB, and/or have concentrated

buoyancy near the tether connections. Only vertical tethers are considered in this study. A

principle sketch of the tether stabilized SFTB is shown in Figure 4-1.

> Figure 4-1 Tether stabilized SFTB

The anchoring of the tethers are by gravity anchors which are designed for large and

permanent tension. The anchors can be gravity and/or suction anchors or rock anchors

drilled and grouted into bedrock.

The horizontal alignment can be either straight or curved. A solution with an arc is often

seen, as the arc action is more efficient than beam action. Also, for longer spans the arc

geometry allows thermal deformation without large axial forces being built in.

One of the big advantage for the tether stabilized SFTB is that it is unaffected by surface

induced loads such as tidal loads, ship impact, wave slam etc. Even a 100-year storm

condition has minor impact on the SFTB, and there is thus no weather restriction on

operation. Also, the environmental impact is minimized by the invisible SFTB with subsea

connection to rock tunnels at each end.

/ , rev.

18

The disadvantage by the tether stabilized SFTB at present stage of the project is the risk

regarding challenging seabed topography and uncertainty in seabed conditions and effect on

anchoring. Gravity based foundation is sensitive to seabed topography and soft soil, whereas

drilled anchors to bedrock are more flexible, but requires heavy installation vessel for

installation and testing.

The concept involves a minimum of structure and hydrodynamic damping, and is thus tuned

out of the period range of wind generated sea to limit wave response.

4.1.2 Pontoon stabilized SFTB

The pontoon stabilized SFTB is supported by pontoons in the waterline. The SFTB can have

either positive or negative net buoyancy, or be neutral in water. Vertical loads such as traffic

or marine growth are taken by adjustment of pontoon waterline. The bridge can hence be

seen as a beam on flexible supports. An illustration is shown in Figure 4-2.

> Figure 4-2 Pontoon stabilized SFTB Horizontally, the pontoon bridge act similar to the tether stabilized bridge by the arc bending stiffness governing wave response. The pontoons will however attract additional forces from wind, tide and surface waves compared to the fully submerged tether stabilized SFTB.

The major advantage by the pontoon stabilized SFTB as compared with the tether stabilized solution is the avoidance of seabed mooring. Also the pontoons add damping, however still

the damping ratio is moderate as related to a surface floating structure. For the present design, the pontoon stabilized concept shows some more wave motion than the tether stabilized SFTB, however still there is no operation restriction even for the 100-

year storm condition. Tuning of resonance periods in heave out of wave excitation range is not obtainable within practical pontoon dimensions.


19 4.2 Proven technology

The two concepts are characterized by implementing existing and well documented

technology into a new structure system, rather than developing components from scratch.

Major elements representing proven technology are summarized in the table below.

> Table 4-1 Examples of existing technology

Concrete tubes

Experience on design, fabrication and

maintenance is taken from offshore oil

and gas platforms as well as from

existing bottom stabilized submerged

tunnels. The design criterion on water

tightness is handled by post tensioning

combined with reduced allowable strain

in reinforcement, as implemented in

floating offshore concrete structures.

Figure: Concert by Katie Melua inside the

shaft of Troll A GBS, 303 m below sea

level.

Joining of tubes

The joining of concrete tubes has

previously been made on Troll A. The

purpose of the connection for the SFTB is

to create emergency gates between the

tubes, and for buoyancy reserves.

Figure: Troll A foundation design. Four

columns are joined at the mid height

/ , rev.

20 Tethers

Tether elements are taken from offshore

TLPs in the form of steel pipes. These are

air filled and buoyancy neutral in water

and with D/T-ratio close to 30. Diameter

up to 44 inch is within proven technology

offshore and so is depth of 1’500 meter.

Figure: Heidrun TLP in operation since

1995. Four tethers per corner, length

approximately 260 m.

Tether foundations, either gravity

based or driven piles, is proven

technology from offshore, including

tether connection details. The solution

depends on seabed properties,

hereunder soil type and slope. For soft

soil conditions the dynamic part of tether

tension may be taken as suction by skirts

on a gravity foundation as is done

offshore. On the other hand, the dynamic

response is a smaller part of total tether

tension for the SFTB than for an offshore

TLP, thus the benefit of suction skirt is to

be reconsidered.

Figure: Gravity foundation for Snorre A

before installation. The tether porches

are protected by yellow covers.

Pontoons in the form of concrete

floating elements have been in operation

on the two Norwegian floating bridges for

more than 20 years. Experience with

concrete elements in the splash zone

may also be taken from offshore floating

concrete platforms like the Troll B semi

and the Heidrun TLP, respectively.

Figure: Nordhordlandsbrua outside

Bergen. In operation since 1994


21 Link between pontoon and bridge

A steel truss structure is well proven

both within the oil and gas industry and

the renewable energy industry.

Especially for bottom fixed offshore wind

turbines such jacket structures are mass

produced for wind turbine generator

foundation.

Figure: Production facility for steel truss

structures.

Marine operations with large

displacement structures

Moving of large displacement concrete

structures is well known from installation

of the offshore GBS platforms. The most

known structures is Gullfaks C

(displacement of 1.4 million tons) and

Troll A which has the record of being the

tallest man-made structure ever moved

(472 m).

Figure: Transportation of Gullfaks C

4.3 Water tightness

The water tightness of the structure is crucial for the integrity of the SFTB. Special

requirements are hence introduced for the SFTB design to ensure that the structure is

watertight for all design stages and events.

For the concrete tubes this goes mainly on the amount of longitudinal prestressing to secure

that the cross section is always in compression, and hence not experience cracking for the

serviceability limit states.

For accidental events and ultimate limit states, the criterion for water tightness is a strain

limitation in the reinforcement ensuring no plastic yielding to occur in the rebars. This is

identical to the design criteria’s for the existing concrete floaters for oil and gas production.

/ , rev.

22 5 CONCEPT ROBUSTNESS

5.1 Definition

The wording “robustness” is central in below evaluation, and a definition is needed:

Robustness is normally meant as the structures ability to sustain unexpected

exposures during operation without undergoing global collapse or overall damage that is out of proportion to the event. The requirement on maintaining global stability in case one element failure is a mean to incorporate robustness in design, being a

normal requirement for offshore structures.

In present stage of the Bjørnafjorden project where the SFTB concepts are under development, the wider definition is forwarded:

Concept robustness is the ability to withstand unexpected situations, both in

upcoming design and in operation, without the concept undergoing major changes. Modified input parameters in the form of wave height, current velocity or seabed

parameters outside the values used in present optimization study now come in as additional “unexpected” events.

This chapter aims to underline the robustness of the concepts. Both the system robustness and the robustness of each of the main components are described.

5.2 Sensitivity studies

Load combinations

During the design phase of the SFTB, the loads are combined according to the rules in N400

and NS-EN 1990. Environmental loads (wave, current, tide and wind) have been considered

as one load group with one load factor. 100 year return period for all environmental loads

has been considered in the combination. The concepts shows sufficient capacity with full

traffic loads during a 100 year storm, and all of the functional requirements are met.

Parameter variation

A common mean to document robustness is to perform sensitivity studies on relevant

parameters. During the design stages of the SFTB several key parameters for the crossing

have been investigated:

Diameter of tubes

Alignment

SFTB length

Depth profile

Environmental conditions

The concept shows remarkable robustness against such changes. To highlight this, the

changes from the first phase to the second phase of the concept development are presented

in short:

Bridge length extended from some 3500 m to approximately 5000m

From T12.5 to T9.5 tunnel profile

From center distance 150 m to 200 m between vertical supports

From 6 to 4 tethers per group for tether solution

New location and depth profile of foundations for tether solution

Improve and uniform tether anchor concept; drilled and grouted rock anchors


23

The utilizations for the main components still remain in the same range as for the first

phase.

The key for such behavior is the ability to limit the responses to the governing environmental

data:

Heave motions tuned by steel area in tethers/waterline stiffness of pontoon and

distance between tether groups/pontoons

Sway motion tuned by center distance between tubes and radius of arch

Tuning of ballast in tubes for optimizing static response

In addition to the parameters described above, parameter variation for the governing design

drivers has been performed during the project.

Sensitivity to changes in wave height and period. Worst wave period for wind sea

and swell combined to find characteristic response.

Study on influence of wave direction.

Pontoon size. Final configuration based on optimization based on both static and

hydrodynamic behavior.

Several options for tether foundation (gravity based for flat rocky seabed, gravity

base with skirts for clay or drilled rock anchors for steep and rocky areas)

The sensitivity of the above parameters is low, ensuring good robustness of the final

solution.

5.3 Relevant experience

The experience by more than 40 years operation of concrete platforms in the North Sea is

relevant in view of the design lifetime of 100 years for the SFTB. The experience from the

oil/gas is the following:

Environmental loads. Generally moderate increase in wave height is seen for some

locations. More serious is the experience with individual extreme waves towards

topside and shafts creating extreme local pressures up to 5 MPa (500 tons/m2). This

is a surface phenomenon in open sea and not considered relevant for the submerged

SFTB in a fjord.

Response characteristics. As part of reassessment programs there has been

situations where new model tests have revealed larger response than used for detail

design, mostly connected to higher order load effects such as “ringing”. For our SFTB

with minimum damping this situation is relevant.

Design technique. The experience is that by reassessing offshore structures under

increased loading much design capacity is gained by going from classical linear

analysis in design into nonlinear regime modeling, especially for local capacity

control due to impact, explosion etc.

In case a reassessment of structure reliability has to be performed due to increased

environmental or operation actions, as built concrete properties are expected to be a benefit.

During fabrication in dock concrete testing is part of the quality control. Extra element tests

to document as built design parameters should be carried out.

/ , rev.

24 5.4 Ballast operation

The SFTB is designed to sustain all relevant loads during the entire lifetime of 100 years. It is

hence not planned to adjust the ballast level to compensate for e.g. marine growth, water

absorption in concrete etc. unless the values are exceeding the large margins are accounted

for in the design.

Flexibility with respect to amount- and location of ballast will however result in the possibility

to adjust for unexpected actions or effects during the construction and operation phase. It is

also possible to tune the ballast to achieve the “correct” design self weight if there is

difference between the self weight used in design and the measured actual weight during

construction.

Ballast handling concerns the global stability requirement of the structure and is thus closely

linked to global collapse of the SFTB. This calls for clear operation routine on ballast

handling. The current design includes no penetrations of the SFTB to sea, and “fail-safe”

ballasting scheme with a controlled volume reservoir.

5.5 Concrete tubes capacity

The driving criterion for the design of the concrete tubes is the water tightness criterion in

SLS. It states that the membrane forces in the concrete shall be below 0, i.e. always

compressive membrane forces in the tubes in SLS, and hence a stadium I concrete behavior.

This criterion governs the amount of prestressing and prestressing level in the concrete

tubes.

Based on experience, the SLS criterion is far on the conservative side, and if needed a

modified crack width criterion may be applied based on factored ULS.

The typical prestressing level to avoid membrane tensile stresses in the tube section in the

SFTB is between 7.5-12 MPa, depending on crossing solution and location in the tubes.

Normal reinforcement comes in addition for ULS/ALS capacity, where the concrete is allowed

to develop cracks and redistribute forces internally (stadium II). The strain in the

reinforcement is limited to 2.17 per mille in ULS to avoid plastic deformations and potentially

open cracks. Safety against leakage is covered by this control.

The design of the concrete tubes is designed by the FE-software ShellDesign. As a control of

the software, and for a better visual presentation of the results, MN capacity curves have

been created based by conventional methods by use of the SFTB geometry and amount of

reinforcement. The concrete capacity for the different limit states is depicted in this MN

capacity curve in Figure 5-1.


25

> Figure 5-1 SFTB tube capacity diagram. Shown for tether solution

Figure 5-1 shows a MN diagram of the concrete tube for vertical loads. The vertical axis

depicts the axial force in the tube while the horizontal axis depicts the bending moment. The

following can be noted:

Axial force in tube: contribution from prestressing shown as horizontal black line.

Variation in axial force due to global bending about vertical axis from wave and

current shown as dotted horizontal lines.

SLS capacity as function of axial force is shown by the light blue lines.

ULS capacity is shown as the dark blue/green graph.

ULS bending capacity is in the range of 4 times the SLS capacity for relevant values

of axial force.

To highlight the influence of each load, the load effects in terms of bending moment about

the horizontal axis in one section of the SFTB is summarized in the MN diagram. The load

effects are shown for both the tether and pontoon solution. The chosen section is in the mid

part of the bridge, located towards a cross bar for both alternatives. The unfactored load

effects for the sections are depicted in Figure 5-2.

/ , rev.

26

> Figure 5-2 SLS loads in section, the upper red line shows the pontoon solution loads and the lower blue line shows the tether solution loads

The following can be noted from the drawing:

SLS capacity sufficient for both alternatives with no tensile membrane forces

The bending moment in the unloaded situation for the tether solution is positive

(compression in top of the section towards support). The bridge is tuned with some

hogging to counteract for long term loads such as marine growth and water

absorption, and also a portion of the traffic load. For the pontoon solution, the

bending moment when unloaded is close to zero, indicating a neutral configuration

Governing loads:

o Tether solution: Traffic load governing for bending moment, wave actions

governing for overall capacity due to sway response which gives

positive/negative axial forces in the tubes, and subsequently reduced

capacity for bending moment

o Pontoon solution: In general higher response with approximately equal

contribution to bending moment from traffic and waves. Higher amount of

prestressing hence needed to fulfill the SLS criterion.

Figure 5-3 shows the corresponding ULS loads and capacity for the considered section.


27

> Figure 5-3 ULS actions in tubes, the upper red line shows the pontoon solution loads and the lower blue line shows the tether solution loads

The labels for each load effect are here omitted, but the sequence for each load is identical

as in Figure 5-2. As seen, the capacity in ULS is the very large compared to the load effects

for both alternatives.

This introduces robustness for the tube, as the SLS criterion with no membrane tensile forces

may be harsh for the dynamic characteristics of the SFTB. Analyses made to control the

water ingress rate during such event with loading beyond the SLS criterion shows that water

will not penetrate through the cracks during the dynamic event of opening and closing a

crack. The short duration of the crack opening and closing time (assumed 3-4 s based on

heave characteristics), low external water pressure and the dynamics of the front in the

crack are included in the analyses.

5.6 Tethers

Tether capacity

The purpose of the tethers is to stabilize the buoyant SFTB vertically and to provide vertical

stiffness to the SFTB. The key is then to have sufficient tether tension when installed, so that

time varying loads and operational loads do not lead to tether slack.

The SFTB is installed with a nominal tether tension of approximately 10 MN.

There are two main criteria for the tether:

1. Avoid tether slack during the lifetime (ULS criterion, EQU)

2. No overstressing of the tethers (ULS criterion, STR)

The load effect on an individual tether is depicted in Figure 5-4. The criterions above can be

found as the horizontal line at 0 tether force (1) and the dotted line at 22 MN tether force

(2).

/ , rev.

28

> Figure 5-4 Load contribution in tethers

Figure 5-4 is further explained:

The red vertical arrow shows the un-factored (“SLS”) condition in the tether.

o As seen, tolerances on the self-weight, buoyancy etc. has a significant

impact. Some of this uncertainty can be further reduced by production follow

up and weighing in dock. A portion of this load is variable buoyancy due to

salinity in water, temperature, etc. which has to be designed for. Inaccuracy

in pretension level achieved during installation is also to be considered in the

tolerance.

o The contribution from water absorption in the concrete and ballast, and

marine growth is also accounted for. No planned maintenance to removing

growth, re-ballast the tubes etc. is needed for the tether integrity during the

lifetime of the SFTB.

o Traffic load is the governing load for tether slack criterion

o Hydrodynamic actions from a 100 year storm give additional load in the

tether to both sides.

The green vertical arrow shows the EQU control, which documents the tether slack

control. This is factored by the EQU control in NS-EN-1990.

o Load factor 0.9 on favorable loads (nominal pretension and tolerances), 1.35

for traffic (governing variable load), 1.05 for marine growth and water

absorption and 1.15 for hydrodynamic contribution (1.6x0.7).

o Tether force not below 0 for the combination -> No tether slack

o Correlation factor is 1.0 for all loads in the combination. In principle this

means that the following must be fulfilled at the same time to have tension

close to 0:

Salinity very low for minimum buoyancy

All tolerances during production to “wrong” side

Full marine growth on the full surface of the concrete tubes and

maximum water absorption in concrete and ballast.

Full traffic load in both tubes (total 54 kN/m).


29 Full load in pedestrian lane. Corresponds to 10 kN/m, approximately

5 persons per square meter during a length of approximately 400

m(!) bridge

All of the above effects during a 100 year storm.

The blue vertical arrow shows the STR control, which covers the tether capacity

control (yielding of tether).

o Extra uplifting forces after the SFTB is installed considered

o The structural tolerances and buoyancy variances are the governing loads

o Hydrodynamic actions and some effects of traffic is addition

o No water absorption, marine growth or traffic present to produce this case

o Capacity sufficient compared to the dotted line.

o Small response compared to Offshore TLP’s. Tethers with yield strength of

235 MPa used. The reason for this selection is that the present concept need

the steel area for stiffness rather than the strength. As comparison, the

tethers of Heidrun TLP has a yield strength of 480 MPa

Tether slack

As a part of a robustness assessment, a separate study has been performed to investigate

the consequences if tether slack should occur. This means that all static contributions to

minimum tether tension is present simultaneously, and the dynamic contribution from the

wave action will govern the slack. The heave period of the SFTB is around 3 s, meaning that

time where the tether experience heave will be very short. The study concludes that the

tether may have some lifting in the tether porch (a few cm). This is not critical as the

porches are designed for this. Transient stresses in the tethers were not seen in the study,

concluding that tether slack due to wave dynamics is not critical.

Loss of tether

An investigation has been made to investigate the effect of unexpected loss of one tether.

This may be due to submarine impact, foundation failure etc. and is considered as an

accidental limit state. The study concludes that the SFTB can be operated normally with loss

of one tether. The tether should be however be replaced after such event to maintain the

robustness of the system.

5.7 Pontoons

The robustness of the pontoon stabilized SFTB is ensured by the following:

Sufficient freeboard for all load combinations

Change in the draft of the pontoon due to unexpected events or loads are visible

Compartmentation of the pontoons. Puncture of up to two compartments will not

lead to damage of the SFTB

Designed for ship impact. The tunnel has sufficient capacity for a design ship impact

Weak link between pontoon and tubes to prevent overloading of the tubes in case of

severe unexpected loads

SFTB dimensioned to be able to operate with loss of one pontoon. Limitation in traffic

is then foreseen until the pontoon is replaced.

Experience on concrete pontoons has been gained through 20 years of operation for the two

floating bridges in Norway.

/ , rev.

30 5.8 Design status

The present design phase has proven technical feasibility of the two SFTB concepts, and

further come up with dimensions as basis for cost estimate. The safety format of Eurocode is

implemented together with the N400 guideline on SFTB.

With major attention to dynamic load effects the design work has been verified by simplified

frequency domain response estimate in Abaqus together with 3rd party verification by

Marintek. The dynamic analysis scheme in time domain by 3Dfloat is thus proven fit for

purpose.

To verify the dynamic characteristics of the SFTB cross-section, especially the flow-induced

forces on low-density SFTB structure, wind tunnel tests have been carried out by Svend Ole

Hansen ApS. Single tube, double tubes and rectangular box have been analysed.

Uncertainty still exists on environmental loads, hereunder wind generated sea and current.

Presumably, conservative waves and load combinations are used in design, therefor no

major modification of structure is foreseen due to variation in environmental input. The

pontoon concept is considered less sensitive to future variation in design input, whereas the

tether stabilized SFTB depends on seabed soil, rock and slope parameters.


31 6 DESIGN PREMISES

6.1 Design codes

For the assessment study for the Submerged Floating Tube Bridge (SFTB), NPRA’s Handbook

N400 will be applicable as a road standard. This manual provides reference to other relevant

standards combined with a document hierarchy with priority order A - I. Within each group,

the order of priority is: 1. Regulations, 2. Standards and 3. Guidelines.

The documents relevant for the current study are listed in order of priority:

A Handbooks - Norms - Handbook N400 sec. 13.12 “Flytebruer og rørbruer”

B Guideline Handbooks

C Other NPRA books / internal reports

D Norwegian engineering standards (NS and Eurocode)

E Norwegian materials and workmanship standards

F Standards for drawings and project documents

G Regulations, policies, standards or publications not covered above

H Other Standards

I Publications from industry associations

Where relevant information is missing in NPRA’s handbooks and Norwegian engineering

standards, the project group will suggest using supplemental standards. These will be in the

category G (Table 6-1). The safety level of the Eurocode system shall be obtained even if

other standards are used.

As per Handbook N400 structural analysis and design is to be in compliance with the

Eurocodes. NS-EN 1992-1-1 for concrete design does not cover specific aspects of marine

structures, and a clarification has been made with Standard Norge that the concrete design

still may follow Eurocode. The background is that a revision of EN 1992-1-1 is under

development including marine structures.

> Table 6-1: Relevant standards

Priority Most relevant standards Description

A Handbook N400, kap. 13.11 Floating and tube bridges

A Handbook N100 Road and Street Design

A Handbook N500 Road tunnels

B Handbook V420 Guidance, design of bridges

C Rapport: Krav til plass og rom i rørbru,

SVV, 2011

Overview of the requirements and

challenges in the current legislation

with respect to space and room in tube

bridges.

D NS-EN 1990 Basis of structural design

D NS-EN 1991 - Eurocode 1 Actions on structures

/ , rev.

32 D NS-EN 1992 - Eurocode 2 Design of concrete structures

D NS3473 Design of concrete structures

D NS-EN 1993 - Eurocode 3 Design of steel structures

G DNV-RP-C205 Environmental Conditions and

Environmental Loads

G DNV-RP-F105 Free Spanning Pipelines

G ISO-21650 Actions from waves and current on

coastal structures

G NORSOK N-003 Actions and action effects

G Estimat på bølge og strøm,

Mulighetsstudie for kryssing av

Sognefjorden Oppedal – Lavik, Sintef

Site-dependent wave and current for

design purpose

G DNV-OS-H101 Marine Operations, General

G DNV-RP-H103 Modeling and Analysis of Marine

Operations

6.2 Functional requirements

6.2.1 Design life

The operational design life for the crossing shall be 100 years, according to handbook N400.

Easily replaceable components, moving parts and outfitting may be designed for a shorter

design life, minimum 20 years.

6.2.2 Reliability class

The bridge structure has the Reliability Class RC3 according to N400. Elements may have

different classification, depending on importance for structural integrity.

6.2.3 Structural requirements

Deflections

The limitation of short time deflections is controlled for infrequent load combinations in the

Serviceability Limit State. No requirements to deflections are given specifically for SFTBs in

N400, hence NPRA has advised the limiting values as per Table 6-2.

> Table 6-2: Limiting deflections

Direction Total deflection

Horizontal L / 200

Vertical L / 350

http://exchange.dnv.com/publishing/Codes/download.asp?url=2011-04/rp-h103.pdfhttp://exchange.dnv.com/publishing/Codes/download.asp?url=2011-04/rp-h103.pdf


33

For deflections in the horizontal direction, L will be taken as the total length between the

abutments. The dynamic contribution shall not exceed L / 350.

For vertical deflections L will be taken as the distance between the vertical supports, i.e.

pontoons or tether groups.

Accelerations and vibrations

Accelerations and vibrations shall be evaluated with respect to user comfort. NPRA has

recommended the following maximum accelerations to ensure pedestrian comfort:

0.5 m/s2 for vertical vibrations

0.3 m/s2 for horizontal vibrations

Water tightness

Structural elements subjected to permanent or potential water pressure difference shall be

watertight. Particularly strict requirements, given below, for tightness according to NS

3473:2003 A.15.5 shall be applied for ensuring water tightness in operation and temporary

conditions. The reason for using this superseded standard is its specific requirements

regarding water tightness which have no equivalent in the Eurocode, a standard not intended

for marine concrete structures.

The minimum depth of the compression zone xc shall not be smaller than the lesser of

0.25 h and 100 mm

If the tensile membrane stress is larger than zero, the minimum compression zone

shall be larger than 200 mm

Water tightness criteria are checked in the Serviceability limit state for characteristic load

combinations.

In Ultimate and Accidental limit state the reinforcement strain shall be limited to the elastic

domain.

6.2.4 Floating stability

Intact stability criteria

Watertight integrity and hydrostatic stability in temporary phases shall comply with

requirements given in DNV-OS-C301.

Damage stability criteria

The SFTB shall provide sufficient buoyancy and stability in accordance with the requirements

in DNV-OS-C301. The extent of the damage shall be assumed to two compartments unless it

is proven that the bulkheads remain intact. The consequences on the tubes shall be

evaluated. The bulkheads are to be designed for plausible impact loads in addition to the

pertinent water pressure difference.

6.2.5 Traffic requirements

Road category

The road standard and tube bridge class shall be selected based on the following

parameters:

– Annual Average Daily Traffic (ADT) : >20 000 ADT

– Design speed limit : 110 km/h

/ , rev.

34 The crossing shall at least satisfy the requirements for primary road class H9.

Bicycle access

The tube bridge shall accommodate a bicycle access in a dedicated gallery separated from

the road traffic compartments. Tube tunnel profile T4 may be adopted provided the

accessibility for emergency vehicles can be demonstrated.

Alignment

The horizontal alignment is chosen based on what is most favourable for the concept

considered. The minimum radius for horizontal alignment without width extension is taken as

2 350 m reflecting a future design speed limit of 120 km/h.

According to N500, the maximum gradient for the tube bridge shall not exceed 5 % including

potential tidal variations (1 year RP).

The vertical alignment shall satisfy a ship clearance of minimum 20 m above the tube bridge

within the fairway(s).

Minimum radii for vertical alignment shall be taken in compliance with Handbook N100 Tab.

C.2 to:

– Minimum radius, crest curve : 2 356 m

– Minimum radius, sag-curve : 4 112 m

Slope discontinuity

Reference is made to Handbook N400, section 13.11.1. The requirements regarding

maximum admissible angular change in joints according to N400 are not deemed relevant for

the SFTB.

Tube bridge cross section

Tube bridge class and tube bridge configuration for circular cross sections are determined

according to Handbook N500 and is based on Annual Average Daily Traffic (AADT) and tube

bridge length. For the anticipated ADT (15 000 in 2040), tunnel class E with tunnel profile

T9.5 with a total road width of 9.5 m is used (Figure 6-1). The requirement for free

headroom measured normal to the road surface from the edge line is 4.60 m. Minimum inner

radius for a circular tube bridge cross section is 5.02 m with centre 1.57 m above road

surface.


35

> Figure 6-1: Tunnel profile T9,5 from N500 and minimum inner tube diameter The road design manual Handbook N100 requires the possibility to pass damaged vehicles

and tunnel class E requires emergency lay-bys at 500 m ±50 m intervals (Handbook N500).

For the 3.0 m lay-by lane tunnel profile T12,5 (Figure 6-2) shall be used. Minimum inner

radius for a circular tube bridge cross section is 6.52 m with centre 1.44 m above road

surface. For plan layout of the lay-by lane reference is made to Handbook N500 sec. 4.6.1.

> Figure 6-2: Tunnel profile T12.5 from N500 for lay-by and minimum inner tube diameter

Emergency evacuation of persons shall be accommodated every 250 m according to N500.

Tube bridge profile for bicycle gallery shall be T4 and need to be accessible for emergency

vehicles. Reference is made to N500.

/ , rev.

36 6.2.6 Navigational channel

The required navigation channel will be determined on the basis of a risk assessment in

dialogue with the bridge designers. The following numbers have been established as a

starting point for the design:

Dimensions of ship clearance (fairway)

Width : 400 m

Depth : 20 m

Outside the fairway

Depth : 15 m at shores

6.2.7 Equipment

Technical rooms in tube bridges

Tube bridges with a length of more than 2 000 m shall provide place for technical rooms in

order to house cabling for electrical installations, emergency power supply,

telecommunications and accumulators. Specifications on operation criteria and placement of

technical rooms are included in NPRA report regarding requirements for space and room in

an SFTB [4].

Ventilation in running bores

Tube bridges need appropriate ventilation for operational phases and accidental phases

(fire). For the case of fire, ventilation should provide overpressure in emergency exits and

exhaust dangerous gasses through separate canals. Maintenance on the ventilation system

should not require taking the tube bridge out of operation, e.g. by using sufficiently

redundant systems.

Drainage in tube bridge

Any water in the tube bridge has to be collected and pumped out of the tube bridge.

Penetrations of the main hull in permanent condition are not permitted. The draining system,

reservoirs, pumping stations and conduits for the traffic compartments shall be designed to

collect and handle surface water, wall washing water, firefighting water and spillage from a

road tanker. If transport of dangerous goods is permitted, the safe drainage of flammable

and toxic liquids shall be given due consideration.

6.2.8 Inspection

All equipment requiring regular inspection or maintenance must be accessible.

6.2.9 Instrumentation

The structural behaviour and any protective systems (corrosion protection etc.) shall be

surveyed. The measurement of water levels in each compartment shall be connected to an

alarm system.


37 6.3 Bjørnafjorden bathymetry and soil conditions

Since results from detailed seabed survey became available first at the end of the project

phase, the available site and soil investigations were very limited, and the assessment study

has been mostly based on the following:

Engineering geological mapping: 18th March 2015 a site visit and engineering

geological mapping of available areas was carried out by Norconsult

Acoustic profiling: An acoustic survey by "sparker" (reflection seismic survey) was

performed along the strait crossing area in 2012; "Bjørnafjorden – Bruforbindelse,

Løsmassekartlegging for vurdering av ankringsforhold", GeoMap March 2012.

Acoustic profiling: Another series of "boomer" profiling in the shallow waters near the

shoreline was performed in 2014; "E39 state municipal plan Aksdal-Bergen, Acoustic

profiling with boomer to map bedrock horizon", GeoMap November 2014.

Test samples of clay material: A total of 4 sample series from the clay from the

sediments in the deep part of the fjord have been recovered and tested.

At the bridge site, Bjørnafjorden has water depths down to about 500 m. The near-shore

areas are characterized by undulating surface, covered by limited soil. There are steep

underwater slopes down to the middle part of the fjord from the shoreline on both sides of

the crossing. The slopes are assumed to have outcropping rock. The middle part is covered

by varying types of sediments, described as moraine, mixed deposits and clay.

/ , rev.

38

> Figure 6-3: Bathmetry (survey April 2016)


39

> Figure 6-4: Sediment thickness (survey April 2016)

/ , rev.

40

> Figure 6-5: Seabed profile (centerline tether-stabilized SFTB)

Figure 6-3 to Figure 6-5 show the bathymetry and seabed conditions across Bjørnafjorden

obtained from the recent survey campaign (April 2016). The crossing alignments are

indicated by the two curved lines, where the tether-stabilized SFTB is represented by the

western curved line and the pontoon-stabilized SFTB by the eastern line curve. The

coordinates refer to easting and northing based on EUREF 89 UTM 32N.

Figure 6-3: Bathymetry in Bjørnafjorden, thick black lines show 50 m countours, and grey

lines are 5 m contours, illustrating variable depth conditions, where the basin is 560 m deep.

Figure 6-4: Isopach results in Bjørnafjorden based on 50 m grid. This illustrates sediment

thicknesses between 0 and 80 m. The deepest areas (560 m) contain the thickest sediment

sequences (80 m). On the slope areas the sediment thickness generally varies between 10-

20 m. In the northernmost areas there is less sediments, and the bedrock is outcropping,

seen as grey areas.

Figure 6-5: Seabed profile along the tension leg SFTB, illustrating variable depth conditions

with steepest slope in the south and a gentler, but more variable slope in the north.

6.4 Environmental conditions

6.4.1 Water level

Sea water level: High 2.32 m (from LAT)

Low -0.32 m (from LAT)

Correction of sea water level related to MSL = 0.9 m:

High: 1.42 m

Low: -1.22 m

Correction of sea water level due to variation in seawater density:

Sea water density: max =1028 kg/m3

mean =1023 kg/m3

min =1018 kg/m3


41

Corrected High sea water level used in analyses: 1.42*1028/1023 = 1.43 m

Corrected Low sea water level used in analyses: -1.22*1018/1023 = -1.21 m

6.4.2 Current velocities

A numerical simulation of the current conditions at the site has been conducted. The

current is found to be strongest in the middle of the fjord. The predicted omnidirectional

extreme current speeds are shown in the table below.

> Table 6-3 Omnidirectional extreme current velocity for given return period

Depth 10 year 100 year 10000 year *

Surface 1.13 m/s 1.33 m/s 1.69 m/s

30 m 0.46 m/s 0.54 m/s 0.69 m/s

*) 10000 year value extrapolated from 10 and 100 year values. To be used for VIV

predictions.

6.4.3 Sea states - wind induced waves

The significant wave height and corresponding wave directions for wind generated waves for

given return periods are presented in Design Basis[9] and given in Table 6-4.

For wind sea the NPRA has specified a 100 y Hs = 3.0 m with period range 4 < Tp < 6 s (for

both crossings). When calculating dynamic response of the tube bridge in a sea state, the

sea state characteristics (Hs, Tp, direction) should be considered constant along the bridge.

> Table 6-4: Highest significant wave height Hs (m) with corresponding peak spectral period range to be checked in dynamic analysis. 330 degrees correspond to a

perpendicular angle of attack on the bridge

Return

period

Scaling

from 100 y

Hs

(m)

Tp, min

(s)

Tp, max

(s)

γ

(-)

Spread

n

Dir

(deg)

1 y 0.67 2.0 4.0 6.0 2 - 4 5 - 10 330o

10 y 0.81 2.4 4.0 6.0 2 - 4 5 - 10 330o

100 y 1.00 3.0 4.0 6.0 2 - 4 5 - 10 330o

1000 y 1.19 3.6 4.0 6.0 2 - 4 5 - 10 330o

10000 y 1.29 3.9 4.0 6.0 2 - 4 5 - 10 330o

6.4.4 Sea states - swell

Swell sea states in the fjord are determined by transferring offshore wave

Documents

· 2016-11-11 · / , rev. 2 Revision Date Reason for Issue Prep. by Contr. by Appr. by 01 04.05.16 Issued for client review Proj.team THS SAH 02 31.05.16 Issued for Approval Proj.team