BritNed Interconnector Project

IRO / CEDA 8 March 2011

09/03/2011 2

Project Information

• BritNed Project: - 1000 MW Capacity Bi-Directional Dual Cable between UK & Netherlands

• Parties involved (offshore operations):

- BritNed Project Team (National Grid UK & Tennet)

- ABB High Voltage Cables Sweden (Main Contractor)

- Tideway Offshore- Global Marine (Cable installation)- Primo Marine (Engineering)

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HVDC sub-sea cable2 x 245 km, laid close

Maasvlakte, NL2 x 7 km Land cable

Isle of Grain, GB2 x 2 km land cable

Kp61 Kp122 Kp183Kp0 Kp244

2 x 2 nos 2 x 7 nosKp4CLB/CLVinterface

Kp 140UK-NL border

JointCable TerminationHDD at landfall (210 m)

Transition Joint sea/land

Project Layout

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Location: Cable & Pre-sweep Areas

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Crossing Locations

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1 Introduction: Project InformationClay & Sand;

no sand waves

Sand waves

Sand; no significant

sand waves

UK section:• 51 km• appr. 72 sand waves

NL section:• 82 km• appr. 116 sand waves

Mobile sandwaves were identified along proposed Cable Route

Work Method: Pre-sweeping

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‘8 year no return for maintenance’

UK + NL: 786,358 m3

UK permit: max 100.000 t 50,000 m3; 5%

NL permit:max 1,000,000 m3; 95%

2008 2003 2008 2003, 2004

• 72 mobile sand waves• Distr. of 36,500 m3:• 22 pre sweep profiles

• Min.: 861 m3

• Max: 6,400 m3

• Av.: 1,800 m3

• 116 mobile sand waves• Distr. of 750,000 m3:• 116 pre sweep profiles

• Min.: 1,200 m3• Max: 24,100 m3• Av.: 6,500 m3

Distribution of volume UK - NL

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Survey Results of 2002 & 2008 were compared

Movements up to 20m

Detection of Mobile Sand Waves

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RipplesWavelength: 0.05 mWave height: 0.005 mTimescale: 10 min.Migration: -Orientation: wind

Sand banksWavelength: 2 – 10 kmWave height: 5 - 30 m

up to 75% of water depthTimescale: > 100 yrsMigration: < 1 m/yrOrientation:5-30 tidal current

Mega ripplesWavelength: 0.3 - 10 mWave height: 0.05 – 0.2 mTimescale: 0.1 yrMigration: 100 -200 m/yrOrientation: ?

Sand wavesWavelength: 30 - 800 mWave height: 1 - 15 m

up to 30% of water depthTime scale: 5 yrsMigration: 5 – 20 m/yrOrientation: tidal current

‘too slow’

‘too low’

relevant to the PBD

Leng

th s

cale

Time scale

Classification

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Options to deal with sand waves

1. ‘No measures’- No pre-sweeping – except for steep slopes- Maintenance trenching/burial required for following years

2. ‘Pre sweep to trough level’: deep trench through sand waves- ‘No maintenance trenching required’- 7 - 8,000,000 m³ pre sweeping

3. ‘Partial pre sweeping’- Lower initial investment for pre sweeping- Maintenance trenching required after ‘X’ years

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‘Pre sweep to trough level’

Current seabed

Volume to pre-sweep prior to cable lay

Cable after trenching

• This requires 7 – 8,000,000 m³ for the whole route• ‘No maintenance required’

Direction of sand wave movement

Seabed after

x years

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Reference levelfor trenching

Direction of sand wave movement Current

seabed

Seabed after

x years

Volume to pre-sweep prior to cable lay

(This volume has to recover after trenching)

After x years: cable protection requires

maintenance in these sections Cable after

trenching

Seabed after

x years

‘Partial Pre-sweeping’

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Execution

TSHD “Pearl River”TSHD “Lange Wapper”

- Length- Breadth- Draught- Dredging depth- Loading Cap.

182.00 m28.00 m8.50 m

120.00 m24,150 m³

- DP / DT positioning- Multibeam installed on board

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Pre Sweep Profile Types

A

B

C

D

Pre sweep profile

Existing sand wave profile

‘Toe to toe’

‘Multi-level’

‘Threshold’

‘Smoothened slope’

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Profile Types & Design Refinement

A ‘Toe to toe’

84 (59%)

B ‘Multi-level’

49 (34%)

C ‘Threshold’

5 (4%)

D‘Smoothened slope’

4 (3%)

• No refinement required• Many close to NL coast

• Refinement required• Best endeavour

5% of designed volume

• Refinement required: ‘centre in the sand wave’

• Decision on board

• Refinement required:Decision on board

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Adjusting the design on board TSHD

Seabed as surveyed in 2008:pre construction survey

Design of the pre sweep profilebased upon the 2008 survey

Seabed as surveyedin 2009 / 2010: in-survey

Migration of the sand wave crestbetween the 2008 pre construction survey

and the 2009 or 2010 in- survey

After in-survey: shift the pre sweep profile design to accommodate for

actual bathymetry (+/- 10% of the allocated volume)

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Rock Berm Dimensions

>25 m>25 m

20 m

130 m

> 0.5 m1.0 m

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FPV “Seahorse”

• Loading capacity : 17,000 ton• Sailing speed: 12 kts• Built: 1999• LOA: 162 m• Draft: 6.40 m

FPV “Tideway Rollingstone”

• Loading capacity : 11,500 ton• Sailing speed: 12 kts• Built: 1995• LOA: 139 m• Draft: 6.60 m

Method Statement – Rock Placement

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Method Statement – Rock PlacementEquipment - Ice Class FPV “Flintstone”

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Method Statement – Rock PlacementPre-lay and post-lay

09/03/2011 21Cable ‘As Found’ August 2009

09/03/2011 22Pre Crossing Rock Berm placed: 10 m width for BritNed on top;10 m both sides of Object; 0.5m separation between BritNed and Object

09/03/2011 23BritNed Cables installed over Pre Lay Rock Berm; Trenched up to 25m from Object

Uraduct

09/03/2011 24BritNed Cables installed over Pre Lay Rock Berm; Trenched up to 25m from Object

09/03/2011 25Post Lay Rock Berm: 65m on both sides of Object; 1.0m layer thickness

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Great acknowledgements to:

• Primo Marine (ir. Wino Snip) for engineering cooperation• ABB for teamwork on site

Acknowledgements