TW BritNed IRO CEDA 110308 - Central Dredging Association · 3/8/2011 · 09/03/2011 10 Options to...
Transcript of TW BritNed IRO CEDA 110308 - Central Dredging Association · 3/8/2011 · 09/03/2011 10 Options to...
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BritNed Interconnector Project
IRO / CEDA 8 March 2011
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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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09/03/2011 3
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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09/03/2011 11
‘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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09/03/2011 14
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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09/03/2011 17
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
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09/03/2011 21Cable ‘As Found’ August 2009
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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
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09/03/2011 23BritNed Cables installed over Pre Lay Rock Berm; Trenched up to 25m from Object
Uraduct
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09/03/2011 24BritNed Cables installed over Pre Lay Rock Berm; Trenched up to 25m from Object
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09/03/2011 25Post Lay Rock Berm: 65m on both sides of Object; 1.0m layer thickness
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09/03/2011 26
Great acknowledgements to:
• Primo Marine (ir. Wino Snip) for engineering cooperation• ABB for teamwork on site
Acknowledgements
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