Dealing with SANDWAVES on BRITNED

1. ‘Breaking the Sand Waves for the BritNed Interconnector’
PO.30 Corridor Design through mobile Seabed
Wino Snip, Wilco van Lieshout, Jaap Smit
Primo Marine, Rotterdam, The Netherlands, www.primo-marine.com
1. Sand Waves along the BritNed Interconnector Route 2. Relevance to Cable Protection 3. Pre Sweeping to Protect
Sand banks
Wavelength: 2 – 10 km
Wave height: 5 - 30 m
up to 75% of water depth
Timescale: > 100 yrs Seabed
Relevant to Migration: < 1 m/yr after
Cable Burial Sand waves
Orient.: 60°- 85° to tidal cur.
Direction of sand wave x years
movement Current
Wavelength: 30 - 800 m
Wave height: 1 - 15 m seabed
up to 30% of water depth
‘Too
Time scale: 5 yrs Slow’
Length scale
Migration: ≈ 5 – 20 m/yr
‘Too Mega ripples
Orientation: ⊥ tidal current
Low’ Wavelength: 0.3 - 10 m
Relevant for the
Wave height: 0.05 – 0.2 m
Timescale: 0.1 yr Trencher? Reference level Volume to pre-sweep
Migration: ≈ 100 -200 m/yr
Ripples Orientation: ? for trenching After x years: cable prior to cable lay
Wavelength: 0.05 m protection requires (This volume has to
Wave height: 0.005 m maintenance in
Timescale: 10 min. these sections
recover after trenching)
Migration: - Cable after
Orientation: ⊥ wind trenching
Time scale
if ≈ 5 m/yr
1. migration then: ≈ 200m/40yrs
if ≈ 500 m
then: line of
lowest points
2. seasonal change in profile
End of winter
1 – 15 m
End of summer
profile
profile
30 - 800 m
Source: van der Berg, 2007
1a. Facts & Figures: the BritNed Interconnector Cables 1b. Facts & Figures: BritNed & the Sand Waves 2. Relevance to Cable Protection 3. Pre Sweeping to Protect
• Cable length 260 kilometres; 2 HV DC cables of 12 cm diameter each • Optimisation between CAPEX and OPEX for Sand Wave Crossings: • Sand Waves can migrate several meters each year • The Mobile Sand Waves will be Pre Swept on their eroding side
• Cable capacity 1,000 MW 8 years of ‘bury and would like to forget – philosophy’, followed by • Sand Waves migration is driven by Tidal Current before the BritNed Cables will be laid and trenched
• Access for all market participants maintenance of the Protective Burial Depth (PBD) • The Design Life of the BritNed Interconnector is 40 years • Pre Sweeping postpones maintenance; without Pre Sweeping maintenance
• Connection between Maasvlakte (NL) and the Isle of Grain (UK) • Pre Sweeping 100% of the Sand Waves would take approx. 8,000,000 m3 • Protective Burial Depth (PBD) of BritNed: approx. 2 m below Reference Level of the PBD would be then required directly in the first years after installation
• Operational at the beginning of 2011 • During Construction only approx. 1,000,000 m3 will be pre swept • During its lifetime the migration of Sand Waves will unbury the BritNed Cables • After Pre Sweeping and Cable Installation the Sand Waves do recover from
• Project costs 600 million euro • Offshore Construction started July 2009 • Surface Waves (Storms) tend to flatten out Sand Waves Pre Sweeping by Forces of Nature [van der Berg 2007]
• BritNed is a joint venture of National Grid (UK) and TenneT (NL) • First Pre Sweeping campaign completed in August 2009 • This recovery will provide additional protection against unburial on the eroding
sides of the BritNed Interconnector
4. Migration Analysis in 3D 5. Rerouting the Cables through the Sand Waves 6. Pre Sweep Volume Distribution
≈ 60 m 3 /m’/5 year • ≈ 12 m3 /m’/year
• ≈ 1.29% of NL
≈ 0.5 m
• target volume
U140 = 9,600 m 3
RPL 1A
RPL 1A
≈ 120 m U140
RPL 3_01 RPL 3_01
Vert. ex. = 10
4. Migration Analysis in 3D
• The BritNed Cable Route was surveyed in 2003 (Route Planning Survey)
and in 2008 (Pre Construction Survey) 2008
• These Bathymetry Data Sets with a 5 year Interval provided a sound basis for 2003
RPL 3_01 RPL 3_01
a Migration Analysis of Sand Wave Areas in 3D
• The mobile areas were categorised; the areas with highest mobility rates
were identified RPL 1A
Pre Sweep Profile
Sand Wave U137
5. Rerouting the Cables through the Sand Waves RPL 1A
• After the Migration Analysis in 3D the BritNed Cables were rerouted – where possible:
1. To reduce the number of Sand Wave to be crossed Hor.: 10 m
2. To avoid more mobile Sections within Sand Wave Areas Vert.: 2.0 m
3. To avoid Crests and route through Troughs Vert. ex: 2.0
4. To clear Wrecks and other Obstacles
5. To optimise crossing angles from a Pre Sweep Profile Design point of view
7. 3D Pre Sweep Profile Design 8. Design Deliverables 9. Design Actualisation Offshore
Vert. ex. = 4.0 Vert. ex. = 4.0
9. Design Actualisation Offshore
6. Pre Sweep Volume Distribution • The 2008 Pre Construction Survey was the basis of the Design of the Pre Sweep Profiles; Pre Sweeping will be performed from August 2009 to April 2010
• The Pre Sweep Volume for BritNed was limited by Licences: • Between Design and Construction some of the Sand Waves will migrate significantly (> 2m shift of the Crest)
1,000,000 m3 on the Dutch side and 100,000 t (approx 50,000 m3) on the UK side • During Pre Sweeping the “3D Pre Sweep Profile Box Design” will be shifted along the Cable Route – this is performed on board of the Dredger
• For the Design this Volume had to distributed over the mobile areas • By shifting the “3D Pre Sweep Profile Box Design” the Design is actualised such that the Pre Swept Volume will match the Assigned Volume +/- 10%
• The Volume Distribution was based on comparison of longitudinal sections of 2008 • This ensures a maximised usage of the available volume for Pre Sweeping and thus a maximum protection of the Cables within the Licences
with those of 2003: the ‘weighted size’ of the ‘eroded area’ between those sections
is used as the basis for the Volume Distribution
7. Pre Sweep Profile Design in 3D
• In a Workshop with the Dredging Contractor and the Cable Lay & Trenching 10. Results of the 2009 Pre Sweeping: Out Survey
Contractor Equipment Specific Design Parameters for the Pre Sweep Profiles were
established
• With these Parameters the rerouting through the Sand Wave Areas was finalised
10. Results of the 2009 Pre Sweeping: Out Survey
• With the final route established the Pre Weep Profiles were designed in 3D
• The first Pre Sweeping Campaign was carried out in August 2009 on the
• Iteration was required to match the 3D Pre Sweep Profile Design with the
Dutch side of the Cable Route
assigned Pre Sweep Volume for each Sand Wave
• All Profiles where dredged in accordance with the Specifications
• Hopper Dredger specific Pre Sweeping Tolerances were taken into account
• Relative smooth and clean Pre Sweep Profiles where achieved by:
1. Frequent Intermediate Surveys by the Trailing Suction Hopper Dredger
8. Design Deliverables 2. Suspending the Suction Head of the Hopper Dredger to fixed levels
• Interfaces between Design and Construction were thoroughly analysed and 3. Using the Jets on the Drag Heads without Suction
discussed before the actual design started to level out the smaller tops left over after the first runs
• In a Workshop the steps from start of Design to Hand Over of Pre Swept Profiles
were analysed, practised and optimised before the actual design started. • The results of the first campaign of 2009 are used for the design of the
• 3D DXF files were the main Deliverables of the Pre Sweep Profiles Design, following Pre Sweeping Campaigns – which is ongoing at this moment
accompanied by “The Blue Table of Pre Sweep Profiles and Volumes” (September 2009)
• These 3D DXF files were directly loaded into the Dredge Computer System on
Vert. ex. = 1.0
board of the Trailing Suction Hopper Dredger
• 2D Drawings were mainly used to explain the Design to the Client and for reference
European Offshore Wind 2009 Conference & Exhibition, 14 – 16 September,
Stockholm, Sweden