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Technical document: Project Description
French section
April 2013
Rapport 109375-R P2 v2.2 1
Nemo Link– Electrical interconnector between United Kingdom and Belgium French section Technical document: Project Description Prepared for Elia Asset NV
Siège Social CREOCEAN Zone Technocéan / Chef de Baie Rue Charles Tellier 17000 La Rochelle - France Tél : 05.46.41.13.13 Fax : 05.46.50.51.02 e-mail : [email protected] web : www.creocean.fr
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Table of contents
1. Location and footprint of the project ............................................................................ 1
1.1. Project overview ..................................................................................................................... 1 1.2. Cable route ............................................................................................................................ 2 1.3. Footprint of the project in French Waters .................................................................................... 7 1.4. Installation programme ........................................................................................................... 8 1.4.1. Schedule of the works ..................................................................................................................................8 1.4.2. Commissioning date .....................................................................................................................................9
2. Technical description of the project ............................................................................ 10
2.1. Outline description of cable system ......................................................................................... 10 2.2. Offshore cable installation ...................................................................................................... 11 2.2.1. Cable route preparation (Pre-lay) ................................................................................................................ 11 2.2.2. Cable laying operation ................................................................................................................................ 16 2.2.3. Burial of the cable ...................................................................................................................................... 16 2.2.4. Cable joints............................................................................................................................................... 18 2.2.5. Cable crossings ......................................................................................................................................... 18 2.2.6. Post-burial of the cable ............................................................................................................................... 18 2.3. Cable operation .................................................................................................................... 20 2.3.1. Maintenance ............................................................................................................................................. 20 2.3.2. Retrieval of buried cable ............................................................................................................................. 24 2.4. Marine equipment for the operations ....................................................................................... 24 2.4.1. Laying operation ........................................................................................................................................ 24 2.4.2. Maintenance ............................................................................................................................................. 26 2.4.3. End of Life Retrieval of the Cable ................................................................................................................. 26
3. Provisions to ensure the safety of shipping and to prevent navigational accidents ............. 27
3.1. Signs and securing the working mobile area ............................................................................. 27 3.1.1. Temporary precautionary mobile zone .......................................................................................................... 27 3.1.2. Accompanying Vessel ................................................................................................................................. 27 3.1.3. Signs ....................................................................................................................................................... 27 3.2. Cooperation with monitoring traffic services in the Channel ......................................................... 28 3.3. Broadcast Notice to Mariners .................................................................................................. 28 3.4. Emergency Coordination Plan (ERCoP) ..................................................................................... 28 3.5. Cable protection ................................................................................................................... 29 3.6. Risk of cable exposure ........................................................................................................... 29 3.7. Geomagnetic interference ...................................................................................................... 29 3.8. Discovery of munitions or explosives ....................................................................................... 29
4. 30
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List of Figures Figure 1.1.1 Schematic overview of the HVDC interconnector between the Belgium
and the United Kingdom Figure 1.2.1 Cable route Figure 1.2.2 NEMO marine cable route in French waters Figure 1.4.1 Schedule for the marine operations of the project Figure 2.1.1 Cable description Figure 2.2.1 Grapnel Figure 2.2.2 Trailing suction hopper dredgers (TSHD) Figure 2.2.3 Identified areas for pre-sweeping operation Figure 2.2.4 Laying of the cable Figure 2.2.5 Example of burial ploughs: grapnel plough and jetting plough Figure 2.2.6 Cable crossing over buried/semi buried infrastructure Figure 2.4.1 Laying and burial of the cable carried out by two vessels in convoy
List of Tables Table 1.2.1 Turning points (A/C) on the cable route in French waters Table 1.2.2 Cable route in French waters Table 1.2.3 500 m cable route corridor in French waters Table 1.3.1 Footprint of the project Table 2.2.1 Identified areas for pre-sweeping operation Table 2.3.1 Electric and magnetic fields from BritNed
Abrreviations M Mile KP Kilometric Point EEZ Exclusive Economique Zone A/C Alter Course TSHD Trailing Suction Hopper Dredger CM Cote marine = Seashore OMI Organisation Maritime Internationale MCA Maritime and Coastguard Agency CROSS Centre Régional opérationnel de surveillance et de sauvetage
(=Regional Operational Centre for Surveillance and Rescue) CNIS Channel Navigation Information Service RIPAM Règlement international pour prévenir les abordages en mer
(=International Regulations for Preventing Boarding) ColReg Collision Regulation HSE Health-Safety – Environment ERCoP Emergency Response Cooperation Plan MARPOL Maritime Pollution SHOM Service Hydrographique et Océanographique de la marine
(=Hydrographic and Oceanographic Service of the Navy) UKHO United Kingdom Hydrographic Office VHF Very High Frequency
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1. Location and footprint of the project
1.1. Project overview The NEMO project, led by National Grid Nemo Link Ltd and Elia Asset SA, consists of the installation of a subsea power cable crossing the North Sea between the United Kingdom and Belgium. This interconnector is designed to connect the two high voltage grid systems in order to increase market liquidity and support security of supply both in Britain and Belgium. The electrical interconnector capacity will be between 700 and 1300 MW. The High Voltage Direct Current (HVDC) interconnector is made up of two high voltage conductors of opposite polarity, and operates as a unique conductor. The use of High Voltage Direct Current (HVDC) provides the most efficient and effective means of transporting electricity over this distance. The interconnector consists of two main parts: a submarine "offshore" part and an "onshore" part on the land or landfall (Figure 1.1.1). The two ends of the cables are connected to a high voltage converter station for converting direct current / alternating current (AC / DC) and for the connection to the high voltage grid system 380 kV. The conversion is performed by the semiconductor components of power electronics.
Figure 1.1.1: Schematic overview of the HVDC interconnector between Belgium and the United Kingdom
Schematic overview of the HVDC interconnector between Belgium and the United Kingdom
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1.2. Cable route The entire route is derived from preliminary cable route engineering, with the survey shown in Figure 1.2.1; and the list of kilometric points of the cable route provided in Table 1.2.1: Turning point
KP Long WGS 84
Lat WGS 84
Easting UTM31N
Northing UTM31N
Depth. (m CM)
Cap
Purpose
West Zeebrugge 0 3.1670 51.3258 511641 5686073 Landing in Belgium
ALTERCOURSE 1.6 3.1469 51.3335 510232 5686924 -6.0 273°
ALTERCOURSE 4.5 3.1060 51.3353 507385 5687119 -8.0 273° Avoid a wreck
ALTERCOURSE 12.1 3.0581 51.3379 504047 5687405 -10.6 272° Avoid a wreck
ALTERCOURSE 17.7 2.9976 51.3416 499833 5687813 -15.2 271° Avoid wrecks/landmine area
ALTERCOURSE 19.5 2.9165 51.3444 494188 5688127 -16.6 278° Avoid a known wreck location
ALTERCOURSE 24.3 2.8920 51.3480 492478 5688535 -21.7 278° Crossing C-POWERcable
ALTERCOURSE 28.1 2.8253 51.3580 487839 5689651 -25.1 263° Crossing Hermes South cable
ALTERCOURSE 30.1 2.7718 51.3516 484105 5688952 -25.7 255°
ALTERCOURSE 32.5 2.7449 51.3442 482234 5688135 -21.5 271° Keep distance of 250 m from cable Pan European Crossing et Hermes2
ALTERCOURSE 32.8 2.7107 51.3449 479849 5688216 -16.8 274°
ALTERCOURSE 42.8 2.7064 51.3451 479549 5688248 -32.0 272° Keep distance 250 m from cable PEC
ALTERCOURSE 43.4 2.5634 51.3505 469593 5688899 -30.7 284°
ALTERCOURSE 45.1 2.5557 51.3524 469060 5689111 -28.9 312° Crossing Tangerine cable
ALTERCOURSE 46.0 2.5411 51.3653 468054 5690553 -29.5 286°
ALTERCOURSE 48.0 2.5364 51.3667 467728 5690702 -33.6 286° Crossing pipe FRANPIPE and cables
Tangerine and Rioja 2ALTERCOURSE 48.0 2.4600 51.3570 462402 5689659 -32.1 270°
ALTERCOURSE 50.0 2.4405 51.3571 461041 5689684 -33.3 256°
ALTERCOURSE 51.1 2.4367 51.3562 460780 5689583 -35.4 232° Avoid known wreck locations Crossing FRANPIPE
ALTERCOURSE 52.5 2.4322 51.3526 460459 5689193 -35.1 256°
ALTERCOURSE 52.8 2.2863 51.3152 450262 5685120 -35.4 264° Keep distance 250 m from Tangerine
ALTERCOURSE 53.3 2.2714 51.3137 449222 5684962 -26.8 277° Crossing cables TAT14 and UK-Belgium 3
ALTERCOURSE 64.2 2.2226 51.3196 445828 5685657 -33.8 277°
ALTERCOURSE 65.3 2.1721 51.3259 442316 5686390 -37.1 270° Keep distance 250 m of Tangerine
ALTERCOURSE 68.8 2.0111 51.3253 431094 5686470 -29.8 270°
ALTERCOURSE 72.4 1.9596 51.3252 427506 5686501 -37.1 271° Crossing UK-Belgium 5 cable
ALTERCOURSE 94.1 1.8600 51.3276 420570 5686870 -40.3 269°
ALTERCOURSE 96.8 1.8216 51.3272 417893 5686869 -40.3 270° Crossing AC1 cable
ALTERCOURSE 103.5 1.7255 51.3269 411202 5686950 -46.3 273°
ALTERCOURSE 107.3 1.6712 51.3301 407423 5687369 -36.2 262° Avoid a steep crest
ALTERCOURSE 107.6 1.6665 51.3295 407098 5687306 -29.1 248° Avoid a Sabelleria reef
ALTERCOURSE 108.2 1.6600 51.3268 406634 5687018 -26.4 264°
ALTERCOURSE 112.3 1.6016 51.3209 402558 5686435 -21.0 269°
ALTERCOURSE 116.2 1.5453 51.3201 398629 5686430 -8.6 257° Avoid a sandbank
ALTERCOURSE 118.7 1.5111 51.3122 396231 5685592 -13.8 263°
ALTERCOURSE 121.2 1.4766 51.3078 393814 5685152 -12.9 266°
ALTERCOURSE 123.0 1.4509 51.3059 392021 5684987 -8.8 275° Avoid a damaged area
ALTERCOURSE 123.7 1.4404 51.3064 391290 5685050 -9.5 285°
ALTERCOURSE 127.3 1.3937 51.3187 388066 5686486 -2.1 283°
ALTERCOURSE 128.2 1.3814 51.3216 387212 5686831 -0.5 279° Keep distance of 250 m from the Thanet windfarm cable
ALTERCOURSE 129.2 1.3651 51.3248 386089 5687211 297°
Richborough 129.4 1.3647 51.3249 386059 5687234 Landing in the United Kingdom
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The offshore section is approximately 130 km, of which 23 km is in French waters, exclusively in the marine area, so without any landing on French territory. The French section is located roughly between PK59 and PK83 (counted from Zeebrugge, Belgium - cf. Figure 1.2.2). Based on the results of the survey, the defined cable route on which the impacts can be studied, is as follows:
Table 1.2.2: Cable route in French waters
PK Latitude N Longitude E X Y Depth (m)
60 2,343° 51,330° 454207 5686696 ‐38,1
61 2,329° 51,326° 453279 5686325 ‐36,8
62 2,316° 51,323° 452350 5685954 ‐35,9
63 2,303° 51,319° 451421 5685583 ‐35,3
64 2,290° 51,316° 450493 5685212 ‐29,3
65 2,276° 51,314° 449519 5685007 ‐34,0
66 2,262° 51,315° 448537 5685103 ‐35,9
67 2,247° 51,317° 447557 5685303 ‐36,4
68 2,233° 51,318° 446577 5685503 ‐37,2
69 2,219° 51,320° 445598 5685705 ‐36,9
70 2,205° 51,322° 444619 5685909 ‐34,7
71 2,191° 51,324° 443640 5686114 ‐31,5
72 2,177° 51,325° 442661 5686318 ‐30,1
73 2,163° 51,326° 441668 5686395 ‐30,2
74 2,148° 51,326° 440668 5686402 ‐31,8
75 2,134° 51,326° 439668 5686409 ‐35,0
76 2,120° 51,326° 438668 5686416 ‐33,8
77 2,105° 51,326° 437668 5686423 ‐28,9
78 2,091° 51,326° 436668 5686430 ‐39,1
79 2,077° 51,326° 435668 5686437 ‐38,3
80 2,062° 51,326° 434669 5686444 ‐37,2
81 2,048° 51,325° 433669 5686451 ‐38,4
82 2,034° 51,325° 432669 5686459 ‐39,2
83 2,019° 51,325° 431669 5686466 ‐31,2
This route is not fixed; the project partners wish to maintain the ability to lay the cable within the 500 m corridor defined rather than the central route. Because of the presence of mobile features such as sand waves, the subsea morphology is likely to change following cable installation.
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Table 1.2.3: 500 m cable route Corridor in French waters
ID Longitude Est Lon_DD
Latitude Nord Lat_DD X_UTM31(m) Y_UTM31(m)
141 2,277669 51,312801 449656 5684850
142 2,276809 51,3125 449596 5684820
143 2,27195 51,3115 449256 5684720
144 2,27091 51,3115 449183 5684720
145 2,24639 51,314498 447478 5685060
146 2,17331 51,323398 442396 5686120
147 2,17158 51,3236 442276 5686140
148 2,06252 51,323398 434676 5686200
149 2,05448 51,323398 434116 5686210
150 2,04501 51,3232 433456 5686200
151 2,044719 51,323398 433436 5686220
152 2,02672 51,3232 432181 5686210
153 2,030649 51,327701 432462 5686710
154 2,172349 51,328098 442335 5686640
155 2,27148 51,316001 449229 5685220
156 2,274189 51,316699 449418 5685300
157 2,29355 51,321399 450772 5685810
158 2,35471 51,337001 455050 5687500
159 2,35553 51,335399 455105 5687320
160 2,35698 51,332599 455203 5687010
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1.3. Footprint of the project in French Waters The project includes several phases to cover the whole life of the project:
The installation of the electrical interconnector (cable laying) The operation and maintenance (monitoring of the burial of the cable, any required repairs
etc.) The decommissionning activity at the end of life of the cable, including the dismantling
operation and the reconditioning of the site. The technical information for these activities is described in the following Section 2. The footprint of the marine offshore work in the French section of the cable route is described below:
Table 1.3.1: Footprint of the project
Activity Surface area Marine cable route preparation
Clearance of a corridor of 100 m to each side of the defined cable route in order to remove the seabed debris and ensure the passage of the machines used for laying the cable. The maximum surface area taken by the grapnel or the ROV on the seabed is 10 m. On the sea surface, the required area for vessels is 500 m radius.
Cable laying On the seabed: The cable will be buried to a target depth of circa 2 -3m. Based on
current burial technology it is estimated that the maximum footprint of the burial machine will be 10 m and the footprint of the trench will be between 1 – 5 m.
Pre-sweeping path width will vary depending on the shape and size of each feature but may be 10 m to 20 m.
On the sea surface:
Typically, a large Cable Lay Vessel (CLV) will be up to 150 m in length and will sit within a 500 m radius precautionary zone. If the lay vessel is closely trailed by a burial vessel, the precautionary zones may link together creating a single zone increased from 500 m to 1000 m.
If the CLV is required to work in a static position and use anchors, this radius increases up to 2000 m.
Operation and maintenance
The precautionary zone for a Cable Lay Vessel carrying out repairs on the cables is 2000 m.
Decommissioning As for the laying cable installation, the precautionary zone around the vessel is 500 m to 1000 m
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1.4. Installation programme
1.4.1. Schedule of the works Cable installation schedules are dependent on a number of factors such as the delivery and availability of the cable and the availability of the installation contractor. The Project route is relatively straightforward with approximately 20 km of shallow water (< 10 m CM) on the United Kingdom side, a deep water (> 10 m CM) section of some 100 km (that crosses UK, French and Belgian waters) and a shallow water section at the Belgian end of approximately 10 km. Figure 1.4.1 below provides an indication of the installation programme for the marine aspects of the project, including a splitting of the works for the shallow water and deep water, and also for the work scheduled within the UK waters and the French-Belgium waters. Month Activity Description 1 2 3 4 5 C1a UK shore end installation
Route clearance, laying and post lay burial of cable
C1b Belgian shore end installation
Route clearance, laying and post lay burial of cable
C2 Offshore installation UK
Pre-sweeping of sand waves, route clearance, cable jointing, simultaneous lay and burial of cable
C3 Offshore installation France and Belgium
Pre-sweeping of sand waves, route clearance, cable jointing, simultaneous lay and burial of cable
Figure 1.4.1: Programme for the marine operation of the project
The programme for the commencement of installation has not yet been agreed but it is likely that installation will begin in 2016 at the earliest. In general, installations in European waters are undertaken in the summer season, broadly between April and October. This period is determined primarily by the high probability of adverse weather occurring outside of this period. The schedule will also be affected by factors such as the potential requirement for ecological mitigation, and the availability of vessels. Installation work in the intertidal areas in the UK and Belgium is expected to take less than one week. It is expected that up to five marine cable joints will be required for the entire cable system and each jointing operation will take approximately five days.
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1.4.2. Commissioning date Installation work is expected to be completed in 2018. The commissioning date has not yet been defined.
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2. Technical description of the project
2.1. Outline description of cable system The chosen cable system will be a bipolar high voltage direct current (HVDC) system with a voltage of 300 kV or 500 kV, depending on the model. The size of this type of cable is about 12 cm. To ensure the bidirectional interconnector, two cables are required (one positive, one negative). They will be connected in the same sheath (model XLPE) or placed alongside each other (MI cable).
A – Mass Impregnated cable (cable MI up to 500 kV)
B – Cable XLPE (up to 300 kV)
Figure 2.1.1: Cable desciption
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The XLPE cable technology consists of a specific insulation. This insulation is layered over a copper or aluminium conductor and is covered with a water tight sheath, usually made of seamless lead for submarine cables, and a further protective plastic coating. Cables intended for the Nemo Link, and more generally for submarine use, have an additional layer of galvanised steel wire armour to increase the cable’s tensile strength so it can better withstand the stresses of submarine installation. This is usually a single layer of wires, wound around the cable and covered in a sleeve of bitumen-impregnated polypropylene yarn to inhibit corrosion. In high activity areas or rocky seabeds, this armour may be made with a double layer.
MI type cable is a proven technology and has been widely used on major interconnector projects in service to date. This MI cable is made of a stranded type single copper core cable that has paper insulation impregnated with high viscosity mineral oil. This cable type is not pressurised like a fluid (low viscosity oil) filled cable and therefore contains no free oil to leak out in the event of a cable sheath rupture. The armour consists of one or two layers of galvanised steel armour wires, which are applied in a helix to provide mechanical strength during cable handling and installation and protection from external damage. The armour wires are bedded into a layer of bituminised jute strings and a layer of polypropylene string is applied over them to bind them which provides abrasion resistance and improves handling.
These two cables technologies, XLPE and MI, guarantee that no oil can leak into the marine environment in case of cable rupture. For the proposed 1000 MW Nemo Link, the most likely cable configuration, or project "base case" is a bipolar HVDC system, with a pair of MI cables bundled together in the same trench. However, depending upon installation technology available, there is a slight possibility that the cables could be installed in separate trenches. The cable, with its protective armour of a single or double layer, will be buried to an average depth of circa 2 -3 m, where possible.
2.2. Offshore cable installation The cable installation on the French section has no onshore landing, so only offshore operations will be required.
2.2.1. Cable route preparation (Pre-lay) Prior to the cable operation, three different activities are required to ensure the passage of the burial machine:
Cable route clearance of any seabed debris or out-of-service telecommunications cables using a grapnel.
Seabed route preparation to ensure the good burial of the cables in mobile seabed. Cable route preparation for the crossing with other cables and set up of protective mast.
2.2.1.1. Cable route clearance
Prior to the start of marine operation, it is essential to ensure the cable route is clear of obstructions that may hinder the operation.
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Seabed debris such as scrap trawler warps or ships’ crane wires that may have been jettisoned by vessels onto the seabed, abandoned communications cables and other debris can be detrimental to the burial machine. Before the start of cable laying operations the cable route shall be cleared of any obstruction identified during the survey. To clear the route of detected and any undetected debris, a small vessel will be mobilised to remove them during an operation known as a ‘pre-lay grapnel run’ (PLGR). Cable installation may be carried out in a number of campaigns, the length of which is related to the cable carrying capacity of the main lay vessel. The PLGR operation may be phased to ensure that the route is clear of any recently dumped debris before each campaign. Out of service communications cables on the Nemo Link cable route will be pulled up with the grapnel and cut at a length of 100 m around the cable route.
Figure 2.2.1: grapnel
The PLGR vessel tows a wire with a specially designed grapnel, along the centreline of the cable route until it encounters debris. The tow winch is fitted with a strain gauge which will detect the rise in tension as an object is hooked. Most old cables and scrap wires are normally found on, or just below, the seabed. The PLGR grapnel will be designed to penetrate the seabed to a depth of approximately 1 m. Any debris encountered will be recovered to the deck of the vessel for appropriate licensed disposal ashore. Should any unexploded ordnance be discovered during this process, a registered Explosives and Ordnance Disposal (EOD) specialist will be available during the installation process to identify any suspicious items and provide advice on the appropriate remediation. The vessel will cut out the abandoned communications cables and recover a section of the cable to open a gap of 100 m through which the burial machine can pass. The two cut ends of the cable at either side of the gap will be fitted with weights to secure them against movement before they are returned to the seabed.
2.2.1.2. Route preparation (Pre-sweeping)
Results of the geotechnical survey show that the seabed is mobile (sandbanks) in the french section of the cable route. In such conditions, it is not necessary to set up a trench prior to the cable laying operation (burial during the cable laying operation).
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The sandbanks are shaped in sand waves which make the burial of the cable difficult, especially with regards to maintaining the depth of cable burial during construction. Pre-sweeping by dredging is used to reduce the height of the sandwaves along the cables route and produce a flatter path for the installation equipment to move along. This also allows for greater control of the burial depth of the cables. It also makes the protection afforded by burial more resistant to sandwave mobility and therefore more durable over time. This pre-sweeping operation will be undertaken just a few days in advance of cable laying operations to ensure the dredged path remains open for the installation to take place. The pre-sweeping is carried out by trailing suction hopper dredgers (TSHD) that move the sand aside, shave off the crest lines of sandwaves and create a flatter path for a burial machine to move along. This method has been used for the cable laying operation of BritNed power cable between UK and the Netherlands. The extracted spoil volumes resulting from pre-sweeping are typical of dredging operations for channel or port maintenance and therefore are relatively small. It is assumed that the spoil will be re-deposited onto the seabed in the immediate vicinity of the pre-sweeping activity. Consequently there is no spoil extraction from the marine environment.
Figure 2.2.2: trailing suction hopper dredgers (TSHD) Path width will vary depending on the shape and size of each particular seabed encounter; the maximum width planned for the trench is 20 m to a depth of 2 m, and with seabed slopes of 10 m width at each side of the trench.
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To reduce transport costs of the spoil volumes (compared to a standard storage site) and promote the rehabilitation of the natural site (without permanently removing the sand), it is preferable that the dredged sand is deposited 1.5 km upstream from the current, and the operation takes place only a few days before the cable laying operation. The sections planned to be levelled by dredging (pre-sweeping) were identified following the bathymetric survey and a particular exercise has been carried out to calculate the corresponding volumes to be dredged. This concerns only the sandbank crests and sand waves along the cable route. On the French section of the cable route, several areas of sand waves have been identified (Figure 2.2.3). Slopes have been measured and no deflection of the cable route is necessary. However four areas suitable for pre-sweeping operations have been identified; they are located at the crossing of the North of South Sandettié bench and South of Fairy bench.
Table 2.2.1: Identified areas for pre-sweeping operation
KP Latitude N Longitude E X Y Volume
(m3) Objective
63-65.4 51.31693 2.28503 450 174 5 685 316 21 330 Protect the cable against the mobility of the sand
waves.
69.2- 73.6 51.32437 2.18433 443 165 5 686 213 60 249 74.5 – 77.5 51.32572 2.11854 438 583 5 686 417 32 614 82.3 - 87 51.32529 1.99369 429 883 5 686 480 24 470
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2.2.2. Cable laying operation The Cable Laying Vessel carries out the transportation and installation of the cable, uncoiling the cable along the route. Depending on the carrying capacity of the CLV and the total length of the project (100 km offshore cable), several successive campaigns will be necessary. It is expected that the cable route requires five connections (cable joints); these locations have not yet been determined. There are three options for the cable installation depending on the type of cable chosen (two cables together in the same duct or two separate cables), requiring the creation of one or two trenches (see Figure 2.2.4).
2.2.3. Burial of the cable The cables will be buried into the seabed either by a plough or trenching machine deployed by the main cable laying vessel directly or by a support vessel following behind. Ploughs are towed machines generally used for simultaneous lay and burial operations where the cable vessel controls cable laying speed to match plough performance and residual tension targets. In sandy materials, the plough opens the trench where the cable is layed, and then the trench will be closed naturally by gravity or leveled by the plough, almost simultaneously. Another burial technique can be used; the water injection plough (jetting machine), which injects water under high pressure to destabilise the sediment layer and enable natural burial of the cable by gravity. The layer of sediment redeposits on the top and then recompacts naturally. This method is efficient in sandbanks because it requires less tension on the cable plough than a classic plough for the same burial depth. However, the jetting machine generates more turbidity than a burial trench and the width of the area depends on the burial depth planned. The following photos illustrate the two types of ploughs, which could be used in the French section:
a Figure 2.2.5: Example of burial ploughs: grapnel plough and jetting plough
Figure 2.2.4: Laying of the cable (from ARCADIS, 2011)
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A – Laying of a double cable in a same trench (<1 m width)
B- Laying of two cables in two close trenches (maximum distance of 2,5 m)
C- Laying of two cables inside two separated trenches with a maximum space of 50 m
Width of the
trench
Burial depth 1 to 3 m
Burial depth 1 to 3 m
Burial depth 1 to 3 m
Width of the trench <2,5 m
Space between the two trenches > 50 m
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2.2.4. Cable joints As a cable laying vessel or barge cannot carry the entire quantity of cable required for the marine cable route, it will be necessary to install the cable in several sections. Joints will be required to join each section of cable. Cable joints will be made on board the cable lay vessel or barge and will take up to a week to complete. During this time the vessel is likely to anchor to maintain position. Once the cable joint has been made on board the vessel, the cable lay will continue as normal. It is expected that up to five offshore joints will be required for the whole offshore cable of the Nemo Link.
2.2.5. Cable crossings Crossing of cables require crossing agreements to be in place between the developer and the cable owners. These agreements detail the physical design of the crossing, the protective measures utilised (according to the requirements of the International Cable Protection Committee (ICPC)), and also outline the rights and responsibilities of both parties to ensure the ongoing integrity of the assets. Indicative diagrams of typical methods of cable crossing are shown in Figure 2.2.6. The footprint of cable crossings will be circa 100 m (rock or concrete mattress along the Nemo Interconnector cable) by 30 m (width of bridge over existing cable). N.B.: if the set up of rocks is required to protect the buried cable, a specific request will be made to the administration including an environmental impact assessment. Along the French section of the cable route, the Nemo Link cable will cross no cable or pipeline in service, so no protective crossing is needed.
2.2.6. Post-burial of the cable In the section where the cable is not buried during the cable laying operation (cable crossing or cable joint for instance), a post burial of the cable should be necessary after the cable laying operation or few days later. No cable crossing is planned in French waters and the position of the offshore joints is not known at this time.
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Figure 2.2.6: Cable crossing over buried/semi buried infrastructure-
(PMSS, 2011 coming from METOC, 2004)
Top section
Cross Section
Pipeline or cable
Limit of jet burial (graded out)
NEMO cable
Pre-cable lay rock Placement (bridge)
Post-cable lay rock placement for the cable
stabilisation and protection
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2.3. Cable operation
2.3.1. Maintenance Following installation, routine maintenance work to the cables is not anticipated. However, some work may be required to maintain the burial of the cable to protect it from adverse interactions with other sea users and marine processes which might damage it. The cable and its installation will be designed to minimise any maintenance requirements.
2.3.1.1. In-service survey operation
Routine survey of a correctly installed and protected marine cable is not normally required as the subsea cables will be designed to require minimum maintenance. However, in areas of high seabed mobility, or if post-installation changes in the natural or manmade environment are perceived to have occurred, a survey of specific areas of the Nemo Link cables may be initiated. The same applies to the cable crossings, where a regular survey may also be a requirement to identify the cable exposures or spanning. The survey will be carried out from a survey vessel using side-scan sonar (SSS) and ROV deployed instruments, such as cable trackers and video cameras, to ensure that the cable is properly buried and protected, especially at the crossing areas.
2.3.1.2. Cable repairs
Even if the cable is buried for the entire cable route, damage caused by third parties such as trawlers or commercial ships' anchors could appear and require a specific repair. Typically the phases of a repair operation are as follows:
Loading of spare cable to the repair vessel (coming from the spare part depot in a nearby harbour, or directly from the manufacturer). The length of the cable is required to be at least three times the depth of the water where the damage has been located (dependent on the length of the damage along the cable whereby longer damaged sections will require more cable upon repair);
Location of the damage; Cable retrieval (using a grapnel); Cable recovery to the surface; Repair of the cables at the junction with the new cable; and Re-deployment onto the seabed and re-burial. As an additional cable length has
been laid, the repaired cable cannot be returned exactly to its previous position and alignment on the seabed. The excess of cable then forms a loop on the seabed which will be reburied by jetting.
A cable repair operation will be expected to have a duration of several weeks or months, depending on the type and extent of damage and operational constraints. The details of repair (date, cause of failure, added cable length, position loop repair, vessel used etc.) will be recorded in the maintenance log book.
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2.3.1.3. Emission from operating cable
There are a number of emissions which may occur to varying degrees during installation or operation of the interconnector. Such emissions include:
electric and magnetic fields; heat; noise; stray currents; and rise of earth potential (ROEP).
Electromagnetic Field (EMF) The geo-magnetic field forms the background that man-made magnetic fields interact with and against which they can be assessed. A desk study undertaken specifically for the BritNed interconnector estimated the geo-magnetic field in the vicinity of the marine section of the cables to be approximately 48 µT (Swedpower, 2003a). This is assumed to be similar to that for the Nemo Link. This is supported by data from the world magnetic model. The geo-magnetic field is thought to play a part in the navigation of marine fauna and is also used by man to navigate, by the use of the magnetic compass. The magnetic field has both magnitude and direction, which vary from place to place. Ships’ compasses are adjusted to compensate for the difference between magnetic and true north, the declination. Electric fields are induced in the sea water as it passes through the geo-magnetic field. The strength of these fields is dependent on the geo-magnetic field strength and also sea water chemistry, viscosity and its flow velocity and direction relative to the lines of magnetic flux. Naturally occurring induced electric fields have been estimated for the North Sea and have been measured at 35 µV/cm (Pals et al., 1982). In 1974, Kalmijn estimated electric fields in the English Channel to reach 25µV/cm twice a day. However, the strength of the electric field in the sea varies continuously because of the varying speeds and directions of the water flow that are consequences of the tides and weather conditions, but it is essentially a static field. Magnetic fields associated with the marine cables The cable will produce a static magnetic field with a low-level time-varying magnetic field superimposed on to it. The predicted line current on the Nemo Link produces a static magnetic field of a magnitude that is substantially the same as the geo-magnetic field. In theory, a cable type called the Integrated Return Conductor (IRC) will have a greatly diminished (or even no) magnetic field, as both conductors are centred on the same axis of the cable. This type of cable may be considered ideal in respect of its magnetic signature and has been used for a twin monopole HVDC link between Scotland and Northern Ireland. However, the IRC cable type is not currently available for transmitting the level of power proposed for the Nemo Link, which will use instead a bipole system with two cables of opposite electrical polarity. The Nemo Link cables will be installed in a bundled configuration, with nominal separation of 0.2 m. The resultant magnetic fields will be very low due to mutual cancellation of the positive and negative poles, and the time-varying component has been calculated to be insignificant. Electric fields associated with marine cables The HVDC voltage on the conductors of the Nemo Link cables produces a static electric field. The marine environment is shielded from this electric field by the lead sheath and other external metallic
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components of the cables, which are bonded to earth. For this reason, electric fields directly caused by the cable conductor voltages external to the cable will be insignificant. Very small electric fields are also produced in the water by the time-varying magnetic field resulting from the electrical ‘noise’ on the cables. The effects of these low-level fields are not considered to be significant. The electric field produced in the cable’s magnetic field will also be influenced by the natural electric field induced in the water by virtue of the earth’s magnetic field at the same location. It is not possible to accurately describe the electric field strength at a given location without a full understanding of the local environmental conditions. A study to estimate electric fields produced by the BritNed (SwedPower, December 2003) cable system (a similar system to the Nemo Link) was based on a maximal water flow parallel to a surface laid cable, which produces the greatest induction, and ignores any interactions with natural fields. The magnetic field strength at the seabed will be reduced for the buried cables. Expected electric and magnetic fields from the Nemo Link following analysis have been confirmed as similar to those from BritNed, which were modelled for several different cable configurations (SwedPower, December 2003).
Cable Configuration
Electric field strength (µV/m)
Magnetic field strength (µT)
1 m from cable
5 m from cable
1 m from cable
5 m from cable
Bundled (0.2 m separation) 61 1.9 72 2.2 Separated (2 m separation) 260 18 310 21
Table 2.3.1: Electric and magnetic fields from BritNed. Source: Swepower, December 2003
Note: The values above are calculated to be the maximum possible electric and magnetic fields to be produced by BritNed. Calculations are based on a seawater current speed of 0.85 m/s, and a cable electrical current of 1,320 A. These results show that the electric and magnetic fields are significantly reduced by grouping cables together (bundled cable): the magnetic field strength decreases to 0.5 μT to 8 m of the cables, which represents only 1% of the intensity of the geo-magnetic field. This will be the "base case scenario” of the NEMO project.
2.3.1.4. Heat
In transporting DC electrical energy, losses occur as a consequence of the internal resistance in the conductor. This resistance is proportional to the length of the cables and inversely proportional to the cross-sectional area of the conductor (i.e. in this case the copper cable core). The energy that is lost is converted primarily into heat. Based on thermal resistivity data collected during the geotechnical survey of the cable route, a calculation has been carried out to assess the impact of cable operation on the seabed temperature, with the following assumptions: two bundled cables with a conductor cross-sectional area of 1440mm², buried at 2.5m and with a seabed temperature of 17°C in the summer and 5°C in the winter.
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The calculations indicate the following:
At a depth of 30 cm below the seabed, localised heating directly above the cables amounts to a rise of 1.2°C,
At a depth of 10 cm below the seabed, localised heating directly above the cables amounts to a rise of 0.7°C,
The 2.5 m burial values detailed above could equate to a local rise in the seabed temperature in the uppermost 30 cm of sediment to a maximum of 18.2°C in the summer and 6.2°C in the winter. This heating effect would be much localised on the immediate area surrounding the buried cable and will not affect the water temperature, especially because the seawater would be at background temperatures very close to the seabed surface.
2.3.1.5. Noise
The noise during the offshore installation is emitted by the vessels and the operation of the installation machines. The offshore spread will typically be moving at a rate of 300m/hour on a 24 hour basis; therefore the noise caused by the installation at any one location will generally be transient and temporary. However, at joint locations the installation spread could be stationary for between 1 and 2 weeks. A study into the noise emitted during subsea power cable installation was commissioned by Cowrie in 2003. During the installation of the cables at North Hoyle Offshore Wind Farm, measurements were made of the noise levels created by trenching of cables into the seabed. Levels were recorded at a range of 160 m from trenching using a hydrophone at 2 m depth. This was necessary because, at the time the measurements were being made, the work was being undertaken in very shallow water. The sound pressure level of this recording was 123 dB re 1 mPa. The trenching noise was found to be a mixture of broadband noise, tonal machinery noise and transients which were probably associated with rock breakage. It was noted at the time of the survey that the noise was highly variable, and apparently dependent on the physical properties of the particular area of seabed that was being cut at the time. Analysis of the data indicates that if a Transmission Loss of 22 log (R) is assumed, a Source Level of 178 dB re 1 mPa @ 1 m results. Noise modelling undertaken using this source then indicates that, for distances up to 5 km from the source, all of the measurements are below 70 dBht (with one isolated exception), and hence below the level at which a behavioural reaction would be expected. It is therefore expected that the impacts from cable laying noise will not be significant. Rise of earth potential (ROEP) A rise of earth potential (ROEP) occurs when an electric current flows in the ground. The electric potential is higher at the point where the current enters the ground, and decreases with distance from that point. ROEP is usually caused by fault currents that may occur at the power converter stations, power plants, or power lines: a fault current flowing through the structure of the station to the point of ground. As the resistance of the soil is finished, the current injected at the point of grounding produces a rising of potential relative to a reference point. Faults on cables may occur when the insulation around the HV conductor fails due to internal breakdown or damage from external sources, and most of the current flow generated returns to the converter of the power station via the cable lead sheath and steel armouring (which are bonded to the earth mat at the converter station). The remaining flow circulates in the water or soil at the fault on the cable, where the seabed plays the role of earth. Calculations show that the temporary rise in the converter station earth mat potential may reach 800 V (SwedPower, 2003b). However, for a member of the public to experience a shock it would be
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necessary for him to physically “bridge” the distance from a location external to the converter station and touch the earth mat (or connected apparatus). This is highly improbable, and therefore the public would not be at risk from faults occurring on the buried sea or land cables that cause a transient ROEP. If the ROEP area extends over adjacent infrastructure, such as pipelines, voltages up to 230 V may be transferred to the infrastructure. However, touch and step potentials will not prove harmful if the minimum separation between cable and pipeline is maintained at greater than 1.7 m (KEMA, 2003).
2.3.2. Retrieval of buried cable At the end of the cable’s life the decision may be to leave the cable in-situ or to remove the cable, and the decommissioning operation will be conducted according to the standard industry protocol. National Grid Nemo Link Ltd recognises the importance of considering the decommissioning process at an early stage and, should decommissioning be undertaken, the operation will be conducted according to the standard industry protocol at the agreed time. At the end of the cable’s life the options for decommissioning will be evaluated. In some situations, the least environmentally damaging option may be to leave the cable in-situ. This option raises the issue of liability for any claims from fishermen or other third parties that come in contact with the cables. This issue will be addressed in the planning stage of cable decommissioning. The objectives of National Grid Nemo Link Ltd during the decommissioning process will be to minimise both the short and long term effects on the environment whilst making the sea safe for others to navigate. Based on current regulations and available technology, National Grid Nemo Link Ltd proposes to remove the cable system where necessary or leave safely in-situ, and to leave the protection in situ (mattresses or rocks). The cable recovery process would essentially be the reverse of a cable laying operation. The cable is retrieved for its entire length (directly with a grapnel or expose first with the jetting device and subsequently picked up by the grapnel) and taken back to the vessel (stored in tanks on the vessel or guillotined into sections). When back in port, the cable recovery vessel would unload the cable onto the quayside for later reuse or recycling. The cable route would be surveyed at the end of the operation to ensure that all cable had been removed.
2.4. Marine equipment for the operations
2.4.1. Laying operation On the offshore part of the project (approximately 100 km of deep water >10 m depth), the vessel will be a Cable Lay Vessel (CLV) which could work on minimum 10 m of water depth. Typically, a large CLV will be up to 150 m in length and will move slowly (1 to 2 knots). The precautionary zone around this vessel is 500 m radius, and this will be larger (potentially up to 2 km) if the CLV has anchors.
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The transport capacity of the CLV limits the length of cable to be laid in a single campaign. Several campaigns are needed to install the entire system but, for the French section, the cable laying operation in French waters will take place on the same day if there are no cable joints on the French section. The best time period for cable laying operations in European waters is during the summer season, broadly between April and October. This period is determined primarily by the high probability of adverse weather occurring outside of this period. In areas with a high level of traffic, other vessels will be notified in advance of the cable laying operations by Notice to Navigation (AVURNAV), also known as “Notice to Mariners”, and VHF radio transmissions. In the area, accompanying guard vessels (known as guard dogs) will be deployed to protect the site and liaise with other vessels sailing or operating nearby (Channel traffic, DST Pas de Calais and fishermen English / French / Belgian).
2.4.1.1. Burial
The classic plough or jetting plough is towed by the CLV (generally in the precautionary zone of the vessel), or by a smaller vessel following the CLV.
Figure 2.4.1: Laying and burial of the cable carried out by two vessels in convoy (d’après ARCADIS, 2011)
The radius of the precautionnary zone around the vessels is 500 m in each lateral side of the vessel but is increased to 2 km in longitudinal (2 vessels in convoy and plough). The accompanying vessels operate within the security perimeter of the CLVs.
Accompanying vessel
Accompanying vessel
CLV Vessel towing the plough
CLV Plough
Plough Precautionnary zone of 500m
Buried cable
Floating cable
Floating cable
Vessel towing the plough
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2.4.1.2. Cable joint
Cable joints of the two ends of cables will be made on board the CLV and will take up to a week to complete. In this time the vessel is likely to anchor to maintain position. If one of the five joints occur within the 23 km French section of the cable route, the cable laying operation will be extended in French waters, and the ship will be stationary (area security 2 km).
2.4.1.3. Post-burial
In the areas where the cable cannot be buried during the cable laying operation (such as at a cable crossing or cable joint), a post-burial operation will be undertaken by a post-burial vessel which could follow the CLV or undertake this operation in the few days afer the laying operation. Cable laying operations and the post-burial operation are two separate operations. Post-burial is not considered to be necessary within the French section of the cable route, unless an extra cable loop coming from a cable joint falls within this section which then needs to be dealt with during post-burial.
2.4.2. Maintenance Monitoring of the buried cable A survey to ensure the proper burial of the cable could be carried out by small specialised vessels, capable of working in such depth of water. It is not necessary to use heavy equipment to undertake these geotechnical and geophysical surveys. Intervention on the cable for repair A repair operation requires generally one vessel, like a CLV. This vessel has to work in a static position to carry out the repair and use anchors. The post-burial of the repaired cable, including the extra cable and the joints, is undertaken with a jetting device from the CLV or an additional vessel specifically for this operation.
2.4.3. End of Life Retrieval of the Cable The decommissionning operation requires the same type of vessel as the cable laying operation. Indeed a CLV is used for all the operations involved in the retrieval and storage of the cable, and an accompanying vessel follows the execution of the works. The decommissionning schedule is the same as for the laying operation, according to the capacity of the vessel to store the cable. The grapnel operation could take place directly from the CLV. The survey at the end of the decommissioning phase could be carried out by the same kind of vessel used for the survey/monitoring of the proper burial of the cable.
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3. Provisions to ensure the safety of shipping and to prevent navigational accidents
Mitigations have been defined to reduce the impact of the cable installation on marine traffic.
3.1. Signs and securing the working mobile area
3.1.1. Temporary precautionary mobile zone Due to the progression and the size of the vessels working in a high marine traffic area, a precautionary zone forbidden to other vessels not associated with installation will be set up around the working area.
3.1.2. Accompanying Vessel The accompanying vessels inside the precautionnary zone (known as guard dogs, and with bilingual workers on board) are deployed to liaise with and maintain the communication between the vessels inside and outside of the precautionary zone. Two accompanying vessels will be mandated for the cable laying operation; one for the protection of the CLV, and the other for the communication (and management of conflict if necessary) with the other vessels. These specific vessels will be chosen according to their speed, availability and on-board equipment. Thus, the protection of the site may be provided by local fishermen who have a good knowledge of the area and of the other users around the working area.
3.1.3. Signs The cable installation vessels shall comply with the regulation and show the lights and shapes prescribed, as required by the International Regulations for Preventing Collisions at Sea (COLREGS or COLREG).
plough Vessel towing the plough
accompanying vessel
Floating cable Buried cable
Precautionnary zone of 500m
accompanying vessel
CLV
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3.2. Cooperation with monitoring traffic services in the Channel The traffic monitoring services (CNIS - Channel Navigation Information Service, on the English side, and CROSS - Regional Operational Centre for Surveillance and Rescue on the French side) ensure the safety of navigation in the Straight of Calais. For that purpose, they ensure and monitor the compliance of vessels with the procedures of the International Maritime Organization (IMO), for vessels navigating in a Traffic Separation Scheme (TSS), and they provide the latest information on navigational conditions and irregularities that may affect the safety of navigation. The obligatory procedure of identification of ships in the Straight of Calais (CALDOVREP), and radar coverage of the area, reduce navigational risk significantly during cable installation. It is necessary to notify in advance the operation plans to the traffic services and thereafter to provide daily reports (Dover Coastguard Station in the United Kingdom and CROSS Gris Nez in France). A communications plan indicating the position of vessels and daily activities will be established with the Dover CNIS and the Gris-Nez to ensure good communication. In the unlikely event of non-authorised vessels transiting the Straight without contacting the CNIS or CROSS despite the CALDOVREP regulations, the guard ships’ shipyard procedure will include a specific process for dealing with these vessels.
3.3. Broadcast Notice to Mariners Information necessary for safe navigation will be announced in accordance with the guidelines of the monitoring maritime traffic services, which broadcast in VHF marine signal a Notice to Mariners with a description of the operation, the identification of vessels used for the operation and specifying the area of operation. This review is regularly updated and an urgent Notice to Mariners (AVURNAV) will be issued in the event of an incident or specific operation (cable connection requires a stationary vessel during this operation). The Kingfisher Information Service, which provides precise information on the position of vessels in offshore waters and of underwater hazards will also be advised to publish details of the installation works and the final cable route in their newsletters. In addition, the new infrastructure (cable) will be indicated upon marine maps and the sailing directions issued by the SHOM and its British counterpart (UKHO). Direct information will be provided to local organisations and fishing associations via a Fisheries Liaison Officer (FLO) on the British side; and via the Regional Committees for fishing, including all of the fleets concerned (Basse-Normandie, Haute-Normandie and Nord-Pas de Calais) on the French side.
3.4. Emergency Coordination Plan (ERCoP) HSE Representatives (Health, Safety and Environment) of the offshore construction company will provide a plan for coordination during an emergency (Emergency Response Cooperation Plan - ERCoP) covering all phases of the project. This plan must be approved by the MCA and CROSS, and shall describe in detail the cooperation with the monitoring of maritime traffic with the emergency services, the nearest ports/harbours and pilots.
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3.5. Cable protection To minimise the risk of collision and/or damage to the cable, especially in the event of an emergency anchorage near the cable, the cable (including its metal shield) is buried to a defined depth based upon the potential penetration of anchors and fishing gear, as well as legal requirements within the Straight of Calais (a strict and absolute 0.5 m along the entire length of the cable route).
3.6. Risk of cable exposure Any incident causing the outcrop of the cable, whereby it becomes susceptible to the anchors or other gear of vessels frequenting the site (such as fishing vessels) will be identified on the surface of the water through the installation of a light buoy and/or the presence of a guard ship (depending on the traffic density in the area of the incident) until the problem is resolved (the cable is re-buried).
3.7. Geomagnetic interference The risks related to the EMF emitted by the power cable upon vessels’ navigational instruments is of concern in relation to vessels navigating the Straight of DST of Pas de Calais, especially in the event of low visibility conditions. The study undertaken for the power cable BritNed (2004), which is of comparable configuration to the Nemo Link, showed that the maximum deviation of the compass caused by the cable was less than 5 degrees (SwedPower , October 2003), which remains at a level acceptable to the authorities managing traffic in the DST. The cable burial depth for the Nemo Link, which is similar to the BrtiNed project, makes the findings of this study relevant here.
3.8. Discovery of munitions or explosives National Grid Nemo Link Ltd undertakes to notify the competent authorites (Préfecture Maritime and DDTM) within the prescribed time (48 hours) of any discovery of a suspicious device, and commits to comply with any instructions given by the named organisations. It also undertakes to provide, at the end of the survey, a list of wrecks and obstructions found in the cable route.
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4.