Scottish Hydro Electric Power Distribution Project ...news.ssen.co.uk/media/224962/Project_Description.pdf · Project Description Information correct as at 01 September 2017 Page

Scottish Hydro Electric Power Distribution

Project Description

Rousay - Westray

Section ID 149

SHEPD

Project Description

Information correct as at 01 September 2017 Page 2 of 21

Contents

Definitions and Abbreviations ................................................................................................................. 3

1. Introduction ................................................................................................................................... 4

2. Background .................................................................................................................................... 5

3. Proposed cable construction ........................................................................................................ 6

4. Pre-installation survey requirements ........................................................................................... 7

5. Project description ...................................................................................................................... 10

5.1. The existing route ................................................................................................................................................... 10

5.2. The proposed route ................................................................................................................................................ 10

5.3. The proposed installation methods ........................................................................................................................ 13

5.4. Proposed cable protection methods ....................................................................................................................... 17

5.5. The proposed delivery programme......................................................................................................................... 18

Appendix A: Proposed cable route ........................................................................................................ 19

Appendix B: Route Position List (RPL) ................................................................................................... 20

Appendix C: Proposed cable installation programme .......................................................................... 21

Project Description


Definitions and Abbreviations

The following definitions are used within this document:

SSEN Scottish and Southern Electricity Networks SHEPD Scottish Hydro Electric Power Distribution plc Cable SHEPD submarine electricity cable network Contractor Submarine cable installation company

The following abbreviations and definitions may be used within this document:

AtoN Aid to Navigation DWA Double Wire Armoured HDD Horizontal Directional Drilling HVAC High-Voltage Alternating Current kV kilovolt MAIB Marine Accident Investigation Branch MHWS Mean High Water Springs ML Marine Licence MLWS Mean Low Water Springs MSL Mean Sea Level ODN Ordnance Datum Newlyn PAC Pre-Application Consultation PPY Poly Propylene Yarn PSD Particle Size Distribution ROV Remotely Operated Vehicle RPL Route Position List SNH Scottish Natural Heritage XLPE Cross Linked Polyethylene

Project Description


1. Introduction

1.1. Scottish Hydro Electric Power Distribution plc (SHEPD) proposes to install a replacement 33kV submarine electricity cable from Scockness in Rousay to East Sous in Westray within the Orkney Isles.

1.2. This document provides a description of the project, detailing the physical construction of the cable, the routing, method of installation and operation of the cable.

1.3. This document should be read in conjunction with the:

Marine Licence Application Form Pre-application consultation report (appended by Cost Benefit Analysis model) Environmental Supporting Information Fisheries Liaison and Mitigation Action Plan (covering all legitimate sea users) Construction Environment Management Plan Operation, Inspection, Maintenance and Decommissioning Strategy

Project Description


2. Background

2.1. The existing 33kV submarine electricity cable connecting Rousay and Westray is shown in Figure 1.

2.2. The existing cable was installed in 1983 and is nearing the end of its operational life. This has been verified through our existing asset records and following recent visual inspections by ROV on the cable. The replacement of this cable is essential to securing SHEPD power supplies to the island of Westray and supports the security of supply to Rousay, Eday, Sanday, Shapinsay and Stronsay.

Figure 1 - Existing cable route

Project Description


3. Proposed cable construction

3.1. Electricity will be transmitted using HVAC submarine cable technology. The typical cable structure is shown in Figure 2.

3.2. The proposed submarine cable consists of a three core design with copper round compacted stranded conductors, XLPE insulation, copper polyethylene laminated tape, polyethylene sheath, PPY, double galvanized steel wire armour, PPY, with one interstitial armoured optical fibre cable. The cable is rated at 33 kV HVAC, with an outer diameter of 127 mm and weight of 28.8 kg/m in water. The proposed cable construction is shown in Figure 3.

3.3. The three core design minimises the resultant electric and magnetic fields produced from the cable during operation. This is further reduced by balancing of the loads within each of the cable’s individual phases.

3.4. The proposed double wired armour (DWA) construction will provide the cable with additional mechanical protection and will also help reduce the resultant electric and magnetic fields generated during operation of the cable in comparison with single wired armour (SWA) cable constructions.

3.5. Fibre optics will be installed integral to the submarine cable for the purpose of cable condition monitoring, control and power system protection.

3.6. The submarine cable conductor specification and power rating has been selected through assessment of historic demand on the existing SHEPD network and with consideration of future customer demand growth on the network.

1 Copper stranded conductors 2 Semiconductive waterblocking tape 3 Non-metallic screen 4 XLPE insulation 5 Non-metallic screen

Figure 3 – Proposed XLPE HVAC submarine cable construction

Figure 2 - Typical XLPE HVAC submarine cable structure

Source: ABB

Project Description


6 Semiconductive waterblocking tape 7 Metallic screen 8 HDPE sheath 9 Non-hydroscopic fillers 10 Binding tape 11 PPY separator layer 12 Inner layer of galvanized steel armouring 13 PPY separator layer 14 Outer layer of galvanized steel armouring 15 PPY outer layers 16 Fibre optic

4. Pre-installation survey requirements

4.1. Sonar mapping surveys of the marine environment have been undertaken in order to confirm the engineering viability and impact of the proposed cable route. The routing has been informed following a review of seabed conditions, bathymetry, stakeholder views, seabed use and any other identified seabed features.

4.2. Initially, geophysical and limited geotechnical pre-installation surveys were undertaken over a 1000 m wide corridor centred along the existing submarine cable route. This survey corridor was selected following a review of potential cable landing points on Rousay and Westray, an assessment of current and proposed sea user actitivites in the area and from previous inspections of the existing cable. A corridor width of 1000 m was selected to provide some flexibility for routing of the cable with consideration of survey cost, environmental species impact from the survey equipment outputs and also disruption to sea users from the survey works. Centring the survey corridor on the existing cable provided the ability to route the proposed cable east or west to allow some micro-routing following assessment of the survey data.

4.3. SHEPD appointed a contractor to conduct the marine surveys. These surveys were undertaken along the route corridor between September 2015 and October 2015. The main objectives of the marine surveys were to identify:

seabed conditions (e.g. sand, rock, mud) to optimise the proposed submarine cable route (avoidance of rock outcrops)

potential geological constraints, such as dykes, rock pinnacles, sand waves, incised channels etc.

locations of potential engineering constraints and/or safety hazards, such as existing pipelines and cables either in service or out of service, wrecks, marine debris, unexploded ordnance etc.

areas of potential biological and ecological importance (such as biogenic and rocky reefs, priority marine features etc) to allow habitat mapping and inform the requirement for additional surveys and assessment.

4.4. The bathymetric data along the surveyed route (Figure 4) has highlighted a significance of rock outcrops at the approach to Westray at the southern tip of the island as the route depth reduces to depths below 20 m. This data has shown the extent of the rock present across the survey corridor to the south-east of Westray and inwards to the shore landfall. This has influenced the proposed cable route design by positioning the cable to the west of the existing cable location whilst remaining within the extent of the marine surveyed corridor. The results from the marine survey and from site

Project Description


visits suggests that there will be further rocky outcrops around the southern tip of Westray, west of the survey corridor which should be avoided for the proposed cable installation.

4.5. Also evident from the marine survey is that there is less rock on the approach to the existing shore end landfall at Rousay in comparison with Westray. Rocky areas are however present at the intertidal areas around the shore at Rousay which was further verified by site visits. This has helped inform the decision to re-use the existing landfall location.

4.6. Across the middle section of the route within the Westray Firth, the water depth is fairly constant with only gradual changes in depth as indicated from the bathymetric data reaching a maximum depth of around 45m. With the fast flowing tidal conditions present, there is minimal sediment overlying the seabed along a significant portion of the route with rock prevalent.

Figure 4 - Bathymetric data between Rousay and Westray

Project Description


4.7. Sub-bottom profiling has been utilised to map the seabed surface to allow identification and extent of the sediment types. These survey methods are useful for identifying areas of the surveyed route that may be suitable for direct cable burial within the seabed or help to inform alternative cable protection methods if deemed required.

4.8. Limited seabed grab samples were taken during the marine survey. The purpose of this sampling was to carry out a benthic characterisation assessment which was combined with PSD analysis of the material at the seabed surface. The geotechnical sampling methods were intrusive in that there is a physical interaction between the sampling device and the sediments. Further and more intrusive geotechnical surveying will be required where direct cable burial is required along the route. This is needed to inform soil conditions and suitability of burial equipment.

4.9. From the sub-bottom profiling, the extent of sediment has been captured above the underlying rock within the survey corridor. This has shown that there are a number of small sections of the route with overlying sediment and cable burial may be possible within these sections. Due to the extent of sediment however, for the majority of these sections any cable burial would be shallow and with the relatively strong tidal currents along the route, there is a risk of the cable becoming exposed over time. There are large sections of the route within the middle section of the Westray Firth without or with very little overlying sediment meaning that direct cable burial is not an option in these areas. Any cable protection needed would require additional external protection such as rock placement, rock filter bags or concrete mattressing directly over the cable.

4.10. The pre-installation surveys have allowed SHEPD to optimise the cable route within the survey corridor and helped to identify feasible installation and cable protection methods. In addition to the pre-installation surveys, further drop-down video and imagery surveys have been undertaken during June 2017 to understand better the range of potential habitats and geographic spread. The methodology in relation to the surveys was developed through discussion with SNH. These surveys have helped further inform the routing and better assess the impact of our proposed cable on the habitats present.

4.11. The additional surveys have also influenced our cable installation design to minimise the impact on the habitats and seabed features present on the route. This has allowed identification of mitigation measures in the form of cable re-routing where appropriate, crossing locations for protective habitats, cable burial in areas suitable and also rock bag placement to stabilise the proposed cable to minimise movement along the seabed.

4.12. The survey outputs from the drop-down video surveys will be shared to evidence the habitats and features present along and in the vicinity of the proposed cable. This will help assess the impact of the proposed cable, cable burial and protection methods proposed to stabilise the cable.

Project Description


5. Project description

5.1. The existing route

5.1.1. The project is to install a replacement 33kV HVAC cable between Rousay and Westray. The existing cable route is installed from the Scockness on Rousay to East Sous on Westray, south of Rapness within the existing route corridor across the Westray Firth. At each shore end landfall, the existing land-based network of overhead line and underground cable connects the submarine cable to the SHEPD network.

5.2. The proposed route

5.2.1. Following a review of pre-installation marine survey data, an optimum route for the cable utilising the marine survey corridor has been identified as shown in Figure 5. The proposed cable will be 10.3km in length between the two transition joints, which are located inshore from the MHWS limit. However, the application length is 12km to allow for obstacle avoidance during cable lay and tolerances with the cable lay operations.

5.2.2. The proposed cable route lies within the 1000 m wide survey corridor1 with some micro-routeing selected to avoid areas of sandwaves, bedrock, boulders and avoid or minimise the impact on sensitive marine features identified from the surveys for the proposed cable.

5.2.3. The proposed cable retains the existing shore end landfall at Scockness, Rousay due to the suitable shallow approach. Alternative landfall positions were reviewed but discounted due to a number of factors. Firstly around the Scockness area, the coast is very rocky in the intertidal areas, with several cliffs around the coast to the north-east of Scockness at Faraclett Head. Further to this there is a lack of bays suitable for a submarine cable landfall.

5.2.4. There are a number of environmental designations onshore and offshore at Rousay. To the north-west of the existing cable landfall is the Rousay Site of Special Scientific Interest (SSSI). Offshore to the North is the Rousay Special Protection Area (SPA) and in the area at the existing cable landfall and to the east is the Wyre and Rousay Sounds Marine Protected Area (MPA). By reusing the existing cable landfall, we have avoided our works impacting on the SSSI area and minimised our route through the SPA. We have carried our surveys within the MPA to identify the priority marine features present in the area and assess the impact of our works and cable on these habitats. This has influenced our cable installation methodology.

5.2.5. Following discussions with both land owners and tenants, this has highlighted the importance of re-using the existing cable landfall. This will minimise disturbance to their activities and land whilst minimising impact on the landscape and reducing reinstatement. Also with the difficult ground conditions this has influenced our proposed land-based works, routeing and landfall options. This is further affected by the limited road network on Rousay for site access, plant and difficulties with operational access. To relocate the landfall on Rousay will require extensive civil works for construction of new roads and temporary site compounds which will have an impact on the local land owners, tenants and areas with potential impact on the SSSI area.

5.2.6. From Rousay, the proposed cable is positioned south of the existing cable. The proposed cable is laid adjacent to the existing cable. An offset will be used to allow safe installation, operation,

1 See section 4.2 for justification of how the corridor was selected.

Project Description


future maintenance and ongoing inspection of the cable(s). The proposed cable route then continues south of the existing cable from Rousay. Prior to the Westray Firth, the proposed cable crosses the existing cable and then continues to the north of the existing cable up to the landfall on Westray.

5.2.7. Similarly to Rousay, on Westray the proposed cable will come ashore at the existing shore end landfall at East Sous. The cable will then contain a ducted road-crossing and short section of underground cabling into the existing SHEPD substation adjacent to the unnamed road.

5.2.8. The proposed cable route and method of installation has been identified based on a combination of desk studies, marine seabed surveys and stakeholder views. Prior to the cable installation, SHEPD’s appointed contractor will undertake a final review of the marine survey to confirm the seabed conditions and finalise the exact cable within the consented corridor and techniques to be employed.

5.2.9. Further to this, a review of the existing shore end landfall locations and land-based infrastucture was undertaken and verified with site visits on both Rousay and Westray to confirm the shore end landing points and the method of cable installation above the MLWS limits. This was needed to assess the suitability for site access and the logistical constraints for plant and machinery but also better understand the environmental and landowner contraints.

5.2.10. A shore end cable installation by Horizontal Direction Drilling (HDD) on Rousay was considered but discounted for a number of reasons. The limited public road network would require significant civil improvements including the unnamed road leading to the Scockness area at the landfall area. Due to the tight turns on this unnamed road and adjacent residential properties, the construction of a temporary or permanent road would be required to permit the large machinery access to the cable landfall area.

5.2.11. The area around the cable landfall on Rousay is composed of difficult hilly terrain. The ground varies from very rocky to softer, sandier areas. For the HDD works a large, level area of approximately 50 m by 50 m is required. The site needs to be stable to support the large drilling machinery during the setup and drilling operations. With the hilly and unstable ground on Rousay, this will require extensive civil improvement works. This will create significant disruption to the local road network.

5.2.12. With the level of archaelogical presence in the area, any civil works could be impacted. This will require extensive pre-construction surveys with agreements from land owners and tenants but also increases the risk of archaeloigcal impacting on the works. The additional works will likely increase the difficulty of acquiring the consents to carry out the works. Within the Scockness and surrounding area there is also the potential for environmental impact in relation to the protected bird species and seals.

5.2.13. On Rousay, modifications to the land based infrastructure by undergrounding a number of sections of overhead line would have operational benefit and improve future access. This would also provide environmental and visual benefits to the local landowners and bird species however due to difficult ground conditions for underground cabling and the sensitive area we propose only minor modifications to the existing overhead line. This includes installation of a new terminal H-pole in line with the existing overhead line but further inshore by 105 m. To reduce the impact of the existing overhead line on bird species in the area, we proposed to install bird flight diverters onto six overhead line spans to reduce the risk of bird strikes with the conductors.

Project Description


5.2.14. As it is proposed to re-use the existing shore end landfall positions, the existing AtoN warning beacon locations will be retained. Any ongoing maintenance and inspections associated with the warning beacon will be carried out on a regular basis to ensure they are in good condition for sea user safety. Further details can be found in the Operation, inspection, maintenance and decommissioning strategy.

5.2.15. The proposed cable route and installation method, including associated land based works, have been informed following a review of the marine survey data and cable inspections, stakeholder feedback from consultations and environmental constraints2 whilst balancing SHEPD’s electricity licence obligations. Details of the proposed RPL for the cable route from MHWS limits from Rousay to Westray is located within Appendix A and Appendix B.

5.2.16. In addition to this, more focussed investigations and studies have been carried out along the proposed cable routes and working areas affected by the works by SHEPD’s archaeological and ecological consultant on both Rousay and Westray. This has helped to firstly identify archaeological/ecological interests and then to help eliminate and/or mitigate any potential

2 Environmental constraints are detailed in Construction environment management plan

Figure 5 - Proposed cable route

Project Description


impacts from the proposed works within cultural heritage sites and/or environmentally sensitive locations.

5.2.17. These investigations have identified archaeological interest (above MHWS) at Rousay including an existing recorded site on Scockness and remains at Mae Sand (south of the existing and proposed cable landfall) and within the nearby loch at the Bay of Ham.

5.2.18. The Rousay Special Area of Conservation Management Plan details restrictions on areas and access. This has been considered within the design by reducing the extent of works on Rousay and careful selection of access route through discussion with landowners and tenants. Following further investigations and consideration of the impact caused by cabling works, the proposals have been revised by reducing the extent of cabling works, duration for the works to be undertaken and reducing access requirements. This restricts the landfall to the existing location.

5.2.19. Further information on this can be found in the Construction environment management plan.

5.3. The proposed installation methods

5.3.1. For the cable laying activities, a standard small cable lay vessel will be used. An additional smaller support vessel is likely to be required at each of the shallower shore locations; this is likely to be a multicat / dive support vessel (DSV). This would require a four point anchor system to be laid out prior to commencing works in the nearshore region. An anchor handling vessel would be required to lay out the anchors. A guard vessel is also likely to be used during the cable lay operations in order to ensure other vessels remain outside the area of operations to reduce collision risk.

5.3.2. Initially, the proposed submarine cable will be surface laid on the seabed across the length of the route. Due to the strong tidal currents along the route it is anticipated that the majority of the proposed cable route will be across rocky seabed with very little overlying sediment. Due to the limited sediment direct cable burial will not be possible. Additionally, there is known to be a short section of the route close to the Rousay landfall that will pass through protected Maerl beds. By initially surface laying the cable this minimises the footprint in these areas however cable mobility on the seabed may increase the footprint and impact on the habit

5.3.3. Consequently, SHEPD plan to lay rock filter bags (each with a seabed footprint of approximately 2.0 m by 2.0 m) in spot locations either side of the cable to pin the cable to the seabed. Only clean washed stone will be used to fill the rock bags and no cementitious material will be used. Each bag shall be no more than 1 m high when installed. The rock filter bags have been proven to provide a habitat for aquatic species and mould to the seabed contours where installed.

5.3.4. Positioning of the rock bags either side of the cable has been proven to stabilise the cable. The risk of installing the rock bags directly onto the cable could cause subsequent damage from point loading dependant on the seabed and lay of the cable. This may also increase the tension on the cable and potential for cable suspensions either side of the rock bag positions which increases mariner safety from potential snagging incidents. A post lay inspection of the cable after it is installed will identify the potential risk to placement of the rock filter bag directly onto the cable.

5.3.5. We will be undertaking a cable stability assessment that will help us to inform the cable stability throughout the proposed route with a view of minimising the amount of rock bags required to pin the cable whilst ensuring cable stability following the installation. This will help inform the rock bag separation required and also the effect of the buried sections on cable stability in these areas.

Project Description


5.3.6. On the Rousay side west of Kili Holm, 12 rock filter bags (2 tonnes in weight) either side of the submarine cable will be placed at roughly 50 m intervals through the Maerl habitat. This is likely to be from 250 m from the shore out to 500 m. The rock bags will help stabilise the cable in this area, thus preventing damage to the Maerl habitat through cable movement on the seabed following installation. As noted above a cable stability assessment will be carried out to minimise the number of rock bags required to ensure the cable stability. Also to reduce the amount of rock bags installed within the Maerl habitat, we proposed to position rock bags immediately outside the Maerl extent. To minimise the impact of our works within the sensitive Maerl bed, the contractor will be provided with the survey site information to identify the extent of the Maerl bed. During any planned anchoring activities, anchoring positions and chain canterays will be controlled such that any ground chain and anchor placement is out with the Maerl habitat.

5.3.7. Within the main channel of the Westray Firth, 900 m from Rousay to 500 m from Westray, 80 rock bags will be placed adjacent to the submarine cable at approximately 185 m intervals to stabilise the cable in known rocky reef areas. This spacing will be refined by the cable stability analysis.

5.3.8. There are three sections along the route with overlying sediment amounting to approximately 2,250 m in length of the proposed cable route. Cable burial is therefore planned in these sections with the use of a post-lay jetting machine. Cable burial will be proposed where there is the potential for cable burial along the route. To achieve the planned depth of burial (0.75 m) in these sections a depth of trench of approximately 1 m will be required; the width of the trench will be approximately 0.6 m. The jetting tool will be industry standard and will operate in a crawler operation to minimise disturbance to the seabed during operations; it will be no more than 6 m wide.

5.3.9. In summary we propose to install a maximum of 92 rock filter bags along the route and our cable installation and burial methodology is summarised in Table 1.

Project Description


Location along the cable route

Installation type Length of cable (m)

No rock filter bags

From transition joint to shore Cable burial from transition joint 50 0

From Rousay landfall out to 250 m

Cable lay with up to 100 m of cast iron pipe protection

250 0

250 m from Rousay landfall up to 500 m

Cable lay with rock placement every 50 m 250 12

500 m from Rousay landfall up to 1,650 m

Cable burial (post-lay jetting) 1,150 0

1,650 m from Rousay landfall up to 2,450 m

Cable lay with rock placement every 185 m 800 12


Cable burial (post-lay jetting) 700 0


Cable lay with rock placement every 185 m 6,150 68


Cable burial (post-lay jetting) 400 0


Cable lay with up to 100 m of cast iron pipe protection

600 0

From transition joint to shore Cable burial from transition joint 50 0

Total (marine cable length) 10,300 92

Table 1 - Cable installation, burial and protection

5.3.10. Due to the suitability of the existing shore end landfall locations and following assessment of alternative areas, the proposed cable will be installed adjacent to the existing cable at both Rousay and Westray.

5.3.11. It is proposed to install the cable by using an open-cut trench method of installation inshore from the MLWS tidal limits at both shore end landfall locations. An open cut trench will be excavated to install and bury the cable. This will utilise traditional terrestrial based plant including excavators at low tide. The typical underground cable trench is illustrated below in Figure 6. To allow for micro-routing during cable installation the landfall excavation works will take place within 100 m either side of the cable landfall positions shown.

5.3.12. At Rousay, to complete the shore end installation works there will be some underground cabling from the transition joint location with the marine cable and minor modifications to the existing 33kV overhead line. Further overhead line undergrounding was originally planned however due to rocky ground conditions and sensitive protected areas on Rousay; the scope of proposed works was reduced.

5.3.13. At the Westray landfall, the marine cable will be installed by an open cut trench method above the MLWS limit. The marine cable will installed within a ducted conduit (Figure 7) through the nearby road with open cut trench continued into the adjacent substation. There will be a short section of underground cabling from the joint location to the existing terminal structure contained within the existing substation.

5.3.14. The trench will be excavated alongside the existing cable using a terrestrial-based mechanical excavator during low spring tide. The excavated material will be placed to one side of the trench for later reinstatement. Using a mechanical winch and cable rollers, the cable will be manoeuvred

Project Description


into the bottom of the trench and then covered with the excavated material using the mechanical excavator. The trench width will be minimised where possible however will be dependent on ground stability but will typically be 1 m wide. The target depth of the trench will be 1.25 m. Temporary trench shoring may be required to prevent collapse of the trench wall. The footprint of the excavator may be up to 5 m, and a working width, including for the temporary storage of removed material, would be in the order of 10 m.

Figure 6 - Open cut cable trench cross section inshore of MWS limit

Figure 7 - Open cut trench section through road on Westray

Project Description


5.3.15. On either shore where sufficient burial cannot be achieved, cast iron split pipe will be fitted around the cable for additional protection in the event of exposure. On Rousay due to the rocky shore there may be more issues achieving cable burial and split pipe protection will be required down to MLWS mark. On Westray we propose to install 100 m of split pipe protection to the cable and on Rousay 100 m from MWHS to MLWS in the event that we can’t achieve sufficient cable burial to protect the cable. The split pipe is an articulated cast iron shell design that locks around the cable and fixed with bolted end clamps. Each shell has 8 mm wall thickness, with an effective length of 391 mm and combined weight in air of 39.96 kg/m.

5.3.16. The intertidal cable will be connected to the terrestrial cable in a transition joint pit buried in the ground located above the MHWS limit at each end. At the Rousay end, from MHWS limit the cable will be buried onshore for approximately 120 m and connect into the existing overhead power line. On the Westray shore, from the MHWS limit the cable will be buried for approximately 5 m through the road and connect in to the existing electrical substation. On completion of jointing, spoil material will be backfilled into the trenches and the shore will be reinstated; grassed areas will be left to re-seed naturally. The upper surface layer will be stripped and stockpiled separately to allow a quicker reinstatement.

5.3.17. At each shore landfall location, to facilitate the onshore works and ensure compliance with The Construction (Design and Management) Regulations 2015, a temporary site area will be required, approximately 25m x 25m in size, to provide welfare facilities and parking during the works. To minimise ground disturbance, ease reinstatement and recovery, the area will be covered with a membrane material and levelled with rock chippings.

5.3.18. SHEPD’s Cost Benefit Analysis Methodology3 is used as supporting evidence that the solution proposed in this project description represents the best value4 installation method of this submarine electricity cable route. The modelling confirms the best value solution for the Rousay – Westray Cable to be:

Initially surface lay the cable along the sea bed Carry out approximately 2,250 m of post lay jetting in areas where sediment is present along

the route Install a maximum of 92 rock filter bags installed adjacent to or directly over the cable where

the cable passes through priority marine features Install a maximum of 200 m of cast iron shells Install a maximum of 8 concrete mattresses

5.3.19. A summary of the Cost Benefit Analysis model can be found in the Pre-application consultation report.

5.4. Proposed cable protection methods

5.4.1. At both shore end landfall locations it is proposed to use an open cut trench method to install the cable inshore from the MLWS tidal limits.

3 http://news.ssen.co.uk/media/147004/4731-ssepd-submarine-cables-doc-july-2016_06.pdf

4 Which we define as best balancing the health and safety, environmental, socio-economic, wider-engineering and

economic considerations whilst meeting regulatory obligation.

Project Description


5.4.2. The cable installation method within the marine environment from each MLWS location will initially be surface laid across the length of the route. The proposed installation methods are less intrusive/disturbing and allow for quicker installation duration. Due to the strong tidal currents along the route, rocky seabed and to minimise impact on protected habitats, it is planned to lay rock filter bags in spot locations either side of the cable at intervals along the route to pin the cable to the seabed.

5.4.3. In areas where there is sediment suitable for burial along the route, accounting for approximately 2,250 m of the proposed route, it is proposed to carry out post lay jetting of the cable to a target depth of cover of 0.75 m to bury the cable.

5.4.4. In the area where the proposed cable crosses the existing cable it is proposed to utilise a combination of rock filter bags and concrete mattressing to cross the cable. It is proposed to install eight concrete mattresses (10 tonne in weight) with dimensions of 3.0 m (breadth) x 6.0 m (length) x 0.4 m (depth) laid adjacent to and directly over the existing cable.

5.4.5. On either shore above the MLWS limit, where sufficient cable burial cannot be achieved, cast iron split pipe will be fitted around the cable for additional protection in the event of exposure. On both Rousay and Westray we propose to install a maximum of 100 m of split pipe protection to the cable.

5.4.6. This change to the cable design methodology follows consideration of comments and/or objections received at the PAC Events as detailed in the Pre-application consultation report.

5.5. The proposed delivery programme

5.5.1. Due to the presence of designated seal haul outs along and close to the proposed cable installation activities, SHEPD have programmed the cable installation to avoid the sensitive periods for harbour (June to August) and grey (October to December) seals.

5.5.2. For this cable, based on the proposed cable design methodology and timing constraints, it is proposed to install the submarine cable during the months of March – May 2018, or before if the necessary consents have been secured. The indicative project delivery dates can be found in Appendix C.

5.5.3. The onshore cabling works on Rousay and Westray will be completed prior to the submarine cable installation whilst the modifications required to the overhead electricity network on Rousay will be carried out during commissioning of the submarine cable to the SHEPD network.

6. Unexploded Ordnance Strategy

6.1.1. The contractor appointed by SHEPD shall undertake an Unexploded Ordnance Survey along the cable route utilising a Magnetometer or Transverse gradiometer. The results shall be used to inform cable routing within the consented corridor such that significant targets are avoided. If it is not possible to avoid a target, it shall be identified using and ROV or Drop Down camera. Where an ordnance or bomb is encountered that cannot be avoided, a specialist contractor shall be engaged to clear or remove the target from the working area.

Figure 6 – Open cut trench shore end installation

Project Description


Appendix A: Proposed cable route

Project Description


Appendix B: Route Position List (RPL)

59.174627 -2.955000

59.175348 -2.952612

59.175531 -2.951935

59.175841 -2.950794

59.175743 -2.947873

59.175631 -2.944566

59.175585 -2.940529

59.177984 -2.932236

59.180518 -2.929782

59.182302 -2.926471

59.183403 -2.924235

59.184726 -2.924060

59.185328 -2.921166

59.186155 -2.916837

59.187077 -2.910130

59.187490 -2.907085

59.188414 -2.902422

59.189413 -2.899803

59.191041 -2.892141

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Project Description


Appendix C: Proposed cable installation programme

Activity Date

Project Start 6 March 2018

Start of marine works 10 March 2018

Completion of submarine cable lay 31 March 2018

Completion of submarine cable installation inclusive of burial and protection

24 May 2018

Documents

Scottish Hydro Electric Power Distribution Project ...news.ssen.co.uk/media/224962/Project_Description.pdf · Project Description Information correct as at 01 September 2017 Page