Irish-Scottish Links on Energy Study (ISLES)

Transcription

1 European Union European Regional Development Fund Investing in your future Irish-Scottish Links on Energy Study (ISLES) April 2012

2 TABLE OF CONTENTS 1 EXECUTIVE SUMMARY INTRODUCTION OUTLINE OF OFFSHORE CONSTRUCTION PROCESS FRONT END ENGINEERING DESIGN (FEED) STUDIES GEOPHYSICAL, GEOTECHNICAL AND SITE INVESTIGATIONS OFFSHORE CONSTRUCTION & DEPLOYMENT SHORE AND SHALLOW WATER CONSTRUCTION COMMISSIONING AND TESTING CONCLUSION OF THE CONSTRUCTION AND DEPLOYMENT PROCESS OPERATION AND MAINTENANCE GENERATION OF PRELIMINARY ISLES ROUTE PLAN CONSTRUCTION SCHEDULE FOR OFFSHORE CABLE LAYING OPERATIONS EFFECT OF CONSTRUCTION METHODS ON THE COST OF OFFSHORE CABLE INSTALLATION SCOPE OF WORK ASSUMPTIONS LOCATION OF THE ISLES CONSTRUCTION PORT RESULTS OF THE COST MODEL OVERVIEW OF OFFSHORE CONSTRUCTION COSTS CONSTRUCTION SCENARIOS COMPARISON OF COSTS FOR CABLE COLLECTION USING A CHEAP TRANSPORT BARGE COMPARED TO A CABLE LAY SHIP FURTHER ANALYSIS OF THE COSTS FOR LAY AND BURIAL OF BIPOLAR CABLES IN A SINGLE TRENCH IMPACT OF A VESSEL S CABLE LAY CAPACITY CONSTRUCTION AND INSTALLATION TECHNOLOGY ISSUES CURRENT CONSTRAINTS REGARDING OFFSHORE CONSTRUCTION AND MAINTENANCE REVIEW OF TECHNOLOGY LIMITS DEPTH OF BURIAL REQUIREMENTS AND ITS EFFECT ON CONSTRUCTION SPEED PROTECTION STRATEGY FOR CABLES BASED ON HAZARD IDENTIFICATION AND SITE SPECIFIC FACTORS PROJECT RISK IDENTIFICATION ADVANCEMENTS AND INNOVATION IN CONSTRUCTION TECHNIQUES: UPGRADE THE TECHNOLOGY TO SPEED UP LOAD OUT, LAY AND TRENCHING MDR0707Rp0028 i Rev F01

3 LIST OF FIGURES Figure 1: ISLES construction, deployment and maintenance activities 3 Figure 2. Reproduced from a presentation titled Importance of metocean data at marine renewable energy sites. James Parker Gardline Environmental Ltd [1] 5 Figure 3. Survey vessel with Multibeam echo sounder [2] 7 Figure 4. Side scan sonar - SSS [2] 7 Figure 5. Typical scope of geophysical survey for an offshore platform, reproduced from Reference [3] 7 Figure 6.Typical scope of geotechnical survey for an offshore platform, reproduced from [3] 8 Figure 7. Typical scope of geotechnical survey for offshore pipeline routes, reproduced from [3] 9 Figure 8. Tubular steel jacket foundation structure - Suited to HVDC platform 11 Figure 9. Jacket delivery by transport barge 12 Figure 10. Jacket lift from transport barge 12 Figure 11. platform topside transport to offshore site with cargo barge and tug. 13 Figure 12. Rambiz floating crane vessel installing a platform topside onto a foundation. 13 Figure 13. platform topside float over with crane barge. 13 Figure 14. platform topside ready to be lowered onto jacket 13 Figure 15: Construction of Gun fleet Sands and installation of Offshore Substation 13 Figure 16: Jack up Crane Vessel during foundation Installation 13 Figure 17. Digging an open trench from headland to the beach 16 Figure 18. Robert Donaghy Cigre Cock Harbour 2010, ESB International 17 Figure 19. Image of HDD rig courtesy of Land and Marine. 17 Figure 20. Open sheet pile trench 17 Figure 21. Auger boring 17 Figure 22. Cable being placed into beach duct from lay barge 18 MDR0707Rp0028 ii Rev F01

4 Figure 23. Cable going into HDD duct - Exit of HDD duct showing cable duct seal. 18 Figure 24. Cable plough being used to bury export cable directly from the beach. 19 Figure 25. Fall pipe vessel 20 Figure 26. Side dumping vessel 20 Figure 27. Protective concrete mattresses in a mattress installation frame 20 Figure 28. IHC EB Seatrac Subsea Rock cutting machine 21 Figure 29. Simec Castor 2 21 Figure 30. LD travocean concrete casting machine 21 Figure 31. Grapnel used for Pre-Lay Grapnel Run. Image courtesy of Denholm Offshore 22 Figure 32. Subsea excavator Carrera 4 used for seabed levelling, and cable deburial image courtesy of Seatools. 22 Figure 33. North Ocean 102 cable lay vessel, used to lay Britned cable link 23 Figure 36: Schematic of the Post lay burial. 25 Figure 37: Schematic of Simultaneous lay and burial. 25 Figure 38. Cable plough 25 Figure 39. Hard soil trencher 25 Figure 40. Pipeline plough 25 Figure 41. PL3 Pipeline plough IHC Engineeering Business 27 Figure 42. Prototype SCAR plough, Ecosse Subsea 27 Figure 43: Typical cable repair vessel Sea Spider 31 Figure 44: Typical damaged cable where a flash-over occurred. 31 MDR0707Rp0028 iii Rev F01

5 LIST OF TABLES Table 1: Inputs to the Costing Structure Table 2: HVDC Link Cable Data Table 3: ISLES power cable network installation cost outlay Table 4: Cable protection scenarios Table 5: Cost outcome for different protection methods Table 6: Naples cable load out costs Table 7: Hartlepool cable load out costs Table 8: Transit cost from Naples using a barge Table 9: Costs for separate laying and trenching of bundled bi-pole cable in a single trench Table 10: Costs for simultaneous lay and burial Table 11: Costs for separate lay into a pre cut trench with pipeline plough Table 12: Cost of the project using cable lay vessels with 4000m 3 / 10,000t Carousel Table 13: Cost using cable lay vessel with 2,500t Carousel Table 14. Technology limits Table 15. Depth of burial and its effect on construction speed Table 16. Types and sources of subsea cable installation risks Table 17: Operating speed scenarios Table 18: Fast operating speed and its effect on cost outlay Table 19: Equipment upgrade speed and its effect on cost outlay MDR0707Rp0028 iv Rev F01

6 1 EXECUTIVE SUMMARY From a construction and deployment perspective, this study finds that the ISLES project is feasible to construct and a preliminary cable network design has been developed. Existing offshore construction vessels, trenching equipment and methods would be employed on this project, all of which have a successful track record in delivery of similar projects. The availability of vessels is such that if ISLES were to be constructed in the same time frame as Round 3 offshore windfarms then cable lay vessel availability would be a tight constraint. Availability of vessels is also highly dependant on the time allocated to build all links of the ISLES network. The environmental implications of construction have been included within this report. In conclusion the overall environmental impact of building the offshore network remain inline with existing offshore construction developments are low consequence and are not deemed to make a case against the development of ISLES. In terms of technology innovation, this study identifies the following two areas which would benefit from further development. The first topic aims to reduce the risk of cable damage during deployment, while the second aims to reduce the costs of cable installation. 1. A technical review of simultaneous burial of bundled bi-pole power cable with a plough or trencher, compared to pre-cutting a trench with a pipeline plough. Currently subsea trenching and lay of bundled cable is a challenging operation. 2. A technical review of the speed of spooling to the vessel at the cable factory and whether automatic systems could be employed to speed up this process. It is recommended that in order to further progress the ISLES project the following activities are undertaken in order to reduce project risk and to enable a more accurate assessment of the cost and schedule for offshore construction. 3. Metocean data study 4. Geotechnical Desk top study MDR0707Rp Rev F01

7 2 INTRODUCTION This report has been arranged into five sections: 1. Outline of offshore construction process 2. Operations and maintenance requirements 3. The proposed offshore cable installation route 4. Cost sensitivities to cable construction and deployment options 5. Construction and installation technology issues The first section details the main work scopes that would be carried out during the planning stages and construction stages of the offshore network. The second section outlines the operations and maintenance requirements of the subsea cable network. The third section discusses the proposed route, seabed conditions and basic trenching implications. Detailed route charts for the proposed route are located in Appendix A. The fourth section details the cost implications of carrying out different construction methodologies to illustrate the various financial pros and cons. Finally, the last section of the report describes the feasibility of constructing the offshore network, highlights the challenges and equipment limits and indicates where technology innovation may assist by improving the business case or reducing the overall project risk. In writing this report the authors would like to express their thanks to the following offshore contractors and companies who have been contacted or consulted in relation to this study 1. Subocean / TPG 2. CTC Marine projects 3. Seacore Fugro 4. VSMC 5. Global Marine Systems Ltd 6. Scaldis 7. Offshore Marine Management 8. Van oord 9. ABB 10. Prysmian 11. Nexans 12. JDR cables 13. International Cable Protection Committee 14. IMCA MDR0707Rp Rev F01

8 3 OUTLINE OF OFFSHORE CONSTRUCTION PROCESS 1. Front End Engineering & Design Studies. (a) Design study route & location (b) Metocean data study 2. Site investigations (a) Desk top study (b) Geophysical survey (c) Geotechnical survey and lab testing (d) Additional geophysical and/or geotechnical surveys and/or laboratory testing as required. 3. Generation of route plan 4. Offshore Construction & Deployment (a) Foundation & Installation (b) Offshore Substation Installation (c) Cable Laying (d) Cable Protection 5. Onshore Construction & Deployment (a) Shore Approach (b) Cable Burial (c) Onshore Substation Installation 6. Commissioning and Testing (a) Cable termination (b) Continuity checks (c) Power transmission 7. Operations and Maintenance (a) Cable route inspection annual survey (b) Offshore platform structural inspection (c) HVDC transmission hardware inspections (d) Cable fault repairs 8. Decommissioning (a) Platform topside removal (b) Jacket removal (c) Pile cut off & removal (d) Cable burial or removal (e) Final survey Figure 1: ISLES construction, deployment and maintenance activities MDR0707Rp Rev F01

9 3.1 FRONT END ENGINEERING DESIGN (FEED) STUDIES Offshore Network Design Study This study phase is carried out by the network developers in advance of commencing any construction activities. Its purpose is to: 1. Develop the detailed design of the network in terms of initial cable lay route corridor selection, substation foundation type, shore approaches and grid connection methods. 2. Establish a more accurate project budget and project schedule. 3. Identify any technical issues which require further investigation or design work. The results of the FEED study would provide sufficient information to enable the network developer to specify survey locations and to define the scope of work for the survey Metocean data review A metocean data review will research the currently available meteorological and oceanographic data for the region, covering: Wind Wave Currents / Tides Weather and sea conditions in the region influence the duration of work that can be carried out by offshore construction vessels and the associated downtime waiting for good weather. Occasionally, cable lay vessels may be required to abandon the cable lay and burial process as a consequence of adverse weather conditions. Wave, wind and high tidal current combinations impose limits to a vessels position keeping ability. Even though ISLES is deemed to be predominantly constructed with vessels fitted with automatic computer controlled Dynamic Positioning (DP) systems, the position keeping ability remains of constant concern to the vessel captain. Metocean data provides much of the input and operational information necessary for the design of offshore platforms. Wave, tide and current combinations will affect seabed scour around platform legs and the base of other subsea structures. The data gathered will indicate extreme conditions and it will allow assessment of the anticipated fatigue life of the platform structure. The data is also of importance in determining operational availability for maintenance, renewable resource assessment and estimation of future operating costs. In addition, the contractor s insurance requirements during construction and in service are governed, to some degree, by the metocean characteristics of the region. The chart reproduced in Figure 2 indicates the metocean data requirements throughout the lifecycle of a marine renewable site. MDR0707Rp Rev F01

10 Figure 2. Reproduced from a presentation titled Importance of metocean data at marine renewable energy sites. James Parker Gardline Environmental Ltd [1] 3.2 GEOPHYSICAL, GEOTECHNICAL AND SITE INVESTIGATIONS The sea bed topography and soil characteristics of the proposed offshore cable route and substation locations need to be accurately determined at the pre-construction phase. This will allow cable trenching contractors to choose the correct burial machine, to estimate the rate of burial and determine the required burial depth. The cost of installation is a function of these parameters. In addition, the same information is required by the substation foundation designers to allow them to engineer the most appropriate substation foundation which, in turn, impacts on the cost of the foundation installation work Geotechnical and geophysical desk top study The desk top study is the first phase of a site investigation, bringing together existing or researched information and identifying potential areas of information conflict or deficiency. The desk top study should include a review of all sources of appropriate information and should collect and evaluate all relevant data for the site, including for example: Existing bathymetric information. Existing geological information and foundation experience in the region. Information and records of seismic activity. MDR0707Rp Rev F01

11 3.2.2 Geotechnical and geophysical site investigations Primarily, the site surveys take the form of separate geophysical and geotechnical surveys carried out along the cable route, though in practice these can be carried out from one integrated vessel. However, it is advantageous to carry out a separate geophysical survey to determine the best route before proceeding to carry out the geotechnical survey. The main reason for this is that the results of the geophysical survey will impact on the final route choice. It is therefore cost efficient to only carry out the geotechnical survey once the best route has been established. The way in which foundations of offshore structures are designed to interact with the seabed will vary according to the type, strength and depth of soil, or the type of rock on which they are to be located. This detailed information must therefore be determined before the design phase. Contractors such as Fugro Survey, Deep Ocean, GEMS and Gardline have a fleet of vessels capable of carrying out this work Geophysical survey Geophysical surveys are non-intrusive, and the purpose is to record bathymetry, map the seabed topography and identify metallic objects, pre-existing infrastructure or hazardous areas on the sea floor. The geophysical surveys are carried out by a survey vessel fitted with a range of survey equipment (Figs 3 and 4). The vessel or vessels would carry out multiple passes of the route to establish the seabed profile for a cable route corridor with a width of 500m to 1000m. On a 150m wide side scan sonar range, parallel survey lines would be spaced 125m apart. For a 750m route corridor, this would require 6 survey lines. Survey lines would need to be oriented to tidal flow directions. It would be usual to carry out the geophysical survey in the seasons before the construction campaign commences to avoid it being on the critical path of any construction activities. It should also be noted that the data processing would take a considerable time for the ISLES project and if the results of the survey indicate that the initial cable route should be modified to avoid certain seabed features then a second geophysical survey may be required. MDR0707Rp Rev F01

12 Figure 3. Survey vessel with Multibeam echo sounder [2] Figure 4. Side scan sonar - SSS [2] Scope of work for geophysical survey for offshore platforms The typical survey scope of work required for offshore platforms is reproduced below from [2]. Figure 5. Typical scope of geophysical survey for an offshore platform, reproduced from Reference [3] Geotechnical survey Geotechnical surveys are carried out to gather detailed information about the site specific soil conditions. Data acquisition methods for geotechnical site investigations along a pipeline or cable route include: Cone penetration testing. Gravity / drop coring (very soft to firm clays). MDR0707Rp Rev F01

13 Vibrocoring (sand). Grab sampling (sand / gravel). The spacing of soil sampling and the need for any in-situ testing along the cable installation route will depend on the variability of soil conditions and the presence of anticipated or known geo-hazards. Minimum laboratory testing should consist of geotechnical classification testing for basic physical properties. Following the results of the surveys, the ability of a plough or trencher to bury the cable can then be more accurately determined. Detailed guidelines for survey testing and sample frequency can be found in Reference [3] Scope of work for geotechnical survey for offshore platforms The typical geotechnical survey scope of work for offshore platforms is shown in Figure 6. Figure 6.Typical scope of geotechnical survey for an offshore platform, reproduced from [3] MDR0707Rp Rev F01

14 Scope of work for geotechnical survey for offshore pipeline routes The typical geotechnical survey scope of work for offshore pipelines is shown in Figure 7. Figure 7. Typical scope of geotechnical survey for offshore pipeline routes, reproduced from [3] Surveys Potential Environmental Impacts Seismic and acoustic surveys generate marine noise. The repeated noise emissions generated by the seismic energy source can result in environmental effects such as: Physical damage to animals close to the source. Direct behavioural effects through avoidance. Indirect behavioural effects by impairing / masking the ability to navigate, find food or communicate, or through affecting the presence of food sources. The main concerns associated with acoustic emissions from seismic surveys and, to a lesser extent acoustic surveys, are potential impacts on plankton, fish and fisheries, and marine mammals. [4] Acoustic surveys use much lower strength emitters and therefore have considerably less impact, however, they are still in excess of background marine noise and can cause changes in behaviour or temporary avoidance by marine life. Additional potential impacts include the physical presence of the survey, which can include a number of vessels and the streamer array. MDR0707Rp Rev F01

15 Borehole sampling to identify platform locations can involve the locating of a jack up barge or suitable vessel on a site to carry out geotechnical investigation. Vessels operating bore holing operations are held in place using Dynamic Positioning systems and thrusters. These increase the noise emission from the vessel in the area and both jack up rigs and DP vessels have a longer physical presence onsite. Both survey and DP vessels can collide with larger marine mammals and DP vessel thrusters can cause injury to marine life. However, such events are very infrequent. All three jurisdictions have specific guidance relating to the operation of these surveys, with conditions such as the presence of Marine Mammal Observers to minimise their environmental impact. These operations are carried out on a routine basis for a number of other industries and applications within the ISLES concept area. Further information on these impacts and their mitigation is outlined in the ISLES Environmental Constraints Report. 3.3 OFFSHORE CONSTRUCTION & DEPLOYMENT Construction port The base port (or ports) in the region will form a logistics hub (or hubs) for the offshore construction and deployment activities of the project. The general requirements of a suitable quayside facility for cable lay and heavy lift vessels can be listed as: Sufficient quayside length to accommodate vessels of length up to 150m with high load bearing capacity and adjacent access. Water access to accommodate vessels up to 45m beam and 6m draft with no tidal or other access restrictions. At least 4 hectares suitable for pre-assembly of substation modules and for cable storage. Overhead clearance to sea of 100m minimum to allow vertical shipment of platform module decks. Primary ports with large shipyard facilities in the region are: Liverpool Cammell Laird Belfast Harland and Wolff Glasgow Govan yard (BAE systems) Secondary ports which may be of importance to the ISLES region are: Dublin Port of Dublin Glasgow King George V Dock (Clydeport) Larne - Port of Larne Port of Mostyn Londonderry Port Holyhead Port MDR0707Rp Rev F01

16 It is anticipated that platform top side modules and jacket structures may be deployed directly to the ISLES sites from their respective fabrication facilities without using the construction port. Alternatively, the jackets and top side modules may be delivered to the base port and subsequently deployed from there. The preferred strategy would be subject to economical analysis during the tender stage, with construction contractors determining the most attractive commercial strategy Offshore platform foundation type Jacket platform structures remain the most common offshore structures used for oil and gas drilling and production in the North Sea. It is anticipated that this design would be the most suitable for High Voltage Direct Current (HVDC) platforms and Voltage Source Converters (VSC) hubs for ISLES. Fixed jacket structures consist of tubular steel members interconnected to form a frame. These structures usually have four legs to achieve stability against overturning in waves, though designs with just three, or up to eight legs are also available. Main piles, which are tubular members, are usually carried with the jackets and driven through the jacket legs into the seabed to anchor the structure to the seafloor. These platforms generally support a superstructure having 2 or 3 decks. The use of these platforms is generally limited to a water depth of about 150~180m in the North Sea. A typical wave of 30m would be considered as the maximum design wave height. Figure 8. Tubular steel jacket foundation structure - Suited to HVDC platform Offshore platform jacket installation The process for deployment of the offshore platform jacket is generally as follows: 1. Jacket transport from fabrication site to offshore location by tugs and cargo barge (Fig. 9). 2. Jacket lift and lay down by floating crane barge (Fig 10) 3. Pile installation & concrete grouting or swaging. The jackets would be fabricated onshore and transported to the offshore project site by a cargo barge, or directly by a heavy lift floating crane barge such as the Scaldis Rambiz (Fig 12). MDR0707Rp Rev F01

17 For rocky sea bed conditions, where piles would need to be drilled, accurate installation of the piles may require the use of a specially designed subsea template positioned on the seabed to guide the piles and drill into the correct location. Once drilled, the piles would normally be grouted in place with a cement grout. Accurate measurement of the installed position of the piles would be found by carrying out a pile top positioning survey using an ROV. The jacket legs can then be lowered into position on the piles by the crane. Assuming the heavy lift floating crane barge would be anchored on location, it would then lift a jacket from the cargo barge. After one jacket is lifted off the cargo barge, the cargo barge is towed outside the construction area and the crane barge would position the jacket on, and into, its pre-installed foundation piles. Figure 9. Jacket delivery by transport barge Figure 10. Jacket lift from transport barge A survey ROV monitors the jacket s position in relation to the pre-installed piles while, at the same time, two ROV systems under water monitor the legs that are being lowered into the piles. After set-down on the seabed, the structure is released from the crane. The crane barge retrieves her anchors and moves to the next location, where the cargo barge with the next jacket is manoeuvred underneath the cranes on the crane barge again for the next installation. For softer soils and sandy seabeds, it would be normal to install piles with the jackets, and vibrate the piles and hammer them to the required depth by using an IHC Hydrohammer. When the crane barge has completed the jacket set down, a grouting support vessel would then move in to connect grout injection equipment to the grout connection points on the jacket. The space between jacket legs and foundation piles is filled up with cement grout to fix the jacket into position and to help it to withstand the wind, waves and tidal currents in the Irish Sea. An alternative method of connecting the jacket to the piles is to use a swaging process instead of the grouting process. This method deforms the jacket sleeve to create a tight frictional joint between the pile and the jacket sleeve Offshore substation topside installation The outline process for deployment of the offshore platform topside is generally as follows: 1. Topside transport from fabrication site to offshore location by tugs and cargo barge (Fig 11). 2. Topside float-over and lay down onto jacket by floating crane barge (Figs12-15). MDR0707Rp Rev F01

18 Once the platform topside has been laid down, it is then ready for cable pull in and mechanical and electrical termination. Figure 11. platform topside transport to offshore site with cargo barge and tug. Figure 12. Rambiz floating crane vessel installing a platform topside onto a foundation. Figure 13. platform topside float over with crane barge. Figure 14. platform topside ready to be lowered onto jacket Figure 15: Construction of Gun fleet Sands and installation of Offshore Substation Figure 16: Jack up Crane Vessel during foundation Installation MDR0707Rp Rev F01

19 3.3.5 Environmental Impacts of offshore platform construction and deployment Platform Environmental Impacts The construction and presence of a platform has a number of potential environmental impacts which need to be considered and mitigated against where possible. The platform, regardless of construction technique, will have a footprint on the seabed. Whilst this is a relatively small area, this impact does mean the siting of the platform should avoid sensitive habitats. In addition the platform will have water column effects which must be assessed to minimise any hydrodynamic impact by its siting. The platform has a presence above the water and should be sited with consideration of visual sensitivities and avoid sensitive sites of ecological importance such as adjacent to seal haul outs etc. The platform and construction operations will occur 24hrs a day. As a result, noise and lighting operation will occur at night. Lighting can attract seabirds and cause fish to aggregate at the platform. It is assumed that any platform siting would be within the site boundary of a renewable energy field or adjacent to one. These areas would be subject to extensive environmental impact assessment for suitability prior to the Isles infrastructure development. It is therefore likely that the findings of these pre-existing assessments could be reapplied to gauge the impact of the ISLES project s platform sitings. Pre-installation and transport to site The platform, its jacket structure, its equipment, as well as construction shipping, etc. will need to be mobilised from a suitable port to the site. Especially during the construction phase, it is anticipated that there will be increased shipping and transits caused by the transport of this equipment and infrastructure. Vessels operating for this phase will need to adhere to best practice in terms of operations, ballast water control, retention of waste, etc. to minimise any potential impacts. Ship transits will need to ensure avoidance of collision with marine life and Marine Mammal Observer guidance should be followed, especially for any vessels onsite required to maintain Dynamic Positioning. Given the level of shipping in the ISLES concept area the relative number of vessels required for these operations is very small and any impacts will be minimal. Piled Platforms The main environmental impact anticipated during the construction of a piled structure is the marine noise generated by driving the piles. Pile-driving noise during construction is of particular concern as the very high sound and pressure levels can potentially affect fish and marine mammals. This can cause temporary exclusion of an area through avoidance. For fish this can prevent species from reaching breeding or spawning sites, finding food, and acoustically locating mates. Whilst the effects caused by construction of the platforms are temporary, there is the possibility of a cumulative effect if associated with the construction of a wind farm using the same method. Long term avoidance reactions might also result in displacement away from potential fishing grounds and lead to reduced catches or changes in fish distribution. However, reaction thresholds and therefore the impacts of pile-driving on the behaviour of fish are thought to be highly localised and the piling for the platform will be of short duration. [5] Marine mammals are particularly sensitive to marine noises and therefore pile driving [6,7]. The following is from a study of the sound field surrounding the installation of two 5MW wind turbines of NE Scotland with the use of impact pile driving. The turbines were in relatively deep water (>40m), 25 km from a Conservation area with a protected population of bottlenose dolphins. MDR0707Rp Rev F01

20 Pile driving noise at ranges from 0.1 to 80km measured during pile driving operation ranged from a peak of 205 db re 1µPa, to being indistinguishable from background noise. Noise levels across frequencies were detectable above background levels to about 70km. Noise levels related to suggested noise exposure criteria for the bottlenose dolphin, harbor porpoise, minke whale, and harbour and grey seals have been assessed. Based on updated hearing impairment thresholds, these measurements suggest no risk of hearing impairment at distances greater than 100m (Bailey et al., 2010) [26]. A number of other studies have indicated similar results. The monitoring of the pile driving installation of foundations at the Horns Rev II windfarm in the North Sea, has an extended piling period of 5 months for construction. During this period monitoring showed that harbour porpoise may have a particular sensitivity with lower recorded numbers during constriction and effects detected up to 2km away, however, assessment was based on hydrophone detection and there is currently inconclusive as to whether the species move away or change behaviour as a response to prolonged piling (i.e. cease clicking) [8] Mitigation There are a number of mitigation measures that can be put in place should site surveys of the platform locations indicate that there may be sensitive fish habitats or marine mammals in the area. Seasonal timing: Construction can be scheduled to not coincide with sensitive times of year such as mating seasons etc. Bubble curtains (confined / unconfined): Placing air bubbles around a pile (bubbles can be confined or unconfined) can act as a means to prevent sound propagation based on the difference in density between air and water. [9] Up to a 15 db reduction can be achieved. [10] Ramp-up / Soft start: Gradually increasing hammer energy levels over time. The goal of this technique is to allow animals in the vicinity to experience a reduced level of sound and evacuate the area before maximum levels are achieved. [11] Cushion blocks / Caps: Materials (wood, micarta, nylon) placed atop piles during impact pile driving activities to reduce source sound levels. Typically sound reduction can range from 4 up to 26 db. [10] Temporary Noise Attenuation Pile (TNAP) design: A hollow walled air-filled or foam-lined steel pile casing is placed around the pile being driven. Noise levels can be reduced by between 8 and 14 db. [10] Acoustic deterrents can be used to ensure marine mammals avoid piling activity during the short construction duration. [12] Gravity Based Platforms Gravity based platforms have a greater benthic (seabed) footprint than piled systems. The base is usually reinforced concrete or a steel chamber sunk as a single item or as a series of support structures. As a result the platform may have a significant seabed footprint that will be covered during the life of the platform. The construction operation has few marine impacts except those outlined in relation to vessel activity. The content of the ballast material should be assessed for ecological impact at the construction stage as historically there have been issues with materials used. [13] Gravity based systems provide a relatively inexpensive and low impact method of platform construction, though they are constrained by suitable siting and oceanographic conditions. MDR0707Rp Rev F01

21 3.4 SHORE AND SHALLOW WATER CONSTRUCTION The connection of the offshore power network to the onshore transmission starts near the shore where the cable lay vessel reaches its operational limit due to depth limitations. The cable landing on the beach and foreshore is usually constructed by one of the following methods: 1. Digging an open trench through the land adjacent to the beach, and down the beach to the low water mark (Figs 17 and 18). 2. Horizontal Directional Drilling under existing infrastructure and the beach to the low water mark (Fig 19). 3. Auger boring under existing infrastructure and the beach to the low water mark (Fig 21). Figure 17. Digging an open trench from headland to the beach Horizontal Directional Drilling (HDD) is often used as an effective means of installing a duct for cables under the surf zone and intertidal zone of a beach. HDD and Auger boring methods are preferred to the digging of open trenches in environmentally sensitive areas. With HDD, the angled drill rig is set up on the shore and drills and lines the hole with pipe casing until a sufficient depth is reached where soil stability can be maintained. A drilling fluid aids the cutting process and flushes the excavated soil out of the hole. Slurries that are biodegradable and do not contaminate the water are available. This method has been widely used for shore connection of subsea cables, river crossings and coast line crossings. The depth of soil cover required for adequate protection using HDD methods are reported in Reference 1. MDR0707Rp Rev F01

22 Figure 18. Robert Donaghy Cigre Cock Harbour 2010, ESB International Figure 19. Image of HDD rig courtesy of Land and Marine. Figure 20. Open sheet pile trench Figure 21. Auger boring Connecting the first end of the offshore cable The first end of the offshore cable is generally installed from the offshore vessel by floating in the cable and pulling it in with a winch or tracked vehicle located on the shore. A pre-installed pull in wire allows the cable to be drawn up a pre-installed beach duct (Figs 22 and 23) which would typically have its outlet below the low water mark and beyond any surf zone. Alternatively, the cable may be floated in and pulled directly up the beach and foreshore before then being buried by cable plough down the beach. MDR0707Rp Rev F01

23 Figure 22. Cable being placed into beach duct from lay barge Underwater installation of the cable in the beach duct can be a time consuming activity and a major area of difficulty if the entrance to the duct has not been designed well or if it is not free from debris and seabed sediments. Figure 23. Cable going into HDD duct - Exit of HDD duct showing cable duct seal. Image courtesy of Global Marine Systems Ltd In summary the shore landing of the cable generally consists of the following operations: Clearing of the end of the duct, where the directional drill section ends on the beach. Pulling of the cable ashore and guiding the cable into the duct. Handling and positioning of the burial tool over the cable from beach duct. MDR0707Rp Rev F01

24 Closing of the trench once burial is completed and clearing of the beach duct. For rocky seabeds and shore approaches where it is deemed necessary to protect the cable by burial, subsea rock cutting machines exist such as the IHC EB Seatrack machine (Figs 28 and 29). Alternatively, stabilising the cable using rock dumping is a common method which may be more cost effective depending on the length of protection required. Figure 24. Cable plough being used to bury export cable directly from the beach Rock dumping Rock dumping can be carried out by a fall pipe vessel (Fig 25) or a side dumping vessel (Fig 26). The fall pipe vessel has the capacity to typically carry a payload of up to 20,000t of rock and it has a large working window as the rock can be accurately placed on its target by a remotely operated fall pipe duct. A fall pipe vessel has a maximum rock size of about 400mm diameter. Deposition of rock with a fall pipe vessel is efficient as the pipe can be placed directly above the cable route to ensure good coverage and minimal wastage. MDR0707Rp Rev F01

25 Figure 25. Fall pipe vessel Figure 26. Side dumping vessel In contrast, the side dumping vessel literally pushes rock overboard with the consequence of less accurate rock placement and higher losses. The side dumping vessel also has the disadvantage that its payload capacity is much lower than a fall pipe vessel, typically 2000t. However, it has the ability to handle rocks of a much greater size. Concrete mattresses are used to lay over exposed cables and pipelines where local levels of protection are required, for example to prevent scour around platform J-tubes and to protect cable crossings. These mattresses can be installed by a multipurpose vessel or cable lay vessel. This approach avoids the costly mobilisation and demobilisation of a dedicated rock dumping vessel for small and specific locations. Mattresses can be installed using a crane (Fig 27) to compensate for the vessel heave motions to ensure exposed cables or pipelines are not damaged during lay down. Various mattress designs exist to suit a variety of seabed soil conditions and flow regimes. Mattresses can also be supplied with fronds to encourage sediments to gather and deposit, thus forming artificial sand banks. Figure 27. Protective concrete mattresses in a mattress installation frame MDR0707Rp Rev F01

26 Another example of a cable protection method is a subsea vehicle from LD Travocean which is capable of continuously casting a concrete protection structure (Fig 30). Figure 28. IHC EB Seatrac Subsea Rock cutting machine Figure 29. Simec Castor 2 Figure 30. LD travocean concrete casting machine MDR0707Rp Rev F01

27 3.4.3 Cable lay operations Before cable laying, a route clearance operation would commence. This would typically be a Pre-Lay Grapnel Run (PLGR) where an anchor handling vessel or multipurpose construction vessel would drag a grapnel (Fig 31) along the route to drag out debris such as old wires, fishing nets, chains, etc There may also be seabed levelling operations to flatten sandwave regions or other steep gradients prior to stabilising any loose seabed soils with rock if required. Figure 31. Grapnel used for Pre-Lay Grapnel Run. Image courtesy of Denholm Offshore Figure 32. Subsea excavator Carrera 4 used for seabed levelling, and cable deburial image courtesy of Seatools. The laying of the offshore power cables for the ISLES network requires a DP operated cable lay vessel with a large capacity cable storage carousel and specialised burial plough and trenching ROV s. The cables are buried to a depth depending on the soil conditions and perceived risk of cable damage. There is a limit to the burial depth imposed by the heating effect of the ground conditions. The heating effect of burial at greater depths leads to limitations of power transmission efficiency, as discussed in greater detail in the Technology Roadmap Report..Section 4.2 discusses the burial depth requirements in more detail. MDR0707Rp Rev F01

28 Figure 33. North Ocean 102 cable lay vessel, used to lay Britned cable link For the Isles region, where water depths are greater than 50 metres and the project is of considerable size (in terms of km of cable to be laid), the use of a cable lay barge for the whole project would be slow and inefficient. This is because the forward progress of a cable lay barge is dictated by speed of anchor winches, and the constant re-mooring operations required to advance the barge to allow the cable lay to progress. In addition to their limited speed, the station keeping ability of a barge is insufficient in areas with higher tidal flows of close to 5 knots, such as the area close to the Mull of Kintyre and near Rathlin Island. This would likely necessitate the use of pre-installed concrete anchor blocks in certain locations because drag anchors may not provide sufficient holding force during periods of inclement weather. Cable barges could be used to great effect for long, shallow beach approaches as draught limitations would prevent DP cable lay ships from working close to the shore. It is anticipated that the cable lay speed of a lay barge would be of the order of 200 m/hr, for comparison the cable lay speed of a DP vessel would be in the order of 750 to 1000m/hr, i.e. 4 to 5 times quicker than the barge. Cables are anticipated to be laid as a bi-pole bundle (Figs 35 and 36) and buried in a single trench. This requires the cables to be stored on the vessel turntable as a pair (Fig 34). The offshore cable laying campaign would commence with the vessel transiting to the cable factory to loadout (spool) the full vessel payload of cable. For a 7000t payload it typically takes 15 to 20 days to load on the cable. An important part of the spooling operation is the inspection of the cable to check for defects. This inspection process is typically a manual operation and it limits the spooling speed and hence determines the time required to carry out the operation. MDR0707Rp Rev F01

29 Once the vessel has spooled the cable onto the vessel turntable at the cable factory it would then transit to the ISLES region to be joined by a full cable lay crew before transiting to the offshore work site. Figure 34. Bundled bipole cable on vessel turntable stored as a pair. Figure 35. Bundled bipole cable being deployed from stern chute of vessel The first operation is to handover the cables individually from the vessel to pre-installed messenger wires which are used to pull the cable up the platform J-tube. (A J-tube is a tube in the form of a J which houses and protects the cable between the seabed and the platform topside. The messenger wires are strong cords routed through the J-tube and looped back onto the topside to allow them to be easily accessed and attached to the cable.) Once pulled up to the platform topside the cable can be mechanically connected. The cable lay vessel will then proceed to lay the cable on the seabed before placing the other end of the cable adjacent to the next platform s J-tube. A remotely operated vehicle is then used to connect this free end of cable to the pre-installed messenger wires so the cable can be pulled in and connected. MDR0707Rp Rev F01

30 The decision whether to lay the cable and bury it afterwards (post lay burial Fig 36) or to bury the cable simultaneously as it is laid (simultaneous lay and burial Fig 37) would be made following the preliminary site study work and after further discussion with offshore contractors. Figure 36: Schematic of the Post lay burial. Image courtesy of IMCA Figure 37: Schematic of Simultaneous lay and burial. Image courtesy of IMCA Cable burial techniques and protection strategies The main construction options available for cable burial are: 1. Separate cable lay and burial campaigns cable buried by cable plough or trencher after it has been laid on the seabed (post lay burial) (Fig 47). 2. Simultaneous lay and burial with cable plough or trencher. 3. Separate trenching and burial campaigns trench pre-cut by a large pipeline plough and cable laid into an open trench followed by backfill by plough or rock dumping. Figure 38. Cable plough Figure 39. Hard soil trencher Figure 40. Pipeline plough MDR0707Rp Rev F01

31 Problems with bundled cable Burial of a bundled bi-pole cable is more difficult than single cables due to unequal cable tensions, and the risk of the cable becoming separated on the seabed. Differential cable tensions are often induced during the laying process and can cause the cable to twist or spiral unpredictably, leaving sections unable to pass through a trenching machine or cable plough. Spiralled cables, where the cable can adopt a form that resembles a coiled telephone cord rather than a uniform co-linear cable bundle, can cause severe problems with the passage of the cable through a cable plough or trencher. There is no remedy for this effect and it can lead to damaged and / or unburied cable. Cable separation, where the ties between the two cables are broken and the cables separate on the seabed, may require the subsea trencher to disengage the cable and reposition ahead of the unbundled length. This causes severe disruption to the cable burial campaign and significantly increases the probability of cable damage during installation. Both cable separation and spiralling would typically require reburial with a smaller, free flying, jetting ROV or additional protective cover provided by rock dumping. Separate lay and burial cable plough or trencher There is a risk of damage to the unburied cable due to the time between lay and burial operations. The plough or trencher can induce tensions into the pre-laid cable due to cable friction as the cable travels through the machine. This can lead to free spans in sandwave areas. In addition a kink can develop in the cable ahead of the machine. This kinking ahead of the trenching machine occurred on Britned and caused time delays to the burial schedule as the trencher had to put the cable down, move ahead of the kink, recommence trenching and the kink had to be buried afterwards by a different machine. The excess of cable which develops ahead of the plough is difficult to manage, but opportunities exist to minimise this by driving the burial machine in a curved path on a straight route. However, the success of this approach is not well documented. Operational risks are always present surrounding launch and recovery of the burial machine from the vessel, especially in high sea states. Landing the machine on the seabed safely over the cable can also be a challenging operation in energetic seas. Simultaneous lay and burial cable plough or trencher This approach offers immediate protection to the cable and cable tension can be managed by the cable lay system as the cable enters the plough or trencher. The cable catenary can be monitored by ROV during the process. MDR0707Rp Rev F01

32 Separate lay and burial pipeline plough Laying the cable into a pre-cut trench may offer a low risk construction method, whereby a pipeline plough is used to create a large V-trench, carrying out the aggressive soil cutting without the presence of the cable bundle. The cable bundle can then be laid into this wide trench and back filled by a second pass with a backfill plough. This approach would mean that the risk of damage to the cable is much reduced compared to the post lay burial technique and the simultaneous lay and burial technique Figure 41. PL3 Pipeline plough IHC Engineeering Business Figure 42. Prototype SCAR plough, Ecosse Subsea A comprehensive report into cable burial techniques was written in 2008 for BERR, titled Review of cabling techniques and environmental effects applicable to the offshore wind farm industry [3]. The cable burial techniques proposed in this report remain equally applicable to the ISLES project, as such there is no added value in repeating essentially the same review within this report: Based on the limited soil and seabed information available at this stage, cable burial by cable plough is the anticipated primary protection strategy for the subsea cable for the ISLES project. Where the cable cannot be buried by plough or trencher, it has been assumed that the cable would be protected by rock dumping Environmental effects of cable Installation Cables and rock armour provide artificial hard substrates in sedimentary areas. Organisms such as anemones colonise the cable and are more abundant than in the surrounding soft sediment sites. If cables are shallow buried, rows of anemones can occur where the cable is located. Echinoderms and sponges can also colonise cables and rock armour. The cable may also subtly affect local hydrodynamic conditions that concentrate shell gravel and or drift material near the cable. Long-term impacts of leaving the cable on the seafloor are likely to include: MDR0707Rp Rev F01

33 Continued abrasion of nearshore rock outcrops by the cable. Potential for additional organisms to colonise the cable. Potential impacts of cable-repair operations. Potential snag hazard for fishing gear [33]. Cables on the surface of the sediments can cause scour in certain hydrodynamic conditions, clearing epifauna in rocky habitats or resuspending sediments in sedimentary habitats. Burying cables can have localised environmental impacts as trenching disturbs the seabed. However, impacts are often short term as sediments are rapidly recolonised after operations. In general, cables have short term, highly localised impacts during laying or burying. 3.5 COMMISSIONING AND TESTING It is usual for the substation to have undergone a factory acceptance test prior to being installed offshore, thus minimising the risk of a subsystems failure. Offshore commissioning consists of electrical termination of the cables at the platform and continuity checks from link to link. An extensive onshore and offshore substation testing programme would follow final electrical hook up of the network. Environmental Impacts During commissioning and testing there may be additional transit vessels providing technicians and supplies to the platforms. In addition, there may be additional lighting and night working which can cause aggregations of marine life or minor noise impacts. There will be associated risks of increased grey waste etc from these activities. It is anticipated that these activities, as all proposed activities in the ISLES construction and operation, will be controlled by Health, Safety and Environmental management systems whish will minimise any such effects. MDR0707Rp Rev F01

34 3.6 CONCLUSION OF THE CONSTRUCTION AND DEPLOYMENT PROCESS From a construction and deployment perspective the study finds that the ISLES project is feasible to construct with existing offshore construction vessels and trenching equipment. Standard methods would be employed on this project, all of which have a successful track record in delivery of similar projects. The availability of vessels is such that if ISLES were to be constructed in the same time frame as Round 3 offshore windfarms then cable lay vessel availability would be a tight constraint. Availability of vessels is also highly dependant on the time allocated to build ISLES itself. Challenging sea states and tidal flows in the region may lead to a short summer offshore construction season. It is anticipated that a minimum 30% bad weather downtime should be allowed for in the planning of the project. A significant proportion of very hard seabed conditions and exposed bed rock will be encountered along the route of the Northern ISLES network which will make cable burial uneconomical, as such either protection by rock dumping or additional cable armouring should be considered. MDR0707Rp Rev F01

35 4 OPERATION AND MAINTENANCE Cable maintenance Cable fault probability The operational expenditure for cable maintenance on the Isles network has been based in part on the anticipated number of cable faults occurring over the 854 km network of the route. It is estimated that a fault rate of between 0.1 to 0.2 annually (i.e. one fault every 5 to 10 years) is realistic based on the analysis of historical figures and taking into account the length of the network and its location in water depths less than 1000m. This is inline with published resources [4]. The average fault rate is 0.1 faults per 1000km per year for water depths less than 1000m, with most faults occurring in water depths less than 200m. Once a cable fault has been identified, the metocean conditions strongly influence the time required to facilitate cable repairs. Timescales to complete the repair can vary from a few weeks to many months depending on the weather conditions and accessibility to the cable fault. The ambient temperature required to handle the cable is ideally more than 5 o C. Therefore, if a fault occurs during the winter season the repair process usually has to wait until a suitable temperature is reached. The prevailing tidal currents and wind speed are also deciding factors in planning the effective repair process in deep waters. The ideal work conditions are considered to be when the wind speed is 12 m/s (26 m.p.h) or less. The use of several different vessels depending on the activities listed below allows some mitigation of the delays due to inclement weather. Locating the cable fault Mobilisation and demobilisation of cable repair vessel Cable de-burial Cable jointing Cable testing Cable lay down and subsequent cable protection Post repair survey However the cable jointing process requires a good weather forecast of approximately 5 clear days for a cable vessel, such as the Sea Spider (Fig 43) to successfully carry out the repair. MDR0707Rp Rev F01

36 Figure 43: Typical cable repair vessel Sea Spider Figure 44: Typical damaged cable where a flash-over occurred. Figure 45: Cable suspended inside the repair container on board the cable repair vessel Environmental impacts of subsea cable maintenance The maintenance of the cable will involve a vessel (or vessels), which may include the use of Dynamic Positioning to locate the vessel over the cable. Using a grapple, the cable is then retrieved from the seabed. The retrieval is likely to remove any settled organisms on the cable. It may cause some localised impacts to the seabed, such as damaging nearby epifauna and redistributing local sediments. The effects are highly localised in the area of lifted cable and therefore not considered to be of concern. Once repaired the cable is dropped to the seabed and armoured with rock dumping. Rock armour smothers the immediate biology on a very localised level. In sediment habitats it creates a reef like substrate, which is then often colonised by sponges and anemones, as mentioned in the cable installation section. In rock habitats, dumped rock covers the existing epifauna but is highly likely to be recolonised. MDR0707Rp Rev F01

37 When the cable is dropped it may be in a slightly different position to before. The exposed area will be recolonised as will the rock armour in the new area. The distance of displacement is often quite small due to the nature of the cable Operational activities and ongoing network surveys Another operational cost outlay, in addition to cable fault repair, will be the requirement to survey the cable network at regular intervals. The network owners will seek to minimise the risk of damage and prevent downtime by ensuring that the cable remains suitably protected during the life of the infrastructure. Ongoing, proactive cable maintenance is favoured over a reactive, fault repair maintenance strategy for cost and network availability reasons. The Offshore Transmission Owner (OFTO) would typically sign agreements with generators on the percentage availability of the network, with excessive downtime incurring lost revenue and financial penalties. A cable survey contract would typically be placed following the completion of the construction work and would comprise a survey vessel, with inspection or work ROV and possibly a trencher to allow reburial of any exposed cables. The final route as-built survey would highlight and record: any lengths of unburied cable free spans sections of shallow cable mobile sediments and scour The scope of the survey and maintenance activities would typically be established on the basis of the as-built route. The as-built survey record then serves as a known benchmark. Areas of mobile seabed and energetic seas would require more frequent survey intervals than well protected deeply buried cable in a hard soil area. For the ISLES network the as built survey of the full route is estimated to take around 100 offshore days. This gives an indication of the survey vessel time that would need to be budgeted for ongoing surveys of the complete route. MDR0707Rp Rev F01

38 5 GENERATION OF PRELIMINARY ISLES ROUTE PLAN The preliminary route for the northern ISLES network has been chosen to minimize depth changes and to avoid known navigation and installation hazards. The route will require further refinement to avoid some environmentally sensitive areas as well as commercial and navigation areas such as tanker anchorages. The seabed conditions vary from hard basaltic rocks to the mud and fine sand of the estuaries and locks. A full geotechnical investigation with frequent sampling will be required as part of the final route assessment. The preliminary route description is located in Appendix A, along with the route plan list. It is tentatively estimated that between 40% and 60% of the routes will be trenchable if so required for example, on seabed stability, trawl/anchor protection, and landfall/shallow water protection. The remaining sections of the route may require further armouring to protect the cable in rocky areas. In addition, there may be a requirement to carryout protective rock / gravel dumping over the cable in those areas where the cable may be especially vulnerable to external forces / actions, or will require additional stability/protection. There are three potential cable crossing points over the Hibernia A Telecommunications Cable. In addition, in the Firth of Clyde there are potential crossings of the Linis-3 and Sirius North cables, giving a total of four crossings in this area. In terms of feasibility of construction, the development of the network does not pose any significant adverse construction and deployment issues, other than vessels operating in strong tidal currents in certain places and needing to work a short summer construction season due to relatively adverse metocean conditions. The receipt of full geotechnical and geophysical information would enable a more informed trenching assessment to be carried out. MDR0707Rp Rev F01

39 6 CONSTRUCTION SCHEDULE FOR OFFSHORE CABLE LAYING OPERATIONS The offshore cable laying and burial process forms the majority of the costs of the offshore campaign. A cable lay and burial project schedule for the Northern Isles network has been developed in discussion with offshore contractors and can be found in Appendix B. It can be noted that the longest of the 12 links that form the route may typically take around a year to complete. This indicates that if the rest of the network was constructed in parallel there would be demand for approximately 10 cable installation vessels for one summer season. Alternatively, the construction of the network would take up to ten years with one vessel. MDR0707Rp Rev F01

40 7 EFFECT OF CONSTRUCTION METHODS ON THE COST OF OFFSHORE CABLE INSTALLATION In this section a cost estimate has been developed for the installation of the offshore power cable and various construction scenarios for cable installation have been explored and discussed. 7.1 SCOPE OF WORK The development of the capital expenditure estimate has been built up from the sequence of steps required for cable laying and installation based on the transmission architecture for the Northern ISLES network (Fig 46). Figure 46. Northern ISLES network architecture MDR0707Rp Rev F01

41 The scope of work that has been considered for the cost estimate is listed below. Cable lay and trenching vessel mobilisations and demobilisations Port transits Power cable load outs from cable manufacturing location Site transits (in field) Pre-lay surveys General offshore cable installation set up activities Power cable lay (including cable pull in at platforms and beach landings) Trenching (burial) of the cable Protection of unburied sections of cable and remedial works by rock dumping with a fall pipe vessel. Post installation survey Electrical termination of power cables at each offshore substation platform. The above mentioned steps are considered in detail and the complete program of work can be found in Appendix C. It should be noted that the cost estimate does not include the following activities: Development and consent costs. Cable and cable joint procurement costs. Substation build and installation costs. 7.2 ASSUMPTIONS The costs presented in this section are based on 2011 costs. In order to establish a reasonable cost basis, the following assumptions have been made as discussed below and outlined in Table 1: The sea bed conditions in the Northern ISLES region have been investigated by RPS Group and it is considered to be 50 % trenchable by plough or trencher. The proportion of cable that can be buried by the plough is a key parameter in the development of the cost estimate for ISLES. As such the cost sensitivity of this assumption has been explored and discussed later in the chapter. Protection by rock dumping by fall pipe vessel has been considered to bury the remaining 50% of the cable and to cover the cable at joints, crossings, J-tube approaches, cable transitions and shore landings. The overall speeds listed in Table 1 have been based on industry experience with technology limitations of laying and burying bi-polar cable in a single trench. This is generally applicable for power cable lay with the Nexans Skagerrak, Prysmian Guilio Verne and North Ocean 102 vessels. Speed of lay is deemed an overall average, taking equipment downtime and other adverse factors into consideration. The vessel fuel costs remain susceptible to fluctuations. The price considered in Table 1 is the price at the time of writing this report. The vessel considered for the study is a generic DP power cable lay vessel with approximately 1800 m 3 of product storage volume and 5000 tonnes of cable storage capacity. MDR0707Rp Rev F01

42 Table 1: Inputs to the Costing Structure General The cost estimate has been developed considering 50% burial of bipolar bundled cable in a single trench. Rock dumping by fall pipe vessel has been considered for the protection of the remaining un-trenched lengths of cable as well as for cable protection at transitions, cable joints and cable crossings. Variables Vessel economical transit speed (knots) 13 Pre lay survey speed (m/hr) 800 Post installation survey speed (m/hr) 500 Average cable lay speed including equipment downtime (m/hr) 200 Trenching speed (m/hr) 175 Spooling speed (m/hr) 300 Weather allowance (proportion of project duration spent waiting for clear weather) 30 % Fuel and cost (MGO) ( per tonne) 651 [7] Day rate for high spec 5000t cable lay and trenching DP vessel inc: WROV spread inc crew Online and offline survey crew Back deck crew Cable lay crew Plough / Trencher crew Navigation / comms data costs per day 155,000 Day rate for a low spec cable transport barge, two tugs and tug crew per day 60,000 The ISLES transmission network is constructed with 500 MW and 1000 MW capacity links, the properties of which are listed in Table 2. The cable and carousel loading capacity has been calculated accordingly. Table 2: HVDC Link Cable Data Transmission Capacity Outer (mm) Diameter Mass/Meter (kg/m) Carousel Capacity Volume (m 3 ) Approximate carousel capacity (Tonne) Single Loading Capacity (k.m.) 500 MW MW LOCATION OF THE ISLES CONSTRUCTION PORT The geographical location of the construction port has been considered for efficient provision of project support activities in terms of cable storage, infrastructure load out and for minimal vessel transit to and from the offshore site. Specific to the ISLES project, the base port considered is an unnamed generic port accessible to the Irish Sea region irrespective of the actual facilities available. It is not in the scope of this study to evaluate the pros and cons of various port facilities in the region or to state a preferred base port for ISLES. MDR0707Rp Rev F01

43 7.4 RESULTS OF THE COST MODEL This section discusses the results of the cost model and the effect of various construction scenarios. Table 3: ISLES power cable network installation cost outlay Method of construction: Bundled bipolar cables buried in a single trench Separate lay and trenching operations Item Construction Element Cost Outlay Proportion Remarks 1 Route Clearance 2.2m 0.5% This is inclusive of a bad weather allowance (30%) 2 Lay & Trenching 260m 66.3% This is inclusive of a bad weather allowance (30%) 3 Jointing 11.1m 2.8% This includes transits and cable spooling cost at the port. 4 Rock Dumping (1) - Cable (50%) 107m 27.3% This is inclusive of a bad weather allowance (30%) 5 Rock Dumping (2) - at fixed locations 5m 1.3% This is inclusive of a bad weather allowance (30%) 6 Shore Landing (5 landfalls) 5m 1.3% Shore Landing cost has been considered as 1 million/ landing. 7 Cable terminations 2m 0.5% This is inclusive of a bad weather allowance (30%) Total Cost: 392m Table 3 shows an estimated cost of 392 million for installation of the ISLES power cable network on a total route length of 854 kilometres, integrating renewable resources of 2300 MW present in the northern ISLES region. This gives an installed cost of 459k / km. For comparison, this is below the lower range of k / km proposed by the national grid in the offshore development information [6]. It is important to note that the cost of 459k / km excludes the following activities: 1. Geophysical and geotechnical surveys. 2. Materials cost. 3. Ancillary vessels cost. MDR0707Rp Rev F01

44 7.5 OVERVIEW OF OFFSHORE CONSTRUCTION COSTS This section provides an outline of the main activities involved in the offshore cable installation process with their contribution to the total cost: Route Clearence 1% Rock Dumping Fixed Locations 1% Shore Landing 1% Jointing 3% Terminations 1% Vessel on DP ops 22% Survey 10% Vessel in port 20% Rock Dumping 50 % Cable 27% Lay & Trenching 66% Weather Allow ance 19% Vessel trenching 9% Vessel in transit 20% Figure 47. Offshore cable installation cost breakdown Figure 48. Offshore cable lay and trenching cost breakdown Laying, trenching and cable load transits are the most time consuming tasks involved in the offshore installation of power cables. In the case of the ISLES project these activities account for approximately 66% of the total offshore campaign cost. It should be highlighted here that the laying operation and burial operation have been considered as separate activities. This assumption leads to a higher cost implication as compared to the option of simultaneous lay and burial. These cost differences are discussed further in section 7.8. Cable collection and load-out from the cable factory is a major contributor to cost outlay in terms of time spent in transit or in port. These two activities account for 40% of the offshore cable lay and trenching campaign. This cost has been developed taking into consideration Pyrsmian s, Arco Felice cable factory near Naples which is the loading port furthest from the ISLES region. This cost takes into consideration the transit of the cable lay vessel to and from the ISLES base port to the cable factory and the time alongside at the quay required for spooling on the cable. A cheaper method of cable load out has also been presented in the following sections by means of a separate offshore barge fitted with cable storage turntables, which can be towed by tugs. This vessel offers a reduced day rate in comparison as highlighted in Table 1. The cost of cable collection and load-out is a significant part of the offshore campaign and leads to the question of whether any technical innovation could create a step change increase in the speed of cable loading. This has been discussed in section 8.6. Rock dumping is an alternative and complimentary cable protection strategy to subsea cable burial by ploughing and is necessary at certain locations. Rock dumping is proposed for 50% of the cable route length where the cable cannot be buried due to rocky sea bed conditions. It is also included near foundations, cable joints and cable crossings, where ploughs and trenchers cannot operate. This activity accounts for approximately 27% of the total cable installation cost. MDR0707Rp Rev F01

45 Cable jointing is the process of joining two cables to complete the length of cable link on a particular route. The need for jointing arises because of insufficient cable length on the cable laying vessel due to the inherent limit of the onboard cable storage capacity. Since the subsea cable jointing process can take more than 5 days and require the forecast of a clear weather window, the route needs to be suitably planned to minimise the number of joints. In the ISLES project base case, 8 cable joints have been planned over the 854km cable route. This jointing process will cost of 11 million (excluding the cost of the joint hardware itself) Shore landing and shore approach work is the stage of the cable laying process where the cable is floated to the beach or landfall and pulled up to the beach manhole to connect the cable to the land based transmission network. This process starts as close to shore as the cable installation vessel can safely reach considering the vessel s draught limitations. Support equipment in the form of pulling winches, cable rollers and other plant is required to pull the cable further onshore. Depending on the location, directional drilling machine may be used to install pipes or ducts to bring the cable under the beach or shore. In consultation with construction contractors, the five shore connections that have been planned are estimated to cost 1 million per shore connection. Downtime caused by weather or metocean conditions falling outside of equipment limits has been estimated to account for 30% of the project duration. This equates to 19% of the overall project costs. Once an offshore cable lay and trenching campaign is underway, the client would be charged for any downtime caused by inclement weather. For the Northern ISLES region, the metocean conditions are also expected to lead to a short summer offshore construction season with challenging conditions and energetic sea states. This downtime cost of approximately 41million provides a tangible incentive to improve equipment operational limits through technical and operational innovation. Other costs in the form of initial route clearance and cable terminations are expected to cost 4 million and contribute 1% of the total project cost outlay. Route clearance is an activity which takes place before the cable laying process to clear debris in the cable installation route corridors. Cable termination relates to the process of electrically terminating the cable at each substation platform into the switchgear unit. MDR0707Rp Rev F01

46 7.6 CONSTRUCTION SCENARIOS The cost model allows a range of cost scenarios to be explored by altering the model variables. This enables the cost implications of differing construction strategies and circumstances to be examined. The following construction scenarios have been considered: Proportion of the cable that can be successfully buried. The case of 50% cable burial has been considered as the base case. However, without undertaking geotechnical site surveys and an in depth burial assessment study of the proposed route, it is not certain that this can be achieved in practice. The cost model has been modified to reflect the cases as shown in Table 4, with any remaining cable protection being provided by rock dumping. Table 4: Cable protection scenarios Scenario Burial Rock Dumping 1 100% 0% 2 75 % 25 % 3 (Base Case) 50 % 50 % 4 25 % 75% 5 0% 100% The cost of 50% rock dumping of the cable equates to a cost of 107 million for 427 kilometres. This is reflected in scenario 3 of Table 4 where the contribution from cable lay and burial is 260 million. This outlines the total cost of protecting the bipolar cable laid over the route length of 854 km. Table 5: Cost outcome for different protection methods Scenario Cable protection by burial (plough) Cable protection by rock dumping Total Cost (Million) % 0 % % 25 % (Base Case) 50 % 50 % % 75 % % 100 % 444 Examination of Table 5 indicates that if 75% trenching can be achieved in the ISLES region, the cost of installing the cable can be reduced by 40m, which is a reduction of 11% from the base case. By contrast, if we assume the region to only be 25% trenchable, then the cost will rise by 38m, or just over 10% form the base case. The range in the cost of the cable laying campaign between the best case (100% cable burial) and the worst case (100% rock dumping) is 155m. This clearly demonstrates how rock dumping, as an alternative strategy to cable burial, increases the project cost. This cost increase supports the case for a technological advancement in trenching techniques and detailed planning to maximise the length of cable that can be buried successfully in the seabed. MDR0707Rp Rev F01

47 7.6.2 Cost implication of the location of cable manufacturing facilities. As was shown in Figure 48, vessel in transit accounts for 20% of the offshore campaign, thereby making it an important element to be considered in more detail. The ISLES project is scheduled to be delivered circa 2020 with no definite cable supplier shortlisted for the project. There are a number of HVDC cable manufacturers located in the EU region as shown in Figure 49. Nexan (Halden) ABB (Karlskrona) JDR (Hartlepool) NKT (Cologne) Prysmian (Arco Felice) Figure 49: Offshore Cable Manufacturers and Supply Route To investigate the transit cost implications, the furthest port of cable loading in Naples (Prysmian) and nearest available cable loading port in Hartlepool (JDR) have been considered for developing the costs as detailed in Tables 6 and 7. This gives an insight into the transit costs involved and their effect on the overall cost estimate. This in turn leads to the question of whether savings in vessel transit costs could provide economic justification for building a new cable factory local to the ISLES region. Table 6: Naples cable load out costs DP cable lay ship with 1800m3 5000t cable storage capacity Transits between ISLES base port and Arco Felice, Naples (Prysmian cables) Cable Type Total length Carousel Capacity No. of Loadings Cost of one load out Cost 500 MW 1242km 125km m 63m 1000 MW 484km 80km 6 5m 30m Total Cost: 93m MDR0707Rp Rev F01

48 Table 7: Hartlepool cable load out costs DP cable lay ship with 1800m3 5000t cable storage capacity Transits between ISLES base port and Hartlepool, UK, (JDR cables) Cable Type Total length Carousel Capacity No. of Loadings Cost of one load out Cost 500 MW 1242km 125km m 42m 1000 MW 484km 80km 6 2.9m 17.4m Total Cost: 59.4m The distances to Naples and Hartlepool from a base port in the ISLES region are approximately 2265 NM and 552 NM respectively. Comparing the total load out cost from these two cable manufacturers yields a cost difference of 33.6 million. This demonstrates that the location of the cable manufacturer s load out facility has a significant impact on the cost of the project. In the Technology Roadmap Report, the cost of building a new cable manufacturing facility in the ISLES region was estimated to be 34m. Compared to the value of the contract for supplying the quantity of cable required for this project, this is a relatively small sum and may be considered, by a cable manufacturer, to be a worthwhile investment if it ensures that this contract is secured by their company. Consideration should be given to the fact that, while the transit cost saving to the ISLES project of procuring cable from a more local manufacturer is significant, a relatively small percentage change in the purchase cost of the cable could easily cancel this saving out. It is therefore entirely possible that a more distant manufacturer, such as Prysmian, may prefer to reduce their margin slightly to be cost competitive, rather than taking the potentially more risky decision to build a new facility. It should also be noted that a project of ISLES scale may be completed in several phases. This could weaken the perceived Return On Investment (ROI) potential for any new manufacturing facility established for ISLES alone. This, combined with the relatively modest savings to be made in transits compared to cable factories on the North Sea (estimated to be approximately 3m), means the proposition of building a new factory may only be attractive to a new entrant into the European market. From the point of view of the ISLES project s management, it is likely to be a policy decision whether to offer any incentive to a cable manufacturer, possibly a new entrant into the European market, to build a factory in the ISLES region. While any attempt to reduce transit costs is very sensitive to the cost of the cable, there are numerous potential benefits of a more local manufacturer. The first is sheer convenience and a reduction in the time required to complete the cable laying process. This could be particularly important if the availability of cable lay vessels becomes a constraint in the future. An associated advantage of this would be a reduction in the real terms cost impact of any increase in the vessel day rates which may occur in the event of increased demand for cable lay vessels. A second benefit occurs if a new entrant into the European HVDC cable market were to bid for the cable supply contract, based on a proposal to build a new factory in the region. This would lead to an increase in competition which would help to ensure a good price for the cable. This is especially important because of the large quantity of cable required for this project and the small number of suppliers. A third set of benefits would be associated with the legacy of this manufacturing site. A local cable manufacturer would be in an advantageous position to supply cable for future renewable offshore power generation projects in the area. While this may not be a direct benefit to the construction of ISLES, it would improve the chances of the network being well utilised and hence make it a more attractive proposition to an OFTO. Furthermore, a range of associated services and expertise may naturally build up in the area, offering the chance to develop a local hub of knowledge and skills of great value to any future offshore projects in the area. MDR0707Rp Rev F01

49 It has been suggested that another possibility for Prysmian, or other cable manufacturers currently located further afield, would be to establish a cable storage facility local to the ISLES region. This may allow the cable manufacturer to compete more strongly with cable factories already located in Northern Europe. A cable storage facility would also allow stock levels of cable to be produced in order to prevent the slow production rate at the factory from being a construction bottleneck. However, the cost of transporting the cable to such a storage facility must also be considered which will greatly diminish the appeal of such an option. It should also be noted that any investment from a cable manufacturer in such a facility would undoubtedly require some form of financial commitment from the project developers in order to provide the necessary incentive and commercial security. 7.7 COMPARISON OF COSTS FOR CABLE COLLECTION USING A CHEAP TRANSPORT BARGE COMPARED TO A CABLE LAY SHIP The previous section shows that cable collection accounts for 20% of the cable lay and trenching campaign cost. In this section, rather than using a DP cable lay vessel, we consider using a barge towed by two tugs to collect the cable from the factory and to transport it back to the ISLES region. As shown in Table 8, using this method the cost of cable from Naples is reduced by over 26 million from the figure of 92 million given in Table 6. This is nearly 8 million more expensive than the cost of collecting the cable from Hartlepool by DP vessel as detailed in Table 7. The combined market day rate for a large transport barge with twin tugs and cable loadout crew of 60 k is much reduced in comparison to the DP vessel day rate but transit speed by barge is slower. Additionally, using a barge as a support vessel for transit of cable load out from a cable factory requires the cable to be transferred to the DP cable lay ship in the ISLES region. This re-spooling onto the DP vessel, combined with the slower speed of cable spooling to a barge at the factory, leads to 34 days of additional cable spooling. This results in an additional cost of 2 million for each cable loadout. This scenario provides a viable alternative to the DP vessel in terms of cable loadout and transit from the cable factory to the ISLES port and it may help be used as an intermediate cable storage strategy to reduce production bottlenecks, however it involves double spooling and this is unattractive since the risk of cable damage will be higher than single spooling. Table 8: Transit cost from Naples using a barge Cable type Total length Cost of tugs and barge per day One cable collection and transit (Days) Cost per cable collection and transit Total cost 500 MW 1242km (10 loading) 60, m 38.4m 1000 MW 484km (6 loading) 60, m 16.2m Mob/Demob Cost 2m Capital Expenditure on cable carousel 10m Total Cost: 66.6m MDR0707Rp Rev F01

50 7.8 FURTHER ANALYSIS OF THE COSTS FOR LAY AND BURIAL OF BIPOLAR CABLES IN A SINGLE TRENCH Separate lay and burial As shown in Figure 48, as a proportion of the 260 million cable lay and trenching campaign, trenching and laying account for 9% and 22% of the cost respectively. As previously stated, a key factor that needs to be taken into consideration is that this costing has been developed for separate cable laying and trenching operations. The next section will consider the cost implications of using a simultaneous cable lay and burial method. Table 9: Costs for separate laying and trenching of bundled bi-pole cable in a single trench. Single trench and bundled bipolar cable Component cost summary for Northern ISLES network Duration (Days) Proportion Survey % Vessel in port % Vessel in transit % Vessel trenching 105 9% Weather allowance % Laying operations % Total Duration 1216 Days Total Cost 260m Simultaneous lay and burial If the construction of the ISLES network were to be carried out using a simultaneous cable lay and burial operation this could offer a cost saving, due to a reduced number of days offshore. Although there are a number of projects that have been constructed using this method, there are certain technical challenges associated with this approach as discussed in section Figure 50: Simultaneous lay and burial MDR0707Rp Rev F01

51 At 230 million (Table 10), simultaneous lay and burial construction has been estimated to cost approximately 30 million less than the 260 million estimated for use of a separate lay and burial construction method (Table 9). Table 10: Costs for simultaneous lay and burial Single trench and bundled bipolar cable Summary for cost schedule of Northern ISLES Duration (Days) Proportion Survey % Vessel in port % Vessel in transit % Weather allowance % Simultaneous cable lay and trenching % Total Duration 1081 Days Total Cost 230m Pre-cutting a trench using a pipeline plough A third construction method which has been considered is to pre-cut a V-trench using a pipeline plough and to lay the cable into the trench afterwards. A separate pass with a backfill plough would return the displaced soil on top of the cable, or for any unburied areas, the cable can be directly protected by rock dumping. The advantage of this approach is to carry out the aggressive trench cutting without the cable being present. This offers a low risk solution compared to trenching bundled cable with a cable plough. Calculations indicate that this would increase the cost compared to separate lay and burial by approximately 15 million. This represents an increase of 2% on the cost of separate lay and burial. By comparison with the costs for simultaneous lay and burial, this is an increase of over 35 million which is an approximate increase of 15%. Table 11: Costs for separate lay into a pre cut trench with pipeline plough Single trench and bundled bipolar cable Trenching speed: 500 m/hr, Backfill speed: 250 m /hr Duration (Days) % Proportion Survey 122 9% Vessel in port % Vessel in transit % Vessel trenching (1 pass to cut trench, 1 pass to backfill) % Weather allowance % Vessel for cable lay % Total duration 1353 Total Cost 266m MDR0707Rp Rev F01

52 It should be noted that the speed considered for the bipolar cable lay operation has been conservatively estimated as 200 m/hr for cable lay and 175 m/hr for cable burial respectively. It is important to note that this is the total average lay rate inclusive of vessel downtime for maintenance etc. This is considered by installation contractors to be approximately half the speed typically achievable for cable burial and installation of a single cable in a single trench. This assumption has the effect of adding an extra 10% to the cost of the offshore campaign. These results lead to the following conclusions: Technology limitations and operational risks associated with achieving simultaneous lay and burial for bi-polar cable in a single trench is mitigated by a low burial speed. This leads to a high installed cost. Therefore, it could be argued that a business case exists for a cable laying company to work towards new technology and innovations in burial machines for bundled cable installation in order to gain a competitive advantage over their rivals. This advantage would be due to an increase in the burial speed and hence a reduction in the cost of their method of construction. Another argument which arises from the above discussion is the alternative approach of burying two cables in separate trenches, which can improve the power transmission efficiency. However, analysis and input from industry suggest this method would be costly in comparison to single trench installation. Taking into account the broad factors of exposure to environmental conditions, additional route clearance and varied seabed conditions, the viability of this strategy is questionable and the overall construction risk is deemed to be higher. MDR0707Rp Rev F01

53 7.9 IMPACT OF A VESSEL S CABLE LAY CAPACITY The cost estimate for the ISLES offshore cable laying campaign has been based on the available cable storage capacity of the cable laying vessel being 5,000 tonnes/1,800 m 3. This choice is based on this being the largest capacity of storage widely available in the market. This capacity storage implies an upper limit on the length of HVDC cable of 125 km for a single link while in bipolar cable configuration it leads to a maximum link length of 62.5 km cable. See Figure 51. Figure t cable carousel on an offshore vessel, laying HVDC bipole bundled cable It is estimated that in the time frame of the construction of integrated electrical transmission activity similar to ISLES Project, such as Round 3 North Sea wind farms, could increase the demand to 5-10 installation vessels for export and array power cable laying. [14] This future demand for power cable installation vessels naturally leads to two questions: 1. Would utilising a larger DP vessel with a 4,000 m 3 / 10,000 tonnes cable storage capacity drive down installation costs? The quantity of jobs available for a vessel of this size may be limited, and future vessel availability may become a constraint. 2. Would utilising a smaller DP vessel with 1000 m 3 / 3000 tonnes cable storage capacity drive down costs? The market for a vessel of this size is already buoyant, for both the telecoms and oil and gas sectors. There are some technical issues to be addressed with some of these vessels in order to make them capable of laying the type of power cables required for ISLES, notably bend radius limits and whether cables are coilable or not. Non-coilable cable would require a rotating turntable rather than a static cable tank. MDR0707Rp Rev F01

54 The cost model has been based on cable installation by DP vessels available in the market with cable storage capacity of 1800 m 3 / 5000t. These vessels have completed some major subsea power cable laying projects including Britned and Norned, covering subsea distances of 494 km and 500 km respectively to form a mono link between two points. New build cable lay vessel with 10,000 t cable capacity The majority of cable lay vessels available have been built for offshore oil and gas or telecommunications cable lay operations. There are comparatively very few large vessels in operation dedicated to large subsea power cable laying projects. This leads to the question of whether industry investment in the manufacture of power cable laying vessels with a carousel capacity of 4000 m 3 / t is warranted. Such a vessel would be a market leader in terms of the length of cable that can be loaded on to it. For current state of the art vessels, this length is limited to about 62.5 km for bi-pole power cable of maximum outer diameter of 145 mm. A 4000 m 3 / t vessel could lay double this length of cable route distances, cutting the number of transits to and from the cable factory and decreasing the number of cable joints. The tentative cost schedule using such vessel for ISLES has been developed as shown in Table 12. This assumes parallel upgrade of shore side cable storage and load out equipment at manufacturing sites. Table 12: Cost of the project using cable lay vessels with 4000m 3 / 10,000t Carousel Day rate for high spec 10,000t cable lay and trenching DP vessel: 185k / day inclusive of: WROV spread inc crew Online and offline survey crew Back deck crew Cable lay crew Plough / Trencher crew Navigation / comms data costs Item Construction Element Cost Outlay Proportion Remarks 1 Route Clearance 2.2m 0.6% 2 Lay & Trenching (50%) and Transits 254m 66.7% Inclusive of 30% bad weather allowance 3 Jointing 5.5m 1.5% 4 Rock Dumping (1) - Cable (50%) 107m 28.1% 5 Rock Dumping (2) - at fixed locations 5m 1.3% 6 Shore Landing (5 landfalls) 5m 1.3% 7 Cable terminations 2m 0.5% Total Cost: 381m MDR0707Rp Rev F01

55 Comparison of Tables 12 and 3 suggests that it is reasonable to assume that ISLES, along with Round 3 windfarm development, makes a sound case for constructing a new build, market leading cable lay vessel of 10,000t capacity. This conclusion is based on the strategic advantage of bringing to the market a vessel capable of installing a network which requires fewer cable joints and which therefore gives improved reliability and availability over the life of the network. The cost difference for installing the network using such a vessel rather than the base case vessel, estimated to be a saving of 11m, isn t a sufficient reason on its own. It should also be noted that the ISLES project is large, but is not large enough to provide the required ROI for a new vessel of this size in isolation. Existing cable lay vessels with 2,500 t cable capacity The economics of using vessels has been investigated, since there are significantly larger numbers of vessels available with a cable tank or turntable of 1000 m 3 / 2500 t capacity. The downside is that only half the cable length can be accommodated when compared to a vessel with a 5000 t cable capacity. It would therefore require twice the number of cable loadouts from the production facility and twice the number of cable joints. The anticipated costs of the project based on the use of these smaller vessels are detailed in Table 13. The standard industry day rate for these vessels is circa 75,000/day. The number of days associated with other elements of the project remains the same, as indicated in Table 13. The total cost outlay for the lay and trenching process is 244 million which is 16m less in comparison to the cost outlay with currently available 5000 tonne vessels. The number of cable joints is assumed to double from the base case of the 5000t capacity system, thus increasing the cable jointing costs. Overall the total cost for project is reduced by 4.5m to million. This approach is not without its own technical and operational issues in terms of reliability risks associated with the increase in cable joints. Table 13: Cost using cable lay vessel with 2,500t Carousel Day rate for high spec 2,500t cable lay and trenching DP vessel: 155k / day Item Construction Element Cost Outlay Proportion Remarks 1 Route Clearance 2.2m 0.6% 2 Lay & Trenching (50%) and Transits 244m 62.9% Inclusive of 30% bad weather allowance 3 Jointing 22.3m 5.8% 4 Rock Dumping (1) - Cable (50%) 107m 27.6% 5 Rock Dumping (2) - at fixed locations 5m 1.3% 6 Shore Landing (5 landfalls) 5m 1.3% 7 Cable terminations 2m 0.5% Total Cost: 387.5m MDR0707Rp Rev F01

56 8 CONSTRUCTION AND INSTALLATION TECHNOLOGY ISSUES 8.1 CURRENT CONSTRAINTS REGARDING OFFSHORE CONSTRUCTION AND MAINTENANCE METOCEAN CONDITIONS Tidal flows Accurate cable lay is challenging in areas with high tidal currents and energetic sea states. For the North Irish Sea and the western isles of Scotland, this results in a short summer season for offshore pre-construction survey and construction work. Of particular note, survey vessels, ROV systems along with cable lay and trenching operations may need to stop and start during peak tidal flows around the Mull of Kintyre and Rathlin Island due to tidal currents and overfalls which are outside the equipment operational limits. Please see Appendix D and also Figure 52 below for information on the tidal streams in the region. Figure 52. Tidal Currents in the ISLES region Cable burial can be very difficult or impossible in areas which feature high tidal current speeds. This problem is caused by the frequent presence of an exposed rocky seabed in these areas and the constant movement of sediment caused by such powerful flows. MDR0707Rp Rev F01

57 The cook straight in between the North and South Islands of New Zealand is an example of this (Fig 53). Here, cable protection relies on a regularly patrolled fishing exclusion zone. Even so, fibre optic cables were displaced by illegal fishing prior to full time boat patrols of the zone, when such incidents ceased. Reproduced from [5] Figure 53. Cables laid in the Cook Strait between the North and South Island of New Zealand, arrows indicate where cables have been displaced. Wave heights Significant wave heights in the North Atlantic are expected to exceed 2m for approximately 50% of the time and 3.5m for 30% of the time. A wave scatter diagram is located in Appendix D. Wave periods of between 8 and 10 seconds excite the natural frequency of heavy lift vessels and should be avoided during the installation of platforms. These wave periods are present 50% of the time in the North Atlantic. Conclusion for ISLES For the ISLES project, the metocean conditions in the region are likely to significantly reduce the offshore construction season. Local tidal effects such as those around the Mull of Kintyre will make cable lay and burial very challenging and may require new techniques or equipment to be developed to improve the process. It is anticipated that between 20% and 40% of project construction days for the offshore campaign will be spent waiting for suitable weather conditions. This cost will need to be taken into account by the project developers. MDR0707Rp Rev F01

58 8.2 REVIEW OF TECHNOLOGY LIMITS The following table highlights the limits of the main construction equipment and vessels used in the offshore cable installation process. Table 14. Technology limits Component SURVEY Survey ship speed of side scan survey Survey ship limiting sea state Limits The speed of survey vessel is limited to 4 knots. Assuming 6 passes on the route corridors, the geophysical survey would take about 40 consecutive days for the 1000km route (24hr working). Considering a bad weather allowance of 30%, this survey may take 4 months to carry out. Wave height: survey work can take place up to 2m significant wave height* before data quality deteriorates. *Average height of highest 33% of waves 2m wave height is exceeded 50% of the time in the North Atlantic (Appendix 3) JACK UP VESSEL Jack Up foundations seabed soils strength HEAVY LIFT VESSEL Heavy lift vessel limiting sea state CABLE FACTORY Cable production factory load out speed Cable storage capacity DP CABLE SHIP Cable lay ship cable storage capacity Cable lay ship speed of lay Cable lay ship limiting sea state for cable lay CABLE JOINTING Cable jointing sea state Sea state 1-2 Cable jointing temperature Above 5 C Cable jointing wind speed 12m/s CABLE LAY BARGE Speed of cable lay ROV Tidal current which ROV can operate DIVING OPERATIONS Where the soil is extremely hard or calcareous. In these cases, the penetration of the spud-can will be minimal allowing only a portion of the spud-can bottom plate to be in contact with the seabed. In this condition, only that part of the spud-can structure in contact with the soil will be supporting the environmental loads, deadweight and operational weight of the Jack Up. It is extremely important to verify that such partial bearing will not cause damage to the spud-can structure. In cases like these, an adequately reinforced tip on the spud-can may be advantageous compared to flatbottomed footings. Wave periods of between 8 and 10 seconds can excite the natural frequency of the vessel and should be avoided. These wave periods are present 50% of the time in the North Atlantic Speed of loadout generally limited by the visual inspection process required to ensure that the power cable is in good condition t 3000t, Many ships available 5000t Various ships available 7000t Few ships available 1000 m/hr Sea state 5 Significant wave height 4m 200m/hr 2 knots (1 m/s) Water depth for air diving operations 40-50m Maximum on bottom time due to tidal mins currents > 0.5 knots TRENCHING VESSEL Trenching vessel speed Up to 1000m/hr ploughing Up to 200m/hr trenching machine MDR0707Rp Rev F01

59 Trenching vessel limiting sea state Trencher can be launched and recovered up to sea state 5. Plough can be launched and recovered in sea state 7. SUBSEA CABLE PLOUGH or TRENCHER Near shore cable burial / beach landing Plough can be brought through air water interface and can be used to plough straight from the beach. Trenching machines require a positive head of about 10m of water above the jetting pumps, so have a shallow water limitation of a minimum of 15m water depth to operate. Seabead topography which limits operations Typical working soil undrainded shear strength Slopes which are more than 15 0 and / or side slopes exceeding 10 0 Must be >5 kpa and < 100 kpa ROCK DUMPING VESSEL Side dumping vessel capacity 2000t. Side dumping rock size 100mm to 1m. Fall pipe vessel payload capacity 20,000t. Fall pipe rock size Up to a maximum of 400mm diameter. MDR0707Rp Rev F01

60 8.3 DEPTH OF BURIAL REQUIREMENTS AND ITS EFFECT ON CONSTRUCTION SPEED The depth to which anchors or fishing gear can penetrate the seabed depends on the soil type and cable burial is the primary protection strategy against this threat. The burial depth and soil type define the trenching speed at the offshore site and hence, the number of vessel days required to install the cable is calculated from these figures. As such, the accurate selection of the burial depth is a critical factor which greatly influences the cost of cable laying and affects the long term maintenance needs and costs. Industry has widely accepted the adoption of the Burial Protection Index as a common methodology to quantify the required depth of burial of a cable or pipeline in a range of soil types, subjected to a range of threats. For a more detailed discussion of selection of burial depth, burial equipment and geotechnical aspects of burial, please refer to [34] and [35] Table 15 below provides an indication of the typical burial depths in differing soils. In addition, the performance of burial machines in a variety of soil types has been indicated. Table 15. Depth of burial and its effect on construction speed Typical Burial Sea Bed Type Depths for power cables Typical burial speed Machine Notes Exposed Bed Rock 0.0 Untrenchable Untrenchable Untrenchable Chalk m/hr rock cutter Stiff Clay Plough 350 to 750m/hr Mechanical cutter 150 to 250 m/hr Hard soil trencher / (Plough) Hard soil trencher may achieve better burial where hard clay is at seabed Clay Plough 250 to 750m/hr Mechanical cutter 150 to 250 m/hr Hard soil trencher / (Plough) Hard soil trencher may achieve better burial where hard clay is at seabed Gravel Plough 200 to 500m/hr Plough / Trencher Coarse Sand Plough 200 to 500m/hr Jetter 100 to 300m/hr Plough / Trencher Trencher not as efficient as plough in sand Silty Sand Plough 50 to 150m/hr Jetter 100 to 400m/hr Plough / Jetting ROV Sand Waves Subject to soil conditions and max slope in the order of 10 to 15 Plough Intertidal Mud Flats Subject to bearing capacity of seabed, currents and water depth Plough / Jetting ROV Beach Sand Subject to plough method and availability of tow force Plough The choice of cable burial tool is defined by the geotechnical characteristics of the seabed and the depth of burial capabilities of the machine. Figure 54 provides an indication of the capabilities of various burial machine types. MDR0707Rp Rev F01

61 For the ISLES project, proper selection of burial machine type cannot be made until the results of geotechnical site surveys are carried out. Figure 54, table courtesy of CTC marine projects 8.4 PROTECTION STRATEGY FOR CABLES BASED ON HAZARD IDENTIFICATION AND SITE SPECIFIC FACTORS The subsea cable protection strategy for the ISLES network will strongly influence the cost of construction activities. The cable needs to be protected from the risk of damage due to various offshore activities like fishing, anchorage or other natural hazard. It is most economical to bury the cable into the seabed where possible to a depth which protects the cable. This burial depth is therefore a key installation design parameter. Cable protection by burial in the seabed has developed from the 1970 s as a viable cable protection strategy. This approach has led to decline in cable faults 3.7 / 1000 km / year in 1979 to 0.44 / 1000 km / year in 1985 and moreover it significantly reduces the cost of additional steel armouring by the cable manufacturer. [15] Where it is not possible to bury the cable other methods, such as rock dumping, concrete mattressing, or cable pinning with grouted anchor bolts, can be carried out as part of the offshore construction campaign. Also, additional steel armouring layers can be incorporated into the manufacture of the cable to further enhance protection if required in rocky areas but this increases costs. The seabed type, strength and topography in the ISLES region will dictate how much of the cable length can be successfully buried. Detailed geotechnical seabed surveys would be carried out during the pre-construction phase to identify where the sea bed is rocky or hard, or where steep inclines may prevent direct burial by a trenching machine. The final choice of cable protection strategy for each section of cable would then be determined by geotechnical specialists with the chosen protection method being based on hazard identification and site specific factors. The following sections define these hazards in general and discuss the common protection strategies that are employed to mitigate their risks. Additionally, the ISLES site specific hazards and protective measures are noted. MDR0707Rp Rev F01

62 This report considers the following hazards: Fishing Anchors Dredging activities Seabed topography Dropped objects Figure 55 highlights the main causes of submarine cable faults and shows that a high proportion of damage is thought to be caused by fishing activities. It should be noted that these proportions have more recently been modified by observations from members of the ICPC (International Cable Protection Committee) and their figures are shown in Figure 56. This more recent finding states that anchor damage is now understood to be a far more frequent cause of damage than thought in earlier years. This conclusion was the result of the use of a more sophisticated approach to monitoring vessel traffic data at the time of a fault occurring on a subsea cable. Figure 55. Trends in submarine cable system faults. Reproduced from Reference [16] Figure 56. Trends in subsea cable damage as modified by use of AIS observation in ICPC Fishing Description of risk to cable Fishing by the trawling method is still considered to be a major contributing factor to subsea cable damage. Specifically, the trawl boards tend to run along the surface of the seabed and can dig in to a depth which depends on the type of fishing gear used and the seabed soil characteristics (Fig 57). Fishing weights can also snag on exposed subsea cables. MDR0707Rp Rev F01

63 Figure 57. Schematic of beam trawler and damage to a snagged cable. (Images courtesy of KIS-CA) Fish bites are less frequent, but remain an occasional source of natural damage to exposed cables which can lead to the plastic or bitumous outer serving being compromised, exposing the inner sheath and allowing water to penetrate the steel armouring. This can leave the cable vulnerable to corrosion. Cable protection strategy To overcome the threat from fishing, burial to an appropriate depth is considered to be the primary protection. If burial to the required depth is not possible, then rock dumping is usually considered a viable option which can prevent the penetration of the trawler board or deflect the hit by the trawler, thereby protecting the subsea power cable. By creating a rock berm to protect the cable from fishing activities the rock installation contractor should make provision for: Depositing enough rock to sufficiently withstand the penetration depth of the trawl board. Creating gradual slopes in order to allow the trawl board to pass over the berm without snagging Providing a suitable grade of rock (size of rock) to pass through fishing nets without damaging them. Rock dumping installation contractors include: Van Oord, Boskalis Westminster, Dredging International and Tideway. Implication for ISLES construction: Commercial fishing occurs throughout the ISLES region, therefore protection against damage is a primary design and installation requirement Interaction with dragged or dropped anchors Description of risk to cable Since 2007, it has been thought that the primary contributing factor causing cable damage is anchors either being dragged over, or dropped on, the power cable (Fig 58). MDR0707Rp Rev F01

64 Figure 58. Damage to subsea cables from dragged anchors. (ICPC) Figure 59. Rock berm designed to protect cable from anchors (Image courtesy of Van Oord NV) Vessels suffering engine failure or loss of steering may choose to drop anchor, or drag an anchor to slow down in an emergency situation. The likely effect of the dragged anchor will be worst if it occurs in stormy or inclement weather, with the possible result being the dragging of a partially penetrating anchor through several kilometres of seabed. The damage to a subsea cable can stretch to many hundreds of metres. Cable protection strategy This hazard is mitigated by burial of the cable to an appropriate depth, just as is the case for fishing. For cases where burial is not achieved as per requirements then rock dumping is again considered a viable option. Anchor penetration in different sea bed soil types can usually be predicted, leading to a cable burial depth requirement. Rock berms intended principally to protect against anchor strike would be designed in such a way as to lift the anchor out of the seabed before it can contact the cable. This leads to a high and wide rock berm, which would be created with a rock size which would encourage the anchor to walk over the berm as opposed to dragging through it. See figure 59. Implication for ISLES: Commercial shipping movement occurs throughout the ISLES region, therefore protection against damage is a primary design and installation requirement Dredging activities Description of risk to the cable Dredging for aggregates occurs in offshore areas where construction sand and gravel is easily obtainable. If subsea power cables are buried in regions where this practice occurs, it can be exposed or snagged due to the ongoing activity. This poses a serious threat to any buried cable. MDR0707Rp Rev F01

65 Shell fish dredging would also pose a similar risk to the cable. Cable protection strategy Ideally prevention of this activity offers the best protection for the cable. To overcome concerns of damage in such regions the subsea cable route should be planned taking dredging activities into consideration. Concerned authorities will also need to issue formal restrictions against dredging activities in the vicinity of the cable route. Implication for ISLES: No dredging for aggregates currently takes place in the ISLES region Exposure of cable due to seabed variations and mobile seabeds Description of risk to the cable Seabed variations are caused by naturally occurring phenomenon, such as wave and tide action, which cause the movement of sediments from one area to another. This sediment mobility either adds to or removes any soil covering the product laid in that area. This action can create large areas of sand ripples or sand waves (Fig 60) to a height of several meters, and can lead to the movement of dunes across the seabed, thereby potentially exposing and re-burying sections of the subsea cables in the process. Any exposed sections of cables would then be prone to additional risk of damage from flow induced vibrations, and over tension. A common failure regime would then be fatigue failure of the cable. Figure 60. Power cable spanning sand waves (reproduced by permission of KIS-CA) Cable protection strategy Burial of the cable to a depth where the underlying seabed is stable would provide an adequate protection strategy in a mobile seabed. However, this depth may be impractical for certain types of burial machine. Analysis would need to be undertaken to properly determine the long term depth of scour. In addition, the sandwave area could be flattened by pre-sweeping the cable route Depending on the nature of the flow regime, and flow speed, the use of rock or gravel dumping may locally stabilise the seabed and be used to bury the cable. MDR0707Rp Rev F01

66 Figure 61. Rock berm designed to support a freespan Image courtesy Van Oord NV Figure 62. Illustration of dropped object on cable Image courtesy Van Oord NV Implication for ISLES: No major sandwave areas are found in the ISLES region. Undulating seabed topography leading to unsupported cables would need to be addressed if encountered during the route survey Other dropped objects The development of the ISLES region to harness the indigenous renewable resource will require a multitude of offshore construction vessels to build the infrastructure. Dropped objects from heavy lift vessels, cable lay ships and survey vessels all pose a threat to the subsea cable. Dropped objects from commercial shipping lines, or fishing vessels operating in the same waters would remain an ongoing risk. MDR0707Rp Rev F01

67 8.5 PROJECT RISK IDENTIFICATION The offshore construction and deployment of the ISLES offshore network requires a huge capital investment which would involve placing many contracts into a complex supply chain. To deliver a project of this nature, the main contractor needs to identify, assess, and manage the project risk in order to achieve both a commercial and operational success. A summary of the risks inherent in an offshore cable installation project are captured in Table 16 below: Table 16. Types and sources of subsea cable installation risks Type of Risk Sources Mitigation Generic Poor design and pre survey report. Poor geotechnical report. Operational Unreliable operations and maintenance team Poor machinery and equipment installation Poor technical feasibility reports and design Depth of lowering of the cable within the trench not achieved in certain places Commercial Wrong financial assumptions Major terms not included in contract Variation according to time and economic condition not provided. Poor feasibility study Employ experienced consultants to carry out comprehensive review of design, geotechnical and pre-survey reports. O&M agreements must have benefits or benefit reduction on O&M company Trenching specification needs to be carefully written. Most contractors only work on a reasonable endeavours basis, i.e. no contractual agreement usually exists to bury the cable. Consult financial consultant for verifying financial report of the project. Compare contract with proven contract Consult legal advisor familiar with industry in drafting contract. MDR0707Rp Rev F01

68 Seabed Conditions It is usual for the cable lay contractor and trenching contractor to base their price and equipment selection on the bathymetric and soil condition data or trenchability statement developed by their client company. This information is provided by the pre-trenching survey of the cable installation route. This information is used by the cable lay contractor and trenching contractor to select the equipment and burial depth, and to estimate the performance of the equipment. Therefore, if the actual trenching operations are affected due to the deviations in the information present in the preliminary trenching assessment, it would be usual for the contractor to charge their client for any delay caused at an appropriate vessel day rate. To minimise such risks, the pre-trenching survey needs to be comprehensive. Also, the offshore scope of work, along with the commercial arrangements, should be detailed enough to include provision for delays or deviations to allow these to be quantified appropriately. Cable Protection / Cable Crossings Where threats are present, an unburied cable remains at risk of failure. Similarly, protection is necessary for any cable crossings on the route. Protection by rock dumping or by the placing of concrete mattresses would normally be carried out after the trenching campaign and after review of the post installation survey. In order to minimise the risk to unprotected cables, the main contractor should schedule this work to immediately follow on from the main cable lay campaign. Equipment Trials Trials for vessels and trenching equipment should be budgeted for by the OFTO if required. This is often overlooked. Trenching and Cable Lay Equipment The proper working geometry and operation of the equipment needs to be checked regularly during the offshore cable laying and trenching campaign. This should be recorded by the offshore technicians and accepted by the client representative. Sea Bed Slopes The angle of seabed slope for some sections of the route may be outside the capabilities of the trencher, In these instances the cable may need to be buried by a different device or protected by rock dumping. MDR0707Rp Rev F01

69 8.6 ADVANCEMENTS AND INNOVATION IN CONSTRUCTION TECHNIQUES: UPGRADE THE TECHNOLOGY TO SPEED UP LOAD OUT, LAY AND TRENCHING Cable loading speed at the cable factory. An area of significant time and hence cost expenditure occurs when all of the cable contained on the vessel has been laid and the vessel must return to the cable manufacturer to reload its turntable. In the case of Prysmian in Naples, it is estimated that the downtime in laying operations can be between days with approximately 9-10 of those days required to re-spool the turntable at the manufacturer s site. In the re-spooling process, cable is transferred from one turntable on the quayside at the manufacturing site to the empty turntable on the vessel. Cable is visually inspected by workers as it is fed onto the vessel. Cable transfer time could be decreased by increasing the spooling speed. However, if spooling speed becomes too fast then inspection quality may be compromised. It may be worthwhile to investigate the use of an automatic defect monitoring video camera system. Another option to reduce down-time caused by spooling could be to allow cable baskets to be interchangeable. For example, an empty basket could be lifted from the vessel deck in dry dock by an overhead crane (Fig 63). A pre-spooled basket could then be lifted onto the vessel to replace the empty one. This is likely to require significant investment in order to construct the dry dock and overhead crane needed to allow such an operation. Another factor which needs to be considered is the ownership of the basket and at what point liability for damage to cable or basket transfers from the cable manufacturer to the vessel operator. For example, if the cable was damaged during spooling from the factory due to a fault with the basket, it becomes unclear who would be liable for the cost of repair. Should it be the owner of the basket (the vessel operator) whose property was at fault or the company who were responsible for operation of the basket at the time (the cable company)? The speed of cable laying, spooling and trenching have a considerable impact on the costing of the overall project which supports the case for upgrading current equipment technology. The various speed scenarios have been highlighted in Table 17. This can be compared to the basis of the current cost structure detailed in Tables 3 and 4. Table 17: Operating speed scenarios Activity units Slow operations case (Base case) Fast operations case Equipment upgrade case Bundled cable lay speed (m/hr) Bundled cable trenching speed (m/hr) Spooling speed (m/hr) The slow operations case led to the overall cost of cable laying, spooling and trenching being 260 million which is 22% and 33% more in comparison to the fast operation and equipment upgrade cases, as shown in Tables 18 and 19 respectively. This again points to the possibility that a company could gain a competitive advantage by working towards increasing the speed of cable installation as this can lead to considerable savings on projects of ISLES scale. MDR0707Rp Rev F01

70 Table 18: Fast operating speed and its effect on cost outlay Single trench and bundled bipolar cable Cable lay speed = 400 m/ hr Trenching speed = 350 m/hr Spooling speed = 600 m/hr Duration (Days) Proportion Survey % Vessel in port % Vessel in transit % Vessel trenching 57 6% Weather Allowance % Vessel on DP ops % Total Duration 935 Days Total Cost 203m Table 19: Equipment upgrade speed and its effect on cost outlay Single Trench and Bipolar Cable Cable lay speed = 1200 m/ hr Trenching speed = 800 m/hr Spooling speed = 1000 m/hr Days % Proportion Survey % Vessel in port 86 11% Vessel in transit % Vessel trenching 30 4% Weather Allowance % Vessel on DP ops % Total Duration 783 Total Cost 173m MDR0707Rp Rev F01

71 Figure 63. Quick turntable change over of pre-spooled cable MDR0707Rp Rev F01

72 1 APPENDIX A ROUTE DESCRIPTION Admiralty Charts reproduced by permission of the Controller of Her Majesty s Stationary Office and the UK Hydrographic Office [17] Coleraine Hub to Hunterston (2 x 1000MW HVDC Cables) Section Conditions Overview Water depths vary from 26m at Coleraine to 120m in the North Channel to the west of the Mull of Kintyre. In the Firth of Clyde, the depths range from 60 to 40 metres until landfall at Farland Head. Landfall at Harland head was chosen as there are two power cable landfalls in this area Landfall should take place between Farland Head and Ardrossan. The route was chosen to keep clear of the Hibernia A telecom cable. Between the Hub and the Mull of Kintyre the seabed is predominately gravel, mud, shells, pebbles and sand with occasional rock outcrops. South and East of the Mull of Kintyre the seabed is gravels, sand, shells with weeds and kelp, becoming mud and sand in the Firth of Clyde. 67 Rev F01

73 Section Conditions 1 to 5 The chosen site for the Coleraine Hub lies in an area of rocks and overfalls. The seabed appears to be predominately gravels, mud, shells, pebbles and sands in the deeper sections below the 50m contour. The shallower depths comprise of rock outcrops, requiring a degree of armouring for the HVDC cables. Careful routing will be required in the vicinity of Point 4 due to the steepness of the cross slope. 68 Rev F01

74 Section Conditions 5 to 9 Water depths vary from 60 metres at Point 5 to 120 metres in the North Channel, shallowing again to 45 metres in the Firth of Clyde. The seabed soils vary from sand, shells and pebbles near Point 5 to mud, fine sand and shells in the deeper parts of the North Channel. As the seabed shallows, the soils composition becomes gravelly sand and shells with kelp and weed south of Sanda Island (Point 8). There are significant overfalls between Points 6 and 8, indicative of a rocky seabed and strong currents. Between Points 8 and 9, there is little seabed information other than indications of sand and mud with isolated rock outcrops. 69 Rev F01

75 Section Conditions 9 to Farland Head Between Points 9 and 10, the water depths lie between 45 and 52 metres, with a seabed changing from sands and muds with rock outcrops, to mud and muddy sands. The route in this Section crosses the Linis-3 and Sirius North Telecom cables in order to remain in reasonable water depths and avoid the 100m deep trench on the east side of the Isle of Arran Between Points 10 and 13, the route traverses a muddy sandy seabed whose depths vary between 50 and 60 metres. Occasional rock outcrops are expected. It is expected that the shoal areas less than 50m in depth are rock outcrops. The route re-crosses the Linis-3 and Sirius North telecom cables between Points 11 and 12. At Point 13 the route lies parallel to the proposed route of the Anglo-Scottish HVDC cable. The seabed conditions between Point 13 and the landfall are similar to the previous section with water depths ranging from 50 to 74 metres before shoaling at the landing. The soils become mud, pebbles and rocks closer to the shore. 70 Rev F01

76 Argyle Hub to Hunterston (500MW HVDC Cable) Section Conditions Overview Water depths vary from 20m at the Argyle Hub to 80m 5 miles south of Skerry Vore, rising to 26m at the Coleraine Hub. Thereafter the route parallels the Coleraine to Hunterston 1000MW power cable route with the same conditions Between the Argyle Hub and the Coleraine Hub, there is little in the way of soils information. The seabed appears to consist predominately of shells, sand, gravel, pebbles and mud in the north, becoming shelly sand and rock out crops in the south. For the remainder of the route see the conditions for the Coleraine to Hunterston 1000MW cable routing. 71 Rev F01

77 Section Conditions 1 to 3 The site chosen for the Argyll Hub lies in 10.5 metres of water in an area of rocks and overfalls. Stevenson s and Mackenzie s Rock lie close by. The cable between the hub and the 50m contour line will require armouring and protection. Below the 50m contour the seabed is predominately sand and shells, with rocky outcrops. Above the 50m contour in the vicinity of Point 2 the seabed is expected to be shelly sand with rock outcrops. Between Points 1 and 2 the depths vary from 86m south of Mackenzie s Rock to 50m at Point 2. Between Points 2 and 3 the depths vary between 38m at its shallowest to 50m at its deepest. 72 Rev F01

78 Section Conditions 3 to 5 Between points 3 and 4 the seabed rises from 55 metres at Point 3 to 24 metres at Point 4 (close to the proposed Islay Wind Farm location). In this section the nature of the seabed is expected to be rockier, possibly requiring the cable to be armoured and protected. The expected length of additional armouring is 10 kilometres. Between points 4 and 5, the seabed deepens to 65 metres before shallowing again at the Coleraine Hub. The seabed is expected to be shelly sands below the 50m contour and rocky above. Additional armouring may be required between the 50m contour and the Coleriane Hub, which amounts to an estimated length of 10.5 kilometres. 5 (Colleraine Hub) to Hunterston. The 500mW cable route parallels the Coleraine Hub to Hunterston 1000mW cable routes. The seabed conditions will be the same. 73 Rev F01

79 Argyle Hub to Coleraine Hub (500MW HVDC Cable) Section Conditions Overview Water depths vary from 20m at the Argyle Hub to 80m 5 miles south of Skerry Vore, rising to 26m at the Coleraine Hub. Between the Argyle Hub and the Coleraine Hub, there is little in the way of soil information. The seabed appears to consist predominately shells, sand, gravel, pebbles and mud in the north becoming shelly sand and rock outcrops in the south. The conditions along this route have already been described for Points 1 to 5 of the Argyle Hub to Hunterston 500mW HVDC cable route. 74 Rev F01

80 Location C OWF to Argyll Hub (500MW HVDC Cable) Overview Water depths vary from 20m at Location C to approximately 200m, 10NM south of Barra Head, rising to 20m at the Argyle Hub. The shallow areas at Location C and the Argyle Hub are predominately rocky. Between Barra Head and Skerry Vore, the seabed in the deeper water appears to be mud, sand and shells. 75 Rev F01

81 Section Conditions 1 to 4 Water depth at the proposed wind farm location is approximately 30m, with rocky outcrops in a shelly, sandy seabed. Between Location C and Point 2 the depths vary between 25 and 30m. The seabed is sand and shells with rock outcrops in the north becoming weed and shells with rock out crops at Point 2. The water depths vary from 30m to 53m between Point 2 & 3. The seabed becomes increasing rocky in the deeper water. The depths increase to the south west to 115m at Point 4. The cable may require armouring over this section between the location and Point 4, some 45.6 kilometres of cable. 76 Rev F01

82 Section Conditions 4 to Argyll Hub Between Points 4 and 5 the water depths increase to approximately 200m before shoaling again to 100m at Point 5. The nature of the seabed becomes mud, sand and shells, with patches of gravels. The route continues to shoal from 100m to 54m at Point 6. The seabed becomes increasingly rocky as shallow water is reached. At the Argyll Hub the seabed is a rocky area with frequent overfalls close to the hub location. Water depths varying from 10 to 20m. The cable may need armouring above the 50m contour for a length of 7.3 kilometres. The route passes through a laden tanker and radio reporting zone between Points 3 and Rev F01

83 Islay Wind Farm to Coleraine Hub (500MW HVDC Cable) Section Conditions Overview This short route has water depths ranging from 25m at each location to 70m, a mile or so south of the Islay Wind Farm. At the wind farm and hub locations the soil conditions are expected to be rocky with some sand and shells. The deeper water seabed is likely to be sands, gravels, pebbles and weeds (possibly kelp). The cable may require armouring in those areas above the 50m contour. 78 Rev F01

84 Coolkeeragh Hub to Coleraine Hub (500MW HVDC Cable) Section Conditions Overview Water depths vary from 36m at the Coolkeeragh Hub to 70m, 5 NM north of Turbot Bank. The greatest depth is 110m, some 3 NM south west of the Coleraine Hub, where the water depth is 25m. For the first half of the route the seabed is predominately rocks, gravels and fine sands with some mud and shells. For the remainder of the route, in the deeper waters to the north east of the Turbot Bank, the seabed appears to be weed, gravels, shells, pebbles and sands, becoming rocky at the Coleraine Hub. The route crosses the Hibernia A telecom Cable at approximately N W. 79 Rev F01

85 Section Conditions 1 to 3 The Coolkeeragh Hub lies in a Rocky area with a depth of 36 metres. The route shoals to 30m at Point 2, increasing in depth to 70m at Point 3. The seabed changes from being rocky in the west to sand, stones and shells in the east. The route crosses the Hibernia A submarine telephone cable just after Point 3. Armouring of the cable should be considered in depths of less than 50m. This would be a length of approximately 39 kilometres in this section. 80 Rev F01

86 Section Conditions 3 to Coleraine Hub Between Points 3 & 4 the depths vary between 57 and 70m. The last part of the section is along a ridge with depths of 121m on the north west side and 134m on the south east side. Alternative routing may be required to go around these trenches. Between Points 4 and 5 the depths shallow from 65m to 23m, with depth varying between 23 and 27m up to the Coleraine Hub. There are significant overfalls in between Point 6 and the Hub. Water depth at the hub is 26.5m In the west, the seabed is predominately gravels, shells, pebbles, and sand with weed (possibly kelp). The seabed becomes rocky above the 50m contour. The area between Point 5 and the Coleraine Hub suffers from overfalls. The cable may need to be armoured in depths of less than 50m, an estimated length of 6.5 kilometres 81 Rev F01

87 Coleraine Hub to Coleraine Land Fall (500MW HVDC Cable) Section Conditions Overview Water depths vary from 26m at Coleraine to 130m, 3 NM SSW of the hub. The depths along the majority of the route vary between 70 and 50 metres until a point 3 NM north of Ranmore Head when the seabed shallows to the landfall. The landfall to the west of Ranmore Head was chosen as Hibernia A telecom cable spur comes ashore at this point. The route crosses the Hibernia A telecom Cable at approximately N W. Along the majority of the route the seabed is predominately gravel, mud, shells, pebbles and sand, becoming rockier when approaching the landfall. 82 Rev F01

88 Section Conditions 1 to 5 Water depth at the Hub is 26.5m in a rocky area with frequent overfalls. Water depths increase to 70m at Point 2 which is just to the east of a 134m deep trench. Between Points 2 and 3 the seabed gradually shelves to 55m. The route crosses the Hibernia A submarine telephone cable just before Point 3. The depths between Points 3 and 5 average around 55m, shoaling to 30m at Point 5. Between the Hub and the 50m contour the seabed is expected to be predominately rock. Below the 50m contour the soils change from rock to gravels, shells, pebbles, and sand with weed (possibly kelp). The seabed gradually becomes shelly sand in the vicinity of Point 3. There are rock outcrops to the east of the route. The seabed is predominately sand and shells becoming pebbles, gravels and sands in the shallower water around Point 5 Consideration should be given to armouring the cable between the Coleraine Hub and the 50m contour to the SSW, a length of 1700 metres. 83 Rev F01

89 Section Conditions 5 to Landfall (Portrush) Between Points 5 and 7 the depths shoal gradually from 30m to 25m at Point 6, then shoal rapidly to 0m at the landfall. The seabed in this section appears to consist of pebbles, gravels, and shells with patches of fine sand. The route follows the Portrush Spur of the Hibernia A submarine telephone cable. 84 Rev F01

90 Location K OWF to Coleraine Hub (500MW HVDC Cable) Section Conditions Water depths vary from 22m at Location K to to 70m, 5 NM north of Turbot Bank. The greatest depth is 110m, some 3 NM south west of the Coleraine Hub. At the Coleraine Hub, the water depth is 25m. The shallow areas at Location K and the Coleraine Hub are predominately rocky. In the deeper waters to the north east of the Turbot Bank, the seabed appears to be weed, gravels, shells, pebbles and sands. The route crosses the Hibernia A telecom cable at approximately N W. Section Conditions 1 to 3 The water depth at the Location K VCS is approximately 20m. The VCS is located on the edge of Hampton s and Turbot Banks which are rock outcrops. Between the Location K VCS and Point 2 the depths deepen to 36m. The water depths deepen along the section to 57m at Point 3 By the VSC, the seabed is sand with stones and shells becoming stoney, shelly sand at Point 3. 3 to Coleraine Hub The route information is the same as Points 3 to the Hub for the Coolkeragh Hub to Coleraine Hub routing. 85 Rev F01

91 Location J OWF to Coolkeeragh Hub (500MW HVDC Cable) Section Conditions Overview This short route has water depths ranging from 27m at the windfarm location to 36m at the Coolkeeragh Hub. The route lies in a predominately rocky area with some fine sandy areas. Cable armouring should be considered for this 7.1km route 86 Rev F01

92 Coolkeeragh Hub to Coolkeeragh Landfall (500MW HVDC Cable) Section Conditions Overview The 34 kilometre route follows the centre of Lough Swilly from the Coolkeeragh Hub, some 8 NM offshore to a landfall at the southern end of the lough. Coolkeeragh Hub is situated in a Rocky area with a depth of 36 metres. The route shallows to 25m at Point 2, and further shallows to 20m at the entrance to Lough Swilly. It is expected that soils will vary from rock and sand pockets at the hub to silty soils at the landfall. Little is known of the soils in this area. 87 Rev F01

93 Route Planning Lists For 1000MW and 500MW HVDC Cables (excludes A/C cables from OWF locations to VSCs) 88 Rev F01

94 Point Long_DD Lat_DD LAT_DMmm LONG_DMmm Azimuth Kms CumKm Note Comments 'N 'W Coleraine Hub Rocky area with frequent overfalls close to the hub location. Water depths varying from 26.5m to 70m 'N 'W 'N 'W Mull of Oa 'N 'W Depth generally around 70m rising to 59m south west of the Mull of Oa and then deepening to 70m. Strong currents and races off the Mull of Oa. Rocky to the north west becoming Gravel, Mud, Shells, Pebbles and Sand with occasional rock outcrops towards the Mull of Kintyre 'N 'W 'N 'W 'N 'W 'N 'W Mull of Kintyre Sanda Island Water depths vary from 60m in the north west deepening to a maximum of 120m in the North Channel before shallowing to approximately 60m at the start of the Firth of Clyde. Soils vary from Sand, Shells, Pebbles in the north west through Mud, Fine Sand and Shells in the North Channel to Gravels, Sand, Shells and Kelp at the entrance to the Firth of Clyde. Strong currents, eddies and overfalls can be found off the Mull of Kintyre and Sanda Island 'N 'W 'N 'W 'N 'W Kildonan Pt Depths vary between 50m and 60 metres. The soils are predominately sands and muds with occasional rocks. The route crosses the LANIS-3 And SIRIUS NORTH telecom cables between Points 9 & 10 and again between Points 11 & 12. The new cable route would parallels the proposed Anglo-Scottish HVDC Cable from Point 13 onwards 'N 'W Rev F01

95 'N 'W 'N 'W 'N 'W 'N 'W 'N 'W Depths vary from 50m at Point 13, with a deepest depth of 74m near Ardrossan, with the landfall at Farland Head. The mean depth is around 60 metres and the seabed consists mainly of mud and sand with some pebbly areas 'N 'W Farland Hd RPS ISLES Provisional 1000mW HVDC Export Cable Route Coleraine Hub to Hunterston Landing - Cable 01 Point Long_DD Lat_DD LAT_DMmm LONG_DMmm Azimuth Kms CumKm Note Comments 'N 'W Coleraine Hub Rocky area with frequent overfalls close to the hub location. Water depths varying from 26.5m to 70m 'N 'W 'N 'W Mull of Oa 'N 'W Depth generally around 70m rising to 59m south west of the Mull of Oa and then deepening to 70m. Strong currents and races off the Mull of Oa. Rocky to the north west becoming Gravel, Mud, Shells, Pebbles and Sand with occasional rock outcrops towards the Mull of Kintyre. 2 Rev F01

96 'N 'W 'N 'W 'N 'W 'N 'W Mull of Kintyre Sanda Island Water depths vary from 60m in the north west deepening to a maximum of 120m in the North Channel before shallowing to approximately 60m at the start of the Firth of Clyde. Soils vary from Sand, Shells, Pebbles in the north west through Mud, Fine Sand and Shells in the North Channel to Gravels, Sand, Shells and Kelp at the entrance to the Firth of Clyde. Strong currents, eddies and overfalls can be found off the Mull of Kintyre and Sanda Island 'N 'W 'N 'W 'N 'W Kildonan Pt 'N 'W Depths vary between 50m and 60 metres. The soils are predominately sands and muds with occasional rocks. The route crosses the LANIS-3 And SIRIUS NORTH telecom cables between Points 9 & 10 and again between Points 11 & 12. The new cable route parallels the proposed Anglo-Scottish HVDC Cable from Point 13 onwards 'N 'W 'N 'W 'N 'W 'N 'W 'N 'W Depths vary from 50m at Point 13, with a deepest depth of 74m near Ardrossan, with the landfall at Farland Head. The mean depth is around 60 metres and the seabed consists mainly of mud and sand with some pebbly areas 'N 'W Farland Hd 3 Rev F01

97 RPS ISLES Provisional 1000mW HVDC Export Cable Route Coleraine Hub to Hunterston Landing - Cable 02 Point LongDD Lat_DD LAT_DMmm LONG_DMmm Azimuth Kms CumKm Note Comments 'N 'W Argyll Hub Rocky area with frequent overfalls close to the hub location. Water depths varying from 10 to 20m 'N 'W Depths very from 20m just south of the hub to 50m at Point 2. Maximum depth along the route is around 80m. once away from the Argyll Hub, the soils vary from shelly sand to shells, sand, gravel, pebbles and mud in the deeper waters becoming sand and shell again at Point 'N 'W The cable route passes over a ridge of shelly sand with a minimum depth of approximately 35 metres. The depths at Point 4 are around 55m 'N 'W Depths vary from 55m in the north to 27 metres at the Coleraine Hub. The route passes over a rocky ridge with a minimum depth of 26 metres to the north of the Islay Windfarm location. The route passes through a 60m trench to the south of the windfarm location. The shallower sections above the 50m contour appear to be rocky ridges. 4 Rev F01

98 Point LongDD Lat_DD LAT_DMmm LONG_DMmm Azimuth Kms CumKm Note Comments 'N 'W Coleraine Hub Rocky area with frequent overfalls close to the hub location. Water depths varying from 26.5m to 70m 'N 'W 'N 'W Mull of Oa 'N 'W Depth generally around 70m rising to 59m south west of the Mull of Oa and then deepening to 70m. Strong currents and races off the Mull of Oa. Rocky to the north west becoming Gravel, Mud, Shells, Pebbles and Sand with occasional rock outcrops towards the Mull of Kintyre 'N 'W 'N 'W 'N 'W 'N 'W Mull of Kintyre Sanda Island Water depths vary from 60m in the north west deepening to a maximum of 120m in the North Channel before shallowing to approximately 60m at the start of the Firth of Clyde. Soils vary from Sand, Shells, Pebbles in the north west through Mud, Fine Sand and Shells in the North Channel to Gravels, Sand, Shells and Kelp at the entrance to the Firth of Clyde. Strong currents, eddies and overfalls can be found off the Mull of Kintyre and Sanda Island 'N 'W 'N 'W 'N 'W Kildonan Pt 'N 'W Depths vary between 50m and 60 metres. The soils are predominately sands and muds with occasional rocks. The route crosses the LANIS-3 And SIRIUS NORTH telecom cables between Points 13 & 14 and again between Points 15 & 16. The new cable route parallels the proposed Anglo-Scottish HVDC Cable from Point 17 onwards 'N 'W Rev F01

99 Point LongDD Lat_DD LAT_DMmm LONG_DMmm Azimuth Kms CumKm Note Comments 'N 'W 'N 'W 'N 'W 'N 'W Depths vary from 50m at Point 17, with a deepest depth of 74m near Ardrossan, with the landfall at Farland Head. The mean depth is around 60 metres and the seabed consists mainly of mud and sand with some pebbly areas 'N 'W Farland Hd RPS ISLES Provisional 500mW HVDC Export Cable Route Argyll Hub to Hunterston Landing 6 Rev F01

100 Point LongDD Lat_DD LAT_DMmm LONG_DMmm Azimuth Kms CumKm Note Comments 'N 'W Argyll Hub Rocky area with frequent overfalls close to the hub location. Water depths varying from 10 to 20m 'N 'W 'N 'W Depths very from 20m just south of the hub to 50m at Point 3. Maximum depth along the route is around 80m. once away from the Argyll Hub, the soils vary from shelly sand to shells, sand, gravel, pebbles and mud in the deeper waters becoming sand and shell again at Point 'N 'W The cable route passes over a ridge of shelly sand with a minimum depth of approximately 35 metres. Amrouring may be required over this ridge due to the rockier nature of the seabed The depths at Point 5 are around 55m 'N 'W Depths vary from 55m in the north to 27 metres at Coleraine Hub. The route passes over a rocky ridge with a minimum depth of 26 metres to the north of the Islay Windfarm location. The route passes through a 60m trench to the south of the windfarm location. The shallower sections above the 50m contour appear to be rocky ridges, and the cable may require armouring 'N 'W Coleraine Hub Rocky area with frequent overfalls close to the hub location. Water depths varying from 26.5m to 70m RPS ISLES Provisional 500mW HVDC Export Cable Route Argyll Hub to Coleraine Hub 7 Rev F01

101 Point LongDD Lat_DD LAT_DMmm LONG_DMmm Azimuth Kms CumKm Note Comments 'N 'W Coolkeeragh Hub Rocky area with a depth of 36 metres, shallowing to 25m at Point 'N 'W 'N 'W 'N 'W 'N 'W Water depths gradually shallowing to 20m at the entrance to Lough Swilly and 0m at the landfall. The seabed is expected to become more silty in the Lough. Very little soils information is available in this area 'N 'W 'N 'W Landfall RPS ISLES Provisional 500mW HVDC Export Cable Route Coolkeeragh Hub to Coolkeeragh Landfall 8 Rev F01

102 Point LongDD Lat_DD LAT_DMmm LONG_DMmm Azimuth Kms CumKm Note Comments 'N 'W Coolkeeragh Hub Rocky area with a depth of 36 metres, shoaling to 30m at Point 'N 'W The route increases in depth to 70m at Point 3 and the seabed changes from being rocky in the west to sand, stones and shells in the east 'N 'W The route crosses the Hibernia A submarine telephone cable just after Point 3. The depths are between 70 and 65m. The last part of the section is along a ridge with depths of 121m on the north west side and 134m on the south east side. Alternative routing may be required to go around these trenches. The seabed is predominately gravels, shells, pebbles, and sand with weed (possibly kelp) 'N 'W Between Points 4 and 5 the depths shallow from 65m to 23m. The seabed becomes rocky above the 50m contour. This area suffers from overfalls 'N 'W The depths in the section vary from 23m to 27m. The seabed is rocky 'N 'W Coleraine Hub Rocky area with frequent overfalls close to the hub location. Water depth at the hub is 26.5m RPS ISLES Provisional 500mW HVDC Export Cable Route Coolkeeragh Hub to Coleraine Hub 9 Rev F01

103 Point LongDD Lat_DD LAT_DMmm LONG_DMmm Azimuth Kms CumKm Note Comments 'N 'W Coleraine Hub Rocky area with frequent overfalls close to the hub location. Water depth at the hub is 26.5m 'N 'W Water depths increase to 70m at point 2 which is just to the east of a 134m deep trench. Below the 50m contour the soils change from rock to gravels, shells, pebbles, and sand with weed (possibly kelp) 'N 'W Between Points 2 and 3 the seabed gradually shelves to 55m. The seabed gradually becomes shelly sand in the vicinity of Point 3. There are rock outcrops to the east of the route. The route crosses the Hibernia A submarine telephone cable just before Point 'N 'W 'N 'W The depths between points 3 and 5 average around 55m, shoaling to 30m at Point 5. The seabed is predominately sand and shells becoming pebbles, gravels and sands in the shallower water 'N 'W 'N 'W Landfall (Portrush) Between Points 5 and 7 the depths shoal gradually from 30m to 25m at Point 6, then shoal rapidly to 0m at the landfall. The seabed in this section appears to consist of pebbles, gravels, and shells with patches of fine sand. The route follows the Portrush Spur of the Hibernia A submarine telephone cable. RPS ISLES Provisional 500mW HVDC Export Cable Route Coleraine Hub to Coleraine Landfall 10 Rev F01

104 Point LongDD Lat_DD LAT_DMmm LONG_DMmm Azimuth Kms CumKm Note Comments 'N 'W Argyll VSC Rocky area with a depth of 17 metres rapidly deepening to 37m at Point 'N 'W Between Points 2 and 3 the depth is fairly constant at 37 metres with a sandy seabed 'N 'W The seabed rise from 37m to 10m at the hub location. The seabed becomes more rocky at the hub 'N 'W Argyll Hub Rocky area with frequent overfalls close to the hub location. Water depths varying from 10 to 20m. RPS ISLES Provisional 500mW HVDC Export Cable Route Argyll VSC to Argyll Hub 11 Rev F01

105 Point LongDD Lat_DD LAT_DMmm LONG_DMmm Azimuth Kms CumKm Note Comments 'N 'W Location C OWF Water depth approximately 30m, rocky outcrops in a shelly sandy seabed 'N 'W Barra Head Between Location C and Point 2 the depths vary between 25 and 30m. The seabed is sand and shells with rock outcrops in the north becoming weed and shells with rock out crops at Point 'N 'W The water depths vary from 30m to 53m at Point 3. The seabed becomes increasing rocky in the deeper water 'N 'W The depths increase to the south west to 115m at Point 4. The seabed remains rocky 'N 'W Between Points 4 and 5 the water depths increase to approximately 200m before shoaling again to 100m at Point 5. The nature of the seabed becomes mud, sand and shells, with patches of gravels. The route passes through a laden tanker and radio reporting zone between Points 3 and 'N 'W The route continues to shoal from 100m to 54m at Point 6. The seabed becomes increasingly rockier as shallow water is reached 'N 'W Argyll Hub Rocky area with frequent overfalls close to the hub location. Water depths varying from 10 to 20m. RPS ISLES Provisional 500mW HVDC Export Cable Route Location C VSC to Argyll Hub 12 Rev F01

106 Point LongDD Lat_DD LAT_DMmm LONG_DMmm Azimuth Kms CumKm Note Comments 'N 'W Islay VSC 'N 'W Coleraine Hub The VSC is in a rocky area with a depth of 24m. The route passes through a trough with a depth of 70m before shoaling again to 26m at the Hub. In the shallow areas at each site the seabed is expected to be rocky. In the deeper section the seabed is expected to be pebbly, gravelly sand with some weeds (possibly kelp). RPS ISLES Provisional 500mW HVDC Export Cable Route Islay VSC to Coleraine Hub Point LongDD Lat_DD LAT_DMmm LONG_DMmm Azimuth Kms CumKm Note Comments 'N 'W Location J VSC 'N 'W Coolkeeragh Hub The VSC is in a rock and sand area with a depth of 27m. The seabed is reasonably level around 33 metres over the route length. The depth at the hub is 36m. The seabed is expected to be fairly rocky RPS ISLES Provisional 500mW HVDC Export Cable Route Location J VSC to Coolkeeragh Hub 13 Rev F01

107 Point LongDD Lat_DD LAT_DMmm LONG_DMmm Azimuth Kms CumKm Note Comments 'N 'W Location K VCS Water depth approximately 20m, on the edge of Hampton s and Turbot Banks 'N 'W Between Location K VCS and Point 2 the depths deepen to 36m. The seabed is sand with stones and shells 'N 'W The water depths deepen along the section to 57m at Point 3. The seabed becomes stoney, shelly sand 'N 'W The route crosses the Hibernia A submarine telephone cable just after Point 3. The depths are between 70 and 65m. The last part of the section is along a ridge with depths of 121m on the north west side and 134m on the south east side. Alternative routing may be required to go around these trenches. The seabed is predominately gravels, shells, pebbles, and sand with weed (possibly kelp) 'N 'W Between Points 4 and 5 the depths shallow from 65m to 23m. The seabed becomes rocky above the 50m contour. This area suffers from overfalls 'N 'W Coleraine Hub The depths in the section vary from 23m to 27m. The seabed is rocky with frequent overfalls close to the Point 5 and the hub location. Water depths at location is 26.5m. RPS ISLES Provisional 500mW HVDC Export Cable Route Location K VSC to Coleraine Hub 14 Rev F01

108 2 APPENDIX B SUBSEA CABLE LAY CONSTRUCTION SCHEDULE 15 Rev F01

109 3 APPENDIX C TIDAL FLOWS IN THE ISLES REGION Reproduced by permission of Bloomsbury Publishing Plc from the Reeds Nautical Almanac. [18] Neville Featherstone, Edward Lee-Elliott, Reeds Nautical Almanac and Bloomsbury Publishing Plc Neville Featherstone, Edward Lee-Elliott, Reeds Nautical Almanac and Bloomsbury Publishing Plc 16 Rev F01

110 Neville Featherstone, Edward Lee-Elliott, Reeds Nautical Almanac and Bloomsbury Publishing Plc Neville Featherstone, Edward Lee-Elliott, Reeds Nautical Almanac and Bloomsbury Publishing Plc 17 Rev F01

111 4 APPENDIX D WAVE HEIGHT SCATTER DIAGRAM Figure 51: Wave scatter diagram reproduced from DNV RP205 Environmental Conditions. [19] 18 Rev F01