4.3.3.    Sand Wave Clearance

113. In some areas within the Proposed Development array area and along the Proposed Development export cable corridor, existing sand waves and similar bedforms may be required to be removed before cables are installed. This is carried out mainly for two reasons, although others may arise:

• many of the cable installation tools require a relatively flat seabed surface in order to work effectively. Installing cables on up or down a slope over a certain angle, or where the installation tool is working on a camber may reduce the ability to meet target burial depths; and
• the cable must be installed to a depth where it may be expected to stay buried for the duration of the Proposed Development operational lifetime (35 years). Sand waves are generally mobile in nature therefore the cable must be buried beneath the level where natural sand wave movement could uncover it. Sometimes this can only be achieved by removing the mobile sediments before installation takes place.
  114. Sand wave clearance may take place throughout the construction phase. If required, sand wave clearance will be completed in areas within the Proposed Development array area along the inter-array cables, OSP interconnector cables and the Proposed Development export cable corridor. Seabed features clearance will involve removal of the peaks of the seabed features by techniques such as dredging, with material replaced in the troughs, thereby levelling the seabed. A specialist dredging vessel may be required to complete the seabed features clearance.
  115. Sand wave clearance may also be undertaken using other methodologies including pre-installation ploughing tools to flatten sand waves, pushing sediment from wave crests into adjacent troughs and levelling the seabed.
  116. The maximum design scenario for sand wave clearance in the Proposed Development array area and Proposed Development export cable corridor is summarised in Table 4.22   Open ▸ . Final values for sand wave clearance will be refined following completion of a geophysical survey campaign prior to construction.
  117. In addition to sand wave clearance, boulder clearance and pre-lay grapnel run may be required to prepare the site for cable installation. This is described as part of cable installation in section 3.1.1.

Table 4.22: Design Envelope: Sand Wave Clearance Parameters

4.3.4.    Boulder Clearance

118. Boulder clearance is commonly required during offshore wind farm site preparation. A boulder is typically defined as being over 200 mm in diameter/length. It is expected that the boulder clearance campaign will be carried out with the use of a DP vessel.
119. Boulder clearance may be required along the inter-array cables, OSP/Offshore convertor station platform interconnector cables and the Proposed Development export cable corridor. Boulder clearance is required to reduce the risk of shallow cable burial resulting in the need for further cables burial works and/or cable protection, as well minimising risk of damage to cables during installation. It may also be required in the vicinity of the foundation locations (including within the jack-up vessel zone around the foundation locations), in order to avoid disruption to installation activities and to ensure stability for the jack-up vessel. Table 4.23   Open ▸ provides the maximum design scenario for boulder clearance in the Proposed Development array area and Proposed Development export cable corridor.
120. The cable route may be pre-ploughed for the removal of discreet boulders. Should more dense boulder fields be encountered, there may be a need for additional techniques. This decision will be informed by the geophysical and pre construction surveys.

Table 4.23: Design Envelope: Boulder Clearance Parameters

4.3.5.    Vessels for Site Preparation Activities

121. Table 4.24   Open ▸ includes all vessels to be used during site preparation activities.

Table 4.24: Design Envelope: Vessels for Site Preparation Activities

3.1. Construction Phase

3.1.1.    Methodology

122. The Proposed Development is likely to be constructed according to the general sequence below, although the final sequence may vary from this:

• step 1 – offshore export cables – landfall installation;
• step 2 – foundation installation and scour protection installation;
• step 3 – OSP/Offshore convertor station platform topside installation/commissioning;
• step 4 – inter-array and interconnector cable installation and cable protection installation;
• step 5 – offshore export cables – offshore installation and cable protection installation; and
• step 6 – wind turbine installation/commissioning.

Figure 4.8: Typical Long Section of Trenchless Technique Method

123. Each stage is outlined in further detail in the following sections.

Step 1 – Offshore export cables – landfall installation

124. Figure 4.9   Open ▸ shows the Proposed Development export cable corridor as it reaches landfall at Skateraw.
125. It is proposed that the cables are installed through the intertidal zone using trenchless technology, such as HDD ( Figure 4.8   Open ▸ ). HDD involves drilling a hole (or holes) along an underground pathway from one point to another, through which the offshore export cables are installed, without the need to excavate an open trench. To achieve this, a drill rig is located onshore, landward of MHWS. A working area will be established containing the drill rig, electrical generator, water tank, mud recycling unit and temporary site office. The drilling installation will commence from above the MHWS, with the HDD exit point (punch out location) located seaward of MLWS between 488 m and 1,500 m below MWHS. As such, no works are planned to take place in the intertidal zone.
126. A drilling fluid, such as Bentonite, is pumped into the drilling head during the drilling process to stabilise the hole and retrieve the drilled material. Once the drilling is complete, cable ducts may be installed from land and pushed out, or towed into position by a vessel offshore and pulled in. The offshore export cables are then pulled through the pre-installed ducts by land-based winches.
127. The HDD punch out may also require the excavation of HDD exit punches out.
128. The HDD works comprise the following main stages:

1. A pilot hole will be drilled from onshore to offshore.
2. Once the pilot hole has been completed, the reaming process will commence, increasing the diameter of the pilot hole to accommodate the safe installation of HDD duct. The reaming process will continue back and forth for a number of passes to achieve a minimum bore diameter. During the drilling procedure, drilling fluid is continuously pumped to the drill head to act as a lubricant. Solids are removed from the returning fluid, and the spoil is transported off site or into the mud pit (landward of the MHWS) to settle.
3. A jack-up vessel or dredger will be used at the at the HDD exit point to create an HDD exit punch out.
4. The last forward HDD reamer exits the seabed at the HDD exit punch out.
5. The HDD reamer is then disconnected from the drill pipe and recovered.
6. The High-Density Polyethylene (HDPE) liner pipe will be pre-assembled and then floated in, connected to the drill pipe, and pulled onshore from the offshore end through the pre-drilled bore into position.
7. Steps a to f are then repeated for all the 220 kV (or 275 kV) offshore export cable circuits.
8. Trenches are then excavated from the HDD entry points above the MHWS to the transition joint bay and ducts installed and backfilled; (covered as part of the onshore submission).
9. HDD construction equipment and plant is then demobilised from site.
10. The ducts are then proved ready for cable pull in and messenger wires are installed.
11. Cables will then be installed in the ducts by pulling onshore through the ducts from the offshore delivery vessel to the transition joint bays.
    129. Once commenced, the HDD drilling activities may be required to operate continuously over a 24-hour period until each bore is complete. Subject to further construction planning and availability of drilling rigs, drilling may be carried out concurrently to accelerate the construction works programme.
    130. There are typically two pull in techniques considered for the HDD landfall installation. The first being direct pull in, where the cable vessel will sit a short stand-off distance from the HDD exit point, where the cable is pulled directly and unreeled from the vessel. The second being floated pull in, where the vessel will stand-off at a suitable water depth for its safe operation and float the cable toward the duct, with a second vessel assisting located above the HDD exit point to guide the cable through the duct.
    131. Bentonite comprises 95% water and 5% bentonite clay which is a non-toxic, natural substance. Bentonite drilling fluid is non-toxic and can be commonly used in farming practices. Every endeavour will be made to avoid a breakout (loss of drilling fluid to the surface). A typical procedure for managing a breakout under water would include:

• stop drilling immediately;
• pump lost circulation material (mica), which will swell and plug any fissures;
• check and monitor mud volumes and pressures as the works recommence; and
• repeat process as necessary until the breakout has been sealed.

Figure 4.9: Location of the Proposed Development Export Cable Corridor

Table 4.25: Design Envelope: Offshore Export Cables (Seaward of MHWS)

Step 2 – Foundation installation and scour protection installation

Jacket foundations

132. Wind turbines and OSP/Offshore convertor station platform foundations will be transported to the Proposed Development array area by vessel from the fabrication site or port facility (see section 3.1.2 for further detail on vessels to be used at the Proposed Development).
133. Jacket foundations could use either piles or suction caissons. Information on the methodology to be followed during suction caissons installation is provided in paragraphs 73 and 74. The piled jacket foundation will be installed into the seabed by either piling or drilling techniques, or a combination of both (drive-drill-drive), depending on seabed conditions. Typically, piles will be piled into the seabed using a vibro/hydraulic hammer until any hard ground is encountered, with drilling techniques deployed to install the remaining length of pile, if required.
134. Piling characteristics are presented in Table 4.26   Open ▸ . In order to complete the piling, the pile is usually lowered to the seabed with the help of a crane while kept in position using a pile gripper. A pile installation frame will be temporarily placed on the seabed to facilitate pile placement and spacing. The frame will be removed and moved to the next location once the piles are installed. The impact of the temporary placement of the frame on the seabed is bound by the maximum design scenario of disturbance caused by placement of scour protection. The hydraulic hammer is then positioned onto the pile and driven to target depth. Although a hammer energy of 4,000 kJ is considered as the maximum design scenario for the purposes of assessment, the realistic maximum average energy used when piling will be lower for the majority of the time (3,000 kJ). It is worth noting that the piles are likely to be pre-piled in advance with the jackets then installed on top at a later date.
135. Piling will commence with a lower hammer energy of 600 kJ, with a slow ramp up of energy up to a realistic 3,000 kJ over a period of 20 minutes. If necessary, this will be followed by a gradual increase to the maximum required installation energy (if higher than 3,000 kJ, but not to exceed the maximum energy of 4,000 kJ) during the piling of the final metres of pile, which is typically significantly less than the maximum hammer energy. The PDE includes for up two piling events occurring simultaneously at wind turbines, with no concurrent piling of OSPs/Offshore convertor station platforms. Table 4.26   Open ▸ provides the maximum deign scenario for the jacket piling.

Table 4.26: Design Envelope: Jacket Piling Characteristics

136. Drilling characteristics are presented in Table 4.27   Open ▸ . If drilling is required (i.e. in the event that pile driving may not be suitable due to hard ground), a sacrificial caisson may need to be installed to support surficial soils during the drilling activities. The caisson would be driven and left in place. The pile would then be lowered into the drilled bore and grouted in place, with the voids (annuli) between the pile and the rock, and between the pile and the caisson, filled with inert grout. The grout would fill the voids by being pumped from a vessel into the bottom of the drilled hole. The process would be carefully controlled and monitored to ensure minimal spillage to the marine environment.
137. Drilling will result in the release of seabed material, which will be deposited adjacent to each drilled foundation location.

Table 4.27: Design Envelope: Jacket Drilling Characteristics

Step 3 – OSP/Offshore convertor station platform topside installation/commissioning

138. The OSP/Offshore convertor station platform topsides will be transported to the Proposed Development by vessel either from the fabrication yard or the pre-assembly harbour, after the foundations are installed. The OSP will be transported by the installation vessel or on a barge towed by a tug. Once on site, the OSP/Offshore convertor station platform will be rigged up, seafastening cut, lifted and installed onto the foundation. The OSP/Offshore convertor station platform will then be welded or bolted to the foundation. The installation vessel will mobilise with all the required equipment including rigging, welding and bolting equipment.
139. All necessary cable connecting and commissioning works are expected to be carried out with the assistance of a jack-up or DP vessel, with assisting support and supply vessels as required. Crew Transfer Vessels (CTVs) likely will be used to transfer personnel to and from the installation vessel.

Step 4 – Inter-array and interconnector cable installation and cable protection installation

140. A range of possible cable installation options may be required in order bury cables to the required target burial depths. While the nature of the seabed sediments within the Proposed Development array area may tend to installation of inter-array and interconnector cables being largely carried out using jetting tools any, or a combination of the options highlighted in Table 4.17   Open ▸ may be required.
141. The same installation and cable protection methodologies apply as described for the offshore export cables in paragraphs 142 to 146. Cable crossing required for the inter-array and interconnector cables are discussed in paragraph 102.

Step 5 – Offshore export cables – offshore installation and cable protection installation

Offshore export cables installation

142. A range of possible cable installation options may be required in order bury cables to the required target burial depths. There are various types of installation tools that may be used to install the offshore export cables, including:

• jet trenching, which injects water at high pressure in the area surrounding the cable using a jetting tool. allowing the cable to sink to the required burial depth;
• deep jet trenching;
• mechanical trenching, which excavates a trench in the seabed in which the cable is layed; and
• cable ploughs, which opens a narrow trench in the seabed using a towed plough, inserting the cable simultaneously.
  143. Pre-sweeping and/or dredging may be required in some areas. This will allow for the selected cable installation method to be used. Trenchless techniques will also be used at landfall as explained in Table 4.25   Open ▸ .

Cable protection installation

144. Cable protection will be used where minimum target burial depths are not achieved during installation and at cable crossings (see section 4.2.5). Cable protection systems are also used as cables approach and enter the wind turbines and OSPs/Offshore convertor station platforms (see section 4.2.5).
145. It is proposed that cable protection will consist of the following cable protection systems:

• rock placement;
• rock bags;
• concrete mattresses;
• cast iron shells; and
• sleeving.
  146. Further information is provided in paragraphs 95 to 100.

Cable crossing installation

147. As explained in paragraph 101, up to 16 cable crossings may be required for the offshore export cables. The crossings would be protected using one of the protection technologies described in paragraph 145. A crossing angle close to 90 degrees relative to the existing cable is the preferred option, however this might differ depending on the final design and protection technology used.

Step 6 – Wind turbine installation/commissioning

148. The wind turbines will be transported to the Proposed Development array area by vessel from the pre-assembly port where sub-assemblies (nacelle, rotor blades and towers), assembly parts, tools and equipment will be loaded onto an installation or support vessel.
149. At the installation location, the wind turbine towers will be lifted onto the pre-installed foundation and transition piece by the crane on the installation vessel. The nacelle and rotor blades will then be lifted into position. The exact methodology for the assembly will be dependent on the installation contractor and wind turbine type.
150. Following installation of the wind turbine, commissioning activities will take place including mechanical completion, electrical completion, HV commissioning and HV energisation.
151. Following energisation, the HV commissioning activities will be completed and the wind turbines will undergo performance and reliability testing.

3.1.2.    Installation Vessels and Helicopters

152. A range of installation vessels will be used for the construction of the Proposed Development. This includes main installation vessels (e.g. jack-up or DP vessels with heavy lifting equipment), support vessels (including Service Operation Vessels (SOVs), tugs and anchor handlers, cable installation vessels, guard vessels, survey vessels, crew transfer vessels and scour/cable protection installation vessels. In addition, it is possible that helicopters will be used for crew transfers.
153. Installation vessel and helicopter parameters are presented in Table 4.28   Open ▸ for activities associated with the construction of the Proposed Development. The table provides an overview of the number of vessels/helicopters (and return trips) for construction of the Proposed Development including within the array area and along the Proposed Development export cable corridor (including landfall) at any one time during the entire construction phase. It should be noted that the numbers presented are an estimated maximum design scenario for assessment purposes and in reality, vessel and helicopter numbers are anticipated to be less than this. The maximum number of vessels is 155 on site at any one time with up to 11,484 return trips.

Table 4.28: Design Envelope: Infrastructure Installation (Proposed Development Array Area and Export Cable Corridor (Including Landfall)) - Vessels and Helicopters

Jack-up vessels/barges make contact with the seabed when their jack-up spud cans (base structure of each leg) are lowered into place. For the purposes of the Application, jack-up vessel parameters are presented in Table 4.29   Open ▸ .

Table 4.29: Design Envelope: Jack-up Vessels

3.1.3.    Construction Ports

154. It is likely that the Proposed Development components will be fabricated at a number of manufacturing sites across Scotland, the UK and Europe, while the substructures could be fabricated in the Middle East or Far East. Components may be transported directly to the Proposed Development from where they are manufactured or may be delivered to a port where they are stored in line with the day to day practice of that port before onward transport to the Proposed Development. This will be determined as part of competitive tendering processes whilst aiming to maximise UK and Scottish content, in line with Supply Chain Plan commitments.
155. All components are anticipated to be transported via sea transport to the Proposed Development for installation via vessels and associated equipment. Therefore, there is not anticipated to be a requirement for large components (e.g. wind turbine blades) to be transported via road.
156. The construction port for the storage, fabrication, pre-assembly and delivery of Proposed Development infrastructure has not yet been confirmed at the time of writing this RIAA, however the majority of large infrastructure will go to site via vessel. Suitable ports will be selected based on the presence of appropriate facilities to handle and process offshore wind farm components. It is anticipated that all activities carried out within port will fall under established port licences and operational controls. For the purposes of this RIAA and in order to assess a maximum design scenario, the assessments consider a maximum number of vessels and vessel movements to/from site, where relevant.
157. Construction personnel will transit to the location of the Proposed Development on the installation vessels or other vessels listed in Table 4.28   Open ▸ . Crew transfers may also take place between the construction port and the site of the Proposed Development via Crew Transfer Vessels (CTVs), Service Operation Vessels (SOVs), or by helicopter operating from a licenced airfield. Crew transfers during construction, operation and decommissioning will launch from existing port sites.

3.1.4.    Construction Programme

158. An outline of the programme for construction of the Proposed Development is provided below. The indicative commencement and completion dates, together with estimated durations of key construction activities, have been used to inform the assessment of construction impacts. Further detail on specific timeframes, durations and sequencing of activities is provided in the maximum design scenario tables that are included in each of the technical chapters.
159. Due to its scale, the Proposed Development will be built out over a period of up to eight years including site preparation works and snagging activities following installation of the wind turbines prior to final commissioning. The majority of activities will occur over various campaigns targeted at the relevant assets. Most activities will have a maximum duration of five years or less. Although construction activities will typically occur sequentially there are expected to be periods where certain construction activities occur concurrently. For example, substructure installation and inter-array cable installation, or commencement of wind turbine installation while foundation installation is being completed.
160. Indicative outline construction programme includes the following:

• commencement of offshore construction (site preparation and landfall activities) expected Q1 2025;
• completion of construction (including snagging) expected Q1 2033;
• key construction activity and estimated durations:

-            site preparation works – will occur for the duration of the construction phase but will not be continuous;

-            landfall installation – up to approximately 15 months;

-            wind turbine substructure installation – up to four years and six months across two installation campaigns;

-            OSPs/Offshore convertor station platforms installation – up to three years across two installation campaigns;

-            Inter-array cables installation – up to five years across two installation campaigns;

-            offshore export cables installation – up to two years and one month;

-            wind turbine installation – up to three years across two installation campaigns; and

-            completion and snagging – up to five years across two campaigns periods.