3.4. Construction Phase

3.4.1. Methodology

  1. Construction of the Array is expected to occur over a period of eight years cumulatively aligning with the following indicative construction series:
  • step 1 – anchoring and mooring installation;
  • step 2 – OSP topsides and fixed jacket foundations installation/commissioning;
  • step 3 – inter-array and interconnector cables installation, including cable burial and/or protection, where required; and
  • step 4 – floating wind turbine and floating foundation installation/commissioning.
  1. The following subsections summarise these steps.

Step 1 – Anchoring and mooring installation

  1. Moorings and anchoring systems will be transported to the Array by vessel and pre-laid at the installation locations (exact locations to be confirmed at final design stage (post-consent)), prior to installation of all other infrastructure. Section 3.4.2 presents further details of vessels involved in installation activities within the Array. It should be noted that some components, such as anchors, mooring chains and clump weights may be wet stored within the Array and close to the final installation locations to optimise delivery schedules. These will not be wet stored for an extended period but they may be queued whilst installation of mooring and anchoring systems and other construction works are ongoing.
  2. There are several anchoring options being considered as described in section 3.2.3, however, these will comprise either driven piles or DEAs alone, or a combination of driven piles and DEAs/suction anchors, depending on seabed conditions. Driven piles will be installed in the seabed using a vibro/hydraulic hammer until any hard ground is encountered. Drilling techniques may be used to install the remaining length of pile, if required.
  3. Anchoring systems will be transported to site using an installation vessel(s) (e.g. heavy lift vessels, or alternative solution) and installed in the seabed using a crane and other equipment as appropriate. The mooring lines will then be connected to the anchoring system using LTM connectors, or similar (see paragraph 44). Once mooring and anchoring systems are installed, mooring lines will be left lying on the seabed until they are hooked up to the floating foundations (step 4; paragraph 115). Ancillaries such as clump weights may be used to temporarily anchor portions of the mooring lines to the seabed to restrict movement prior to hook up.
  4. If DEAs are selected as an anchoring method for floating foundations (see Anchoring Option 2 and 3; Table 3.7   Open ▸ and Table 3.8   Open ▸ ), it is assumed that these will be lifted from the installation vessel using a crane and positioned on the seabed. The DEAs will then be pulled using an anchor handling tug or similar, in order to embed the anchor in the seabed. The anchor will likely be pulled 30 m to 60 m during the installation process, subject to further ground investigations and anchor design. This process will be undertaken in a controlled manner to ensure that DEAs are installed at the correct position and to appropriate depth.
  5. If suction anchors are selected as an anchoring method for floating foundations (see Anchoring Option 4; Table 3.9   Open ▸ ), it is assumed that a crane will be used to lift the suction anchor from the installation vessel towards the seabed. Once the steel caisson reaches the seabed, water is sucked out of each bucket via a pipe which runs through the stem above each caisson. The resulting suction force allows the buckets to penetrate into the seabed. Once the bucket has penetrated the seabed to the desired depth, the pump is turned off. A thin layer of grout is then injected under the bucket to fill the air gap and ensure contact between the soil within the bucket, and the top of the bucket itself.
  6. Table 3.27   Open ▸ presents the piling characteristics required for the installation of the anchoring and mooring systems, if driven piles are selected as an anchoring method (see Anchoring Option 1, 3, 4 and 5; Table 3.6   Open ▸ , and Table 3.8   Open ▸ to Table 3.10   Open ▸ ). It is assumed that a crane will be used to lower the pile to the seabed and will be kept in position using a pile gripper. To enable pile placement, a pile installation frame may be temporarily placed on the seabed, which will be moved to the next location once the piles are installed. A hydraulic hammer will be positioned onto the pile, driving it to target depth. A hammer energy of 3,000 kJ has been considered as the MDS for the purposes of assessment.
  7. Piling will commence with a lower hammer energy of approximately 450 kJ and will slowly ramp up energy up to a maximum 3,000 kJ, if required, over a period of 20 minutes. Detailed geotechnical data of the Array will be reviewed to inform a driveability assessment which will in turn inform maximum realistic hammer energy required for piling. The findings of this study will allow the final hammer energies used to be optimised so that piling progress can be maintained whilst minimising required hammer energy. It is anticipated that the maximum hammer energy stated in Table 3.27   Open ▸ will only be required at some piling locations. Up to two piling events occurring simultaneously at wind turbines (or wind turbine and OSP locations) are considered within the PDE. No concurrent piling of OSP foundations is proposed. The maximum design envelope for the driven piles associated with the wind turbine anchoring is presented in Table 3.27   Open ▸ .
  8. If scour protection is required, this will be installed at a later stage following installation of the anchoring systems.

 

Table 3.27:
Maximum Design Envelope: Wind Turbine Anchoring – Piling Characteristics

Table 3.27: Maximum Design Envelope: Wind Turbine Anchoring – Piling Characteristics

 

  1. If hard ground is encountered which makes pile driving unsuitable, drilling may be required. In this case, a sacrificial caisson may be installed to support surficial soils during the drilling activities; this would be driven into the seabed 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 be pumped from a vessel into the bottom of the drilled hole. The process would be subject to control measures and monitoring to ensure minimal spillage to the marine environment. Drilling characteristics are presented in Table 3.28   Open ▸ .
  2. Seabed material (drill arisings) will be released as a result of drilling activities. This material will be deposited adjacent to each drilled foundation location within the Array.

 

Table 3.28:
Maximum Design Envelope: Wind Turbine Anchoring – Drilling Characteristics

Table 3.28: Maximum Design Envelope: Wind Turbine Anchoring – Drilling Characteristics

 

Step 2 – OSP topsides and fixed jacket foundations installation/commissioning

  1. The OSP jackets will be fixed to the seabed using driven piles. Driven piles will be transported to the Array by vessel from the fabrication site or port facility, and installed in the seabed at the installation locations (exact locations to be confirmed at final design stage (post-consent)), using methods described previously in paragraphs 95 to 100. Should drilling techniques be required, this will follow the same methodology as described in paragraphs 102 and 103.
  2. Piling will commence with a lower hammer energy of 660 kJ, and will slowly ramp up energy up to a maximum 4,400 kJ over a period of 20 minutes. No concurrent piling is proposed across multiple OSPs. Concurrent piling may occur between an OSP and a turbine location.
  3. Once the driven piles have been installed, the OSP jackets will be delivered to site by barge or delivery vessel, lowered to the seabed using a crane, and installed over the pre-installed driven piles. Once in place the jackets would be grouted onto the piles.
  4. The maximum design envelope for the driven piles associated with the OSPs foundations is presented in Table 3.29   Open ▸ . Drilling characteristics are presented in Table 3.30   Open ▸ .

 

Table 3.29:
Maximum Design Envelope: OSP Options – Piling Characteristics

Table 3.29: Maximum Design Envelope: OSP Options – Piling Characteristics

 

Table 3.30:
Maximum Design Envelope: OSP Options – Drilling Characteristics

Table 3.30: Maximum Design Envelope: OSP Options – Drilling Characteristics

 

  1. Once the jacket foundations are installed, the OSP topsides will be transported to the Array via vessel either from the fabrication yard or the port facility. It is likely this will be transported by the installation vessel or on a barge towed by a tug. Once on site, the OSP topside will be rigged up, seafastening cut, lifted and installed onto the foundation. The topside and foundation will then be welded or bolted together. Rigging, welding and bolting equipment will be available on board the installation vessel.
  2. It is expected that commissioning works will be carried out using a jack-up or DP1 vessel. Assisting support and supply vessels will be used as required and Crew Transfer Vessels (CTVs) will be used for transfer of personnel to and from the installation vessel.

Step 3 – Inter-array and interconnector cables installation

  1. A cable lay vessel will be used for installation (lay) of inter-array cables and interconnector cables ( Figure 3.17   Open ▸ ) using various equipment such as a carousel or reels, tensioners and cable lay spread. Inter-array cables and interconnector cables are typically surface laid prior to cable burial or installation of external cable protection post lay. Cable lay and cable burial can also be performed simultaneously.
  2. There are several options which may be used to bury cables to the minimum target burial depth. Equipment that may be used to bury the static portion of the inter-array and interconnector cables include:
  • Jet trenchers or mass flow excavators which inject water at high pressure into the sediment surrounding the cable. Jet trenching tools use water jets to fluidise the seabed which allows the cable to sink into the seabed under its own weight.
  • Mechanical trenchers, usually mounted on tracked vehicles, which use chain cutters or wheeled arms with teeth or chisels to cut a trench across the seabed.
  • Cable ploughs are usually towed either from a vessel or vehicle on the seabed. There are two types of plough:

–           a displacement plough which creates a V shaped trench into which the cable can be laid; or

–           a non-displacement plough which simultaneously lifts a share of seabed whilst depressing the cable into the bottom of the trench. As the plough progresses, the share of the seabed is replaced on top of the cable.

  1. Paragraph 74 describes cable crossings potentially required for the inter-array and interconnector cables.
  2. Junction boxes will be installed from a construction support vessel (CSV) with adequate craneage and laid on the seabed. The junction boxes will then be secured by the structure’s design (e.g. gravity anchors which are buried in the sediment with burial depth dependent upon various factors such as weight, geometry and soil characteristics) or through suction anchors, depending on ground conditions. Once in position the inter-array cables will be pulled into the junction boxes and secured by Remotely Operated Vehicles (ROVs).
  3. The inter-array cables will run from the floating foundation to the junction box as described in paragraph 63.

Figure 3.17:
Indicative Schematic of Inter-Array Cable Installation from Vessel

Figure 3.17: Indicative Schematic of Inter-Array Cable Installation from Vessel

 

Step 4 – Floating wind turbine and floating foundation installation/commissioning

  1. Floating foundations will be fabricated and assembled at a fabrication yard. The floating foundations will be wet stored within harbour limits of the fabrication yard / integration port.  A supply of floating foundations will be assembled in advance of turbine delivery to optimise the integration programme. The floating foundations will then be towed or dry transported on a barge or delivery vessel to the final wind turbine assembly yard using anchor handling tugs ( Figure 3.18   Open ▸ , step 1). The wind turbines (comprising nacelle, rotor blades, hub and towers) will be assembled and integrated onto the floating foundations at the final wind turbine assembly yard ( Figure 3.18   Open ▸ , step 2). The schedule for integration of wind turbines with floating foundations will be optimised so that there is limited requirement for wet storage at this stage. It is not anticipated that integrated floating wind turbines will be queued at the wet storage area awaiting tow to the Array, instead, they will be towed to the installation location within the Array as soon as they are pre-commissioned, by up to two anchor handling tugs, or similar, (exact locations to be confirmed at final design stage (post-consent)) ( Figure 3.18   Open ▸ , step 3). Most floating substructures will employ a ballasting system to control their draft and level of submergence when transported or in operation. The ballasting methodology shall be dependent on the final substructure design and water depth of the final integration port. Some concepts allow the control of the ballast inside different compartments in the structure to modify the response of the floating wind turbine during operation, effectively applying an active control on the volume and mass of the ballast distribution. Active ballast will require special equipment hosted on board (e.g. pumps, pipes, valves). The ballasting material may vary across concepts but generally consists of sea water for the part of the ballast that will be changed for transportation or operation. Permanent ballast (i.e. ballast that won’t be modified during the design life of the foundation) is usually made of solid material (gravel, sand, iron ore etc.) and would be placed and sealed prior to load-out. At the installation location, the integrated floating wind turbines will be installed and hooked up to the pre-installed mooring system ( Figure 3.18   Open ▸ , step 4). Depending on the foundation concept, the final placement and positioning of the floating wind turbines prior to commissioning may require the adjustment of the ballast configuration.
  2. Following hookup of the pre-existing mooring system to the integrated floating wind turbines, dynamic inter-array cables are ‘pulled-in’ to the integrated floating wind turbines using a cable laying vessel and connected to the wind turbine. Buoyancy modules, and tether clamps with clump weights, will be installed as required in order to maintain the dynamic inter-array cable configuration. Following connection to the necessary cabling, a process of testing and commissioning will be undertaken.

Figure 3.18:
Indicative Schematic of Floating Wind Turbine and Floating Foundation Installation and Towing Operations During the Construction Phase

Figure 3.18: Indicative Schematic of Floating Wind Turbine and Floating Foundation Installation and Towing Operations During the Construction Phase

 

3.4.2. Installation Vessels and Helicopters

  1. A number of installation vessels will be used during the construction phase including main installation vessels (e.g. DP1 vessels with heavy lifting equipment), support vessels (including Service Operation Vessels (SOVs)), tugs and anchor handlers, cable installation vessels, guard vessels, survey vessels, CTVs and scour/cable protection installation vessels. Helicopters may also be used for crew transfers.
  2. Table 3.31   Open ▸ presents the maximum design envelope for vessels and helicopters used for the construction phase. The number of vessels/helicopters on site at any one time and the total vessel/helicopter movements (return trips) during the entire construction phase are presented in this table. The vessel numbers presented in Table 3.31   Open ▸ are an estimated maximum design scenario for the purposes of the assessment, and it is anticipated that vessel and helicopter numbers will be less than those presented in reality. The maximum number of vessels is 87 on site at any one time with up to 7,834 return trips.

 

Table 3.31:
Maximum Design Envelope: Infrastructure Installation – Vessels and Helicopters

Table 3.31: Maximum Design Envelope: Infrastructure Installation – Vessels and Helicopters

 

  1. Jack-up vessels or barges touch down on the seabed when their jack-up spud cans (base structure of each leg) are lowered into place. Jack-up vessel parameters are presented in Table 3.32   Open ▸ .

 

Table 3.32:
Maximum Design Envelope: Jack-up Vessels

Table 3.32: Maximum Design Envelope: Jack-up Vessels

 

3.4.3. Construction Ports

  1. Fabrication of components for the Array infrastructure is likely to occur at a number of manufacturing sites including those located within Scotland, the United Kingdom (UK), Europe, the Middle East and the Far East. It is likely that components will be transported to final assembly yards on the east coast of Scotland for final fabrication or integration before being towed to the Array.
  2. It is anticipated that all components will be transported to the Array for installation via sea transport using vessels and associated equipment. It is not anticipated that large components (e.g. wind turbine blades) will be transported via road.
  3. At time of writing this Array EIA Report, the Applicant is yet to determine which construction port(s) will be used for the storage, fabrication, pre-assembly and delivery of the Array infrastructure. The Applicant will determine suitable ports based on the facilities available to handle and process components for the Array. Port selection will take into account logistics to reduce towing distance of foundations and integrated turbines as far as practicable. The Applicant anticipates that established port licences and operational controls will cover all activities associated with the Array which are carried out within port. In order to assess a MDS, the assessments within this Array EIA Report consider a maximum number of vessels and vessel movements to/from site, where relevant from the east coast of Scotland or England.
  4. Construction personnel will transit to the Array location on the installation vessels or other vessels listed in Table 3.31   Open ▸ . CTVs, SOVs, or helicopters operating from a licenced airfield may be used to transfer crew between the port facility and the Array location during construction, operation and decommissioning.

3.4.4. Construction Programme

  1. The indicative construction programme for the Array is provided below. This indicative construction programme, including the estimated commencement and completion dates, and estimated durations of activities, has been used within the technical chapter assessments of construction impacts.
  2. As described at paragraph 116, there is no intention to wet store integrated turbines within the limits of the final integration and marshalling port. The location of the final integration and marshalling port is currently unknown. The Applicant are currently developing a fabrication, delivery and integration strategy and engaging with a number of port and harbour operators to identify an optimised approach.  In the absence of an integration and marshalling yard it is not possible, at this stage, to consider the potential site-specific impacts on relevant receptors. The Ossian construction programme will be managed to reduce the requirement for storage of integrated pre-commissioned turbines within port. A stock of floating foundations will be accumulated, and mooring lines and cables would be installed within the array in advance of turbine integration. The Applicant aims to minimise any wet storage requirements by towing integrated turbines to their final location within the array as soon as they are ready, subject to suitable weather conditions for transfer. Enabling works, including integration, and marshalling activities, required within the final integration port to cover turbine pre-commissioning, testing and storage (if required) will be covered by the consenting requirements applying to them (including any requirements for environmental assessment) and will be managed by the port or harbour authority with support where appropriate from the Applicant.
  3. The Array will be built out over a period of up to eight years including site preparation works. Separate campaigns will be undertaken for the relevant assets and are likely to occur concurrently across the eight year construction period. It should be noted that the activities listed below will not occur continuously throughout the eight year period, rather, the programme indicates the period within which these activities could occur. Increased construction activity is anticipated within the spring to autumn months, with limited works undertaken at site during the winter period.
  4. The indicative construction programme is as follow
  • Commencement of offshore construction phase (site preparation activities) expected Q2 2031;
  • Completion of construction expected Q4 2038;
  • Key construction activity and estimated durations:

–           Site preparation activities – estimated seven year duration between Q2 2031 and Q4 2037. These works will not be continuous;

–           Floating turbine mooring and anchoring installation – estimated seven year duration between Q2 2031 and Q4 2037. These works will not be continuous;

–           OSP topsides and fixed jacket foundations installation/commissioning – will occur for the duration of the construction period but will not be continuous;

–           inter-array and interconnector cables installation – will occur for the duration of the construction period but will not be continuous; and

–           floating wind turbine and floating foundation installation/commissioning – estimated seven year duration between Q2 2032 and Q4 2038. These works will not be continuous.

3.4.5. Recommended Safe Passing Distances and Aids To Navigation

Safety zones, recommended safe passing distances and Notice to Mariners

  1. The Applicant will communicate with other mariners of safe clearance distances around construction, installation, maintenance and decommissioning activities during the construction and operation of the Array as per standard practice and guidance.

Statutory safety zones

  1. Volume 1, chapter 2 describes the legislation for establishment of statutory safety zones. The Applicant intends to apply for the following safety zones for the Array:
  • temporary (or rolling) 500 m safety zones surrounding the location of all surface piercing structures where construction work is being undertaken by a construction vessel;
  • 50 m safety zones around all partially completed or completed surface piercing structures which are not yet fully commissioned during the construction phase; and
  • 500 m around any structure where major maintenance is ongoing (major maintenance works are defined within the Electricity (Offshore Generating Stations) (Safety Zones) (Application Procedures and Control of Access) Regulations 2007.
  1. The Applicant will apply for statutory decommissioning safety zones during the decommissioning phase (as appropriate) which are not anticipated to exceed the standard 500 m safety zone.

Recommended safe passing distances

  1. The Applicant may use recommended safe passing distances during the construction, operation and maintenance and decommissioning phases for the safety of third party vessels. Notice to Mariners (NtMs) will be used to communicate these to sea users during all phases of the Array.

Aids to navigation

  1. The floating wind turbines and OSPs will be lit and marked to aid navigation. The Lighting and Marking Plan (LMP) for the Array will be defined post consent in consultation with the Northern Lighthouse Board (NLB), Maritime and Coastguard Agency (MCA), the Civil Aviation Authority (CAA) and the Ministry of Defence (MOD).
  2. Marine aids to navigation will be provided throughout the lifetime of the Array in accordance with the requirements of the NLB, MCA and MOD, and in adherence to Civil Aviation Publication (CAP) 393 Article 223 (CAA, 2016 (as amended)), unless otherwise agreed. Monitoring and maintenance of all navigational aids associated with the Array will be undertaken so that the relevant CAA availability targets are met.