Interconnector cables

  1. Interconnector cables connect OSPs to one another and provide redundancy should there be any failures within the electrical transmission system. It is expected that these cables will be a combination of HVAC and HVDC. The maximum design envelope is presented in Table 3.20   Open ▸ .
  2. Up to 236 km of interconnector cables will be installed within the Array. It is anticipated that cables will be protected via burial methods and will be buried at a minimum target depth of 0.4 m (subject to CBRA). External cable protection will be used in areas where minimum target burial depth cannot be achieved, as described in paragraph 61. Site preparation activities may also be required to provide relatively flat seabed surface for installation of cables and enable burial of interconnector cables to target depths. These are discussed in section 3.3.

 

Table 3.20:
Maximum Design Envelope: Interconnector Cables

Table 3.20: Maximum Design Envelope: Interconnector Cables

 

External cable protection

  1. Where minimum target cable burial depth cannot be achieved, external cable protection methods will be employed to restrict movement and prevent exposure over the lifetime of the Array. This will protect cables from activities such as fishing, anchor placement or dropped objects, and limit effect of heat and/or electromagnetic fields. External cable protection systems include concrete mattresses, rock placement, cast iron shells or polyurethane/polyethylene sleeving. The final solution(s) chosen at final design stage (post-consent) will be dependent upon seabed conditions and any potential interactions with human activities which may occur within the Array. Table 3.21   Open ▸ presents the maximum design envelope for external cable protection for inter-array cables and interconnector cables.

 

Table 3.21:
Maximum Design Envelope: External Cable Protection Parameters

Table 3.21: Maximum Design Envelope: External Cable Protection Parameters

 

                        Concrete mattressing
  1. Concrete mattresses comprising high strength concrete blocks and ultraviolet (UV) stabilised polypropylene rope may be used as a means of external cable protection for inter-array and interconnector cables and at cable crossings (paragraphs 68 and 74). The standard size of units is 6 m x 3 m x 0.3 m, however, size, density, and shape of units may be modified (within the parameters presented in Table 3.21   Open ▸ ), for example, by tapering edges of units for use in high current environments, or using denser concrete, so that they are engineered for and bespoke to the locality in which they are installed.
  2. Concrete mattresses are installed above the cables using a Dynamic Positioning (DP1) vessel and free-swimming installation frame. The mattresses are lowered to the seabed and the installation frame is released in a controlled manner once in the correct position to deploy the mattress on the seabed. This installation process is repeated for each mattress along the length of cable that requires external protection. Dependant on expected scour, mattresses may be gradually layered in a stepped formation on top of each other.
                        Rock placement
  1. Rock placement may also be utilised as a form of external cable protection for inter-array and interconnector cables and at cable crossings (paragraphs 68 and 74). Rock is placed on top of cables either by creating a berm or using rock bags ( Figure 3.16   Open ▸ ).
  2. Installation of rock placement in the form of berm creation will utilise a vessel with equipment such as a ‘fall pipe’ so that rock can be placed close to the seabed. Rock may be placed to a maximum height of 3 m and 20 m width (see Table 3.21   Open ▸ ). The berm created via rock placement will be designed to provide protection from anchor strike and anchor dragging, and to reduce risk of snagging by towed fishing gear as far as practicable in line with best practice guidance. Depending on expected scour, the cross-section of the berm may vary, and the length of the berm will be dependent on the length of the cable which requires protection.
  3. Alternatively, pre-filled rock bags may be used which will be placed above the inter-array and interconnector cables or cable crossings using installation beams. Rock bags consist of various sized rocks contained within a rope or wire net which are lowered to the seabed and deployed on to the seabed once in the correct position (similar to installation of concrete mattresses, see paragraph 70). Rock bags have typical dimensions of 0.7 m in height and 3 m diameter; the number of rock bags which may be required will be dependent on the length of cable which requires protection.

 

Figure 3.16:
Rock Cable Protection Methods (Left: Rock Placement; Right: Rock Bags)

Figure 3.16: Rock Cable Protection Methods (Left: Rock Placement; Right: Rock Bags)

 

Cable crossings

  1. Up to 12 inter-array cable crossings and up to 12 interconnector cable crossings may be installed across the Array. Cable crossings may comprise several different methods as demonstrated in Table 3.22   Open ▸ , and additional cable protection will be installed at cable crossings. Table 3.22   Open ▸ presents the maximum design envelope for cable crossings, and accounts for additional protection required.

 

Table 3.22:
Maximum Design Envelope: Cable Crossing Parameters

Table 3.22: Maximum Design Envelope: Cable Crossing Parameters

 

3.3. Site Preparation Activities

  1. Prior to the construction phase of the Array, a number of site preparation activities will be required to be undertaken. It is assumed that site preparation works will continue throughout the construction phase as required, therefore, these works may be undertaken at any point within the construction programme. A summary of site preparation activities is provided in sections 3.3.1 to 3.3.5.

3.3.1. Pre-Construction Surveys

  1. Pre-construction surveys, including geophysical and geotechnical surveys, may be carried out to provide further information of:
  • seabed conditions and morphology;
  • soil conditions and properties;
  • presence or absence of any potential obstructions or hazards; and
  • to inform detailed design for the Array.
  1. Geophysical surveys will be undertaken within the Array to provide further information of Unexploded Ordnance (UXO), bedforms and mapping of boulders, bathymetry, topography and sub-surface layers. Geophysical survey techniques to be employed include Multibeam Echosounder (MBES), magnetometer, Side-Scan Sonar (SSS), Sub-Bottom Profiler (SBP) and Ultra-High Resolution Seismic (UHRS).
  2. Geotechnical surveys will be carried out at specific locations within the Array and will employ techniques such as Cone Penetration Tests (CPTs), vibrocores, box cores, piston cores and boreholes.

3.3.2. Clearance of Unexploded Ordnance

  1. The possibility exists for UXO originating from World War I or World War II to be present within the Array. Due to the health and safety risks posed by UXO and potential interactions with planned locations of installed infrastructure and vessel activities, it is necessary for UXO to be surveyed and managed carefully before the construction phase and installation of offshore infrastructure commences.
  2. A desk-based study of the Array (Ordtek, 2022) reviewed the relevant military history in the vicinity of the Array and the likelihood of encountering UXO. Based on known military activity, the desk-based study concluded that there was a low background risk of UXO within the Array, and the likelihood of encountering different types of UXO within the Array was considered to be unlikely, meaning that it would be unusual for UXOs to be encountered within the Array. However, due to existing evidence of use in the wider area, potential for unrecorded activities such as munitions dumping, and potential for burial and migration of UXO due to natural seabed processes, the potential presence of UXOs cannot be discounted (Ordtek, 2022). Further assessment of UXOs has been undertaken within the relevant topic chapters (volume 2, chapters 7 to 20) on the basis of the desk-based study (Ordtek, 2022).
  3. Methodologies considered within the PDE to avoid/clear UXOs are as follows:
  • avoid and leave in situ;
  • micrositing of offshore infrastructure to avoid UXO;
  • relocation of UXO to avoid detonation;
  • low order technique (e.g. deflagration); and
  • high order detonation (with associated mitigation measures).
  1. Due to the health and safety risks that UXOs pose, the Applicant would seek to either avoid UXOs entirely, avoid UXOs via micrositing, or relocate UXO where practicable. If methods cannot be employed to avoid UXOs, a specialist contractor will clear UXOs in advance of further site preparation and construction works taking place. The preferred clearance method for UXO is use of a low order technique with a single donor charge of 0.25 kg Net Explosive Quantity (NEQ) for each clearance event. Up to 0.5 kg NEQ clearance shot will be required for neutralisation of residual explosive material at each location. Detailed design work would be required to confirm planned locations of infrastructure, prior to conducting any UXO surveys. The Applicant has assumed that up to 15 UXOs may require clearance based upon the desk-based study (Ordtek, 2022) and experience from other offshore wind farms in the region such as the Seagreen 1 Offshore Wind Farm. As a risk remains that unintended high order detonation may occur, 10% of clearance events have been assumed to have the potential to result in high order detonation (see volume 2, chapter 10).
  2. Table 3.23   Open ▸ presents the maximum design envelope for UXO clearance.

 

Table 3.23:
Maximum Design Envelope: Unexploded Ordnance Parameters

Table 3.23: Maximum Design Envelope: Unexploded Ordnance Parameters

 

3.3.3. Sand Wave Clearance

  1. Existing sand waves may need to be cleared in some areas of the Array prior to the installation and burial of inter-array and interconnector cables. There are two main reasons for undertaking sand wave clearance:
  • To provide a relatively flat seabed surface for cable installation and so that cable burial tools can work effectively: if cables are installed up or down a slope over a certain angle, or where the cable burial tool is working on a camber, the ability to meet target burial depths may be impacted.
  • In order for cables to be buried at the target burial depth and remain buried for the operational lifetime of the Array (35 years): as sand waves are generally mobile in nature, the cable must be buried beneath the level where natural sand wave movement could uncover it. Therefore, for this to be achieved, mobile sediments may need to be removed before cables are installed and buried.
  1. No large bed forms were observed as being prevalent across the site. It is expected based on geophysical data that if sand wave clearance is required it will be undertaken in specific discrete areas of the Array (e.g. along inter-array and interconnector cables) and could occur throughout the construction phase.
  2. Sand wave clearance techniques could include pre-installation ploughing which flattens sand waves and pushes sediment from wave crests into adjacent troughs to level the seabed. It is not anticipated that large scale dredging would be required within the site boundary.
  3. Table 3.24   Open ▸ presents the maximum design envelope for sand wave clearance. A geophysical survey campaign will be completed prior to construction which will allow the final parameters for sand wave clearance to be defined.

 

Table 3.24:
Maximum Design Envelope: Sand Wave Clearance Parameters

Table 3.24: Maximum Design Envelope: Sand Wave Clearance Parameters

 

3.3.4. Boulder Clearance

  1. Boulder clearance may be required in some areas of the Array prior to installation of offshore infrastructure, in particular, along inter-array cables and interconnector cables. A boulder is defined as being over 256 mm (Wentworth Scale) in diameter and/or length. A DP1 vessel is likely to be used to undertake the boulder clearance campaign.
  2. Boulder clearance is required to aid cable installation and increase the success rate for achieving minimum target burial depth during cable burial, therefore, reducing the risk of further cables burial works and/or the need for cable protection. Boulder clearance also reduces the risk of cable damage during installation and subsequent burial. It may also be required in the vicinity of the OSP jacket foundation locations (including within the jack-up vessel zone around the OSP foundation locations), to avoid disruption to installation activities and to ensure stability for the jack-up vessel. The maximum design envelope for boulder clearance in the Array is presented in Table 3.25   Open ▸ .
  3. Boulders may be cleared using a plough or boulder grab, however, the geophysical and pre-construction surveys, and the parameters of any boulders present (e.g. size, density and location of boulders), will inform the methodology to be used. It is possible that more than one method of boulder clearance may be deployed across the Array. Cleared boulders will be relocated to an appropriate location within the site boundary.

 

Table 3.25:
Maximum Design Envelope: Boulder Clearance Parameters

Table 3.25: Maximum Design Envelope: Boulder Clearance Parameters

 

3.3.5. Vessels for Site Preparation Activities

  1. The maximum design envelope for vessels to be used during site preparation activities is presented in Table 3.26   Open ▸ .

 

Table 3.26:
Maximum Design Envelope: Vessels for Site Preparation Activities

Table 3.26: Maximum Design Envelope: Vessels for Site Preparation Activities