2.3.7. Scour Protection for Foundations

  1. Natural hydrodynamic and sedimentary processes can lead to seabed erosion and ‘scour hole’ formation around anchor and mooring systems, and foundation structures. Scour hole development is influenced by the shape of the foundation structure, seabed sedimentology and site-specific metocean conditions such as waves, currents, and storms. Employing scour protection can mitigate scour around foundations. Commonly used scour protection types include:
  • concrete mattresses: cast of articulated concrete blocks, several metres wide and long and linked by a polypropylene rope lattice, which are placed on and/or around structures to stabilise the seabed and inhibit erosion;
  • rock: layers of graded stones placed on and/or around structures to inhibit erosion, or rock filled mesh fibre bags which adapt to the shape of the seabed/structure as they are lowered on to it; or
  • artificial fronds: mats which are several metres wide and long and composed of continuous lines of overlapping buoyant polypropylene fronds that create a drag barrier, preventing sediment in their vicinity being transported away. The frond lines are secured to a polyester webbing mesh base which is secured to the seabed by a weighted perimeter or anchors pre-attached to the mesh base.
  1. Rock placement is the most frequently used scour protection method. This involves the placement of crushed rock around the base of some types of foundation structures.
  2. The type and volume of scour protection required will vary depending on the foundation types considered, and the final parameters will be decided once the design of the foundation structure is finalised. This decision will consider a range of aspects including geotechnical data, meteorological and oceanographical data, water depth, foundation type, maintenance strategy, and cost.

2.3.8. Inter-array Cables

  1. Inter-array cables carry the electrical current produced by wind turbines to an OSP. So as not to hinder the movement of the floating wind turbine substructures, it is proposed that dynamic inter-array cables will be used. There are several cable designs which may be used, however, the most likely to be used for the Array is a ‘lazy-s’ configuration which allows extension of the cables in response to the floating substructure movements. Buoyancy modules are attached to the dynamic inter-array cable to support the weight of the cable and provide the ‘lazy-s’ configuration in the water column, and bend restrictors help to reduce the fatigue in the inter-array cables. Bend restrictors are typically used where the cable exits the floating substructure and at touchdown points of the cable on the seabed.
  2. From the point at which the dynamic cable transitions to static, the section of static cable on the seabed will be protected in line with the output of the Cable Burial Risk Assessment (CBRA), typically using either cable burial methods or external cable protection (comprising graded rock, concrete mattresses, or similar). Where crossing pre-existing cables, pipelines or exposed bedrock, inter-array cables will be protected with a hard protective layer (such as rock or concrete mattresses), as required. Where cable protection is required, the protection measure will be dependent on several factors such as seabed conditions, seabed sedimentology and the physical processes at the Array. A schematic of the dynamic/static inter-array cabling system is presented in Figure 2.6   Open ▸ .

Figure 2.6:
Typical Schematic of the Dynamic/Static Inter-array Cable System (Subject to Detailed Design Configuration)

Figure 2.6: Typical Schematic of the Dynamic/Static Inter-array Cable System (Subject to Detailed Design Configuration)

 

  1. Different approaches and techniques are available for installation of the inter-array cables laid on the seabed. The final choice of installation will be subject to review of the seabed conditions and the CBRA. The following list details some of the installation tools that will be considered to achieve cable burial where required:
  • 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 lift 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. The cable installation methodology and potential cable protection measures will be described in detail in the Project Description chapter of the Array EIA Report and finalised at the final design stage (post-application).
  2. Sand wave and boulder clearance may also be required prior to cable installation to maximise the potential for cable burial. The anticipated MDS of seabed affected, and volumes of material required will be described in the Project Description chapter of the Array EIA Report.
  3. The maximum design envelope for inter-array cables is presented in Table 2.8   Open ▸ .

 

Table 2.8:
Maximum Design Envelope: Inter-Array Cables

Table 2.8: Maximum Design Envelope: Inter-Array Cables

 

2.4. Offshore Construction Phase

  1. Construction of the Array is expected to occur over a period of nine years cumulatively aligning with the following indicative construction series:
  1. seabed preparation activities (including, sand wave and boulder clearance, Unexploded Ordnance (UXO) clearance and pre-construction surveys);
  2. anchoring and mooring installation;
  3. wind turbine and OSP integration (in the case of floating OSPs) with floating foundation at harbour;
  4. Towing of integrated floating substructure and turbine to site;
  5. wind turbine and OSP foundation installation/commissioning (in case of using floating OSPs), including scour protection where required;
  6. OSP installation/commissioning (in case of using fixed type OSP);
  7. inter-array and interconnector cables installation, including cable burial or protection, where required; and
  8. wind farm commissioning.
  1. Pre-construction surveys, including geophysical and geotechnical surveys, may be carried out to provide further information of UXO, bedform and mapping of boulders, bathymetry, topography and sub-surface layers. The possibility exists for UXO originating from World War I or World War II to be encountered within the Array during the construction phase. Due to the health and safety risks posed by UXO, it is necessary for UXO to be surveyed and managed carefully. If UXOs cannot be avoided through micrositing, for example, or relocated, UXO will be cleared by a specialist contractor in advance. Detailed design work would be required to confirm planned locations of infrastructure, prior to conducting any UXO surveys.
  2. Sand wave and boulder clearance may be required, in particular for the inter-array and interconnector cable laying, to provide a relatively flat seabed surface, remove mobile sediments and boulder obstructions to maintain required cable burial depth, and reduce the risk of damage to cables. Boulder clearance will also ensure minimal disruption to installation of mooring and anchoring systems and foundations, and jack-up vessel activities.
  3. The offshore construction phase may be supported by various vessels, including Anchor Handling Tug Supply (AHTS) vessels, Service Operation Vessels (SOVs), Crew Transfer Vessels (CTVs), jack-up or floating Heavy Lift Vessels (HLV), support vessels, cable lay vessels, pre-lay survey vessels, Remotely Operated Vehicle (ROV) deployment vessels, rock installation vessels, service and commissioning support vessels, and guard vessels.
  4. Moorings and anchoring systems will be transported and pre-laid at the installation location (yet to be confirmed), prior to installation of the floating substructures and wind turbine generators. Floating substructures and wind turbine generators (comprised nacelle, rotor blades, hub, and towers) will be assembled and integrated at the final assembly yard and wind turbine assembly yard and towed to the installation location using a AHTS vessel, or similar. If floating OSPs are chosen, the floating OSP substructures and OSP topsides will also be assembled and integrated at an assembly yard and towed to the final installation location. At the installation location, the integrated floating wind turbines and floating OSPs (if chosen) will be installed and hooked up to the pre-installed mooring system. Following connection to the necessary cabling, a process of testing and commissioning will be undertaken.

2.5. Operation and Maintenance Phase

  1. The overall operation and maintenance strategy will be finalised once the operation and maintenance base location and technical specification of the Array are known, including wind turbine type, electrical export option and final project layout. Routine operation and maintenance works will be conducted using SOV, helicopter, drones, and/or CTV. For infrequent major operation and maintenance works, including major component replacements, turbines and OSPs (in the event that floating substructures are used for the latter) will be decoupled from their mooring and anchoring systems and towed to a suitable port facility. HLVs and/or jack-up vessels will be used for infrequent major maintenance campaigns associated with the OSPs, in the event that fixed bottom foundations are used. ROVs will be used to inspect foundations, mooring and anchoring systems, and cabling.
  2. Offshore operation and maintenance will comprise of both preventative and corrective activities. The details of estimated annual and total operation and maintenance activities will be detailed within the Project Description chapter of the Array EIA Report.

2.6. Decommissioning Phase

  1. Under Section 105 of the Energy Act 2004 (as amended), developers of offshore renewable energy projects are required to prepare a Decommissioning Programme for approval by Scottish Ministers. A Section 105 notice is issued to developers by the regulator after consent or marine licence has been issued for the given development. Developers are then required to submit a detailed plan for the decommissioning works, including anticipated costs and financial securities. The programme will consider good industry practice, guidance and legislation relating to decommissioning at that time. The programme will be consulted on by stakeholders and will be publicly available.
  2. The Array EIA Report will provide an overview of the anticipated decommissioning events and an assessment of the potential significant effects of this phase on receptors.

2.7. Designed in Measures

  1. The following designed in measures will be included within the Array design and will be considered in assessment in the Array EIA Report. These are summarised in Appendix 2:
  • spacing between wind turbines within the Array will be sufficiently distant (at least 1,000 m) so that wake effects or changes to the wave field will be mitigated;
  • spacing between wind turbines within the Array will be identified taking into account navigational risk and considered through the Navigational Risk Assessment (NRA) process;
  • scour protection will be deployed around Array mooring and anchoring systems and foundations where required;
  • implementation and monitoring of cable protection around Array cables through the development of and adherence to a Cable Plan (CaP);
  • implementation and monitoring of cable protection (via burial, or external protection where adequate burial depth as identified via CBRA is not feasible) with any damage, destruction or decay of cables notified to MCA, Northern Lighthouse Board (NLB), Kingfisher and UK Hydrographic Office (UKHO) no later than 24 hours after discovered;
  • development of and adherence to an appropriate Code of Construction Practice (CoCP);
  • the development of, and adherence to, an Environmental Management Plan (EMP), including a Marine Pollution Contingency Plan (MPCP) and Invasive Non-Native Species Management Plan (INNSMP);
  • the development of, and adherence to a Decommissioning Programme;
  • implementation of soft-start and ramp-up measures for piling activities and UXO clearance;
  • the development of and adherence to a Vessel Management Plan (VMP), or equivalent;
  • the development of and adherence to a Marine Mammal Mitigation Plan (MMMP) to outline the additional mitigation to be implemented for piling and UXO clearance;
  • the development of and adherence to a Piling Strategy (PS) (or equivalent, after consultation with stakeholders) which will set out the mitigation measures including soft-start and ramp-up measures;
  • use of low order deflagration (ideally, and where possible) for UXO clearance;
  • ongoing consultation with the fishing industry and appointment of a Fisheries Liaison Officer (FLO);
  • development of a Fisheries Management and Mitigation Strategy (FMMS);
  • adherence to good practice guidance with regards to fisheries liaison (e.g., Fishing Liaison with Offshore Wind and Wet Renewables Group (FLOWW), 2014, 2015);
  • timely and efficient distribution of Notice to Mariners (NtM), Kingfisher notifications and other navigational warnings of the position and nature of works associated with the Array;
  • use of guard vessels, as appropriate;
  • implementation of a Navigational Safety Plan (NSP), or equivalent;
  • liaison with Fisheries Industry Representatives (FIRs), as appropriate;
  • compliance with MGN 654 (MCA, 2021) and its annexes where applicable;
  • development of, and adherence to, a Development Specification and Layout Plan (DSLP), or alternative;
  • development of, and adherence to, a Lighting and Marking Plan (LMP) or equivalent;
  • development of, and adherence to, an Emergency Response Cooperation Plan (ERCoP);
  • notification to the UKHO of the proposed works and appropriate marking on UKHO Admiralty charts;
  • buoyed construction area in agreement with NLB;
  • application for safety zones of up to 500 m during construction, periods of major maintenance and decommissioning;
  • use of advisory safety distances around vessels undertaking construction, major maintenance, and decommissioning activities;
  • marine coordination and communication to manage project vessel movements;
  • compliance of project vessels with international marine regulations as adopted by the Flag State, including the International Regulations for Preventing Collisions at Sea (COLREGs) (International Maritime Organization (IMO), 1972/77) and the International Convention for the Safety of Life at Sea (SOLAS) (IMO, 1974);
  • marking and lighting of the site in agreement with NLB and in line with International Association of Marine Aids to Navigation and Lighthouse Authorities (IALA) Recommendations (IALA, 2021a) and Guidance (IALA, 2021b);
  • compliance with regulatory expectations on moorings and anchoring systems for floating wind and marine devices (Health and Safety Executive (HSE) and MCA, 2017);
  • blade clearance of at least 22 m above the water line, accounting for pitch and roll as per MGN 654;
  • fitting of aviation lighting on offshore wind turbines to all required turbines in accordance with Civil Aviation Publication (CAP) 764 (Civil Aviation Authority (CAA), 2016) and will be set out for consultation within a Lighting and Marking Plan (LMP) to be approved post-consent by Marine Scotland;
  • Helicopter hoist status lighting will be located on the nacelle of the wind turbine in accordance with CAP 437 (CAA, 2021) and will be set out for consultation within a LMP, or equivalent plan, to be approved post-consent by Marine Scotland;
  • the implementation of Archaeological Exclusion Zones (AEZs) around sites identified as having a known important archaeological potential to ensure that all offshore infrastructure will be located to avoid any known wrecks (50 m to 100 m buffer);
  • archaeological input into specifications for and analysis of future preconstruction geophysical surveys;
  • archaeologists to be consulted in the preparation of any preconstruction ROV or diver surveys and in monitoring/checking of data, if appropriate;
  • all anomalies of possible archaeological potential will be reviewed against the final layout and design. If they are likely to be impacted, these anomalies would undergo further archaeological investigation. Should these anomalies prove to be of archaeological importance then future AEZs may be implemented following consultation with Historic Environment Scotland (HES);
  • archaeological input into specifications for and analysis of future preconstruction works. This might include the presence of a geoarchaeologist on board the survey vessel and a provision for sampling, analysis and reporting of recovered cores;
  • commitment to preparation and consultation with Historic Environment Scotland, on an Offshore Written Scheme of Investigation (WSI) and Protocol of Archaeological Discoveries (PAD) prior to any interaction with the seabed;
  • archaeologists to be consulted in advance of pre-construction site preparation activities and, if appropriate, to carry out watching briefs of such work;
  • micro-siting of wind turbine foundation anchors and mooring lines to avoid known wrecks;
  • mitigation of unavoidable direct impacts on known sites of archaeological importance. Options include i) preservation by record, ii) stabilisation and iii) detailed analysis and safeguarding of otherwise comparable sites elsewhere;
  • distance of the Array from the locations of sensitive seascape, landscape and visual receptors;
  • ongoing engagement with oil and gas operators to coordinate activities and facilitate coexistence where possible and appropriate to do so;
  • increasing the share of local/national content through meet the supplier events and contracting requirements;
  • refinement of commitments underpinning the Supply Chain Development Statement (SCDS);
  • Best Practicable Means (BPM) will be used to reduce the impacts of airborne noise upon sensitive receptors; and
  • monitoring of airborne noise related complaints and undertaking appropriate remedial action.