Scour protection for foundations

  1. Foundation structures for wind turbines and substations are at risk of seabed erosion and ‘scour hole’ formation due to natural hydrodynamic and sedimentary processes. The development of scour holes is influenced by the shape of the foundation structure, seabed sedimentology and site-specific metocean conditions such as waves, currents and storms. Scour protection may be employed to mitigate scour around foundations. There are several commonly used scour protection types, including:
  • concrete mattresses: several metres wide and long, cast of articulated concrete blocks which are linked by a polypropylene rope lattice which are placed on and/or around structures to stabilise the seabed and inhibit erosion;
  • rock placement: either layers of graded stones placed on and/or around structures to inhibit erosion or rock filled mesh fibre bags which adopt the shape of the seabed/structure as they are lowered on to it; or
  • artificial fronds: mats typically several metres wide and long, composed of continuous lines of overlapping buoyant polypropylene fronds that create a drag barrier which prevents sediment in their vicinity being transported away. The frond lines are secured to a polyester webbing mesh base that is itself secured to the seabed by a weighted perimeter or anchors pre-attached to the mesh base.
    1. The most frequently used scour protection method is ‘rock placement’, which entails the placement of crushed rock around the base of the foundation structure.
    2. The amount of scour protection required will vary for the two foundation types being considered for the Proposed Development. The final choice of scour protection will be made after design of the foundation structure, taking into account a range of aspects including geotechnical data, meteorological and oceanographical data, water depth, foundation type, maintenance strategy and cost. Scour protection PDE parameters for foundations with piled jackets and suction caissons are presented in Table 1.15   Open ▸ .

 

Table 1.15:
Scour Protection Parameters – Wind Turbine Foundations and OSP/Offshore Convertor Station Platform

Table 1.15: Scour Protection Parameters – Wind Turbine Foundations and OSP/Offshore Convertor Station Platform

 

Subsea cables

  1. The type of cable laying vessel that will be used to lay subsea cables on the seabed has not been selected at this time. Therefore, the maximum design envelope accounts for both the use of a Dynamic Positioning (DP) vessel and vessels which require the use of anchor during cable laying activities (see Table 1.16   Open ▸ to Table 1.19   Open ▸ ).

Inter-array cables

  1. Inter-array cables carry the electrical current produced by the wind turbines to an offshore substation platform or an offshore convertor station platform. A small number of wind turbines will typically be grouped together on the same cable ‘string’ connecting those wind turbines to the substation/convertor platform, and multiple cable ‘strings’ will connect back to each offshore substation/convertor platform.
  2. The inter-array cables will be buried where possible and protected with a hard protective layer (such as rock or concrete mattresses) where adequate burial is not achievable, for example where crossing pre-existing cables, pipelines or exposed bedrock. The requirement for additional protection will be dependent on achieving target burial depths which will be influenced by several factors such as seabed conditions, seabed sedimentology, naturally occurring physical processes and possible interactions with other activities including bottom trawled fishing gear and vessel anchors. There is the potential for seabed preparation to be required prior to cable installation with methods such dredge and deposit of sediments material, use jet trenchers, mechanic trenchers or grapnels currently being considered. The cable installation methodology and potential cable protection measures will be finalised at the final design stage (post-consent). The PDE for inter-array cables is presented in Table 1.16   Open ▸ .
Table 1.16:
Design Envelope: Inter-Array Cables

Table 1.16: Design Envelope: Inter-Array Cables

 

Interconnector cables

  1. Interconnector cables will be required to connect the OSPs/Offshore convertor station platforms to each other in order to provide redundancy in the case of failures within the electrical transmission system. The cables are likely to consist of a cross-linked polyethylene (XLPE) insulated aluminium or copper conductor cores.
  2. These cables will be either HVDC or a combination of HVDC and HVAC. Table 1.17   Open ▸ provides the maximum design scenario for interconnector cables.
  3. The interconnector cables will have a minimum target burial depth of 0.5 m. If burial is not possible due to ground conditions or target burial depths not being achievable, then cable protection techniques will be employed (paragraph 78). The total length of interconnector cables will not exceed 94 km. There is the potential for seabed preparation to be required prior to cable installation with methods such dredge and deposit of sediments material, use jet trenchers, mechanic trenchers or grapnels currently being considered.

 

Table 1.17:
Design Envelope: Interconnector Cables

Table 1.17: Design Envelope: Interconnector Cables

 

Offshore export cables

  1. Offshore export cables are used for the transfer of power from the OSPs/Offshore convertor station platforms to the transition join bay at landfall where they become onshore export cables. Up to eight offshore export cables will be required (applicable to both Combined and HVDC Options).
  2. The offshore export cables will have a maximum total length of 872 km, comprised of up to eight cables connecting the OSPs/Offshore convertor station platforms to landfall at Skateraw. Each of these offshore export cables will be installed in a trench up to 2 m wide with a target burial depth of between 0.5 m and 3 m per cable.
  3. Although the Proposed Development export cable corridor has been identified, the exact route of the offshore export cables is yet to be determined and will be based upon geophysical and geotechnical survey information. This information will also support the decision on requirements for any additional cable protection. Flexibility is required in the location, depth of burial and protection measures for the offshore export cables to ensure physical and technical constraints, changes in available technology and Project economics can be accommodated within the final design.
  4. The proposed method for the installation of the offshore export cables through the intertidal zone at landfall at Skateraw is by using a trenchless technique burial method ( Figure 1.9   Open ▸ ). Following punch out of offshore export cables, onwards installation to the wind farm will be completed by using jetting, trenching and ploughing as summarised in Table 1.18   Open ▸ , noting pre-sweeping/dredging may be required in some areas.

 

Table 1.18:
Design Envelope: Offshore Export Cable Method of Installation

Table 1.18: Design Envelope: Offshore Export Cable Method of Installation

  1. The maximum design scenario for the offshore export cables is described in Table 1.19   Open ▸ .

 

Table 1.19:
Design Envelope: Offshore Export Cables

Table 1.19: Design Envelope: Offshore Export Cables

 

Cable protection

  1. Cable protection will be used to prevent movement or exposure of the cables over the lifetime of the Proposed Development when target cable burial depth is not achieved due to seabed conditions. This will protect cables from other activities such as fishing or anchor placement, dropped objects, and limit the effects of heat and/or induced magnetic fields. Cable protection may comprise sleeving, cast iron shells, concrete mattressing or rock placement. The preferred solution for protection will depend on seabed conditions along the route and the need to protect cables from other activities which may occur in that area.
  2. The maximum design scenario for inter-array, interconnector and offshore export cables, are presented in Table 1.20   Open ▸ .

 

Table 1.20:
Design Envelope: Cable Protection Parameters

Table 1.20: Design Envelope: Cable Protection Parameters

 

Concrete mattressing

  1. Concrete mattresses are constructed using high strength concrete blocks and U.V. stabilised polypropylene rope. They are supplied in standard 6 m x 3 m x 0.3 m units of standard density, however modifications to size, density, and shape (tapered edges for high current environments, or denser concrete) can be engineered bespoke to the locality.
  2. The mattresses can be installed above the cables with a standard multicat type DP vessel and free-swimming installation frame. The mattresses are lowered to the seabed and once the correct position is confirmed, a frame release mechanism is triggered and the mattress is deployed on the seabed. This single mattress installation is repeated for the length of cable that requires protection. The mattresses may be gradually layered in a stepped formation on top of each other dependant on expected scour. Concrete mattressing can be used for cable protection and at cable crossings.

Rock placement

  1. Rock placement on top of cables to provide additional protection is carried out either by creating a berm or by the use of rock bags (see Figure 1.8   Open ▸ ).

Figure 1.8:
Rock Cable Protection Methods (Left: Rock Placement; Right: Rock Bags)

Figure 1.8: Rock Cable Protection Methods (Left: Rock Placement; Right: Rock Bags)

  1. Rock placement is achieved using a vessel with equipment such as a ‘fall pipe’ which allows installation of rock close to the seabed. The rock protection design for the Proposed Development will be within a maximum height of 3 m and 20 m width (see Table 1.20   Open ▸ ), with an approximate slope of 1:3 both sides of the cable. This shape is designed to provide protection from anchor strike and anchor dragging, and to allow over trawl by fishing vessels. The cross-section of the berm may vary dependent on expected scour. The length of the berm is dependent on the length of the cable which requires protection.
  2. Alternatively, pre-filled rock bags can be placed above the cables with specialist installation beams. Rock bags consist of various sized rocks contained within a rope or wire net. Similar to the installation of the concrete mattresses, they are lowered to the seabed and when in the correct position, are deployed on to the seabed. Typically, each rock bag is 0.7 m in height and has a diameter of 3 m. Rock placement can be used for cable protection and at cable crossings. The number of rock bags required is dependent on the length of cable which requires protection.

Cable crossing

  1. Up to 16 cable crossings may be required for the offshore export cables. The offshore export cables will cross each of the Neart na Gaoithe cables and will avoid crossing each other. This will be facilitated by the installation of standard cable crossing designs, likely to be comprised of ducting, concrete mattresses or rock as described above. Offshore export cables will avoid crossing interconnector cables. The maximum design scenario for cable crossing is presented in Table 1.20   Open ▸ .
  2. It is also possible that up to 78 inter-array cable crossings will be required. Additional cable protection will be required at these crossings, and these crossings and the required protection are accounted for in Table 1.20   Open ▸ . The design will look to minimise cable crossings with up to 78 inter-array crossings predicted in total.

 

Table 1.21:
Design Envelope: Cable Crossing Parameters (Inter-Array Cables and Offshore Export Cables)

Table 1.21: Design Envelope: Cable Crossing Parameters (Inter-Array Cables and Offshore Export Cables)