Brims Tidal Array

Technology:

BTAL’s preferred technology for the site is the non-surface piercing OpenHydro Open-Centre Turbine, with an appropriately developed Rochdale Design Envelope to allow consideration of alternative turbine technologies within the Project Description. However, there may be a requirement for some surface piercing elements, in particular for offshore hubs/substations. Consent will be sought for a design envelope sufficiently broad to potentially include surface piercing turbine systems, as an alternative option.

The OpenHydro Open-Centre Turbine (OCT) is a bi-directional (bi-directionality ensures that the turbine can extract energy in both the ebb and flood flow directions) shrouded horizontal axis turbine (HAT). It is a simple device comprised of four key components: a direct-drive permanent magnet generator, a hydrodynamic duct, a horizontal axis rotor, and a subsea gravity base type support structure. The turbines designed for this site would have an outer diameter of up to 20m, resulting in a turbine height of up to 27m above the seabed. The size of the subsea base will depend on the site design characteristics but is likely to have a length between each foot in the order of 30m - 50m. Only the feet will be in contact with the seabed and the area of the feet on the seabed will be minimal. Each device will be capable of generating at least 1 M W but actual output will be determined through detailed design and will depend on actual individual site conditions. The location, spread and layout of devices and infrastructure will be determined through detailed planning, informed by the EIA, NRA and stakeholder consultation. The OCT design has no need for a gearbox or other complicated components and is based on a philosophy of zero maintenance between overhauls. The turbine is also seawater-cooled and lubricated by seawater, which means there is no requirement for oils or lubricants.

Alternative technologies that are included within the design envolope are all horizontal axis turbines (HAT). A number of such turbines are currently being developed which may be either shrouded or unshrouded. Devices typically have three or more blades which rotate around a nacelle and may be of fixed or variable pitch. Power take-off and generation configurations vary, from utilising direct-drive solutions with no gearbox, through to marinised wind turbine nacelles. Commercial scale devices in development have rotor diameters of up to around 20m. Some illustrative examples are demonstrated in Figures 4.6-4.10 in the Scoping Report for the Brims Tidal Array.
 

Foundations:

The support structure for the OpenHydro OCT is an unpinned gravity base structure, which is installed along with the turbine as one assembly. A general concept view of the OpenHydro subsea base arrangement is provided in Figure 4.2 below. It is likely to have three to four ‘legs’ terminating in cones to provide resistance to sliding forces imparted on the turbine by the currents. The likely length of each side of the subsea base is 30 – 50m and final dimensions would be determined during a detailed design process. 

There are a range of potential support structures/foundation types which may be associated with HAT’s. These include monopile foundations (drilled socket in the seabed), multi-leg / braced structure (with pinned piles) or gravity bases (pinned or unpinned). If an alternative technology is selected for build out on the site, the final choice of support structure would be made post-consent, at a more advanced stage of the detailed design process. This approach is analogous to established practice for other offshore renewable energy developments.
 

Export Cable:

Connection of the offshore arrays to the grid will be through one or more export cables to shore. As part of the site design process, export cable routes from the tidal array to landfall and any grid connection options will be identified based on technical and environmental criteria, as well as through discussions with the electricity network operator, regulatory authorities and stakeholders. The type and routing of the cables will be subject to further detailed electrical design for the Project.

All cables would be armoured to protect against abrasion, and where there is a lack of sediment they would be laid directly onto the seabed. In some areas pre-cast concrete mattresses or a similar form of protection may also be required on top of the cable. In inshore waters and towards landfall points, where there is sufficient sediment, cables may be buried.

For Phase I of the Project it is envisaged that there will be a requirement for between 1 to 3 export cables, with one cable leading from each sub-sea hub or offshore (surface piercing) substation back along the seabed towards the most suitable landfall location. It is assumed that mechanical excavation will be required to dig a trench into which piping will be l aid or cable directly placed. Another option may be directional drilling at the landfall site. There are a number of landfalls currently being considered including either side of Aith Hope, the coastline south of the Melsetter Estate and the coastline around Brims Ness. The final selected landfall will be closely linked to the decision of where to site the onshore substation. Broad indicative Onshore Cable Corridor Search Areas are indicated in Figure 3.1 above but this will beconfirmed by SHE-T who will be responsible for the development of the onshore substation. Tidal turbines will be inter-connected in arrays subsea. A number of factors, including turbine parameters and available technical solutions will influence the number, length, spacing and configuration of inter-array cables. It is anticipated that the majority of the cables would be laid in line with the prevailing tidal flow directions, although clearly there will be a need to interconnect within the arrays, and some cables may run across the prevailing flow directions.

The design of the offshore electrical infrastructure continues to be developed and as such there may be a requirement to have one or several offshore electrical hubs either mounted on the seabed or on a surface piercing structure. The current technical maturity of such infrastructure is not sufficient to allow a definite description to be made at this point. 

A surface piercing structure may be part of a turbine installation or a separate structure altogether. Typically, such a platform might be supported by a jacket structure, braced monopile or alternatively it could be a moored surface piercing structure. A surface piercing offshore substation would normally be unmanned with access by helicopter or vessel. In the event such a structure becomes a requirement, these may be considered for use as navigational aids, for example, to provide site marking, if appropriate. Alternatively, it may be possible to group a number of turbines together using subsea hubs, using technology currently in development. This is because, at present, the technology limits the voltages at which devices can be connected subsea, which in turn limits the MW rating of cable which can be installed. An offshore platform would bring multiple lower voltage cables together, with the power then being exported to shore on fewer, high voltage cables. The final proposal will be based on both technical and environmental assessment.
 

Onshore Infrastructure:

There are two main options for constructing a landfall:

• Directionally drilled from a near-shore location to beyond the surf zone and the offshore cable pulled through the drilled duct to shore; or
• Cable burial up an existing beach in an open trench.

In the scenario where horizontal directional drilling (HDD) is required, ideally, the location of the drilling site would be as close to the onshore substation as possible in order to minimise the distance of any onshore electrical cabling. The exact location for cable landfall has not yet been determined but Figure 3.1 above illustrates the AoS.

This is currently outside the scope of the project. It is expected that SHE-T will be responsible for the onshore substation. However, it is an important part of the development and a typical description is therefore provided below for completeness. It is currently anticipated that the following infrastructure could be included:

• Compound to house a grid transformer and connection terminations;
• A control building compound for housing switchgear, SCADA etc.; and
• An operational/personnel/office compound area.

The exact locations for the onshore infrastructure have not been defined at this stage of the design process and will be determined by SHE-T but the likely search area is shown in Figure 3.1 above. It is currently anticipated that a 33/132kV substation is required. The likely development area such a substation would require for an installed capacity of 200MW is of the order of 70m x 50m. It is likely that the area requirement for Phase I will be less than this. The actual footprint of the substation will be determined by a number of factors such as:

• Whether air or gas substation design is chosen. Air insulation requires considerably more space and i s usually used in outdoor substations. Inert gas is more commonly used where more of the infrastructure is housed indoors; and
• Whether the cables are brought vertically or horizontally into the building. The volume of the substation is likely to remain consistent in order to accommodate control and electrical equipment however the footprint of the building will be greater if the building is constrained to a single story or low rise.  The footprint may be reduced if the building is of a greater height allowing more equipment to be stacked.

It is expected that SHE-T carry out the required engineering and environmental assessments on the identified location. This substation will form the connection point for the Project to export power into the national grid.

Vessel Spread:

There are a wide range of installation and removal methodologies currently being trialled in the testing of tidal energy converters and support structures ranging from the use of moored and tugged barges, anchored crane barges, through to dynamically positioned (DP) heavy lift construction vessels. It is likely that this deployment will use DP vessels for the installation of the foundations and devices. A specialist cable laying vessel will be required to lay the export cable(s), this may require more than one vessel. Installation of the possible substation and cables could involve the use of heavy lift or DP vessels. Maintenance activities may also use a DP vessel fitted with a crane.