Subhub Tidal Platform

“Subhub supports every stage of a tidal turbines life from integration ashore to deployment, installation and retrieval for maintenance and finally decommissioning”

Subhub Tidaly Platform ashore showing its primary role as a tripod gravity based foundation structure to support tidal turbines.

What is Subhub Tidal Platform

Subhub is a submersible, gravity based, foundation platform that supports every need of tidal turbines through life.

  • Its unique ballasting system enables it to be self installing and self aligning with the flow
  • It requires limited amount of support from commonly available tugs and barges
  • No other moorings required since it encapsulates its own solid ballast
  • Therefore only requires a single, quick offshore operation to install it making deployment an extremely low cost exercise
  • Importantly it has high availability of marine operations because it has wide weather, wave and tidal windows.
  • The hull form is carefully designed to accelerate the flow to extract the most energy from the site and overcome tidal shear
  • It’s performance is relatively uneffected by cross flow angles
  • Significant environmental benefits of no-noise installaiton with zero aesthetic impact on the seascape

Benefits of Subhub

  • Low cost  deployment using harbour tugs
  • Cheap to install and maintain using fixed barge
  • Upwards of 25% increase in energy yield of the site
  • Approximately 5% increase in site capacity factors

Integration Ashore

Full systems integration is conducted at the quayside. This is much safer and logistically easier than connecting up tidal turbine systems offshore. Most importantly it means the cost of deployment is substantially reduced since there is NO reliance put on large and specialist installation structures. Only harbour tugs and multicats are required which are commonly available and about 10x less expensive to anchor handlers and/or DP2 vessel and 15x less expensive to jack-ups and crane vessels.

Integration of the tidal turbines and cross beam whilst alongside so they can commissioned and fully tested prior to deployment.

Deployment

Subhub is a fully integrated platform. That means it has everything onboard that it requires through its life. Therefore it can be deployed in a single offshore operation with very little support required from vessels. In fact, commonly available harbour tugs is all that is required to deliver the Subhub tidal platform to the tidal site.

Subhub is designed to be towed offshore in high sea states and this has been proven in the deployment of the demonstrator in near gale and rough conditions during its transit from Belfast Harbour to Strangford Lough. Commercial pressures can force schedules and having large operating weather windows is important for maintaining deployment rates and costs.

Installation

The installation and retrieval stages of any tidal turbine is the riskiest part of any offshore operation and this is where Subhub excels. Instead of using a heavy lift crane barge or DP2 installation vessel, which are assets lent from the O&G industry, Subhub uses buoyancy and its excellent stability characteristics when submerged to dive to the seabed. QED Naval have proven this installation and retrieval in both model and full scale.

One of the key advantages of this installation method is its simplicity, reversibility and robustness against against environmental conditions. The model and demonstrator has been proven in a broad range of environmental conditions.

Subhub tethered to the installation barge which is used to maintain Subhub’s position allowing Subhub to align itself to the flow. The umbilicals contains service lines and data cables to enable the remote control of the Subhub for installation and retrieval.
Subhub in the “neutral condition” ready for installation.
Subhub in the “dive condition” on its controlled descent to the seabed.
Subhub in its “operating condition” on the seabed. Only the buoys indicate its position and clearly the installation barge would not normally be in position during operation.

Retrieval for Maintenance

Retrieval for maintenance is as simple as installation and means maintenance costs can be minimised. All that is required is a generator (400V, 3 phase) to power the Hydraulic Power Units (HPU) and an air compressor to provide Low Pressure (LP) Air. The hydraulic systems control the seawater and air inlet valves and the LP air is used to displace the water within the Subhub’s numerous seawater ballast tanks.

Subhub in the “surfacing condition” a process that takes less than 30 minutes.

Decommissioning or Re-Use

Decommissioning of the tidal field is a matter of retrieving the Subhub and towing it away along with all the rest of the electrical infrastructure contained onboard the platform. Once at the yard the platform can be assessed by Marine Warranty Survey’s and considered for refit and re-use or cut up and the steel sold as a commodity.

All the electrical and ballast system  can be stripped out and serviced and re-used. The electrical and mechanical connectors are high grade stainless steel and have long service lives so they could be sold or re-used.

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