“Scottish Enterprise has supported QED Naval on the commercialisation of their Subhub Tidal Platform technology with a £469,000 SMART R&D grant. This £1.9m project aims to address all the technical and commercial risks associated with Subhub technology and has culminated in the build and successful testing of the ballast system.”
The SMART R&D project which is now complete culminated 6 years of technical and commercial development by QED Naval including the SMART Feasibility Study commencing in September 2012 and completing with the SMART R&D project in May 2019.
Performance and Extreme Loading Assessment
The initial stage of Subhub platform development was to define the extreme wave and tidal loads during operation on the seabed; this entailed developing fluid loading CFD models to replicate the Subhub in a flow followed by wave conditions then combining them along with the thrust and torque loads of the tidal turbines.
The results from these models defined the required On-Bottom-Weight to secure the Subhub to the seabed with some conservative factors of safety. They also defined important performance parameters confirming significant increases in average flow rates over the disc area of the tidal turbines, typically 5%/19%/5% increases for the port, centre and starboard turbines respectively.
FloWave Tank Testing (Edinburgh)
Performance Model (PM) installed in the FloWave test tank
The results from these numerical models were then validated using physical models built by QED then tested at the FloWave tank testing facility at Edinburgh University which commenced on 14th November 2014. This test tank can replicate any wave and tidal condition anywhere in the world and our work was well supported by the centre’s staff.
The outcomes were incredibly valuable and formed a key part of our knowledge on the extreme loading from the combination of the waves and tides. In real terms, there was very little data on this highly non-linear loading environment so this work was invaluable in de-risking the design.
QED Naval worked in collaboration with Schottel Hydro to develop a model scale turbine to assess the performance aspects of the Subhub. The turbine was tested with and without the Subhub then compared to previously existing data with very impressive results. The testing validated the numerical results very well in terms of wave and tidal loading and performance which, broadly speaking, produced a 10%/40%/10% increase in power output in the port, centre and starboard turbines respectively.
Operations Model Testing
The Operations Model was designed in accordance with our prescribed process and built to validate the stability analysis and assess the different installation methods. It spearheaded the development of the pneumatically controlled ballast system and data acquisition system with tank pressure and level sensors.
Sheltered Water Testing (Leith Docks)
The sheltered water trials of the Operations Model started in July 2015 and was conducted at Forth Estuary Engineering (now Dales Marine) at Leith Docks, an ideal facility for testing a 1/20th scale model which weighed in at just under 2 tons when fully equipped.
Several design changes in the tank structure, pneumatic pipe system and seawater inlets valve were made and informed the future design before successful installation and retrieval trials were performed. This work culminated in fundamental design changes in the Subhub’s stability and installation methodology.
Towing trials were also conducted within the dock to assess the drag loads, towing speeds and stability during transit to the site.
High Flow Trials (Fort William)
Following the success of the sheltered water trials in Leith, QED Naval moved their equipment to The Underwater Centre (TUC) at Fort William (sadly now closed). Again this offered ideal conditions for high flow trials on the Subhub Operations Model which ran from October to November 2015. Within a few metres of the hard stand shore the measured flow rates were in excess of 1m/s (which was the scaled equivalent of 2m/s at community scale). The water depths varied from 3.0m to 7.0m depending on the state of the tide which represents the upper end of the scaled depth parameters for Subhub of 3D to 7D, where D is the diameter of the turbine. The Basis Of Design (BOD) for the Subhub is 2D to 5D.
The results were extremely positive since they successfully demonstrated the speed and control of the installation and retrieval process, less than 15 minutes in both cases. At any stage of the process, the Ops Model could be slowed and/or reversed within seconds demonstrating that the Subhub is self-installing and self-aligning to the flow therefore no requirement for complex alignment operations on the seabed.
Using the Subhub’s buoyancy as an installation and retrieval method is simple and flexible over a broad range of wave and tidal flow conditions. For example, for a short period when a squall ran up the Great Glen bringing with it reasonable sized waves, it showed that installation and retrieval process was unhindered by waves equivalent to a significant wave of 1.2m at community scale.
Detailed Design and Design Reviews
The lessons learned from the Performance and Operations model testing were applied to the Subhub Community Demonstrator. The initial BOD was applied to a Subhub design for testing at EMEC in collaboration with Tocardo. However, given ROC feed in tariffs ended in April 2017, Tocardo withdrew from this partnership to focus on their own platform.
Consequently, QED Naval formed a new partnership with Schottel Hydro which required a new BOD based around testing in Strangford Lough using Queen’s University Belfast tidal test site; ideal testing conditions given its ease of access and varied site conditions specifically water depths ranging from 10m to 25m. Flow rates on the QUB site ranged from 1.5m/s to 3.0m/s. Interestingly, there is a significant variation in the seabed’s makeup from sand to stony to rock.
Upon completing the detailed design analysis whereby all the high priority risks were mitigated, SgurrEnergy (now Wood) completed an independent design review which led to the publication of a report approving the Subhub design. This milestone meant that the design could move to the production of manufacturing outputs and financial close in terms of this stage of the R&D funding rounds.
Procurement and Manufacture
The Bill Of Materials database is maintained by QED’s Office Manager; it shows that 60% of the items are procured in Scotland and has indirectly supported over 200 jobs.
The tender to fabricate the Subhub was won by Belfast based company Cimpina who have excellent facilities within their fabrication hall located on Queen’s Island, providing good access to quayside facilities for launch. The team at Cimpina are all highly qualified through DNV GL standards.
The build took just under 6 months to complete. QED Naval completed Factory Acceptance Tests (FATs) in August 2017 where each tank was individually pressurised to determine its water tightness making minor repairs to welds where required by Cimpina.
A short period of hot work to complete outfit items was followed by the painting of the Subhub.
In January 2018 Subhub left the fabrication shed and was moved to a nearby secured area to complete outfit and furnishing.
Outfit, Commissioning and Onshore Testing
Outfitting the Subhub’s interior included fitting the ballast system pipework, data cabling, sensors, cable trays and watertight penetrations though the bulkhead. The connectors for ballast system had to be catalogued and fitted to ensure the lines ran to their respective tanks. The hydraulic valves system needed to filled and bled. The ballast control container was fully outfitted at the same time.
In September 2018, the appendages arrived at the lay down area: replica turbines, cross beam support structure and subsea manifold. These were manufactured in Scotland by AJS Production Ltd; the bolt work and connectors for the ballast system also had to be fitted.
The final parts to be assembled were the outboard ballast system: pipework, umbilicals and connectors.
All parts were assembled and commissioned ready for onshore testing. Plant hire was procured locally and full installation and retrieval simulations were performed ashore with increasing levels of simulated water depth.
Load Out and Launch
On 17th December 2018 QED Naval appointed Mar-Train Heavy Haulage Ltd to move the Subhub to Belfast Harbour’s D1 Quay in readiness for launch. This was a major operation involving street and bridge condition surveys by Mar-Train and bridge contractors.
The launch contractors and stevedores for the lift were Doyle Shipping Services who have been very supportive of the project and have provided storage for our operational equipment. DSG project managed the launch in association with Belfast Harbour with QED Naval providing lift plans.
Prior to launch, QED Naval sought approval from Marine Coastguard Agency (MCA) for permission to operate in UK waters. QED Naval employed Prolines, a Marine Warranty Surveying company to submit a load line exemption form prior to launch operation and a sailing survey was completed by MCA and MWS surveyors.
On the 18th January 2019, DSG contracted Kavanagh cranes to complete the Subhub lift into the water at D1 Quay using a dual lift from 350t and 500t cranes; the Subhub was weighed at various stages during the build and outfit to de-risk the lift. The final weigh-in was witnessed by MWS and H&W Operations Manager to certify the method used; the lift ship weight was estimated to be 143.9 tons with the actual lift weight on the day being 146.2 tons therefore within 1.6% of the final value.
MCA worked closely with the towing agents Inyanga who contracted Marine Towing Services (MTS Taktow) to complete the towing operation from Belfast Harbour to Strangford Lough. A final meeting with Belfast Harbour and their pilots took place on 18th January 2019 who took control of the overall towing operation within Belfast Harbour’s jurisdiction. The rear tow Farset from Ferran’s and Son’s, dropped off the rear tow with the pilot at buoy 15 and returned to Belfast Harbour by midnight.
Arrival in Strangford Lough was after 1400 hours on the rising flood tide through Strangford Narrows. The pilot from Cuan Marine was picked up at the entrance of the Lough and he guided the vessels through to a mooring that was pre-laid within the sheltered water anchoring area in Castle Ward Bay.
Ballast System Performance Trials
The sheltered water and flow trials of the Ops Model were repeated with the Subhub Community Demonstrator but with the full ballast control system. The installation and retrieval process was repeated under different wind, wave and tidal conditions which delivered successful results with the hydraulically operated valves, pressure and vents systems proving to be reliable and easy to control. The speed of installation was proportionate with the Ops Model that being 15 minutes to a depth of 12m or 3D.