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Future of Energy | Catching the Flow

MeyGen

MeyGen, the world’s largest tidal stream array, is about to be expanded. This report looks closer at a scheme that is operating at the edge of human understanding.

Britain’s moonshot is how the designers of the MeyGen tidal stream project describe the first phase of a scheme that since April this year has been officially classed as a commercial power station and which is supplying energy to the UK Grid.

The technology and engineering inventiveness needed to design and install the first four of possibly hundreds of turbines sited at 31m depth under the sea have involved “working at the edge of human understanding”, says Robert Bird Group UK director John Ward.

The consultant’s team has designed structures to sit in the fast flowing waters of a 2km channel between the north east tip of Scotland and the island of Stroma where the Atlantic Ocean meets the North Sea.

“This is cutting edge territory, very exciting for engineers and an opportunity for the UK to be at the forefront of a completely new industry,” says Ward.

“And like the moonshots which led to major new engineering developments, the spin offs from our work – in fields such as fluid mechanics, fatigue analysis and software development – have applications around the world in building and infrastructure design as well as tidal energy.”

The UK’s tidal stream potential is 8.5GW, according to MeyGen developer, owner and operator Simec Atlantis. Worldwide the potential is 99GW of clean, secure and predictable energy.

This is cutting edge territory, very exciting for engineers and an opportunity for the UK to be at the forefront of a completely new industry

But the government in Westminster has yet to appreciate the potential of the world beating work that has been going on so far away from London. It pulled funding from all tidal power development in 2016 with no prospect of review until 2021.

MeyGen

MeyGen

The scale of the massive ballast blocks, each weighing 200t, becomes apparent when stood next to them

Fortunately, support from the Scottish government, a European grant and a determined developer mean that MeyGen is moving to the next phase of its planned roll out via Project Stroma.

Having proved the technology and become the world’s first tidal range utility, the next step is to upscale and bring down the costs with the aim being an undersea army of turbines delivering just shy of 400MW of rated capacity.

To that end the first four turbines are to be joined by two larger ones next year, each capable of generating 2MW. More powerful generators and larger rotor diameters will increase energy generating capacity.

And rather than the individual export cables of the original four turbines, the new units will shareva cable via a connection hub. The innovation will reduce infrastructure and installation costs.

“Project Stroma will be an important enabler for the extension of the MeyGen site by a further 80MW and ultimately to the full site capacity of 400MW,” says Atlantis chief executive officer Tim Cornelius.

Nearby sites in the Pentland Firth offer further growth potential in the UK, Cornelius says. “And larger rotor diameter turbines and subsea connection hubs will open new markets for Atlantis in places like France, South Korea, Japan and the Channel Islands.

MeyGen’s first phase, which proved the potential of future development, completed its construction and commissioning phase in March this year. To date it has generated 8GWh of energy to the Grid. The array also exported a world record 1.4GWh of electricity to the Grid in a single month, which would have powered 5,420 average UK homes.

The 3.5km long site was chosen for its significant tidal races with the shallow waters of the inner sound creating a depth averaged tidal current of up to 4.4m/s. But because of the irregular seabed, which drops to 50m in places at a treacherous “cod hole”, the flow is highly turbulent with strong eddies and vortices.

The combined effects of this energetic marine environment produce very powerful hydrodynamic forces, which the turbines and supporting structures have to be able to withstand. Added to that, the design requirement was for a 25 year maintenance-free life.

Tidal stream energy is a completely new industry. Unlike tidal range barrages such as the one proposed for Swansea where energy is produced by the relatively slow rise and fall of the tides, tidal stream captures the energy from the flow of water going past a device at substantial speed.

“Like the wind capturing the air, we are capturing the energy of moving water,” Ward explains. “Unlike wind, it’s regular and it’s reliable because tides are reliable. For every 2MW of wind infrastructure the average output is 250kW of energy. For every 2MW of tidal infrastructure you can guarantee 1.8MW of power. What this gives the tidal stream industry is fantastic potential to bring guaranteed, predictable power to the Grid that can be a base load power source which wind cannot.

“I believe 15% of the world’s energy supply could be supplied by tidal stream.”

The challenge is that the infrastructure has to be placed in areas with strong currents. “That’s the engineering problem, high waves and challenging locations.”

Ward and Robert Bird designer Laura Legnani knew that the Meygen site’s storm surge current could reach 5.2m/s, with all equipment designed for a 50 year wave 15m high. “The challenge was to work in what is considered quite shallow water where waves produce strong reactions,” Legnani says.

The chosen concept for the first phase of the MeyGen (Phase 1A) project was a gravity based structure comprising a 140t fabricated steel tripod weighted down by two 200t steel ballast blocks per leg. The 190t structure is 25m by 20m in plan and includes a 15m high cylindrical pylon which then supports the 100t, 18m diameter turbine above the seabed.

The turbine is designed to yaw about the pylon to align its rotor blades with the tidal flow. By careful consideration of the prevailing tidal streams and most onerous wave directions an isosceles tripod configuration was chosen for the base with the longest leg facing west.

Robert Bird worked with Imperial College London and the Danish Hydraulic Institute in the early project stages to better understand the turbine hydrodynamics and oceanic interaction. The team helped develop the draft International Electrical Commission standard for marine energy devices.

Like the wind capturing the air, we are capturing the energy of moving water

Based on research and first principle studies, in-depth dynamic analysis procedures were developed including multi-axial fatigue analysis to efficiently design the structure along with behavioural assessment of the combined substructure turbine system under dynamic loading. Bespoke software tools were developed and validated by the team to support the work.

“Being the world’s first turbine array, many design aspects lay outside existing codes. In particular the behaviour of structures in marine environments with large waves and strong currents was not fully understood and was poorly documented,” Legnani explains. A 1:36 scaled tank test was performed at the HR Wallingford fast-flow facility in Oxfordshire to calibrate the basis of hydrodynamic loading assumptions.

The design and installation concepts were field tested at the European Marine Energy Centre test site in Orkney.

“The typical solution for a fixed marine structure is a tubular jacket, but in such relatively shallow water that would have interfered with the turbine operation and attracted significant drag,” she says.

“Robert Bird developed an alternative low profile structural form to enable development of the shallow water MeyGen site.

MeyGen

MeyGen

The giant 15m high pylons were placed from a barge using remotely operated autonomous vehicles

“This low profile form also minimises frontal area that is exposed to environmental drag, thereby reducing the size of ballast required and enabling the turbine to be placed in a high tidal energy region.”

Articulated feet with spherical bearings were developed to allow the tripod base to self level on the irregular seabed. Ballast blocks for the tripod legs were stressed together, with the lifting eye for installation welded to the connecting pin, which as well as holding together the plates also transferred lifting loads. Buildability and turbine maintenance was a major focus of the design.

“The combination of depth and strong currents meant it was not safe for divers to go so deep, and we had only a 40 minute operating window to work in, on neap tides when the waves were no more than 2m,” Legnani explains.

“For construction we largely removed the need for humans though for maintenance it is possible for divers to work on turbines which are only 16m down.”

Remote operating autonomous vehicles (ROAVs) were the answer for overseeing assembly of each unit with everything placed from a barge.

“The ROAV went down with the tripod first, watching where it landed. One leg was put down, then the second, then the third. The ballast blocks were shaped to slot in over the legs.

“In the next tide window, the turbine was then dropped in from the surface to automatically wet-mate with the pylon and with the ROAV giving the crane operator line of sight.

“There’s 10m of useful visibility in the sea off Stroma,” Legnani says.

Each of the phase one turbines has a dedicated subsea array cable laid directly on the seabed and brought ashore via a horizontal, directionally drilled borehole. The turbines feed into an onshore power conversion unit building at the Ness of Quoys where the low voltage supply is converted to 33kV for export via a 14.9MW grid connection into the local distribution network.

Ward is hopeful that post-2021 the UK Government will back tidal stream power with a decade of grants similar to the support given to wind energy when it first began development. For every £1 of public funding, another £6 to £7 of private investment would be generated and the UK would remain at the forefront of this new technology.

“The next phases should all be about cost reductions for commercial scale,” he says. “There are substantial technical improvements to come.

“We have first advantage. It would be a travesty if the UK’s lead in the world, as so often happens, is cut off through lack of vision.”

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