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Electric currents

Beneath the waves off the Scottish north coast lies the latest development in tidal turbine technology. Declan Lynch reports.

Usually engineers like to avoid working in fast sea currents where foundations are difficult. But that is exactly what renewables developer Atlantis Resources project manager David Collier is looking to do. Collier is leading Atlantis’ push into tidal turbine technology, and the faster the current the more energy is created.

Atlantis, a private research company, is currently trialling a prototype tidal turbine, the AK1000, north of the Scottish coast near the Orkney Islands. It is a revolutionary design which utilises twin turbines to harness energy from sea currents, and a gravity base to keep it in position.

“Marine energy is 25 years old but I do believe that the time is now for the industry to grow,” says Collier. Tidal turbine technology has not had the fanfare of other renewable sources such as wind power but Atlantis has been busy developing the technology since 2002.

“There are two key advantages over wind. Firstly, tidal flow is predictable, and secondly we know where the flow is coming from. This is interesting for utility companies because they can plan,” says Collier.

Two challenges

There are two significant challenges to installing tidal turbines. First, creating the right type of turbine to harness energy from the waves and second, installing and keeping the turbines in position in very inhospitable environments.

To harness the tidal energy, Atlantis has developed a twin turbine system to generate energy from high and low tides. Collier and his team developed a number of different prototypes, including a turbine inside a venturi tube which produces more energy. But the twin turbine delivers the best value for money.

The Atlantis-developed twin turbine consists of six composite blades - three on each side - 7.8m long. The total rotor diameter is 18m, with a nacelle, or casing, which houses the engine, measuring 12m.

Choosing the best method to install the units on the seabed was another difficult decision for Atlantis.

“We looked seriously at a subsea drilling platform but with mixed success. Wherever you look, you must solve the problem of initially attaching a platform to the rockbed,” he explains.

The strong currents mean that drilling on the rocky seabed bed is very difficult because it’s almost impossible to keep a vessel still.

“We decided that the gravity base is the best solution after much discussion and analysis,” he says.

The gravity base, although expensive, is self installing and should prove the cheapest overall. To help develop the base Atlantis enlisted the help of consultant Robert Bird Group.

Strong currents

The steel gravity base must withstand currents of up to 4m/s as well as the overturning load from the attached turbine unit.

“There’s no code for what we are designing,” says Robert Bird associate director John Ward. “We’ve used some information from the Norwegian Design Codes - DNV - to meet requirements for strength, fatigue and corrosion but much of the design has been from first principles.”

Ward and his team have been working on the detailed design of the gravity base since October 2009. The design must withstand currents and waves as well as the huge overturning force induced by the turbine.

“Simulating wave and current interaction was a steep learning curve. We spent plenty of time developing finite element analysis to produce the design,” he says.

“There are two key advantages over wind. Firstly tidal flow is predictable and secondly we know where the flow is coming from”

David Collier, Atlantis

Design of the structure is governed by constraints around the need to lift the gravity base into position. To place the structure in such inhospitable conditions the team has to rent a huge tug boat normally used in the oil and gas sector. It has a maximum lift of 200t, and it doesn’t come cheap, costing upwards of £100,000 per day to rent.

Engineers had to produce a design with enough self weight to resist forces which would make it slide along the seabed. It also has to resist overturning forces from the sea and turbine. RBG produced a computer model of the gravity base which was used to analyse critical areas - in particular the connection with the turbine - called the stab connection.

Waves and currents crashing into the turbines produce a lot of movement on the gravity base. The computer programme allowed Ward and his team to identify fatigue-critical joints due to the movement between the main structure and stiffeners. In these places RBG designed in full penetration butt welds, and in other joints they are using fillet welds which are much cheaper but not as strong.

Three legged skeleton

RBG and Atlantis developed a three legged skeleton gravity base weighing 75t with grips underneath to help prevent any sliding. On each leg are two steel boxes, one on top of the other, that act as ballast, the lower weighing 200t, the upper weighing 160t.

“We looked at many different ideas for the ballast, in particular some form of concrete, but when you place it in water it loses a third of its weight due to its inherent buoyancy. Again, taking all the factors into account, steel came out to be the best option,” added Collier.

The finished structure is a marine-specified S355 strength steel. The base is painted and has cathodic protection to guard against rust. The main chamber supporting the turbine is unpainted - due to the difficulties of working in confined spaces - but is fully sealed from the outside.

Isleburn Engineering fabricated the base in Aberdeen using steel from Corus produced in Scunthorpe. Collier describes the base as a pre-commercial prototype which cost about £3M. The base and turbine together weigh 1,300t and have a combined height of 22.5m.

The 18m diameter turbine has a rated capacity of 1MW at water current speeds of 2.65m/s - enough to power 1,000 homes. Tidal turbines are anticipated to produce energy 33% of the time, compared to 20% to 25% with wind power.

Platform supply vessel

Atlantis unveiled the AK1000 tidal turbine at Invergordon, Scotland in August. Using a platform supply vessel, the Skandi Skolten, Atlantis installed the gravity base structure in a location just off the Orkney Isle of Eday over seven days. Once in position it was connected to the European Marine Energy Centre (EMEC) on Orkney, itself.

Over the next year, energy output from the tidal turbine will be independently monitored by EMEC. Atlantis will use the information to eventually produce a commercial product. Development work has been tough going, with the project currently suspended in order to change faulty turbine blade.

Atlantis will also be monitoring the ecological effects of the turbines. Studies show fish do not tend to swim in fast flowing channels with rock seabeds - the ideal environment for the turbines -so Atlantis hopes the blades will not harm any wildlife. Atlantis also expects that the energy taken out of the currents by the turbine will be such a nominal amount that it won’t affect the ecosystem of the seabed.

Atlantis wants to market a fully operational tidal turbine with gravity base at £5M per fully installed unit. It has been looking around the world for suitable locations with fast currents, rock seabeds and minimum of depths of 35m. So far it has identified areas off the coast of Korea and India. Some of the best locations are to be found north of Scotland.

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