Telescopic turbines defy deep waters

Corus is to carry out feasibility testing for an offshore wind turbine with a telescopic tower that digs its own foundation.

The DTI has awarded Corus Bi-Steel £100,000 for a project that aims to reduce turbine installation times and keep down costs. Scale models are to be built and tested during the summer.

Current offshore wind turbine designs have been growing in size, weight and difficulty of installation as power requirements increase, and tend to cost £1m per MW, said Hugh Bowerman, technical manager for Corus Bi-Steel.

Specialist equipment

Towers need to be built in less sheltered areas than before, perhaps 15-25km offshore. So they have to cope with higher stresses from waves and currents in the deeper water as well as wind.

Monopile foundations, consisting of 300-tonne structures 60-70m high and 5m wide, have to be dug or hammered into a pre-prepared area of the seabed, which needs specialised and expensive installation vessels. The tower then has to be built on top, with all the problems of using cranes in a rolling sea.

‘We stood back and asked, ‘Is there a different approach?’, said Bowerman. If the turbine could be built in one piece inshore, installation difficulties would be greatly reduced. A starting point was designing a self-burying foundation, a ‘flat disc’-type structure fitted with water jets on the underside to blow out sand and gravel. One concern is controlling the jets to ensure that the foundations settle firmly and level. A 1/5-scale model 3m in diameter is being built, and should be trialled at a site such as Pembroke Dock this summer.

Corus hopes to pre-install foundations with telescopic towers that would be winched to their full height rather than put up by crane. The study aims to develop viable techniques for erecting and ‘locking off’ the tower sections once raised, perhaps with a tightening method scaled up from processes used in other industries. A separate scale model (perhaps one quarter to one third size) will be made to test these concepts.

Dry dock assembly

While Bowerman was confident about the technical solution, making the design economic remains a challenge. Setting up a ‘production line’ in close proximity to the turbines’ destination could mitigate this.

The turbines could be manufactured in a dry dock, then transported by barge to the site at less cost than a specialised installation vessel.

The study phase should end in about 12 months’ time, and if successful Corus will aim to rapidly commercialise the technology. Corus Bi-Steel will manage the project assisted by consultant Global Maritime, Offshore Data and Danish wind energy expert NEG Micon.

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