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Current affairs

Spotlight - Strong currents make piling off the north coast of Scotland potentially deadly. Jon Young reports.

Atlantic tidal currents of up to 15 m/h off the coast of Orkney make this an attractive site for renewable power generation. But the same currents can also be deadly to anyone who falls in, notes RPS director for ports, harbours and renewables Michael Shaw. 'If someone fell overboard, they would be out of sight within minutes, ' he says.

It was in this harsh environment that RPS, with the help of a pair of jack up barges, battled for three months to install European Marine Energy Centre's rst tidal turbine.

Reassured by the presence of rigid-hulled inatable boats (RIBs) circling the two barges, workers cracked on with drillling piles some 11m into the seabed to support the turbine.

RPS's marine engineering team undertook the detailed design of the platform structure that now supports the turbine, resting on a pair of piles socketed into the rock seabed.

'We had to drill two 1.2m diameter piles 11m into the rock on the seabed, ' says Shaw. 'We had only basic site investigation because of the cost and practicalities, so we had to build in contingency plans.' The pairs of monopiles had to cope with the hydrodynamic loading and turbulence associated with the tidal location.

Unpredictable eabed conditions caused some initial problems for the contractors, who were forced to nd a replacement barge after one was badly damaged on the seabed.

'The first barge hit a a totally uncharted rock while its legs were partially down, the legs bent and that delayed the project for a number of weeks because there were very few jack up barges available, ' says Shaw. 'But we were able to get a second barge and complete the project before Christmas.' Shaw says in the end the project was completed using 'two jack up barges, one with the crane and one as the temporary gate for the piles.' He adds that the project was more difficult than it might seem on the surface. 'There were tides of up to eight knots [15km/h] and several storm events. So this seemingly simple project became very complicated when you take in the exposure.' An access platform was also needed as part of the project for the inspection and maintenance of the turbine. In addition it houses the electrical plant and is a base for the winches which lift the turbine out of the sea for test and maintenance purposes.

OpenHydro designed the turbine, and chief executive ofcer James Ives says that the next stage - to be completed during 2007 with engineering assistance from RPS - is a seabed-located device.

If this device proves successful, the plan is to develop, in conjunction with Nova Scotia Power, a large-scale farm in the Bay of Fundy on the north east part of the Gulf of Maine.

McLaughlin and Harvey completed the turbine installation early this year.

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