Published at 13:57, Friday, 21 May 2010

FOR many years offshore wind turbines have been fixed on top of a single large diameter tubular tower driven into the seabed.

These are often called Monotowers.

The design, ie the diameter and thickness of the Monotower, is heavily influenced by the installation water depth, and for waters up to 30 metres deep typically span six metres in diameter.

With most of the available shallow water now in use, designs are now focusing on water depths of 30 to 50 metres or more.

Improvements to the initial generator and blade design now allow them to produce more power. All of this means that the towers are subject to an increased loading and as a consequence the Monotower concept has probably reached its practical size limit.

Future offshore windfarms are likely to follow the lead of offshore oil and gas platforms which use a substructure resting on and secured to the ocean floor.

These structures extend through the water column to some metres above the sea’s surface to allow the topside accommodation/process plant/drilling facilities etc to be placed on top. Similarly for wind turbines a lighter substructure is likely to be placed and secured on the seabed and a tower with the blades and generator installed and secured on top of it.

Recently Canadian oil company Talisman Energy (UK) with Scottish & Southern Energy carried out a project given the name DOWNVInD, (Distant Offshore Windfarms with No Visual Impact in Deepwater) to look at which future direction structures for offshore wind energy generation might take. The project
resulted in the design and installation of two 5 MW wind generators, the largest yet installed in the offshore environment, with a rotor diameter of 126 metres.

They are located in the Moray Firth in Scotland in 42 metres water depth, near to Talisman’s existing Beatrice Alpha oil platform. The electricity generated is used on the platform, which in turn
reduces the amount supplied via a subsea umbilical from the national grid.

Oil States MCS Ltd based in Barrow, has, for many years, owned and operated the unique Hydra-Lok® cold forming or “swaging” system to secure offshore substructures in place.

The system secures the structures by expanding the driven pile radially into a surrounding sleeve built into the substructure, thus connecting the structure to the seabed.

The Hydra-Lok® system was chosen to connect the Beatrice wind generator substructures to their piled foundation.

The swaging method is easily monitored and has the significant advantage of being much quicker than alternative methods, which saves expensive installation vessel time.

In addition Oil States MCS Ltd owns and operates a levelling system for offshore structures, named Latch-Lok™, which facilitates accurate levelling of substructures.

Wind towers need to be installed as near vertical as possible in order to optimise their performance and that of their supporting structures.

The substructure should be installed as near to level as can be achieved, a typical value being ± .25°, often even less.

The use of the Latch-Lok™ tool allows for this and has the advantage of being able to be combined with the Hydra-Lok® tool so that a structure is levelled and secured in a single, accurate and rapid operation.

It is this combination, as shown inset above, that was used to level and secure the DOWNVInD wind generators.

Published by http://www.nwemail.co.uk