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Braving the waves.

How do you reach windfarms far out to sea safely to carry out maintenance tasks? Richard Lucas looks at some of the possible solutions

The wind blows hard and often in the middle of the North Sea. For a generation and more, it's made oil platforms inhospitable places. Over the coming years, it's likely to provide the energy for the next generation of UK power infrastructure.

But the giant windfarms that are planned a long way from the shoreline will be rather more at the mercy of the weather than the hydrocarbon energy infrastructure they're designed to replace. For wind turbines have a reputation, not wholly undeserved, for unreliability.

The difficulties of operating windfarms so far from land are part of the reason why the take-up from the third round of offshore licences is so far pretty unimpressive. Huge blocks in the North Sea, plus others in the Bristol Channel and to the west of the Isle of Wight, were allocated several years ago. But the actual developments have dragged a little: the biggest, the Dogger Bank scheme that is due to generate 9GW and could go further, may get into the planning stage next year.

"One of the issues has been that the Round 3 sites are very different from the Round 1 sites which were close to the shore," says Breanne Gellatly, delivery manager for the Offshore Wind Accelerator programme at the Carbon Trust. "They're further from the shore, the waters are deeper and they're in harsher ocean conditions. The existing solutions for operations and maintenance (O&M) for offshore windfarms wouldn't be enough: you'd lose a lot of availability of your turbines because you wouldn't be able to maintain them properly."

The Offshore Wind Accelerator programme has brought together a consortium of windfarm operators with some public money, channelled through the Carbon Trust, to investigate some of the issues in deepwater developments.

O&M issues bulk large in this. Gellatly says that on current offshore turbines there's a distinction to be made between planned and unplanned maintenance, "and it's running at one 'planned' to every five 'unplanned' interventions". That, she adds, should improve as turbine technology becomes more reliable. But there's still a logistical problem of getting maintenance technicians out to the far-distant structures.

Which is why one strand of the Accelerator programme has concerned itself with access. A call for ideas produced more than 400 submissions. That's been refined down to a dozen that are being actively pursued and are at different stages, several of them being trialled on offshore installations.

The systems on trial indicate that the O&M problem is a two-stage difficulty. The first part is to do with the remoteness of the proposed windfarms, and some of the answers to this can borrow from the offshore oil and gas experiences of the past 40 years, where there have been developments in fast access boats and in supply ships that moor close to grouped structures.

So among ideas that are being pursued under the Accelerator scheme are fast boats with innovative 'suspension' systems that are derived from automotive practice and that separate the deck from the hull, and a surface-effect ship that puts an air cushion between twin hulls for the fast transit to the site and then releases the cushion to provide a stable platform for the transfer. Leading the pack in terms of readiness is the Norwegian Fjellstrand shipyard which has an order for six of its WindServer modified-hull vessels.

But there is a downside to this strand of thinking: weather conditions can be very different at the port of departure from those at the windfarm 100 miles away and also, says Gellatly, "you could still be transiting the operators for hours and by the time they get there they're too seasick to work". So an alternative arrangement might have a 'mother ship' on station near the windfarm with a small armada of 'daughter ships' nestled inside it, in the manner of landing craft within an amphibious support vessel. One of the Accelerator projects proposes just such a concept.

But it's the second part of the windfarm problem that defines many of the current crop of answers: the difficulty of transferring personnel from a vessel whose position is dependent on wave, tide and wind on to a fixed structure. On inshore wind turbines, systems have been developed that can handle a 'significant wave height' or SWH of 1.5m--defined as the mean wave height, trough to crest, of the highest one-third of all waves in a particular sea state.

That won't do for windfarms far out to sea, giving access on, at best, only 200 days a year. The target SWH for Round 3 devices is 3m, which might raise accessibility to 300 days. But Gellatly and her colleagues aren't happy with that either, because SWH is only one unidirectional component in the complexity that is a sea state. They are pressing for a broader spider-graph-style measurement of fitness for purpose, called a P-Plot, that plots performance against waves from 360[degrees]. An extra layer of difficulty is that the transfer systems are not meant to attach themselves on to the turbine tower for fear of damaging the structure.

As with the ships, there is a range of ideas under discussion for transfer systems. They go from a basic bridge system that has yet to be tried on a vessel to a complex package of hydraulics that compensate for ship motion. In the same area, but not under the Accelerator programme, is the Maxccess system (see box below). There is also a rather different technology, from the German Momac company. It uses systems derived from robotics to manoeuvre a 'work platform' at the end of a boom to the point where a technician can just step across on to the turbine tower's ladder.

Gellatly isn't apologising for having so many systems under investigation. "Every site is different," she says. "This is the beauty of doing real sea trials. You might have two fields that are equally far from shore but that have different sea conditions."

Search for a safe and elegant solution

Dr Tony Trapp is a serial innovator who has, with like-minded colleagues, built up two businesses in the subsea industry. Now he's turned his attention to offshore wind and the maintenance access problem, and his Osbit Power firm has brought several past collaborators together again.

Work on what is now the Maxccess system started in 2010 and proceeded quickly to trials in a wave tank and then harbour trials in connection with the Greater Gabbard windfarm off the coast of East Anglia. A second 12-tonne device was deployed at Sheringham off north Norfolk at the start of this year, and this summer a bigger 18-tonne Maxccess system has gone into a year's trial on the Walney windfarm being developed off Barrow-in-Furness.

Trapp says: "There is a huge safety issue here, and that ostensibly is the driver. What we're expecting people to do with the previous systems is not very safe and it's not a properly engineered solution. We push hard and rely on friction and if that doesn't work then we buy a bigger boat and push harder. But safety isn't the only driver: it's also about getting more operating days a year."

"Properly engineered", though, is what motivates Osbit Power. "The hardest and the most beautiful engineering is an elegant mechanical solution, and that is what we have done," he says. "The standard engineering approach that almost everyone adopts is that we've got something that moves and therefore we need to compensate for it, with heave compensation, motion sensing, powerful complex hydraulics, servo valves and a lot of software. Much better and more difficult is to find a simple mechanical solution which is often not the easiest to make, but is the lowest cost, lightest, smallest size, lowest maintenance ..."

Unlike systems being looked at under the Offshore Wind Accelerator scheme for Round 3 windfarms, Maxccess works by stabilising the bow of the ship using what Trapp terms a "constrained bridle" that attaches to the turbine tower but detaches very quickly if there is danger of damage. The ship is still free to pitch, roll and yaw. The present device has, on the East Anglian inshore fields, sometimes been the only operational system in bad weather, he says.

Work is now under way to scale up for the bigger waves of the more-distant windfarms, but Trapp isn't only interested in that. "We've got a solution that we think can also open up the oil and gas market as well for accommodation vessels coming up to rigs," he says.
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Title Annotation:OFFSHORE
Author:Lucas, Richard
Publication:Professional Engineering Magazine
Date:Sep 1, 2013
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