This article is taken from the January/February issue of Renewable Energy Focus magazine. To register to receive a digital copy click here.
In part 2 of this article, we looked at active control systems.
A contrasting solution, however, championed by Osbit Power Ltd, based in Northumberland, UK, is an entirely passive system. Here, relative motion between the vessel and the turbine tower is accommodated by a mechanical linkage which adjusts itself passively, with no servos or electronic control system.
Intended for support vessels of 15-25m overall length, the MaXccess T12 system is an extending gangway that simply clamps onto the turbine buffer bars so embarking/disembarking technicians have a solid connection to step over. Vessel motion is absorbed by the articulating gangway – this allows for +/− 10 degrees movement in pitch (+/−15 deg structural limit); +/−15 degrees in roll (20 deg structural); and +/− 10 degrees in yaw.
Skippers can approach a turbine from a range of headings to suit prevailing sea conditions, lock onto one of the bars then finally position the vessel square onto the ladder. Because it avoids the expense and complication of an active compensation system, the all-mechanical MaXcess is described by Osbit as a practical and low-cost solution.
By allowing the service vessel to pitch, roll and yaw freely, it secures a large reduction in ‘weather days’, providing safe access in conditions when craft using ‘bump and jump’ would be back in harbour waiting for calmer weather, says the company's sales and marketing manager Ben Webster.
Moreover, MaXccess mitigates the particular hazard experienced in current conventional operations in which sudden sharp bow motions occur when friction between the bow fender and the turbine bars is lost whenever a high wave succeeds in pushing the bow away, says Webster. MaXcess avoids this by maintaining a separation between the vessel's bow fender and the turbine bars.
A horizontal force of up to 16 tonnes is available to accomplish this, while the clamping system can sustain a vertical force of 12 tonnes (hence the T12 designation.) Overall, this affords vessels of up to about 65 tonnes displacement the capability to operate in significant wave heights of up to some two metres although, as Webster tells REFocus:
“Although everyone talks in terms of significant wave height, which is the average height of the top third highest waves, it is the very highest, or peak, waves that are going to do the damage. We have a clever linkage that allows for the movement of the vessel while preventing the clamp from rolling off the tube in these high waves. That was the most difficult part of the system to engineer.”
In removing the need for full-power thrust into the boat landing, MaXcess reduces fuel usage, fender damage and other vessel wear and tear, he adds. Features include a traffic light system to signal safe transfer conditions, night-time illumination, a fail-safe manual retraction option in case the normal hydraulic supply is lost, video monitoring, instrumentation to record clamp position and pressure, slide position, horizontal load (forward and aft) and vertical load; and data logging.
A touch-screen control unit provides graphical display of clamp status plus system loads and diagnostics. This enables operators to know when access is safe and when it may become necessary to unclamp the system.
MaXcess T12 weighs 1.5 tonnes and is bolted to the foredeck via a sub-frame. It requires a deck space of 4m (from the front of the bow fender) by 1.5m width. Webster says systems have been sold to Siemens Wind Power and Statoil; the former using it for five months at the Great Gabbard wind farm. Lloyds Register of Shipping CLAME design approval has been secured.
Osbit Power is now working on a more powerful system, one capable of handling 18 tonnes vertically and suitable for larger vessels of up to some 110 tonnes displacement. A current timetable expects trials to start this Spring, testing a system able to broaden the wave height operating window to some 2.5m Hs. This would make it suitable for operation at wind farms more distant from shore.
Looking further ahead and for even better Round 3 project operational access, the company has won a UK DECC grant to develop a new-generation system that will use a fully-gimballed gangway to accommodate an even wider range of operating conditions. This too, Ben Webster confirms, will be a passive system.
Which type of system wins out, active or passive, as the service infrastructure of choice for far-offshore wind farms like those planned for Round 3 in the UK, remains to be seen. However, it does seem that such appliances will help make that ‘leap of faith’ transfer between vessel and tower less fraught than it might seem today.
About: George Marsh Engineering roles in high-vacuum physics, electronics, flight testing and radar led George Marsh, via technology PR, to technology journalism. He is a regular contributor to Renewable Energy Focus.