Feature

Offshore wind O&M: accessing all areas


George Marsh

Part 1. With the next round of offshore wind farms set for deeper, more harsher seas, George Marsh reports on the latest designs in turbine access systems to get O&M staff safely from ship to turbine without fear in even the most hostile environments.

This article is taken from the January/February issue of Renewable Energy Focus magazine. To register to receive a digital copy click here.

Having technical personnel step from a gyrating wind farm service vessel (WFSV) onto a seabed-rooted turbine tower, or vice versa, in the turbulent offshore environment can be likened to requiring a cowboy to dismount from a bronco while it is still bucking wildly. For each transiting technician, making that step or jump across the void should be more than a leap of faith; it should be a safe, controlled operation. Therein, however, lies a considerable technical challenge.

Practice to date has been to butt the WFSV tightly against friction bars on the tower and hold it there with forward propulsion. Friction between specially configured bow fenders on the vessel and the bars constrain vertical motion of the vessel's bow so that stepping onto the turbine ladder can at least be contemplated.

This ‘bump and jump’ method just about works for the smaller, lighter vessels that are the offshore workhorses of today and for waves up to some 1.5m significant wave height (Hs). However, for the larger craft needed for more distant offshore service and where, for any reasonable operational window, waves of up to 2.5m or more will have to be catered for, this method is likely to run out of traction – literally. So what alternatives are there?

The options

A system is needed which, irrespective of what the vessel is doing, will position a boarding step/platform at a fixed point in space next to or touching the target ladder so that the technician can step over from one to the other as easily (sic) as making the transition on land. This can be achieved with a gangway extending from the craft's deck that continually adjusts its angles so the required position is maintained.

This is difficult because vessel motion involves six degrees of freedom – pitch, roll, yaw, heave, surge and sway – whilst any motion is likely to become amplified at the end of a structure that extends from the vessel's deck. While this challenge can be met today, there is a trade-off between performance and cost so that, in practice, systems may not compensate all six motions and may not compensate any of them perfectly.

There are two schools of thought about what type of system is best:

  • one that uses servos and an electronic control system to actively position the boarding platform; or
  • one in which a link is made with the tower so that the device adjusts passively.

Part 2 of this series of articles will look at Active positioning, and Part 3 Passive Solutions.

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.

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Comments

Kim Hedges said

31 May 2013
Just as stores and equipment can be zip wired between moving ships, could a similar high wire bosun chair type device not be devised?

High masted ship, with an extension to allow greater height, wire under dynamic tension, connection to Turbine Tower. Initial hook up to a remote controlled wire lifting loop mounted to the sides of the tower, the wire taken up high (from ship level perspective), then locked into place up high next to tower door (similar to a lighthouse staging). Computer program monitoring expected 6 ways of travel.

2nd line from high line attached with powered remote control from both operator on ship and operator on bosun chair device. As expected surge/wave peaks, fast speed lift occurs straight up 50 metres. Then slower sideways movement to the tower. The last 10 metres to the tower, the wire rope is rigid, so cannot snap.

Retrieval in reverse.

Alternatively, how about a helicopter until we figure out how to 'Beam me up Scotty?'

landrews said

30 May 2013
I have one method for solving integrity of energy no storage needed without losing landscape and environment
Activities such as wind power, but not necessarily placed outdoors, working 24/24h
See my model wind energy. simple - mild-effective-inexpensive, can be placed anywhere in the southernmost islands north pole ( the Arctic and Antarctica )(even cold weather)
It is located in a closed cycle -not too noisy - not interfere with the direction of the wind
Page at www.trongdong.weebly.com

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