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?
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.