Knuckleboom cranes for nacelles

Palfinger Marine’s nacelle knuckleboom Palfinger Marine’s nacelle knuckleboom

Inside a recent, large wind crane contract for the Borkum II windfarm was an interesting element: while half Palfinger’s order was for stiffboom cranes, the other half was for folding knuckleboom units to be installed on the nacelles.

The Trianel farm is located in the North Sea approximately 40km north of the island Borkum and consists of 32 Senvion 6.2M152 turbines. These will generate about 200MW hours of electricity, but of course, downtime in such an energetic environment is always an issue, therefore ease of maintenance is high on any operator’s list.

However, unlike the 32 electro-hydraulic stiff boom cranes (PSM400) that Palfinger is also supplying for the towers’ service platforms, these nacelle cranes probably won’t be used on a regular basis. However, they do make a useful addition for safe and fast lifting of the turbine’s slightly smaller elements – such as spare jaw gears for the slewing drive, coolers or lubricants for maintenance, explained Rupert Reischl of Palfinger’s wind cranes department.

These components usually have to be brought up into the nacelle from the service platform or from the support vessel’s own helideck. Compared with the rope winch or chain tackle alternatives, these cranes are a lot more flexible in terms of operating area and manoeuvrability. Further, they can also provide man-riding capabilities for inspections outside the wind turbine.

But nacelle cranes require a particularly compact design as space is limited. So, these PK40002 knucklebooms design fold down into a space around 2.7m in height, 3m in length, and around 1.4m wide.

While they might not be able to handle an entire gearbox, these ‘continuous slewing’ cranes are still pretty capable: the maximum lift is 129.1kN or 13,160kg, or at 4.5m outreach, that’s 80.6kN or 8,220kg; given the maximum hydraulic extension of 20.4m its 9.4kN or 960kg.

The PK40002 design comes with a hydraulic power-pack generating the necessary 300bar pressure, but like the other Palfinger wind cranes, they can be integrated into the turbine’s own power system.

Of course, as little used, remote equipment, these power packs themselves need to be robust: and come equipped with fluid level indicator, thermometer and filler-breather filter. More, the PK series also has a regenerative hydraulic circuit which yields for almost a third faster boom extension speed.

Finally, they have Palfinger’s Power Link system which yields a constant lifting moment, resulting in maximum power at the hook, even given unfavourable boom angles

Considering the 25 year-plus lifespan of these wind turbines, nacelle cranes could be considered a worthwhile investment.

By Stevie Knight

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