By Hans Schneider

Longer blades, tougher logistics

With the notable exception of Siemens Wind Power’s 3.6 MW turbines, the blades of today’s offshore wind turbines are typically pre-assembled into a single rotor component before being loaded onto a vessel and sailed out to their final destination.

However, with rotor diameters approaching 200m for the largest announced wind turbines, some of today’s commonly seen installation techniques are due for change – and quayside rotor assembly is one of them. Already now, in fact, even the smallest of such rotors makes some pretty heavy demands when it comes to quayside loading space, cranes and so on.

What would it be like to assemble, transport and install something far larger? There’s the reach problem, for example. If a rotor is so big that when it is assembled that you can’t reach its centre, then you need auxiliary cranes at the quayside to move the rotor closer to the vessel.

And what about storm survival measures? As soon as a rotor is assembled it needs to be secured against storms – also while being moved. The bigger the rotor, the greater the survival measures required.

Then there’s loading. It’s no easy task to position rotors of this size on a vessel.

To take a full load of turbines on board (SEA INSTALLER and SEA CHALLENGER take four at a time), you would often need to move the vessel around to take the first two on board, then change position to load the other two. That takes more time, and there are safety implications, too.

Getting in and out of port with a 130m transverse load, for example, is no walk in the park, either. Even with a 75m beam when carrying the blades of today’s Siemens Wind Power 6 MW machine, you have to be extra careful!

So continuing down the full rotor track with the much larger blade sizes planned for tomorrow’s wind farms means that things are only going to get worse.

Changes in the wind

To handle the demands of larger turbines, the industry has had to radically revisit some of the installation mantras we’ve had in place for many years.

For example, a key principle of cost reduction has been to minimise the number of lifts – the fewer lifts and installations offshore the better.

That’s why some installation projects ended up with today’s full rotor method, Bunny Ear configurations, and other approaches. But we have also discovered that the logistical challenges at port level, in particular, destroy the business cases behind such approaches. So now there’s a philosophical change underway.

Installation advantages

Single-blade installation waves goodbye to the minimal lifts principle, but also to the practice of assembling rotors on land, instead delaying assembly until the installation vessel is on location. Once in position, individual blades are lifted and attached to the hub one by one.

A2SEA has thoroughly analysed single-blade versus rotor installation for its Westermost Rough installation project, an exercise that has contributed to understanding how to optimise things both from logistics and safety points of view. Single blade installation is the likely choice for the project.

Together with other leading industry voices, A2SEA now recommends single-blade installation for a variety of reasons. Perhaps the most compelling of these concerns the huge assembly, maneuvering and transport demands placed by a 200m rotor diameter.

That’s the equivalent of two soccer pitches being lifted, rotated this way or that, and loaded onto a waiting vessel at the quayside. No easy task, I’m sure you’ll agree.

Transporting single blades instead of rotor ‘stars’ also reduces storage requirements aboard the transport vessel, in turn cutting down the number of trips to and from port.

Installation challenges

A lot of effort has gone into yoke systems, jack systems, and other infrastructure to support single-blade installation. And we have determined the key requirements for successful installation. For example, where rotors can be lifted at their centre in a single lift, raising three individual blades requires three separate lifts.

For each lift, it is necessary to rotate the hub to accommodate the blade at an accessible angle. Such lifts are not particularly difficult, and don’t present additional safety concerns.

Initially however, the major turbine manufacturers needed additional design work to ensure that their hubs could rotate to accommodate single blade installation. Siemens, for example, has developed a special tool for turning the hub of its 6 MW offshore giants.

As far as lifting goes, single blades don’t need taller cranes, and the lifting operation is less complex than a full rotor lift. Lifting a full rotor requires two cranes – one for holding one end and another holding at the hub, and you need quite a heavy auxiliary crane to keep the end in position once it is raised. With single blades, it’s more about balance.

All in all, there’s nothing that can’t be done with full rotor installation – but it’s not going to be the easiest or safest or most affordable path for the future.

Single-blade installation is, most likely, going to make the task of installing tomorrow’s giant turbines a far easier and lower-cost proposition.

 

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