Single lift message spreads to wind turbine generators

A new method of installing wind turbine generators in a single lift is being promoted among potential clients and turbine manufacturers by Seaway Heavy Lifting. The contractor is hoping to conduct a full-scale test next season using its new DP monohull, Oleg Strashnov. Meg Chesshyre opens this months marine renewables sector review with a look at the Seaway concept.

With a significant volume of work elevated high above sea level, offshore assembly and installation of a wind turbines towers, nacelles and blades is always challenging, explains Seaway Heavy Liftings (SHL) chief technical officer Wim van der Velde. Strict wind speed and wave height limitations apply to reduce the inherent risks to appropriate levels.

In our view, it would be better to assemble the entire unit onshore in many cases, load out the completed unit and carry out the installation in an all-in-one operation at location. This would be faster and more efficient. It would also enhance safety and reduce testing and commissioning offshore. Essentially, the assembled unit can be pre-tested at the fabrication yard. Our vessels and equipment are now available to provide this integrated service, based on one-phase installation. The benefits to clients include enhanced safety, shorter and predictable construction times and cost-savings.

In Seaways proposed all-in-one operation, complete WTGs will be picked up from the quayside using Oleg Strashnov's 800te auxiliary hook, transported four or five at a time to location and installed in DP mode.

The assumption is that SHL can pick up the wind turbine generators (WTGs) in port, fully assembled in a single lift, with loadout in one day, and transport them to the field up to five at a time. This installation method requires some small adjustments to the existing WTG tower and the jacket, hence the discussions with the manufacturers. The new method speeds up offshore installation considerably, says van der Velde, pointing out that a WTG can be installed offshore within eight hours. It also eliminates the need for people working at heights, and the use of a DP vessel means there is no seabed interference.

A shock absorbing system will ensure the WTG lands softly.

The WTGs will be picked up by Oleg Strashnov from the quayside using its 800t auxiliary hook and loaded on the deck. The bottom flange of the tower will rest on a predetermined grillage where it will be secured and sea fastened. The tower will also be supported by a framework taking the horizontal forces at a higher level (this structure is currently under design and is subject to the specifics of the WTG). Depending on the size of the WTGs, Oleg Strashnov can carry four or five WTGs and will sail with an average speed of 11-12 knots.

We know this is the key to more productive working offshore. Wim van der Velde, Seaway

When approaching the offshore site the vessel will activate its DP system. While approaching the jacket structure the lift rigging will be deployed, the steering lines attached and the access system will be started. At location the jacket will be prepared to receive the WTG, the sea fastening will be released and the WTG lifted from its grillage and installed onto the jacket. The jacket will be equipped with a shock absorbing system to ensure that the WTG lands softly. This system is also equipped with docking/guide pins ensuring that both flanges line up correctly. The two flanges will be secured by a hydraulic locking device. Oleg Strashnov will relocate to its next location while a commissioning crew will stay behind working from a separate vessel to complete the bolting and the commissioning activities.

Oleg Strashnov at work on the Riffgat wind farm project in the German Bight.

A few minor modifications are required to the receiving jacket and WTG tower. The jacket will require some bolt holes to be equipped with thread to enable mounting of the hydraulic locking devices and secondary docking pins, and primary docking cones with shock absorbers. The tower will need the installation of primary docking pins, and two trunnions or pad-eyes near the bottom flange to lift the complete assembly.

As an alternative to in-port assembly, SHL has developed an installation method that uses a jackup at the wind farm as offshore assembly base. However, this jackup does not have to relocate and will remain at the same location throughout the project. Weather-sensitive jacking operations are not required and the feeding is taken care of Oleg Strashnov.

We have a strong reputation for reliability, but we are also strong in terms of innovative thinking, declares Wim van der Velde. We want to offer our clients additional options. Certainly, one-phase installation, following full assembly and testing ashore, is a significant opportunity for all concerned. Naturally, we have no monopoly on these ideas, but, for the moment, we are the only wind farm installation contractor with the necessary vessels and equipment available - right now - to deliver a fully assembled transport and installation service. We know this is the key to more productive working offshore.