Offshore wind turbines are awkward creatures to transport. With their heavy towers and nacelles, and their long, fragile blades, shipping turbines by sea is the only practical option. The wind industry’s complex supply chain adds to the challenge. As regional demand rises, manufacturers open new local facilities. Components built there must then come together at the project staging area with components produced elsewhere.

This is the situation Siemens faced four years ago when planning for the future of the European offshore wind turbine market that it now dominates. Its new blade factory in Hull was on the horizon and it already had multiple nacelle, tower and blade production facilities in northern Europe. In 2012, it turned to the Deugro organisation for expert advice on shipping.

“We analysed the logistics that would be needed to support the demands of future offshore projects,” says Richard Thomsen, managing director of Deugro Danmark. “Siemens wanted one or more vessels that could sail between different factories picking up components before delivering them to the project load-out port.” Based on this requirement and Deugro’s advice, Siemens put out a tender in 2014. After much hard thinking and market investigation, Deugro’s final recommendation was to build two bespoke wind-specific cargo carriers.

That was unusual – shipping between factory and project base relies almost exclusively on conventional vessels – but not unknown. Exceptions include Vestas’ two Bladerunner boats that take blades from its research and development facility on the Isle of Wight to Southampton’s port for transshipment, and Enercon’s E-Ship 1, which, fittingly, harnesses wind power to help transport wind turbine components. Siemens, Deugro and shipbuilder Concordia worked together to specify, design and build the vessels that would become the Rotra Vente and the Rotra Mare. Specially designed for nacelle transport, the Rotra Vente can hold up to eight 7MW nacelles under sliding deck covers, but can also carry towers and blades.

“Both vessels are built to handle our current wind turbine portfolio, mainly large direct-drive offshore turbines like the SWT-6.0-154 and SWT-7.0-154,” says Bernd Eilitz, press manager at Siemens. “Another important factor was the limitations of the harbour sites such as the dimensions of Hull’s Alexandra Dock sluice chamber and the Cuxhaven [roll-on-roll-off] ramp.”

Sister ships

The Rotra Mare is nearly identical in size to its sister ship, though it lacks its sliding deck covers and is completely open. Optimised as a blade and tower carrier, it holds its cargo in two tiers. This can be either two layers of six blades or a bottom layer of towers with six blades on top. Hydraulic lifting devices at both ends of the open cargo deck help load the top tier of blades. Due to their greater weight, the towers must always be carried on the bottom tier. Allowing crane and roll-on-roll-off (ro-ro) loading was central to the specifications. With nacelles weighing up to 430t, minimising crane use significantly enhances speed and safety as well as cutting out delays due to high winds. Compared with normal craning and standard cargo vessels, Siemens estimated cost savings of 15–20%.

The Rotra Mare was converted from the Flintercrown container vessel while the Flintercoral became the Rotra Vente. Partially built in China, the Flintercoral was never completed due to a glut of container ships following the 2008 global financial crash. Languishing in a Vietnamese shipyard, it was sold to Concordia and towed to the Netherlands in 2010 where it was laid up. “Choosing an existing vessel managed to tick both boxes,” says Thomsen. “Given the tight timeframe, modifying rather than building from scratch would be faster and also considerably cheaper.”

Those modifications were substantial. Starting in late 2014, workers at Concordia’s Werkendam shipyard stripped out the Flintercoral’s cargo hatches, container hold structures, its upper hull sides and part of the bow section. Holland Shipyards then took over, starting the conversion project in late 2015. First, a heavy duty ro-ro deck was installed that could take the weight of the nacelles, along with a higher hull side to protect the nacelles from wind and seawater.

Then came a roof to cover the deck and further protect the valuable nacelles. Four movable sections can be rolled forward or aft with the two slightly lower middle sections sliding under the outer sections. Retracting the roof sections gives plenty of room for oversized cargo and permits crane loading.

A mobile forecastle design is an unusual feature of both vessels’ ro-ro capability, chosen instead of the more conventional bow door and watertight door combination. That involved chopping off the original bow section and converting it to a mobile structure. “One reason is that the original vessel design made this the most feasible answer,” says Thomsen. “Overall it was the best compromise and is a very clever, fast and flexible solution.”

To link to the shore, a 70t loading ramp extends up to 6.8m beyond the Rotra Vente’s bow. This can move through a vertical range of 5.6m to suit differing quay heights and tidal movement. That vertical range is partly enabled by 12 new ballast tanks on the lower decks that let the vessel trim its buoyancy as cargo is loaded and unloaded. It’s equipped with an automatic heeling tank system and a manual forward-and-aft trimming system that can pump 2,000m of ballast water every hour.

With no requirement for any kind of special port ramps or other onshore capability, the two vessels can dock at almost any quay – vital when they must deliver turbines to a changing roster of project bases. “Of course there has to be a safe berth with not too much tidal range or heave,” says Thomsen. “But the flexibility and manoeuvrability of the vessels means we only need normal docking facilities.”

Efficiency drove the Rotra Vente’s final modification, adding a new propeller to match its service speed of 15kt. Fuel savings will pay back this extra cost in under a year. From starting work to delivery, Rotra Vente’s extensive modifications only took ten months. The vessel entered service in November 2016 with the Rotra Mare joining it in March 2017.

Flexible delivery

Both vessels are now busy supplying turbines to the booming offshore industry in the North Sea and the Baltic. As well as the Siemens factories in Denmark and the UK, staging areas like Great Yarmouth are frequent destinations. The new nacelle factory at Cuxhaven will also soon be on the schedule. “The beauty of this system is that it is extremely flexible,” says Thomsen. “You can deliver what is required right now, according to project or manufacturing logistical needs. Within a few days, you can pick up and deliver a full load and be back for the next one.”

Speed depends on the specific itinerary but Thomsen says: “On the normal sea lanes like Esbjerg to Hull, it’s very fast indeed and that helps makes the overall journey very short.”

Workers now wheel turbines from the end of the factory production line straight to the quayside and on to the vessel. Loading each nacelle takes under an hour. At the other end, they can be driven rapidly to another area of the project base port with no need for time-consuming multiple crane lifts.

“Once you use [ro-ro] on a vessel, you can use this transport mode throughout the system until the components are finally loaded on to the installation vessel by crane,” says Thomsen. Industry-standard selfpropelled modular transporters (SPMTs) are the method chosen by Siemens to move the heavy nacelles. Siemens contracts heavy transportation specialists like ALE to take care of this delicate work. However, ro-ro loading isn’t generally the solution used for the Rotra Mare as blades must always be craned on to the vessel’s upper tier. This ability to crane loads on and off both vessels would make it possible to deliver components directly to an offshore wind farm if required.

When it comes to specific projects and time savings, Deugro is understandably cagy about revealing any competitive information. “All we can say is that our expectations have been met,” says Thomsen, referring to the predicted cost reduction of 15–20%.

With the service operation vessels that Siemens pioneered now becoming standard equipment in wind, will these two new vessels also start a trend? As ever, that depends on the market. “Since jack-up [installation] vessels are relatively slow, and distances and component volumes in offshore wind projects are growing, there could be a demand for further vessels like these in the medium term,” says Eilitz. “But we can only speak for Siemens Gamesa Renewable Energy and these two vessels have been tailor-made to meet the demands of our specific offshore logistics concept.”

The beauty of this system is that it is extremely flexible. You can deliver what is required right now, according to project or manufacturing logistical needs.

Designed with an eye to the future, the vessels’ dimensions and cargo capacity should have a good chance of handling components for evolved versions of today’s machines and the next 10MW+ turbines. “Both transport vessels have been specified according to our demand for blade and nacelle dimensions of the current and the next product generation regarding weights, blade lengths and so on,” says Eilitz. “Once we have developed the next turbine generation, it might be possible that there will be some adaptations due to the larger dimensions and masses of the components.”

If European project demand does eventually drop, then moving further afield could be an option. This was considered during the build with capability added to allow the use of heavy fuel when working outside Europe. The ship currently burns the lighter marine diesel oil that meets local emissions requirements. The vessels are prepared for global operation but it seems unlikely in the foreseeable future. Instead, they will be kept busy as northern Europe continues its offshore wind boom.

“As a result of close partnership, this project successfully lowered the cost of logistics,” says Thomsen. “I think the wind industry will call for more dedicated, focused solutions like this to optimise transportation, and this is an excellent example of how it should be done.”

Siemens’ pioneer SOVs pave way for far offshore support

Siemens paved the way for faroffshore crew support with its groundbreaking service operation vessels (SOVs). Instead of long daily trips from land bases, crews stay at the offshore site, lengthening the working day. Heave-compensated gangways make turbine transfer much safer and Siemens currently charters the ESVAGT vehicles Froude, Faraday and Njord to support work at North Sea farms such as Dudgeon, Butendiek and Baltic 2.

Now the company has another two new SOVs in operation: Windea La Cour and Windea Leibniz. The new design with its X-STERN hull shape means ships can be positioned facing towards the weather, allowing them to operate safely in adverse conditions and reduce energy use.

The 7MW leviathan

The Rotra Mare and Rotra Vente will carry the Siemens 7MW offshore turbine now being installed at projects like Iberdrola’s East Anglia One. An evolution of the company’s SWT-6.0-154, this massive machine features:

  • 75m-long blades cast in a single piece
  • 154m rotor diameter
  • permanent magnet generator
  • gearbox-free direct-drive system.