Fresh breeze for offshore wind farms

The cost of wind energy has fallen sharply across Europe in recent years, in part because of larger turbines, economies of scale, increased competition, lower capital costs, a steep learning curve and competitive auctions. The price per megawatt-hour has dropped up to 50 per cent within as little as three years. This year for the first time two offshore wind projects were awarded in Germany without entailing any government subsidies – underscoring the maturity of the technology as it competes with other energy sources. And while prices for all renewables are expected to fall, the drop in the case of a technology as young as wind energy is likely to be even more pronounced.

Dynamic market development

The operational forecast for the European offshore wind sector predicts an increase in megawatt output of more than 50 per cent in the year 2020 compared to 2016. About 40 new sites will be available, equivalent to a 33 per cent increase in the number of turbines.

As wind turbines grow in size, the installation vessels must follow suit, and the assembly and installation routines must be adapted accordingly. Jack-up height and crane reach are key parameters.

This remarkable upturn in the offshore wind market is not limited to Europe. Countries such as the US, China and Taiwan are seeing similar developments. As offshore wind energy is still in its infancy in these countries and construction and installation capabilities are limited, wind energy costs are still significantly higher than those of other energy forms.

But offshore wind offers several advantages that offset the higher costs. Wind farms can be sited in waters near densely populated coastal areas, avoiding the high prices of land. Compared to land-based wind turbines, offshore units benefit from higher wind speeds resulting in higher power output. Furthermore, wind energy combines well with other alternative energy sources, such as solar PV, as it can produce energy in bad weather and darkness when the loads are high. This helps balance the production profile throughout the year. Offshore wind produces energy at 35 to 55 per cent of installed capacity, compared to only 10 to 20 per cent for photovoltaic installations located in the northern hemisphere. The not-in-my-backyard (NIMBY) effect is also negligible when the nearest turbine is located miles away at sea.

High-tech maritime support wanted

All main industry segments served by the DNV GL Group — Renewables, Oil and Gas and Maritime — are in some way connected with offshore wind. “You need to understand the relative importance of each discipline and how they affect each other to have a positive impact on the bigger picture. We look forward to supporting the entire supply chain needed to operate an offshore wind farm such that it is safe, efficient and environmentally friendly,” explains Marte Riiber de Picciotto, Head of Section, Renewables Advisory. “The digital revolution is allowing us to connect our equipment and systems so that they operate faster, smarter and better. In this new world we are moving towards data-driven decision-making, which is also incorporated in our rules and standards,“ she adds.

One of the crucial issues during installation and operation is the marine supply chain. Transporting turbines to offshore locations, erecting, installing and maintaining them throughout their service lives and finally dismantling them requires highly specialized maritime assets ranging from crane barges and tugs to jackup vessels, crew boats, floatels and fast service vessels.

There are various types of offshore wind farm (OWF) installation vessels, including towed “dumb” barges fitted with cranes, sheerlegs crane barges, semisubmersibles, heavy-lift vessels, DP2 heavy-lift cargo vessels, leg-stabilized crane vessels, and selfpropelled jack-ups. After project delays had caused an oversupply of these assets for several years, the industry is currently seeing a shortage of capable vessels, especially after the market introduction of 7 MW and 8MW turbines.

The latest generation of jack-up installation vessels is dimensioned for 9 MW and larger wind turbines and water depths of more than 40 metres. But that is not the end of it: next-generation concept designs are expected to offer enhanced flexibility and to combine a variety of capabilities such as high transit speed and DP3 positioning, along with open deck spaces, long-reach, highcapacity cranes, and rapid and reusable sea fastenings.

The installation of nacelles and blades is one of the main challenges because of the extreme working heights which amplify wave and wind-induced ship motion. Various installation methods are used to handle these challenges. On wind turbines larger than 5 MW the nacelle is typically installed first, followed by the entire rotor consisting of the pre-assembled hub and blades. However, since transporting the nacelle and rotor for turbines of this size exceeds the capacity of most crane vessels, installers today often install the hub first, then the blades one by one. But future crane vessels might be large enough to once again pre-assemble the rotor prior to installation.

Handling the motion of the ship and the crane load is another challenge during installation. Taglines connected to the deck of the ship or the boom of its crane are a typical remedy. Other solutions, such as specialized yokes, add mass to the crane to reduce motion when lifting blades. Siemens reports that a yoke developed in-house now enables installations at wind speeds of up to 14 m/s, and Liftra’s Blade Dragon device has been certified by DNV GL for 18 m/s winds. Using lifting yokes and taglines or combinations thereof can avoid the cost of specialized cranes.

Feeder vessels can be used to support the installation process as well. They help maximize the productivity of the crane vessel while accelerating transit times. This means that for future projects, design engineers must account for the on-site transfer of components from one vessel to the other.

Once all the turbines of an offshore wind farm have been installed, workboats are needed for laying cables and other supporting tasks and, later on, during regular operation, crew boats and other specialized ships play an essential role in maintaining the wind turbines. “For major component replacements as part of the operation and maintenance phase, jack-up vessels are still practically the only option,” Eknes Arnstein, Business Director Special Ships points out. “To reduce costs, offshore wind farm operators may switch from ad hoc contracting to sharing jack-ups across similar portfolios,“ he recommends.

Safe transit between ship and wind turbine

When maintenance technicians visit a wind turbine, one of the most challenging moments is the transfer from the crew boat to the wind turbine platform. The step-over method, while technically unproblematic, is limited to calm weather, but there are various ways to ensure transit safety even in high waves. As wind turbines and maintenance ships grow in size, more complex, DP-supported access systems with gangways ensure safe passage. In January 2017 DNV GL responded to the need for more sophisticated solutions by launching its Walk2Work class notation for motion-compensated gangways.

The number of specialized suppliers and vessels is still small, and owners, contractors, and subcontractors are still learning how to collaborate more efficiently. “It will take some time and experience but with more offshore wind farms being built, economies of scale will emerge, including cost-saving measures such as sharing crew transfer vessels and helicopters, and coordinating jack-up barges across assets and operators for major component replacements. Furthermore, emerging countries will profit from more experienced ones,” de Picciotto is confident.