Some call it the “high wind” or “offshore paradox.” The further offshore a wind project, the more challenging it is to install safely and cost-effectively. But despite the difficulty, a few global wind developers have their sights set on deeper waters and higher winds, which hold the promise of greater power generation.
This means offshore construction typically happens in rougher seas and high winds. It also means equipment, such as cranes and jack-up vessels, must have the capacity to reliably lift and hold taller towers, longer blades, and heavier turbines, regardless of wind or current.
Undeterred, equipment manufacturers in the UK have been working diligently to meet offshore demands. Their labor has resulted in a new, heavy-lift jack-up vessel, a foldable offshore crane, and a wind turbine shuttle that also serves as an installation vessel.
Meet the “SOUL”
SOUL is a jack-up vessel and crane in one engineered to support the installation of today’s 6 to 8-MW turbines as well as the next-generation of 10 to 15-MW units. According to its designers, Norway and Dutch shipbuilding companies Ulstein and SeaOwls, the SOUL weighs about 10% less than conventional jack-up designs, thanks to its unique cruciform layout.
Scaling-up to meet the requirements of turbines that are only a couple more megawatts in size seems a simple enough — just build a bigger crane and vessel. However, the disproportionate weight increase compared to the necessary gain in variable deck load poses a design challenge for jack-up vessels.
“We noticed this created uncertainty with wind-farm operators and contractors in terms of how to install larger, future generations of offshore wind turbines because floating vessels are simply not a viable alternative in harsh offshore conditions,” says Erik Snijders, Founder and Managing Director at SeaOwls. “So we went back to a jack-up design and a square platform with the legs spaced out as much as possible.”
Snijders says that rotating the SOUL’s platform by 45 degrees let the design provided a natural bow shape with two large legs. A crane is then positioned on the vessel’s center line.
“With the main crane built around a stern leg, it creates more of an optimal main deck reach and better over-the-side lifting capabilities,” explains Bram Lambregts, Deputy Managing Director at Ulstein Design & Solutions BV. “And because the hull now houses much larger leg footings, bearing pressures on the seabed are significantly reduced.” He says that the wake of the legs’ pads (or spud cans) means there is no interference with the inflow to the propulsion thrusters.
The new SOUL series will come in several sizes for the efficient transport of up to six 10 to 15-MW wind turbines. Lambregts maintains all loading and install operations can occur without the need of ballast water, which is sometimes added to the cargo holds of vessels to increase vessel stability at sea.
“It’s interesting how a seemingly simple twist in the design let us make such improvements in operational aspects,” adds Lambregts. “We look forward to seeing SOUL at work offshore.”
A leg up — and around
Although the SOUL is an innovative development, Ulstein and SeaOwls say they are talking with professionals of the wind industry who are interested to understand their needs. In the meantime, we are working hard with the development of SOUL.
For jack-up vessels that require a separate crane, and one capable of withstanding harsh offshore conditions, Huisman recently designed and built a 1,500 mt (1 metric ton = 1,000 kg) Leg Encircling Crane (LEC) just for wind projects. It is currently the largest LEC in the world.
As its name suggests, the crane is built around a jack-up leg and, when required, the boom can be stored around another leg to save valuable deck space. The LEC also has a small tail swing to allow for optimal use of free deck space. For example, the 1500 mt LEC has a required leg opening of 11m and a tail swing of only 14.5m at operator cabin level. The operator cabin mounts to the side of the crane.
So that the crane weight is equal or less than its lifting capacity, Huisman uses a design philosophy that has proven its value in the offshore oil and gas industry. David Roodenburg, Director Strategy and Business Development at Husiman, explains: “By using high-grade steel and an intelligent design, a low construction weight is achieved that provides for an increased remaining payload on the jack-up vessel compared to conventional leg encircling cranes.”
The Leg Encircling Crane comprises of a steel A-frame, bolted on a pedestal via the slew bearing, a lattice type boom, and various hoist tackles to control the boom and the lower blocks
Roodenburg says Huisman is one of only a few companies that has successfully constructed and employed large-diameter slew bearings. “By applying Huisman’s segmented slew bearing, the overturning moment is transferred into the vessel structure in a highly structured way.” Roodenburg adds that the design reduces the amount of steel needed yet safely transfers forces into the vessel structure.
“The crane’s slew bearing, built from multiple segments, allows for easy inspection and maintenance. Furthermore, a slew bearing requires no kingpin to transfer the crane’s horizontal loads,” he says.
“Plus, all of the LEC’s main equipment, except for winches, are located inside a closed housing to protect it from the harsh marine environment,” Roodenburg says. The enclosed construction also protects the internal rollers and raceways.
The crane that works well in offshore wind-farm construction may prove unsuitable for turbine maintenance. To better meet the needs of the offshore O&M industry, Huisman has also developed a lightweight crane specifically for the maintenance of offshore turbines.
The Foldable Offshore Crane sports a foldable boom, which results in a small footprint in its storage position and a simple crane to transport. “The folding mechanism leads to a much lighter unit compared to conventional cranes, which simplifies inspection and maintenance because the lifting hooks are safely stored inboard,” says Roodenburg.
He says the ideal maintenance jack-up vessel has a small deck, is cost-effective, quick to mobilize, and can lift up to 600 mt to a height of over 160m. “So the Foldable Offshore Crane is well suited for work of this capacity on jack-ups because it is capable of exchanging wind-turbine components to around 160m above deck level from a vessel with a jack-up height of 0 to 70 m,” he says.
Also slightly tilting the hinge in the boom adds extra boom clearance to the crane. “This simplifies lifting of large loads and reduces the required boom length to reach above the middle of a nacelle,” says Roodenburg. Furthermore, the crane can be retrofitted on existing jack-ups, which is particularly useful for operators in need of more hook height to answer the growing demands of the offshore wind industry.”