When the Block Island Wind Farm off Rhode Island’s coast powers up in January, it will be the first offshore wind installation in the U.S. While these are new sight on this side of the Atlantic, Europe’s been building such mechanical farms for years—and they have some amazing, specialized hardware to help get the job done.
Take the Fred Olsen shipping company’s Brave Tern, a 15,000-ton ship borrowed for this job. In a way, Brave Tern is real-world Transformer, changing from a ship to a construction platform by extending its legs to rise out of the water. Ships like this are called windcarriers because they can both transport and assemble wind turbines. The U.S. simply doesn’t have anything quite like it.
A TRANS-ATLANTIC CHALLENGE
It’s a minor miracle that this ship is in the Western Hemisphere in the first place. No windcarrier had ever crossed the Atlantic and planning the Tern’s maiden Atlantic voyage this past July was a major operation. Brave Tern can withstand storms and heavy seas pretty well, but there isn’t much space between the waterline and the main deck.
“We have to expect water on deck, so-called ‘green sea,’ when sailing in heavy seas,” project manager Eskil Røsets told Popular Mechanics. “Mitigating the effect of any green sea was an important part of the planning.”
As it braved the high seas, the Tern carried five housings that contain a turbine’s working components, or nacelles, each of which is the size of a school bus. To prevent storms and green sea from damaging this sensitive cargo, workers used detail computer models of storms to prepare for every contingency. After these tests, the team created sea fastening modules, each weighing 50 tons, to hold the nacelles securely in place and also protect them from the wrath of the water.
Ships usually sail the shortest route across the Atlantic, but in this case, safety was paramount. The Brave Tern team balanced information regarding wind, currents, and possible storms, and settled on a route via the Azores and Bermuda. This minimized the risk of running into extreme weather, and it worked perfectly.
A MECHANICAL TRANSFORMATION
On arrival, the Brave Tern‘s first order of business was transforming from a massive ship into a construction platform. First, the ship maneuvers into position, guided precisely by GPS, using underwater jets known as tunnel thrusters. Block Island’s several support barges made this parking job even more difficult. Røsets described the operation as “parking a huge ship in a small garage.”
Once in place, the three-hundred-foot long legs are lowered by hydraulic jacks. Each leg has its own conical foot called a “spud can” that’s 14 feet across. When the spud cans push through the mud and make contact with the seafloor, the ship is “pinned” and the positioning system is turned off.
Next, a process of “pre-loading” ensures that the platform is stable and does not lurch suddenly while being raised. Two legs diagonally opposite each other lift slightly, putting the ship’s weight on two spud cans and compacting the sea bed beneath. The same process is then carried out for the opposite diagonal pair, until a firm footing is established. Finally, the vessel is raised out of the water, at a blazing speed of 16 inches-per-minute.
“This may not seem like much,” says Røsets. “But considering that the vessel and all its equipment may weigh more than 19,000 tons, it is a quite decent speed.”
While all this is going on, crews prepare for the next installation. Røsets says there is not much sense of movement on the ship, apart from in the initial pinning phase. There is some noise from friction between the legs and the hull, but the whole transition from floating to elevated operation is incredibly smooth, and sometimes the vessel will be moved daily, so transforming becomes routine.
“Even after experiencing many of these operations,” says Røsets, “it is still a quite unique feeling going from a floating mode with the sea level about 3.5 meters below the main deck, to an elevated mode where the sea level is 20-30 meters below you.”
GETTING TO WORK
Now the construction crew can finally start building.
The pre-fabricated concrete foundations had already been secured on the sea bed with steel piles able to withstand the type of storm that only comes once every thousand years, and a steel tower built on the foundations. Brave Tern‘s main crane, able to hoist eight hundred tons, puts each nacelle in place atop the tower where is it bolted into place. Then comes the tricky bit.
A special gripper known as a “blade yoke” lifts up each of the composite turbine blades, weighing 58,000 pounds precisely into position. Because of their shape, the blades can catch the wind and get pulled out of the blade yoke; but the blade will crack if gripped too tightly, so it is a delicate operation. The positioning process is guided by a worker standing inside the turbine hub. Two workers with power tools then secure the turbine blade with giant bolts three feet long — a hundred and twenty eight of them. The total assembly process for a nacelle and the blades takes one or two days in good conditions.
After a little more than two weeks, the crew completed the Block Island project on August 18, 2016. All five turbines stand 600-feet tall, from the the tip of the turbine blade, and each one will generate six megawatts. The combined output from the five turbines can power 17,000 homes. For the U.S., it’s a start but a relatively small one compared to wind farms like Holland’s Gemini array—filled with 150 turbines. That’s because the European Union has made a commitment to supply 20 percent of its electricity with renewable energy by 2020. The U.S. has shown far less enthusiasm.
However, Røsets says more offshore wind projects are planned along the U.S. coast and in the Great Lakes. That will mean more work for the wind carriers, and in the next few years ships like the Brave Tern could become a common sight in North American waters.
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Source: Popular Mechanics
