The American Bureau of Shipping recently did a hazard identification study as the first step in preliminary planning and advice for WindFloat, a proposed semisubmersible platform for large offshore wind turbines.
Principle Power Inc., Seattle, acquired intellectual property rights to that wind energy concept in April. It’s one of several offshore floater concepts under development, said ABS.
WindFloat is a three-column semisubmersible structure capable of supporting a wind turbine for marine application in a power range of 3.6-10 Mw. Its design dampens wave and turbine-induced motion so it can be placed in more than 50 m of water to take advantage of “superior” wind resources, said company officials.
Key components are the columns, truss, wind turbine tower, and water entrapment heave plates at the base of each column to minimize pitch and yaw. A closed-loop active ballast system mitigates wind-induced thrust forces, restoring the system to optimal efficiency after changes in wind velocity and direction. Total displacement with ballast would be 8,000 tons, with a hull draft of 20 m. The company specifies a rotor diameter of 120-150 m. A conventional catenary mooring system using prelaid drag-embedded anchors would hold it in place.
Study details
The ABS study focused on risks associated with design. “What if you have a collision? What if you damage the hull legs? What are the potential problems with the tow out of such a tall structure? These are just some of the areas we considered,” said Ahmad Shafaghi, director of energy risk consulting at the ABS Consulting affiliate.
WindFloat has been in development for 6 years, with extensive design work and numerical modeling, stress and fatigue analysis, as well as multiple wave tank testing campaigns. “When designing the unit we looked at the synergies from oil and gas concepts—from fixed platforms to spars to a semisubmersible,” said Dominique Roddier, coinventor of WindFloat and chief technology officer at Principle Power.
Rigs available
Considering the time and effort invested in designing and testing a new offshore facility, however, an outside observer can’t help wonder why offshore wind pioneers don’t simply use an already proven product—some of the many jack up rigs stacked along the Texas and Louisiana coasts. Offshore rigs could be converted into as stable of platforms for marine wind farms as they have been for drilling. Plus they’ve been tested far beyond the capabilities of a wave tank, having survived the real world of ocean currents and storms.
It wouldn’t be the first time someone thought of putting a used rig to other uses. Back in the late 1880s Era Aviation Inc., then the aviation subsidiary of Houston drilling contractor Rowan Cos. Inc., was under contract for perimeter security flights at Vandenberg Air Force Base, Calif., site of satellite launches for military and commercial organizations as well as testing of intercontinental ballistic missiles. As was its practice with all clients, Rowan sent a copy of its annual report to the air base where the cover photo of a Gorilla jack up rig caught the eye of a general who thought the unit would make a grand launching pad for rockets once the rig mast was removed.
Rowan executives were receptive to the idea since drilling was in an even bigger slump back then than now. The Air Force’s idea was to erect a 5 million lb rocket over the rig’s moon pool in the Vandenberg harbor and then tow the rig out beyond the 3-mile limit of California’s offshore authority. There the rig would set up as it would at a drilling site; the rocket would be readied and fired by remote control from Vandenberg’s onshore launch command.
Air Force officers toured jack ups to assess the feasibility of using them for offshore launches. It was estimated then a jack up launch platform could be custom-built for $100 million, compared with $3.3 billion for one Vandenberg launch pad that was never used. Nothing came of the plan, however. In time the market improved, and Rowan’s rigs went back to work.
In 1999, however, Sea Launch Co. LLC—a consortium of US, Russian, Ukrainian, and Norwegian investors—began launching satellites for commercial customers from the converted semisubmersible Ocean Odyssey. The Long Beach, Calif.-based company says it has made 31 launches from an equatorial position in the Pacific Ocean. In June, however, it filed for bankruptcy protection under Chapter 11.
