OFFSHORE OIL SCIENCE BRINGS OFFSHORE WIND IN

Here’s a good phrase: monopile tower technology. It describes wind turbines with a single (mono) tower. (Not to be confused with mano, Spanish for hand and indicative of manly effort.) Monopile tower technology is inadequate for the tempestuous environment of deep ocean waters where wind energy potential is enormously rich. So far the most mano a oceano efforts, despite the wind industry’s engineering machismo, has not conquered wind’s deep water riches.

So it’s time to turn to the scholarly approach. The textbooks have the solution. They were written by the manly men of the oil industry from the 1930s to the 1980s when the exploration of seabed oil riches moved from lake bottoms in Louisiana to the ferocious North Sea.

First, offshore oil drillers have established the cost-benefit relationship between bottom-founded platforms and energy produced. It comes down to the depth cubed, based significantly on the cost of the steel needed to create stability.

Offshore oil did not come to solutions particularly applicable to wind until after oil prices crashed in the late 80s. Christopher D. Barry, P.E., naval architect and co-chair, Society of Naval Architects and Marine Engineers: “Floating platforms represent the most straight forward solution.”

One key to deepwater offshore wind platforms’ challenge is that they are designed to fold their blades under extreme wind conditions. The primary necessity is that they retain their buoyancy in huge wave conditions.

Barry explains the rather technical physics. Essentially, there is less to absorb a hit from a huge offshore wave in a floating platform than in an attached platform. Barry likes the “semi-submersible” tripod design.

Alternative designs explained carefully by Barry: Spar, guyed towers, buoyancy supported towers, and tension leg platforms. All have advantages but add to costs.

Delivering the power from far offshore to the onshore grid presents problems the oil industry may have only partially solved. Flexible oil carrying hoses may serve as models for flexible cables but challenges pertaining to the translation of DC to AC electricity, which Barry explains in some detail for the electrical engineers, may remain to be managed.

Based on his thorough understanding of the ground the offshore oil industry has conquered, Barry considers the challenges faced by deep water offshore wind worth the undertaking. Naval architectural expert Barry: “It is clear that there is a significant energy resource in offshore relatively deep water wind sites, and thanks to this accidental synergy between offshore oil and renewable energy that there are many viable concepts to exploit these resources.”

Offshore oil platforms. (click to enlarge)

Buoyancy Supported Offshore Wind Energy
Christopher D. Barry, June 13, 2008 (Alternative and Renewable Energy Development Institute Newsletter via Renewable Energy World)

WHO
Christopher D. Barry, P.E., naval architect and co-chair, Society of Naval Architects and Marine Engineers; Michael J. Dvorak, Mark Z. Jacobson and Cristina L.Archer, co-authors, California Offshore Wind Energy Potential;

WHAT
According to California Offshore Wind Energy Potential, offshore wind is an untapped potential mother lode of power generation. It faces many of the same challenges of working in deep ocean waters that offshore oil drilling technology has already solved.

Fixed offshore wind turbines. (click to enlarge)

WHEN
- The oil industry began drilling in lakes in the 1930s.
- From the challenges of getting at oil in the harsh conditions of the North Sea when oil prices were very low, in the 1980s, the oil industry found solutions the wind energy can put to advantageous use.

WHERE
- Although over 1100 megawatts of offshore wind production have already been developed in Europe, the challenges of deep water offshore development have yet to be conquered.
- As much as 200 terawatts of capacity may lie off California’s coast – but 90% is in parts of the Pcific with depths greater than 50 meters, heretofore considered out of reach.

WHY
- As water depth doubles, platform height (bottom to water line) doubles and amount of steel in it doubles. If the platform is bigger, the wave forces and overturning forces get bigger at a roughly depth to force cubed relationship, especially in the Pacific.
- In a floating system, most of the wave hit comes at the water plane, where the body is at the surface. The buoyancy can be anywhere, so the force of the waves is reduced if the body is submerged.
- The semi-submersible tripod design optimizes buoyancy versus stability and allows for on-site tuning of the ratio of column diameter to can shape according to particular marine environs. It also incorporates more concrete in the floating structure (less expensive) and less steel in an anchoring structure (more expensive).
- AC power transmitted in cables through a conductive medium would likely be highly inefficient. Platforms will have to be equipped with step up transformers and rectifiers to produce high voltage DC for transmission ashore.

Floating offshore wind turbines. (click to enlarge)

QUOTES
- Barry, on cost: “…the offshore oil faced this problem in the late 80's, especially when the price of oil fell dramatically, and developed a number of options to economically exploit small oil fields in deep water. Most of these concepts are even more applicable to wind power… Floating platforms represent the most straight forward solution.”
- Barry, on alternatives to the semi-submersible tripod floating tubine: “There are also three essentially “hybrid concepts”, guyed towers, buoyancy supported towers, and tension leg platforms. These generally have much reduced motions, though probably at higher costs.”