Offshore wind turbine technology: Harnessing the Saudi Arabia of wind

The potential to harness 149,000 megawatts of energy within 50 nautical miles of Maine’s coastline has earned the Gulf of Maine the moniker ‘The Saudi Arabia of wind'.

By Rikki Stancich

The State of Maine plans to develop 5 Gigawatts of offshore wind energy by 2030, by installing 1,000 5-megawatt wind turbines 20 miles from shore at an estimated cost of US$20bn.

WindEnergyUpdate caught up with Habib Dagher, Director of the University of Maine’s Advanced Structures and Composites Center, to discuss the manufacture of floating wind turbine technology in deep waters.

Habib Dagher heads up a team of researchers and is collaborating with more than 30 companies on the design, manufacture and testing of floating wind turbine technology in deep waters 60–900 meters offshore. He also met recently with U.S. Energy Secretary Steven Chu to discuss a proposal to establish a national deep-water offshore wind research center at the University of Maine, which aims to establish itself at the fore of alternative energy research.

WindEnergyUpdate: What are the challenges associated with offshore floating wind turbine technology off the coast of Maine?

Habib Dagher: The major challenge is identifying the most cost effective technology for different water depths and meteorological conditions. We are attempting to place wind turbines in water depths in excess of 70 metres. The State of Maine waters are the deepest on the East Coast of the United States and the water depths also vary considerably, so we are looking at a wide variety of technologies suited to different water depths.

WindEnergyUpdate: Which designs are you testing?

Habib Dagher: We’re looking at three major groups, including SPAR design, similar to that of the Norwegian StatoilHydro wind project. We actually signed a collaborative agreement with StatoilHydro two weeks ago to cooperate on the development of a SPAR design for the Gulf of Maine.

We are also looking at Tension Leg Platforms (TLPs) and semi-submersibles. All three technologies have applications in different depths and conditions.

We are open to a lot of different ideas because there are a variety of meteorological conditions that need to be taken into account.

WindEnergyUpdate: What are the pros and cons of each design?

Habib Dagher: SPARs are simple structures and therefore are easy to manufacture, which reduces logistics. The design is very stable and can reduce acceleration and motion in different sea conditions. They can be towed to the site horizontally, then, once righted with the ballast the rest of the vertical structure is installed and then the turbine. But the challenge is that you are dealing with two floating structures. Also, it requires waters with a depth of 150 metres plus to right the structure, as well as a deep channel to tow to site, which could limit location possibilities.

Tension Leg Platforms can be used in shallower depths, but they are less stable. This is because they use tension in the cables to right -up. However, the longer the cable gets, the less stable the platform becomes. The turbine can be installed in the dry dock and the structure can then be towed to the site. It’s not limited to deep water near the shore.

Semi-submersibles can be used in any depth, can be manufactured in the dry dock and towed to site. But because it is submerged in the wave environment it is subject to a lot of wave motion, so is less stable.

WindEnergyUpdate: Which of these have been successfully tested offshore?

Habib Dagher: A TLP has been erected off the coast of Italy by Blue H; the SPAR design has been tested by StatOilHydro; and the semi-submersible has had tank testing. There is quite a lot going on in this field.

WindEnergyUpdate: Have you identified areas where greater cost efficiency and efficiency gains can be had?

Habib Dagher: The key issue is constructability – construction methods that enable us to manufacture near water, on shore, and that can be deployed by throwing components to the final position.  This reduces – and could ultimately eliminate - the need for costly jack-up barges.

We are focusing on deployment logistics, maintenance logistics and construction logistics. When you are operating 20 miles offshore, a key cost driver is developing material and maintenance systems that reduce down time - that enable you to swap turbines quickly and repair damages near the shore. A lot of traditionally philosophical maintenance systems are now being evaluated.

Blades and blade technology is another key area, when it comes to up-sizing turbines beyond 5 megawatts.  If you are manufacturing the turbines near shore, you have to consider the logistics of moving large structures. Here, we are focusing on materials – more advanced composites, such as nanotechnology and hybrid composites.

WindEnergyUpdate: The University of Maine is about to open a pre-approved test site to the industry. Can you provide more details about when that will be ready?

Habib Dagher: The State of Maine is about to open a deep offshore wind test site and the University of Maine is running a test site that will be open to the wind industry to test new technology. We are inviting companies to bring their designs in to test –it is a great opportunity, as it will allow companies to by-pass the usual permitting costs. Structures for testing will need to be permitted (usually it’s about a 60-day permitting period). Between 1-5 sites will be pre-approved in state waters for deep water floating offshore technology evaluation. The sites will be selected by the State of Maine on December 5, 2009.


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