The scope of offshore wind in the United States is small—but it’s not expected to stay small for long.
At present, the Block Island Wind Farm off the coast of Rhode Island is the only operating offshore installation in the United States. However, more offshore facilities are expected as more construction permits are granted, according to Brian Ray, chief engineer for industrial bearings at The Timken Co. If the 12 active offshore commercial leases in the U.S. are fully built, that would significantly boost the nation’s total wind energy generation in a short amount of time.
“Onshore turbines typically generate power about 20% to 30% of the time,” says Doug Lucas, advanced engineering technologist for The Timken Co. “Offshore, it is more like 50% to 60% because the winds are more sustained.”
The largest challenges for the U.S. offshore wind market will be developing the supply chain, streamlining regulatory processes and improving power transmission, according to Kyle Kingman, president of Offshore Power, LLC. This took some time in Europe, but Kingman anticipates the U.S. will learn from Europe’s experience and benefit from cross-market availability.
State of the wind
Today’s typical offshore turbines can generate 5 megawatts (MW) of electrical power, although some newer turbines can generate 10–12 MW. Unlike their onshore counterparts, where component sizes are limited by the capabilities of road or rail transport, offshore turbines can be manufactured close to port facilities and transported to the construction site aboard ships.
“Larger turbine sizes come with many obvious benefits, but also some inherent challenges. The challenge with increased turbine size is mainly in transportation and installation costs,” Kingman says. “Barges and lifting vessels capable of handling the larger parts are more expensive to hire. Vessels and crews compliant with the Jones Act, Section 27 of the Merchant Marine Act of 1920—which requires all vessels carrying goods within the United States to be primarily owned and crewed by U.S. citizens, as well as built and registered in the United States—are more limited in availability. Port facilities with a suitable location, sufficient size and adequate lifting capacity are needed. With that need alone, you virtually eliminate most of the U.S. East Coast, with few exceptions.”
These factors may ultimately limit the size of offshore turbines, he adds, but solutions tend to arise in parallel with technical challenges.
Floating versus fixed
The survey Forecasting Wind Energy Costs & Cost Drivers found that a fixed-bottom offshore turbine with a 125-meter hub height and 190-meter rotor diameter could produce 11 MW of power, compared with 9 MW for a similarly-sized floating turbine. The survey revealed that reducing energy production costs for fixed-bottom turbines will depend mainly on increasing turbine capacity ratings, improving designs for foundations and support structures, and reducing financing costs and project contingencies. Foundations, support structures and installation processes are expected to play an even greater role for floating turbines.
Locating wind farms farther offshore increases transportation costs, but it can address nuisance concerns from onshore communities, pose less risk to birds, and offer access to stronger, steadier winds. Floating wind systems may not need heavy lift vessels to travel further offshore, and they can be decommissioned more easily than fixed platforms, according to Kingman.
Currently, there is a lack of interconnection points where electrical power can be brought to shore, Kingman notes. Land-side infrastructure requires significant upgrades to handle the injection and to balance the transmission load. In Europe, Kingman works on the North Sea Link project, which connects electrical power systems between Norway and the U.K. This project, on track for completion in 2021, will be the largest subsea interconnector in the world.
Technology plays an increasing role in turbine maintenance. On-turbine sensors continuously monitor temperature, vibration, and lubricant conditions and relay the results to operational facilities onshore. Flying drones and remote sensors perform inspections that are not practical for human technicians. Furthermore, sonar systems and autonomous underwater vehicles have greatly improved the quality of geophysical surveys for site selection and underwater power transmission cable installation.
Engineers who continue to expand these solutions, and others, could help open up a rich source of renewable energy offshore in the United States.