Floating Offshore Wind Turbines Explained
NREL webinar diagram - March 17, 2020
floating vs. fixed bottom foundations
best offshore locations - U.S. and global
floating turbine types
turbine spacing & air turbulence
electric cable network
offshore & land-based turbine sizes
turbine, foundation, and installation costs
Overview of Floating Offshore Wind
National Renewable Energy Laboratory Webinar
March 17, 2020
Walt Musial
Principal Engineer
Offshore Wind Research Platform Lead
National Renewable Energy Laboratory
Golden, Colorado USA
Webinar video available at NREL YouTube channel.
Floating wind turbine types
(from diagram above)
Spar: stability through ballast (weight) installed below its main buoyancy tank.
Challenges -- Deep draft limit port access.
Semisubmersible: achieves static stability by distributing bouyancy widely at the water plane.
Challenges -- Higher exposure to waves; more structure above the waterline.
Tension-leg platform (TLP)
Achieves static stability through mooring line tension with a submerged bouyancy tank.
Challenges -- Unstable during assembly; high vertical load moorings.
Best global sites for offshore wind energy production
NREL webinar map - March 17, 2020
Seafloor depth and proximity to population centers are factors in offshore windplant site selection.
Most existing global offshore wind turbine generators are installed on fixed bottom support structures
> NREL webinar diagram - March 17, 2020
U.S. coastal floor depths are shallower in Atlantic and Great Lakes regions
Lighter shades of blue indicate shallower floors. U.S. Department of The Interior - Bureau of Ocean Energy Management (BOEM) regulates 15 Lease Areas (red) giving developers exclusive site control of up to 25 gigaWatts (GW) capacity. BOEM has also identified 13 Call Areas (orange) - potential wind energy areas that under public review.
NREL webinar map - March 17, 2020 (Tap/click to enlarge).
U.S. offshore windspeeds are best near NE coast, northern California and Oregon
NREL webinar map - March 17, 2020
Darker colors indicate the best potential offshore wind energy sites, where average annual windspeeds are greater than 10 meters per second (22 miles per hour). Offshore wind electric energy may double the present U.S. annual electric energy consumption.
Floating foundations are required for depths greater than 60 meters
NREL webinar map - March 17, 2020
Dark-blue areas indicate potential offshore windpower areas where floor depths are greater than 60 meters. Great Lakes offshore wind plants may require adequate distance from shore to eliminate visual impact. Lake freezing is a design concern.
Floating wind turbine components
NREL webinar map - March 17, 2020
Offshore wind turbines sizes are larger
Growth of wind turbine sizes since 1980. Green grass at bottom left half of chart indicates land-based projects. NREL webinar diagram - March 17, 2020
Undersea electric cable grid design
NREL webinar diagram - March 17, 2020
Array voltage will soon increase to 66 kiloVolts (kV) to lower cost.
Electrical array cable cost increases with turbine spacing but decrease with turbine size.
Exact turbine spacing is a trade-off between wake losses and array cable cost.
Other factors such as navigation safety may play a role.
Offshore wind project electric collection network design is similar to an onshore electric utility distribution network, but performs the opposite function. Onshore powerlines and buried cables distribute electric power from a substation to end-users. Offshore submarine cables collect electric power from wind turbine generators for delivery to a floating substation, where the voltage is boosted and the electric power is delivered to shore via submarine transmission cables.
Higher windspeeds produce more electricity
Block Island Wind entered service as the first U. S. commercial offshore wind electric generating project in 2016. NREL offshore map above shows average annual windspeed at Block Island is 9.5 - 9.75 meters per second. At this windspeed, a 6 MW wind turbine at this site generates about 3.5 MW. Periods of windspeeds greater than the annual average generate the most electricity. At rated windspeed of about 12 meters per second, power output does not exceed the 6 MW rating, as rotor blades furl to prevent overspeed.
NREL webinar chart - March 17, 2020 (Tap-click to enlarge)
Total generating capacity at Block Island Wind is 30 MW (megaWatts) The project consists of 5 fixed-bottom turbines rated 6 MW each. Location is about 3.8 miles from Block Island off the Rhode Island Coast.
Challenge: turbine size and spacing affects airflow turbulence
Simulated wake turbulence downwind of turbines. NREL webinar diagram - March 17, 2020
Wake losses are the reduction of wind turbine generated electric power due to windspeed reduction and airflow turbulence caused by an upwind turbine
NREL webinar diagram - March 17, 2020
As wind turbine heights and rotor diameters increase, spacing must also increase to compensate for turbulence and wake losses.
Challenge: floating wind turbines rock and tilt
Modified from NREL webinar diagram - March 17, 2020
Wave motion affects floating offshore wind turbine power generation. Tilting toward the wind causes net windspeed to increase at rotor height, and to decrease when tilting motion is away from way from wind.
Cost of floating wind turbine plus on-site installation
Stehly, Tyler; and Philipp Beiter. 2020. 2018 Cost of Wind Energy Review. National Renewable Energy Laboratory, Golden, Colorado. NREL/TP-5000-74598. Funding provided by the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy Wind Energy Technologies Office. NREL webinar chart - March 17, 2020
NREL’s 2018 Cost of Wind Energy Review estimated turbine electric generator will be 24.3 percent of the total installed cost of floating offshore wind plants. Section 5.2 of this report explains factors which produced this estimate, including global floating offshore wind plant construction - excerpt below:
2018 Cost of Wind Energy Review (NREL)
(excerpt)
Given the relatively limited number of offshore wind projects in the United States and the lack of publicly available data, we obtained the CapEx estimates using ORCA (Beiter et al. 2016). The representative turbine characteristics (i.e., turbine capacity, rotor diameter, and hub height) used as inputs to the model were obtained from the “2018 Offshore Wind Technologies Market Report” (Musial et al. 2019). The capacity-weighted average turbine installed globally in 2018 was 5.5 MW with a 140-m rotor diameter at a 94-m hub height. We used these turbine parameters in combination with the spatial parameters presented in Table 12 for the fixed-bottom and floating reference sites to calculate CapEx.
The ORCA model yields a total installed CapEx value of $4,444/kW for the fixed-bottom reference site and $5,355/kW for the floating reference site. It should be noted that the CapEx estimates for floating offshore wind in this analysis assume a 5.5-MW turbine and are not necessarily optimized for floating offshore wind applications, therefore, they may negatively impact CapEx estimates. Progression to larger turbines is likely to coincide with deployment of commercial-scale floating wind technologies (Spyroudi 2016).