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| Fig. 1: Schematic of Hywind Tampen. Five of the eleven turbines supply power to Gulfaks and the remaining six lead to Snorre. [4] (Image source: M. Singleton, following the EFTA Surveillance Authority. [1]) |
Floating 150 km from the coast of Norway in the North Sea, the world's largest floating wind farm, Hywind Tampen, opened all propellers to the wind in May 2023 and began supplying power. The Hywind Tampen project was initiated and carried out by Equinor, an international company spanning 30 countries whose main initiative is addressing carbon emissions by means of renewable energy generation. [1] The energy company also contributes to oil and gas production. The 11 Equinor-owned floating turbines substitute the need for gas turbines and supply 88 MW of power to two neighboring oil companies, Snorre and Gullfaks, fulfilling approximately 35% of their power demand (384 GWh/yr). [1-3] The ESA reports that the wind turbines eliminate what would be 200,000 tons per year of carbon dioxide (CO2) produced by their alternative, gas turbines. [1,4] A ring structure optimized for maximum carbon dioxide emission reduction connects each 8.6 MW turbine, with six turbines leading to Snorre and five turbines supplying Gullfaks, as depicted in Fig. 1. [1] The wind turbines harness the power of the wind by means of 167m-long propellers, who convert kinetic energy to electrical power. [1]
Pressure fluctuations on the propeller due to changes in wind velocity affect the aerodynamic properties, such as the lift, drag, and pitching moment, which can be described by the Bernoulli principle. [5] The Bernoulli principle is derived from energy conservation: the total energy of a fluid along a streamline must remain constant and can be expressed as [6]
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(1) |
where the second term on the lefthand side is the change in pressure, ρ is the air pressure, and v is the air velocity along the propeller surface. [5] Written in this form, Eq. (1) states that the static pressure and changing pressure must remain constant. [5] As air flows over the curved portion of the propeller, it speeds (which must be met by a pressure decrease), while the air on the lower surface travels slower (pressure increase). [5] The difference in pressures on the two surfaces of the blade creates lift. The average power of each turbine can be calculated as
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(2) |
where ρ is the air density (taken to be 1.225 kg/m3 at sea level, 15°C), D is the diameter of the turbine rotors, and U is the average wind speed. [5] The efficiency of wind turbines should also be considered - the theoretical maximum power efficiency can be described by the Betz limit of 59.3%. [7] For Hywind Tampen, the average wind speed is approximately 10 m/s and the average power of each turbine is [7]
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(3) |
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| Fig. 2: Diagram of a spar-buoy structure, as used in the construction of Hywind Tampen. (Image source: M. Singleton, after Mathern et al. [8].) |
which is consistent with the power Equinor reports of 8.6 MW, within 4% error, which is likely due to a mixture of fluctuating wind speeds and the efficiency of each turbine not reaching the theoretical maximum set by the Betz limit. Hywind Tampen's wind turbines are connected to the sea floor via spar-buoy concrete substructures, as depicted in Fig 2. [8]
The development of Hywind Tampen relied heavily upon state grant funding. The Hywind Tampen windfarm cost a total of approximately 5 billion NOK, 43% of which (2.3 billion NOK, $207 million USD) was funded by the EFTA Surveillance Authority (ESA) through ENOVA SF. [1,3] ESA reports that this grant is the largest to date given by ENOVA and, without this state-issued aid, Hywind Tampen would not have realized. [1] As a preliminary study, the Levelized Cost of Electricity (LCOE) can be used to calculate the amount of energy produced by Hywind Tampen while factoring in building costs and the lifetime of the farm (reported to be 19 years, to be abandoned in 2041 [1]), which provides both a financial viability estimate and can be used to compare with other power generation concepts. [1,9] The LCOE is the total lifetime costs of the power generation divided by the total lifetime electricity generation. [10] The general formula for the LCOE is given by
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(4) |
where t indexes the year, I0 is the initial investment cost, M is the maintenance cost, F is the fuel costs, d is the discount rate, and P is the annual electrical generation capacity. [9,11] For windfarms, the fuel cost is zero. The investment cost of Hywind Tampen is 4,505.50 million NOK ($406 million USD). [1] The discount rate is 8%, and P is 384 GWh/yr. [1] Using an average operations and maintenance cost of 30 MWh/yr calculated from 7 independent studies of offshore windfarms, the estimated annual maintenance cost of Hywind Tampen is 135 million NOK. The LCOE for Hywind Tampen is reported by the ESA to be 1.38 NOK/kWh. [1]
For reference, the estimated LCOE for an 8MW fixed-bottom (onshore) wind turbine is $77 USD/MWh (estimate from Wiser and Bolinger) and a global average of $129 USD/MWh for 8MW floating wind turbines. [12,13] The main difference in LCOE between onshore and offshore wind is the higher cost of offshore annual maintenance. Hywind Tampen's LCOE is reasonable for typical offshore wind farms, falling at $120 USD/MWh.
One of the most advantageous aspects of deploying offshore floating wind turbines is the ability to reach to deeper waters. The buoy structure of Hywind Tampen's eleven turbines (see Fig. 2) allows for a larger portion of the turbine structures to float above water, with tethers anchoring them to each other and to the sea floor, than fixed-bottom turbines. However, since floating wind turbines have not been standardized (and are rather still uncommon across the globe), there is a lot of uncertainty about the stability and risk of investing in these farms. One study, in particular, was conducted of seven "experiments" in which model floating wind turbines were tracked and reviewed. [14] With particular relevance to Hywind Tampen is the 2009 Spar at NMRI experiment conducted at the National Maritime Research Institute (NMRI) by Kyoto University in Japan with a spar buoy structure. [14] To simulate oceanic effects (i.e., forces from thrust and water currents) the group applied simulated waves and a constant wind force on the propellers. [14] While other experiments used different or additional aerodynamic parameters (for example, simulated wind fields with fans and regular and irregular wave tests), the Spar at NMRI experiment is limited by the simulation and how well the simulations can model naturally occurring ocean dynamics. Numerous studies have been conducted to determine and review the stability of offshore floating wind turbine structures. [15-18] A study by the offshore oil and gas industry validates the stability of floating wind farms, claiming they have "long-term survivability", but warns that adaptations will be necessary. [15] In this study, a Norway designed spar buoy from StatoilHydro is simulated by the Offshore Code Comparison Collaboration to support and sustain a three-bladed upwind variable-speed variable blade-pitch-to-feather-controlled 5MW turbine. [15]
© Madison Singleton. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "The Hywind Tampen Project," EFTA Surveillance Authority, 11 Mar 20.
[2] I. Herrera Anchustegui, "Is Hywind Tampen's State Aid Approval a Kickstart for the Norwegian Offshore Wind Industry?" Eur. State Aid Law Q. 19, No. 2, 225 (2020),
[3] M. Ibrion and A. R. Nejad, "On a Road Map for Technology Qualification, Innovation and Cost Reduction in Floating Offshore Wind: Learning from Hywind and Norwegian Approach," J. Phys.: Conf. Ser. 2507, 012008 (2023).
[4] I. R. Dahl, B. W. Tveiten, and E. Cowan, "The Case for Policy in Developing Offshore Wind: Lessons from Norway," Energies 15, 1569 (2022).
[5] J. F. Manwell, J. G. McGowan, and A. L. Rogers, Wind Energy Explained: Theory, Design and Application (Wiley, 2010).
[6] G. A. Lindsay, "Pressure Energy and Bernoulli's Principle," Am. J. Phys 20, 86 (1952).
[7] E. Tenggren, et al., "A Numerical Study on the Effect of Wind Turbine Wake Meandering on the Power Production of Hywind Tampen," J. Phys. Conf. Ser. 1669, 012026 (2020).
[8] A. Mathern, C. von der Haar, and S. Marx, "Concrete Support Structures for Offshore Wind Turbines: Current Status, Challenges, and Future Trends," Energies 14, 1995 (2021).
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[12] B. H. Buck and R. Langan, eds., Aquaculture Perspective of Multi-Use Sites in the Open Ocean (Springer, 1917).
[13] T. Stehly and P. Duffy, "2020 Cost of Wind Energy Review," U.S. National Renewable Energy Laboratory, NRL/TP-5000-81209, January 022.
[14] G. Stewart and M. Muskulus, "A Review and Comparison of Floating Offshore Wind Turbine Model Experiments," Energy Procedia 94, 227 (2016).
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[16] S. Bashetty and S. Ozcelik, "Review on Dynamics of Offshore Floating Wind Turbine Platforms," Energies 14, 19 (2021).
[17] J. M. Jonkman, "Dynamics of Offshore Floating Wind Turbines - Model Development and Verification," Wind Energy 12, 459 (2009).
[18] A. N. Robertson and J. M. Jonkman, "Loads Analysis of Several Offshore Floating Wind Turbine Concepts," U.S. National Renewable Energy Laboratory, NREL/CP-5000-50539, October 2011.