Floating Offshore Wind Turbines

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in italiano

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di Paul Sclavonous - Department of Mechanical Engineering
Massachussets Institute of Technology (MIT)

Paul Sclavounos - MIT
Wind is a rapidly growing renewable energy source, increasing at an annual rate of 30 per cent with the vast majority of wind power generated from onshore wind farms. Their growth is however limited by the lack of inexpensive land near major population centers and the visual impact caused by large wind turbines. Wind energy generated from floating offshore wind farms is the next frontier with over 75 per cent of the worldwide demand for electricity coming from coastal population centers. Vast sea areas with stronger and steadier winds are available for wind farm development and 5 MW wind turbine towers located 30 kilometers from the coastline are invisible. Current offshore wind turbines are supported by monopoles driven into the seafloor or other bottom mounted structures at coastal sites a few miles from shore and in water depths up to about 25m. The primary impediment to their growth is the prohibitive cost of the foundation system and wind turbine installation process as the water depth increases. This article discusses the technologies and the economics associated with the development of motion resistant floating offshore wind turbines drawing upon a seven year research effort at MIT. Innovative floater concepts are discussed, inspired by developments in the oil and gas industry for the deep water exploration of hydrocarbon reservoirs. The interaction of the floater response dynamics in severe weather with that of the wind turbine system is addressed and the impact of this coupling on the design of the new generation of multi-megawatt wind turbines for offshore deployment is discussed. The primary economic drivers affecting the development of utility scale floating offshore wind farms are also addressed.

The large scale generation of affordable electricity from environmentally friendly and renewable sources of energy is one of the major challenges humanity will face during the 21st century. It is also widely recognized that technology will play a critical role towards achieving this goal. Wind is a rapidly growing renewable energy source capable of generating utility scale power from onshore wind farms with a current worldwide installed capacity in excess of 80 GW. The next frontier of wind energy will feature multi-megawatt floating wind turbines deployed in gigawatt scale offshore wind farms placed at a distance from the coastline where tall towers are invisible and in water depths ranging from 30 to several hundred meters. Assuming perfect visibility, and that the radius of earth is 6,370,000 meters, a tower with height H will not be visible at a distance L from the shoreline given by the expression.

It follows that a 100m tall tower (above sea level) supporting a 5 MW wind turbine will not be visible if it is 35 km from shore, assuming that blades spinning in and out of the horizon at that distance are barely visible. In reality this marginal zone of visual impact may be 30 km or less depending on the atmospheric conditions and the sensitivity of the human eye. The offshore environment favors the transportation of the large components of 5 MW wind turbines on ships and their assembly at a coastal facility. Water depths ranging from 30 to several hundred meters prevent the installation of monopoles or other traditional bottom mounted structures due to their excessive cost. This challenge may be addressed by designing the floaters and mooring systems discussed in the present article for the support of multi-megawatt wind turbines, inspired by analogous offshore platforms developed for oil and gas exploration in water depths exceeding 3,000 meters. A sample of three such offshore platforms currently deployed by the oil industry is illustrated in Figure 1.

Inspired by these oil industry developments, wind turbine “floaters” can be designed for the support of a 5 MW wind turbine ballasted so that they float stably without the support of a mooring system. This would enable the full assembly of the floater and wind turbine at a coastal facility equipped with land based cranes, storage facilities and a streamlined manufacturing and assembly process. Moreover, large wind turbine blades may be easily transported on ships to the coastal manufacturing facility. When fully assembled on the water, the floating wind turbine would be ready to be towed to the offshore wind farm site. The particulars of the 5 MW wind turbine system used in the study are presented in Figure 2.

Three types of wind turbine floaters have been studied at the Laboratory of Ship and Platform Flows (LSPF) at MIT. The Tension Leg Platform (TLP) is connected to the seafloor by steel tethers which are kept at a high enough tension following their connection to the wind turbine floater so that the system cannot tilt due to the effect of the wind and the waves. The only floater response allowed by a TLP is a horizontal back and forth oscillatory displacement akin to a soft “wavequake” excited by the ambient waves.

The connection of the TLP tethers to the floater may be carried out by initially ballasting the floater with water and deballasting it after the tethers have been connected to their seafloor foundation in order to generate a large enough pretension. At the seafloor the tethers would be connected to a block of concrete filled with scrap steel or a heavy mineral sufficiently heavy to withstand the tether pretension and its fluctuations in severe storms. The cost of this concrete foundation is a key economic driver of the TLP concept. Figure 3 illustrates a TLP wind turbine floater ballasted with concrete.

The second type of wind turbine floater being studied is a Spar Buoy connected to the seafloor with taught catenary mooring lines made out of steel or a synthetic material. Unlike the TLP, the catenary moored Spar Buoy can tilt due to the wind and wave action. However, this tilt can be kept to a minimum – typically less than a few degrees – by selecting a configuration of catenary mooring lines with a pretension and connection points to the buoy selected to minimize floating wind turbine responses due to the waves. As with the TLP, the Spar Buoy supporting a wind turbine floats stably prior to its attachment to the catenaries, and their connection offshore is a routine matter. The catenaries would also be connected to the seafloor with gravity anchors. An advantage of the Spar Buoy is that the static plus dynamic tension of the catenary anchors may be lower than the corresponding TLP tether tension, therefore the cost of the Spar mooring system and anchors is likely to be lower. Figure 4 illustrates an optimized Spar Buoy wind turbine floater.

A hybrid floating wind turbine system that combines the advantages of the TLP and Spar Buoy concepts is illustrated in Figure 5. By properly selecting the initial tension of the catenaries, their composition out of steel/synthetics, their angle relative to the seafloor, the radius of the rim where their fairleads are connected to the buoy and the vertical position where the rim is attached to the buoy, the static plus dynamic tension of the catenaries at the gravity anchors can be minimized therefore reducing significantly the cost of the foundation system. At the same time the static and dynamic tilt of the tower can be kept very low and well within the thresholds specified by wind turbine manufacturers.

The design and optimization of a hybrid TLP – Spar Floating Wind Turbine system supporting a wind turbine ranging from 3-5 MW is currently under way at MIT funded by Enel Produzione. A safe and cost optimized system is being developed for water depths ranging from 30 to several hundred meters and the wave and wind conditions encountered around the Italian coastline. Key cost drivers of the hybrid wind turbine floating system are the maximum tension of the mooring lines which in turn drives the uplift capacity of the anchors and the steel weight of the floater which depends on the payload, namely the weight of the turbine tower, nacelle and blades. The approximate weight of a 3 MW onshore wind turbine system is 350 metric tons while the weight of a 5MW turbine is 700 tons. For a floater buoy with a draft of 20-30m the buoy-to-turbine weight ratio is 1:1. So a steel buoy weight of 350 and 700 tons is necessary for the support of a 3 MW and 5 MW turbine, respectively. For the wave environment encountered around the Italian coastlines the maximum uplift capacity per anchor is estimated at 150 metric tons in water. Eight anchors will be needed per wind turbine floater, two on each of its four sides. These figures are input to the economic analysis associated with the construction, deployment and installation of the floating wind turbine system.

The mean thrust of the wind on the rotor of a 3 MW turbine operating at its rated power is approximately 60 tons. The corresponding mean thrust on a 5 MW turbine is 80 tons and both forces act horizontally at a distance of 100m above sea level. The moment resulting from these forces will cause the turbine tower to tilt.
This tilt can be controlled by the appropriate design of the mooring system and is kept smaller than a few degrees. The wind speed is rarely steady instead it fluctuates stochastically around its mean value. The resulting oscillatory forces and moments are small and cause negligible dynamic displacements of the floating wind turbine system due to its high inertia. The typical significant wave height around the Italian coastline was estimated by Enel to be in the range of 3-4 meters. The effect of waves of such height on the dynamic responses of the floater buoy which has a diameter of 8-10 meters, a draft of 20-30 meters and is constrained by taught mooring lines is very small. Yet, the floating wind turbine system needs to be designed so that it withstands the most severe seastate to be encountered during its lifetime which is assumed to be 20-30 years.
Consistent with the practice followed by the oil industry for the design of offshore platforms, the survival seastate for which the floating wind turbine system needs to be designed for is the worst seastate that would be encountered over a time period equal to twice its lifespan or about 50 years. The significant wave height of the 50 year storm around the Italian waters does not exceed 10m. Dynamic response analyses carried out for an optimized hybrid floater in a 10 meter seastate yielded maximum nacelle accelerations less that 10 per cent of the gravitational acceleration g. These are well below the threshold acceleration of about 50 per cent of g considered critical by wind turbine manufacturers.

The transmission of electricity from a floating offshore wind farm consisting of 100 or more units to shore may be carried out by initially collecting the power generated by each unit to a central offshore sub-station via electric cables laid out on the seafloor. The sub-station will contain the necessary electrical equipment and may be housed in a bottom mounted tower or a floating structure, depending on the water depth. The high voltage transmission of the power to shore would then take place via a central cable laid out under the seafloor. Impedance losses associated with the transmission of power using AC current are considered acceptable over distances of the order of 50 kilometers. Electricity transmission technologies using high-voltage DC current over large distances with minimal losses also exist yet at a higher cost. These technologies are aimed at utility scale floating wind farms strategically placed at offshore sites selected so that they can sell electricity to one or more major markets depending on the local electricity demand and prices.

Economic drivers of floating offshore wind farms are the cost of the wind turbine, floater, mooring system and anchors and the cost of operation and maintenance. The approximate cost of a 3 MW on shore wind turbine is 6 millions dollars or 2 millions/MW. The floater buoy supporting this turbine has a steel weight of 350 tons. The gravity anchors have a cumulative weight of 1,200 tons, or 150 tons/anchor, and each of the eight mooring lines must withstand a maximum tension of 150 tons and have a fatigue life that exceeds 25 years, the lifespan of the offshore wind farm. These figures are sufficient for the generation of estimates of the cost of materials, construction, assembly and deployment of the floating wind turbine system. Interconnection costs are estimated at 15-20 per cent of the total capital costs described above.
Operation and Maintenance is a significant cost component of wind farms and it must be kept at a minimum for floating wind turbines. An attractive feature of the hybrid floating wind turbine concept is that it may be ballasted, disconnected from the mooring system and towed to shore or a large offshore platform for major maintenance or part replacement. Routine maintenance a few days per year may take place onsite where accessibility by a small boat should be easy in calm weather. Operation & Maintenance costs are often partially integrated with the cost of the wind turbine offered by the manufacturer who may commit to the operation of the offshore wind farm over an initial period of several years. Wind is more persistent offshore than onshore and the associated capacity factor may approach or exceed 40 per cent. Moreover, the wind velocity 100 m above the smooth sea surface is higher than the corresponding wind velocity onshore due to the absence of obstacles and boundary layer effects.

A 20 per cent higher average wind speed in the offshore environment leads to an increase of the power generated by an offshore wind farm by approximately 73 per cent. A 50 per cent higher average wind speed translates to more than a threefold increase in power. Therefore, the revenue from an offshore wind farm may be significantly greater than that from an onshore farm with the same rated power. This increase may be sufficient to more than offset the increase in the marginal costs associated with the operation in the offshore environment. Investments in utility scale floating offshore wind farms are of comparable scope to investments in oil and gas reservoirs. A barrel of oil has a mass of 130 kg and a specific energy of 12 kWh/ kg therefore it contains energy equivalent to 1.5 MWh. Assuming that an offshore wind farm operates at a capacity factor of 40 per cent – namely 40 per cent of its rated power is on average converted into electricity 24 hours a day – then its energy production per day is 9.6 MWh which is equivalent to the energy content of 6.4 barrels of oil. The conversion efficiency of the wind power resource using wind turbines and of oil & gas using combustion engines & combined cycle gas turbines into useful energy, e.g. mechanical energy, heat or electricity, are assumed comparable and range from 40- 50 per cent. Using the same conversion efficiency, a utility scale floating offshore wind farm with a rated capacity of 1 GW and a life of 30 years will generate over its lifespan energy comparable to that contained in a 70 millions barrel oil field. For a cost of 3 millions dollars per rated MW of floating wind turbine capacity, the 3 billions dollars investment in a 1 GW floating offshore wind farm translates into an equivalent cost of 43 dollars barrel of oil for the development of the 70 millions barrel oil field. This cost is not far from the costs quoted by the oil & gas industry associated with the exploration and extraction of hydrocarbons from sub sea reservoirs.

The investment risk in oil & gas exploration is primarily associated with the drilling of dry wells and the volatility of the oil and gas prices which are driven by a global market forces, at least for crude oil. The investment risk in floating offshore wind farms is associated with the volatilities of the wind speed and the electricity prices. The wind speed volatility can be well quantified in offshore sites where sufficiently long records of wind speed profiles are available. The electricity prices are often affected by regional market forces and may be predictable especially when fixed price long term power purchase agreements are negotiated. Upon the completion of the development of the floating wind turbine technology outlined in the present article, the investment risks outlined above will be assessed and quantified towards for the structuring of the finance of large scale offshore wind farm projects using the proper mix of debt, equity and other securities.

Floating Offshore Wind Turbines are an innovative new technology for the generation of utility scale electricity form an inexhaustible environmentally friendly source of ocean energy. Drawing upon technological developments achieved by the oil industry over the past several decades for the exploration and extraction of oil and gas from hydrocarbon reservoirs in very deep waters, optimized wind turbine floaters and mooring systems have been developed at MIT, yielding very good response properties even in severe seastates and promising economical attributes for water depths ranging from 30 to several hundred meters. Therefore the case for floating offshore wind farms to emerge as a significant contributor to the electricity generated worldwide becomes quite compelling.