|Fig. 1: The three leading prototype designs for floating offshore wind platforms.  (Courtesy of the U.S. Department of Energy.)|
Despite the almost 52 GW of installed wind power capacity in the US, the wind industry faces extensive challenges and must constantly innovate more efficient, lower cost options to be competitive. Offshore wind farms offer some compelling advantages for the future of wind energy development, but to date these projects have been stymied in the US by regulatory and interest group opposition. After years of litigation, it appears that the first offshore wind farm in the United States is imminent, though the uncertainty of the production tax credit casts doubt on the future of wind projects. Regardless many major American developers are positioning to be able to take advantage of the offshore wind market once it is tapped. Europe leads the world in offshore wind farms (with over 1300 turbines for an installed capacity over 3.5 GW) and many companies are eager to apply the expertise and lessons learned from Europe in the US market. 
Worldwide, conventional fixed-bottom deployment of offshore wind has progressed but is still limited by a number of constraints. One of the most exciting emerging technical advances for the wind industry is the development of floating wind turbine platforms. With a number of advantages over conventional offshore wind, floating wind turbine concepts have spawned from start ups worldwide and at least three different designs are being actively tested off the coast of Portugal, Norway, and the United Kingdom. These prototypes are still early stage, far from mass production and commercialization, but are gaining attention and earning investment dollars.
The first and most immediately compelling advantage of floating offshore wind is access to incredible wind resource over deep waters.  Currently we can only access a small fraction of the offshore wind resource worldwide due to depth constraints. In the United States, for example, the west coast is largely impractical for offshore wind despite ample wind resource due to the rapid drop off of the continental shelf.
Offshore wind is recognized for its proximity to load centers but often still encounters significant NIMBY ("Not In My Back Yard") resistance. Population centers tend to cluster near the coastlines, so offshore wind minimizes the distance from generation to load centers, without competing for valuable land.  Opponents argue, however, that turbines negatively impact the skyline (visual pollution) or result in disruptive noise. Floating turbines address these concerns by allowing wind farms to be pushed farther offshore and out of sight.
Finally, there are also several manufacturing advantages to floating platforms, such as using less material in construction and reducing the need for specialty marine engineering expertise. One major cost driver for conventional offshore wind are the heavy lift vessels required to erect the turbine. Very expensive special purpose ships are required to transport the parts on site and perform the assembly. Floating turbine platforms, however, are designed to be assembled in port and towed into position using simple barges or tugboats. This can result in major cost savings and greatly increased flexibility in construction.
While the benefits of floating offshore wind are compelling, there remain significant obstacles to widespread deployment. The engineering challenges are extremely complex and many of the concepts have yet to be rigorously tested.  Various early stage computer models attempt to predict turbine performance with varying degrees of success.  Prototypes are gathering valuable data to drive further design refinement, but the longest has been operational for only three years so the actual realistic lifetime of a floating wind turbine is still unknown. Long-term survivability of floating platforms has been demonstrated by the oil industry, however, so multiple decade performance should be technically feasible. 
Pitching and rolling are huge concerns as these movements add tremendous stress to turbine components and can possibly even threaten to cause the spinning blades to impact the water. Designs are addressing this concern with solutions like automatically feathering blades to steady the tower by adjusting the magnitude of the wind force on the blades, but anomalies like storms and rogue waves are difficult to account for.
From an infrastructure standpoint, floating wind turbines would require substantial upgrade to existing ports. In order to assemble a platform in port there must be dedicated cranes, laydown lots, dock space, repair boats, towing channels, and staging areas, to name a few specific shortfalls. Once an assembly area has been constructed production can scale rapidly, but these facilities would need to be built out in every region looking to deploy floating offshore wind. As opposed to a conventional offshore wind farm, whose parts can be shipped around the world, a complete floating turbine can likely be towed a limited distance.
Hywind - Statoil: Statoil pioneered the first floating wind turbine in June 2009.  The Hywind project uses a floating spar design that consists of heavy ballasts hung low on a single long cylindrical tank that provides the buoyancy. This design provides a strong righting moment and high inertial resistance to pitch and roll, and the deep draft resists heave motion. This platform is typically anchored with a catenary system of mooring lines, which uses more line but requires less complex and robust anchor systems because the line is under less tension.  A major consideration of the spar buoy design is that in order to achieve stability it requires a massive amount of material and extremely deep draft. The Statoil Hywind turbine is 100 meters deep and weighs 1500 tons, but has provided over three years of data to help guide the development of a leaner second generation platform, Hywind II, due to be deployed in the next few years. 
WindFloat - Principle Power: The WindFloat is the leading example of a design that uses multiple distributed buoyancy chambers to form a wide stable base with relatively low draft. The platform is semi-submerged and typically employs a catenary mooring system to maintain station. Compared to the spar buoy design the WindFloat uses less material and can be deployed in shallower waters, but exposes more of the structure to surface wave action so it may need more anchors.  A 2.0 MW Principle Power WindFloat prototype is currently being tested off the coast of Agucadoura, Portugal.
Pelastar - Glosten: The third design that is being explored for floating wind turbines relies on mooring line tension for righting stability. In this system the turbine tower is fixed to a submerged buoyant platform that is secured to the seabed with taut vertical mooring lines. Compared to a catenary mooring line system, using taut mooring lines requires less mooring cable but necessitates more robust anchors due to the increased load placed on them.  By submerging most of the stability structure below the water tension leg platforms minimize wave loading, which reduces pitch and roll.
Floating wind turbines have potential to unlock huge offshore wind energy resources in a cost effective manner, but many concepts must be proven before this industry can scale. Currently the market for floating offshore wind is very fragmented and there is not yet a clear design winner. Considerable investment will be needed for this technology to scale and so strategic partnerships with larger energy companies are likely (Principle Power has partnered with EDP, Repsol, and Vestas). On top of all the technical and financial challenges for floating wind, regulatory hurdles add another layer of complexity and uncertainty. Floating wind turbines are still years from mass deployment but they offer a lot of promise as the next step change in the wind energy industry. Wind power struggles to be economically competitive with fossil fuel generation and floating wind turbines may be a crucial component to this effort.
© Alex Pratt. 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.
 The European Offshore Wind Industry: Key 2011 Trends and Statistics," European Wind Energy Association, January 2012.
 "A National Offshore Wind Strategy: Creating an Offshore Wind Energy Industry in the United States," U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, February 2011.
 C. P. Butterfield, W. Musial and J. Jonkman, "Overview of Offshore Wind Technology," U.S. National Renewable Energy Laboratory, NREL/CP-500-42252, October 2007.
 A. Cordle and J. Jonkman, "State of the Art in Floating Wind Turbine Design Tools," U.S. National Renewable Energy Laboratory, NREL/CP-5000-50543, October 2011.
 W. Musial, S. Butterfield and A. Boone, "Feasibility of Floating Platform Systems for Wind Turbines," U.S. National Renewable Energy laboratory, NREL/CP-500-34874, November 2003.
 I. Sample, "Windfarms That Float - or Fly - Could Be the Future of Energy Feneration," The Guardian. 28 Feb 12.
 "New Modeling Tool Analyzes Floating Platform Concepts for Offshore Wind Turbines," U.S. National Renewable Energy Laboratory, NREL/FS-5000-50856, February 2011.