|Fig. 1: Onshore wind farm. (Source: Wikimedia Commons)|
|Fig. 2: Offshore wind farm. (Source: Wikimedia Commons)|
Wind energy has long been used as a means to generate electricity. However, because of the phenomenon known as wind shadow, where turbines deplete the strength of winds downstream from it, there has been growing interest in developing wind farms offshore. Since there are stronger, more consistent winds out in the ocean it would only make logical sense to harness the technology of wind turbines used on land and put them to use offshore. From the time of the introduction of the first offshore wind farm in 1992 off the coast of Denmark, many other nations around the world, including the United States, have joined in developing and utilizing offshore wind farms as a form of renewable energy. However, a vast majority of the worlds wind farms still reside in Europe. In fact, according to the Global Wind 2014 statistics, approximately 90% of all offshore wind farms were located in Europe (North Sea, Atlantic Ocean, or Baltic Sea). 
The following will compare how onshore wind farms (Fig. 1) and offshore wind farms (Fig. 2) compare in terms of turbine structure and costs.
While one may think an onshore wind turbine could be used at sea without modification, there are actually modifications that need to be made to it. The first, is that because of the difficulties and costs of maintenance and repair at sea, offshore wind turbines need to have extremely high reliability and be resistant to corrosion. Additionally, turbines need to be designed to take advantage of the stronger winds at sea. This is done by increasing the cut-out wind speed, which essentially is the speed at which the wind turbine will automatically shut down to prevent any damage to the turbine structure.
Turbine foundations are also different when out in sea versus on land. When on land, the process is fairly simple, using either rods drilled into the ground or a reinforced concrete pad set in the ground. When out in the ocean however, there are three main methods used for shallow and medium depth waters:
Monopole: Steel tube drilled 10 to 40 meters into sea bed where a transition piece is fitted on top that connects to the base of the turbine tower.
Tripod: A triangular or four-legged structure made of steel that is secured to sea bed through piles and is better suited at medium depths.
Gravity foundation: relies on its own weight to secure the turbine through a large, heavy base made of reinforced concrete. However, the sea floor must be uniform and level. 
To compare the cost of electricity generated between onshore and offshore wind farms, the metric LCOE (levelized cost of energy, where LCOE is in $/MWh) is used. This is calculated as:
where OpEx is operational expenditures ($/kW/yr), CapEx is capital expenditures ($/kW), FCR is the fixed charge rate, and AEP is the average annual energy production (MWh/MW/yr). 
Using data provided by the National Renewable Energy Laboratory, the cost breakdown of onshore wind farms can be broken down as follows: total CapEx = 1690, OpEx = 51, FCR = 9.6%, AEP = 3494.  We then find that:
Using the same data provided by the National Renewable Energy Laboratory, the cost breakdown of onshore wind farms can be broken down as follows: total CapEx = 4615, OpEx = 179, FCR = 10.3%, AEP = 3608.  We then find that:
These data were gathered from new wind farm projects in 2015. For the onshore portion, the data were collected from the 68 projects installed in the U.S. during that year. However, no offshore wind projects were installed in the U.S in 2015, so the data for offshore were collected from international projects and proposed U.S projects. 
As we can see, installing offshore wind farms is far more costly where the majority of the difference comes in the differences in capital expenditures needed for the offshore wind farms. The increase in annual energy production is not nearly enough to offset this cost. As technology continues to develop and we find more efficient ways to deploy wind turbines in the ocean, the LCOE will decrease and hopefully narrow the current gap between onshore and offshore wind farms.
© Andrew Liang. 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.
 C. Ng and L. Ran, Offshore Wind Farms: Technologies, Design and Operation (Woodhead Publishing, 2016).
 P. A. Lynn, Onshore and Offshore Wind Energy: An Introduction (Wiley, 2011).
 C. Mone et al., "2015 Cost of Wind Energy Review," U.S. National Renewable Energy Laboratory, NREL/TP-6A20-66861, May 2017.