How are offshore wind turbines installed?
It’s Fun Fact Friday and we’re diving back into energy, taking a look at how offshore wind turbines are installed!
Offshore installations of wind farms have been growing rapidly over the last two decades as the price of these installations has dropped more than expected. According to a study on ing.dk, by 2016, price per MWh was lower than the lowest estimate for projected prices in 2050, creating a wind power boom. Offshore installations are more difficult and more costly than their land-based counterparts, but they produce more power and don't take up precious real estate. These benefits aren’t without their challenges as the turbines must survive waves, corrosion, and storms while running continuously for their lifetime. Many installations take place miles from shore with total heights of up to 200 meters, creating huge construction challenges from transportation of materials to seafloor mounting. Let’s take a closer look at how these installs are completed.
Why are offshore wind farms better than onshore?
Offshore wind farms are more expensive and more difficult to construct than onshore, so why not stick to land? The most important reason is the amount of wind. Offshore installations make sense because they match demand and produce more electricity - according to Wikipedia, “The advantage of locating wind turbines offshore is that the wind is much stronger off the coasts, and unlike wind over land, offshore breezes can be strong in the afternoon, matching the time when people are using the most electricity. Offshore turbines can also be located close to the load centers along the coasts, such as large cities, eliminating the need for new long-distance transmission lines.” With power generation in particular, turbines can be built to a larger scale off the coast, which produces more electricity per turbine. In fact, they have grown larger and larger in the quest for higher efficiency. Check out the graphic below comparing the Vindeby turbines from 1991 to the Burbo Bank Extension turbines used in 2016: Vindeby turbines have a diameter of 35 meters with a power output of 0.45 MW, while the Burbo Bank Extension turbines have a diameter of 164 meters and power output of 8 MW, an increase of almost 18 times!
Image Credit: Offshore Wind Industry
How do offshore turbines stay in place?
With a height of 113 meters, or over 370 feet, how do these turbines stay in place? The hardest part is elevating the turbines while anchoring them to the seafloor. Up to 15 meters, a steel cylinder called a monopile is fixed to the sea bed and buried up to 30 meters deep. For an ocean depth of 30 meters, a gravity foundation is used. According to Iberdrola, this type of foundation consists of a “large concrete or steel platform with a diameter of approximately 15 metres and a weight of approximately 1,000 tons.” Installations can be made deeper than 30 meters, usually using a jacket, or foundation with a lattice framework like an antenna tower, with 3 or 4 legs that are anchored to the bottom. Of course, the composition of the seafloor has to be taken into account as well depending on the type of foundation.
Image Credit: Iberdrola
For even deeper installations, floating platforms are now becoming a reality. According to Popular Mechanics, Windfloat Atlantic recently built a floating wind farm, “WindFloat Atlantic platforms are anchored to the seabed with chains at a depth of about 300 feet, which is farther out than the maximum depth that can currently possibly be reached by traditional bottom-fixed offshore wind turbines.” Going farther out can get stronger and steadier wind, leading to more energy collected. Other benefits include the ability to install turbines without an anchoring pad on the sea floor and positioning the turbines far enough out that they can’t be seen from shore, preserving ocean views. Windfloat Atlantic also shows much of the installation happening on the edge of the water, before the turbines are presumably towed to their final destination. Check out the short video below on wind turbine foundations.
Video Credit: DOB-Academy studio
How are offshore wind turbines assembled?
We’ve covered why offshore wind farms produce more power and how they are fixed in place, but what about the construction of these behemoth windmills? Except in rare cases like the floating turbines mentioned above, much of the construction of these wind farms is done on location offshore, including for larger turbines that cannot be mounted on a floating platform. Installation of these structures is typically done by a jackup rig like Wärtsilä’s Wind Turbine Installation Vessel INNOVATION, shown below. This vessel has two cranes at 1500t and 40t, and can jack with 8000 tons of cargo on board. According to Wärtsilä, “The unit is equipped with a diesel-electric power plant: electric power for propulsion and other services is derived from six generator sets located in three separated compartments. They provide total onboard electrical power of 34.4MW and feed two 6600V high-voltage propulsion switchboards equipped with vacuum circuit breakers.” The vessel has a DP2 dynamic positioning system with four azimuth thrusters and three tunnel thrusters to keep it steady during open water installations. What does all that mean? It means dynamically positioned installation vessels can lift huge pieces of equipment quickly and safely, staying steady in wind and currents. In shallower water, jackup rigs can also put legs directly on the seafloor and lift themselves out of the water, creating a stable installation platform.
Image Credit: Wärtsilä
With the cost of wind power dropping, more offshore wind farms will be on the radar. Orders are increasing for specialty installation vessels to build, maintain, and decommission wind farms. According to Recharge, the second half of 2019 saw benchmark prices for wind installations at $78/MWh. There’s a lot more in store for wind power and the vessels and crews that support it.
About OneStep Power: We are a safety company that provides electrical testing services to dynamically positioned DP2 and DP3 vessels. Our testing is non-destructive, repeatable, and reliable. If you have any questions on electrical testing for increased safety and efficiency, please feel free to reach out, we are happy to answer!