How does offshore power generation work?
As power needs grow and nations push for more renewable energy, we look offshore to generate the power we need. Wind turbines have moved offshore due to higher wind speeds and more consistent gusts, along with the ability to construct turbines as big as we can physically build them. Floating solar and wave energy converters (WECs) also produce power from offshore sunlight and the movement of ocean waves. With these installations growing in size and getting farther from shore to take advantage of space and energy production, how do you get all this energy to shore to the people who ultimately need it? As it turns out, it can take hundreds of miles of cable, large substations, and power transfer vessels to get the power where it needs to go. Some offshore power can even be directly converted into electricity for hybrid vessels or fuel like ammonia for offshore refueling. Today, we’re going to take a look at how gigawatts of power is transformed and transported.
Offshore power generation
There are well-known benefits to wind power: the energy is unlimited and turbines don’t release greenhouse gases as they operate. Farther offshore there are even more benefits: there is stronger and more consistent wind, turbines can be installed far enough out that they aren’t visible from shore, and parts are more easily transported by ships than on land. When wind first moved into the water, turbines were installed in very shallow water, but technology has improved allowing turbines to be installed miles offshore, so far that even turbines more than 200 meters tall aren’t visible from shore. Increases in technology have also allowed offshore turbines to almost double in size in a about 10 years, from the 3.6 MW, 120 meter diameter units used in the Irish Sea farm “West of Duddon Sands” completed in 2014, to the 13 MW, 220 meter diameter turbines planned for Vineyard Wind 1 in Massachusetts, USA. The layout is also growing: the largest planned offshore wind farm will have a total capacity of 8.2 GW, offshore South Korea, or about 7 times the size of the largest currently operating wind farm, Hornsea 1 at 1.2 GW.
Hornsea 1 Wind Farm, the largest operating wind farm in the world. Image from Ørsted.
These numbers can be scaled down for floating solar and wave power. Wave energy converters generally make small amounts of energy and most of them are in the testing phase. Although there are estimates that theoretically 2.64 trillion kilowatt hours of wave energy exists in the United States, it wouldn’t all be captured as energy. It is an exciting field with many different types of converters that can supply smaller amounts of electricity locally. Floating solar is also in the early stages, with Singapore building one of the largest floating solar farms in the summer of 2021. It covers approximately 45 football fields and is a 60 megawatt peak system created to power five water treatment plants. Check out more about offshore wind, solar, and wave power in our 3 types of renewable offshore power generation article.
We’ve talked about turbines and power numbers from offshore green energy sources, but how do we get those gigawatts of wind power turned into useful energy? The process is similar for all forms of large scale offshore power generation, so we’ll use wind turbines as an example. According to Iberdrola, the force of the wind turns the blades of the turbine at a relatively slow speed of about 7-12 turns per minute. The blades are fixed to the turbine shaft attached to a gearbox that speeds up the movement to around 1,500 revolutions per minute, or about the speed of a car engine while cruising. A generator then converts that energy from the spinning gearbox output into electricity, which is fed through the tower and into a converter that turns the direct current (DC) into alternating current (AC). A transformer raises the voltage to 33 kV to 66 kV to be transported across the wind farm to a substation that then increases the voltage again to 150 kV to be sent to shore. We included a screenshot from a fantastic graphic from Iberdrola below that shows this process visually, check out their article on how offshore wind farms work - they provide an excellent in-depth description!
Process of turning wind energy into electricity. Image screenshot from Iberdrola.
Offshore substations move the power generated from wind farms to shore through underwater cables. Early substations for offshore wind were small, maybe 400 tons and operated unmanned. Substations have grown and now are 10,000 to 22,000 tons, according to Wind Power Engineering. They can be attached to monopiles, jackets, floating, and some even self install, eliminating the need for heavy lift installation vessels. They can include boat landings, helicopter decks, and may serve as a logistics headquarters during the build-out of a wind farm. Substations like the one at UK’s London Array collect all the power from the wind farm, convert it from 33,000 volts to 150,000 volts and send it to shore. Self-installing stations like the units by Alstom can float out to the location and either use a suction can method to attach to the seabed or connect to a pre-installed jacket. Substations typically use condition-monitoring systems and try to do preventative maintenance so nothing goes wrong. They are also making changes as the industry improves, such as gas-insulated transformers instead of oil-insulated. These stations are not usually manned with a permanent crew.
Underwater substations are in the works from Aker Solutions, a part of the bid for offshore wind farms in Scotland, according to offshoreWIND.biz. Traditional substations are installed on some kind of mounting system or floating at or above the water’s surface. Underwater substations can use seawater for natural cooling and increased reliability. There is also less maintenance and less materials cost. With the innovations in robotics and sensors, there’s really no reason an underwater substation couldn’t be managed remotely and inspected by underwater drones. The substations would also make it easier to install wind farms where jacketed, above ground substations aren’t feasible or cost effective.
Render of an underwater substation. “Source: Aker Offshore Wind”. Image from offshoreWIND.biz.
The power cables that carry this power are a big part of building offshore power infrastructure. The cables are buried on land and either buried or covered offshore. Burying cables offshore requires specialized cable laying vessels that have large baskets, also called carousels to load up thousands of tons of cable up to 3,000 kilometers in length. They also have special sheaves, usually at the stern of the ship that feeds cable into the water. An underwater plough simultaneously digs a trench for the cable and lays it at the bottom. Cable vessels have dynamic positioning systems to stay precisely on course and avoid obstacles like large rocks, fishing areas, and coral reefs. Cables can also be laid at the bottom of the ocean if digging a trench isn’t feasible, but they must be covered to avoid being hit by anchors, fishing equipment, or even attacked by sharks and spies (check out How do undersea data cables work for more on sharks and spies). If the cables can’t be buried, they will usually be covered by a fallpipe vessel whose sole purpose is to deliver rocks to the seabed. The rocks cover cables and pipelines to protect them as well as bolster wind turbine foundations, provide ballast for offshore rigs, and cover any underwater structures that need protection. That’s how cables are protected, but how are they connected?
“Cross section of the submarine power cable used in Wolfe Island Wind Farm.” By Z22 - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=28019547. Image from Wikipedia.
How power cables are connected
There are three main types of power cables, according to Ørsted. Array cables link the turbines together and connect them to an offshore substation. Export cables then transfer that energy from the offshore substation to the onshore substation for distribution on land. The onshore substation then transmits the energy through transmission lines, the power lines that are on metal structures, wooden poles, or sometimes buried underground that transfer power to homes and businesses. Many times, onshore power cables are buried underground to minimize visual impact and maximize protection. According to Ørsted, this process requires them to “carry out a lot of work before deciding where the cables will be buried, including environmental and technical assessments and considerations from local stakeholders.” Burying cables isn’t always as simple as digging a trench and laying cable though, as cables have to go under roads, railroads, and even buildings. For this a technique called Horizontal Directional Drilling (HDD) is used to lay the cable underground without disturbing the structures above, after which the land is restored to its previous condition.
Horizontal Directional Drilling (HDD) under a parking lot structure. Image from Ørsted.
Power transfer vessels
Cables aren’t the only method of power transmission - PowerX in Japan is developing a power transfer vessel that would transport electricity to shore. The Power ARK 100 will be automated and loaded with 100 batteries rated at 222 mW storage capacity and a cruising speed of 7 knots. Range is 100-300 km on electric power and can be extended with sustainable biodiesel fuels for longer journeys. This will give more flexibility on where the power can be delivered, similar to an LNG transport vessel that would bring fuel to a port.
“Japan's PowerX is developing the world's first vessel to transport electricity generated by offshore wind farms to shore. (Courtesy: PowerX)”. Image from Renewable Energy World.
There are up to twice as many wind resources offshore as onshore, according to Iberdrola. The wind industry is exploding around the world as countries seek to decarbonize and lower their reliance on fossil fuels and imported power. The massive amounts of power generated offshore have to be transported to shore with substations, undersea cables, and even onshore cables. Some offshore wind farms are being designed to provide electricity directly to electric and hybrid vessels as well as create green hydrogen that could fuel ships offshore. Autonomous power transfer vessels full of batteries could be the future of power delivery, especially as battery technology improves. We look forward to the growth of the offshore wind industry!
If you’d like to learn more about wind power in the United States, check out The State of US Offshore Wind where we talk about how the wind industry is picking up and some of the planned wind farms.
OneStep Power tests dynamically positioned vessels to save money on fuel, maintenance, and manage risk. We work on all types of DP2 and DP3 vessels such as offshore construction vessels (OSVs), wind turbine installation vessels (WTIVs), and crew transfer vessels (CTVs). If you want to make sure your vessels can run in closed bus, saving fuel and maintenance costs that can be in the hundreds of thousands of dollars per year, then reach out to us, we’re happy to help.
Happy Fun Fact Friday!