More Development Needed to Scale Up Floating Offshore Wind

[By Live Oftedahl]

"We cannot just wait for the technology to improve. We need to build experience and not just watch others do it first. If we are to remain in the driver's seat, we must develop offshore wind on a large scale. It will be expensive at the start, but in the long term the costs will decrease," says Professor Erin Bachynski-Poli?, at the Department of Marine Engineering, NTNU.

Norway has been a leader in offshore wind technology, but now other countries such as France, Japan and the USA are catching up, according to the researcher.

Bachynski-Poli? is one of NTNU's foremost experts on wind turbine constructions, and primarily works with how wind turbines behave in waves, currents and wind conditions. That is, how wind turbines can produce well while not being damaged by wear and tear or extreme external conditions.

Large-scale investment from the government

In February, the government launched a "large-scale investment in offshore wind." In the areas Sørlige Nordsjø II and Utsira Nord, offshore wind turbines were to be set up which together should be able to produce 1.5 gigawatts at full capacity.

That this was not ambitious enough were signals coming from several quarters.

In May, the government raised the level of ambition and launched a "powerful investment in offshore wind" of 30 gigawatts. They declared that 1,500 offshore wind turbines should be put into production by 2040 by covering an area of ??approximately 1 percent of Norwegian sea areas.

30 gigawatts corresponds roughly to what Norway uses in energy per year as of today.

The government called the offshore wind investment "a new major industrial boost for Norway," and compared it to the oil development that started in 1969.

During the press conference about this large-scale venture, Minister of Business Jan Christian Vestre said that this could become an industrial adventure and a green reindustrialization of our country.

"The largest turbines today can produce up to 15 megawatts at full capacity. When the government says 1,500 wind turbines, which will produce a total of 30 gigawatts, they are basing themselves on the fact that each individual turbine must be able to produce 20 megawatts," says Bachynski-Poli?.

As of today, such large wind turbines do not exist.

How can we design better?

"A challenge with wind turbines that will produce up to 20 megawatts is that the blades become very heavy. Innovation is required for aerodynamic forces and not self-weight to become dominant," comments Bachynski-Poli?.

How far have we come in solving the challenges of floating offshore wind and structures?

"We see that it is possible to create floating offshore wind farms, for example in Hywind Scotland, but we have to reduce the costs if it is to be economically profitable to develop large fields," says the professor.

Now the platform and the turbine structure are made separately. Different suppliers account for different parts. This means that the entire construction of the wind turbine is not optimised.

"One of the most important things that needs to be in place in order to reduce costs and build more efficiently, is that the turbine part and the platform should be designed together."

Smaller size – less steel

It can also be an advantage to make turbines that also work well at lower wind speeds rather than very large turbines that work best at higher wind speeds.

It requires different designs of the blades and the construction in general.

"Another question is how we can cut the size and reduce the amount of steel. This will make the turbines more flexible and dynamic. It requires that we have good control over the movements, and that we can change the angle of the turbine's blades to a greater extent and adjust the resistance from the generator in the turbine to stabilize the structure," she says.

At the Department of Marine Engineering at NTNU, research is carried out, among other things, on creating good algorithms and how good measurement data can be created.

Floating wind provides more energy

Floating offshore wind is super interesting because it produces more energy than bottom-fixed or land-based turbines.

"On land, the wind turbines produce on average approximately 25-30 per cent of maximum capacity. Offshore bottom-fixed wind turbines have between 45 and 50 percent utilization rate, while floating offshore wind is up to 60 percent for long periods. It simply blows both more and harder at sea," says Bachynski-Poli?.

In 2009, the first floating offshore wind turbine, the Hywind demo, was built and set up outside Stord. Norway was the first to come out with a multimegawatt turbine. Compared to today's turbines, the Hywind demo was small with a production capacity of 2.3 megawatts.

Hywind Scotland was launched in 2017, and is one of several demo parks in the world. These turbines produce a maximum of 8 megawatts and had a utilization rate of 57 percent on average from April 2019 to March 2020.

Norwegian industry has been the first to create small offshore wind farms. Now the realization of large fields such as Utsira Nord and Sørlige North Sea is the next challenge.

The design and construction of such turbines is a multidisciplinary task. Disciplines from, among other things, marine engineering, aerodynamics, electrical engineering, regulation engineering and geoengineering are needed to create these giant structures and make them function optimally under various conditions.

How can we cut costs?

Some of the keys to succeeding with such a venture will be to create designs that make offshore wind more economically profitable, to train enough candidates to work with offshore wind, to restructure parts of the oil industry to establish and operate these offshore wind fields, as well as to research good enough turbine solutions.

"We have not tested the simulation tools enough for them to capture all physical effects. If you are to create something completely new, you must ensure that the algorithms capture the physical reality to the greatest extent possible. Further development requires better comparison with physical experiments," says Bachynski-Poli?.

It simply takes a lot of model testing and full-scale testing to be able to create good simulation models. When these are in place, costs will also be reduced. As of today, a lot of data is missing.

"If costs are to be cut, more accurate simulation tools are required. Floating wind will not achieve competitive prices if the safety margins are too large," says Bachynski-Poli?.

Since floating wind turbines are unmanned and have little risk of pollution, the risk is primarily economic. 

"That is why they want to push down the safety margins in order to reduce costs. Then you have to be sure of the design."

When can it get serious?

With the exception of the demo turbines and demo parks that already exist, when can we seriously start developing offshore wind farms in Norwegian waters?

"Maybe not until five or six years from now in Utsira Nord, but it might happen faster if you make the processes go faster," says Bachynski-Poli?.

If one succeeds in creating efficient floating platforms, this can be standardized and used in wind farms all over the world, while fixed offshore wind requires local adaptations because the bottom conditions vary within these farms.

"Offshore wind is part of the solution in future renewable energy production, but not the whole solution. Floating offshore wind will be a more efficient and stable source of energy than wind turbines on land and bottom-fixed wind turbines," concludes Bachynski-Poli?.

Offshore wind turbine structures

Illustration: Erin Bachynski-Poli?

• SPAR is a long and thin construction, gets stability from heavy ballast far below. Requires relatively deep water - both where it is assembled and where it is set up (preferably in a fjord). Hywind  is a spade. 
• SEMI-SUBMERSIBLE gains stability from the pillars that are far apart. You don't need as much water depth as a spar, and it is relatively easy to install. But it is more susceptible to stress from waves. WindFloat is an example. 
• TENSION LEG PLATFORM (TLP) gets stability from the tight vertical tension rods. It has relatively small movements, but is difficult and expensive to install due to the high tension in the tie rods and expensive anchors.
• BARGE TYPE Gains stability from the large area in the water plane. It is relatively cheap to make, and can be deployed in very shallow water, but is exposed to heavy loads from waves. Sevan has a concept that can be called a barge type.

This article appears courtesy of Gemini News and may be found in its original Norwegian edition here.