Weight including turbine: 5300 ton
Height below water: 100 m
Diameter below water: 8.3 m
Diameter at water line: 6 m
Turbine: Siemens 2.3 MW
Turbine weight: 138 ton
Mooring: 3 lines secured with anchors
Depth range: 120-700 m
An ambitious attempt to put turbines into deep water is ongoing in Norway. Entering its second winter, the Hywind project has made its owners hungry for more.
By Karsten Prinds
As is often the case, the most interesting things lie below the surface.
Above the surface there is a regular Siemens 2.3 MW wind turbine. It stands alone in the waves of the North Sea off Karmøy, Norway. But it is supported by one of the largest and heaviest structure of any offshore wind turbine.
The Hywind floats with the support of a 100 meter high steel cylinder, which is 8 meters wide. It weighs 5300 tonnes. Of this, 1500 tonnes is steel, which makes it equivalent to a submarine – just raised up and loaded with rocks. The Norwegian oil and gas company Statoil constructed the Hywind and Asset Manager Sjur Bratland is enthusiastic about the concept:
- We believe that we can use our skills from the oil and gas industry for offshore wind. And if we succeed, we will open up a whole new market for the wind industry, he says.
Statoil sees market potential in countries like Norway, Ireland, Scotland, the east coast of USA, Spain, Portugal, Japan and others. These countries have oceans where water depths plummet and traditional support structures are challenged or impossible to use.
Tugboats are used during installation. First the support structure is raised in deep waters close to shore. After installation of the turbine, tugboats transport the whole structure to site in one piece. Photo: Øyvind Hagen, Statoil.
But so far the challenge for Hywind has been the price. The structure is made of the same amount of steel as five monopiles, and in addition there is ballast of about 3600 tonnes in the form of olivine rock. The total cost including installation and two years operation has been NOK 400 million (€51 million).
- We had to use significant safety margins in the design so the dimensions can be optimized. We are expecting that the Hywind concept in terms of Cost of Energy can equate to a grounded support structure for 15-20 meters depth, says Sjur Bratland.
- The goal for us at this stage was to verify the concept and prove that a floating concept could carry a standard wind turbine, he continues.
And according to Statoil this goal has been achieved. Technicians from Siemens have inspected the turbine thoroughly and it has not shown any unexpected wear or damage. The Hywind tilts up to 3 degrees in the worst weather, Statoil’s measures show. But more important is the frequency – it takes 20-30 seconds to sway from one side to the other.
- With this low frequency and low pitch there is no damage or additional wear to the turbine compared to other support structures, says Sjur Bratland.
The low tilt and frequency is achieved through a motion control system and all the ballast at the bottom of the 100 meter long cylinder. Olivine rock was chosen because it is common in Norway, it has high density and it absorbs water. On other sites the ballast will consist of other materials common to the area.
The next step is more installations with optimized design. Where these will be has not yet been decided. But Sjur Bratland is confident:
- The first oil from the North Sea was also very expensive, and it is the same with Hywind. But it gives us lots of knowledge for new and more affordable floating turbines.