Going Far Out With AC at Gemini
A wind farm 85km out to sea with an AC cable connection with the main land? No, that’s impossible. Definitely not a good idea over such a long distance, you’d better get that idea out of your mind. That was what the developers of the Gemini Offshore Wind Farm were told frequently. But they have now proven that it can be done. It just required innovation.
Words by Dennis Rijsbergen.
There are wind farms farther from the coast than Gemini, for example some of the projects off the German coast. Yet Gemini is different, says Geert van Ek, project manager for Gemini. “Wind farm projects that far into the sea can usually connect to offshore stations provided by the grid owner. The grid is available for offshore wind farm projects. At Gemini however we had to take care of the whole infrastructure including Offshore High Voltage Stations and Land High Voltage Stations in order to have a grid connection secured.”
Exceeding the Maximum
And that perhaps is the greatest immediate challenge for Gemini. Gemini will feature two export cables that bring 220kV AC electricity ashore in Eemshaven, in the north of the Netherlands. Both cables are about 120km long. Parts of the cable connection have a copper core and other parts an aluminum core. At Eemshaven they will be connected to a LHVS transformer station. This increases the voltage to 380kV which is used to deliver the renewable energy to the national grid of Tennet. In the meantime, the required types of export cable have been developed exclusively for each section of the Gemini project. It is proof that, with the power of innovation, much is possible. “Also unique is that we have an offshore HDD (Horizontal Directional Drilling) from one underwater location to another. With HDD it is quite common to drill from land into sea, but we chose it on this project because we are dealing with intersections of the cable.”
Building in Harmony
The construction of Gemini gives an example of perseverance and innovation within the Dutch offshore industry. “The sheer size of the Gemini offshore wind farm, the rough weather and challenging local conditions near shore, the environment, the direct connection to the electrical grid, these are things that have been experienced in other projects one at a time, but Gemini is a combination of everything at once. There are also a lot of factors which are not under our direct control and which we need to take into account. For example, for the cable laying activities we are working in the vicinity of a nature reserve. That means we are forced to do the work within a period of a few months so we will not disturb nature.”
Spanning the Country
The same applies for driving the foundations into the seabed. That’s only allowed in the second half of the year, starting from July. “If you are unable to finish all 150 foundations within that time, you’ll have to wait half a year to be able to continue. That would be problematic for the further development of the project and that is why we deploy two offshore installation vessels,” says Michiel Pieterman, Package Manager at Van Oord. Van Oord is building the huge offshore wind project. Van Oord’s new DP2 cable-laying vessel Nexus, was delivered in December by Damen Shipyards. After delivery, Nexus berthed at Damen Shiprepair Vlissingen, in Zeeland Seaports, where Van Oord is building and installing the additional cable laying equipment required for the project. Several companies based in Zeeland Seaports have secured orders as a result of the Gemini project. For example, BOW Terminal is responsible for storage and load transfers of 75 pile foundations. “We have a permanent availability of heavy lift capacity up to 1,500t with a Gottwald MK1500 and a Liebherr LR1600, therefore we are in the position to lift 75 pile foundations of 850t each for this project,” said a spokesman for BOW Terminal. “The Gemini project is our 25th energy-related project since our start in October 2010 and our first project for the Dutch offshore wind market.”
Papendrecht, the Netherlands-based VBMS has been awarded a contract to support Van Oord with installation of two sections of the cable, covering a total distance of 11km between the shallow and deep water parts of the route. VBMS will carry out its scope with the vessel Stemat Spirit.
High Power
The offshore wind industry is evolving constantly as each individual wind farm requires unique innovations. For example, the turbines placed at Gemini are amongst the most powerful offshore wind turbines in the market at this moment. They deliver 4MW – 0.4MW more than their predecessor. By mid-2017, Gemini will provide a total of 785,000 households with energy. “We may not be the biggest in terms of number of turbines, but, when you consider how much wind we catch, we are amongst the largest when it comes to production. Gemini is located in a very productive section of the North Sea, with more wind than average and the highest wind speeds in the Dutch sector”, Mr. Van Ek states.
Investment Friendly Climate
A project on such a scale requires huge investment – around EUR 2.8 billion. Mr. Van Ek says that investor confidence in innovative industries such as wind power requires stable legislation more than anything. “Changes in legislation can have a major impact on market prices and investors need sufficient comfort in order to invest in operating the wind turbines over long periods.”
At sea, industrial installations have to endure a lot, Mr Pieterman adds. “The salt water corrodes the metal parts. In order to avoid that as much as possible, we ensure that inside the masts there is a conditioned environment. This prevents rust and dirt, which makes maintenance a lot easier and cheaper. Therefore, the technique must be completely up to date.”
And the same goes for the legislation, according to Mr Van Ek. “It is very hard to build offshore wind farms when legislation is changing all the time. Whether or not you have political and public support is vital in the construction of projects such as Gemini. And to attract large investments, you need a stable policy on renewable energy.”
Visit Van Oord at EWEA 2015 at stand C2-B12, VBMS at stand C1-B8, BOW Terminal at stand C3-B50, Zeeland Seaports at stand C3-B50 and Tekmar at stand C1-B41.
