As we build Gemini, we will apply our proven construction and risk mitigation strategies on a greater scale. Working closely with our turbine supply and construction partners, Siemens and Van Oord, we will help the Netherlands achieve their renewable energy targets by powering 785,000 Dutch households. That’s intelligent energy.
Gemini will employ best-in-class technology to optimize safety and efficiency throughout construction and operations.
Vessel location at sea is tracked through the deployment of an automatic identification transponder system (AIS). To ensure the safety of the various installation and maintenance vessels, each ship’s transponder transmits its position, heading, speed and registered maritime identification number every 2 to 10 seconds. This data is received by ships in the vicinity and can be plotted automatically on a digital map or radar screen.
Project management on the open sea
A web-based site management tool enables a real-time interactive map application. The tool features dynamic real-time wind park asset status and the tracking of personnel and vessels (AIS) overlaid on a nautical chart. All data is stored for the lifetime of the project, creating a clear audit trail of site activity.
Each wind turbine self-starts when the average wind speed reaches 3 to 5 m/s (metres per second). The output increases proportionately with the wind speed up to 12 to 13 m/s, at which point the power is regulated at a rated power of 4 MW.
When wind speed exceeds the maximum operational limit of
25 m/s, the turbine will shut down by pitching its blades (turning them parallel to the air flow). When the wind speed drops back below the maximum limit, the turbine’s systems automatically reset and begin to generate electricity again.
Current permits allow for daylight helicopter deployment, which will allow delivery of service teams directly to the turbine platform. The project has been designed and will be built for
24/7 helicopter access.
When steel structures are submerged in seawater, corrosion occurs. To prevent this, Gemini will employ an advanced impressed current cathodic protection system. Four electrical anodes will be premounted equidistant around the exterior circumference of the transition piece, and four more will be mounted to the interior. The combination of seawater, steel and the direct electrical current passing through the anodes will create a neutral, non-corrosive environment on the TP and the MP to the mudline.
Wildlife protection was very important at the Gemini construction site. To minimize the potential impact that construction noise could have on wildlife, project crews implemented a sound mitigation protocol that included deploying acoustic deterrents and slowly ramping up pile-driving activity at each MP location.
Van Oord’s offshore installation vessel, Aeolus, was used to transport and install MPs and transition pieces (TPs), as well as towers, nacelles and blades. Aeolus can be jacked above sea level on four legs, can work in water depths to 55 m and can install three MPs and TPs before returning to port for restocking.
Guided by GPS and carrying a load of 26,000 tonnes, Van Oord’s flexible fallpipe vessel, Stornes, deposited 2,000 tonnes of scour protection material onto the seabed before each monopile (MP) was hammered in. This material forms a rock pad approxi-mately 30 m in diameter and 1.5 m deep that prevents erosion around the MP.
Transition piece installation
Once the MP was secured, the TP was lifted and installed. The TP was seated and adjusted until its verticality was confirmed; grouting then solidified the 5-m joint connecting the TP to the MP. All external attachments, including the boat landing, ladders and the working platform, had been connected to the TP before it was installed.
After the infield cable had been laid onto the seabed, a remotely operated vehicle (ROV) was deployed to initiate the burying of the cable. The ROV simultaneously trenched and guided the cable into place on the seabed. The cable settled to the bottom of the 1.5-m trench, which filled itself in through natural sedimentation within a few days.
After installation of the TP and before erecting the turbine tower, the electrical interconnection cable was laid. A protection system safeguards the cable over the scour rocks. The cable enters the MP and was raised internally to the disconnecter panel at the bottom of the TP, above an airtight platform.
Two offshore high-voltage substations were strategically positioned in the wind park layout. They each aggregate the electricity produced by 75 wind turbines, transform the power from 33 kV to 230 kV and then transfer it via the export cable to the land station.
Upon arrival on site, Aeolus determined the location of each turbine (within GPS coordinates). Each custom-built MP was upended to vertical using Aeolus’s 990-tonne main crane assisted by a second smaller crane. As the MP was lowered to the seabed, its location was adjusted to within centi-metres of the GPS location.
Once the MP was in place, an impact hammer drove it between 30 and 40 m into the seabed while engineers monitored for verticality until the MPs top reached 5 m above the lowest astronomical tide. Each MP is tapered. The bottom diameter varies from 6.6 m to 7 m, while the top narrows to
5.5 m to reduce wave loads and ensure a uniform flange and appendages.
The Nexus – built specifically for Gemini – transported 5,000 tonnes (up to 40 km) of electrical cable on a massive carousel. The cable was carefully unreeled and laid onto the seabed with the laying crew on board controlling the cable slack required to manage seabed unevenness and fluctuations.
The nearshore cable-laying barge, Vetag 8, was used to carry the export cable to the shallow water area and tidal flats located between Eemshaven and Rottumeroog en route to the land station. The vessel is non-propelled, ballastable and towed at the forward end.
The joining of the export cable sections has been executed on a specialized vessel, the MPB Scheldeoord. Connections were made by hand and encased with ‘offshore joints’. Each joint took a crew of four up to five days to complete.
On location, the crew spooled approximately 3,000 m of export cable onto the Nessie V. The Nessie V is a cable transporter that reeled off the export cable directly ahead of the trencher and worked in tandem with the Nessie II. With two 2.75-m-wide tracks, Nessie V created very low ground pressure, which reduced the environmental impact of cable-laying in soft soil conditions.
Wind turbine generator
Once the interconnection cables had been installed, the Aeolus was deployed to transport, lift and assemble the final components. First the tower was lifted on and bolted to the TP. This was followed by lifting and securing the 140-tonne nacelle and finally the three blades, which each required separate lifts. Once all the main parts had been installed, the electrical cables could be connected and the turbine was ready to be commissioned.
The innovative aero elastically tailored blade design absorbs gusts of high wind by twisting the blade and spilling some wind, allowing for greater energy capture, longer system lifetime and stable electricity flow to the customer.
Service technicians can enter the turbine via the heli-hoist platform above the nacelle or through conventional tower access where a gangway system enables safe access in rough sea conditions.
The land station is transforming the electricity from the export cables to a higher voltage (380 kV) and is connecting the wind park to the TenneT high-voltage grid. Gemini is producing enough electricity to power more than 785,000 Dutch households, which is roughly equal to the total population of the Netherlands’ three northernmost provinces.
The Nessie II is a trencher used in intertidal areas. It has been designed specifically to minimize impact on ecologically sensitive coastal areas and has been approved for use as part of an environmentally conscious cable-laying method.
THE MAKING OF GEMINI WIND PARK