The Tripile
Advantages
- Simple design
- Improved stiffness compared with monopiles
- Automated levelling process
Disadvantages
- Heavy construction
- Large amount of steel
- Labor-intense manufacturing (TP)
The idea is simple: Support your offshore wind turbine on three legs rather than one, and it will be more stable.
In many ways, the tripile is an obvious improvement of the renowned monopile design. Conceived and developed by the German wind turbine manufacturer Bard, the first tripile structure was installed in 2008.
Bard is a new player in the offshore wind industry, and it has overseen a rapid development in both support structures and turbines. The first installation in 2008 was of a single support structure and turbine in Hooksiel, Germany, but by the following year, installation began of one of the world’s largest offshore plants, Bard Offshore 1.
The Bard 5.0 MW turbines, supported by tripile structures at 40 m water depth at the offshore plant 'Bard Offshore 1'. Photo: Bard Offshore
Now four other offshore plants making use of Bard turbines and tripile foundations are in the works: Veja Mate in Germany and three plants in the Netherlands (Bard Offshore NL 1, EP Offshore NL 1, and GWS Offshore NL 1). A total of 260 turbines are planned (Company website, retrieved August 2011).
Structure
The tripile design. Illustration: LORC
The tripile consists of three piles and a transition piece. The piles are cylindrical steel tubes, similar to monopiles, and are approximately 3 m in diameter. Each pile can – depending on water depth and soil conditions – be up to 90 m high and weigh up to 400 tonnes. Between 30 and 45 m of the pile rests in the soil, depending on the soil properties.
Above water, a Transition Piece (TP) connects the three piles. The transition piece has a flange on top upon which the wind turbine tower is mounted, and its three legs rest within the accompanying piles. There is a Ducorit UHCP grout-filled annulus of 5 m in height and 13 cm in thickness between the transition piece leg and the pile.
The TP is constructed from welded steel plates. It weighs 490 tonnes and features far more welding than is present in monopile transition pieces. The transition piece is fitted with a work platform and stairs. The boat landing is mounted on one of the piles.
A transition piece on the quay at Cuxhaven, Germany. To get an idea of the scale, note the person standing in front of the right leg. Photo: Bard Offshore
The support structure contains between 1100 and 1690 tonnes of steel in total. The tower weighs 450 tonnes and the nacelle 280 tonnes.
Installation
One challenge during installation is to position the three piles accurately. With the assistance of a seabed template and the Global Positioning System, the piles are hammered down one by one.
The hammering of monopoles into the seabed with hydraulic hammers is, however, extremely noisy. Rising concerns about the health of fish and sea mammals means that new installation restrictions are going to be enforced in Europe in order to mitigate the noise.
Afterwards, the tops of the piles rise above the sea, allowing subsequent operations to be performed above water. This contrasts with monopiles, where a large part of the transition piece is below the mean sea level.
A transport vessel arrives carrying the transition piece. The transition piece is lowered onto the piles, with each leg-end aimed into a pile. To ensure that the transition piece is levelled, a hydraulic solution has been developed by the industrial tools company Enerpac. The so-called Synchronous Lifting System consists of three hydraulic cylinders per pile, or nine cylinders per support structure.
The hydraulic cylinders adjust the vertical spacing between pile and transition piece leg, thus enabling quick and precise levelling (Enerpac: Press release, 2009).
Once levelled, the annulus between transition piece leg and pile is filled with grout, and the cylinders remain on the structure for several days while the grout settles. There is thus no bolted or welded connection between the piles and the transition piece: The loads and forces are transferred by the grout alone (Enerpac: ‘Perpendicular at sea’, 2009).
The tower, turbine, and rotor are then mounted atop the transition piece.