FACT BOX
Suited for 20-50 meters depth
Weighs approximately 700 tonnes (without piles)
Advantages
- Good resistance against overturning
- Good overall stiffness
Disadvantages
- Complex main joint with risk of fatigue
- Large impact from wind and waves
The name is well known in other contexts: any serious photographer owns a tripod, people from surveyors to soldiers use tripods, and antennas are supported by tripods. In offshore wind farms, however, the tripod is a new and rarely used structure.
The structure is common in the offshore oil and gas industry. But so far only the German wind farm Alpha Ventus uses tripods to support six of their wind turbines, the Areva Multibrid 5000. Tripods are to be installed at Borkum Riffgrund West II, Germany (43 pcs.), Cote d’Albatre, France (21) and Galveston, USA.
But this is still only a small minority of support structure designs.
Two kinds of tripod. The photographers’ choice (left) and the energy companies’ choice (right). Photos: AV Hire London and Alpha Ventus.
The structure
There is nothing small about the dimensions, though. The tripod is made out of 700 tonnes of steel, and three piles 40 meters in length are needed to secure it. The structure consists of a central column, diagonal bracings, and three supporting sleeves with mud mats. Through each sleeve is placed a pile, which is driven into the seabed and connected to the sleeve with concrete or grouting.
Instead of using sleeves with mud mats and piles, the tripod can also be founded with suction buckets. But this has not been used in wind farms yet.
Figure 1: Terminology. Illustration: LORC, Alpha Ventus
The three feet give the tripod good stiffness and stability against overturning. This makes it more suitable for larger water depths than the monopile. The depth ranges from 20 to 50 meters.
But compared to jackets, the tripod is more prone to wave loads because the large diameter of the steel tubes results in a large surface area. And the main joint at the central column poses an engineering challenge – it is receptive to fatigue and complex to design.
Installation
The Tripod is installed much like the post-piled installation of the jacket. The tripod is loaded onto a barge and sailed to site. A heavy-lifting vessel like the Taklift 4 lowers the tripod to the seabed guided by ROV’s (Remotely Operated Underwater Vehicles) or divers.
After lowering the tripod, the piles arrive and a vessel with a hydraulic hammer drives the piles into the seabed through the sleeves of the tripod. When the three piles have been driven, the connection between piles and sleeves is filled with grout or concrete. The tripod is thus secured to the seabed almost like nailing and gluing.
The hammering of piles 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.