Water depths: 25 - 120 meters
Weighs up to 828 tonnes
- Very good resistance to overturning
- Light and efficient construction
- Many man-hours of welding
- Complicated transportation to site
When power companies began to look at deeper waters for installing wind turbines they had to consider alternative support structures. Thus, the jacket structure entered the sector and moved the boundaries. Until 2007, other structures such as the monopile and gravity-based structures had only been able to put wind turbines at a water depth of 20 meters (Barrow, UK, 2006).
But the Beatrice demonstrator project changed that. Making a leap from 20 to 45 meters water depth, it strongly suggested that the jacket structure had something to offer in terms of large depths.
The concept of jackets is inherited from the oil and gas industry. Jackets have been used for supporting rigs at a depth of more than 100 meters.
A jacket is made up of three or four main legs, connected to each other by bracings. All elements are tubular unlike onshore lattice structures which are usually made from angular profiles.
Figure 1: Terminology used on the jacket. Illustration: LORC, Alpha Ventus
The bracings and legs are connected up into tubular joints – a critical component of the construction. The tubular joints have the shape of the letter ‘K’ at the legs, the letter ‘X’ where the bracings cross, and the letter ‘Y’ at the top and bottom.
At present the tubular joints are welded – often referred to as welded nodes. Because of the many joints, there is a large amount of welding which is performed manually. It takes a lot of man-hours to complete the jacket.
The welded nodes are the weak points in the construction in terms of fatigue. Fatigue occurs under dynamic loading and leads to material damage of the steel structure. It is especially at the weldings that fatigue is most likely to occur.
A solution to this is to use cast nodes instead. Cast nodes decrease the fatigue problem of the structure since the number of welding seams is decreased. However, producing cast nodes is more difficult than welding.
Jacket legs can have a diameter of more than one meter – in some cases up to 1.4 meter – so the mold for the cast node itself has a significant size. Bracings usually have a diameter of less than one meter. The technique for casting in these dimensions is advanced.
At present, cast nodes exist in prototypes onshore – such as in this concept from the German company Weserwind.
The Transition Piece
Like the monopile, a jacket needs a transition piece to support the wind turbine tower. The transition piece also includes the working platform just below the wind turbine tower. But unlike the monopile concept, the transition piece on jackets does not have to level the construction. Leveling is done at the seabed.
Figure 2: Terminology used on and around the transition piece. The TP connects the jacket legs to the tower. Illustration: LORC, Alpha Ventus
Workers on the platform of a transition piece for the Ormonde offshore wind farm. Photo: Ben Garden
The transition pieces so far installed have a wide top, they are typically 9 meters tall (pages 43-45, Talisman Energy 2006), and weigh 160 tonnes.
Besides the legs, bracings, and joints, the jacket consists of other elements, called secondary steel. Secondary steel includes:
- Work platform
- Ladders and stairs
- Access systems, i.e. boat landing
- J-tube and cables
- Corrosion protection systems
Secondary steel typically weighs approximately 150 tonnes.
At the seabed, the structure is often attached into the ground using piles, but gravitation bases or suction anchors are also possibilities.
The foundation of a jacket with piles can be carried out as either post-piled or pre-piled.
In the more traditional way of installing a jacket, the post-piling process, the piles are driven through sleeves at the bottom of the jacket legs. The piles themselves might be hammered or vibrated into the seabed after the lowering of the jacket.
Figure 2: The post-piling process. Mainly used in the oil and gas industry.
Typically, the connection between the sleeves and the piles is secured with grouting. The gap between the sleeve and the pile is filled out with a special grout material, which transfers loads from the jacket leg to the pile.
The connection can also be secured using swaging, a cold forging process, where the diameter of the inner tube (the pile) gets expanded until it establishes a safe connection to the sleeve. The inner tube is expanded using a die (a specialized tool used in manufacturing industries to cut or shape material using a press) or using high pressure water. The swaging process is described thoroughly by Oil States Industries.
Today post-piling is seldom used in wind farms. Only in the Beatrice wind farm, where jackets have been used for wind turbines for the first time, was the installation carried out using post-piling. However, in the oil and gas industry post-piling is widely used. This is because the oil and gas industry typically only requires installation of a single structure, whereas wind farms require installation of many similar structures.
During the pre-piled installation of a jacket, a template is used when hammering or vibrating the piles into the soil. Only after the piling process is the jacket lowered to the bottom of the sea, where the spikes at the end of the legs fit into the piles.
Figure 3: Pre-piling the jacket. A template is needed, but it is faster for repeated installations.
The connection between the spikes (jacket legs) and the piles is made with grouting or swaging, but also other connection concepts, like Pile Quick Coupling from Leenaars BV, have recently entered the market.
At Alpha Ventus the jackets were pre-piled. Notice the spikes on the bottom of the structure. Photo: Alpha Ventus
Pre-piling is considered to be a faster method than post-piling. With pre-piling, smaller vessels can be used for the piling and the large vessels can be employed very efficiently – they use little time to set the jackets into the pre-installed piles. With post-piling, the expensive large vessels have to spend more time with each jacket.
In addition, post-piling requires sleeves mounted on the jacket as well as so-called mud-mats. The mud-mats transfer loads to the seabed and act as the supporting foundation while the piles are being installed. With pre-piling, the piles can underpin the structure. At Beatrice – a post-piled structure – the sleeves alone weighed 160 tonnes (Seidel, 2007), so a considerable amount of steel can be saved by using pre-piling. The cost for the pre-piling template needs to be taken into account and compared to the cost for the sleeves, but at a reasonable number of installations there is a cost benefit of using a template instead of mounting sleeves on every jacket.
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.
A third method – though not one used in wind farms so far – is using suction anchors instead of piles. This was proposed at Beatrice, but ultimately Talisman Energy chose a piled solution.
And finally, one could use a hybrid construction with gravity bases at the bottom – similar to having concrete feet for the structure. This has not yet been seen on wind farms.
As described above, jackets demand thorough preparation of the seabed. In most cases, a template is used to drive down the piles ahead of installation. The template is essential for driving the piles at an accurate distance from each other. Using post-piling makes the template unnecessary.
The jacket has to be level within the standard margins of 0.5 degrees. Usually, a Remotely Operated Vehicle (ROV) is used to measure the height of the piles when they have been installed with the template. These measures are taken into account when producing the jacket so that height deviations from the piles can be leveled out by manufacturing the jacket legs and spikes in lengths that compensate for the deviations. The Jacket with the transition piece mounted is then placed on the piles. For this procedure a vessel with a heavy lift crane is needed. The connections between piles and jacket are then grouted (or swaged).
Transporting the finished jackets to site is literally a big thing. At Alpha Ventus, three jackets were transported on one barge and this is the record. Being around 50 meters tall and weighing between 500 and 800 tonnes, not every barge and crane can install the jacket. At Alpha Ventus, the gigantic Thialf from Heerema Marine Contractors was used. At Beatrice and Ormonde, the Rambiz from Scaldis did the job.
The transition piece is installed before transport to sea and then the tower and turbine in sequence. But at Beatrice a remarkable solution was chosen: the transition piece, tower, and turbine with rotors was assembled on the quay and sailed to site in one piece. However, this method has not been repeated at other sites.
A graphical walk-through of the pre-piled installation at Alpha Ventus can be found here. The post-piled installation at Beatrice is explained in this video:
Notice that the installation at Beatrice is considered laborious and has been optimized in later wind farm installations.
In future one might see transition pieces without the working platform and the bracings at each corner of the jacket as a result of more integrated designs. This is suggested by Repower and Weserwind with this prototype of a narrow transition piece which only weighs approximately 65 tonnes.
Another possibility is that the jacket and tower are merged into one construction, rendering the transition piece at sea level superfluous. This is the concept from the Dutch company 2-B Energy. They operate with the design of a three-legged jacket or lattice structure from seabed to nacelle.