Large monopiles are currently dominating the offshore wind market, but as the shift to deeper water and larger turbines continues, jackets and tripods may be chosen more often as foundation type. For the installation of these foundations CAPE Holland can contribute its fast experiences and proven technology from the oil and gas market, but also from the first Offshore Wind Farm projects it has been working on.
The basic jackets are steel lattice structures which are relatively light and can transfer enormous loads in axial direction. They are however very expensive to make as they have a huge number of complex welds everywhere the steel tubes meet. They also require a complex method of Transport and Installation as these structures are huge to transport, handle and they always need at least 3 anchor points, although most have more.
The jacket foundation for offshore wind turbines is generally used in water depths over 60m where the monopile seems to have reached its limit.
Most jackets that are axially loaded (“just” carry weight) have special skirts (vertical welded sleeves through which the foundation piles are driven and fixated), sometimes even 2 or 3 per leg depending on the structure. The structure is then first positioned on the seabed, and the piles are then driven through the skirts and fixated by means of grouting whereby the pile is cemented to the structure. This is all done in the same installation campaign.
For the laterally loaded jackets for the wind turbines, it is more common to use the pre-piling method, which means that the piles are installed first in a different campaign, from installing the jackets which will be done at a later stage. This way the installation of the piles can already be completed before the jackets are on location. The jacket legs are stabbed into the piles after which they are grouted in place. In order to make sure the piles are installed in the right location and for the legs to fit smoothly, a special installation frame, called a template, is built which also keeps the piles vertical during driving. These huge templates are very complex as they need to be levelled on the seabed to make sure all the piles are installed in the right location and at the required verticality.
After the piles have been installed, they are measured, and their final position is carefully recorded and if required the jackets can be adjusted to deal with out of tolerance stick up height of the pile.
A pile positioned in a jacket sleeve, after being released by the ILT and before it is replaced by the impact hammer, may cause a risk of the jacket toppling over, particular on soft top layers of the soil.
With the traditional installation method this means that every pile placed in the sleeve, needs to be driven with the impact hammer, before the next pile is placed, possibly having to start with a smaller hammer before the heavier final hammer is used. Only with sufficient self weight penetration and sufficient long pile sleeves can all piles be placed before the impact hammer is deployed
THE SOLUTION
With the pile connected via the CAPE VLT to the crane, the pile does not lean against the jacket at any time and the CAPE VLT will not be disconnected until the pile is stable. So even with an unstable seabed, the jacket is not affected by the pile installation and all piles can be installed to stability before the impact hammer is deployed to drive all piles to final penetration (if required) with only lowering the impact hammer once.
For pre-piling it is essential that the spacing between the piles and their stick-up is very precise and that the piles are within their verticality tolerance to ensure proper fitment of the jacket and its overall verticality. In order to achieve this with the traditional installation method, a piling template placed on the seabed is required. These templates are very complex as they need to be accurately levelled and need to support the piles quite tightly. As the water depth and turbine sizes increase, so do the spacing between the legs and the diameter of the pin piles. Where 2 or 2.5m diameter is quite normal for skirt jacket piles, for the pre-piling the pile diameters are generally larger, with on certain projects already reaching 5m diameter. These larger sizes require even larger templates. The immense scale of the templates together with the complex hydraulics and sensors for the levelling and pile monitoring means that not only the costs of these templates are significant, also the transport and handling requires huge deck space, crane capacity and handing equipment
THE SOLUTION
As the CAPE VLT has a firm connection to the pile and can assure a controlled installation including making it possible to make corrections to the verticality with the crane or vessel, the template does not need to support the pile preventing it from falling over, nor does it need to guide for verticality. This means that the template could be significantly simplified to just a positioning frame for the set down of the piles which would mean that the frame does not have to be so tall and can weigh substantially less.
A further big advantage is that the CAPE VLT could also be used to lift and position the template on the sea floor which means that after driving the last pile, the template can be lifted to deck without requiring an extra lift saving further on the total installation time.
Although the noise during impact driving of pin piles and jacket piles driving is not quite as high as with monopiles, it is quite often above the set levels for the various geographical areas. Particularly with the increasing pile sizes for the pin piles, the noise emissions during impact driving will only go up. With monopiles it is much easier to apply a noise mitigation close to the pile wall, whereby with jacket or pin pile driving the jacket or template are generally in the way of deploying a nearby noise mitigation, making this more complex.
THE SOLUTION
As the installation with the CAPE VLT is unlikely to require any noise mitigation anyway, the advantage with jacket and pin pile installation is that the deeper the pile is penetrated, the less surface area of the pile is exposed to the water column, so less sound energy is emitted from the pile wall, so even if the piles need to be impact driven to final penetration, pre-driving with the CAPE VLT will reduce the overall noise impact.
For the pin pile installation, a narrow tolerance window of the final stick up is required between the piles to assure verticality of the jackets once installed. A survey after installation of the piles determines what adjustments have to be made to jacket or what shims need to be used to resolve any out of tolerance stick–up. It is therefore very important to impact drive carefully for the last few centimetres not to overshoot the target as the pile can not be raised up with an impact hammer.
THE SOLUTION
Stick up tolerance can be better controlled with the CAPE VLT then with traditional impact driving due to the fixed connection of the pile to the CAPE VLT. The crane hook can be stopped at the right elevation while the CAPE VLT is still vibrating and corrections down as well as up can be made. If for any reason the target stick-up is over shot, the pile can be brought back up again to the correct level. Once the correct elevation has been reached, the CAPE VLT will be stopped and set up will immediately occur to ensure the tolerance is met.
