Jacket & concrete foundations: which is the most cost effective option?
Marc Seidel, offshore engineering expert at turbine maker REpower Systems, and Jon Wren, programme manager at service company and supplier, OGN Group, offer their insights and experiences with jacket and concrete/gravity base foundations as the industry...
This interview has been updated.
Interview by Elisabeth Jeffries
Q: What are the pros and cons of jacket foundations?
MS: There are a number of advantages. Applicability to a wide range of water depths is one of those advantages as they can be used at up to 60 metres. But there are other benefits: they have a high structural stiffness; the turbine behaves nearly like an onshore turbine with virtually no wave-induced vibrations. They are also light-weight; compared to other structures, they are in fact the lightest. This is positive from the point of view of material cost and installation.
They also have a potential for a large supplier base because they are not complicated and don’t need large wall thicknesses. Many suppliers with sufficient space can build them. Finally, they can be used in nearly all conditions. They can put up with high waves, while soil conditions are much less relevant than for a monopile.
JW: We see that jacket foundations are the most cost effective option for offshore wind developments in the UK round 3 arena with the costs of fabrication and installation being driven down through innovations, such as the ones OGN have planned with our Triton jacket, and our planned new automated fabrication facility.
One of the big advantages of jacket foundations is the oil and gas experience and expertise developed over the last 40 years in making and installing them. Jackets are steel fabrications and require less preparation at the seabed than is the case for other types, such as concrete foundations (also known as gravity base).
We have demonstrated in the oil and gas industry that they are long-term viable options and very appropriate to waters over 25 metres in depth, where the monopile foundations, used in the shallow waters of rounds 1 and 2 off the UK coast, are no longer viable.
Jackets will, in the opinion of much of the UK industry, continue to be the preferred option until maybe in the distant future when the UK Offshore wind industry moves to very deep waters when floating platforms, tethered to the seabed become necessary.
Q: What are the pros and cons of concrete (gravity base) foundations?
MS: There are quite a few advantages: concrete is cheap and there is no noise emission during installation. They are already supplied by a number of experienced European construction companies using well known proven concrete technology. Plus, they are highly resistant to damage by salt water, their maintenance costs are low and they are fully removable.
On the other hand, scour protection is normally needed; they are also a new approach for deep water, and there are complex permit issues while they’re only usable in limited water depth depending on location. Detailed soil investigation is required as is seabed preparation.
JW: Concrete foundations do have some advantages. They are hollow structures that can be floated to their location offshore where they are then filled with water and sunk. This is less expensive than using a ship to transport them, which is the case for steel foundations like jackets.
However, experience at some offshore wind farms shows that the overall cost of using concrete foundations has worked out higher than jackets. One reason for this is because you need to dredge an area of the seabed to make it flat to accommodate the concrete foundation. That’s not the case for jackets.
Q: Are the jacket foundations that you have used actually the company's own proprietary applications - are there distinct features which would be bespoke for different wind farms?
MS: Jacket designs are always site specific as they depend on site conditions (wind, waves and soil). In addition, some design features (such as the transition piece) are specific to companies and designers.
REpower has its own design, which was built in Bremerhaven onshore as a prototype. There is no offshore application yet.
Standardisation of jackets is only possible for construction principles. There are difficulties in more general standardisation due to variations in water depths and differences in loading and pile capacities – which for example, does require that the footprint varies.
JW: When working in the oil and gas sector, all our jacket foundations are bespoke – our own design. In the offshore wind turbine sector, we’re working to standardise the design as much as possible with our Triton design.
This design is for a three legged jacket, which has been created with the aim of reducing both fabrication and installation costs. It is suitable for mass production and able to be installed in a wide weather window.
Q: How does the installation of jacket foundations help develop the industry and what types of evolution/innovations do you foresee in this area?
MS: The good experience from our projects with jackets does of course increase confidence in this technology. At present, developments focus on design details like pilings and grouted connections rather than the concept itself. Future projects will see many improvements particularly to such details. Interesting innovations are emerging, such as the keystone twisted jackets, which have few members and welds. In keystone twisted jackets, most of the weight is in the cheap piles.
JW: Jacket foundations can be installed on a variety of seafloor conditions, from rock through to sand. They can be anchored by driven or drilled piles and also using suction buckets. This, together with the ability to work in deeper water, makes them suitable for many different locations.
The industry is developing several different types of foundations, all with the aim of reducing manufacturing and installation costs and as yet is loath to put its eggs all in one basket.
It doesn’t all come down to jackets though; the costs for all foundations are high as these are complex structures and have to be installed and then work for 25 years in a hostile environment. However, the industry is working to reduce these costs and we have our own plans to help reduce the costs of fabricating and installing jackets.
We have just obtained planning permission to build a new state of the art automated factory on our site in North East England to supply jackets to the offshore wind industry. We hope to have the plant running in 2015 with the aim of eventually employing several hundred people. We firmly believe that jackets are the optimum foundation for deeper water and the larger turbines that are the basis for the UK round 3 developments.