DEVELOPMENTS IN FOUNDATION TECHNOLOGY
THE ROLE OF INNOVATION There is an ever-increasing view that Innovation will lead to large savings in the cost of foundations for deeper water projects. A great deal of funding is being provided by the likes of DECC and the Carbon Trust to develop innovative foundation solutions to drive costs down. The danger is that the search for the ultimate will defer decisions or worst still will result in adopting solutions that are not suitable for the harsh conditions found far out in the North Sea.
COST ANALYSIS
Some developers view the cost of the foundation in isolation, believing that making a marginal saving on steel weight will make a significant impact in the overall installed cost. This approach exposes the developer to unpredictable offshore installation risks and cost escalations.
A better approach is to view the installed costs of the foundation separately to the tower and nacelle as they have their own unique criteria laid down by the turbine manufacturers. You can knock a steel jacket about during installation but certainly not a turbine. The foundation and the turbine require different installation vessels type and such an approach will reduce installed costs.
By developing foundation solutions that extend the offshore installation weather windows will also reduce the costs and also the overall number of vessels required.
Novel solutions = potential increased risks.
Currently there are a number of deeper water wind farms in Europe that are being developed using steel jacket foundations rather than monopiles. In parallel there are many novel solutions being proposed for deeper waters but this represents another area of potential risk.
FLOATING STRUCTURES
Floating type structures appear to be a straight forward and obvious solution but they carry a significant operational risk. Offshore oil & gas has around 20 floating production platforms (FPSOs) deployed in the North Sea, all have mooring integrity issues and some have suffered major mooring failures. Drilling Rigs too have suffered similar issues and failures. Over the past 10 years the HSE has implemented a rigorous, expensive inspection regime the cost of which has had to be absorbed by the oil companies. The question is can the marginal wind farms OPEX sustain such costs with large numbers of moorings being proposed?
It is well known in the oil & gas industry that mooring systems in waters of less than 100 metres experience tension waves in the mooring lines increasing the rate of wear and tear.
CORROSION
Corrosion is also a concern. For example, let’s assume a site located in 60m water depth with say 200 turbines supported on a floating structure secured by six mooring lines and anchors. This amounts to 1200 mooring lines each a potential failure point; this is far in excess of the number of FPSO mooring lines current deployed in the North Sea and represents a significant threat to the operational viability of the hypothetical wind farm.
The turbine manufacturers place very strict verticality criteria for the installed towers and this is at odds with the floating
concepts which will move around with the sea conditions amplifying horizontal accelerations high up at the nacelle.
Novel ideas and innovation have their place and role to fulfil but they need to
be firmly placed under the microscope and fully risk assessed using the valuable experience of the oil & gas industry.
THE ROLE OF INDUSTRIALISATION The majority of foundations for offshore wind turbines have, up until now, been of a monopile design. These are comparatively simple structures with the only real design variable being the diameter and wall thicknesses of the pile. Hence when considering the construction of such items it is fairly easy to apply known and well proven automation techniques to the handling and welding of these structures. The move of offshore wind farms into deeper water will ultimately necessitate a move away from monopile foundations to more complex structures such as lattice framework jackets, which have now
A moored floating wind turbine support structure
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www.windenergynetwork.co.uk
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