Industrial prefabrication of three- and four-legged jacket foundations and using standardised components and innovative manufacturing processes could significantly reduce costs, says a leading German steel company
There is growing recognition that as the offshore wind industry expands into deeper waters, so greater use is likely to be made of jacket-type foundations. But there is a problem: manufacturing jackets is an expensive and time-consuming business. New ways to industrialise jacket construction and reduce manufacturing costs are required.
Conventional jacket foundations of the type currently used in the offshore wind industry are expensive to manufacture because the process is labour-intensive and because specialised welding is required.
That being the case, a key focus area for jackets is making them easier and more economical to mass produce. One way of ensuring that is to break down jackets into ‘nodes’ that can easily be mass produced and assembled – ideally using low-cost techniques such as robot welding – an approach described by German company Salzgitter Mannesmann Renewables, part of the massive Salzgitter steel production group, at Global Offshore Wind 2019 in London in June.
Salzgitter Mannesmann Renewables head Georg Michels and manager technical sales and R&D co-ordinator offshore wind Stephan Brauser told the conference to be competitive with monopiles, a 30% reduction in the cost of manufacturing and installing jacket foundations is required.
The innovative supply chain concept that Salzgitter Mannesmann Renewables has developed has the potential to reduce manufacturing costs significantly, they believe. It makes use of ‘kits’ consisting of standardised tubes, K, Y and Z nodes, and leg/brace sections supplied by the steel company. Using automated welding and non-destructive testing these kits would be quickly and efficiently assembled by fabricators.
The process they envisage makes extensive use of robotised welding of nodes. “Tubular nodal welds are a cost driver in the construction of jackets,” they explained. “Robot welding of the nodes accelerates the process and can ensure that all of the welds are of a uniformly high standard.”
As they also explained, robotised welding has other advantages. Because the welds made by robots also have a uniform shape, and welds can be made inside structures, fatigue is less likely to be an issue during a jacket’s lifetime.
When it comes to assembling components, orbital welding of the type proposed by Salzgitter Mannesmann Renewables is a cost-effective way of producing butt joints and leads to a significant reduction in time and costs compared with manual welding.
The company has undertaken a study of the cost reduction potential of this kind of approach compared to conventional jacket production for a 100-turbine offshore windfarm using three-legged jackets.
For a conventional point-to-point jacket, 72 welds are required but all were complex tubular welds. For a jacket produced from nodes, 72 welds were also required but all were circumferential welds. Both processes required 48 K/Y nodes, and 24 X nodes, but the point-to-point process required manual welding throughout and complex tubular welds.
In contrast, the process developed by Salzgitter Mannesmann Renewables used less complicated circumferential welds and around half of the welds used were orbital and semi-automatic in nature.
Mr Michels and Mr Brauser said the process they have developed using standard tubes and components and robotic welding of nodes reduced primary steel tube costs by 15%; and reduced jacket assembly costs by 10%. Assembly time was reduced by at least 10%. Overall, they said, this results in jackets that are around 15% less expensive.