Report finds on-shore construction is most cost effective wind turbine building method

Last December, the University of Delaware published a high profile report which concluded that the most cost effective method of constructing offshore wind turbines is to build them in port.

So, what are the key findings of the report?  What methodology did the report authors use to arrive at these findings?  What are the broader implications for the offshore wind industry - and how best can developers act on the conclusions?

The key purpose of the report 'Industrializing Offshore Wind Power with Serial Assembly and Lower-cost Deployment' is to highlight the results of a project carried out by a team of engineers and contractors to develop a method to move offshore wind installation toward lower costs, faster deployment, and lower environmental impact.  As part of the project, a team including staff from the University of Delaware - as well as from a wide range of other companies and organisations in the offshore wind energy space, including Mammoet, Weeks Marine, SPT Offshore, Moffatt & Nichol, Atlantic Grid Development, EEW Steel, CG Power Solutions, Clipper Marine, Saipem Group, Signal International, Steel Suppliers Erectors, Universal Foundation/Aalborg University and XKP Visual Engineers - employed a combination of methods, some of which it describes as 'incremental' and some of which are 'breaks from past practice, to yield 'multiple improvements.'

In all, a total of three designs were evaluated based on a detailed engineering investigation - a 'base case' 5MW turbine on a jacket with pin piles and a 10MW turbine on a conventional jacket with pin piles, both assembled at sea, as well as a third design consisting of a 10MW turbine on a tripod jacket with suction buckets and complete on-shore turbine assembly.   A key conclusion is that a combination of a larger turbine, onshore assembly and the use of suction buckets helps to 'substantially reduce the capital cost of offshore wind projects.'  In particular, the report finds a 'notable' capital cost reduction of 31% by changing from a 5MW to a 10MW turbine, and a further 9% capital cost reduction via assembly on land followed by single-piece installation at sea.

The team also extend this analysis by proposing a fourth design that estimates 'further cost reduction' when the equipment and processes outlined in design three are 'optimized, rather than adapted to existing equipment and process.'  Interestingly, the team calculate that reductions in the overall cost of energy for each of the four designs are approximately the same in each case.

In arriving at the estimated costs for the third design, the design team - consisting of 'experienced offshore structure designers, heavy lift engineers, wind turbine designers, vessel operators and marine construction contractors' - also analysed 'accepted structures such as suction buckets used in new ways,' as well as 'innovations conceived but previously without engineering and economic validation, combined with new methods not previously proposed.'  Moreover, the analysis of the second and third designs is 'based on extensive engineering calculations and detailed cost estimates' - and the team is confident that 'all design methods can be done with existing equipment, including lift equipment, ports and ships, except that design four assumes a more optimized ship.'

"The cost of offshore wind is already dropping, but can be reduced considerably more by using the methods we have developed," says the reports' lead author, Willett Kempton, a Professor at the University of Delaware's School of Marine Science and Policy within the College of Earth, Ocean, and Environment - and Research Director at the University's Center for Carbon-free Power Integration.

Following a detailed comparison of each of the outlined design methods based on cost and deployment speed, the team selected the third design - which consists of a conventional turbine and tubular tower mounted on a tripod jacket above three suction buckets.  The rotor blades are also mounted on the tower itself, not on the hub, and the whole structure is built 'from the bottom up' in port with assembled structures queued in the port ready for deployment.

When weather conditions are favourable, these fully-assembled structures are then lifted off the quay and lashed to a vessel before being shipped to the installation site. In this instance, the vessel analysed is a 'shear leg crane vessel with dynamic positioning like the existing Gulliver' - or could also be a 'US-built crane barge.' To complete the process, the entire structure is lowered to the bottom on-site by the crane vessel- with smaller service vessels assuming responsibility for pumping the suction buckets and blades hoisted into place by 'small winches operated by workers in the nacelle without lift vessel support.'

In Kempton's view, the key advantages of this design include a 'significant' reduction in the cost and time at sea of the expensive lift vessel, the fact that a jack-up vessel is not required and a shorter weather window requirement for each installation. The project team also point out that turbine structure construction is 'continuous with a queue feeding the weather-dependent installation process,' pre-installation geotechnical work is 'faster and less expensive' and 'there are no sound impacts on marine mammals.'

Commenting on the findings of the report, Kempton also observes that, despite their undeniably large size, it is completely possible to 'build complete offshore wind turbine structures, including subsea support and casinos, on the quay, in the port.' He is also keen to stress that the conclusions are 'not just a concept' - but also provide 'engineering design for all the heavy operations and calculate the costs.'  Ultimately, Kempton is confident that in-port construction 'reduces cost, speeds deployment, improves safety for workers, reduces environmental impact and has other advantages.'

"Although this has never been done before, it can be accomplished with existing equipment," he added.

By Andrew Williams

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