By William Steel
DONG Energy and partners recently completed one of the most comprehensive programmes of offshore monopile testing testing ever undertaken.
In total, 28 monopiles — the most typical foundation architecture used to support offshore wind turbines — were tested to assess and validate a new structural design method developed by the PISA (Pile Soil Analysis) academic working group. The programme produced results which are being taken as indicative of significant cost reduction potential for the offshore wind industry.
In a press release from DONG Energy, Alastair Muir Wood, Lead Geotechnical Engineer DONG Energy, and Technical Manager for the PISA Project, said: “We’re very pleased with the test results, which confirm that traditional design methods in these soils are very conservative. The results indicate that in these site conditions there may be opportunities for savings identified by reducing the quantity of steel in the foundation. In other words, there’s a savings potential that will contribute to reducing the cost of electricity.”
The programme was led by Dong Energy, the Danish state-owned utility, and Environmental Scientifics Group Ltd (ESG), and carried out within PISA — a joint industry project led by DONG Energy and run through the Carbon Trust’s Offshore Wind Accelerator RD&D programme, based at Oxford University.
A principal objective of the PISA initiative has been to establish significant savings in offshore wind turbine production costs through development of new foundation designs. It’s envisioned that this goal can be realised, at least in part, by demonstrating the validity of monopile designs which, owing to their structural features, require less steel to construct than is typical in today’s designs.
Producing a monopile using less steel whilst retaining the strength required to support tall, heavy structures of wind turbines as well as resilience to operate in harsh sea conditions over an expected life-span of 20 years or more is no easy feat. But there is significant financial incentive for developing new foundation technologies.
Capital costs of offshore wind power are estimated to be around twice that of onshore projects; in part due to the expenses relating to foundations, which themselves are some 2.5 times as expensive offshore versus onshore. Offshore foundations account for 15% to 20% of total project costs — an amount split between materials (~16%) and installation (~6.5%), according to a 2009 EWEA report.
It is those costs associated with materials that PISA aimed to reduce in its monopile research. Such reductions would be reflected in offshore wind’s eventual levelised cost of energy (LCOE) — one of the utility industry’s primary metrics for assessing the cost of electricity, and a measure that massive amounts of wind industry investment is committed to in the hopes of reducing.
Within the programme, three types of monopile were evaluated, with a 2m diameter monopile being one of the largest ever tested. The monopiles were tested in several ways. Important stress tests were performed in which the structures were exposed to increasing amounts of strain to assess how much force they could take before failure. For instance, lateral strain was tested by literally pulling the piles sideways, into the soil, until they failed.
In the largest test conducted, some 250 instruments gathered real-time data from the foundation structures whilst loads greater than the weight of 37 London-style double decker busses were applied.
The testing took place onshore in Cowden, England, and in Dunkirk, France. These sites were chosen in part on account of the soil and earth compositions in these locations — clay in Cowden, and dense sands in Dunkirk — being good analogues of typical conditions occurring in much of the North Sea, where the companies involved in the testing have much of their offshore activities situated.
While the potential cost reductions for shallow, fixed foundations (especially monopiles) are expected to be modest in the coming future according to the International Renewable Energy Agency (IRENA Wind Power Cost Analysis, June 2012), the findings presented by Dong Energy et al., bode well for an industry in which even modest reductions in costs are of significance.