profile Dr Byron Byrne
New code could make monopiles less expensive
Research work by a team of academics is helping to develop a revised design code for monopiles
structures, where piles generally have a smaller diameter than those for offshore wind turbines and can be much longer. Over time, however, developers have come to realise that the design guidance they have been using, extrapolated as it is from the oil and gas sector, doesn’t always capture the performance of the structures they have installed. Using design guidance from the offshore oil and gas industry, they believe, has led to monopiles for turbines being longer than they really need to be, and constructed from thicker steel plate than may be necessary. This is particularly important given that the proportion of project capital that goes on the foundations in an offshore windfarm can be as much as 20 per cent. If the cost of foundations can be reduced by 25 per cent then the overall cost of a windfarm could fall by around 5 per cent. To address the issue, Dong Energy and The Carbon Trust put together a research project to develop a new design methodology for offshore wind foundations, in order to overcome the shortcomings of the current methods, and help drive down manufacturing and installation costs. The other funding partners of the project are Iberdrola, RWE, SSE, Statoil, Stattkraft, Vattenfall and Alstom. The aim is to develop a new design code based on numerical analysis, benchmarked against field testing.
T The lead academic for the pile soil analysis (PISA) project
is Professor Byron Byrne, an Australian who came to Oxford University in the UK to pursue a PhD on foundations for offshore structures, who is now fellow and tutor in engineering science at St Catherine’s College. Working with colleagues at Oxford, Imperial College London and University College Dublin, Professor Byrne is investigating how offshore wind turbine foundations can be designed more effectively in the future. He explained that, in the offshore oil and gas sector, a pile for a jacket might be 600-1,000mm in diameter and 40-80m long; those used in the offshore wind sector are quite different – they might be 5-8m in diameter and perhaps 40m in length. Monopile foundations for the next round of projects might need to be 10m in diameter. “Offshore wind monopiles are subject to very different types of loads to those used in jackets in the oil and gas sector,” Professor Byrne explained. “If a more appropriate design methodology can be developed significant cost savings could accrue.” What kind of cost savings might be possible depend very much on a number of factors, such as the size and weight of the turbine used, soil type and the cost of manufacturing and so forth, but Professor Byrne
64 I Offshore Wind Journal I 3rd Quarter 2014
he design guidance used for monopiles for offshore wind turbines originates in the offshore oil and gas sector. It was developed primarily for use with foundations for jacket-type
and his colleagues believe that for a typical 6 megawatt turbine, a reduction of the monopile steel by 35 per cent could be possible. Professor Byrne explained that the most popular method of analysis for laterally loaded piles – and the method adopted in the offshore design codes – is based on the ‘Winkler model’ and is commonly termed the ‘p-y’ approach. This method of analysis has successfully been used in the offshore oil and gas industries for many decades. However, current versions of the p-y approach do not appear to be well-suited to predicting the response of piles with the geometry and loading seen in the offshore wind industry. “We hope that the new design methodology developed during the project will provide greater confidence in the prediction of pile response under the lateral loading seen by offshore wind turbines,” Professor Byrne explained. “A more accurate design method could allow monopiles to be installed in deeper waters than is currently possible using the existing standards, and potentially with larger turbines on top. This could make monopiles a suitable foundation solution for a number of the UK Round 3 projects.” Initial analyses by Imperial College have demonstrated that short, stiff piles proposed for future windfarm installations behave in a rigid fashion, unlike the slender flexible piles which were used to derive the original p-y approach.
P
rofessor Byrne said that, so far, the academic team working on the PISA project has developed the new calculations for monopiles for two reference soil types (one a stiff clay and the other with a dense sand typically found in projects developers plan to undertake) and has issued these to the project partners. The PISA consortium has also contracted for the test piles to be manufactured – in June the PISA project partners were invited to NLMK Dansteel’s facilities near Copenhagen to witness the rolling and testing of the PISA steel. The scale piles were due to be installed by the end of September at the two sites, at Cowden in the UK and Dunkirk in France, in order that testing can get under way late in 2014 or early 2015. “We should have the data we need by late January 2015,” said Professor Byrne. The aim of the working group is to find technical solutions to be implemented in time for the design and construction of the large Round 3 offshore wind projects in the UK. The working group will be publishing its final report in mid-2015, but once the immediate scope of the project is completed it could continue after the submission of the new design methodology through PhD studentships focused on numerical modelling at Oxford and Imperial College. OWJ
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