Wind Support is currently undertaking research on the behaviour of piles in chalk. The first phase of tests has been successfully completed and the results are now being analysed. However, there is still a lot more that can be done and Wind Support is seeking the collective sponsorship of the offshore wind energy industry for the next phases of the project.
In order to achieve the EU renewable energy target for the UK, more than 6,000 offshore wind turbine will be installed over the next decade. One of the main obstacles to the economic development of offshore wind farms is the high capital cost. Optimization of the foundation design could lead to a substantial cost-saving. To fully optimise the design, a good understanding of the soil-structure interaction behavior is essential. If knowledge of the particular soils and rocks encountered is lacking, conservative design assumptions need to be made, which tends to result in an over-design.
The types of ground material where further research would be most beneficial include weak rocks, especially chalk and mudstone. Of these two materials, chalk has the greatest research needs, partly due to its wide variability and the difficulty in predicting its behaviour. Furthermore, chalk is widespread across the areas being developed for offshore wind farms in the southern North Sea, the English Channel and the Baltic.
In shallow waters, monopiles are generally the preferred type of foundation for offshore wind turbines, whilst in deeper waters, piled jackets or tripods are normally used. In both cases, the piles are generally installed by driving using an impact hammer, as this tends to be the most cost-effective.
The latest state-of-the art for the design of driven piles in chalk is very limited. There is a distinct lack of a systematic investigation of the behaviour of piles subject to axial and lateral loads under static and cyclic loading conditions. Current industry guidelines (CIRIA C574) include an ultimate shaft resistance of 20 kPa in all chalk conditions except high density Grade A, which is likely to be very conservative, especially some time after pile driving when soil set-up has taken place. Furthermore, there are no established methods for analysing lateral load-displacement behaviour.
DNV, Germanischer Lloyd and Lloyds Register all agree that there is a compelling need for a comprehensive programme of large-scale pile tests in order to improve our understanding of the engineering behaviour of driven piles in chalk.
Wind Support is proposing to undertake the required pile testing in several phases, so that the research effort can be applied in a controlled and cost-effective manner. For Phase 1, a programme of tests was carried out (between November 2011 and March 2012) in order to investigate some of the most pressing issues related to driven piles in low to medium density chalk, such as:
ultimate shaft resistance (including set-up effects)
initial stiffness under static lateral loading (including set-up effects)
lateral load-deflection behaviour under cyclic loading
post-cyclic ultimate lateral load resistance
The pile tests for Phase 1 were carried out at a disused chalk pit in Kent, where the Chalk is low to medium density Grade B.
Five 0.76m diameter open-ended tubular steel piles. Three of the piles are 'short' and two are 'long', with embedment depths of 4m and 10m, respectively. The 'short' piles are representative of 'semi-rigid' monopiles, while the 'long' piles are representative of 'flexible' jacket/tripod piles. The piles were instrumented with strain gauges (vibrating wire type), LVDTs and pore pressure cells.
For Phase 1, the following load conditions were applied:
Static uplift loading to failure
Static lateral loading to working load levels
Static uplift loading to failure after static lateral loading to working load levels
Cyclic lateral loading with post-cyclic static lateral loading to failure
The expected benefits from the Phase 1 pile tests include:
New recommendations on the ultimate shaft resistance of driven open-ended steel piles in low to medium density Grade B chalk;
Improved understanding of the lateral load-deflection behaviour under static and cyclic loading, including initial stiffness, ultimate lateral resistance, stiffness degradation due to cyclic loading, creep effects, and set-up effects;
New recommendations on the derivation of realistic P-y curves for low to medium density chalk.
The Phase 1 research was collectively sponsored by DONG Energy Power A/S, Centrica Renewable Energy Ltd, Dudgeon Offshore Wind Ltd (Warwick Energy Ltd), Fugro Geoconsulting Ltd, and Lloyd's Register.
The results of Phase 1 will be used to determine or confirm the most important requirements for later phases.
Proposed Future Phases
It is proposed to carry out further testing on the same piles during a future Phase 2. These tests could include cyclic uplift loading, static and cyclic axial loading in compression. For Phase 3, it is proposed to undertake the same types of tests but on different types of chalk at other sites in order to develop a comprehensive database.
Request for Financial Support
Wind Support Limited was founded in 2003 by John Carey. John has a PhD in Geotechnical Engineering and has worked almost exclusively on offshore wind energy projects since 1998.
Wind Support is currently seeking financial support from the offshore wind energy industry in order to carry out further phases of research. If your company is interested in making a contribution, please contact Dr. Francesca Ciavaglia for further information.