Cyclic Behaviour of Monopile Foundations for Offshore Wind Farms

Lead Research Organisation: University of Cambridge
Department Name: Engineering

Abstract

Offshore wind farms are gaining popularity in the UK due to the current interest in the need for greener energy sources, security of energy supply and to the public's reluctance to have wind farms on-shore. Offshore wind farms often contain hundreds of turbines supported at heights of 30m to 50m. The preferred foundations for these tall structures are large diameter monopiles due to their ease of construction in shallow to medium water depths. These monopiles are subjected to large cyclic, lateral and moment loads in addition to axial loads. It is anticipated that each of these foundations will see many millions of cycles of loading during their design life. In coastal waters around the UK, it is common for these monopiles to pass through shallow layers of soft, poorly consolidated marine clays before entering into stiffer clay/sand strata. One of the biggest concerns with the design of monopiles is their behaviour under very large numbers of cycles of lateral and moment loads. The current design methods rely heavily on stiffness degradation curves for clays available in the literature that were primarily derived for earthquake loading on relatively small diameter piles with relatively small numbers of cycles of loading. Extrapolation of this stiffness deterioration to large diameter piles with large numbers of cycles of loading represents the key risk factor in assessing the performance of offshore wind turbines. Further research is therefore required. The proposed project aims to understand the behaviour of large diameter monopiles driven through clay layers of contrasting stiffness and subjected to cyclic lateral and moment loading. Centrifuge model tests will be conducted taking advantage of recent developments at the Schofield Centre that include a computer-controlled 2-D actuator that can apply both force or displacement controlled cyclic loading to monopiles in-flight. In addition it is possible to carry out in-flight installation of the monopiles to simulate the insertion of these monopiles into the seabed. New equipment will be developed for the in-flight measurement of soil stiffness and dynamic response comparative to the state-of-the-art equipment which is now used in the field. The main outcome of the project will be a better understanding of the response of the monopiles in layered soil systems to large number of loading cycles (lateral and moment loads). The results will be directly compared to the current design practices and guidelines for improved design will be developed. The outcome of this project will allow an accurate estimation of the behaviour of offshore monopile foundations under very large numbers of cycles of loading, thus leading to a confident estimation of the life cycle of the foundation. This is critical in determining the economic viability of an offshore wind farm given that the capital costs are high and the revenue stream is relatively low but continues for the life of the wind farm.

Publications

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Futai M (2018) Dynamic response of monopiles in sand using centrifuge modelling in Soil Dynamics and Earthquake Engineering

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Haiderali A (2016) Improving the lateral capacity of monopiles in submarine clay in Proceedings of the Institution of Civil Engineers - Ground Improvement

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Haiderali A (2013) Lateral and Axial Capacity of Monopiles for Offshore Wind Turbines in Indian Geotechnical Journal

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Haiderali, A. And Madabhushi, S.P.G. (2012) Three-Dimensional Finite Element Modelling of Monopiles for Offshore Wind Turbines in Proc. World Congress on Adv. Civil Environ. Mater. Res. Seoul

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Kirkwood, P.B. And Haigh, S.K. (2013) Centrifuge Testing of Monopiles for Offshore Wind Turbines in Proc. 23rd International Offshore and Polar Engineering

 
Description Centrifuge testing was carried out to investigate the lateral behaviour of the monopile. Experiment results indicated the DNV (2010) design methodology greatly underestimated the lateral stiffness of the foundation, resulting in underestimation of the system's natural frequency. As a result, there was a strong need to characterise the p-y curves.
To characterize the experimental p-y curves, the DNV design methodology based on Matlock's (1970) recommendations for soft clays was modified and this in turn enabled better estimation of the monopile's lateral stiffness. LPILE results corresponded very well with the experimental curves within the maximum permanent rotation at mudline of 0.5° as specified by Achmus et al. (2009) and estimated the ultimate lateral load very well. However, the modified DNV methodology overestimates lateral stiffness beyond a mudline displacement of 0.25 m (i.e. permanent rotation at mudline of 1.0°). This indicates that the modified DNV methodology may be suitable for monopile wind-turbine designs but may be unconservative for other applications where ultimate lateral stiffness is of greater importance such as in the design of anchor piles. A review of literature indicates that the reference deflection, yc at which 50% of the ultimate soil reaction is mobilised can vary quite widely and is dependent on the soil. Since the natural frequency band for "soft-stiff" design is very narrow, failure to properly estimate the pile-soil stiffness may result in designs with natural frequencies closer to the wind turbine frequency. Therefore, it is suggested that designers to carry out lateral pile loading tests on the soil in question, whether via field tests or centrifuge tests, to confirm the appropriate constant that should be utilised to calculate yc.
Based on the study carried out in LPILE, the increase in lateral stiffness by the shear force is marginal as the lateral resistance improving/mobilised soil reaction reduction effects are limited to depths below the rotation point that account for 25% of the monopile's length. Though the lateral resistance improving effects are marginal, the pile toe shear force should be considered as it increases the bending moments experienced by the monopile, especially at depths closer to the pile toe. An equation to characterise the pile toe shear force was developed based on experimental results and by defining the shear reference deflection, yc shear, based on Skempton's (1951) approach. Inclusion of the suggested shear force curve into the LPILE analysis improved bending moment estimation along the pile helped confirm the validity of the shear force curves. Not only were the calculated bending moment curves closer to the experimental curves, the stiffness of the calculated load-displacement curves increased marginally and were still extremely close to the experimental curves within the maximum permanent rotation at mudline of 0.5° as specified by Achmus et al. (2009). Based on the analysis results, it is advisable that the Modified DNV criteria be used in conjunction with the suggested shear force curve to improve estimation of monopile lateral stiffness and bending moments.
Exploitation Route Despite the suggestions made, there is a need for further research to ascertain the reason why the constants for defining yc varies widely across different soils and how different soil characteristics affect the constants. From the experiment results, the constant for defining yc appears to be unaffected by overconsolidation ratio. Research in this is necessary as experiment results show that that the major contributor to accurate estimation of the monopile's lateral stiffness is not the pile toe shear force, but the p-y curves utilised in design. If resources are available, another area for research with regards to pile toe shear force would be determine whether there should be a difference between yc_shear and yc, including the reason behind it.
This research also led to a very fruitful collaboration with Prof Marcos Futai from Brazil which resulted in a publication in Soil Dynamics and Earthquake Engineering. This paper highlights the need to carryout the dynamic response analysis at correct prototype stresses and strains using centrifuge modelling.
Sectors Construction,Education,Energy,Environment

 
Description The main impact of this research has been to improve the current design procedures being used for offshore monopiles that are widely used to support offshore wind farms in the UK coast. The publications that arose from this research help the offshore wind industy to improve their monopile design procedures. Also our industrial collaborators Dong Energy, McAlpine, Renewable Energy systems (RES) and others have had access to the experimental data that will allow them to calibrate their own analytical design tools.
First Year Of Impact 2012
Sector Construction,Education,Energy,Environment
Impact Types Economic

 
Description Visiting Professor Marcos Futai from Brazil to continue this work over a period of one year. 
Organisation Universidade de São Paulo
Country Brazil 
Sector Academic/University 
PI Contribution Dr Stuart Haigh and myself collaborated with Prof Marcos Massao Futai of University of Sao Paulo, Brazil on determining the dynamic response of offshore monopile foundations along with an MPhil student Ms Jing Dong at University of Cambridge. Further Centrifuge testing has been carried out and a publication is currently submitted to Journal of Soil Dynamics and Earthquake Engineering, based on this collaboration.
Collaborator Contribution Prof Marcos Massao Futai has played a key role in helping us conduct the centrifuge tests along with the MPhil student Ms Jing Dong.
Impact This collaboration is not multi-disciplinary.
Start Year 2016