Rationalising offshore wind-turbine pile design and assurance in difficult ground

Academic/Research team: Prof. Richard Jardine (PI), Dr Stavroula Kontoe (CI), Roisin Buckley (Research Assistant)

Industrial Collaborators: SPR/Iberdrola, Geotechnical Consulting Group (GCG)

Funding: UK Innovate

Duration: October 2014- September 2017

The research work is part of a major Joint Industry Project (JIP) involving Scottish Power Renewables (SPR)/Iberdrola, Imperial College (ICL) and Geotechnical Consulting Group funded by UK Innovate. The JIP aims to provide more secure and cost-effective turbine support structures for windfarms installed in difficult ground offshore Northern Europe, giving particular attention to Chalk dominated sites.

The project is led by SPR as part of their Offshore Wind Farm (OWF) Development at Wikinger, in the Baltic Sea, offshore Germany. The testing phase of the project was completed in January 2015 and involved, for the first time, remotely operated seabed static pile load tests, using a unique testing system. The tests also included dynamic stress wave monitoring at three Chalk and Glacial Till dominated locations. The research team at Imperial College also undertook a parallel pilot onshore campaign at an established Chalk site in Kent, with two 101mm OD highly instrumented Imperial College Piles (ICPs) and seven un-instrumented 139mm OD driven steel tubes. Supporting laboratory tests were undertaken on de-structured chalk, modelling driven pile installation.

The overall aims were:

(i) to understand the low resistances observed during driving,

(ii) systematically quantify ‘friction fatigue’ during installation and set-up over pauses of several months and

(iii) assess axial response under one-way cycling.

The successful Baltic and Kent experiments demonstrated: previously unrecognised, but very strong capacity growth (or set-up) over time; the need to avoid multiple-retests and conduct ageing and cyclic tests on “fresh” piles; and potential scale effects applying to set-up and cyclic degradation.

Publications:

Barbosa P., Geduhn, M., Jardine, R. J., Schroeder, F. C. & Horn, M. (2015). Offshore pile load tests in chalk. In: Proc. 16th Eur. Conf. Soil Mech. Geotech. Eng., Edinburgh, Scotland, pp. 2885-2890

Buckley R. M., Jardine, R. J., Kontoe, S., Parker, D. & Schroeder, F. C. (2017). Ageing and cyclic loading behaviour of piles driven in a low to medium density chalk. Géotechnique, available online ahead of print: https://doi.org/10.1680/jgeot.17.P.012.

Buckley R. M., Jardine, R. J., Kontoe, S. & Lehane, B. M. (2017). Effective stress regime around a jacked steel pile during installation ageing and load testing in chalk. Can. Geotech. J. under review.

 Buckley R. M., Jardine, R. J., Kontoe, S., Liu, T., Ushev, E., Lehane, B., Pine, T., Schroeder, F. C. & Barbosa, P. (2017) Field investigations into the axial loading response of displacement piles in chalk. In: Proc. 8th Int. Conf. on Offshore Site Investigation and Geotechnics, London, UK

Buckley R. M., Kontoe, S., Jardine, R. J., Maron, M., Schroeder, F. C. & Barbosa, P. (2017). Common pitfalls of pile driving resistance analysis - A case study of the Wikinger offshore windfarm. In: Proc. 8th Int. Conf. on Offshore Site Investigation and Geotechnics, London, UK