At present, the monopile is the leading foundation of choice in the offshore wind industry, with over four-�fifths of all offshore wind turbines founded on this solution. In a drive towards greater efficiency and power production from the harnessing of more reliable, stronger winds, larger diameter rotor and blade systems are being deployed in deeper waters further from the shore. With this, the support foundation is subject to greater magnitude cyclic lateral loads and overturning moments at the seabed which need to be managed. Current design for laterally loaded monopiles follows the philosophy adopted in the offshore oil and gas industry, of which the piles are slender and present �significantly different failure mechanisms to the now larger diameter monopile counterparts. As a result, current design methodology is no longer considered to be suitable.
This thesis presents the in-depth analysis of the results from a series of physical model experiments performed in the geotechnical centrifuge at The University of Sheffield. The behaviour of the monopile subject to both monotonic ultimate limit and long-term cyclic serviceability limit loading scenarios is explored, with the aim to build upon the growing knowledge of monopile performance. Comprehensive instrumentation is located on the extremity of the model pile allowing for the capture of interesting local soil-pile interaction behaviour, as well as the traditional global monopile characteristics.
The test programme is divided into two parts. Firstly, a suite of monotonic pushover tests are performed, which reveal a progressive change in pile failure mechanism with increasing load magnitude. At low load, pile deformation and soil mobilisation are restricted to the region close to the mudline, which is synonymous of a flexible type failure. As the applied load increases, a transition towards a rigid, rotational failure is observed, with the onset of a `toe-kick' and associated additional base soil mobilisation, moment and shear resistance. In an attempt to address the shortcomings of current design methodologies, an analytical multi-spring framework model is developed to incorporate the additional soil resistance mechanisms and is validated against experimental results.
Following from this, a series of cyclic lateral loading experiments are performed which explore a range of loading scenarios, including long-term low magnitude,
short-term high magnitude, and several different varying magnitude load packages. Observations show contrasting accumulation trends for high and low magnitude cyclic loading, with a power- and logarithmic-law model respectively providing the best prediction of the accumulation behaviour. Local soil-pile interaction behaviour reveals a transition of representative failure behaviour from a flexible to rigid type, which corresponds to the observed change in the selected accumulation trend model. Locked-in stresses within the soil are seen to progressively develop on unload with the application of cycles and the a distribution of these is dependant on the magnitude of the applied load.
With the application of more realistic cyclic load packages of varying magnitude, interesting interaction behaviour between the successive load packages, both at a global and local level, is seen to take place. The nature of this interaction is dependant on the magnitude and cycle count of the previous load history. A newly proposed cyclic rotation accumulation contour design model is able to predict the progression of the permanent rotation at the mudline, a critical design parameter, with the applied cyclic varying load packages.
