Axial response of offshore jacket piles supporting wind turbines

This thesis addresses the design methods for offshore piles, and specifically the role of the end bearing of open-ended piles in contributing to capacity. In the design of offshore jacket structures, the response of open-ended tubular piles is governed by their interaction with the soil along the shaft and at the base. To estimate the total static capacity, the shaft and base capacities are estimated separately and added together. These estimates are based on empirical relationships correlated against the results of soil and pile tests. Many design methods exist for open-ended piles. However, for this research the design methods investigated are the API (2014), ICP (2005) and UWA (2005) methods. The target of these methods, is to estimate the load-carrying capacity of the designed pile, after an axial pile head displacement of 0.1D. Over the last few years, much research has focused on more accurately capturing this value, and much less on the distribution of the axial load between shaft and base. It has been found that, in general, the design methods investigated, do estimate the total capacity well. However, the capacity distribution can be improved. Recent pile tests performed using double walled piles in both sands and clays have revealed more about the behaviour of the soil plug, which contributes to the base capacity in open-ended piles, and are analysed here to gain a better understanding of the overall distribution. The role of the plug, in open-ended piles, is of significant concern for designers in determining its contribution to capacity. Simplifying assumptions are often made that the pile is either fully plugged or unplugged, and these assumptions directly impact foundation stiffness and capacity. In this research project, each of the design methods investigated employ a different criterion for plugging and therefore result in different foundation solutions.
To study this problem, a one-dimensional, finite element analysis procedure has been written in MATLAB, which allows a detailed examination of the load distribution between the pile shaft and the base. This procedure can model the behaviour of open-ended piles, specifically isolating the internal and external shaft frictions, and the end bearing on the annulus. Using our existing knowledge of the components and their interactions, we can adopt the design parameters from the design methods to deduce their finite element variants and associated capacities. In addition, by examining the factors contributing to capacity in more detail, recommendations can be made to improve the finite element procedure leading to the development of modified finite element variants of the design methods for clays, sands and layered soils. Despite these scientific variations and modifications, the variability of soil, and considering the low number of reliable open-ended pile tests in compression, which accurately distribute the applied load between shaft and base, it has been difficult to derive robust conclusions. The results have found however, that while no significant improvements were observed when the database comparisons of total capacity were analysed, the modified finite element variants of the design methods did produce pile head responses that achieved acceptable capacities based on an improved model of the soil plug.

Keywords: Jacket piled foundations
Open-ended piles
soil plug load transfer diagram
Soil plug
API, ICP, UWA
Advisor: Houlsby, G
Burd, H
Taylor, P