No abstract

This paper describes a one-dimensional (1D) computational model for the analysis and design of laterally loaded monopile foundations for offshore wind turbine applications. The model represents the monopile as an embedded beam and specially formulated functions, referred to as soil reaction curves, are employed to represent the various components of soil reaction that are assumed to act on the pile. This design model was an outcome of a recently completed joint industry research project – known as PISA – on the development of new procedures for the design of monopile foundations for offshore wind applications. The overall framework of the model, and an application to a stiff glacial clay till soil, is described in a companion paper by Byrne and co-workers; the current paper describes an alternative formulation that has been developed for soil reaction curves that are applicable to monopiles installed at offshore homogeneous sand sites, for drained loading. The 1D model is calibrated using data from a set of three-dimensional finite-element analyses, conducted over a calibration space comprising pile geometries, loading configurations and soil relative densities that span typical design values. The performance of the model is demonstrated by the analysis of example design cases. The current form of the model is applicable to homogeneous soil and monotonic loading, although extensions to soil layering and cyclic loading are possible.

This paper presents experimental work aimed at improving understanding of the behaviour of rigid monopiles, in cohesionless soils, subjected to lateral cyclic loading. It involves 1g laboratory model tests, scaled to represent monopile foundations for offshore wind turbines. The test programme is designed to identify the key mechanisms governing pile response, and is divided into four main parts: (a) investigation of loading rate effects; (b) hysteretic behaviour during unloading and reloading; (c) pile response due to long-term single-amplitude cyclic loading; and (d) multi-amplitude cyclic loads. The results show that the pile response conforms closely to the extended Masing rules, with additional permanent deformation accumulated during non-symmetric cyclic loads. This ratcheting behaviour is characterised by two features: first, the ratcheting rate decreases with cycle number and depends on the cyclic load magnitude, and second, the shape of the hysteresis loop tightens progressively, involving increased secant stiffness and decreased loop area. Test results involving multi-amplitude load scenarios demonstrate that the response of the pile to complex load scenarios can be analysed and understood using the conclusions from single-amplitude cyclic loading. Such test results should be sufficient for deriving the principles of new modelling approaches.