Abstract
Based on advanced 3D finite element modelling, this paper analyses the stress paths experienced by soil elements in the vicinity of a monopile foundation for offshore wind turbines subjected to cyclic loading with the aim of informing soil laboratory testing in support of monopile foundation design. It is shown that the soil elements in front of the laterally loaded monopile are subjected to complex stress variations, which gradually evolve towards steady stress cycles as the cyclic lateral pile loading proceeds. The amplitude, direction and average value of such steady stress cycles are dependent on the depth and radial distance from the pile of the soil element, but it also invariably involves the cyclic rotation of principal stress axes. Complementary laboratory testing using the hollow-cylinder torsional apparatus was carried out on granular soil samples imposing cyclic stress paths (with up to about 3 × 104 cycles) which resemble those determined after 3D finite element analysis. The importance of considering the cyclic rotation of principal stress axes when investigating the response of soil elements under stress conditions mimicking those around a monopile foundation subjected to cyclic lateral loading is emphasised.
Highlights
Renewable offshore wind energy has been playing a crucial role in meeting the increasing demand for green energy and reducing the global carbon footprint
This paper shows how an advanced 3D finite element model can be used to investigate the stress paths induced by cyclic storm loading in different soil elements in front of a laterally loaded pile
It is shown that advanced laboratory tests, able to impose the cyclic rotation of principal stress axes, can provide a more representative characterisation of the mechanical response for elements located in front of cyclic laterally loaded monopile foundations
Summary
Renewable offshore wind energy has been playing a crucial role in meeting the increasing demand for green energy and reducing the global carbon footprint. Procedures to derive accurate soil reaction curves These can be implemented in a 1D finite element idealised monopile system, which becomes cost- and time-effective when analysing a large number of loading and limit state scenarios for individual wind turbines. The current laboratory practice, to determine and calibrate constitutive soil parameters for the design of offshore wind monopile foundations, typically relies on cyclic triaxial and simple shear tests. The performed 3D finite element model employs the latest developments in cyclic soil constitutive modelling proposed by Liu et al [12], which can accurately predict the expected soil response under a large number of loading cycles using the concept of hardening memory surface [13]. It is shown that advanced laboratory tests, able to impose the cyclic rotation of principal stress axes, can provide a more representative characterisation of the mechanical response for elements located in front of cyclic laterally loaded monopile foundations
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