3D FE-Informed Laboratory Soil Testing for the Design of Offshore Wind Turbine Monopiles

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.

Large diameter laterally loaded piles in sand: Numerical evaluation of soil stress paths and relevance of laboratory soil element testing

“…The 3D FE model has been built in the Opensees software (McKenna, 1997) following a procedure already adopted and thoroughly tested by Corciulo et al (2017), Kementzetzidis et al (2019), Cheng et al (2021) and Liu et al (2021). The overall dimensions of the soil domain are 70 m × 35 m × 47 m to minimise the effect of the boundary conditions on the stress distribution in the soil adjacent to the monopile, as shown in Corciulo et al (2017) and Liu et al (2021).…”

Experimental study on the lateral cyclic response of the piles in sand

“…The second phase commences when the soil reaches the maximum possible density. During this phase, a ratcheting deformation occurs in the soil particles based on the cyclic lateral movements of the pile (see Figure 24) [16,58]. It is important to emphasize that both convective grain migration and densification take place simultaneously.…”

“…This parameter is specified as the ratio between the energy which has been dissipated during one cycle over the total elastic energy, i.e. [58] 1 4…”

Model Tests on Cyclic Responses of a Laterally Loaded Pile Considering Sand Anisotropy and Scouring

“…For the specification of anisotropic material characteristics of sand, many laboratory tests, including plain strain, direct shear, triaxial, and hollow cylinder tests, and the related constitutive models are presented [9][10][11][12]. For the issue of effects of soil anisotropy on the soil-structure interaction, there are works on the dynamic performance of piles [13,14], the static vertically loaded piles [15], which are restricted in the elastic analysis with a transversely isotropic media, and the stability of pile-stabilized slope [16], which only considers the strength anisotropy of soil for limit analysis.…”