Dynamic and monotonic response of Monopile Foundations for Offshore wind turbines using centrifuge testing

Abstract

The seismic response of monopile foundations is a growing area of research as the offshore wind industry expands worldwide, including in earthquake prone regions of the world. This paper presents dynamic centrifuge tests aimed at investigating the dynamic response of monopiles in both dry and saturated sandy soils. The latter case includes soil liquefaction under strong input motions, with measured excess pore pressures indicating liquefaction. The natural frequency of the monopile-soil system is experimentally determined by measuring the response to a sine sweep motion. Strong earthquakes are then applied at this frequency and its harmonics. This paper discusses the response of the monopile in terms of the peak accelerations observed in the dry and saturated tests, as well as using response spectra and amplification ratios. The dynamic bending moments along the pile are also measured to infer the bending moment profile with depth. Finally, two identical monopiles are pushed-over in each of the centrifuge tests to establish the pre and post-earthquake monotonic response, including the lateral stiffness and capacity, which are compared for the dry model tests and the saturated case.

Seismic excitation of offshore wind turbines and transition piece response

“…44 Given the complexity of the grouted connection model and the associated high computational cost, the selection of a limited number of relevant earthquake motions was essential. As such, two earthquake records were selected for this study, enabling direct comparison with published work on offshore turbines under seismic loads 17,18,45 : (i) Kobe, Japan, Nishi-Akashi, January 17, 1995, and (ii) Chi-Chi Taiwan-03, CHY082, September 20, 1999. These were also selected because of their significantly contrasting characteristics, enabling further insight into the dynamic behaviour of the structure under varying seismic loads:…”

“…Further research into the effect of excess pore water pressure and liquefaction will be needed to extend this research in the future. 43,45 The analytical model introduced by Lotsberg et al 50 to compute the vertical displacement of the transition piece relative to the monopile under bending (Equations 4 to 6) was compared with the displacements obtained using the numerical model. The application of the model to the experimental tests performed by Wilke 19 neglected axial load, and therefore Equation 6 was modified to account for self-weight:…”

A simplified procedure for the prediction of liquefaction‐induced settlement of offshore wind turbines supported by suction caisson foundation based on effective stress analyses and an ML‐based group method of data handling

“…Most of the previous research has been exclusively related to either the inertial interaction of ground founded OWTs or the kinematic seismic response of foundations 2,21,22 ; previous studies have mostly focused on how the eccentric the rotor‐nacelle‐assembly (RNA) is to the tower top and how the rotary inertia due to blades can influence the structural response of the wind turbines, or they have focused on the forces induced on the piles connected to rigid pile caps following ground motion. A few studies have considered the soil‐structure interaction problems of grounded OWT systems in liquefiable soils, but many issues are still uncertain 23,24 ; Haddad et al 25–30 …”

“…3 of the wind turbines, or they have focused on the forces induced on the piles connected to rigid pile caps following ground motion. A few studies have considered the soil-structure interaction problems of grounded OWT systems in liquefiable soils, but many issues are still uncertain 23,24 ; Haddad et al [25][26][27][28][29][30] In this study, a comprehensive series of nonlinear dynamic finite element (FE) effective stress analyses modeling the skirted circular foundations supporting OWTs with varied embedment ratios subjected to 20 outcropping rock motions were carried out.…”