Seismic vulnerability of offshore wind turbines to pulse and non‐pulse records

Abstract

The increasing number of wind turbines in active tectonic regions has attracted scientific interest to evaluate the seismic vulnerability of offshore wind turbines (OWTs). This study aims at assessing the deformation and collapse susceptibility of 2MW and 5MW OWTs subjected to shallow-crustal pulse-like ground motions, which has not been particularly addressed to date. A cloudbased fragility assessment is performed to quantify the seismic response for a given intensity measure and to assess the failure probabilities for pulse-like and non-pulse-like ground motions. The first-mode spectral acceleration S a (T 1 ) is found to be an efficient response predictor for OWTs, exhibiting prominent higher-mode behavior, at the serviceability and ultimate conditions. Regardless of earthquake type, it is shown that records with strong vertical components may induce nonlinearity in the supporting tower, leading to potential failure by buckling in three different patterns: (i) at tower base near platform level, (ii) close to tower top, and (iii) between the upper half of the main tower and its top. Type and extent of the damage are related to the coupled excitation of vertical and lateral higher modes, for which tower top acceleration response spectra S a,i(Top) is an effective identifier. It is also observed that tower's slenderness ratio (l/d), the diameter-to-thickness ratio (d/t), and the rotor-nacelle-assembly mass (m RNA ) are precursors for evaluating the damage mode and vulnerability of OWTs under both pulse-like and nonpulse-like ground motion records.

Seismic Analysis of 10-MW Off Shore Wind Turbine with Large-Diameter Monopile in Consideration of Seabed Liquefaction

“…However, in the nonlinear response history analysis (RHA), such a direct superposition of the damping ratios would overestimate the total damping of the OWT system. As suggested by Ali et al (2019) and De Risi et al…”

A Simplified Blade Model for Reliable Seismic Assessments of Wind Turbines

“…The FEM of the 61.5 m fixed root blade is modelled using elastic beam-column elements in OpenSees software [23]. This element type requires simpler material and cross-sectional definition, and it is often used for nonlinear seismic assessment of wind turbine towers due to computational efficiency [7,10]. The airfoil area (A) and moments of inertia (Ix, Iy, Iz) are used to define the cross-sectional properties, whereas density (ρ), Young's modulus (E) and shear modulus (G) are employed for material definition.…”

“…Rigid links are used for connecting blades-to-hub-to nacelle and tower with the RNA. More details on the approach used for structural modelling of the wind turbine can be found in [7,10,25]. The blade properties are used following DS-1, presented in Table 3.…”

“…However, the local blades details, such as airfoil shape, twist angle, and material distribution, are not only challenging to find but are also numerically complicated and computationally expensive. As a result, it is a common practice to either assume these details [5][6][7]10] and/or use the lumped mass approach to represent the RNA in finite element models (FEM) of wind turbines [1,11].…”

“…Several studies have reported the design and optimisation of composite blades using complex FE shell or solid element models to evaluate their strength and modal response [12][13][14][15]. Such high-fidelity FE models are difficult to implement with reasonable computational efficiency in earthquake engineering problems, particularly when a large number of numerical analyses of the whole rotor-towerfoundation-system is needed under a representative set of earthquake records [1,7,10,11].…”