On the Assessment of Equivalent Hysteretic Damping on Offshore Wind Turbines Subjected to Seismic Excitations

Abstract

The increasing global demand of renewable and clean energy has led to the exponential growth, development, and interest in the offshore wind energy industry and expansion towards earthquake-prone areas. Offshore wind turbine structures, typically supported by a tubular monopile foundation, are increasing in size to meet the increasing human demands. In the constant ongoing debate in search of a balance between design accuracy and efficiency, general consensus is yet to be found in search of accurate yet simplified representation of soil-pile interaction. A Winkler foundation principle has shown to be a good compromise regarding this discussion but current knowledge is mainly based on (pseudo-static) small soil-strain inducing wind- and wave load-cases. Adopting this principle, the soil-structure interaction mechanism is represented by local lateral soil reactions (springs with distributed stiffness in mechanical formulation). Strong ground shaking induces shearing and volumetric variation of the soil particles. Through hysteresis the soil material exhibits energy dissipation: hysteretic damping. Accounting for hysteresis is considered computationally more demanding than when the soil continuum is assumed elastic but this is an assumption which only holds when soils undergo very small soil strains.
This thesis explores the amount of energy dissipated as a result of hysteresis during seismic response of an offshore wind turbine and the applicability of such damping in a local linear visco-elastic manner. In order to obtain insight in this nonlinear energy-dissipation mechanism associated with the hysteretic offshore wind turbine model under seismic excitation, a Python code was developed that calculates the energy dissipation of each load-cycle separately. The developed energy dissipation assessment algorithm is effective in application of arbitrary hysteretic response and unloading-reloading rules.
The hysteretic nature of the soil-pile interaction springs in question are calibrated against the widely applied API p-y, force-displacement curves. Unloading-reloading rules are specified to define the load-cycles. Boulanger et al. describes such unloading-reloading rules for pile application under seismic loading. The applicability of these backbone curves and unloading-reloading rules remains questionable in application of rigid monopile foundations. Despite not representing the accuracy of true soil-monopile interaction, obtained results in this research may support the exploration of innovative unloading-reloading rules.
The developed energy dissipation algorithm is proven to be a powerful tool in identifying the amount of energy dissipation over a total timeseries. Reasonable agreement in peak (maximum observed), Ultimate Limit State, deflection and bending moment seismic response at mudline and tower top has been found between a hysteretic supported model and equivalent elastic models using a single (load-dependent and depth-dependent) equivalent damping coefficient in parallel with each soil spring. Representing the hysteretic energy dissipation mechanism using viscous dampers with constant damping coefficients has therefore proven to be an effective modelling strategy to account for the damping mechanism of plastic unloading-reloading rules without accounting for hysteresis. The effectiveness of an equivalent elastic modelling strategy reduces when the response undergoes substantial permanent plastified displacements. A typical property which is unable to be simulated under the application of an elastic modelling strategy.