Cross-wind modal properties of offshore wind turbines identified by full scale testing

Damgaard, Mads; Ibsen, Lars Bo; Andersen, Lars Vabbersgaard; Andersen, Jacob

doi:10.1016/j.jweia.2013.03.003

Cited by 142 publications

(94 citation statements)

References 16 publications

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“…Kallehave et al (2015) observed that the measured natural frequency of monopile-based OWTs decreased with increasing wind speeds and related it to increasing displacement levels. The same conclusion was reached by Damgaard et al (2013) when analysing the reduction in natural frequency with increasing acceleration levels.…”

Section: Observed Foundation Behavioursupporting

confidence: 73%

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Effect of foundation modelling on the fatigue lifetime of a monopile-based offshore wind turbine

et al. 2017

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Abstract. Several studies have emphasized the importance of modelling foundation response with representative damping and stiffness characteristics in integrated analyses of offshore wind turbines (OWTs). For the monopile foundation, the industry standard for pile analysis has shown to be inaccurate, and alternative models that simulate foundation behaviour more accurately are needed. As fatigue damage is a critical factor in the design phase, this study investigates how four different soil-foundation models affect the fatigue damage of an OWT with a monopile foundation. This study shows how both stiffness and damping properties have a noticeable effect on the fatigue damage, in particular for idling cases. At mud-line, accumulated fatigue damage varied up to 22 % depending on the foundation model used.

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Section: Observed Foundation Behavioursupporting

confidence: 73%

“…For OWTs in operation, several authors (Damgaard et al, 2013;Shirzadeh et al, 2013;Tarp-Johansen et al, 2009;Versteijlen et al, 2011) found foundation damping between 0.25 and 1.5 % of critical damping depending on load level and soil profile.…”

Section: Observed Foundation Behaviourmentioning

confidence: 99%

Effect of foundation modelling on the fatigue lifetime of a monopile-based offshore wind turbine

et al. 2017

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“…Geometric dissipation is negligible for frequencies less than 1 Hz [6,8,11], and the majority of wind and wave loads have frequencies below 1 Hz (e.g. Refs.…”

Section: Introductionmentioning

confidence: 99%

“…This type of soil damping should be more specifically labeled OWT monopile foundation damping (or generally referred to in this paper as "OWT foundation damping") due to the specific formulation and mechanism of hysteretic material soil damping within the OWT soil-structure foundation system. Some researchers [3,6,11,14] have examined the signals from instrumented OWTs during emergency shutdown (sometimes referred to as a "rotor-stop test"), ambient excitation, and overspeed stops [7] to estimate OWT natural frequency and damping. Subsequently, OWT foundation damping values from 0.25 to 1.5% have been estimated from the residual damping after aerodynamic, hydrodynamic, structural, and nacelle tuned mass damping have been accounted for in numerical modeling.…”

Section: Introductionmentioning

confidence: 99%

Foundation damping and the dynamics of offshore wind turbine monopiles

et al. 2015

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a b s t r a c tThe contribution of foundation damping to offshore wind turbines (OWTs) is not well known, though researchers have back-calculated foundation damping from "rotor-stop" tests after estimating aerodynamic, hydrodynamic, and structural damping with numerical models. Because design guidelines do not currently recommend methods for determining foundation damping, it is typically neglected. This paper investigates the significance of foundation damping on monopile-supported OWTs subjected to extreme storm loading using a linear elastic two-dimensional finite element model. The effect of foundation damping primarily on the first natural frequency of the OWT was considered as OWT behavior is dominated by the first mode under storm loading. A simplified foundation model based on the soil-pile mudline stiffness matrix was used to represent the monopile, hydrodynamic effects were modeled via added hydrodynamic mass, and 1.00% Rayleigh structural damping was assumed. Hysteretic energy loss in the foundation was converted into a viscous, rotational dashpot at the mudline to represent foundation damping. Using the logarithmic decrement method on a finite element free vibration time history, 0.17%-0.28% of critical damping was attributed to foundation damping. Stochastic time history analysis of extreme storm conditions indicated that mudline OWT foundation damping decreases the maximum and standard deviation of mudline moment by 7e9%.

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“…These operational modal analysis methods, and also the wind turbines they are applied on, have developed ever since and more experience is gained at each step [8]. In the last couple of years, an interest has been developed into obtaining estimates for the structural parameters of offshore wind turbines using large sets of measurement data obtained from actual installed OWTs [4,5,12].…”

Section: Introductionmentioning

confidence: 99%

Identifying Structural Parameters of an Idling Offshore Wind Turbine Using Operational Modal Analysis

Ogno

2014

Dynamics of Civil Structures, Volume 4

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The design of modern day offshore wind turbines (OWTs) relies on numerical models, which are used for simulating the dynamic behavior in different operational and environmental conditions. From these results one can estimate ultimate and fatigue loads, which are needed for determining the design life of the turbines. The dynamic behavior, and thus the lifetime, of the turbines are influenced for a large part by its structural properties, such as the natural frequencies and damping ratios. Hence, it is important to obtain accurate estimates of these modal properties. For this purpose Operational Modal Analysis (OMA) techniques are used to estimate the modal domain of the OWT. As, for instance, the loads in the structure and the damping ratios are inversely related, higher damping values will results in lower loads, and hence in more optimized and less costly support structures. In this paper the data-driven Stochastic Subspace Identification (SSI) method is used to evaluate the modal domain of the OWT by using output-only measurements obtained from an installed 3.6 MW offshore wind turbine. In order to better satisfy the OMA assumptions of having a Linear Time-Invariant (LTI) system and white noise uncorrelated input, the analyses are performed in case of idling turbines, thereby avoiding the effect of rotational harmonic components, changing system properties due to yawing and pitching actions, as well as strong aerodynamic nonlinearities. In this paper we focuss on the first four global eigenfrequencies that were found and the associated damping ratios. Even though a sensor mix of several strain gauges and a single bi-directional accelerometer are available, the best results were obtained by only using the accelerometer on the nacelle. Keywords Offshore wind turbines • Vibrations • Operational modal analysis • Stochastic subspace identification • Modal damping estimation IntroductionIn order to achieve the goal of 20% renewable energy in 2020, as set by the European Union, large scale developments of different types of renewable energy are in full progress throughout Europe. One of the more promising ways of generating renewable electricity on a large scale is provided by wind energy. However, as many of the favorable onshore locations are usually also densely populated and therefore not suited for large wind farms, much of these farms will have to be built offshore. Unfortunately this technology is still more expensive than the conventional power generation methods, therefore continuous design optimization and technical innovations are required to lower the Levelised Cost of Electricity (LCOE).Currently the design of modern wind turbines relies on aero-servo-elastic simulation tools that are used for simulating the dynamic behavior of the OWT in different operational and environmental conditions. In these models the structure is typically modeled as a flexible (nonlinear) multi-body model, that is coupled with a steady-state aerodynamic model and a controller model. The design of the support structure o...

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Cross-wind modal properties of offshore wind turbines identified by full scale testing

Cited by 142 publications

References 16 publications

Effect of foundation modelling on the fatigue lifetime of a monopile-based offshore wind turbine

Effect of foundation modelling on the fatigue lifetime of a monopile-based offshore wind turbine

Foundation damping and the dynamics of offshore wind turbine monopiles

Identifying Structural Parameters of an Idling Offshore Wind Turbine Using Operational Modal Analysis

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