Kim Branner

Application and Analysis of Sandwich Elements in the Primary Structure of Large Wind Turbine Blades

The present work studies the advantages of applying a sandwich construction as opposed to traditional single skin composites in the flanges of a load carrying spar in a future 180 m wind turbine rotor. A parametric finite element model is used to analyze two basic designs with single skin and sandwich flanges, respectively. Buckling is by far the governing criterion for the single skin design. Introducing a sandwich construction results in a globally more flexible structure making tower clearance the critical criterion. Significant weight reduction up to 22.3% and increased buckling capacity is obtained. Moreover, the study showed that proper choice of core material is important to prevent face wrinkling. Geometric nonlinear analysis showed sensitivity to imperfections. A consistent submodeling technique is presented for verifying the response from the global model in any section of interest.

Damage tolerance and structural monitoring for wind turbine blades.

The paper proposes a methodology for reliable design and maintenance of wind turbine rotor blades using a condition monitoring approach and a damage tolerance index coupling the material and structure. By improving the understanding of material properties that control damage propagation it will be possible to combine damage tolerant structural design, monitoring systems, inspection techniques and modelling to manage the life cycle of the structures. This will allow an efficient operation of the wind turbine in terms of load alleviation, limited maintenance and repair leading to a more effective exploitation of offshore wind.

Updating Finite Element Model of a Wind Turbine Blade Section Using Experimental Modal Analysis Results

This paper presents selected results and aspects of the multidisciplinary and interdisciplinary research oriented for the experimental and numerical study of the structural dynamics of a bend-twist coupled full scale section of a wind turbine blade structure. The main goal of the conducted research is to validate finite element model of the modified wind turbine blade section mounted in the flexible support structure accordingly to the experimental results. Bend-twist coupling was implemented by adding angled unidirectional layers on the suction and pressure side of the blade. Dynamic test and simulations were performed on a section of a full scale wind turbine blade provided by Vestas Wind Systems A/S. The numerical results are compared to the experimental measurements and the discrepancies are assessed by natural frequency difference and modal assurance criterion. Based on sensitivity analysis, set of model parameters was selected for the model updating process. Design of experiment and response surface method was implemented to find values of model parameters yielding results closest to the experimental. The updated finite element model is producing results more consistent with the measurement outcomes.

Load calculation methods for offshore wind turbine foundations

Calculation of design loads for offshore wind turbine (OWT) foundations is typically performed in a joint effort between wind turbine manufactures and foundation designers (FDs). Ideally, both parties would apply the same fully integrated design tool and model for that purpose. However, such solutions are rather limited as it would require exchanging confidential data and the need of sophisticated modelling capabilities for all subsystems of the OWT. In practice, this leads to an iterative and sequential load calculation process involving different design tools. In this process, the wind turbine manufacturer provides the FD with dynamic responses obtained from aeroelastic simulations at a predefined interface. These responses are subsequently expanded to the corresponding dynamic responses in all structural parts of the foundation. In this article, a novel load calculation method, for the expansion to dynamic foundation responses based on an inverse dynamics algorithm, is introduced and described in detail. Furthermore, a summary of load calculation methods currently applied for the design of bottom-mounted OWTs foundations is provided and compared with the proposed method. While emphasis is given to jacket-type foundations, the methods are considered applicable for other bottom-mounted foundation types as well. All load calculation methods are applied and evaluated for an exemplarily fatigue design scenario from the perspective of an FD in order to establish more confidence in these methods. The article concludes with an assessment and recommendation for all presented load calculation methods.

Materials for Wind Turbine Blades: An Overview

A short overview of composite materials for wind turbine applications is presented here. Requirements toward the wind turbine materials, loads, as well as available materials are reviewed. Apart from the traditional composites for wind turbine blades (glass fibers/epoxy matrix composites), natural composites, hybrid and nanoengineered composites are discussed. Manufacturing technologies for wind turbine composites, as well their testing and modelling approaches are reviewed.

Uncertainty modelling and code calibration for composite materials

Uncertainties related to the material properties of a composite material can be determined from the micro-, meso- or macro-scales. These three starting points for a stochastic modelling of the material properties are investigated. The uncertainties are divided into physical, model, statistical and measurement uncertainties which are introduced on the different scales. Typically, these uncertainties are taken into account in the design process using characteristic values and partial safety factors specified in a design standard. The value of the partial safety factors should reflect a reasonable balance between risk of failure and cost of the structure. Consideration related to calibration of partial safety factors for composite material is described, including the probability of failure, format for the partial safety factor method and weight factors for different load cases. In a numerical example, it is demonstrated how probabilistic models for the material properties formulated on micro-scale can be calibrated using tests on the meso- and macro-scales. The results are compared to probabilistic models estimated directly from tests on the macro-scale. In another example, partial safety factors for application to wind turbine blades are calibrated for two typical lay-ups using a large number of load cases and ratios between the aerodynamic forces and the inertia forces.

Condensation of long-term wave climates for the fatigue design of hydrodynamically sensitive offshore wind turbine support structures

Cost-efficient and reliable fatigue designs of offshore wind turbine support structures require an adequate representation of the site-specific wind–wave joint distribution. Establishment of this wind–wave joint distribution for design load calculation purposes requires typically a correlation of the marginal wind and wave distribution. This is achieved by condensation of the site-specific wave climate in terms of wave period or wave height lumping, subsequently used as input for a correlation with the corresponding wind climate. The quality of this resulting wind–wave correlation is especially important for hydrodynamically sensitive structures since the applied met-ocean parameters have a non-linear influence on calculated fatigue design loads. The present article introduces a new wave lumping method for condensation of the wave climate. The novelty is predominantly based on refined equivalence criterions for fatigue loads aiming at preservation of the fatigue damage distribution over either the wave height or wave period distribution. This new method is assessed in comparison with different other traditional wave lumping methods on the basis of the site-specific wave climate for the offshore wind farm project Gemini which has kindly been made available by the developer Typhoon Offshore. It is shown that the new method allows for a significantly better preservation of the hydrodynamic fatigue in comparison to the traditional methods.

Blade Materials, Testing MethodsAnd Structural Design

A major trend in wind energy is the development of larger wind turbines for offshore wind farms. Since access to offshore wind turbines is diffi cult and costly, it is of great importance that they operate safely and reliable. The wind turbine rotor blades, which are the largest rotating component of a wind turbine, are designed for an expected lifetime of 20 years. During this period of time, the blades will be subjected to varying loads. Large wind turbine blades are made of composite materials and can develop a number of interacting failure modes. High structural reliability can be achieved by designing the blades against the development of these failure modes. This chapter provides an overview of experimental and modeling tools for the design of wind turbine blades, with particular emphasis on evolution and interaction of various failure modes. This involves knowledge of materials, testing methods and structural design.

Damage Detection in Wind Turbine Blade Panels Using Three Different SHM Techniques.

A comparison of three different damage detection methods is made on three nominally identical glass reinforced composite panels, similar to the load carrying laminate in a wind turbine blade. Sensor data were recorded in the healthy state and after the introduction of damage by means of a four–point bending quasi–static test. Acceleration sensors, PZT transducers and the piezoelectric excited Lamb waves were used for the measurements of the panels. All three methods are based on the comparison of the healthy and damaged structure. The first method is statistical covariance–driven damage detection using a subspace–based algorithm, where one damage indicator for all three panels was computed. The second method is based on PZT transducers and the A0 mode of Lamb waves propagating in the panel, making use of the reflection of the signal at damage in the panel. The third method is based on the estimation of modal parameters of the intact and damaged panel using pLSCF and following their deviations. The results from these three damage detection methods are compared and discussed.

Reliability Assessment of Fatigue Critical Welded Details in Wind Turbine Jacket Support Structures

This paper describes a probabilistic approach to reliability assessment of fatigue critical welded details in jacket support structures for offshore wind turbines. The analysis of the jacket response to the operational loads is performed using Finite Element Method (FEM) simulations in SIMULIA Abaqus. Fatigue stress cycles are computed on the jacket members by applying tower top loads from an aeroelastic simulation with superimposed marine loads and in accordance to the IEC-61400-3 guidelines for operational conditions. The combined effect of the hydrodynamic loads and the rotor loads on the jacket structure is analyzed in a de-coupled scheme, but including the structural dynamics of the support structure.The failure prediction of the welded joints, connecting the individual members of the support structure is based on SN-curves and Miners rule according to ISO 19902 and DNV-RP-C203/DNV-OS-J101. Probabilistic SN-curves and a stochastic model for Miners rule is used to estimate the reliability of selected critical welded details in the jacket structure taken into account the uncertainty in the fatigue stresses.Copyright