Damage Mechanisms on Supporting Structures in Wind Turbines

Jordi Griera, winter semester 2020/21

Wind turbines can suffer different types of damage. The aim of this article is to analyse the damage mechanisms on supporting structures in wind turbines.

Introduction

Wind turbines can be divided into those depending on aerodynamic drag and those depending on aerodynamic lift in order to convert energy.

Drag based wind turbines have a very low power coefficient, around 0,16 maximum ^[1]. Lift based wind turbines use airfoils (blades) that interact with the incoming wind. The blade intercepting the incoming wind generates a drag force component in the flow direction and a lift force component perpendicular to the drag force. The blade is designed in order to be able to generate more lift force than drag, then the lift force causes the necessary driving torque.

Wind turbines can also be divided according to its axis orientation. Vertical axis wind turbines (VAWTs), also known as Darrieus turbines after the French engineer who invented them in the 1920s and horizontal axis wind turbines (HAWTs) or propeller type are the dominant wind turbine type. ^[1][2]

Finally, wind turbines can be divided into those built with the aim of generating electricity from high-speed winds or from low-speed winds. ^[3]

This article will focus on the damage mechanisms on supporting structures of horizontal axis wind turbines.

Overview of damage mechanisms in wind turbines

Wind turbines basically consist of the following parts: Foundation, tower, nacelle, and blades.

The supporting sections of a wind turbine are the foundation and the tower. The tower can be made of steel, concrete or both steel and concrete (known as hybrid tower). The type of tower chosen depends on the tower height, bladed diameter, etc.

Steel construction

Wind turbines towers are mostly tubular steel structures. Tubular steel structures are relatively light and thanks to its circular cross section the bending stiffness is the same in all directions. Furthermore, they have a good torsional stiffness, and they are relatively easy to install. ^[4]

Tubular steel towers are made from steel sheets welded together.

The Non-Destructive Test (NDT) techniques used in wind turbines tubular steel towers are Phased Array Ultrasonic Testing, Ultrasonic Pulse Echo, Time of Flight Diffraction (ToFD), visual examination, and Eddy Current.

Phased Array Ultrasonic Testing

It is an advanced application of ultrasonic testing technology, used for weld inspections, crack and flaw detections, thickness measurements and corrosion inspections. Conventional ultrasonic testing (also known as Ultrasonic Pulse Echo) uses a single transducer that sends ultrasonic waves to find defects inside the material. Phased Array probes have multiple transducers, which generate delayed pulses. It permits the variation of the beam angle, the focal point and the focal spot, which makes this technic really versatile. ^[6]

Ultrasonic Pulse Echo

Ultrasonic Pulse Echo is a NDT technique that uses ultrasonic pulse waves to find defects in materials. These waves have a frequency greater than 20.000 Hz. When an ultrasonic wave arrives to a boundary between two media, part of the energy goes through the boundary while another part is reflected. The percentage of energy reflected and transmitted depends on the acoustic impedance. Further information can be found in the following article. ^[7]

Time of Flight Diffraction (ToFD)

Time of Flight Diffraction technique is used for a variety of applications; the most common is the rapid weld testing of circumferential and axial weld seams. Based on the use of longitudinal waves, the ultrasonic sensors are placed on each side of the weld. One sensor sends the ultrasonic beam into the material, the other receives reflected and diffracted ultrasound from anomalies and geometric reflectors. Measuring the time of flight of the diffracted beams enables accurate and reliable flaw and crack detection. ^[8][9]

Eddy Current

Single frequency Eddy Current technique is the most commonly used for surface and near surface crack detection. Normally the applied current in the sensor is sinusoidal with a frequency that may change from few hundred Hz to a few MHz. The selected frequency is based on the material and depth of the defect to be detected. The best response is obtained when the sensor induces the greatest amount of eddy current density near the defect. ^[10][11]

Concrete construction

Pure concrete towers are not as common as tubular steel structures; however, they have been used in some wind power plants. Concrete towers are made of reinforced concrete. Some advantages of these types of towers are the stiffness, the robustness, and the maintenance properties. ^[4]

Max Bögl Wind AG developed a mobile fabrication for concrete construction, which allows a more flexible construction of wind farms for its international customers. “We developed a mobile fabrication concept and adapted our german manufacturing plants for a serial production of the modular concrete segments worldwide. This allows us to achieve the same high level of quality at any location in the world.”

This modular assembly and disassembly of the mobile fabrication allows the firm to participate in projects all over the world avoiding expensive long-distance and heavy-load transportation. ^[12]

There are several NDT techniques that apply to concrete constructions; the most relevant are the following ones. Normally more than one technique is used.

Ultrasonic Pulse Velocity (UPV) measurement

This technique is really useful to analyse the strength of the concrete. The velocity of the ultrasonic waves suffers a large increase after the first casting days. Theoretically there is a direct link between the wave velocity in an elastic media and the Young’s modulus. As long as there are empirical relations between modulus and strength, this technique can provide a direct measure of the strength.

The variation range of velocity decreases with the fresh concrete curation. Therefore, the sensitivity of the UPV technique decreases every day after the fresh concrete has been placed. The sensitivity of the UPV to the rate of the concrete strength is extremely high in the first days after que concrete placement but decreases after five or seven days of drying. ^[13]

UPV measurements are typically performed using a pair of transducers, normally piezoelectric transducers, in contact with the material (concrete in this case) through a coupling medium. The piezoelectric is excited by a spike shape electrical voltage signal and produces ultrasonic waves, which make it to vibrate at its resonant frequency. These vibrations excite the material and generate waves that are transmitted through the material to the receiving transducer. The UPV is computed from the time of flight between both transducers and the separation between them. ^[14]

Rebound Hammer measurement

Rebound Hammer measurements consist in a direct mechanical solicitation in the surface of a concrete structure. The rebound value is correlated with the hardness of the near-surface concrete. The Rebound Hammer is composed of a spring hammer with a defined mass and a latching mechanism. After the release the mass hits a plunger that contacts the test surface. The rebound distance of the spring hammer to the plunger or other rebound values are measured either digitally or on a graduated scale. The hardness is correlated to the rebound values. ^[13][15]

Ground Penetrating Radar (GPR)

Ground Penetrating Radar is a NDT technique used to determine voids, inclusions, etc. in the underground. Based on an electromagnetic impulse-echo method, a pulse generator emits short electromagnetics waves via a transmitter at a high repetition rate. Part of the energy of the electromagnetic waves is reflected at interfaces between layers with different permittivity. The receiver registers the reflected signals. ^[16]

Hybrid concrete and tubular steel towers

There are some cases when the turbine is really large with high hub height this hybrid tower is required. This type of tower consists of concrete sections in the lower part and tubular steel sections on the upper part. ^[4]

In hybrid towers both techniques explained in steel constructions and concrete constructions are used.

Foundation

There are different types of foundations according to the wind turbine power, geotechnical conditions of the emplacement, and the tower type.

When the soil has enough bearing capacity, all the loads from the wind turbine are transferred to the ground by spread footing, for example, a slab foundation.

When the soil has not sufficient bearing capacity, then piling is required. Those piles support the footing and transfer the loads to a more rigid ground, normally the bedrock where the piles can be anchored. ^[4]

The foundation of wind turbines is the part exposed to the most extreme environmental conditions. ^[18]

There are several different crack types that could occur in wind turbines foundation. Some can occur before the hardening of the concrete foundation, due to plastic shrinkage or constructional movement. After the hardening, cracks according to physical, chemical, thermal or structural loads can occur. ^[4]

Cracks can lead to a water leakage through the foundation and around the reinforcements. The consequences are frost damage, reinforcement corrosion and leaching concrete.

Cracks at the top part of the foundation are found by visual inspection and can be fixed with a carefully application of a resilient groove covering these cracks. But the lower parts of the foundation are not accessible. Depending on the soil properties, i.e., chloride concentration, the conditions for corrosion will be fulfilled in a major grade or not.

Crack damage in the foundation is a really common issue in wind turbines.

It should be considered that the main reasons for this type of damage are due to poor structural design, poor workmanship performance or inappropriate material selection. Although cracks cannot be avoided in this type of structures, their size can be limited with a careful design and construction.

The quality control in wind turbines foundation can be divided into:

Fresh croncrete test

Sampling the concrete and on-site testing to determine the slump, the air content, etc. Flow tests can be made during concrete placement. For compressing strength evaluation, the sampling of some concrete specimens can be made. The durability assessment is made through a Rapid Chloride Permeability Test (RCPT).

Non-Destructive evaluation

Nowadays NDT methods are increasing due to the technical improvements in hardware and software, data collection and analysis, ability to repeat the test and the speed of NDT methods.

Ultrasonic Pulse Echo (UPE) tomography, Impact Echo (IE) and Ground Penetrating Radar (GPR) are the most commonly used methods.

Ultrasonic Pulse Echo (UPE)

Ultrasonic Pulse Echo in a NDT method used to scan the sub-surface of concrete elements. It uses acoustic stress waves to analyze the properties of the sub-surface layers. It locates the defects by identifying anomalies of acoustical impedance different from the concrete acoustic impedance. ^[7]

Impact Echo (IE)

Impact Echo is also an acoustic NDT method use to analyse the concrete structural members. Developed by Sansalone and Carino in 1986 ^[19], this test is based on the generation of stress waves by a short duration mechanical impact on the concrete surface. This test permits to find the depth of the structural member, voids, objects and discontinuities. ^[19]

Ground Penetrating Radar (GPR)

Explained in 2.2.3. Ground Penetrating Radar is a NDT method that provides a reliable and cost-effective data from the sub-surface of the concrete. This tool permits to scan and image the subsurface. Based on electromagnetic waves, it can detect dialectic properties discontinuities within the concrete.

There are other methods used such as crack monitoring via fiber-optic sensors, to find big changes in cracks, tilt monitoring, involving tiltmeters to assess the foundation tilting in real time and vibration monitoring, using accelerometers to have a real time monitoring of the wind turbine.

Sources

1. Ackermann, T.: Wind Power in Power Systems. John Wiley & Sons, Ltd, England (2005). Pages 7-22.
2. Eriksson, S., Bernhoff, H., Leijon, M.: Evaluation of different turbine concepts for wind power. Renewable and Sustainable Energy Reviews 12 (2008). Pages 1419-1434.
3. Moh, M., Saad, M., Asmuin, N.: Comparison of Horizontal Axis Wind Turbines and Vertical Axis Wind Turbines. IOSR Journal of Engineering Volume 4, Issue 08 (2014). Pages 27-30.
4. Hassanzadeh, M.: Cracks in onshore wind power foundations. Causes and consequences. Elforsk rapport (2012). Pages 11-56.
5. Retrieved from http://www.steelwindtower.com/wind-turbine-tower-comparison-pros-and-cons-explained/ on 23.01.2021
6. Retrieved from https://www.mme-group.com/advanced-ndt/phased-array/ on 17.01.2021
7. Scholtz, L. Ultrasonic Pulse-Echo Method. Wiki of the Chair of Non-Destructive Testing. (2016).
8. Retrieved from https://www.olympus-ims.com/en/applications/introduction-to-time-of-flight-diffraction-for-weld-inspection/ on 17.01.2021.
9. Heymes, G. Imaging Techniques for Non-Destructive Testing. Wiki of the Chair of Non-Destructive Testing. (2017).
10. Sophian, A., Tian, G Y., Taylor, D., Rudlin, J.: Electromagnetic and eddy current NDT: a review. Insight Vol 43, No 5 (2001).
11. Knörzer, H. Fundamentals of eddy current testing. Wiki of the Chair of Non-Destructive Testing. (2017).
12. Retrieved from https://www.mbrenewables.com/en/production-and-fabrication/on 23.01.2021.
13. Breysse, D.: Non-destructive evaluation of concrete strength: An historical review and a new perspective by combining NDT methods. Construction and Building Materials 33 (2012). Pages 139-163
14. Yaman, I. O., Inci, G., Yesiller, N., Aktan, H. M.: Ultrasonic Pulse Velocity in Concrete Using Direct and Indirect Transmission. Aci Materials Journal. Title no 98-M48 (2001). Pages 450-457
15. Schick, F. Rebound hammer. Wiki of the Chair of Non-Destructive Testing. (2019).
16. Moser, T. Radar. Wiki of the Chair of Non-Destructive Testing. (2015).
17. Retrieved from https://www.altenergymag.com/content.php?post_type=1478 on 23.01.2021.
18. Ostachowicz, W., Malinowski, Pawel H., Soman, R., Wandowski, T.: Damage assessment in wind turbine technology. E3S Web of Conferences 14 (2017).
19. Margareta, A. Impact-Echo (Overview). Wiki of the Chair of Non-Destructive Testing. (2011).