How to maintain the system while avoiding costly shutdowns and repairs
Wind energy is a major contributor of power in the renewable energies market. Wind energy production around the U.S. has been growing at a rate of 20 to 30 percent per year - one of the fastest growing market segments in the power industry. To produce more power, rotor blades are being manufactured in longer lengths with blade arcs reaching up to 262 feet (80 meters). Rotor blades will continue to increase in size as the demand for more cost efficient renewable energy grows. With the growing demand and larger components, it's even more important to maintain the in-service condition of the system to avoid costly shutdowns and repairs.
Periodic inspections of these major components are performed to help prevent failures (which lead to costly shutdowns and repairs). However, these inspections do not always identify the conditions leading up to a failure. Inspection techniques and equipment to identify the failure are available, but owners and operators need to know what is required to select the proper technique.
The manufacturing process, resin addition, and the bonding process are variable fabrication processes that sometimes lead to defects such as delamination between layers of composite materials, wrinkles or waviness between layers, and lack of bond or debond of the composite layers.
Non-destructive testing (NDT) techniques for the detection of manufacturing flaws are usually conducted by the manufacturer prior to delivery. The methods employed include basic visual inspection of each component to locate obvious defects and confirm compliance with design specs. Penetrant inspections are performed to validate joint integrity on any welded joints, and to ensure the component is free of cracks not visible during basic visual inspections. More advanced techniques are also performed and include ultrasonic inspection and infrared thermograph to inspect the full volume of each component. An ultrasonic test can be carried out to investigate if any damage is present sub-surface on thicker sections of the blades. Ultrasonic inspections reveal these flaws quickly, reliably and effectively and are the most often used NDT method for composites. The adhesive joints are critical in the blade structure. Infrared scanners are used to examine the blade throughout its length, measuring exactly the same points each time. The scanner is able to see through the laminate and check the adhesive joint and records temperature differences in the adhesive during flexing, possibly identifying flaws. If flaws are found, they can almost always be repaired immediately.
The three wind turbine components that have the highest incidence of failure are the gearbox, generators, and rotor blades. These components also have the highest cost for replacement leading to extended and unplanned shutdowns. Some of the challenges related to the inspection and maintenance of wind turbines in the in-service condition are accessibility to large rotor blades, blade materials, and complex surface areas. Potential defects in these three areas are:
The driving mechanism of a wind turbine is subject to much greater dynamic load due to the irregular force of the wind. Virtually all components of a wind turbine are subject to damage and require regular maintenance and NDT to minimize failure. Inspection techniques range from advanced methods including ultrasonic inspection, phased array, acoustic emission and drone inspections to normal maintenance such as lubricating the moving parts, checking the oil level in the gearbox and testing the electrical system related to the generator. In most cases a particular NDT technique can be utilized on multiple components of a wind turbine depending on the area of interest. Two general areas of interest for NDT would be the surface condition or the volumetric condition of a component.
Ultrasonic Inspections – Ultrasonic inspections can be performed in the conventional shear or longitudinal mode to check the integrity of the composite materials at specific locations. Most components can be inspected with ultrasonics, which is primarily used for part thickness measurements, bolting inspections and weld inspections using shear waves. The main difficulty in ultrasonic measurement is the attenuation in the specific resin/fiber combination used to make the blade. It is preferred to take measurements in microseconds and convert data to thickness once a suitable standard has been made.
Phased Array Inspections – Phased array is an ultrasonic technique utilizing multiple elements to cover more area with a single scan. Phased array can be used in the same applications as conventional Ultrasonic Testing (UT) but is desired for its fast scanning, reliable data and encoding capabilities. Phased array offers a high resolution scan able to detect fatigue cracks, material inclusions, adhesive joint failures and disbanding of composite materials among many other defects.
Acoustic Emissions (AE) – AE is one of the few NDT techniques used in the in-service condition. This technique is able to locate and monitor both high damage regions and flaws in a wind turbine structure. AE testing usually provides an immediate indication relating to the strength or risk of failure of a component. Other advantages of AE include fast and complete volumetric inspection using multiple sensors or permanent sensors mounted for process control.
Drone Inspections – Visual inspections using drones have become popular due to the increasing size and access limitations of wind turbines. Drone inspection is a cost-effective and efficient method for on-site visual inspection and assessment of wind turbine blades. Using drones' high resolution visual and thermal imaging capability you can locate leading edge erosion, damage to the skin layer, broken/missing bolts or any other visual defects from the safety of the ground.
Remote Visual Inspection (RVI) – RVI will allow for internal inspection of components to ensure the qualityof the internal surfaces, condition of gears, lubricants, loose/missing parts and any evidence of improper operations. This technique is relatively simple and a quick way to locate damage that could lead to larger, more costly problems.
Magnetic Particle Testing/Liquid Penetrant Testing/Positive Material Identification (MT/PT/PMI) – Magnetic particle inspection and liquid penetrant testing are widely used across all industries for surface inspection of metallic components. The MT technique will detect fatigue cracks in gears, pins, bearings, structural components or welds in ferromagnetic materials. PT inspections should be used onstainless steel, aluminum and composite materials for the detection of these flaws. A positive materials identification test is a valuable tool when a material's hardness, toughness, and corrosion resistance plays a major role in the proper operation of the component. Verification of materials related to the gearbox, such as high speed bearings and gear teeth,can identify and prevent future unnecessary wear or damage to internal components.
Additional NDT techniques are available for inspections of wind turbines, but this blog covers the most popular techniques used. As wind turbines get bigger and the demand for renewable energy grows, Intertek will continue to develop and utilize new, more efficient techniques to evaluate the condition of wind turbines. Our main goal for any NDT inspection is to detect and/or prevent failure of components that could lead to costly repairs, unplanned shutdowns or harm to humans and the environment.
The goal of Asset Integrity Management (AIM) is to effectively manage corporate assets in order to gain maximum value, profitability and returns while safeguarding personnel, the community, and the environment. At Intertek our AIM team delivers trusted and innovative technical solutions that ensure the quality, safety and reliability of our clients' assets.
Terry Haigler brings a wealth of knowledge to Intertek with 13 years of experience in the power generation sector in both nuclear and non-nuclear inspections. He specializes in ultrasonic inspection technique looking for in-service damage of piping welds and systems constructed of stainless steel (304, 316), carbon steel, P22, P11, and P91 materials. All of his inspection services are performed in accordance with ASME Section V, XI, or ASME B31.1. Additional experience includes metallurgical replications looking for creep damage and extensive experience with NDE techniques including MT, PT, VT, and eddy current inspections.