NDT: safeguarding essential dams14 November 2023
As America’s reliance on dams for water management and energy generation grows, Nondestructive Testing (NDT) emerges as the silent hero safeguarding these critical structures. John Pariseau delves into the world of NDT, revealing its indispensable role in maintaining the resilience and safety of hydraulic structures. Discover how standardized training, third-party certifications, and a commitment to excellence are shaping the future of NDT, ensuring the longevity of dams and the preservation of vital resources
America’s history is intrinsically linked to its vast water resources. Managing and harnessing those resources ensures a reliable and sustainable source of water for agricultural irrigation, industrial processes, and potable drinking water. In the process, food security and economic growth became certainties. Additionally, dams contribute to flood control, safeguarding communities from devastating natural disasters and providing navigational waterways for commercial shipping. Beyond water management, the capacity of dams to generate clean and renewable hydropower drives innovation and supports diverse energy production. The recreational opportunities and tourism potential they offer also add significant value, contributing to both local economies and the preservation of natural habitats. As the backbone of water management and energy generation, it becomes even more evident that dams stand as vital assets in enhancing national resilience and fostering sustainable development.
Dams utilize various styles of water control gates which are commonly referred to as hydraulic structures. The style of these gates is quite diverse from tainter gates, also known as radial gates, used for smooth water regulation while minimizing erosion to miter gates which are used for navigational locks. Additionally, there are sluice gates, stop logs, bulkheads, vertical lift gates and so on. One common element with all hydraulic structures is the significant amount of fatigue loading they must withstand during their service life. Hydraulic structures will shake themselves apart while in service. Because of these high fatigue loads, as well as many hydraulic structures being considered fracture critical, many are now designed and built to the AWS D1.5 bridge code. This ensures designs with fatigue life in mind, reinforces material toughness requirements and puts a higher emphasis on quality.
Nondestructive testing (NDT)
One of the techniques employed to evaluate the gates’ structural integrity and detect any signs of wear, corrosion, or fatigue is nondestructive testing (NDT). These inspections provide engineers with critical data on the gates’ condition, allowing them to address any potential issues promptly.
Quality in steel construction is a primary focus for hydraulic structures. NDT is one of the primary methods for verifying both the quality of construction as well as the structures performance while in-service. NDT is the process of inspecting, testing, or evaluating materials, components, or assemblies for defects without damaging or destroying the object being examined. There are currently 16 recognized NDT Methods, those most commonly used for welded steel construction of hydraulic structures are Ultrasonic Testing (UT), Radiographic Testing (RT), Magnetic Particle Testing (MT), Liquid Penetrant Testing (PT) and Visual Testing (VT). Of these, only two are considered volumetric test methods, Ultrasonic Testing and Radiographic Testing. Volumetric test methods are capable of examining the through thickness of a test object regardless of the objects thickness. This becomes incredibly important especially with structural welding where because of the nature of the welding process many types of discontinuities can occur deep within the weld which if left undiscovered can result in failure.
Ultrasonic Testing utilizes high frequency soundwaves which propagate through the test object. Because of the short wavelength associated with higher frequencies used with UT ,the soundwaves are incapable of transmitting through gases or low-density materials due to their significant difference in acoustical impedance. This results in soundwaves reflecting from air voids or materials of significant density differences back to the transmitting transducer providing a signal on the display. UT is incredibly sensitive to locating cracks and other linear discontinuities. Examinations can be performed relatively quickly, accurately, cost-effectively, and there are no safety issues, in addition only one side of the test object needs be accessible when using the pulse-echo technique.
Radiographic Testing utilizes ionizing radiation either in the form of gamma radiation from a decaying radioisotope or X-radiation from an X-ray generator. The ionizing radiation penetrates through the test object and is received by either a radiographic film for conventional RT, by a digital detector array (DDA) for digital radiography, or an imaging plate (IP) for computed radiography. Regardless of the image capture device, the final product is a 2D plan view image of the test object. Discontinuities are identified by a lighter or darker image on the film. Because the radiation travels at a constant rate through a homogeneous material, any discontinuities located within the test object will have a lower density (cracks, porosity, slag, etc.), which will exhibit a darker colored image due to more radiation reaching the film or higher density (tungsten inclusions, excessive weld reinforcement, etc.). This results in a lighter image due to less radiation reaching the film. RT works well with simple geometry structures such as CJP welds in butt-joint configurations and is capable of locating various types of discontinuities, however because of the small change in density with tight linear discontinuities, such as hairline cracks, they are often not identifiable on radiographic images. RT, however, does excel at locating rounded indications such as porosity and slag inclusion. While it does require access to both sides of the test object, the source of radiation is on one side and the detector on the opposite side. Additionally, RT has significant safety issues due to the use of ionizing radiation and special licensing and training is required for RT technicians. Also, because of the high safety issue, RT can only be performed during off hours when workers are not present. When comparing the two-test methods UT and RT, one would likely choose UT due to the higher sensitivity to linear indications, speed of testing, and having no safety issues.
Technicians and certification
One element that is often overlooked is the skill of the technician. In NDT, technicians are certified to specific methods as Level I, II or III. Level I technicians have limited experience, knowledge and training and are required to be observed by either Level II or III technicians during examination of test objects. Level II technicians have an increased skill and knowledge level and are capable of performing NDT tests without direct oversight. Level III technicians are at the supervisory level and are typically tasked with developing NDT procedures, training, and certification practices for their company.
One unique element of NDT is that certification to Level I, II, or III is typically the responsibility of the company in which they are employed. Central certification programs do exist but are rarely utilized with the exception of the ASNT Level III program.
It is under this mentorship where the choice between NDT methods may be swayed. Instructors often understand that UT is a far more complex test method with an almost uncountable number of variables which can skew the test results. This requires a higher level of training, experience, and equipment familiarity that frankly most UT technicians do not receive when trained by their employers. RT on the other hand is relatively simplistic to perform, the image interpretation requires a higher skill level but because of its permanent nature, the image interpretation is often performed by individuals with a higher skill level. With this in mind, UT should be the preferred method, but RT is often more readily utilized.
In addition to these factors, the certification document used in NDT is the ASNT Recommended Practice SNT-TC-1a: Personnel Qualification and Certification in Nondestructive Testing. Though this document recommends minimum training, experience, and examination requirements for employers to certify their technicians, it is not a requirement for the employers to follow these recommendations. This stems from a time when quality standards were unique to each manufacturer and the qualification/certification requirements of NDT technicians were modified to fit that manufacturer’s needs. Because this process continues today, the education and skill level of NDT technicians can swing wildly from employer to employer.
As America strives to balance responsible water management and sustainable energy generation, the expertise of NDT professionals becomes ever more vital. The competence of NDT technicians serves as the foundation of accurate inspection and precise data collection. It is critical to elevate the expertise of NDT professionals through comprehensive and standardized training programs. These programs ensure that technicians are equipped with the necessary knowledge and skill to perform high-quality inspections, interpret results accurately, and make informed decisions based on their findings.
These credentials not only validate the expertise of NDT technicians but are also imperative for the ability of the inspector to do accurate exams. Certifications validate technicians’ proficiency in the latest NDT equipment and methodologies, guaranteeing their capability to perform accurate inspections. In the absence of rigorous training and certification, the reliability of the exams can be questioned, potentially leading to serious consequences and safety of dams and hydraulic structures can be seriously compromised. As NDT methods advance, training must adapt to encompass changing technologies and applications of NDT, ensuring continued quality and the preservation of these essential water management systems.
The value of certification extends beyond individual expertise. Third-party organizations offering NDT training play a pivotal role in standardizing the knowledge and skills of NDT technicians across the industry. These courses provide a comprehensive framework that aligns with industry best practices and regulatory standards. Technicians who undergo such training programs not only enhance their proficiency, but also gain a broader understanding of the unique challenges posed by complex structures. Additionally, third-party organizations continuously update their curricula to reflect the latest advancements in NDT technologies, ensuring that NDT technicians remain at the forefront of their field.
Third-party certifications instill a sense of trust and accountability in the NDT process. When an external body validates the competencies of technicians, it fosters confidence among stakeholders, including engineers, project managers, and regulatory authorities. This confidence is crucial, especially in the context of hydraulic structures, where even the slightest oversight can have far-reaching consequences.
The importance of third-party certification courses is magnified when we consider the critical role of dams and hydraulic structures in ensuring water supply, flood control, and power generation. In the face of growing infrastructure demands, relying on certified NDT technicians becomes an indispensable part of maintaining the safety and resilience of these vital structures.
In addition to access to quality accreditation programs, new generations of skilled inspectors can be nurtured through mentorship and apprenticeship opportunities. Collaborating with industry leaders and educational institutions creates a pathway for aspiring NDT professionals to gain hands-on experience and guidance from seasoned experts. Such collaboration fosters a continuous transfer of knowledge and expertise, raising the bar for NDT inspection practices.
Incorporating NDT training
To empower structural engineers with a deeper understanding of NDT methods, educational institutions should incorporate NDT training into their core engineering programs. By providing engineers with specialized NDT knowledge, they can better understand the complexities of specific examinations such as Ultrasonic Testing, Radiographic Testing, and Magnetic Particle Testing and their application during the design, construction, and operational phases of hydropower projects.
The value of NDT is not always fully understood by the infrastructure industry and engineers. Engineers are tasked with multifaceted responsibilities, from design and construction to ensuring the structural integrity of these facilities. Yet, the intricate nature of NDT methods and their contributions to the maintenance and longevity of hydraulic structures can sometimes be overlooked. Engineers may focus more on the tangible aspects of construction, neglecting the critical role that NDT plays in uncovering hidden flaws and defects after commissioning. Bridging this knowledge gap is pivotal. By informing engineers about the intricate nature of NDT and its ability to safeguard against unseen vulnerabilities, it is possible to equip professionals to make more informed decisions, thereby bolstering the resilience of dams and hydraulic structures in the face of ever-evolving challenges.
By fostering mentorship and apprenticeship opportunities, integrating NDT education into engineering curricula, and embracing advanced NDT techniques, the hydropower industry can ensure the continued resilience and safety of these critical infrastructures.
The utilization of high-quality NDT inspection at dams and hydropower projects is essential to maintain their safety and longevity. Investing in robust NDT practices allows stakeholders to mitigate risks, enhance safety measures, and extend the operational life of these critical infrastructures. The early detection of structural issues and potential failures through NDT techniques enables engineers to take proactive measures, safeguarding these vital structures.
By requiring thorough NDT expertise and training, we can foster sustainable water management practices and ensure the longevity of dams and their component structures, safeguarding communities, and the environment for generations to come. With a commitment to best practices and continuous improvement, NDT inspection stands as a reliable pillar supporting the sustainable development of hydropower projects worldwide. Investing in these initiatives can elevate the quality of inspections, strengthen structural safety, and uphold the integrity of hydraulic structures.
John Pariseau is an ASNT Level III in UT, RT, MT, PT, and VT as well as an AWS Senior Certified Welding Inspector, NACE CIP Level 3, and ICC Master Special Inspector. He is the Executive Director of the National Inspection Academy, a 501(c)3 not-for-profit inspection training company specializing in Nondestructive Testing.