Defect Sizing in Pipeline Welds: What Can We Really Achieve?

Background/Objectives: Grain structure of Austenitic Stainless Steel (ASS) weld causes difficulty in defect detection while inspecting using Time of Flight Diffraction (TOFD) method. This study aims at comparing segmentation methods to overcome this difficulty based on defect characterization. Methods/Statistical Analysis: This study makes use of TOFD and Radiographic images of ASS weld pads fabricated with defined linear and volumetric defects. The Region-Based Level Set algorithm and Discontinuity Based Segmentation algorithm were explored for achieving flaw segmentation and quantitative characterization and validation of the result with that of standard radiographic results. Findings: The efficiency of the algorithms was analyzed by comparing and validating the size of defect with that of standard radiographic results in the form of error percentage. The consistency of error percentage in defect sizing (up to 11%) achieved by RegionBased Level Set algorithm for all the test images given in the database indicate that, this algorithm is the best as compared to Discontinuity Based Segmentation algorithm(error percentage up to 47%) for defect segmentation and characterization in TOFD images. Application/Improvements: The segmentation algorithm enables automation of measurement process and enhanced detection and characterization of defects at the initial stage. Further, it reduces human fatigue caused by operator while defect detection as the volume of data increases. The future direction is to fully automate the system in order to save time for interpretation and to modify the algorithm to segment images with multiple defects. Keywords: Defect Characterization, Discontinuity Based Segmentation, Flaw Segmentation, Region-Based Level Set Segmentation, Time of Flight Diffraction (TOFD), Weld Defect

Computational tools based on probabilistic fracture mechanics have been developed to enable reliability-based fitness-for-service assessments of flawed girth welds. The same tools are readily adapted for establishing maximum allowable defect sizes to achieve targeted weld reliability. Sensitivity studies have shown that of the various input parameter uncertainties, measured defect height often has the greatest impact on the probabilities of both fracture and plastic collapse. A reduction in sizing uncertainty should thus dramatically improve predicted reliabilities. The increasing use of mechanized ultrasonic testing (UT) in pipeline construction, driven by the demands of engineering critical assessment (ECA) -based acceptance criteria, highlights the need to quantify this uncertainty, particularly for systems incorporating pulse-echo (P/E) and time-of-flight diffraction (TOFD) methods and phased-array (PA) technology. EWI collected third-party independent data and statistically characterized the systematic and random errors in girth weld defect sizing, as measured by mechanized UT using P/E and TOFD methods, as well as PA ultrasonic technology, in support of pipeline reliability assessments.

It is difficult to accurately size the defects that are oriented at an angle (that is not normal to the wave) using conventional amplitude based ultrasonic techniques. Since Time of Flight Diffraction (TOFD) is based on the diffraction of ultrasound at defect edges, defect sizing using this technique is amplitude independent. However, most of the TOFD based assessment relies on manual sizing, whose accuracy depends on quality of image and the operator’s experience. Also, the utilization of TOFD for sections less than 15 mm has reportedly several difficulties. In this paper, we report our attempts to size the vertical and inclined defects using an in‐house TOFD system built to inspect thin sections (6–10 mm). To improve sizing, automated defect sizing techniques termed Embedded Signal Identification Technique (ESIT) and Point Source Correlation Technique (PSCT) were developed. A ray tracing based model was also developed for a) optimizing the experimental parameters for thin sections, b) interpreting the r...

Ultrasonic flaw sizing is important issue for remaining life prediction of industrial plants. Time-of-Flight Diffraction (TOFD) method is recognized as one of the most accurate flaw sizing method. However, to measure the crack height accurately, the crack must be symmetrically placed between transmitter and receiver. When the crack do not exist such position, the B-scan is required for accurate crack sizing. By the way, if additional transmitter or reciever is added to the system, the crack location and height can be estimated without B-scan. In this study, we examined the 4 sensor TOFD method which utilizes two transmitters and two receivers. In the result, the crack location and height can be estimated by 4 sensor TOFD method. However, the estimation errors were large due to the low SN ratio of received signal. Then we applied the pulse compression technique to the 4 sensor TOFD method for improving SN ratio of received signal. In the result, the SN ratio was enhanced about 11 dB by the pulse compression. The average location on error is reduced from 2.49 mm to 1.13 mm, and the average depth estimation error is reduced from 0.28 mm to 0.21 mm.

Signal quality enhancement using higher order wavelets for ultrasonic TOFD signals from austenitic stainless steel welds

New non-destructive examination methods have been available for many years. These include time of flight diffraction and phased array methods. These are routinely used for inspection of welds. Work is ongoing at ISO and CEN for standardization of these methods. First the time of flight diffraction was drafted and now ISO and CEN are working on a standard for phased arrays. These drafts are used for welding inspections. The acceptance criteria is in draft form for time of flight diffraction which will be followed by that for phased array. Standards are already available for base material phased array inspection such as the ASTM document “Standard Guide for Evaluating Performance Characteristics of Phased Array Ultrasonic Examination Instruments and Systems”. Several ASME code-cases have been issued in recent years on weld inspection. Qualification for the ASME code is also planned for this novel technique.

Reliability assessment of manual ultrasonic time of flight diffraction (TOFD) inspection for complex geometry components

Defect detection using the phased-array laser ultrasonic crack diffraction enhancement method

Defect sizing is required for a quantitative assessment of the quality and reliability of safety relevant components and materials using ultrasonic non-destructive testing. The SAFT (Synthetic Aperture Focussing Technique) and the TOFD technique (Time Of Flight Diffraction) are such promising sizing candidates, extracting more information from the raw ultrasound echo data and the corresponding crack tip response. In this work the phased array technique is used to inspect a clad mock-up model of a pressure vessel section. The full scale model contains artificial test reflectors which are located in the weld and in the cladding region as well. The defects—representing typical flaws at a very early stage—are analysed with different frequencies, beam angles and directions of incidence. For the reconstruction of reflector indications a SAFT algorithm is applied to the phased array measurement results. Additionally the reflectors are analysed by means of the TOFD technique, using different beam angles at the same time. Both analysis methods are performed using different directions of incidence considering the complex cladding structure underneath the inner surface of the mock-up model. A direct comparison of the SAFT and TOFD techniques shows that, besides the clarity of the results, the detection and sizing capabilities of SAFT are far better.

The ultrasonic Time-of-Flight Diffraction (TOFD) technique is a well developed technique for sizing defects in thick sections (thickness >10 mm). Attempt has been made here to extend this technique for thin sections (6-10mm). An automated defect sizing algorithm using the Embedded Signal Identification Technique (ESIT) was developed for separating partially superimposed signals often encountered in thin sections and the results were compared with the manual sizing method. Both EDM notches and more realistic fatigue cracks in thin section were used to evaluate the proposed technique.

The high-stress, high-energy nature of generator retaining rings makes it essential that defects are detected early to avoid propagation and catastrophic failure. This paper explores how combinations of automated ultrasonic and eddy current methods can be tailored to give the most complete retaining ring inspections possible, reducing the time and inconvenience involved with the inspection. As part of a management programme to ensure the integrity of the rings, ultrasonic inspection is periodically carried out, often with the rings in situ on the rotor. Conventional pulse-echo inspection using low frequency shear waves and Time-of-Flight Diffraction (TOFD) inspection have been the primary search techniques for cracking. More recently, ID creeping waves, which have the ability to detect and resolve shallower defects than those found with conventional shear waves, have been introduced to the inspection. This paper compares and contrasts the use of shear waves and ID creeping waves for the detection of stress corrosion. Array eddy current can be used to complement the ultrasonic inspection, providing recordable data on surface breaking and sub-surface defects not achievable with conventional dye penetrant methods.

In modern high-strength concrete, steel fibers are commonly added to the concrete mix to improve its mechanical properties and structure. The steel fibers typically have a specific length and are randomly distributed to increase the tensile and bonding strength of concrete structures. However, the presence of steel fibers can impact the propagation of ultrasonic waves in concrete, causing alterations in ultrasonic wave transmission characteristics and potentially affecting the ability to assess the quality and safety of construction projects. A successfully produced high-strength concrete (> 60 MPa) from cement, sand, silica fume, and superplasticizer. Using this concrete mix, the authors fabricated beam specimens with dimensions of 150 × 150 × 600 mm for both cases with and without steel fiber reinforcement. Based on the Time-of-Flight Diffraction (TOFD) ultrasonic testing method, the authors conducted a series of experiments on high-strength concrete beam specimens. The results of ultrasonic wave propagation experiments indicated that the presence of steel fibers at a 2% content increased the time of wave propagation. On the other hand, curing conditions, specimen maintenance time, and transmission path distance affected the ultrasonic wave propagation time in high-strength concrete. Understanding the influence of steel fiber content on the propagation time of ultrasonic waves in high-strength concrete can contribute to improving the inspection and quality assessment processes of construction projects. Additionally, it helps enhance the understanding of the properties of high-strength concrete reinforced with steel fibers.

Frontiers in NDE research nearing maturity for exploitation to ensure structural integrity of pressure retaining components

Mechanized or Automated Ultrasonic Testing of pipeline girth welds is now in common use in the on - and offshore industry. Automated ultrasonic testing (AUT) is globally seen as more than just an alternative to the standard radiographic inspection technique not only because it does not poses safety hazards but also because it is faster, more reliable and has better detection capabilities of critical Lack of Fusion defects in pipeline girth welds. One of the reasons to use the AUT technique is due to its possibility to use acceptance criteria which are based upon ECA (Engineering Critical Assessment) instead of the so-called "Good Workmanship". Usually the AUT systems are mounted on a band strapped around the pipe. From the weld, ultrasonic data is collected from which the defect sizes and positions can be determined by experienced operators using dedicated software algorithms. This paper discusses the RTD Rotoscan system, which is the first worldwide AUT inspection system for new construction pipeline girth welds. The principles of AUT, the conventional RTD Rotoscan as well as the RTD phased array Rotoscan and its advantages in comparison with conventional (multi probe) AUT are discussed. Furthermore challenges regarding the use of AUT on Austenitic welding having a corrosion-resistant alloy layer are presented. This paper describes also latest improvements made on AUT during the last years in order to optimize inspection philosophy and minimize the system's "Uncertainties". In addition a brand new method, RTD IWEX, is briefly described that allows the detection and sizing of weld imperfections in 3D.

Structural reliability is an engineering method that focuses on the calculation and prediction of the probability of failure for a structural system. Structural reliability is used by many pipeline operators to assess the probability of failure, and these probability of failure estimates are then used to manage safety, environmental, and operational risks for threats such as corrosion, cracking, and third-party damage. In the structural reliability approach, the probability of failure is obtained from a multi-dimensional integral, for which the solution is commonly estimated numerically using Direct Monte Carlo (DMC) simulation. DMC is straight-forward and robust but requires a significant amount of computational effort to estimate small probabilities, which are typical of threats to pipeline integrity. Subset Simulation is an approach that improves on the efficiency issues of DMC by representing the rare event probability as the product of a number of more frequent events, which are each estimated separately. Previous work published at IPC in 2020 by Bandstra and Fraser [1] showed very close agreement between DMC and Subset Simulation for a single defect reference case with a single limit state. This study extends that initial work by applying Subset Simulation to various failure models for a variety of pipeline defects. Subset Simulation is applied to the CSA corrosion model, PRCI MAT-8 crack model, and the EPRG dent failure model, and the performance is evaluated by comparing the results to DMC. For each of these comparisons, simulations are performed across a large-scale grid of validation cases that consider a range of pipeline and defect sizes. An approach that utilizes Subset Simulation to handle multiple limit states is also presented and applied for relevant failure models and the results are evaluated against DMC.