Girth weld inspection of pipeline construction projects has been common practice in the pipeline industry for many years. Nondestructive inspection (NDT) techniques are used to identify material and weld flaws in order to assess the integrity of the pipeline as part of the quality control measures taken during the initial construction of the pipeline.
Traditionally, radiography has been used for this purpose. Radiographic techniques common to the industrial application, however, does have some intrinsic limitations such as the use of hazardous radiation, but also limitations related to the characterization of detected imperfections (type of defect, sizing, positioning, etc.)
An improvement was achieved with the introduction of an inspection methodology based on narrow directional ultrasonic beams. Taking away the radiation hazard and allowing for better detection and sizing of lack of fusion defect in particular proved to be a valuable development for the pipeline industry.
Although automated ultrasonic inspection (AUT) of girth welds based on the so-called “zonal discrimination” concept is the current benchmark, the interpretation of the data is not straightforward and it requires a trained and experienced operator.
Inspired by imaging technologies used in the medical field and the field of seismic exploration, a methodology was developed to construct images of imperfections in girth welds called IWEX, short for inverse wave field extrapolation. IWEX images can reveal the location, the orientation and the size of imperfections in a true relation to the weld configuration in a clearer and more interactive 3D format. Enhancements in modern computer and sensor technology enabled the practical use of IWEX imaging under environmental conditions. To evaluate the performance of IWEX imaging under conditions commonly experienced in Canada during winter, a field trial was organized to run IWEX in parallel with the zonal discrimination concept on TransCanada’s Chinchaga Lateral Loop 3 pipeline project.
In this article, some basic principles of IWEX imaging will be presented and contrasted with the traditional zonal discrimination concept. Furthermore, the experiences from the field trial will be discussed with the perceived value of the IWEX imaging technology. Finally, next steps and a future vision will be addressed.
Principles of IWEX Imaging
In general, the principle of finding imperfections in girth welds with ultrasound is based on the properties that ultrasound can propagate through the material and it reflects at boundaries. A defect is like a mirror for ultrasound, reflecting the ultrasound back to the sensor where a response can be recorded if a defect is present. Depending on the shape and size of the ultrasonic probe, the ultrasonic beam can be very narrow (approximately 2 mm), much like a laser pointer. The zonal discrimination concept is based on a series of these narrow beams. One beam is used to inspect a certain depth zone of the weld whereby the orientation of the beam corresponds with the weld bevel configuration, in order to detect lack of fusion defects in the zone. Consequently, the orientation and position of a possible defect in relation to the ultrasonic beam is critical for detection and also for defect sizing.
As opposed to narrow beams, ultrasound can also be generated with a wide almost spherical wave front. This is achieved by the use of small elements that can transmit and receive the ultrasound. With an array of these elements, ultrasound can be transmitted and received at different positions such that an upward traveling wave front can be recorded. IWEX ultrasonic imaging makes use of this principle.
In order to construct an image, an ultrasonic array is used whereby a single element is fired as a transmitter. The wave front generated by this element propagates through the material and it will reflect at any boundary, including a defect. Then, all the available elements are recording the upward traveling reflected and diffracted waves. This process can be repeated for each element as transmitter elements. The resulting recordings contain a data set from all possible combinations of transmitter-receiver elements, and therefore such a data set is referred to as full matrix capture.
The full matrix capture data set is much like a fingerprint of the area under investigation. It contains the reflections from the material boundaries, the geometry and defects if present. If the array is placed over the area, the ultrasonic wave fronts generated by the individual elements will reflect at possible imperfections from multiple directions. Furthermore, the back wall of the pipe behaves like a large mirror, such that an imperfection is also reached from underneath. Hence, the data set contains information related to the orientation, position and size of the defect.
The next step is to construct an image from the full matrix capture data set. This can be done by IWEX. This concept is based on wave field physics that allows extrapolating measured wave fields forward in space and later in time, but also opposite: backwards in space and earlier in time.
The image is built of pixels; each pixel corresponds with a location in the area under investigation. The measured wave fields in the data set are back propagated to a pixel location. In case an imperfection was present at that pixel location, the back propagated wave field would give a contribution or image amplitude. If no defect was present, the pixel would not get significant image amplitude. To construct the entire image this process is repeated for all defined pixel locations of the image.
System Setup and Data Display
The IWEX system resembles the appearance of a standard AUT system. It consists out of a frame with two ultrasonic array probes, to cover the weld from both sides (see Figure 1). In case required, additional conventional probes can be added to the frame, for example for the detection of transverse cracks.
The frame is connected to a scanner bug that can be mounted on a guiding band, which is placed around the circumference of the pipe close to the weld.
On top of the bug, the IWEX box is mounted. This box forms the heart of the system; it collects the data and processes it into images. The images are then transferred via an umbilical cable to a computer where the results can be viewed and analyzed.
During an inspection, the IWEX system moves around the guiding band around the circumference. At each circumferential position, typically with steps of 1 mm, a 2D IWEX images is obtained. A 3D volume of the data is obtained by cascading the 2D images with respect to the circumferential positions.
The data display was designed to perform quick data analyses in two steps. The first step is to identify the presence of any imperfections at a first glance. In case an imperfection is identified, it can be further analyzed in the second step (i.e., height sizing, length sizing, etc.).
During the first analysis step, the presence of an imperfection can be revealed using several projections of the 3D volume to corresponding planes. The data display shows two projection planes to the upstream and downstream sides of the weld, at the far left and right side of the screen. These projections reveal at which side an imperfection is located. Furthermore, the data display shows three top down projection planes covering the cap area, the volume area and the root area. These projections reveal if an imperfection is located close to the inner or outer surface or within the volume (see Figure 2).
In case an imperfection is identified, the operator can move a cursor to the circumferential position during the second analysis step. At the top center of the screen, the 2D IWEX image is displayed corresponding to the circumferential position. At the top left and right of the screen, also two cross section images are displayed obtained from only the upstream probe and downstream probe, respectively. These images can be used to differentiate imperfections from geometry (cap or root reinforcement) that may be imaged close or on top of each other.
In the 2D view, the dimensions of the indication can be determined directly from the image, using a draw box. A software tool allows the operator to draw a box that snaps to a certain drop off level of the indication, relative to the maximum image amplitude of the selected area. The height of the draw box is then displayed on the screen and it represents the height of the indication. A similar routine can be used to determine the length of an indication. In that case, the projection strips are used to select an area with a draw box.
Besides the 2D cross section at the top of the screen, a selected area of the weld can be displayed in an interactive rotational 3D view (see Figure 3), which can also be enabled real time during the scanning of the weld. The 3D visualization enables the operator to view the indication from multiple directions.
Benefits of IWEX Imaging
IWEX Imaging provides a user friendly imaging interface where virtually anyone can now visualize and understand the data being presented. Flaw image measuring tools used in this software are common to industry practice with the ability to standardize sizing algorithms to reduce the chance of oversizing or under sizing weld flaws. Current AUT applications still have extensive components and functions that need to be used correctly to enable a person to understand what is being presented. Without proper training and understanding of the welding processes interpretation of the data being presented may result in misinterpretation.
With the ability to simplify the user interface the time required to properly train a technician to interpret data can be significantly reduced without an adverse effect on the data interpretations made. In addition, the ability for a NDE technician to show a welder or welding technician a weld flaw in a 3D
image firstly reduces the time it takes for the people receiving the information to understand what the problem is, and secondly it instills a level of confidence in the NDE applications accuracy as misinterpretation of data is extremely reduced.
IWEX Under Field Conditions
Many of large diameter new build pipeline projects are constructed during winter seasons where construction personnel and equipment are subject to very harsh working conditions. Long hours, significant welding production rates and subzero temperatures are hard on equipment and personnel.
Safety, efficiency, quality and productivity are some key elements in the construction of any project. If a proposed technology cannot prove to meet, exceed and/or advance current industry practices in each of these areas, then next steps will be difficult to gain traction. TransCanada has had the unique opportunity to see the development of IWEX over the past few years and were looking forward to the chance of field testing the equipment once it had reached a level of development that could be considered fit for testing outside of laboratory conditions.
During the winter field trial performed with TransCanada on the Chinchaga Lateral Loop 3 in northern Alberta, IWEX was exposed to a real world pipeline construction environment. This component was crucial to determine the benefits and limitations that could be experienced when utilizing this system outside of a laboratory setting.
IWEX was comparably tested against common zonal discrimination AUT techniques and showed positive results in actual detection of weld flaws and accuracy of sizing. In addition productivity tests were performed to determine if IWEX could maintain a suitable production rate that would be required to maintain real time feedback to the construction teams in an effort to proactively reduce repair rates.
Although inspection cycle times were slower than traditional applications IWEX was able to keep up with mechanized welding production rates known to the current construction market. Subzero temperatures (minus 20 to minus 35 C) did not create any noticeable adverse effects to the equipment being used and due to the size of the equipment being common to industry standard personnel were not subject to any additional physical strain.
The field trial was considered successful and TransCanada will continue working with IWEX toward full scale testing for possible implementation in the near future. As with all new technologies, acceptance in the industry is crucial. A full scale test that includes comparison and verification will contribute to this.
Furthermore, qualification and validation programs will be considered to evaluate the IWEX technology against existing codes and standards such as DNV FS101, API 1104 and CSA.
Niels Pörtzgen, Ph.D., is technical authority of the pipeline department at ApplusRTD. A veteran of the NDT/NDE industry for 15 years, he provides consultation in automated ultrasonic inspection challenges mainly for new construction pipeline girth welds.
Jason Althouse is the senior NDE technologist for TransCanada, working as the SME for all NDE applications used throughout the company’s operations in Canada and Mexico. A veteran of the NDT/NDE industry for 17 years, his tasks include project development involvement, quality training and audits, and research and development of emerging technologies, among others.