Robotic non-destructive testing (NDT) is a method of inspection used to assess the structural integrity of petroleum, natural gas, and water installations. Crawler-based robotic tools are commonly used for in-line inspection (ILI) applications in pipelines that cannot be inspected using traditional intelligent pigging tools (or unpiggable pipelines).
Robotic NDT tools can also be used for mandatory inspections in inhospitable areas (e.g., tank interiors, subsea petroleum installations) to minimize danger to human inspectors, as these tools are operated remotely by a trained technician or NDT analyst. These systems transmit data and commands via either a wire (typically called an umbilical cable or tether) or wirelessly (in the case of battery-powered tetherless crawlers).
Applications
Robotic NDT tools help pipeline operators and utility companies complete required structural integrity data sets for maintenance purposes in the following applications:
Petroleum and public utility pipelines Pipe walls Girth welds Nuclear cooling systems Storage tanks Floor plates Shell plates Welds
Pipeline conditions that may prevent or hinder a flow-driven pig inspection include:
Some pipe fittings (e.g., small-radius bends, tees, butterfly valves, reducers) may be impassable for bulky inspection pigs. Technicians can manually adjust robotic tool travel speed, orientation, and configuration to navigate fittings that might trap or damage a free-flowing pig. Product flow may not be conducive to pig travel. Technician control of self-propelled crawler travel reduces the risk of velocity-based sensor malfunction. Real-time tool monitoring allows the technician to adjust the tool run immediately if readings become unacceptable, including adjusting tool settings to re-scan missed areas or repairing damaged components. Most robotic tools employ non-contact examination methods – technicians are not forced to manage a layer of couplant. Limited tool access may impact use of traditional tools – smart pigs require special entry and exit points (called launchers and receivers, respectively), which may be permanently or temporarily installed. Some crawlers can be inserted via removed fittings or cut-out spools as small as 24” in length, providing greater flexibility in launch and retrieval options – these tools do not require special fixtures. Some crawlers are designed to enter and exit natural gas lines via hot taps, which can be placed at pipeline operator convenience without taking the line out of service. Even in pipelines that could feasibly accept a traditional smart pig, the ability of crawlers to perform short inspections inside specific areas of concern is much more efficient for pipeline operators than arranging a lengthy pig run just to reach the same small area.
Robotic NDT tools also offer safety advantages in inhospitable areas:
Tank shell inspection crawlers typically climb the sides of the tanks, avoiding the danger to the inspectors and time/expense to the tank owner of providing fall protection or/and scaffolding. Similarly, tank floor inspection crawlers that can be lowered into the tank via portholes on the tank roof eliminate the hazards of confined space entry and the time/expense involved in air quality monitoring. Tools capable of working while submerged eliminate the hazards, difficulty, and expense of draining the inspection area. When used in storage tank inspections and subsea applications, these tools also eliminate hazards associated with diving.
Robotic ILI crawler variants
Tethered tool overview
Tethered robotic inspection tools have an umbilical cable attached to them, which provides power and control commands to the tool while relaying sensor data back to the technician. Tethered crawlers have the following advantages over untethered crawlers:
Technicians can use the tether to help retrieve the crawler in an emergency or to perform repairs Unlimited power supply from the umbilical cable allows technicians to examine potential defects as necessary without concern for battery life The umbilical cable supplies real-time control and sensor data to technicians, allowing re-inspection of questionable findings if necessary as well as alerting technicians immediately to tool malfunctions (i.e., minimizing false calls or/and missed anomalies) Most tethered ILI crawlers are small enough to be inserted via removed fittings/flanges or small cuts in a pipeline, minimizing inconvenience to the pipeline operator Bi-directional capabilities require only one access point for pipe inspections
Tethered crawlers have the following disadvantages against untethered crawlers:
The length and weight of the umbilical cable limits the distance these tools can travel Pipelines and tanks typically must be taken out of service to accommodate ILI tool entry and travel
Untethered ILI crawler overview
Untethered robotic ILI crawlers are powered by onboard batteries; these tools transmit sensor data wirelessly to the tool operator or store the data for downloading upon tool retrieval. Untethered crawlers have the following advantages over tethered crawlers:
Untethered tools have a greater effective distance without the limitations imposed by an umbilical cable Pipelines can be sealed with untethered tools inside – the pipe can often remain in service during the inspection Bi-directional capabilities require only one access point for pipe inspections
Untethered crawlers have the following disadvantages against tethered crawlers:
Untethered robotic ILI crawlers can get stuck, requiring excavation and pipe cutting to retrieve the tool Data-recording robotic ILI crawlers do not supply real time data to operators, which can require additional inspection runs to analyze possible findings Untethered robotic ILI crawlers typically require large launchers to deploy and retrieve
Inspection technologies
Robotic NDT tools employ suites of inspection sensors. This section describes common sensor types; most tools combine several types of sensor depending on factors such as robot size, design, and application.
Electromagnetic Acoustic Transducers (EMAT) – milled steel
Main article – Electromagnetic acoustic transducers
Electromagnetic acoustic transducers (EMAT) induce ultrasonic waves into uniformly-milled metal inspection objects (e.g., pipe walls, tank floors). Technicians can assess metal condition and detect anomalies based on the reflections of these waves – when the transducer passes over an anomaly, a new reflection appears between the initial pulse and the normal reflection.Ultrasonic Testing. The Hashemite University NDT Center. Accessed 2 March 2016.
Direct beam EMAT, where the tool induces ultrasonic waves into the metal at a 0° angle (or perpendicular to the metal surface), is the most common inspection method. Direct beam inspections determine metal thickness as well as detect and measure the following defects:
Metal loss on the internal surface (e.g., pitting corrosion, general metal loss) Metal loss on the external surface (e.g., pitting corrosion, gouges), including a residual thickness measurement in defect areas Mid-wall pipe mill anomalies (e.g., laminations, non-metal inclusions), including depth measurement
Angle beam inspections, where the tool induces ultrasonic waves into the metal at an angle relative to the metal surface, can be performed concurrently with direct beam inspections to confirm anomaly detections. An angle beam transducer only registers echoes from anomalies or reflectors that fall into the beam path; unlike direct beam, it does not receive reflections from the opposite wall of normal steel.
The combination of angle beam and direct beam methods may find additional anomalies and increase inspection accuracy. However, the angle beam method has a lower tolerance for surface debris than the direct beam method. Angle beam inspections discover crack-like anomalies parallel to the pipe axis and metal loss defects that are too small to detect via direct beam, including the following:
Stress corrosion cracking Mechanical damage (e.g., scores, feed marks, scratches) Pitting corrosion
Besides its uses in unpiggable pipelines, the non-contact nature of EMAT tools makes this method ideal for dry applications where liquid couplant requirements may make traditional UT tools undesirable (e.g., natural gas lines).
EMAT – girth welds
Weld integrity is a crucial component of pipeline safety, especially girth welds (or the circumferential welds that join each section of pipe together). However, unlike the consistent molecular structure of milled steel, welds and their heat-affected zones (HAZs) have an anisotropic grain structure that attenuates ultrasonic signals and creates wave velocity variances that are difficult for ILI tools to analyze.
One angle-beam EMAT method employs a set of nine frequency-time (FT) scans on each side of the girth weld, where each frequency corresponds to a different input wave angle.In-Line Inspection Technology to Detect, Locate, and Measure Pipeline Girth Weld Defects. California Energy Commission, 2015. Web. Accessed 1 March 2016. The following figure shows a diagram of the inspection area covered by this method, where the green area represents the propagation of shear waves in the weld and surrounding metal.
The tool merges each set of FT scans into a single frequency-time matrix scan to display weld conditions, with anomalies color-coded by severity. This method of girth weld scanning is designed to detect the following weld defects:
Planar defects (e.g., lack of fusion, cracks) Volumetric defects (e.g., porosity, nonmetallic inclusions)
Magnetic Flux Leakage (MFL)
Main article – Magnetic flux leakage
Magnetic flux leakage (MFL) tools use a sensor sandwiched between multiple powerful magnets to create and measure the flow of magnetic flux in the pipe wall. Structurally-sound steel has a uniform structure that allows regular flow of the magnetic flux, while anomalies and features interrupt the flow of flux in identifiable patterns; the sensor registers these flow interruptions and records them for later analysis. The following figure illustrates the principle of a typical MFL inspection tool; the left side of the diagram shows how an MFL tool works in structurally sound pipe, while the right side shows how the tool detects and measures a metal loss defect.de Raad, J.A., and J.H.J. Stalenhoef. MFL and PEC Tools for Plant Inspection. December 1998. Web. Accessed 1 March 2016.
MFL tools are used primarily to detect pitting corrosion, and some tool configurations can detect weld defects. One advantage of MFL tools over ultrasonic tools is the ability to maintain reasonable sensitivity through relatively thick surface coatings (e.g., paint, pipe liners).Drury, J.C, and A. Marino. A Comparison of the Magnetic Flux Leakage and Ultrasonic Methods in the detection and measurement of corrosion pitting in ferrous plate and pipe. October 2000. Web. Accessed 1 March 2016.
Video inspection
Main article – video inspection
Robotic NDT tools employ cameras to provide technicians an optimal view of the inspection area. Some cameras provide specific views of the pipeline (e.g., straight forward, sensor contact area on the metal) to assist in controlling the tool, while other cameras are used to take high-resolution photographs of inspection findings.
Some tools exist solely to perform video inspection; many of these tools include a mechanism to aim the camera to completely optimize technicians’ field of vision, and the lack of other bulky ILI sensor packages makes these tools exceptionally maneuverable. Cameras on multipurpose ILI tools are usually placed in locations that maximize technicians’ ability to analyze findings as well as optimally control the tool.
Laser profilometry
Main article – surface metrology