Engineers and stakeholders responsible for pipelines need to understand how the emerging technology of robotic laser profiling will impact system maintenance and specification decisions by documenting the in-service structural integrity of different pipeline materials.
By John Salik, Eng., and Oliver Conow | Images courtesy of C-Tec (www.ctecworld.com)
Pipelines are a costly investment. If installed and maintained properly, pipelines provide many decades of trouble-free service. Accurate knowledge of the condition of a pipeline is integral to avoiding unforeseen repair and replacement expenditures. Modern sewer and aqueduct inspection is increasingly done using pipe profiling robots that move within pipes and acquire inner geometry data. Any deviations from the installed inner diameter (I.D.) are of vital importance to infrastructure engineers who determine the extent of repairs. Laser-profiling robotic technology provides engineers with an accurate picture of the required maintenance, structural integrity and life-cycle costs of installed pipelines. In turn, this knowledge informs the specifying engineer of the most reliable and cost-effective pipe material for future projects.
Advances in laser testing technologies
Currently, mandrel testing comprises the bulk of pipeline testing in North America. While mandrel testing is an acceptable method, the newer robotic laser inspection systems provide a more thorough and accurate picture of the pipe’s structural integrity.1 A growing number of non-contact, robotic pipe-inspection systems are available and use a variety of measurement methods, including:
• Image processing methods such as pixel counting
• Laser-point distance triangulation
• Laser time-of-flight measurement
How three laser profiler models work
Understanding how each laser profiler works, its advantages and disadvantages, is imperative for engineers responsible for pipe specification, installation, maintenance or testing.2 Robotic inspection systems that use lasers for distance measurements (from the pipe’s center) are explained in this article. Each method assembles measurement data into a 2-D cross-section of the pipe’s interior wall. Stacking these images sequentially renders a 3-D profile. Profilers may be stationary or mobile-data collectors and vary in how distance data are collected.
1. Rotational profilers use two lasers in rotation to determine distances using triangulation. The laser’s dot projections originate from the center of the pipe with a small but known angle. Using a camera, the laser points are imaged with a calibrated digital camera. Because the distance between the points increases as the distance to the pipe wall varies, digital images are used to find the distance between laser projections by pixel counting and triangulation. While this method yields one measurement (pipe center to wall), the camera-laser system is robot-mounted so that it can rotate along the pipe axis while being held stationary. Because it measures one point at a time, forward motion results in radial distance measures that form a spiral (See Figure A). While no true cross-sectional profile can be obtained from such a system in motion, 3-D profiles may be generated by either significantly increasing the rotational speed relative to forward motion (so that its sampling spiral has a finer pitch), or by mathematical estimation of the best-fitting local cylinder that represents the pipe.
2. LIDAR3 systems use a scanning laser that moves back and forth in a single plane. Distance measurements are acquired by measuring the time it takes for the laser to bounce off a target and return to its origin. Because the light propagation speed is constant, distance can be determined from the so-called “time of flight.” The scanning motion results in a plane that projects along the interior pipe wall (See Figure B). Because the laser’s angular step remains constant, the orthogonal measurements from the pipe’s center to the wall are taken only two at a time (per sweep), but at many non-uniform distances from the robot. When placed in rotation, many pairs of distances are acquired so that a ring of measurements is formed. This measurement ring forms a 2-D cross section, and with many sections obtained simultaneously, a 3-D pipe profile can be created. Here, we note three things. First, the cross-sectional profiles are not uniformly acquired, but with a sufficiently small scanning angle, this may not be an issue. Second, the robot is held statically within the pipe in order to acquire the pipe profile. Finally, measurements are made with respect to line-of-sight to the laser origin, so that any sufficiently large deformations or obstacles will block the laser from acquiring data downpipe. Typically, obstructions (laser shadows) are not a problem as sufficient data are available for estimates of deformation. Because spacing between profiles increases downpipe, certain features could be missed.
3. Continuous-ring profilers use a planar laser whose light rays emanate radially outward in a continuous fashion from a fixed focal point. The laser plane is perpendicularly aligned to the pipe axis. Incident rays on the interior wall readily illuminate its orthogonal cross section. Using a calibrated high-definition digital camera, the illuminated ring is imaged along the pipe’s axis and then analyzed. Because of the camera calibration, the digitized image contains usable spatial information (known relation between pixels and actual distance). By counting the number of pixels from the center of the pipe to the incident laser, many radial distance measurements are obtained simultaneously along the pipe wall. When the camera-laser is in motion, the camera frame rate assures that the illuminated ring is imaged at fixed intervals along the pipe (See Figure A.) The result is many cross-sectional samples that generate a uniform 3-D profile.
What should engineers look for in laser profilers?
With a growing variety of pipe inspection robots available, many methods have emerged that provide an objective measure of a pipe’s structural integrity. Engineering analysts are naturally concerned with pipe integrity from their client’s perspective, and these preferences have resulted in three primary profiling modes, all based on a pipe’s cylindrical geometry:
A. Deformation Measurement: This profiling mode focuses on local deviations from the pipe’s specified I.D.
B. Corrosion Measurement: This profiling mode records deviations in average local I.D. that are caused by sectional surface changes resulting primarily from degeneration due to age or chemical reaction.
C. Liner Thickness Measurement: This profiling mode addresses deviations in local I.D. before and after a pipe liner has been placed, thus requiring a comparison of the two profiles.
Agencies, owners and stakeholders responsible for pipeline systems are interested in
• Reduction of life-cycle costs
• Efficient planning and pipeline condition assessment
• Mitigation of risk
Three major stakeholders who share responsibility for modern pipelines and who can benefit from robotic pipe inspection are:
1. Contractors – At the point of installation, as a method of quality control, contractors can determine pipeline conditions quickly and accurately so that they can perform necessary corrections before the project is completed and handed over to its owner.
2. Engineers – With reliable and repeatable pipe profiles in hand, engineers can perform a comprehensive structural integrity assessment anytime to manage maintenance budgets.
3. Proprietors – Pipe condition reports are project quality assessments to assure the owners of the new installation’s value for the costs incurred.
Why are repeatability and accuracy important?
It is of vital concern to clients to obtain pipe inspection reports that contain valid measurements that are reliable enough to generate the same data in repeat testing. While it is the role of a regulatory body to define inspection standards,4 it is the role of an independent testing facility to provide assurance that the testing system can generate accurate and repeatable data.
When measuring pipe deformations, for example, it is important to know to what extent and where deformations occur, so the better the system’s repeatability simply translates to increased reliability. Statistical analysis tells us that the more the measurements, the better the estimate. When measurements are too far from what is expected in the ellipse model (which happens regularly), trained profilers know that this is likely due to dirt, debris, obstacles and water in the pipe. Consequently, not all measurements can be used for deformation measurement. If a profiler has very few measurements to make in the first place, the data collected may not be of sufficient once the unusable measurements are thrown out.
Accuracy is of great importance. The cost of early detection is much lower than of the cost of pipe failure. Measurement accuracy is a cost-mitigating factor that may be the pivotal point in deciding if pipe replacement is the only option. In cases where structural integrity is not of grave concern, deformation accuracy can be used as a quality control measure to assure that the pipe installation was done properly. Pipe testing accuracy is a leveraging tool in a project’s cost/benefit analysis and also provides important data to ensure that installation was performed to specifications and your maintenance plan is on track.
Despite its relative infancy, laser profiling promotes fiscal confidence through reliable reports. This technology minimizes the possibility of missing serious pipe defects (unlike older measurement systems) and avoids cost overruns that result from scheduling work crews for ongoing test-and-fix cycles. Laser profiling provides an objective means of controlling the costs of installation, maintenance and repair over the system’s service life.
1 Laser profilers cannot be used to detect cracks or measure crack width inside RCP; video micrometers measure dimensions of a pipe feature, like cracks.
2 Output from laser profiling systems can vary greatly. For example, the results from a full-ring laser profiler and a spinning laser system can be significant. DOTs and engineers must carefully assess the repeatability, accuracy and calibration of testing systems.
3 LIDAR = Light Detection and Ranging or “laser radar”
John Salik, Eng., previously worked as chief scientific advisor for C-Tec in Laval, Quebec, and has twelve years of experience in satellite communication, inspection robotics and computer vision. He currently works as a lead research engineer at the Centre de Recherche Enviro, Laval, and serves as taskforce leader on ASTM Committee F36.20, working to establish standardized laser profiler performance metrics.
Oliver Conow is currently the client support manager at C-Tec, Laval, Quebec, for the past six years and specializes in electromechanical designs. He also serves in business strategy development and does presentations on laser profiling technology.