Stephen R Cox and David A Harman look at the use of ultrasonics for measuring sludge and sediment levels in pipelines
In early 2013, SGS PfiNDE received a call from a client pipeline company that transports a light hydrocarbon product with entrained particulate and sediment through a 150-mile section of pipeline in Minnesota, USA. The company encountered a challenge when, after cleaning the first 20-mile section of the pipeline, the cleaning tool became blocked by, what was determined to be, the excessive buildup of sediment in front of the cleaning tool. Due to the large amount of sediment in this particular line, the tool had gathered enough sludge to completely stop any further progress of the cleaning pig.
Although the company knew the approximate location of the pigging tool used to clean the pipeline, the engineers faced three challenges. First, they needed to know the amount and characteristics of the sediment blockage. Second, they needed to know the location of the blockage in relation to the stopple tees. Third, they needed to isolate and contain the sediment, determine the sludge level and ensure that not only could the tool be removed, but that when the line was put back in service, the company would also have no further issues with flow. Initially, the use of X-ray technology to solve the issues was discussed. However, because of the diameter of the pipe, and due to the fact that it was full of sediment and product, the parties agreed that the use of x-ray technology would not only prevent them from meeting the their timeline goals, they would also not receive all of the information needed to solve their problem. After further discussions, it was decided that a technique could be developed using ultrasonic inspection technologies to determine not only location of the blockage, but also the amount of sediment in the line, its true length and other characteristics necessary to facilitate an expedited repair and return to service.
Ultrasonic inspection technologies require only a single technician with minimal equipment, results are displayed in microseconds and readily available for interpretation, and they do not pose a safety hazard. Whereas radiographic imaging requires multiple personnel with extensive equipment requirements, takes several minutes for each shot along multiple-foot sections of the pipeline, with additional time required for film development prior to interpretation of the results, and poses a safety hazard to all nearby personnel. Since the transducer can transmit and receive sound through the entire volume of materials, with an ultrasonic inspection system, a single transducer is connected to the ultrasonic instrument and is used to take measurements from any location with access to the outside surface on the circumference of the pipe.
SGS inspection technicians determined that the use of an ultrasonic scope that allowed the user to manually set the material velocity, as opposed to data logger equipment with pre-set velocity values, was required due to the problems encountered by varying velocities of the steel pipe, sludge and fluid. The equipment selected allowed the inspectors to drop the velocity down to that of water, which is considerably lower than carbon steel. The technicians also used a low-frequency, 2.25-megaHetrz transducer, which allowed significant penetration through the pipe, the fluid, and the blockage.
The technicians knew the velocity of the steel used in the pipeline was 0.230in/μs (.230 x 105). In addition, they knew that the velocity of water (as opposed to oil) is approximately one-fourth that of steel, or 0.053 in/μs (0.053 x 105).
As a result, they initially set the velocity on the ultrasonic scope to water. Once the ultrasonic system was coupled to the pipe, and the pipeline diameter of 24in was taken into account, the team made the necessary adjustments in velocity settings to match the known distance from the top outside surface to the known distance of the inside bottom surface. This adjustment brought the velocity setting closer to the true velocity of petroleum product in the pipeline, which normally ranges from 0.050 to 0.055 in/µs (0.050–0.055 x 105).
When inducing sound into the top of the pipe, at a location upstream of the impacted pigging tool that was clear of sediment, the technician was able to transmit ultrasonic signals through the wall thickness of pipe and the entire volume of product in the line, and receive a return signal from the bottom (180 degrees) of the inside pipe wall. Additionally, the technician was able to transmit ultrasonic signals through the entire volume of bottom wall thickness.
Utilising ultrasonic inspection technology, the technicians were able to obtain a full-volume reading.
Ultrasonic readings were taken along the entire section of pipeline containing the sediment around the circumference, enabling technicians to plot out the exact location of the sediment and determine the volume.
After final interpretation of the inspection results, it was determined that approximately 150ft of pipeline was solidly impacted with sediment build-up.
Based on the results, the pipeline company was able to accurately develop a strategy to effectively and efficiently cut and remove the specific section of the impacted pipeline.
Typically, pipeline companies consider the use of ultrasonic inspection technologies, such as shear wave, phased array, FAST and TOFD, only for use in high-end weld investigations, and while looking for cracks, dents and internal defects. Used properly, by a well trained, skilled technician, ultrasonic testing is appropriate for finding discontinuities below the surface of the material, such as internal corrosion, manufacturing flaws in pipeline materials, and flaws in welds.
However, with today’s generation of ultrasonic technology, inspection techniques can be developed to provide safe and virtually instantaneous results that can be used quickly, efficiently and economically obtain additional information such as fluid levels and checking for the presence of sludge, as well as many other instances where, traditionally, a more costly and time-consuming technique like radiographic inspections would have to be utilised.
Stephen R Cox and David A Harmanare with SGS PfiNDE in Oklahoma, USA.