Another tool to help the direct assessment of external pipeline corrosion appears to be gaining the attention of operators. Long-range guided wave ultrasonic testing (LRGWUT) technology is being viewed more widely as another tool to help. The increasing visibility of the technique can be attributed to capability improvements, an increased understanding of the decisions that must be thoughtfully made about where to use the technology and, equally important, the importance of employing professional data interpretation.
LRGWUT is being looked at to provide another arrow in the external corrosion direct assessment (ECDA) quiver. The technology is not new to pipeline inspection, yet has had its most notable success on in-plant inspections when it is necessary to detect metal loss defects while the pipe under scrutiny is surrounded by insulation or other coverings.
In its more recent use in field pipeline corrosion detection, it has been found to be particularly advantageous for use at rail and road crossings, especially with cased pipe. It also has shown usefulness when used on above-ground pipe sections and when applied to non-piggable portions of the pipeline. As such, it can serve as a verification tool in conjunction with other ECDA methods and as a principal instrument for specific pipeline investigations.
LRGWUT systems utilize single or multiple circumferential probe rings of either electromagnetic or piezoelectric transducer configurations. The transducers surround the pipe, needing only minimal clearance with which to operate. The equipment, available for pipe diameters ranging from 3-42 inches, can generate three wave types--longitudinal, torsional or flexural--sending the waves in both directions from the probe transducer to inspect the entire pipe wall.
The transmitted wave detects echoes from corrosion defects in the pipe and reflects the signal back to the transducer from the defects or circumferential pipeline welds. The reflections can then be analyzed independently for each direction. A typical assessment range of +/-100 feet can be expected in either direction from a single test point under normal conditions. Under ideal situations, the technique has proven effective at lengths of more than 300 feet forward or backward from the transducer.
The distance of signal propagation depends largely on the pipeline size and wall thickness as well as the condition and type of the pipeline's coating and the compactness of the materials surrounding the pipe. Increased signal attenuation from compacted soil, certain types of coatings, increased soil depth and obstacles such as concrete can be expected and can limit the range to less than 30 feet in either direction.
Field Testing
While laboratory tests using LRGWUT systems have been conducted with mixed outcomes, the most usable results come from field testing on pipelines in service or on controlled field applications. IONIK was recently involved in three projects that effectively tested the applicability of guided wave technology.
A field demonstration was conducted at the operations facilities of a major pipeline operator on reclaimed 4-inch and 6-inch diameter pipe sections, approximately 300 feet in length. Conditions were provided to test the technology on bare pipe in air, bare pipe surrounded by compacted soil cover, on pipe-in-pipe to simulate cased crossings and on water-filled pipe-in-pipe to replicate flooded casings or underwater conditions. In all cases, minute anomalies were machined into the pipe in either longitudinal or circumferential directions. Using LRGWUT technology, the imperfections and areas of metal loss were accurately pinpointed (Graph).
For the second case, LRGWUT was used as part of a multi-year study to assess the overall state of integrity on more than 120 miles of a natural gas distribution system, of which approximately 10 miles were under paved surfaces with numerous road and rail crossings. In that capacity, the technique was used as part of a multi-technology toolbox in an ECDA assessment of coal-tar wrapped and fusion bonded epoxy-coated pipe varying from 4 to 12 inches in diameter. The tool was used to supplement pipeline current mapper (PCM) and alternating current voltage gradient (ACVG) sensing in designated locations where verification was necessary or access was limited.
The assessment included data collected and interpreted by IONIK to ascertain the extent of coating damage, corrosion depth measurements and soil conditions. The guided wave technique was used to inspect 6-inch and 8-inch cased road crossings at I I sites necessitated by the technical limitations of the other indirect inspection tools at those locations. The length of the measurements varied between 22 and 100 feet.
The guided wave technology was able to detect several minor indications, likely related to corrosion at failed coating. The smallest area of metal loss was 4% of external metal loss on an 8-inch line, to give an idea of the technique's sensitivity. It was further able to ascertain that there were no moderate (10-19% metal loss) or severe (>20% metal loss) anomalies that required further investigation.
The LRGWUT technique was also used as a complementary technology in the Direct Assessment process at two cased road locations on an 8-inch coal tar-coated carbon dioxide pipeline in close proximity to a major urban area (Photo). The two digs were 46 total feet (6 feet forward in the flow direction and 40 feet backward) and 45 total feet (40 feet forward, 5 feet backward). The digs detected no severe indications of more than 20% metal loss, but did reveal two minor anomalies--one under 9% and one moderate imperfection of between 9-20%. The dig locations, and consequently the anomalies, were accurately pinpointed through sub-meter GPS readings.
Interpretation Is Key
The data generated by a LRGWUT tool is valuable for identifying anomalies on pipelines, particularly in hard-to-inspect locations. As is the case with all the techniques available for that purpose, correct interpretation of results is critical in the ECDA process. One could use an analogy of a NASCAR owner who funds a high-performance race car and then turns over its operation to an inexperienced driver. The results are most likely going to be undesirable.
Similarly, in looking for detection of minute pipeline anomalies, the guided wave tool can provide the data needed for detection, but needs to be set up correctly in the field, closely monitored during its operation and the data interpreted correctly by a knowledgeable technician.
New generations of the technology are available that are able to focus wave energy precisely on areas where there is metal loss. Additionally, multi-mode technology, simultaneously using longitudinal and torsional waves, is being introduced to add a higher degree of accuracy. Once again, experienced technicians are required to make in-process adjustments on the instrument and to correctly interpret the results. With recent capability improvements, proper applications and professional interpretation, LRGWUT technology is bidding to increasingly find a spot in the ECDA toolbox. P&GJ
Dirk L. van Oostendorp, José H. Garcia and Mark A. Ortiz, IONIK Consulting, Houston, TX
Authors:
Dirk L. van Oostendorp, Regional Manager-The Americas, for IONIK Consulting has more than 26 years experience in energy and infrastructural engineering projects, including onshore and offshore pipelines. This includes project management, corrosion control, pipeline integrity, inspection technologies and risk assessment. He regularly lectures at NACE venues on pipeline integrity, corrosion and diagnostics.
José H. Garcia, Senior Materials Engineer, has more than 21 years of energy industry experience in both operations and services, with extensive work in the areas of inspection, NDT quality assurance and materials selection. He has also provided technical support on the repair of OCTG, wellhead equipment, pipelines, pressure vessels, process equipment, as well as rotating equipment.
Mark A. Ortiz, Senior Pipeline Engineer, has more than 12 years international experience in both pipeline engineering and operating company environments!. He has worked with advanced computer modeling software (PipeSim Net, Pipephase, Caesar 11), is a NACE Corrosion Technologist and experienced in cost estimating, pipeline construction and repair, and operations management.
