The objective of this project is to demonstrate that Magnetostrictive Sensor (MsS) torsional guided wave technology can be used to monitor the initiation and growth of corrosion in natural gas pipelines.
Southwest Research Institute (SwRI) – project management and research products
ClockSpring Company LP – field testing assistance
El Paso Corporation – access to in-ground pipeline and pipeline samples for testing
San Antonio, Texas 78238
Magnetostrictive sensor (MsS) technology involves the launching of a mechanical “guided wave” pulse of a relatively low frequency (typically under 100 kHz) along a pipeline. Signals reflected from defects or welds are detected at the launch location in the pulse-echo mode. This technology can quickly examine a long length of piping for defects such as corrosion and circumferential cracking from a single test location, and is already used commercially for inspection of aboveground piping in refineries and chemical plants.
An MsS probe consists of a thin ferromagnetic strip (typically nickel) bonded to the pipe with a number of coil-turns (typically twenty or so) wound over the strip. When activated with a pulse of electric current, the coil generates torsional guided waves (T-waves) in the strip that subsequently couple to the pipe-wall and propagate along the pipe. The guided waves can travel up to 30.5 m (100 feet) from the fixed location of the probe. When the guided waves are reflected from welds or defects in the pipe and arrive at the sensor location, they generate electric voltage signals in the coil that can be related to the defects. Such a probe is relatively inexpensive and could be permanently mounted and buried on a pipe at a modest cost to allow long-term periodic data collection for tracking of small changes in the pipe wall thickness caused by corrosion and cracking, particularly over areas known to be susceptible to corrosion; for example, low points in a gas pipeline where liquids condense.
The potential impact of a successful method of determining extent and location of buried pipeline corrosion from a fixed signal location would be significant if successfully achieved. This would minimize the effort and costs associated with internal inspection or excavation based inspection methodologies. The enhancement in ease of pipeline condition monitoring would permit more frequent system evaluation resulting in an increase in overall safety and integrity of the natural gas infrastructure.
The objective of this project was to demonstrate that the MsS technique can be used to monitor initiation and growth of corrosion in a buried pipeline. The objective was accomplished in two steps. The first step was to conduct preliminary work at the SwRI facilities to evaluate the MsS monitoring technology and to optimize it for field demonstration. This included evaluation and selection of adhesive bonding material for permanent installation, evaluation of the methodology on samples of aboveground pipe and buried pipe with and without coating, and development of procedures for sensor installation, testing, and data analysis. The second step involved working with ClockSpring and El Paso Corporation to demonstrate the technology on an actual pipeline.
The results of the laboratory evaluation showed that five-minute epoxy was suitable for adhesively bonding the sensor strips in the field. Defects on the order of 0.5 percent of pipe wall cross-sectional area loss could be detected over a 4.6-meter (15-feet) range in uncoated, above-ground piping, while on uncoated and buried piping the detection capability decreased to defects on the order of two percent over approximately a 7.6-meter (25-feet) range at 0.6-meter (two feet) of soil cover depth. The decrease was due to increased wave attenuation caused by soil. Coating such as bitumen further increases wave attenuation and further limits the detection capability.
Two field tests were carried out; one conducted on a 610-mm (24-inch) OD, 6.4-mm (0.25-inch) wall thickness transmission gas pipeline in Cleveland, Texas and the other on a 762-mm (30-inch) OD, 11.4-mm (0.45-inch) wall thickness transmission gas pipeline in El Paso, Texas. Both lines had bitumen coating for corrosion protection. The pipelines were excavated in a location near to a known defect area and the MsS probes were installed on the excavated line after removing the coating locally for installation. On the 762-mm (30-inch)-OD pipeline, the probes were also buried by refilling the excavated area with soil, followed by additional testing after one-day and 21-day intervals. SwRI is collecting more MsS data periodically over the next year or so.
This project showed that the MsS probe indeed can be installed on a pipe and buried for long-term monitoring of pipe condition changes. It was determined that the application of MsS to monitoring of bitumen-coated pipelines is limited because of very high wave attenuation caused by the bitumen-coating and surrounding soil, and a resulting reduction in defect detection sensitivity and monitoring range. Based on these results, it was recommended that the MsS monitoring methodology be used in benign, relatively low-attenuation sections of pipelines (for example, sleeved sections of pipeline frequently found at road crossings and pipelines with fusion epoxy coating).
This project is completed and no further tasks remain.
Final Report [PDF] September, 2003