Pipeline &
Gas Journal
March issue 2000:
Intelligent Pig Inspection Of
Uncoated Seamless Pipelines
by Terry R. Shamblin,
Senior Engineer,
Columbia Gas Transmission, Charleston, WV
Many gas pipelines in operation in the United States today were constructed before coating and cathodic protection (CP) practices were at current standards. A number of these vintage pipelines had no coatings and had CP added at dates long after their construction, thus allowing initial corrosion growths. With continual public and industrial growth and development on and around these pipelines and normal maintenance concerns, there is a need to conduct periodic integrity assessments to ensure public safety and maintain pipeline efficiency. One of the best tools now available to measure or gauge pipeline integrity is the intelligent or smart pig.
While there are various technologies offered by smart pig vendors, MFL, or magnetic flux leakage, is the one most commonly utilized for in-line inspections of natural gas pipelines. With the latest advances in MFL smart pigs, it is possible to accurately analyze corrosion on uncoated seamless steel pipelines.
The ability to accurately identify corrosion on such structures provides the mechanism to evaluate the pipeline’s integrity with analytical tools such as RSTRENG, and plan a successful remediation program. A successful remediation program can save the pipeline operator millions when compared to the expensive alternative of replacing the pipeline segment.
Pipeline Integrity Assessments and Risk Management Programs are common bywords in today’s pipeline world. With ever-rising material, construction, and environmental costs, all pipeline operators are looking for better ways to maximize utilization of current assets, prolong facility life and reduce costs. In many instances, integrity assessments are taking the place of expensive pipeline replacements.
This article examines the inspection, evaluation and remediation of seven sections of uncoated, seamless, steel pipelines in the eastern U.S. These line sections were inspected in the last two years with MFL intelligent pigs.
The line segments inspected in these surveys were installed in the period of 1947-52. Cathodic protection was not added to any of the line sections until the late 1950s and early 1960s. Even then, it was a hit and miss program of adding CP to uncoated pipelines. The segments lie in the states of West Virginia, Pennsylvania and New York. They were installed in varying climates and geological environments. The lines operate at stress levels varying from 40-70% Specified Minimum Yield Strength (SMYS). The segment lengths inspected vary from 1.5 miles to 54 miles. Wall thicknesses vary from 0.277 to 0.375 inches. The lines segments are either Grade B steel (SMYS = 35,000psi) or X-45 (SMYS = 45,000 psi.)
Inspection
The line sections were inspected with MFL intelligent pigs within the past two years. Line No. 2 was the first survey conducted of the seven and was inspected with a low or standard resolution pig while the other sections detailed were inspected with high resolution pigs.
Standard resolution tools contain only a few sensors, wider sensor pads, and record data in an analog mode onto a magnetic tape recorder. The newer generation high resolution pigs contain a larger number of smaller sensors, record data digitally onto solid state memory chips and gather much larger volumes of usable data. Anomaly examinations from excavations on these pipeline sections reflected higher degrees of accuracy for the sections inspected with high resolution tools.
As noted earlier, the combination of uncoated pipe and the seamless construction make it quite difficult to grade the inspection logs for these surveys. Although the high resolution pig logs can be graded to as low as 10% wall loss, the data from these inspections was graded only to a minimum of 20%. The additional 10% normally had little affect on analytical strength calculations and the uncoated crust on the exterior of some of these sections usually equated to a 10-15% metal loss, thus complicating the grading process for corrosion losses of that order. The irregular surface on the interior of seamless pipe produces sensor vibration adding “noise” or clutter to the MFL signal plus the varying wall thickness makes it more difficult to properly saturate the pipe wall with the magnetic flux.
The data collected from the in-line inspections of these pipelines included pipe tallies reflecting all pipe joint lengths, mechanical defects, metal loss anomaly details and appurtenance locations. Metal loss anomaly details included peak depth measured as a percentage of the pipe wall thickness, axial length, accurate location along the axis of the pipeline and orientation reported as clock position. Although the high resolution pigs provided the ability to distinguish internal corrosion from external corrosion, no internal corrosion was noted on any of these inspections.
Evaluation
Data obtained from the in-line inspections were analyzed to determine the affect of corrosion metal loss to the operating integrity of the pipeline. First, all anomalies reported in excess of 80% wall loss were listed for bellhole inspections. Second, looking at both length and depth, a representative number of anomalies were identified and analyzed with RSTRENG, an industry-accepted analytical program, to determine their affect on operating pressures. RSTRENG calculations for these anomalies used conservative values for length and depth, i.e., full depth along the entire length of the anomaly and they were assessed in descending order of length and depth. Once thresholds for the minimum length and depth combinations that would lower operating pressures were reached, then RSTRENG calculations were terminated on the reported data.
Not all anomalies reported above 20% wall loss were analyzed, only those length and depth combinations that affected the Maximum Allowable Operating Pressure (MAOP) of the pipeline segment. Generally, wall losses exceeded 50% depth before they fell into this category, although some that were graded in the 30% and up depth range were included. These anomalies were also listed for bellhole inspection and field evaluations. Field evaluations included measuring the identified anomalies and comparing the actual values to the reported values. A larger number of them were re-analyzed with RSTRENG to determine their actual affect to operating pressure.
Remediation
The remediation process for the pipeline segments was very simple. Those anomalies field-verified as having negative effects on operating pressures were removed. Repairs such as weld over sleeves or composite sleeves were not utilized due to the external condition of the pipeline segments. Generally, the light corrosion present on the external wall prevented usage of such repair methods. Anomaly repairs involved replacement of a single joint of pipe or a portion of a joint with an equal length of pre-tested pipe. For those areas with a large number of pressure reducing anomalies in successive pipe joints, longer segments were replaced and hydrotested upon completion. Replacement lengths on these pipeline segments ranged from five to 1,300 feet.
Program Results
The following details reflect the results of inspection, evaluation and remediation for each segment. Costs outlined for vendor inspection and remediation costs include only those contract costs for inspection and pipe replacements. They do not include any costs for modification of the pipeline segments to make them piggable and installation of launcher/receiver facilities. On all of these segments, temporary launcher/receiver facilities were used. Costs mentioned for total line segment replacements are purely estimates based on past experience.
Line 1
Line No. 1 is a 16-inch pipeline located in south central West Virgina lying in a rural Class 1 mountainous terrain. The line was constructed in 1947 and is only 1.5 miles in length. The line is 0.312 wall, Grade B pipe operating at 72% SMYS. This line was inspected with a high resolution MFL pig. Anomalies identified on this segment were all reported to have less than 70% wall loss with the majority of them falling in the 30-39% range. Excavations of the worst anomalies proved the smart pig data to be accurate and although the line’s MAOP is at 72% SMYS, only 120 feet of pipe required replacement. The total cost for inspection and pipe replacement was approximately $84,000. The total estimated cost to replace the line would be approximately $1.95 million.
Line 2
Line No. 2 is a 16-inch pipeline located in north central Pennsylvania, lying in a very rural Class 1 setting with rolling hills and mountainous terrain. A portion of the line traversed an abandoned strip mine area and excavations in that area showed the pipeline lying in a loose shale environment. The line was constructed in 1952 and the portion that was inspected is 22 miles in length. The line is 0.375 wall, Grade B pipe operating at 44% SMYS. This line segment was the first of the seven mentioned in this study to be inspected and was surveyed with a low or standard resolution pig. The anomalies identified in the inspection results reflected wall loss depths from 20-80% with the majority of the graded anomalies falling in the 30-39% range.
Excavations on this line proved the pig results to be undercalled in some instances. Actual pit depths found in some of the excavations were more extensive than reported. This line is operated at approximately 44% SMYS, so there was tolerance for corrosion. Adjustments to reported data were made based on field findings and a total of 1,300 feet of pipe were replaced to re-establish the integrity of the pipeline segment. Total inspection and replacement costs for this line segment were approximately $379,000. The total estimated cost to replace the pipeline segment would be $28.6 million.
Line 3
Line No. 3 is a 20-inch pipeline segment located in southwestern Pennsylvania and lies in a predominately Class 1, rural, rolling hills environment. The line was constructed in 1947 with 0.375 grade B pipe and operates at 71% SMYS. The segment inspected is 33 miles long and with the rolling environment traverses many low lying wet areas. It was surveyed with a high resolution MFL pig. Corrosion levels were quite extensive on this pipeline. Anomalies graded on this line ranged from 20% to greater than 80% with the majority of them falling in the 40-49% range.
Field excavations proved the reported pig results to be accurate. The corrosion quantities and higher operating stress on this pipeline segment required extensive replacements to maintain operating pressure integrity. Approximately 17,100 feet of this line has or will be replaced. Approximate inspection and replacement costs will total $5.9 million. An estimate of the total cost to replace the entire pipeline segment would be approximately $49.5 million.
Line 4
Line No. 4 is a 14-inch pipeline segment located in north central Pennsylvania and lies in a congested rolling hills class 1, 2, and 3 area. The line segment was constructed in 1948 with 0.312, X-45 pipe and operates at 50% SMYS. The line segment is 11 miles long and traverses wetlands, residential areas, apple orchards and other prime farmland. The line was inspected with a high resolution MFL pig. Corrosion levels were high on this pipeline section. A large number of anomalies were graded on this pipeline and they ranged from 20-79% with a majority of them in the 40-59% range and a large number of them in the 60-79% range. With the proximity of this line in the high class location areas as mentioned, major replacements were required to re-establish operating pressures for this section. Approximately 15,872 feet of pipe were replaced. The total inspection and replacement cost for this portion was approximately $7.1 million. Approximate estimated costs to replace this pipeline segment would be $15.4 million.
Line 5
Line No. 5 is a continuation of line 4. This segment of the pipeline follows the northeastern edge of Pennsylvania and ends at the New York border. The line traverses class 1 and 2 areas through rolling hills and into mountainous areas on the northern end. Again, it is vintage 1948, 14-inch, 0.312, X-45 pipe and operates at 50% SMYS in this segment. The southern end of the section travels through many wetlands and farmlands and goes into the mountains on the northern end. Corrosion levels on the southern end of the inspected segment were extensive but reduced significantly the farther north the line traveled. The length of the segment inspected was 54 miles and was surveyed with a high resolution MFL pig. Numerous anomalies were graded on this report and ranged from 20->80% with the majority of them falling into the 40-49% category. To re-establish operating pressures approximately 10,325 feet of pipe were replaced. The approximate cost of inspection and pipe replacement was $5.2 million. Approximate costs to replace the entire segment would be $75.6 million.
Line 6
Line No. 6, although considered a different line number, is a continuation of line 5 north of the Delaware River. It is a 14-inch line constructed in 1948 of 0.312, X-45 pipe. The line is 6.1 miles long and, again, was inspected with a high resolution MFL pig. The line travels through a very rural mountainous area and as noted in the description of line 5, exhibited lower corrosion levels on the northern end of this pipeline system. Few anomalies were graded and they ranged from 20-75% with the majority of them falling into the 40-49% range. Only 85 feet of pipe were required to be replaced to re-establish pipeline integrity in this section. Inspection and replacement costs were approximately $105,000. Estimated costs to replace this pipeline would be approximately $7.3 million
.
Line 7
Line No. 7 is a section of 8-inch pipeline located in southeastern New York state lying west to east through a class 1 rocky rolling hills terrain and traversing a few wetlands. The line is composed of 0.277 wall, grade B pipe constructed in 1949 and operated at 40% SMYS. The line was inspected with a high resolution MFL pig.
Corrosion was extensive on this pipeline segment in areas, predominantly in the wetland zones. Anomalies graded in this inspection ranged from 20-79% with the vast majority of them ranging from 20-40%. Pipe replacements to re-establish MAOP on this segment totaled 320 feet. The approximate costs for inspection and pipe replacement totaled $118,000. Estimated costs to totally replace the line segment are approximately $8 million.
Conclusions
Uncoated steel pipelines should not be considered automatic candidates for replacement. In today’s business environment of making every dollar count, significant cost benefits can be realized by prolonging a pipeline’s life with a properly planned inspection and replacement program. P&GJ
This article was presented at NACE International 1999. P&GJ