On a Missouri River levy, near St. Joseph, Mo., workers replace with new pipe a segment of that which had contained a wall anomaly. Photograph from Platte Pipe Line Co.
Platte Pipe Line's use of a new inspection tool to identify seam-weld defects revealed that the transverse field inspection (TFL) tool had less impact on pipeline operations and provided greater assurance than hydrostatic testing for the long-term operational reliability of the pipeline system.
Use of the tool was prompted by an operational failure on the Platte system in July 1997. This failure was attributable to operational pressure-cycle fatigue crack growth of a hook-crack in the longitudinal seam weld. The defect had grown to failure over 24 years since the pipe had been hydrostatically tested in 1973.
In response to this failure, the US Department of Transportation (DOT) Office of Pipeline Safety (OPS) issued a corrective order to verify the integrity of this pipeline through hydrostatic testing or an approved in-line inspection program.
Manufacturing defect
Platte Pipe Line is a 932-mile system that transports crude oil from Casper, Wyo., to Wood River, Ill. This system consists primarily of 20-in. OD, 0.312 or 0.344-in. WT pipe.
Platte was built in 1951 and 1952 using three pipe manufacturers and types of longitudinal seam welds: A.O. Smith flash-welded pipe (550 miles); Youngstown electric resistance welded pipe (ERW; 255 miles); and Kaiser single submerged arc welded (SAW) pipe (127 miles).
Fig. 1 shows the system.
The failure Platte experienced in July 1997 was in the flash-welded longitudinal seam of a section from A.O. Smith. The hook-crack, which is an original pipe manufacturing defect, was subjected to operational pressure cycle fatigue causing crack growth that extended in service until it failed at the operating pressure well below previously established maximum operating pressures.
Fig. 2 presents a photograph of the fracture surface.
In response to this failure, OPS required a program to ensure that other similar defect types, which potentially existed in the A.O. Smith flash-welded pipe, would not produce other service failures.
One method of achieving this objective is hydrostatically to test the portion of the system composed of this type of pipe to a minimum pressure to remove potentially critical defects.
Platte recognized the difficulty and limitations of hydrostatic testing this system and decided to pursue another option.
Although ultrasonic in-line inspection tools are currently available for detecting cracks in pipelines, none of these tools was readily available for a 20-in. OD pipeline and would not be available in the near future.
Therefore, Platte decided to work with Pipeline Integrity International (PII) to develop the existing TFI technology for this application. Over 4 months, the TFI inspection tool was developed, built, tested, and placed in Platte Pipe Line.
Program development
The development program was to determine the capabilities of the TFI tool that had never been used in a pipeline system to identify seam-weld defects. Therefore, the performance specifications (detection, discrimination, and sizing capabilities) were unknown.
The capability of an in-line inspection program is measured in three ways:
- The tool's ability to detect.
- The ability of the data-analysis techniques to discriminate.
- The ability to size the pipeline anomaly identified by the tool within measurable and predictable accuracy-tolerance levels.
Platte developed a program with consistent parameters so that all defects examined could be evaluated and reconciled in the same manner. Since this would be a new program, it was developed under a team approach.
The program was validated and finally implemented throughout autumn and winter conditions within the bounds of an operating pipeline. Procedures were written for each of the field activities so that the program was applied consistently.
Reporting and documentation were also critical. Details on the project management follow presently.
Magnetic flux leakage (MFL) inspection tools used for corrosion-caused metal loss impose a strong magnetic flux field in the axial direction of the pipe. Magnetic flux lines of force will be deflected if metal loss is present in the pipe wall, the properties of the pipe wall steel change, or ferrous materials are next to the pipe wall.
The primary sensors within the inspection vehicle detect a change in magnetic flux density ("leakage") and the inspection vehicle records the signal amplitude of the density change.
This signal amplitude and related signal characteristics allow data analysis to predict the nature of the anomaly that caused the signal response and to predict the severity of pipe wall metal loss.
Pipe wall anomaly orientation has a sizeable impact on signal response and affects the capability of the inspection tool accurately to detect, discriminate, and size pipe wall anomalies.
For a conventional MFL inspection tool that induces a parallel mode magnetic flux, those anomalies with an orientation transverse to the pipe axis provide the best potential for a valid and true signal response, while conversely those anomalies orientated parallel to the pipe axis provide the least.
This can be illustrated by placing a board in a river. If the board is placed parallel to the flow of the water (flux), few ripples are noticed. If the board is placed perpendicular to the flow of the water, major ripples can be seen.
An inherent limitation of MFL tools used for corrosion-caused metal loss is that the technology is "blind" to narrow, axially oriented cracks, specifically, cracks within a longitudinal seam-weld.
In addition, the accuracy for sizing narrow, axially oriented corrosion can be limited especially where the corrosion may extend significantly in the axial direction. In the case of corrosion-caused metal loss, the inspection vehicle may detect the metal loss, but the dimensions of the corrosion, especially the depth, may be reported as less severe than actual depth of corrosion.
The tool will most likely not even detect the crack anomaly.
A TFI tool induces a transverse magnetic field and magnetizes the pipe circumferentially as opposed to axially. An advantage of this approach is that anomalies orientated axially, specifically longitudinal seam-weld cracks and longitudinal corrosion, can be more reliably and accurately detected and sized, similar to placing the board perpendicularly in the river.
Although the potential advantages of this magnetic flux technique have been obvious to inspection-tool manufacturers and service companies since the mid-1970s, it is only recently that developments and technological advances have finally permitted the viability of this approach.
Here is a description of Platte's program to evaluate the feasibility of this inspection technology and to verify the program relative to hydrostatic testing.
Initial stage
These were initial objectives for the TFI development program:
- Develop a 20-in. diameter, 16-bit, 400+ sensor TFI inspection tool.
- Conduct laboratory testing and "pull test" verification, as required, ensuring the validity of the TFI approach.
- Inspect a designated section of the Platte pipeline and validate inspection results through excavation and anomaly measurement.
The 20-in. inspection tool was built in early 1998. The subsequent "pull through" testing was carried out at PII's UK facilities in April 1998.
A total of 17 pull-through tests were conducted on actual pipe sections with seam-weld cracks and manufactured seam-weld cracks. The testing results indicated this technology was acceptable for an inspection of a portion of the Platte pipeline.
Field operations
Approximately 12 cleaning tool runs, 12 geometry runs, and 22 TFI tool runs were required to complete the cleaning and inspection of the entire 932-mile, 20-in. pipeline system. This inspection was completed over 6 months.
Early in the inspection-tool development program, field-anomaly assessment was discovered to represent a difficult and critical challenge to the success and validity of the TFI approach.
Field tests provided strong indications that conventional sheer-wave ultrasonic technology was unable reliably to detect and size cracks with the degree of confidence required for a project of this nature.
A unique nondestructive examination (NDE) approach was identified to provide increased accuracy and consistency of results. The NDE technique selected for this project is referred to as FAST (flaw assessment and sizing technique).
This relatively new technique for the pipeline industry uses a multimodal ultrasonic sensor probe developed for use in the nuclear industry. That industry was experiencing difficulties performing examinations using conventional shear-wave techniques and sought a better examination method.
Shear-wave examinations within thin wall pipe using the traditional pulse-echo methods resulted in sensitivity problems: The noise often masks the defect signal response causing inaccurate defect sizing. Due to the difficulties with conventional shear-wave techniques and the need to comply with increasingly stringent regulations, an alternative ultrasonic technique was sought.
FAST was born from this quest. This technique was very successful at identifying crack-like anomalies. Comparison between the FAST NDE results and destructively examined pipe samples proved this technique accurate as a mode of field crack identification.
Additionally, the NDE program included wet magnetic-particle inspection to reveal surface breaking anomalies and standard sheer-wave ultrasonic test for complex geometry.
An overview of FAST follows.
Shear-wave examination techniques were initially employed to identify anomalies on the Platte pipeline as identified by the TFI tool. Ultrasonic shear-wave probes using conventional angles of 45°, 60°, and 70°, 5 Mhz, 0.25-in. diameter were used. A typical ultrasonic test instrument was calibrated to a reference standard similar to an ASME-type calibration block.
Limitations to this process include the following:
- It is amplitude-based with potential to allow critical flaws to go undetected.
- It is operator dependent and may not always be repeatable from one examiner to the next.
- Flaw orientation is critical to detectability.
- Minor manufacturing anomalies made evaluations more difficult.
The primary advantages of FAST are the access to readily available technology and readily available technicians.
Application of the newly developed ultrasonic inspection technique on Platte was based on results obtained during a trial demonstration performed on actual anomalies identified by the TFI tool. A good correlation was established between the actual defect dimensions (destructively examined) and the data obtained using the FAST technique.
A technique was developed specifically for the examination of the flash-welded longitudinal-seams due to the unique geometry of the weld. The inspection area was divided into two zones to make the inspections precise and easier to interpret.
Zone 1 consists of the upper half of the weld area. Zone 2 overlaps the lower part of the Zone 1 examination area and continues to the weld's internal surface.
This two-zone approach utilizes the OD creeping wave and the 70° longitudinal wave. The OD creeping wave detects flaws at or near the surface of the weld. The 70° longitudinal wave overlaps this area and covers the weld area to the ID surface.
Field tests also showed that pipe external surface preparation and cleaning was a critical component in ensuring accurate and consistent anomaly field assessment.
And the tests showed that detailed procedures were developed for pipe cleaning, field assessment of defects using the FAST UT approach, NDE reporting requirements, and training for all NDE project staff.
At the onset of this project, analysis of the TFI data could be defined as "low resolution analysis of high resolution data." Inspection-tool capabilities and tolerances relative to detection of pipe wall anomalies proved to be exceptional.
Computer-enhanced interpretation of results, however, was not possible at this early stage of development. Inspection-data analysis of results was therefore manual, consisting of several repetitive and laborious analysis passes through the vast accumulation of data.
Platte estimates that each mile of pipeline exhibited 500 pipe wall anomalies identified in the seam weld; few of which were actual cracks. A typical inspection segment distance of 70 miles resulted, therefore, in 35,000 anomaly signal responses being examined several times before issuance of a final pipe-section-anomaly excavation list.
An on site Platte representative provided input during the final analysis pass stages and acted as liaison to field NDE staff. This real-time analysis approach allowed the rehabilitation team gradually to eliminate unnecessary excavations.
Data quality control
MFL data analysis for corrosion-caused metal loss is based upon years of development, pull-test data, and field-measurement feedback. The study of analyzing conventional MFL data began with the manual analysis of inspection paper-log rolls and has progressed to automatic analysis utilizing the latest computer enhanced interpretation techniques.
Since this technology has a significant track record, very few discrepancies between the predicted results and the subsequent field NDE results are expected.
A documented data-quality control procedure was required that answered three critical questions:
- Was the data-analysis team correct in defining the relative location and type of the anomaly in question?
- Was the survey team accurate in pinpointing the location of the anomaly for excavation?
- Was the NDE field technician accurate in defining the nature and significance of the anomaly in question?
This procedure relied on constant feedback and monitoring of NDE results as follows:
Step 1: NDE results were documented in short form within a spreadsheet and daily forwarded to the Platte-PII data analysis team. Results were continually correlated relative to location and nature of the anomaly and against the TFI representative signal response.
Step 2: NDE field technicians were required to examine an area on 2 ft either side of the anomaly location as identified within the excavation list to ensure that the correct anomaly was being investigated.
Step 3: NDE field technicians were required to notify the analysis team immediately if the location of the anomaly varied from the location identified within the excavation list.
Step 4: If the validity of the field findings was questioned, the following data-confirmation procedure was then undertaken:
- The analysis team confirmed the location as identified during the data-analysis procedure.
- The field survey team was requested to resurvey the specific location.
- The NDE team was requested to re-examine the pipe section utilizing a different NDE technician to provide an NDE audit.
Results of each stage of the quality-control process was documented, approved by the technical manager, and filed for future reference. Only after these conditions were satisfied was the result declared official and entered into the NDE database.
Field testing
A 50-mile section of the Platte pipeline containing A.O. Smith-manufactured flash-welded pipe, downstream of the Gurley, Neb., pump station was selected for the initial field-testing.
Platte selected 50 TFI signal responses, in 10 excavation sites, for excavation and NDE. TFI signal responses were selected, not solely based upon signal amplitude and perceived severity, but rather to reflect a wide variety of signal-response characteristics including uniqueness of signal shape or curvature, within the weld seam, proximity to a girth weld, and number of channels affected.
The results of the field examinations proved that the TFI inspection tool was effective in detecting a wide variety of pipe wall anomalies including corrosion, mill-related discontinuities, and specifically, longitudinal seam-weld defects (lack of fusion and hook-crack).
All sections excavated were removed as a cylinder and subjected to destructive metallurgical testing in order to confirm NDE findings and inspection tool signal responses.
At this point, although confidence that the TFI inspection tool could detect seam-weld cracks was achieved, similar confidence in inspection tolerances relative to anomaly discrimination and sizing was not present.
A repair program based upon TFI inspection tool technology would require that every longitudinal-seam weld anomaly signal response be excavated for NDE examination.
Procedures, specifications
The importance of proper procedures can be described as follows:
- To ensure the safety of all work crews.
- To ensure compliance with engineering and jurisdictional requirements.
- To standardize data acquisition.
- To permit effective data management.
- To standardize data analysis.
- To maintain effective quality control.
The field procedures developed for this project had to be achievable, repeatable, and controllable. In order to maintain consistency, a program involving certification-qualification, project training, and general auditing was developed.
More than 27 written procedures were developed to address safety; site requirements; surface preparation; repair, site excavation, and anomaly identification; NDE methods and application techniques; pipe segment marking and engraving; digital image acquisition and storage; and field reporting.
Before implementation of this program, a meeting was held with the Central Region OPS office to outline details of the program. Since NDE accuracy and reliability were critical components within the program, the FAST technique was tested in the field and an OPS representative was able to get a hands-on assessment of the technique.
The overall implementation of the program, as proposed to the OPS, included a validation section and a hydrostatic test section.
In the validation section, all anomalies identified by the TFI tool were removed and destructively examined to prove the NDE technique accurately characterized all anomalies and that all crack-like anomalies were correctly classified during the TFI data analysis.
The purpose of the subsequent hydrostatic test was to prove that this program was at least as good as a hydrostatic test because hydrostatic testing was the current standard for pipeline-integrity verification.
The success of the development program suggested that the TFI program was a viable option for the detection of longitudinal seam-weld defects. For development of a formalized program, criteria were required for the assessment, excavation, and repair of the anomalies.
The TFI data assessment was primarily a manual process that included identification of MFL signals that impacted the longitudinal seam weld. Based on development of a discrimination model, all crack-like signals that impacted the seam-weld were then identified for excavation. Upon excavation, all locations were subjected to NDE.
A location was then identified for repair (pipe replacement) when the NDE identified a crack-like seam weld anomaly with a measured depth greater than 10% of the wall thickness. This 10% threshold was used to provide a tolerance for the NDE measurements in the field to discriminate between actual defects and "noise" associated with the seam-weld geometry.
Validation
The validation program was conducted downstream of the location for the development program, a 73-mile section between Chapel block valve and milepost 324. This validation program consisted of the following:
- Analysis of the TFI data to identify all seam-weld anomalies.
- Excavation of all crack-like seam weld anomalies.
- Nondestructive evaluation of locations reported by the TFI tool.
- Pipe replacement of all locations where NDE identified a seam-weld crack with a measured depth greater than 10% of WT.
This validation program resulted in 262 excavations. Of these, 63 were removed from service through pipe replacements. Several of these seam-weld defects were subjected to destructive evaluation.
The purpose of this evaluation was to develop and validate the TFI data-assessment methods and the NDE measurements in the field. This destructive evaluation included pressure testing pipe cylinders removed from service and metallographic examination.
An example of one of these locations subjected to destructive evaluation is identified as Repair Number 640. The TFI tool reported this location as an 11.3-in. seam-weld anomaly.
Upon excavation, this location was subjected to NDE and classified as an ID hook-crack with a depth of 50% WT. This pipe section was then removed from service and pressure tested by welding end-caps onto either end of the pipe section containing this defect and pressurizing the cylinder with water.
This defect subsequently ruptured at a pressure level of 1,550 psig (95.5% specified minimum yield strength, SMYS). Fig. 2, showing this fracture surface, identifies the location of the hook-crack (original pipe manufacturing defect) and the subsequent crack growth (pressure cycle fatigue).
It should be noted that this type of destructive evaluation and testing of defects removed from the pipeline provided continuous information for further confirmation of this program.
The defect described is a perfect example of what is referred to as a "just-surviving"defect. That is, if the pipeline system were hydrostatically tested to stress levels between 90% and 95%, this defect could have just survived the test and could have extended while in service such that it reached a critical size to fail at the operating pressure.
This is one major advantage of the TFI program over standard hydrostatic testing.
All of the features identified as seam-weld anomalies by the TFI tool were identified for excavation. These locations were excavated, subjected to NDE, and removed from service based upon the NDE results.
Once all of these locations were addressed, this 73-mile section was successfully hydrostatic tested with no failure to stress levels between 90 and 95% SMYS. This successful hydrostatic test provided confirmation that the TFI program was a viable option and supported continuing the program.
Program implementation
Subsequent to validation of the program and the successful hydrostatic test, another meeting with the OPS discussed results and plans to implement the program on the remaining 450 miles of pipe manufactured by A.O. Smith.
The implementation phase began in late summer 1998 and was completed in March 1999. Throughout this program, 1,262 excavations were made to locate and remove 381 crack-like features.
An additional benefit to the nitrogen purge and cutout method of repair was that unrelated integrity issues such as creek crossings, exposed pipe locations, abandoned casings, and third-party damage could be addressed more cost effectively.
While the excavation to crack-like feature ratio was high during the implementation phase, subsequent work in refining the TFI data review process has already been proven to reduce this ratio. These results can be used significantly to reduce the number of unnecessary excavations.
Development and validation aspects of this program demonstrated that the TFI program was appropriate for Platte, and field work was initiated.
Project management
Because in-line inspection and repair affect pipeline operations in many ways, project management worked hand-in-hand with field operations, pipeline control center, pipeline scheduling, and owner representatives.
Weekly meetings ensured all parties were informed about upcoming activities and impending scheduling activities, whether project-related or pipeline-related. Daily communication between these parties was also held as needed.
Because the owner wanted to complete repairs and reduce the impact on shippers, the construction team was already in the field, completing a previous line section, as the data became available for the next line section to be repaired.
This schedule caused the work plans, scopes of work, material acquisition, contractor requirements, and landowner contact lists to be in constant flux. To maintain organization, effective communication was paramount. To maintain cost control, timesheets were collected daily, and the project cost accountant adjusted the project paid and committed amounts daily.
With input from the project manager, the forecast project totals were revised and updated weekly.
Repair methodology
The repair method for this project cut out crack-line seam weld anomalies identified by NDE. For this, a section of the pipeline was purged with nitrogen.
A typical schedule involved surveying, completing excavations, NDE, and preparatory work on a 100-mile section over 25-30 days, followed by a 10-day pipeline service downtime to purge, complete all cut outs, and recommission the line.
Following repairs, a clean-up crew remained on site to return the right-of-way to the original condition, while the excavation crew moved on to the next line section. At any given time, 10-15 crews were working at different locations.
To take advantage of the line being evacuated to complete the repairs, additional line section maintenance work was also undertaken. Examples of these activities included burying or replacing existing spans, lowering exposed line sections, and installing or repositioning mainline block and check valves.
Excavations
The pipeline excavation and repairs were completed through use of tight quality assurance and control procedures to ensure the location of each anomaly was established with confidence. Following excavation, the coating was removed around the anomaly, and the immediate area was sandblasted to prepare for the nondestructive examination of the pipe.
Nondestructive examination was performed to determine if the anomaly met the established cutout criteria. If the anomaly met these criteria, the NDE technician also marked the pipe cut locations and performed a lamination scan of the new girth weld area. Pretested pipe was cut and strung on the right-of-way for each cutout location. If necessary, the pipe was bent and retested before to stringing.
All efforts were made to maximize the preparatory work and minimize the required pipeline service downtime. A set of written procedures ensured that each step was being completed in the same manner from one crew to the next.
Nitrogen purge
The nitrogen purges were designed to displace the line contents at approximately the same flow rate as normal operations.
For this, such operational conditions as variable-frequency drive speed restrictions, topography, and available pumping units were taken into account. Nitrogen suppliers were bid based on hydraulic conditions.
When the nitrogen pigs passed field block valves or intermediate pump stations, the nitrogen pump truck was repositioned ahead of the valve to continue pumping. This allowed the field or station valve to be closed, and the evacuated line section could be vented.
This extra step was taken to minimize downtime by getting cutout crews working on an evacuated line section shortly after normal operations ceased.
After the nitrogen had been vented to atmosphere, the cutout crews would begin work. Typically, between five-to-eight cutout crews were used during the repair phase.
Recommissioning; records
After all repairs were completed, recommissioning pigs were inserted at the head end of the evacuated pipeline. In-line receiving facilities were installed at the downstream end of the evacuated pipeline, and fractionation tanks were set up to vent air in the pipeline.
The line was refilled with oil from system tanks, and the pipeline section was proof-tested for 1 hr to ensure system integrity prior to resuming normal operations.
Throughout the entire project, the project management team maintained rigorous record-retention standards.
Examples of these records include purchase orders, pipe material test reports, pipe hydrostatic test records, pipe reconciliation, installation records, X-ray maps, X-ray sheets, welder qualifications, NDE technician qualifications, NDE records, NDE summary spreadsheets, and written procedures utilized.
Drafting personnel updated the alignment sheets and as-built all facilities modified as part of the repair program.
Benefits
The benefits of this method included:
- In the case of Platte, the 10-day duration of pipeline outages was far preferable to a possible 4-month continuous outage required with a hydrostatic test program.
- The defects that are subject to growth because of pressure cycling were removed with this technology. And with this method "just-surviving" defects were removed to avoid eventual failure if left in the system, thus improving the asset.
- The use of nonintrusive repair methods for cracks, third-party damage, and corrosion could eliminate downtime altogether.
- The TFI technology is less costly than ultrasonic-vehicle technology.
- The TFI technology is less costly than hydrostatic testing.
- An inspection program provides information on the status of the integrity of a pipeline system.
The authors
Ed A. Yasinko has since 1995 been a pipeline consultant under contract to Platte Pipe Line Co. to oversee the reconstruction and modernization of the Platte system. He was president and chief executive officer of Producers Pipelines Ltd. and Westspur Pipeline Co. 1990-95. Yasinko is a graduate (1958; with distinction) from the University of Saskatchewan in mechanical engineering. He is a member of the Association of Professional Engineers, Geologists & Geophysicists of Alberta, the Association of Professional Engineers & Geoscientists of Saskatchewan, and a member of Canadian Standards Association International Technical Committee on Oil & Gas Pipeline Systems (Z662).
Patrick Vieth is a senior group leader with Cortest Columbus Technologies Inc., Dublin, Ohio. Before joining CC Technologies, he held positions with Battelle and Kiefner & Associates Inc. Vieth holds a BS in mechanical engineering from Ohio State University and is a member of NACE and ASME.
Dean D. Dick is a project manager for AEC Express Holdings Inc., and formerly a project manager for Marathon Ashland Petroleum Co. He holds a BSEE (1988) from the University of Wyoming.
Troy Pierantoni is a senior project engineer for Rooney Engineering Inc. and was previously with Marathon Pipe Line Co. He holds a BS in civil engineering from the University of Wyoming and is a registered professional engineer in Wyoming.
Phil Nidd is vice-president for AGRA Pipeline Professionals Inc. He has 18 years' experience in pipeline-integrity assessment and rehabilitation project management. Nidd is a member of NACE and the American Society for Metals.
Based on a presentation to the International Pipeline Conference 2000, Oct. 1-5, 2000, Calgary.