High Precision With Ultrasonic Pigging
Ultrasonic pig measurement section.
Being subject to appropriate inspections, pipelines and piping/pipework offer an almost unlimited service life. Strength and actual service capabilities must be assessed at regular intervals. Especially for thin-walled pipes made of high-strength steel, it is of particular importance to conduct regular inspections and examinations to detect any flaws and defects before they actually start to cause problems. Here, ultrasonic and magnetic flux leakage (MFL) pigs have been established as proven and suitable tools for inspection applications.
The utilization of these technologies requires interdisciplinary expertise and know-how. Bearing in mind that such “inspections” shall provide reliable measurements for the assessment of a pipeline‘s condition, several aspects related to material quality, mechanical properties and corrosion conditions must be considered.
The target to obtain reliable results for the establishment of the best techno-economical concept for a pipeline’s service life and condition makes it indispensable to have a broad range of engineering disciplines and experience involved. This involves raising the necessary questions on starting to assess a pipeline’s operational condition, applying the appropriate inspection technology, and the interpretation of recorded data on site (preliminary report) in detail based on determined threshold values (final report), as well as assessment of individual results. Finally, a reliable and safe concept for proper operation and/or rehabilitation has to be developed. This approach will be described later on a crude oil pipeline on which TÜV SÜD provided services for a well-known energy supplier.
Reliability of pipelines is of paramount importance in the energy sector. Verification of the integrity and strength of buried pipelines is aimed at preventing unacceptable defects from occurring on pressurized parts, which under continuous operational loads, may result in leakage or fractures causing costly and unscheduled service interruption.
Assessment Of Defects
Anomalies such as wall thinning which affect the safety and integrity of a pipeline are referred to as "direct" defects. By contrast, indirect defects such as damage to the insulation of a buried pipeline do not involve any direct hazards, although they may evolve into direct defects over time, such as corrosion leading to wall thinning.
Thinning of a pipe wall results in increased stresses in the base material, which, in turn, may limit the service life of the pipeline. Wall thinning is defined as a reduction in wall thickness on the inner or outer wall surface of the pipe, which may have been caused at an early stage during metal sheet manufacturing or later during pipeline operation. It may lead to leakage caused by corrosion or to cracking as a result of material fatigue or because maximum stress limits are exceeded. Wall thinning is frequently caused by milling in the course of pipe production.
Stress analysis is based on the assumption that the actual load-bearing capacity and fatigue strength of a defect can be determined by means of a strength analysis. By taking into account the consequences of dynamic operations such as pipeline startup and shutdown, conclusions can also be drawn as to the remaining service life of pipelines with these defects.
Measuring Wall Thickness
The actual wall thickness of the pipes is measured with intelligent pigs that operate either by applying the ultrasonic or the magnetic flux leakage (MFL) principle. Ultrasonic pigs use the pulse-echo method, in which a transducer generates an ultrasonic pulse that travels through a coupling medium (e.g. oil), is partly reflected from the inner wall, passes through the pipe wall, is then fully reflected from the outer wall surface and returns to the transducer either as frontwall or backwall echo.
The transmission times of the echoes and the differences between the frontwall and the backwall echo are recorded. The transmission times of the echoes and the sound velocity in the coupling medium and in the pipe wall allow exact statements to be made on wall thickness. Utilizing this procedure, the theoretical accuracy of measured wall thickness is approximately +/- 0.2 mm. Using an appropriate number of ultrasonic transducers, scanning frequency and pig velocity, 100% measurement of pipe wall thickness can be performed. This method reliably detects all flaws with a diameter of 10 mm and larger. The depth of anomalies, however, can only be measured exactly from a diameter of 20 mm upwards.
As the ultrasound needs a coupling medium to overcome the distance between the transducer and the pipe wall, ultrasonic pigs can only be used in pipelines transporting liquids. If used in gas pipelines, the diagnostic system requires a suitable liquid batch, i.e. the pig is being propelled through the pipe in a batch of liquid medium, separated by batching/sealing pigs ensuring no bypassing and entry of liquid from the batch column into the gas transported within the pipeline.
Ultrasonic pigs consist of several units (batteries, data recording and ultrasonic units and transducer carriers) which are flexibly interconnected. Pigs can travel through pipes with a bend radius of up to 1.5 D. At present, there are ultrasonic pigs available in sizes between 6-56 inches and suitable for service pressures of up to 100 bars. Depending on their equipment, pipelines with a length of up to 500 km can be inspected without interruption.
German Pipeline Case Study
As part of the re-assessment of an oil pipeline of about 100 km in length, an energy supplier in southern Germany had to furnish evidence that its pipeline, which had been operated for several decades, still complied with the requirements and state-of-the-art. The operator thus had to prove the integrity of the piggable and unpiggable pipeline sections to the approval authority. TÜV SÜD Industrie Service furnished proof of pipeline integrity by evaluating the data collected in various pig runs and finally interpreting them in an integrated approach. Measuring the wall thickness of the pipeline using an ultrasonic pig formed part of the comprehensive examination.
In the case of the crude oil pipeline, 112 ultrasonic transducers with vertical incidence were used. The pigging system plotted the echo amplitudes of every ultrasonic probe over the entire running time, making it possible to determine whether the maximum echo signal received was caused by direct reflection of the signal or by a multiple echo. The system also permitted relatively accurate statements about the depth of near-surface defects in the inner pipe wall (inclusions, laminations). The distance between the transducers in circumferential direction was about 8 mm, the scanning distance in longitudinal direction about 3 mm, and the pig traveled at a speed of ≤ 2.4 m/s.
The scanning probes for the pulse-echo method had a diameter of 10 mm. This setup ensured full coverage of the pipe wall in circumferential direction and overlapping scanning in longitudinal direction. All transducers recorded the inspection data, so that full coverage throughout the circumference and the length of the pipeline was assured. This type of inspection identified all flaws in the pipe associated with wall thinning. The measurement was carried out with +/- 0.2 mm accuracy in the direction of wall thickness and +/- 10 mm in longitudinal direction. Hence, the pipes were inspected in far greater detail than immediately after their production.
Evaluation Of Results
The report summarizing the measured wall thickness of the 100-km pipeline listed a total of 1,554 instances of wall thinning, 241 laminations or inclusions, 114 dents, 16 sites of varying wall thickness (production defect), 308 spots of inhomogeneity (surface and weld defects) and 381 installations. This high number of anomalies demonstrates the extreme sensitivity of the pigging system, which detected flaws with diameters from approxinmately 10 mm to several meters in longitudinal and circumferential direction and a depth of 0.8 mm upwards.
TÜV SÜD Industrie Service then sorted this large number of reductions in wall thickness by classifying depth and length/width and subjected them to strength analysis. To do so, the pipeline professionals used software to determine the actual stress in the remaining wall thickness for each individual defect and compare it with the acceptable stress for the material in question. They re-examined all defects for which excessive stresses were determined, evaluated them and conducted strength analyses. To verify the measured results and identify the root causes of wall thinning, the experts recommended exposing some of the defects and inspecting them on site.
In this context, particular attention was to be paid to the following question: Had these defects been caused during pipe production, i.e. did they already exist before the initial hydrostatic pressure test was performed during pipeline construction, or did they occur during pipeline operation?
Where the pigging results indicated that further inspections were needed, the experts exposed the defects and repeated exact measurements. The actual wall thicknesses and the actual defect sizes measured on these pipe sections were then taken into consideration in strength analysis. All defects that limited strength were documented in an overview plan which reflected the hydraulic situation of the pipeline. If the tolerable stresses were exceeded at maximum service overpressure, the pipes affected by these defects had to be rehabilitated/repaired or replaced. This was done during the next scheduled shutdown of the pipeline. For this purpose, the oil pipeline was emptied and filled with nitrogen, so that defective pipes could safely be removed from the pipeline and replaced by new and tested pipes.
When evaluating the feature list (list of defects), the experts also isolated all defects that were located near areas of subsequent insulation of circumferential welds. These areas are considered highly vulnerable to corrosion as the pipe insulation (bitumen or PE) applied by the pipe manufacturer is replaced at these areas by insulation that is subsequently applied at the construction site. After these defects were analyzed on the computer screen and examined for possible signs of corrosion, they were listed separately and provided with specific comments including "Corrosion suspected", "To be monitored", "Grinding" or "No corrosion."
Where corrosion was suspected, further actions were taken; for example, new insulation was applied. The comment "To be monitored" meant that a defect had to be re-examined within the scope of the next pigging inspection (inline inspection) to ensure timely detection of dangerous increases in corrosion, if any.
Defects with a depth of < 0.8 mm were not detected by the diagnostic devices and therefore not included in the assessment. Generally, these defects have no strength-reducing effect and are detected on time, i.e. before they reach critical sizes, in future inspection runs. The experts confirmed that flaws deeper than 0.8 mm involving active corrosion were unlikely in view of the insulation of the pipe, the effective cathodic corrosion protection, the inspected corrosion coupons that had been buried and exposed to the same conditions as the pipeline to determine the rate of corrosion, and the sufficient input of cathodic protection current.
The objective of the next pig run was to verify that the actions taken to prevent corrosion were effective and that none of the corrosion defects had grown. In addition, the experts recommended intensive measurement of the cathodic corrosion protection at regular intervals to ensure that adequate cathodic protection potential continued in the future.
Based on the results from the pig run, the experts exposed four defects over a distance of about 100 km and compared the sizes of these defects and the residual wall thicknesses with the data recorded by the pigs. The high degree of compliance between the measured values and the values recorded by the pigs confirmed the recording accuracy of the ultrasonic measurement system.
Rehabilitation Of Wall Thinning
To avoid the need for pipe replacement and its complex preparations within the scope of pipeline rehabilitation in cases where pipeline strength is reduced by wall thinning, hot sleeves have proved their worth in practice. These sleeves are fitted around the defect using heat shrinkage and can absorb additional stress caused by internal pressure which the pipeline would otherwise be unable to absorb due to the loss of material. These sleeves enable the effectiveness of stress absorption to be verified easily. The measure avoids costly service interruptions for rehabilitation.
Pipeline inspection with pigs in conjunction with the professional evaluation and interpretation of pigging results can ensure the operational strength of a pipeline. In the case under examination here, an expert opinion was prepared to verify the integrity of the mineral-oil pipeline required by law. The energy supplier's operating license was extended and the energy supplier was informed in detail about the pipeline's state of repair. Using this information, the energy supplier took targeted actions aimed at ensuring the continued uninterrupted service of the pipeline.
Pipeline Inspection With Measuring Pigs
Defects in pipelines can be detected with special intelligent pigs that are carried along passively by the flow of the respective medium in the pipeline. Distinctions are made between the following groups of intelligent pigs:
- Geometry pigs are used to detect deviations from the ideal circular shape of the pipe, such as dents and bulges, ovality, wrinkles, changes in internal diameter, peaking;
- Wall-thickness pigs are used to measure the wall thickness (wall thickness transitions, anomalies caused by manufacturing, inside and outside corrosion, peeling, laminations);
- Crack detection pigs, used to inspect pipe walls for cracks, striation marks, notches, grooves or weld defects in longitudinal directions; and
- Magnetic flux leakage pigs which verify the tightness of a pipeline.
Hans-Joachim de la Camp is head of the pipelines department at TÜV SÜD Industrie Service GmbH in Munich, Germany, and is an authorized inspector. He can be reached at 089 5791-1858
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