Table of Contents ECNDT '98
Session: Pipeline Industry
Operational Experience with Inline Ultrasonic Crack Inspection of German Crude Oil PipelinesH. Willems, O.A. Barbian
Pipetronix GmbH, Karlsruhe-Stutensee, Germany
Deutsche Transalpine Oelleitung GmbH, München, Germany
Corresponding Author Contact:
|TABLE OF CONTENTS|
Pipelines are considered the safest and most economic way of transporting gas or liquids over long distances. Material degradation such as corrosion or cracking, however, can lead to premature failure with potentially catastrophic impact on man and environment. On the other hand, many pipelines are still in sound condition at the end of their design life thus enabling safe future operation. In any case non-destructive inspection techniques for the detection and sizing of material damage are required, if the integrity of pipelines is to be reliably assessed. This is achieved by means of so-called intelligent pigs which allow to inspect up to several hundred pipeline kilometers in one run with respect to special damage types such as e.g. corrosion damage. For the important issue of crack detection, however, intelligent pigs have in the past not been available as the electronics and data processing components required for this application were not at hand before. With the tool referred to in this article, this gap has successfully been closed.
The integrity of older pipelines, in particular, has to be proved by the operating companies at periodic intervals. In the past, integrity testing was usually performed by means of hydrostatic testing. This type of test is expected to remove all critical cracks, i.e. cracks that could cause failure under normal operating conditions. However, since no information on sub-critical cracks is obtained, the estimation of the safe future service life becomes rather uncertain. Moreover, hydrotesting can cause crack growth of near-critical cracks, thus reducing the expected safety margin /1/. Additionally, hydrostatic tests are expensive and time consuming as the line has to be taken out of service.
Instead of hydrotesting, the German regulations for pressurized vessels also allow the application of nondestructive techniques provided that at least equivalent results are obtained. This means that hydrostatic retesting can be replaced by nondestructive inline inspection, if defects which would lead to rupture during a hydrotest can be detected with sufficient reliability. The most critical defects associated with pipeline ruptures are axial crack-like defects (fatigue cracks, stress corrosion cracking (SCC), crack-like weld defects). In order to replace hydrostatic retesting by intelligent inline inspection, it was therefore decided to develop an inline inspection tool for the detection of this type of defects. TAL (Transalpine Oelleitung GmbH) as a Germany based operator of crude oil pipelines was, together with the German TÜV, one of the partners involved in the specification for the crack detection tool and later on in its application.
In order to reliably detect crack-like defects with sufficient margin to critical sizes, the following defect specification was defined for the crack detection tool:
The dimensions given are derived from (conservative) fracture mechanics calculations. In addition to the defect specification, some important tool related specifications are listed below (valid for 24" tool):
Based on these specs the crack detection tool UltraScan CD (Fig.1) was developed between 1991 and 1994. It consists of three modules (battery module, ultrasonic electronics & data processing module, data storage module) and the sensor carrier, all connected by universal joints. A second crack detection tool, which provides up to 896 ultrasonic channels, is available for sizes from 36" to 56" since 1996.
The UltraScan CD tool is based on the ultrasonic technique since only ultrasound allows for in-line detection of external as well as internal cracks with the required sensitivity and resolution. The inspection is performed using the well-proven 45°-shear waves technique /2/. The shear waves are generated in the pipe wall by angular transmission of ultrasonic pulses through a liquid coupling medium. In oil pipelines, the oil itself serves as coupling medium. For the inspection of gas pipelines, the UltraScan tool is run in a slug of liquid in order to provide the ultrasonic coupling /3/.
Because axial cracks are to be detected, the ultrasonic pulses are transmitted in circumferential direction in order to obtain maximum acoustic response. The ultrasonic reflection signals as received in pulse-echo technique are amplified using a logarithmic amplifier which provides a dynamic range of 80 dB. In a first data reduction step, the digitized data are filtered by means of the ALOK-technique /4/. In a second step, only signals complying with certain patterns expected for axial crack-like defects are recognized and stored on the on-board mass storage units. As a result of both steps a total data reduction factor of the order of 10³ is achieved.
The sensor carrier of the crack detection tool is designed such that the complete pipe circumference is uniformly scanned in both clockwise and counterclockwise directions using 480 - 840 sensors (for pipe diameters 24" to 56") for crack detection (Fig. 2). This arrangement provides multiple wall coverage which ensures that relevant reflectors are detected by several sensors. Additionally, two sensors per skid serve to continually measure the actual wall thickness and to detect girth welds, the latter information being used both to produce pipe tallies and to precisely locate defects with respect to the nearest girth weld. All sensors are mounted in a highly flexible polyurethane carrier thus maintaining a constant distance from the pipeline wall and a defined angle of incidence.
After an inspection run is completed, the recorded data are pre-processed offline taking advantage of built-in detection redundancy, thus considerably reducing the amount of indications to be looked at by the interpretation group. The evaluation of the data is performed on personal computers using advanced data visualization software. After completion of data interpretation a features list is compiled in a condensed form, containing the indications requested by the client. Relevant features are fully documented including colored images of the corresponding ultrasonic B- and C-scans along with the necessary information needed for precise excavation. The final report includes a features list, fully documented features and a complete pipe tally with full information on each individual pipe joint (distance, wall thickness, pipe length, location of longitudinal weld, etc.).
More than 1800 km of operating pipelines have been inspected with the crack detection tool since its commercial introduction in 1994. In some cases the tool was successfully used to detect and size in particular SCC-damage in gas pipelines as well as in oil pipelines /5,6/. As far as crude oil pipelines in Germany are concerned, the main reason for inline crack inspection is to replace hydrostatic retesting as a means of integrity testing. The German pipeline sections of TAL consisting of approx. 450 km (sizes of 26" and 40") were completely inspected with the crack detection tool. Inspections with wall thickness pigs and caliper pigs had been performed already earlier. After having completed an extended verification program, the TÜV experts formally stated the equivalence of the inline inspection program and approved the integrity of the pipelines considered (see also /7/).The verification program included hydrotesting of a 22 km / 26" section. In accordance with the findings of the crack inspection, no rupture occurred.
For a successful ultrasonic inline inspection, it is necessary to perform a proper cleaning program prior to the run in order to remove any debris or wax. Wax free or low wax crudes are recommended for the run as otherwise wax may accumulate in front of the sensors and block the ultrasonic coupling.
Due to the high sensitivity of the tool, not only crack-like but a range of different types of reflectors in the pipe wall are being detected. A very important step in the process of data evaluation is therefore discrimination of various types of indications, specifically discriminating between injurious and non-injurious reflectors. In most cases, the type of indications detected by the tool can be classified in the following way:
Based on the results of the data interpretation, 30 verification digs were carried out by TAL in order to verify the overall performance of the crack detection tool. The result can be summarized as follows:
Non-injurious defects with longitudinal orientation such as e.g. elongated, surface-parallel inclusions or laminations produce to some extent crack-like features and are therefore detected and recorded by the online data processing. In more detail, however, the B-scan patterns look different and the overall information given by the tool is such that discrimination can positively be made. In Figure 3, a C-scan of a transition from a relatively inclusion free pipe joint (left-hand side of the scan) into a joint with a high inclusion/lamination density (right-hand side of the scan) is shown. Such dirty pipe joints resulting from improper manufacturing are relatively often found in older pipelines. Since for the TAL pipelines also the wall thickness inspection data are available (which enable an easy identification of surface-parallel reflectors), such features can be verified indirectly by using the information from the wall thickness inspection data. A surface-breaking lamination, however, which has to be treated as a crack-like defect, can be identified as such due to the associated corner reflection. C-scans of the same surface-breaking lamination as detected by the different tools are shown for comparison in Fig. 4.
As opposed to crack-like indications, notch-like indications are identified in the B-scans by a more homogeneous shape as well as amplitude pattern. Such indications are in particular caused by
usually resulting from manufacturing or transportation of the pipe joints.
Typically, the depths of these defects are below 1 mm. They are however a preferential site for crack initiation due to the stress concentrating shape. An example for a notch-like indication is shown in Fig. 5. It was caused by a 1.5 mm deep scratch as revealed by the verification dig. Even such relatively small defects are picked up by typically 4 to 8 sensors. Fig. 5b shows B-scans of this scratch as obtained from three adjacent sensors.
The origin of these defects is in all cases attributed to the manufacturing process. In particular, neither fatigue cracks or nor SCC were found. (However, large amounts of SCC damage have been detected and verified in non-European pipelines /5,6/).
An example of a cold crack detected in a 40" crude oil pipeline is shown in Fig. 6a. The crack was located in the middle of the weld seam. Length was 175 mm, depth was 4-5 mm. The crack consists of a sequence of intercrystalline smaller cracks with mainly axial orientation. The crack was very difficult to find by manual inspection and amplitude based depth sizing would have been totally wrong. Here, the advantages of the automated inspection system providing multiple defect detection, high signal dynamic by logarithmic data compression and context-related data visualisation (see B-scans in Fig. 6b) are obvious.
The results obtained from inspection of almost two thousand kilometers of operational pipelines (crude oil and gas) confirm that the reported crack detection tool meets the requirements for reliable in-line crack inspection. The new tool can detect axial crack-like defects with lengths > 30 mm and depths > 1 mm in the base material as well as in the weld area with high reliability. Offline recognition and classification of the recorded ultrasonic indications is ensured by the multiple detection concept which allows for confident discrimination of the variety of reflector types encountered in pipeline steels.
Based in particular on the positive results of extensive verification work, the use of ultrasonic inline inspection (crack detection, wall thickness measurement) has been approved by the German TÜV as a means of pipeline integrity testing.
The crack detection tool (UltraScan CD) was developed in close co-operation with the Fraunhofer-Institute for Nondestructive Testing (IzfP) in Saarbrücken and the Research Centre Karlsruhe (FZK). The constant support received from TAL during commissioning and validation of the UltraScan CD tool is highly appreciated.