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7th Int. Conference on Acoustic Emission, 12-15 Sept.2012
Damages Monitoring for Oil and Gas Pipeline Using AE Technique
Sergio BUDANO1, Antonio LUCCI1, Roberto PIANCALDINI1 Luca PRANDI2 Giuseppe GIUNTA2
1Centro Sviluppo Materiali, Rome, Italy [email protected], [email protected], [email protected]
2eni SpA, gas and power division, San Donato Milanese, Italy [email protected], [email protected]
Abstract A full-scale pilot line has been built, buried and submitted for a long-term hydraulic fatigue test in CSM Sardi-nia full-scale lab. The internal cycling pressure reproduces what happens in pipeline when it is in service condi-tions. On outer pipe surface some damages (dent and gouge type) have been intentionally created in order to re-produce the mechanical interaction between ground movement machines and pipes. The damages are supposed to be preferential site for crack initiation and growth during service operation. Other potential places for defect initiation are also considered, including seam and girth welds. In order to monitor the possible crack growth, the pipeline has been instrumented using several AE sensors, connected to two different AE monitoring systems. The test is still continuing, but it has allowed identifying and solving some relevant technical issues for a relia-ble AE monitoring for pipeline application, such as the minimum number of sensors to be installed, proper in-stallation, and set up procedure and data analysis. Keywords: long-term AE monitoring, buried pipeline troubles, gas and oil transport 1. Introduction In the framework of the eni gas&power research project oriented to develop a reliable Acous-tic Emission (AE) system for monitoring critical sections of transmission pipeline, a specific study has been carried out jointly with Centro Sviluppo Materiali (CSM) aimed at investigat-ing AE features and applicability on steel pipelines widely used in the oil and gas industry. Around the world, gas and oil companies are engaged to improve the safety of the pipeline to transport non-renewable energy resources at high pressure over thousands of kilometers. The presence of discontinuities in the equipment submitted to internal pressure may cause failure. In the case of oil as transported medium, the leakage of the pipe causes serious environmental damage in the surrounding ground. For high-pressure gas pipelines, the crack instability on the pipe wall could bring a bigger disaster with the burst of the pipeline and, consequently, the explosion of the filling gas. Currently, the safety and reliability of the pipelines are guar-anteed by periodic examinations carried out by pig inspections. Nevertheless, the non-destructive testing (NDT) using pig cannot be scheduled frequently and is sometime ob-structed for geometrical mismatching. These problems can be overcome by Acoustic Emis-sion Technique (AET). The passive NDT, such as AET, is advantageous over the active NDT, and can be used for both inspection and monitoring. During the inspection we can obtain good results in term of evaluation of the reliability for the equipments after the in-service period. Defining the moni-toring as very long time inspection for operating pipeline, the following problems can arise: the remote test area could not permit a continuous presence of technicians (i.e. desert or
frozen areas, etc.); the test time could be very extended due to the stationary process involved; the pipeline section to be monitored could be very long; the inspection environment could be harsh in the trench.
A long time service is needed to monitor the damage evolution due to quasi-static loads and simultaneously severe environmental conditions. Regarding the monitoring of pressurized
7th Int. Conference on Acoustic Emission, 12-15 Sept.2012
vessels and pipes, the literature reports the successes of the AET to detect both the leakage and the crack initiation and propagation of the defects/geometrical discontinuities located on the pipes, with high resolution as well [1 - 3]. On this subject the literature shows that the most significant activities have been carried out by Petrobras [4 - 6]. Petrobras dedicated more than 20 years to study the AE technique and to test vessels, reactors, valves, risers, off-shore structure, etc., but the literature does not report clearly the activity carried out for in-service monitoring of buried pipeline. The aspect of the monitoring of the leakage-flow for in-service pipeline has been studied. From the point of view of the AE installation, Anastaso-poulos optimized the sensor frequency, their inter-distance, the procedure to prepare the ex-posed surface of the pipe for the sensor coupling and the test pressure [7]. Anyway the test time was relatively short (some days) and the sensors were installed on an unburied pipeline. The installation problems met during the AE monitoring phase of a pipeline devoted to the gas&oil transport at high pressure have not been mentioned in the literature. The present paper reports the procedure and the technical solution found during experimental AE monitoring of buried pipelines submitted to cycling pressure for more than one year.
Figure 1 – CSM full-scale lab tests (Perdasdefogu Sardinia – IT) aerial view. 2. CSM background and experimental activities A large and safe area located in Perdasdefogu-Sardinia (IT) is the base of the CSM full-scale test laboratory, see figure 1. Many test equipments and facilities are available to carry out burst test, fatigue, bending and hydraulic tests and check the performances reproducing the service condition of pipeline and CGC (gas compressed cylinder).
7th Int. Conference on Acoustic Emission, 12-15 Sept.2012
Pipeline plants have been built to test a full-scale pipe in order to replicate the service condi-tion in term of buried pipeline, cathodic protection system, loads due to internal pressure (static and cycling), different filling fluid transported (air, gas, water). Among the aims of the project it is to evaluate the reliability assessment of buried pipes based on Acoustic Emission Monitoring Technique in presence of the external defects caused by third party damage. This is a typical damage caused by the impact of a excavator bucket on the pipe shell. This third party damage is considered very severe for the line reliability, espe-cially when environment assisted cracking phenomena are present, that are typical for under-ground environment. Such defects are replicated on the pipeline wall by an ad hoc Mechani-cal Damage Simulator (MDS) that is available at CSM laboratory. This rig is able to repro-duce the external defects on the pipe wall surface (see figure 2). The literature defines these defects as “gouge”, “dent” and “dent & gouge”.
Figure 2 – CSM Simulator of Mechanical External Damage and view of the damage
The experimental pipeline laid in CSM full-scale lab has been designed selecting the pipes made of both API 5L X80 and X100 steel grade. These steels are considered as new genera-tion of high-strength steel grades and could be used in high-pressure gas transport projects. The selected pipes were manufactured through two different processes: the first is from wide plates by UOE process and Longitudinal Submerge-arc Welding (LSAW); the second from reduced plate width using Helicoidally Submerge-arc Welding (HSAW) process. The pipe dimensions are: external diameter D=1.24m, wall thickness in the range 16.6-18.4mm, and length 10-11m. In order to set the pipes into the MDS simulator, each pipe was cut into 2 sectors. Finally, two pipelines were assembled and identified as “Section A” of 161m total length and “Section B”, total length of 33m, see figure 3. The ends of the pipelines were closed by specially designed end-cups. The layout of AE monitoring was set up to focus on 19 dent & gouge defects and 40 butt welds. In order to accelerate the growth speed at damages (dent & gouge) the buried pipelines were submitted to both overvoltage cathodic protection and pressure cycling in the range of values 65-72% and 79-88% of specified minimum yield strength (SMYS), for section A and B re-spectively. The picture of the pipeline in the trench, before being buried, is shown in the figure 4.
7th Int. Conference on Acoustic Emission, 12-15 Sept.2012
Figure 3 – Section A and Section B installed on the Line/Trench B. Each pipeline is independently controlled in pressure.
Figure 4 – Trench B – Section A. Pipeline instrumented and ready to be buried.
In order to investigate the relationship between the defect growing and the AE monitoring, we have planned 2 monitoring periods divided by stops, in which: i) the line was unpressu-rized and unburied; ii) the dent & gouge defects were submitted to NDT inspections (magnet-ic particle and ultrasonic examinations). The state of the experimental activity of Section A and the Section B after 15 months is summarized in the Table 1:
Table 1 – Status of the Line B after 15 months of pressure cycling period (id).
SECTION Period “id”
Months Pmax (bar)
Pmin (bar)
Cycles Cathodic Protec-
tion CP (Volt)
Number of Stops
For NDT A
1 12 150 135 2280
≈1.0 2 B 125 110 2988 A
2 3 150 75 338
≈2.5 1 B 1333
7th Int. Conference on Acoustic Emission, 12-15 Sept.2012
3. Acoustic emission installation For AE monitoring of the Line/Trench B, a total of 35 AE sensors are used. In particular, Section B (L=33m) has 6 AE sensors installed to monitor 8 dent & gouge defects and 7 butt-weld. Section A (L=166m) has 17 AE sensors plus other 12 AE sensors connected to 2 dif-ferent brands of AE systems, respectively. Section A contains 10 dent & gouge defects, and 22 butt weld. The main problems in design of AE monitoring of pipelines are related to:
1. buried pipe in aggressive soil; 2. sliding movements; 3. long-term inspection.
Effect due to interaction with the ground The AE sensor installation procedure has to take into account different items: to guarantee the best contact between the AE sensor and the pipe surface; to protect the AE sensor from both the ground and the humidity. In order to satisfy these needs, a suitable box for containing the sensor has been designed. The box has been selected on the market choosing the product with International Protection Rating, equal to IP67. This rate is referred to both the Solid Particle Protection (9) and the Liquid Ingress Protection (9). These values are the best for on-shore application. The de-signed box was patented by eni g&p [8], see figure 5. Inside the box the acoustic sensor and the preamplifier are installed. The anchor plate has been designed to be used as interface between the box and the pipe wall surface. This contact area must assure the humidity insulation and the adherence of the materials. In this way a neoprene layer have been taped on the pipe surface. The AE sensor is located inside a cylin-drical holder (see “AE sensor”, figure 6). A screw was designed to press a calibrated spring, which is in contact with the sensor. In this way a constant force between the sensor and the pipe shell is guaranteed. Lastly, two belts are allocated to fix the box around the pipeline circumference, figure 5. A discharger has been installed to save the AE line from external high voltage perturbation (i.e. lighting), figure 6.
Figure 5 – Box designed for AE sensor anchored by
belts to the pipeline
Figure 6 – Photo of the components inside the box
7th Int. Conference on Acoustic Emission, 12-15 Sept.2012
The AE sensors have been connected to the AE devices by using standard BNC connector and RG58 cable. The maximum length of the cable was just greater than 100m. 4. Buried pipeline - Problems During the 1st period of AE monitoring of the pipelines, Section A and Section B were sub-mitted to service conditions for 1 year under cyclic pressure and overvoltage cathodic protec-tion. This period was helpful to simulate an AE monitoring in a remote area as well as to ac-celerate the damage growth in dent & gouge defect areas. A test procedure for checking pe-riodically AE data acquisition systems were scheduled. It consisted in acoustic pulsing cali-bration and in measuring both attenuation and speed of sound. These measurements were compared to the values acquired before starting the line monitoring. After 1 year the AE measure chains has undergone by 30% of degradation compared to the start-up value. This condition determined the expiration of stop n. 2. The trench was dug up and all the AE data chains were inspected. The following main problems were noticed. Some AE cables and connectors of the data chains were damaged. Traces of humidity were found inside the boxes as well as oxidation on the neoprene
coupling. The boxes slide against the ground due to the cycling pressure (figure 8). This pipeline
sliding could be assimilated to the snake movement. The sliding produced the damage of the belt anchor, figure 9.
Some AE sensor installation failed. For one brand the damage of the AE sensors were found on the rubber ring in the contact area, figure 10.
The 2nd sensor brand failed too. PZT crystal detached from the internal support, figure 11. The silicone gel used as coupling media between the sensor surface plate and the steel pipe wall was very degraded. It fully lost its performance.
Both electronic devices selected to carry out the AE monitoring met failure. Although improved, the network located in this far area did not allow the systematic
downloading of the data to the remote server.
Figure 8 – Sliding of the boxes
Figure 9 – Damage of the belt anchor due to the slid-ing of the pipeline.
sliding
Damage – the plate was bended
7th Int. Conference on Acoustic Emission, 12-15 Sept.2012
Figure 10 – AE Sensor. Surfaces damaged Figure 11 – Plate detached .
5. Solutions found Although both CSM and eni g&p have great experience in the pipeline gas transport, the problems found required to do some careful consideration to understand the cause of the troubles and the approach to find the best solution to improve the reliability of the AE moni-toring technique. The main issues discussed and closed are the following. The presence of the humidity inside the box. This source is the cause of the presence of
the rusting on the contact surfaces between the sensor and the steel. The humidity seeps from the ground to the neoprene layer. The solution consisted:
in replacing the neoprene layer/sheet with the silicone one. This alternative improves the water-tightness of the contact surfaces between the anchor plate and the pipeline surface.
in protecting all the external contact areas exposed between the anchor plate and the wall pipe steel. Apsacoat priming coating has been used, figure 12.
The sliding of the pipeline, located in the slight slope, is a usual condition in the pipe-line networks. The present case study took into account this situation. The solution found to avoid the damage of the box due to the sliding of the pipe consisted in a flap installed laterally to the wall anchor plate, figure 12. This solution should allow the ground to step over the box.
Figure 12 – Flap installed laterally to the wall anchor plate and Apsacoat protection (green priming coating).
Actually no alternative solution was found for the sensors. After applying the modification, new test parameters have been defined and the experimental activity started for the 2nd Period. The measurements of the AE wave attenuation and the sound speed carried out sending the pulsing procedure during this period did not reveal any
7th Int. Conference on Acoustic Emission, 12-15 Sept.2012
problems on the AE chains. After 3 months the pressurizing phase was stopped to perform again the dynamic NDT examination of the dent & gouge defects. 6. Preliminary results At the end of the 1st period the NDT showed low growth of the damage on the dent&gouge defects. Afterwards the test parameters, pressure cycle and CP, were changed, as shown in Table 1. After the 1st stop in the 2nd period (about 3 months), the pipelines were unloaded in order to carry out active NDT on the dent&gouge defects. Case Study An interesting map of a dent&gouge defect located on the section B were discovered. In par-ticular we focused on two areas identified as B11aA and B10aA, Figure 13 and Figure 14, re-spectively. These areas are characterized by: the position of the dent & gouge on the spiral weld, 2 different crack morphologies:
1st, presence of many small cracks with length shorter than a<5mm, Figure 13b. 2nd, two very long cracks along the seam spiral weld, Figure 14b.
Figure 13 – dent & gouge defect Id. B11aA – The defect was carried out over the spiral weld. a) General aspect of the damaged area. b) Spread of small cracks discovered by NDT (magnetic particles)
7th Int. Conference on Acoustic Emission, 12-15 Sept.2012
Figure 14 – dent & gouge defect Id. B10aA – The defect developed over the spiral weld. a) General aspect of the damaged area. b) Two long cracks discovered by NDT (magnetic particles) The acoustic emission data have been evaluated by the NI-AEHistory software platform. CSM developed this user-friendly software package devoted to the pipeline monitoring. The AE source localization developed inside NI-AEHistory refers to the patented eni g&p algo-rithm [9]. The system uses the AE information as input data to evaluate the reliability as-sessment of the pipeline. The AE source location procedure considers both the 1D and 2D valuation. In this way the AE monitoring collected during the pressure cycling (monitoring time t=500hours) of the pipeline buried and identified as Section B, revealed elevated AE hit density both in the dent & gouge defect areas named B11aA and B10aA, Figure 15 and Fig-ure 16. The results highlight higher hit numbers (Hits=589) localized near the B11aA ab-scissa, Figure 15, and the second with the sum of the AE events (Hits=136) localized around B10aA area, Figure 16. This first result seems to be in accordance with the differenc-es in term of crack density observed during the NDT carried out in these two areas, Figure 13 and Figure 14.
Figure 15 –– Pipeline Section B. Linear localization of AE sources. Hit amplitude vs. pipeline length. Higher density has been localized nearest the defects dent & gouge defect Id. B11aA (red line). Green line represents the AE sensor position.
a b
zoom
Defect DENT & GOUGE B11a_A
7th Int. Conference on Acoustic Emission, 12-15 Sept.2012
Figure 16 – Pipeline Section B. Linear localization of AE sources. Hit amplitude vs. pipeline length. Higher density has been localized nearest the dent & gouge defect Id. B10aA (red line). Green line represents the AE sensor position. Nevertheless, the NDT analysis revealed the different crack morphology, in term of both den-sity and length. Then, the localized AE burst have been evaluated considering their energy contents. The sum of the AE Energy (EAE), collected during the monitoring time of 500h, has been evaluated for both these damaged areas. The graphs report the pressure value asso-ciated to the AE hit, Figure 17 and Figure 18.
Figure 17- Pipeline Section B. Damaged area B11aA. Sum of Energy evaluated for the localized AE sources. Monitoring time t=500h.
Defect DENT & GOUGE B10a_A
Pressure range
AE Energy
7th Int. Conference on Acoustic Emission, 12-15 Sept.2012
Figure 18 - Pipeline Section B. Damaged area B10aA. Sum of Energy evaluated for the localized AE sources. Monitoring time t=500h.
The sum of energy EAE, evaluated for the AE events localized, equals to: EAE=1.38·108
[dB·s] and EAE=4.52·107 [dB·s], respectively for the B11aA and B10aA damaged areas.
But considering the total number of hits (Hits) localized, the Energy for each hit is equal to:
EAE=2.34·105 [dB·s] and EAE=3.32·105 [dB·s].
These values are comparable. As the fracture mechanism in the steel is the same, in fact the steel grade is the same (X80), the expectation is that the AE energy released by the crack sources should be equivalent. 7. Conclusions The Acoustic Emission Technique applied for pipeline monitoring involves: long inspection time, aggressive environment mainly due to the burying in soil and the absence of the opera-tors. These aspects must be overcome to establish the reliability of the AE system. After one year monitoring of full-scale buried pipelines, the following problems are raised: The sliding of the pipeline due to the cycling pressure (or landslide in service) causes
serious damage to the box designed to contain/protect the AE sensors, preamplifier and the discharger.
The insulation and the strength of the AE sensors must be improved to guarantee their functionality for long time.
The silicone gel used for the sensor coupling loses its performance after long time and in presence of humidity. An alternative water resistant gel shall be selected.
The reliability of the AE electronic devices must be improved for in-service application.
AE Energy
Pressure range
7th Int. Conference on Acoustic Emission, 12-15 Sept.2012
AE monitoring provides very good perspectives to be used successfully as investigative tech-nique for the reliability assessment for buried pipelines. The case study has shown the AE monitoring is able to detect the fracture processes: crack initiation and propagation. The AE Energy level associated to the damage development ap-pears to be the same for both crack initiation and propagation. Acknowledgments This research was carried out in the framework of the Project MAST-AE, founded by ENI SpA gas&power division. The authors would like thank Dr. M. Gabriele, CSM, for their ex-pertise and support for the development of software platform NI-AEHistory and signal processing. CSM would like to thank the team of Full Scale Testing laboratories in Sardinia. References 1. G. Giunta, L. Prandi, S. Budano, A. Lucci. Fracture Mechanisms Evaluation in Gas
Transportation Pipes by Acoustic Emission Analysis. ASNT- Fall Conference and Quali-ty Testing Show, Palm Springs Convention Center, Palm Springs, California, 24–28 Oct. 2011
2. S. Budano, R. Piancaldini, A. Lucci, G. Giunta. Gas pipeline full scale burst test monitor-ing by AE technique. 54th Acoustic Emission Working Group Meeting Acoustic Emis-sion PRINCETON, NJ – USA - MAY 21 & 22, 2012
3. G. Giunta, S. Budano, A. Lucci, L. Prandi. Pipeline Health Integrity Monitoring (PHIM) Based on Acoustic Emission Technique. Proceedings of the ASME 2012 Pressure Ves-sels & Piping Division Conference PVP2012-78545 July 15-19, 2012, Toronto, Ontario, CANADA
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5. S.D. Soares and J.C.G. Teixeira Could Acoustic Emission Testing Show a Pipe Failure in Advance? Petrobras - R&D Center, Ilha do Fundão – Q. 7 - Cidade Universitária Rio de Janeiro – RJ Brazil, 21949-900
6. R.R. da Silva, D. Mery, S.D. Soares Evaluation of Acoustic Emission Signal Parameters for Identifying the Propagation of Defects in Pressurized Tubes IV Conferencia Paname-ricana de END Buenos Aires – Octubre 2007
7. A. Anastasopoulos, D. Kourousis, K. Bollas, Acoustic Emission Leak Detection of Liq-uid Filled Buried Pipeline J. Acoustic Emission, 27 (2009), 27-39
8. G. De Lorenzo, G. Giunta, Method for the elastic installation of detection devices on pipelines and suitable device therefore, Patent PCT/EP2009/00297
9. G. De Lorenzo, G. Giunta, A. Montini; Method for the remote detection, localization and monitoring of critical faults in pipelines - Patent PCT/2009/129959
