Oil pipeline technical condition monitoring system based on acoustic emission method

Assuring oil pipelines no-failure operation involves timely detection and repair of parent metal, welds and pipe coating defects.

The most critical for this plan is to control technical condition of oil pipeline river, railway and road crossings close to build up areas. These segments typically are the highest load ones and environmental damage in case of oil spill largely increases compared to mainline.   

Part of the problem is resolved by traditional inspections especially using acoustic emission method (AE). Our experience which includes about 2000 km of inspected oil and gas pipelines confirms acoustic emission method reveals not only crack type defects, such as growing welding defects or laminations, but also metal corrosion, dents, yielding areas.

It is significant that AE inspection records not only unacceptable defects requiring immediate repair but also those defects which are not critical yet but have a tendency to grow.

However it is quite challenging to evaluate such growing defects breakage based on the acoustic emission. For instance, some types of metal defects are characterized by a period when despite stable defect growth the acoustic emission level temporarily decreases but after a while heavy breakage is possible. Even having identified such a defect using traditional nondestructive testing methods (ultrasonic, x-ray, visual, etc.) and having confirmed the defect did not exceed rejection criteria a mistake is possible in identifying its hazard rate and evaluating the situation due to the fact the defect growth level at the moment of inspection is unknown.

At this stage of AE inspection method development there are several conditions providing for evaluation of the defect development and disruption based on the acoustic emission pattern, such as matching an AE parameter change with the comparison curve and recording modifications in its change pattern, i.e., it is important to know the hindsight of the defect acoustic emission pattern.

In view of the above a conclusion can easily be drawn that for convinced forecasting of defects development and, thus, of the oil pipeline technical condition either its continuous monitoring or time controlled inspection is required under identical conditions. And the option of time controlled inspection is absolutely adequate due to the fact in actual practice the defect development is quite a lengthy process.

Traditionally the main constraining factor of AE inspection use is the need to perform a large scope of excavation to provide for AE detector installation on the pipe body where the trenching cost is comparable with AE inspection cost.

The most challenging is to excavate trenches in the area of oil pipeline water crossings, flood plains, etc., i.e. in the most potentially hazardous areas.

A wish to install a certain fixed monitoring system at such crossing to avoid trench excavation/backfilling costs every time makes perfect sense.  There are several options of such technical condition monitoring system available:

The highest technology, thus making it the most expensive option, is installation of a fixed continuous monitoring system. In process of the pipeline construction or refurbishment fixed acoustic emission detectors are installed at its underwater and flood plain areas (at each 70 to 150 meters depending on specific conditions and objectives). Detectors are terminated to communication cables with fixed hardware located in a special shelter. The monitoring is performed continuously in process of the oil pipeline operation.   

The area of such system inspection is up to 700~ 800 meters. The system may provide for automatic alarm in case of a critical situation or abrupt change of recorded parameters.

Obviously the system hardware shall be reliably protected from external effects, provided with uninterrupted power supply and with reliable communication with the control center.

Since 1993 such fixed monitoring systems have been successfully operated in Transneft system at river crossings. Over 20 systems were constructed, however, due to very high cost, requirement of continuous maintenance, etc. continuous monitoring systems are economically justified when installed only at most critical oil pipeline segments. 

From our point of view in the context of large-scale use a more attractive option is AE non-continuous monitoring system proposed by NTC NefteGazDiagnostika.

In the same way as for vibration monitoring at different compressor stations, pump jacks, etc. only fixed vibration sensors are installed with information from them being gathered on a periodic predetermined base it is proposed to: install only fixed acoustic emission detectors at the monitored oil pipeline segment. The detectors cables are terminated above ground to a junction box and mobile hardware is connected only for the time of AE inspection performance.

In such situation the information may be gathered on any predetermined periodic base, without additional cost for trenching and, importantly, at identical inspection conditions. Each inspection data are saved on electronic media. Comparative analysis of those data provides for evaluating the inspected pipeline condition not only at the moment of the inspection but also for tracking development of potential defects and, thus, preventing a hazardous situation from happening.

In addition, in such situation, one AE detector may be used to inspect practically any number of crossings and as only detectors are installed as fixed given their relatively low price it is quite feasible to install this system option not only at crossings but also at the mainline without monitored area size limitation.

It should be noted in both options of the monitoring system AE detector terminations may be used as measuring electrode to measure pipeline electric potentials and to perform electrocorrosion inspections including evaluation of coating condition and cathodic protection systems operation efficiency. 

The idea of non-continuous monitoring system was supported by OAO Lukoil-Komi where NTC NEFTEGAZDIAGNOSTIKA engineered and installed non-continuous AE monitoring system for water injection line Kolva river syphon crossing.

The system includes a package of 12 low frequency AE detectors integrated with preamplifiers installed at onshore segments of two parallel pipelines.

Detectors are installed in special metal pits, filled with epoxy based compound and clamped against the pipe metal with an equal gauged force. Compound acts as contact acoustic medium and in parallel is a reliable sealing component and insulator.

Cables are terminated through special protective tubes above ground to sealed junction boxes.

The distance between detectors is 70 meters with the exception of 200 meters for pipeline underwater segments.

In process of the system testing an interesting side-effect was revealed, due to acoustic surface development and gaged AE detectors clamping force their sensitivity significantly improved compared to the classic magnet mounted installation. In addition inner noise level decreased.

This system has been operated since July 2001, it proved its survivability and parameters stability. When location maps obtained during periodic inspections are compared AE sources behavior is clearly visible.

A patent for Pipeline Technical Condition Acoustic Emission Monitoring Method was received on this system.

The monitoring system trial operation data formed the basis for guidelines Field Pipelines Potentially Hazardous Segments Technical Condition Monitoring using AE Method (MU 01-005-2003, approved by RF GGTN on 19.03.2003) developed by NTC NEFTEGAZDIAGNOSTIKA jointly with OOO Tekhnicheskaya Diagnostika on NK LUKOIL request.

In 2003 AE non-continuous monitoring system was installed at a 5 kilometer mainline segment of the oil pipeline in OAO LUKOIL-Permneft.

In 2004 AE non-continuous monitoring systems were installed at 14 water crossings (each segment is 1 km long) at OOO LUKOIL-PERM pipelines.

In 2005 AE non-continuous monitoring system with underwater detectors was installed at Yazva River (Perm Krai).  

All installed systems are well-behaved.

Obtained results render it possible to talk about the selected approach validity and make it possible to recommend such systems installation at pipeline owners.

Patent for "Method of acoustic emission monitoring the technical condition of the pipelines"