Pipeline technical inspection without smart pigs involved

When it comes to field pipeline operation, one of the most topical and complex issues is the identification of unauthorized oil withdrawal or, in other words, unauthorized tie-ins into pipelines. This criminal activity of third parties causes significant economic damage to the enterprise since it entails penalties for environmental contamination, additional expenses on the localization and elimination of the consequences of accidents and incidents at pipelines and loss of profit from oil sales.
The article “Acoustic Field Pipeline Diagnostics Method”, written by S.V. Evseev, First Deputy General Director and Chief Engineer of “NTC Neftegazdiagnostika”, LLC, with co-authors, was published in Issue No. 10-2015 of “Engineering Practice”. Having studied this article, the experts of the Pipeline Operation Directorate of “Orenburgneft”, PJSC proposed to purposefully assess the efficiency of the acoustic and resonance pipeline diagnostics method when identifying unauthorized tie-ins.

THEORETICAL BACKGROUND OF THE METHOD

The acoustic and resonance method is based upon vibration of specific pipe elements caused by pressure pulsation in a pipeline and acoustic frequency signal emissions, with such signals pervading the medium. The assessment of the technical condition of a pipeline is carried out in accordance with the developed criteria which link the vibro-emission properties of a defect with the possibility of leakage.
This method allows carrying out the diagnostics of pipelines of aboveground, underground, trench and trenchless pipe laying, with a diameter of 80 mm or more, which are in use, with an internal pressure exceeding 0.25 MPa and only provided that there is a medium flow in the pipeline. The main advantage of this method lies in the absence of any need to change the pressure in the course of diagnostics – the pipeline operates in the normal course.
To understand the physics of this process, local thinning-outs in a pipe can be regarded as a membrane. The problem of membrane oscillations in the pipe is solved in a number of fundamental works on hydroacoustics. They show that the main parameter of oscillation – frequency – depends on the ratio of thicknesses of an undisturbed part of the pipe and a defect, and the linear dimensions of the latter; the less the defect is, the higher is the oscillation frequency. The assessment showed that a 200-300 mm defect has its own oscillation frequency of around 1,000 Hz. Taking into account a wide variety of corrosion defects in pipelines, the most probable frequency range of emission signals pervading the medium is from 100 to 5,000 Hz.
The pipe surface consists of separate elements (intervals) of vibration. One of the parameters of vibration of these elements is vibration’s own frequency which primarily depends on the square of the element. When using the acoustic and resonance method, signals are studied in the frequency range from 500 to 3,000 Hz. Pressure pulsation in the medium serves as a force factor which makes the pipe element vibrate. It is primarily caused by the turbulence of the medium motion. In such a case, pressure pulsation in the medium should be sufficient to “oscillate” the elements. Therefore, the fundamental conditions for the diagnostics with the use of this method include the medium’s flow rate being around 1 m/s and the pressure of at least 0.25 MPa.
Pressure pulsation in the medium is a series of pulses with unique frequency. When the frequency of a pulse is equal to the oscillation frequency of a defect or is close to it, the latter vibrates with the highest amplitude (resonance effect). In such a case, there occurs the most intense radiation (emission) of signals into the environment (air), the metal of the pipe and the medium. The oscillation amplitude of the defect and the signal energy, in particular, emission, depend on the wall thickness where the defect is, Picture 1.

Picture 1. Identifying the places where the pipe walls thins out

PILOT-SCALE TESTS

In June 2016 the experts of “Orenburgneft”, PJSC, in liaison with NTC Neftegazdiagnostika”, LLC, carried out the pilot-scale tests of the acoustic and resonance diagnostics method of two sections of the oil pipeline “Kurmanaevskaya Preliminary Water Discharge Unit – Bobrovskaya Oil Treatment Unit” (hereinafter referred to as “Kurmanaevskaya PWDU – Bobrovskaya OTU”

Table 1. Technical characteristics of the pipeline

The experts of the POD selected the sections for the tests based upon their compliance with the required technical characteristics as to the pressure and presence of a medium flow. Furthermore, the personnel of “Orenburgneft”, PJSC had earlier eliminated unauthorized tie-ins at the pipeline sections that were selected for the tests. The experts of “NTC Neftegazdiagnostika”, LLC had not been aware of the number and location of those tie-ins prior to the tests. 
The technical characteristics of the pipeline selected for the tests and its scheme are specified in Table 1 and Picture 2 respectively.

Picture. 2. Scheme of the pipeline “Kurmanaevskaya PWDU– Bobrovskaya OTU”.
 

The tests were carried out at two sections of the oil pipeline (Pictures 3 and 4):

Picture. 3. Scheme of section under study No. 1

Picture. 4. Scheme of section under study No. 1

1.    Section No. 1. KP 50+18 – KP 52+62; length – 244 m, diameter – 273.8 mm.
2.    SectionNo. 2. KP 67+50 – KP 71+50; length – 400 м, diameter – 325.9 mm.

Picture 5. Measurement

The holes were produced by the Pipeline Operation and Repair Shop in the framework of the existing land allocation. The measurement of distance between the holes was carried out on a commission basis with the use of a GPS navigator Garmin.   When connecting devices, the insulating layer of the pipeline is removed at a minimum (50 50 mm) and then restored (Picture 5). The measurement time period per section is around 5 minutes.

MEASUREMENT RESULTS, DATA INTERPRETATION

When the field works were completed, the experts of “NTC Neftegazdiagnostika”, LLC analyzed the obtained data with the use of specialized software and arrived at the following conclusions: 
Seven defects were identified at Section No. 1: 5 near-critical and 2 critical ones. 
The location of the possible defects is established with the accuracy of 1-2 meters. The results are shown in Picture 6.

Picture 6. Report on the results of the acoustic and resonance diagnostics of Section No. 1 of the oil pipeline “Kurmanaevskaya PWDU – Bobrovskaya OTU”which is owned by “Orenburgneft”, PJSC
 

Seven defects were identified at Section No. 2: 4 near-critical and 3 critical ones. 
The location of the possible defects is established with the accuracy of 1-2 meters (Picture 7).

Picture 7. Report on the results of the acoustic and resonance diagnostics of Section No. 2 of the oil pipeline “Kurmanaevskaya PWDU – Bobrovskaya OTU”which is owned by “Orenburgneft”, PJSC.

To verify the provided data, one organized an additional flaw-detective inspection (AFDI), a visual measurement inspection (VMI) and an ultra-sound inspection (USI) with the hole drilling at the above-mentioned points. The inspections were carried out with the use of certified equipment and the involvement of a non-destructive laboratory expert (Pictures 8-10).

Picture 8. AnomalyNo.1. Circular weld defect – joint displacement

MAIN RESULTS AND SUMMARY

The Pipeline Operation Directorate of “Orenburgneft”, PJSC organized and carried out the pilot-scale tests of the acoustic and resonance diagnostics method at two sections of the pressure oil pipeline “Kurmanaevskaya PWDU – Bobrovskaya OTU”. The applicability of the above-mentioned method to the identification of unauthorized tie-ins was assessed.

Therefore, the defect verification showed a 75% defect identification convergence, whereas no full flaw-detective inspection of Section No. 2 at a distance from 330 to 332 m was carried out, which means that there is no defect there. All known defects were identified, including two eliminated unauthorized tie-ins.
“Orenburgneft”, PJSC plans to continue using the acoustic and resonance diagnostics method in the future in order to early identify critical defects in pipelines, including the location of unauthorized tie-ins into pipelines.

    
Kolesnikov Dmitry Vladimirovich – Deputy Head of the Pipeline Operation Directorate – Chief Engineer of the POD of “Orenburgneft”, PJSC
    
Savitskaya Elena Iosifovna– Head of the Engineering and Pipeline Reliability Enhancement Department of “Orenburgneft”, PJSC
    
Kislinsky OlegYuryevich– Chief Engineer of the Engineering and Pipeline Reliability Enhancement Department of “Orenburgneft”, PJSC

1. Acoustic and Resonance Field Pipeline Diagnostics Method / S.V. Evseev, et al. // Engineering Practice. – 2015. – No. 10.