Abstract: The article considers the non-contact magnetometric diagnostics (NCMD) of buried pipelines.

In Russia, China and other countries the NCMD is used for gas and oil pipelines, water conduits, heating system pipelines and other service buried pipelines.

The main task of the NCMD is the detection of the potentially dangerous sections of buried pipelines, on which, as a rule, it is impossible to carry out the in line inspection.

Based on the 20 years experience in the metal magnetic memory (MMM) method in 2000 Energodiagnostika Co. Ltd. began development the NCMD of buried pipelines using the specialized multi-channel highly sensitive flux-gate sensors as a unit of TSC-type instrument. The NCMD is based on measurement of distortions of the magnetic field of the earth, conditioned by the variation of the pipe metal magnetization in SCZs and in zones of developing corrosion-fatigue damages.

Test results of buried pipelines by the NCMD are presented. A complex diagnostics of buried pipelines based on the NCMD with identification of potentially hazardous segments and subsequent NDT in prospect holes is offered.

Keywords: stress concentration, buried pipelines, magnetic memory of metal, field welded joints, stress-strain?state, non-destructive testing.

1 Introduction

The non-contact magnetometric diagnostics (NCMD) of buried pipelines or pipelines covered with a thick layer of insulation is a relatively new type of technical diagnostics, which is ambiguously perceived in the scientific and technical community of traditional non-destructive testing.

However, the analysis of the fifteen-year practice of this direction development, the statistics and the results of the carried out inspections suggest a promising development of this direction of the technical diagnostics.

The NCMD is based on detection of distortions of the magnetic field of the earth (H_Е), intensity conditioned by the variation of the pipe metal’s magnetic permeability in stress concentration zones (SCZ) and in zones of developing corrosion-fatigue damages.

The pattern of the H_Е field intensity variations is conditioned by the pipeline strain occurring in it due to the exposure to a number of factors: the residual process and installation stresses, the working load and self-compensation stresses at outdoor air and environment (soil, water, etc.) temperature fluctuation.

Based on the 20 years experience in the metal magnetic memory (MMM) method in 2000 Energodiagnostika Co. Ltd. began development of the NCMD of buried pipelines using the specialized multi-channel highly sensitive flux-gate sensors as a unit of TSC-type instrument.

The main task of NCMD is detection of the potentially dangerous segments of buried pipelines.

Currently the NCMD, based on the MMM method, is carried out according to regulatory documents [1-5]. The

experience of the NCMD of pipelines buried under the soil layer at the depth of 2 to 3 m or deeper is described, for example, in paper [6]. The main objective of all diagnostic methods and tools during the state assessment of continuously operated pipelines is the search for and identification of potentially hazardous segments with developing damages. The inspection results must provide an answer to the question: “Where and when to expect damage or an accident?” If this problem is solved, in this case there is a possibility to timely replace or repair a potentially hazardous segment.

2 Technique and equipment for NCMD of buried pipelines

2.1 NCMD technique and requirements to operator

During the NCMD of buried pipelines the same parameters of the MMM method are used. During the NCMD three components of the magnetic field of the earth (Н_Е),? will be measured above buried pipeline (Figure 1).

The criteria and the program software, developed at Energodiagnostika Co. Ltd. based on the metal magnetic memory method [7-9], are applied for decoding of the information about the pipelines condition by variations of the magnetic field of the earth recorded at the depth of 200 to 300 mm from the earth surface.

In particular, clear relation between the variation frequency of all the three magnetic field components and the pipe standard size (diameter, wall thickness and pipe length between the joints) was found. These qualitative diagnostic parameters, detected during NCMD, characterize the SCZ – the sources of various-type damages development – within the pipe metal macro volume.

Figure.1. NCMD performance: During NCMD three components of the magnetic field of the earth H_E will be measured above buried pipeline.

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Special training is strongly recommended for NCMD performance.

A center for experts training in the NCMD operates at Energodiagnostika Co. Ltd. Certification Body (Reutov, Moscow region). The training program incorporates the course of training in the metal magnetic memory method (8 working days) and the additional course of training in NCMD (2 working days). During the last six years more than 200 experts in NCMD from 40 diagnostic companies were trained in Russia.

The following tasks are the most complex at practical learning to use the NCMD:

– selection of the optimal expert movement speed along the route without loosing of authentic data (magnetic parameters) on the pipeline condition;

– determination of the pipeline axis with a pipeline finder and of the pipeline field location using a GPS-navigator;

– route preparation for the inspection and offsetting from interferences encountered on the experts travel path along the route (power lines, automobile roads, metal barriers, buildings and structures, etc.);

– processing of inspection results and classification of magnetic anomalies by categories of the pipeline damages development hazard;

– selection of top priority sites for soil opening (“prospecting”).

In the course of experts training in the NCMD Energodiagnostika Co. Ltd. Certification Body issues recommendations on the distinctive features of magnetic anomalies and diagnostic parameters that allow to distinguish maximum stress concentration zones (before the damage development) from the zone of the developing corrosion damage. The existing criteria allow to detect defected welded joints and to distinguish them from the joints in a satisfactory condition.

1.2. NCMD equipment.

The non-contact magnetometric diagnostics of underground pipeline segments is performed using the highly sensitive sensors and TSC type instruments manufactured by Energodiagnostika Co. Ltd. (Moscow).

Figure 2 shows the instrument complex for the NCMD manufactured by Energodiagnostika Co. Ltd. on a commercial basis.

The NCMD performance by using the instrument complex in urban conditions and in cross country ones are presented on the Fig.2,b and Fig.3.

Figure 2. The instrument complex for NCMD:

a-Appearance of the scanning device: 1 – road wheel; 2 – length-counting unit; 3 – Type 11 sensor attachment unit; 4 – Type 11 sensor; 5 – handle; 6 – measuring instrument mounting unit; 7 – folding support leg; 8 – universal head;

b – the NCMD performance by using the instrument complex in urban conditions.

Figure 3. The NCMD performance by using the instrument complex in cross country conditions.

At magnetograms decoding the most complex task is classification of magnetic anomalies by types of damages. Some companies, while trying to make an impact on the customer during the solution of this problem, indulge in wishful thinking. During the additional pipeline inspection in prospect holes the detected defects are further presented as defects found by NCMD before the segment opening. In fact, the current level of NCMD development, as a rule, does not allow to state before prospecting the type of defect that corresponds to the detected anomaly.

At magnetograms decoding it is necessary to consider the specific conditions and structural features of inspected pipelines. For example, operating conditions and, accordingly, the condition of gas pipelines located in southern regions of the country noticeably differ from gas pipelines located in the north.

2. Results of comprehensive testing and analysis.

2.1. NCMD of duried oil pipelines.

Figure 4 shows the fragment of the magnetogram recorded during the NCMD an oil pipeline (? 377′10, pressure 2.5 MPa). This oil pipeline was in operation from 1965. Anomaly of the magnetic field intensity was marked on the diagram. After soil excavation additional visual detected the damage of plastic coat and pitting corrosion up to 4 mm depth was detected in the prospect hole.

Figure 4. NCMD of interfield oil pipeline (?377′10, medium pressure P=2.5 MPa): 1- distribution of three components of geomagnetic field intensity H_E; 2- distribution of the magnetic field gradient dH_E/dx; 3 – the section of buried pipeline with anomalous magnetic field distribution. Anomaly was proposed for additional examination in the prospect hole. Delamination of pipe insulation and development of external pitting corrosion with 4 mm deep pits was found in the prospect hole on the section with anomalous magnetic field distribution.

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Oil pipeline (?325 mm) inspection was performed in March 2013 in urban conditions in the town of Dong Ying (China). Figure 5,a, shows the magnetogram recorded during NCMD, on which two sections are marked. The first section corresponds to the manifold cover location, which caused an obstacle during the inspection performance. The second section with typical anomalous distribution of the magnetic field intensity was recommended for additional inspection in the prospect hole. After the pavement opening, the additional visual examination in the prospect hole resulted in detection of the oil pipe damaging with diameter of about 10mm (Figure 5,b).

In Russia and China criminal pipeline tie-ins are an illegal challenge to economic security of the countries. NCMD is one of the technical solutions to detect such criminal tie-ins.

NCMD of the oil pipeline (?529mm) was carried out in Huandao (China) in 2014. Figure 6,a, presents the NCMD results and highlights the section of anomalous distribution of the Н_Е magnetic field intensity and the field gradient dH_Е/dx. This section was recommended for additional inspection in the prospect hole. The additional visual examination in the prospect hole detected a criminal oil pipeline tie-in.

а)

1. b)

Figure 5. Results of oil pipeline (?325 mm) NCMD in urban conditions:

1. a) Magnetogram of the Н_Е field intensity distribution with highlighted anomalies: 1-manifold cover location; 2-anomaly detected during NCMD and recommended for pavement opening and carrying out of the additional inspection in the prospect hole; 3-oil pipeline damage location;
2. b) Results of additional oil pipeline inspection in the area of detected anomaly after the pavement opening. After the pavement opening, the additional visual examination in the prospect hole resulted in detection of the oil pipeline damaging (a hole with diameter of about 10mm).

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Figure 6. Results of oil pipeline (?529 mm) NCMD:

1. a) Magnetogram of the Н_Е field intensity distribution with the highlighted anomaly;
2. b) Results of additional oil pipeline inspection in the area of detected anomaly. A criminal tie-in was detected in the prospect hole after the pipeline opening.

2.2. NCMD of buried gas pipelines.

Figure 7 presents the NCMD results of the low pressure gas pipeline bend section (?30’’ (~?762′12 mm), P=1.2 MPa) (Azerbaijan) with the highlighted section of anomalous distribution of the Н_Е magnetic field intensity and the field gradient dH_Е/dx.

This section was recommended for additional inspection in the prospect hole.

Inspection by the contact MMM method was carried out in the prospect hole (Figure 8,a) at the first stage. Inspection by the MMM method detected two SCZs (SCZ No.1 and SCZ No.2) marked in Figure 8,b. Insulation was opened and ultrasonic testing (UT) was performed in the areas of detected SCZs.

A 50mm long discontinuity flaw was detected at the depth of 9mm during UT performance in the SCZ No.1. UT in the SCZ No.2 detected a 100mm long discontinuity flaw at the depth of 11mm. Based on the current regulatory documents, both discontinuity flaws were unacceptable.

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Figure 7. Results of gas pipeline bend section (?30’’ (~?762′12mm), P=1.2 MPa) NCMD. The magnetogram shows the anomalous distribution of the Н_Е magnetic field intensity and the field gradient dH_Е/dx and highlights the anomaly (1).

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a)

b)

c)

Figure 8. Results of gas pipeline bend section (?30’’ (~?762′12mm), P=1.2 MPa) inspection in the prospect hole in the detected anomaly:

1. a) Prospect hole at the route bend section;
2. b) Results of the additional inspection in the prospect hole by the contact MMM method without the insulation removal. The diagram highlights the detected SCZ: 1- SCZ No.1; 2- SCZ No.2;
3. c) Arrangement scheme of SCZ detected by the MMM method without the insulation removal. In both of the SCZ 80 to 120 mm long discontinuity flaws were detected UT at the depth of 8 to 11mm.

2.3. NCMD of main gas pipeline potentially hazardous welded joints.

At present JSC “Gazprom” pays great attention to ensuring reliability of main gas pipeline (MGP) field welded joints both during the construction of new gas pipelines and in conditions of their overhaul [11].

However, a significant amount of main gas pipelines with circumferential welded joints produced in field conditions is in continuous operation (for 30 years or longer). It is known that the circumferential welded joints on gas pipelines are much less reliable than the factory welded joints. We also know that 30-40 years ago methods and means of field welded joints non-destructive testing were insufficiently developed compared to the present time. Special studies during the quality control of field welded joints after the long-term operation of MGPs often detect unacceptable defects according to both old and new sorting standards [12].

Field welded joints are typically produced during welding of stressed gas pipelines “closing” ends. Therefore circumferential field welded joints with high level of residual stress have high potential energy, which is released in the process of their failure. And such failures often occur on continuously operated gas pipelines both in Russia and other countries.

In addition, individual field welded joints have high levels of residual stresses formed during their fabrication. That is why their sudden failures occur in continuously operated main gas pipelines. And such failures often occur in the pipelines, both in Russia and in other countries.

For example, failure of buried ?500mm gas pipeline operating under pressure of 12MPa occurred in the Polish gas system in January 2014. Gas explosion and subsequent fire destroyed the nearby houses and other buildings. The accident investigation established that a circumferential field welded joint was the source of the gas pipeline failure (Figure 9). At the time of failure the pipeline was in operation for more than 40 years.

Figure 9. Failure of a ?500mm gas pipeline circumferential field welded joint after 40 years of operation.

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24 km of underground pipeline segments were inspected using BMD under a contract with the Polish company “Gazsistema” during the period from 2011 to 2013. Based on the results of BMD, it was recommended to carry out additional examination in prospect holes of 10 maximum anomalies.

Figure 10 shows the fragment of the magnetogram recorded during the inspection in the non-contact mode of one of the considered pipeline segments. In accordance with the used technique, the magnetic anomaly marked in figure 10 characterizes the circumferential welded joint location.

After opening the underground pipeline segments circumferential field welded joints were found in all areas with magnetic anomalies previously identified based on the BMD results. Additional inspection of shells and field welded joints was carried out using the metal magnetic memory method and ultrasonic testing.

Based on the complex control results, the condition of gas pipeline shells metal after 40 years of their operation was satisfactory based on existing standards, and in eight of the ten field joints the MMM method identified the SCZ, in which the ultrasonic method detected unacceptable defects typically in the form of lack of fusion in the weld root and in fusion zones with the pipe base metal.

Figure 10. Magnetogram of gas pipeline segment non-contact inspection: 1- distribution of three components of geomagnetic field intensity H_E; 2- distribution of the magnetic field gradient dH_E/dx; 3 – anomaly corresponding to the welded joint with high level of stress concentration.

Let us consider in more detail the inspection results of one of such joints.

Figure 11 shows the “prospecting” (opening) location of a gas pipeline segment with the detected magnetic anomaly (Figure 10).

Based on the visual examination of this segment, the condition of the gas pipeline shell metal was satisfactory. However, the circumferential field welded joint inspection by the MMM method detected the area with significant inhomogeneity of the magnetic field and its gradient (dH/dx) distribution. In areas of maximum magnetic field gradient values, corresponding to the zones of maximum stress concentration, the additional UT was carried out. Based on the results of the UT in the SCZ identified by MMM method, unacceptable defects were detected in the form of discontinuities located at different depths of the welded joint. Similar magnetograms with anomalies, characterizing high stress concentration, were found in prospect holes in eight out of the ten inspected circumferential field welds.

Unacceptable defects that apparently formed during welding of joints in the course of pipelines installation create high stress concentration in local areas of potential damage

Figure 11. Gas pipeline segment after its opening in the magnetic anomaly zone for additional inspection by the MMM method and ultrasonic testing (a) and the diagram of inspection by the MMM method of a circumferential field welded joint of the ?400 mm gas pipeline pipes: 1 – location of circumferential (field) welded joint; 2- distribution of the SMLF gradient dH_L/dx along the welded joint; 3 – the SCZ corresponding to the maximal value of the field gradient. Unacceptable defects. in the form of discontinuities located at different depths of the welded joint were detected by additional UT.

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It should be noted here that, according to the approximate estimate, the 24-km length of gas pipelines, inspected by the non-contact magnetometric method, contained about 800 field girth welded joints. Only 10 sections with detected magnetic anomalies, characterizing the maximum stress concentration on girth welded joints, were recommended for opening. This makes just 1 to 1.2% of the total number of field butt joints.

References