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NON-CONTACT METHOD FOR THE INSPECTION OF UNDERGROUND PIPELINES
Summary
The Non Contact Magnetic Tomography pipeline survey technology enables the survey of pipelines of all configurations, underground and submerged, while they remain working at normal operating pressures and without the preparation procedures needed by in line intelligent pig techniques. The main features are summarised below:
Assessment of defects is based not on geometry but of the level of stress caused by them
Data is collected by a non-contact scanning magnetometer and is subsequently analysed by a patented software algorithm.
The pipeline remains in service throughout the survey thus eliminating cleaning, purging and downtime costs.
The survey report defines the location and extent of geometric and corrosion defects such as:
ß Dents, corrugations, scores, out of roundness and differences in wall
thickness
ß Loss of metal – including internal and external corrosion defects of
any nature
ß Delamination
ß Defects within welded joints eg cracks, pores, lack of weld penetration, displacement of edges and metal flakes
ß Crack-like defects in any orientation
ß Sections with deviations of a level of stress deformed conditions caused by, for example, sagging, landslips, washouts or transitions under roads.
ß Local corrosion under scaled insulating coating
The location of defects are defined within an accuracy of +/- 1.5m thus facilitating further investigation or repair
The final report defines the degree of danger of defects and recommends maximum safe working pressures and also the length of time it is safe to operate defective sections of pipe at those pressures.
The system can be used to survey trunk pipelines (across any terrain including water) and service pipelines within cities. Accuracy is not affected by either the close proximity of other pipelines or city traffic.
Data Collection and Analysis
Fluctuations in magnetic field strength are used to automatically display, register and record in memory defects in the metal as the magnetometer is carried along the longitudinal axis of the pipeline by an operator. All data is then subsequently analysed to produce an integrated indicator of the hazard level of the defects detected. This is presented in chart form and references an identification of each defect and its location (obtained using GPS).
Flaws in the metal are located by relating the magnetic permeability of the pipeline to stress raisers and are defined by analysing the interconnection of stress concentration with a change in the polarity of the components of the earth’s magnetic field.
The effect of interfering fields is minimised by equipping the magnetometer with high sensitivity converters and dedicated software with filtration algorithms.
Identifiable Defects
The following internal and external defects, their parameters and their location can be accurately defined:
∑ Crack-like defects in any orientation (laps, scabs, scratches, stress corrosion, cracking and exfoliation)
∑ Weld defects (laps, pores, cracks, lack of fusion, lack of penetration, displacement, metal flakes, residual thermal stress within the heat affected zone)
∑ Compression marks, corrugations, scores, out of roundness and changes in wall thickness caused by corrosion pits and filiform corrosion
∑ Loss of metal – including internal and external corrosion defects of any nature
∑ Delamination
∑ Sections with deviations of a level of stress deformed conditions caused by, for example, sagging, landslips, washouts or transitions under roads.
∑ Local corrosion under scaled insulating coating
∑ Indents
∑ Buckles
∑ Deviation from the specified laying axis
Tolerance and Accuracy
Minimum length of detectable defects: >10mm
Opening of detectable defects: >300 microns
Depth of detectable defects: >5% of the pipe wall thickness
Measurement tolerance:
Crack length: +/-20%
Crack depth: +/- 30%
Wall thickness loss: <25%
Detection rate: 2m/sec max
Location and orientation of defects: No restriction
Process Limitations
Operating temperature: None defined. Has been used at -50 to + 63 deg C
Pipeline diameter: Min 56mm
Max 1420mm
(based on actual survey experience to date. Extensions to either stated limit may be possible)
Pipe wall thickness: Min 2.8mm
Max 22mm
(based on actual survey experience to date. Extensions to either stated limit may be possible)
Detection rate: 2m/sec max
Distance between magnetometer and pipeline: 15 x pipe dia max (refers only to measuring pipeline depth and axis deviation)
Cannot guarantee the detection of those defects which do not cause a change in the level of stress deformed condition – ie blow holes, pitting
Summary of Procedure
1. Analysis of pipeline design, executive and operational documentation
2. Visual inspection of the pipeline route
3. Preparatory works
4. Pipeline inspection
5. Inspection data processing
6. Assess technical condition of pipeline section
7. (Mark out test pits if required)
8. (Additional NDT on exposed pipe if required)
9. Finalise results, conclusions and present report
Experience
Launched commercially in 2002 the technology has to date been used to successfully survey more than 15500km of pipelines located both underground and under water in the Russian Federation, Uzbekistan, Ukraine, Syria, Argentina, Croatia and Malaysia. Monitoring throughout this period has demonstrated an efficiency rate and reliability level of not less than 80% which is equal to the detection rate expected of in pipe inspection intelligent PIG methods. For example, a survey of gas pipelines owned by BASH-TRANSGAZ showed a defect detection reliability rate of 82% as confirmed by subsequent pipe openings. This increased to 87% when inspecting oil pipelines owned by TAFTNET and where many hazardous weld defects were identified and subsequently verified. This level of detection was maintained when inspecting pipelines owned by ALROS and other companies.
Illustrations
1. Non contact scanning magnetometer
2. Reference chart showing pipeline in relation to surface structures
3. Presentation format of detailed survey results
4. Summary Report of Results, Conclusions and Recommendations (Sample Extract)
FAQS
Q. Can piping encased by a heat insulation layer such as air conditioning ducts be inspected. Normally the outer surface of the heat insulation layer is made of spiral aluminium sheet. The insulating materials are either polymer foam or rock wool.
A. Yes. The materials mentioned have no effect on the accuracy of inspection.
Q. Outer metal coatings are sometimes fitted as a protection for the product-carrying pipeline. Does this have an effect on the accuracy of the inspection.
A. We have inspected pipeline sections located under main roads (pipes in casing and pipe in pipe). The points of contact (eccentricity of casing) are identified by the system and, provided the product-carrying pipe is operating under pressure all anomalies can be identified.
Q. If one only pipe among a complex network of piping requires inspection (eg as seen at chemical plants) can the system behave reliably
A. We have not worked on such plants but would expect complex networks would interfere with interpretation and that differentiation would be difficult
Q. If a pipeline is affected by a geomagnetic effect (telluric effect) can the pipeline be inspected?
A. Yes. The telluric effect does not impact on the accuracy of the inspection.
Q. If a pipeline has an inductance effect from a nearby overhead high voltage AC power line can the pipeline be inspected.
A. Yes. The power line effect is identified as a hindrance (anomaly) and theoretically the anomaly could be interpreted falsely as an intersection with a power line.
Q. Does the inspection include fittings (eg washout, valves, flanges)
A. All fittings on the pipeline are inspected. There is no difficulty in differentiating defects and fittings if the pipeline technical scheme is available or the customer provides sufficient initial technical investigation. Without this information there is a theoretical possibility that mistakes in interpreting the nature of anomalies can be made,
Q. Can pipeline sections located under steel reinforced concrete be inspected.
A, Yes. We have successful experience in inspecting municipal pipeline distribution systems in such conditions.
Q. Ductile iron piping is not normally joined by welding but with spigot joints. This will result in electrical discontinuities along the pipeline and each section of pipe may become electrically isolated. Will this impact on the inspection accuracy.
A. Cast iron piping with spigot joints has been successfully inspected without encountering problems. However we have not inspected ductile iron pipes and would need to carry out tests to determine accuracy.
Q. Can the system inspect buried or aboveground pressurised bullet vessels used for gas storage (nominally 1 – 2,5m dia by 5 – 10 m length).
A. The system has no difficulty inspecting such vessels up to 1.5m dia but for items larger than this we would recommend the use of a contact method of inspection.
Hitech Engineering Materials and Systems Ltd
Tudor House Mews, Westgate, Grantham, NG31 6LU, Lincs, UK
Te. +44 (0)1400 283523 Fax +44 (0)1400 282648
enquiries@hitech-me.co.uk
Q. If a pipeline has a cathodic protection system can the survey be carried out without shutting down the cp system.
A. The presence of a cp system does not affect the accuracy of the inspection. There is no requirement to shut down this system.
Q. Does the technique need an intermittent contact with the pipeline and if so, how far apart and what kind of contacts (cable, clamps
etc).
A. There is no contact with the pipeline during inspection
Q. Can the Transtek system work on vertical piping such as offshore oil/gas risers from underwater to the platform.
A. We have no experience of this inspection. Testing would be required.
Q. Can the system inspect sections of vertical pipe after the elbows of horizontal pipes.
A. We have successfully tested such pipeline sections up to 15m. Testing would be required for greater heights.
Q. What are the common difficulties encountered by the system when conducting work at the site.
∑ Customer cannot provide technical documentation relating to the pipeline and its history
∑ Operator access to terrain above the pipeline is hindered by bushes, undergrowth and similar obstacles
∑ Construction of structures above the pipeline