Diagnostics of pipelines, gas and oil pipelines
Fig.1 shows the general inspection scheme for all industrial pipelines using the specialized magnetometric instrument – the Tester of Stress Concentration (TSC-type). The instrument has a screen, a memory unit for recording the measurement results and a scanning device in the form of a trolley, on which the sensors measuring the Hp magnetic field and the pipelines length are installed. The inspection does not require the preliminary preparation of the surface. In some cases the pipelines inspection may be carried out without the insulation removal.
Fig.1. The scheme of a pipeline inspection using a two-channel sensor: 1 - the scanning device with the length-measuring sensor; 2, 3 - flux-gate transducers (the number of transducers may vary from two to sixteen depending on the inspection tasks and the dimension-type of the pipeline); 4 - the connecting cable; 5 - the TSC-type instrument with a memory unit and a screen for displaying of the graphical information.
Fig.2 shows the fragment of the Hp magnetic field distribution along the segment of the ⌀168х16 pipeline with a visible swag. The diagram of the Hp field distribution corresponds to the actual strain of the gas pipeline.
Fig.2. The fragment of the Hp magnetic field distribution along the gas pipeline segment: 1 - gas flow.
Ageing of oil- and gas-trunk pipelines places the task of their functioning safety and reliability assurance in the list of the most important state problems. At present the general length of trunk pipelines in Russia is over 300 thousand kilometers. And about 40% of gas pipelines and 60% of oil pipelines have been in operation for more than 20 years.
It is obvious that the traditional approach to maintaining the pipelines operability by performing overhauls of individual pipe segments mainly with complete insulation coating and pipes replacement cannot provide safety and reliability of gas-trunk pipelines because of their long length and distinctly different state. Therefore operation and repair "according to the actual state", i.e. shifting to selective "spot" repairs of elements and segments based on results of the 100% diagnostic inspection of many-kilometers-long pipelines becomes the main strategy for providing high reliability of trunk systems.
The following concept of long-term operated gas- and oil pipelines reliability and safety assurance is suggested:
1. Analysis of damages, results of non-destructive and destructive metal testing and of the carried out replacements of worn-out segments based on the available statistics of an operating organization.
2. The 100% inspection of all gas and oil pipeline segments using up-to-date NDT methods and means (in-pipe diagnostics, non-invasive magnetometric diagnostics, the metal magnetic memory method, acoustic emission) allowing carrying out the early diagnostics of damages and detecting stress concentration zones (SCZ), which are the main sources of developing damages.
3. Additional inspection with traditional NDT means (USD, X-ray, investigation of the metal’s metal mechanical properties and structure) is performed on gas and oil pipeline sections with detected SCZs after the "prospecting" operation.
4. For certain most stressed sections with SCZs left in operation a confirmatory strength calculation is carried out taking into account the pipelines’ metal damaging and wear nature.
5. The 100% fittings examination using the metal magnetic memory method and other NDT methods.
6. Summarizing of the results of the complex 100% inspection and developing of measures aimed at assuring gas and oil pipelines reliability with making the replacement schedule of the physically worn-out pipe segments being most susceptible to damaging.
The proposed concept is based on the assessment of the real gas and oil pipelines’ lifetime since such an assessment most optimally combines the operational experience (former damaging statistics) and the early diagnostics of future damages using up-to-date methods.
Energodiagnostika Co. Ltd. developed a measuring complex (fig.3) for non-invasive magnetometric inspection of gas and oil pipelines buried under the soil layer of 2m and deeper. During the operator’s movement along the route at a speed of not less than 2 km per hour the segments, operating in the most stressed conditions and being susceptible to damaging, are detected. Prospecting and additional inspection are carried out on these segments in order to detect specific defects.
Fig.3. Non-invasive magnetometric inspection of gas and oil pipelines buried under the soil layer.
For on-line 100% inspection of welded joints and walls of large-diameter (⌀530÷1420mm) gas and oil pipelines a scanner-flaw detector was developed, which allows carrying out the state assessment of the entire pipe surface at a speed of 100 running meters per hour and faster (fig.4 and fig.5). Insulation removal and the pipe surface dressing are not required, and the natural magnetization of the metal, formed in the course of operation (the magnetic memory of metal), is used.
Fig.4. Inspection of gas and oil pipelines by the contact method.
Fig.5. Inspection of gas and oil pipelines by the contact method.
Fig.6 presents the inspection results of the strained gas pipeline segment (⌀1420х18,7mm) at Urengoi GTP.
Fig.6. Results of the strained gas pipeline segment inspection: 1 - SC zone.
The Methodical Guideline (MG) on non-invasive magnetometric diagnostics (NIMD) of gas and oil pipelines using the TSC-type instruments is developed. The proposed MG incorporates the description of the NIMD principle allowing detecting and localizing stress-strain state and revealing the presence of various-type damages in the metal of pipelines buried under the ground, water and other media.
NIMD is based on measurement of the distortions of the magnetic field of the earth (Нearth) conditioned by changing of the pipe metal’s magnetic permeability in SCZs and in the corrosion-fatigue damaging development zones. The Нearth variation nature (frequency, amplitude) is conditioned by the pipeline strain occurring in it due to the effect of a number of factors: residual process and assembly stresses, working load and self-compensation stresses at the ambient air and environment (soil, water, etc.) temperature fluctuations.
At decoding of magnetograms and classification of magnetic anomalies the criteria developed based on the more than 20-years experience in the metal magnetic memory method application at direct inspection of pipelines are used.
The "MMM-System" software product is used for results processing and detection of segments operating in the most stressed conditions.
Fig.7 and fig.8 show the fragments of the inspection results of individual segments of gas and oil pipelines, buried under the soil layer at the depth of two meters.
Fig.7 shows the results of a buried gas pipeline ⌀530, st.20) inspection. The diagrams of the Hp field distribution along the three components (normal and tangential (longitudinal and transverse)) indicate the presence of the local variation of the magnetic field with the maximum gradient in the SC zone (see the bottom part of the magnetogram).
Fig.7. The results of a buried gas pipeline segment inspection.
Fig.8 shows the distribution of the resulting Hp magnetic field over the oil pipeline ⌀219х8mm, steel 20) buried under the soil layer at the depth of about 2 m. The marked SC zones are characteristic of the pipelines operating in conditions of the lack of self-compensation combined with high residual stresses after fabrication and installation.
Fig.8. The distribution of the resulting Hp magnetic field over the oil pipeline.