In-tube magnetic examination of lined tubuing

1.The present article substantiates 100 percent practicability of inspecting lined tubing with protective bushing by magnetic examination, utilizing in-tube magnetic detectors.

Internal and external factory corrosion-resistant coating and protective bushing of the welded joints are designed to ensure 100 percent protection of steel tubing against internal and external corrosion of piping. Service life of internally insulated tubing is increased averagely by 8–10 times versus non-protected pipelines. Currently developed techniques for protecting welded joints with bushing ensure high quality afield weld interconnections, including West Siberia environment. The mentioned factors determine exploitability of lined tubing with protected welded joints.

However, in the course of operation it will be necessary to periodically inspect condition of piping and in due time take measures to eliminate possible defects and damages occurred in operation, or resulted from external impacts. In this event, the most effective technique for checking condition is 100 percent in-tube inspection by running intellectual diagnostic shells inside the pipeline. At present the following two examination methods are used: ultrasonic and magnetic and, correspondingly, ultrasonic and magnetic detectors.

For lined tubing with protected welded joints, the ultrasonic method is a problem, because there is additional diffusion and absorption of the ultrasonic signal over the lining, thus reducing accuracy of measurements and complicating processing of the measurement results. Around the welded joint area, if the protective bushing exists, it is impossible to inspect condition of the tube lining, which is shielded by a protective bushing.

Among possible methods to inspect lined tubing and protective bushing, the most advanced is the magnetic inspection method. Magnetic fields are not diffused over the lining and ensure metal magnetization around the bushing, facilitating to detect leakages of magnetic flow under detect conditions.

Bench tests were performed at test facilities of Rosen Europe, RTRC (ROSEN Technology and Research Center), Lingen, Germany. The bushing was welded to the pipe of the same diameter. The bench tests were performed by running magnetic in-tube shell inside the pipeline with a test coil.

The in-tube magnetic inspection program consists of two stages.

Stage One: it was analyzed the practicability of running magnetic examination shell in the lined, bushing-protected pipe without jamming, damage of lining and deformation and / or displacement of the bushing.

Stage Two: it was analyzed the practicability of detecting defects of the pipe wall, if interior and exterior coatings are available, as well as around the bushing.

2.The first stage examinations include two runs of high-resolution magnetic inspection shell, without use of tube wall thickness digital gauges. The runs of the shell were performed without clean-up of the tube internal surface and in “dry” mode. Such conditions ensured the worst-case shell running conditions and maximum shear load on the bushing. The shell was running in the pipe at constant speed and with the use of electric hoist. Actual in-tube conditions during the shell run are better, because the tube wall is not “dry”, it sufficiently reduces impacts on the lining.

During the primary tests practicability of in-tube magnetic examination was proved:

- magnetic shell runs without jamming;

- no defects or displacements of the bushing;

- interior coating remains serviceable;

- axial load during the run of the magnetic shell is less than the threshold, with 2.7 assurance factor.

Results of the first examination stage are essential. If any of the mentioned requirements were not met, it would mean impossibility to examine lined tubes by the in-tube inspection method.

The second stage examination procedures include four runs of the high-resolution magnetic shell, with the use of electronic tools to inspect the tube wall thickness.

To test practicability of detecting defects of lined tubing, various defects of different depth, wide and length were surfaced to simulate on-tube defects that cause stress concentrations (pitting, extended corrosion areas, cavities, cracks and mechanical damages). Accuracy of detection and measurement of defects parameters may be influenced both by interior covering and uneven exterior surface of the tube. Any tube heterogeneity, including lining, may cause “noises”, i.e. localities of diffused magnetic field. The defect detection specification is aimed at creating the efficient magnification level to be sufficiently higher after diffusion over defects, and the “noise” to be unobserved.

It was determined the specific location density of instruments capable of detecting high-resolution cross-sectional and lateral defects. For lined tubes the determined density of sensors location is not more than 2.9 mm. There was performed the check examination of a tube’s defects around the longitudinal weld.

It was also examined practicability of inspecting condition of the tube and bushing walls. During this trial the artificial defects were not surfaced, since it could require removing the welded joint. The task was to achieve a distinct picture of the tube and bushing walls.