8" X 3 Km Gas Flowline Intelligent Pigging

[march1971]

We just concluded the intelligent pigging of an 8"x3km gas flowline that was installed in 1987 but the final IP report from the contractor revealed no defect. This line is a schedule 100 pipe with Nominal wall thickness of about 15.1mm and has a bullet pierce of 16mm length x 11mm width x 5mm depth at 3 O'clock position with the bullet still in place on the external. The IP tool did not detect this defect. The tool in question is an MFL tool. The pigging was carried out via auxillary pump using water as the medium because the line was not originally designed for pigging.
I am wondering what went wrong and need clarification on the following questions that is going on in my mind:
(1) Is it possible for a gas line to be in service for about 20 years without having a single mtal loss defect?
(2) What is the minimum defect size that can be detected by an MFL tool.
(3) What could be the reason why such a bullet point defect size of 16mm x11mm x5mm was not detected.
(4) Is it not possible for an MFL tool to detect such a defect(bullet point)

 

[stanweld]

Regarding Question 1, yes it is possible that no measureable metal loss due to corrosion will be observed after 20 years in in gas service.

Regarding Questions 2, 3 & 4, the minimum defect size will be dependent on the inspection device's limitations and its calibration as will its ability to detect the bullet indentation described.

 

[BigInch]

It is possible that the defect was below the accuracy level of the instrument and the survey methods. A 5mm void would usually be detected, but if not done very carefully a 5mm defect of small length (diameter), such as you have, might not be noticed. The defect sensitivity detection level should have been specified by the survey contract. Check to see if it was specified in the contract and evaluate if the contractor's equipment or method was not appropriate to suit that specification.

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