8" x 3 km Flowline Intelligent Pigging 1

[march1971]

We just completed the IP of an 8" x 3km gas flowline that was installed in 1987. This line has an external bullet ponit defect measuring 16mm long x 11mm wide x 5mm deep at 3 O'clock position with the bultt in place and is a schedule 100 pipe with Nominal wall thickness of 15.1mm. The final IP report revealed no single defect on the line(neither external not internal) not withstanding that the bullet point has not been repaired. The IP tool in question is an MFL tool and auxillary pumping was empolyed using water as the fluid.
My questions are:
(1) Is it possible for a line to be in service for about 20 years without have a single metal loss defect?
(2) Is defects such as bullet point not detectable by MFL tool?
(3) What is the minimum defect size that an MFL tool can detect?
(4) What are the possible reasons why this defect was not detected?

 

[stanweld]

Do not cross post your question.

 

[brimmer]

1. It could be that your line has no corrosion, but unlikely. I don't know what type of external coating is on the line, or if there is CP etc., and what are the characteristics of the gas, is it wet or dry, does it have H2S etc., do you have/need a chemical program, do you complete maintenance pigging etc. If you have maintained the line well, it may not have corrosion, but chances are as it is a flow line it would have developed some.
2. Not sure exactly what you mean when you say bullet point, but certainly a defect of that size and depth should have been easily detectible by the pig.
3. Depends on the tool specifications/vendor. I assume it is a high resolution MFL tool, these can detect corrosion defects even less than 10% and 1 cm size pits quite easily.
4. Possible reasons, was the line cleaned before the run? If it was not the tool will loose sensitivity if there are internal corrosion deposits (magnets loose contact). Was the run valid (was the tool working the entire time)? Did it loose power at any stage in the run? The wall thickness it quite thick for an 8", is the tool capable of magnetizing (saturating) this wall thickness at this diameter of pipe? The pig was calibrated correctly before hand. You might not want to state the name of the ILI vendor, but some are much better than others...