The influence of the outer pipe during internal MFL detection in subsea steel pipe-in-pipe.

• The MFL signals of subsea steel pipe-in-pipe are studied. There is a magnetic compression effect on the MFL signals of the outer pipe to the inner pipe. • The gap between pipes is the main factor affecting the magnetic compression effect. • The compression signal ratio (S y) displays an approximate negative exponential function with gap between pipes. Pipe-in-pipe system is crucial for subsea oil and natural gas transportation. The accurate identification and characterization of defects in pipe-in-pipe is of great significance for energy transmission security and environmental protection. Although magnetic flux leakage (MFL) testing methods are commonly used to detect and assess pipeline defects, the outer metal pipe inevitably affects the detection signal of the inner pipe during the testing process, which results in the MFL signal inaccurately reflecting the size of the defect. Here, we mainly discussed the relationship between the outer pipe and the MFL defect signals. The magnetic compression effect (MCE) of the outer pipe in current MFL testing and its relevant theory were analyzed, which manifested in the compression of the MFL signal in its peak value. The influence of different gaps between pipes (S) and inner and outer wall thickness ratios (N) on the MFL signal was examined via finite element simulation. The compression signal ratios (S y) and peak signals of the defects were compared, indicating that S was the main factor affecting MFL signal variation in pipe-in-pipe, while N had a relatively minor impact on the signal. S y displayed roughly the same negative exponential function variation with an S increase for defects at various depths. The efficacy of the simulation results was verified via MFL pipe-in-pipe defect experiments. These findings are useful for quantitatively evaluating MFL detection in subsea pipe-in-pipe. [ABSTRACT FROM AUTHOR]