Magnetic Flux Leakage application on a New/Used Oilfield Tubular Inspection (Part-4) By : Mohammed Youssef ASNT-Level III, TUBESTAR - Saudi Base

New/Used oilfield tubulars such as casing, tubing, drill pipe and coiled tubing must meet stringent manufacturing specifications set by the American Petroleum Institute (API). Lower grade tubulars are typically inspected for defects using magnetic flux leakage (MFL) devices at the pipe mill.

MFL inspection uses powerful electromagnetic yokes to magnetize the tubular circumference while encircling the outer diameter. Arrays of MFL sensors then scan the length, detecting flux leakage that occurs at areas with corrosion, cracks, or wall loss. Reference notches of defined depth, dependent on tube grade and inspection sensitivity needs, are etched to calibrate and set the detection thresholds.

As the tube passes through slowly rotating magnetization coils, a minimum 100% surface area is screened by the MFL hauls searching for both transverse and longitudinal imperfections. Resulting data feeds into computers running analysis software, recording any flaws, and mapping their locations. The tubular is finally partially demagnetized before storage or shipment.

By identifying defects early, MFL allows fabricators to catch and rework or scrap substandard sections. This ensures oilfield tubulars meet quality levels and prevents potential in-service failures leading to costly disruptions.

 Specialized MFL sensors detect this leakage flux, and can identify areas where corrosion has reduced wall thickness or other defects like cracking or pitting are present. Advanced MFL tools also measure the intensity of leakage fields, allowing estimation of the size, depth and profiles of the flaws.

Instruments (Device) :

 To facilitate thorough inspection, the EMI method employs four essential functions, outlined as follows:

·         Longitudinal System

The longitudinal system is an electromagnetic inspection (EMI) to detect longitudinal defects.  The system consists of a rotating head, a logic chassis, and an operator sound alert circuit.

·         Transverse System

The transverse system inspects casing/drill pipe with a stationary head and scanner. The section of casing/drill pipe under inspection is magnetized with a longitudinal field.  Transducers held against the outside surface of the casing give complete surface inspection as pinch rollers move the casing through the inspection head.  This EMI system detects transverse imperfections.

·         Wall Thickness

Computerized system that features non-contact encircling transducers to monitor wall thickness of tubular goods. These transducers presume the wall thickness through a deductive technique of measuring the amount of axial magnetic flux that is not consumed by the pipe. The transducers are hall effect elements that are spaced around the pipe. They are connected to indicate the minimum wall thickness per 1/4 of pipe. The transducer signals are sent to the computer system.

·         Grade comparison.

Grade comparison System using principles based on differences in the electromagnetic characteristics of grades of OCTG. This System is typically an integral component of an EMI inspection system and might not be available separately.

Standardization:

 Reference Pipes shall contain notches, drilled holes and wall reduction

The output result displays the screen divided into six channels, as depicted in the figure below:

• Two channels dedicated to Longitudinal analysis.
• Two channels allocated for Wall Reduction (Hall effect)
• And two channels are designated for Transverse examination.