Richard J. Davis

The Feasibility of Using the MFL Technique to Detect and Characterize Mechanical Damage in Pipelines

Mechanical damage is the single largest cause of pipeline failures for gas transmission pipelines and a leading cause of failures for liquid pipelines today. Outside forces (usually construction equipment) can deform the natural cylindrical shape of a pipeline, scrape away metal and coating, and/or stress and cold work the steel changing its microstructure and altering its mechanical properties. Both the geometric deformation and the amount of residual stress, plastic deformation and cold working contribute to the severity of the defect. Having an NDE in-line inspection (ILI) technique for both detection and characterization of mechanical damage defects is important.

The Design of a Mechanical Damage Inspection Tool Using Dual Field Magnetic Flux Leakage Technology

This paper describes the design of a new magnetic flux leakage (MFL) inspection tool that performs an inline inspection to detect and characterize both metal loss and mechanical damage defects. An inspection tool that couples mechanical damage assessment as part of a routine corrosion inspection is expected to have considerably better prospects for application in the pipeline industry than a tool that complicates existing procedures. The design is based on study results that show it is feasible to detect and assess mechanical damage by applying a low magnetic field level in addition to the high magnetic field employed by most inspection tools. Nearly all commercially available MFL tools use high magnetic fields to detect and size metal loss such as corrosion. A lower field than is commonly applied for detecting metal loss is appropriate for detecting mechanical damage, such as the metallurgical changes caused by impacts from excavation equipment. The lower field is needed to counter the saturation effect of the high magnetic field, which masks and diminishes important components of the signal associated with mechanical damage. Finite element modeling was used in the design effort and the results have shown that a single magnetizer with three poles is the most effective design. Furthermore, it was found that for the three-pole system the high magnetization pole must be in the center, which was an unexpected result. The three-pole design has mechanical advantages, including a magnetic null in the backing bar, which enables installation of a pivot point for articulation of the tool through bends and restrictions. This design was prototyped and tested at Battelles Pipeline Simulation Facility (West Jefferson, OH). The signals were nearly identical to results acquired with a single magnetizer reconfigured between tests to attain the appropriate high and low field levels.

The Effects of Flux Leakage Magnetizer Velocity on Volumetric Defect Signals

The inspection requirements of the hundreds of thousands of miles of pipeline worldwide necessitates the use of high inspection velocities. Unfortunately, high inspection velocities can compromise the ability to detect and characterize defects [1,2]. These velocity effects need to be quantified in order to have a complete understanding of MFL inspection capabilities. This paper presents an explanation and a summary of these effects based on two and three dimensional finite element analysis and experimental results. Selected finite element and experimental results are also shown. The specific problem addressed is large diameter (>12 inches), ferromagnetic steel pipe material and inspection velocities up to 10 miles per hour. The defects are volumetric metal loss generally caused by corrosion.

The Effects of Remanent Magnetization on Magnetic Flux Leakage Signals

The Magnetic Flux Leakage (MFL)Technique is the most commonly used technique to inspect large diameter transmission pipelines [1–5]. A typical MFL inspection system uses permanent magnets to apply an axially oriented magnetic field to the ferromagnetic pipe material. The magnetic field is perturbed by a metal-loss region (usually caused by corrosion) to produce flux leakage outside the pipe, which can be measured by field sensors. The magnetization system in an MFL inspection system should ideally produce a magnetic field that is strong enough to cause a measurable amount of magnetic flux to leak from the pipe material at metal-loss regions, uniform from inside to the outside surface of the wall thickness so that the measured signal is more linearly related to metal-loss depth, and consistent in magnitude along the length of a pipe so that flux leakage measurements can be compared at different locations during an inspection run.

THE EFFECTS OF MAGNETIZER VELOCITY ON MAGNETIC FLUX LEAKAGE SIGNALS

In many magnetic flux leakage applications, the nondestructive inspection constraints suggest the use of high inspection velocities. However, high inspection velocities can compromise the ability to detect and characterize defects. In general, velocity effects can be detected at speeds exceeding a few miles per hour [1]. These effects need to be quantified in order to have a complete understanding of the capability of the inspection system. This paper presents the application and results of axis- symmetric finite element modeling for the examination of the effects of the magnetizer speed on flux leakage signals. The specific problem addressed is large diameter pipeline steels, and a velocity range of from 0 to 15 miles per hour. The modeling examines the interaction of the magnetizer, pipe material and metal loss defects. Experimental confirmation of selected results is also provided.

NONDESTRUCTIVE MEASUREMENT OF MAGNETIC PERMEABILITY

Conventional techniques for measuring magnetic permeability using permeameters require either a thin strip or a ring to be cut from a sample of the material. Obviously, this type of measurement is destructive in nature and cannot be used for in-situ permeability measurements. In this paper, we describe a technique that can be used to measure the permeability of flat plates nondestructively. The method uses a closed-form solution for the on-axis field that is transmitted through the ferromagnetic plate by an axisymmetric coil with a rectangular cross-section, energized by a DC (or very low frequency) current.

Pipeline Mechanical Damage Characterization by Multiple Magnetization Level Decoupling

Mechanical damage, caused by mechanical forces that deform the natural cylindrical shape of a pipe, can be detrimental to the operational integrity of a pipeline. Mechanical damage can either remain benign for the operational life of the pipeline or lead to failure. Mechanical damage is the leading cause of pipeline failures in the United States. Therefore, it is important to both detect mechanical damage defects and characterize parameters such as microstructure change, residual stress, and the extent of removed metal.

Low Frequency Eddy Currents with Magnetic Saturation for In-Line Detection and Sizing of Stress Corrosion Cracks

Under previous programs for the Pipeline Research Committee, Battelle has developed and field tested a low frequency eddy current instrument for characterizing stress corrosion cracks in pipelines. While a significant improvement over conventional magnetic particle inspections, the eddy current method as it was developed in these programs is limited to use for inspections of pipelines from the outside surface. This paper examines the possibility of using this low frequency eddy current equipment to detect and size stress corrosion cracks from the interior of the pipeline by magnetically saturating the pipeline to reduce its permeability and thereby increase penetration of the eddy currents into the pipe.