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Drum Demagnetisation

EM Cable Services now offers a demagnetisation service on all types of wireline drums where magnetisation may cause an issue utilizing the Benchmark drum demagnetizing panel ADDMA101.

All we need is the drum, complete with the cable length to perform the demagnetisation.

This can be performed on all Multi or Mono type conductor wirelines.

Demagnetism Causes and Theory

Causes of Magnetism

Magnetic fields exist as concentrations of magnetic flux lines. These concentrations are polarized in magnetized metal, with the flux lines running between north and south poles. In fields produced by electrical currents, the flux lines circulate around the current at right angles to its direction of flow. In very simple terms, the degree to which the fields are attractive or repelling depends on the density of the flux line concentration (i.e. the number of lines passing through a unit of space). The strength of a field around an electrical current is directly proportional to the strength of the current. Thus, field strength around a DC current remains relatively constant, whereas field strength around an AC current alternately builds and collapses with each current flow reversal. Flux line circulation around the current also reverses with each flow alternation.

  1. Permeability: the ease with which the metal accepts induced magnetic flux. Metals which do not carry magnetic lines of flux, such as aluminium and copper, will not become magnetized.
  2. Retentively: the ability of the metal to retain induced magnetic flux. As a general rule, softer ferromagnetic metals are less retentive than harder ferromagnetic metals.
  3. Reluctance: the opposition offered by ferromagnetic metal to the establishment of magnetic flux. The relationship between reluctance and magnetic flux lines in a metal is analogous to the relationship between resistance and current flow in an electrical circuit.
  4. Ferromagnetic: hoisting equipment can become magnetized either mechanically or electrically. Mechanical magnetisation occurs through simple exposure to the earth's magnetic field during normal operation, extended storage in one orientation, or transport from one geographical location to another. Repeated stroking in the same direction or the shocks and jars associated with production, maintenance, and repair processes can also magnetize equipment. Electrical magnetisation occurs whenever direct current (DC) or alternating current (AC) flows through the equipment, trucks, or skids.

Demagnetising

Magnetic fields exist as concentrations of magnetic flux lines. These concentrations are polarized in magnetized metal, with the flux lines running between north and south poles. In fields produced by electrical currents, the flux lines circulate around the current at right angles to its direction of flow. In very simple terms, the degree to which the fields are attractive or repelling depends on the density of the flux line concentration (i.e. the number of lines passing through a unit of space). The strength of a field around an electrical current is directly proportional to the strength of the current. Thus, field strength around a DC current remains relatively constant, whereas field strength around an AC current alternately builds and collapses with each current flow reversal. Flux line circulation around the current also reverses with each flow alternation.

Exhibit A

Ferromagnetic metals generally retain some magnetism after the magnetising force is removed. This leftover magnetism is called residual magnetism, and its field strength depends on a number of factors, among them the permeability, retentively, and reluctance exhibited by the host metal as well as the strength of the magnetising force. As shown here, electrical current will be induced in conductors that cut through the flux lines of a residual magnetic field. The induced current will be greatest when the conductor moves through the field at right angles to the lines, and will change direction whenever the movement of the conductor through the field is reversed. No current will he induced into conductors that move through a field parallel to its magnetic flux lines.

Exhibit B

Magnetism and Well Logs

Effect on Cable Conductors

Current can be induced into logging cable conductors whenever cable drum rotation moves them through magnetized areas in the hoisting equipment. The strength of the induced current depends on:

  1. The rotational speed of the drum (which determines the number of flux lines cut per unit of time)
  2. The density of the field
  3. The angle at which the conductors cut through the lines.

The flow direction of an induced current depends on the polarity of the magnetic field that produced it. Because several magnetized areas can exist simultaneously in different densities and polarity orientations, induced current flows can vary in strength and change direction during each drum rotation.

SP logging signal currents and induced currents will add algebraically. Therefore log traces will be the net result of interaction-1 between the two currents during each cable drum rotation. Conceivably, during one drum rotation, one magnetized area could induce opposing current that is much greater than the logging signal current.

In this case, the resulting galvanometer log trace would indicate a subsurface condition completely opposite to that detected by the logging instrument. Another magnetized area could induce current that adds to the strength of the signal current, and the associated log trace would signify much greater spontaneous potential than was actually detected. The magnetized areas in two or more drive train components, such as the drive chain, sprocket, and drum flange, could come into proximity with each other. The field strength and polarity resulting from the interaction of these fields could produce a totally unique effect on signal current that would not appear again on the galvanometer log until the same areas line up in the same way several drum rotations later.

Logging cables, cable drums, drive train components, skids and trucks are all checked for magnetism during their respective fabrication phases as well as just before shipment to the field. In spite of these precautions, mechanical stresses imposed by hoisting and spooling as well as the welding and hammering associated with normal repair and maintenance can magnetize hoisting equipment to levels that affect SP logs. Accordingly, trucks and skids should be checked regularly for induced magnetism.

Demagnetising - Theory

Understanding how iron and steel become magnetized is the first step towards developing successful demagnetisation practices and procedures. The key to understanding is the hysteresis curve shown in exhibit A. This curve portrays the induction and elimination of magnetism by an outside force into previously magnetized metal. On the graph in the figure, induced magnetism is plotted along axis B and the magnetising force is plotted along axis H.

As the magnetising force increases to the right along axis H, the induced magnetic field gathers strength along the dotted line until it reaches saturation point (a). This point marks the maximum limit of flux line density possible in the metal. Regardless of any additional increase in the magnetising force, no further increase in density is possible other than that which may occur in ambient non-magnetic air or material.

As the magnetising force returns to zero along axis H, the induced field strength decreases from (a) along the hold line to point (b). The distance between this point and zero represents the strength of the residual field that remains after the magnetising force has been removed. The residual field can only be eliminated by applying a magnetising force having a polarity opposite to that of the initial magnetising force. As this second force increases to the left along axis H, the residual magnetism decreases along the curve between points (b) and (c). The demagnetising force required to eliminate the residual field is called coercive force and its magnitude is represented by the distance between point (c) and zero on the H axis.

As the reverse magnetising force increases, induced magnetism grows from zero towards saturation at point (c). By again reversing the polarity of the magnetising force and increasing its strength along the H axis to the right, the intensity of the newly induced field decreases along the curve running between points (d) and (i). Point (e) on this curve represents residual field intensity and point (f) indicates the coercive force required to eliminate the residual field and demagnetize the metal.

Exhibit A Hysteresis

Demagnetising - Practice

Hysteresis curves illustrate two facts that are basic to demagnetising procedures and equipment.

First, two magnetic fields having different flux line directions cannot occupy the same space at the same time. The simultaneous imposition of two such fields into the same space produces a resultant field which is the vectorial sum of the flux line directions in the two fields. However, if two fields are impressed successively, the last field will eliminate the remnant field from the previous magnetisation (if the last field is strong enough to establish itself in material occupied by the remnant field). Secondly, changes in induced fields lag the changes in the magnetising faces that produce them, and induced field strengths do not reach the field strengths of the magnetising forces.

Thus, it is possible to eliminate an established magnetic field with a magnetising force of the appropriate flux line orientation and field intensity. Demagnetising procedures apply magnetising forces that undergo periodic intensity reductions and polarity reversals. The graph in exhibit B shows how these forces eliminate induced magnetism. As in exhibit A, the graph in exhibit B plots induced magnetic flux along axis B. Demagnetising force plots along axis H.

The curve in the lower left quadrant of the graph traces the applied demagnetising force; associated hysteresis curves appear in the upper left quadrant; and induced magnetic field strength is shown in the upper right quadrant. Note the direct correlation between intensity and polarity changes in the demagnetising force and the induced magnetic field. This correlation illustrates the progressive erosion of induced flux line density by the coercive forces and field intensity reductions occurring with each polarity reversal of the demagnetising force.

The frequency of polarity reversals also bears on the effectiveness of a demagnetising force. As reversal frequencies increase, the penetration of the demagnetising force decreases. Therefore, demagnetising forces having relatively low frequency reversals are more effective for deep seated magnetic fields in larger equipment items. The procedures in this publication specify the 60 Hz reversals in common single phase line voltage for field demagnetisation efforts.

Exhibit B flux curve