Corrosion Inspection Tools
A variety of tools and interpretation techniques are employed to monitor corrosion because a large amount of information is required for interpretation from both single and multiple casing. Four types of tools are considered here (Watfa. 1989):
1. Electromagnetic casing corrosion detection.
2. Multifinger caliper tool (mechanical).
3. Acoustic tool.
4. Casing potential profile tool.
The Electromagnetic Corrosion Detection
In essence these tools consist of a number of electromagnetic flux transmitters and receivers that are linked by the casing string(s) in much the same way as the core in a transformer links the primary and secondary coils.
For a qualitative measure of the average circumferential thickness of multiple casings (Watfa, 1989), the phase shift between the transmitted and received signals is measured. The phase shift related to the thickness of the casing is as follows:
Ф = ‘Intern] where:
t — combined thickness of all casings.
a = combined conductivity of all casings.
(i = combined magnetic permeability of all casings.
/ = tool frequency.
By increasing /, the depth of investigation can be reduced to include only the inner casing and values of a and ft can be determined. Increasing / still further provides an accurate measure of the ID of the inner casing string. All three measurements can be made simultaneously to provide an overall view of material losses.
For a more detailed analysis of the inner casing string a multi-armed, pad tool can be used which generates a localized flux in the inner wall of the casing by means of a central, high-frequency, pad-mounted signal coil. Flux distortions measured at the two adjacent receiver or ‘measure’ coils, are indicative of inner pipe corrosion.
In a second measurement, electromagnets located on the main tool body generate a flux in the inner casing. Again, the presence of corrosion will induce a flux leakage, which is measured by the two measure coils. This measure is a qualitative evaluation of total inner casing corrosion.
The multi-finger caliper tool consists of a cluster of mechanical feelers that are distributed evenly around the tool. Each of these feelers gives an independent
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Fig. 6.2: Typical installation of galvanic anodes. (After XACE. Houston, TX. Control of Pipeline Corrosion, fig. 8-6.) |
measurement of the radius. The small size of feelers allows small anomalies in the inner casing wall to be detected and measured. The multi-finger caliper gives an accurate construction of the changes in the internal diameter of the casings.
The acoustic tool consists of eight high-frequency ultra-sonic transducers. The transducers act as receiver and transmitter, and two measurements are obtained from each transducer. These measurements are: internal diameter, which is measured from the time interval of signal emission to the echo return, and the internal casing thickness.
Casing Potential Profile Curves
Corrosion damage to the casing can be detected easily using the casing potential profile tool. This tool measures the voltage drop (1R drop) across a length of casing (e. g., 25-ft) between two contact knives (see Fig. 6.1).
Logging (from bottom to the top) is done at intervals equal to the spacing of the knife contractors. Voltage (IR) drops are then plotted versus depth (casing potential profile). As shown in Fig. 6.1. readings on the left (-) side of zero indicate that current flows down the pipe, whereas positive values ( + ) show that flow is upward. Consequently, the curve sloping to the left from bottom indicates corroding zone (anode), where electrons are leaving the casing.