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Measurements
The measurement specification for the MERLIN CP monitors was drawn up with input from pipeline operating companies and independent CP consultants. It includes all the measurements conventionally taken at test stations, cp posts and riser posts:
Channels |
Pipe-to-Soil 1 |
Pipe-to-Soil 2 or Instant OFF |
Coupon Current or Bond Current |
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DC |
AC |
DC |
AC |
DC |
AC |
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1 |
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2 |
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3 |
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Monitoring channels are fully independent. This means that a 2-channel MERLIN can be configured to monitor either two pipe-to-soil inputs (the second input could be a sleeve or foreign pipe) or, if a coupon is fitted, one pipe-to-soil input and an instant OFF input. The MERLIN unit incorporates an internal relay, which is opened just before channel 2 is read.
All Merlin CP monitors measure DC and AC voltages on each channel simultaneously. This has become extremely important as operators now recognise that induced AC on pipelines can have detrimental effects.
If a coupon is fitted at the test post, channel 3 can be used to calculate the DC and AC current by measuring the voltage across a shunt of known value (Abriox can supply suitable shunts and cable assemblies). Once the shunt value and coupon surface area have been entered in the CPSM3 software, the data can be displayed either as the current in mA or as the current density in A/m².
High and low alarms can be set for each measurement channel (for DC and AC values). A MERLIN unit will alarm as soon as a measurement is detected outside the acceptable “window” set by the user.
AC Monitoring
It is now recognised that cathodically-protected pipelines (and other metal structures) may be affected by AC from nearby power lines or traction systems.
This phenomenon is thought to be of particular concern in the case of pipelines with high quality (eg fusion-bonded epoxy) coatings that run parallel to high-voltage overhead transmission lines.
There is also evidence to suggest that AC-assisted corrosion can occur below the 15VAC threshold.
Abriox’s MERLIN system has been designed to enable pipeline operators to monitor levels of AC on their pipeline – and to monitor their effect (if any) on the cathodic protection system. The MERLIN CP Monitor measures simultaneous DC and AC levels on all channels.
AC parameters that can readily be monitored include:
• AC voltage (pipe-to-soil)
• AC current flowing through a coupon (measured across a shunt)
When the shunt value and the coupon surface area are entered, the CPSM3 software automatically calculates the current density in A/m2 and will present the results in both tabular and graphical form.
The MERLIN system has been used to monitor test posts with known or suspected high levels of AC. In diagnostic mode (hourly measurements) it has been possible in some cases to identify and mitigate the source of transient or regular variations in AC levels.
The British Standards Institute has issued a new draft technical specification on the Evaluation of AC corrosion likelihood of buried pipelines: application to cathodically-protected pipelines (DD CEN/TS 15280). BSI points out that, in the presence of AC interference, the criteria given in EN 12954:2001 (Cathodic protection of buried or immersed metallic structures. General principles and application for pipelines) are not sufficient to demonstrate that the steel is being protected against corrosion.
The new technical specification provides limits, measurement procedures and information to deal with long-term AC interference and evaluating the likelihood of AC corrosion.