Groundbed failure and foreign-line interference on a coated crude line

Diagnosis first. A five-fold rise in a deep MMO bed points to the environment, not the anodes: MMO anodes are not consumed at anything like this rate in 12 years. Likely causes are gas blockage in the well (the classic deep-bed failure), the coke breeze drying out, a dropped water table, a cable or junction fault, or a collapsed casing.

Separate them with cheap tests before committing capital:

• Rectifier DC volts and amps, to confirm the resistance is in the circuit and not a meter error.
• Anode-to-soil potential and a temporary portable reference cell reading at the wellhead.
• Cable resistance and a junction inspection; a single bad splice can look like a bed failure.

Ranked options

1. Re-wet and vent the well (pump water, release gas). Lowest cost; often restores most of the output.
2. Localised repair - reconnect a failed anode string or replace a splice.
3. New groundbed, with a proper vent and a location chosen for soil resistivity and distance from km 17.

Criterion: cost per amp of current actually restored to the pipe, verified by a potential shift at the remote end - not nameplate amps. A cheap repair that adds amps no one can measure on the pipe is not a repair.

Area and current density. The under-protected stretch is about 12 km by 1.92 m of circumference, roughly 23,000 m². At 6 A that is about 0.26 mA/m² averaged over the whole surface - a normal figure for a good coating, yet the criterion is not met. The current is therefore not arriving; it is discharging near the drain point and being lost to earth.

The distinguishing measurement is a step test. Raise the rectifier output in increments and log IR-free potentials at a fixed remote location (say km 19, near the far end of the affected zone).

• If remote potentials barely move while the local one swings, the groundbed/soil path is the limit - the current is not travelling.
• If remote potentials respond proportionally but the ON/OFF shift stays under about 100 mV and gradients are steep near the drain, coating holidays or shielding are driving the demand.

Supporting evidence: pipeline current mapping or Pearson-type coating surveys along km 8–20, and the CIS gradient shape. A shallow, uniform gradient with a small ON/OFF difference across the whole stretch is much more consistent with a weak, high-resistance groundbed than with a few large holidays, which give sharp local signatures. History is also evidence: the same coating met -0.85 V at 19 A.

Characterise the interference properly first. A 120 mV negative shift is cathodic pickup, which is usually protective but can mean overprotection, coating disbondment or hydrogen on their line, and it nearly always has an anodic return zone somewhere away from the crossing. That return zone is the real hazard.

Joint test

• Synchronised current interruption on Station 2, with loggers on the foreign line at the crossing and at least 500 m either side in both directions.
• Record both ON and instant-OFF potentials, so IR drop is removed and polarity of the shift is unambiguous.
• Survey their line outward until the shift falls below a few millivolts, and look specifically for a positive (anodic) excursion where current leaves their pipe.

Mitigation options, in order of preference:

1. Adjust Station 2 output or relocate the new groundbed away from km 17, so the interference disappears at source.
2. A resistance bond between the two lines, with a shunt to measure and cap the drained current.
3. Galvanic anodes local to the affected section of their line, if a bond is not acceptable.

Formalise: a written agreement covering test procedures, interruption schedules, notification of rectifier changes, records of both parties' potentials, and a periodic joint review. Do not let the matter live in correspondence.