Despite the fact that the TR was not energised, the circuit through this TR and ground bed was still closed, thus allowing the AC to be half wave rectified, with the ground bed subsequently passing a DC component, and hence inadvertent CP to the pipeline.

An additional complication to the assessment revealed that the presence of trains on the circuit lead to the erratic nature of the interaction. This was due to fluctuations and imbalance of current flowing in the overhead circuits as the trains changed speed and moved from one section of track to the next.

Potential monitoring of the Tees-Wiske pipeline eventually allowed train movements to be predicted with some accuracy, even prior to sightings. An interesting observation of this effect showed that certain train movements could not be detected through potential monitoring, however, such vehicles were diesel powered, not electric!

To overcome these effects, it was necessary to either remove or control this interaction, such that formal CP could be applied to the pipeline without the excessive potentials that may lead to coating damage in the future.

Corrosion of the pipeline was not a concern, as the current discharge was occurring at the ground bed. However, the high negative potentials were a cause of concern as they could lead to damage of the coating and a shortening of its life. Three approaches were considered;

1. Current earthing,
2. AC current filtering,
3. A polarisation cell.

Although, earthing of the currents is possible through the use of anodes, installed at selective locations along the line, this approach was considered as a last resort.

The polarisation cell was introduced across the insulating flange, that is, between the protected pipeline and the pump station earthed pipework. No noticeable effects were reportable.

The AC filter, in the form of a "capacitor bundle", developed by Dynamics (Bristol) Ltd. was installed across the DC output of the TR, that is, between the protected pipeline and the ground bed, and following observation of potentials with the electric train movements. The effects of interaction were reduced significantly and to acceptable levels.

With the interaction under control, it was then possible to superimpose the formal CP system, and thus bring the line potential into a reasonably steady profile of ca. –1.3V (CSE) – below the standard –850mV criterion.

The CP system has now been formally commissioned with further monitoring visits scheduled to ensure adequate and controlled protection is achieved. 

Discussion

Both these project examples show how corrosion monitoring, both in survey and permanent system forms, can assist the engineer assess the presence and mitigate against the effects of stray current.

The objective is always to prevent such stray currents causing harmful and possibly catastrophic corrosion failure. 

Acknowledgements

The authors wish to express their thanks to the following parties for permission to produce this paper, these are: