PROCEDURE 20

OFFSHORE PIPELINE SACRIFICIAL ANODES SURVEY.

This procedure will determine the activity of sacrificial anodes

INSTRUMENTS AND EQUIPMENT

1. A survey vessel capable of exact positioning and locating the pipeline.
2. Diving services.
3. Three channel data-logger.
4. Armature winding wire.
5. Booms and paternoster.
6. Three silver/silver-chloride electrodes.

METHOD

20.1 Attach the trailing end of the armature wire to the subject pipeline metal.

20.2 Attach the other end of the armature wire to the common pole of channel 1 on the data-logger, on board the survey vessel, via the reel-off equipment.

NOTE The common pole on the data-logger normally shares a common potential with all channels. If this is not the case, then it is necessary to bridge all channels to achieve this condition.

20.3 Attach electrode 1 to the positive pole of channel 1 of the data-logger via the 'sinker arrangement' which will enable it to be trailed along the sea-bed.

20.4 Attach electrode 2 to the positive channel 2 of the data logger via the over line floater arrangement which will enable it to be trailed behind the vessel directly above the sunken electrode.

20.5 Attach electrode 3 to the positive pole of channel 3 of the data-logger via the floater and boom arrangement which will allow it to be positioned in the water remote from the influence of the subject pipeline.

20.6 Start the data-logger recording the voltages on all channels.Move the vessel over the pipeline, at a constant speed, logging the position accurately against time.

NOTE. The accuracy of the survey depends on the accuracy of the position of the boat when features are recorded on the data-logger.

20.7 Drop weighted location marker buoys at all significant deviations between the remote electrode voltage and the sunken electrode voltage.

NOTE. Active anodes should cause a peak voltage between the pipeline metal and the close electrode.

20.8 Examine 2 locations, with divers, where there are peak voltages on channel 1 of the data logger.

20.9 Examine 2 locations where there are depressed voltages on channel 1.

NOTE. This could indicate coating missing but the pipeline should be protected by the current passing onto the metal from the anodes.

GENERAL NOTES.The surface electrode and the remote electrode are to act as controls which will be used to analyse the voltages recorded between the close electrode and the pipeline metal.

The variation in voltages between the remote electrode and the pipeline should be removed from the total voltage between the close electrode and the pipeline.

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