Decommissioning and abandonment

Decommissioning and abandonment

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The water contains a number of inhibitors to ensure the continuing integrity of the pipeline for an indefinite period. Doses are much higher than would be expected for pre- commissioning, which lasts only a few weeks. Corrosion inhibitors and oxygen scavengers prevent aerobic and acidic corrosion. The biocide prevents the growth of sulphate-reducing bacteria (SRB), which in turn will produce anaerobic corrosion.

Trenching

TRENCHING

TRENCHING

ƒ Lower pipeline below surface of seabed ƒ Backfill ƒ Methods covered in detail elsewhere ƒ Why bury now when not at installation? ƒ Open trench ƒ Scour of seabed ƒ Is wall corroded? ƒ Reduced strength ƒ But no longer pressurised

ƒ Spalled concrete in nets ƒ Crossings at live pipelines

The pipe may need to be both lowered into a trench and then buried to leave a smooth seabed. This is as discussed in the other modules. However, we need to ask how it is possible to bury the pipeline now when it was not undertaken earlier during installation. Perhaps the pipe was only trenched and not buried or some scour has since occurred. What if there is corrosion of the wall and the steel strength is reduced.? It may not be a problem since it does not have to withstand operating pressure anymore. Will deteriorated concrete spall off and end up in fishermen’s nets? Also, where the pipeline crosses another live line, detailed analysis is required. Can the line be lowered sufficiently or is it necessary to cut the line?

TRENCHING ISSUES