Page 356 - Pipelines and Risers

PipeIine Inspection, Maintenance and Repair 321

On-Line Leak Detection
- continuous mass balance of the pipeline;
- continuous volumetric balance corrected for temperature and pressure of the pipeline;
- continuous monitoring of rate of change of pressure;
- continuous monitoring of rate of change of flow;
- low pressure alarms;
- high pressure alarms;
- high flow alarms;

Off-Line Leak Detection
- visual inspection of the pipeline route;
- running of a leak detection pig (see Chapter18.3.3);
- methane-in-water sensing by Remotely Operated Vehicle (ROV).

Several other methods of on-line leak detection are available, some of which will also indicate
the location of a suspected leak. However, in general a good deal of intermediate pressure,
temperature and flow information is required with attendant telemetry and for this reason such
methods are not generally suitable for offshore use.

18.13 Operational Pigging
The conflicting balance of sensitivity to leaks and false alarms will determine the sensitivity
of an on-line leak detection system. Large leaks can normally be detected more rapidly than
small ones. To maintain the user’s confidence in the system, avoiding false alarm should have
a higher priority than attempting to shorten the leak detection time or reducing the minimum
detectable leak rate.

Operational pigging is performed to maintain pipeline integrity. With regular operational
pigging the pipeline should be maintained at its optimum throughput capacity and a higher
efficiency will be achieved. Typically the following purposes will be served with regular
pigging:

- prevention of scale build-up;
- cleaning of the pipe wall;
- removal of internal debris;
- removal of liquids (condensate and water);
- enhancement of the performance of corrosion inhibitors;
- provision of a means to verify the occurrence of corrosion.

Pig Type and Frequency
Operational pig runs are performed in pipelines using cup or bi-directional pigs to remove
water drop-out, soft wax, sand deposits, scale and other debris build-up. The operational