Page 355 - Pipelines and Risers

326 Chapter 18

Pressure, Temperature and Flow Control
Instrumentation must be provided at the control center to register the pressure, temperature
and rate of flow in the pipeline. Any variation outside the allowable transients must activate
an alarm in the control center.

To ensure protection to the pipeline against over (and under, for example, when there is
leakage) pressurization and excessively high temperatures, automatic primary and secondary
trips should be installed at the compressor station. Details as to their location and their high /
low pressure and high temperature settings are required as input into the Operations Manual.

Relief Systems
Relief systems such as relief valves, are typically required to ensure the maximum pressure of
the pipeline does not exceed a certain value. Relief valves must be correctly sized,
redundancy provided, and they must discharge in a manner that will not cause fire, health risk
or environmental pollution.

High Integrity Protective Systems (HIPS) may be considered when the conventional relief
methods are unsuitable for ultimate plant protection. However, the application of a High
Integrity Protective System must be justified and its design must be agreed with the relevant
Regulatory Authority. The following main principles apply:

0 A clear economic advantage must be demonstrated over the conventional approach to
justify the increased complexity and dependence on rigorously controlled maintenance
associated with HIPS;
0 HIPS must be designed with appropriate redundancy and testing frequency to ensure
higher reliability than conventional protection systems;
0 Economic comparisons should take into account life cycle maintenance and testing costs;
0 HIPS must respond quickly enough to prevent over pressure if downstream systems can
be suddenly blocked-in. This is one reason why HIPS lend themselves to protection of
large volume systems, including pipelines, rather than small sections of plant;
HIPS isolation valves must have a tight shut-off. Otherwise, partial capacity relief valves
will be needed after the HIPS isolation valves to accommodate leakage rates should the
HIPS isolation valves fail.

Leak Detection
The pipeline must have an integrity monitoring system capable of detecting leak. A leak
detection system in itself has no effect on the leak expectancy of a pipeline and will only
make the operator aware of the occurrence of a leak, enabling him to take remedial actions in
order to limit the consequences of the release. The leak detection system requirements will
vary depending on the pipeline system in question (e.g. offshore or onshore, length etc.)
however, the following should be considered at the design stage and/or implemented during
operation.