Page 359 - Pipelines and Risers

330 Chapter 18

- an emergency;
- major maintenance;
- production shutdown.

An emergency shutdown of the pipeline is achieved by closing the appropriate Emergency
Shut-down (ESD) Valves. The ESDVs valves will be closed automatically by one of the
following:

- fusible plug loops (which can be tripped) in case of fire;
- low pressure trips in case of a pipeline leak;
- high pressure rrips in case of high pressure in the pipeline;
- low instrument air supply;
- terminal ESD valve as per the shutdown sequence.

A pipeline ESD valve should also be able to be closed manually at the control room and
locally at the valve itself. The closing of the ESD valve should be linked to the prime mover
shutdown.

18.1.5 Pipeline Depressurization
For most pipelines, in the event of pipeline rupture, depressurization of the line must be
carried out immediately in order to reduce the amount of escaped gas. For onshore pipelines,
closure of line sectioning valves, each side of the rupture, may further limit the amount of
product inventory escaping.

The time taken to fully depressure a pipeline to atmospheric pressure will depend on several
factors, not least of which will include, the size and type of pipeline inventory, the operating
pressure at time of rupture, the rate of flow escaping and the maximum vent rate at the end
station.
For long, large diameter gas trunklines the time taken to fully depressure a line can easily be
in the order of several days.

The procedures for emergency depressurization are an essential part of the Pipeline Operating
Manual and should state, along with the actions required, the maximum achievable
depressurization rate during emergency blowdown.

18.2 Inspection by Intelligent Pigging

18.2.1 General
In Europe, the use of intelligent pigs has increased from, on average, about 2% of the
pipelines per year at the beginning of the eighties to about 8% in the nineties. The inspection
capabilities of intelligent pig contractors have continuously improved by developments on