Page 357 - Pipelines and Risers

328 Chapter 18

pigging frequency is different for each pipeline and varies with changes in flow conditions,
gas composition and corrosion condition in the pipeline. Depending on the results of the
pigging evaluation and the corrosion monitoring assessment, the pigging frequencies will be
reviewed and updated regularly. As an example, for the Balingian gas trunkline network
operated by Sarawak Shell Berhad (SSB) in Malaysia, the following cleaning pigging
frequency is required
Pipeline I Pigging Frequency
Balineian oil (12. 16 & Winch) 1 1 in 3 months
I Temana oil (8 & 12-inch) I 1 in 3 months I
~
Samarang oil (8, 12 & 18-inch) 1 in 2 weeks
Balingian gas (12 & 18-inch) 1 in 6 months
Loconia gas (30.32 & 36-inch) 1 in 6 months

Several types of pigs can be used. The selection of pig type will depend on the purpose of the
pigging run. The following gives a brief description of some of the main types of pig used in
normal operation. Non routine or intelligent pigging is addressed within Section 18.3.

Cleaning pigs - cleaning pigs are available fitted with a number of sealing cups (omni-
directional) or sealing discs (bi-directional). The cleaning devices attached to the pig body
range from carbon or stainless steel wire brushes that are spring loaded to the pipe wall to
oversized circular wire brushes interfering on the pipe wall. For internally lined pipelines,
nylon bristle brushes can be used. There are also scrapers moulded to resemble plough blades
in polyurethane, or for non-lined pipes, hardened steel blades profiled to suit the pipeline
inner diameter. All pigs are designed for the brushes or blades to cover the circumference of
the pipe surface.

Foam pigs - pipeline cleaning foam pigs are made of hard polyurethane and covered with
abrasive coating or wirebrush bands. Pigs manufactured of soft open cell polyurethane foam
are used for water absorption in swabbing and drying service.

Spheres - spherical moulded tools made of polyurethane or neoprene of which the larger sizes
are inflatable. The larger diameter spheres have facilities (inlets) whereby the spheres may be
inflated to slightly greater diameters. Main application is in pipelines that have not been
designed to accept standard pigs and / or in two phase pipelines to remove liquid hold-up and
product separation.

pigging Operations
The detailed procedures required for carrying out a routine pigging run should be contained or
referenced within the Pipeline Operating Manual. Typically, the pipeline operations
department should carry out the following activities:

a. Check whether the pig trap isolation valves have been leak tested in the previous six
months. The six month durations is good common practice. If not then leak testing is
recommended prior to commencing a pig run;