Page 153 - Pipeline Pigging Technology

Pipeline Pigging Technology

Running conditions

Export gas will be treated to sales and transportation specifications at
Sletpner and Trott, and it is not planned to carry out any conventional
operational pigging. All conventional pigging will therefore be limited to the
precommissioning and commissioning phases. All water used for flooding
and pigging will be filtered, and strict control will be applied to prevent the
ingress of foreign matter.

Medium: This will vary depending on the type and purpose of the
operation. The dewatering train is composed of slugs of methanol and diesel/
water-based gels, propelled by gas. All other pigging will be with water which
is filtered to 50micron (maximum).

Speed: Pig speed during the precommissioning and commissioning phases
will be 0.6-0.8m/sec (2.0-2.6ft/sec). This will give a run time of between 16
and 12 days, respectively.

Pressure: The line pressure during pigging will be 25-30bar (360-435psi)
maximum. This will fall to approximately 4bar (58psi) at Sletpner.

Temperature: The temperature during pigging will be equal to the ambi-
ent, i.e. 5°-7°C (41 °-45°F).

PIG WEAR AND TEAR

Mechanical pigs

A mechanical pig is designed to have firm contact with the pipe wall. Fig.3
shows the build-up of a typical precommissioning or commissioning pig with
polyurethane discs on a steel body. The guide discs normally have a diameter
slightly less than the internal pipeline diameter, while the seal discs are
oversized. Firm contact with the pipe wall implies wear. Dependent upon
several factors, such as pipeline length, pipeline roughness, amount of debris,
force between the disc and the pipe wall, propelling medium, etc., the seal
discs may wear down to less than the pipeline internal diameter, thereby
causing by-pass.

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