An introduction to pipeline pigging - 1

[Freeman]

AN INTRODUCTION TO PIPELINE PIGGING - 1

INTRODUCTION
In the world we live in today, pipelines are used to transport all manner of powders and fluids from one point to another. From the food industry to oil and gas transportation, millions of lives are affected by the ability to maintain flow through pipelines that cross over land and under sea to deliver a product. Coupled with the economic drive to maintain product flow, the importance of this task has spawned a whole industry to ensure pipeline integrity.
DEFINITIONS
Before we can further discuss the topic of pipeline pigging we must make some definitions;

PIG: A tool propelled through a pipeline to perform a function.

PIG TRAIN: Several pigs launched in a set order to perform a specific task.

PIG TRAP: Pressure vessel used for launching and receiving pigs.

UTILITY PIG: Non-autonomous pig used to carry out simple tasks, e.g. batching,
cleaning etc.

INTELLIGENT PIG: A generic name given to a pig that utilises power to perform a
specific task or log specific information. Intelligent Pigs are also
referred to as inline inspection tools.

GEL PIG: Pig created by gelling a given medium.

[Freeman]

HOW PIGS WORK:
Pigs come in various varieties, the most common being mandrel, single bolt, solid cast, foam, articulated, and spheres.
Mandrel Pigs

[link Point to another website Only the registered members can access] The various elements of the pig are attached, using multiple bolts, to a solid body or mandrel.
Single Bolt Pigs

[link Point to another website Only the registered members can access] Similar to the mandrel pig, the elements are fitted to a single bolt, rather than a solid body.

Solid Cast Pig

[link Point to another website Only the registered members can access] Both sealing and guide elements are cast as a single component in polyurethane.

Foam

[link Point to another website Only the registered members can access] Open cell polyurethane foam is available in light, medium, and heavy density, dependant on the application. Foam pigs can be polyurethane coated to improve wear resistance and stability.

Articulated

[link Point to another website Only the registered members can access] Made up of two or more pigs joined with a universal coupling, articulated pigs form the basis for many intelligent pigs.
Spheres

[link Point to another website Only the registered members can access] Spheres can be solid or inflatable with air, water, or glycol and are used for product
separation, liquid removal, and in meter proving.

[link Point to another website Only the registered members can access]

[Freeman]

Pigs are designed so that sealing elements provide a positive interference with the pipewall. Once inserted into a line, pigs are driven through the line by applying pressure in the direction of required movement. A pressure differential is created across the pig, resulting in movement in the direction of the pressure drop. In operational lines, this pressure is applied by the line product, whereas, in un-commissioned lines, the propelling medium can be chosen to suit the task being carried out, e.g. water for flooding or dry air or nitrogen gas for dewatering.
Note: Sufficient flow is also required to ensure pig movement at a suitable velocity.

Once the force behind the pig becomes greater than the opposing frictional force, the pig will move in the direction of the applied force (pressure).
The pressure at which the pig begins to move is known as the "break-out" or "stiction"
pressure. This tends to be greater than the pressure required to maintain movement and is characterised by a pressure rise followed by a pressure drop to a plateau for the pig launching operation.
Depending on the design of the sealing element, pigs can either be run in a single direction, or run backwards or forwards through a line. Pigs that can only be run in one direction are known as unidirectional pigs, and have polyurethane sealing elements of the cone or cup design. These types of pigs are generally used in established lines known to be piggable.

[link Point to another website Only the registered members can access] Sealing elements in the bidirectional pigs are flat, providing an identical seal in either
direction, and therefore, giving more adaptability in previously unpigged lines.

Additional sealing elements can be added to pigs, leading to better sealing properties along with a higher pressure differential required to drive the pig.
Support for the pig to ensure it remains central in the line can be provided by either support discs, or, for large diameter pigs, centralizing wheels.
As a general guide, the following table summarizes pig characteristics:
Component Size Sealing Disc Diameter 102% to 105% Pipeline ID Support Disc Diameter 99% Pipeline ID Pig Assembly Length 1.5 ´ Pipeline ID

[Freeman]

TYPES OF PIGS

Batching Pig: Also known as a swabbing pig, the batching pig is designed to act as a simple barrier between dissimilar fluids or to provide a sweep of a line.
Gauging Pig: The inclusion of a simple gauge plate, made of a soft metal (generally
aluminium), on batching pigs provides the function of confirming the integrity of the flow
area of the pipe. Any major intrusions into the line will cause damage to the gauge plate,
highlighting there is a problem, though not highlighting where.
Cleaning Pig: Pigs can be configured with various tools to aid cleaning. Circular brushes,
spring-mounted brushes, scrapers, or plough blades for waxes and sludges, or more
aggressive tools such as carbide "pins" for removal of scales.
Magnetic Pig: Inclusion of powerful rare earth magnets on the circumference of the pig
mandrel allows the pig not only to lift ferrous debris from the line, but can also provide the secondary function of activating pig signallers.
Note: Foam pigs also allow the addition of gauge plates, brushes, abrasives etc., although these are either fitted into the pig using bolts, or by direct casting into the polyurethane coating.
Intelligent Pigs: Research and development into inline inspection tools began in the late
1960's. Advances in technology have lead to pigs that can carry out complex tasks and data logging as they traverse the line. Mapping, geometry measurement, ***** detection,
measurement of metal loss, and many other tasks can be carried out. Intelligent pigging is now an industry within an industry.
Gel Pigs: For certain tasks and in certain conditions, a viable alternative to running
mechanical pigs is the use of gel pigs. Rather than use a solid barrier between fluids, a gelled substance can perform the same task. Various mediums can be gelled, including water (fresh and salt), glycol, methanol, solvents, diesel, and crude. The gels can be designed specifically to a required viscosity or cast as solid with chemical components designed to break down the gel after a given time or when a set temperature or pH has been reached.
Gels have their limitations and are not generally suitable for long runs, in dry pipelines, or
where the propelling medium is gas because they tend to suffer from "gas cutting" or excess bypass.

PIG SELECTION

Selection of the correct pig depends on several factors such as line type, length, material,
condition, propelling medium, and the function to be performed. Advice should be sought
from pig manufacturers and those experienced in the science and art of pigging.

[CarlosBatista]

Thank very much