Introduction to Pipeline Pigging

I have decided to do a deeper research on the Pipeline Inspection Gauge (PIG). Since I have been told that the pipeline pigging process is the most important part in de-commissioning of a pipeline. I have explained briefly on the PIG. Now I would like to add more information regarding PIG.

The function of the Fluid Separation PIG is to separate different type of fluid inside the pipeline during the pigging process. The Fluid Separation PIG will be included in the PIG train during the pigging process. Usually the Fluid Separation PIG are used for a general purpose of pipeline cleaning before applying the hydrotest. The usual fluid that is being used during the pigging process are listed below:

1) Fresh water - for desalination of sea water inside the pipeline
2) Glycol Slugs - dehydration and hydrate inhibition for product introduction
3) Nitrogen Gas - act as a driver medium of the PIG

The cleaning task the PIG can perform are because of the positive interface the PIG has between the pipeline wall surface and the PIG itself. The cleaning effect of the PIG can be enhanced by placing a brush or scrapers on the PIG itself. Some debris that has been produced inside the pipeline are ferrous material which may be removed by installing a magnet on the PIG. The forward action of the PIG will produce a turbulence effect on the fluid which may help in removing small and solid debris from the line while wax and sludges will be removed using PIG that has been installed with brushes and also scrapers.

There are 4 purpose of pigging which are:

1) Pre-commissioning
Pigging is used in the pre-commissioning process to remove the construction debris like welding chip in the pipeline. After removing the debris from the pipeline, hydrotest will be conducted on the pipeline. In this stage, three different PIGs are used namely the cleaning, batching and also gauging PIG. Some pipeline may require further pigging for the purpose of de-watering and also drying operation depending on the medium to be transported in the line.

2) Commissioning
Commissioning pigging is use to separate product in the pipeline from the medium in line using a batching PIG.

3) Operational Pigging
This type of pigging are usually use to mechanically and chemically cleaning the pipeline that has been used from debris like hydrocarbon waxes. Using this method, the flow of the product can still be maintain while minimizing the back pressure produced by the PIG. Batching PIG are used to separate the chemical used for the purpose of cleaning by attaching it in the PIG train. The In Line Inspection (ILI) PIG is use for the purpose of routine maintenance plan.

4) Decommissioning
Decommissioning are usually being used during the process where the pipeline has reached its end life or the pipeline will be used for transporting other type of products. In decommissioning, both mechanical and also chemical cleaning method will be used for the cleaning purpose. For a certain cases, the pipeline will be dewatered for pipeline recovery before it is being reuse.

PIGGING MEDIUMS
There are three normal type of transporting medium being used commercially during the pipeline pigging process.

1) Liquid
- consists of incompressible liquid
- may provide maximum control over the PIG's speed and also provide lubrication for PIG seals
- maximize seal effectiveness and life
- sealing equipments must be compatible with fluid medium, prevailing pressure and flow conditions

2) Gas
- provide more propelling power to drive the PIG
- appropriate consideration towards safety implications during pigging process
- adequate amount of gas required to maintain sufficient pressure to drive the PIG
- possibility of wear increased on the sealing discs

3) Multiphase Fluid
- same safety precaution must be considered as gas propelling medium
- consider slugging effect and associated forces at the receiving end
- temporary pipework should be secured and permanent facilities and equipment should be rated

CRITERIA IN DETERMINING THE TYPE OF PIG
1) Purpose
- type, location and volume of the substance to be removed or displaced in the conventional pigging applications
- type of information to be gathered from an intelligent PIG run
- objectives and goals of the PIG run

2) Line Contents
- content of the line during pigging
- available against required pressure
- velocity of the PIG

3) Characteristics of the pipeline
- minimum and maximum internal line sizes
- maximum distance PIG must travel
- minimum bend radius and bend angles
- other additional features like valve type, branch connections and also elevation profile

TYPE OF PIGS
1) Utility PIGs
> Cleaning PIG - to remove solid or semi solid deposits or debris from pipeline








> Sealing PIG - provide good seal to sweep liquids from line or produce interface between two dissimilar products in pipeline
> Mandrel PIG - have a central body(mandrel) that can be attached with various components for specific duty








> Foam PIG - molded from polyurethane foam with various polyurethane strips permanently bonded between them








> Solid Cast PIG - molded in one piece, usually from polyurethane








> Spherical PIG / Sphere - solid decomposition being inflated into optimum diameter with glycol and/or water








2) Geometry PIGs












PIGs that are designed to record the pipeline conditions like dents, wrinkles, ovality and also bend radius angle. It may be used to detect corrosion in the pipeline by measuring the inside surface of the pipe

3) In Line Inspection (ILI) Tools / Smart PIGs










Provide information on the condition of the pipe to be analyzed by the engineer and the technician. The information that the ILI / Smart PIG can provide are:
> pipe diameter and geometry
> curvature monitoring
> pipeline profile
> temperature or pressure recording
> bend measurement
> metal loss or corrosion detection

4) Plugs

Consider as a specialist PIG that isolate section of pipeline while remedial work is undertaken. This type of PIG may withstand up to 200 bar of pressure. Plugs are grip into the pipeling and it does have a separate sealing system. High Friction PIGs may provide barrier for depressurized systems.

5) Gel PIGs
A series of gelled liquid system developed for the usage of pipeline inspection either as part of the initial commissioning or for further maintenance of the pipeline. Most gel PIGs are water based while some are consists of solvents and other chemical that has been gelled. Gelled diesel are usually used in the gas pipeline. As a liquid, the PIG may entered any type of pipeline which will accept liquid. It can be used together with the other conventional PIGs or to replace the batching PIG in the PIG train. The gel type of PIG may improve the overall performance of the pipeline inspection using PIG as well as to reduce the risk of PIG sticking to each other. There are four main types of gel PIGs which are:

1) Batching or Separator Gel
2) Debris Pickup Gel
3) Hydrocarbon Gel
4) Dehydrating Gel

Gel PIGs will not wear like the conventional PIGs but the gel PIG may susceptible to dilution and also gas cutting. Care must be taken while designing the PIG train that may include the gel PIGs to reduce fluid bypass among the PIGs and to replace a conventional PIG at the back of the train when displacing a gas. Some formulated gels have also been used to seal valves during hydrotesting and also have been developed with controlled gellation and viscosity for temporary pipeline isolation purposes.

Principle of pipeline application of the Gel PIGs are:

1) Product Separation
2) Debris Removal
3) Line Filling / Hydrotesting
4) Dewatering and drying
5) Condensate removal from Gas Lines
6) Inhibitor and Biocide Laydown
7) Special Chemical Treatment
8) Removal of Stuck PIGs

PIG TRAPS/LAUNCHERS/RECEIVERS
PIG traps are used to insert the PIGs into a pipeline before launching, receiving and finally removing it without interrupting the product flow in a pipeline. Usually a PIG trap are not generally a proprietary products and the specification are drawn up by the user but the closure of the PIG trap are proprietary and important part of the pigging system. Safety is the main consideration in choosing the PIG trap closure at which the closure must be built with safety lock to prevent the closure be opened while the trap is pressurised.