Importance of Pigging Execution Planning for Gas Pipelines
Table of Contents
1 Introduction to Pigging Execution planning
Extensive pigging planning is required for launching a safe, efficient, and successful pigging operation on an oil or gas pipeline. Pigging planning starts even before designing and laying down the pipeline for transportation of oil or gas. Although pipeline design does not rely entirely on pigging operation requirements, several factors related to pigging are considered before designing gas pipelines, such as pipeline sizing, material, section length, etc. The importance of pigging planning is evident because it is a routine operation in oil pipelines. In contrast, in gas pipelines, pigging is carried out after 4 to 10 years depending upon the quality of fuel passing through the line.
Moreover, pigging plays an essential role in the maintenance of the pipeline. The oil and gas industry has proved to be the major driver for pigging. The first pig was used in 1930 for cleaning pipelines. Later on, in 1960, the first corrosion inspection pig was launched. The pig could detect any cracks and leakages in the pipeline while mapping the exact location (Sousa, Pereira, & Matos, 2021).
With 100 years, the accumulated knowledge about pigging operations has resulted in prior planning methodologies essential for efficient and successful pigging operations. There are some fundamental pigging design aspects and principles which are universal for all pipelines. However, in some cases, such as extensive slurry buildup, there may come operational challenges, the knowledge of which beforehand is essential. To carry out the successful pigging program, the pipeline operators must also work closely with the service company or pig supplier to mitigate any risks. Moreover, pipeline configuration in place also needs to be checked before starting a pigging operation. Pipeline design issues can be mitigated by adopting unique designs of the pigs. If this method is not feasible, the pipeline design might need to be modified. The selection of pigs is also included in pigging execution planning. The pigs selected must be robust enough to bear the environmental pressure inside the pipeline. The standard procedures to launch the pig must be followed to ensure the safe launch and receive of pigs. A range of technical evaluations are needed before pigging operation, and extensive planning and preparations can only be the key to successful pigging. The overall process of planning, preparing, and execution is time-consuming and complex (Ran, Zhou, & Yang, 2013). This article will try to address the most critical planning and preparation steps needed for pigging operations.
2 General Planning before starting Pigging operation
This section briefly describes the documentation and preliminary requirements before starting a pigging operation as an essential part of planning. Generally, specialized contractors are hired for pigging operations because they have expertise in this field (Cameron, Paviglianiti, Grzyb, & Caesar, 2008). The contractors will work with the plant operators, engineers, and managers to devise the most efficient, safe, and successful pigging exercise. Enlisted below are the general requirements for a contractor provided by an oil or gas company:
After a complete inspection of the installed system, contractors first need to provide design documents.
Following documents must be provided by the contractor for evaluation before they start pigging operations.
Complete design calculation reports
Equipment and closure datasheet which will contain data about launcher and receiver trap
Material requisitions for launcher trap, receiver trap, valves, instruments, electrical insulation joints
The complete layout of installations, flow sheets, and drawings
The oil and gas company should provide launcher and receiver trap diagrams. The representative diagrams are shown in figure 1 and figure 2. The drawing dimensions should be mentioned in millimetres or inches. If using vertical or inclined launchers, the drain should be close to the valve. The pig passage indicator shown in the launcher and receiver diagram should indicate the foam pigs. The position of the pig passage indicator should be mentioned either top or within the side surface. One crucial point is not to assemble any branches under the pipeline, as it can result in extra load, and the pipeline becomes prone to failure.
Following general considerations for trap design must be followed as a part of pigging operation planning.
As per recommended practice in trap design, the bottom of the barrel must be at least one meter above the floor. This procedure provides provision for the installation of fittings for drainage or other accessories. Moreover, there should be installing supports to the load handling equipment at the entrance and exit of the pig.
Pipeline design code must be followed while selecting the pipeline sizing for the pig launcher and receiver.
The pipeline design must be planned to have launcher and receiver installations parallel to the floor. If the pipeline design does not allow this or space restrictions, vertical or inclined receivers and launchers are usually allowed.
It is also a recommended practice to keep the barrel diameter of the launcher or receiver 3.5 inches larger than the pipeline diameter. It allows easy launching and receiving of the pig. Another thing to keep in mind is that for a piggable system, the inside diameter of fittings and pipes located between barrel reduction and the original pipeline should be equal or greater than the inside diameter of the smallest pipeline. The barrel reduction is usually kept at 30 degrees to the main pipeline.
Another step of pigging operation planning is communicating clearly that no clamp connections should be used, especially in a gas pipeline. Only flanged connections should be used.
During pipeline design or inspection, one important thing to consider is the distance between two consecutive bends. The distance between them should be three times the nominal diameter of the pipe to avoid pigs stuck in the pipeline.
The barrel reducer for the launcher should be eccentric, while the barrel reducer for the pig receiver shall be concentric. The illustrations for both the reducers are shown below:
Figure 1: Types of Barrel reducers
3 Design planning before starting pigging operations
Prior planning is essential regarding closure requirements for pig traps, pressure equalization systems, manometers, vents, valves, thermal relief valves, pig passage indicators, nameplates, pipe derivations/bends, flanges, pig foam receiving basket, and lifting system (Fisher, 1998). This section describes the general design considerations that must be taken into account while planning a pigging operation.
The closure of pig traps is usually designed according to ASME BPVC Sec-VIII-01. In gas pipelines, the closure trap must include a pressure warning device. It enables the operators to keep the pressure under control and avoid closure openings. The closure system should be designed such that it can be opened or closed immediately. The sealing material used for closure must be resistant to operational temperature, pressure, and depressurization jolts.
The pressure equalization system helps regulate pressure at two ends of a pig. The pressure equalization line (PEL) is a prerequisite for pigging operation, and it consists of a block and a throttling valve. The PEL has an internal diameter of 1 inch for the pipeline diameter up to 6 inches. Above this diameter PEL, the internal diameter is 2 inches. The throttling valve used in the PEL is needle type. PEL should be installed as close as possible to bypass and barrel line.
Pressure monitoring during pigging is an essential step in gas pipelines. Pressure indicators should be installed, one near the closure and the second one after the reducer. It is also recommended to install a third pressure indicator on the barrel along with vent valves. The third monitoring device should be able to measure both vacuum and pressure.
Vents are an essential part of pigging operation and avoid the buildup of excessive pressure in the pipeline. It is a recommended practice to install two values, one upstream while the second downstream. Nitrogen injection is also a common practice for safe pigging operations. A check valve is installed so that pipeline contents may not flow back towards the nitrogen injection tank. Apart from atmosphere vents, the barrel pressure relief should be directed to flare.
As a part of pigging operation, planning the selection of types of valves is an important parameter. The valves with a nominal diameter equal to or greater than 12 inches should be operated hydraulically, pneumatically, or electromechanically.
A thermal relief valve for pipelines transporting liquid should be installed in the barrel. For gas pipelines, the pressure relief valve is recommended.
The pig passage indicators help the operators identify whether the pig has launched from the launcher or reached the receiver end. Moreover, the passage indicator should be capable of detecting the passage of foam pigs. An intrusive active ultrasonic type pig passage indicator should be used for gas pipelines with treated gas flowing inside.
The nameplates are usually used for identifying receiver and launcher information such as manufacturer, manufacturing year, design code, design temperature, design pressure, corrosion allowance, fluid compatibility, and capacity.
As a part of planning, the flanges must comply with ASME B16.5, ASME B16.47, and ASME BPVC section VIII-01. The inside pipe diameter and flange diameter should be the same. The mechanical characteristics of the flange should be equal to or better than the pipeline.
4 Fabrication and supply planning before pigging operation
Detailed documentation is a prerequisite for fabrication and supply planning before pigging operations. The document in the form of a single data book should be prepared by the contractor and provided to the oil or gas company. The databook should contain a detailed drawing of the pig trap, mechanical testing report of pig trap, material specification sheet for pipes, plates, fittings, flanges, gaskets, etc. The contractor should also provide material certificates, weld maps, welding procedure specifications, and identification of welded joints. The contractor should also include procedures for non-destructive testing, hydrostatic tests, stress-relieving, and specification for coatings and paint (Banaji et al., 2004). The other details such as weld size report, visual inspection report, dimensional inspection report, post-weld heat treatment report, hydrostatic testing report, and non-destructive testing on the welded joint report should also be provided.
5 Safety Planning
Throughout the oil and gas industry history, accidents related to pigging have happened because it is a dangerous maintenance operation. Although several advancements in the process have been made, such as automation of valves and mechanical interlocking, accidents still happen. This is because the process cannot be fully automated, and labour is involved in pig loading and receiving. Statistics have shown that over 70% of accidents in the oil and gas industry happened due to human error (Nagaraj, 2013).
An adequately designed pigging system reduces the safety requirement for the whole operation. However, during pigging, the launching and receiving of the pig require several sequential valve operations. For pig launching, there is a vessel parallel to the main pipeline, which makes it possible to launch or receive the pig without interrupting the flow operation. There is a closure door to load or offload the pig. There are chances of a severe injury or even death if the following procedure is not adopted before opening the closure safely.
Before opening the vessel, it must be isolated from the main high-pressure gas pipeline.
The pressure from the vessel must first be released, and contents drained.
It should be ensured that there are no toxic gases inside the vessel, such as Hydrogen Sulfide.
The closure must be properly closed before the start of launching or receiving the pig.
6 Pigability Assurance- An Important Planning Step
Pigability assurance is a term used in the oil and gas industry to evaluate how suitable is the economical and safe operation of gas-driven pigging operation (Clark & Nestleroth, 2004). Pigability is dependent on the pipe itself, attached components, and their attributes. The Pigability of a pipeline is defined in the following table:
The most important part of pig ability assurance is whether the internal dimensions of the pipeline are suitable for pigging or inline inspection operation. The pipeline’s operating pressure and flow conditions dictate whether the inline inspection or pigging can safely be performed. In gas pipelines, the pressures less than 400 psi are considered insufficient to drive the pig. The effect of low pressure inside the pipeline is more pronounced in hilly areas because the gas column will not drive the pig. Therefore, the pressure might need to be increased that is safely bearable by the pipeline. Pigability is also affected by the nature and corrosivity of the gas. If smart pigs are used, the temperature inside the pipeline must be bearable by electronic components.
Moreover, the pipelines contain dirt, debris, and corroded areas with corrosion buildup. This needs to be first cleaned for accurate inline inspection. Pigability assurance is a complete planning step that involves a threat analysis assessment available in ASME B31.8S. The following diagram shows the threat assessment analysis for pigability assurance of a pipeline for inline inspection.
Figure 2: Pigability assurance using threat analysis flowchart
7 Long-distance pigging planning
Several factors have to be considered while planning a long-distance pigging operation, including mechanical performance, construction quality, the manufacturing quality of pipeline, and attributes. Long-distance pigging performance is affected by other factors, such as the type of pigging equipment, segment technology, and terrain (Ran et al., 2013). Each of this factor impact safety, cost, and efficiency of the pigging process. The gas pipelines can be laid thousands of kilometres. Pigging such pipelines safely and efficiently is a big challenge. Considerable planning before the launching of the pigging operation is required. Pressure testing can be done on a specific section of pipelines; however, when the pipeline goes through thousands of kilometres, it may pass through deserts, mountains, plateaus, and rivers; the pipeline profile will depend on the terrain environmental conditions there. Some authors have proposed segmental pressure testing before pigging (Deng, Zhou, Zhang, Chen, & Jing, 2017). However, the procedure is complex, time-consuming and more research is needed to optimize such testing.
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