Inspecting, cleaning or batching between different products in a pipeline would be exceedingly difficult without the use of a pig, especially if the task is required without the interruption of product flow. But how do you select the best pig for the intended purpose?
Let’s review the benefits and negatives
Pipeline spheres are hollow cast polyurethane balls that can traverse a short radius bend that some ridged body type pigs will not. They can be sized to match any size pipe ID from heavy to thin wall line pipe. Spherical pigs are filled and inflated with a combination of water and glycol mix and are typically sized with a hand pump to the ID of the thickest known pipe wall thickness.
Spherical pigs are used primarily for the removal of liquids from natural gas pipelines, but can be utilized for the calibration of liquid proving meters. Since spherical pigs are not capable of removing solids from the pipe wall, they are typically used on liquid rich gas gathering lines or on plant piping where multiple bends can exist in a short distance. The basic benefit of a spherical pig is its capability to remove liquids with very little differential pressure required. However, that same benefit can be an issue because spheres have only one sealing surface. Rich natural gas systems that generate large volumes of condensate on both onshore and offshore pipelines are a challenge because they are continually condensing liquids from the gas stream. These systems require frequent pigging operations to maintain and control condensate levels to prevent flooding of downstream slug catchers and process facilities.
Spherical pigs can be costly since they require additional design considerations such as the installation of expensive flow tees installed at laterals so they can traverse the taps and continue to maintain a seal. The flow tees prevent the pig from entering the side outlet or from stalling in the tee while the low bypasses.
Additionally, since a spherical pig has a single-line seal it inherently will not pass off-takes that a pig with its multiple seals would easily traverse. Even though spherical pigs can be fairly efficient with liquid removal efforts in rich natural gas pipelines, they can’t be equipped with on-board transmitters. Those transmitters are used for tracking purposes where proper sizing is required to prevent stalling when undersized, or create excessive wear when oversized, to improve their efficiency to remove liquids.
Conventional pigs equipped with cups and/or discs provide much more flexibility to be utilized for liquid and debris removal, cleaning, gauging and batching of pipeline systems. However, the configuration of the pig is essential to provide its intended purpose. Conventional pigs do require higher differential pressures. This can be regarded as a benefit because the higher the differential pressure required to move the pig results in greater efficiency for the removal of liquids and debris from the pipeline system.
Conventional pigs provide a very predictable improvement in the flow efficiency of natural gas and crude oil pipelines. When comparing conventional cup/disc pigs versus a spherical pig used to remove the accumulated liquids on a 200-mile natural gas pipeline in Oklahoma yielded a 90 percent increase in the flow efficiency. The
flow efficiencies were validated by monitoring the flow rates and differential pressures before and after pigging with a conventional pig in comparison to a spherical pig. The spherical pig yielded an increase of the flow efficiency by 15 percent whereas the conventional pig provided a 90 percent increase in
Additional studies were conducted on a 55-mile, 12-in. natural gas pipeline to compare the effectiveness of the conventional pigs when compared to spherical pigs. On this particular project, the first spherical pig stalled in the pipeline where a secondary sphere was required to “bump and run” the stalled sphere. Based on observations and monitoring of the flow rates and differential pressure, no improvement was yielded with the spherical pig.
The subsequent launch of a 12-in. disc/cup pig improved the flow efficiencies by 95 percent on the 55-mile line segment. Additionally, a pressure drop of 10 psig was observed compared to the 100 psig when using spherical pigs. The greater sealing characteristic of the disc/cup pig was able to remove the stalled spherical pigs and increased the flow rate by 20 million cubic feet per day (MMcfd) using the same compressor horsepower output.
Improving and maintaining high-pressure pipeline flow efficiencies are essential in optimizing a gas-gathering system. Pipeline systems where spherical pigging procedures are ineffective, conventional pigs can be particularly important especially in locations of severe elevation changes and low gas velocities. However, the pigging frequency must be maintained by monitoring the flow rates and differential pressures to sustain the optimum flow efficiency, which can be obtained through an automated pigging system.
Automated pigging systems have the ability to load multiple pigs — launching the pigs at preset times or preset differential pressures from the launcher to the receiver. The use of automated pig launchers, where a number of pigs are pre-loaded into a launcher and launched as and when required, overcomes the HS&E exposure and additional manpower requirement as the launchers are opened less frequently. However the initial cost of these systems can be prohibitive. This rate of return on investment can be offset by the increase in flow efficiencies, fewer internal corrosion issues and less manpower based on the frequency of pigging operations executed.
Multiple pig launchers can be and are used in many areas, such as areas with remote access, hard to reach locations, unmanned platforms, or any pipeline requiring frequent pigging. WeldFit identified the need for multiple pig launching systems that are set in the horizontal position such as a standard pig launcher where it is capable of launching a single pig at a predetermined launch frequency. The WeldFit SureLaunch pigging system is not pig specific so that any pig type can be loaded and launched based on its intended purpose (i.e. cleaning, liquid removal, batching.)
The need for frequent and continuous pipeline pigging programs has been graphically highlighted
recently with some high-profile oil and gas incidents. Many of these can be linked to internal corrosion and liquid build up inside the pipeline resulting in a pipeline leak or failure. Many of these incidents may have been avoided with a proper pigging program.
The Pipeline and Hazardous Materials Safety Administration’s (PHMSA) newly proposed rule-making and regulations could require gathering pipelines located in “moderate consequence areas” and “high consequence areas” to be made piggable to reduce their internal corrosion risk threat and perform in-line inspections integrity assessments. The WeldFit SureLaunch automated pigging system is proven to carry out many pigging operations reducing cost of manpower by reducing deployment of pigging crews, increasing pipeline flow efficiencies, reducing HS&E exposure and reducing internal corrosion growth rates. Automated pigging will also play a big role in helping operators maintain compliance with regulatory agencies by reducing the internal corrosion risk threat and performing mandatory in-line inspections.
Russ Sander is project manager, Pigging Technologies Division of WeldFit Energy Group.