Pipeline pigging is the process of propelling a pig through a pipeline from a pig launcher and receiving the pig at the pig receiver end of the pipeline. Various types of pigs are propelled through the pipeline depending on the intended function which could be cleaning, dewatering, chemical batching, dimensioning or inspection.
Pipeline failure could cause the operator huge economic and safety consequences. Pipeline pigging is an important aspect of pipeline integrity management and is carried out in various phases of the project to ensure that the pipeline can be operated safely through its design life. Pipeline pigging operations can be carried out while the pipeline is in operation thereby avoiding downtime costs.
What are Pigs?
Pipeline Petroleum Services Inc.
Pigs are devices that are inserted through a pig launcher (trap) and driven through the pipeline by the liquid or gas flowing in the pipeline to be received at the other end of pipeline where a pig receiver (trap) is provided to safely receive the pig. Pigs are designed to pass through pipeline bends, full bore valves, barred tees and other special items found in the pipeline system without being obstructed. Pigs are generally manufacture out of steel with a number of rubber cups to provide stability and for a seal to be formed aganst the pipeline internal wall. The flexibility of the rubber cups provides a seal to create the required pressure drop and a resultant force to propel the pig through the pipeline while allowing the pig to travel through minor dimensional variations in the pipeline.
Pigs are used to perform a variety of functions in the pipeline. Pipeline pigging operations are carried out during the construction and commissioning phases of the project as well as during the normal operation of the plant facilities. The selection of pig and the pigging operation to a large extent depends on the pipeline fluid, operating parameters, pipeline size and length. Each pipeline pigging program is hence very specific to suit a particular need during construction phase or during operation and inspection.
Pipeline Pigging during Construction Phase
Cleaning during Pipeline Construction
Prior to hydrotesting the pipeline, it needs to be cleaned and gauged to ensure its geometry is in accordance with the project specification. If debris is left in the pipeline prior to gauging or caliper pigging, it may give a false indication about the pipeline geometry and defects. Debris is usually left during pipeline construction and it could be potentially damaging to the pipeline or the connected equipment if not cleared prior to putting the pipeline in service. One or more trains of brush pigs may be used depending on the quantity of debris anticipated in the pipeline. However, if the pipeline is internally coated with for example, fusion bonded epoxy (FBE), use of brush pigs will damage the internal coating of pipeline and hence cannot be used. If the amount of debris is considerable, it may require more than one pig run to ensure that pipeline is fully cleaned of the debris. Recommended speeds for cleaning pigs are between 0.5 to 1 m/s.
Gauging
Gauging pigs measure or gauge the inside diameter of the pipelines and are used during the construction (pre-commissioning) phase of the project. They are typically provided with aluminum gauging plates whose diameter is 95% of the pipeline internal diameter. The aluminium plate material deforms if it meets any obstructions like dents, a reduction in diameter within the pipeline or buckles. Aluminium being softer than steel will not cause any damage to the inside surface of the pipeline. If the gauging pig traverses the pipeline without any damage to the plate, it is an indication that the pipeline does not have any significant reductions or ovalities which could be of concern. Gauging pigs are used after debris removal to verify that there are no obstructions or significant ovalities in the pipeline. They may be fitted with electronic tracking devices to record the position along the pipe where the plates have been impacted by an obstruction. It is preferred to run a bi-directional gauging pig so that it can be retracted by reversing the flow in the pipeline, should the gauging pig get stuck in the pipeline. A gauging pig is run prior to running the caliper pig as a part of the baseline survey before handing over the pipeline to owner.
Caliper Pigging
Caliper pigs measure internal profile variations like dents, ovality and internal diameter transitions with the primary objective being to detect mechanical damage and/or ensure that a less flexible metal loss inspection pig or intelligent pig can pass through the pipeline without meeting any obstructions. Caliper pigs are normally designed to be flexible and can pass minor ID reductions.
Most of the Caliper pigs are equipped with mechanical sensors (fingers) that follow the inner profile of the pipe wall. Typically, these pigs can detect dents and ID reductions of between 1% and 2% of the pipe diameter. Mechanical caliper pigs can give false readings when they negotiate debris or solid wax, hence it is vital to have the pipeline cleaned and gauged prior to running the caliper pig. Reputed contractors that offer services with mechanical caliper pigs are Pipetronix, Enduro Pipeline Services and TD Williamson (TDW).
Dewatering
Displacement pigs are used to displace air or fluid in the pipeline. When used in a pipeline to remove the test water after completion of hydrotest, they are called dewatering pigs. After completion of hydrotest, dewatering involves running a series of pigs through the pipeline using compressed air. Dewatering on liquid pipelines may be performed as the final commissioning stage where addition of water to the production during the start-up phase is acceptable. In such cases the pipeline may be left filled with water for considerable amount of time after hydrotest and disposed prior to final commissioning. It is important to test the quality of water and to use treatment chemicals that will not allow microbiological growth to take place in the pipeline during this period. For gas lines the presence of residual water is unacceptable and the line needs to be fully dewatered to avoid the potential of corrosion and formation of hydrates in the pipeline. If treated sea-water has been used to hydrotest the pipeline, the pigging operation will consist of batch of fresh water between the first two pigs followed by air as shown in schematic below. After the dewatering operation is completed, a significant amount of water could be still left in the pipeline.
Drying
As mentioned above, a considerable amount of water may still be left in the pipeline after performing the dewatering operation. The remaining water can be removed using a series of foam pigs to swab up residual water. However for dry gas pipelines where no amount of water is tolerated, the pipeline is blown with dry air until the dew point of air measured at the receiving end gives an indication that the pipeline is in dry condition.
Smart Pigs or Intelligent Pigs for Baseline Survey
A baseline survey or in-line inspection (ILI) using smart pigs or intelligent pigs is carried out to ascertain and record the condition of the pipeline prior to operation. A baseline survey allows subsequent inspection results to be compared against the baseline results to assess the defect growth over the design life of the pipeline. In absence of baseline ILI, it is difficult to establish whether a defect was existent at the time of handover to operations or due to subsequent corrosion in the pipeline resulting in incorrect assessments of defect growth in the pipeline. Regular ILI during Operation and Inspection phase, facilitates the corrosion rates to be established and mitigation measures to be put in place to preserve the integrity of the pipeline.
Pipeline Pigging during Operation and Inspection
Operational pigging is performed to ensure integrity of the pipeline through its design life. Operational pigging plays and important role in flow assurance. Operational pig runs are performed in pipelines using cup or bi-directional pigs to remove water drop-out, soft wax, sand deposits, scale and other debris build-up. With regular operational pigging the pipeline should be maintained at its optimum throughput capacity and a higher efficiency will be achieved. Typically the following purposes will be served with regular pigging:
- Prevention of scale build-up.
- Cleaning of the pipe wall.
- Removal of internal debris, as accumulation of debris can provide a favourable environment for corrosion mechanisms such as microbial induced corrosion. For example wax build-up on the interior walls of the pipeline can result in reduction of pipeline internal diameter and hence the flow throughput.
- Removal of water from oil pipelines and removal of condensate from gas lines.
- Ensure effective performance of corrosion inhibitors, biocides and other chemical treatments.
- To verify pipeline wall conditions and detect any occurrence of corrosion.
The operational pigging frequency is different for each pipeline and varies with changes in flow conditions, gas composition and corrosion condition in the pipeline. Depending on the results of the pigging evaluation and the corrosion monitoring assessment, the pigging frequencies are required to be reviewed and adapted on a regular basis.
Cleaning during Operations
Pipelines are used to transport hydrocarbons or water from one location to other. In many cases the crude oil may contain water that settles at low points in the pipeline and causes accelerated corrosion. In other cases pigging may be required to remove dirt, sand, wax or other substances harmful to the pipeline if left to accumulate. Cleaning scrapers are selected depending on the type of cleaning required.
Cleaning pigs are available fitted with a number of sealing cups (omnidirectional) or sealing discs (bi-directional). The cleaning devices attached to the pig body range from carbon or stainless steel wire brushes that are spring loaded to the pipe wall to oversized circular wire brushes interfering on the pipe wall. For internally lined pipelines, nylon bristle brushes can be used. There are also scrapers moulded to resemble plough blades in polyurethane, or for non-lined pipes, hardened steel blades profiled to suit the pipeline inner diameter. All pigs are designed for the brushes or blades to cover the circumference of the pipe surface.
Batching during Operations
Batching is the process of moving different products through the pipeline. Batching pigs are used in pipeline pigging to separate the products or batch chemicals in a pipeline and are made of flexible cups to provide a seal for effective product separation. Batching is often used for moving a batch of corrosion inhibitor or biocide between two pigs to wet the internal surface of the pipeline to provide protection against corrosion Displacement pigs can be used for batching operation.
Risks in Pipeline Pigging
A pigging operation can never be guaranteed due to serveral unforeseen circumstances that may be encountered while pigging the pipeline such as the pig stalling in the pipeline or accidents while launching or receiving the pig. Hence it is cruicial that each pipeline pigging operation be subject to a thorough risk assessment to identify potential hazards and to have the necessary mitigations in place prior to commencement of pigging operation.
The position of valves can be a major risk during pipeline pigging operation. Full bore valves or through-conduit gate valves with internal diameter equal to that of pipeline are used to ensure piggability. Despite taking of various piggability aspects during design, a partially opened valve could cause damage to both the valve and the pig. Hence it is important to check the position of valve during pigging.
Pipeline Pigging - Typical Sequence for Pig Launching
- Check that the end closure door is secured and all valves on the pig trap are in closed position. Only the bypass valve 20 is in open position. Confirm by reading on the pressure gauge that the the pig launcher is in the depressurised state.
- Drain valves 5 and 6 should be in closed position. Turn the spectacle blinds at drain valves 5 and 6 to open position and first open valve 19. Then open drain valves 5 and 6 to drain any fluids in the pig launcher to the closed drain system.
- After completing the draining operation, close drain valve 19. Turn spectacle blinds at drain valves 5 and 6 to closed position and close both the drain valves.
- Connect Nitrogen supply to nitrogen purge connection on check valve 12. Open valves 10 and 11 and pressurise the pig launcher to a pressure of 2 to 3 barg. Check if there are any leaks. Read the pressure gauge and confirm the required pressure is reached before the purging is stopped. Close valves 10 and 11.
- Ensure that the tell-tale valve 9 is closed before removing the blind flange on the valve. First open valve 7 and then open valve 9 to check if there is any H2S gas or hydrocarbon gas in the pig trap via valve 9.
- Close valve 14, turn the spectacle blind at valve to open position, open valves 13 and 14 and depressurise the pig trap through this vent line.
- Read the pressure gauge and confirm that the pressure is reduced to atmospherice before closing valves 13 and 14.
- Repeat the pressurising and depressurising process until the H2S gas or hydrocarbon gas content is reduced to sufficiently low acceptable level.
- Open the end closure door and insert pig in the pig trap.
- Close the door and ensure that the door is securely closed
- Open valves 10 and 11 and pressurise the pig trap with nitrogen to a pressure of 2 to 3 barg. Read the pressure gauge and confirm the required pressure is reached before purging is stopped. Close valves 10 and 11 and check for any leaks.
- Once it is confirmed there are no leaks, depressurise the pig trap through valves 13 and 14. Read the pressure gauge to confirm the pressure is reduced to atmospheric before closing valves 13 and 14.
- Repeat the pressurising and depressurising process until the oxygen content in pig trap is reduce to a sufficiently low and acceptable level. Then close valve 7.
- Now first open mainline valve 16 and then valve 4.
- Open valves 1 and 2 and pressurise the pipework and barrel between valve 23 and valve 17. Check for any leaks while pressurising.
- Read the pressure gauge and confirm that the pressurisation is complete.
- Open valve 23 first and then mainline valve 17. Close valves 1 and 2.
- Throttle bypass valve 19 to launch the pig. Ensure that the pig has passed the barred tee by confirmation of pig signaller downstream of barred tee.
- After this confirmation full open the bypass valve 19. Close mainline valve 17 and kicker line valve 23.
- Depressurise the pig trap through valve 13 and 14.
- Read the pressure gauge and confirm that the pressure si reduced to atmospheric before closing valves 13 and 14.
- Close valve 4 and then valve 16.
- Open valves 10 and 11 and pressurise the pig trap with nitrogen to a pressure of 2 to 3 barg. Read the pressure gauge and confirm the required pressure is reached before purging is stopped. Close valves 10 and 11 and check for any leaks.
- Close valve 14, turn the spectacle blind at valve to open position, open valves 13 and 14 and depressurise the pig trap through this vent line.
- Read the pressure gauge and confirm that the pressure is reduced to atmospherice before closing valves 13 and 14.
- Repeat the pressurising and depressurising process until the H2S gas or hydrocarbon gas content is reduced to sufficiently low acceptable level.
- Close valves 13, 14, 7 and 9. Disconnect the Nitrogen supply and reinstate all spades to closed position.