Pipeline Dewatering Pigs
Royal Poly Products — Manufactured in Western Australia
Water is one of the most damaging substances that can be left inside an operational pipeline. In gas transmission systems, residual water creates the conditions for hydrate formation, internal corrosion, and flow restriction. In newly commissioned lines, hydrotest water must be fully removed before the pipeline can accept product. In any pipeline, standing water at low points promotes under-deposit corrosion and microbiologically influenced corrosion (MIC) that can dramatically shorten asset life.
Pipeline dewatering pigs are the standard tool for removing water efficiently and thoroughly from pipeline interiors. Royal Poly Products manufactures a range of dewatering pigs suited to Australian pipeline conditions, available in foam, disc, and cup configurations across the full range of pipeline diameters.
What Is Pipeline Dewatering?
Pipeline dewatering is the process of using one or more pigs to sweep water out of a pipeline bore after hydrostatic pressure testing or during operational maintenance. The pig travels through the line driven by compressed air, nitrogen, or natural gas, pushing water ahead of it to a collection point at the far end of the run.
In many cases, a single dewatering run is sufficient to remove the bulk of standing water. In longer pipelines, or those with complex topography including significant elevation changes and low points, multiple pig runs — sometimes using gel slugs between pigs — may be required to achieve the specified dryness level.
Dewatering is typically followed by a drying program where the remaining moisture in the pipeline wall and at low-point dead-legs is removed to meet commissioning dryness specifications, expressed as a dew point or water content target.
Consequences of Inadequate Dewatering
Leaving residual water in a pipeline has serious operational and integrity implications:
• Internal corrosion: Water trapped in low points or under deposits is a primary driver of internal corrosion. Even small volumes of residual moisture can initiate localised corrosion cells that cause significant wall loss over time.
• Product contamination: In liquid pipelines, residual water can contaminate the first product batch, causing quality issues and potentially triggering receiving facility rejections.
• Delayed commissioning: Failure to meet dryness specifications after hydrotesting delays the commissioning milestone and incurs additional project costs.
Effective dewatering is not just about moving water out of the pipeline — it is about confirming that the pipeline meets the dryness specification required for safe and reliable product introduction.
Our Dewatering Pig Range
Royal Poly Products manufactures dewatering pigs in a range of configurations to match pipeline size, geometry, and the volume of water to be displaced.
Foam Dewatering Pigs
Foam pigs are the most commonly used dewatering tool due to their flexibility, compliance with pipeline geometry, and ability to absorb and carry residual moisture as they travel. The foam material wicks water from the pipe wall surface and from small irregularities in the bore, contributing to a more thorough dewatering result than a rigid pig can achieve.
For dewatering applications, foam pigs are typically selected from the intermediate to high density range, depending on pipeline diameter and drive pressure availability. Bare or criss-cross coated configurations are most common, as aggressive surface coatings are generally not required for dewatering duties.
In longer pipelines or those with significant elevation changes, a train of two or more foam pigs — spaced along the pipeline with gel slugs between them — ensures thorough water removal. Each pig sweeps the residual moisture left by the pig ahead, progressively drying the bore from launch to receipt.
Disc Dewatering Pigs
Bi-directional disc pigs provide a firm, consistent seal against the pipe bore and deliver reliable drive performance across a wide range of pipeline pressures and flow conditions. Their rigid disc contact with the pipe wall means they carry less absorbed water than foam pigs but provide a more positive displacement of standing water — particularly effective in nominally horizontal pipelines without significant low-point accumulations.
disc pigs are available in swabbing and high-seal configurations, with the high-seal variants offering maximum bypass prevention in larger diameter lines.
Cup Dewatering Pigs
Cup pigs use circumferential sealing cups to create a positive seal against the pipe bore. They deliver strong, consistent displacement force and are well suited to dewatering in larger diameter pipelines where maintaining adequate differential pressure behind the pig is important. Cup pigs can also be used in combination with foam pigs in a multi-pig dewatering train.
Dewatering Pig Trains
For pipelines requiring thorough dewatering to tight dryness specifications, a multi-pig train approach is the industry standard. A typical dewatering train for a gas pipeline pre-commissioning program might include:
• Gel slug — a plug of viscous pipeline gel that provides a more thorough interface wipe and assists water displacement from low points and dead-legs
• Intermediate foam pig — collects residual water carried past by the gel slug
• Second gel slug — further conditions the bore
• Trailing foam pig — final sweep before the drying phase begins
The exact configuration of a dewatering train is determined by the pipeline length, diameter, topographic profile, and the dryness specification that must be achieved before commissioning can proceed.
Royal Poly Products manufactures all pig components for dewatering trains and can supply pipeline gels to complete the dewatering package.
Dewatering After Hydrostatic Testing
Hydrostatic pressure testing involves filling a pipeline section with water, pressurising it to the required test level, holding pressure for the specified duration, and then confirming that no pressure loss has occurred — indicating the integrity of the welds and pipe body. After a successful test, the pipeline must be dewatered before it can proceed to the drying phase and ultimately to gas-up or product introduction.
Dewatering after a hydrotest typically involves driving the water out of the test section using the following sequence:
• Follow-up pig runs — the number of runs depends on the pipeline length and topographic profile — to progressively reduce residual water content.
• Gel pig train if required to achieve a high-specification dryness result.
• Transition to the drying phase once bulk water removal has been confirmed.
Royal Poly Products supports pre-commissioning project teams by supplying the right pig products for each stage of the dewatering and drying sequence, with the lead times and local availability that Australian projects require.
Transition to Pipeline Drying
Dewatering removes bulk water from the pipeline. Drying removes the residual moisture that remains in the pipe wall surface and at low points after dewatering is complete. For gas transmission pipelines, drying is typically achieved by circulating dry air, nitrogen, or methanol slugs through the line until the outlet dew point meets the commissioning specification.
Drying pigs — typically low-density foam pigs in bare or coated configurations — are used to assist the drying process by sweeping residual moisture forward and maintaining contact with the pipe wall surface as the drying medium passes through.
Royal Poly Products supplies both dewatering and drying pigs as part of a complete pre-commissioning pig package, ensuring consistent product quality and availability across all stages of the program.
Pig Signalers for Dewatering Runs
Confirming pig progress during a dewatering run is essential — particularly for long pipelines where a pig that has stalled or become lodged at a low point could leave significant volumes of water undetected in the line. Royal Poly Products supplies pig signalers compatible with our pig range, enabling reliable above-ground tracking of pig position throughout the dewatering run.
Frequently Asked Questions
What is the difference between dewatering and drying a pipeline?
Dewatering and drying are sequential stages of the same process but address different conditions in the pipeline. Dewatering removes the bulk free water from the pipeline bore — the standing liquid that fills low points and accumulates after a hydrotest. Dewatering pigs physically push this water out of the line to the pig receiver. Drying follows dewatering and addresses the residual moisture that remains as a film on the pipe wall surface and in low-point dead-legs after the bulk water has been removed. Drying is achieved by circulating a dry medium — compressed air, nitrogen, or methanol — through the pipeline with foam pigs assisting the moisture sweep, until the outlet dew point meets the commissioning specification. Both stages are required before a gas pipeline can be safely introduced to product.
How many dewatering pigs are needed for a typical pipeline?
The number of dewatering pigs required depends on the pipeline length, diameter, topographic profile, and the dryness specification to be achieved. For short, nominally horizontal pipelines with a simple geometry, a single dewatering pig train of two to three pigs may be sufficient. For longer pipelines, pipelines with significant elevation changes and multiple low points, or pipelines with tight dryness specifications, a multi-pig train including gel slugs between pig stages is the standard approach. A typical medium-length gas pipeline dewatering program might use four to six foam pigs arranged in a train with two or three gel slugs, followed by further drying pig passes as part of the transition to the drying phase. Royal Poly Products can assist with dewatering train design based on your pipeline profile and specification.
Can dewatering be performed while the pipeline is in service?
In some cases, yes — operational dewatering of liquid accumulations in gas pipelines is a routine maintenance activity conducted with the pipeline in service. Low-density foam pigs are commonly used for this purpose, driven by the pipeline’s own gas flow. However, post-hydrotest dewatering following pressure testing requires the pipeline to be taken out of service, as the drive medium for the dewatering run is typically compressed air or nitrogen rather than the pipeline product. The approach for operational dewatering should be carefully planned and approved by the pipeline operator’s operations team, particularly for high-pressure transmission systems.
What happens if a dewatering pig gets stuck in the pipeline?
A stuck pig is typically detected by a loss of differential pressure behind the pig train or the absence of a signal at a downstream pig signaller. When a stuck pig is suspected, the first step is to attempt to restart pig movement by adjusting drive pressure — sometimes a pig will stall temporarily in a low point or at a diameter change and can be freed by a controlled pressure increase. If the pig remains stuck, its location is identified using above-ground pig locating equipment, and it is retrieved through planned excavation and pipe access. The risk of a stuck pig is minimised through thorough pre-run planning, including bore gauging before the dewatering run, careful pig sizing, and appropriate pig selection for the pipeline geometry.
What drive medium is used for dewatering pigs?
The most common drive media for dewatering pigs in Australian pre-commissioning programs are compressed dry air and nitrogen. Nitrogen is preferred where product contamination or oxidation risk is a concern — particularly in high-pressure gas pipelines where any residual oxygen in the line after dewatering could create a hazardous condition during gas-up. Compressed air is widely used for water pipelines and lower-risk gas pipeline applications. Water can also be used as a drive medium in some configurations, where a secondary pig train separates the drive water from the test water already in the line. The choice of drive medium is specified in the project’s commissioning procedure and should be agreed with the pipeline owner before the dewatering program begins.
How do pipeline gels assist dewatering?
Pipeline gels are high-viscosity polymer solutions injected as slugs between pig stages in a dewatering train. They serve several important functions: they provide a more effective wipe of the pipe bore than the direct pig-to-water interface, sweeping moisture from wall irregularities and weld roots that the pig body itself may not contact; they absorb free water from low points and dead-legs that resist displacement by pigs alone; they maintain a positive separation between the drive medium and the water being displaced; and they assist the transition between dewatering and drying by conditioning the bore surface. Gel slug volume is calculated based on the pipeline diameter, length, and the number of significant low points along the route.
Can dewatering pigs handle pipelines with elevation changes?
Yes, but pipelines with significant elevation changes require careful dewatering program design. In sections where the pipeline descends steeply, gravity can cause water to run ahead of the pig and re-accumulate in downstream low points, reducing the effectiveness of the dewatering run. In uphill sections, the pig must overcome both the friction of the bore and the hydraulic head of the water column ahead of it. These challenges are addressed through appropriate drive pressure management, multi-pig train configuration to maintain a continuous slug of water ahead of the train, and the use of gel plugs to prevent water bypass around the pig. A detailed topographic profile of the pipeline is an essential input to dewatering program design for any pipeline with significant elevation variation.
Do dewatering pigs work in pipelines without pig launchers and receivers?
Dedicated pig launcher and receiver assemblies are strongly recommended for any pigging operation, including dewatering. They provide controlled, safe pig insertion and retrieval under pressure and ensure that the pig and displaced water are handled in a contained, predictable manner. Where launcher and receiver facilities are not present, alternative insertion and retrieval methods may be possible depending on the pipeline configuration and operating conditions, but these approaches are more complex, carry greater risk, and require careful engineering assessment. Royal Poly Products recommends that any pipeline intended for regular pigging be designed with appropriate launch and receive facilities from the outset.
How is dewatering volume tracked during a run?
Water displaced by the dewatering pig train is collected at the pig receiver and measured to confirm that the expected volume has been removed from the pipeline. The theoretical volume of water in the pipeline — based on the pipe internal diameter, length, and fill level — is calculated in advance and used as a benchmark. Significant shortfall between expected and received volume may indicate that water has bypassed the pig train and accumulated in low points, or that a pig has stalled before completing the full run. Pig signaller data from points along the route helps confirm full transit and supports reconciliation of the displaced volume.
What pig material is compatible with methanol used in dewatering programs?
Where methanol slugs are used as part of the dewatering or drying program, the foam pig material must be confirmed as compatible with methanol to avoid swelling, degradation, or breakdown of the foam structure during the run. Royal Poly Products’ polyurethane foam pigs have been assessed for methanol compatibility across our standard density range. Please advise our team when placing your order if methanol will be used in the dewatering or drying program so that we can confirm the appropriate foam formulation for your application.
About Royal Poly Products
Royal Poly Products is an Australian manufacturer of pipeline pigs and pigging solutions, based in Jandakot, Western Australia. The company designs and manufactures a full range of foam pigs (low, intermediate, medium, and high density), bi-directional disc pigs, cup pigs, solid cast pigs, dual diameter pipeline pigs, and pipeline gels, supporting pipeline operators across Australia and internationally.
ISO 9001:2015 certified and a recipient of the Western Australian Export Award (Emerging Exporter 2025) and the Australian Export Award, Royal Poly Products combines local manufacturing expertise with global quality standards to deliver reliable pigging products for gas, water, oil, and industrial pipeline applications.
Website: royalpolyproducts.com
Phone: +61 08 6117 9204
Address: Unit 5/41 Biscayne Way, Jandakot WA 6164, Australia
Email: sales@royalmechgroup.com