Pipeline Dewatering After Hydrostatic Testing: Best Practices and Pig Selection

1. What Is Pipeline Dewatering?

Pipeline dewatering is the process of removing water from a pipeline bore using foam pig runs after hydrostatic pressure testing. Hydrostatic testing — a mandatory requirement under AS 2885 for Australian gas and liquid petroleum pipelines — fills the pipeline completely with water and pressurises it to test pressure to confirm structural integrity. Once the test is complete, all of that water must be removed before the pipeline can be placed into service.
Dewatering is performed by inserting foam pigs into the pipeline at the launcher and using pump pressure or pipeline product pressure to drive the pigs through the bore. As each pig travels through the pipeline, it pushes the water ahead of it toward the receiver, where it is collected and disposed of. Multiple pig runs are required to progressively reduce the water volume from the full pipeline bore to the trace moisture level required for gas service.
Pipeline dewatering is used across oil, gas, water, mining, desalination, and process pipelines — any pipeline that undergoes hydrostatic pressure testing before commissioning, and any pipeline that has been flooded during maintenance, repair, or an emergency.

★ Key point:

Dewatering is not a single pig run — it is a progressive sequence of multiple runs using increasing pig aggressiveness and decreasing water volume recovery at each pass. Attempting to dewater a long pipeline with a single pig run almost always results in inadequate water removal.

2. Why Dewatering Is Mandatory Before Gas Introduction

Residual water in a gas pipeline causes severe operational problems that can damage equipment, disrupt supply, and create safety hazards. The consequences of inadequate dewatering in a gas pipeline include:

Gas hydrate formation

At the pressure and temperature conditions inside a natural gas pipeline, residual water reacts with gas molecules to form gas hydrates — ice-like crystalline compounds that block the pipeline bore entirely. Hydrate blockages are extremely costly to remove and can take days or weeks, during which the pipeline is out of service. Thorough dewatering and drying is the only reliable prevention.

Internal corrosion

Water in contact with carbon steel pipe wall causes rapid internal corrosion. In a gas pipeline carrying sour gas (containing H₂S or CO₂), corrosion rates with free water present can be orders of magnitude higher than in dry gas. Dewatering and drying removes the water that enables corrosion before the pipeline is placed into service.

Gas quality failure

Gas quality specifications — including those set by the Australian Energy Regulator and individual network operators — include limits on water vapour content (dew point) of gas entering transmission networks. A pipeline that has not been adequately dewatered and dried will fail gas quality testing at the inlet measurement station, preventing gas introduction until the pipeline is re-dried.

Equipment damage

Water slugs carried through gas pipelines by product flow can damage compressors, regulators, flow meters, and control valves. The kinetic energy of a water slug at gas pipeline velocities is sufficient to cause mechanical damage to rotating equipment and instrument diaphragms. Adequate dewatering eliminates this risk before operations begin.

Key point:

Under AS 2885.3, Australian pipeline operators are required to demonstrate that a new gas pipeline has been dewatered and dried to a specified standard before it can be placed into service. The dewatering pig sequence and water volume recovered at each run must be documented as part of the commissioning record.

3. How Much Water Is in Your Pipeline?

Before planning a dewatering operation, calculate the total water volume in the pipeline. This determines the number of pig runs required, the waste water disposal volume that must be planned for, and the pump capacity needed at the launcher. Use this reference table:

To calculate the exact water volume for your pipeline: Volume (litres) = π ÷ 4 × (internal diameter in metres)² × pipeline length in metres × 1000. For a pipeline with multiple wall thicknesses or bore changes, calculate each section separately and sum the results.

Key point

Always calculate the total water volume before beginning a dewatering operation. Inadequate waste water disposal capacity at the receiver is one of the most common causes of commissioning delays in Australian pipeline projects — and one of the easiest to avoid with pre-planning.

4. The Standard Dewatering Pig Sequence

The standard dewatering sequence for a new Australian gas pipeline after hydrostatic testing uses five pig runs, escalating from low density to medium density before returning to low density fully coated for the final drying pass:

This 5-run sequence is the standard for a new medium-length gas pipeline (typically 5–30km). Longer pipelines, pipelines with complex profiles, or heavily contaminated pipelines may require additional runs. Shorter pipelines or simple water main commissioning may be completed in 2–3 runs.

Key point:

The golden rule of dewatering: always record the water volume recovered at the receiver after each pig run. A progressive reduction in volume — 70%, 20%, 7%, 2%, trace — confirms the sequence is working. If water volume does not reduce between runs, investigate for low points trapping water before continuing.

5. Step-by-Step Dewatering Sequence Explained

Run 1 and 2 — Low density bare foam pig (LD-BR): bulk dewatering

The first one or two pig runs use low density bare foam pigs (LD-BR) for bulk water displacement. The low density pig is chosen for this phase for two reasons: first, it confirms the bore is clear of obstructions before a more aggressive pig is run; second, it generates adequate differential pressure to displace the bulk water volume without the risk of stalling that comes with a heavier pig on the first run in an unconfirmed bore.
Each LD-BR run pushes a large volume of water ahead of it to the receiver. The pig may arrive degraded after the first run through a newly constructed pipeline — this is normal, as the pig absorbs construction debris and is driven hard by the high water volume behind it. The pig is inspected on retrieval for integrity and the presence of construction debris, which informs the next stage of the sequence.

Run 3 — Medium density bare foam pig (MD-BR): progressive dewatering

Once the bulk water has been displaced by the LD-BR runs, the sequence escalates to a medium density bare foam pig (MD-BR). The firmer MD pig generates greater wall contact and differential pressure, allowing it to push water out of low points and pipeline sections that the lighter LD pig may have bypassed.
The volume of water recovered at the receiver after this run should be significantly lower than after the first two runs — typically 5–10% of total pipeline volume. If a large volume is still being recovered, an additional MD-BR run is indicated before progressing.

Run 4 — Medium density criss-cross pig (MD-XX): combined cleaning and dewatering

The fourth run uses a medium density criss-cross coated foam pig (MD-XX). At this stage in the sequence, the remaining water volume is small and the focus begins to shift from bulk liquid removal to pipe wall cleaning. The criss-cross coating creates a wiping action that removes silt, mill scale, and fine debris from the pipe wall surface — debris that would otherwise remain after dewatering and interfere with the final drying pass.
The pig should arrive at the receiver with visible debris — silt, wax, or mill scale — on its surface. This is the expected result of the cleaning action. If the pig arrives clean with minimal debris, the pipeline was in better condition than typical for new construction — which is a positive sign.

Run 5 — Low density fully coated pig (LD-FC): final drying pass

The final run uses a low density fully coated foam pig (LD-FC). The solid polyurethane shell of the FC pig provides the best liquid seal of any foam pig configuration — it picks up and carries residual moisture from the pipe wall surface that the previous pigs have left behind.
The LD-FC pig should arrive at the receiver dry — no visible moisture on the pig surface, and no free liquid in the receiver drain. This confirms the pipeline has been dewatered and dried to the standard required for gas introduction. If the pig arrives damp, a second LD-FC run is required before the pipeline can be declared dry.

Important — confirm dry before declaring complete

In some pipeline specifications, visual inspection of the final drying pig is not sufficient. A dew point measurement of the gas immediately after introduction, or a moisture content test of air or nitrogen pushed through the pipeline after drying, may be required to confirm the pipeline is dry to the specified standard. Check the project specification before declaring dewatering complete.

6. Pig Selection Guide for Dewatering

Use this guide to select the right pig for each stage of your specific dewatering operation:

7. Dewatering Challenges — Low Points, Dead Legs, and Elevation

The most common reason a dewatering sequence fails to achieve the required dryness level is the pipeline profile. Water naturally accumulates at the lowest points of the pipeline — and foam pigs do not always remove all of this water in a single pass.

Low points

In a pipeline with significant elevation changes, water accumulates at low points (sags) in the profile. As a foam pig traverses a low point, it must push the accumulated water uphill to the next high point before it can continue. If the differential pressure driving the pig is insufficient to lift the water column at a severe low point, the pig may stall or bypass — leaving water behind.
For pipelines with known severe low points, the dewatering sequence should include additional medium density pig runs — and consideration should be given to running pigs in the opposite direction if the pipeline geometry allows, to approach low points from the downhill side.

Dead legs and branch connections

Unprotected tee connections and dead leg sections of pipeline are not swept by a pig running in the main bore. Water trapped in dead legs remains after the main bore dewatering is complete. For gas pipelines with dead legs, options include: flushing the dead leg separately with a smaller foam pig or gel slug; purging with dry nitrogen; or accepting that the dead leg cannot be dewatered by pigging and managing the moisture through alternative means.

Elevation gain

A pipeline that rises significantly in elevation from the launcher to the receiver presents a specific challenge: the water volume to be displaced must be lifted against gravity by the pig. This requires higher differential pressure than a flat or downhill run, and may require a higher density pig (MD or HD) to generate adequate drive force. Calculate the hydrostatic head of the water column against the available pump pressure before specifying pig density for a steep uphill dewatering run.

Key point:

Always obtain the pipeline profile (elevation vs chainage) before planning a dewatering operation. Low points, dead legs, and elevation changes must be identified and their impact on the dewatering sequence assessed before the first pig run. Surprises in the pipeline profile are the most common cause of stuck pigs and failed dewatering operations in Australian pipeline commissioning projects.

8. Gel-Assisted Dewatering

For pipelines where conventional foam pig dewatering has failed to achieve the required dryness level — or where the pipeline profile is known to have severe low points and dead legs — a gel-assisted dewatering sequence can be used to achieve a higher standard of water removal.
A dewatering gel is a viscous pipeline gel formulated to seek out and absorb residual water from low points, dead legs, and rough pipe wall surfaces that foam pigs cannot effectively reach. The gel is pumped into the pipeline as a slug between foam pigs — the front foam pig prevents the gel from mixing with residual product or air ahead of it, and the rear foam pig drives the gel through the pipeline under differential pressure.
As the gel slug travels through low points and dead legs, it picks up residual water by absorption and viscous displacement. The gel arrives at the receiver laden with the water it has collected, which is then disposed of along with the gel itself.

Royal Poly Products supplies pipeline dewatering gels specifically formulated for Australian gas, water, and oil pipeline applications, alongside the foam pig sequence required to deploy them effectively. Contact the technical team for gel volume recommendations specific to your pipeline.

Key point:

Gel-assisted dewatering is particularly effective in pipelines with multiple low points, complex profiles, or where dew point specifications are very tight. The combination of foam pigs and gel typically achieves a drier result than foam pigs alone in fewer total pig runs.

9. Dewatering Water Disposal — Australian Requirements

The water removed from a pipeline during hydrostatic testing and dewatering must be disposed of in accordance with Australian environmental legislation. This is not a minor consideration — a 20″ pipeline 50km long contains nearly 10,000 kilolitres of test water, and its disposal requires pre-planning and regulatory compliance.

Onshore pipelines — key requirements

• In Western Australia, the discharge of water to ground or surface water requires approval from the Department of Water and Environmental Regulation (DWER) under the Environmental Protection Act 1986
• In all states, hydrotest water that has been in contact with the pipeline interior may contain rust, scale, mill scale, and potentially elevated metals concentrations — it is typically not suitable for direct discharge to watercourses without treatment or testing
• Test water may require pH adjustment or treatment before discharge if it has been chemically conditioned during testing (e.g. with corrosion inhibitor or oxygen scavenger)
• Disposal options typically include: approved evaporation ponds, licensed waste water treatment facilities, land application under approval, or tanker removal to an approved disposal facility

Planning disposal before commissioning begins

The most common mistake in dewatering project planning is leaving water disposal arrangements until after the pig runs have begun. Water disposal must be arranged before the dewatering sequence starts — including obtaining any necessary environmental approvals, confirming the disposal method, and ensuring adequate capacity at the receiver for the total water volume. Failing to plan disposal in advance can halt a commissioning project at the most critical stage.

Key point:

Calculate the total water volume (see Section 3 table) and arrange approved disposal before beginning the dewatering sequence. Environmental non-compliance during pipeline commissioning can result in project suspension, fines, and significant reputational damage for the pipeline owner and contractor.

10. Common Dewatering Mistakes

Attempting to dewater with a single pig run

A single foam pig run will never fully dewater a long pipeline. The pig pushes the bulk of the water ahead of it, but leaves significant residual water in low points, rough pipe wall surface areas, and dead legs. Multiple progressive runs are always required — the exact number depends on pipeline length, bore, and profile.

Starting with medium density on the first run

Starting a dewatering sequence with a medium density pig before a low density pig has confirmed the bore is clear risks a stuck pig if the hydrotest has disturbed internal debris or if weld splatter was not removed during construction. Always start with LD-BR for the first dewatering run.

Not recording water volumes at each run

Failing to measure and record the water volume recovered at the receiver after each pig run means the commissioning team has no objective measure of dewatering progress. Without this data, it is impossible to determine when the dewatering sequence is complete — or to demonstrate regulatory compliance under AS 2885. Install a flow meter or use a calibrated measurement vessel at the receiver for every pig run.

Declaring the pipeline dry after a single drying pass

A single LD-FC drying pass arriving dry at the receiver does not prove the pipeline is fully dry to the standard required for gas service. At least two consecutive dry pig arrivals should be confirmed before declaring dewatering complete. For tight dew point specifications, a moisture content measurement of purge gas through the pipeline is the most reliable confirmation method.

Inadequate waste water disposal planning

Arriving at the receiver with thousands of kilolitres of hydrotest water and no approved disposal method brings commissioning to an immediate halt. Plan disposal before the first pig run — not after.