Pipeline Commissioning:The Complete Pigging Sequence

1. What Is Pipeline Commissioning?

Pipeline commissioning is the process of bringing a newly constructed pipeline into operational service — from the completion of construction through to the introduction of the first product and
the handover to the pipeline operator. In Australia, commissioning is governed by AS 2885 (Pipelines — Gas and Liquid Petroleum) for gas and liquid petroleum pipelines, and by the relevant state water authority standards for water transmission mains.
Commissioning is not a single event — it is a structured sequence of activities, each of which must be completed and verified before the next can begin. Pigging is central to most of these
activities. Without a correctly executed pigging sequence, a pipeline cannot be safely or legally placed into service.

The commissioning pigging sequence typically begins after hydrostatic pressure testing is complete and ends with the introduction of the first product — gas, water, oil, or other pipeline product — into a pipeline that has been confirmed clean, dry, and free of construction debris.

★ Australian regulatory requirement

Under AS 2885.3 (Pipelines — Gas and Liquid Petroleum: Operation and Maintenance), Australian pipeline operators are required to demonstrate that a new pipeline has been gauged, hydrostatically tested, dewatered, and cleaned to specified standards before it can be placed into gas or liquid petroleum service. The pigging sequence described in this guide satisfies these requirements when correctly executed and documented.

2. Why Pigging Is Central to Pipeline Commissioning

Newly constructed pipelines contain significant contamination from the construction process. This includes:

Pipeline pigging addresses all of these contamination sources in sequence. A correctly executed commissioning pigging program removes construction debris, removes hydrotest water, cleans the pipe wall, and dries the bore to the standard required for the intended pipeline product — before the first product molecule enters the line.

★ Consequence of inadequate commissioning pigging

Pipelines that are inadequately commissioned result in debris entering the product stream — blocking regulators, meters, and valves downstream. In gas pipelines, residual moisture causes internal corrosion and hydrate formation that can block the line entirely. In water pipelines, construction debris causes water quality failures. The cost of a poorly commissioned pipeline — both in remediation and in regulatory non-compliance — far exceeds the cost of a correctly executed pigging sequence.

3. The Complete 10-Step Commissioning Pigging Sequence

The following table shows the complete commissioning pigging sequence for a new Australian gas pipeline from construction completion through to product introduction:

Note that step 10 — gas introduction — is not a pig run. It is included to show where the pigging sequence ends and the operational phase begins. The pig runs are steps 1–2 and 4–9. Step 3
(hydrostatic testing) and step 10 (gas introduction) do not involve pigs.

4. Step-by-Step Sequence Explained

Step 1 — Gauge plate run (pre-flood)

The gauge plate run is the first pig run of the commissioning sequence and one of the most important. A low density bare foam pig (LD-BR) is fitted with a metal gauge plate — typically an aluminium disc with a diameter equal to 95% of the pipeline's nominal internal diameter — and run through the pipeline before any water is introduced.
The purpose is to confirm that the pipeline bore is clear of obstructions that would prevent subsequent pig runs or damage the pipeline. Any restriction, weld intrusion, or construction debris that protrudes into the bore deforms the gauge plate as the pig passes. The plate is inspected on retrieval — an undamaged plate confirms the bore is clear; deformation indicates an obstruction that must be located and removed before the commissioning sequence proceeds.
Royal Poly Products' Smart Gauge Plate service takes this further — an instrumented gauge pig that records contact data electronically during the run, allowing the exact location of any bore restriction to be identified from the data download without manual excavation along the entire pipeline length.

Step 2 — Flood pig

The flood pig is run simultaneously with the introduction of water for hydrostatic pressure testing. A low density bare foam pig is inserted into the launcher ahead of the test water, and the pipeline is then filled with water behind the pig. The pig travels ahead of the water front, preventing air from being trapped at high points in the pipeline profile.
Air pockets trapped during flooding cause inaccurate pressure test results and can lead to pressure test failure. The flood pig ensures a complete and accurate fill of the pipeline with water before the pressure test begins.

Step 3 — Hydrostatic pressure test

No pigs are run during the hydrostatic pressure test itself. The pipeline is pressurised to the test pressure specified in AS 2885 (typically 1.25 times the maximum allowable operating pressure, or MAOP) and held for the specified test duration — normally between 4 and 24 hours depending on the pipeline class and regulatory requirements.
The pipeline is inspected for leaks during the test. A successful test — stable pressure over the full test duration with no leaks — is a mandatory precondition for commissioning. Following the test, the pipeline is depressurised and the dewatering pigging sequence begins.

Steps 4 and 5 — Dewatering passes 1 and 2

Dewatering is the process of removing the hydrotest water from the pipeline using foam pig runs. The sequence begins with a low density bare foam pig (LD-BR) for the first dewatering pass, followed by a medium density bare or criss-cross pig (MD-BR or MD-XX) for the second pass.
The first dewatering pig displaces the bulk of the water volume from the pipeline to the receiver, where it is collected and disposed of appropriately. In long pipelines with complex profiles,
multiple low density pig runs may be required before the bulk water is cleared. The second pass removes the residual water that the first pig left behind at low points and in dead legs.
The volume of water recovered at the receiver after each pig run is recorded. A significant reduction in water volume between runs indicates the dewatering sequence is progressing correctly.

Steps 6 and 7 — Cleaning passes

After the bulk water is removed, cleaning pig runs are performed to remove construction debris, silt, wax, and mill scale from the pipe wall. Medium density criss-cross foam pigs (MD-XX) are the standard choice for new pipeline cleaning — they generate adequate wall contact for
effective debris removal while remaining flexible enough to negotiate the full pipeline geometry.
The cleaning sequence continues until pigs arrive at the receiver clean — no significant debris visible on the pig surface or in the receiver drain. In heavily contaminated pipelines (common in
older construction or where dust and sand have entered the line during construction), multiple cleaning passes may be required, and an escalation to MD silicon carbide (MD-SC) or wire brush pigs (MD-CW) may be appropriate if mill scale or hard deposits are present.
Each pig is inspected on retrieval. The type, volume, and nature of debris recovered gives the commissioning team a picture of the internal pipeline condition and informs decisions about
whether additional cleaning passes are required.

Steps 8 and 9 — Drying passes

After the cleaning sequence is complete, drying pig runs remove the residual moisture from the pipe wall. Low density fully coated foam pigs (LD-FC) are the standard choice for drying — the solid polyurethane shell provides the best liquid seal of any foam pig configuration, maximising water pick-up on each pass.
The drying sequence is particularly critical for natural gas pipelines, where residual moisture causes several serious problems: internal corrosion of the carbon steel pipe wall, formation of
gas hydrates (ice-like crystalline compounds that can block the pipeline entirely at low temperatures or high pressures), and contamination of the gas product stream with water vapour that fails gas quality specifications.
The drying sequence is considered complete when the pig arrives at the receiver dry — no visible moisture on the pig surface and no free liquid in the receiver drain. In some specifications, a dew point measurement of the gas introduced immediately after the drying sequence is also required to confirm the pipeline is dry to the specified moisture standard.

Step 10 — Product introduction

With the pigging sequence complete, the pipeline is ready for product introduction. For natural gas pipelines, nitrogen purging of residual air is typically performed before gas introduction to prevent the formation of a flammable gas-air mixture during the fill. The gas front is introduced progressively, monitored for pressure and flow, and the pipeline is brought up to operating pressure under controlled conditions.
The commissioning pigging sequence is now complete. The pipeline is handed over to the operator for operational service.

5. Gel Pig Commissioning Sequences

For pipelines with particularly heavy contamination — severe mill scale, significant construction debris, or pipelines where conventional foam pigs have failed to achieve the required cleanliness — a gel pig sequence can be incorporated into the commissioning program to achieve a higher standard of cleaning.
A gel pig sequence uses slugs of viscous pipeline gel pumped between foam pigs. The gel encapsulates and suspends fine debris particles — including mill scale fines and silt that foam pigs push along the bore rather than capturing — and carries them to the receiver. The following sequence is typical for a gel-assisted commissioning clean:

Royal Poly Products supplies both the foam pigs and the pipeline gels for gel commissioning sequences, ensuring full compatibility between the gel formulation and the pig materials. The gel volume is calculated based on the pipeline bore, length, and the cleaning objective —
contact the technical team for a specific gel volume recommendation for your project.

★ When to use a gel pig sequence

Gel pig sequences are recommended when: (1) the pipeline has been open to the elements for an extended construction period and significant silt or dust contamination is expected; (2)
conventional foam pig runs have failed to achieve the required cleanliness standard; (3) the pipeline specification requires a higher than normal cleanliness standard (e.g. for chemical, pharmaceutical, or high-purity gas service); or (4) corrosion inhibitor application is required as part of the commissioning sequence.

6. Pre-Commissioning Checklist

Before beginning any commissioning pigging sequence, verify the following items are complete. This checklist is based on AS 2885.3 requirements and Royal Poly Products' commissioning
experience across Australian gas, water, and oil pipelines:

7. Water Pipeline Commissioning — How It Differs

The commissioning pigging sequence for water transmission mains and distribution pipelines differs from the gas pipeline sequence in several important ways:

No drying requirement

Water pipelines do not need to be dried after hydrostatic testing — they will be carrying water in service. The commissioning sequence for water mains ends after the cleaning passes, without
drying pig runs. However, any disinfection or flushing requirements specified by the relevant water authority must still be completed before the pipeline is placed into potable water service.

Disinfection pigging

For potable water pipelines, a disinfection pig run using a chlorinated water slug or pipeline gel containing disinfectant may be required by the relevant water authority (Water Corporation WA, Sydney Water, Melbourne Water, SA Water, etc.) before the pipeline is approved for potable water service. This is typically performed after the mechanical commissioning pigging sequence is complete and before the final bacteriological sampling.

Material compatibility

Foam pigs and gels used in potable water pipelines must be manufactured from materials that are approved for contact with drinking water. Royal Poly Products' foam pigs used in potable
water applications are manufactured from polyurethane materials that comply with relevant Australian drinking water standards. Always specify the intended product when ordering pigs for water pipeline commissioning.

Cleanliness standards

The cleanliness standard for water pipeline commissioning is typically specified by the relevant water authority and may include requirements for turbidity, colour, pH, and bacteriological quality of flushed water samples taken at the end of the commissioning sequence. Pigging sequence adequacy is judged by whether the water quality samples meet these standards —
not simply by the visual condition of the pig on retrieval.

8. Common Commissioning Pigging Mistakes

Skipping the gauge plate run

The gauge plate run is sometimes skipped to save time or cost, particularly in short pipelines. This is a significant risk — construction debris and weld intrusions that would be detected by the
gauge plate run will be encountered by subsequent pig runs, potentially causing a stuck pig deep in the pipeline. The cost of recovering a stuck pig is always far greater than the cost of a gauge plate run. Never skip Step 1.

Running medium density pigs before bore is confirmed clear

Starting the dewatering sequence with medium density pigs before a low density bare pig has confirmed the bore is clear is one of the most common causes of stuck pigs in Australian pipeline commissioning. The low density pig may bypass partially (arriving degraded but intact) where a medium density pig would stall. Always start with LD-BR for the first dewatering pass.

Inadequate water disposal planning

Hydrotest water volumes in long pipelines can be enormous — a 20" diameter pipeline 50km long holds approximately 2,500 kilolitres of test water. Dewatering this volume requires significant waste water disposal capacity at the receiver end. Inadequate planning for test water disposal causes commissioning delays, environmental incidents, and sometimes regulatory
non-compliance. Plan water disposal before the commissioning sequence begins.

Not recording pig run data

Pig run data — launch time, signaler passage times, receiver arrival time, pig condition on retrieval, water volume recovered, debris type and volume — is required for AS 2885 compliance documentation and provides essential information for assessing whether the
commissioning sequence has been successful. Failure to record this data leaves the operator without the documentation needed to demonstrate regulatory compliance.

Declaring the pipeline dry too early

A single drying pass is rarely sufficient to dry a pipeline to the standard required for gas service. The pig may arrive dry on the surface while residual moisture remains in low points, dead legs, and rough pipe wall surface areas. The drying sequence should be continued until at least two consecutive pig runs arrive dry at the receiver — not just one.