Data analysis underpins
successful pig rescue
on small-diameter pipeline
High flowrate pump
deployed to move
The ability to pig a pipeline is crucial to maintenance and other operations such as pre-commissioning on the pipeline. Usually high-length, low-diameter pipe- lines are difficult to pig, and therefore
produce a complicated pigging operation.
There are several parameters that cause the
pig(s) to get stuck. These include cleanli-
ness of the pipeline; accumulated amount of
debris and corrosion product; pipeline dents
or buckles; and erosion of pig disks.
Recently, a pigging operation was per-
formed in a high-length, small-diameter offshore pipeline which led to a stuck pig condition. The offshore pipeline that provides the
case study here was a 112-km, 4-in. system
that is located in the South Pars gas field.
The goal here is to summarize the decision-
making approach and operational activities
which were taken to rescue the stuck pigs.
After a review of the available data, it was
concluded that the main reason for pigs getting stuck in this pipeline was the accumulated
amount of debris. The condition of the pigs
showed that the object which caused the pigs
to get stuck has not been a sudden obstruction such as a dent, but a constant object like
debris and sediments.
Furthermore, according to the pipeline’s
repair history and being filled with seawater
due to damage, the roughness of the pipe-
line inner wall was a matter of discussion. It
was concluded that production of corrosion
product inside the pipeline and formation of
sediments was very likely.
It was decided to rescue the pigs by means
of a high flowrate pump which could push the
debris out of the pipeline and make the pigs
move forward. In the end, some preventive
strategies were developed in order to reduce
the risk of pigs getting stuck or stalled in
Flooding, cleaning and gauging (FCG)
operations are the first step of pipeline pre-commissioning operation. Significant quantities of corrosion debris can build up in long
pipelines, even if corrosion rates are rather
low, for instance when the line contains moist
air for long periods. This is because of the
large surface area of metal exposed. If corro-
sion becomes appreciable, ver y large amounts
of debris are produced. If debris is left in the
line, it may get through into the plant and in
extreme cases may lead to off-specification
products. It is important, therefore, that
pipelines and equipment are kept as clean
as possible to give maximum effectiveness
to the corrosion protection measures during
operation. For the above reasons, pipelines
for the transportation of liquid or gaseous
products require thorough cleaning.
Operators and contractors have been running pigs successfully for years. Every so
often, a problem occurs and one becomes
stuck, stalled or damaged in the pipeline due
to various factors. The approach to rescue a
stuck, stalled or damaged pig can vary, and
often depends on the condition of pipeline
and pigging operation [ 1].
Although a more than 100-km, 4-in. pipeline
is rare, this type of pipeline is ver y common in
South Pars gas field. Stuck pig conditions have
been happening during pigging operations in
4-in. pipelines in other phases of this gas field.
These similar conditions happen during FCG
or even dewatering operations when the pigs
get stuck or stalled in the pipeline.
The most common problems which happen
in similar pipelines in this area which lead
to a stuck pig condition can be summarized
• Collision of vessel anchors with pipeline
which casue damage to and buckling of
• High length and low diameter of the
pipeline which make the pipeline diffi-
cult to pig. It is especially difficult to use
brush pigs in this situation, because these
types of pigs scrape the loose debris and
produce a large amount of accumulated
debris which blocks the pipeline
• High erosion of pig disks in contact with
pipe inner wall, since the contact surface
of erosion is negligible compared to high
length of the pipeline;
• Long interval between time of laying and
commissioning the pipeline which leads
to production and accumulation of debris.
When a pig becomes stuck, it is important
to identify the cause [ 2]. Retrieving the pig is
the first priority. Usually one of two conditions
exist: fluid bypasses around the pig, or there is
a complete blockage of the flow. A strategy for
both scenarios is recommended, as follows.
Troubleshooting pigs with a bypass:
1. Increase the flow rate and line pressure,
but do not exceed safe limits of the pipeline.
2. Remove pressure from the line and vent
or drain toward the launcher. Removing
pressure allows the pig to relax to its original shape and may cause it to back up in
the pipeline. Allow pressure to dissipate.
After 15-30 minutes, re-pressure the line
Pipeline and Process Services
Inner condition of damaged piece of pipeline (detached during first repair) [ 17]. Images courtesy IPEC.