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Data analysis underpins successful pig rescue on small-diameter pipeline

 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 pipelines 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 cleanliness of the pipeline; an accumulated amount of debris and corrosion product; pipeline dents or buckles; and erosion of pig disks.

Recently, a pigging operation was performed 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.

Inner condition of a damaged piece of pipeline (detached during the first repair) [17]. Images courtesy IPEC.

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 pipeline 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 flow rate 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 future operations.

FIRST STEPS

Flooding, cleaning, and gauging (FCG) operations are the first steps 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 corrosion becomes appreciable, very 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].

SIMILAR CASES

Although a more than 100-km, 4-in. pipeline is rare, this type of pipeline is very common in the 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 as follows:

• Collision of vessel anchors with pipeline which causes damage to and buckling of the pipeline

• High length and low diameter of the pipeline that make the pipeline difficult 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 the 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 exists: 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 the 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 in an attempt to drive the pig through the restriction. Repeat two to three times.

3. Run a soft swab up behind the stuck pig to try to attain a positive seal. Repeat step 1 above.

4. Consider ways of backing the pig out of the line, then returning it to the pig launcher. This requires pressurizing from the opposite end of the pipeline.

Troubleshooting pigs which have no bypass:

1. Increase the 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 in an attempt to drive the pig through the restriction. Repeat two to three times.

3. Consider ways of backing the pig out of the line, and returning it to the pig launcher. This requires pressurizing from the opposite end of the pipeline.

4. In a potable water line when pigging with a foam pig, super-chlorinate (3,000 to 5,000 ppm) in a slug form to dissolve the pig. The line must be thoroughly flushed and tested after super chlorination.

Al-Khaledi et al. [3] used a similar method for retrieving a stuck pig in a non-metallic pipeline by injecting a chemical mixture into the pipeline and holding for a while in order to dissolve the sludge within a short duration of exposure. They concluded that a mixture of toluene, diesel, and demulsifier is capable of breaking and dissolving the sludge once they get in contact.

One of the methods to predict the pigging condition before starting the operational job is modeling and simulation of the operation as O’Donoghue [4] and Gupta et al. [5] have done. O’Donoghue uses modeling based upon MATLAB and uses the dynamic modeling environment SIMULINK in order to study the motion of pig trains. Gupta et al. use OLGA to simulate the pigging operations & PVTsim at its backend. They also perform continuous monitoring of pressure and temperature through SCADA system.

Other pigging product vendors[6] have said that in addition to increased differential pressure across the pig or reverse flow, launching a second pig in the hope that it will free the stuck pig and push it out of the pipeline is one of the methods used for removing stuck pigs. Therefore, careful planning and the correct selection for running a second pig is critical for the success of this contingency pipeline pigging procedure. A poly pig or foam pig may be used as a rescue pig and ideally, it should be fitted with magnets and a transmitter to enable pig tracking and location.

Once the poly pig reaches the stuck pig it will deform, but will also create a seal around the stuck pig in the pipeline. The stuck pig may then move forward, or both pigs will remain in the pipeline. The advantage with this method is that the exact location of the poly pig or foam pig can be located and in turn, the position of the stuck pig in the pipeline can be determined. However, the disadvantage is that if the stuck pig fails to move, the poly pig or foam pig will have created a seal around the stuck pig and flow through the pipeline will be prevented, causing a pipeline blockage due to the stuck pigs. Furthermore, if the differential pressure across the pigs is increased sufficiently, the poly pig or foam pig may be destroyed and while this will enable flow to continue, the tracking magnets and transmitter would be left inside the pipeline. Pigtek Ltd. suggests using an alternative by using Pigtek Advanced By-Pass Pig, which can be used as a rescue pig. This specialist pig design has one or more bypass valves fitted internally to the pig body, which are set to open at a predetermined pressure.

By planning a proper pig rescue plan, taking a look at other pig stuck cases can be helpful. Tabatabaei et al. also experienced a pig stick situation in a long pipeline, and have shared their learned lessons in an article [7].

For the pipeline in this case, different procedures for moving the stuck pig were examined. Some of these procedures included pressurizing the pipeline with gas or liquid, performing sudden fluid discharge, inserting other pigs inside the pipeline, etc. Furthermore, necessary engineering and safety considerations when following up the pig rescue procedures were discussed.

Accumulated debris on inner wall of damaged of the pipeline (detached during the first repair)[17].

A part of pigs’ disks torn out during operation received in water discharge during first FCG operation [17].

Another approach to avoid pig stuck situation in pipelines which suffer from severe wax deposits is to clean the pipeline using wisely-planned pigging: starting with very flexible pigs and gradually progressing toward firmer and more abrasive designs, so that the risk of a stuck pig and plugged line is minimized [8].

Nesbitt [9] outlines the benefits of separator gel pigs for rescuing the stuck mechanical pigs which are stopped due to the large bypass of fluid. In that study, a successful application of these gel pigs is described with extended details when two stuck spheres in a 14-in. oil export line were removed.

Another interesting experience was with the Marlin TLP [10] oil export pipeline wax management strategy; and the circumstances that led to a stuck pig incident. The lessons learned from this incident included both managerial and technical solutions, and underscored the importance of: a management of change (MOC) procedure; high-quality pig traps; and efficient condition monitoring equipment.

Combe and Hair [11] presented a 13-step flowchart for recovery of stuck or stalled pigs in offshore pipelines. They believe that when a pig is stalled in a pipeline, there are several “operational” options available which can be tried to get the pig moving. These are all non-intrusive and as a result are reasonably low risk.

SOUTH PARS PIPELINE

The pipeline in this case study is designed to transport mono-ethylene glycol (MEG) produced and recovered in a refinery to an offshore platform which produces sour gas; then transports that gas toward the refinery by means of a 32-in. pipeline. The 4-in. pipeline is installed as a piggyback line on the 32-in. pipeline.

A pile of metallic-type debris received in discharging water during first FCG operation[17].

The pipeline was laid at the beginning of 2013 in South Pars gas field. In September 2015, during the start of pre-commissioning operations, it was revealed that the 4-in. pipeline had a wet buckle since water came out of the pipeline after cutting the pull head. The damaged location was found near landfall; but, the exact time of damage was not clear. After that, the pipeline went through several pigging operations and repairs.

During these pigging operations, several conditions were analyzed and miscellaneous results were achieved. But, the main operation examined here is the second FCG and pig rescue operation took place in April 2017.

As the pipeline had been damaged a long time ago and been exposed to seawater for a long term, huge amounts of mud and debris had entered the pipeline. This was found out from contaminated discharging water during first FCG operation.

FCG OPERATION

FCG operation on the pipeline was started on April 15, 2017, and three pigs of FCG train were launched into the pipeline by using treated fresh water. Water injection was continued in order to receive the pigs at the platform end of the pipeline.

PIG STUCK

On April 17, 2017, and after injecting more than 1,100 m3 of treated fresh water (almost 20% more than pipeline volume), no pigs were received and it was inducted that there was a problem. Afterwards, inserting shocks was started from platform end of the pipeline. The shocking process was comprised of closing the outlet valve at the platform while continuing water injection to increase the pipeline pressure, and then, instantly opening the outlet valve in an attempt to push the attempt to jerk and dislodge the stuck pig. As a result of shocking, the first pig of the train was received in the platform upper bend on April 18, 2017. Inserting more shocks did not result in receiving more pigs, and thus the pig rescue operation had to be launched.

PIPELINE CONDITION ANALYSIS

After receiving only one pig, the situation needed to be investigated. By noticing the pressure readings, and after ignoring sudden pressure drops and rises (which are due to temporary reasons such as launching pigs, pump stop, increasing the flow or inserting shocks), it was understood that pipeline pressure had dropped gradually in a period of constant flowrate of around 21 m3/hr (equal to pig velocity of 0.7 m/s). This meant that at the beginning of FCG operation, an external object such as accumulated debris may have made an obstruction in the pigs’ way, which in turn led to high resistance in front of the water flow. This resistance gradually decreased after a long distance. This gradual decrease in the resistance, which is observed as a decrease in pipeline pressure, indicated that the obstruction had been demolished slowly and made the path easier for pigs to move along the pipeline.

According to the condition of the received pig and gradual pressure drop, it was concluded that a huge amount of debris or external object(s) had been in front of the pig and had led the pig to become collapsed under pipeline pressure. So, in order to rescue the two remaining pigs, it was decided to significantly increase the flowrate to make a high flow and move the stuck pigs.

First pig of the FCG train in received condition.

PIG RESCUE

After investigation on operation information and analysis of recorded data, the following approach was considered and taken into action for pig rescue. During the operation, every step was decided and acted upon after accurately considering the pipeline received data and evaluating the condition.

In such pipelines in which there is evidence of the presence of probable accumulated debris in front of the pig train, the shocking process can have an appropriate effect on the pipeline condition. Closing the outlet of the pipeline, while keeping the water injection going, results in an increase in pipeline pressure. As the outlet is opened, a pressure decrease happens in front of the pig train which produces a high flow around the stuck pigs. This high flowrate not only pushes the pigs and tries to move them, but also causes the accumulated debris to be distributed along the pipeline. This means that the pile of debris which had blocked the pigging route will be scattered and the blockage of the pipeline will be resolved. So, in some steps of pig rescue plan, the shocking process was incorporated into the plan.

Finally, in order to accomplish the final step of this approach, a special pump truck device was provided, and by increasing the flow rate up to 40 m³/hr continuously (equal to pig velocity of 1.4 m/s), the pipeline pressure reached to 230 bar, and as a result, the second and third pigs were rescued after injection of 340 and 870 m³ water respectively. The aforementioned high flowrate and pressure, which is required simultaneously for the rescue operation, is a difficult condition to provide in such a pipeline.

THEORY AND PRACTICE

After the pigs had been retreived, the collected data from the operation was gathered and compared with theoretical equations. It has to be mentioned that the theoretical calculations only show the pressure loss due to friction; while the practical results include not only pressure loss due to friction, but also required pressure for pig movement and elevation difference (around 10 m from launcher point at landfall location to receiver point at platform top of jacket). Thus, these values (required pressure for pushing the pigs and elevation difference) have been added to the pressure loss value which is calculated from theoretical equations. For theoretical calculations, the roughness of the pipeline was calculated as 0.05 as for minimum of slightly corroded carbon steel and maximum of new carbon steel [13].

Second pig (received condition) [17].

PREVENTIVE STRATEGIES

Based on evaluated references and in the field experiences, the following strategies can be presented in order to prevent a stuck pig condition:

• Propelling a poly pig before the start of any pigging operations is a recommended strategy in order to both evaluate the condition of and pre-clean the pipeline.

• Chemical cleaning is a way of cleaning a pipeline without the need for launching brush pigs. The chemical mixture needs to be designed based on possible contents of pipeline debris to be capable of breaking and dissolving the sludge within a short duration of exposure. It is mostly recommended for short length pipelines which have accumulated wax and debris. For use in high length pipelines, a chemical mixture can be inserted as a batch between two pigs so that it can help the pigs confront the wax and debris inside the pipeline. Also, a chemical cleaning program removes deposits or solids such as black powder, millscale, or other solid elements from the inside of the pipe in order to enhance its cleanliness for in-line inspection, and improve its efficiency [14].

• Modeling and simulation in analytical environments (such as ANSYS, OLGA, SIMULINK, MATLAB, etc.) is another technical way of predicting the conditions inside the pipeline. Many works have been performed in this area which can be referred to for more information such as O’Donoghue, Gupta et al. and IDAC and BP Case Pig Analysis Case Study [15].

• Regular cleaning of the pipeline usually leads to gradually remove the debris and deposits from the pipeline, making each pigging operation easier to perform. During the life of a line, operational pigging is a cheap effective way of maintaining flow and minimizing back pressure [5]. The reason is that once in service, deposits can begin to settle on the pipe wall. If cleaning is not performed on a regular basis during production operations, these deposits can build up, encouraging corrosion growth and potentially preventing inhibitors that can protect the system from being properly applied. In addition, cleaning at this stage prepares the pipeline for further pigging operations [16].

On this matter, the ROSEN Group suggests the following steps to maintain the pipeline, increase operational performance, and extend the operational lifecycle of the pipeline [16]:

• Post-construction cleaning

• Pre-commissioning

• Operational cleaning

• Change of service cleaning

• Cleaning prior to cessation of production

• Decommissioning cleaning.

CONCLUSION

According to the described condition of the operation, it can be concluded that the main reason for pigs getting stuck is an accumulated amount of debris, which can be either loose debris or sticky debris. By noticing the condition of the secondly received pig, it could be observed that not only the gauge plate had been broken, but also front disks of the pig had been rubbed and damaged in the exact same half side of the pig. Nose damage to the pig had occurred such that even the front nut had been eroded and destroyed up to its half. This meant that the reason leading the pig to this condition had been a constant and resistant factor or object, not an instantaneous factor. The condition of the third received pig also approved this scenario because the disks were not torn, but eroded asymmetrically. This showed that the object which had caused the gauge plate to break had not been a sudden obstruction such as a dent, but a constant object like debris and sediments. So, it was decided to rescue the pigs by means of high flowrate which could push the debris out of the pipeline and make the pigs move forward.

Furthermore, since the pipeline had been filled with seawater for two years and had a history of two repairs, the roughness of pipeline inner wall was a matter of discussion, and production of corrosion products inside the pipeline and formation of sediments was therefore likely.

ACKNOWLEDGMENT

The authors would like to thank IPEC for permission to publish this article. The authors also express their sincere appreciation to the client for cooperation and full support during the operation of this project in the South Pars gas field.