Foam pigs solve pipe cleaning problems offshore Brazil

P.C.R. Lima, S.J. Alves Neto
Petroleo Brasileiro S.A.
Rio de Janeiro

Pipeline systems in which conventional pigs cannot be run are common in such complex offshore installations as are found in Brazil's Campos basin. These systems may contain changing pipe diameters or wet christmas trees and manifolds.

A new concept for using low cost, low-density foam pigs for both liquid removal in wet-gas pipelines and paraffin removal in oil and multiphase pipelines has been successfully tested offshore Brazil.

Preliminary experimental work conducted in a 4-in., 157-ft test loop that included a small-scale glass manifold and a 6-in. steel manifold proved these pigs effective. These tests evaluated almost all kinds of flexible polyurethane foams manufactured in Brazil.

Additionally, three field tests were conducted.

The first low-density foam pig operation was performed on a 127-mile, 16-in. OD wet-gas offshore pipeline in which the foam pig showed even higher liquid-removal efficiency than conventional inflatable spheres.

The second operation took place on a 6-mile, 12-in. OD multiphase production offshore pipeline, which had never been pigged during 9 years of operation, and resulted in the removal of approximately 200 tons of paraffin.

The third successful case was the cleaning of a 2-mile flexible flow line of a subsea-completed satellite well, in which the foam pigs were sent through a 2.5-in. gas-lift line, through a wet christmas tree not designed to be pigged, and then back through the 4-in. production flow line.

Although the present discussion focuses on condensate and paraffin removal in pipelines, the principles can be applied to several kinds of operations including general pipeline cleaning, product removal or separation in pipeline, corrosion evaluation, and chemical product application.

Lab test

The developed pigs are manufactured from polyurethane foam block, mainly supplied by Brazilian flexible-foam industries. The two polyurethane materials, polyeter and polyester, present significant differences in chemical and mechanical properties.

Foam density range varies from 1.1 to 2.4 lb-m/cu ft. The pigs are shaped with a special device designed and constructed by Petroleo Brasileiro S.A. (Petrobras), Rio de Janeiro.

The physical properties considered for foam selection were density, tensile strength, wear resistance, tearing resistance, resilience, elasticity modules, and type (closed or open) and size of foam cells.

Fig. 1 presents the field-scale test loop, especially constructed to evaluate performances of foam pigs. The 157-ft test loop consisted of a 4 in., Sch. 80 steel pipe, equipped with a 6-in. pig launcher, a variable diameter spool piece, a transparent section, and a tank to receive the liquid displaced by the pig.

The liquid phase was either water or light refined oil. The air was supplied by a 49,500 scfd, 200-psi maximum discharge pressure compressor.

The laboratory tests performed were for liquid removal efficiency, effect of piping reductions, and wear resistance.

Fig. 2 shows the shape, dimension, material, and type of foam of a pig used in the tests.

The liquid removal efficiency test was performed by initially putting a known volume of water in the pipe. The foam pig was then run using compressed air. The overall pig efficiency was evaluated by measuring the volume of removed water.

Table 1 shows the result of the efficiency tests.

The effect of pipe diameter reduction was quantified by measuring the necessary differential pressure to run the foam throughout the system. Visual inspection was also made to check for possible damages.

Table 2 shows the test results.

The wear resistance tests consisted of measuring the foam loss after several runs in the loop without any liquid in it (dry). The test results are as follows: total distance run, 3.7 miles; average foam velocity, 68.9 fps; final diameter/initial diameter = 0.817.

It is worth mentioning that these tests were performed using a 6-in. pig passing through a 4-in. loop, and the final pig diameter was 4.9 in. If a smaller diameter pig were used, the wear would be less.

Two test manifolds were constructed to observe the behavior of foam pigs passing through a complex piping system. Fig. 3 shows the small scale glass manifold; Fig. 4 shows a field scale steel manifold used in this work.

It was observed that the foam pig always follows the main flow stream, even through a 90 side arm tee.

Condensate removal

Several field tests were performed in three Petrobras gas pipelines. Table 3 shows the main characteristics of these lines.

In the first two cases the foam pigs presented higher condensate-removal efficiency than that of conventional polyurethane spheres.

In the third case, it was impossible to compare performance because variations in ID prevented the running of conventional spheres. The pipeline's differential pressure decreased, however, to that of a dry gas line after the pig operations.

Several types of foams were tested, and the pig wear observed in these field tests was much smaller than in the lab. This occurs because the pipeline is wet and the foam pig velocity is lower than that of the laboratory test.

Paraffin removal

Fig. 5 presents the general arrangement of the PAT-2/PXA-3 offshore pipeline, a 6-mile, 12-in. multiphase production pipeline that links two production platforms. It had never been pigged during its 9 year operation.

The pressure loss before the cleaning operation indicated a performance typical of a 7-in. ID pipeline. A measure of the free volume of the line indicated the presence of 7,168 cu ft of paraffin inside it.

A cleaning operation with the use of 18 foam spheres and cylindrical pigs, in gradually increasing diameter mode, removed approximately 200 tons of paraffin.

The volume of paraffin inside the pipeline was reduced to only 847 cu ft, and the differential pressure returned to normal. The first pig run in the pipeline was a 6 in., 1.1 lb-m/cu ft foam sphere; last was a 13 in., 2.1 lb-m/cu ft cylindrical foam pig.

Foam pigs are now used regularly in the pipeline for paraffin buildup prevention and, consequently, to maintain low differential pressure.

The RJS-274 satellite well is located in the Moreia field at Campos basin and produces through a 4-in. flexible flow line. Declining production rate of the well indicated a problem of paraffin deposition in the line.

Before the cleaning operation, the prevailing pressure drop was typical of a 2-in. ID pipe, despite its 4-in. original diameter. The use of a conventional pig in the system was considered impossible because the only access to the flow line from the wellhead side is through a 2.5 in. gas-lift line and through a wet christmas tree not designed to be pigged.

Fig. 6 shows this path.

The encouraging results shown by foam pigs in the laboratory manifold tests led to a field test being performed in the well.

Eight spherical and cylindrical foam pigs were run in sequence pushed by gas, removing a large amount of hard paraffin. The first pig run in the flow line was a 3-in., 1.4 lb-m/cu ft foam sphere and the last was a 6-in., 2.4-lb-m/cu ft cylindrical foam pig.

After the operation, a new production test indicated a significant increase in the flow line performance from 2 in. to 3.6 in.

Foam pig receiver

Reception of low density foam pigs requires special attention because they differ significantly from conventional ones:

• A special pig holder must be placed in the pig receiver (Fig. 7);

• The pig must be cleaned immediately after receiving because iron sulfide in foam can generate spontaneous combustion after contacting air.

  The low density foam pig technology developed, tested, and currently in use by Petrobras allows:

• Condensate, paraffin, liquid, and product removal from pipeline system with significant changes in pipe diameter

• Pigging of a complex piping system

• Pigging of a pipeline subject to severe clogging by paraffin

• Drastically reducing the cost of pig operations and pipeline offshore installations. The cost of condensate removal from a wet-gas pipeline decreased 100 times with the use of low density foam pigs.

Acknowledgment

The authors thank Petrobras for permission to publish this article and all Petrobras colleagues who contributed to and supported this research.

Bibliography

Lima, P.C.R., "Liquid Removal Process in Pipelines through a Moving Piston," U.S. Patent No. 5,389,155; 1995.

Wheaton, E.O., "Method of Cleaning Pipe Lines," U.S. Patent No. 2,906,650; 1959.

Girard, H.J., "Foamed Plastic Pig for Pipelines," U.K. Patent No. 1,270,378; 1972.

Knapp, K.M., "Method and Apparatus for Removing Paraffin from a Fouled Pipeline," U.S. Patent No. 5,032,185; 1991.

The Authors