A sticky problem

PPSA member John Smart of John Smart Consulting Engineers in Houston addresses the challenge of sediment in oil and gas pipelines.

Black powder, or sediment in pipelines, is generated by corrosion reactions in the pipeline and can either accumulate in the line or move from fluid drag in pipelines once the velocity reaches a certain minimum value. Or, black powder can be swept clean by pigging. Knowing the velocity required to move loose powder can help plan the cleaning of a pipeline. This helps to prevent sticking of pigs by solids accumulation in front of a pig. The bed height to which black powder can accumulate under existing flow conditions can help plan the amount of debris expected to be removed from a line and therefore the method of cleaning. Particulates consisting of iron oxides, sulfides, carbonates, sand, salt, clay, asphaltenes, etc. can be generated in a pipeline if water exists in the line and the water can wet the walls of the pipe. Corrosion can also occur in nominally dry pipelines under upset conditions and during hydrotesting.  Solids will accumulate as sediment if the velocity is not high enough to drag the particles along or if they are “glued” in place by hydraulic oil or corrosion inhibitors, or form crusty deposits.  Sediment deposits will cause increased pressure drop through the pipeline and can lead to blockage of the line, especially during pigging. Build-up of solids in front of a pig can cause the pig to become stuck. For a 12-inch pipeline with ½ inch (1.25 cm) deep deposits in the bottom of the line only, the volume of solids is about 22 ft3 per mile, weighing over 4000 pounds per mile (1130 Kg/KM). If the depth of the deposit were one inch, the weight of iron oxides/sulfides would be over 18,000 pounds per mile (5000 Kg/KM). Build-up of solids in front of a cleaning pig can cause a plug to develop in the line, which can stick the pig.

Planning ahead
Knowledge of whether or not the flow conditions can sweep solids through a pipeline ahead of a cleaning pig is thus of great importance when using pigs to clean a line, and of fundamental importance in designing the pig cleaning process. Sticking a pig in a pipeline is a great problem on land, but absolutely cannot be tolerated in offshore lines. In deep water, it would be extremely difficult to handle these solids accumulations and would likely require replacement of the line if they could not be removed. Furthermore, when an offshore platform loses its sales line, the platform must be shut in, cutting off production. Pipelines must be cleaned for a number of reasons, including the need to maintain operating efficiency, to be able to successfully run ILI tools, and to distribute corrosion inhibitors over the entire internal wall of a pipeline in wet multiphase gathering systems.  Solids can include produced sand, corrosion products such as iron carbonate, iron oxides and iron sulfide, metallic iron, weld spatter, salt, asphaltenes and scale. The term “black powder” is a good generic name for these solids, as sometimes they don’t seem to have a lot in common other than being black. When a disc, scraper, or wire brush pig is run inside a pipeline, solids are scraped off the wall of the pipe and pushed in front of the pig. Whether or not these particles fall to the bottom and stop moving or are swept along by fluid velocity determines the extent that of solids will accumulate ahead of the pig. 

Water velocity to move sand
The velocity to move rounded sand particles with a shape factor of 1.5 in water at 60oF is shown in Figure 1. Pipe diameter is given in inches. The velocity required depends upon pipe diameter but is fairly constant for sand particles greater than about two-mils (50 microns) effective diameter. The meaning of this velocity is that for a pipeline being pigged, if the fluid velocity is greater than that shown, particulates in front of a pig will not pile up to form blocking deposits.

Figure 1. Velocity to move Sand particles in Water Iron Compounds in 20oAPI Oil

 Figure 2. Velocity to move iron compounds in 20oAPI oil

The calculated velocity to move iron compounds in 20oAPI oil by velocity is shown in Figure 2. Oil velocities from 10 feet/sec to over 15 feet per second are required, plus the 10-15% safety margin for up-hill flow. Some types of pigging programmes designed to accommodate solids is still desirable. These include:
• Progressive pigging
• Use of fluid by-pass and jetting in front of the pig
• Use of gel pigs to suspend the particulates depending on how much solids is expected to be in the line
• Use of less aggressive cups and discs on the pig

Other operating parameters such as how long the pipe can remain out of service are also important factors. Use of heavier more viscous oils will result in much longer particle settling times back to the bottom of the pipe if stirred up by turbulence in front of the pig.

Iron Compounds in No. 2 Diesel
Diesel is sometimes used for cleaning pipelines as the compound is relatively inexpensive and available, and much of it can be recovered or sold. The velocity for entrainment of the same iron compounds is shown in Figure 3, with velocities similar to 20oAPI oil. In eight-inch lines, 1 mil (40 micron) particles will move at 1.9 feet/sec., and at 6.1 feet/sec for five-mil particles. For 18-inch lines, one-mil particles will move at 3.8 feet/sec., and 8.6 feet per second for five-mil particles. 30-inch lines require 5.8 feet per second for one-mil particles and 10.3 feet per second for five-mil particles.

Figure 3. Iron Compounds in No. 2 Diesel. The properties of the diesel used for calculations were 32oAPI diesel at 60oF, with a viscosity of 200 cp.

Lines using No. 2 diesel should also be treated with caution as larger particles will collect in front of the pig and will need to be pushed by it. This means that diesel is not a particularly good fluid to use when cleaning very dirty pipelines containing black powder. Also, with the safety margin added in for up-hill flow, these numbers may be a bit optimistic. Therefore, by-pass pigs with jetting nozzles, progressive pigging or gel pigging is more conservative for lines with iron oxides, sulphides and carbonates in them.

Black Powder in Natural Gas Pipelines
Figure 4 shows the velocity to move black powder in natural gas pipelines operating at 60oF and 1000 psi. Gas velocity is 10.4 feet per second in eight-inch lines, up to 13.6 feet per second in 30-inch lines. Natural gas pipelines can vary in their flow velocity depending on the season, and may cycle above and below this number. If the pipeline was operated in a corrosive condition, even just enough to cause mill scale to flake off the inside wall, large amounts of black powder could result that could possibly be delivered to customers, or, at the least, require substantial filtering before delivery. These results explain why gas pipeline compressor stations need to have filters installed in front of compressors, to catch black powder coming down the pipeline. Pipeline operators report that as black powder is transported down a pipeline, it fractures and becomes very fine, with a size in the range of one micron. Extremely fine powder like this can pass through normal pipeline filters. They also report that extremely fine powder such as one micron can be tolerated by reciprocating compressors, but can damage turbine compressors, even with filters in place.

Figure 4. Black Powder in Natural Gas Pipelines

These velocities also illustrate the cleaning requirements when crude oil pipelines are converted to natural gas service. One technique to keep the amount of black powder flowing down a pipeline is to treat the line with corrosion inhibitor. Corrosion inhibitors put a tacky film of the wall of the pipe to which the black powder sticks, making it immobile.

Figure 5 shows the effect of pipeline operating pressure on the velocity required to move 1 mil diameter and finer black powder particles in an eight-inch pipeline

Figure 5. Velocity to move black powder particles in an eight-inch pipeline as a function of line operating pressures

Challenges ahead
Conversations with pipeline operators reveal that when pipelines are cleaned for such purposes as ILI inspections, using pigging with and without chemical cleaning initially results in a clean line. However, after some time, black powder is usually found coming down the line again when the lines are dry. Further, many gas pipelines are not dry, but contain compressor lubricating oil and possibly other liquids such as glycol which can adhere solids to the wall of the pipe and keep them from moving. One common technique to keep black powder from moving is to treat the line with a corrosion inhibitor. Inhibitors are tacky compounds and can “glue” black powder in place.

Further, operators report, when black powder solids move down a pipeline, they tend to fracture and become very fine, in the range of one micron or less These particles are more easily moved down a line. One advantage for fine powder in pig cleaning is that it settles much more slowly than coarse powder, and can be kept fluidised more easily by turbulence. However, it is much more difficult to filter.

The most important problem with black powder movement is that it can destroy equipment such as gas compressors and turbine generators by the abrasive action of the particles on the equipment.