Comparing Speed-Control Pig Cleaning With Hard-Bodied Cleaning Pig

June 2014, Vol. 241, No. 6

Nigel Money, Technical Director, Pipelines 2 Data Ltd., UK, Gary Smith, President, Inline Services Inc., USA and Andrew Pulsifer, Enable Midstream Partners L.P., Houston, TX

The Southeast Supply Header (SESH) is a 42-inch and 36-inch natural gas pipeline originating at Carthage, TX and running 446 miles to a terminus at Coden, AL. SESH links the onshore natural gas supply basins of East Texas and northern Louisiana to markets predominantly served by offshore natural gas supplies from the Gulf of Mexico.

Enable Midstream cleans these pipelines annually to enhance gas quality, maintain throughput efficiency, remove potentially deleterious liquids and debris, reduce the risk of internal corrosion and ensure the best performance from inline inspection (ILI) inspection runs.

It is commonly accepted in the pipeline industry that ordinary cleaning pigs do not effectively clean a pipeline when run in many elevated gas flow volumes and velocities. Gaslines are prone to velocity surges or speed excursions due to the compressibility of gas. When run at higher gas flow velocities, cleaning pigs tend to hydroplane over liquids, leaving liquids and debris in the pipeline.

This viscous and dynamic action tends to increase with the speed of the pig. At elevated pig speeds, hydroplaning often occurs, consequently leaving potentially significant volumes of liquids in the line. In order to be effective, cleaning pigs should be run at a constant predictable lower speed. The industry-accepted optimal pipeline cleaning pig speed is less than 10 mph.

There is also concern that at elevated speeds, cleaning pigs tend to jump over or across internal circumferential girth welds and spiral weld areas, thus leaving residual liquids and debris in the pipeline at the weld areas. Therefore, it is necessary to decrease the speed of cleaning pigs by reducing gas flow in order to achieve maximum cleaning effectiveness.

Consequentially, reducing gas flow and throughput while cleaning a pipeline, can result in a significant loss of gas flow revenue for a pipeline operator as well as disrupt gas delivery reliability to the downstream client base.

A speed-controlled pig (SCP) was developed under a joint technology development agreement among Enable Midstream Partners, Inline Services and Pipelines 2 Data Ltd. (P2D) to provide a tool that could effectively clean pipelines without the need to sacrifice gas delivery or losing transportation revenue by reducing normal gas flow volumes or gas velocity.

The SCP tool was designed to provide a method to safely bypass natural gas while controlling tool speed. The tool can clean effectively at speeds of 6-10 mph while the natural gas velocity in the pipeline continues flow rates between 15-35 mph.

The SCP tool is equipped with a number of features, such as a transmitter for ease of tool tracking, and dual odometers for accurate distance measurement to points of interest. A logger records pipeline slope, pressure, temperature and tool orientation, while an inertial measurement unit (IMU) detects roll, pitch, yaw and vibration.

Pressure differential is continuously recorded and can be used to calculate drag and the condition of the internal surface of the pipe; the presence of liquids, debris and black powder can be inferred from the changes in ride characteristics, tool dynamics and drag.

With recent changes in the gas market, and as production facilities and pipelines age, operators are required to handle more liquid-laden gas in pipelines. It is widely recognized that more liquids and debris are being delivered in gas from producers.

Liquids and debris tend to build up in low spots in a pipeline. The IMU shows the location, angle of slope and the extent of elevation change in a pipeline. The slope data is valuable for locating low spots in a pipeline or profiling elevation changes such as a river crossing where liquids may tend to reside. Removing these liquids and debris from pipelines can enhance gas quality, improve pipeline operation efficiency, prevent internal corrosion and reduce the risk of failed ILI inspection runs.


Figure 2: An SCP profile of data from a 5,532-feet river crossing on a 42-inch pipeline.

The SCP tool is also equipped with powerful neodymium iron boron (NdFeB) rare earth magnets, which gather and remove magnetic types of metallic objects, welding debris and black powder.

Deformation and debris-monitoring sensors can also be mounted on the SCP tool, the latter of which can be used to help locate where problem inputs may be contaminating a pipeline with deleterious solids. With certain adaptations, the SCP tool is capable of recording actual parametric conditions, in situ sampling of the composition of the gas and liquids to help determine the possible presence of microbiologically influenced corrosion (MIC), evaluating the internal surface condition of the pipe and collecting other relevant data to determine the actual areas that may be affected.

Comparison Run
In October 2012, Enable ran a comparison of the performance of the SCP cleaning tool vs. a standard hard-bodied cleaning pig in the Panola-to-Vernon section of its 42-inch CP pipeline.

This section of pipeline is 93.2 miles long. It crosses the Red River and the Pierre, Saline, Bistineau and Black Lake bayous, a number of perennial streams and many other intermittent streams, ponds, lakes and wetland areas. On construction, the potential effects to sensitive water bodies were avoided through the implementation of horizontal directional drilling (HDD) installation techniques.

While most of the terrain is gently rolling, there are some steep inclines in and out of ravines and under streams. There are seven main line valves spaced out along the line. The line had previously been cleaned with the SCP tool in December 2011 with gas flow velocity running as high as 24.5 mph in this pipeline section.

On Oct. 9, 2012, the SCP tool was run in the Panola-to-Vernon 42-inch pipeline section. Although cups and discs on the SCP tool can be mounted in any number of preferred cleaning configurations, in this case the tool was configured with three dimension-changing (DC) cups, two seal discs and a heavy-duty full encirclement brush for maximum cleaning capability.

In this instance, the SCP instrumentation was set to control the cleaning tool speed in the range of 6-8 mph. Tool launch from Panola was accomplished with a gas flow volume of 1.046 Bcf and a pressure of 1,070 psi, producing a starting gas flow velocity of 12.3 mph. Additional gas flow volume came on stream, rising to 1.563 Bcf and providing gas flow velocity of 22.03 mph for the majority of the run.

Actual gas flow velocity depends on gas flow volume, pipe inside diameter, pressure, temperature, specific gravity, compressibility, efficiency factors and operating conditions. Average gas flow velocity calculations in this pipeline section are based on the thinnest (0.427-inch) pipe wall. Actual gas flow velocity was higher in all heavier wall thickness areas which in this case are comprised of 0.500-inch, 0.562-inch, 0.617-inch, 0.625-inch, 0.750-inch and 1.000-inch sections. The pipe is double submerged arc welded line pipe.

Tool speed depends on gas bypass, tool dynamics, drag, the extent and lubricity of the liquids, the extent and mass of debris, pipeline configuration, slope or elevation changes, internal wall conditions, coated or uncoated pipe, girth weld or spiral weld caps and spiral welded pipe. Drag also relies on the tool weight, the number, design, physical dimensions, stiffness and hardness of cups, discs and brushes, and the extent of cup, disc and brush wear.

Drag is calculated using the recorded pressure differential across the tool. Black powder in the pipeline, particularly when very dry, can cause a high coefficient of friction, which can change drag, tool dynamics and speed behavior and cause excessive cup and disc wear which can further exacerbate irregular tool movement.

The SCP tool was closely tracked by Enable field crews and the supervisory control and data acquisition (SCADA) system as it traveled from launcher to receiver. The SCP tool speed was controlled to an average of 7.85 mph. The tool demonstrated excellent speed control over the full gas flow volume range for the duration of the run.

The actual amount of gas bypassed by the SCP tool will depend on pressure differential across the tool, drag and the other factors affecting tool dynamic behavior. It is estimated that 64% of the natural gas was bypassed by the SCP to achieve a uniform cleaning speed in this pipeline section.

The tool was safely brought into the trap at less than 7 mph and was received in excellent condition.

Cleaning results: The SCP tool brought in about 30 gallons of liquids measured into the Vernon storage tank. When the trap was opened, additional liquids were present in the trap that could not be measured; some liquids had exited through the side opening 12-inch receiver bypass line. There was also significant magnetic debris on the tool’s high-strength magnets.


Figure 3: Debris on the SCP tool.

Figure 4: Metallic objects, welding debris and black powder buildup on the high-strength magnets.

On Oct. 10, 2012, a standard hard-bodied cleaning pig was run in the same Panola- to-Vernon section. The device was difficult to track at the normal pipeline gas flow speed. Gas flow velocity during this cleaning pig run was reduced by Enable to an average of 16.9 mph. The highest calculated gas flow velocity was 17.4 mph. The average speed of the hard-bodied cleaning pig for the run was 17.16 mph. The pig arrived at the receiver at speeds in excess of 17 mph. The pig was in good condition.

Cleaning results: The standard hard-bodied cleaning pig brought in about one quart of liquids and no debris.

Conclusions

1. A standard cleaning pig when run at high gas flow volume and high velocity may not thoroughly clean the pipeline.

2. SCP cleaning tool performance is proven when compared to regular cleaning pigs.

• The SCP tool is a more effective pipeline cleaning and maintenance tool at elevated gas velocities/volumes and demonstrated a superior ability to clean pipelines because it runs at speeds friendly to cleaning.

• Liquids and debris movement is enhanced by the lower speed of the SCP tool, which does not skate (aquaplane or hydroplane) over liquids, leaving deleterious debris and liquids in the pipeline.

• The SCP tool reduces the effect of cups or discs jumping over girth welds, which occurs at higher speeds; more tool weight means the tool cleans better, and the tight seal design of multiple cups, discs and brushes seal and clean better.

• Gas movement through the speed control bypass vanes on the front of the tool creates turbulence immediately in front of the tool, thus creating additional drag to help slow the tool speed.

• All of the bypass gas flow is jetting out in front of the tool and aimed at the pipe wall. The resulting turbulence provides excellent entrainment velocities and keeps liquids and debris in suspension and moving out ahead of the tool to the receiver.

• Unwanted liquids are delivered to the receiver for sampling and analysis and not left in the pipeline.

• Powerful NdFeB rare earth magnets gather welding debris and metallic objects.

3. Gas Flow Delivery

• The SCP tool allows the pipeline operator to maintain full gas flow volume and velocity while more effectively cleaning the pipeline

• Typical gas flow delivery comparison can be shown in the Enable 93.2-mile, 42-inch natural gas pipeline project. When using a standard cleaning pig, it would have been necessary to reduce the 22-mph gas velocity by reducing gas flow from 1.563 Bcf/d to about 0.667 Bcf/d to achieve the desired cleaning effectiveness at the reduced cleaning pig speed of about 7.85 mph.
By using the SCP cleaning tool, it was not necessary to reduce gas flow in the pipeline to achieve optimum line cleanliness. The equivalent 7.85-mph tool speed was achieved without reducing gas flow. This resulted in the ability to continue to deliver the additional 0.896 Bcf/d for the duration of the pig run, or 11 hours, 53 minutes. By using the SCP tool an additional 0.443 Bcf was available to be delivered to the customer.

4. Fuel Costs

A clean pipeline delivers natural gas more efficiently; less horsepower is required to move an equivalent volume of gas. Additional throughput is achieved in a clean pipeline per compression horsepower required, thereby resulting in considerable savings in fuel costs

5. Additional Savings

• If we assume that cleaning a pipeline has a certain real value or cost, and by comparison, if cleaning the pipeline at normal high gas flow volumes and high velocity with a standard cleaning pig only achieves 50% cleaning efficiency, the operator would need to run the pipeline at least twice with a standard cleaning pig for it to come close to achieving 100% cleaning efficiency.

However, the operator would never get to 100% cleaning efficiency at normal high-velocity pipeline flow speeds with a standard cleaning pig. Therefore, real savings arise in only having to run the SCP tool one time to achieve maximum effective cleaning.

• The SCP tool provides optimum customer satisfaction with higher quality gas delivered.

• Reduces maintenance costs, such as filter replacements, saves money.

• The process helps avoid ILI re-run charges.

6. Standards of cleanliness and assurance of ILI piggability

• There is no standard for “when is a pipeline clean.”

• The SCP cleaning tool removes contaminants and debris – the ideal breeding ground for conditions leading to internal corrosion.

• The SCP provides assurance the pipeline is clean enough for ILI integrity management surveys. It not only cleans the pipeline but also helps to prevent failed ILI tool runs by finding problems ahead of time.

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