Pipeline & Gas Journal
March issue 2000:

1. Displacement of the crude oil
2. Internal inspection (i.e., smart pigging) to assess the integrity of the line
3. Internal cleaning
4. Hydrotesting
5. Mechanical modifications (i.e., stations, new valves, pig traps, repairs, etc.)
6. Internal cleaning after hydrotesting
7. Drying to a low dewpoint
8. Corrosion inhibitors
9. Commissioning with natural gas or product

To operate the pipeline trouble-free after conversion, a proper cleaning is perhaps one of the most important issues to address. If the deposits accumulated over many years of crude oil service - and flash rust produced from hydrotesting is not removed - operational problems can occur which have literally shut down the line.

Gel Pig Technology
Gel pig technology has proven to be an extremely effective method in assuring the degree of cleanliness and dryness required for a successful conversion. Gel technology can be used for efficient displacement of the crude oil, removal of the large quantities of debris typically found in crude oil pipelines, and drying the pipeline to acceptable levels. Results are attained with considerably less time and cost, since the cleaning and drying of long lines is typically achieved in just a single pass.

The use of gel technology for a pipeline conversion generally involves running a rather complex cleaning train containing special gels for sweeping out fluids, isolating stages, or removing large amounts of debris. The gels themselves can be made from water, hydrocarbons (i.e., diesel, kerosene, etc.), solvents, or various chemicals. There are often several different types of gel within a cleaning or drying train. Proper removal of any residual gel left behind by the cleaning or drying train must be incorporated into its design. This assures that there are no potential problems associated with gel residue once the line is in a new service.

It is imperative that a company be consulted that has a proven track record and is experienced in the design and execution of gel pig services for conversions. Potential catastrophic results could occur when misapplications or the proper chemistry and design are not used.

A properly designed gel pig train may employ both chemical and mechanical techniques for the removal of unwanted fluids and pipeline debris. A train typically contains several mechanical pigs specifically engineered for the task. Mechanical pigs provide scraping action to help loosen deposits from the pipe wall. Solvents, inhibitors and other chemicals are often included in the design of the train. The gel pig fluids can suspend and effectively carry large amounts of debris from the pipeline. It would take mechanical pigs or solvents alone hundreds, or even thousands of runs to remove the same quantity of debris that is removed with a single gel pig train.

A drying train can be designed which uses gel for more efficiently sweeping residual water from the pipeline. Methanol is incorporated into the train for drying the line to a targeted dewpoint in a single run. A dewpoint of -40° F or less has been achieved with this method. The drying train can be run immediately following a gel cleaning train or independently after cleaning or hydrotesting. In long pipelines, methanol can be gelled to reduce friction and wear on the mechanical pigs.
Some of the advantages of gel pig services for cleaning existing crude oil pipelines for conversion to a new service include:

• Ability to remove large quantities of debris
• Reduced risk of sticking pigs
• Efficient cleaning at relatively low velocities
• Decreased potential contamination of gas or product
• Cleaning or drying long pipelines in a single pass
• Analytical confirmation as to the degree of cleanliness or dryness
• Decreased environmental exposure - minimal launching and receiving sites
• Relatively simple disposal options
• Reduced costs
• Reduced time
• Custom design for removal of specific deposits
• Easy and effective application of inhibitor Requirements

Prior To Gel Pig Services
Prior to beginning the conversion of the pipeline, some key information is required for a proper gel cleaning and drying design. Deposits from crude oil lines have historically contained large amounts of inorganic debris (i.e., sand, millscale, rust, iron oxides, other metals and minerals), in addition to varied amounts of organic deposits (i.e., crude oil residue, paraffin, asphaltines, maltenes, etc.). The material to be removed from a specific line must be identified and quantified.

It is recommended that actual pipeline samples (i.e., approximately 2- to 3-foot sections), solid contaminants and oil samples be taken for analysis. For long lines, several samples of the pipe and deposits should be taken from different areas. The amount of debris in the line is estimated from the analysis and the composition of the solids to be removed is determined. Solubility testing is performed to determine the best solvent to remove the organic portion of the deposits, and the most effective base fluid for gels.

In attaining data, an alternative to pipeline sample analysis is cleaning a small section of the pipeline with a mini gel train. Evaluation of a proper gel pig service results in reliable information as to: (1) The quantity of debris removed from the pipeline, (2) The quantity of debris potentially remaining in the line, (3) A positive indication as to whether the line is clean, and (4) The composition of the removed deposits. The final design for the project can then be optimized based on data collected during the smaller “test cleaning”.

Case History
There have been thousands of miles of crude oil pipeline successfully converted to natural gas or finished product service in the United States using gel pig technology. The following case history describes a recent conversion which ultimately used this technique to clean a pipeline, previously in crude oil service, for trouble-free natural gas operations.

In early 1996, a major gas transmission company decided to convert an existing crude oil pipeline to natural gas service in the central United States. The pipeline was comprised of more than 900 miles of 16-, 20-, 22-, and 24-inch line. Techniques for cleaning the pipeline for conversion were evaluated, including gel pig technology. However, the owner initially elected not to use the gel pig method, based on a perceived cost saving.

The selected method for cleaning included a train comprised of mechanical pigs and solvent for removal of the residual crude oil deposits and organic debris. A single solvent train ran the entire length of the pipeline. After the initial solvent cleaning, the line was hydrotested. Upon completion of the hydrotest, the line segments were dewatered, cleaned and dried using dehydrated air.
The line was commissioned with natural gas and placed in service in August 1997. Expectations were to deliver clean, dry natural gas to several major transmission companies, reaching volumes in excess of 200 MMscfd by October 1997.

Shortly after start-up, the line began experiencing problems from excessive debris and moisture. Large filtration units were placed at several strategic locations along the line, but would quickly plug with deposits (primarily iron oxides) on a daily basis. Changing filters could take a crew up to eight hours per filtration unit.

The debris was creating erosion problems with valves, instruments and compressors, safety concerns, damage to major equipment, poor gas quality and delivery problems. On-line pigging was attempted with no success. Mechanical pigs would become severely damaged from extreme wear, while removing very little debris.

By the end of September 1997, it was apparent that it would be necessary to clean the operational gas line. BJ Process and Pipeline Services (BJ-PPS) was contracted to move ahead with the task of cleaning the pipeline operationally, using BJ Gel Pig Services. A job of this magnitude typically requires several months of engineering and preparation, but due to the emergency status, it was completed over the next three days with the job actually beginning within just one week.
The entire line was cleaned in four separate sections utilizing a complex cleaning train design for each. This included diesel-based BJ Debris Removal and Separator Gel Pig Fluids, a breaker wash, an inhibitor application and a variety of mechanical pigs (brush, cup, disk and spheres).
Due to limitations imposed by their gas deliveries, it was not possible to start at the beginning of the line and continue to the other end. Instead, the first section cleaned was in the middle of the line, and then sections on each end were completed.

Mechanical pigs had to be exchanged at several diameter changes along the pipeline. A high pressure, high volume two-phase separator was used for receiving the pig trains and diverting the fluids and solids to frac tanks. Samples of the gel pig trains were taken at the receiving sites and each transfer site for analysis. From these samples, the amount of debris removed from each section and the amount of debris potentially remaining in each section was estimated.
The total combined design value for the four trains was based on the potential removal of up to approximately 500,000 pounds of debris from the entire line. All four trains were pumped to design, by BJ-PPS, without incident. The trains, driven with natural gas on-line, were tracked and monitored via electronic transmitters and tracking personnel around the clock. The last of the four trains was received on Nov. 3, 1997. The cleaning had been completed in less than a month, including 10 days of standby due to the owner’s preparation time and their operational issues.
As the trains were received, it was obvious that a significant amount of debris was being removed. Tons of solids were removed from the pig traps as mechanical pigs were exchanged or received. The quantity of solids removed from traps with mechanical pigs pales in comparison to the total amount suspended in the gel.

Organic and inorganic deposits from the original crude oil deposits were removed from the line by the diesel-based gel. Iron oxides from the hydrotest were also removed.
Analysis of the gel samples estimated a cumulative total of 1,032,538 pounds of debris were removed from the line. The total remaining debris was estimated at approximately 53,100 pounds of debris, which was primarily located in a 151-mile section.

However, due to time constraints, the owner decided to go into service without additional gel cleaning. Based on the gel evaluation, the line was approximately 95 percent clean at this time, but the pipeline owner was aware that the line could still contain a considerable amount of debris in certain areas.

After approximately two to three weeks in service, the debris problems subsided, but the moisture content had increased in the section still suspected of containing relatively high amounts of residual debris. During the gel cleaning a significant amount of free water and methanol had been removed from the line ahead of the gel cleaning train, particularly in this section. It was originally thought that the line was relatively dry and contained no free liquids.

Remaining debris can increase the moisture content of the gas, due to water trapped in the solids.
The pipeline owner decided to make a second run, in just the section of concern, to remove the remaining debris and moisture. This included a methanol drying train immediately following the cleaning train. All other parts of the line had acceptable dew point readings and did not require additional cleaning or drying.

The second run on this section, similar in design but smaller, began on Dec. 3, 1997 and was completed on Dec. 15, 1997. The second train removed an estimated 60,124 pounds of additional debris. This amount was just slightly higher than the 53,100 pounds estimated from the results of the first cleaning train. The final analysis indicated that the line should be relatively clean, with very little remaining debris (98.7 percent clean). The methanol drying train during the second run left no significant moisture, as the drying fluid received was greater than 97 percent methanol.
The pipeline was placed back into service immediately following the second cleaning train and has been operating since with no incidents due to debris or moisture. In the first month of service after cleaning, the moisture content was well below 7 lb/MMscf, the acceptable level of moisture for marketable gas. Flow rates were quickly increased to capacity without incident.
There was no presence of residual gel or free fluids remaining in the line after the cleaning. Low points and areas of concern were physically checked confirming absolutely no accumulation of fluids.

The operational gel cleaning was extremely successful in eliminating the problems with excessive debris and moisture. However, these results could have been achieved during the actual conversion project prior to commissioning with gas, at a significant cost and time savings, by using similar techniques. This would have also alleviated many of the consequential costs and problems experienced after conversion.

Summary
As markets for crude oil, natural gas and finished products continue to change and evolve, there will be an increased trend toward converting existing pipelines for new services. The use of gel pig technology has proven to be a very effective tool for purging, cleaning, drying, inhibiting, and commissioning these converted pipelines. As the length of the pipeline increases, the gel pig method becomes increasingly more attractive than alternative methods.

A large, complex gel pig cleaning train can be used to successfully displace the existing crude oil or product and remove excessive amounts of debris in typically just one pass through the pipeline. Removal of deposits typically found in an existing crude oil pipeline must be properly addressed if the line is expected to operate without incident. This includes removal of both organic (i.e., crude oil residue, paraffin, etc.) and inorganic deposits (i.e., sand, dirt, iron oxides, sulfides, etc.).
Gel technology dramatically reduces the cost, time and environmental exposure required for completing a conversion. Unlike traditional methods (i.e., dry pigging, pigging with solvents, high velocity flushing, etc.), the gel pig method provides measurable, reliable information as to the degree of cleanliness attained. From a detailed analysis of the gel train, the amount of debris removed, the composition of the debris, and the amount of debris remaining can be accurately estimated.

Thousands of miles of crude oil pipeline have been successfully converted to natural gas and finished products service in the United States over the past decade. When properly designed and implemented, gel pig technology provides the most technically and economically feasible method to convert existing pipelines to a new service. P&GJ