A successful cleaning chemical formulation process for hydrocarbon transportation pipelines has been developed and applied in pipelines in the U.S. Case studies are included in this article.
An elaboration on the research leading to the new product and the laboratory testing that substantiated the formulation can be found in a longer paper presented by the author at the PPIM 2011 Conference conducted in Houston in February.
Internal pipeline deposits are a common observation. These deposits can restrict flow to the extent where process shut-downs and offline cleaning programs become necessary. A plethora of solids can be found in pipelines and can originate from a variety of sources with different formation mechanisms. One method of characterization of the solids utilizes the media that the pipeline carries as the classifier. It should be noted, however, that this is just a general rule and is governed by the fact that very few pipelines actually transport 100% of any single phase. For example, practically all oil export pipelines transport associated water between 0.2 and 5.0% by volume. This small amount of associated water can result in deposits such as mineral scales even though the dominant phase is hydrocarbon and would not be associated at all with such deposits. The general rules however follow:
• A crude transportation pipeline becomes fouled with organic scales. Crude oil contains asphaltenes, naphthenic acids and paraffinic components, all of which during transportation can precipitate and adhere to pipeline walls (Craddock et al., 2007).
• A gas transportation pipeline can also form organic scale deposits, condensed from gaseous state. Exotic mineral-based scales may also form of zinc, lead, arsenic or mercury. Furthermore, if the gas is even slightly wet and sour conditions are present, sulfide scales will form (Nasr-el-Din et al., 2001).
• Water transportation pipelines most commonly form corrosion deposits – e.g. iron carbonate (FeCO3), iron oxide / hydroxide (FeO, Fe2O3, FeOOH) and iron sulfide (FeS, FeS2 etc). A key factor of iron sulfide scales is that they are oil wetting (oleophillic) and when even low concentrations of liquid hydrocarbon are present in the water then a mixture of iron scale and hydrocarbon scale results (Wylde and Duthie, 2008).
• Multiphase fluid and gas transport pipelines can have a combination of all the deposits described above. These deposits can often be the most compositionally complex resulting in complex removal and mitigation solutions.
The most common method of cleaning pipelines is through pigging. This can be performed while the pipeline is either online or offline. Recently, focus has been made on chemically-assisted pigging operations, as it has been recognized that in the absence of chemical additives, both inorganic and organic scales can become compacted within the pipe, which can restrict the lateral movement of the pig. The addition of chemical surfactants assists in breaking up, softening and transportation of these adherent deposits (Cordell and Vanzant, 2003).
A literature search yielded very few results regarding compositional information of commercially available chemicals used during chemically-assisted pigging operations – only references pertaining to very specific deposits such as paraffins could be found (Poole et al., 2008). A review of oil degreasing and solid removal technology yielded several parameters that can be deemed essential to efficient chemical cleaning during pigging operations (Buzelin and de Campos Lima, 2008; Javora et al., 2008; Bordalo and Oliveira, 2007). The key parameters described below show the different types of surfactant properties required in an ideal cleaning product:
• Wetting: the action of a surfactant to reduce surface tension of a media. This reduction is achieved by molecular attraction towards a dissimilar surface. For pipeline cleaning, wetting agents help to remove hydrocarbon deposits from oil wet scale therefore allowing access to inorganic materials.
• Emulsification: surfactants enable the formation of a stable emulsion of two or more immiscible liquids – similar to micellular solubilization just with larger solubilized particles. During pipeline cleaning it is necessary to emulsify hydrocarbon and solid particles removed in order to prevent redeposition downstream.
• Solubilizers: surfactants that can affect otherwise insoluble materials. When a surfactant concentration is high enough, micelle structures can form which incorporates the insoluble materials and brings them into an apparent solution. The best example of this is where it is required to recombine hydrocarbon and water.
• Detergency: the ability of a surfactant to remove particles from a surface. In pipeline cleaning it is a mandatory requirement to release hydrocarbons and other solids from a pipeline wall upon wetting to promote rapid release (Thompson, 1994; Lange, 1994). Detergency is an essential component to mobilize hydrocarbon phases after wetting to remove them from the pipeline wall.
• Dispersion: surfactants that retain insoluble particles in suspension by preventing aggregation of particles with one another. Ideally, particles are small and this will lead to a more stable dispersion. Similar to emulsification this property of surfactants prevents redeposition of solid particles by maintaining them in suspension (Friedli, 2001).
When these functionalities are combined to form an ideal pipeline cleaning chemical, the resultant product could become a complex mixture of five to seven chemical components. The result, however, is a more purpose driven solution as the product gives good wetting, solvency and detergency and results in more readily freeing solids from the walls of pipelines. Surface active components provide emulsification and dispersion characteristics that render insoluble deposits and multiphase liquids as a single entity enabling more efficient transport and less secondary precipitation. It is also commonplace to add an antifoam chemical into a finished formulation as surfactants can display high foaming potential when agitated. Often, glycol or specialty antifoams are incorporated to bring this under control.
Although relatively well documented, using straightforward organic solvents for pipeline cleaning is less efficient than the previously described specialty blends. Base solvents do not provide an effective means to transport the mixed composition slurries that would result after the organic components are dissolved – this means secondary deposition is very likely. Furthermore, it is common for aggressive organic solvents (such as toluene and xylene) to be incompatible with pig bodies.
Now let us look at some case studies of successful employment of the new cleaning formulation.
Case History 1
Specialty formulation 1 was deployed to clean a natural gas transport pipeline in TX, USA. This was deployed by working in partnership with the Integrity Service Division of apipeline pigging company,
The 12-inch, 9-mile section of this East Texas pipeline had previously been cleaned with a series of brush and seal pigs. One of the pigs became lodged in the pipeline resulting in minimal fluid flow. A decision was made to use a chemically-assisted treatment in an attempt to dislodge the stuck pig.
A total of 168 gallons (4 bbl) of specialty formulation 1 was pumped neat followed by 1,680 gallons (40 bbl) of clean water. The product was pushed with a pig toward the lodged pig. The lodged pig was moved to the south then the flow was reversed and both pigs arrived at the northern junction and were removed from the pipeline. The water from the line contained large amounts of iron and hydrocarbons. When the two pigs arrived at the trap, a large amount of solids were recovered from the pipeline (Figure 1) and a small sample was obtained for analysis.
Figure 1: Photographs of pig trash and pigs after removal at the southern end of the pipeline described in case history 1.
The second stage of the treatment was a 6% concentration treatment consisting of 250 gallons (6 bbl) of specialty formulation 1 pumped in neat followed by 5,040 gallons (120 bbl) of clean water into the southern end of the pipeline. A pig pushed the product the entire 9-mile length of the line to the northern trap. The water from the line contained large amounts of dispersed solids and hydrocarbons. This was followed by the arrival of the pig bringing even more solids. A further sample was obtained for a deposit analysis.
Table 1: Quantified elemental composition of the EDX analysis from case history 1; A = first stage sample, B = second stage sample – NB: C and O were not included in this quantification.
The pig trash samples were analyzed via EDX. Summaries have been plotted in Figure 2 and the quantified compositions in Table 1. The analyses showed a dominance of iron sulfide with an associated 25 to 35% organic material. This process yielded the following conclusions: 1) large volumes of soluble hydrocarbons were dissolved and removed; 2) removal of the hydrocarbons resulted in solids mobilization. Iron sulfide scale is very oil wet and can often be combined with 25wt% or greater organic material, therefore successfully removing the oil, grease and wax will help to mobilize the iron scale (Wylde and Duthie, 2008).
Figure 2 : EDX spectra of the two pig trash samples from case history 1; A = first stage sample, B = second stage sample.
Case History 2
Three parallel gas pipelines in South Louisiana required an oil soluble product with odor control to chemically assist a pigging campaign. The pipelines were comprised of a 24-inch line and two 30-inch lines all of which were 64 miles long. A cleaning program was required prior to an intelligent pigging run as the lines had been in service for 32 years and not undergone any offline pigging. Clearly, cleanliness and cleaning efficacy were a high priority.
Aromatic solvent was used to dilute specialty formulation 3 to 10% activity and this mixture was pumped into the pipelines and pushed with a series of brush pigs at a planned 11 mph. For each pipeline, 500 gallons (11.9 bbl) of mixture was used.
Table 2: XRD analysis results on the inorganic component of pig trash in case history 2.
A significant amount of unexpected oil, grease and solids were removed from the system. A sample of the pig trash was analyzed using weight loss, EDX and x-ray diffraction (XRD). Figure 3 shows a photograph of the pig trash as it appeared in the trap. The thermal weight loss showed an organic content of 55.4% and this appeared to be heavy end hydrocarbons such as paraffins and asphaltenes. The XRD analysis can be seen in Table 2 and this shows the inorganic portion to be poorly crystalline. The presence of magnetite (Fe3O4) suggests corrosion was occurring. EDX analysis has been summarized in Figure 4 and Table 3 and this shows the inorganic portion was largely composed of iron and oxygen. As well as magnetite (identified by XRD), there could also be other iron oxides (FeO, Fe2O3) and iron oxy-hydroxide (FeOOH). Other detected minor elements included silicon (siliceous material in the form of sand, silt or clay), sulfur (suggesting iron sulfide minerals) and manganese (supports a corrosion origin for the solids).
Figure 3: Photograph of the pig trash sample from case history 2.
Table 3: Tabulated EDX results showing quantified elemental composition for the pig trash recovered in case history 2.
Figure 4: EDX spectrum of the pig trash sample after ashing.
Both arsenic and mercury were detected in an acid digestion of the sample followed by inductively coupled plasma mass spectrometry (ICP-MS). This has been summarized in Table 4 and showed the pig trash had the potential to generate arsine gas. However, this did not occur due to the use of specialty formulation 3 to assist with cleaning instead of previously more acidic products.
Table 4: ICP analysis on the pig trash sample from case history 2.
The overall cleaning operation of the pipelines ensured success of the intelligent pig run and maximized value to the overall operation.
Case History 3
This final case history subject is a U.S. West Coast offshore in-field pipeline. This was a 10-inch ID carbon steel pipeline, 9,330 feet in length. The pipeline was required to be brought back into service after being mothballed for several years. There was a legislative requirement to determine the pipeline integrity and therefore its ability to transport multiphase production of 8,000 bpd of fluid (60% water cut and 14° API oil).
Existing deposits in the pipeline had already been determined through intelligent pigging prior to mothballing, thus the cleaning campaign needed to be aggressive hence a chemically-assisted pigging strategy was devised. The cleaning campaign was applied in three stages: 1) preflush using 300 gallons (14.3 bbl) of specialty formulation 1 injected neat, followed by 5,400 gallons (128.6 bbl) of treated seawater; 2) cleaning run 1 using 3,000 gallons (71.4 bbl) of aromatic solvent, followed by 1,000 (23.8 bbl) gallons of specialty formulation 1 injected neat, followed by 9,000 gallons (214.3 bbl) of treated seawater; and 3) cleaning run 2 was the most aggressive stage and used 1,000 gallons (23.8 bbl) of aromatic solvent, followed by 2,500 gallons (59.5 bbl) of neat specialty formulation 1, followed by 22,500 gallons (535.7 bbl) of treated seawater.
The final stage was never planned to be as aggressive as it was. This was optimized in response to observations during the campaign. Massive deposits were removed during the preflush and during the first cleaning run the entire 30-foot trap was filled with trash. A further 20 foot of material had to be removed before the pig could be retrieved.
Following the final cleaning stage, smaller volumes of trash were recovered. Had chemical cleaning not been performed before running the intelligent pig, the information gathered would have not given the information necessary to satisfy the legislative requirements. Photographs of a typical pig after removal from the pipeline can be seen in Figure 5, along with a photo of the intelligent pig after it had completed its run in Figure 6.
Figure 5: Photographs of the brush pig after removal during run 2 in case history 3.
Figure 6: Removal of intelligent pig after it was run – note cleanliness.
The lessons learned from the case histories are: 1) the specialty pipeline cleaning chemicals showed high efficacy of treatment, 2) solids removal was more efficient than previous treatments that did not use chemical cleaning chemicals to assist pigging, 3) arsine and hydrogen sulfide gas generation can be controlled using specialty cleaning formulations, and 4) the specialty formulations described here are recommended for use in any offline cleaning application and have particular applicability prior to intelligent pigging campaigns.
Based on a paper presented at the Pipeline Pigging and Integrity Management Conference organized by Clarion Technical Conferences and Tiratsoo Technical and held in Houston, February 16-17, 2011.
The author: Dr. Jonathan Wylde graduated with a BSc (Hons) degree in geology (1999) and PhD (2002), both from the University of Bristol. He joined Clariant Oil and Mining Services in 2002 as a senior scale chemist in Aberdeen, Scotland. He relocated to Houston in 2007 as Technical Manager for North America. In 2009, he became the Business Manager for Canada based in Calgary, Alberta. In 2011, he relocated back to Aberdeen, Scotland where he is currently the UK Business Manager. URL: www.noram.clariant.com/, or contact the author through the agent: 713-970-2188.
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