Enhanced Pipeline Cleaning using Combined Mechanical and Chemical Techniques

Formation of debris (organic or inorganic scales, paraffin waxes, asphaltenes etc), or corrosion products (iron oxides, sulphide and carbonates commonly referred to as “black powder”) in pipelines can adversely impact system through-puts and increase the energy required to maintain design flow rates.

Most “pure” deposits, when considered in isolation, have readily identifiable treatment options, although a number of cleaning techniques can be considered. Some of these are identified in the following paragraphs.

1. Paraffin wax, asphaltenes, and organic solids may be removed chemically with solvents, dispersants, hot oil, and exothermic chemical reactions. Mechanical removal is done by pigging or by jetting with fluids.
2. Inorganic, acid-soluble scales may be removed chemically with inhibited acids, or mechanically by pigging, milling, or jetting.
3. Few inorganic, acid-soluble scales are removed readily by chemical means. Mechanical means include pigging, milling, and jetting.
4. Iron oxide, iron sulphide, and iron carbonates (black powder) may be removed chemically with acids or gelled fluids. Mechanical means include pigging, milling, and jetting.
5. Loose debris such as sand or well fines can often be removed with gelled fluids. Mechanical means include pigging and jetting..
6. Emulsions will generally require some form of chemical treatment since most are not removed readily by mechanical means.

It is common for a number of debris types to co-exist or to be mixed with hydrocarbons, which can complicate the cleaning and removal process.

To develop an effective debris removal program, a detailed review of the pipeline operational history should be undertaken. Chemical analysis of transported fluids, treatment programmes, and where possible, analysis of the deposit itself should be performed. Results obtained will assist in the development of a suitable removal program. Additionally, an appreciation of the system’s configuration with reference to “as built” system drawings of the pipeline and its components will also be invaluable in developing the overall cleaning program.

An often overlooked but important factor in the development of any cleaning program is the volume of debris present within the system and how this may behave during the removal process. It is not uncommon for pipeline systems to contain many tonnes of deposit, as indicated in the two case studies below. Having an appreciation of the material volume and its properties is essential to properly specify the properties of the driving and transport fluids required to effectively remove debris from the system.

Pipeline Cleaning - Mechanical
Mechanical pigs are commonly deployed into a pipeline to scrape debris from the pipewall and remove loosened deposits. Pigs can be fitted with brushes or scraper elements to agitate, dislodge, or abrade deposits adhering to pipeline walls or to dislodge debris that has accumulated in system low points. Pigs can also be fitted with magnets to assist in removal of magnetic debris such as iron oxides. Unfortunately, the capability of such pigs to transport debris tends to be limited since transport of the debris relies upon the mechanical action of the pig, which is concentrated in the immediate area surrounding the pig itself. Most debris removal by mechanical pigging results from the “bulldozing” action occurring at the leading surface of the pig.

As a pig transits along the pipeline, a plume of debris is typically pushed ahead of it. If the deposits are of a high specific gravity or the flowing product has a limited debris carrying capacity, particularly as is the case in gas pipelines, the debris settles rapidly both in front and behind the pig.

If this volume of debris ahead of the pig becomes significant, there is a potential for the pig to become stuck behind the accumulation.

Alternatively, the pig may pass over the debris accumulation as the sealing elements deflect, allowing the pig to continue along the pipeline while leaving behind a potentially significant amount of deposit.

Consideration must be given to the transport properties of the fluid in the pipeline to help ensure that any dislodged debris is efficiently carried from the pipeline system.

The success of mechanical pigging is highly dependent upon the properties of the debris in the pipeline, the pig design, and the transport capacity of the carrier/driving fluid in the line. Problematic high specific-gravity materials or deposits with high adhesive properties, or complex mixtures of emulsified solids/hydrocarbon and water are not readily addressed solely by this technique.

Pipeline Cleaning - Chemical
Chemical cleaning technology is proven and can be used to remove hydrocarbons, grease, corrosion products, and scales from pipeline systems. In subsea pipeline systems, particularly for those in deepwater where the ambient temperature is low, the effectiveness of most chemical treatments is often compromised.

Paraffin waxes can have a melting point ranging from ~36°C to approximately 102°C. Deposited paraffin can also contain gums, resins, asphaltic material, crude oil, sand, silt, corrosion products, scale, and in many instances, water. Due to the wide range of paraffin wax carbon chain lengths and the almost infinite number of impurities that can be included, the effectiveness of any chemical treatment can vary significantly from one wax to another.

Scale is a solid mineral deposit usually formed wherever water production occurs. Scale types can be divided into water-soluble, acid-soluble and acid-insoluble scales although in practice, a “pure” scale consisting of a single component is rare. Usually, scale deposits are complex and contain a mixture of inorganic components, corrosion products, as well as organic constituents. Mixing of incompatible waters or changes in temperature and pressure of the produced fluid are the most common phenomena resulting in precipitation of insoluble species and resulting scale formation.

Although true chemical solvents have limited application in the removal of some debris types, the quantity of solvent required can prove prohibitive. The use of specially formulated debris transport gels (DTG) in combination with mechanical pigs has proven to be an effective method for removing significant debris volumes from pipelines. These gels can be specifically engineered to provide appropriate transport properties based on known deposit characteristics, deposit volume, and prevailing pipeline flow conditions.

Example Case Studies
Case 1—A 438-km overland gas pipeline had been converted to liquids service. When in gas service, the line had not exhibited any debris problems. Following conversion, transported fluids were continually contaminated by fine ‘black powder’ material. Debris-removal options were evaluated. The use of mechanical pigs was considered, but ineffective transport of the debris posed pig-wear issues and the volume of black powder to be removed presented a challenge. Consequently, a hydrocarbon-based DTG was used in conjunction with the cleaning pigs.

A pig train was designed to minimise potential dilution effects from both forward and reverse bypass of fluid around the pigs while considering the need to minimise contamination of the transported products ahead of and behind the pig train.

The estimated volume of debris in the pipeline was between 70,000 kg and 175,000 kg, so the pig train was designed to carry a minimum of 180,000 kg of debris. Approximately 72,750 kg of ‘black powder’ was transported from the pipeline in one pass. The pipeline was cleaned to the required specification.

Case 2—A 300-km section of a crude oil transportation pipeline had been removed from service and left dormant for a number of years; the pipeline owner was converting the pipeline to gas service. The pipeline contained an unknown amount of crude oil, sand, silt, and corrosion deposits. The objective was to remove liquid/solid organic material and solid inorganic material, and to clean the pipeline so that it could enter gas-transportation service.

There was a concern that the liquid crude oil that had remained in the pipeline would have lost a significant amount of the lighter fractions and be in a viscous state. Additionally, the original crude oil was known to contain paraffin and asphaltene components. Significant corrosion products were also anticipated in the pipeline.

A pig train comprising nine pigs (six sealing and three brush-type) and containing parcels of DTG and surfactant parcels was designed to effect the cleaning operation.
The pig discs performed very well with minimal wear over the 300-km run and 2,000 cubic meters of crude oil was displaced from the pipeline along with an estimated 397,000 kg of debris consisting of organic solids, sand and black powder. The pipeline was cleaned to desired specifications and left in a condition suitable to accept process gas.

Conclusions
The use of mechanical or chemical cleaning techniques in isolation may not produce adequate cleaning results in pipeline systems. Where a complex mixture of materials is present, a combination of chemical and mechanical techniques can produce superior results. The use of specialised fluids with enhanced solid transport properties, specialised dispersants, or specialised surfactants capable of releasing emulsion-bound materials can vastly improve the success of routine mechanical pigging operations. Consideration to the volume and type of deposit to be removed is essential, as is a thorough understanding of the pipeline system itself.