EP0029243B1

EP0029243B1 - Method for cleaning and sanitizing the interior of pipelines

Info

Publication number: EP0029243B1
Application number: EP80107123A
Authority: EP
Inventors: Robert James Purinton, Jr.
Original assignee: Dow Chemical Co
Current assignee: Dow Chemical Co
Priority date: 1979-11-20
Filing date: 1980-11-17
Publication date: 1985-05-22
Also published as: NO157744C; NO803492L; EP0029243A1; CA1145902A; NO157744B; DE3070680D1

Description

This invention pertains to a novel method for treating the interior surfaces of pipelines using aqueous, cross-linked gelled pigs.
Pipeline efficiency and volume of product being conveyed through the pipeline can be lost by a build-up of scale on the interior surface of the pipe. Mechanical pigs and/or gelled chemical pigs have previously been used to remove scale from the interior surface of a pipe. Mechanical pigs are, normally solid bullet-shaped devices which have wire brushes or abrasive surfaces to physically abrade the scale adhered to the interior surface of the pipe. Gelled chemical pigs, on the other hand, remove surface deposits on the interior surface of a pipe by dissolution and/or by picking up loose debris as they pass through the pipeline.
In many instances, scale also contains bacteria which attack the product to be conveyed by the pipeline. For example, sulfate-reducing bacteria can generate copious quantities of gaseous hydrogen sulfide from certain crude oils, which causes severe corrosion of pipeline walls and also contaminate the product flowing in the line. Hydrogen sulfide is also a noxious, toxic gas which makes it difficult and dangerous to handle from a personnel standpoint and also from a pumping standpoint. Gases in a liquid can cause pumps to cavitate, lose prime, or to function less efficiently. Bacteria are also known to consume hydrocarbons, resulting in a loss of the product.
Because water is an undesirable foreign matter in any oil or gas pipeline, the treatment of pipelines with gelled pigs also includes their drying. One method of drying a pipeline is described by G.D.H. Crawford, Gas Journal, vol. 341, No. 5549, 282 (March 18, 1970). In Crawford, the bulk of the water is removed from a pipeline by a conventional mechanical pig and then swabbing the interior of the pipeline by passing a quantity of methanol through the pipeline sandwiched between a pair of pigs. Crawford found it necessary to use this technique to remove residual water from pipelines carrying natural gas having a high proportion of methane. Residual water was said to form hydrates with the methane under certain conditions of temperature and pressure and lead to serious transmission difficulties.
From U.S. Patent No. 4,003,393 is known a method of removing residual fluids and solids by a gel-like mass comprising an organic liquid and a metal salt of an aliphatic substituted orthophosphate ester.
From EP-A 15012 it is known to remove fluids and debris from pipelines by pushing a plug with a scraper through the pipeline.
A new method of cleaning the interior surfaces of pipelines has been discovered. The new method comprises passing an aqueous gel-like mass through the line directly driven by a fluid under pressure, the gel-like mass consisting of an aqueous, cross-linked gelled galactomannan gum, or derivative thereof.
The aqueous gelled cross-linked pigs used in this invention are superior to other pigs which utilize, for example, polyacrylamides and the like for the gel matrix. This superiority is shown in their shear stability, ease of hydration in water, and the convenience with which the gelled pigs are broken when the job is complete. This facilitates waste disposal and enhances the commercial viability.
It has also been discovered that pipelines can be cleaned and simultaneously sanitized by passing an aqueous gelled pig containing at least on bactericide through the interior of a pipeline. By the term, "sanitize" is meant that the bacteria level of the pipeline surface in contact with the aqueous gelled pig is reduced. Normally, the bacteria level is reduced essentially zero or some other very low value.
The aqueous gelled pigs of the present invention are easily formulated, are easy to use, and eliminate the need for completely filling a pipeline with an antibacterial solution and thus represent an advancement in the art of pipeline cleaning.
The present invention also resides in a method of drying the interior surface of a pipeline by sequentially passing through the pipeline an aqueous cross-linked gelled pig, a fluid mobility buffer comprising a non-crosslinked gelled alkanol of from one to three carbon atoms, and dessicating amount of a liquid alkanol of from one to three carbon atoms. The mobility buffer permits the user to derive the benefits of both the gelled aqueous pigs and a liquid desiccating alkanol.
The aqueous-based pig compositions comprise water, a galactomannan gum or derivative thereof as a thickening agent, and a crosslinker. The pig compositions may optionally contain a bactericide as an additive. Other additives may include an abrasive in solid particulate form (e.g. sand) to promote the cleaning ability of the pig as it passes through the pipeline, or other conventional additives which stabilize the pig.
Galactomannan gums and derivatives thereof are well known thickeners for water and water- based fluids. Examples of such gums include natural gums (e.g. guar gum, locust beam gum, endosperm seed gums, and the like) and derivatives thereof (e.g. hydroxyalkyl galactomannans, carboxyalkyl galactomannans, hydroxyalkyl carboxyalkyl galactomannans, and the like). These are known classes of compounds and essentially any member can be used in the present invention. The most common commeri- cally employed galactomannan gums are guar, hydroxypropyl guar, hydroxyethyl guar, hydroxyethyl carboxymethyl guar, and carboxymethyl guar gums. Because the aforementioned gums are readily commercially available, these gums are preferred thickeners, and of these, guar gum and hydroxypropyl guar gum are the most preferred. It should be noted that in some references the galactomannan gums are referred to as polysaccharide and polysaccharide derivatives. Such thickeners are normally used in amounts of from 4.8 to 18 kg/m³ (40 to 150 pounds per 1000 gallons) of water (i.e. from 0.5 to 1.8 percent by wt.). They are preferably used in amounts of from 4.8 to 9.6 kg/m³ (40 to 80 pounds per 1000 gallons) of water (i.e. from 0.5 to 1.0 percent by wt.). The actual amount used, however, can be adjusted to convenience by the practitioner.
Aqueous compositions containing the above thickeners are normally cross-linked using a polyvalent metal ion. The cross-linker is normally added as a soluble salt or as a soluble organometallic compound in an amount sufficient to achieve the desired amount of cross-linking. Borates, organotitanates, and organo-zirconium salts are commonly used. The cross-linking ability of such compounds is pH dependent in many instances (e.g. the borate systems). This factor presents a convenient mechanism for dealing with the thickened fluids in a non-crosslinked form until the properties of a cross-linked fluid are desired. In the non-crosslinked state, the thickened aqueous fluids are normally pumpable at conventional pressures. Substantially elevated pressures are required to pump the fluids in the cross-linked state.
The galactomannan gums are cross-linkers are, as noted, known classes of compounds and are disclosed in U.S. Patent Nos. 3,058,909; 3,974,077; 3,818,991; 3,779,914 and 3,696,035. Reference is also made to the disclosure in the text by Davidson and Sittig, "Water-soluble Resins" Second Edition (1968) and the test by Smith and Montgomery, "The Chemistry of Plant Gums and Mucilages", Biograph Series No. 141 (1959).
Normally, the pig is formulated outside of the pipeline as a pumpable mass and the cross-linker or cross-linker/activator is added to the pumpable mass as it is being pumped into the pipeline. This "on-the-fly" approach has several procedural advantages, not the least of which is ease of placement at convenient low pressures. In this manner, the pig forms a cross-linked gel network after it enters the pipeline and conforms to the general shape and size of the pipeline. To illustrate, an aqueous pig comprised of a borate cross-linked polysaccharide (or polysaccharide derivative) gel is a preferred pig composition where the pig may be subjected to considerable shear. Such pig formations are conveniently prepared and used by first blending boric acid (from 0.9 to 1.8 kg (2 to 4 pounds)) with an aqueous slurry or solution of the polysaccharide or polysaccharide derivative (from 27 to 36 kg (60 to 80 pounds)) to form a pumpable homogenous mass. Sufficient base (e.g. aqueous NaOh) is then metered in to change the pH to a basic pH (pH 8.5-10 normally) as the homogenous aqueous mass is being pumped into the pipeline. The desired quantities of boric acid and polysaccharide or derivative are present per 3.78 m³ (1000 galons) of water in each instance in the pipeline. The gel-time of these borate-crosslinked systems is easily adjusted by the quantity of base added (cross-linking occurs faster at higher pH values).
After the gelled pig has been formed in the pipeline, it is normally driven through the pipeline by the driving force of a fluid under pressure. This fluid may be a gas or a liquid and will vary depending upon the needs of the user. For example, if the user wishes to leave the pipeline in a dry, empty state, one would normally use a dry inert gas such as, for example, nitrogen, carbon dioxide, ethane, propane or liquified petroleum gas. If the user desires to refill the pipeline with a liquid product such as, for example, crude oil or gasoline the pig could be driven with the liquid product, so long as the product does not adversely affect the properties of the pig before its purpose was complete or substantially complete in the pipeline.
Normally the pigs are formulated and used at ambient temperatures or below and are pumped through the pipelines at pressures sufficient to move the pig at a reasonable rate. Temperatures are therefore normally below about 60°C (140°F). Pressures are normally below about 140.6 kg/cm² (2000 psig). The predominant number of pipelines will normally be cleaned at pressure less than 35 kg/cm² (5000 psig). Linear flow rates of up to about 91.5 m/min. (5 feet/second) are normally satisfactory, from a commerical cleaning standpoint Rates of from 13.7 to 22.8 m/min. (0.75 to 1.25 ft/sec.) are preferred.
The size and shape of the pipeline is basically irrelevant because the gelled pigs are able to be pumped through the pipe over long distances and their shapes will adjust to fit the size of the pipeline during use. This makes the gelled pig extremely effective because stalactites and stalagmites in the pipeline do not cause its destruction by ripping and tearing it apart as they do solid mechanical pigs.
The aqueous gelled pigs can be used alone or as an element of a pig train in the pipeline cleaning process. In the latter instance, the aqueous gelled pig is preceded and/or followed by other chemical pig segments or mechanical pigs. Such chemical pig segments could be of the same or different compositions. For example, the pig train could be formed having an aqueous gelled pig according to the instant invention as the leading segment to remove loose scale and other debris from the pipe followed by a bactericide-containing gelled pig for sanitizing the pipe. Accordingly, this combination would be very effective in cleaning as well as sanitizing pipelines. Segments of the pig train could likewise include fluids (liquids or gases) or non-cross- linked gels containing various additives such as corrosion inhibitors, and the like.
After the aqueous cross-linked gelled pig has passed through the pipeline, it can be recovered and disposed of as such or "breakers" can be added to the pipeline causing the cross-linked gelled pig to break-up and lose its structure and viscosity. As noted above, this is a very desirable property because it facilitates waste disposal. Additionally, in many instances the aqueous gelled pig is of essentially inconsequential volume relative to the volume of the "product" following it and therefore does not adversely affect the materiai which follows. For example, a few hundred gallons of a pig used according to the instant invention and discharged into the hold of a ship transporting crude oil would not adversely affect the properties of the thousands of gallons of crude oil also present in the tanker. Bactericides which are compatible with water- based formulations are likewise a known class of compounds. Typically such compounds are aldehydes, organic quaternary ammonium compounds or water-soluble salts of halogenated (particularly chlorinated) phenols. Examples of such bactericides include formaldehyde, gluter- aldehyde, dodecyl trimethyl ammonium chloride, and octadecyl tris (2-hydroxyethyl) ammonium chloride. Bactericides that are effective against sulfate-reducing bacteria are particularly useful in the instant invention because of the serious problems such bacteria can create, particularly in oil pipelines. If the particular thickener used is subject to bacterial attack, it may be desirable to also include a bactericide as a preservative for the formulated pig.
The fluid mobility buffer of the invention used for drying the interior surface of a pipe comprises a non-crosslinked gelled alkanol of from one to three carbon atoms. Preferred alkanols are methanol, ethanol, and isopropanol. Mixtures of alkanol can be used, if desired. The thickening agent for such alkanols can be galactomannan gums or derivatives thereof but are preferably hydroxy (lower alkyl) celluloses and are more preferably hydroxyethyl or hydroxypropyl cellulose. Such thickeners may be included in the alkanol in substantially any concentration that has the effect of gelling the alkanol and thereby lowering its volatility and enabling the gelled material to be pumped as a viscous slug through the pipeline. Concentrations of from 6 to 24 kg/m³ (50 to 200 pounds) of thickener per 3.78 m³ (1000 (gallons) of alkanol are normally used in making the buffer. Sodium hydroxide or other strong base can also be added to the buffer as a viscosity enhancer.
The buffer of the present invention separates the aqueous gelled pig from the liquid alkanol and prevents interfacial mixing of these two components which would destroy or substantially reduce the effectiveness of each. The gelled alkanol does not appear to cause degradation of the gelled aqueous pig, such as by dehydration, even though the gelled alkanol has the capacity to take up substantial quantities of water as it passes through the pipeline.
The desiccant used for drying the interior surface of a pipe comprises a liquid alkanol of from one to three carbon atoms. Preferred alkanols are methanol, ethanol and isopropanol. The alkanol(s) is used in an amount sufficient to dry the pipeline to the desired degree of dryness, i.e. a desiccating amount.
It is preferred that the alkanol desiccant and the buffer be the same, but they may be different at the convenience of the user. For example, one would ordinarily prefer to follow gelled methanol with liquid methanol, but it would likewise be satisfactory to follow gelled methanol with ethanol or isopropanol.
It is normally convenient to follow the liquid desiccant with a mechanical swab or with a crosslinked hydrocarbon gel, such as the gelled hydrocarbon pig described in U.S. Patent No. 4,003,393 or an ungelled hydrocarbon pig described in U.S. Patent No. 4,152,289, but the use of an inert gas is also operable.
The aqueous cross-linked gelled pig; the buffer, and the desiccant are normally driven through the pipeline by the driving force of a fluid under pressure such as hereinbefore described. If the user desires to refill the pipeline with a liquid product, the "pig train" could be driven with the product so long as there was a satisfactory interface between the product and the alkanol such that the product did not adversely affect the desiccating ability of the alkanol before the job was complete or substantially complete in the pipeline.
The following examples will further illustrate the invention.

Example 1

A 64.4 km (40 mile) pipeline was cleaned by passing through it sequentially (a) 7.6 m³ (2000 gallons) of a borate cross-linked aqueous gelled pig (pH 8.5-10) having 45.4 kg (100 pounds) of hydroxypropyl guar per 7.6 m³ (2000 gallons) water, (b) 3.8 m³ (1000 gallons) fresh water, (c) 60.8 m³ (16,000 gallons) of 15 percent hydrochloric acid, (d) 30.4 m³ (8000 gallons) of a commercial passivator and neutralizer, and (e) several polyurethane pigs for gas migration control. This train was driven through the pipeline with fresh water pumped at 340 I (90 gallons) per minute. Samples of the pig were taken as it passed through the pipeline. Data obtained from such samples showed that the gelled aqueous pig retained its integrity throughout the 64.4 km (40 mile) journey. An excellent cleaning job resulted from this treatment.
In Example 1 the pig train was driven through the pipeline at a rate of approximately 0.3 m (1 foot) per second. This is a very satisfactory rate from a commercial standpoint, but rates up to about 1.5 m (5 feet) per second have been used with success. The higher linear velocity trains normally require somewhat longer pigs or pig segments to achieve the same degree of cleaning (thought to be primarily a function of contact time) and to minimize the tendency of pig segments to mix if turbulent flow is encountered.

Example 2

The effectiveness of a bactericide-gelled pig was evaluated by passing ten gallons of an aqueous gelled pig containing 250 ppm of a commercial quaternary ammonium bactericide (Dowell M 76) through a 19.5 m (65 foot) test loop of one inch steel and a section of one inch polyvinyl chloride pipeline contaminated with river water laden with bacteria. The gelled pig was driven through the pipeline with fresh water at approximately six inches per second. The gel discharged from the pipeline and the pipeline was flushed with approximately 151 I (40 gallons) of fresh water. Samples were taken from the river water, from the gelled pig, and from the flush water. Culture tests revealed an extremely high level of bacteria of greater than one million bacteria/cubic centimeter in the river water; no bacteria were detected in the gelled pig, and less than 10 bacteria per cubic centimeter were detected in the flush water. The gelled pig was prepared by blending 27 kg (60 pounds) of hydroxypropyl guar and 1.35 kg (3 pounds) of boric acid and 250 ppm of the bactericide per 3.78 m³ (thousand gallons) of water and adjusting the pH of the solution to a pH of from 9-10 with aqueous sodium hydroxide. The system cross- linked as the pH became basic.

Example 3

Approximately 18.3 m (60 feet) of a 2.54 cm (1-inch) steel pipeline, containing 7.6 m (25 feet) of clear polyvinyl chloride sections was filled with water, evacuated with compressed air, and then dried by passing through it the following pig train:

(1) A crosslinked gelled water pig was added first. It was prepared by mixing 45.4 I (12 gallons) water, 354 grams of hydroxypropyl-guar, 16.5 g. boric acid and 350 milliliters of a 5 percent solution of sodium hydroxide in water. The sodium hydroxide was added on-the-fly as the pig was being pumped into the line. The pig crosslinked quickly (2-5 seconds) after entering the line to a firm gel.
(2) A gelled methanol pig-prepared by blending 13.2 I (3.5 gal.) methanol, 191 g of hydroxypropyl cellulose (average molecular weight of approximately 1 million) and 24 g. solid sodium hydroxide- was then charged.
(3) Methanol-56.7 I (15 gal.)

The pig train was then driven through the line at 0.3-0.6 m (1-2 feet) per second with compressed nitrogen (approximately 6.47 m³; 231 standard cubic feed used).
Visual inspection of the line prior to drying showed the walls wet with water and small puddles in low points on the line. After drying with the pig train, the surface walls had a dew point of -28°C (-19°F) as measured by the Bureau of Mines Dew Point Tester (manufactured bv Chandler Enqineerinq Company).

Claims

1. A method of cleaning the interior of a pipeline by moving a gel-like mass through the line directly driven by a fluid under pressure, characterized in that the gel-like mass consists of an aqueous cross-linked, gelled galactomannan gum, or derivative thereof.

2. The method of Claim 1 wherein said galactomannan gum, or derivative thereof, is a guar gum or a hydroxypropyl guar gum.

3. The method of Claim 1 or 2 wherein said galactomannan gum, or derivative thereof, is crosslinked with borate, organotitanate, or organozirconium ions.

4. The method of Claim 3 wherein said galaco- mannan gum, or derivative thereof, is cross- linked with borate ions.

5. The method of any one of the preceding claims wherein said gum, or dervative thereof, is present in said gelled pig in an amount of at least about 5.4 kg/1000 1 (45 pounds per 1000 gallons) of water.

6. The method of Claim 1 wherein said gelled pig comprises hydroxypropyl guar gum in an amount of from 5.4 to 9.6 kg per 1000 I (45 to 80 pounds per 1000 gallons) of water crosslinked with borate ions and buffered at a pH of from 8.5 to 10.5.

7. The method of any one of the preceding claims, including at least one bactericide in said aqueous gelled pig in an amount sufficient to substantially reduce or eliminate bacteria within said pipeline.

8. The method of Claim 7 wherein said bactericide is effective against sulfate-reducing bacteria and is selected from aldehydes, organic quaternary ammonium compounds or water-soluble salts of halogenated phenols.

9. The method of Claim 7 wherein said bactericide is selected from formaldehyde, gluteral- aldehyde, dodecyl trimethyl ammonium or octadecyl tris (2-hydroxyethyl) ammonium chloride.

10. The method defined by Claim 1 wherein said gelled pig is passed through said pipeline at driving pressures of up to about 140.6 kg/cm² at a rate of up to about 91.5 m/min.

11. The method defined by Claim 1 wherein said pipeline and its contents are at temperatures of up to about 60°C.

12. The method defined by Claim 7 wherein said pipeline and its contents are at about ambient temperature.

13. The method defined by Claim 10 wherein said aqueous gelled pig is passed through said pipeline at a rate of about 13.7 to about 22.8 m/min.

14. The method of any one of the preceding claims wherein said aqueous gelled pig is at least one element of pig train having a plurality of chemical pig elements.

15. The method defined by Claim 14 wherein at least the leading element of the said pig train is said aqueous gelled pig.

16. The method defined by Claim 14 wherein at least one of said aqueous gelled pig elements is preceded and/or followed by an aqueous gel or aqueous liquid.

17. The method of any one of the preceding claims, including the steps of sequentially passing through said pipeline:

(a) an aqueous crosslinked gelled pig,

(b) a fluid mobility buffer comprising a non-crosslinked gelled alkanol of from one to three carbon atoms, and

(c) a desiccating amount of a liquid alkanol of from one to three carbon atoms.

18. The method of Claim 17 wherein (b) is an alkanol thickened with hydroxyethyl or hydroxypropyl cellulose.

19. The method of Claim 17 or 18 wherein (c) is methanol, ethanol, or isopropanol.

20. The method of Claim 17 wherein said alkanol in (b) and (c) is the same in each instance and is methanol, ethanol, or isopropanol.

21. The method of Claim 17 wherein

(a) is an aqueous crosslinked gelled pig comprising an aqueous gelled guar gum or hydroxypropyl guar gum crosslinked with borate, titanate zirconium ions,

(b) is a fluid mobility buffer comprising methanol, ethanol, or isopropanol thickened with hydroxypropyl cellulose, and

(c) is methanol, ethanol or isopropanol.

22. The method of Claim 21 wherein (a) is a hydroxypropyl guar gum crosslinked with borate ions, (b) is methanol thickened with hydroxypropyl cellulose, and (c) is methanol