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Avoid Casing Pipe for better Corrosion Protection andEconomy

Anup Kumar Bishayee

Abstract

Using of casing pipe over the carrier pipe is long standing at a crossing of

roadway, railroad for transportation of fluid through pipeline. This allows not

only an extra strength to the carrier but also facilitate replacement of carrier

when required. With constant evolution of the technology a fact confront with

the aged conception of using casing that the Cathodic Protection, a system

which opposes corrosion in underground steel pipe, resisted at casing. In view

of corrosion engineering the casing pipe creates a shielding to the carrier and

the Corrosion current affects carrier pipe in a significant manner. Now-a-days

a various methods develop to protect pipeline from external damages and

using those techniques pipeline can be strengthening without using casing. It

is very difficult and much costlier to assess a cased pipeline over an uncased

pipeline. This evolution raise an argument, if an uncased pipeline crossing

meets the entire requirement then why goes with casing, where more cost

involvement, more technicalities, more difficulty to maintain involve? Pipelineoperators, investors, contractors and corrosion engineers are trying to array

their logic to resolve this argument.

Biography:

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Avoid Casing Pipe for better Corrosion Protection and Economy

Cased pipeline crossings pose particular challenges, not only because the carrier pipes within thecasings are vulnerable to corrosion, but because assessments are hampered by the difficulty and

high cost of accessing these pipelines compared to those that are not cased. Tony Keane,

Executive Director, NACE International.

Casings have historically been used at road and railroad crossings to accommodate higher dead

loads (overburden for deep pipe) and live loads (traffic). They also help to protect from third-party

damage of the pipeline. The legacy reasoning was to provide the capability to remove or replacethe carrier pipe without disturbing the roadway or railroad. In actual practice, this is not widely

attempted. Greater strength or wall thickness, concrete coatings and other methods provideprotection to the pipe from mechanical damage from the external load. Higher depth may alsoobtain by Horizontal Directional Drilling (HDD) technique.

Advantages of Cased crossing:

1. Mechanical protection for carrier pipe from external live and dead loads.2. Future removal and replacement of carrier pipe.

3. Frost-line insulation from transported commodity in temperature sensitive soil.

4. Sub base and crossing protection in the event of a carrier pipe leak.5. Protection from third party damage.

Disadvantages of Cased crossing:1. Higher cost to owner due to the following requirement:

Larger bore-hole for casing

Two installations (one for casing and one for carrier pipe)

Insulators and spacers between carrier and casing pipe

End seals

Installations and maintenance of casing vents

Anup Kumar Bishayee, SCC, DEE, B. Tech. (Electrical), engaged since1996 with High Pressure Gas Transmission pipeline constructiondepartment of Greater Calcutta Gas Supply Corporation Limited, a StateOwned organization. He took part in a number of successful pipelineprojects conducted by the corporation. Sri Bishayee is AssistantSecretary of Indian Institute of Gas Engineers. Coal Bed MethaneProduct Water and LPG Reticulate System are his two publicationspublished in the International Seminar of IIGE and in the NationalSeminar of Institute of Engineers (India) respectively in the year 2004.

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Annular space grouting

2. Potential shielding of pipeline Cathodic Protection system.3. Potential shorting of pipeline Cathodic protection system.

4. Potential exposure of carrier pipe to corrosive atmosphere due to condensation inside

casing.

Threat towards Insulation:

It is necessary that the casing pipe should be electrically insulated from the carrier pipe for

adequate cathodic protection of the latter. The carrier pipe to soil potential should differs with thecasing pipe to soil potential is a qualitative indication of satisfactory insulation between the two

pipes. If both the potentials indicate same and varying similarly as the varying impressed potential,

a short circuit is indicated.

The short circuit between carrier and casing pipe diverts cathodic protection current from its

intended path by involving a metallic shield over the carrier. Cathodic protection current then is

able to flow straight through the casing walls to those portions of the carrier pipe in contact with

any electrolyte inside the casing. Natural earth movements caused by freeze/thaw, heating/coolingand wetting/drying, settlement due to construction of the crossing along with liquid temperature

cycles within the pipeline and superimposed load cycles from trains, cause the pipe and casing tomove differentially. These movements over time, despite the best efforts to insulate them from

each other, often cause the pipe and casing to touch one another causing a short. This short

grounds the cathodic protection system and eliminates the protection, allowing the pipe to corrode.In extreme cases this differential movement can cause significant stress on the carrier pipe at the

casing end. While it would seem easy to say; when it shorts fix it, the problem is that some shorts

are intermittent, may be in the middle of a crossing, are difficult to excavate and combined withthe fact that there are hundreds of thousands of pipeline casings you begin to understand the

problem.

In many cases a pipeline casing is sealed at each ends to prevent groundwater infiltration or flow.When equipped with required vents, the casing will contain water after a period of time because

the pipeline will be relatively cold and will condense moisture from the air. This presents a

problem of atmospheric corrosion that the cathodic protection will not contend with.

The reason that the cathodic protection system is ineffective within a casing is that the metal

casing pipe shields the carrier pipe from the protecting cathodic protection current. Also because ofthe casing, determining what the status of the pipeline within the casing is difficult to conclude

without running a smart pig, which may or may not be possible due to the pipelines design.

Remedy:The status of insulation at cased crossing should be measured at each annual survey. If it is not

possible to keep it insulated, steps should be taken to eliminate the conditions conductive to

corrosion on the carrier pipe within the casing. Filling the annular space between casing and thecarrier pipe with casing compound (greases containing chemical inhibitors) or unrefined petroleum

(low in sulfur content) will stifle any corrosion tendency.

Threat towards Coating:

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Acidic and near neutral pH environments that develop under disbonded shielding pipeline coatings

can lead to corrosion and possibly environmentally assisted cracking. One may realize, however,

that as the coating deteriorates and as its permeability to O2 increases, the corrosion rate deep inthe crevice (or in the blister) could become substantial as CP is shielded there. Generally, any

changes in the properties of a coating are considered as a coating failure. Soil stress and other

mechanical damage can also create serious disbondment problems with pipeline coatings. Damageof the coating due to these stresses may lead to pipeline corrosion failure and costly repair.

Remedy:In the cased piping the fail-safe technology of coating may apply which allows higher CP current

and higher protection also. The geo-textile mesh-backed (GTMB) tape system of Fusion bond

Epoxy (FBE) system is the fail-safe coating system where the electro-chemical reaction at the

cathode surface cause the electrolyte to become more alkaline (Higher pH value about 0.9) aroundthe cathode and will reduce or stop corrosion.

Key Challenges:The key challenge in assessing cased pipelines is that the Direct Assessment (DA) method to

determine the condition of pipeline with respect to External Corrosion, Internal Corrosion and

Stress Corrosion Cracking, are not effective if there is no electrical path. But even if an electrolyte

is introduced into the annulus, the casing acts as a shield, precluding meaningful results about thelevel of cathodic protection or coating condition. This is a very significant issue against cased

crossing because DA is an indispensable tool for pipeline assessment that cannot be assessed by

Hydro testing or In-line Inspection (ILI).

Because the pipeline will be in atmospheric and immersion service where the coating is not only

the first line of defense but sometimes is the only line of defense.

The design, installation, maintenance, repair and monitoring should be conforming API RP 1102,

NACE SP0200, hydro testing, ILI should conforming ASME B31.8S and API Standard 1163.NACE SP0502 conforms the External Corrosion Direct Assessment (ECDA) method.

Conclution:

Recent and historical results have shown that cased pipeline present a very low but not negligiblerisk of corrosion failure. The cost associated with assessing cased pipeline is far outweighing the

cost of assessing uncased pipeline when the risk of corrosion is often lower. Though the cased

pipeline have been safely in service for many decades, but considering its additional design andconstruction cost, additional maintenance and assessing cost and moreover considering the

additional loads on Cathodic protection system, casing should simply be avoided. In some

conditions where casing could not be avoided due to low depth, local regulations etc., the use ofthe latest standards and technologies, especially regarding the coating system of the carrier pipe

must be followed.

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Acknowledgement:

1. Cathodic Protection by A.W. Peabody

2. ASCE Manuals and Reports on Engineering Practice3. Coating used in conjunction with Cathodic Protection- Technical Committee

report, NACE International Publication, 6A100, July 2000.

4. Protecting pipelines at crossing: are casing obsolete?- Discussions with a panel ofNACE International experts.

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