extending pipe life

A number of factors affect the longevity of a pipeline.These include quality of construction and protectivecoating systems, cathodic protection, nature of the

environment, operating conditions, and quality and fre-quency of pipeline maintenance to name btJt a few.

No one factor influences the long-term integrity of apipeline more than the effectiveness of its coating system.Pipeline leaks, ruptures and ultimately a pipeline's integrityand useful life can be directly attributed to coating deterio-ration or failure.

In the year 2000 it was estimated that of the 2.2 mil-lion miles of pipeline in the USA, 24% was more than 50years old. This ageing of pipeline infrastructures and resul-tant coating deterioration has been the impetus for thedevelopment of methods and technologies to rehabilitateolder facilities and extend the life of existing pipelines. Newcoating systems and application techniques have now beendeveloped to enable old and existing pipeline systems tobe upgraded in a cost-efficient manner, and to the latestindustry standards for protecting facilities from externalcorrosion and environmental damage.

This article outlines the functions of a pipeline coating,factors leading to coating failure, the consequences of afailed coating system and discusses the advancementsmade in liquid coating systems and application methodsfor remediation of pipeline infrastructures.

Corrosion protection ofburied pipelinesCorrosion protection is required to maintain the integrity ofa buried pipeline system and coatings are the primary

protection for a pipeline. As a buried pipeline is subject tocorrosive attack if it is in contact with a wet environment,coating the pipeline to isolate it from this corrosive envi-ronment is an obvious approach to corrosion control. Sinceno coating system is defect free, cathodic protection isused to provide supplementary protection.

Most countries have regulations that require pipelinesto be coated and in general stipulate that a coating pos-sess the following properties:

.Electrically isolate the external surfaces of the pipelinefrom its environment.

.Have sufficient adhesion to resist underfilm migrationof electrolyte.

.Be sufficiently ductile to resist cracking.

.Resist damage due to soil stress and normal handling.

.Be compatible with cathodic protection.

.Resist deterioration due to the environment and service

temperature.Cathodic protection is fundamental to preserving a

pipeline's integrity. Cathodic protection is a method of cor-rosion control that is achieved by supplying an externaldirect current that neutralises the natural corrosion currentarising on the pipeline at coating defects. Current requiredto protect a pipeline is dependent on the environment andthe number and size of the coating defects. Clearly, for aparticular environment, the greater the number and size ofcoating defects, the greater the amount of current requiredfor protection.

Coating plays an integral part in the functioning of apipeline's cathodic protection system. Where a coating

WORLD PIPELINES NOVEMBER/DECEMBER 2002 55

extending pipe life

$/Km

60,000[G:>

50,000

System 1:1) Pipe Diameter

762mm to 914mm2) Soil Resistivity

1000 to 5000Q -cm3) Clay Soil

40,0001

@30,000

System 2:

1) Pipe Diameter762mm to 914mm

2) Soil Resistivityabove 50 0000 -cm

3) Rock, Sandy Soil

20,000

10,0001I

nl10

Atm2Current Requirement

cracking. These two scenarios could leadto catastrophic failure ot,the pipeline.

In the presence of shielding, over-the-line electrical corrosion protection sur-veys are suspect in their ability to accu-rately assess the status of corrosion pro-tection of a pipeline. Other more costlymethods such as pigging, hydrostatictesting, or discrete excavations would beneeded. Regardless of the method ofintegrity assessment used, remedial pro-grams will be required if corrosion isfound. The remedial options are generallylimited to additional cathodic protection,recoating, pipe replacement or a combi-nation of the aforementioned choices. Inthe case of a disbonded coating causingshielding, additional cathodic protectionwould not be an effective option, and thusthe only alternative is either rehabilitationby recoating or pipe replacement.

Liquid coatingsAs can be seen from the above discus-sions, in many cases recoating of existing

Figure :1.. Cathodic protection costs as a function of coating performance. facilities is the most viable option for reha-

bilitating pipelines. In some cases, addi-tional cathodic protection current is unable

to provide corrosion protection or the cost becomes pro-hibitive and will not guarantee corrosion protection to thefacility. Coating rehabilitation costs are substantially lowerthan pipe replacement and coating technology is now avail-able that enables the facility to be upgraded to and exceedexisting standards.

Liquid coatings are ideal for pipeline rehabilitation andcoating repairs. They are well suited for field application, beit for large recoating projects or for short pipe sections, in-the-ditch or out.

Liquid coating formulations are now available that are100% solids (no VOCs), do not require a primer and pos-sess high one-<:oat build capabilities. In addition, these for-mulations are available in both spray and brush grades togive pipeline operators flexibility in choosing the applicationtechnique best suited for the particular project.

Plural component airless spray equipment was devel-,'..

oped and enhanced to allow for easy spraying of theseformulations in the field, both in and out of the ditch. Inaddition, automated line travel spray equipment wasdesigned to allow large-scale pipe recoating projects tobe undertaken.

For smaller pipeline rehabilitation sites, spray gradeformulations utilising airless spray equipment or brushgrade formulations using brushes and paint rollers arenow available.

100% solids liquidurethane coatingsIn the 1980s, 100% solids liquid urethanes began to beused in North America for coating of girth welds, valves, fit-tings and pipe on new construction projects. Then in themid-1980s, the first large scale recoating program forpipeline rehabilitation occurred, using these urethanes andspecially designed automated line travel coating equip-ment. Approximately 11.5 km of buried 34 NPS diameterpipe was raised from the ditch, blast cleaned and recoatedto a thickness of 0.635 mm.

system has badly deteriorated, cathodic protection require-ments and costs can increase exponentially. Until the late1970s, before the widespread use of epoxy and similarhigh adhesion coatings, practitioners of cathodic protectiontypically used a design factor of 3% bare area for a pipelinecoating. For tape and enamel systems in use at that time,this design factor was realistic. Today's pipelines, with highadhesion systems, typically require two to three orders ofmagnitude less current.

Many of the older 'low adhesion' coatings deterio-rated over time to the point of total ineffectiveness.Industry experience has shown that this has happenedwith mastics, asphalt enamels, tal)es and to a lesserextent coal tar enamels. Figure 1 illustrates how costsfor cathodic protection can accelerate as coating deteri-orates. Curves #1 and #2 represent approximately 30%coating deterioration where the curves ascend almostvertically. Additionally, cathodic protection becomes diffi-cult to maintain or even achieve.

In 1995, the National Energy Board of Canada (NEB)held an inquiry into stress corrosion cracking (SCC) onCanadian oil and gas pipelines. In its report filed in1996, the NEB reported that most of the SCC relatedfailures occurred on pipelines coated with polyethylenetape. The report stated that some polyethylene tapes areprone to disbonding because of tenting created by thetape that occurs between the pipe surface and the lon-gitudinal weld seam. Also, disbondment occurs wherethe tape is overlapped betVjeen successive wraps oftape or where soil stress has caused the coating to moveor wrinkle.

Because of the high electrical insulating property ofpolyethylene and the relatively long path under the dis-bonded tape, sufficient cathodic protection current can-not reach the pipe to prevent corrosion. This phenomenais called shielding. When shielding does occur it canresult in pitting corrosion. Of greater concern, as a resultof shielding, is extensive general corrosion or the forma-tion of an environment susceptible to stress corrosion

56 WORLD PIPELINES NOVEMBER/DECEMBER 2002

extending pipe life

In general, these polyurethane coatings exhibited thefollowing characteristics:

Disadvantages: -i.Slower curing than urethane.

.Curing stopped at temperatures below 5 ac.

.Higher temperature formulation were more difficult tospray.

Recent developmentsIn the late 1990s and early 2000s, epoxy coating formula-tions approached the application advantages of urethaneproducts and possessed performance properties thatexceeded those of fusion bond epoxy (FBE). In addition, for-mulations were further developed to meet particular oper-ating conditions encountered by pipeline operators forpipeline recoating and repairs.

Advantages:.Aexibility in formulating.

.High build-single coat application.

.Fast curing at lower ambient temperatures.

.High abrasion and gouge resistance.

.Reasonable cathodic disbondment resistance attemperatures below 50 ac.

.Reasonable tensile adhesion.

High temperature epoxyWith the industry trend towards higher operating pipelinesystems, coatings based on the newest (zero VOC) nova lactechnology have been developed that cure to a highly cross-linked coating for ultra high temperature service on buriedor submerged pipeline facilities. These systems possesssuperior adhesion and excellent resistance to cathodic dis-bondment at temperatures up to 150 °C.

Disadvantages:.Moisture sensitive.

.Poor cathodic disbondment resistance at temperaturesabove 50 ac.

.Poor hot water adhesion.

.Poor impact resistance at sub zero temperatures.

100% solids epoxy /urethane coatingsIn the late 1980s and 1990s, pipeline oper-ating temperatures began to increase.Epoxy/urethane formulations were intro-duced which had significantly improved per-formance properties over urethanes athigher in-service temperatures.

Advantages:.Improved cathodic disbondment resis-

tance up to 80 °c.

.Improved tensile adhesion.

.Improved hot water adhesion.

Figure 2. Insitu rehabilitation.

Disadva ntages:.Slower curing than urethane.

.Curing stops at temperatures below 5 DC.

In the late 1990s, almost 120 km oflarge diameter (34 -42 NPS) pipe wererecoated in North America with these for-mulations, using automated line travel

spray equipment.

100% solids

epoxy coatingsIn the mid-1990s epoxy and novalac-epoxycoatings wereformulated for use in the pipeline industry.These formulations have improved perfor-mance characteristics and properties overthe earlier coating systems.

Advantages:.Improved cathodic disbondment resis-

tance at temperatures up to 95 °C.

.Improved tensile and hot water adhesion.

.Improved sag resistance. Figure 3. Insitu rehabilitation inspection.

WORLD PIPELINES NOVEMBER/DECEMBER 2002 57

extending pipe life

Figure 4. Automated rehabilitation equipment.

protection with superior adhesion and resistance tocathodic disbondment at temperatures up to 80 aG.

ConclusionThe development of liquid coating technology has had asignificant effect on advancing recoating for pipeline reha-bilitation and repair.

.Liquid pipeline coatings have made recoating a mostefficient and cost-effective approach for re-establishingthe integrity of pipelines and extending the life of agedinfrastructure.

.These coating systems exhibit better corrosion protec-tion performance properties as compared to standard'mainline' coatings and provide higher integrity to therecoated facility.

.New liquid coating enhancements are now available foruse in difficult operating conditions and environments.

.In addition, recoating reduces maintenance, integritymonitoring and cathodic protection costs.

Low temperature cure epoxyNew epoxy formulations have the ability to cure at temper-atures down to a DC and still provide the performance prop-erties expected from the use of epoxies. These systemsare ideal for coating applications in cool weather(autumn/winter) and provide excellent corrosion protectionproperties for pipeline facilities operating at temperaturesup to 65 DC.

References1. National Energy Board Report of the Inquiry-$tress Corrosion Cracking

on Canadian Oil and Gas Pipelines, November 1996.2. Banach, J., Pipeline Coatings-Evaluation, Repair, and Impact on Corrosion

Protection Design and Cost, 'Corrosionj87', Paper No. 29, NationalAssociation of Corrosion Engineers, San Francisco, Califomia, 1987.Damp surface epoxy

An epoxy is now available that may be used for coatingpipelines where the surface is damp due to high humidityor condensation. This system provides excellent corrosion Enquiry no: 30