extending pipe life - SPC

1 extending pipe life A number of factors affect the longevity of a pipeline . protection for a pipeline . As a buried pipeline is subject to These include quality of construction and protective corrosive attack if it is in contact with a wet environment, coating systems, cathodic protection, nature of the coating the pipeline to isolate it from this corrosive envi- environment, operating conditions, and quality and fre- ronment is an obvious approach to corrosion control. Since quency of pipeline maintenance to name btJt a few. no coating system is defect free, cathodic protection is No one factor influences the long-term integrity of a used to provide supplementary protection.

2 pipeline more than the effectiveness of its coating system. Most countries have regulations that require pipelines pipeline leaks, ruptures and ultimately a pipeline 's integrity to be coated and in general stipulate that a coating pos- and useful life can be directly attributed to coating deterio- sess the following properties: ration or failure..Electrically isolate the external surfaces of the pipeline In the year 2000 it was estimated that of the mil- from its environment. lion miles of pipeline in the USA, 24% was more than 50..Have sufficient adhesion to resist underfilm migration years old. This ageing of pipeline infrastructures and resul- of electrolyte.

3 Tant coating deterioration has been the impetus for the .Be sufficiently ductile to resist cracking. development of methods and technologies to rehabilitate older facilities and extend the life of existing pipelines. New .Resist damage due to soil stress and normal handling. coating systems and application techniques have now been .Be compatible with cathodic protection. developed to enable old and existing pipeline systems to .Resist deterioration due to the environment and service be upgraded in a cost-efficient manner, and to the latest temperature. industry standards for protecting facilities from external Cathodic protection is fundamental to preserving a corrosion and environmental damage.

4 pipeline 's integrity. Cathodic protection is a method of cor- This article outlines the functions of a pipeline coating , rosion control that is achieved by supplying an external factors leading to coating failure, the consequences of a direct current that neutralises the natural corrosion current failed coating system and discusses the advancements arising on the pipeline at coating defects. Current required made in liquid coating systems and application methods to protect a pipeline is dependent on the environment and for remediation of pipeline infrastructures. the number and size of the coating defects. Clearly, for a particular environment, the greater the number and size of Corrosion protection of coating defects, the greater the amount of current required buried pipelines for protection.

5 Corrosion protection is required to maintain the integrity of coating plays an integral part in the functioning of a a buried pipeline system and coatings are the primary pipeline 's cathodic protection system. Where a coating WORLD PIPELINES NOVEMBER/DECEMBER 2002 55. extending pipe life cracking. These two scenarios could lead $/Km to catastrophic failure ot,the pipeline . 60,000[ In the presence of shielding, over-the- G:> line electrical corrosion protection sur- veys are suspect in their ability to accu- 50,000 rately assess the status of corrosion pro- tection of a pipeline . Other more costly System 1: 1) pipe Diameter methods such as pigging, hydrostatic 40,0001 762mm to 914mm testing, or discrete excavations would be 2) Soil Resistivity needed.]

6 Regardless of the method of 1000 to 5000Q -cm 3) Clay Soil integrity assessment used, remedial pro- 30,000 @ grams will be required if corrosion is System 2: found. The remedial options are generally 1) pipe Diameter limited to additional cathodic protection, 20,000 762mm to 914mm recoating, pipe replacement or a combi- 2) Soil Resistivity above 50 0000 -cm nation of the aforementioned choices. In 3) Rock, Sandy Soil the case of a disbonded coating causing shielding, additional cathodic protection 10,0001. I. would not be an effective option, and thus the only alternative is either rehabilitation by recoating or pipe replacement. nl 10. Liquid coatings Current Requirement Atm2 As can be seen from the above discus- sions, in many cases recoating of existing Figure protection costs as a function of coating performance.

7 Facilities is the most viable option for reha- bilitating pipelines. In some cases, addi- tional cathodic protection current is unable system has badly deteriorated, cathodic protection require- to provide corrosion protection or the cost becomes pro- ments and costs can increase exponentially. Until the late hibitive and will not guarantee corrosion protection to the 1970s, before the widespread use of epoxy and similar facility. coating rehabilitation costs are substantially lower high adhesion coatings, practitioners of cathodic protection than pipe replacement and coating technology is now avail- typically used a design factor of 3% bare area for a pipeline able that enables the facility to be upgraded to and exceed coating .

8 For tape and enamel systems in use at that time, existing standards. this design factor was realistic. Today's pipelines, with high Liquid coatings are ideal for pipeline rehabilitation and adhesion systems, typically require two to three orders of coating repairs. They are well suited for field application, be magnitude less current. it for large recoating projects or for short pipe sections, in- Many of the older 'low adhesion' coatings deterio- the-ditch or out. rated over time to the point of total ineffectiveness. Liquid coating formulations are now available that are Industry experience has shown that this has happened 100% solids (no VOCs), do not require a primer and pos- with mastics, asphalt enamels, tal)es and to a lesser sess high one-<:oat build capabilities.

9 In addition, these for- extent coal tar enamels. Figure 1 illustrates how costs mulations are available in both spray and brush grades to for cathodic protection can accelerate as coating deteri- give pipeline operators flexibility in choosing the application orates. Curves #1 and #2 represent approximately 30%. technique best suited for the particular project. coating deterioration where the curves ascend almost Plural component airless spray equipment was devel- vertically. Additionally, cathodic protection becomes diffi- ,'.. oped and enhanced to allow for easy spraying of these cult to maintain or even achieve. formulations in the field, both in and out of the ditch.

10 In In 1995, the National Energy Board of Canada (NEB). addition, automated line travel spray equipment was held an inquiry into stress corrosion cracking (SCC) on designed to allow large-scale pipe recoating projects to Canadian oil and gas pipelines. In its report filed in be undertaken. 1996, the NEB reported that most of the SCC related For smaller pipeline rehabilitation sites, spray grade failures occurred on pipelines coated with polyethylene formulations utilising airless spray equipment or brush tape. The report stated that some polyethylene tapes are grade formulations using brushes and paint rollers are prone to disbonding because of tenting created by the now available.