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One of the major issues in the pipeline industry is coating disbondment. A very large percentage of the external corrosion and stress corrosion cracking (SCC) has been and continues to be observed under disbonded coatings that shield CP. This has been an ongoing issue with coated and cathodically protected pipelines since the beginning of using these two technologies. The various coating types and their typical failure modes under various environmental conditions as well as their compatibility with Cathodic Protection (CP) when disbondment occurs play a major role in selection criteria for coating and re-coating programs.
With the continued development and improvements of the “Electro-Magnetic Acoustic Transducer” (EMAT) technology to locate disbonded coatings without the need of exposing the pipeline gives operators economically sound information about their pipeline systems. The “EMAT” technology is also capable of identifying the various coating types and coating condition. Correlating the EMAT coating information with CP readings enables pipeline operators to distinguish between CP shielding disbonded coatings and coating defects that result in “CP open” coating holidays or coatings that are non-shielding when disbondment occurs. A combination of EMAT coating information with environmental data (e.g. climate soil types wet/dry cycles) and various CP survey data will allow for a better selection and use of coatings. This information will be critical to the development of new coating products to effectively control development of SCC and external corrosion when disbondment occurs. This set of information thus constitutes an excellent basis for coating types to be used for coating rehabilitation programs SCC susceptibility models and Pipeline Integrity Management Systems (PIMS).
This paper will outline different coating failure scenarios and will give examples of selection criteria for adequate coating repairs. It will describe the drivers for the coating type selection and how the EMAT in-line inspection results can support the decision process.
The tendency of pipeline girth weld coatings to shield cathodic protection (CP) current was studied in the laboratory. Epoxy, epoxypolyurethane, polyurethane, and wax were investigated. Results showed that the liquid coatings, when applied extra thin to accelerate the kinetics of absorption and current transmission, all disbonded by blistering, and all allowed CP current to be transmitted.
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