ABSTRACT

As older pipeline coatings deteriorate and disbond, companies quickly realize the high cost of maintaining these lines. Cathodic protection requirements increase, and corrosion and stress corrosion cracking (SCC) can occur on the pipeline surface. In an effort to reduce the costs associated with maintaining these aging pipelines, an economical means of maintaining the integrity of these lines has been investigated. After four years of large-scale pipeline recoating, it has been demonstrated that pipeline recoating programs can be an attractive alternative to replacing corroded pipe.

INTRODUCTION

In 1996, TransCanada Transmission ("The Company") initiated a trial program dedicated to investigating the technical feasibility of large-scale mainline recoating. The program was necessitated from field observations and pipe-to-soil potentials which indicated that the coating on some of The Company's older pipelines had substantially deteriorated (see Figure 1). Since pipelines with substantial coating deterioration require extensive amounts of cathodic protection (CP) current and subsequent frequent monitoring and maintenance, the condition of the coating on these lines elevated concerns about the cost and effectiveness of additional CP. As well, because The Company's pipeline system in the western region consists of six pipelines within a common right-of- way that are connected to a common CP system, it was feared that the CP current requirements to achieve protection on poorly coated pipelines could exceed the allowable current densities on the adjacent, newly coated fusion bond epoxy (FBE) lines, thus leading to premature coating disbondment.

As a trial project, The Company chose to rehabilitate 1.6 km of NPS 34 pipeline at a location where additional cathodic protection was no longer considered an attractive alternative. The options available were to either replace the pipe or rehabilitate it. The pilot project was driven by the need to promote and evaluate the technical and economical feasibility of large scale pipeline recoating technology as a means of rehabilitation. The Company also wanted to ensure the continued integrity of the line, lower its operating costs, and study the application of long-term solutions to problem areas. The trial project proved successful and at a cost of $1.2 MM, it was deemed to be more economical than a pipe replacement.

With the success of the trial project, The Company has since embarked on an annual program of mainline recoating. In total, The Company has recoated over 42 kilometers of NPS 34 pipe and 31 krn of NPS 42 pipe. Recoating has proven to be the most effective approach to maintaining pipeline integrity on some portions of The Company's system. In-line inspection data has allowed The Company to target the program to those sections of pipe with the largest densities of corrosion features, thus indicating those areas with insufficient CP. As well, the newer coating systems resist cathodic and mechanical disbondment and have higher temperature resistance than the older coatings. Recoating also enables The Company to reduce its maintenance costs related to SCC, pigging, hydrostatic testing, and corrosion control monitoring. Refer to Table 1 on the history of The Company's Mainline Recoating Program. The field application of pipeline recoating entails many activities that employ typical pipeline construction techniques. The pipe is taken out of service and the overburden (with the exception of 6") removed with conventional excavating equipment. The pipe is removed from the ditch and the ditch is then backfilled. The Company requires the ditch to be backfilled during the recoating operations as a safety measure, as well as to allow more

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