External soil side corrosion in the field joint area has been experienced on a number of Shell Canada Limited (Shell) pipelines. This corrosion has occurred in spite of the application of cathodic protection and protective coatings.
Shell has experienced this problem most noticeably on thermally insulated pipelines although some uninsulated pipelines have been affected.
The problem to date has resulted in entire pipeline segment replacements. extensive repair of joint areas of pipelines and advanced pipe line inspection programs. This problem may have significant implications for all pipeline operators.
This paper will discuss causes of this problem along with proposed solutions. In this discussion a brief review of Shell's joint corrosion experiences are given including historical use of coating materials and construction practices. Test results and comparisons are given on generic coating systems used to coat pipeline field-joints. Proposed remedial measures are offered which primarily focus on field joint coatings installation practices. Reference will be made to the apparent ineffectiveness of cathodic protection in providing protection for thermally insulated pipelines. Comments ill be presented on the use of the instrumented pig inspection method and the associated difficulties with the inspection and interpretation of results for joint area corrosion.
It is common practice to apply an external plastic coating to pipelines to control soil-side corrosion. To provide the protection at coating flaws or so-called holidays, cathodic protection is applied. Recommended practices exists for both methods.(1) In some jurisdictions (eg. Alberta) this is also a regulatory requirement.
These coatings can be either shop-applied prior to pipeline construction or field-applied "over-the-ditch" during construction.
Shop-applied coatings are supplied to the job-site with a coating cut-back to facilitate welding in the field. Once welded together this joint cut-back area must be coated using a compatible coating with the shop-applied coating.
Furthermore, for gas gathering pipeline systems thermally insulated pipelines are often required. Therefore, the field-joint coating system must also incorporate insulation as well as protective coatings. Figure 1 is a schematic showing this concept.
Based on inspection surveys using instrumented pigs, Shell has discovered extensive external corrosion of a number of pipelines. This corrosion has been concentrated at the field-joint area but external corrosion away from the joint area has also been found. Both insulated and uninsulated pipelines have been involved, however, it appears that insinuated pipelines present a greater potential for this problem.
Table 1 presents an overview of Shell's experiences wlth joint corrosion to date.
Based on Shell's experiences, the ability of cathodic protection to protect a pipeline with certain disbanded coatings may be limited. All of the pipelines on Table 1 were under cathodic protection. It appears that the shielding effect of the disbonded coating prevents the required electrical potential shift to attain cathodic protection of the pipe (2.3,4). This problem is only compounded for thermally insulated pipelines.
As shown on Table 1, Shell has made extensive use of heat-shrink sleeves. Various types of sleeves were used which utilize different adhesive systems and methods of installation.
