Offshore Pipeline Coating for Field Joints - Why??

摘要

Offshore steel pipelines are typically shop coated with a robust anticorrosive coating such as fusion bonded epoxy or a multilayer polyolefin system. Cathodic protection (CP) in the form of sacrificial anodes is also normally installed to help provide corrosion control. Since, these offshore pipelines are welded together in either single of double joint segments on the offshore lay vessel; there is a need to apply a coating at the weld locations. These coatings are commonly called field joint coatings. The typical field joint coatings used for offshore pipelines include liquid applied materials such as epoxies, urethanes, epoxy/urethanes, heat shrinkable sleeves or sometimes fusion bonded epoxy among other materials. During the installation of an offshore pipeline, the application of the field joint coating is often described as a time consuming step. The time to properly apply a field joint coating can range up to 15 to 20 minutes per joint. Although, this doesn't seem long per joint; however considering that a deep water pipeline could have hundreds or even thousands of joints, the field joint coating installation time can be quite long. As the typical pipeline lay vessel day rate can evolve into the hundreds of thousands of dollars, there can be a significant economic incentive to eliminate the offshore pipeline field joint process. Of course, any reduction in the corrosion integrity of the pipeline is not acceptable. A major issue with potentially eliminating the field joint coatings is the need to upgrade the pipeline's CP system to account for the additional bare steel (uncoated) surface area. This paper will highlight, from a conceptual standpoint, advantages and disadvantages with eliminating the field joint coating for the main pipeline. The implications for the CP system will also be covered. The intent of this paper is to challenge the typical practice of coating all field joints and to initiate a dialogue among the industry corrosion and pipeline engineers.