Investigation of Duplex Stainless Steels for Pipeline Repairs

Abstract

Hundreds of miles of production pipelines make up the network at the Prudhoe Bay oilfield in Northern Alaska, part of the vast petroleum reserves which feed one million barrels of crude oil per day to the 800-mile Trans Alaska Pipeline System. Part of the inevitable layout on a land-based pipeline of this size is the need to cross roads and undergo similar directional changes. When the pipeline was constructed this hurdle was overcome by burying the pipe for short sections. At present the system has hundreds of buried road crossings. One of the continued high-cost maintenance issues with the pipeline is the corrosion that occurs at the elbows used for direction and elevation changes. The pipelines are constructed primarily out of high tensile strength carbon steels. While carbon steel is sufficient for most of the pipeline, the elbows present new challenges such as the cumulative effects of erosion and pitting corrosion. One proposed solution was to replace the carbon steel with duplex stainless steel in these areas of higher susceptibility. Duplex stainless steel has a high general corrosion resistance and better velocity-assisted corrosion resistance than carbon steel. Coupled with its recent decrease in cost, this solution makes the use of duplex stainless steel a potential candidate for the above application, reducing costs of the repair and inspection for the entire pipeline. Technical issues which are significant in joining carbon steel to duplex stainless steel include the proper selection of the weld metal alloy, weld cooling rates, and corrosion of the carbon steel due to the joining of dissimilar alloys. This paper addresses these issues of welding and corrosion that were investigated for the use of duplex pipe and components in the carbon steel system.