Pipeline casings are widely used at locations where a pipeline crosses beneath railroad tracks, highways, and other areas where external pressure and stress can impact pipeline integrity, or areas where rehabilitation or replacement of the pipeline could significantly disrupt other systems. The casing is intended to protect the pipeline but recent recommendations from the Pipeline and Hazardous Materials Safety administration (PHMSA) and NACE International indicate that in many instances casings provide little if any benefit. Conversely the casing can also create an environment that can trap water in the area surrounding the pipeline. The water and water vapor inside the casing can lead to atmospheric and galvanic corrosion issues. Additionally the presence of water can cause electrolytic shorting between the pipeline and the casing that could affect the level of cathodic protection on the carrier pipe.
A variety of water displacing substances are used to prevent water and water vapor related problems inside pipeline casings. Many are wax-based, or petrolatum-based agents that may provide an effective barrier between the pipe and potentially corrosive elements. These substances fill the casing and typically become semisolid. The material is subject to degrade over time. It is difficult to measure or monitor on an on-going basis the effectiveness of these barrier agents due to the difficulty of detecting voids that can develop inside areas of the casing. Additionally, as the fill material settles, areas of the casing can be exposed to galvanic corrosion agents. In recent years an increasing number of "vapor phase corrosion inhibitor" substances have been developed for use as displacement agents for casing applications. A unique characteristic of this type of inhibitor is the ability to protect the pipeline at the liquid phase, liquid to gas phase , and gas phase within the interstitial area without filling the entire casing. This allows the operator to monitor the corrosion rate inside the casing environment with electrical resistance (ER) probes.
This paper is a case study on the use of corrosion rate monitoring probes to determine the rate of material loss inside the pipeline casing where a vapor phase corrosion inhibitor substance is used. The probe can be suspended in the protective vapor and provide continual measurements of probe material loss. The data can be transmitted using remote monitor technology and accessed by the technician from any web-enabled device. The availability of this data allows the technician to track and trend the rate of corrosion, develop predictive analysis using near real-time data, and proactively address any acceleration of the corrosion rate inside the casing that could be due to inadequate moisture displacement.
This study focuses on combining elements of several evolving technologies to enhance pipeline integrity in casing installations. The vapor phase corrosion inhibitor provides very effective displacement, and the combination of the ER probe and a web-based remote monitor system provides accurate, near real-time performance data.
