ABSTRACT

Various facilities such as gas compressor stations, storage tank stations, and gathering and distribution stations have buried metallic structure that are both coated and uncoated and of different metal types. These facilities are identified as congested area facilities. For example, facility such as a gas compressor station may have coated pipelines, uncoated screw piles, copper grounding wires, and copper groundrods. Each of these structures has different structure-to-earth resistance and current demand for protection. Increasingly, these structures are electrically interconnected to mitigate the lightning hazard, and to achieve optimal lightning protection throughout the facilities. However, this has an adverse effect on the cathodic protection (CP) system of the facilities because a large fraction of the CP current could be diverted to uncoated carbon steel and copper groundwires and groundrods. In fact, in some congested area facilities, less than one percent of CP current is delivered to the active buried structures such as buried pipelines transmitting oil or natural gas. Further, the approaches available to estimate the CP current distribution for the transmission pipelines are not directly applicable for the facilities with congested buried metallic structures with different structure-to-soil resistances. An analysis was conducted to develop guidelines to design and install and optimal CP system for such facilities, and to remediate insufficient CP to the below grade pipelines at the existing facilities. The analysis included developing a three-dimensional computer model to estimate CP current distribution in congested areas with coated and uncoated buried pipelines, screwpiles, groundwires and copper groundrods. The model was exercised to determine the optimal anode bed structure to deliver sufficient CP current to active buried structures such as transmission pipelines within the facility. The model was used to develop recommendations for optimal CP systems for newer facilities, and remediating insufficient CP at the existing facilities, and anode bed structure that will result in optimal CP system for a given facility.

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