PG&E's existing natural gas pipeline under the Freshwater Slough in Eureka, CA was at risk of failure due to exposure from scour. A replacement pipeline was to be installed using horizontal directional drilling (HDD) during the summer of 2005. During the pilot bore, an inadvertent drilling fluid return occurred in the environmentally sensitive slough habitat. Construction was halted by the State Lands Commission and alternative construction methods were evaluated. The evaluation indicated that HDD posed the least environmental impact; however, the crossing needed to be redesigned to reduce the risk of hydrofracture for the second installation attempt.This paper discusses the re-design of the Freshwater Slough crossing and the construction of the revised pipeline in 2008. A thorough hydrofracture analysis of the challenging soils led to a revised bore geometry that minimized hydrofracture risk by providing sufficient depth below the slough. A down-hole pressure monitoring device was specified and used during the pilot bore to further minimize the hydrofracture risk. Despite the occurrence of some further inadvertent fluid returns on the subsequent attempt, the pipeline was successfully installed, due mostly to cooperation and coordination between the various environmental agencies, including the California Coastal Commission, California Department of Fish and Game, National Oceanic Atmospheric Administration, the US Army Corps of Engineers, and the California Coastal Commission.
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