Geohazard assessment lifecycle for a natural gas pipeline project

Abstract

This paper is a walkthrough of the geohazard risk assessment performed for the Front End Engineering Design (FEED) of a planned large-diameter natural gas pipeline, extending from Eastern Europe to Western Asia for a total length of approximately 1,850 km. The geohazards discussed herein include liquefaction-induced pipe buoyancy, cyclic softening, lateral spreading, slope instability, groundwater rise-induced pipe buoyancy, and karst. The geohazard risk assessment lifecycle was comprised of 4 stages: initially a desktop study was carried out to describe the geologic setting along the alignment and to conduct a preliminary assessment of the geohazards. The development of a comprehensive Digital Terrain Model topography and aerial photography data were fundamental in this process. Subsequently, field geohazard mapping was conducted with the deployment of 8 teams of geoprofessionals, to investigate the proposed major reroutes and delve into areas of poor or questionable data. During the third stage, a geotechnical subsurface site investigation was then executed based on the results of the above study and mapping efforts in order to obtain sufficient data tailored for risk quantification. Lastly, all gathered and processed information was overlain into a Geographical Information database towards a final determination of the critical reaches of the pipeline alignment. Input from Subject Matter Experts (SME) in the fields of landslides, karst and fluvial geomorphology was incorporated during the second and fourth stages of the assessment. Their experience in that particular geographical region was key to making appropriate decisions based on engineering judgment. As the design evolved through the above stages, the pipeline corridor was narrowed from a 2-km wide corridor, to a 500-m corridor and finally to a fixed alignment. Where the geohazard risk was high, rerouting of the pipeline was generally selected as a mitigation measure. In some cases of high uncertainty in the assessment, further exploration was proposed. In cases where rerouting was constrained, mitigation via structural measures was proposed. This paper further discusses the cost, schedule and resource challenges of planning and executing such a large-scale geotechnical investigation, the interfaces between the various disciplines involved during the assessment, the innovative tools employed for the field mapping, the classifications developed for mapping landslides, karst geology, and trench excavatability, determining liquefaction stretches and the process for the site localization of the Above Ground Installations (AGI). It finally discusses the objectives of the FEED study in terms of providing a route, a ± 20% project cost estimate and a schedule, and the additional engineering work foreseen to take place in the detailed engineering phase of the project.