Safeguarding a Buried Pipeline in a Landslide Region

Abstract

Due to economic or other reasons, natural gas transmission pipelines are sometimes constructed in regions susceptible to geohazards such as mud flow, soil creep, and landslide. Although it is possible to design a pipeline to withstand these ground movements, it may not be cost effective to install kilometers of high-grade thick-wall steel pipe traversing a mountainous region. Furthermore, some areas may not appear to be prone to soil movement or may not show visible signs of slope instability at the time of pipeline design and construction. In this case, a section of buried pipeline on a hillside in Colombia was affected by gradual ground movement resulting in buckling of the pipe at the pipe bridge located at the foot of the hill. Repairs were carried out by replacing the buckled section of pipe with a new pipe and cutting the pipe at the top of the hill to relieve tension developed in the buried pipe. Survey markers were installed across the hillside to monitor future ground surface movement. Following the repair, as part of the pipe integrity management plan, the pipeline operator wanted to know where to install strain gauges to monitor pipe stresses, where to install inclinometers to monitor future land slips, and the amount of ground movement allowable before intervention is required. This paper describes how the pipeline operator's requests were achieved by applying multiple analysis techniques. First, the available geotechnical and geological data from the site was reviewed. A representation of the slope was created in a finite element model and a strength reduction procedure was used to estimate a number of significant slip surfaces corresponding to various slope stability factors. The soil movement profile was then normalized and applied as movement perturbation to a three-dimensional finite element pipeline model, which had the appropriate nonlinear interaction between the buried pipe and the surround backfill soil, and the pipe support and connection constraints at the pipe bridge. The pipe stress analysis revealed several critical locations where the peak stress as a function of soil movement was then determined. The computed results gave a good correlation to the observed pipe bridge failure thus giving confidence in the pipeline model and provided useful information for the implementation of the monitoring works.