Failures and Integrity of Pipelines Subjected to Soil Movements

Abstract

Experiences by several operators and oil and gas transportation companies of failures in pipelines revealed that soil movements and load transfer from soil to pipeline play an important role in pipeline integrity. Buried pipelines are structures that interact with the soil that is founding them. When implanted in unstable areas, pipelines are subjected to additional loads transmitted by the movement of the ground. This movement is frequently very slow, sometimes taking up to 50 years before the onset of pipeline failures. In these cases, ensuring pipeline integrity depends, in part, upon correct quantification of pipe-soil interaction.Geotechnical remedial works to control soil (fast) slides or (slow) creeping are often costly, so that the proper placement of sensors and appropriate monitoring of deformations is an important variable to decide an intervention for stress relief. To this purpose, the key tool is, certainly, the soil-pipe computer simulation.Strain monitoring is an essential tool for keeping the integrity of pipelines buried in potentially unstable soils. Vibrating wire strain gauges evenly spaced at the wall of selected pipe sections are frequently used to assess the axial and bending loads transmitted to the pipe. Limit values are then set as alarm levels to indicate when to carry out remedial works and / or stress relief. There is a limitation to this approach when it is applied to existing facilities already in service for some time. The stress condition of the pipeline when the gauges are placed is usually unknown, so that the measurements have an error equal to the original stress state. Two ways to alleviate this difficulty are discussed: one is the use of inertial tools that allow comparing the present geometry of the pipeline with a previous ?as built? condition. Lateral displacements and changes in curvatures can be introduced in a model and assess stresses developed in the pipeline since the time of burying it in the ground. Another, more direct way, is to use a measurement technique for residual stresses, such as the ?blind hole? method. Difficulties and limitations associated with the choice of the sections to instrument, calibration procedures for measuring live stresses and ensuring service integrity of the affected section after the procedure are discussed.Rational implementation should be based on soil-pipe computer simulations and geotechnical surveys. These combined technologies allow not only avoiding in service incidents, but also designing low cost and effective mitigation measures. Real-case numerical simulations have been developed to optimize instrumentation points with strain gauges, inclinometers and displacement sensors. The nonlinear finite-element-based multi-material computer simulation to be presented in this chapter assesses the behavior of the pipe subjected to soil movement in both directions, transverse and longitudinal to the pipeline. Soil strata is discretized in layers with properties based in geological reports. The simulation is proving useful for defining practical rules for prior geotechnical monitoring and for placing and sizing works to isolate pipe from soil, such as corrugated casing.