A Case of Sinkage of a 34” Gas Pipe During Pull-in Operation, São Paulo, Brazil

Abstract

The objective of this paper is to provide information aiming at a better understanding of the mechanisms that led to the sinkage of a gas pipe during a pull-in operation, also performing a survey of all relevant information regarding this operation and soil investigation. This study also comprises an analysis of the available soil information and the identification of possible scenarios of behaviour to feed mathematical models. A first mathematical model was developed to assess the axial horizontal force Q that causes a horizontal closed end pipe of diameter D and length ℓ to move inside a sandy soil mass when buried at a depth z. Further on, two feasible mechanisms were studied to explain the pipe sinkage during the pull-in operation. The first mechanism was associated with compressive volumetric strains that take place within the sand layer beneath the pipe. These strains are accounted for the relative soil-pipe shearing displacements. The second mechanism assumes that the soil beneath the pipe is submitted to a plastic failure under a vertical load corresponding to the vertical component of the pulling force applied to the pipe-head. This model comprises an elastic pipe resting on a rigid-plastic soil that fails under Mohr-Coulomb envelop of failure. Lack of soil information, however, hampered the full understanding of the process that led to the sinkage of the pipe.