Cyclic Ratcheting Resistance of Buried Pipelines

Abstract Upheaval buckling (UHB) is a common issue for high pressure and high temperature (HPHT) pipelines. The out-of-straightness (OOS) of the buried pipelines combined with the high axial compressive forces induced by the extreme operating conditions causes the pipeline to move upwards till the downward soil resistance is sufficient to resist any further movement. As the pipeline mobilise upwards, the gap underneath is soon filled up by soil particles from around the pipe. As a result, when pipelines cool down during temporary suspension in operation, they cannot return to their original position. As pipelines undergo many thermal cycles of start-up and shout-down during the lifetime, a pipeline OOS feature can progressively move upwards and eventually lead to failure even if it was designed safely against UHB in the original profile. This phenomenon of cyclic pipe movements upwards is known as pipeline ratcheting, which is the driving mechanism behind many UHB pipeline failure cases. This paper presents insight into ratcheting and results of small scale experiments that were conducted at the Schofield Centre, University of Cambridge, to model the ratcheting response of pipelines buried in saturated sands. The results obtained have shown that the critical upward mobilization at which ratcheting is initiated is a function of soil critical state friction angle and pipeline diameter. The pipeline downwards force on the soil during shut-down is the other parameter affecting the initiation of ratcheting. This paper presents valuable insight on ratcheting and mitigating upheaval buckling of pipelines to the design engineers.