Assessment of subsea pipelines crossing seismic fault

One of the causes of hazardous substance leak from the offshore pipelines can be attributed to potential pipeline rupture due to seismic fault movement. Some of the consequences of the ruptured pipeline due to fault movement include significant environmental damage, costs associated with clean up and asset loss, and damage to reputation of the operator. Objective of this study was to review approaches to estimate the response of subsea pipelines to fault movement during a seismic event and to compare existing methodologies. When subjected to seismic fault movement, pipelines/umbilicals undergo strains due to bending and axial tensile or compressive force (depending on the fault type). Under normal fault rupture, for example, the pipe failure mechanism, particularly for thick wall pipe (low D/t value) would likely be tensile rupture since the largest pipe strains are tensile in this case. For thin wall pipe (high D/t value), wrinkling may be the governing failure mechanism. There are two modeling approaches available to determine the response of the pipelines subjected to fault movement: 1) analytical approach and 2) finite element modeling approach. In analytical approach, the tensile strains developed in the pipe is estimated by predicting the deformed shape of the pipeline and the total elongation of the pipeline. From the comparison of the analytical approach and the finite element analysis, the results show that although analytical approach is convenient to use, it has significant limitations as such that it is not able to capture or account for buckling (wrinkling) where the pipelines are subjected to the compression forces due to the fault movement, the effect of soil pressure, internal fluid pressure and etc. A better alternative to analytical approach is the finite element modeling approach where a pipeline or umbilical is explicitly modeled in its entirety and full dynamic simulation is performed considering large displacements, soilstructure interaction, contact, pipe internal pressure, strain rate effects and etc. This paper discusses and quantifies effects of the factors affecting the performance of the pipelines subjected to seismic fault movement. Furthermore, the paper presents critical factors that can be optimized in order to enhance the performance of pipelines. Finally, the mitigation recommendations are presented that can improve the capability of the pipeline to sustain differential movement along the fault without developing a leak source to contaminate the environment.