Seismic Fault Displacement of Buried Pipelines Using Continuum Finite Element Methods

Abstract

In deep waters, pipelines are usually installed exposed on the seabed, as burial is generally not required to ensure on-bottom stability. These exposed pipelines are nevertheless susceptible to seismic geohazards like slope instability at scarp crossings, soil liquefaction and fault movements which may result in failure events, although larger diameter pipelines are generally known to have good tolerances to ground deformation phenomena, provided the seismic magnitudes are not too onerous. Regardless of the pipeline size, these seismic geohazard issues are usually addressed during the design stage by routing the pipeline to avoid such hazardous conditions, where possible. However, extreme environmental conditions like hurricanes or tropical cyclones, which are typically experienced in the Gulf of Mexico and Asia-Pacific regions, are also factors which can cause exposed pipelines to be susceptible to large pipeline displacements and damage. Secondary stabilisation in the form of rock dump is sometimes employed to reduce the hydrodynamic loads from high turbidity currents acting on the pipeline. However, rock dumping (or burying the displaced pipeline) on a fault line could again pose a threat to its integrity following a seismic faulting event. The traditional method of assessment of a buried pipeline subjected to seismic faulting is initially carried out using analytical methods. Due to the limitations of these techniques for large deformation soil movement associated with fault displacement, non-linear finite element (FE) methods are widely used to assess the pipeline integrity. The FE analysis typically idealises the pipeline using discrete structural beam-type elements and the pipeline-soil interaction as discrete non-linear springs, based on the concept of subgrade reactions proposed by Winkler. Recent research from offshore pipeline design activities in the arctic environment for ice gouge events have however suggested that the use of the discrete Winkler element model leads to over-conservative results in comparison to the coupled continuum model. The principal reason for the conservatism is related to the poor modeling of realistic surrounding soil behaviour for large deformation events. This paper discusses the application of continuum FE methods to model the fully coupled seabed-buried pipeline interaction events subject to ground movements at active seismic faults. Using the continuum approach, a more realistic mechanical response of the pipeline is established and can be further utilised to confirm that calculated strains are within allowable limits.