Response of Offshore Pipelines to Landslides

Abstract

According to statistics for the Mississippi Rived Delta (MRD), landslides accounted for a significant amount of damage to offshore pipe. This study focuses on the pipeline response to offshore landslides for various orientations of the pipeline with respect to the direction of seafloor movement. Specifically analytical solutions are presented for offshore pipelines laid parallel to and perpendicular to the direction of seafloor movement. The influence of the landslide spatial extent (plan area) and, burial depth are discussed. Finally the case of an oblique orientation of a pipeline with respect to the direction of seafloor movement is discussed in relation to a case study of observed damage in the MRD.The conclusions of the study include: 1) For common landslide sizes in the MRD, one expects less pipe strain for pipe laid parallel to the direction of seafloor movement ( longitudinal seafloor movement). 2) The expected failure mode for such pipe is tensile rupture at the head of the landslide. The pipeline may also experience beam buckling around the toe, but is unlikely to wrinkle due to low D/t ratio common to off-shore pipe. 3) If a tie-in is close to the landslide zone, failure is more likely to occur at the tie-in rather than on the pipeline. 4) Regardless the pipeline orientation, for the same soil strength, pipe diameter and wall thickness, a X-60 steel pipe is able to survive a landslide about twice the size as that for a Gr-B pipe. 5) Due to continuous deposition in the MRD, pipelines with long service lives tend to be deeply buried. When subjected to landslides, they are subject to larger forces at the pipe-soil interface, and larger pipe strains. Hence these older, more deeply buried lines are particularly vulnerable especially those made of low steel grade.