Response of Offshore Pipelines to Landslides
Authors: Liu Jack , O&apos;Rourke Michael 
Published: July 2010

According to statistics for the Mississippi Rived Delta (MRD), landslides accounted for a significant amount of damage to off-shore 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: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).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.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.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.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.

Introduction

Offshore pipelines in the Gulf of Mexico have suffered significant damage during past hurricanes. They were 480 locations of pipe damage caused by Hurricane Andrew, and over 600 locations caused by Hurricane Katrina and Rita. The damage mechanisms or causes can be categorized into three groups:damage, typically around platforms, due to falling objects, platform collapse or anchor drag;landslides, occurring mainly around the MRD;hydrodynamic forces including instability, exposure and free spanning, occurring mainly in shallow water (i.e., 61m or less).

Minerals Management Service (MMS) provides information on 38 failures that were caused by landslides from several hurricane events (Nodine et al, 2007). According to the list of 38 failures, 25 failures were due to pulling apart at risers/tie-ins, and 6 were due to rupture of submerged pipelines. This list clearly indicates that pipeline rupture and pulling apart at tie-ins are two major failure modes for offshore pipelines subjected to landslides.

Hooper and Suhayda (2005) pointed out that Mississippi River sediment issuing into the Gulf of Mexico is deposited in front of the three major passes (Southwest Pass, South Pass and Pass A Loutre). The deposition rates can be as high as 0.3-0.6m/yr, creating natural slopes with slope angles of a few degrees or less. Soil borings reveal that Mississippi Delta consists of weak under-consolidated clay. The shear strength of soil at the seafloor surface is about 2.4kN/m2, with a 1.3kN/m2 per meter increase with depth (Nodine et al, 2006). These gentle slopes are unstable under strong waves and currents. Triggered by hurricanes, landslides in the MRD area are generally localized features, on the order of hundreds meters long (plan dimension in the direction of seafloor movement) by hundreds of meters across and typically 15-30m deep (Gilbert et al, 2007).

O'Rourke and Liu (2010) have provided detailed information about pipe-soil interaction forces, strain-based design criteria for pipelines, and close-form solutions for pipe response to earthquake-induced permanent ground deformation. This paper utilizes those relations to analyze pipe response to offshore mudslides in the MRD, and investigates two common failure modes in detail.