Seismic Vulnerability of the Subsea Pipeline
Authors: Kershenbaum N.Y. 
Published: October 2008

Unburied marine pipeline vulnerability under seismic impact, a new approach of investigation, and conclusion / recommendations for certain analyzed cases are the subjects of this paper. The paper presents a method of investigation, analysis results, and real applications of unburied subsea pipeline behavior data (stress I strain / deviation forms) due to sea-bottom seismic faults along with high temperature / pressure product pumping. The field-estimated maximum surface displacement as a function of the earthquake magnitude has been used in the analysis. Normally, unburied subsea and onshore pipelines take a sinusoidal form as a result of axial expansion and force interactions with the seafloor through surface friction. Previous papers have considered pipeline "snaking" phenomenon effects such as buckling failure and the self-compensative expansion attribute. The self-compensative expansion attribute provides some benefits such as the elimination of pipeline expansion joints, but carries a hidden disadvantage in that it makes such pipelines more vulnerable to seismic slip faults. This investigation concerns the unburied offshore "snaked" pipeline behavior under various types of seismic ground-slip faults including strike, normal, reverse, and longitudinal faults. The analysis results covers a wide area of applications for subsea pipeline construction and operation in seismically active fields. An original physical/mathematical model of the phenomenon is presented along with the computer software. The new analysis method has been applied in practice to realize safe, subsea pipeline construction and operation with a significant cost reduction.

Introduction

Modem subsea pipeline construction assimilates the new fields of development which often propagates to seismic active zones. The zones include South-East, Far-East, America West and West-North Coast offshore regions. The new development fields in offshore pipelines associated with high probability of earthquakes have a potential hazard to lost operational stability, breach integrity, and being burst. Pipeline seismic hazard analyses require an assessment of the future earthquake potential, style of faulting (slip type), and the estimated soil displacement values. This data base determines the unburied pipeline resulting movement, deformation, and strains imposed by a seismic fault and enable to make approximate estimation of the buried pipeline conditions as a result of the earthquake. Refs. I presents comprehensive classifications of the ground faults as being either strike-slip, normal-slip, or reversed-slip, depending on the predominant component of movement. Refs. 2 addresses technical issues involved in regressional analyses of the empirical relationships between earthquake magnitude and soil displacement / soil foundation rupture width and length. This study results allows for seismic seafloor displacement as a certain (empirically developed) function of the earthquake magnitude value (at a 95% significance level). Refs. 3 presents the cyclic load test results, estimating a pipeline embedment in weak sediment under vertical loading. Behavior of a buried pipeline under large ground slip imposed by the structure vertical displacements is investigated in Refs A. Pipeline is represented as a sequence of straight beam finite elements supported on the bottom by bearing soil springs, and on the top by uplift soil springs. The local buckling and wrinkling, and conditions of surrounding soil, were found to have great influences on behavior of pipelines. Refs. 5 considers a buried pipeline transverse movements due to displacement in the ground caused by landslides, that is very similar to a seismic strike slip-fault form of