Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

Behavior of Buried Subsea Gas Pipeline Using Vector Form Intrinsic Finite Element Method
Zhenmian Lia,b, Yang Yua,b*, Siyuan Chenga,b, Xin Liua,b, Yaofeng Youa,b, Yang Chena,b, Lixin Xua,ba State Key Laboratory of Hydraulic Engineering Simulation and Safety, Tianjin UniversityTianjin, Chinab Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration

Shanghai, China

Seismic fault movement is one of the threats to subsea buried pipelines
which cover a long distance for oil or gas transportation. Especially, forcontinental shelf, the earthquake is active where one plate squeezes withanother. The main hazards of seismic faults to subsea pipelines include:(1) cause or exacerbate spanning; (2) Strength or fatigue failure under
the combined action of seismic waves, environmental loads and soilliquefaction; (3) Global buckling or local collapse. Due to the externalhydrostatic pressure, buckling propagation is very likely to occur along
the pipeline and causes overall failure. Therefore, seismic analysis ofsubsea pipeline is of important scientific and engineering value.

ABSTRACTLong-distance pipelines often inevitably run through seismic faults
which may result in great threats to pipeline safety, such as twist
deformation, wrinkling and tensile rupture. Buckling behavior of buried
subsea gas pipeline under a strike-slip fault was investigated in this paperusing the vector form intrinsic finite element (VFIFE) method. A specialnonlinear soil spring model was developed for the VFIFE shell elements
and numerical strategies were proposed to solve the multifold
nonlinearities of geometric, elastoplastic material, and self-contact
collision. Two different pressure conditions, the pressurized pipeline andthe unpressurized pipeline, were considered respectively as the initialstates. Effects of the crossing angles and the diameter-thickness ratios onbuckling modes and axial strains were discussed. The results shows thatthe bending deformation of subsea pipeline is lateral buckling with twobends on both sides of the fault plane. For buckling failure judgement,
the maximum flattening parameter is more suitable for the unpressurized
pipeline while the maximum tensile strain for the pressurized pipeline.
The unpressurized pipeline is more prone to fail with local collapse andbuckling propagation. For thick pipelines, the deformations are
dominated by the bending caused by the seismic fault and the main
failure pattern is a dent of the compression side on the bends which evendevelop into a self-contact collision of the inner wall. For thin pipelines,
local collapse and buckling propagation are induced by a small fault
displacement and dominated by the external pressure. Both parallel andorthometric buckling propagation are observed. Whether the pressurized
or unpressurized pipeline, a smaller crossing angle leads to more serious
lateral deformations so that the critical fault displacement is smaller.
Several design suggestions for offshore pipeline considering seismic
faults are proposed. The results can be used to guide the seismic design
and buckling prevention research of subsea pipeline crossing seismic
faults.

Compared with onshore pipelines, there are far fewer reports on seismic
research for offshore pipelines. At the current editions, the internationalcodes, DNVGL-OS-F101 (2017) and API-RP-1111 (2015), only
emphasize the hazards of seismic loads and the importance of the seismic
design for subsea pipelines but not yet discuss them in depth. Actually,
as early as 1978, Nath and Soh (1978) studied the responses of the subsea
pipeline under lateral earthquakes. Kershenbaum et al. (2000)
investigated an unburied offshore “snaked” pipeline under various typesof seismic faults and found that longitudinal seismic faults have lesseffects on a straight pipeline than a “snaked” pipeline. Arifin et al. (2010)
used the soil-pipe element method to simulate the behaviors between thesoil and buried pipeline system and found that the pipeline system will
be safe under the seismic wave impact and more attention should begiven to fault hazards. Xu et al. (2015) considered several nonlinear
characteristics and systematically analyzed the behaviors of the pipeline
with different design parameters under strike-slip faults. Azimov et al.
(2017) discussed and quantified the influencing factors of pipeline
performance under seismic faults and proposed several measures toimprove the ability to maintain differential movement. Considering
temporal-spatial variations, Chen et al. (2020) conducted the transient
dynamic responses of subsea pipeline under seismic ground motions.
Recently, Triantafyllaki et al. (2020; 2021) evaluated the risk of buckling
failures of unburied subsea pipelines in the strike-slip fault and theinverse fault using three-dimensional elastic-plastic simulations andfound that proper adjustment of pipeline routes can significantly prevent
pipeline rupture. In general, there is no seismic evaluation and design
methods for subsea pipeline that can be practically used in engineering.
Therefore, the current common practice basically refers to the onshore

KEY WORDS: subsea pipeline, buckling, seismic fault, pipe-soil
interaction, the vector form intrinsic finite element method (VFIFE).

INTRODUCTION

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