FINITE ELEMENT ANALYSIS OF BURIED PIPELINES CROSSING REVERSE FAULT

Response evaluation of buried steel pipelines at active fault crossings is among the top seismic design priorities. This is because the axial and bending strains induced to the pipeline by step-like permanent ground deformation may become fairly large and lead to rupture, either due to tension or due to buckling. Surface faulting has accounted for many pipe breaks during past earthquakes, such as the 1971 San Fernando (USA), the 1995 Kobe (Japan), the 1999 Izmit (Turkey), the 1999 Chi-Chi (Taiwan) events and more recently, the 2004 Mid Niigata earthquake in Japan. Literature review suggests that previous researches in the analysis of pipeline subjected to fault motion have been mainly focused on the case of strike-slip fault. Certainly, a 3D large scale finite element analysis is a powerful method and allows a rigorous solution of the problem with minimizing the number of necessary approximations. The aim of present work is to examine and compare the mechanical response of continuous (welded) buried steel pipelines crossing active reverse faults by three dimensional FEM. General-purpose finite element program ABAQUS is employed to simulate accurately the mechanical behaviour of the steel pipe, the surrounding soil medium and their interaction, considering the non-linear geometry of the soil and the pipe through a large-strain description of the pipeline-soil system and the inelastic material behaviour for both the pipe and the soil. For 3D FEM continuum model, an elongated prismatic model is considered, where the pipeline is embedded in the soil. Four-node reduced-integration shell elements (type S4R) are employed for modeling the pipeline cylinder, whereas eight-node reduced-integration brick elements (C3D8R) are used to simulate the surrounding soil. The analysis is conducted in two steps; gravity loading is applied first and subsequently fault movement is imposed. Seismic fault plane is assumed to be located at the middle cross-section of the pipeline. The steel pipeline was of the API5L-X65 type, with a bi-linear elasto-plastic stress–strain curve given by Ramberg-Osgood model. The mechanical behavior of soil material is described through an elastic–perfectly plastic Drucker-Prager constitutive model. A contact algorithm is considered to simulate rigorously soil–pipeline interaction which accounts for large strains and displacements. The analysis proceeds using a displacement-controlled scheme, which increases gradually the fault displacement. Quasi-static analyses were carried out by applying fault offset components to soil block in the continuum FE models through a smooth loading function of time. Buried steel pipelines have been analyzed for reverse fault motion to study the influence of design parameters viz. crossing angle, backfill properties, burial depth, pipe surface property, pipe material and cross-section properties on maximum compressive strain, and buckling of the pipeline. The following main conclusions were obtained based on the studied response of pipeline subjected to reverse fault motion using the FEM model.
- For the steel pipeline subjected to reverse fault motion, compressive strain was always found to be more critical than the tensile strain.
- The capacity of the buried pipeline to accommodate the reverse fault offset could be increased by adopting: a loose granular backfill, a shallower burial depth, near-parallel orientation with respect to the fault line, a smooth and hard surface coating, and increasing pipe-wall thickness.
- Finally, the obtained information can provide either guidance for developing improved earthquake-resistant design or countermeasures to mitigate damage to pipelines crossing active reverse faults.