Razi University

Buried steel pipelines are lifelines for oil, gas and water, and their seismic response is governed by the surrounding ground. This dissertation examines buried steel pipelines across three stages of an earthquake using nonlinear finite element models in ANSYS. In Part 1, a three-dimensional soil–pipe model was verified against a large-scale strike-slip fault test and extended to base-displacement excitation, and a beam-on-nonlinear-spring model was assessed for the same loading. In Part 2, the effects of earthquake intensity, record frequency content (El Centro, Kobe and Sarpol-e Zahab), two-layer profiles, finite horizontal layers and inclined layers crossing the pipe on the transverse seismic response were studied with an extended Drucker–Prager model calibrated against reference modulus-reduction and damping curves. In Part 3, the response of an X65 pipeline to localized post-earthquake loss of soil support was analysed for 22 positions of the degraded region, with and without operating pressure, together with its sensitivity to soil properties and layering; the vertical soil–pipe interaction capacity was then determined with five Mohr–Coulomb formulations. In Part 1, the three-dimensional model reproduced the pipe deformation of the large-scale fault test with an error below 5%, and base-displacement excitation, in both the three-dimensional and the spring models, gave the displacement imposed on the pipe directly at every instant. In Part 2, reached a peak seismic stress below 6.5% of yield. The crown displacement is a column-scale index: in loose sand it consists mainly of accumulated plastic deformation (about 73% permanent at 0.3g), and in two-layer profiles it is controlled by the lower layer, whereas the pipe stress and the soil plastic strain are governed by the soil within about one to two diameters of the pipe. The stiffness and strength contrast between the layers acts in opposite directions on these two groups of indices: loose over dense sand increases the pipe stress by up to about 20% and reduces the displacement, whereas dense over loose sand increases the displacement up to eightfold. The critical record depends on the layering sequence, so PGA alone is not a sufficient selection criterion. A finite horizontal layer below the pipe changes the stress mainly through a local effect with a characteristic length of about one diameter, and a loose inclined band crossing the pipe raises the pipe stress up to 5.2 times. Under localized loss of support, internal pressure changes the governing mechanism: without pressure the stress amplification factor reaches 13.8 while the wall stays at 9% of yield, whereas at 7 MPa it is only 1.49 but the wall reaches 97% of yield and ovalisation 3.16% of the diameter. The two regimes are separated by a threshold of about 1 MPa, and 73% of the transition occurs below 0.5 MPa; in the pressurised regime the soil yields first, with soil plastic strain, settlement and separation amplified 69, 13.7 and 12.5 times relative to the empty pipe. At equal radial distance a void beneath the pipe causes 3.6 times the settlement of a void beside it, the direction of ovalisation reverses between the two, and the critical section moves by 45–90° with pressure. The influence extends to about 3.5 diameters, and a zone of about two diameters, where the wall reaches 89–97% of yield, is proposed as the critical zone for i  ection. The settlement caused by the loss of support is highly sensitive to soil properties: 10 kPa of cohesion reduced it by about 90% and removing dilatancy increased it about 2.9 times, whereas a 44% reduction of the modulus changed it by only 4–13%; in looser sands the wall stress exceeded the specified minimum yield strength, and poor compaction of the haunch zone raised the settlement by up to 80% of the dense–loose difference.