A rupture of buckled steel pipes on the tensile side of a cross-section is studied in this paper as the most plausible case of ultimate failure for the pressurized buried pipelines under monotonically increasing curvature. Finite element simulation of full-scale bending tests on two pressurized X80 pipes with different yield-to-tensile strength (Y/T) ratios were conducted. The Y/T ratio and internal pressure were identified as the crucial factors that have a coupled effect on the ultimate failure mode of buckled pipes. That is, the high values of Y/T ratio and internal pressure mutually trigger the rupture of buckled pipes on the opposite side of the wrinkling.
Steel pipelines are so ductile and can accommodate a large amount of post-buckling deformations while preserving their operational safety and structural integrity. To benefit from this outstanding quality and prevent the buckled (wrinkled) pipelines from premature rupture, the postbuckling behavior of the steel pipes should be well understood.
Rupture is one of the major failure limits to the integrity of pipelines that endangers the environment as well as the public safety and property. Comprehensive experimental and numerical studies on the fracture of buckled steel pipes (Das, 2003; Sen, 2006; Mohajer Rahbari, 2017) show that under increased monotonic curvature, successive buckles (wrinkling) are formed on the compressive side of the wall, and the occurrence of rupture at the wrinkling location is unlikely because of the ductile nature of steel material. Rupture of wrinkling can occur once buried pipelines are subject to a very rare and changing boundary conditions accompanied by extremely large plastic deformations toward tearing the wrinkled wall (Ahmed, 2011). However, experiments have shown that the increasing curvature can easily trigger the postbuckling rupture of the tensile wall on the opposite side of the wrinkling (Sen, 2006; Mitsuya et al., 2008; Tajika and Suzuki, 2009; Igi et al., 2011; Tajika et al., 2011; Mitsuya and Motohashi, 2013; Mitsuya and Sakanoue, 2015). This mode of failure seems very likely to be the rupture limit of the wrinkled pipes, as it occurs following the same regime of monotonic bending deformations that have previously made the pipe buckle.
Tan, Leichuan (China University of Petroleum) | Li, Ningjing (China Petroleum Pipeline Engineering Corporation) | Gao, Deli (China University of Petroleum) | Ren, Shaoran (China University of Petroleum) | Adeeb, Samer (University of Alberta) | Wang, Zhengxu (China University of Petroleum) | Gu, Yue (China University of Petroleum) | Li, Wenlong (China University of Petroleum) | Chen, Xuyue (China University of Petroleum)
ABSTRACT: Hydraulic fracturing practices in shaly unconsolidated sandstone reservoirs readily result in complex fractures due to high shale content, strong plasticity, and fracture toughness. This paper introduces a new method for manufacturing shaly unconsolidated sandstone that is supported by experimental results. In addition to the laboratory experiment, a Particle Flow Code (PFC) numerical model was established based on the relevant physical properties, mechanical parameters, and fluid-solid coupling theory. The fracture propagation law of shaly unconsolidated sandstone was comprehensively assessed. Shale content was found to significantly influence fracture propagation. Straight fractures tend to transform into circuitous pinnate fractures accompanied by seepage fracture zones as shale content increases. There is clear stress chain directivity accompanied by uneven distribution of stress after stress field loading, which produces shear stress as the fractures expand. The shear force created by shale can exceed easily the shear strength, leading to shear failure, uneven stress distribution and uneven compaction. The results of this study may provide a workable basis for optimizing the hydraulic fracturing process in shaly unconsolidated sandstone reservoirs.
Shaly unconsolidated sandstone reservoirs can be considered as the transition zone from unconsolidated sandstone to mud shale with typical characteristics of relatively poor physical properties, resulting in low oil and gas production rate and bad recovery. Due to the weak cementing properties, the shaly unconsolidated sandstone often indicates distinctly different mechanical properties from general sandstone, such as the significant elastoplastic features in hydraulic fracturing process. In view of such kind of low permeability reservoirs, whether the fracturing technology can create effective artificial fractures is the primary criterion to determine the success or failure of the fracturing measure.
Hydraulic fracturing technology has been widely used in the oil and gas industry for many decades, which can be regarded as a definitely impactful measure to rise the yield. Warpinski et al., 1979 pointed out that when fracturing is carried out in unconsolidated sandstone, there will be forming a very complex fracture. It was argued by Settar et al., 1989 that there will be fluid filtration phenomenon when unconsolidated sandstone is fractured, and the emerged fractured zone could be the cardinal reason for this. After recent years development, the physical simulation experiments of general sandstone are very developed both in theory and practice nowadays. Researchers (Chen et al., 2000; Khodaverdian et al., 2000; Gao et al., 2016; Tan et al., 2018; Feng and Gray, 2018) used sandstone specimens with the size of 300 mm×300mm×300 mm to simulate and analyze the effects of stress field, rock fracture toughness and internal branches on the fracture propagation. Bohloli et al., 2006 considered the influence of various types of fracturing fluid on the hydraulic fracturing technology, arguing that this technology is very suitable for the unconsolidated sandstone layer, and the fractures produced under the strong stress condition are relatively short. Jia et al., 2007 conducted the true three axis simulation experiment using fine sand and cement mixed with the proportion of 1:3, finding there is great influence of inclination angle, borehole azimuth and perforation on the fracture extension law. Jasarevic et al., 2010 and Feng et al., 2016 observed that micro fracture existed in the hydraulic fracturing of the unconsolidated sandstone. Meanwhile, the filtration zone can be easily formed as well.
Any catastrophic rupture scenarios of a steel pipe should be taken into considerations in the design and during the maintenance stage as the loss-of-containment may be accompanied by either property damage or fatal accidents. Ductile fracture of wrinkled (buckled) steel pipes on the tensile side of the cross-section is studied in this research as the most plausible case of ultimate failure for pressurized buried pipelines being subjected to monotonically increasing curvature. The results from two full-scale bending tests on X80 line pipe specimens that are pressurized up to 60% of specified minimum yield strength (SMYS) are considered as an input for the current study. The specimens possess the same dimensions and are made of X80 steel grade with different yield strength to tensile strength ratios (Y/T) of 90% and 83%. The specimen with higher Y/T ratio ruptured on the tensile side of the cross-section while experiencing post-buckling deformations. However, the specimen with lower Y/T ratio was unloaded after the formation of the local buckling.
Finite element analysis (FEA) of the full-scale tests were conducted and verified using the experimental data. The power law is calibrated to model the post-necking plasticity of steel using material test data, and, cumulative fracture criterion in conjunction with general fracture strain locus for the pipelines’ high-strength steel is implemented to predict the ductile fracture initiation in the pipe's wall. It is shown that the FE model accurately reproduces the load-displacement response and final rupture of the specimen with the higher Y/T ratio. For the other specimen, numerical simulation shows no rupture until the inner surface of the buckle comes into contact with itself which reveals that the lower Y/T ratio reduces the chance of rupture. Further numerical studies postulate that both Y/T ratio and internal pressure have a coupled effect on the rupture of wrinkled pipes and play a key role in triggering that kind of failure. That is, higher values of Y/T ratio and internal pressure increases the probability of the rupture of wrinkled pipes.