ITA/AITES accredited materialDynamic response of underground gas storage salt cavern under seismic loads
Introduction
Natural gas is the most environmentally friendly fossil fuel because its combustion generates the lowest CO2 emissions. In addition, it is available in larger quantities than crude oil. Therefore, it is increasingly replacing crude oil as one of the main energy sources in China. Notwithstanding limited domestic gas production, natural gas consumption has increased significantly, thereby increasing China’s dependency on natural gas imports. This in turn increases the importance of natural gas storage to ensure supply security. Underground salt caverns are considered the most suitable storage facilities for natural gas because of their capacity for fast delivery and ability to quickly switch from injection to production (Nazary et al., 2013, Mortazavi and Molladavoodi, 2012, Wang et al., 2013, Wang et al., 2012, Wang et al., 2011, Hartmann et al., 2012). Therefore, underground salt cavern gas storage facilities have become one of the hot topics in engineering in China.
Fig. 1 presents the depths, shapes, and relative volumes of typical salt cavern gas storage facilities in the world (John, 2006). Such facilities are characterized by wide depth distribution (from a few hundred to more than two thousand meters), different shapes, massive volume (typically several hundred thousand to over one million cubic meters), and locations in complex geological formations. These characteristics make salt cavern gas storage facilities vulnerable to earthquakes. Once the cavern fails during an earthquake, high pressure and escaping natural gas can cause fires and explosions. Therefore, studies on the dynamic response and safety evaluation of salt cavern gas storage facilities under seismic loads are necessary.
Earthquakes can trigger additional geological disasters. Structures are subjected to excessive loads in an instant, thereby damaging surface and underground structures, and causing such structures to fail. However, more studies focus on the effects of earthquakes on surface structures, such as buildings, pipelines, and dams, than on underground structures. This is due to the fact that failures of and damages to surface structures are more easily observable than those of underground structures, especially when the underground structure is very deep. However, some recent earthquakes, such as those that took place in 1995 in Kobe, Japan, in 1999 in Chi-Chi, Taiwan, in 1999 in Kocaeli, Turkey, in 2008 in Wenchuan, China, and in 2011 in Yushu, China, have seriously damaged underground structures (Iida et al., 1999, Hashash et al., 2001, Fahimifar and Vakilzadeh, 2009, Chen et al., 2012), thereby prompting engineers to pay more attention to the seismic designs of underground structures.
Existing studies about underground structures under seismic loads mainly focus on tunnels, underground powerhouse caverns, subways, and underground oil and gas pipelines. For example, Dowding and Roten (1978) investigated the failure of tunnels under seismic loads and stated that earthquakes affect a tunnel in three ways: surface soil liquefaction and landslides, large fault displacement, and vibration (dynamic response of the tunnel). Zhu et al., 2011, Zhu et al., 2012, Zhu and Zhao, 2013 studied the effects of normally and obliquely incident waves on joints and pointed out that joints had a notable influences on the energy transmitted by the seismic waves. Li (2013) and Li et al., 2013, Li et al., 2014) proposed the thin-layer interface model to study seismic wave propagation normally normal to a filled joint and discussed the effect of the incident angle, the ray path derivation, and some other parameters on the vibration response of underground structures. Their results showed that oblique seismic waves dramatically influenced the dynamic response of an underground rock cavern group. Takashi and Norikazu (1996) investigated the failure of a subway in Kobe during the 1995 Hyogo-ken Nambu earthquake and conducted corresponding numerical simulations. The results indicated that the maximum shear stress reached almost twice the limit shear strength of the rock masses that surrounded the subway. Takashi and Norikazu thought that the shear stress produced by the earthquake was the critical factor that caused the failure of the subway’s intermediate pillar. Davis and Bardet (2000) conducted field investigations on 61 corrugated metal pipes shaken by the 1994 Northridge earthquake. They found that 28 of the small-diameter corrugated metal pipes performed well, whereas 32 large-diameter corrugated metal pipes exhibited results that ranged from no damage to complete collapse. However, studies on the dynamic response of salt cavern gas storage facilities in an earthquake are rare.
In this paper a dynamic elastoplastic damage constitutive model is proposed to investigate the damage characteristics of rock salt during an earthquake. A numerical model of salt cavern gas storage facilities in China is established using the FLAC3D software. The dynamic response and safety of a salt cavern gas storage facility are studied based on the proposed damage constitutive model and numerical simulations. Encouraging results were obtained, which can serve as technological references for the seismic design of salt cavern gas storage facilities.
Section snippets
Dynamic elastoplastic damage constitutive model
The damage constitutive model is the foundation for evaluating the safety of rock masses that surround salt cavern gas storage facilities during an earthquake. A dynamic elastoplastic damage constitutive model is proposed based on the Drucker–Prager criterion to determine the damage sustained by rock salt under seismic loads. Unlike the quasistatic excavation process of a cavern, under dynamic cyclic loads rock salt is characterized by greater hardening and fatigue damage. According to Guo et
Input angle of seismic wave
Earthquake occurrence time and place are highly uncertain, which means that the input angle of seismic waves on a salt cavern gas storage facility varies greatly. According to literature (Zhu et al., 2011, Zhu et al., 2012, Zhu and Zhao, 2013, Li, 2013, Li et al., 2013, Li et al., 2014), the input angle of a seismic wave significantly influences the displacement, maximum principal stress, and plastic zone, among others, of rock masses that surround caverns.
In the numerical simulations, the
Conclusions
- (1)
A dynamic elastoplastic damage constitutive model is proposed according to the failure characteristic of a rock mass under seismic loads. The model is implemented by the embedded FISH language of FLAC3D. The three-dimensional numerical simulations of a salt cavern gas storage facility in China are conducted by using FLAC3D and the proposed damage constitutive model. Boundary conditions and seismic wave processing are discussed.
- (2)
The proposed damage constitutive model and numerical simulations are
Acknowledgments
The authors wish to acknowledge the financial supports of National Basic Research Program of China (973 Program) (Grant No. 2009CB724600) and National Natural Science Foundation of China (Grant No. 41272391). Thanks to Professor Jaak J.K. Daemen for his constructive suggestions and modifications.
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