Zhang, Yuhao (Chinese Academy of Sciences) | Shi, Xilin (Chinese Academy of Sciences) | Ma, Hongling (Chinese Academy of Sciences) | Yang, Chunhe (Chinese Academy of Sciences) | Ye, Liangliang (Chinese Academy of Sciences) | Han, Yue (Chongqing University) | Zhang, Nan (Chongqing University)
ABSTRACT: Reasonable operating pressure is the key to ensure the safe operation of salt cavern gas storage. The determination of upper and lower limit operating pressure is particularly important. Take into account the characteristics of a bedded salt mine, a set of methods is proposed for determining the upper and lower limit operating pressure. Upper limit operating pressure is determined by formation pressure gradient, cavern roof depth, and cavern wall compact principles. The lower limit operating pressure is determined by the mechanical calculation results (volume shrinkage, key point displacement and plastic zone distribution) and other people's research results. The specific research object is a bedded salt gas storage cavern in China's Jintan. The upper limit operating pressure is determined as 18 MPa by the above proposed method. The simulation model is established by FLAC3D software, based on the physical mechanical experimental parameters of the cavern in the mining area and combined with the data of sonar measurement. The model is used to simulate the mechanical behavior of the cavern in low pressure operating conditions. By analyzing volume shrinkage, plastic zone volume, key point displacement and plastic zone distribution, the lower limit operating pressure is designed as 8 MPa. The research results can provide a reference for operating pressure of bedded salt cavern gas storage.
The stable supply of natural gas is important for the stable development of China's society. Increasing the proportion of natural gas in the energy structure is an important measure to improve the environment and manage urban haze from coal power plants and automobiles. Because of the geological conditions and energy reserves of China, it is urgent to build underground natural gas storage. In 1999, salt caverns were first considered for natural gas storage in China. In order to ensure that the West-East Gas Transmission Project and the Sichuan-to-East Gas Transmission Project can better address the Yangtze River Delta gas supply demands. China plans to build salt cavern gas storage in Jintan, and Yingcheng. In 2005, the first salt cavern for natural gas storage was completed and put into operation in Jintan. Compared with other countries’ deposits, the salt rock layer presents the characteristics that “the number of salt rock layers is large, the thickness of single layers is thin and the content of insoluble impurities is high“ (Yang et al., 2009). The natural gas storage design, construction, and operations in these bedded salt formations cannot simply copy the experience of storage in salt domes and thick bedded salt strata. This study on bedded salt rocks is mainly focused on the mechanical properties of layered salt rocks in China. A large number of on-site sampling tests and numerical simulations show that the interlayer is usually the first part to be impaired (fractured or sloughed) (Yin et al., 2006, Li et al., 2006). It can be seen that interlayers are important for the stability of salt cavern gas storage.
Based on remolded clay sediment obtained from a certain area in the South China Sea, a series of tri-axial unconsolidated untrained compression tests on clay sediment containing tetrahydrofuran hydrate were performed under the conditions of different hydrate saturation and confining pressure. The stress-strain curve of clay sediment containing hydrate consists of three stages: elastic (when strain is less than 1.5%), plastic (when strain is between 2% and 6%) and strain hardening (when strain is greater than 6%) stage, and very different from that of clay sediment which behaves only plastic failure. Moreover, the undrained shear strength of clay sediment containing hydrate increases about 1~7 times than that of clay sediment and decreases by 50% after hydrate dissociation.
Gas hydrates, as a potential energy resource, are usually occurring as ice-like solids and widespread in sand and clay sediments in deep marine continental margins and permafrost regions. The mechanical properties of hydrate-bearing sediments are one of the most important information and parameters for analyzing the stability of seabed and foundation during the exploration of gas hydrates. Because of the low temperature and high pressure conditions which hydrates form in natural sediments, the synthesis and tri-axial compression tests in laboratory on hydrate-bearing sediments have been more difficult to simulate in situ environment and perfectly be accomplished than clay and sand sediments. The study on mechanical property of sediments containing gas hydrate has always been a hot research topic since 2000.
Until now, most of studies on mechanical properties of hydrate-bearing sediments are focus on sand sediment containing methane hydrate and based on triaxial compression test results in laboratory. Previous researches (Winters 2007, Waite 2004, Hyodo 2007/2013, Masui 2005/2007, Miyazaki 2011, Masui 2005/2007) show that the strength of hydrate-bearing sediments depends on hydrate saturation, confining pressure, grain size distribution, density, temperature, strain rate, synthesis method and other factors. However, there are seldom researches (Yun 2007, Song 2014) on mechanical properties of clay sediment containing hydrate, and the part of reason is because gas hydrate is more difficult to be synthesized in clay sediment in laboratory. Yun compared triaxial test results of sand, silt and kaolinite containing tetrahydroguran hydrate and concluded that the stress-strain behavior of clay sediment containing hydrate is very different from sand sediment containing hydrate and strongly relative to the grain size and cementation. Therefore, the further study on shear strength and stress-strain behavior of clay sediment containing hydrate is necessary and should to be taken more attention.
Crustal stress, and especially its orientation, is one of the key parameters in geotechnical mechanics and engineering. Because of the complexity of traditional crustal stress testing methods, especially in deep holes, the determination of crustal stress orientation is difficult, which is hindering the development of crustal stress testing technology. Therefore, the quick, effective, and accurate measurement of crustal stress in deep boreholes has important practical significance. This paper presents a method for the fast, effective, and accurate determination of deep crustal stress orientation by using panoramic stereopair imaging technology and stressed borehole geometric shape. Firstly, borehole panoramic stereopair images of the borehole wall are acquired using a biconical mirror, and the geometric features of the wall, are characterized by constructing a digital elevation model (DEM) of the borehole wall. The borehole geometric shape under crustal stress state is then analysed. In general, the circular borehole will become elliptical in shape. This paper presents the standard equation of the ellipse and establishes the relationship between the elliptical shape parameters and crustal stress. Lastly, from this relationship and the borehole geometric shape obtained by stereopair imaging, the orientation of crustal stress is determined utilizing the positioning capability of this technology. Simulations show that the borehole geometries and orientations are in agreement with the actual crustal stress orientations without the help of petrophysical parameters. This is a direct method of crustal stress orientation assessment based on the analysis of borehole geometric shape obtained by using the latest borehole stereopair imaging technology. This constitutes a new way of measuring deep crustal stress, which overcomes technological problems and promote the development of crustal stress measurement technology.
Jiang, Q. (Chinese Academy of Sciences) | Feng, X-T. (Chinese Academy of Sciences) | Fan, Q. (China Three Gorges Project Corporation) | Fan, Y. (China Three Gorges Project Corporation) | Xu, J. (China Three Gorges Project Corporation) | Chen, H. (China Three Gorges Project Corporation)
Classical rock support methods for underground engineering emphasize the importance of investigating and controlling the unloading deformation of surrounding rock, such as in the New Austrian Tunnelling Method (NATM) and Norwegian Method of Tunnelling (NMT). Current engineering practice in underground tunnels and caverns under high geostress conditions indicates that stress-induced collapse and rock bursts are the major engineering hazards. Prior to this kind of failure, crack initiation and propagation occurs. Thus, a new support concept, termed the crack-restraining method, has been developed based on the NATM and NMT for underground hard-rock engineering under high geostress conditions. This method includes three key technical points: (1) The length of the rockbolts is slightly greater than the maximum depth of cracking; (2) the optimal supporting time of the rockbolts is decided based on the crack evolution and deformation tendency; (3) shotcrete with high tensile strength is used to ameliorate the stress state of the rock mass' surface. The aim of these supporting efforts is to restrain the subsequent cracking and enhance the stress-bearing capability of the rock mass. Application of this approach to the Baihetan underground cavern (the largest hydraulic cavern in China) and other deep underground tunnels indicated that this crack-restraining support method is appropriate for determining the rockbolt length and supporting time.
Increasing attention is being given to deep-level excavation and mining due to the gradual exhaustion of shallow mineral resources and recent discoveries of deep mineral resources. Most South Africa gold mines have reached depths of more than 2000m, and the mining depth at Mponeng almost 4350m (Kwiatek, Plenkers, and Dresen, 2011); the Creighton mine in Canada has reached 2400m in depth (Snelling, Godin, and McKinnon, 2013); the Kolar gold mine has progressed to 2400m (Mishra and Panigrahi, 1999). Greater depths are also being attained in civil engineering projects –some traffic tunnels and hydraulic tunnels have been excavated at depths greater than 1000m (Zangerl et al., 2008, 2010, 2014). All these engineering projects have shown that the unloading behaviour of hard rock under high geostress is very complex. The support scheme for hard rock under the conditions encountered in these projects obviously presents a great technical challenge.
Liu, Xiaobo (University of Tulsa) | Chen, Jingyi (University of Tulsa) | Zhao, Zhencong (University of Tulsa) | Lan, Haiqiang (Chinese Academy of Sciences) | Liu, Fuping (Beijing Institute of Graphic Communication)
In reality, it is very important for us to deal with the irregular free surface and attenuation in numerical modeling of seismic wave propagation, which can help us accurately understand seismic wave characteristics. In this study, we develop a seismic wavefield simulation method based on 2D time domain second-order viscoelastic isotropic equations in regards of both surface topography and attenuation. First, the 2D second-order viscoelastic equations and free surface boundary condition are converted from Cartesian coordinates to curvilinear coordinates. The free surface boundary conditions are accurately applied to viscoelastic wave equations by using a boundary modified difference operator for the mixed derivatives. Second, the Convolutional Perfectly Matched Layer (CPML) boundary condition is applied to absorb the unwanted reflections from the model edges. Regarding the numerical algorithm, the finite difference methods are used to approximate elastic wave equations. We use first-order forward difference at surface, whereas the second-order central difference is used in the main computing domain and other boundaries of the model. The numerical test results show that irregular free surface is successfully inserted into seismic wave equations in viscoelastic media and illustrate the influence of attenuation.
Presentation Date: Tuesday, September 26, 2017
Start Time: 3:30 PM
Presentation Type: ORAL
Wang, Xiaokai (Chinese Academy of Sciences and the University of Texas–Austin) | Yang, Changchun (Chinese Academy of Sciences) | Li, Xueliang (Chinese Academy of Sciences) | Chen, Wenchao (Xi'an Jiaotong University) | Zhao, Haixia (Xi'an Jiaotong University)
The producing oil-field need the second/third time seismic data acquisition to evaluate the remaining production of crude oil. In the repeated data acquisition, the non-random well-pump noise affects the quality of seismic data. In order to know the characteristics of well-pump noise, we design a small single-point receiver array, and use this array to receive sufficient time duration of the typical wellpump noise. After analyzing the obtained well-pump noise, we find it has some characteristic differences with reflections, such as limited bandwidth, locating in lowfrequency area, low velocity, dispersion and bad spatial correlation. Taking into account these differences, we propose an iterative method for well-pump noise attenuation in the time-frequency domain. By applying the proposed method to synthetic signal and real field data, we demonstrate the superior performance of our method.
Presentation Date: Wednesday, September 27, 2017
Start Time: 11:25 AM
Presentation Type: ORAL
Migration-based seismic location methods can reliably and automatically image weak seismic sources with low signal-to-noise ratios, such as microseisms and microtremors. Compared with traditional diffraction stacking, cross-correlation stacking can extract more redundancy in the data and suppress noise better, since it delays and stacks cross-correlograms of pairwise receivers. In this work, we propose to double the correlation process of relative-correlation stacking, which is based on the differential travel-time from a master event and a target event to receivers. By doubling the correlation process, the origin time decouples itself to the relative location problem, and the stacking process becomes less sensitive to travel-time perturbations. We also generalize different correlation-based imaging methods by approximating the beamforming algorithm with cross-correlation process. Numerical examples demonstrate the feasibility of the proposed method and the generalization.
The seismic source location problem emerges in seismology areas with different scales, such as tremor and earthquake location (earthquake seismology), microseismic monitoring in reservoirs (exploration seismology), and rock burst monitoring in mines and tunnels (engineering seismology). As a counterpart of the conventional travel-time inversion method, migration-based source location method has been proposed due to its robustness and automatism for locating very weak passive seismic events (e.g. Cesca and Grigoli, 2015). Most of these source imaging methods are based on diffraction stacking operator, which stacks the waveforms with corresponding travel-time curves (e.g. Gajewski et al., 2007; Zhebel et al., 2010; Anikiev et al., 2014). Cross-correlation stacking is another stacking operator for imaging seismic sources, and it can be regarded as a subsidiary of seismic interferometric imaging (Schuster, 2004).
Presentation Date: Tuesday, September 26, 2017
Start Time: 9:45 AM
Location: Exhibit Hall C/D
Presentation Type: POSTER
Wang, Rui (Chinese Academy of Sciences, University of Chinese Academy of Sciences) | Wang, Yu (Chinese Academy of Sciences) | Wang, Shuo (Chinese Academy of Sciences) | Tang, Chong (Chinese Academy of Sciences, University of Chinese Academy of Sciences)
This paper addresses the problem of three-dimensional (3-D) way-point tracking for a biomimetic underwater vehicle (BUV) propelled by undulatory fins: RobCutt-II. Based on the specific mechanical design and control system configuration, the RobCutt-II can perform diversified locomotion patterns, especially submerging or surfacing vertically in the water. In order to deal with the challenging problem of motion control of the BUV in underwater operation, a switching control for 3-D way-point tracking is proposed. Moreover, we design an experiment simulating the actual underwater operation process. The RobCutt-II first dives to the desired depth vertically, then swims successively to the way-points along the horizontal plane, and finally keeps depth at the last way-point. Simulation and experimental results demonstrate the feasibility and effectiveness of the proposed switching control.
Natural selection and evolution make biological systems diversify into nearly every possible habitat and preserve remarkable adaptations for locomotion. Mimicking the unique undulatory propulsion mode of cuttlefish, biomimetic underwater vehicles (BUVs) propelled by undulatory fins have many advantages such as higher maneuverability, stronger disturbance rejection, and quieter actuation than conventional underwater vehicles equipped with jets or axial propellers (Neveln, Bai, Snyder, Solberg, Curet, Lynch and MacIver, 2013). Hence these BUVs are expected to be widely used in marine and military fields (Blidberg, 2001).
As growing research on the wave-like propulsion mechanism and hydrodynamics analysis demonstrate a variety of prospective utilities in undersea vehicles, researchers and engineers have developed many kinds of biomimetic robotic prototypes with fin propulsion. Curet et al. designed a robotic knifefish with an undulatory propulsor using 32 servo motors to drive the long-fin (Curet, Patankar, Lauder and MacIver, 2011). Hu et al. developed a robotic undulating model and proposed a control scheme that enabled it to mimic fin-ray undulation kinematics of live fish (Hu, Low, Shen and Xu, 2014). Rahman et al. designed a squid-like underwater robot with two undulating side fins simulating stingrays or cuttlefishes (Rahman, Sugimori, Miki, Yamamoto, Sanada and Toda, 2013). However, although many types of BUVs have been developed, few BUVs have been put into practical applications as far as we know. There are several possible reasons for this fact. In addition to the complex hydrodynamics and the model uncertainty in underwater environment, researchers seldom consider the closed-loop motion control such as way-point tracking for these BUVs, which are of primary importance for most applications.
The present study investigates experimentally the wave-dissipating performance of twin-plate breakwater under oblique random waves. Through extensive model tests, the effects of the relative plate width and relative wave height on the dissipating performance are investigated under various incident wave angles. The results show that the dissipating performance changes insignificantly within the measured range of 15°~ 60° incident angles. The transmission coefficient kt under oblique waves fluctuates around the value under normal waves. In addition, the transmission was directly affected by the relative plate width, i.e., the transmission was 001 < kt < 0015 for B=L < 005 and 0005 < kt < 0010 for B=L ≥ 005. In particular, under oblique waves the wave patterns after the twin-plate breakwater exhibited significant three-dimensional characteristics, which were different from those under normal incident waves. To validate the present experimental results, the measured transmission coefficients are compared to those of Neelamani and Gayathri (2006) under normal incident waves, and the comparison shows good agreement.
Conventional breakwaters have been widely used in coastal engineering all over the world. However, the cost for the construction of conventional breakwaters increases exponentially with the increase in the water depth. Furthermore, the conventional breakwaters obstruct natural currents, which could lead to hydroenvironmental issues. The permeable breakwater, on the one hand, seldom affects the current direction and hence maintains the balance of the sediment transport in the harbor. On the other hand, such a breakwater cannot stop strong currents and/or sediment transport, which might affect the operation of the harbor. The early permeable breakwater, which was proposed by Wiegel (1961), consisted of several rows of narrowly spaced piles and was gradually developed into a pile-foundation permeable breakwater with the upper part serving a wave dissipating function. In recent years, various permeable breakwaters were developed and applied in engineering projects, i.e., the horizontal plate breakwater (Patarapanich and Cheong, 1989; Nagata et al., 2003; Liu et al., 2008; Kee, 2009), T-type breakwater (Neelamani and Rajendran, 2002), suspended pipe breakwater (Mani and Jayakumar, 1995), and floating breakwaters (He et al., 2012). The horizontal twin-plate breakwater is probably the most effective one among these permeable breakwaters. It is well known that most of the wave energy concentrates near the water surface. The horizontal twin-plate breakwater consists of two horizontal plates, and the upper plate is placed near the water surface to dissipate the wave energy effectively. Besides, the breakwater has little effect on the propagation of the tide and current, which is preferable for natural coastal environments.
In this paper, a procedure is developed to analyze long-distance free-spanning pipelines on an uneven seabed through the incorporation of the Vector Form Intrinsic Finite Element (VFIFE or V-5) method with the bubble model, which is also called the UWAPIPE model. A dynamic contacting scheme is proposed to model variations of the contact situation between a seabed and a deforming pipeline on it. Specifically, a high-efficiency Message Passing Interface (MPI) parallel scheme is adopted to reduce the computing time of the procedure. Through the use of the procedure, the configuration of a 10 km long-distance pipeline lying on a real irregular seabed in the South China Sea is simulated, and the results are compared with a Remotely Operated Underwater Vehicle (ROV) survey and Det Norske Veritas (DNV) recommendations. The effects of the submerged weight and internal pressure of the pipeline are evaluated. Subsequently, the unevenness ratio is defined, and its relation to the pipeline bending moment is presented. The effects of the seabed undulation and adjacent spans are proved to be significant for the examined case.
Submarine pipelines commonly cross uneven seabed, and free spans could form due to the seabed undulations in practical engineering. In the conventional security assessment for free-spanning pipelines, only the span height and length are considered, while the effects of the seabed configuration and the resulting bending stresses are ignored (Rezazadeh et al., 2010). This would produce errors in the analysis of vortex-induced vibration, global buckling, axial walking, and even the integrity of the pipeline. To consider the effect of the uneven seabed, the pipeline configuration of the static equilibrium, including the free span scenarios, penetration in the soil, and corresponding stress distribution under different work conditions (e.g., as-laid, water-filled, and operational) must be obtained first (DNV, 2006; Larsen et al., 2002; Soreide et al., 2005).
The pipe-soil interaction plays a significant role in the evaluation of the global pipeline stability. In many previous studies, a series of nonlinear springs are used to model the soil resistances in the axial, lateral, and vertical directions, respectively (Gao et al., 2011; Larsen et al., 2002; Pereira et al., 2008; Sun et al., 2009; Wang et al., 2015). While the simplicity of the spring model is acknowledged, it is difficult to predict the actual pipeline embedment and the corresponding vertical soil resistance through the use of the idealized springs because they cannot reflect the actual soil deformation, especially the plastic component. Moreover, the spring model commonly relates the lateral and axial resistances to the pipeline submerged weight instead of to the varying vertical soil resistance (Tian and Cassidy, 2008).