The Stress-Strain Rules Of Underground Water-sealed Oil Storage Caverns And The Optimal Designs

Underground Water-sealed Oil Storage Cavern (UWOC) is an advanced way of storing oil after the World War Two, which has the advantage characteristics of low cost, safe covert, land conservation, non-environmental pollution and low fuel consumption. Thus, it is widely used as the transfer and reserve oil depot. The construction of the large-scaled UWOC is imperative with the intensification of the contradiction between oil supply and demand home and abroad. Although there is some experience in the UWOC construction, we still are in the exploration stage and has not developed a theoretical system to guide the practice. Therefore, it is significant to carry out the study about UWOC project.There are a lot of factors influencing the stress-strain feature of the cavern groups in the UWOC project. The ones are object geological environment such as rock mass structure and its mechanical properties as well as stress. The others are subject factors such as cavern space and excavation program. On the base of the object geological environment, in order to carry out the optimal design and construction for the project, the study of the stress-strain rules of the caverns influenced by the cavern space and excavation program is a practical subject.With the example of UWOC project in Huangdao, the stress-strain rules of the caverns and the optimal design is studied in this paper. First, the rack mass structure and its mechanical properties as well as initial stress are analyzed from the view of the influencing factors. According to the two main controlling factors of cavern space and excavation program, the stress-strain rules are analyzed by the FLAC3D numerical simulation software and the optimal design are proposed. The main content of this paper are as follows.(1) Rock mass structures and mechanical propertiesFirst, the type of the discontinuities and their engineering geological significances are analyzed and their development features are depicted by the accurate measuring line method. And then the method of deep seated discontinuities identification based on digital borehole images is used to study their deep development features. On the base, the mechanical parameters of the rocak mass are calculated by the Hoek-Brown estimation method.Results obtained indicate that:There are some differences between the development features of the discontinuities in surface and deep. The amount of the discontinuities is reduced along the vertical direction and the dominant direction has some deflection. However, the number of the groups is same.(2) Initial geo-stress fieldThe initial geo-stress field feature is initially analyzed by the measuring data of the 25 experiment sections in 3 boreholes in site. The FLAC3D numerical simulation model is constructed considering the main faults and the initial geo-stress field is calculated by the damping least squares algorithm. Combining the results of the two analysis ways, the stress spatial distribution feature is obtained.Results obtained indicate that:The initial geo-stress is main the level tectonic stress, and the direction of the maximum level principal stress is N73°W. The distribution of the stress is controlled by the faults and topography. There is a low level principal stress zone near the fault, and the maximum level principal stress is 6.43～12.4MPa, and the minimum level principal stress is 4.56～8.48MPa between the elevation of 60～-150m. and the direction of the level principal stress has some deflection influenced by the faults. Meanwhile, there is a high level principal stress zone far from the fault, and the maximum level principal stress is 8.43～18.2MPa, and the minimum level principal stress is 6.56～15.2MPa.(3) The stress-strain rules of the caverns and the optimal designAccording to the two main controlling factors of cavern space and excavation program, the stress field, displacement field and plastic zone are analyzed by the FLAC3D numerical simulation software under the conditions of different cavern spaces and excavation programs. And the stress-strain rules are obtained by comparing and analyzing the calculated results of the different conditions. Therefore, the reasonable cavern space and excavation program are proposed according the judgment indexes of project.Results obtained indicate that:①The cavern space has little effect on the both side caverns but it has a great influence on the middle cavern, and the minimum principal stress increases and the maximum principal stress decreases with the cavern space growing on the wall between two caverns.②The stress concentration degree and the deformation amount around the cavern are not sensitive to the change of the cavern space. But the plastic zone and its scale are very sensitive to the space chage.③the excavation order has little effect on the stress and strain fields but has large effect on the plastic zone distribution. And the prior excavated cavern has a small plastic zone but the later one has a large plastic zone.④The reasonable space between the caverns is 1.4 times of the average cavern span. The clear distance between caverns is 28m. and the clear distance between cavern and construction roadway is 21m. And the jump excavation is the reasonable excavation way, that is, the two sides caverns are excavated first, and then the middle cavern is excavated. There are two innovation points in this paper.①The method of deep seated discontinuities identification based on digital borehole images is proposed and it provides a basement for the study of the deep-seated discontinuities development features.②The UWOC stress-strain rules with cavern spaces and excavation orders changing in granitic gneiss area is proposed and provide a theoretical fundamental for the design and construction.