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Estimation Of Material Parameters Probability Distribution And Failure Probability For Salt Rock Underground Storage Cavern

Posted on:2012-08-30Degree:MasterType:Thesis
Country:ChinaCandidate:W Q ZhangFull Text:PDF
GTID:2212330338463870Subject:Structural engineering
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In recent years many research show that salt rock exhibits pronounced time-dependent deformation or creep under relatively low stress level and has very low permeability and porosity. Salt rock creeps to a very large strain without fracturing and tends to be self-healing. So salt rock is considered as an ideal material for the storage of natural gas, petroleum and wastes, especially nuclear wastes, e.g. It is very important to study site selection and construction of salt rock underground storage cavern for the sake of the safety of nation energy storage. For the safety of salt rock underground deposit, uncertainties and their influences should be considered, and failure probability of operation period should be analyzed.For the safety of geotechnical structure, influences of randomicity should be considered because of kinds of existing uncertainties. Before the system failure probability of salt rock oil and gas underground deposit is estimated, parameters' probability distribution of correlative materials must be determined. By reason of the complexity of underground deposit, taking account of reality of project, distribution of random variables of geotechnical materials are determined by maximum likelihood estimation method, distribution density functions are analyzed by Bayes formula.Based on finite element model, equation of limit state surface is transformed into explicit equation by response surface method. Combined with Monte Carlo method, computation efficiency is improved. In this paper, failure probabilities of one-cavern and two-cavern numerical model are calculated, the main points are summarized as follows:(1) Single-cavern numerical computation model is established, and Mohr-Coulomb criterion is determined as failure criterion. The system reliability is calculated based on the computation of reliability of some key points. The element with highest failure probability is determined first, then failure mode and system reliability are computed.The result of the analysis and computation shows that the controlled failure mode is shear failure when the storage pressure is at a low level. Reliability of middle part of deposit improves as storage pressure increase. At the same time, while the storage pressure is higher than a certain value, the reliability reduced greatly. In conclusion, the storage pressure should be controlled strictly.(2) Concentrated underground storage caverns are often used as an efficient way of energy storage. However, reasonable distance between cavities is indispensable for the safety and normal operation of underground storage caverns. Many factors such as complex geological and operation conditions, high variability of material parameters will influence the safety of caverns.In this paper, taking account of stability and sealability, Mohr-Coulomb criterion and damage-dilatancy criterion are used, and then plastic area and damaged area under different conditions are calculated by a two-cavern numerical model with different distances and operating pressures. Failure probabilities of elements nearby the caverns are also calculated by response surface method combining with Monte Carlo method.The results indicates that at normal operation conditions, plastic area and failure probabilities of elements nearby the caverns increase obviously as distance between caverns decreasing. At the same time, damaged area increases remarkably as distance between caverns decreasing, the damaged area even connects the two caverns when operation pressure is low. Calculation results also show that when operation pressures of two caverns are different, the influence on failure probabilities under different pressures will deminish as distance between caverns increasing. Considering for the calculation results of stability and sealability, taking two times of the diameter of cavern as the minimum distance between caverns is reasonable.
Keywords/Search Tags:salt rock, response surface method, failure probability, distance between caverns
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