| The priority of energy strategic reserves in the 13 th Five-Year Plan should be expediting the construction of petroleum reserve base and natural gas storage to enhance the emergency load capability of natural gas in order to implement decisions and arrangements of the 13 th Five-Year Energy Plan formulated by the Party Central Committee and the State Council deeply.Underground salt cavern reserve possesses higher security and economy compared with onshore tanks and marine tanks, 60% of international strategic energy reserves are located underground. Salt could adapt the change of pressure in chamber well because of its peculiar properties of extremely low permeability, excellent ductility and great self-healing features after damaged. Therefore, underground salt cavern is the worldwide-recognized ideal reserve place of petroleum and gas.Surrounding rock of salt cavern would be certainly subjected to periodic load because of chronic gas injection and production, then fatigue damage happens. In a real-world situation, gas injection occurs periodically and production is not continuous, time intervals with non-stress exist among the circles during the operation of gas storage cavern, which reflects a more realistic state of fatigue, this phenomenon exists in not only the operation of gas storage cavern, but also the construction and operation of other projects. Therefore, the interval fatigue experiment inserted in gradient time intervals was designed based on classic fatigue experiment of salt, mechanical properties and its acoustic emission characteristics were studied utilized dislocation theory and classic fatigue damage theory, the influence mechanism of time intervals inserted in fatigue was explored.(1) Classic fatigue experiments and the interval fatigue experiments inserted in gradient time intervals were conducted, the stress-strain curves of interval fatigue appear sparse rule compared with those two kinds of tests.(2) The count method of fatigue circles named ‘O-β-α’ was proposed, which makes research of time intervals during fatigue in one same sample be possible. Insertion of time interval separates the original continuous development tendency of residual strain, the residual strain in circles following an interval(α circles) is generally larger than that in circles before the intervals(β circles), the axial, radial and volumetric residual deformation all behave as such rule.(3) The insertion of a time interval with no stress in the fatigue process accelerates the accumulation of residual strain: The longer the interval, the faster the residual strain accumulates during the fatigue process and the shorter the fatigue life of salt rock.(4) The post-peak elasticity modulus in α circle is different from which in β circle, the volatility is significantly and the average of elasticity modulus in circles after interval is larger than which in circles before interval generally, while Poisson’s ratio does not change obviously with interval.(5) α circles obviously produce a greater number of acoustic emission counts than β circles. Acoustic emission activity becomes more active in α circles during intervals of no stress and the longer the interval, the more obvious this phenomenon is.(6) The residual stress urges the inverse movement of dislocation during intervals, which is beneficial to the regression of dislocation and the generation of new glide plane. The reverse softening caused by the Bauschinger effect makes the inner structure of salt more unconsolidated, which accelerates the accumulation of plastic deformation.(7) A qualitative relationship between the accumulated damage variable and the time interval is established, an acceleration effect conclusion of time interval is obtained. A prediction model of salt’s fatigue life is proposed based on time interval, which has great reference value to fatigue evaluation subjected to different upper or lower stress and time interval.(8) The acceleration of fatigue life caused by interval is not existing all the time, a threshold around 900 seconds in the duration of the time interval exists. The fatigue life of salt would not decrease any longer when the duration of the time interval is larger than 900 seconds, a rising trend of fatigue life appears inversely. |
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