Hydrate-containing marine soil hollow cylinder torsional shear test device and test method

Abstract

The invention belongs to the technical field of physical property measurement of geotechnical engineering foundations, and discloses a torsional shear testing device and a torsional shear testing method for a hydrate-containing marine soil hollow cylinder. The hollow cylinder torsional shear testing device for the hydrate-containing ocean soil comprises a hollow cylinder torsional shear axe, an inner pressure chamber control system, an outer pressure chamber control system, a temperature control system, a gas-water control system, an axial-torsional shear control system and a data acquisition and control system. The inner pressure chamber control system, the outer pressure chamber control system and the temperature control system are used for adjusting the temperature and pressure conditions in the hollow cylindrical torsional shear axe; therefore, static mechanical tests and dynamic mechanical tests under complex stress states such as directional shear tests, non-coaxial tests, inclined consolidation tests and the like of different principal stresses of hydrate ocean soil under different decomposition modes / decomposition time / decomposition gradients are realized.

Description

Technical Field

[0001] The present invention belongs to the technical field of geotechnical engineering basic physical property measurement, and relates to a hollow cylinder torsional shear test device and test method for hydrate-bearing marine soil. Background Technique

[0002] Accurately obtaining the mechanical parameters of hydrate-bearing marine soil under the true stress state is the primary prerequisite for the safe and efficient exploitation of hydrate resources, and is also the key condition for ensuring the reliable design and operation of offshore structures such as production drilling risers and suction piles (buckets) of floating offshore platforms. Existing publication numbers CN114252351B (ring shear test device), CN112710554B (triaxial shear and seepage integrated test device), CN118746502B (hydrate-bearing marine soil and structure interface shear property test device), and CN104833582B (hydrate-bearing marine soil triaxial test device) provide effective means for obtaining the mechanical parameters and deformation characteristics of hydrate-bearing marine soil. However, the stress state of hydrate-bearing marine soil under these instrument conditions is relatively simple, and it is impossible to truly restore the complex stress state of the hydrate-bearing marine soil reservoir, which is difficult to support the subsequent commercial exploitation of hydrate resources, and increases the probability of geological disasters (such as triggering tsunamis and earthquakes) and ecological environment chain reactions (such as exacerbating the greenhouse effect, etc.).

[0003] In the long-term exploitation project of hydrate resources, complex marine environmental loads such as storms and waves continuously transfer through production drilling risers to the shallow hydrate-bearing marine soil reservoir, which makes the hydrate-bearing marine soil always in a complex stress state, and even the principal stress direction rotates. Some research (Brosse et al. The undrained shear strength anisotropy of four Jurassic to Eocene stiff clays. Géotechnique, 2017, 67(8): 653-671.) points out that the shear strength of soil may show a trend of first decreasing and then slightly increasing with the rotation of the principal stress direction, showing strength anisotropy behavior. The hollow cylinder torsional shear test is considered to be the most suitable indoor test equipment for studying the strength anisotropy of soil, and is also one of the few test means that can accurately control any principal stress direction. However, the existing hollow cylinder torsional shear instrument based on freeze-thaw-unsaturated dynamic load published in CN202411942283.1 can only achieve temperature control, cannot meet the conditions for the occurrence of hydrate-bearing marine soil such as low temperature and high pressure, and lacks the unique gas source environment of hydrates. Therefore, the research and development of hollow cylinder torsional shear test devices for hydrate-bearing marine soil at home and abroad is still blank.

[0004] Therefore, the present invention innovatively develops a hollow cylinder torsional shear test device and test method for hydrate-containing marine soil, accurately depicting the macroscopic mechanical properties of hydrate-containing marine soil under complex stress states, which is a key prerequisite for promoting the development of special soil mechanics of hydrate-containing marine soil, and providing a solid scientific basis and technical support for resource development and the prevention and control of regional geological disasters. Summary of the Invention

[0005] The purpose of the present invention is to fill the domestic and foreign gaps in the hollow cylinder torsional shear test of hydrate-containing marine soil, and provide a hollow cylinder torsional shear test device and test method for hydrate-containing marine soil.

[0006] The technical solution of the present invention:

[0007] A hollow cylinder torsional shear test device for hydrate-containing marine soil, comprising a hollow cylinder torsional shear axe 1, an internal pressure chamber control system 2, an external pressure chamber control system 3, a temperature control system 4, a gas-water control system 5, an axial-torsional shear control system 6, and a data acquisition and control system 7;

[0008] The hollow cylindrical torsional shear ax 1 includes a high-pressure reaction vessel 1-1, an upper bearing platform 1-2, an upper pressure head 1-3, an annular metal water-permeable stone 1-4, an annular filter paper 1-5, an inner membrane 1-6, an outer membrane 1-7, a lower pressure head 1-8, an inner membrane fixator 1-9, an O-ring gasket 1-10, an O-ring seal 1-11, a connecting bolt 1-12, a lower bearing platform 1-13, an operating platform 1-14, a sealing bolt 1-15, and hydrate-containing marine soil 1-16; the high-pressure reaction vessel 1-1 is hermetically connected to the operating platform 1-14 through the sealing bolt 1-15 to provide a sealed environment for the hollow cylindrical torsional shear ax; the upper bearing platform 1-2 is located above the high-pressure reaction vessel 1-1 and is rigidly connected to the inner wall surface of the high-pressure reaction vessel 1-1 through a metal connecting rod, and is provided with bolt holes and an O-ring seal groove; the upper end of the upper pressure head 1-3 is provided with a bearing platform structure to provide an upper restraint for the inner membrane 1-6, and it is connected to the upper bearing platform 1-2 through the connecting bolt 1-12 to jointly provide an upper restraint for the hydrate-containing marine soil 1-16. The upper pressure head 1-3 is internally provided with an exhaust water channel and an oil discharge channel; the lower end of the inner membrane 1-6 is fixed to the lower pressure head 1-8 through the inner membrane fixator 1-9 and the O-ring gasket 1-10, and its upper end is fixed to the upper pressure head 1-3 through the O-ring seal 1-11 and the connecting bolt 1-12; the lower end of the outer membrane 1-7 is fixed to the lower pressure head 1-8 through the O-ring seal 1-11, and its upper end is also fixed to the upper pressure head 1-3 through the O-ring seal 1-11; the space between the outer membrane 1-7 and the high-pressure reaction vessel 1-1 forms an outer pressure chamber, and the internal space of the inner membrane 1-6 forms an inner pressure chamber after being filled with silicone oil. The pressures of the inner pressure chamber and the outer pressure chamber are respectively adjusted through the inner pressure chamber control system 2 and the outer pressure chamber control system 3; the lower pressure head 1-8 is connected to the lower bearing platform 1-13 through the connecting bolt 1-12 to jointly provide a lower restraint for the hydrate-containing marine soil 1-16. The lower pressure head 1-8 is internally provided with an inner pressure chamber silicone oil channel and an air intake water channel; the annular metal water-permeable stone 1-4 is arranged on the hydrate-containing marine soil 1-16, and its surface is provided with anti-slip blades to prevent the annular metal water-permeable stone 1-4 and the hydrate-containing marine soil 1-16 from detaching during the torsional shear test, which may affect the test results.

[0009] During the test, first fix the lower end of the inner membrane 1-6 on the lower pressing head 1-8. Place the inner supporting membrane barrel inside the inner membrane 1-6 and turn the upper end of the inner membrane 1-6 outwards onto the inner supporting membrane barrel, so that the inner surface of the inner membrane 1-6 closely adheres to the inner supporting membrane barrel. Fix the lower end of the outer membrane 1-7 on the outside of the lower pressing head 1-8. Place the outer supporting membrane barrel inside the outer membrane 1-7 and turn the upper end of the outer membrane 1-7 outwards onto the outer supporting membrane barrel, so that the inner surface of the outer membrane 1-7 closely adheres to the outer supporting membrane barrel. After sequentially placing the annular metal permeable stone 1-4 and the annular filter paper 1-5 on the lower pressing head 1-8, a hollow cylindrical sample-making space is achieved through the above steps. Then, layer by layer fill the matrix material of the hydrate-containing marine soil 1-16 into it. After the filling is completed, place the annular filter paper 1-5 and the annular metal permeable stone 1-4 on the hydrate-containing marine soil 1-16 in sequence, and then place the upper pressing head 1-3. Fix the upper end of the inner membrane 1-6 on the upper pressing head 1-3 through the O-ring 1-11, and then remove the inner supporting membrane barrel. Fix the outer membrane 1-7 on the upper pressing head 1-3 through the O-ring 1-11, and then remove the outer supporting membrane barrel. Fix the hydrate-containing marine soil 1-16 together with the lower pressing head 1-8 on the lower bearing platform 1-13 through the connecting bolt 1-12. Adjust the height of the lower bearing platform 1-13 through the axial-torsion servo at the bottom of the device until the upper pressing head 1-3 is tightly connected to the upper bearing platform 1-2. The O-ring 1-11 and the connecting bolt 1-12 inside the upper bearing platform 1-2 together serve to form a closed inner pressure chamber.

[0010] The inner pressure chamber control system 2 includes an inner pressure chamber volume controller 2-1, an inner pressure chamber silicone oil storage tank 2-2, an inner pressure chamber pressure regulating valve 2-3, valve a 2-4, valve b 2-5, valve c 2-6, an upper oil discharge pipeline 2-7, and an oil discharge valve 2-8. The inner pressure chamber volume controller 2-1 converges with the inner pressure chamber silicone oil storage tank 2-2 through the pipeline after valve b 2-5 and then through the pipelines after the inner pressure chamber pressure regulating valve 2-3 and valve a 2-4 in sequence. The converging pipeline reaches the inside of the inner pressure chamber through the inner pressure chamber silicone oil channel provided inside the lower pressing head 1-8. The inner pressure chamber sensor 7-5 and valve c 2-6 are arranged on the converging pipeline, jointly providing pressure guarantee for the inner pressure chamber of the hollow cylinder torsional shear axe 1. The upper oil discharge pipeline 2-7 and the oil discharge valve 2-8 are sequentially connected to the oil discharge channel arranged inside the upper pressing head 1-3, aiming to ensure that the inner pressure chamber is filled with silicone oil.

[0011] The outer pressure chamber control system 3 includes an outer pressure chamber volume controller 3-1, an outer pressure chamber silicone oil storage tank 3-2, an outer pressure chamber pressure regulating valve 3-3, valve d 3-4, valve e 3-5, and valve f 3-6. The outer pressure chamber volume controller 3-1 is connected to the outer pressure chamber silicone oil storage tank 3-2 through the pipeline after valve e 3-5 and converges with the pipeline after the outer pressure chamber pressure regulating valve 3-3 and valve d 3-4 in sequence. The converging pipeline reaches the outer pressure chamber through the wiring channel inside the operation console 1-14. The outer pressure chamber sensor 7-6 and valve f 3-6 are arranged on the converging pipeline, jointly providing pressure guarantee for the outer pressure chamber of the hollow cylinder torsional shear axe 1.

[0012] The temperature control system 4 mainly provides a suitable temperature environment for the synthetic hydrate-bearing marine soil 1-16, and includes a water area device 4-1, a water area jacket 4-2, and a heat preservation pipeline 4-3. The water area device 4-1 is hermetically connected to the water area jacket 4-2 through the heat preservation pipeline 4-3. The water area jacket 4-2 is located inside the inner wall of the high-pressure reaction axe 1-1. The two work together to achieve temperature control and provide temperature guarantee for the hollow cylinder torsional shear axe 1.

[0013] The gas-water control system 5 includes a hydrate gas storage tank 5-1, a pressure regulating valve 5-2, a gas-free water volume controller 5-3, valve g 5-4, valve h 5-5, valve i 5-6, an exhaust water pipeline 5-7, and an exhaust water valve 5-8. After passing through the pressure regulating valve 5-2 and valve g 5-4, the hydrate gas storage tank 5-1 is divided into two branches. One branch is connected to the intake water channel on the lower platen 1-8 after passing through valve h 5-5 and the gas-water pressure sensor 7-4, jointly providing hydrate gas for the synthetic hydrate-bearing marine soil 1-16 and providing the gas pressure required for synthesizing hydrates. The other branch is connected to the inlet end of the gas-free water volume controller 5-3 after passing through valve i 5-6. The gas-free water volume controller 5-3, valve h 5-5, and valve i 5-6 work together to degas and saturate the hydrate-bearing marine soil 1-16. The exhaust water pipeline 5-7 is connected to the exhaust water channel inside the upper platen 1-3, and the exhaust water valve 5-8 is arranged thereon. The specific operation is as follows: after synthesizing the hydrate-bearing marine soil 1-16, close the pressure regulating valve 5-2, valve g 5-4, and valve h 5-5. After adjusting the pressure control of the gas-free water volume controller 5-3 to be consistent with the pore pressure inside the hydrate-bearing marine soil 1-16, open valve h 5-5 and valve i 5-6. By opening and closing the exhaust water valve 5-8 connected to the exhaust water channel inside the upper platen 1-3 and the exhaust water pipeline 5-7 multiple times, a channel for hydrate gas and gas-free water is provided during the process of degassing and saturating the hydrate-bearing marine soil 1-16.

[0014] The described axial-torsional shear control system 6 includes an axial-torsional shear servo control system 6-1, a guide ring 6-2, a dynamic sealing ring 6-3, and an axial-torsional shear force transfer rod 6-4, mainly applying complex shear stress to the hydrate marine soil 1-16 specimen. Specifically, the upper end of the axial-torsional shear force transfer rod 6-4 is connected to the lower bearing platform 1-13 through a connecting bolt 1-12, its lower end is connected to the axial-torsional shear servo control system 6-1, and it passes through the guide ring 6-2 and the dynamic sealing ring 6-3 through the operating platform 1-14. The dynamic sealing ring 6-3 can still ensure the effectiveness of sealing under a pressure of 35 MPa.

[0015] The described data acquisition and control system 7 includes various sensors, a data acquisition instrument 7-9, and a host computer 7-10, mainly realizing the automatic control and acquisition of the hydrate marine soil hollow cylinder torsional shear test device and ensuring the reliability of test data. Specifically, an axial displacement sensor 7-1 and a torsional angle sensor 7-2 are fixed on the axial-torsional shear force transfer rod 6-4, respectively used to monitor the axial displacement and torsional angle during the test of the hydrate marine soil 1-16; an axial force and torque sensor 7-3 is fixed on the metal connecting rod connected to the upper bearing platform 1-2, used to monitor the axial force and torque during the test of the hydrate marine soil 1-16; a gas-water pressure sensor 7-4 is used to monitor the pore pressure inside the hydrate marine soil 1-16, located on the pipeline connected to the air and water inlet channel on the valve h5-5 and the lower platen 1-8; an internal pressure chamber sensor 7-5 is used to monitor the confining pressure inside the internal pressure chamber, located on the pipeline connecting the valve c2-6 and the lower platen 1-8; an external pressure chamber sensor 7-6 is used to monitor the confining pressure inside the high-pressure reaction autoclave 1-1; a temperature sensor 7-7 is fixed on the metal connecting rod connected to the upper bearing platform 1-2, used to monitor the temperature inside the high-pressure reaction autoclave 1-1; these sensors are connected to the data acquisition instrument 7-9 through a data acquisition line 7-8, and the test data is input into the host computer 7-10 after being converted by the signal of the data acquisition instrument 7-9, realizing the automatic acquisition and control of data.

[0016] Preferably, the material of the high-pressure reaction autoclave 1-1 is 7075 aviation aluminum alloy, and the upper bearing platform 1-2, the upper platen 1-3, the lower platen 1-8, the lower bearing platform 1-13, and the operating platform 1-14 are all made of 1-1316L stainless steel. The water jacket 4-2 has a spiral groove design inside to ensure rapid heat exchange of the hollow cylinder torsional shear autoclave 1.

[0017] Preferably, the hydrate gas in the hydrate gas storage tank 5-1 can be methane gas with a purity of 99.99%, carbon dioxide gas with a purity of 99.99%, or xenon gas with a purity of 99.99%.

[0018] Preferably, all pipelines involved are made of soft stainless steel pipelines, and 1 / 16 stainless steel seamless jacketed pipes and high-pressure pneumatic pipelines with good flexibility are used.

[0019] Preferably, the inner pressure chamber volume controller 2-1, the outer pressure chamber volume controller 3-1, and the gas-free water volume controller 5-3 are all spiral pumps controlled by a microprocessor, which can accurately control and measure the fluid pressure and volume changes, and can provide a pressure of not less than 35 MPa.

[0020] Preferably, the sensors can be used in environments with high and low pressure differences (5 kPa - 35 MPa) and high and low temperature differences (-20 - 65 °C), and the measurement accuracy is ±0.1%.

[0021] Preferably, the inner pressure chamber control system 2, the outer pressure chamber control system 3, the temperature control system 4, the gas-water control system 5, and the axial-torsional shear control system 6 can all be manually or automatically controlled by a computer. The data acquisition and control system 7 of the instrument adopts two control methods of stress control and strain control. Among them, stress control uses the stress increment change rate as the control method, while strain control uses the displacement change rate as the control method.

[0022] A test method for a hydrate-containing marine soil hollow cylinder torsional shear device, comprising:

[0023] S1. Line connection: After the pipelines of the hydrate-containing marine soil hollow cylinder torsional shear device are connected, each sensor in the data acquisition and control system 7 is connected to the data acquisition instrument 7-9 and the upper computer 7-10 through the data acquisition line 7-8 in turn, and the digital control system is connected through the data acquisition control software in the upper computer 7-10, that is, the operation of the inner pressure chamber control system 2, the outer pressure chamber control system 3, the temperature control system 4, the gas-water control system 5, and the axial-torsional shear control system 6 is controlled by the upper computer 7-10. It must be noted that since the displacement control is relatively stable, each system is controlled by displacement to prevent test accidents. After ensuring the stable operation of all systems, zero the sensors.

[0024] S2. Specimen Preparation, Installation and Airtightness Detection: Raise the high-pressure reaction kettle 1-1 of the hollow cylinder torsional shear axe 1. After removing the lower pressing head 1-8 from the lower bearing platform 1-13 and placing it in an empty area, first fix the lower end of the inner membrane 1-6 to the lower pressing head 1-8 through the inner membrane fixer 1-9, O-ring gasket 1-10 and connecting bolt 1-12. Place the inner bearing membrane barrel in the inner membrane 1-6 and turn the upper end of the inner membrane 1-6 outwards onto the inner bearing membrane barrel, so that the inner surface of the inner membrane 1-6 closely adheres to the inner bearing membrane barrel; fix the lower end of the outer membrane 1-7 to the outside of the lower pressing head 1-8, place the inner and outer bearing membrane barrels in the outer membrane 1-7 and turn the upper end of the outer membrane 1-7 outwards onto the outer bearing membrane barrel, so that the outer surface of the outer membrane 1-7 closely adheres to the outer bearing membrane barrel; after placing the annular metal permeable stone 1-4 and the annular filter paper 1-5 on the lower pressing head 1-8 in sequence, the hollow cylinder sample preparation space is realized through the above steps. After placing the annular metal permeable stone 1-4 and the annular filter paper 1-5 in sequence, layer by layer fill the matrix material of the hydrate-containing marine soil 1-16 to the designed height. After placing the annular filter paper 1-5 and the annular metal permeable stone 1-4 on the upper surface of the matrix material in sequence, place the upper pressing head 1-3. After fixing the upper end of the inner membrane 1-6 to the upper pressing head 1-3 through the O-ring seal 1-11, remove the inner bearing membrane barrel. After fixing the outer membrane 1-7 to the upper pressing head 1-3 through the O-ring seal 1-11, remove the outer bearing membrane barrel. Fix the hydrate-containing marine soil 1-16 together with the lower pressing head 1-8 to the lower bearing platform 1-13 through the connecting bolt 1-12. Adjust the height of the lower bearing platform 1-13 through the axial-torsional servo at the bottom of the device until the upper pressing head 1-3 is tightly connected to the upper bearing platform 1-2. The O-ring seal 1-11 and the connecting bolt 1-12 inside the upper bearing platform 1-2 together serve to form a closed inner pressure chamber. Subsequently, lower the high-pressure reaction kettle 1-1 and fix it to the operating platform 1-14 through the sealing bolt 1-15. To eliminate the disturbance of each sensor during the process of filling the matrix material of the hydrate-containing marine soil 1-16, zero the sensors again to improve the accuracy of the test.

[0025] S3. Synthesis of hydrate-containing marine soil specimens: First, fill the inner pressure chamber with oil. Open valve c2-6, control valves b2-5 and a2-4 to inject silicone oil into the inner pressure chamber. At the same time, adjust the inner pressure chamber volume controller 2-1 to its maximum range. After about 0.5 times the volume of the inner pressure chamber of silicone oil is discharged from the upper oil discharge pipeline 2-7, it indicates that the inner pressure chamber is full. Then close valve a2-4 and the oil discharge valve 2-8. Apply an internal pressure of 5 kPa by controlling the displacement of the inner pressure chamber volume controller 2-1. Second, fill the hollow cylinder torsional shear apparatus 1 with oil. Open valve f3-6, control valves e3-5 and d3-4 to inject silicone oil into the hollow cylinder torsional shear apparatus 1. At the same time, adjust the outer pressure chamber volume controller 3-1 to its maximum range. After the silicone oil is full, close valve d3-4. Similarly, apply an external pressure of 5 kPa by controlling the displacement of the outer pressure chamber volume controller 3-1. At the same time, control the inner pressure chamber volume controller 2-1 and the outer pressure chamber volume controller 3-1 to apply a pre-pressure of 500 kPa to the matrix material of the hydrate-containing marine soil 1-16 in the hollow cylinder. At the same time, turn on the water area device 4-1 and adjust the temperature to 2 °C. The temperature adjustment process should last about 6 hours. After the temperature inside the hollow cylinder torsional shear apparatus 1 is stable, close valve i5-6. Then, open the pressure regulating valve 5-2, valve g5-4 and valve h5-5 in sequence to provide the required hydrate gas (methane gas with a purity of 99.99% or carbon dioxide gas with a purity of 99.99% or xenon gas with a purity of 99.99%) and the designed pore pressure (7900 kPa) for the hydrate-containing marine soil 1-16. At the same time, correspondingly control the pressure values of the inner pressure chamber volume controller 2-1 and the outer pressure chamber volume controller 3-1 to ensure that the reading of the gas-water pressure sensor 7-4 is always less than that of the inner pressure chamber sensor 7-5 and the outer pressure chamber sensor 7-6, and the readings of the inner pressure chamber sensor 7-5 and the outer pressure chamber sensor 7-6 are always equal. Finally, the reading of the gas-water pressure sensor 7-4 is 7900 kPa, and the readings of the inner pressure chamber sensor 7-5 and the outer pressure chamber sensor 7-6 are both 8000 kPa. Then close valve g5-4 and valve h5-5. The temperature and pressure conditions at this time are considered to meet the formation of methane hydrate solids inside the hydrate-containing marine soil 1-16. When the reading of the gas-water pressure sensor 7-4 is stable, it is considered that the synthesis of the hydrate-containing marine soil 1-16 is completed. Record the final gas pressure and use the gas consumption method to determine the true hydrate saturation of the hydrate-containing marine soil 1-16.

[0026] S4. Saturation and Consolidation of Hydrate Marine Soil Specimens: After the formation of hydrates inside the hydrate-containing marine soil 1-16 is completed, it is necessary to expel the hydrate gas in the specimen with nitrogen. The purpose is to prevent the excess hydrate gas from reacting with the water without gas during the water saturation process to form hydrates again under stable temperature and pressure conditions. Adjust the water pressure without gas of the water volume controller without gas 5-3 to the pore pressure inside the hydrate-containing marine soil 1-16. Open valve i5-6 and valve h5-5. After the readings of the gas-water pressure sensor 7-4 become stable, open the exhaust water valve 5-8 multiple times until the exhaust water valve 5-8 discharges water without gas that is three times the pore volume inside the hydrate-containing marine soil 1-16, and ensure that the pore water pressure coefficient B of each hydrate-containing marine soil 1-16 specimen exceeds 0.95. Then, the hydrate-containing marine soil 1-16 is considered a water-saturated specimen. According to the specific experimental conditions, simultaneously adjust the inner pressure chamber volume controller 2-1 and the outer pressure chamber volume controller 3-1 to the target values, and consolidate the hydrate-containing marine soil 1-16. When the axial displacement of the hydrate-containing marine soil 1-16 is less than 0.05% / min, it is considered that the consolidation is completed and the test can continue.

[0027] S5. Hollow Cylindrical Torsional Shear Test on Hydrate-Containing Marine Soil: During the test, maintain the temperature inside the hollow cylindrical torsional shear apparatus 1 constant to ensure that the hydrates inside the hydrate-containing marine soil 1-16 do not decompose during the torsional shear process. During the test, use the upper computer 7-10 to adjust the axial-torsional shear control system 6, the inner pressure chamber control system 2, and the outer pressure chamber control system 3, and apply axial force and torque to both ends of the hydrate-containing marine soil 1-16 specimen in the shape of a hollow cylinder respectively. Apply inner and outer confining pressures to the inner and outer pressure chambers respectively. By independently changing the magnitudes of the four external loads (axial force W, torque T, inner confining pressure P i and outer confining pressure P o ), it is possible to control the magnitudes and directions of the principal stresses inside the hydrate-containing marine soil 1-16 specimen.

[0028] The unit stresses (axial stress σ z , radial stress σ r , tangential stress σ θ , and torsional shear stress τ θz ) of the hydrate-containing marine soil 1-16 specimen should be calculated according to the following formulas:

[0029]

[0030] In the formulas, R and r are the outer diameter and inner diameter of the hydrate-containing marine soil 1-16 specimen respectively.

[0031] Based on these unit stress components, the maximum, minimum, and intermediate principal stresses of the hydrate-containing marine soil 1-16 specimen are:

[0032]

[0033] For convenience, generally the following four parameters related to the magnitude and direction of the principal stresses are adopted for test control and data processing:

[0034]

[0035] Wherein, the mean principal stress s and the deviator stress t are stress parameters; α is the principal stress direction angle, which is the angle between the maximum principal stress σ1 and the vertical direction; b is the intermediate principal stress coefficient, and its value ranges from 0 to 1. Using the stress path module of the instrument, stress or strain control for any path of the mean principal stress, deviator stress, principal stress direction angle, and intermediate principal stress coefficient can be directly carried out.

[0036] Here, the undrained direct shear test is taken as an example for detailed description. The undrained direct shear test refers to a shear test carried out under a constant principal stress direction angle α, and the mean principal stress s and the intermediate principal stress coefficient b are kept unchanged during the shear process. The cases of α = 90° and α = 0° respectively correspond to the tensile and compression tests of the hydrate-containing marine soil 1-16 specimen. At this time, the hydrate-containing marine soil 1-16 specimen adopts the strain control mode. For the hydrate-containing marine soil 1-16 specimen with α = 45°, the deviator stress t is directly controlled to increase linearly in the stress path module of the instrument to achieve the purpose of shearing the specimen. For tests with other principal stress angles, since both torsional shear strain and axial strain develop, the strain control method is also adopted. Most tests stop when the deviator strain reaches 20%. However, in order to observe in detail the failure mode of the specimen, especially the development process of the shear band, larger final strain values are set for individual hydrate-containing marine soil 1-16 specimens.

[0037] S6. After the test is completed, close the water area device 4-1. Correspondingly, the pressure of the inner pressure chamber volume controller 2-1 and the outer pressure chamber volume controller 3-1 is relieved by the pressure control, ensuring that the reading of the gas-water pressure sensor 7-4 is always less than that of the inner pressure chamber sensor 7-5 and the outer pressure chamber sensor 7-6, and the readings of the inner pressure chamber sensor 7-5 and the outer pressure chamber sensor 7-6 are always equal, so as to avoid bursting of the inner membrane 1-6 and the outer membrane 1-7 and polluting the silicone oil liquid. Finally, the reading of the gas-water pressure sensor 7-4 is 50 kPa, and the readings of the inner pressure chamber sensor 7-5 and the outer pressure chamber sensor 7-6 are both 60 kPa. Open the valve a2-4 controlling the silicone oil storage tank 2-2 of the inner pressure chamber and the valve d3-4 controlling the silicone oil storage tank 3-2 of the outer pressure chamber in sequence to drain the silicone oil in the inner and outer pressure chambers of the hollow cylinder torsional shear axe 1. After removing the sealing bolt 1-15, raise the high-pressure reaction axe 1-1, remove the hydrate-containing marine soil 1-16 in sequence, and clean the test bench. After closing each system through the upper computer 7-10, exit the software and turn off the upper computer 7-10, and save and process the test data.

[0038] Through the above test method, the undrained directional shear mechanical test of hydrate-containing marine soil under the hollow cylinder torsional shear state can be realized.

[0039] Preferably, in step S5, by adjusting the stress path module of the test program, static and dynamic mechanical tests under complex stress states such as directional shear tests, non-coaxial tests, and inclined consolidation tests with different magnitudes of the intermediate principal stress can be realized.

[0040] Preferably, in step S5, the temperature and pressure conditions inside the hollow cylinder torsional shear axe 1 are adjusted by using the temperature and pressure control system, so as to realize static and dynamic mechanical tests of hydrate-containing marine soil under complex stress states under different decomposition methods / decomposition times / decomposition gradients.

[0041] Advantages of the present invention:

[0042] (1) Innovatively develop the first set of hollow cylinder torsional shear test device for hydrate-containing marine soil, filling the domestic and foreign R & D gaps of hollow cylinder torsional shear test devices for hydrate-containing marine soil, improving the realization of the hollow cylinder torsional shear test method for hydrate-containing marine soil, with convenient instrument operation and a systematic and simple test method.

[0043] (2) The temperature and pressure conditions inside the hollow cylinder torsional shear axe can be adjusted by using the internal pressure chamber control system, the external pressure chamber control system and the temperature control system, so as to realize static and dynamic mechanical tests of hydrate-containing marine soil under different decomposition methods / decomposition times / decomposition gradients under complex stress states such as directional shear tests, non-coaxial tests, and inclined consolidation tests with different magnitudes of the intermediate principal stress. It has remarkable characteristics such as multi-function and strong comprehensiveness, providing hardware support for ensuring the reliable design and operation of offshore facilities such as offshore platforms and the future commercial exploitation of hydrates in China. Description of the Drawings

[0044] Figure 1 is a schematic structural diagram of the hollow cylinder torsional shear axe of the present invention;

[0045] Figure 2 is a schematic structural diagram of a hollow cylinder torsional shear test device for hydrate-containing marine soil according to the present invention;

[0046] Figure 3 is a schematic overall system diagram of a hollow cylinder torsional shear test device for hydrate-containing marine soil according to the present invention;

[0047] Figure 4 is a schematic diagram of the test results of Example 1 of a hollow cylinder torsional shear test device for hydrate-containing marine soil according to the present invention.

[0048] In the figure: 1 is a hollow cylindrical torsional shear axe; 2 is an inner pressure chamber control system; 3 is an outer pressure chamber control system; 4 is a temperature control system; 5 is a gas-water control system; 6 is an axial-torsional shear control system; 7 is a data acquisition and control system. 1-1 is a high-pressure reaction axe, 1-2 is an upper bearing platform, 1-3 is an upper pressure head, 1-4 is an annular metal water-permeable stone, 1-5 is an annular filter paper, 1-6 is an inner membrane, 1-7 is an outer membrane, 1-8 is a lower pressure head, 1-9 is an inner membrane fixator, 1-10 is an O-shaped gasket, 1-11 is an O-shaped sealing ring, 1-12 is a connecting bolt, 1-13 is a lower bearing platform, 1-14 is an operating platform, 1-15 is a sealing bolt, 1-16 is hydrate-containing marine soil; 2-1 is an inner pressure chamber volume controller, 2-2 is an inner pressure chamber silicone oil storage tank, 2-3 is an inner pressure chamber pressure regulating valve, 2-4 is valve a, 2-5 is valve b, 2-6 is valve c, 2-7 is an upper oil discharge pipeline, 2-8 is an oil discharge valve; 3-1 is an outer pressure chamber volume controller, 3-2 is an outer pressure chamber silicone oil storage tank, 3-3 is an outer pressure chamber pressure regulating valve, 3-4 is valve d, 3-5 is valve e, 3-6 is valve f; 4-1 is a water area device, 4-2 is a water area jacket, 4-3 is a heat preservation pipeline; 5-1 is a hydrate gas storage tank, 5-2 is a pressure regulating valve, 5-3 is a non-gas-water volume controller, 5-4 is valve g, 5-5 is valve h, 5-6 is valve i, 5-7 is an exhaust water pipeline, 5-8 is an exhaust water valve; 6-1 is an axial-torsional shear servo control system, 6-2 is a guiding ring, 6-3 is a moving sealing ring, 6-4 is an axial-torsional shear force transmission rod; 7-1 is an axial displacement sensor, 7-2 is a torsional angle sensor, 7-3 is an axial force and torque sensor, 7-4 is a gas-water pressure sensor, 7-5 is an inner pressure chamber sensor, 7-6 is an outer pressure chamber sensor, 7-7 is a temperature sensor, 7-8 is a data acquisition line, 7-9 is a data acquisition instrument, 7-10 is a host computer. Detailed implementation mode

[0049] The following further describes the detailed implementation mode of the present invention in combination with the attached drawings and technical solutions.

[0050] The present invention provides a hollow cylindrical torsional shear test device for hydrate-containing marine soil, including:

[0051] As Figure 1As shown in the figure, the hollow cylinder torsional shear axe 1 includes a high-pressure reaction axe 1-1, an upper bearing platform 1-2, an upper pressure head 1-3, an annular metal permeable stone 1-4, an annular filter paper 1-5, an inner membrane 1-6, an outer membrane 1-7, a lower pressure head 1-8, an inner membrane fixator 1-9, an O-ring gasket 1-10, an O-ring seal 1-11, connecting bolts 1-12, a lower bearing platform 1-13, an operating platform 1-14, a sealing bolt 1-15, and hydrate-containing marine soil 1-16; the high-pressure reaction axe 1-1 is hermetically connected to the operating platform 1-14 through the sealing bolt 1-15 to provide a sealed environment for the hollow cylinder torsional shear axe 1; the upper bearing platform 1-2 is located above the high-pressure reaction axe 1-1 and is rigidly connected by a metal connecting rod. It is provided with bolt holes and an O-ring seal groove. The upper end of the upper pressure head 1-3 is provided with a bearing platform to provide an upper constraint for the inner membrane 1-6. It is connected to the upper bearing platform 1-2 through the connecting bolts 1-12 to jointly provide an upper constraint for the hydrate-containing marine soil 1-16. The upper pressure head 1-3 is internally provided with an exhaust water channel and an oil drainage channel; the bottom end of the inner membrane 1-6 is fixed to the lower pressure head 1-8 through the inner membrane fixator 1-9 and the O-ring gasket 1-10, and the upper end is fixed to the upper pressure head 1-3 through the O-ring seal 1-11 and the connecting bolts 1-12; the lower pressure head 1-8 is connected to the lower bearing platform 1-13 through the connecting bolts 1-12 to jointly provide a lower constraint for the hydrate-containing marine soil 1-16. The lower pressure head 1-Preferably, the material of the high-pressure reaction kettle 1-1 is 7075 aviation aluminum alloy, and the upper bearing platform 1-2, the upper pressure head 1-3, the lower pressure head 1-8, the lower bearing platform 1-13 and the operating platform 1-14 are all made of 1-1316L special stainless steel;

[0052] As Figure 2 shown, the internal pressure chamber control system 2 includes an internal pressure chamber volume controller 2-1, an internal pressure chamber silicone oil storage tank 2-2, an internal pressure chamber pressure regulating valve 2-3, valve a 2-4, valve b 2-5, valve c 2-6, an upper oil discharge pipeline 2-7 and an oil discharge valve 2-8; the internal pressure chamber volume controller 2-1 and the internal pressure chamber silicone oil storage tank 2-2 are respectively connected to the internal pressure chamber through the internal pressure chamber pressure regulating valve 2-3, valve a 2-4 and valve b 2-5, and the combined pipeline passes through the internal pressure chamber silicone oil channel provided inside the lower pressure head 1-8 to the inside of the internal pressure chamber. The combined pipeline is provided with an internal pressure chamber sensor 7-5 and valve c 2-6 to jointly provide pressure guarantee for the internal pressure chamber of the hollow cylindrical torsional shear axe 1. The upper oil discharge pipeline 2-7 and the oil discharge valve 2-8 are sequentially connected to the oil discharge channel provided inside the upper pressure head 1-3 to ensure that the internal pressure chamber is filled with silicone oil; the internal pressure chamber volume controller 2-1 is a screw pump controlled by a microprocessor, which can accurately control and measure the fluid pressure and volume change, and can provide a confining pressure of not less than 35 MPa;

[0053] As Figure 2 shown, the external pressure chamber control system 3 includes an external pressure chamber volume controller 3-1, an external pressure chamber silicone oil storage tank 3-2, an external pressure chamber pressure regulating valve 3-3, valve d 3-4, valve e 3-5 and valve f 3-6; the external pressure chamber volume controller 3-1 and the external pressure chamber silicone oil storage tank 3-2 are respectively connected to the external pressure chamber through the external pressure chamber pressure regulating valve 3-3, valve d 3-4 and valve e 3-5, and the combined pipeline passes through the wiring channel inside the operating platform 1-14 to the external pressure chamber. The combined pipeline is provided with an external pressure chamber sensor 7-6 and valve f 3-6 to jointly provide pressure guarantee for the external pressure chamber of the hollow cylindrical torsional shear axe 1; preferably, all the pipelines involved are soft stainless steel pipelines, and all use the 1 / 16 stainless steel seamless jacket tube with better flexibility produced by Swagelok Company of the United States and the high-pressure pneumatic pipeline produced by Parker Company of the United States. All valves use the high-pressure piston regulating valve produced by Swagelok Company of the United States; the external pressure chamber volume controller 3-1 is a screw pump controlled by a microprocessor, which can accurately control and measure the fluid pressure and volume change, and can provide a confining pressure of not less than 35 MPa;

[0054] As Figure 2As shown, the temperature control system 4 mainly provides a suitable temperature environment for the synthetic hydrate marine soil 1-16, including a water area device 4-1, a water area jacket 4-2, and a heat-insulating pipeline 4-3; the water area device 4-1 is hermetically connected to the water area jacket 4-2 through a flexible stainless steel pipeline, and the water area jacket 4-2 is located inside the high-pressure reaction kettle 1-1. The two work together to achieve temperature control and provide temperature guarantee for the hollow cylinder torsional shear tool 1; the water area jacket 4-2 has a spiral pattern design inside to ensure rapid heat exchange of the hollow cylinder torsional shear tool 1.

[0055] As Figure 2 shown, the gas-water control system 5 includes a hydrate gas storage tank 5-1, a pressure regulating valve 5-2, a gas-free water volume controller 5-3, a valve g5-4, a valve h5-5, a valve i5-6, an exhaust water pipeline 5-7, an exhaust water valve 5-8. After passing through the pressure regulating valve 5-2 and the valve g5-4, the hydrate gas storage tank 5-1 is divided into two branches. One branch passes through the valve h5-5 and the gas-water pressure sensor 7-4 and is connected to the intake water channel on the lower platen 1-8 to jointly provide hydrate gas for the synthetic hydrate marine soil 1-16 and provide the gas pressure required for synthesizing hydrates; the other branch passes through the valve i5-6 and is connected to the inlet end of the gas-free water volume controller 5-3. The gas-free water volume controller 5-3, the valve h5-5, and the valve i5-6 work together to drive and saturate the hydrate marine soil 1-16 with gas; the specific operation is that after synthesizing the hydrate marine soil 1-16, the pressure regulating valve 5-2, the valve g5-4, and the valve h5-5 are closed. After adjusting the pressure control of the gas-free water volume controller 5-3 to be consistent with the internal pore pressure of the hydrate marine soil 1-16, the valve h5-5 and the valve i5-6 are opened. By opening and closing the exhaust water valve 5-8 connected to the exhaust water channel inside the upper platen 1-3 through the exhaust water pipeline 5-7 a little at a time, a channel for hydrate gas and gas-free water is provided during the process of driving and saturating the hydrate marine soil 1-16 with gas; preferably, the hydrate gas in the hydrate gas storage tank 5-1 can be methane gas with a purity of 99.99%, carbon dioxide gas with a purity of 99.99%, or xenon gas with a purity of 99.99%; the gas-free water volume controller 5-3 is a spiral pump controlled by a microprocessor, which can accurately control and measure the fluid pressure and volume change and can provide a confining pressure of not less than 35 MPa.

[0056] As Figure 2As shown in the figure, the axial-torsional shear control system 6 includes an axial-torsional shear servo control system 6-1, a guide ring 6-2, a dynamic sealing ring 6-3, and an axial-torsional shear force transfer rod 6-4, mainly applying complex shear stress to the hydrate-containing marine soil 1-16 specimen. Specifically, the upper end of the axial-torsional shear force transfer rod 6-4 is connected to the lower bearing platform 1-13 through a connecting bolt 1-12, the lower end is connected to the axial-torsional shear servo control system 6-1, and it passes through the guide ring 6-2 and the dynamic sealing ring 6-3 through the operation platform 1-14. The dynamic sealing ring 6-3 can still ensure the sealing effectiveness under a pressure of 35 MPa; the axial-torsional shear control system 6 can be manually or automatically controlled by a computer;

[0057] As Figure 2 shown in the figure, the data acquisition and control system 7 includes various sensors, a data acquisition instrument 7-9, and a host computer 7-10, mainly realizing the automatic control and acquisition of the hydrate-containing marine soil hollow cylinder torsional shear test device, and ensuring the reliability of test data. Specifically, the axial displacement sensor 7-1 and the torsional angle sensor 7-2 are fixed on the axial-torsional shear force transfer rod 6-4, and are respectively used to monitor the axial displacement and torsional angle during the test of the hydrate-containing marine soil 1-16; the axial force and torque sensor 7-3 is fixed on the metal connecting rod connected to the upper bearing platform 1-2, and is used to monitor the axial force and torque during the test of the hydrate-containing marine soil 1-16; the gas-water pressure sensor 7-4 is used to monitor the pore pressure inside the hydrate-containing marine soil 1-16, and is located on the pipeline connected to the air and water inlet channel on the valve h5-5 and the lower platen 1-8; the internal pressure chamber sensor 7-5 is used to monitor the confining pressure inside the internal pressure chamber, and is located on the pipeline connecting the valve c2-6 and the lower platen 1-8; the external pressure chamber sensor 7-6 is used to monitor the confining pressure inside the high-pressure reaction kettle 1-1; the temperature sensor 7-7 is fixed on the metal connecting rod connected to the upper bearing platform 1-2, and is used to monitor the temperature inside the high-pressure reaction kettle 1-1; these sensors are jointly connected to the data acquisition instrument 7-9 through the data acquisition line 7-8, and the test data is input into the host computer 7-10 after being converted by the signal of the data acquisition instrument 7-9, realizing the automatic acquisition and control of data. Preferably, the said sensors can all be used in an environment with high and low pressure differences (5 kPa - 35 MPa) and high and low temperature differences (-20 - 65 °C), and the measurement accuracy is ±0.1%. The data acquisition and control system 7 of the instrument adopts two control methods: stress control and strain control. Among them, stress control uses the stress increment change rate as the control method, while strain control uses the displacement change rate as the control method.

[0058] Example 1

[0059] The present invention provides a test method for a hydrate-containing marine soil hollow cylinder torsional shear test device, specifically a method description for an undrained directional shear test of a hydrate-containing marine soil hollow cylinder under complex stress states, including the following steps:

[0060] S1. Line connection: After the pipelines of each system are connected, the sensors in the data acquisition and control system 7 are sequentially connected to the data acquisition instrument 7-9 and the upper computer 7-10 through the data acquisition line 7-8. The digital control system is connected through the data acquisition control software in the upper computer 7-10, that is, the operation of the inner pressure chamber control system 2, the outer pressure chamber control system 3, the temperature control system 4, the gas-water control system 5, and the axial-torsional shear control system 6 is controlled by the upper computer 7-10. It must be noted that since the displacement control is relatively stable, each system is controlled by displacement to prevent test accidents. After ensuring the stable operation of all systems, zero the sensors again.

[0061] S2. Specimen preparation, installation and airtightness detection: Raise the high-pressure reaction axe 1-1 of the hollow cylinder torsional shear axe 1 system, remove the lower pressure head 1-8 from the lower bearing platform 1-13 and place it in the empty area. First, fix the lower end of the inner membrane 1-6 to the lower pressure head 1-8 through the inner membrane fixer 1-9, the O-ring gasket 1-10, and the connecting bolt 1-12. Place the inner bearing membrane barrel and turn the inner membrane 1-6 outwards. Immediately turn the outer membrane 1-7 up and down about 2 cm and fix it inside the outer bearing membrane barrel. Fix the lower section of the outer membrane 1-7 to the lower pressure head 1-8 using the O-ring seal 1-11. Place the annular metal permeable stone 1-4 and the annular filter paper 1-5 in sequence, and then layer-fill the matrix material of the hydrate-containing marine soil 1-16 to the designed height. After placing the annular filter paper 1-5 and the annular metal permeable stone 1-4 on the upper surface of the matrix material, place the upper pressure head 1-3 on the upper surface of the matrix material. Fix the upper end of the inner membrane 1-6 to the upper bearing platform at the upper end of the upper pressure head 1-3 through the O-ring seal 1-11, then remove the inner bearing membrane barrel. Fix the outer membrane 1-7 to the upper pressure head 1-3 through the O-ring seal 1-11, and then remove the outer bearing membrane barrel. Fix it to the lower bearing platform 1-13 through the connecting bolt 1-12. Adjust the height of the lower bearing platform 1-13 through the axial-torsional servo until the upper pressure head 1-3 is tightly connected to the upper bearing platform 1-2. The O-ring seal 1-11 and the connecting bolt 1-12 inside the upper bearing platform 1-2 together serve to form a closed inner pressure chamber. Subsequently, lower the high-pressure reaction axe 1-1; fix it to the operating platform 1-14 through the sealing bolt 1-15. To eliminate the disturbance of each sensor during the process of filling the matrix material of the hydrate-containing marine soil 1-16, zero the sensors again to improve the accuracy of the test.

[0062] S3. Synthesis of hydrate-bearing marine soil specimens: First, fill the inner pressure chamber with oil. Open valve c2-6 on the pipeline of the inner pressure chamber silicone oil channel inside the lower platen 1-8 where the inner pressure chamber volume controller 2-1 and the inner pressure chamber silicone oil storage tank 2-2 are convergently connected, valve b2-5 controlling the inner pressure chamber volume controller 2-1, and valve a2-4 of the inner pressure chamber silicone oil storage tank 2-2 to inject silicone oil into the inner pressure chamber. At the same time, adjust the inner pressure chamber volume controller 2-1 to its maximum range. After the silicone oil is discharged from the upper oil discharge pipeline 2-7 by an amount equal to 0.5 times the volume of the inner pressure chamber, it indicates that the inner pressure chamber is full. Then close valve a2-4 of the inner pressure chamber silicone oil storage tank 2-2 and the oil discharge valve 2-8. Apply an inner pressure of 5 kPa by controlling the displacement of the inner pressure chamber volume controller 2-1. Second, fill the hollow cylinder torsional shear tool 1 with oil. Open valve f3-6 on the wiring channel inside the operation table 1-14 where the outer pressure chamber volume controller 3-1 and the outer pressure chamber silicone oil storage tank 3-2 are convergently connected, valve e3-5 controlling the outer pressure chamber volume controller 3-1, and valve d3-4 of the outer pressure chamber silicone oil storage tank 3-2 to inject silicone oil into the hollow cylinder torsional shear tool 1. At the same time, adjust the outer pressure chamber volume controller 3-1 to its maximum range. After the silicone oil is full, close valve d3-4 of the outer pressure chamber silicone oil storage tank 3-2. Similarly, apply an outer pressure of 5 kPa by controlling the displacement of the outer pressure chamber volume controller 3-1. At the same time, control the pressure of the inner pressure chamber volume controller 2-1 and the outer pressure chamber volume controller 3-1 to apply a pre-pressure of 500 kPa to the matrix material of the hollow cylindrical hydrate-bearing marine soil 1-16. At the same time, turn on the water area device 4-1 and adjust the temperature to 2 °C. The temperature adjustment process should last about 6 hours. After the temperature inside the hollow cylinder torsional shear tool 1 is stable, close valve i5-6 controlling the volume of the anhydrous water controller 5-3. Then, in sequence, open the pressure regulating valve 5-2 of the hydrate gas storage tank 5-1, valve h5-5 on the pipeline connecting the hydrate gas storage tank 5-1 and the intake water channel on the lower platen 1-8, and valve g5-4 to provide the required hydrate gas (methane gas with a purity of 99.99% or carbon dioxide gas with a purity of 99.99% or xenon gas with a purity of 99.99%) and the designed pore pressure (7900 kPa) for the hydrate-bearing marine soil 1-16. At the same time, correspondingly control the pressure values of the inner pressure chamber volume controller 2-1 and the outer pressure chamber volume controller 3-1 to ensure that the reading of the gas-water pressure sensor 7-4 is always less than that of the inner pressure chamber sensor 7-5 and the outer pressure chamber sensor 7-6, and the readings of the inner pressure chamber sensor 7-5 and the outer pressure chamber sensor 7-6 are always equal. Finally, the reading of the gas-water pressure sensor 7-4 is 7900 kPa, and the readings of the inner pressure chamber sensor 7-5 and the outer pressure chamber sensor 7-6 are both 8000 kPa. Then close valve g5-4 and valve h5-5. The temperature and pressure conditions at this time are considered to meet the formation of methane hydrate solids inside the hydrate-bearing marine soil 1-16.After the reading of the gas-water pressure sensor 7-4 stabilizes, it is considered that the hydrate-bearing marine soil 1-16 is completely formed. Record the final gas pressure and use the gas consumption method to determine the true hydrate saturation of the hydrate-bearing marine soil 1-16.

[0063] S4. Saturation and consolidation of the hydrate-bearing marine soil specimen: After the hydrate formation inside the hydrate-bearing marine soil 1-16 is completed, it is necessary to expel the hydrate gas in the specimen with nitrogen to prevent the excess hydrate gas from reacting with the gas-free water during the water saturation process to form hydrates again under stable temperature and pressure conditions. Adjust the gas-free water pressure of the gas-free water volume controller 5-3 to the pore pressure inside the hydrate-bearing marine soil 1-16. Open valve i5-6 and valve h5-5. After the reading of the gas-water pressure sensor 7-4 stabilizes, slightly open the drain valve 5-8 several times until the volume of gas-free water is three times the pore volume of the hydrate-bearing marine soil 1-16, and ensure that the pore water pressure coefficient B of each hydrate-bearing marine soil 1-16 specimen exceeds 0.95. Then it is considered that the hydrate-bearing marine soil 1-16 is a water-saturated specimen. According to the specific experimental conditions, adjust the inner pressure chamber volume controller 2-1 and the outer pressure chamber volume controller 3-1 to the target values simultaneously, and consolidate the hydrate-bearing marine soil 1-16 under this condition. When the axial displacement of the hydrate-bearing marine soil 1-16 is less than 0.05% / min, it is considered that the consolidation is completed and the test can continue.

[0064] S5. Hollow cylinder torsional shear test on hydrate-bearing marine soil: During the test, maintain the temperature inside the hollow cylinder torsional shear tool 1 constant to ensure that the hydrates inside the hydrate-bearing marine soil 1-16 do not decompose during the torsional shear process. During the test, use the upper computer 7-10 to adjust the axial-torsional shear control system 6, the inner pressure chamber control system 2, and the outer pressure chamber control system 3 to apply axial force and torque to both ends of the hollow cylinder-shaped hydrate-bearing marine soil 1-16 specimen respectively, and apply internal and external confining pressures to the inner and outer walls respectively. By independently changing the magnitudes of the four external loads (axial force W, torque T, internal confining pressure P i and external confining pressure P o ), it is possible to control the magnitudes and directions of the principal stresses inside the hydrate-bearing marine soil 1-16 specimen.

[0065] The stress of the elemental volume of the hydrate-bearing marine soil 1-16 specimen (axial stress σ z , radial stress σ r , tangential stress σ θ , torsional shear stress τ θz ) shall be calculated according to the following formula:

[0066]

[0067] where R and r are the outer diameter and inner diameter of the hydrate-bearing marine soil 1-16 specimen respectively.

[0068] Based on these stress components of the elemental volume, the maximum, minimum, and intermediate principal stresses of the hydrate-bearing marine soil specimens 1-16 are as follows:

[0069]

[0070] For convenience, generally four parameters related to the magnitude and direction of the principal stresses are used for test control and data processing:

[0071]

[0072] Wherein, the mean principal stress s and the deviator stress t are stress parameters; α is the principal stress direction angle, which is the angle between the maximum principal stress σ1 and the vertical direction; b is the intermediate principal stress coefficient, with a value between 0 and 1. Using the stress path module of the instrument, stress or strain control for any path of the mean principal stress, deviator stress, principal stress direction angle, and intermediate principal stress coefficient can be directly carried out.

[0073] Here, the undrained directional shear test is taken as an example for detailed elaboration. The undrained directional shear test refers to a shear test carried out under a constant principal stress direction angle α, and the mean principal stress s and the intermediate principal stress coefficient b are kept unchanged during the shear process. The cases of α = 90° and α = 0° respectively correspond to the tensile and compression tests of the hydrate-bearing marine soil specimens 1-16. At this time, the hydrate-bearing marine soil specimens 1-16 adopt the strain control mode. For the hydrate-bearing marine soil specimens 1-16 with α = 45°, the deviator stress t is directly controlled to increase linearly in the stress path module of the instrument to achieve the purpose of shearing the specimen. For tests with other principal stress angles, since both torsional shear strain and axial strain develop, the strain control method is also adopted. Most tests stop when the deviator strain reaches 20%. However, in order to observe in detail the failure mode of the specimen, especially the development process of the shear band, larger final strain values are set for individual hydrate-bearing marine soil specimens 1-16.

[0074] S6. After the test is completed, turn off the water area device 4-1. Accordingly, the pressure control relieves the pressure of the inner pressure chamber volume controller 2-1 and the outer pressure chamber volume controller 3-1, ensuring that the reading of the gas-water pressure sensor 7-4 is always less than that of the inner pressure chamber sensor 7-5 and the outer pressure chamber sensor 7-6, and the readings of the inner pressure chamber sensor 7-5 and the outer pressure chamber sensor 7-6 are equal at all times, to avoid bursting of the inner membrane 1-6 and the outer membrane 1-7 and contamination of the silicone oil liquid. Finally, the reading of the gas-water pressure sensor 7-4 is 50 kPa, and the readings of the inner pressure chamber sensor 7-5 and the outer pressure chamber sensor 7-6 are both 60 kPa. Open the valve a2-4 controlling the silicone oil storage tank 2-2 of the inner pressure chamber and the valve d3-4 controlling the silicone oil storage tank 3-2 of the outer pressure chamber in sequence to drain the silicone oil in the inner and outer pressure chambers of the hollow cylinder torsional shear axe 1. After removing the sealing bolt 1-15, raise the high-pressure reaction axe 1-1, remove the hydrate-containing marine soil 1-16 in sequence, and clean the test bench. After closing each system through the upper computer 7-10, exit the software and turn off the upper computer 7-10, and save and process the test data.

[0075] Through the above test method, the undrained directional shear mechanical test of hydrate-containing marine soil under the hollow cylinder torsional shear state can be realized.

[0076] Preferably, in step S5, by adjusting the stress path module of the test program, static and dynamic mechanical tests under complex stress states such as directional shear tests, non-coaxial tests, and inclined consolidation tests with different intermediate principal stress magnitudes can be realized.

[0077] Preferably, in step S5, the temperature and pressure conditions inside the hollow cylinder torsional shear axe 1 are adjusted by using the temperature and pressure control system, so as to realize static and dynamic mechanical tests under complex stress states such as directional shear tests, non-coaxial tests, and inclined consolidation tests of hydrate-containing marine soil 1-16 under different dissociation modes / dissociation times / dissociation gradients with different intermediate principal stress magnitudes.

[0078] The above-described embodiments are only a preferred solution of the present invention, but they are not intended to limit the present invention. Those of ordinary skill in the relevant technical fields can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, all technical solutions obtained by means of equivalent replacement or equivalent transformation fall within the protection scope of the present invention.

Claims

1. An apparatus for torsional shear testing of hydrate-bearing marine soil in hollow cylinders, characterized in that, The hydrate-containing marine soil hollow cylinder torsional shear test device includes a hollow cylinder torsional shear axe (1), an internal pressure chamber control system (2), an external pressure chamber control system (3), a temperature control system (4), a gas-water control system (5), an axial-torsional shear control system (6), and a data acquisition and control system (7); The hollow cylinder torsional shear axe (1) includes a high-pressure reaction axe (1-1), an upper bearing platform (1-2), an upper pressure head (1-3), an annular metal permeable stone (1-4), an annular filter paper (1-5), an inner membrane (1-6), an outer membrane (1-7), a lower pressure head (1-8), a lower bearing platform (1-13), an operating platform (1-14), and hydrate-containing marine soil (1-16); The high-pressure reaction axe (1-1) is hermetically connected to the operating platform (1-14) to provide a sealed environment for the hollow cylinder torsional shear axe (1); An inner membrane (1-6) and an outer membrane (1-7) are fixed between the upper bearing platform (1-2) and the lower bearing platform (1-13) through an upper pressure head (1-3) and a lower pressure head (1-8) respectively, for providing constraints for the hydrate-containing marine soil (1-16); The annular metal permeable stone (1-4) and the annular filter paper (1-5) are arranged on the upper and lower surfaces of the hydrate-containing marine soil (1-16); The internal pressure chamber control system (2) includes an internal pressure chamber volume controller (2-1), an internal pressure chamber silicone oil storage tank (2-2), an internal pressure chamber pressure regulating valve (2-3), an upper oil discharge pipeline (2-7), and an oil discharge valve (2-8); The pipeline of the internal pressure chamber volume controller (2-1) after passing through valve b (2-5) converges with the pipeline of the internal pressure chamber silicone oil storage tank (2-2) after passing through the internal pressure chamber pressure regulating valve (2-3) and valve a (2-4) in sequence; The converging pipeline leads to the inside of the internal pressure chamber through an internal pressure chamber silicone oil channel provided inside the lower pressure head (1-8). An internal pressure chamber sensor (7-5) and a valve c (2-6) are arranged on the converging pipeline, jointly providing pressure guarantee for the internal pressure chamber of the hollow cylinder torsional shear axe (1). The upper oil discharge pipeline (2-7) and the oil discharge valve (2-8) are connected to an oil discharge channel provided inside the upper pressure head (1-3) in sequence; The external pressure chamber control system (3) includes an external pressure chamber volume controller (3-1), an external pressure chamber silicone oil storage tank (3-2), and an external pressure chamber pressure regulating valve (3-3); The pipeline of the external pressure chamber volume controller (3-1) passes through valve e (3-5) and converges with the pipeline of the external pressure chamber silicone oil storage tank (3-2) after passing through the external pressure chamber pressure regulating valve (3-3) and valve d (3-4) in sequence. The converging pipeline leads to the external pressure chamber through a wiring channel inside the operating platform (1-14). An external pressure chamber sensor (7-6) and a valve f (3-6) are arranged on the converging pipeline, jointly providing pressure guarantee for the external pressure chamber of the hollow cylinder torsional shear axe (1); The temperature control system (4) provides a suitable temperature environment for the synthetic hydrate-containing marine soil (1-16); The described gas-water control system (5) includes a hydrate gas storage tank (5-1), a pressure regulating valve (5-2), a gas-free water volume controller (5-3), an exhaust water pipeline (5-7), and an exhaust water valve (5-8); the hydrate gas storage tank (5-1) is divided into two branches after passing through the pressure regulating valve (5-2) and valve g (5-4). One branch passes through valve h (5-5) and a gas-water pressure sensor (7-4) and is connected to the intake water channel on the lower platen (1-8) to jointly provide hydrate gas for the synthetic hydrate-bearing marine soil (1-16) and provide the gas pressure required for hydrate synthesis; the other branch passes through valve i (5-6) and is connected to the inlet end of the gas-free water volume controller (5-3). The gas-free water volume controller (5-3), valve h (5-5), and valve i (5-6) work together for the gas expulsion saturation of the hydrate-bearing marine soil (1-16); the exhaust water pipeline (5-7) is connected to the exhaust water channel inside the upper platen (1-3), and an exhaust water valve (5-8) is provided thereon; The described axial-torsional shear control system (6) includes an axial-torsional shear servo control system (6-1), a guide ring (6-2), a dynamic sealing ring (6-3), and an axial-torsional shear force transfer rod (6-4), which mainly applies complex shear stress to the hydrate-bearing marine soil (1-16) specimen; the upper end of the axial-torsional shear force transfer rod (6-4) is connected to the lower bearing platform (1-13), its lower end is connected to the axial-torsional shear servo control system (6-1), and it passes through the guide ring (6-2) and the dynamic sealing ring (6-3) through the operating platform (1-14) in the middle; The described data acquisition and control system (7) includes various sensors, a data acquisition instrument, and a host computer, which mainly realizes the automatic control and acquisition of the hydrate-bearing marine soil hollow cylinder torsional shear test device; various sensors are jointly connected to the data acquisition instrument (7-9) through a data acquisition line (7-8), and the test data is input into the host computer (7-10) after being converted by the signal of the data acquisition instrument (7-9) to realize the automatic acquisition and control of the data.

2. The hydrate-bearing marine soil hollow cylinder torsional shear test device according to claim 1, wherein The specific structure inside the hollow cylindrical torsional shear ax (1) is as follows: The upper bearing platform (1-2) is connected to the upper wall surface of the high-pressure reaction ax (1-1); The upper pressure head (1-3) is provided with a bearing platform structure at its upper end, providing upper restraint for the inner membrane (1-6). It is connected to the upper bearing platform (1-2), jointly providing upper restraint for the hydrate-containing marine soil (1-16). The upper pressure head (1-3) is internally provided with an exhaust water channel and an oil drainage channel; The lower end of the inner membrane (1-6) is fixed to the lower pressure head (1-8), and its upper end is fixed to the upper pressure head (1-3); The lower end of the outer membrane (1-7) is fixed to the lower pressure head (1-8), and its upper end is fixed to the upper pressure head (1-3); The space between the outer membrane (1-7) and the high-pressure reaction ax (1-1) forms an outer pressure chamber, and the inner space of the inner membrane (1-6) forms an inner pressure chamber; The lower pressure head (1-8) is connected to the lower bearing platform (1-13), jointly providing lower restraint for the hydrate-containing marine soil (1-16). The lower pressure head (1-8) is internally provided with an inner pressure chamber silicone oil channel and an air intake water channel; The annular metal permeable stone (1-4) is arranged on the hydrate-containing marine soil (1-16), and its surface is provided with anti-slip blades.

3. The hydrate-containing marine soil hollow cylindrical torsional shear test device according to claim 2, wherein The temperature control system (4) includes a water area device (4-1), a water area jacket (4-2), and a heat preservation pipeline (4-3); The water area device (4-1) is hermetically connected to the water area jacket (4-2) through the heat preservation pipeline (4-3). The water area jacket (4-2) is located inside the inner wall of the high-pressure reaction ax (1-1). The two cooperate to achieve temperature control, providing temperature guarantee for the hollow cylindrical torsional shear ax (1).

4. The hydrate-containing marine soil hollow cylindrical torsional shear test device according to claim 3, wherein The upper bearing platform (1-2) is rigidly connected to the upper wall surface of the high-pressure reaction ax (1-1) through a metal connecting rod.

5. The hydrate-containing marine soil hollow cylindrical torsional shear test device according to claim 4, wherein The sensors are specifically as follows: The axial displacement sensor (7-1) and the torsional angle sensor (7-2) are fixed on the axial-torsional shear force transfer rod (6-4) and are respectively used to monitor the axial displacement and torsional angle during the test of hydrate-bearing marine soil (1-16); the axial force and torque sensor (7-3) is fixed on the metal connecting rod connected to the upper bearing platform (1-2) and is used to monitor the axial force and torque during the test of hydrate-bearing marine soil (1-16); the gas-water pressure sensor (7-4) is used to monitor the pore pressure inside the hydrate-bearing marine soil (1-16) and is located on the pipeline connected to the air and water inlet channels on the valve h (5-5) and the lower platen (1-8); the internal pressure chamber sensor (7-5) is used to monitor the confining pressure inside the internal pressure chamber and is located on the pipeline connecting the valve c (2-6) and the lower platen (1-8); the external pressure chamber sensor (7-6) is used to monitor the confining pressure inside the high-pressure reaction autoclave (1-1); the temperature sensor (7-7) is fixed on the metal connecting rod connected to the upper bearing platform (1-2) and is used to monitor the temperature inside the high-pressure reaction autoclave (1-1).

6. A testing method for the hydrate-bearing marine soil hollow cylinder torsional shear device according to claim 5, characterized in that, The steps are as follows: S1. Circuit connection: After the pipelines of the hydrate-bearing marine soil hollow cylinder torsional shear device are connected, each sensor in the data acquisition and control system (7) is sequentially connected to the data acquisition instrument (7-9) and the upper computer (7-10) through the data acquisition line (7-8), and the digital control system is connected through the data acquisition control software in the upper computer (7-10), that is, the operation of the internal pressure chamber control system (2), the external pressure chamber control system (3), the temperature control system (4), the gas-water control system (5) and the axial-torsional shear control system (6) is controlled by the upper computer (7-10); S2. Specimen preparation, installation and airtightness detection: Raise the high-pressure reaction kettle (1-1) of the hollow cylindrical torsional shear ax (1), remove the lower pressing head (1-8) from the lower bearing platform (1-13). First, fix the lower end of the inner membrane (1-6) on the lower pressing head (1-8). Place the inner bearing membrane barrel inside the inner membrane (1-6) and turn the upper end of the inner membrane (1-6) outwards onto the inner bearing membrane barrel, so that the inner surface of the inner membrane (1-6) closely adheres to the inner bearing membrane barrel; Fix the lower end of the outer membrane (1-7) on the outside of the lower pressing head (1-8). Place the outer bearing membrane barrel inside the outer membrane (1-7) and turn the upper end of the outer membrane (1-7) outwards onto the outer bearing membrane barrel, so that the inner surface of the outer membrane (1-7) closely adheres to the outer bearing membrane barrel; After sequentially placing the annular metal water-permeable stone (1-4) and the annular filter paper (1-5) on the lower pressing head (1-8), a hollow cylindrical sample preparation space is formed. Layer by layer, fill the matrix material of the hydrate-containing marine soil (1-16) to the designed height. After sequentially placing the annular filter paper (1-5) and the annular metal water-permeable stone (1-4) on the upper surface of the matrix material, place the upper pressing head (1-3). Then, fix the upper end of the inner membrane (1-6) on the upper pressing head (1-3), and then remove the inner bearing membrane barrel. After fixing the outer membrane (1-7) on the upper pressing head (1-3), remove the outer bearing membrane barrel. Fix the hydrate-containing marine soil (1-16) together with the lower pressing head (1-8) on the lower bearing platform (1-13). Adjust the height of the lower bearing platform (1-13) through the axial-torsion servo until the upper pressing head (1-3) is tightly connected to the upper bearing platform (1-2); Subsequently, lower the high-pressure reaction kettle (1-1) and fix it on the operation table (1-14); S3. Synthesis of hydrate-containing marine soil specimens: Open valve c (2-6), control valve b (2-5) and valve a (2-4) to inject silicone oil into the inner pressure chamber. At the same time, adjust the inner pressure chamber volume controller (2-1) to the maximum range. After the silicone oil discharges an amount of oil equal to 0.5 times the volume of the inner pressure chamber from the upper oil discharge pipeline (2-7), it indicates that the inner pressure chamber is full. Then close valve a (2-4) and the oil discharge valve (2-8); Apply an internal pressure of 5 kPa to the displacement-controlled inner pressure chamber volume controller (2-1); Open valve f (3-6), control valve e (3-5) and valve d (3-4) to inject silicone oil into the hollow cylinder torsional shear apparatus (1). At the same time, adjust the outer pressure chamber volume controller (3-1) to the maximum range. After the silicone oil is full, close valve d (3-4); Similarly, apply an external pressure of 5 kPa to the displacement-controlled outer pressure chamber volume controller (3-1); At the same time, the pressure-controlled inner pressure chamber volume controller (2-1) and the outer pressure chamber volume controller (3-1) apply a pre-pressure of 500 kPa to the matrix material of the hydrate-containing marine soil (1-16). At the same time, turn on the water area device (4-1) and adjust the temperature to 2 °C; The temperature adjustment process should last for 6 hours. After the temperature inside the hollow cylinder torsional shear apparatus (1) is stable, close valve i (5-6), and then open the pressure regulating valve (5-2), valve g (5-4) and valve h (5-5) in sequence to provide the required hydrate gas and the designed pore pressure for the hydrate-containing marine soil (1-16); At the same time, control the pressure values of the inner pressure chamber volume controller (2-1) and the outer pressure chamber volume controller (3-1) to ensure that the reading of the gas-water pressure sensor (7-4) is always less than that of the inner pressure chamber sensor (7-5) and the outer pressure chamber sensor (7-6), and the readings of the inner pressure chamber sensor (7-5) and the outer pressure chamber sensor (7-6) are always equal. Finally, the reading of the gas-water pressure sensor (7-4) is 7900 kPa, and the readings of the inner pressure chamber sensor (7-5) and the outer pressure chamber sensor (7-6) are both 8000 kPa. Then close valve g (5-4) and valve h (5-5); When the reading of the gas-water pressure sensor (7-4) is stable, it is considered that the hydrate-containing marine soil (1-16) is generated. Record the termination air pressure and use the gas consumption method to determine the true hydrate saturation of the hydrate-containing marine soil (1-16); S4. Saturation and Consolidation of Hydrate Marine Soil Specimens: After the formation of hydrates inside the hydrate-containing marine soil (1-16) is completed, the hydrate gas in the hydrate-containing marine soil (1-16) specimen is removed with nitrogen gas; the gas-free water pressure of the gas-free water volume controller (5-3) is adjusted to the pore pressure inside the hydrate-containing marine soil (1-16), valves i (5-6) and h (5-5) are opened, and after the readings of the gas-water pressure sensor (7-4) are stable, the exhaust water valve (5-8) is opened multiple times until gas-free water three times the pore volume inside the hydrate-containing marine soil (1-16) appears, and it is ensured that the pore water pressure coefficient B of each hydrate-containing marine soil (1-16) specimen exceeds 0.95, then the hydrate-containing marine soil (1-16) is considered a water-saturated specimen; according to the specific experimental conditions, the inner pressure chamber volume controller (2-1) and the outer pressure chamber volume controller (3-1) are adjusted to the target values simultaneously, and the hydrate-containing marine soil (1-16) is consolidated. When the axial displacement of the hydrate-containing marine soil (1-16) is less than 0.05% / min, the consolidation is considered complete; S5. Hydrate-bearing marine soil hollow cylinder torsional shear test: During the test, the temperature inside the hollow cylinder torsional shear apparatus (1) is maintained constant to ensure that the hydrates inside the hydrate-bearing marine soil (1-16) do not decompose during the torsional shear process; during the test, the upper computer (7-10) is used to adjust the axial-torsional shear control system (6), the internal pressure chamber control system (2), and the external pressure chamber control system (3), and axial force and torque are respectively applied to both ends of the hydrate-bearing marine soil (1-16) specimen, and internal and external confining pressures are respectively applied to the internal pressure chamber and the external pressure chamber. By independently changing the axial force W, torque T, internal confining pressure P i and external confining pressure P o the magnitude of the principal stress inside the hydrate-bearing marine soil (1-16) specimen can be controlled in terms of magnitude and direction; The unit stress of the hydrate-bearing marine soil (1-16) specimen is calculated according to the following formula. The unit stress includes the axial stress σ z , the radial stress σ r , the tangential stress σ θ , and the torsional shear stress τ θz : , where R and r are the outer diameter and inner diameter of the hydrate-bearing marine soil (1-16) specimen, respectively; Based on these elemental stress components, the maximum, minimum, and intermediate principal stresses of the hydrate-containing marine soil (1-16) specimen are respectively: , four parameters related to the magnitude and direction of the principal stress are adopted for test control and data processing: , where the mean principal stress s and the deviator stress t are stress parameters; α is the principal stress direction angle, which is the angle between the maximum principal stress σ1 and the vertical direction; b is the intermediate principal stress coefficient, with a value ranging from 0 to 1; After the test, the water area device (4-1) is closed, and the pressures of the inner pressure chamber volume controller (2-1) and the outer pressure chamber volume controller (3-1) are controlled to be unloaded, ensuring that the readings of the gas-water pressure sensor (7-4) are always less than those of the inner pressure chamber sensor (7-5) and the outer pressure chamber sensor (7-6), and the readings of the inner pressure chamber sensor (7-5) and the outer pressure chamber sensor (7-6) are always equal, to avoid bursting of the inner membrane (1-6) and the outer membrane (1-7) and contamination of the silicone oil liquid; finally, the reading of the gas-water pressure sensor (7-4) is 50 kPa, and the readings of the inner pressure chamber sensor (7-5) and the outer pressure chamber sensor (7-6) are both 60 kPa. The valve a (2-4) controlling the silicone oil storage tank (2-2) of the inner pressure chamber and the valve d (3-4) controlling the silicone oil storage tank (3-2) of the outer pressure chamber are opened in sequence to drain the silicone oil in the inner and outer pressure chambers of the hollow cylinder torsional shear axe (1); the high-pressure reaction axe (1-1) is raised, and the hydrate-containing marine soil (1-16) is removed in sequence, and the test bench is cleaned; after closing each system through the upper computer (7-10), the software is exited and the upper computer (7-10) is shut down, and the test data is saved and processed.

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