Low-temperature and high-pressure equipment for simulating deep-sea hydrate exploitation and pile-soil interaction and testing method

Abstract

The invention discloses low-temperature and high-pressure equipment for simulating deep-sea hydrate exploitation and pile-soil interaction and a testing method, and belongs to the field of ocean geotechnical engineering. The low-temperature and high-pressure equipment comprises a pressure-resistant test cabin, a thermal insulation layer, a lighting camera device, an outer circulating cooling water bath sleeve, a decompression mining module, an actuator, a pressurization system, a control cabinet, an air bag and a constant-temperature water tank. The pressure control system and the temperature control system are used for simulating a deep sea low-temperature and high-pressure environment, hydrate-related tests can be carried out in the pressure-resistant test cabin body, hydrate sediments with required saturation are generated, hydrate decomposition can be realized through the temperature and pressure control system, and the test efficiency is improved. The actuator applies horizontal and vertical static loads and dynamic loads through force control or displacement control so as to simulate the load borne by the model pile under different working conditions, illumination and camera shooting record the change conditions of the soil surface and the pile end, the simulation effect is good, and hydrate sediment generation and pile-soil interaction are efficiently achieved.

Description

Technical Field

[0001] This invention belongs to the field of marine geotechnical engineering and relates to the field of temperature control technology for high-pressure experimental equipment. It is particularly applicable to the temperature control system of a reactor that needs to maintain both low temperature and high pressure environments in deep-sea combustible ice mining simulation experiments. It relates to a high-pressure reactor that can simulate the temperature and pressure environment of the deep sea. Background Technology

[0002] Natural gas hydrates (also known as combustible ice) are considered one of the most promising clean energy sources for the future, with reserves in the South my country Sea reaching 80 billion tons of oil equivalent. Therefore, developing natural gas hydrate resources is of significant strategic importance for ensuring my country's energy security. Although my country has successfully conducted trial production of natural gas hydrates in the South China Sea, achieving safe, efficient, and economical commercial extraction remains a major challenge. Hydrates in the South China Sea are typically buried in shallow, low-permeability muddy reservoirs on the seabed, often under high temperature and pressure conditions. Extraction of these hydrates can easily lead to geological instability, thus increasing the risk of landslides.

[0003] Traditional reactor temperature control uses a single-loop system, which suffers from low cooling efficiency and large temperature fluctuations (±2℃ or more) under high pressure, failing to meet the requirements of methane hydrate phase change experiments. The high-pressure chamber insulation material has a high thermal conductivity, resulting in significant cooling loss and high energy consumption. To address these issues, there is an urgent need for an integrated, high-precision device capable of accurately simulating the deep-sea environment in a laboratory setting and achieving precise temperature and pressure control to improve the reliability and efficiency of parameter analysis. This invention is based on an in-depth analysis of the shortcomings of existing technologies and aims to solve key bottlenecks in deep-sea geotechnical engineering through innovative design. Summary of the Invention

[0004] This invention addresses the shortcomings of previous reactors, such as low cooling efficiency under high pressure, difficulty in effectively simulating the deep-sea environment, and high energy consumption for heat preservation in the high-pressure chamber. Based on the traditional reactor, it improves upon the problem of the difficulty in maintaining high pressure and low temperature environments in deep-sea simulation experiments. It achieves a temperature control accuracy of ±0.1℃ under 20MPa pressure, reduces energy consumption by more than 30%, and thus more closely approximates the real deep-sea temperature and pressure conditions, more realistically simulating the formation and decomposition process of hydrates.

[0005] The novel technical solution of this invention: A low-temperature high-pressure device for simulating deep-sea hydrate mining and pile-soil interaction includes a pressure test chamber 1, an insulation layer 2, an illumination and camera device 3, an external circulating cooling water bath jacket 4, a decompression mining module 6, an actuator 7, a pressurization system 9, a control cabinet 10, an air tank 11, and a constant temperature water tank. The pressure test chamber 1 is equipped with sealed end caps at the top and bottom, and all parts are connected by high-strength screws. An external circulating cooling water bath sleeve 4 is provided on the outer periphery of the pressure test chamber 1, and an insulation layer 2 is provided on the outer periphery of the external circulating cooling water bath sleeve 4. The constant temperature water bath A13 is located outside the pressure test chamber 1. The external circulating cooling water bath sleeve 4 is connected to the external constant temperature water bath A13 through a circulation pipeline and a circulation pump 20 to form a cooling circulation. Temperature control is achieved by setting a temperature sensor 5 in the external circulating cooling water bath sleeve 4 for feedback. The pressure test chamber 1 is equipped with an illumination and camera device 3, a decompression mining module 6, an actuator 7, an air tank 11, a temperature sensor 5, and a pressure sensor 16 on the sealed end cap at the upper part of the interior. The illumination and camera device 3 observes the conditions of the pressure test chamber 1. The decompression mining module 6 is used to simulate the deep-sea hydrate mining process and is connected to the decompression interface of the upper sealed end cap through a pipeline. The actuator 7 is connected to the model pile and is driven by a motor 12, which can move the model pile up, down, left, and right within the pressure test chamber 1. The model pile is buried to a certain depth in the soil inside the pressure test chamber 1. The air tank 11 is used for pressure control, accompanying the solution in and out of the pressure test chamber 1 to maintain the internal confining pressure and avoid errors and concentration changes. The temperature sensor 5 and the pressure sensor 16 are used to detect the internal temperature and pressure of the pressure test chamber 1. The constant temperature water bath B14 is located outside the pressure test chamber 1 and is used to provide a low-temperature pressurizing medium for the pressurization system 9 and control its temperature. The pressurization system 9 includes a booster pump set 15, a shut-off valve 17, a pressure gauge 19, and a safety valve 18. The booster pump set 15 is the power source, and the pressure gauge 19, shut-off valve 17, and safety valve 18 are installed sequentially on its outlet pipeline. All components are integrated into a cabinet. The pressurization system 9 is connected to the pressurization port of the sealing end cover on the upper part of the pressure test chamber 1 through pipelines for injecting the pressurizing medium. The control cabinet 10 is remotely installed in the operating room and is connected to the temperature sensor 5, the lighting camera device 3, the motor 12, the servo flow valve 8, the booster pump group 15, and the pressure sensor 16 via cables to form a closed-loop control. The pressure reduction extraction module 6 includes a pressure reduction extraction cylinder, a filter, a servo flow valve 8, and a flow sensor. The pressure reduction extraction cylinder serves as the medium collection inlet and is connected to the pressure test chamber 1 via a pipeline. The filter is installed downstream of the outlet of the pressure reduction extraction cylinder to filter impurities in the gas-liquid mixture. The servo flow valve 8 is connected in series in the pipeline after the filter and serves as the actuator for flow regulation. The flow sensor is installed near the servo flow valve 8 to detect the flow rate of the mixture.

[0006] A test method for simulating deep-sea hydrate mining and pile-soil interaction, the test method comprising the following steps: Step 1: Hydrate formation: In the pressure test chamber 1, ISO standard sand and deionized water are mixed evenly according to the required saturation standard. During the sample loading process, pore pressure sensors and temperature sensors are set up at a specified height to obtain a uniform unsaturated sand model with a diameter of 800 mm and a height of 500 mm. Then, the porosity, water saturation and gas saturation of the sand model are obtained. The model pile is connected to the actuator and buried at the specified location. Sand model volume: Where: D is the inner diameter of the pressure test chamber, and H is the height of the sand model; Water saturation: In the formula: For the volume of water, Pore ​​volume; Theoretical hydrostatic pressure at various points within the reservoir: In the formula: The density of water, The water head height at each point, Pressure at the reference plane; Seal and inspect the pressure test chamber 1: Cover the sealing end cap of the pressure test chamber 1 and perform sealing treatment. Inject high-pressure nitrogen into the pressure test chamber 1 through the bottom channel and observe the pressure fluctuation. If the pressure remains stable within 12 hours, the airtightness is considered to be good. Excess gas method for hydrate formation: After venting nitrogen, carbon dioxide gas is injected into the bottom channel of the pressure test chamber 1 to pressurize the sand model to the pressure value corresponding to the required simulated sea depth. An external water bath cooling method is used, with the cryogenic bath activated. The refrigerant circulates through the external circulating cooling water bath jacket 4 to cool the pressure test chamber 1. A pressurized medium temperature control system is used inside the pressure test chamber 1. The pressurized medium inside the pressure test chamber 1 is first drawn into the pressurization system 9 under constant temperature and atmospheric pressure, and then pressurized and pumped into the pressure test chamber 1. The temperature inside the pressure test chamber 1 is lowered and maintained to a certain temperature to generate hydrates. During the generation process, the amount of hydrate generated is controlled by the amount of carbon dioxide gas injected. The hydrate generation status is judged by observing the pressure changes inside the pressure test chamber 1. The generation of hydrates will cause a pressure drop. Water saturation: After the hydrate is formed, ionized water is slowly injected into the pressure test chamber 1 through the bottom channel to displace the residual gas, so that the sand model is water saturated; this can fully simulate the pressure and temperature environment at the corresponding depth of the seabed and the formation process of the corresponding hydrate. After the hydrate is formed, the control cabinet is used to set the corresponding speed and frequency to perform static and dynamic load pull-out on the model pile, and the failure strength and displacement of the model pile are measured. Step 2, Decomposition of hydrates: The simulated hydrate decomposition is carried out in two aspects: local depressurization of the well wall and overall depressurization of the soil layer inside the pressure test chamber 1. The depressurization of the well wall is achieved by adjusting the pressurization system 9 to control the outlet pressure at the top of the pressure test chamber 1, thereby reducing the liquid pressure in the pores of the sand model to achieve depressurization decomposition of hydrates. After the hydrates stabilize, an initial pressure is set for the pressurization system 9, which is greater than the gas pressure inside the pressure test chamber 1 to prevent gas production before extraction. The pressurization system 9 is then adjusted to gradient mode, and the target pressure and depressurization time are set and run to carry out the decomposition test at the predetermined depressurization rate. After a certain period of depressurization, when the outlet flow rate approaches zero, it indicates that the hydrates have been completely decomposed, and the extraction test ends. The decomposition of hydrates in the overall soil layer can be carried out through three methods: First, decompression mining, which involves changing the pressure inside the pressure test chamber 1 to a value that allows the hydrates to undergo a phase change, thus decomposing the solid hydrates stably stored in the pores of the sand model, producing water and a large amount of gas, and absorbing heat from the environment; second, heating mining, which involves changing the temperature inside the pressure test chamber 1 to a temperature that meets the conditions for the hydrates to undergo a phase change, further promoting decomposition; and third, simultaneous decompression and heating to ensure effective decomposition of the hydrates. After decomposition, the actuator is adjusted to perform corresponding actions on the model pile to detect the stability of the model pile. After decomposition is completed, the servo flow valve 8 can be opened to expel water and gas, and the amount of gas discharged is measured to determine the hydrate decomposition status. Model pile-soil interaction: The following are the formulas for calculating the vertical and horizontal tensile bearing capacity of the model pile foundation before and after hydrate decomposition: Before hydrate decomposition: Vertical tensile strength: In the formula: This refers to the skin friction of the pile. For pile end friction; The diameter of the pile; This refers to the cross-sectional area at the pile tip. Side friction resistance: In the formula: ; , ; End resistance: , For the depth of pile tip embedment L Vertical effective stress at the location Horizontal tensile strength: , The effective unit weight of the soil. The depth and diameter of the pile embedment; After the hydrates decompose: Vertical tensile strength: , , The total stress at depth z Horizontal tensile strength: After decomposition The formula simplifies to: In the formula: For effective stress Furthermore, the gas production is calculated during the measurement process as follows: Cumulative gas production measured in practice Q The instantaneous gas production rate during the depressurization decomposition process was calculated, assuming... t The cumulative gas production at each moment is ,exist After a certain period of time, the cumulative gas production is Then in The trace amount of gas produced during the time period is: The gas production rate per unit time can be obtained by differentiating the product with respect to time. When a unit volume of hydrate decomposes, it produces 164 volumes of gas and 0.87 volumes of water under standard conditions, according to the ideal gas law. The correction shows that in t Constant pressure and temperature Under the following conditions, the volume of gas produced per unit of hydrate decomposition is: ; For pressure under standard conditions, This represents the theoretical volume of gas produced per unit volume of hydrate after decomposition under standard conditions. This refers to the temperature under standard conditions.

[0007] The beneficial effects of this invention: This invention, through a dual-cycle design of "pre-cooling medium + jacketed refrigeration," eliminates the pressure effect on the system. The system minimizes interference from cooling efficiency, achieving precise temperature control to meet the stringent requirements of methane hydrate phase change experiments. The use of a supercritical drying aerogel felt insulation layer significantly reduces energy consumption. Employing a PID+PLC control strategy, it features multiple safety mechanisms including over-temperature protection and low-level alarms, ensuring stable operation over extended periods. Pressure control accuracy is ±1% FS, with automatic pressurization, pressure holding, pressure replenishment, and pressure release functions, supporting real-time display and data storage of pressure curves. The actuators are driven by servo motors, enabling two-dimensional stress-strain cyclic loading and unloading of the soil inside the reactor under complex environments, more realistically simulating pile movement. The system integrates pressure, temperature, mechanical loading, and visualization observation functions, comprehensively simulating the multi-physics coupling environment during deep-sea methane hydrate extraction. Attached Figure Description

[0008] Figure 1 This is the overall front view of the reactor described in this invention.

[0009] Figure 2 This is a side view of the overall reactor described in this invention.

[0010] Figure 3 This is a schematic diagram of the overall reactor described in this invention. Detailed Implementation

[0011] The specific embodiments of the present invention will be described in detail below with reference to the technical solutions and accompanying drawings.

[0012] A low-temperature high-pressure device for simulating deep-sea hydrate mining and pile-soil interaction includes a pressure test chamber 1, an insulation layer 2, an illumination and camera device 3, an external circulating cooling water bath jacket 4, a decompression mining module 6, an actuator 7, a pressurization system 9, a control cabinet 10, an air tank 11, and a constant temperature water tank. The pressure test chamber 1 is equipped with sealed end caps at both the upper and lower parts, and all parts are connected by high-strength screws. An external circulating cooling water bath sleeve 4 is installed around the outer periphery of the pressure test chamber 1, and an insulation layer 2 is installed around the outer periphery of the external circulating cooling water bath sleeve 4. The constant temperature water bath A13 is located outside the pressure test chamber 1. The external circulating cooling water bath sleeve 4 is connected to the external constant temperature water bath A13 through circulation pipelines and circulation pump 20 to form a cooling cycle. Temperature control is achieved by installing a temperature sensor 5 in the external circulating cooling water bath sleeve 4 to provide feedback.

[0013] The pressure test chamber 1 is equipped with an illumination and camera device 3, a decompression mining module 6, an actuator 7, an air tank 11, a temperature sensor 5, and a pressure sensor 16 on the sealed end cap at the upper part of the interior. The illumination and camera device 3 observes the conditions of the pressure test chamber 1. The decompression mining module 6 is used to simulate the deep-sea hydrate mining process and is connected to the decompression interface of the upper sealed end cap via pipeline. The actuator 7 is connected to the model pile and is driven by a motor 12, enabling the model pile to move up, down, left, and right within the pressure test chamber 1. The model pile is buried to a certain depth in the soil inside the pressure test chamber 1. The air tank 11 is used for pressure control, accompanying the solution in and out of the pressure test chamber 1 to maintain the internal confining pressure and avoid errors and concentration changes. The temperature sensor 5 and the pressure sensor 16 are used to detect the internal temperature and pressure of the pressure test chamber 1.

[0014] The constant-temperature water bath B14 is located outside the pressure test chamber 1 and is used to provide the low-temperature pressurizing medium for the pressurization system 9 and control its temperature. The pressurization system 9 includes a booster pump assembly 15, a shut-off valve 17, a pressure gauge 19, and a safety valve 18. The booster pump assembly 15 is the power source, and the pressure gauge 19, shut-off valve 17, and safety valve 18 are sequentially installed on its outlet pipeline; all components are integrated into a single cabinet. The pressurization system 9 is connected via pipeline to the pressurization port of the sealed end cap on the upper part of the pressure test chamber 1 for injecting the pressurizing medium. The constant-temperature water bath A13 provides stronger cooling capacity, circulating cooling for the water bath jacket of the test chamber. The two water baths operate independently yet collaboratively, controlled by a PLC.

[0015] The control cabinet 10 is remotely installed in the operating room and is connected to the temperature sensor 5, the lighting and camera device 3, the motor 12, the servo flow valve 8, the booster pump group 15, and the pressure sensor 16 via cables to form a closed-loop control.

[0016] The pressure-reducing extraction module 6 includes a pressure-reducing extraction cylinder, a filter, a servo flow valve 8, and a flow sensor. The pressure-reducing extraction cylinder serves as the medium collection inlet and is connected to the pressure test chamber 1 via a pipeline. The filter is installed downstream of the outlet of the pressure-reducing extraction cylinder to filter impurities in the gas-liquid mixture. The servo flow valve 8 is connected in series in the pipeline after the filter, serving as the actuator for flow regulation. The flow sensor is installed near the servo flow valve 8 to detect the flow rate of the mixture.

[0017] Furthermore, the actuator 7 is driven by a servo motor and includes a two-axis controllable feed motor, a lead screw, guide rails, a sliding table, and a mounting frame. The mechanical mounting frame of the actuator 7 is fixed inside the pressure test chamber 1. Its loading head acts on the soil inside the pressure test chamber 1. The axial servo motor, through a transmission mechanism such as a lead screw, passes through the mechanical seal device on the lower sealing end cover of the pressure test chamber 1 and connects to the axial sliding table inside the pressure test chamber 1. It is also connected to the control cabinet 10 outside the system via a cable. The radial servo motor is directly fixed to the axial sliding table driven by the vertical motor. Through another set of lead screws and guide rails, it drives the horizontal loading head. Its cable moves together with the axial sliding table inside the pressure test chamber 1 and is finally led out of the pressure test chamber 1 through a multi-channel watertight connector on the lower sealing end cover. The actuator 7 has a vertical (axial) loading force of 5kN and a displacement of 300mm, a horizontal (radial) loading force of 3kN, and an adjustment range of no less than 480mm. The axial servo motor of actuator 7 is installed outside the reactor, without occupying internal space, while the horizontal servo motor is installed inside the reactor, forming a pressure-balanced structure. Actuator 7 operates at a speed of approximately 10 mm / s, with a position control accuracy of 0.1 mm. Actuator 7 can perform various types of actuation, including static and dynamic loads, such as single-axis, interpolation, circular, oblique reciprocating, and curve actuation modes, and can also perform force control detection.

[0018] Furthermore, the lighting and camera device 3 is used to observe the experimental process inside the pressure test chamber 1. A miniature camera is installed on the sealed end cap at the top of the pressure test chamber 1, along with a lighting lamp. The height inside the pressure test chamber 1 is 800mm, and with 500mm of test soil sample added, after deducting the camera's height, the observation range is only about 200mm. Therefore, a small analog macro camera with a pressure resistance of 20MPa or higher is selected and encapsulated. Two sets of lighting lamps, with a power of 2... 5 = 10W, using LED light source packaging.

[0019] Furthermore, the insulation layer 2 is made of aerogel material.

[0020] Furthermore, the pressure test chamber 1 is a structure formed by integral forging and machining, with double radial O-ring seals and high-strength screws locking the sealing end caps. The chamber material is selected as 2570 stainless steel, with a design pressure of 20MPa and a design verification and factory test pressure of 25MPa. Based on the third strength theory, the wall thickness of the chamber is calculated to be 40mm, and the thickness of the sealing end cap is 120mm, which is verified using finite element analysis.

[0021] Furthermore, to eliminate pressure fluctuations generated by the output of the booster system 9, a volumetric pressure stabilizer with an anti-pulsation device is installed between the booster pump outlet and the pressure test chamber 1 to reduce the impact of pressure fluctuations on the experiment. The pressure stabilizer is made of 316 stainless steel, has a rated pressure of 30MPa, a built-in filter, and its outlet is connected to a pressure sensor 16 and a safety valve 18.

[0022] Furthermore, the pipeline between the pressurization system 9 and the pressure test chamber 1 includes stainless steel pipes, joints, shut-off valves 17, etc., made of 316 stainless steel, with a pressure resistance of 1000Psi.

[0023] A test method for simulating deep-sea hydrate mining and pile-soil interaction, the test method comprising the following steps: Step 1: Hydrate formation: In the pressure test chamber 1, ISO standard sand and deionized water were mixed uniformly according to the required saturation level. During the sample loading process, pore pressure sensors and temperature sensors were installed at a specified height to obtain a uniform unsaturated sand model with a diameter of 800 mm and a height of 500 mm. The porosity, water saturation, and gas saturation of the sand model were then obtained. The model pile was connected to the actuator and buried at the designated location.

[0024] Sand model volume: Where: D is the inner diameter of the pressure test chamber, and H is the height of the sand model; Water saturation: In the formula: For the volume of water, Pore ​​volume; Theoretical hydrostatic pressure at various points within the reservoir: In the formula: The density of water, The water head height at each point, The pressure at the reference plane.

[0025] Seal and inspect the pressure test chamber 1: Cover the sealing end cap of the pressure test chamber 1 and perform sealing treatment. Inject high-pressure nitrogen into the pressure test chamber 1 through the bottom channel and observe the pressure fluctuation. If the pressure remains stable within 12 hours, the airtightness is considered to be good. Excess gas method for hydrate formation: After venting nitrogen, carbon dioxide gas is injected into the bottom channel of the pressure test chamber 1 to pressurize the sand model to the pressure value corresponding to the required simulated ocean depth. An external water bath cooling method is used, with the cryogenic bath activated. The refrigerant circulates through the external cooling water bath jacket 4 to cool the pressure test chamber 1. A pressurized medium temperature control system is used inside the pressure test chamber 1. The pressurized medium, under constant temperature and atmospheric pressure, is first drawn into the pressurization system 9, pressurized, and then pumped into the pressure test chamber 1. The temperature inside the pressure test chamber 1 is lowered and maintained to a certain temperature to generate hydrates. The amount of hydrate generated is controlled by the injection rate of carbon dioxide gas. The hydrate formation status is determined by observing the pressure changes inside the pressure test chamber 1; hydrate formation causes a pressure drop.

[0026] Water Saturation: After the hydrates have formed, deionized water is slowly injected into the pressure test chamber 1 through the bottom channel to displace the residual gas, saturating the sand model with water. This fully simulates the pressure and temperature environment at the corresponding depths of the seabed and the formation process of the corresponding hydrates.

[0027] After the hydrate is formed, the control cabinet is operated to set the corresponding speed and frequency to perform static and dynamic load pull-out on the model pile, and the failure strength and displacement of the model pile are measured.

[0028] Step 2, Decomposition of hydrates: The simulated hydrate decomposition was conducted in two ways: local depressurization of the wellbore and overall depressurization of the soil layer within the pressure test chamber 1. Depressurization of the wellbore was achieved by adjusting the pressurization system 9 to control the outlet pressure at the top of the pressure test chamber 1, thereby reducing the liquid pressure in the pores of the sand model and achieving depressurized decomposition of the hydrate. After the hydrate stabilized, an initial pressure was set for the pressurization system 9, which was greater than the gas pressure inside the pressure test chamber 1 to prevent gas production before extraction. The pressurization system 9 was then adjusted to gradient mode, and the target pressure and depressurization time were set and run to conduct a decomposition test at the predetermined depressurization rate. After a certain period of depressurization, once the outlet flow rate approached zero, it indicated that the hydrate had completely decomposed, and the extraction test ended.

[0029] The decomposition of hydrates in the overall soil layer can be carried out through three methods: First, decompression mining, which involves changing the pressure inside the pressure test chamber 1 to a value that causes a phase change in the hydrates, thus decomposing the solid hydrates stably stored in the pores of the sand model, producing water and a large amount of gas, and absorbing heat from the environment. Second, heating mining, which involves changing the temperature inside the pressure test chamber 1 to a temperature that meets the conditions for a phase change in the hydrates, further promoting decomposition. Third, simultaneous decompression and heating are carried out to ensure effective decomposition of the hydrates. After decomposition, the actuator is adjusted to perform corresponding actions on the model pile to detect the stability of the model pile. After decomposition is completed, the servo flow valve 8 can be opened to expel water and gas, and the amount of gas discharged is measured to determine the hydrate decomposition status.

[0030] Model pile-soil interaction: The following are the formulas for calculating the vertical and horizontal tensile bearing capacity of the model pile foundation before and after hydrate decomposition: Before hydrate decomposition: Vertical tensile strength: In the formula: This refers to the skin friction of the pile. For pile end friction; The diameter of the pile; This refers to the cross-sectional area at the pile tip. Side friction resistance: In the formula: ; , .

[0031] End resistance: , For the depth of pile tip embedment L Vertical effective stress at the location Horizontal tensile strength: , The effective unit weight of the soil. The depth and diameter of the pile embedment; After the hydrates decompose: Vertical tensile strength: , , The total stress at depth z Horizontal tensile strength: After decomposition The formula simplifies to: In the formula: For effective stress Furthermore, the gas production is calculated during the measurement process as follows: Cumulative gas production measured in practice Q The instantaneous gas production rate during the depressurization decomposition process was calculated, assuming... t The cumulative gas production at each moment is ,exist After a certain period of time, the cumulative gas production is Then in The trace amount of gas produced during the time period is: The gas production rate per unit time can be obtained by differentiating the product with respect to time. When a unit volume of hydrate decomposes, it produces 164 volumes of gas and 0.87 volumes of water under standard conditions, according to the ideal gas law. The correction shows that in t Constant pressure and temperature Under the following conditions, the volume of gas produced per unit of hydrate decomposition is: .

[0032] For pressure under standard conditions, This represents the theoretical volume of gas produced per unit volume of hydrate after decomposition under standard conditions. This refers to the temperature under standard conditions.

[0033] Experimental plan: Hydrate formation, pile foundation bearing capacity and depressurized mining study: (1) Fill with sand and compact it 318.48 kg of sand and 29.68 kg of water were thoroughly mixed. 43.52 kg of this mixture was weighed and evenly filled into the reactor (pressure test chamber 1). The sand was compacted to a thickness of 50 mm using a flat-headed hammer. This process was repeated, adding 348.16 kg of wet sand into the high-pressure reactor in multiple batches. After compaction, the sand model thickness was 400 mm. During the sand filling process, pore pressure sensors and temperature sensors were laid in layers when the sand model reached the specified height.

[0034] (2) Vacuuming the closed vessel Using a lifting device, slowly lift the vessel lid to a suitable height and move it to the top of the high-pressure reactor with the sand model installed. Apply a layer of silicone grease to the sealing ring of the vessel lid. After finely adjusting and straightening the vessel lid using the guide rod, slowly lower the vessel lid until it covers the reactor body. Tighten the sealing bolts on the vessel lid symmetrically, connect the vacuum pipeline, and check that all valves are closed. Open the vacuum valve and turn on the vacuum pump to perform vacuuming inside the reactor. Observe the vacuum gauge to determine whether the vacuuming is complete. When the vacuum gauge pointer stabilizes at around -0.05 MPa and no longer fluctuates, the vacuuming is considered complete. Close the vacuum valve and disconnect the vacuum pump power.

[0035] (3) Constant pressure injection After the vacuuming is complete, open the valve at the top of the high-pressure reactor and connect it to... Gas cylinder, injecting into the vessel The pressure was raised to 3.5 MPa, and subsequent operations were carried out after the pressure stabilized.

[0036] (4) Cooling synthesis Based on the stable existence temperature of carbon dioxide hydrate, a constant-temperature water bath was used to lower the internal temperature of the high-pressure reactor to 4.0~5.0℃. During the experiment, temperature and pressure were recorded in real time. As the temperature decreased, hydrate formed, and the pressure inside the reactor gradually decreased. Once the pressure inside the reactor reached a certain value, the hydrate was re-injected. Pressurize to 3.5 MPa and monitor the pressure change inside the reactor until the pressure stabilizes. When the pressure inside the reactor remains stable and no longer fluctuates, it is determined that the hydrate has been formed.

[0037] (5) Water saturation The generated hydrate sand model was saturated with water, and the pressure inside the vessel was increased to 10 MPa using a pressurization system until the pressure stabilized.

[0038] (6) Model pile operation After the pressure stabilizes, the model pile is vertically manipulated at a speed of 10 mm / s and a displacement of 50 mm. The changes in the pull-out force of the model pile during the manipulation are observed and the data are recorded.

[0039] (7) Pressure reduction mining: The pressure near the mining cylinder is reduced to 1.5MPa by the pressurization system. The pressure reduction mining module is opened to mine the internal gas. After a certain period of pressure reduction, the hydrate decomposition rate and the outlet gas production rate reach equilibrium, so that the pressure reaches equilibrium and stability. At this time, the pressure curve is basically flat. After the outlet flow rate approaches zero, it indicates that the hydrate has been completely decomposed and the mining test ends.

Claims

1. A low-temperature, high-pressure device for simulating deep-sea hydrate mining and pile-soil interaction, characterized in that, The low-temperature high-pressure equipment includes a pressure test chamber (1), an insulation layer (2), a lighting and camera device (3), an external circulating cooling water bath jacket (4), a pressure reduction mining module (6), an actuator (7), a pressurization system (9), a control cabinet (10), an air tank (11), and a constant temperature water bath; The pressure test chamber (1) is equipped with sealed end caps at the top and bottom, and each part is connected by high-strength screws. An external circulating cooling water bath sleeve (4) is set on the outer periphery of the pressure test chamber (1), and an insulation layer (2) is set on the outer periphery of the external circulating cooling water bath sleeve (4). The constant temperature water tank A (13) is located outside the pressure test chamber (1). The external circulating cooling water bath sleeve (4) is connected to the external constant temperature water tank A (13) through a circulation pipeline and a circulation pump (20) to form a cooling cycle. Temperature control is achieved by setting a temperature sensor (5) in the external circulating cooling water bath sleeve (4) to provide feedback. The pressure test chamber (1) is equipped with an illumination camera (3), a decompression mining module (6), an actuator (7), an air tank (11), a temperature sensor (5), and a pressure sensor (16) on the sealed end cap at the upper part of the interior. The illumination camera (3) is used to observe the condition of the pressure test chamber (1). The decompression mining module (6) is used to simulate the deep-sea hydrate mining process and is connected to the decompression interface of the upper sealed end cap through a pipeline. The actuator (7) is connected to the model pile and is driven by a motor (12) to move the model pile up, down, left, and right in the pressure test chamber (1). The model pile is buried to a certain depth in the soil inside the pressure test chamber (1). The air tank (11) is used to perform pressure control operations and maintains the internal confining pressure by accompanying the solution in and out of the pressure test chamber (1) to avoid errors and concentration changes. The temperature sensor (5) and the pressure sensor (16) are used to detect the internal temperature and pressure of the pressure test chamber (1). The constant temperature water bath B (14) is located outside the pressure test chamber (1) and is used to provide a low temperature pressurizing medium to the pressurization system (9) and control its temperature; the pressurization system (9) includes a pressurization pump group (15), a shut-off valve (17), a pressure gauge (19) and a safety valve (18); the pressurization pump group (15) is the power source, and the pressure gauge (19), shut-off valve (17) and safety valve (18) are installed in sequence on its outlet pipeline, and all components are integrated in a cabinet; the pressurization system (9) is connected to the pressurization hole of the sealing end cap on the upper part of the pressure test chamber (1) through a pipeline for injecting pressurizing medium; The control cabinet (10) is remotely installed in the operating room and is connected to the temperature sensor (5), lighting camera (3), motor (12), servo flow valve (8), booster pump group (15), and pressure sensor (16) via cables to form a closed-loop control. The pressure reduction extraction module (6) includes a pressure reduction extraction cylinder, a filter, a servo flow valve (8), and a flow sensor. The pressure reduction extraction cylinder serves as the medium collection inlet and is connected to the pressure test chamber (1) via a pipeline. The filter is installed downstream of the outlet of the pressure reduction extraction cylinder to filter impurities in the gas-liquid mixture. The servo flow valve (8) is connected in series in the pipeline after the filter and serves as the actuator for flow regulation. The flow sensor is installed near the servo flow valve (8) to detect the flow rate of the mixture.

2. A test method for simulating deep-sea hydrate mining and pile-soil interaction using the low-temperature, high-pressure equipment of claim 1, characterized in that... The testing method includes the following steps: Step 1: Hydrate formation: In the pressure test chamber (1), ISO standard sand and deionized water are mixed evenly according to the required saturation standard. During the sample loading process, pore pressure sensors and temperature sensors are set up at a specified height to obtain a uniform unsaturated sand model with a diameter of 800 mm and a height of 500 mm. Then, the porosity, water saturation and gas saturation of the sand model are obtained. The model pile is connected to the actuator and buried at the specified location. Sand model volume: ; Where: D is the inner diameter of the pressure test chamber, and H is the height of the sand model; Water saturation: In the formula: For the volume of water, Pore ​​volume; Theoretical hydrostatic pressure at various points within the reservoir: ; In the formula: The density of water, The water head height at each point, Pressure at the reference plane; Close the pressure test chamber (1) and check: Cover the pressure test chamber (1) with the sealing end cap and perform sealing treatment. Inject high pressure nitrogen into the pressure test chamber (1) from the bottom channel and observe the pressure fluctuation. If the pressure remains stable within 12 hours, the airtightness is considered good. Excess gas method for hydrate generation: After nitrogen is discharged, carbon dioxide gas is injected into the bottom channel of the pressure test chamber (1) to pressurize the sand model to the pressure value corresponding to the required simulated sea depth. The external water bath cooling method is adopted. The low temperature bath is turned on. The refrigerant is circulated to the pressure test chamber (1) through the external circulating cooling water bath jacket (4) to cool down the pressure test chamber (1). The pressure test chamber (1) adopts a pressurized medium temperature control system. The pressurized medium in the pressure test chamber (1) is first drawn into the pressurization system (9) under constant temperature and normal pressure. After pressurization, it is pumped into the pressure test chamber (1). The temperature inside the pressure test chamber (1) is reduced and maintained to a certain temperature to generate hydrate. During the generation, the amount of hydrate generated is controlled by the amount of carbon dioxide gas injected. The hydrate generation is judged by observing the pressure change inside the pressure test chamber (1). The generation of hydrate will cause the pressure to drop. Water saturation: After the hydrate is generated, ionized water is slowly injected into the pressure test chamber (1) through the bottom channel of the pressure test chamber (1) to displace the residual gas, so that the sand model is water saturated; it can fully simulate the pressure and temperature environment at the corresponding depth of the seabed and the corresponding hydrate generation process. After the hydrate is formed, the control cabinet is used to set the corresponding speed and frequency to perform static and dynamic load pull-out on the model pile, and the failure strength and displacement of the model pile are measured. Step 2, Decomposition of hydrates: The simulated hydrate decomposition is carried out in two aspects: local depressurization of the well wall and overall depressurization of the soil layer inside the pressure test chamber (1). The depressurization of the well wall is achieved by adjusting the pressurization system (9) to control the outlet pressure at the top of the pressure test chamber (1), thereby reducing the liquid pressure in the pores of the sand model to achieve depressurization decomposition of hydrate. After the hydrate is generated and stabilized, an initial pressure of the pressurization system (9) is set. This pressure is greater than the gas pressure inside the pressure test chamber (1) to prevent gas production before mining. The pressurization system (9) is further adjusted to gradient mode, and the target pressure and depressurization time of the pressurization system (9) are set and run. The decomposition test at the predetermined depressurization rate can be carried out. After a certain period of depressurization, when the outlet flow rate approaches zero, it indicates that the hydrate has been completely decomposed and the mining test ends. The decomposition of hydrates in the overall soil layer can be carried out in three ways: First, decompression mining, that is, changing the pressure inside the pressure test chamber (1) to reduce the pressure value to a value that can cause the hydrate to undergo a phase change, so that the solid phase hydrates stably stored in the pores of the sand model decompose, producing water and a large amount of gas and absorbing heat from the environment; Second, heating mining, that is, changing the temperature inside the pressure test chamber (1) to make the temperature reach the condition for the hydrate to undergo a phase change, further promoting the decomposition; Third, decompression and heating are carried out simultaneously to effectively decompose the hydrates. After decomposition, the actuator is adjusted to perform corresponding actions on the model pile to detect the stability of the model pile; After the decomposition is completed, the servo flow valve (8) can be opened to remove water and gas, and the amount of gas discharged can be measured to judge the hydrate decomposition status. Model pile-soil interaction: The following are the formulas for calculating the vertical and horizontal tensile bearing capacity of the model pile foundation before and after hydrate decomposition: Before hydrate decomposition: Vertical tensile strength: ； In the formula: This refers to the skin friction of the pile. For pile end friction; Pile diameter; This is the cross-sectional area of ​​the pile tip; Side friction resistance: ； In the formula: ; , ; End resistance: ； , For the depth of pile tip embedment L Vertical effective stress at the location Horizontal tensile strength: ； , The effective unit weight of the soil. The depth and diameter of the pile embedment; After the hydrate decomposes: Vertical tensile strength: ； , , The total stress at depth z Horizontal tensile strength: ； After decomposition The formula simplifies to: In the formula: This is the effective stress.

3. The test method according to claim 2, characterized in that, The gas production rate is calculated as follows during the measurement process: Cumulative gas production measured in practice Q The instantaneous gas production rate during the depressurization decomposition process was calculated, assuming... t The cumulative gas production at each moment is ,exist After a certain period of time, the cumulative gas production is Then in The trace amount of gas produced during the time period is: The gas production rate per unit time can be obtained by differentiating the product with respect to time. ； When a unit volume of hydrate decomposes, it produces 164 volumes of gas and 0.87 volumes of water under standard conditions, according to the ideal gas law. The correction shows that in t Constant pressure and temperature Under the following conditions, the volume of gas produced per unit of hydrate decomposition is: ； For pressure under standard conditions, This represents the theoretical volume of gas produced per unit volume of hydrate after decomposition under standard conditions. Temperature under standard conditions.

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