CN115235800B - Micro centrifuge model and method for simulating axial pipe-soil interaction under offshore deep water condition - Google Patents

Micro centrifuge model and method for simulating axial pipe-soil interaction under offshore deep water condition Download PDF

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CN115235800B
CN115235800B CN202210853340.3A CN202210853340A CN115235800B CN 115235800 B CN115235800 B CN 115235800B CN 202210853340 A CN202210853340 A CN 202210853340A CN 115235800 B CN115235800 B CN 115235800B
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pipe
soil
model
test
pipe section
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CN115235800A (en
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刘路路
刘晓燕
蔡国军
李晓昭
卢萌盟
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China University of Mining and Technology CUMT
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
    • G01M99/00—Subject matter not provided for in other groups of this subclass
    • G01M99/007—Subject matter not provided for in other groups of this subclass by applying a load, e.g. for resistance or wear testing
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
    • G01L11/00—Measuring steady or quasi-steady pressure of a fluid or a fluent solid material by means not provided for in group G01L7/00 or G01L9/00

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Abstract

The invention discloses a micro centrifuge model and a method for simulating axial pipe-soil interaction under offshore deep water conditions, which can measure axial displacement of a pipeline and simulate vertical force or vibration acting on the pipeline, wherein: the axial motion is driven by a servo motor coupled to a chain and sprocket assembly that causes the pipe to move axially relative to the model soil; the vertical movement of the hydraulic cylinder simulates a vertical force or vibration acting on the pipe. Wherein the centrifuge model test is performed under 10-g model gravity, eliminating end effects during axial slip by using a first virtual pipe section and a second virtual pipe section; by placing additional guide tubes that are telescopic on both sides of the hydraulic cylinder, the pipe sway during the test is minimized. Therefore, design data of the underground soil body are accurately and reasonably provided, and the method has great theoretical significance and practical value for verifying the safety and reliability of geotechnical engineering design under the offshore deep water condition.

Description

Micro centrifuge model and method for simulating axial pipe-soil interaction under offshore deep water condition
Technical Field
The invention relates to a micro centrifuge model and a method for simulating axial pipe-soil interaction under offshore deep water conditions.
Background
Offshore deepwater pipeline installations are quite different from shallow water pipeline installations, and since trenching is difficult under offshore deepwater conditions, often allowing the pipeline to partially embed into the sea floor by its own weight, one of the major challenges facing pipeline engineers is to assess the pipeline axial friction under high pressure and temperature cycles caused by frequent starts and shutdowns. These repeated operations cause the pipe to undergo cyclic expansion and contraction, and successive cycles of expansion and contraction may cause extensive axial displacement of the pipe, which can cause damage to the pipe end connections. During the start-up cycle, the expansion of the pipe is counteracted by the moving pipe-soil interface friction, which may be unevenly distributed and time-dependent, and the interaction between the expansion and the pipe-soil friction may cause compressive stresses along the pipe, which in turn causes the pipe to flex laterally.
In addition, pore pressure and stress measurement is very challenging under offshore deep water conditions, the drainage-non-drainage transition behavior is not well characterized, and detailed mechanical characterization of soil is difficult due to the low ground stress of the pipeline during operation.
In order to verify the safety and reliability of the design scheme, a field test or a simulation test may be performed, wherein the field test requires measurement at a specific field, the test condition is uncontrollable and high in cost, therefore, the simulation test is usually selected, for example, the safety and reliability of the design scheme is verified through a centrifugal machine model, the existing centrifugal machine model cannot eliminate the end effect during the axial sliding of the pipeline, the survey result is inaccurate, the resolution ratio is low, and the requirement of the simulation survey under the offshore condition cannot be met.
Disclosure of Invention
Aiming at the problems, the invention provides a micro-centrifuge model and a method for simulating axial pipe-soil interaction under offshore deepwater conditions, solves the problems that the ground stress is very low during pipeline operation under the offshore deepwater conditions, and detailed mechanical characteristic characterization is difficult to perform on soil, can eliminate the end effect during pipeline axial sliding, and has accurate test results and high resolution.
Name interpretation:
1. offshore deep water: the depth of ocean water is 50-600m;
2. 10-g Condition (g represents Earth's gravity): 10 times of the conditions of the gravity of the earth;
3. 1-g Condition (g represents Earth's gravity): 1 gravity condition;
4. total pressure sensor: the total pressure acting on the lower suspension simulation tube during the experiment was tested.
In order to achieve the technical purpose and achieve the technical effect, the invention is realized by the following technical scheme:
the micro centrifuge model for simulating the axial pipe-soil interaction under the offshore deep water condition comprises a model box capable of containing model soil and a driving system arranged above the model box, wherein a rigid frame for supporting the driving system is arranged above the model box:
the driving system comprises a servo motor, a hydraulic cylinder and a chain sprocket assembly, a sliding table is arranged between the bottom of the driving system and a rigid frame, the bottom of the hydraulic cylinder penetrates through the sliding table and the rigid frame and extends into a model box, the bottom of the hydraulic cylinder is sequentially connected with a fourth force sensor and a lower suspension simulation pipe, a pulse signal generated by a centrifuge control chamber controls the servo motor, the servo motor drives the sliding table to move horizontally through the chain sprocket assembly, the sliding table is provided with a second linear displacement sensor along the horizontal direction, a first linear displacement sensor is vertically arranged on the hydraulic cylinder, and two first force sensors used for monitoring horizontal loads applied to a lower suspension simulation pipeline are symmetrically arranged on the sliding table;
the lower suspension simulation pipe comprises a test pipe section, wherein one end of the test pipe section is provided with a first virtual pipe section, the other end of the test pipe section is provided with a second virtual pipe section, the test pipe section is symmetrically provided with two first pore pressure sensors, two total pressure sensors are symmetrically arranged between the two first pore pressure sensors, the end part of the first virtual pipe section, which is close to the test pipe section, is provided with a second force sensor, and the end part of the second virtual pipe section, which is close to the test pipe section, is provided with a third force sensor;
and a second pore pressure sensor is arranged in the model soil.
Preferably, two telescopic additional guide pipes are symmetrically arranged on two sides of the hydraulic cylinder along the horizontal direction, penetrate through the sliding table and the rigid frame, and are connected with the lower suspension simulation pipe at the bottoms.
Preferably, the first linear displacement sensor is arranged between the hydraulic cylinder and an additional telescopic guide tube.
Preferably, two linear guide rails are arranged on the upper portion of the rigid frame in parallel along the horizontal direction, a sliding block capable of moving horizontally along the linear guide rails is arranged at the bottom of the sliding table, a through hole is formed between the two linear guide rails along the horizontal direction, and the hydraulic cylinder and the two telescopic additional guide pipes penetrate through the through hole and can move horizontally along the through hole.
Preferably, a first mounting bracket is arranged in the first virtual pipe section, the second force sensor is fixed in the first virtual pipe section through the first mounting bracket, a second mounting bracket is arranged in the second virtual pipe section, and the third force sensor is fixed in the second virtual pipe section through the second mounting bracket.
Preferably, the bottom of the mold box is provided with at least one water valve.
Preferably, the first virtual pipe section, the test pipe section and the second virtual pipe section are all stainless steel pipes.
The method for simulating the axial pipe-soil interaction under the offshore deep water condition adopts any one of the mini-centrifuge models for simulating the axial pipe-soil interaction under the offshore deep water condition to simulate, and comprises the following specific steps:
step 1, coating lubricating grease on the inner wall of a mold box, placing a sand layer with a set thickness at the bottom of the mold box, covering a layer of geotextile on the sand layer, and adding degassed water into the mold box;
step 2, putting the permeable stone of the second pore pressure sensor into the degassed water, boiling for 10-15 minutes to saturate the permeable stone, then attaching the permeable stone to the second pore pressure sensor in the degassed water, and keeping the permeable stone under the water for a set time to ensure complete saturation; when the liquid level of the clay slurry in the step 1 reaches a specified liquid level, placing a saturated second pore pressure sensor into the model soil, wherein the second pore pressure sensor is ballasted by polystyrene foam;
step 3, after the slurry is placed, preloading the soil sample under the condition of 1-g, wherein the preloading pressure is 1.1kPa, after preloading for 7 days under the condition of 1-g, transferring the model soil into a model box, opening a water valve, and further solidifying under the condition of 10-g, wherein the final thickness of the solidified model soil is 150-200mm;
step 4, under a displacement control mode, embedding the lower suspension simulation pipe into model soil in a model box at a specified speed by a hydraulic cylinder until the required embedding depth is reached;
step 5, after the embedding stage is finished, starting a load control mode, and setting the vertical load of the hydraulic cylinder as the penetration resistance recorded at the final embedding depth of the lower suspension simulation pipe;
step 6, after dissipation is finished, starting a servo motor, driving a chain sprocket assembly connected to a sliding table by the servo motor, controlling the axial movement of the lower suspension simulation pipe, and simultaneously carrying out sensor data acquisition in the axial sliding process of the lower suspension simulation pipe;
and 7, after the test is finished, closing the micro centrifuge model, finishing the test and carrying out subsequent data processing, wherein the calculation formula of the pipe-soil interface resistance F on the test pipe section is as follows:
F＝T-R f -R 1 -R 2
wherein T is the total acting force acting on the lower suspension simulation tube and is measured by the first force sensor; r is f The resistance caused by the soil body in front of the first virtual section is measured by a second force sensor; r is 1 A pipe-soil interface resistance of the first virtual segment, measured by the second force sensor; r is 2 The pipe-soil interface resistance for the second virtual segment is measured by a third force sensor.
Preferably, in all centrifugal tests, the maximum sedimentation amount of the second pore pressure sensor is not more than 10mm, the distance from the final position to the soil surface layer of the model is 2.5D-3D, and D is the diameter of the lower suspension simulation tube; in step 4, the embedding depth is half of the pipe diameter D.
Preferably, in step 4, the fourth force sensor and the first linear displacement sensor feed back the measured load and displacement to the servo motor, and if the load and the vertical displacement of the lower suspension analog pipe do not reach a set value, the servo motor corrects the load or the movement of the hydraulic cylinder according to the error value.
The invention has the beneficial effects that:
when the contact stress, the pore pressure and the sliding resistance of the pipeline need to be measured, the micro-centrifugal machine model can be used for measuring the axial displacement of the pipeline and simulating the vertical force or vibration acting on the pipeline, wherein: the axial motion is driven by a servo motor coupled to a chain sprocket assembly that causes the pipe to move axially relative to the model soil; the vertical movement of the hydraulic cylinder simulates a vertical force or vibration acting on the pipe.
Wherein the centrifuge model test is conducted under 10-g model gravity, eliminating end effects during axial slip by using a first virtual pipe section and a second virtual pipe section; by placing additional guide tubes on both sides of the cylinder, which are telescopic, the pipe sway during the test is minimized. Therefore, design data of the underground soil body are accurately and reasonably provided, and the method has great theoretical significance and practical value for verifying the safety and reliability of geotechnical engineering design under the offshore deep water condition.
Drawings
FIG. 1 is a schematic structural diagram of a microcentrifuge model for simulating a survey axial pipe-soil interaction according to the invention;
FIG. 2 is a top view of a microcentrifuge model for simulating a survey axial pipe-soil interaction according to the invention;
FIG. 3 is a schematic view of the construction of a model pipe according to the invention;
FIG. 4 is a schematic illustration of the forces acting on the model tube during axial sliding of the present invention;
the reference numerals of the drawings have the following meanings: 1-a driving system, 2-a first linear displacement sensor, 3-a second linear displacement sensor, 4-a servo motor, 5-a first force sensor, 6-a hydraulic cylinder, 7-a telescopic additional guide pipe, 8-a slide block, 9.1-a sliding table, 9.2-a linear guide rail and 9.3-a rigid frame; 10-model box, 11-lower suspension simulation tube, 11.1-first virtual tube section, 11.2-first mounting bracket, 11.3-second force sensor, 11.4-first pore pressure sensor, 11.5-total pressure sensor, 11.6-test tube section, 11.7-third force sensor, 11.8-second mounting bracket, 11.9-second virtual tube section, 12-fourth force sensor, 13-water valve, 14-second pore pressure sensor, 15-chain sprocket assembly and 16-centrifuge control room.
Detailed Description
The present invention will be better understood and implemented by those skilled in the art by the following detailed description of the technical solution of the present invention with reference to the accompanying drawings and specific examples, which are not intended to limit the present invention.
As shown in fig. 1 and 2, a microcentrifuge model for simulating axial pipe-soil interaction under offshore deep water conditions comprises a model box 10 and a driving system 1 arranged above the model box 10, wherein the model box 10 is used for accommodating model soil (namely soil under offshore deep water conditions) during simulation experiments, and a second pore pressure sensor 14 is arranged in the model soil and used for researching the stress of the model soil. Model case 10 top is provided with rigid frame 9.3 (rigid frame 9.3 is the supporting framework, the through-hole that runs through from top to bottom has been arranged on its structure, be provided with sliding stand 9.1 between the bottom of actuating system 1 and the rigid frame 9.3, actuating system 1 is used for the drive pipeline to carry out axial displacement, vertical displacement etc, including servo motor 4, pneumatic cylinder 6 and chain sprocket assembly 15, actuating system 1 passes through chain sprocket assembly 15 of servo motor 4 drive connection on sliding stand 9.1, the operation of the whole centrifuge model of control, chain sprocket assembly 15 is supported by slider 8 and linear guide 9.2 of installing on rigid frame 9.3, and is concrete:
the bottom of pneumatic cylinder 6 runs through sliding stand 9.1 and rigid frame 9.3 and extends to in model case 10 and the bottom of pneumatic cylinder 6 is connected with fourth force transducer 12 and hangs down simulation pipe 11 in order, and fourth force transducer 12 of installation is used for monitoring the vertical load that acts on hanging down simulation pipe 11 on pneumatic cylinder 6, and pneumatic cylinder 6 drives and hangs down simulation pipe 11 and carry out vertical direction's motion, and vertical displacement and power can be applyed to hanging down simulation pipe 11 to pneumatic cylinder 6.
The axle of the servo motor 4 can be connected to the chain and chain wheel assembly 15 through a gear, the pulse signal generated by the centrifuge control chamber 16 controls the servo motor 4, the speed of the servo motor 4 is controlled by changing the frequency of the input pulse, the moving direction of the pipeline is controlled by the rotating direction of the servo motor 4, the servo motor 4 drives the sliding table 9.1 to move horizontally through the chain and chain wheel assembly 15, in fig. 2, the head and tail ends of the chain and chain wheel assembly 15 are connected with the servo motor 4 to form a conveying device, wherein, a part of the conveying direction of the chain and chain wheel assembly 15 is the horizontal direction in fig. 2, and the horizontal movement of the hydraulic cylinder 6 and the two telescopic additional guide pipes 7 to the left or the right is realized through the clockwise or anticlockwise conveying direction of the chain and chain wheel assembly 15.
Sliding table 9.1 is provided with second linear displacement sensor 3 and is provided with first linear displacement sensor 2 along the vertical on the pneumatic cylinder 6 along the horizontal direction, and first linear displacement sensor 2 is used for monitoring the vertical direction displacement of hanging down simulation pipe 11, and second linear displacement sensor 3 is used for monitoring the horizontal direction displacement of hanging down simulation pipe 11.
Two first force sensors 5 for monitoring horizontal loads applied to the lower suspension simulation pipe 11 are symmetrically arranged on the sliding table 9.1, and the first force sensors 5 are used for monitoring the horizontal loads applied to the lower suspension simulation pipe 11.
As shown in fig. 3, the underslung simulation pipe 11 includes a test pipe segment 11.6, one end of the test pipe segment 11.6 is provided with a first dummy pipe segment 11.1, and the other end is provided with a second dummy pipe segment 11.9, preferably, the first dummy pipe segment 11.1, the test pipe segment 11.6 and the second dummy pipe segment 11.9 are stainless steel pipes, and the first dummy pipe segment 11.1 and the second dummy pipe segment 11.9 can effectively eliminate an end effect during axial sliding of the underslung simulation pipe 11.
Two first pore pressure sensors 11.4 are symmetrically arranged on the test pipe section 11.6, two total pressure sensors 11.5 are symmetrically arranged between the two first pore pressure sensors 11.4, that is, in fig. 3, the test pipe section 11.6 is sequentially provided with the first pore pressure sensor, the total pressure sensor and the first pore pressure sensor from the left side to the right side, and the two first pore pressure sensors 11.4 are used for researching the dynamic response of the seabed.
The end of the first virtual pipe section 11.1 close to the test pipe section 11.6 is provided with a second force sensor 11.3, and the end of the second virtual pipe section 11.9 close to the test pipe section 11.6 is provided with a third force sensor 11.7. Preferably, the first virtual pipe section 11.1 is provided with a first mounting bracket 11.2 inside, the second force sensor 11.3 is fixed inside the first virtual pipe section 11.1 by the first mounting bracket 11.2, the second virtual pipe section 11.9 is provided with a second mounting bracket 11.8 inside, and the third force sensor 11.7 is fixed inside the second virtual pipe section 11.9 by the second mounting bracket 11.8. To ensure that the force second 11.3 and third 11.7 sensors can capture all the load on the entire underhung simulation tube 11.
In order to reduce the shaking when the lower suspension simulation tube 11 slides axially, two telescopic additional guide tubes 7 are symmetrically arranged on two sides of the hydraulic cylinder 6 along the horizontal direction, the telescopic additional guide tubes 7 penetrate through the sliding table 9.1 and the rigid frame 9.3, the bottoms of the telescopic additional guide tubes are connected with the lower suspension simulation tube 11, and when the lower suspension simulation tube 11 moves, the two telescopic additional guide tubes 7 move synchronously. The first linear displacement sensor 2 may be arranged between the hydraulic cylinder 6 and an additional telescopic guide tube 7.
Preferably, the mold box 10 is provided with at least one water valve 13 at the bottom, and in fig. 1, the mold box 10 is provided with two water valves 13 at the bottom, and the two water valves 13 are respectively arranged at two sides of the bottom of the mold box 10 wall, so that double drainage (i.e. simultaneous drainage from the top and bottom of the mold box 10) is performed in the consolidation stage, and the consolidation process is accelerated.
In order to improve the accuracy of the horizontal movement of the lower suspension analog tube 11 and approach the horizontal direction as much as possible, it is preferable that two linear guide rails 9.2 are arranged on the rigid frame 9.3 in parallel along the horizontal direction, a slide block 8 capable of moving horizontally along the linear guide rails 9.2 is arranged at the bottom of the slide table 9.1, a through hole is arranged between the two linear guide rails 9.2 along the horizontal direction, and the hydraulic cylinder 6 and the two telescopic additional guide tubes 7 penetrate through the through hole and can move horizontally along the through hole. When the chain and sprocket assembly 15 drives the driving system 1 to move horizontally, the two linear guide rails 9.2 can be further limited.
Correspondingly, the method for simulating the axial pipe-soil interaction under the offshore deep water condition adopts any one of the above miniature centrifuge models for simulating the axial pipe-soil interaction under the offshore deep water condition to simulate, and the specific steps comprise:
step 1, performing test preparation work, preparing simulated samples as required, wherein a rectangular rigid model box with the length of 750mm and the width of 388mm can be adopted as the model box 10, in the test process, in order to reduce the friction of the container wall, lubricating grease is coated on the inner wall of the model box 10, a sand layer with a set thickness, for example, a sand layer with the thickness of about 20mm, is placed at the bottom of the model box 10, a geotextile is covered on the sand layer to serve as a drainage layer to promote consolidation, degassed water is added into the model box 10 to promote the clay slurry to be placed under water, and air in the slurry is reduced to the maximum extent.
Step 2, putting the permeable stone of the second pore pressure sensor 14 into the degassed water, boiling for 10-15 minutes to saturate the permeable stone, then attaching the permeable stone on the second pore pressure sensor 14 in the degassed water, and keeping the permeable stone under the water for a set time (about 1 hour) to ensure complete saturation; when the level of the clay slurry in step 1 reaches a specified level, such as 50mm, a saturated second pore pressure sensor 14 is placed in the model soil, and the second pore pressure sensor 14 is ballasted with a small piece of polystyrene foam to prevent it from settling to the bottom of the slurry during consolidation.
Preferably, in all centrifugal tests, the maximum sedimentation amount of the second pore pressure sensor 14 is not more than 10mm, the distance between the final position and the soil surface layer of the model is 2.5D-3D, and D is the diameter of the lower suspension simulation pipe 11.
Corresponding to the size of the model box 10, the lower suspension simulation pipe 11 can be a stainless steel molded pipe element with the diameter of 35mm and the length of 400mm; wherein the length of the test pipe segment 11.6 is 250mm, and the lengths of the first virtual pipe segment 11.1 and the second virtual pipe segment 11.9 are both 75mm; four sensors (two first pore pressure sensors 11.4 and two total pressure sensors 11.5) were mounted on the test tube section 11.6 at 42mm intervals. The two total pressure sensors 11.5 can adopt a PDB-PB total pressure gauge (a microminiature pressure sensor with the measuring range of 50kPa-3MPa, japan TML original package). The two first pore pressure sensors 11.4 may be KPE-PB pore pressure gauges measuring 200kPa.
The measuring range of the first linear displacement sensor 2 and the second linear displacement sensor 3 can be 200mm, and the first force sensor 5, the second force sensor 11.3, the third force sensor 11.7 and the fourth force sensor 12 can be LCM200 micro force sensors with the measuring range of 2224kN. Two first force sensors 5 are used for monitoring horizontal load applied on the lower suspension simulation tube 11, and the other two second force sensors 11.3 and the third force sensor 11.7 are respectively arranged on the lower suspension simulation tube 11, wherein the second force sensors 11.3 are used for testing resistance R caused by soil in front of the first virtual section 11.1 f And a pipe-soil interface resistance R acting on the first virtual segment 11.1 1 (ii) a The third force sensor 11.7 tests the pipe-soil interface resistance R acting on the second virtual section 11.9 2 。
Step 3, after the slurry is placed, preloading is carried out on a soil sample under the condition of 1-g, the preloading pressure is 1.1kPa, after 7 days of preloading under the condition of 1-g, the model soil is transferred into the model box 10, a water valve 13 is opened, further consolidation is carried out under the condition of 10-g, the final thickness of the model soil after consolidation is 150-200mm, the bottom of the side wall of the model box 10 is provided with the water valve 13, so that double drainage (namely drainage from the top and the bottom surface) can be carried out in the consolidation stage, and in the specific test stage, the water valve 13 needs to be closed to simulate the actual site conditions.
And 4, under a displacement control mode, embedding the lower suspension simulation pipe 11 into model soil in the model box 10 at a specified speed by the hydraulic cylinder 6 until the required embedding depth is reached, wherein the embedding depth is half of the pipe diameter D.
Preferably, in step 4, the fourth force sensor 12 and the first linear displacement sensor 2 feed back the measured load and displacement to the servo motor 4, and if the load and the vertical displacement of the lower suspension analog pipe 11 do not reach a set value, the servo motor 4 corrects the load or the motion of the hydraulic cylinder 6 according to the error value.
Step 5, after the embedding stage is finished, setting the vertical load of the hydraulic cylinder 6 as the penetration resistance recorded at the final embedding depth of the lower suspension simulation pipe 11; to completely dissipate the excess pore pressure generated during the burying process (dissipation is indicated by pore pressure sensor readings), a dissipation time of about 0.5h is specified.
Step 6, after dissipation of the void, allows the excess void pressure generated during burying to dissipate completely (dissipation is indicated by the void pressure sensor readings). And starting the servo motor 4, driving a chain and chain wheel assembly 15 connected to the sliding table 9.1 by the servo motor 4, controlling the lower suspension simulation tube 11 to move axially, and simultaneously carrying out sensor data acquisition in the axial sliding process of the lower suspension simulation tube 11.
Step 7, after the test is completed, closing the micro centrifuge model (including closing the servo motor 4 and the sensors), ending the test, and performing subsequent data processing, as shown in fig. 4, wherein a calculation formula of the pipe-soil interface resistance F on the test pipe section 11.6 is as follows:
F＝T-R f -R 1 -R 2
wherein T is the total force acting on the lower suspension simulating tube 11, measured by the first force sensor 5; r f The resistance caused by the soil body in front of the first virtual section is measured by a second force sensor 11.3; r 1 The pipe-soil interface resistance of the first virtual segment, measured by the second force sensor 11.3; r is 2 The pipe-soil interface resistance of the second virtual segment is measured by the third force sensor 11.7.
Eliminating end effects during axial sliding by using a first virtual pipe section and a second virtual pipe section; by placing additional guide tubes that are telescopic on both sides of the hydraulic cylinder, the pipe sway during the test is minimized. The method has the advantages of accurate test result, high resolution, capability of eliminating the end effect during the axial sliding of the pipeline, simple equipment use, full automation, no need of manual intervention during the test, and great theoretical significance and practical value for verifying the safety and reliability of geotechnical engineering design under offshore deep water condition.
The above description is only a preferred embodiment of the present invention, and not intended to limit the scope of the present invention, and all modifications of equivalent structures and equivalent processes, which are made by using the contents of the present specification and the accompanying drawings, or directly or indirectly applied to other related technical fields, are included in the scope of the present invention.

Claims (3)

1. The method for simulating the axial pipe-soil interaction under the offshore deep water condition is characterized in that a micro centrifuge model for simulating the axial pipe-soil interaction under the offshore deep water condition is adopted for simulation:
a micro centrifuge model for simulating axial pipe-soil interaction under offshore deep water conditions comprises a model box (10) capable of containing model soil and a driving system (1) arranged above the model box (10), wherein a rigid frame (9.3) for supporting the driving system (1) is arranged above the model box (10):
the driving system (1) comprises a servo motor (4), a hydraulic cylinder (6) and a chain and chain wheel assembly (15), a sliding table (9.1) is arranged between the bottom of the driving system (1) and a rigid frame (9.3), the bottom of the hydraulic cylinder (6) penetrates through the sliding table (9.1) and the rigid frame (9.3) and extends into a model box (10), a fourth force sensor (12) and a lower suspension simulation pipe (11) are sequentially connected to the bottom of the hydraulic cylinder (6), a pulse signal generated by a centrifuge control chamber (16) controls the servo motor (4), the servo motor (4) drives the sliding table (9.1) to horizontally move through the chain and chain wheel assembly (15), the sliding table (9.1) is provided with a second linear displacement sensor (3) along the horizontal direction, a first linear displacement sensor (2) is vertically arranged on the hydraulic cylinder (6), and two first force sensors (5) used for monitoring horizontal load applied to a lower suspension simulation pipe (11) are symmetrically arranged on the sliding table (9.1);
the lower suspension simulation pipe (11) comprises a test pipe section (11.6), a first virtual pipe section (11.1) is arranged at one end of the test pipe section (11.6), a second virtual pipe section (11.9) is arranged at the other end of the test pipe section (11.6), two first pore pressure sensors (11.4) are symmetrically arranged on the test pipe section (11.6), two total pressure sensors (11.5) are symmetrically arranged between the two first pore pressure sensors (11.4), a second force sensor (11.3) is arranged at the end part, close to the test pipe section (11.6), of the first virtual pipe section (11.1), and a third force sensor (11.7) is arranged at the end part, close to the test pipe section (11.6), of the second virtual pipe section (11.9);
a second pore pressure sensor (14) is arranged in the model soil;
the method comprises the following specific steps:
step 1, coating lubricating grease on the inner wall of a mold box (10), placing a sand layer with a set thickness at the bottom of the mold box (10), covering a layer of geotextile on the sand layer, and adding degassed water into the mold box (10);
step 2, putting the permeable stone of the second pore pressure sensor (14) into the degassed water, boiling for 10-15 minutes to saturate, then attaching the permeable stone to the second pore pressure sensor (14) in the degassed water, and keeping the permeable stone under the water for a set time to ensure complete saturation; when the liquid level of the clay slurry in the step 1 reaches a specified liquid level, placing a saturated second pore pressure sensor (14) into the model soil, wherein the second pore pressure sensor (14) is ballasted by polystyrene foam;
step 3, after the slurry is placed, preloading the soil sample under the condition of 1-g, wherein the preloading pressure is 1.1kPa, transferring the model soil into a model box (10) after pre-pressing for 7 days under the condition of 1-g, opening a water valve (13), and further solidifying under the condition of 10-g, wherein the final thickness of the solidified model soil is 150-200mm;
step 4, under a displacement control mode, the hydraulic cylinder (6) embeds the lower suspension simulation pipe (11) into model soil in the model box (10) at a specified speed until reaching a required embedding depth;
step 5, after the burying stage is finished, starting a load control mode, and setting the vertical load of the hydraulic cylinder (6) as the injection resistance recorded at the final embedding depth of the lower suspension simulation pipe (11);
step 6, after dissipation is finished, starting the servo motor (4), driving a chain and sprocket assembly (15) connected to the sliding table (9.1) by the servo motor (4), controlling the lower suspension simulation tube (11) to axially move, and simultaneously carrying out sensor data acquisition in the axial sliding process of the lower suspension simulation tube (11);
and 7, after the test is finished, closing the micro centrifuge model, finishing the test and carrying out subsequent data processing, wherein the calculation formula of the pipe-soil interface resistance F on the test pipe section (11.6) is as follows:
F＝T-R f -R 1 -R 2
wherein T is the total acting force acting on the lower suspension simulation tube (11) and is measured by the first force sensor (5); r is f The resistance caused by the soil body in front of the first virtual section is measured by a second force sensor (11.3); r 1 Is the pipe-soil interface resistance of the first virtual section, measured by a second force sensor (11.3); r is 2 The pipe-soil interface resistance of the second virtual segment is measured by a third force sensor (11.7).
2. The method for simulating axial pipe-soil interaction in offshore deep water conditions according to claim 1, wherein the maximum settlement of the second pore pressure sensor (14) is no more than 10mm in all centrifugal tests, the distance from the final position to the surface of the model soil is 2.5D-3D, and D is the diameter of the underslung simulation pipe (11); in step 4, the embedding depth is half of the pipe diameter D.
3. Method for simulating axial tube-soil interaction in offshore deep water conditions according to claim 1, characterized in that in step 4, the fourth force sensor (12) and the first linear displacement sensor (2) feed back the measured load and displacement to the servo motor (4), and if the load and vertical displacement of the underslung simulation tube (11) do not reach the set values, the servo motor (4) corrects the load or motion of the hydraulic cylinder (6) according to the error value.
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