Method for determining critical gas operating pressure for thermoplastic liner collapse

Abstract

The present invention discloses a method for determining the critical gas operating pressure at which a thermoplastic liner collapses. The method comprises obtaining the volume of the interlayer between the steel pipe and the inner liner of a steel pipe, the operating temperature of the steel pipe, the operating time of the steel pipe, and the gas composition of the steel pipe; testing the gas permeability coefficient of the steel pipe inner liner based on the operating temperature and the gas composition; inputting the interlayer volume, operating temperature, operating time, and the gas permeability coefficient of the inner liner into a pre-established interlayer permeability pressure model between the steel pipe and the inner liner to obtain the interlayer permeability pressure between the steel pipe and the inner liner; determining the radial collapse pressure of the steel pipe inner liner; and inputting the interlayer permeability pressure and radial collapse pressure into a pre-established critical collapse failure criterion model to obtain the critical gas operating pressure at which the thermoplastic liner of the steel pipe collapses. The present invention can determine the critical gas operating pressure of a thermoplastic-lined steel pipeline.

Description

Technical Field

[0001] The invention belongs to the technical field of thermoplastic lined composite pipeline performance testing, and in particular relates to a method for determining a critical gas operating pressure for collapse of a thermoplastic liner. Background Art

[0002] Corrosion of carbon steel pipelines used in oilfield surface gathering and transportation systems is becoming increasingly severe, with frequent corrosion failures causing significant economic losses and environmental pollution. Using a trenchless method, thermoplastic pipes (such as high-density polyethylene (HDPE), high-temperature polyolefin (HTPO), and nylon (PA)) are inserted into corroded steel pipelines, creating a "pipe-in-pipe" thermoplastic-lined steel pipe structure. This structure not only isolates the steel pipe from corrosion from the transport medium but also repairs some corrosion defects in the steel pipeline, significantly extending its service life. This corrosion protection technology has become widely adopted in oilfields both domestically and internationally. During post-repair service, gases dissolved in the petroleum medium, such as CH₄, H₂S, and CO₂, can penetrate the inner surface of the thermoplastic pipe through adsorption, diffusion, and other permeation phenomena. These gases, which penetrate into the interlayer between the thermoplastic liner and the steel pipe, gradually accumulate. During pipeline outages, internal pressure fluctuations, or sudden negative pressure events, the external pressure generated by the accumulated gases in the interlayer can cause the thermoplastic pipe to collapse and fail. Harsh oil and gas gathering and transportation conditions, such as high temperature and high pressure environments, will accelerate the gas permeation process, increase the total amount of gas permeation in the interlayer, and lead to a greater risk of radial collapse failure of thermoplastic plastic pipes.

[0003] When the transport gas composition, operating temperature, thermoplastic material and specifications have been determined, the majority of oilfield users are generally concerned about how to determine the critical gas operating pressure of thermoplastic-lined steel pipelines, so as to guide the optimization of pipeline operating pressure parameters and control the total amount of gas penetration into the interlayer, thereby avoiding the collapse and failure of thermoplastic-lined pipes from the source. Summary of the Invention

[0004] In response to the problems existing in the prior art, the present invention provides a method for determining the critical gas operating pressure of thermoplastic plastic lining collapse, which can determine the critical gas operating pressure of thermoplastic plastic lined steel pipelines, thereby guiding the optimization of pipeline operating pressure parameters, controlling the total amount of gas penetration into the interlayer, and thus avoiding collapse and failure of thermoplastic plastic lined pipes from the source.

[0005] In order to solve the above technical problems, the present invention is implemented through the following technical solutions:

[0006] A method for determining a critical gas operating pressure for collapse of a thermoplastic liner, comprising:

[0007] Obtaining the volume of the interlayer between the steel pipe and the inner lining layer of the steel pipe, the operating temperature of the steel pipe, the operating time of the steel pipe, and the conveying gas composition of the steel pipe;

[0008] Testing the gas permeability coefficient of the steel pipe lining according to the operating temperature and the conveying gas composition;

[0009] Inputting the interlayer volume, the operating temperature, the operating time, and the gas permeability coefficient of the lining layer into a pre-established interlayer permeability pressure model between the steel pipe and the lining layer to obtain the interlayer permeability pressure between the steel pipe and the lining layer;

[0010] determining a radial collapse pressure of an inner liner of the steel pipe;

[0011] The interlayer penetration pressure and the radial collapse pressure are input into a pre-constructed critical collapse failure criterion model to obtain a critical gas operating pressure for collapse of the thermoplastic plastic lining of the steel pipe.

[0012] Furthermore, the interlayer osmotic pressure model is specifically as follows:

[0013]

[0014] Where, P a is the interlayer penetration pressure; P is the critical gas operating pressure; R is the molar gas constant; T is the pipeline operating temperature; V is the interlayer volume; C is the gas permeability coefficient of the lining layer; D is the diameter of the thermoplastic plastic pipe; l is the wall thickness of the lining layer; t is the operating time.

[0015] Furthermore, the critical collapse failure criterion model is specifically:

[0016] P a =P+P c

[0017] Where, P c is the radial collapse pressure of the thermoplastic liner of the steel pipe.

[0018] Furthermore, the gas permeability coefficient of the steel pipe lining is tested according to the operating temperature and the conveying gas composition, specifically including:

[0019] preparing a thermoplastic film sample from the same raw material as the thermoplastic lining layer of the steel pipe;

[0020] The gas permeability test temperature is set to the operating temperature, and a gas permeometer is used to test the gas permeability coefficient of the transported gas component in the thermoplastic plastic film sample, which is the gas permeability coefficient of the steel pipe lining layer.

[0021] Furthermore, the preparation process of the thermoplastic plastic film sample is the same as the preparation process of the thermoplastic plastic lining layer of the steel pipe.

[0022] Furthermore, determining the radial collapse pressure of the steel pipe lining layer specifically includes:

[0023] Conduct environmental simulation tests on thermoplastic lined steel pipe test samples;

[0024] Conduct radial pressure tests on the inner lining of the thermoplastic lined steel pipe test sample, and record the changes in the interlayer pressure between the steel pipe and the inner lining of the thermoplastic lined steel pipe test sample;

[0025] According to the change of the interlayer pressure between the steel pipe and the inner lining layer of the thermoplastic lined steel pipe test sample, a curve of the change of the interlayer pressure between the steel pipe and the inner lining layer of the thermoplastic lined steel pipe test sample is drawn;

[0026] The radial collapse pressure of the steel pipe inner lining is obtained according to the interlayer pressure variation curve.

[0027] Furthermore, when the radial pressure test is performed on the inner lining layer of the thermoplastic lined steel pipe test sample, the pressure rate is controlled at 0.1 MPa / s to 0.5 MPa / s.

[0028] Furthermore, the calculation formula for the volume of the interlayer between the steel pipe and the inner lining layer of the steel pipe is:

[0029] Interlayer volume = inner lining surface area × interlayer gap thickness

[0030] The thickness of the interlayer gap is 0.05mm to 0.50mm.

[0031] Furthermore, when there is fluctuation or negative pressure in the operating pressure of the steel pipe, the radial collapse pressure should be corrected with a correction factor of 0.5 to 0.8.

[0032] Compared with the prior art, the present invention has at least the following beneficial effects: the present invention provides a method for determining the critical gas operating pressure for collapse of a thermoplastic plastic liner, which tests the gas permeability coefficient of the steel pipe lining according to the operating temperature and the conveying gas composition; inputs the interlayer volume, operating temperature, operating time and the gas permeability coefficient of the lining into a pre-constructed interlayer penetration pressure model between the steel pipe and the lining to obtain the interlayer penetration pressure between the steel pipe and the lining; determines the radial collapse pressure of the steel pipe lining; inputs the interlayer penetration pressure and the radial collapse pressure into a pre-constructed critical collapse failure criterion model to obtain the critical gas operating pressure for collapse of the thermoplastic plastic lining of the steel pipe. On the one hand, the present invention takes into account the gas permeation problem that causes the collapse of the thermoplastic liner pipe, and on the other hand, it also takes into account the anti-collapse performance of the thermoplastic liner pipe itself, and uses the interlayer permeation pressure model between the steel pipe and the liner to obtain the interlayer permeation pressure between the steel pipe and the liner, and finally uses the critical collapse failure criterion model to obtain the critical gas operating pressure of the thermoplastic liner of the steel pipe for collapse. The calculation results are intuitive and reliable, and have strong guidance. The method of the present invention is used to calculate and determine whether there is a risk of collapse and failure of the thermoplastic liner pipe after it has been running for a period of time. By testing the gas permeability coefficients of different transporting gas components in thermoplastics, the present invention can infer the critical operating partial pressures of different gases that cause the collapse of the liner pipe under actual operating conditions.

[0033] The present invention has a reasonable design, intuitive testing, and easy operation. It can also simulate the service conditions of full-size composite pipe products in oil and gas transportation working conditions to the greatest extent. The temperature, pressure, gas components, and liquid medium can all be adjusted, and the test sample can be rotated. The test data obtained from the test is accurate and reliable.

[0034] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, preferred embodiments are given below and described in detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In order to more clearly illustrate the technical solutions in the specific embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the specific embodiments. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0036] Figure 1 It is a gas permeation model for thermoplastic-lined steel pipe interlayer;

[0037] Figure 2 Schematic diagram of the collapse pressure testing system for thermoplastic lined steel pipes;

[0038] Figure 3It is the radial collapse pressure test curve of thermoplastic lined pipe.

[0039] In the figure: 1-transfer joint; 2-pressure gauge; 3-thermoplastic plastic lined pipe; 4-sealing plug; 5-flange; 6-fastening screw; 7-steel base pipe; 8-water inlet; 9-pressure hose; 10-pressure pump; 11-nut; 12-inlet valve; 13-pressure relief valve; 14-pressure sensor connection port; 15-data acquisition system; 16-pressure sensor; 17-pressure pipe; 18-gas cylinder; 19-transmission belt; 20-motor; 21-environmental test chamber; 22-support base; 23-exhaust gas treatment system; 24-inner lining; 25-steel pipe. DETAILED DESCRIPTION

[0040] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of them. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0041] As a specific embodiment of the present invention, a method for determining the critical gas operating pressure for collapse of a thermoplastic liner comprises the following steps:

[0042] Step 1: Obtain the interlayer volume V between the steel pipe and the inner liner of the steel pipe, the operating temperature T of the steel pipe, the operating time t of the steel pipe, and the conveying gas components of the steel pipe; generally, the conveying gas components include CH4, CO2, O2, N2 and H2S.

[0043] Specifically, the calculation formula for the volume of the interlayer between the steel pipe and the inner liner of the steel pipe is:

[0044] Interlayer volume = inner lining surface area × interlayer gap thickness

[0045] The thickness of the interlayer gap is 0.05mm to 0.50mm.

[0046] Step 2: Test the gas permeability coefficient C of the steel pipe lining layer according to the operating temperature T and the conveying gas composition. The gas permeability coefficient is the volume of gas that passes through the unit thickness and unit area of ​​the sample per unit time at a constant temperature and unit pressure difference when the permeation is stable.

[0047] As a preferred embodiment, according to the operating temperature T and the conveying gas composition, testing the gas permeability coefficient C of the steel pipe lining layer specifically includes the following steps:

[0048] Step a: preparing a thermoplastic film sample from the same raw material as the thermoplastic lining layer of the steel pipe;

[0049] Among them, the preparation process of the thermoplastic plastic film sample is the same as the preparation process of the thermoplastic plastic lining layer of the steel pipe.

[0050] Step b: Set the gas permeation test temperature to the operating temperature T, refer to GB / T 1038, and use a gas permeometer to test the gas permeability coefficient of the transported gas components in the thermoplastic plastic film sample, which is the gas permeability coefficient C of the steel pipe lining layer.

[0051] Step 3: Input the interlayer volume V, operating temperature T, operating time t and gas permeability coefficient C of the liner into the pre-built interlayer permeability pressure model between the steel pipe and the liner to obtain the interlayer permeability pressure P between the steel pipe and the liner a ;

[0052] Interlayer osmotic pressure P a The calculation formula is f(T, t, C, P, V). Specifically, the interlayer penetration pressure model is:

[0053]

[0054] In the formula, where P a is the interlayer penetration pressure; P is the critical gas operating pressure; R is the molar gas constant; T is the pipeline operating temperature; V is the interlayer volume; C is the gas permeability coefficient of the lining layer; D is the diameter of the thermoplastic plastic pipe; l is the wall thickness of the lining layer; t is the operating time.

[0055] Step 4: Determine the radial collapse pressure P of the steel pipe lining c .

[0056] As a preferred embodiment, the radial collapse pressure P of the steel pipe liner is determined. c , specifically including:

[0057] Conduct environmental simulation tests on thermoplastic lined steel pipe test samples;

[0058] Perform a radial pressure test on the inner lining of the thermoplastic lined steel pipe test sample, and record the change in the interlayer pressure between the steel pipe and the inner lining of the thermoplastic lined steel pipe test sample; preferably, when performing the radial pressure test on the inner lining of the thermoplastic lined steel pipe test sample, the pressure rate is controlled at 0.1 MPa / s to 0.5 MPa / s;

[0059] According to the change of the interlayer pressure between the steel pipe and the inner lining layer of the thermoplastic lined steel pipe test sample, a curve of the change of the interlayer pressure between the steel pipe and the inner lining layer of the thermoplastic lined steel pipe test sample is drawn;

[0060] The radial collapse pressure of the steel pipe lining layer is obtained according to the interlayer pressure change curve, such as Figure 3 As shown in the figure, the pressure corresponding to the highest peak of the interlayer pressure change curve is the radial collapse pressure P of the steel pipe lining. c .

[0061] More specifically, Figure 2 As shown, in this embodiment, the thermoplastic-lined steel pipe test sample is composed of a thermoplastic-lined pipe 3 inserted into a steel base pipe 7, with the gap between the thermoplastic-lined pipe 3 and the steel base pipe 7 being the interlayer. Preferably, the length of the thermoplastic-lined steel pipe test sample is at least five times greater than the outer diameter, and the end of the thermoplastic-lined pipe 3 is 15 mm to 30 mm longer than the steel base pipe 7.

[0062] like Figure 2 As shown, in this embodiment, a method for determining the critical gas operating pressure P of the thermoplastic lining of a steel pipe for collapse is provided. c The test system includes test samples, end sealing system, interlayer pressure system, interlayer pressure monitoring system and oil and gas working environment simulation test system.

[0063] In the test system, both ends of the thermoplastic-lined steel pipe test specimen are inserted into the thermoplastic-lined pipe 3 via sealing plugs 4, with sealing rings used to seal the ends. A transfer joint 1 is threadedly connected to the sealing plugs 4. Transfer joint 1 is equipped with a pressure gauge 2, an air inlet valve 12, and a pressure relief valve 13 to control the internal pressure of the test specimen during environmental simulation tests. Transfer joint 1 also has internal through-holes for connecting to external gas cylinders, vacuum pumps, and pressure circulation pumps.

[0064] In the testing system, the end faces of the steel base pipe 7 are sealed with flanges 5 and sealing rings. The flanges 5 at both ends of the thermoplastic-lined steel pipe test specimen are connected by fastening screws 6 and secured with nuts 11. The steel base pipe 7 is provided with a water inlet 8 and a pressure sensor connection port 14. A pressure pump 10 is connected to the water inlet 8 via a pressure hose 9, enabling pressure testing of the test specimen's interlayer. A data acquisition system 15 is connected to the pressure sensor connection port 14 via a pressure sensor 16 to monitor interlayer pressure changes in real time and collect pressure curves.

[0065] During use, after one end of the thermoplastic lined steel pipe test sample is sealed with a sealing plug 4, the liquid medium for simulating oil field transportation (such as oil, water, oil field on-site liquid, simulated liquid configured in the laboratory) is placed into the test sample, and the other end of the test sample is sealed with a sealing plug 4. The transfer joint 1 connected to the sealing plug 4 is connected to the pressure pipe 17 and the gas cylinder 18 through an internal through hole. Close the pressure relief valve 13 and open the air inlet valve 12 to pressurize the interior of the test sample. After the test sample is pressurized to the set value, close the air inlet valve 12, place the entire test sample filled with liquid medium and maintaining a certain internal pressure on the support seat 22, and push it into the environmental test box 21. The environmental test box 21 is heated by air to achieve the functions of heating, heat preservation and air circulation constant temperature. The transfer joint 1 connected to the sealing plug 4 can be connected to the motor 20 through the transmission belt 19. Starting the motor 20 to drive the test sample to rotate can ensure that the entire inner wall of the thermoplastic lined pipe is in contact with the liquid medium, and realize dynamic simulation test. Preferably, the motor speed is 10 to 50 r / min, which can ensure that the entire inner wall of the thermoplastic lined pipe is in contact with the liquid medium, and realize dynamic simulation test. The test cycle should be 2 days, 7 days or 14 days. After setting the corresponding test parameters (such as pressure, temperature, time and speed), the oil and gas working environment simulation test of the thermoplastic lined steel pipe can be completed. Then, connect Figure 2 The interlayer pressure testing system and interlayer pressure monitoring system shown can be used to perform radial collapse pressure testing of thermoplastic lined pipes. The gas after the test is safely processed through the exhaust gas treatment system 23.

[0066] In summary, the critical gas operating pressure P for collapse of the thermoplastic lining of the steel pipe in this embodiment is c The specific test steps are as follows:

[0067] 1) Insert the intended thermoplastic pipe into the steel base pipe according to the traditional internal insertion process to prepare the thermoplastic lined steel pipe test sample, or directly cut the thermoplastic lined steel pipe used in the oil field as the test sample;

[0068] 2) Design the test sample end sealing system based on the test sample specifications (outer diameter, inner diameter, wall thickness), and set up an interlayer pressure system and an interlayer pressure monitoring system in the middle part of the steel base pipe layer of the test sample;

[0069] 3) After placing the oil-water medium into the test sample, seal it with the end sealing device and place it in the environmental test chamber to establish the field application conditions of the thermoplastic lined steel pipe (heating to a certain temperature, dynamic operation) and carry out environmental simulation tests;

[0070] 4) After the environmental simulation test is completed, the test sample is connected to the interlayer pressure system and the interlayer pressure monitoring system to carry out the interlayer pressure test, record the interlayer pressure change, analyze the interlayer pressure change curve, and determine the radial collapse pressure P of the thermoplastic lined pipe under the simulated working environment. c .

[0071] Preferably, when there is fluctuation or negative pressure in the operating pressure of the steel pipe, the radial collapse pressure should be corrected with a correction factor of 0.5 to 0.8.

[0072] Step 5: Input the interlayer penetration pressure and radial collapse pressure into the pre-built critical collapse failure criterion model to obtain the critical gas operating pressure for the collapse of the thermoplastic lining of the steel pipe;

[0073] Combine Figure 1 As shown, when P a >P+P c When , the lining collapses and fails. Therefore, specifically, the critical collapse failure criterion model is obtained as follows:

[0074] P a =P+P c

[0075] Where, P c is the radial collapse pressure of the thermoplastic liner of the steel pipe.

[0076] In order to explain the interlayer penetration pressure model between the steel pipe and the inner liner in more detail, the following is a detailed description:

[0077] The formula governing gas permeation can be derived from Fick's first law:

[0078]

[0079] Where: Q is the gas permeability; A is the surface area; P is the critical gas operating pressure; l is the wall thickness of the thermoplastic lined pipe; C is the gas permeability coefficient; t is time.

[0080] like Figure 1 As shown, since the steel pipe is a solid wall structure, the gas pressure P in the interlayer a It is determined by the gas permeation dQ through the thermoplastic liner, and the relationship between them can be expressed as:

[0081]

[0082] Where: Q is the gas permeation; P is the critical gas operating pressure; P ais the interlayer penetration pressure; l is the wall thickness of the lining layer; C is the gas permeability coefficient of the lining layer; D is the diameter of the thermoplastic plastic pipe; R is the molar gas constant; T is the pipeline operating temperature; V is the interlayer volume.

[0083]

[0084] From formula (3), we can get:

[0085]

[0086] set up:

[0087]

[0088]

[0089] Then formula (4) is:

[0090]

[0091] When t=0, P a =0, so E=C, substituting the above parameters into the equation:

[0092]

[0093] Example 1

[0094] Determine the critical gas operating pressure at 60°C for the lining collapse of a DN100×3.5mm nylon-lined steel pipe transporting CH4 gas as follows:

[0095] (1) Gas Permeability Test: Prepare nylon film (sheet) samples with a thickness of 0.1 to 0.5 mm using the same raw materials as the nylon lining. Set the test temperature to 60°C and refer to GB / T 1038. Test the permeability coefficient C of CH4 gas in the nylon film (sheet) sample using a gas permeometer.

[0096] (2) Establish the calculation formula of interlayer permeation pressure: As shown in formula (5), the parameters l (pipe wall thickness 3.5 mm), C (gas permeability coefficient, measured), D (lining pipe diameter 100 mm), R (molar gas constant, known), T (pipeline operating temperature 333 K), and V (interlayer volume = lining pipe surface area × interlayer gap) can be tested or calculated. Therefore, the interlayer permeation pressure (P a ) and pipeline operating pressure (P) can be derived from formula (5): That is, after the diameter and wall thickness of the nylon lined pipe, operating temperature, and conveying gas composition are determined, the interlayer penetration pressure (P a ) is a function related to the pipeline operating pressure (P) and operating time (t).

[0097] (3) Nylon lined pipe radial collapse pressure test: A nylon lined steel pipe test sample with a length of 1200mm was cut on site. The steel base pipes at both ends were machined to cut off about 15 to 30mm at each end. A water inlet was opened in the middle of the steel base pipe layer of the nylon lined steel pipe test sample. A pressure sensor connection port was opened perpendicular to the water inlet. According to the specifications of DN100mm nylon lined pipe, a sealing plug was designed and installed. The intermediate joint equipped with the air inlet valve, pressure relief valve and pressure gauge was connected to the sealing plug, and then the flange was inserted. The flanges at both ends of the test sample were fastened with a tightening screw. The entire test sample was placed on the support seat and pushed into the environmental test chamber. The environmental test chamber was controlled to heat up to 60℃ and the environmental simulation test timer was started. After the set test time (e.g. 168h) was reached, the environmental simulation test was stopped. The test sample was placed in the environmental test chamber and kept at a constant temperature of 60℃. The interlayer pressure system and the interlayer pressure monitoring system were connected. Turn on the pressure pump and pressurize the interlayer at a rate of 0.2 MPa / s, and record the change in interlayer pressure. Figure 3 When the pressure changes as shown, stop pressing. The inflection point of the pressure change curve is the radial collapse pressure P of DN100mm×3.5mm nylon lined pipe at 60℃. c .

[0098] (4) Calculation of critical gas operating pressure for nylon lined pipe collapse: When the interlayer pressure (P a )>Pipeline operating pressure (P)+Nylon lining pipe anti-collapse pressure (P c ), the liner collapses and fails. The critical gas operating pressure P that causes the collapse of the nylon lined pipe at 60°C is obtained.

[0099] Example 2:

[0100] The critical gas operating pressure at which the inner lining of a DN80×2.5mm polyethylene-lined composite steel pipe for transporting CO2 gas and oil-water media collapses at 50°C is as follows:

[0101] (1) Gas Permeability Test: Prepare polyethylene film (sheet) samples with a thickness of 0.1 to 0.5 mm using the same raw materials as the polyethylene liner. Set the test temperature to 50°C and refer to GB / T 1038. Test the CO2 gas permeability coefficient C through the polyethylene film (sheet) sample using a gas permeometer.

[0102] (2) Establish the calculation formula of interlayer permeation pressure: As shown in formula (5), the parameters l (pipe wall thickness 2.5 mm), C (gas permeability coefficient, measured), D (lining pipe diameter 80 mm), R (molar gas constant, known), T (pipeline operating temperature 323 K), and V (interlayer volume = lining pipe surface area × interlayer gap) can be tested or calculated. Therefore, the interlayer permeation pressure (P a ) and pipeline operating pressure (P) can be derived from formula (5): That is, after the diameter and wall thickness of the nylon lined pipe, operating temperature, and conveying gas composition are determined, the interlayer penetration pressure (P a ) is a function related to the pipeline operating pressure (P) and operating time (t).

[0103] (3) Radial collapse pressure test of polyethylene lined pipe: A polyethylene lined composite steel pipe test sample with a length of 1200mm was cut on site. The steel base pipes at both ends were machined and cut off by about 15 to 30mm. A water inlet was opened in the middle of the steel base pipe layer of the polyethylene lined composite steel pipe test sample. A pressure sensor connection port was opened perpendicular to the water inlet. According to the specifications of the DN80mm polyethylene lined pipe, a sealing plug was designed and installed. The transfer joint equipped with the air inlet valve, pressure relief valve and pressure gauge was connected to the sealing plug. After the connected sealing plug was installed with the inner sealing ring, it was inserted into one end of the polyethylene lined pipe. The prepared oil-water solution was poured from the other end and sealed with the connected sealing plug. Then the flange was inserted and the flanges at both ends of the test sample were fastened with the tightening screws. The entire test sample filled with liquid medium was placed on the support seat and pushed into the environmental test chamber. The transmission belt of the motor was connected to the transfer joint. Start the motor to drive the test sample to rotate, and at the same time control the environmental test box to heat up to 50℃, and start the environmental simulation test timing. After reaching the set test time (such as 200h), stop the motor. Keep the test sample in the environmental test box and keep the temperature at 50℃, connect the interlayer pressure system and the interlayer pressure monitoring system. Turn on the pressure pump, pressurize the interlayer at a rate of 0.3MPa / s, record the interlayer pressure change, and when the interlayer pressure appears as follows Figure 3 When the pressure changes as shown, stop pressing. The inflection point of the pressure change curve is the radial collapse pressure P of the DN80mm×2.5mm polyethylene lined pipe at 50℃. c .

[0104] (4) Calculation of critical gas operating pressure for collapse of polyethylene liner pipe: When the interlayer pressure (P a )>Pipeline operating pressure (P)+PE liner anti-collapse pressure (P c ), the liner collapses and fails. The critical gas operating pressure P that causes the collapse of the nylon lined pipe at 50°C is obtained.

[0105] Example 3

[0106] The lining of a DN150×5.5mm polyethylene-lined composite steel pipe was tested and determined to be at risk of collapse failure after five years of transporting oil, water, and CH4 gas at room temperature and 4MPa operating pressure. The details are as follows:

[0107] (1) Gas Permeability Test: Prepare polyethylene film (sheet) samples with a thickness of 0.1 to 0.5 mm using the same raw materials as the polyethylene liner. Set the test temperature to room temperature and refer to GB / T 1038. Use a gas permeometer to test the permeability coefficient C of CH4 gas in the polyethylene film (sheet) sample.

[0108] (2) Establish the interlayer permeation pressure calculation formula: As shown in formula (5), the parameters l (pipe wall thickness 5.5 mm), C (gas permeability coefficient, measured), D (lining pipe diameter 150 mm), R (molar gas constant, known), T (pipeline operating temperature 273 K), and V (interlayer volume = lining pipe surface area × interlayer gap) can be tested or calculated. Therefore, the interlayer permeation pressure (P a ) and pipeline operating pressure (P) can be derived from formula (5): That is, after the diameter and wall thickness of the nylon lined pipe, operating temperature, and conveying gas composition are determined, the interlayer penetration pressure (P a ) is a function related to the pipeline operating pressure (P) and operating time (t).

[0109] (3) Polyethylene lined pipe radial collapse pressure test: A polyethylene lined composite steel pipe test sample with a length of 1500mm was cut on site. The steel base pipes at both ends were machined and cut off by about 15 to 35mm. A water inlet was opened in the middle of the steel base pipe layer of the polyethylene lined composite steel pipe test sample. A pressure sensor connection port was opened perpendicular to the water inlet. According to the specifications of the DN150mm polyethylene lined pipe, a sealing plug was designed and installed. The transfer joint equipped with the air inlet valve, pressure relief valve and pressure gauge was connected to the sealing plug. After the connected sealing plug was installed with the inner sealing ring, it was inserted into one end of the polyethylene lined pipe. The prepared oil-water solution was poured into the other end and sealed with the connected sealing plug. Then the flange was inserted and the flanges at both ends of the test sample were fastened with the tightening screws. The CH4 gas cylinder was connected to the transfer joint, the pressure relief valve was closed, the air inlet valve was opened, and the pressure was increased to 4MPa. After closing the air inlet valve, place the entire test sample filled with liquid medium and maintaining a certain internal pressure on the support seat, push it into the environmental test chamber, and connect the motor's conveyor belt to the transfer joint. Start the motor to drive the test sample to rotate, and at the same time control the temperature of the environmental test chamber to room temperature. After the set test time (such as 240h) is reached, stop the motor and open the pressure relief valve to release the internal gas pressure of the test sample. Keep the test sample in the environmental test chamber and keep the room temperature constant, connect the interlayer pressurization system and the interlayer pressure monitoring system. Turn on the pressure pump, pressurize the interlayer at a rate of 0.3MPa / s, record the change in interlayer pressure, and when the interlayer pressure appears as follows Figure 3 When the pressure changes as shown, stop pressing. The inflection point of the pressure change curve is the radial collapse pressure P of the DN150mm×5.5mm polyethylene lined pipe at room temperature. c .

[0110] (4) Prediction of collapse failure risk of polyethylene lined pipe: When calculating and determining the interlayer pressure (P a ) and tested the anti-collapse pressure of polyethylene lined pipe (P c ), if P a -P c > pipeline operating pressure (4MPa), it can be determined that after 5 years of transporting oil, water and CH4 gas at room temperature and 4MPa operating pressure, the DN150×5.5mm polyethylene lined pipe has the risk of collapse failure; on the contrary, if P a -P c ≤ pipeline operating pressure (4MPa), it can be determined that there is no risk of collapse failure.

[0111] The above content is only for explaining the technical idea of ​​the present invention and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the protection scope of the claims of the present invention.

[0112] Finally, it should be noted that the above-described embodiments are only specific implementation methods of the present invention, which are used to illustrate the technical solutions of the present invention, rather than to limit them. The scope of protection of the present invention is not limited thereto. Although the present invention has been described in detail with reference to the above-described embodiments, those skilled in the art should understand that any person skilled in the art can modify or easily conceive of changes to the technical solutions described in the above-described embodiments within the technical scope disclosed by the present invention, or replace some of the technical features therein with equivalents. Such modifications, changes, or replacements do not deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should be included in the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be subject to the scope of protection of the claims.

Claims

1. A method for determining the critical gas operating pressure for collapse of a thermoplastic liner, characterized in that: include: Obtaining the volume of the interlayer between the steel pipe and the inner lining layer of the steel pipe, the operating temperature of the steel pipe, the operating time of the steel pipe, and the conveying gas composition of the steel pipe; Testing the gas permeability coefficient of the steel pipe lining according to the operating temperature and the conveying gas composition; The interlayer volume, the operating temperature, the operating time, and the gas permeability coefficient of the lining layer are input into a pre-established interlayer permeability pressure model between the steel pipe and the lining layer to obtain the interlayer permeability pressure between the steel pipe and the lining layer; the interlayer permeability pressure model is specifically: Where, P a is the interlayer permeability pressure; P is the critical gas operating pressure; R is the molar gas constant; T is the pipeline operating temperature; V is the interlayer volume; C is the gas permeability coefficient of the lining layer; D is the diameter of the thermoplastic plastic pipe; l is the wall thickness of the lining layer; t is the operating time; determining a radial collapse pressure of an inner liner of the steel pipe; The interlayer penetration pressure and the radial collapse pressure are input into a pre-established critical collapse failure criterion model to obtain the critical gas operating pressure for collapse of the thermoplastic liner; the critical collapse failure criterion model is specifically: P a =P+P c Where, P c is the radial collapse pressure of the thermoplastic liner of the steel pipe.

2. The method for determining the critical gas operating pressure for collapse of a thermoplastic liner according to claim 1, characterized in that: The testing of the gas permeability coefficient of the steel pipe lining layer according to the operating temperature and the conveying gas composition specifically includes: preparing a thermoplastic film sample from the same raw material as the steel pipe inner lining; The gas permeability test temperature is set to the operating temperature, and a gas permeometer is used to test the gas permeability coefficient of the transported gas component in the thermoplastic plastic film sample, which is the gas permeability coefficient of the steel pipe lining layer.

3. The method for determining the critical gas operating pressure for collapse of a thermoplastic liner according to claim 2, characterized in that: The preparation process of the thermoplastic plastic film sample is the same as the preparation process of the thermoplastic plastic lining layer of the steel pipe.

4. The method for determining the critical gas operating pressure for collapse of a thermoplastic liner according to claim 1, characterized in that: Determining the radial collapse pressure of the steel pipe lining layer specifically includes: Conduct environmental simulation tests on thermoplastic lined steel pipe test samples; Conduct radial pressure tests on the inner lining of the thermoplastic lined steel pipe test sample, and record the changes in the interlayer pressure between the steel pipe and the inner lining of the thermoplastic lined steel pipe test sample; According to the change of the interlayer pressure between the steel pipe and the inner lining layer of the thermoplastic lined steel pipe test sample, a curve of the change of the interlayer pressure between the steel pipe and the inner lining layer of the thermoplastic lined steel pipe test sample is drawn; The radial collapse pressure of the steel pipe inner lining is obtained according to the interlayer pressure variation curve.

5. The method for determining the critical gas operating pressure for collapse of a thermoplastic liner according to claim 4, characterized in that: When the radial pressure test is performed on the inner lining layer of the thermoplastic lined steel pipe test sample, the pressure rate is controlled at 0.1 MPa / s to 0.5 MPa / s.

6. The method for determining the critical gas operating pressure for collapse of a thermoplastic liner according to claim 1, characterized in that: The calculation formula for the volume of the interlayer between the steel pipe and the inner lining layer of the steel pipe is: Interlayer volume = inner lining surface area × interlayer gap thickness The thickness of the interlayer gap is 0.05mm to 0.50mm.

7. The method for determining the critical gas operating pressure for collapse of a thermoplastic liner according to claim 1, characterized in that: When there is fluctuation or negative pressure in the operating pressure of the steel pipe, the radial collapse pressure should be corrected with a correction factor of 0.5 to 0.8.

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