A method for predicting the service life of flexible composite pipes

Abstract

This invention provides a method for predicting the service life of flexible composite pipes. It is a method for predicting the expected life of flexible composite pipes by conducting high-temperature accelerated testing on them in a simulated pipeline transport medium. By employing an accelerated temperature testing method, the test data of the internal pressure strength at which the flexible composite pipe fails in a simulated pipeline transport medium, based on the time-temperature conversion principle, is converted into predicted data of the pipeline design temperature (lower temperature), expected life (longer time), and the internal pressure strength at which the flexible composite pipe fails in the transport medium. Through data fitting, a fitting equation is obtained between the internal pressure strength failure value and the failure time of the flexible composite pipe. Different failure strength thresholds are set for different pipe materials, and the expected life of the pipe is calculated.

Description

[Technical Field]

[0001] This invention relates to the field of pipeline technology in the oil and gas industry, and in particular to a method for predicting the service life of flexible composite pipes. [Background Technology]

[0002] With the rapid development of the petroleum and natural gas industry, a large number of pressure pipelines are needed in fields such as oil and natural gas extraction, oil and gas gathering and injection, and sewage treatment. Oil and gas well fluids are generally corrosive, causing carbon steel pipelines to frequently suffer severe corrosion. Flexible composite pipes, a non-metallic pipe material developed in the last decade for oil and gas gathering and transportation, are now widely used in the transportation of highly corrosive fluids such as crude oil, natural gas, and sewage.

[0003] Flexible composite pipes typically have a three-layer structure. The innermost layer is made of thermoplastic plastic (such as high-density polyethylene, polypropylene, nylon, etc.), and this layer is in direct contact with the fluid being transported. The middle layer is made of reinforcing fibers (such as polyester fibers, aramid fibers, or carbon fibers), which bear the internal pressure and axial load of the pipe. The outermost layer is a protective layer, usually made of polyethylene.

[0004] Although flexible composite pipes have good corrosion resistance, they are susceptible to corrosion when exposed to water, H2S, CO2, and Cl. - In oil and gas field gathering and transportation environments with corrosive media, numerous failure cases have still occurred. The service life of flexible composite pipes is mainly determined by the pipe material characteristics, the type of transported medium, environmental parameters, and stress state. Accurately predicting the service life of flexible composite pipes is of significant theoretical and practical engineering value for eliminating potential pressure pipeline accidents, developing effective maintenance plans, and fully utilizing the effective strength of the pipe material.

[0005] GB / T 18252-2000 proposes an extrapolation method for determining the long-term hydrostatic strength of thermoplastic pipes, but this method requires one year to complete the test. Invention patent ZL201410721347.5 discloses a method for predicting the lifespan of flexible composite pipes using the hydrostatic residual strength method based on linear regression.

[0006] However, the test media used in the above-mentioned testing methods are all clean water. Clean water environments cause very limited corrosion and structural damage to flexible composite pipes, and flexible composite pipes often have a longer service life in clean water environments. However, oil and natural gas fluids generally have a certain degree of corrosiveness. Corrosive media can cause aging of thermoplastic pipes, changes in their chemical structure and properties, thereby reducing or eliminating their performance. The test media used in existing methods do not conform to the actual working conditions in the oil and gas industry. Moreover, existing methods typically use curves fitted from short-term test data for extrapolation to calculate the long-term performance of the pipes. The pipe life predicted using existing methods will be dangerously biased. [Summary of the Invention]

[0007] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for predicting the service life of flexible composite pipes, so as to solve the problems that the prior art cannot establish a corresponding quantitative relationship between short-term test data and the long-term expected performance of the pipe, and does not consider the impact of the corrosiveness of the pipeline transport medium on the service life of the pipe.

[0008] To achieve the above objectives, the present invention employs the following technical solution:

[0009] A method for predicting the service life of flexible composite pipes includes the following steps:

[0010] Step 1: Determine the test temperature based on the operating temperature and the increased temperature of the composite pipe. The increased temperature is determined by the pipe's design life and the test time.

[0011] Step 2: Place the composite tube sample filled with the test medium in a constant temperature water bath or environmental test chamber until the temperature of the composite tube sample is the same as the ambient temperature, and obtain the composite tube sample after adjustment.

[0012] Step 3: Conduct a burst test on the composite pipe sample after adjustment to determine the average value of the single burst failure time and the average value of the single burst strength.

[0013] Step 4: After adjusting the state, the composite pipe sample is subjected to hydrostatic failure test in an environmental test chamber to determine the failure time of the composite pipe sample under various test pressures.

[0014] Step 5: Using the time-temperature conversion principle, the average value of the single explosion failure time is converted into the baseline prediction time, and the failure time under each pressure is converted into the failure prediction time.

[0015] Step 6: Using the average value of the single burst strength and the test pressure, as well as the baseline prediction time and the failure prediction time, obtain the fitting formula for the failure time and failure pressure.

[0016] Step 7: Predict the service life of the flexible composite pipe using the fitting formula obtained in Step 6.

[0017] A further improvement of the present invention is that:

[0018] Preferably, in step 1, the formula for calculating the test temperature is:

[0019] T test =T life +△T (1)

[0020] In equation (1), T lifeT represents the design operating temperature of the flexible composite pipe, in °C. test The test temperature is expressed in °C, and ΔT represents the temperature increase, expressed in °C.

[0021] The formula for calculating the temperature increase ΔT is as follows:

[0022]

[0023] In equation (2), α: time-temperature conversion coefficient, taken as α = 0.05 to 0.2;

[0024] t life Pipe design life;

[0025] t test : Test time, t test With t life The units should be the same.

[0026] Preferably, in step 2, the test medium is a mixture of oil and salt water.

[0027] Preferably, in step 3, the specific process of the burst test is as follows: connect the composite tube sample to the pressurization device, so that the composite tube test bursts and fails between 30 seconds and 60 seconds, and record the strength value of a single burst and the time of a single burst.

[0028] Preferably, the number of single blasting tests is greater than or equal to 3.

[0029] Preferably, in step 4, the specific process of the hydrostatic failure test is as follows: connect the composite tube sample from step 2 to the pressurizing device, place the composite tube sample in a constant temperature water tank or environmental test chamber, and apply the test pressure to the set pressure P using the pressurizing device. n Simultaneously, timing begins and continues until the composite tube sample fails, at which point the pressure P is recorded. n The expiration time is specified below.

[0030] Preferably, during the test, the pressurizing device pressurizes the pressure to a set pressure P. n The time range is 5-30 minutes.

[0031] Preferably, in step 4, the composite pipe sample undergoes at least one hydrostatic failure test with a failure time of less than 100 hours, at least one test with a failure time of more than 600 hours, and at least two tests with a failure time between 100 hours and 600 hours.

[0032] Preferably, in step 5, the formula for converting the average value of the single blasting failure time into the benchmark prediction time is:

[0033]

[0034] in, Used as the baseline for prediction time; The test failure time is represented by ΔT; the temperature increase is represented by α; and the time-temperature conversion coefficient is represented by α.

[0035] The formula for converting the failure time under each pressure into the failure prediction time is as follows:

[0036]

[0037] in, For failure prediction time, ΔT represents the test failure time; ΔT represents the temperature increase; α represents the time-temperature conversion coefficient.

[0038] Preferably, in step 7, the burst strength threshold P of the pipe is determined. threshold Calculate according to the following formula:

[0039] P threshold =F p ×F f ×MOP (7)

[0040] In equation (7):

[0041] P threshold — Pipe failure strength threshold, MPa;

[0042] MOP—Maximum operating pressure of the pipe, MPa;

[0043] F p —The pipeline pressure safety factor is generally taken as 1.5 to 2.0;

[0044] F f —Pipe fluid coefficient, taken as 1.0 to 1.5; if the fluid being transported is water, F f Take 1.0. If the fluid being transported is an oil-water two-phase medium, F f Take 1.25. If the transported fluid is a three-phase medium of oil, gas, and water, F f Take 1.5; set P threshold Substitute these values ​​into the fitting formula in step 6 to obtain the predicted service life of the flexible composite pipe.

[0045] Compared with the prior art, the present invention has the following beneficial effects:

[0046] This invention provides a method for predicting the service life of flexible composite pipes. It is a method for predicting the expected life of flexible composite pipes by conducting high-temperature accelerated testing on them in simulated pipeline transport media. By employing an accelerated temperature testing method, the test data of the internal pressure strength at failure of the flexible composite pipe in a simulated pipeline transport medium, based on the time-temperature conversion principle, is converted into predicted data of the pipeline design temperature (lower temperature), expected life (longer life), and the internal pressure strength at failure of the flexible composite pipe in the transport medium. Through data fitting, a fitting equation is obtained between the internal pressure strength failure value and the failure time of the flexible composite pipe. Different failure strength thresholds are set for different pipe materials, and the expected life of the pipe is calculated. The method provided by this invention is based on test data, has a short test cycle, is easy to implement, is closely integrated with the service conditions of the pipe material, and has high predictive reliability. [Attached Image Description]

[0047] Figure 1 This is a fitting curve of the burst strength value versus time for the flexible composite pipe provided in Embodiment 1 of the present invention;

[0048] Figure 2 This is a fitting curve of the burst strength value versus time for the flexible composite pipe provided in Embodiment 3 of the present invention.

Detailed Implementation Methods

[0049] The present invention will now be described in further detail with reference to the accompanying drawings:

[0050] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. The terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance. Furthermore, unless otherwise explicitly specified and limited, the terms "installed," "connected," and "linked" should be interpreted broadly. For example, they can refer to a fixed connection or a detachable connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal connection of two elements. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0051] This invention employs an accelerated testing method with increased temperature. Test data on the internal pressure strength of a flexible composite pipe failing in a simulated pipeline medium, taken at a higher temperature and shorter time, is converted, based on the time-temperature conversion principle, into predicted data on the pipe's design temperature (lower temperature), expected lifespan (longer time), and the internal pressure strength of the flexible composite pipe failing in the transport medium. Through data fitting, a fitting equation is obtained between the internal pressure strength failure value and the failure time of the flexible composite pipe. Failure strength thresholds are set for different pipe materials, and the expected lifespan of the pipe is calculated. The specific implementation steps are as follows:

[0052] Step 1: Determining the test temperature

[0053] Test temperature T test The temperature is determined by the pipeline design operating temperature and the increase temperature ΔT, as shown in equation (1):

[0054] T test =T life +△T (1)

[0055] In equation (1), T life The design operating temperature of the flexible composite pipe is expressed in °C.

[0056] T test Test temperature, in °C;

[0057] △T: Temperature increase, in °C, ΔT is determined by equation (2):

[0058]

[0059] In equation (2), α: time-temperature conversion coefficient, taken as α = 0.05 to 0.2;

[0060] t life Pipe design life;

[0061] t test : Test time, t test With t life The units should be the same;

[0062] The test temperature T mentioned in step 1 test It should be at least 10°C lower than the oxidation induction temperature and melting point temperature of the thermoplastic lining of the composite pipe;

[0063] Furthermore, the time-temperature conversion coefficient α in formula (2) of step 1, formula (4) and formula (5) of step 5 takes a value of 0.05 to 0.2. If the inner lining of the composite pipe is polyethylene, α takes a value of 0.112; if the inner lining of the composite pipe is polypropylene, α takes a value of 0.1; if the inner lining of the composite pipe is polyvinylidene fluoride, α takes a value of 0.12; if the failure of the inner lining of the composite pipe is a non-toughness mode, α takes a value of 0.05.

[0064] Step 2: Prepare the sample

[0065] Several samples are randomly selected from the same batch of flexible composite tubes that meet the product quality requirements and fabricated into short sections with a free length of L0. The free length L0 of each sample between two sealing joints should not be less than three times the outer diameter of the sample, and should not be less than 500 mm. The samples used in this step are from flexible composite tubes with the same manufacturing process, the same raw materials, the same nominal pressure, the same nominal diameter, the same wall thickness, and the same manufacturing batch.

[0066] Step 3: Fill the composite tube with the test medium and condition it.

[0067] The flexible composite pipe sample with sealed joints at both ends is filled with the test medium, which is a simulated pipeline transport medium composed of oil and salt water. The sample is then placed in a temperature-controlled constant-temperature water bath or environmental test chamber, ensuring that the external environment of the flexible composite pipe is water or air. The sample is placed under the test temperature conditions specified in Table 1 for the time specified therein. The test temperature is T, determined in step 1. test Within the specified time, the test tubes undergo conditioning, which involves placing the tubes at room temperature into the environmental test chamber and ensuring that the overall temperature of the tubes is fully consistent with the ambient temperature of the chamber.

[0068] Table 1 Sample conditioning time under test temperature conditions

[0069] Pipe average wall thickness e, mm State settling time, h e≤8 2±0.1 8＜e≤16 4±0.1 8＜e≤32 8±0.1 e＞32 10±0.1

[0070] Step 4: Pipe burst test at test temperature

[0071] After the conditional treatment in step 3, the specimen is subjected to a burst test. The specimen is connected to a pressurizing device, and the pressurization rate is controlled to cause the specimen to burst and fail within 30 to 60 seconds. The single burst strength value and the corresponding failure time are recorded. At least three valid single burst tests should be conducted. The arithmetic mean of the single burst strength values ​​is denoted as P0, and the arithmetic mean of the single burst failure time is denoted as P1.

[0072] The relative deviation range of a single blast strength value is -15% to +15%, which is considered valid data. If it exceeds this range, the single value should be discarded or the blast test should be repeated.

[0073] The relative deviation is calculated according to formula (3).

[0074] Relative deviation = [(single blast value - average value) / average value] × 100% (3)

[0075] The valid failure mode for pipe bursting should be cracking or bursting failure of the flexible composite pipe body. During the test, failure of the joints at both ends of the specimen and the pipe body coming out of the joint are invalid specimens. Test data of invalid specimens should be discarded or retested.

[0076] The simulated pipeline transport medium used in step 4 is a mixture of crude oil and brine. The concentration of the brine is consistent with that of the formation water in the pipeline gathering fluid, and the volume ratio of crude oil to brine is consistent with that of the formation water in the pipeline gathering fluid.

[0077] Step 5: Hydrostatic failure test of pipe at test temperature

[0078] Connect the sample, after condition adjustment in step 3, to the pressurization equipment and place it in a temperature-controlled constant temperature water bath or environmental test chamber. The temperature of the constant temperature water bath or environmental test chamber is set at the test temperature T. test Then, depending on the specifications and dimensions of the sample, apply the test pressure evenly and smoothly to the set pressure P within the shortest possible time, between 5 and 30 minutes. n (4≤n≤10, where n is a positive integer), reaching the set pressure P n Timing begins after the sample fails, i.e., the failure time. (4≤n≤10). The set pressure P for the hydrostatic failure test loading. n The inequality is set to 4 to 10 discrete values ​​between 0.6P0 and 0.2P0, i.e., 0.2P0 ≤ P. n ≤0.6P0. This step is used to obtain the failure time under different pressures.

[0079] The effective hydrostatic failure mode of the pipe should be cracking or bursting failure of the flexible composite pipe body. During the test, failure of the joints at both ends of the specimen and the pipe body coming out of the joint are invalid specimens. The test data of invalid specimens should be discarded or supplemented.

[0080] The test medium is inside the flexible composite tube, and the external environment is air or water. During the test, the deviation of the internal pressure of the flexible composite tube should be controlled within the range of -2% to +2% of the set test pressure, and the external environment should be maintained at ±2℃ of the test temperature.

[0081] The failure mode of the composite pipe described in steps 4 and 5 should be cracking or bursting failure of the flexible composite pipe body. During the test, failure of the joints at both ends of the sample and the pipe body coming out of the joints are invalid samples. The test data of invalid samples should be discarded or the bursting test should be repeated.

[0082] In step 5, P should be adjusted. n The difference between the magnitude and discrete values ​​should ensure that at least one test is conducted where the hydrostatic failure time of the pipe is less than 100 hours, at least one test is conducted where the hydrostatic failure time of the pipe exceeds 600 hours, and at least two tests are conducted where the hydrostatic failure time of the pipe is between 100 hours and 600 hours. The hydrostatic failure time of the tests should be discretely distributed to ensure the reliability of the regression.

[0083] In step 5, P should be adjusted. n The difference between the magnitude and the discrete value ensures that at least one time point in the hydrostatic failure time of the pipe is present. Calculated according to formula (5) It exceeds the design life of the pipe.

[0084] Step 6: Convert experimental values ​​into predicted values

[0085] Using the time-temperature conversion principle, step 4 is performed at the test temperature T. test The burst value P0 of the flexible composite pipe corresponds to the test failure time. Convert to predicted temperature T life Benchmark prediction time Calculated according to formula (4), The corresponding pressure value is P0.

[0086]

[0087] Using the time-temperature conversion principle, step 5 is performed at the test temperature T. test The hydrostatic strength P of the flexible composite pipe n Corresponding test failure time Convert to predicted temperature T life Failure prediction time Calculated according to formula (5), The corresponding pressure value is P n n is a positive integer, 4 ≤ n ≤ 10.

[0088]

[0089] Step 7: Numerical fitting of the predicted data using linear regression.

[0090] In a Cartesian coordinate system, with and The logarithm of P0 is used as the x-axis, with P0 and P1 as the coordinates. n Plotting the logarithm of the equation as the ordinate, and using linear regression to perform numerical fitting, we obtain the fitting equation (6). n is a positive integer, 4≤n≤10.

[0091] ln(P)=aln(t)+b Equation (6)

[0092] Step 8: Pipe life estimation

[0093] Pipe expected life is defined as the time when the predicted failure strength value of the pipe equals the pipe failure strength threshold, where P is the burst strength threshold. threshold Calculate according to formula (7):

[0094] P threshold =F p ×F f ×MOP formula (7)

[0095] In equation (7):

[0096] P threshold — Pipe failure strength threshold, MPa;

[0097] MOP—Maximum operating pressure of the pipe, MPa;

[0098] F p —The pipeline pressure safety factor is generally taken as 1.5 to 2.0;

[0099] F f —The fluid coefficient in pipelines is generally taken as 1.0 to 1.5; if the fluid being transported is water, F f Take 1.0. If the fluid being transported is an oil-water two-phase medium, F f Take 1.25. If the transported fluid is a three-phase medium of oil, gas, and water, F f Take 1.5;

[0100] P, as determined by equation (7), threshold Substituting into equation (6), we obtain equation (8):

[0101] ln(P threshold Equation (8) = alun(t) + b

[0102] Calculate P in formula (8) threshold The corresponding time value t life , t life This means that the batch of flexible composite pipes was manufactured at a predicted temperature of T. life The expected lifetime in the simulated test medium.

[0103] Example 1

[0104] The service life of a polyester-reinforced high-density polyethylene (PE100) flexible composite pipe with a nominal diameter of DN=80mm, a nominal pressure of PN=4.0Mpa, and an average outer diameter of 105mm is predicted, and the conditions for transporting the fluid are shown in Table 2.

[0105] Table 2. Conditions for liquid transport via pipeline in Example 1

[0106]

[0107] The specific steps are as follows:

[0108] Step 1: Determining the test temperature

[0109] The inner lining of the flexible composite pipe is high-density polyethylene (PE100). The oxidation induction temperature of PE100 is 125℃, the melting point is 130℃, and the time-temperature conversion coefficient α is 0.112. The test parameters determined by equations (1) and (2) are shown in Table 3.

[0110] Table 3. Experimental parameters and related calculation parameters used in Example 1.

[0111]

[0112] Step 2: Prepare the sample

[0113] Ten flexible composite tubes from the same batch that meet the product quality requirements are randomly selected and made into short sections with a free length of 1000 mm between two sealing joints. One end of the sample is sealed, and the other end has a metal hole, which can be used to inject the test medium and can be sealed to the pressure testing machine through the joint.

[0114] Step 3: Fill the pipe with the test medium and condition it.

[0115] Take the three samples prepared in step 2 and fill the flexible composite tube samples with sealed joints at both ends with the test medium listed in Table 2. Place the samples in a temperature-controlled constant temperature water bath, the temperature of which has been adjusted to 83℃±2℃ before the samples are placed in the bath. Since the average wall thickness of the samples is 12.5mm, the samples are placed in the constant temperature water bath for 4h±0.1h.

[0116] Step 4: Pipe burst test

[0117] Three samples, after being conditioned in step 3, were connected sequentially to the pressurizing equipment for a burst test. The pressurization rate of the equipment was controlled to cause the samples to burst and fail within 30 to 60 seconds. The single burst strength value and the corresponding failure time were recorded. The test results are shown in Table 4. From Table 4, it can be seen that P0 = 18.6 MPa.

[0118] Table 4 Results of the explosion test of the sample in Example 1

[0119] Sample number Single blast strength value, MPa deviation Expiration time, s Failure morphology 1# 18.6 0.00％ 50 Pipe burst in the middle 2# 19.2 3.23％ 54 Pipe burst in the middle 3# 18.0 -3.23％ 52 Pipe burst in the middle average value 18.6 52

[0120] Step 5: Hydrostatic failure test of pipe at test temperature

[0121] The sample, after condition adjustment in step 3, is connected to the pressurizing device and placed in a temperature-controlled constant-temperature water bath. The temperature of the water bath was adjusted to 83℃±2℃ before the sample was placed in. Then, the test pressure is applied evenly and steadily to the set pressure P over a period of 9 to 12 minutes. n Reaching the set pressure P n Timing begins after the sample fails, i.e., the failure time. The relevant parameters and results of the experiment are shown in Table 5. n = 1, 2, 3, 4.

[0122] Table 5. Hydrostatic test conditions and results in Example 1

[0123]

[0124]

[0125] Step 6: Convert experimental values ​​into predicted values

[0126] According to formula (4), the test failure time corresponding to the burst value P0 = 18.6 MPa of the flexible composite pipe at the test temperature of 83℃ in step 4 is calculated. Prediction time converted to a predicted temperature of 60°C Year.

[0127] According to formula (5), the hydrostatic strength P of the flexible composite pipe at a test temperature of 83℃ in step 5 is determined. n Corresponding test failure time Prediction time converted to a predicted temperature of 60°C Listed in Table 5. n = 1, 2, 3, 4.

[0128] Step 7: Numerical fitting of the predicted data using linear regression.

[0129] In a Cartesian coordinate system, with and The logarithm of P0 is used as the x-axis, with P0 and P1 as the coordinates. n Plotting the logarithm of the values ​​on the ordinate, and using linear regression to fit the values, we obtain the fitting equation (9). n = 1, 2, 3, 4.

[0130] ln(P)=-0.0851ln(t)+2.300 Equation (9)

[0131] In equation (9):

[0132] P—Predicted failure blasting strength value, in MPa;

[0133] t — the time corresponding to the failure blasting strength value, in years; e.g. Figure 1As shown.

[0134] Step 8: Pipe life estimation

[0135] Take MOP = 4.0 MPa, and the pipeline pressure safety factor as F. p =1.5, the transported fluid is an oil-water two-phase medium, F f Take 1.25. Calculate the burst strength threshold P of the pipe according to formula (7). threshold =7.5MPa, P threshold Substitute into equation (9) to calculate P. threshold The corresponding time value t life =28.5 years. Therefore, in a simulated oil-water medium transport environment at 60℃ with a maximum operating pressure MOP = 4.0 MPa, the expected lifespan of this batch of flexible composite pipes is 28.5 years.

[0136] Example 2

[0137] The service life of a polyester-reinforced high-density polyethylene (PE100) flexible composite pipe with a nominal diameter of DN=80mm, a nominal pressure of PN=4.0Mpa, and an average outer diameter of 105mm is predicted, and the conditions for transporting the fluid are shown in Table 2.

[0138] The same steps 1 to 7 as in Example 1 are used.

[0139] Step 8: Pipe life estimation

[0140] Take MOP = 4.5 MPa, and the pipeline pressure safety factor as F. p =1.5, the transported fluid is an oil-water two-phase medium, F f Take 1.25. Calculate the burst strength threshold P of the pipe according to formula (7). threshold =8.4MPa, P threshold Substitute into equation (9) to calculate P. threshold The corresponding time value t life =7.1 years. Therefore, in a simulated oil-water medium transport at 60℃ with a maximum operating pressure MOP of 4.5MPa, the expected lifespan of this batch of flexible composite pipes is 7.1 years.

[0141] Example 3

[0142] The service life of aramid-reinforced polyvinylidene fluoride (PVDF) flexible composite pipes with a nominal diameter of DN = 100 mm, a nominal pressure of PN = 5.5 MPa, and an average outer diameter of 122 mm is predicted. The conditions for transporting the fluid are shown in Table 1.

[0143] Table 6. Conditions for liquid transportation via pipeline in Example 3

[0144]

[0145] The specific steps are as follows:

[0146] Step 1: Determining the test temperature

[0147] The inner lining of the flexible composite pipe is polyvinylidene fluoride (PVDF). The melting temperature of PVDF is 168.7℃ and the thermal decomposition temperature is about 320℃. The time-temperature conversion coefficient α is taken as 0.12. The test parameters determined by equations (1) and (2) are shown in Table 7.

[0148] Table 7. Experimental parameters and related calculation parameters used in Example 3.

[0149] <![CDATA[Test temperature T test > Increase temperature △T Time-temperature conversion coefficient α <![CDATA[Design life t of the pipe life > <![CDATA[Design test time t test > 98℃ 18℃ 0.12 25 years 65 days

[0150] Step 2: Prepare the sample

[0151] Ten flexible composite tubes from the same batch that meet the product quality requirements are randomly selected and made into short sections with a free length of 1000 mm between two sealing joints. One end of the sample is sealed, and the other end has a metal hole, which can be used to inject the test medium and can be sealed to the pressure testing machine through the joint.

[0152] Step 3: Fill the pipe with the test medium and condition it.

[0153] Take the three samples prepared in step 2 and fill the flexible composite tube samples with sealed joints at both ends with the test medium, the composition of which is shown in Table 6. Place the samples in an environmental test chamber with temperature control. The temperature of the environmental test chamber was already stable at 98℃±2℃ before the samples were placed in. Since the average wall thickness of the samples is 11mm, the samples were placed in the environmental test chamber for 4h±0.1h.

[0154] Step 3: Pipe burst test

[0155] Three samples, after being conditioned in step 3, were connected sequentially to the pressurizing equipment for a burst test. The pressurization rate of the equipment was controlled to ensure that the samples burst and failed within 30 to 60 seconds. The single burst strength value and the corresponding failure time were recorded. The test results are shown in Table 8. From Table 8, it can be seen that P0 = 28.4 MPa.

[0156] Table 8 Results of the explosion test of the samples in Example 3

[0157] Sample number Single blast strength value, MPa deviation Expiration time, s Failure morphology 1# 28.6 0.70％ 60 Pipe burst in the middle 2# 27.1 -4.58％ 56 Pipe burst in the middle 3# 29.4 3.52％ 52 Pipe burst in the middle average value 28.4 56

[0158] Step 4: Hydrostatic failure test of pipe at test temperature

[0159] After the condition adjustment in step 3, the sample was connected to the pressurizing device and placed in an environmental test chamber with temperature control. The temperature of the environmental test chamber was stabilized at 98℃±2℃ before the sample was placed in. Then, the test pressure was uniformly and steadily applied to the sample to the set pressure P over a period of 9 min to 14 min. n Reaching the set pressure P n Timing begins after the sample fails, i.e., the failure time. The relevant parameters and results of the experiment are shown in Table 9. n = 1, 2, 3, 4, 5, 6.

[0160] Table 9. Hydrostatic test conditions and results in Example 3

[0161]

[0162]

[0163] Step 5: Convert experimental values ​​into predicted values

[0164] According to formula (4), the test failure time corresponding to the burst value P0 = 28.4 MPa of the flexible composite pipe at the test temperature of 98℃ in step 3 is calculated. Prediction time converted to a predicted temperature of 80°C Year.

[0165] According to formula (5), the hydrostatic strength P of the flexible composite pipe at a test temperature of 98℃ in step 4 is determined. n Corresponding test failure time Prediction time converted to a predicted temperature of 80°C Listed in Table 9. n = 1, 2, 3, 4, 5, 6.

[0166] Step 6: Numerical fitting of the predicted data using linear regression.

[0167] In a Cartesian coordinate system, with and The logarithm of P0 is used as the x-axis, with P0 and P1 as the coordinates. n Plotting the logarithmic values ​​as the ordinate, and using linear regression for numerical fitting, we obtain the fitting equation (10). n = 1, 2, 3, 4, 5, 6.

[0168] ln(P)=-0.1211ln(t)+2.653 Equation (10)

[0169] In formula (10):

[0170] P—Predicted failure blasting strength value, in MPa;

[0171] t — the time corresponding to the failure blasting strength value, in years; e.g. Figure 2 As shown.

[0172] Step 7: Pipe life estimation

[0173] Take MOP = 5.0 MPa, and the pipeline pressure safety factor as F. p =1.5, the transported fluid is an oil-water two-phase medium, F f Take 1.25. Calculate the burst strength threshold P of the pipe according to formula (7). threshold =9.4MPa, P threshold Substituting into equation (10), calculate P threshold The corresponding time value t life =30.1 years. Therefore, if the maximum operating pressure MOP = 5.0 MPa in a simulated oil-water medium transport at 80℃, the expected life of this batch of flexible composite pipes is 30.1 years.

[0174] Example 4

[0175] The service life of aramid-reinforced polyvinylidene fluoride (PVDF) flexible composite pipes with a nominal diameter of DN = 100 mm, a nominal pressure of PN = 5.5 MPa, and an average outer diameter of 122 mm is predicted, and the conditions for transporting the fluid are shown in Table 6.

[0176] The same steps 1 to 6 as in Example 3 are used.

[0177] Step 7: Pipe life estimation

[0178] Take MOP = 4.0 MPa, and the pipeline pressure safety factor as F. p =2.0, the transported fluid is an oil-water two-phase medium, F f Take 1.25. Calculate the burst strength threshold P of the pipe according to formula (7). threshold =10.0MPa, P threshold Substituting into equation (10), calculate P threshold The corresponding time value t life =18.1 years. Therefore, if the maximum operating pressure MOP = 4.0 MPa in a simulated oil-water medium transport at 80℃, the expected life of this batch of flexible composite pipes is 18.1 years.

[0179] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for predicting the service life of flexible composite pipes, characterized in that, Includes the following steps: Step 1: Determine the test temperature based on the operating temperature and the increased temperature of the composite pipe. The increased temperature is determined by the pipe's design life and the test time. Step 2: Place the composite tube sample filled with the test medium in a constant temperature water bath or environmental test chamber until the temperature of the composite tube sample is the same as the ambient temperature, and obtain the composite tube sample after adjustment. Step 3: Conduct a burst test on the composite pipe sample after adjustment to determine the average value of the single burst failure time and the average value of the single burst strength. Step 4: After adjusting the state, the composite pipe sample is subjected to hydrostatic failure test in an environmental test chamber to determine the failure time of the composite pipe sample under various test pressures. Step 5: Using the time-temperature conversion principle, the average value of the single explosion failure time is converted into the baseline prediction time, and the failure time under each pressure is converted into the failure prediction time. In step 5, the formula for converting the average value of the single blasting failure time into the benchmark prediction time is as follows: （4） in, Used as the baseline for prediction time; The test failure time; △ T To increase the temperature; α This is the time-temperature conversion coefficient; The formula for converting the failure time under each pressure into the failure prediction time is as follows: Equation (5) in, For failure prediction time, The test failure time; △ T To increase the temperature; α This is the time-temperature conversion coefficient; Step 6: Using the average value of the single burst strength and the test pressure, as well as the baseline prediction time and the failure prediction time, obtain the fitting formula for the failure time and failure pressure. In a Cartesian coordinate system, with and The logarithm of the value is used as the x-axis, with P 0 and P n Plotting the logarithm of the value on the ordinate, and using linear regression to fit the numerical equation, we obtain the fitting equation (6); n is a positive integer, 4≤n≤10; Equation (6) in, P 0 represents the arithmetic mean of the intensity values ​​of a single blast. P n To set the pressure; Step 7: Predict the service life of the flexible composite pipe using the fitting formula obtained in Step 6; Pipe burst strength threshold P threshold Calculate according to the following formula: P threshold = F p × F f ×MOP （7） In equation (7): P threshold — Pipe failure strength threshold, MPa; MOP —Maximum operating pressure of the pipe, MPa; F p —The pipeline pressure safety factor is generally taken as 1.5~2.0; F f — The pipeline fluid coefficient is taken as 1.0~1.5; if the fluid being transported is water, F f Take 1.0 if the fluid being transported is an oil-water two-phase medium. F f Take 1.

25. If the transported fluid is a three-phase medium of oil, gas, and water, F f Take 1.5; P threshold Substitute these values ​​into the fitting formula in step 6 to obtain the predicted service life of the flexible composite pipe.

2. The method for predicting the service life of a flexible composite pipe according to claim 1, characterized in that, In step 1, the formula for calculating the test temperature is: T test = T life +△ T （1） In equation (1), T life The design operating temperature of the flexible composite pipe is expressed in °C. T test The test temperature is expressed in °C, Δ. T To increase the temperature, the unit is °C; The increase in temperature △ T The calculation formula is: （2） In equation (2), α Time-temperature conversion coefficient, taken as α =0.05~0.2; t life Pipe design life; t test Test time, t test and t life The units should be the same.

3. The method for predicting the service life of a flexible composite pipe according to claim 1, characterized in that, In step 2, the test medium is a mixture of oil and salt water.

4. The method for predicting the service life of a flexible composite pipe according to claim 1, characterized in that, In step 3, the specific process of the burst test is as follows: connect the composite pipe sample to the pressurization device, so that the composite pipe sample bursts and fails between 30 seconds and 60 seconds, and record the strength value of a single burst and the time of a single burst.

5. The method for predicting the service life of a flexible composite pipe according to claim 1, characterized in that, The number of single blasting tests is greater than or equal to 3.

6. The method for predicting the service life of a flexible composite pipe according to claim 1, characterized in that, In step 4, the specific process of the hydrostatic failure test is as follows: connect the composite tube sample from step 2 to the pressurizing device, place the composite tube sample in a constant temperature water tank or environmental test chamber, and apply the test pressure to the set pressure using the pressurizing device. P n Simultaneously, timing begins and continues until the composite tube sample fails, at which point the pressure is recorded. P n The expiration time is specified below.

7. The method for predicting the service life of a flexible composite pipe according to claim 6, characterized in that, During the test, the pressurizing equipment pressurized the pressure to the set pressure. P n The time range is 5-30 minutes.

8. The method for predicting the service life of a flexible composite pipe according to claim 1, characterized in that, In step 4, the composite pipe sample undergoes at least one hydrostatic failure test with a failure time of less than 100 hours, at least one test with a failure time of more than 600 hours, and at least two tests with a failure time between 100 hours and 600 hours.

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