Method for detecting volume of S-shaped marine flexible riser annulus double-liquid-column end blocking air bag

Abstract

The invention provides a method for detecting the volume of an S-shaped marine flexible riser annulus double-liquid-column end blocking air bag, belongs to the technical field of oil and gas pipeline safety detection, and is suitable for accurate detection of the volume of multiple sections of air bags in a deep-sea oil and gas flexible riser annulus. According to the method, nitrogen is injected into the annulus in a stepped mode for three times, the nitrogen injection volume and the balance pressure are recorded under different pressures, and the volume and the liquid column propelling distance of each air bag are calculated by combining a thermodynamic gas state equation and a liquid column propelling principle, so that accurate measurement of the distribution of the complex air bags in the annulus is achieved. The detection system comprises a high-pressure gas cylinder, a pressure regulator, a valve, a pressure sensor, a temperature sensor, a gas flow meter and the like, and is hermetically connected to the annulus inlet of the flexible riser through a pipeline. The method has the advantages of being easy and convenient to operate, high in detection precision and wide in applicability, dynamic monitoring of the annulus state of the marine flexible riser can be achieved, and important data support is provided for safe operation of a deep sea oil and gas conveying system.

Description

Technical Field

[0001] The present invention belongs to the technical field of oil and gas pipeline safety detection, especially for flexible riser detection in deep-sea environment, and specifically relates to a double liquid column airbag volume detection method applied to the annulus of S-type marine flexible riser. The method is based on nitrogen step-by-step injection technology, and can accurately measure the airbag volume in the annulus of the flexible riser, providing an important detection means for the safe operation of deep-sea oil and gas transportation systems, and effectively extending the service life of the flexible riser. Background Art

[0002] Marine oil and gas flexible risers are key equipment for deep-sea oil and gas transportation. They are usually made of multi-layer composite materials with good flexibility and pressure resistance. However, during long-term use, the environmental pressure and temperature differences inside and outside the riser, as well as the characteristics of the medium inside the pipe, may cause small molecular gases (such as water vapor, carbon dioxide, hydrogen sulfide, etc.) to gradually penetrate the lining layer and gather in the annulus to form air pockets. If these air pockets are not controlled, they will generate additional pressure, posing a potential threat to the riser structure and affecting the safety and service life of the equipment. Therefore, it is particularly important to accurately detect the volume of the air pockets in the annulus of the flexible riser.

[0003] In the prior art, invention patent CN112798043B discloses a nondestructive detection device and method for the annulus of a marine oil and gas flexible hose, which realizes the collection and analysis of the gas in the annulus by means of gas sample collection, annulus pressure testing, etc. However, this method is mainly aimed at the detection of a single gas pocket in the annulus, and it is difficult to cope with the complex situation of alternating distribution of multiple liquid columns and gas pockets in the S-type riser.

[0004] In practical applications, in order to cope with the impact of ocean currents, flexible risers are usually arranged in an S shape, so that the liquid column gathers at the curved part of the riser, forming a structure with multiple liquid columns and air bags alternating. The traditional volume calculation method has low accuracy under this structure and cannot fully and accurately quantify the volume of each air bag, and thus it is difficult to fully reflect the actual operating status of the riser.

[0005] In order to solve the above technical bottlenecks, the present invention proposes a volume detection method based on step-by-step nitrogen injection. This method uses three step-by-step nitrogen injections, utilizes the interaction between gas and liquid column under different pressure conditions, and is based on the thermodynamic gas state equation to indirectly deduce the volume of each air bag in the annulus. The present invention not only fills the gap in the existing technology in the detection of complex air bags and liquid column distribution, but also improves the accuracy of riser annulus status monitoring, providing technical guarantee for the safety and reliability of deep-sea oil and gas pipelines. Summary of the invention

[0006] The present invention discloses a method for detecting the volume of double liquid column end barrier airbags in the annulus of an S-type marine flexible riser, aiming to solve the technical problem in the prior art that it is difficult to accurately measure the volume of airbags in the annulus of an S-type flexible riser with multiple liquid columns and airbags alternately distributed. The present invention analyzes the change in gas volume and the advancement of liquid columns at different pressures by step-by-step nitrogen injection, thereby realizing accurate detection of the volume of each airbag in the annulus, and providing reliable data support for the safety assessment and service life extension of the flexible riser.

[0007] The technical solution of the present invention is as follows:

[0008] 1. Nitrogen injection in stages

[0009] The detection method of the present invention is based on three-step nitrogen injection, and the pressure balance and the volume of nitrogen injected are recorded after each injection. By gradually increasing the injection pressure of nitrogen in the annulus, the movement of the liquid column and the change of the remaining space in the annulus are measured layer by layer using the pressure transmission characteristics of the gas.

[0010] 2. Calculation of airbag volume

[0011] During the nitrogen injection process, the volume of the airbag is calculated based on the ideal gas state equation and the change in nitrogen volume under different pressures. The difference in the volume of the airbag after two nitrogen injections is used to obtain the distance the liquid column has advanced, thereby quantifying the volume of each airbag.

[0012] 3. Temperature and dissolution factor control

[0013] During the test, the annular temperature remains constant and the influence of nitrogen dissolution is not considered to ensure the accuracy of the results. The airbag volume and liquid column advancement are finally derived through the segmented calculation of the ideal gas state equation and the calibration of the nitrogen compressibility factor.

[0014] 4. Dynamic Data Analysis

[0015] According to the pressure-volume relationship of three nitrogen injections, the spatial distribution characteristics of the gas bag and the liquid column are analyzed layer by layer. The present invention establishes a database of the gas bag blocking the end of the riser annulus liquid column to facilitate subsequent dynamic monitoring and safety analysis.

[0016] Through the above method, the present invention realizes the accurate detection of the complex airbag distribution of the S-type marine flexible riser, effectively fills the gap in the existing technology in the volume calculation of multi-segment liquid columns and airbag structures, greatly improves the accuracy and reliability of the annulus detection of marine oil and gas flexible risers, and provides strong technical support for the safety of deep-sea oil and gas transportation. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0018] Figure 1 It is a schematic diagram of the flexible riser annulus remaining volume detection process in an embodiment of the present invention, showing the overall process and equipment connection method of three-step nitrogen injection detection.

[0019] Figure 2 This is a flow chart of the calculation of the remaining volume of the annulus in an embodiment of the present invention, which illustrates the specific steps of data collection, calculation and volume derivation.

[0020] Figure 3 Schematic diagram of the air-liquid phase distribution in the flexible riser annulus in the initial state in an embodiment of the present invention, showing the initial distribution of the air pockets and liquid columns in the annulus.

[0021] Figure 4 This is a schematic diagram of the air-liquid phase distribution in the flexible riser ring after the first nitrogen injection in an embodiment of the present invention, showing the changes in the distribution of the air pockets and liquid columns after the initial nitrogen injection.

[0022] Figure 5 This is a schematic diagram of the air-liquid phase distribution in the flexible riser ring after the second nitrogen injection in an embodiment of the present invention, showing further changes in the air pocket and the liquid column after the second nitrogen injection.

[0023] Figure 6 This is a schematic diagram of the air-liquid phase distribution in the flexible riser ring after the third nitrogen injection in an embodiment of the present invention, showing the final distribution state of the air bag and the liquid column after the third nitrogen injection.

[0024] Explanation of the numbers in the figure: 1-high pressure gas cylinder; 2-pressure regulator A; 3-valve A; 4-valve B; 5-pressure regulator B; 6-valve C; 7-valve D; 8-valve E; 9-gas flow meter; 10-valve F; 11-pressure sensor; 12-temperature sensor; 13-valve G; 14-flexible riser. DETAILED DESCRIPTION

[0025] In order to better understand the present invention, the implementation of the S-type marine flexible riser annulus double liquid column end barrier airbag volume detection method is described in detail below with reference to the accompanying drawings. The present invention measures the airbag volume change at different pressures through three step-by-step nitrogen injection operations, thereby obtaining a high-precision volume detection result.

[0026] Example 1: S-type flexible riser ring air bag volume detection process

[0027] 1. Device connection and initial setup

[0028] (1) Device connection

[0029] according to Figure 1 , follow the steps below to connect the detection equipment to the flexible riser:

[0030] Connect the high-pressure gas cylinder (1) to the annulus inlet of the flexible riser to be tested through the pressure regulator A (2) and valve A (3) to ensure a stable supply of nitrogen.

[0031] Then, valve A is connected to the pressure sensor (11), the temperature sensor (12) and the gas flow meter (9) to ensure real-time monitoring of the pressure, temperature and nitrogen flow during the detection process.

[0032] Check the status of each valve and ensure that all connection points are well sealed to prevent gas leakage during the test.

[0033] (2) Air tightness test

[0034] Open the valve 1 of the high-pressure gas cylinder and reduce the pressure of nitrogen to 2 barg through the pressure regulator A (2).

[0035] Open valve A (3) and valve B (4) and observe whether the reading of pressure sensor 11 remains stable to confirm the air tightness of the system.

[0036] If the pressure change does not exceed 0.5%, it means the air tightness is good; otherwise, check the sealing part and readjust the connector.

[0037] (3) Initial pressure record

[0038] Close all valves and open valve G (13) to exhaust the air in the pipeline.

[0039] After the air is vented, the initial pressure of the flexible riser annulus is recorded as a basis for subsequent volume calculations.

[0040] 2. Step-by-step nitrogen injection process

[0041] Inject nitrogen into the annulus in three steps, with different pressures each time, and record the change in annular pressure to deduce the volume of the gas bag:

[0042] (1) First nitrogen injection

[0043] Adjust the pressure of regulator B (5) to 0.75 barg, open valve A (3), valve C (6) and valve E (8), and slowly inject nitrogen into the annulus.

[0044] Observe the reading of pressure sensor 11. When the pressure stabilizes at 0.75 barg, stop nitrogen injection and close pressure regulator B and related valves.

[0045] The accumulated readings of the gas flow meter 9 are recorded for subsequent calculation of the airbag volume.

[0046] (2) Second nitrogen injection

[0047] Adjust the pressure of regulator B to 1.5 barg and repeat the injection operation.

[0048] When the pressure reaches 1.5 barg and stabilizes, stop the injection and record the accumulated flow value of the gas flow meter.

[0049] (3) The third nitrogen injection

[0050] The pressure of regulator B was adjusted to 2.0 barg and the third nitrogen injection operation was performed.

[0051] When the reading of the pressure gauge 11 stabilizes at 2.0 barg, stop the injection and record the accumulated reading of the gas flow meter.

[0052] After the nitrogen injection process is completed, the annular gas is vented to normal pressure through valve G, and finally the equipment is disassembled from the flexible riser and the site is cleaned up.

[0053] 3. Calculation of the volume of the double liquid column end barrier airbag

[0054] The volume of each airbag is calculated by using the thermodynamic gas state equation and the liquid column propulsion formula based on the volume change relationship under different pressure conditions. The following example takes two sections of airbags and liquid columns as an example. The annular temperature remains unchanged. Based on the amount of nitrogen injected under three different pressures, the remaining volume of the annular space is calculated. Figures 4 to 6 shown.

[0055] In the initial state, the pressure balance formula of airbag A and airbag B is:

[0056] P A0 +ρgh0=P B0 (1)

[0057] After the first nitrogen injection and pressure stabilization, the pressure balance formula between airbag A and airbag B is:

[0058] P A1 +ρg(h0-2h1)=P B1 (2)

[0059] After the second nitrogen injection and pressure stabilization, the pressure balance formula between airbag A and airbag B is:

[0060] P A2 +ρg[h0-2(h1+h2)]=P B2(3)

[0061] After the third nitrogen injection and pressure stabilization, the pressure balance formula of airbag A and airbag B is:

[0062] P A3 +ρg[h0-2(h1+h2+h3)]=P B3 (4)

[0063] Where P A0 is the initial pressure of the annular air bag A, Pa; P B0 is the initial pressure of the annular air bag B, Pa; P A1 , P A2 , P A3 Corresponding to the three nitrogen injection pressures in airbag A, Pa; P B1 , P B2 , P B3 They correspond to the pressure of the airbag B after being compressed by three nitrogen injections, Pa; h0 is the initial page height difference between the two ends of the liquid column A, m; h1, h2, h3 correspond to the propulsion distance of the liquid column A after three nitrogen injections, m; ρ is the density of the liquid column, kg / m 3 ; g is the acceleration due to gravity, m / s 2 .

[0064] In this case, the airbag B is a fixed amount of gas in a closed container. The nitrogen injection process causes isothermal compression. According to Boyle's law, the pressure of the compressed airbag can be expressed by the initial pressure and initial volume, as shown below:

[0065] P B0 V B0 =P B1 V B1 =P B2 V B2 =P B3 V B3 (5)

[0066] According to the three liquid column advancement distances, the volume of airbag B after each nitrogen injection can be expressed in turn:

[0067] V B1 =V B0 -Ah1 (6)

[0068] V B2 =V B0 -A(h1+h2) (7)

[0069] V B3 =V B0 -A(h1+h2+h3) (8)

[0070]

[0071] Where V B0 is the initial volume of the annular air bag B, m 3 ; V B1 、V B2 、V B3 are the volumes of the compressed air bag B after three nitrogen injections, m 3 ; R1 is the inner diameter of the hose, m; R2 is the outer diameter of the hose, m; k is the porosity of the riser annulus; A is the bottom area of ​​the riser annulus, m 2 .

[0072] Without considering the dissolution of nitrogen, according to the ideal gas state equation PV = nRT, the volume of the airbag A after three nitrogen injections is converted to:

[0073]

[0074] Subtract the volumes of airbag A converted three times from each other to obtain the liquid column advancement distances h2 and h3 after the second and third nitrogen injections:

[0075]

[0076] Where V A1 、V A2 、V A3 The volume of airbag A is calculated after three nitrogen injections without considering nitrogen dissolution, m 3 ; Z is the compressibility factor of nitrogen; The corresponding standard volume of nitrogen injected into the riser annulus three times, Sm 3 ; is the molar volume of nitrogen under standard conditions, Sm 3 / mol; R is the gas constant; T is the average thermodynamic temperature of the riser annulus, K; P A1 , P A2 , P A3 Corresponding to the three nitrogen injection pressures in airbag A, Pa; V 01 is the volume of the annulus outlet connecting conduit, m 3 ; V 02 is the volume of the pipeline connecting the catheter outlet and the detection equipment, m 3 .

[0077] Combining and reorganizing the above formulas, we can get:

[0078] P A0 +ρgh0=P B0 (15)

[0079]

[0080]

[0081] The second propulsion distance h2 and the third propulsion distance h3 are calculated by formula (10-14). By combining the above relationship, the initial page height difference h0 at both ends of the liquid column A, the first propulsion distance h1, and the initial pressure P of the airbag B can be calculated. B0 and the initial volume V of airbag B B0 .

[0082] Example 2: Pressure Control and Data Analysis

[0083] 1. Pressure graded control

[0084] During the test, the nitrogen injection pressure is gradually increased to ensure safety. Each nitrogen injection pressure is less than the safety pressure limit of the riser to avoid balloon expansion or rupture due to excessive pressure.

[0085] 2. Dynamic data monitoring and early warning

[0086] Based on the real-time collected data, the monitoring system dynamically analyzes the changes in the airbag volume and the position of the liquid column. Once the airbag volume expands abnormally or the position of the liquid column changes abnormally, the system will automatically send out an early warning signal to remind the operator to take appropriate measures.

[0087] 3. Analysis of simultaneous multiplication of pressure and volume

[0088] When the second nitrogen injection is carried out, if the pressure and the volume of nitrogen injection are observed to increase by the same multiple as the first nitrogen injection, it indicates that the liquid column in the annulus has not been significantly pushed forward by the injection of nitrogen. In this case, the residual volume of the annulus remains relatively stable, which provides us with a basis for judgment that the gas-liquid distribution state in the annulus has not changed significantly.

[0089] 4. Liquid column propulsion identification and calculation volume evaluation

[0090] If during the second nitrogen injection process, even if the annulus has reached the set pressure level, the required nitrogen injection volume exceeds expectations, this abnormal phenomenon directly points to the liquid column moving forward under the action of nitrogen pressure. By comparing the difference between the second nitrogen injection volume conversion result and the first nitrogen injection conversion result, the volume occupied by the liquid column moving forward can be accurately calculated, which is of great significance for evaluating the remaining volume of the annulus.

[0091] 5. Optimization strategy within the safe pressure range

[0092] Under the premise of ensuring operational safety, gradually increasing the nitrogen injection pressure can effectively promote the further advancement of the liquid column in the annulus. This strategy not only helps us to have a more comprehensive understanding of the gas-liquid distribution in the annulus, but also minimizes the impact of the liquid column on the estimation of the annular residual volume, thereby providing a more accurate and reliable annular residual volume calculation result.

[0093] Implementation Effect

[0094] Through the implementation method of the present invention, the actual volume of the airbag in the annulus of the S-type marine flexible riser can be effectively detected, and the data of the liquid column advancement distance and the airbag distribution state can be obtained. The present invention improves the accuracy of airbag volume detection, is suitable for state monitoring and maintenance of deep-sea oil and gas transportation systems, and provides technical support for the safe operation and extended service life of the riser.

Claims

1. A method for detecting the volume of double liquid column end barrier airbags in the annulus of an S-type marine flexible riser, characterized in that: The following steps are involved: (1) Record the initial pressure of the flexible riser annulus through the detection device and ensure the sealing performance; (2) Perform nitrogen injection in three steps: For the first nitrogen injection, inject nitrogen to 0.75 barg and record the nitrogen volume and annular equilibrium pressure; The second nitrogen injection was to inject nitrogen to 1.5 barg, and the nitrogen volume and annular equilibrium pressure were recorded; The third nitrogen injection was carried out by injecting nitrogen to 2.0 barg, and the nitrogen volume and annular equilibrium pressure were recorded; (3) Based on the nitrogen injection volume under different pressure conditions and the thermodynamic gas state equation, the volume of the airbag is derived; (4) Calculate the difference in the three nitrogen injection volumes to determine the advancement distance of the liquid column in the annulus, thereby quantifying the volume of each gas pocket in the annulus.

2. The detection method according to claim 1, characterized in that: The use of the thermodynamic gas state equation in step (3) is based on the condition of maintaining a constant temperature in the annulus and ignoring the dissolution of nitrogen to ensure the accuracy of the calculation of the air bag volume.

3. The detection method according to claim 1 or 2, characterized in that: The detection system comprises a high-pressure gas cylinder, a pressure regulator A, a valve A, a gas flow meter, a pressure sensor, a temperature sensor and a flexible riser. The high-pressure gas cylinder is sealed and connected to the annulus inlet of the flexible riser through the pressure regulator and the valve.

4. The detection method according to claim 3, characterized in that: The gas flow meter is used to record the cumulative flow of nitrogen during each nitrogen injection process, the pressure sensor is used to record the annular space equilibrium pressure, and the temperature sensor is used to record the annular space temperature, so as to ensure the accuracy of the data during each nitrogen injection process.

5. The detection method according to claim 1 or 2, characterized in that: The abnormal changes of liquid column position and gas bag volume are identified by the synchronous doubling of pressure and volume during the three-step nitrogen injection process, thereby realizing dynamic monitoring of the gas-liquid distribution state in the annulus.

6. The detection method according to claim 1 or 2, characterized in that: If a significant difference in the volume and pressure of the nitrogen injection is observed during the second or third nitrogen injection process, the propulsion distance of the liquid column and the actual volume of each airbag can be determined by analyzing the nitrogen injection volume conversion results.

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