A pigging method for a long-distance gas-liquid mixed transportation submarine pipeline in ultra-deep water

Abstract

This invention provides a method for cleaning ultra-deepwater long-distance gas-liquid mixed transport subsea pipelines, belonging to the field of marine oil and gas engineering and flow safety assurance technology. The method includes the following steps: accurate data analysis and operational condition simulation; calculation of the minimum ethylene glycol injection volume required for each stage; prediction of the maximum slug flow rate and its impact on the platform separator and trap; determination of the specific duration, gas volume control range, and gas source switching node for each cleaning stage based on the simulation results, while developing an emergency plan; sequential execution of the four stages: production gas carrying liquid stage, recirculating gas purging and carrying liquid stage, recirculating gas cleaning stage, and production gas cleaning stage; and depressurization via a balancing valve, removal of the cleaning tool, and product analysis. This invention achieves controllable risks, reduced time, and minimized production loss in pipeline cleaning operations through a multi-stage gas source switching strategy, integrated real-time online simulation and early warning technology, and optimized slug flow control and ball launch / receiver procedures.

Description

Technical Field

[0001] This invention relates to the field of marine oil and gas engineering and flow safety assurance technology, and in particular to a method for cleaning subsea pipelines for ultra-deepwater long-distance gas-liquid mixed transportation. Background Technology

[0002] As offshore oil and gas resource development advances into deep and ultra-deep water, long-distance subsea pipelines have become crucial infrastructure for oil and gas transportation. During operation, these pipelines gradually accumulate condensate, formation water, paraffin wax, corrosion products, and construction residues. These accumulated liquids and sediments not only increase transport friction and reduce pipeline efficiency but can also induce localized corrosion, hydrate blockage, and slug flow impacts, seriously threatening the pipeline's structural integrity and flow safety. Therefore, regular pipeline cleaning is a necessary measure to maintain efficient and safe pipeline operation.

[0003] However, the ultra-deep water environment presents unprecedented challenges to pipeline cleaning operations: 1. High-pressure, low-temperature environment: Natural gas hydrates can easily form inside pipelines, causing blockages; 2. Complex flow patterns: Long-distance pipelines with large elevation differences are prone to severe slug flow during pipeline cleaning, which may exceed the instantaneous processing capacity of the platform's oil and gas processing system, leading to process shutdown; 3. High requirements for production continuity: Deepwater gas fields usually undertake important gas supply tasks, and pipeline cleaning operations must minimize the impact on the stable export of natural gas; 4. Lack of operational experience and data: There are no successful precedents in China for cleaning long-distance mixed-transport pipelines in ultra-deep water environments, and there is a lack of mature and reliable operational plans and risk control systems.

[0004] Traditional pipeline cleaning methods are mostly designed for pipelines of conventional depth or shorter lengths, often employing simple pneumatic propulsion or shutdown operation modes. These methods are unsuitable for the complex operating conditions and high-risk characteristics of ultra-deep, long-distance pipelines. Therefore, there is an urgent need for a systematic, controllable pipeline cleaning method that can balance safety and throughput. Summary of the Invention

[0005] In view of this, in order to overcome the shortcomings of the existing technology, the present invention aims to provide a safe, efficient, and minimally disruptive method for cleaning ultra-deepwater long-distance gas-liquid hybrid subsea pipelines. This method achieves controllable risks, reduced time, and minimized production losses in pipeline cleaning operations through innovative multi-stage gas source switching strategies, integrated real-time online simulation and early warning technologies, and optimized slug flow control and launch / receiver procedures.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows: a method for cleaning an ultra-deepwater long-distance gas-liquid mixed transport subsea pipeline, comprising the following steps: Step S1: Collect historical pipeline operation data, use PMS software to establish a transient model of pipeline cleaning operation, simulate various working conditions, calculate the minimum ethylene glycol injection volume required for each pipeline cleaning stage, and predict the maximum slug flow rate and its impact on the platform separator and trap. Step S2: Based on the results of the working condition simulation, determine the specific duration, gas volume control range and gas source switching node for each pigging stage, and formulate emergency diversion plans for slug flow exceeding limits, hydrate risk intervention plans and pigging jam emergency response plans. Step S3 involves sequentially executing four stages: production gas carrying liquid stage, circulating gas purging and carrying liquid stage, circulating gas cleaning stage, and production gas cleaning stage, to carry out the cleaning operation. The first step, the gas-liquid production stage: using the natural gas produced from the gas field's production wells as a power source, to conduct preliminary tracing of the target subsea pipeline; The second step, circulating gas purging and liquid carrying stage: shut down the single-zone production well and switch to the platform's self-circulating dry natural gas system as the gas source; The third step, the recirculating gas pigging stage: the pig is launched into the pipeline, and the recirculating gas from the second step is used as the propellant; in the later stage of the pigging operation, the slug flow is controlled in real time. The fourth step is the production gas pigging stage: When the pig runs to the second half of the pipeline or the simulation determines that the main slug flow has passed, the driving gas source will be switched back to production gas. The production gas will be used to continuously drive the pig to complete the remaining journey until it enters the platform's pig collection cylinder. Step S4: Depressurize by using the balance valve on the ball receiving cylinder, open the blind flange of the ball receiving cylinder to remove the pig and perform product analysis.

[0007] Furthermore, in step S3, during the production gas-liquid carrying stage, by controlling the well opening sequence and gas volume of the production wells, the gas flow is used to carry and remove 40%-60% of the free accumulated liquid in the pipeline. At the same time, this stage has begun to restore some pipeline transport capacity.

[0008] Furthermore, in step S3, during the circulating gas purging and liquid-carrying stage, the stable flow rate and low water content of the circulating gas are used to deeply purge the pipeline, remove residual liquid film and light impurities, reduce the overall liquid content of the pipeline, and reduce the amount of slug flow generated during the production gas cleaning stage.

[0009] Furthermore, in step S3, during the circulating gas cleaning stage, the pig removes deposits such as wax and scale adhering to the pipe wall, and balances the cleaning speed with the intensity of slug flow generation by controlling the amount of gas pushed into the pig.

[0010] The entire operation integrates and applies online simulation and monitoring technology for underwater production systems. In particular, before and during the operation, PMS software is used to perform multi-condition dynamic simulation of the entire pigging process to predict the pig's location, pipeline pressure / temperature field, liquid distribution, and slug flow pattern and arrival time. Based on the results of the working condition simulation, the gas flow rate, gas source switching timing, ethylene glycol injection rate, and production well operation strategy at each stage are optimized and adjusted in real time to achieve prediction and proactive control of the operation process.

[0011] Furthermore, in step S2, the emergency diversion plan for sluice-block flow exceeding limits includes: Simulation early warning and active adjustment: Based on the maximum expected inflow volume simulated by PMS, the liquid level control setting of the slug trap is increased in advance to improve the processing capacity of the slug trap; Emergency diversion treatment: When real-time monitoring indicates that the instantaneous inflow volume is about to exceed the production system's processing capacity, the incoming liquid stream is immediately switched to a closed-loop discharge system for temporary storage. After the cleaning operation is completed, the temporarily stored liquid is gradually pumped back to the processing system, thereby ensuring the stability of the main process flow. Speed ​​control: By reducing the amount of gas pushed into the pig, the operating speed of the pig is actively slowed down to smooth out the instantaneous peak of liquid inflow; The hydrate risk intervention plan includes: The PMS software's simulation system provides early warnings of hydrate formation risks, guiding operators to adjust the ethylene glycol injection volume in advance to prevent hydrate blockage.

[0012] Furthermore, before launching the pig, the launching tube is first slowly pressurized until the pressure inside the launching tube is balanced with the pressure in the subsea pipeline, and then the inlet and outlet valves of the launching tube are opened to launch the pig. Before the pig reaches the receiving drum, close the inlet valve of the receiving drum to 70%-80% to create a throttling effect; after confirming that the pig has entered the receiving drum, close the inlet valve of the receiving drum and confirm the isolation effect by operating the balance valve of the receiving drum.

[0013] Furthermore, before the launch operation, if the pressure difference between the launch tube and the subsea pipeline is less than 0.1 MPa, it is considered that the pressure in the launch tube and the subsea pipeline are in balance. During the return operation, the balance valve of the return tube is closed to depressurize the return tube. If the pressure in the return tube does not rise within 30 minutes, the isolation is confirmed to be effective.

[0014] Compared with existing technologies, the pipeline cleaning method for ultra-deepwater long-distance gas-liquid mixed transport subsea pipelines described in this invention has the following advantages: (1) Originality and breakthrough: This invention has successfully achieved the first pipeline cleaning operation of ultra-deep water long-distance mixed transport subsea pipeline in China, and formed a complete and replicable operation method and risk control system, filling the technological gap in this field in China; (2) Significant economic benefits: Through the "four-step" gas source optimization strategy and the whole process of time-precise management, the impact of pipeline cleaning operation on natural gas production is minimized. Compared with the traditional complete shutdown plan, the production loss can be reduced by more than 40%, ensuring the stable gas supply for downstream users. (3) Operation safety is controllable: The PMS online simulation realizes the "transparency" and "predictability" of the operation process. Combined with slug flow emergency treatment and hydrate risk early warning, the three core risks in the pipeline cleaning operation (slug impact, hydrate blockage, and system overpressure) are effectively controlled, and the operation safety is improved. (4) High success rate and high reliability: The optimized launch and reception program and the cleaning speed control strategy ensure the smooth launch, operation and reception of the cleaning device, and avoid operational failures such as stuck ball and broken ball. (5) High value for technology dissemination and promotion: The methodology, data package and experience formed by this invention can be directly applied to other ultra-deepwater pipelines, providing key technical support and practical basis for establishing China's deepwater oil and gas pipeline integrity management standards. Attached Figure Description

[0015] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 These are schematic diagrams illustrating the simulated processes under various operating conditions described in the embodiments of the present invention; Figure 2 Example graph of hydrate formation curve simulated by PMS software; Figure 3 A flowchart illustrating the safety control process for the entire operation; Figure 4 A schematic diagram simulating the real-time position of the pigging tube when inputting various parameters to the PMS. Figure 5 This is a schematic diagram illustrating the four-step operation process for switching gas sources and volumes. Figure 6 This is a schematic diagram illustrating the specific logic of the four-step process; Figure 7 Example diagram of emergency diversion process modification for sluice blockage flow; Figure 8 This is a schematic diagram of the emergency diversion process for sluice blockage flow. Detailed Implementation

[0016] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0017] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0018] like Figures 1-8 As shown, this invention relates to a method for cleaning long-distance gas-liquid mixed-transport subsea pipelines in ultra-deepwater conditions. This method is applicable to long-distance gas-liquid mixed-transport subsea pipelines in ultra-deepwater conditions (depth greater than or equal to 1500 meters) and includes the following steps: Step S1: Collect historical pipeline operation data, use PMS software to establish a transient model of pipeline cleaning operation, simulate various working conditions, calculate the minimum ethylene glycol injection volume required for each pipeline cleaning stage, and predict the maximum slug flow rate and its impact on the platform separator and trap. Step S2: Based on the results of the working condition simulation, determine the specific duration, gas volume control range and gas source switching node for each pigging stage, and formulate emergency diversion plans for slug flow exceeding limits, hydrate risk intervention plans and pigging jam emergency response plans. Step S3 involves sequentially executing four stages: production gas carrying liquid stage, circulating gas purging and carrying liquid stage, circulating gas cleaning stage, and production gas cleaning stage, forming the core "four-step" operation procedure for cleaning operations. The first step, the gas-liquid carrying stage: using the natural gas produced by the gas field's production wells as a power source, the target subsea pipeline is initially cleared; by controlling the well opening sequence and gas volume of the production wells, high-speed airflow is used to carry and remove most of the free liquid in the pipeline. Preferably, airflow is used to carry and remove 40%-60% of the free liquid in the pipeline, creating favorable conditions for subsequent pipeline cleaning operations. At the same time, this stage has begun to restore some pipeline transportation capacity. The second step is the circulating gas purging and liquid-carrying stage: the single-zone production well is shut down and the platform's self-circulating dry natural gas system is switched as the gas source; the pipeline is deeply purged using the stable flow rate and low water content of the circulating gas to further remove residual liquid film and light impurities, significantly reduce the overall liquid content of the pipeline, and reduce the amount of slug flow generated during the production gas cleaning stage.

[0019] The third step, the circulating gas cleaning stage: A pig (such as a foam pig ball) is launched into the pipeline, and the circulating gas from the second step continues to be used as the propellant. This stage is the core process of mechanical cleaning of the pipeline wall. In the later stages of the cleaning operation, the slug flow is controlled in real time. The pig removes deposits such as wax and scale adhering to the pipe wall. This stage balances the cleaning speed and the intensity of slug flow generation by precisely controlling the amount of gas used to push the pig ball. Specifically, when the simulation time exceeds 15% and the pig ball indicator at the underwater manifold has not yet activated, the amount of gas used to push the pig ball is increased, thus increasing the speed of the pig ball until the indicator activates. When the slug flow trap level reaches 90% of the high alarm level, the amount of gas used to push the pig ball is reduced, thus reducing the cleaning speed until the trap level returns to the normal setting value.

[0020] The fourth step, the production gas pigging stage: When the pig has traveled to the latter half of the pipeline or, based on simulation, the main slug flow has been cleared, the gas source will be switched back to production gas. Utilizing the continuous and stable flow field of the production gas, the pig will complete its remaining journey and ensure its smooth and complete entry into the receiving device (platform pig receiver). This switch helps to quickly restore normal pipeline operation using the production gas flow, shortening the operation cycle.

[0021] Step S4: Depressurize by using the balance valve on the ball receiving cylinder, open the blind flange of the ball receiving cylinder to remove the pig and perform product analysis.

[0022] The entire operation integrates and applies online simulation and monitoring technologies for underwater production systems, specifically including: (1) Before and during the operation, PMS software is used to perform multi-condition dynamic simulation of the entire pigging process to predict the pig position, pipeline pressure / temperature field, liquid distribution and slug flow pattern and arrival time. (2) Based on the results of the working condition simulation, the gas flow rate, gas source switching timing, ethylene glycol injection rate and production well operation strategy of each stage are optimized and adjusted in real time to achieve accurate prediction and active control of the operation process.

[0023] (3) The simulation system of PMS software provides early warning of hydrate formation risk, and guides operators to adjust the amount of ethylene glycol injected in advance to prevent hydrate blockage.

[0024] Furthermore, to address the slug flow generated by pipeline cleaning, a slug flow control technology combining process modification and emergency response is adopted, specifically including: (1) Pre-treatment before cleaning in step S1: Through the operation of the production gas-carrying liquid stage, the initial liquid accumulation in the pipeline is reduced to the maximum extent.

[0025] (2) Emergency diversion plan for slug flow exceeding limits in step S2: Simulation early warning and active adjustment: Based on the maximum expected inflow volume simulated by PMS, the liquid level control setting of the slug trap is increased in advance to improve the processing capacity of the slug trap; Emergency diversion treatment: When real-time monitoring indicates that the instantaneous inflow volume is about to exceed the production system's processing capacity, the incoming liquid stream is immediately switched to a closed-loop discharge system for temporary storage. After the cleaning operation is completed, the temporarily stored liquid is gradually pumped back to the processing system, thereby ensuring the stability of the main process flow. Speed ​​control: By reducing the amount of gas pushed into the pig, the operating speed of the pig is actively slowed down to smooth out the instantaneous peak of liquid inflow; Furthermore, the pig launching and receiving procedure was optimized to ensure operational reliability, specifically including: (1) Launching procedure (preparatory work before launching): Before launching the pig, the pressure between the launching tube and the main pipeline is slowly balanced through the balancing pipeline. After the pressure is basically the same, the main isolation valve is opened to prevent the pressure difference from causing damage to the pig or launch failure. Specifically, when the pig is put into the launching tube, the pressure inside the tube is 0 and the pressure in the subsea pipeline is 10 MPa. If the launching tube valve is opened directly, the pressure will rise too fast and the pressure difference will be too high, causing the pig to break. It is necessary to slowly pressurize the launching tube through the 2” balancing valve of the launching tube. After the pressure inside the launching tube is balanced with the pressure in the subsea pipeline, the inlet and outlet valves of the launching tube are opened to launch the pig. In actual application, when the pressure difference between the pressure inside the launching tube and the pressure in the subsea pipeline is less than 0.1 MPa, it is considered that the pressure inside the launching tube is balanced with the pressure in the subsea pipeline. (2) Pig retrieval procedure (in the final stage of pig production): Before the pig reaches the retrieval cylinder, close the retrieval cylinder inlet valve to 70%-80% to create a moderate throttling effect; after confirming that the pig has entered the retrieval cylinder, quickly close the retrieval cylinder inlet valve and confirm the isolation effect by operating the retrieval cylinder's balance valve. This method can effectively prevent the pig from rebounding or getting stuck at the retrieval cylinder inlet. During the pig retrieval operation, close the retrieval cylinder's balance valve to depressurize the retrieval cylinder; if the pressure inside the retrieval cylinder does not rise within 30 minutes, the isolation is confirmed to be effective.

[0026] (3) Emergency response plan for stuck pig: If the pressure difference between upstream and downstream of the pig increases to 10 Bar, it can be considered that the pig is stuck in the pipeline. The method to unblock it is to push the pig back into the launching tube directly. The operation of pushing the pig back into the launching tube is the existing technology and will not be described here.

[0027] The invention will be described in detail below using a subsea pipeline cleaning example from an ultra-deepwater gas field (taking the Lingshui 17-2 gas field as an example). This example involves a gas-liquid mixed transport pipeline approximately 58 kilometers long in the western section and approximately 42 kilometers long in the eastern section, with an operating water depth exceeding 1500 meters. The original design involved a complete shutdown for cleaning operations, with a single operation lasting 4 days and a normal daily production of 900 cubic meters. Using the four-step cleaning method, 2000 cubic meters of natural gas can be produced normally during the cleaning process, effectively avoiding a 44% production loss.

[0028] I. Preparation and Simulation Analysis Before the Operation 1. Data Preparation and Operational Simulation: Collect historical pipeline operation data and establish a transient model of the pipeline cleaning operation using PMS software. Simulate various operating conditions (different push speeds, different gas source combinations) and calculate the minimum ethylene glycol injection volume required for each stage to ensure hydrate suppression (see...). Figure 1 The simulation also predicted the maximum potential slug flow rate and its impact on the platform separator and trap. A specific example of the simulation results for the eastern region is shown in [link to simulation]. Figure 2 ).

[0029] 2. Scheme Formulation and Emergency Response Plan: Based on simulation results, determine the specific duration, gas volume control range, and gas source switching nodes for each stage of the "four-step" process (see gas source switching for details). Figure 5 At the same time, detailed emergency diversion plans for slug flow exceeding limits, hydrate risk intervention plans, and ball jamming emergency response plans were formulated (see full process flowchart). Figure 3 ).

[0030] II. Implementation of the "Four-Step" Cleaning Operation (See process) Figure 6 ) Step 1: Gas-Carrying Liquid Production (approximately 4 hours): Select a high-yield gas well and inject gas into the target pipeline at a relatively high flow rate (e.g., 80% of the design flow rate). Monitor the pressure drop along the pipeline in real time using a PMS to assess the effectiveness of liquid removal. This stage aims to remove most of the free liquid from the lower part of the pipeline.

[0031] 2. Second step: Circulating gas purging of liquid (approximately 2 hours): Smoothly switch to the platform circulating gas system to purge at a stable low to medium flow rate. During this stage, the pipeline is further dried, and the pressure curve tends to stabilize, indicating that the liquid volume in the pipeline has been significantly reduced.

[0032] 3. Third step: Recirculating gas pigging (main pigging stage, duration depends on pipeline length): Launch foam pigs, continuing to be propelled by recirculating gas. The PMS software runs online throughout the process, displaying the simulated position of the pigs in real time (see...). Figure 4 ), calculate the amount of liquid accumulation ahead and provide an early warning of the slug flow arrival time. Based on the warning, operators should lower the liquid level of the slug flow trap 15-30 minutes before the slug flow arrives to reserve processing capacity.

[0033] 4. Fourth Step: Production Gas Pigging (Final Stage, Until Pig Retrieval): Once the PMS simulation shows that the pig has passed the midpoint of the pipeline and the main sluice block has formed, the gas source is switched back to natural gas from one or more production wells. The production gas continues to propel the pig for the remaining stroke until it enters the platform's pig retrieval barrel. This switching achieves a seamless transition between pigging operations and the resumption of production.

[0034] III. Real-time control of slug flow (see system) Figure 8 ) During the later stages of the pigging operation (end of step three, beginning of step four), if the PMS issues a warning or the field instruments indicate that the trap level is rising too rapidly, immediately execute the following: Open the emergency diversion valve to divert some of the incoming liquid into the closed-loop tank, and then use a temporary transfer pump to transfer it to the sludge and oil tank for storage (see...). Figure 7 ).

[0035] At the same time, appropriately reduce the gas volume of the pig (for example, reduce it by 10%-20%) to reduce the speed of the pig from about 2 m / s-3 m / s to 1 m / s-2 m / s, so as to prolong the liquid arrival time and reduce the instantaneous intensity.

[0036] Once the liquid level in the trap returns to a safe range, stop the diversion and resume the normal processing procedure.

[0037] IV. Execution of serve and receive operations Launching procedure: After confirming that the pressure in the launching tube and the pressure in the subsea pipeline are balanced by the balancing valve (pressure difference < 0.1 MPa), quickly open the launching valve to complete the launch.

[0038] Pig retrieval operation: Before the expected arrival of the pig, close the hydraulic ball valve at the inlet of the pig retrieval cylinder to 70% opening. The pig smoothly enters the retrieval cylinder under throttling action. After confirming its arrival by sound detection and differential pressure, quickly close the inlet valve completely. Then, depressurize through the balancing valve, open the blind flange of the retrieval cylinder, remove the pig, and perform product analysis. Product analysis results show that the effluent includes accumulated liquid, trace amounts of wax, and corrosion products, providing a basis for subsequent corrosion protection.

[0039] Verification of assignment results: The method of this invention successfully completed this ultra-deepwater long-distance subsea pipeline cleaning operation. The cleaning sphere was recovered intact, and the cleaned-out materials included accumulated liquid, trace amounts of wax, and corrosion products, which, after analysis, provided a basis for subsequent corrosion prevention. Throughout the operation, the platform maintained continuous production, and natural gas output remained at a high level, with only a slight reduction due to optimized operations; the production loss was far less than that of traditional methods. No safety incidents such as hydrate blockage or equipment overpressure occurred during the entire process, verifying the safety, efficiency, and reliability of the method of this invention.

[0040] 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 cleaning a subsea pipeline carrying gas-liquid mixtures over long distances in ultra-deepwater water, characterized in that... Includes the following steps: Step S1: Collect historical pipeline operation data, use PMS software to establish a transient model of pipeline cleaning operation, simulate various working conditions, calculate the minimum ethylene glycol injection volume required for each pipeline cleaning stage, and predict the maximum slug flow rate and its impact on the platform separator and trap. Step S2: Based on the results of the working condition simulation, determine the specific duration, gas volume control range and gas source switching node for each pigging stage, and formulate emergency diversion plans for slug flow exceeding limits, hydrate risk intervention plans and pigging jam emergency response plans. Step S3 involves sequentially executing four stages: production gas carrying liquid stage, circulating gas purging and carrying liquid stage, circulating gas cleaning stage, and production gas cleaning stage, to carry out the cleaning operation. The first step, the gas-liquid production stage: using the natural gas produced from the gas field's production wells as a power source, to conduct preliminary tracing of the target subsea pipeline; The second step, circulating gas purging and liquid carrying stage: shut down the single-zone production well and switch to the platform's self-circulating dry natural gas system as the gas source; The third step, the recirculating gas pigging stage: the pig is launched into the pipeline, and the recirculating gas from the second step is used as the propellant; in the later stage of the pigging operation, the slug flow is controlled in real time. The fourth step is the production gas pigging stage: When the pig runs to the second half of the pipeline or the simulation determines that the main slug flow has passed, the driving gas source will be switched back to production gas. The production gas will be used to continuously drive the pig to complete the remaining journey until it enters the platform's pig collection cylinder. Step S4: Depressurize by using the balance valve on the ball receiving cylinder, open the blind flange of the ball receiving cylinder to remove the pig and perform product analysis.

2. The method for cleaning ultra-deepwater long-distance gas-liquid mixed transport subsea pipelines according to claim 1, characterized in that: In step S3, during the production gas-liquid carrying stage, by controlling the well opening sequence and gas volume of the production wells, the gas flow is used to carry and remove 40%-60% of the free accumulated liquid in the pipeline. At the same time, this stage has begun to restore some pipeline transport capacity.

3. The method for cleaning ultra-deepwater long-distance gas-liquid mixed transport subsea pipelines according to claim 1, characterized in that: In step S3, during the circulating gas purging and liquid-carrying stage, the stable flow rate and low water content of the circulating gas are used to deeply purge the pipeline, remove residual liquid film and light impurities, reduce the overall liquid content of the pipeline, and reduce the amount of slug flow generated during the production gas cleaning stage.

4. The method for cleaning ultra-deepwater long-distance gas-liquid mixed transport subsea pipelines according to claim 1, characterized in that: In step S3, during the circulating gas cleaning stage, the pig removes deposits such as wax and scale adhering to the pipe wall. By controlling the amount of gas pushed into the pig, the cleaning speed and the intensity of slug flow generation are balanced.

5. The method for cleaning ultra-deepwater long-distance gas-liquid mixed transport subsea pipelines according to claim 1, characterized in that: The entire operation integrates and applies online simulation and monitoring technology for underwater production systems. In particular, before and during the operation, PMS software is used to perform multi-condition dynamic simulation of the entire pigging process to predict the pig's location, pipeline pressure / temperature field, liquid distribution, and slug flow pattern and arrival time. Based on the results of the working condition simulation, the gas flow rate, gas source switching timing, ethylene glycol injection rate, and production well operation strategy at each stage are optimized and adjusted in real time to achieve prediction and proactive control of the operation process.

6. The method for cleaning ultra-deepwater long-distance gas-liquid mixed transport subsea pipelines according to claim 1, characterized in that, In step S2, the emergency diversion plan for sluice flow exceeding the limit includes: Simulation early warning and active adjustment: Based on the maximum expected inflow volume simulated by PMS, the liquid level control setting of the slug trap is increased in advance to improve the processing capacity of the slug trap; Emergency diversion treatment: When real-time monitoring indicates that the instantaneous inflow volume is about to exceed the production system's processing capacity, the incoming liquid stream is immediately switched to a closed-loop discharge system for temporary storage. After the cleaning operation is completed, the temporarily stored liquid is gradually pumped back to the processing system, thereby ensuring the stability of the main process flow. Speed ​​control: By reducing the amount of gas pushed into the pig, the operating speed of the pig is actively slowed down to smooth out the instantaneous peak of liquid inflow; The hydrate risk intervention plan includes: The PMS software's simulation system provides early warnings of hydrate formation risks, guiding operators to adjust the ethylene glycol injection volume in advance to prevent hydrate blockage.

7. The method for cleaning ultra-deepwater long-distance gas-liquid mixed transport subsea pipelines according to claim 4, characterized in that: Before launching the pig, the launching tube is first slowly pressurized. After the pressure inside the launching tube is balanced with the pressure in the subsea pipeline, the inlet and outlet valves of the launching tube are opened to launch the pig. Before the pig reaches the receiving drum, close the inlet valve of the receiving drum to 70%-80% to create a throttling effect; after confirming that the pig has entered the receiving drum, close the inlet valve of the receiving drum and confirm the isolation effect by operating the balance valve of the receiving drum.

8. The method for cleaning ultra-deepwater long-distance gas-liquid mixed transport subsea pipelines according to claim 7, characterized in that: Before the launch operation, if the pressure difference between the launch tube and the subsea pipeline is less than 0.1 MPa, it is considered that the pressure in the launch tube and the subsea pipeline are in balance. During the return operation, the balance valve of the return tube is closed to depressurize the return tube. If the pressure in the return tube does not rise within 30 minutes, the isolation is confirmed to be effective.

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