Subsea pipeline nearshore landfall shallow trenching method

Abstract

The application provides a kind of seabed pipeline nearshore landing section shallow water area post-ditching construction method, and belongs to the technical field of marine engineering construction. In view of the technical problems that the existing pre-ditching method and the conventional post-ditching method are difficult to implement in the shallow water area with a water depth of 2m to 6m in the nearshore landing section, the application uses the pipe-carrying barge retained in the construction site after the pipe-laying operation as a shallow post-ditching operation platform, establishes a four-point tension composite balance positioning system composed of two shore-side winches, a pipe-laying ship and an auxiliary tug, divides the post-ditching operation area into multiple construction sections, starts the cyclic operation from the first construction section, drags the pipe-carrying barge to the starting position of the current construction section through the four-point tension composite balance positioning system, lowers the post-ditching equipment through the hoisting equipment to perform the post-ditching operation, drags the pipe-carrying barge to the next construction section after completing the current construction section, and finally completes all the construction sections. The application is mainly used for the post-ditching burying construction of the seabed pipeline in the shallow water area of the nearshore landing section.

Description

Technical Field

[0001] This invention relates to the field of marine engineering technology. More specifically, this invention relates to a method for constructing a trench in shallow waters near the shore landing section of a subsea pipeline. Background Technology

[0002] As an important component of marine oil and gas resource development and transportation systems, subsea pipelines are typically buried below the seabed to ensure their structural stability during operation. This reduces the impact of external forces such as waves, currents, ship anchoring, and fishing gear towing on pipeline stability, thereby minimizing operational risks and extending pipeline service life.

[0003] In existing projects, the main methods for laying submarine pipelines include pre-dug trenches and post-dug trenches. Pre-dug trenches have a certain application basis in some shallow water areas with relatively simple construction conditions. However, in deep water areas or when the designed burial depth is large, the construction difficulty increases significantly, and the stability of the trench is difficult to guarantee. Especially in sandy or silty strata, collapses and rapid backfilling are prone to occur, requiring repeated trench clearing and resulting in low construction efficiency.

[0004] Post-dredging trenching refers to the process where the pipeline is first laid, and then trenching equipment is used to flush or cut the laid pipeline, gradually lowering it to the designed burial depth. This method offers good construction continuity and high operational efficiency, and is widely used in sea areas where water depth permits. In areas with greater water depth (usually greater than 6 m), where the draft and operating space of the construction vessels meet the requirements, DP positioning deep-water post-dredging vessels can be used in conjunction with conventional post-dredging equipment, resulting in relatively stable burial effects. However, in shallow water areas such as nearshore landing sections (usually less than 6 m), conventional post-dredging construction faces significant challenges due to water depth limitations. On the one hand, vessels that meet the draft requirements and conditions for subsea pipeline construction are scarce; on the other hand, shallow water areas are significantly affected by waves and currents, resulting in rapid siltation and making it difficult to guarantee construction quality.

[0005] At the same time, simply using pre-dug trenches in such shallow nearshore areas also has its shortcomings. When the water depth exceeds about 2 meters, as the water depth increases further, it becomes difficult to effectively excavate and control the shape of the trench due to limitations in construction equipment capabilities and operating conditions. In addition, under conditions of greater water depth, the excavated trenches are significantly affected by waves, currents, and other factors, making it difficult to guarantee the shape and long-term stability of the trench, resulting in rapid siltation and making it difficult to meet the pipeline design burial depth and long-term stability requirements.

[0006] Therefore, under the condition of nearshore landing section with water depth less than 6m, the existing pre-dug trench laying method and conventional post-dug trench laying method are difficult to effectively solve the pipeline laying construction problem. There is still a lack of a submarine pipeline laying construction scheme that can be applied to shallow nearshore landing section and take into account construction feasibility, laying depth and long-term stability. Summary of the Invention

[0007] One object of the present invention is to solve at least the above-mentioned problems and / or defects, and to provide at least the advantages described below.

[0008] One objective of this invention is to solve the problem that existing pre-dug trenching and conventional post-dug trenching methods are difficult to implement in shallow water near-shore conditions.

[0009] One object of the present invention is to provide a method for post-ditching construction in shallow water areas for nearshore landing sections of submarine pipelines, comprising the following steps: Step S1: Use the pipe transport barge that remains at the construction site after the pipe laying operation is completed as a shallow water trenching operation platform, and fix the hoisting equipment on the deck of the pipe transport barge. Step S2: Establish a four-point tension composite balance positioning system in the shallow water area of ​​the nearshore landing section. The system includes two shore-side anchor winches set on the shore side and a pipelaying vessel and an auxiliary tugboat located at sea. Each shore-side anchor winch is connected to the nearshore side of the pipe transport barge through a shore-side anchor cable. The pipelaying vessel and the auxiliary tugboat are connected to the sea side of the pipe transport barge through the anchor cables of the pipelaying vessel and the auxiliary tugboat, respectively. Step S3: Divide the post-ditching operation area along the seabed pipeline axis into multiple construction sections. Starting from the first construction section, perform the following operation process cyclically: The pipeline transport barge is towed to the starting position of the current construction section using two shore-side anchor winches, a pipelaying vessel, and auxiliary tugboats. While maintaining the barge in this position, the post-dredging equipment is lowered directly above the subsea pipeline using hoisting equipment. The post-dredging equipment is then towed along the subsea pipeline axis using the hoisting equipment to perform post-dredging operations for the current construction section. After completing the post-dredging operations for the current construction section, the pipeline transport barge is towed along the subsea pipeline axis to the starting position of the next construction section using two shore-side anchor winches, a pipelaying vessel, and auxiliary tugboats. This process is repeated until all construction sections are completed.

[0010] Preferably, in the method of post-ditching construction in the shallow water area of ​​the nearshore landing section of the submarine pipeline, the inclination angle of the anchor cable on the shore side is controlled between 20° and 35°.

[0011] Preferably, in the method for constructing a trench in the shallow water area of ​​the nearshore landing section of the submarine pipeline, the pipeline transport barge is equipped with a tension monitoring system. The tension monitoring system is used to monitor the tension of the two shore anchor cables, the pipelaying vessel anchor cable, and the auxiliary tugboat anchor cable in real time. When the tension of any of the two shore anchor cables, pipelaying vessel anchor cables, or auxiliary tugboat anchor cables exceeds the preset safety value, adjust the tension of the anchor cable exceeding the preset safety value until the tension of the anchor cable is no greater than the preset safety value.

[0012] Preferably, in the method of trenching construction in the shallow water area of ​​the nearshore landing section of the submarine pipeline, at least one of the two shore-side anchor winches is kept under tension.

[0013] Preferably, in the method for constructing a trench in the shallow water area of ​​the nearshore landing section of the submarine pipeline, the pipeline transport barge is equipped with a positioning and monitoring system. The positioning and monitoring system is used to monitor the position of the transport and management barge in real time, and to determine the longitudinal offset of the transport and management barge along the axis of the seabed pipeline and the lateral offset perpendicular to the axis of the seabed pipeline based on the monitored position. When the lateral offset exceeds ±0.5m or the longitudinal offset exceeds ±1.0m, the two shore-side anchor winches, the pipelaying vessel, and the auxiliary tugboats will work to reduce the lateral and longitudinal offsets until the lateral offset is no greater than ±0.5m and the longitudinal offset is no greater than ±1.0m.

[0014] Preferably, in the above-mentioned method for constructing a trench in the shallow water area of ​​the nearshore landing section of the submarine pipeline, the pipeline transport barge is equipped with an attitude monitoring system. The attitude monitoring system is used to monitor the heel angle of the transport and management barge in real time; When the heel angle exceeds 3°, the two shore-side anchor winches, the pipelaying vessel, and the auxiliary tugboat will work to restore the heel angle of the pipe transport barge to no more than 3°.

[0015] Preferably, in the method for constructing a trench in the shallow water area of ​​the nearshore landing section of the submarine pipeline, the hoisting equipment is equipped with a swing amplitude monitoring system. The swing amplitude monitoring system is used to monitor the swing amplitude of the hoisting equipment in real time; when the swing amplitude exceeds the preset safety angle, the trenching operation of the trenching equipment is stopped.

[0016] Preferably, in the method for constructing a shallow water trench for the nearshore landing section of the submarine pipeline, the pipeline transport barge is equipped with real-time wave and current velocity monitoring equipment; the real-time wave and current velocity monitoring equipment is used to monitor the significant wave height, peak period and near-bottom current velocity in real time. Construction shall be carried out under the following sea conditions: significant wave height not greater than 1.2m, peak period not greater than 6s, near-bottom current velocity not greater than 1.5m / s, and average wind speed not greater than 12m / s. When any of the parameters of significant wave height, peak period, near-bottom current velocity, or average wind speed does not meet the sea state conditions, the post-dredging operation of the post-dredging equipment shall be stopped, and the pipe transport barge shall be maintained in its current position by two shore-side anchor winches, a pipe-laying vessel, and auxiliary tugboats.

[0017] Preferably, in the method for constructing a trench in the shallow water area of ​​the nearshore landing section of the submarine pipeline, the water depth in the shallow water area of ​​the nearshore landing section is 2m-6m.

[0018] Preferably, in the method for constructing a shallow water trench for the nearshore landing section of a submarine pipeline, the movement of the pipeline transport barge is controlled by the following means during the process of towing it to the starting position of the current construction section: When moving towards the sea side, the pipelaying vessel and the auxiliary tugboat tighten their respective anchor cables at the first cable-reeling speed, and the two shore-side anchor winches release their respective anchor cables at the second cable-laying speed. The ratio of the first cable-reeling speed to the second cable-laying speed is adjusted in real time according to the current moving speed of the pipe-carrying barge, so that the pipe-carrying barge moves at a constant speed along the axis of the seabed pipeline. As the vessel moves toward the shore, the two shore-side anchor winches tighten their respective anchor cables at a third retrieval speed, while the pipe-laying vessel and auxiliary tugboat release their respective anchor cables at a fourth release speed. The ratio of the third retrieval speed to the fourth release speed is adjusted in real time according to the current moving speed of the pipe transport barge, so that the pipe transport barge moves at a constant speed along the axis of the seabed pipeline.

[0019] The present invention has at least the following beneficial effects: The present invention provides a method for post-ditching construction of nearshore landing sections of submarine pipelines in shallow water areas, which solves the problem that existing pre-ditching and conventional post-ditching methods are difficult to implement in shallow nearshore conditions. This method is suitable for shallow water areas with a water depth of 2-6m. It can utilize existing vessels during the pipeline laying period to complete the trenching operation without the need to dispatch additional construction vessels, thereby achieving effective pipeline laying in shallow nearshore landing sections, taking into account construction feasibility, burial depth requirements, and long-term pipeline stability.

[0020] Other advantages, objectives and features of the present invention will become apparent in part from the following description, and in part from those skilled in the art through study and practice of the invention. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the plan layout of the shallow water area post-ditching system provided by the present invention.

[0022] Figure 2 This is an elevation view of the shallow water area post-ditching system provided by the present invention.

[0023] Attached reference numerals: 1. Pipeline transport barge; 2. Lifting equipment; 3. Post-dredging equipment; 4. Umbilical cable; 5. Towing rope; 6. Subsea pipeline; 7. Seabed; 8. Sea level; 9, 10. Shore-side anchor winch; 11. Pipeline laying vessel; 12. Auxiliary tugboat; 13, 14. Shore-side anchor cable; 15. Pipeline laying vessel anchor cable; 16. Auxiliary tugboat anchor cable; 17. Land pipeline Detailed Implementation

[0024] The present invention will now be described in further detail with reference to the accompanying drawings, so that those skilled in the art can implement it based on the description.

[0025] like Figures 1 to 2 As shown, this invention provides a method for post-ditch excavation construction in shallow water areas for nearshore landing sections of submarine pipelines, comprising the following steps: Step S1: Use the pipe transport barge 1, which remains at the construction site after the pipe laying operation is completed, as a shallow water trenching operation platform, and fix the hoisting equipment 2 on the deck of the pipe transport barge; Step S2: Establish a four-point tension composite balance positioning system in the shallow water area of ​​the nearshore landing section. The system includes two shore-side anchor winches set on the shore side and a pipelaying vessel 11 and an auxiliary tugboat 12 located at sea. Each shore-side anchor winch is connected to the nearshore side of the pipe transport barge through a shore-side anchor cable. The pipelaying vessel and the auxiliary tugboat are connected to the sea side of the pipe transport barge through the pipelaying vessel anchor cable 15 and the auxiliary tugboat anchor cable 16, respectively. Step S3: Divide the post-ditching operation area along the seabed pipeline axis into multiple construction sections. Starting from the first construction section, perform the following operation process cyclically: The pipeline transport barge is towed to the starting position of the current construction section using two shore-side anchor winches, a pipelaying vessel, and auxiliary tugboats. While maintaining the barge in this position, the post-dredging equipment is lowered directly above the subsea pipeline using hoisting equipment. The post-dredging equipment is then towed along the subsea pipeline axis using the hoisting equipment to perform post-dredging operations for the current construction section. After completing the post-dredging operations for the current construction section, the pipeline transport barge is towed along the subsea pipeline axis to the starting position of the next construction section using two shore-side anchor winches, a pipelaying vessel, and auxiliary tugboats. This process is repeated until all construction sections are completed.

[0026] The following detailed description of the post-ditching construction method for the nearshore landing section of the submarine pipeline according to the present invention is provided in conjunction with specific embodiments.

[0027] Figure 1 In the diagram, the pipeline transport barge moves from the sea towards the land. To indicate that the pipeline transport barge is moving along the pipeline axis, the shore-side anchor cables 13 and 14 of the pipeline transport barge on the sea side, as well as the anchor cable 15 of the pipelaying vessel and the anchor cable 16 of the auxiliary tugboat, are represented by dashed lines, while the shore-side anchor cables 13 and 14 of the pipeline transport barge, as well as the anchor cable 15 of the pipelaying vessel and the anchor cable 16 of the auxiliary tugboat, are represented by solid lines.

[0028] This embodiment takes a near-shore landing section of a submarine pipeline as an example. The water depth ranges from 2m to 6m in this section. The pipeline axis direction has been determined, and the pipeline has been laid and towed ashore.

[0029] First, the pipeline transport barge remaining at the construction site after pipelaying is completed will be used as a platform for shallow-water post-dredging operations. This barge was used to transport the pipeline during the pipelaying phase and will remain at the construction site after the pipelaying work is finished. Lifting equipment will be fixed to the deck of the pipeline transport barge for the subsequent lowering and retrieval of post-dredging equipment.

[0030] Secondly, a four-point tension-balanced positioning system was established in the shallow waters near the shore landing section. This system consists of two shore-side anchor winches, a pipelaying vessel, and an auxiliary tugboat located at sea. Each shore-side anchor winch is connected to the near-shore side of the pipe transport barge via a shore-side anchor cable. The pipelaying vessel is connected to the sea side of the pipe transport barge via its anchor cable, and the auxiliary tugboat is connected to the sea side of the pipe transport barge via its anchor cable. This four-point positioning system enables the positioning and movement control of the pipe transport barge.

[0031] Then, the post-dredging operation area is divided into multiple construction sections along the axis of the seabed pipeline. Starting from the first construction section, the following operation process is repeated cyclically: the pipe transport barge is towed to the starting position of the current construction section using two shore-side anchor winches, a pipe-laying vessel, and auxiliary tugboats. Specifically, the tension of each anchor cable is adjusted to move the pipe transport barge along the pipeline axis to the predetermined position, and the tension of each anchor cable is maintained to keep the pipe transport barge in its current position. Subsequently, the post-dredging equipment is lowered to directly above the seabed pipeline using hoisting equipment. Next, the post-dredging equipment is towed along the axis of the seabed pipeline using hoisting equipment. The post-dredging equipment flushes or cuts the laid pipeline, gradually lowering the pipeline to the designed burial depth, completing the post-dredging operation of the current construction section. The post-dredging equipment is towed by hoisting equipment to maintain it directly above the seabed pipeline, at a distance of approximately 1 meter. During the post-ditching operation, the post-ditching equipment sprays high-pressure water from above both sides of the pipeline to wash away the soil on both sides of the pipeline, forming a trench. The pipeline then sinks into the trench by gravity, thus completing the post-ditching and laying of the pipeline.

[0032] In the four-point tension composite balance positioning system, the movement and positioning of the transport barge are achieved by adjusting the tension of each anchor cable. The detailed process is as follows: When the pipeline transport barge needs to be moved to a predetermined position along the pipeline axis, the coordinated deployment and retraction of the four sets of anchor cables are controlled according to the direction of movement. If the transport barge needs to move towards the far shore, i.e., towards the location of the pipelaying vessel and auxiliary tugboat, the pipelaying vessel and auxiliary tugboat increase the tension of their respective anchor cables by tightening them, while the two shore-side anchor winches reduce the tension of their respective anchor cables by appropriately loosening them. This allows the transport barge to move smoothly towards the far shore along the pipeline axis under the condition that the tension of the anchor cables on the sea side is greater than that on the shore side. If the transport barge needs to move towards the near shore, i.e., towards the location of the shore-side anchor winches, the two shore-side anchor winches increase the tension of their respective anchor cables by tightening them, while the pipelaying vessel and auxiliary tugboat reduce the tension of their respective anchor cables by appropriately loosening them. This allows the transport barge to move smoothly towards the near shore along the pipeline axis under the condition that the tension of the anchor cables on the shore side is greater than that on the sea side.

[0033] During the aforementioned movement, the tension monitoring system collects the tension values ​​of each anchor cable in real time. When the difference between the tension of the anchor cable on the sea side and the tension of the anchor cable on the shore side reaches the preset movement tension difference, the current retrieval speed is maintained. When the tension difference is greater than the preset movement tension difference, the retrieval speed of the tightening anchor cable is reduced or the retrieval speed of the loosening anchor cable is increased, so that the tension difference falls back to the preset movement tension difference range. When the tension difference is less than the preset movement tension difference, the retrieval speed of the tightening anchor cable is increased or the retrieval speed of the loosening anchor cable is decreased, so that the tension difference rises to the preset movement tension difference range. The movement speed is adjusted by controlling the retrieval speed of the anchor cables to ensure smooth movement of the transport barge and avoid instantaneous acceleration caused by sudden changes in tension. At the same time, the tension monitoring system ensures that the tension of each anchor cable does not exceed the preset safety value. When the tension of any anchor cable reaches the preset safety value, further tightening of that anchor cable is stopped.

[0034] Once the management barge has moved to its designated position, maintaining this position requires adjusting the tension of each anchor cable to achieve a dynamic equilibrium. The tension monitoring system collects real-time tension values ​​for each anchor cable. When the difference between the sum of the tensions of the two shore-side anchor cables and the sum of the tensions of the pipelaying vessel's anchor cables and the auxiliary tugboat's anchor cables exceeds a preset equilibrium tension difference, the tightening speed of the anchor cable with lower tension is increased, or the tightening speed of the anchor cable with higher tension is decreased, until the tension difference is no greater than the preset equilibrium tension difference. In this state, the four-directional tensions on the management barge cancel each other out, keeping the barge stationary. When external environmental forces, such as waves or currents, act on the management barge, the tension of each anchor cable automatically undergoes slight changes to resist these forces. Upon detecting these changes, the tension monitoring system fine-tunes the tightening and loosening speeds of each anchor cable to restore their tension to equilibrium. Due to the catenary buffering effect of the anchor cables, the tension change is gradual, maintaining the position of the management barge.

[0035] After completing the trenching work for the current construction section, the pipe-laying barge is towed along the submarine pipeline axis to the starting position of the next construction section using two shore-side anchor winches, a pipe-laying vessel, and auxiliary tugboats. This process is repeated until all construction sections are completed.

[0036] Through the above steps, without the need to add a new dedicated shallow water construction vessel, the post-ditching operation in shallow water areas with a depth of 2m to 6m was completed using existing pipe-laying resources.

[0037] The difference between this invention and the prior art is that: First, this invention utilizes the pipe-carrying barge remaining at the construction site after pipe-laying operations as a shallow-water trenching operation platform, and fixes lifting equipment on the barge. Existing technologies require the deployment of dedicated shallow-water construction vessel teams or reliance on dynamically positioned vessels for operations, while this invention achieves the reuse of existing pipe-laying resources without adding new dedicated vessel teams.

[0038] Secondly, this invention establishes a four-point tension composite balance positioning system in the shallow waters of the near-shore landing section. This system consists of two anchor winches on the shore, a pipelaying vessel, and an auxiliary tugboat, all connected to the pipe transport barge via anchor cables, forming a multi-directional tension balance. Existing technologies struggle to utilize dynamic positioning systems in shallow waters and lack effective positioning alternatives. This invention, however, solves the problem of stable positioning for operating platforms in shallow waters through a four-point tension system.

[0039] Third, this invention divides the post-ditching operation area along the seabed pipeline axis into multiple construction sections. Through a segmented cyclical operation method, a four-point tension system coordinates the movement of the pipeline transport barge to the starting position of each construction section, completing the trenching operation. Existing technologies lack effective control methods for segmented cyclical operations in shallow water areas, while this invention achieves controlled movement of the pipeline transport barge and segmented operations through the coordinated release and retraction of the four-point tension system.

[0040] Based on the above differences, the present invention can complete trenching operations in shallow water areas with a depth of 2m to 6m without the need for additional special vessel groups, reducing the construction preparation period, avoiding multiple re-anchoring, and reducing project costs.

[0041] In a preferred embodiment, in the method for constructing a shallow water trench for the nearshore landing section of a submarine pipeline, the inclination angle of the anchor cable on the shore side is controlled between 20° and 35°.

[0042] This embodiment provides a detailed description of the anchor cable inclination angle control in the shallow water area trenching construction method for nearshore landing section of the submarine pipeline of the present invention.

[0043] In this embodiment, a shore-side winch is installed on the shore side, and its anchor cable is led out from the shore-side winch and enters the water, connecting to the near-shore side of the transport barge. After entering the water, the anchor cable forms a catenary shape in the seawater. By controlling the length of the anchor cable, the angle of entry into the water is controlled between 20° and 35°.

[0044] When the anchor cable's entry angle into the water is between 20° and 35°, the anchor cable forms a sufficient catenary buffer structure in the seawater. This catenary structure utilizes the anchor cable's own weight and water resistance to absorb the instantaneous impact of waves and currents on the transport barge, keeping the transport barge stable under dynamic sea conditions.

[0045] Specifically, when waves act on the management barge, the barge experiences a displacement tendency, causing a change in the catenary shape of the anchor cable and consequently, a change in the cable tension. Because the anchor cable has sufficient length and an appropriate angle of entry into the water, the tension change is gradual, avoiding instantaneous tension peaks. Simultaneously, the catenary shape of the anchor cable allows it to absorb some energy through shape changes when under tension, reducing the impact force transmitted to the shore-side anchor winch.

[0046] By controlling the anchor cable's inclination angle in the water between 20° and 35°, this invention achieves stable positioning of the transport barge in shallow waters without using a dynamic positioning system, by utilizing the catenary buffering characteristics of the anchor cable itself.

[0047] In a preferred embodiment, in the method for constructing a shallow water trench for the nearshore landing section of a submarine pipeline, the pipeline transport barge is equipped with a tension monitoring system. The tension monitoring system is used to monitor the tension of the two shore-side anchor cables, the pipe-laying vessel anchor cable, and the auxiliary tugboat anchor cable in real time. When the tension of any of the two shore-side anchor cables, the pipe-laying vessel anchor cable, or the auxiliary tugboat anchor cable exceeds a preset safety value, the tension of the anchor cable exceeding the preset safety value is adjusted until the tension of the anchor cable is no greater than the preset safety value.

[0048] This embodiment provides a detailed description of the tension monitoring system in the post-ditching construction method for the nearshore landing section of a submarine pipeline according to the present invention.

[0049] In this embodiment, the transport and management barge is equipped with a tension monitoring system. This tension monitoring system is connected to each anchor cable and is used to monitor the tension values ​​of the two shore-side anchor cables, the pipelaying vessel anchor cable, and the auxiliary tugboat anchor cable in real time. The tension monitoring system consists of tension sensors, a data acquisition unit, and a display unit. The tension sensors are installed at the anchoring points of each anchor cable and can continuously collect the tension data of each anchor cable.

[0050] During the positioning and movement of the transport barge, the tension monitoring system displays the tension values ​​of each anchor cable in real time. Operators monitor the tension of each anchor cable according to preset safety values. When the tension monitoring system shows that the tension of any of the two shore-side anchor cables, the pipelaying vessel anchor cable, or the auxiliary tugboat anchor cable exceeds the preset safety value, the operator adjusts the tension of the anchor cable exceeding the preset safety value.

[0051] The adjustment method is as follows: If the tension of a certain shore-side anchor cable exceeds the preset safety value, the corresponding shore-side anchor winch will be controlled to appropriately release the anchor cable, reducing the tension until the tension of the anchor cable does not exceed the preset safety value. If the tension of the anchor cable of the pipelaying vessel or the auxiliary tugboat exceeds the preset safety value, the pipelaying vessel or the auxiliary tugboat will be controlled to appropriately release the corresponding anchor cable, reducing the tension until the tension of the anchor cable does not exceed the preset safety value.

[0052] During the adjustment process, the tension of other anchor cables remained stable to ensure that the positioning of the transport barge was not affected. Through this method, the tension of each anchor cable was consistently maintained within the preset safety value, avoiding safety risks such as anchor cable breakage or barge instability caused by excessive tension.

[0053] In a preferred embodiment, in the method for constructing a shallow water trench for the nearshore landing section of a submarine pipeline, at least one of the two shore-side anchor winches is kept under tension.

[0054] This embodiment provides a detailed description of the control of the stress state of the shore-side anchor cable in the shallow water area post-ditching construction method for the nearshore landing section of the submarine pipeline of the present invention.

[0055] In this embodiment, each shore-side anchor winch is connected to the near-shore side of the pipeline transport barge via a shore-side anchor cable. As the pipeline transport barge moves along the pipeline axis, the actions of the two shore-side anchor winches are controlled to keep the shore-side anchor cable of at least one shore-side anchor winch under tension.

[0056] Specifically, when the pipeline transport barge needs to move from the current construction section to the next, the two shore-side anchor winches, the pipelaying vessel, and the auxiliary tugboat work together to adjust the tension of the anchor cables, allowing the pipeline transport barge to move along the pipeline axis. During the movement, at least one of the two shore-side anchor winches maintains its shore-side anchor cable under tension.

[0057] If the movement is from the nearshore side to the sea side, at least one of the two shore-side anchor winches will tighten its shore-side anchor cable to maintain it under tension, while the pipelaying vessel and auxiliary tugboat will move in coordination by raising and lowering their respective anchor cables. If the movement is from the sea side to the nearshore side, at least one of the two shore-side anchor winches will release its shore-side anchor cable, but will still maintain it under appropriate tension to prevent the pipelaying barge from shifting momentarily due to inertia.

[0058] By maintaining the anchor cable of at least one shore-side anchor winch under tension, the pipeline transport barge is constantly restrained by the shore during its movement, preventing it from experiencing sudden acceleration or loss of control due to loss of shore-side restraint. Simultaneously, this restraint is coordinated with the traction forces applied by the pipelaying vessel and auxiliary tugboats, ensuring a smooth and continuous movement of the pipeline transport barge.

[0059] In a preferred embodiment, in the method for constructing a shallow water trench for the nearshore landing section of the subsea pipeline, the pipeline transport barge is equipped with a positioning monitoring system. The positioning monitoring system is used to monitor the position of the pipeline transport barge in real time, and determine the longitudinal offset of the pipeline transport barge along the axis of the subsea pipeline and the lateral offset perpendicular to the axis of the subsea pipeline based on the monitored position. When the lateral offset exceeds ±0.5m or the longitudinal offset exceeds ±1.0m, the two shore-side anchor winches, the pipelaying vessel, and the auxiliary tugboat work to reduce the lateral and longitudinal offsets until the lateral offset is no greater than ±0.5m and the longitudinal offset is no greater than ±1.0m.

[0060] This embodiment provides a detailed description of the positioning monitoring system and offset control in the post-ditching construction method for the nearshore landing section of the submarine pipeline of the present invention.

[0061] In this embodiment, the transport and management barge is equipped with a positioning monitoring system. This system uses high-precision positioning equipment to monitor the barge's position coordinates in real time. Based on the monitored position coordinates, the system determines the longitudinal offset of the barge along the seabed pipeline axis and the lateral offset perpendicular to the seabed pipeline axis.

[0062] Specifically, firstly, a baseline for the subsea pipeline axis is determined, and the theoretical position of the pipeline transport barge is set as the baseline position. The positioning and monitoring system continuously collects the actual position coordinates of the pipeline transport barge and calculates the difference between the actual position and the baseline position. The difference along the subsea pipeline axis is recorded as the longitudinal offset, and the difference perpendicular to the subsea pipeline axis is recorded as the lateral offset.

[0063] When the positioning monitoring system shows a lateral offset exceeding ±0.5m or a longitudinal offset exceeding ±1.0m, offset adjustment is initiated. Operators control the movements of the two shore-side anchor winches, the pipelaying vessel, and auxiliary tugboats, adjusting the tension of each anchor cable to move the pipe transport barge, thereby reducing the lateral and longitudinal offsets.

[0064] The adjustment method is as follows: If the lateral offset exceeds ±0.5m, tighten the anchor cable on the opposite side of the offset direction and loosen the anchor cable in the offset direction appropriately, so that the pipeline transport barge moves laterally towards the reference position until the lateral offset is no greater than ±0.5m. If the longitudinal offset exceeds ±1.0m, coordinate the actions of the shore-side anchor winch with the pipelaying vessel and auxiliary tugboats, tightening or loosening the corresponding anchor cables to move the pipeline transport barge along the pipeline axis until the longitudinal offset is no greater than ±1.0m.

[0065] Through the above methods, the lateral offset of the transport barge is always controlled within ±0.5m, and the longitudinal offset is always controlled within ±1.0m, ensuring the alignment accuracy between the post-dredging equipment and the subsea pipeline.

[0066] In a preferred embodiment, in the method for constructing a shallow water trench for the nearshore landing section of a submarine pipeline, the pipeline transport barge is equipped with an attitude monitoring system. The attitude monitoring system is used to monitor the heel angle of the pipeline transport barge in real time. When the heel angle exceeds 3°, two shore-side anchor winches, the pipe-laying vessel, and auxiliary tugboats work to restore the heel angle of the pipeline transport barge to no more than 3°.

[0067] This embodiment provides a detailed description of the attitude monitoring system and tilt angle control in the shallow water area trenching construction method for nearshore landing section of the submarine pipeline of the present invention.

[0068] In this embodiment, the transport and management barge is equipped with an attitude monitoring system. This system is used to monitor the heel angle of the transport and management barge in real time. The heel angle refers to the angle of inclination of the transport and management barge in the direction perpendicular to the pipeline axis, reflecting the lateral stability of the barge.

[0069] The attitude monitoring system consists of a tilt sensor and a data acquisition unit. The tilt sensor is installed on the deck of the transport and management barge and can continuously collect the barge's heel angle data. The data acquisition unit transmits the collected heel angle data to the control console in real time for operators to monitor.

[0070] During the positioning and post-ditching operations of the transport and management barge, the attitude monitoring system continuously displays the barge's heel angle. When the attitude monitoring system shows a heel angle exceeding 3°, attitude adjustment is initiated. Operators control the movements of the two shore-side anchor winches, the pipelaying vessel, and auxiliary tugboats to adjust the tension of the anchor cables, thereby reducing the barge's heel angle.

[0071] The adjustment method is as follows: If the transport barge tilts to one side with a heel angle exceeding 3°, the anchor cable on the opposite side of the tilt direction should be tightened appropriately, while the anchor cable on the tilt direction should be loosened appropriately. By adjusting the tension and distribution of each anchor cable, the transport barge will be restored to a level attitude. During the adjustment process, the operator continuously observes the heel angle data from the attitude monitoring system until the heel angle is restored to no more than 3°.

[0072] Through the above methods, the heel angle of the pipeline transport barge is always controlled within 3°, ensuring the lateral stability of the barge during operation and avoiding safety risks such as the swinging of lifting equipment and the deviation of the trenching equipment from the pipeline axis due to excessive heel angle.

[0073] In a preferred embodiment, in the method for constructing a trench in the shallow water area of ​​the nearshore landing section of the submarine pipeline, the hoisting equipment is equipped with a swing amplitude monitoring system; the swing amplitude monitoring system is used to monitor the swing amplitude of the hoisting equipment in real time; when the swing amplitude exceeds a preset safety angle, the trenching operation of the trenching equipment is stopped.

[0074] This embodiment provides a detailed description of the swing amplitude monitoring system and the swing amplitude control of the hoisting equipment in the post-ditching construction method for the nearshore landing section of the submarine pipeline of the present invention.

[0075] In this embodiment, the hoisting equipment is equipped with a swing amplitude monitoring system. This system is used to monitor the swing amplitude of the hoisting equipment in real time. Swing amplitude refers to the horizontal swing range of the hoisting equipment's sling, usually measured by the swing angle of the sling relative to the vertical direction. When the swing amplitude is too large, it is difficult to accurately center the trenching equipment lowered by the hoisting equipment, and there is a risk of equipment collision or sling breakage.

[0076] The swing angle monitoring system consists of an angle sensor and a data acquisition unit. The angle sensor is installed at the boom or sling connection of the lifting equipment and continuously collects data on the swing angle of the sling. The data acquisition unit transmits the collected swing angle data to the control console in real time for operator monitoring. The preset safety angle is pre-set according to the rated parameters of the lifting equipment and operational requirements.

[0077] During the lowering of the trenching equipment by the hoisting equipment and the advancement of the trenching equipment, the sway monitoring system continuously displays the sway of the hoisting equipment. When the sway monitoring system shows that the sway exceeds the preset safety angle, the trenching operation of the trenching equipment is immediately stopped.

[0078] Specifically, when the swing amplitude exceeds the preset safety angle, the control console issues a stop command, the trenching equipment stops advancing or spraying operations, and the hoisting equipment remains in its current state, ceasing any lowering or retrieval operations. Trenching operations can only resume after the swing amplitude has decreased to within the preset safety angle.

[0079] By using the above methods, the swing amplitude of the hoisting equipment is always controlled within the preset safety angle, avoiding safety risks such as the trenching equipment deviating from the pipeline axis, equipment collision, or sling breakage caused by excessive swing amplitude.

[0080] In a preferred embodiment, in the method for constructing a shallow-water trench for the nearshore landing section of a submarine pipeline, the pipeline transport barge is equipped with real-time wave and current velocity monitoring equipment. This equipment is used to monitor the significant wave height, peak period, and near-bottom current velocity in real time. The construction method is carried out under the following sea conditions: significant wave height not exceeding 1.2m, peak period not exceeding 6s, near-bottom current velocity not exceeding 1.5m / s, and average wind speed not exceeding 12m / s. When any of the parameters—significant wave height, peak period, near-bottom current velocity, or average wind speed—does not meet the sea conditions, the trenching operation is stopped, and the pipeline transport barge is maintained in its current position using two shore-side anchor winches, a pipelaying vessel, and auxiliary tugboats.

[0081] This embodiment provides a detailed description of sea condition monitoring and operation window control in the shallow water area trenching construction method for nearshore landing section of the submarine pipeline of the present invention.

[0082] In this embodiment, the transport barge is equipped with real-time wave and current velocity monitoring equipment. This equipment consists of wave sensors and current velocity sensors. The wave sensors monitor the significant wave height and peak period in real time, while the current velocity sensors monitor the near-bottom current velocity in real time. The monitoring equipment transmits the collected data to a control console in real time for operator monitoring.

[0083] The construction method shall be carried out under the following sea conditions: significant wave height not exceeding 1.2m, peak period not exceeding 6s, near-bottom current velocity not exceeding 1.5m / s, and average wind speed not exceeding 12m / s. The above parameters constitute the operating window conditions, and post-ditching operations can only be carried out when all parameters meet the conditions.

[0084] During the operation, wave and current velocity monitoring equipment continuously monitors significant wave height, peak period, and near-bottom current velocity. Simultaneously, average wind speed data is acquired through meteorological monitoring equipment. Operators then compare the monitored data with the conditions within the operational window.

[0085] If any of the parameters—significant wave height, peak period, near-bottom current velocity, or average wind speed—does not meet the operating window conditions, the post-dredging operation of the post-dredging equipment shall be immediately stopped. Simultaneously, the pipe-laying barge shall be maintained in its current position using two shore-side anchor winches, the pipe-laying vessel, and auxiliary tugboats. Specifically, the tension of each anchor cable shall be adjusted to keep the pipe-laying barge in a fixed position, preventing any movement or operational operations.

[0086] Post-ditching operations can only resume after all monitored parameters meet the operating window conditions again. During the suspension, the transport and management barge remains in its positioning state, and the post-ditching equipment remains at the current depth and will not advance further.

[0087] By employing the above methods, post-ditching operations are always carried out under sea conditions that meet the operational window requirements, thus avoiding the safety risks caused by forcing operations in adverse sea conditions.

[0088] In a preferred embodiment, the method for constructing a trench in the shallow water area of ​​the nearshore landing section of the submarine pipeline involves excavating a trench after the pipeline has landed. The water depth in the shallow water area of ​​the nearshore landing section is 2m-6m.

[0089] Specifically, when the water depth is less than 2m, it is difficult for pipeline transport barges to enter the area for operation, and pre-ditching is used in this area. When the water depth is greater than 6m, the draft conditions of conventional post-ditching vessels can meet the operational requirements, and conventional post-ditching is used in this area. When the water depth is between 2m and 6m, neither conventional post-ditching nor pre-ditching is feasible, therefore the shallow-water post-ditching construction method of this invention is adopted.

[0090] Within a water depth range of 2m to 6m, the draft of the pipeline transport barge is sufficient to meet the requirements for shallow water operations. The pipeline transport barge is positioned and moved using a four-point tension composite balance positioning system. The post-dredging equipment is lowered to directly above the subsea pipeline using hoisting equipment and advances along the pipeline axis to complete the post-dredging operation on the laid pipeline.

[0091] By applying the construction method to a water depth range of 2m to 6m, this invention fills the gap in construction methods within this water depth range, forming a complete construction chain from pre-dredging in the intertidal zone, post-dredging in shallow water areas to conventional post-dredging in deep water areas.

[0092] More specifically, in one embodiment, the collaborative control logic of the four-point tension composite balance positioning system is as follows: (1) Tension Balance State: When the pipeline transport barge is in a stationary position, the control target is to achieve dynamic balance of tension in the four anchor cables. Let the sum of the tensions of the two anchor cables on the shore be T_shore, and the sum of the tensions of the anchor cables of the pipelaying vessel and the auxiliary tugboat on the offshore side be T_sea. Then, control |T_shore - T_sea| ≤ ΔT_balance, where ΔT_balance is the preset balance tension difference, which is taken as 2-5 tons. When the difference exceeds ΔT_balance, increase the tightening speed of the anchor cable on the side with less tension or decrease the tightening speed of the anchor cable on the side with more tension until the balance condition is met.

[0093] (2) Axial movement control: When movement along the pipeline axis is required, a target tension difference ΔT_move is set according to the direction of movement. Taking movement towards the sea as an example, the control target is T_sea - T_shore = ΔT_move, and ΔT_move is set to 5-10 tons. By monitoring the movement speed of the transport barge in real time, a PID control algorithm is used to adjust the ratio of the cable winding speed to the cable unwinding speed, so that the movement speed of the transport barge is controlled within the range of 1-3 m / min, avoiding sudden speed changes that cause tension peaks.

[0094] (3) Lateral drift suppression: When the positioning monitoring system detects a lateral drift exceeding ±0.5m, the tension of the two shore anchor cables is adjusted differentially to generate a torque to suppress the lateral drift. Specifically, if the transport barge drifts to the left, the right shore anchor cable is tightened and the left shore anchor cable is loosened until the lateral drift returns to the allowable range.

[0095] Furthermore, the linkage control logic of each monitoring system is as follows: (1) Priority setting: Sea state monitoring (significant wave height, peak period, near-bottom current velocity, average wind speed) is the first level of safety condition. When any parameter exceeds the operation window, the trenching operation will be suspended unconditionally, and the system will automatically switch to "positioning maintenance mode" to maintain the current position of the transport and management barge by the four-point tension system.

[0096] (2) Tension and positioning linkage: During operation, the tension monitoring system and the positioning monitoring system verify each other. When the positioning monitoring system shows that the offset exceeds the limit, it first determines whether it is caused by abnormal anchor cable tension; if the tension is normal, it is adjusted according to the offset control logic; if the tension is abnormal, the tension abnormality is dealt with first and then the position is adjusted.

[0097] (3) Attitude and Lifting Linkage: The attitude monitoring system is linked with the lifting equipment swing amplitude monitoring system. When the lateral tilt angle exceeds 3° or the swing amplitude of the lifting equipment exceeds the preset safety angle, the system automatically stops the advancement operation of the trenching equipment and issues an audible and visual alarm. Operation can only resume after the attitude is restored and the swing amplitude is reduced to a safe range.

[0098] In a preferred embodiment, in the method for post-ditching construction of the nearshore landing section of the subsea pipeline, the movement of the pipeline transport barge is controlled in the following manner during the process of towing it to the starting position of the current construction section: when moving towards the sea, the pipelaying vessel and the auxiliary tugboat tighten their respective anchor cables at a first retrieval speed, and the two shore-side anchor winches release their respective anchor cables at a second release speed. The ratio of the first retrieval speed to the second release speed is adjusted in real time according to the current movement speed of the pipeline transport barge, so that the pipeline transport barge moves at a uniform speed along the axis of the seabed pipeline; when moving towards the shore, the two shore-side anchor winches tighten their respective anchor cables at a third retrieval speed, and the pipelaying vessel and the auxiliary tugboat release their respective anchor cables at a fourth release speed. The ratio of the third retrieval speed to the fourth release speed is adjusted in real time according to the current movement speed of the pipeline transport barge, so that the pipeline transport barge moves at a uniform speed along the axis of the seabed pipeline.

[0099] This embodiment provides a detailed description of the movement control method of the pipeline transport barge in the shallow water area trenching construction method for the nearshore landing section of the submarine pipeline of the present invention.

[0100] In the four-point tension composite balance positioning system, when the pipeline transport barge is moved along the axial direction of the seabed pipeline to the starting position of the construction section, it is necessary to coordinate the release and retrieval speeds of the four sets of anchor cables to keep the pipeline transport barge moving at a constant speed, so as to avoid the anchor cable tension fluctuations or loss of control of the pipeline transport barge due to sudden speed changes.

[0101] When the management transport barge needs to move towards the sea, the pipelaying vessel and auxiliary tugboat tighten their respective anchor cables at a first retrieval speed, generating a traction force towards the sea. Simultaneously, the two shore-side anchor winches release their respective anchor cables at a second release speed, reducing the restraint force towards the shore. The ratio of the first retrieval speed to the second release speed is adjusted in real time according to the current movement speed of the management transport barge. The movement speed is collected in real time by a positioning monitoring system installed on the management transport barge. When the movement speed is lower than the preset target speed, the ratio of the first retrieval speed to the second release speed is increased, relatively increasing the traction force towards the sea; when the movement speed is higher than the preset target speed, the ratio of the first retrieval speed to the second release speed is decreased, relatively decreasing the traction force towards the sea. Through the above adjustments, the movement speed of the management transport barge is maintained within the range of 1 to 3 meters per minute, ensuring smooth movement.

[0102] When the management transport barge needs to move towards the shore, the two shore-side anchor winches tighten their respective anchor cables at a third retrieval speed, generating a traction force towards the shore. Simultaneously, the pipelaying vessel and auxiliary tugboat release their respective anchor cables at a fourth release speed, reducing the restraint force towards the sea. The ratio of the third retrieval speed to the fourth release speed is adjusted in real time according to the management transport barge's current speed. When the speed is lower than the preset target speed, the ratio is increased, relatively increasing the shore-side traction force; when the speed is higher than the preset target speed, the ratio is decreased, relatively decreasing the shore-side traction force. Through these adjustments, the management transport barge's speed is maintained within the range of 1 to 3 meters per minute.

[0103] During the aforementioned movement, the tension monitoring system collects the tension values ​​of each anchor cable in real time. When the tension of any anchor cable exceeds the preset safety value, further tightening of that anchor cable is stopped, and it is appropriately loosened until the tension returns to within the safe value before continuing movement. Simultaneously, the positioning monitoring system monitors the lateral deviation of the transport barge in real time. When the lateral deviation exceeds ±0.5 meters, the tension of the two shore-side anchor cables is adjusted differentially to generate a torque that suppresses the lateral deviation, ensuring that the transport barge maintains lateral stability while moving along the pipeline axis.

[0104] Through the aforementioned movement control method, the transport and management barge can move at a constant speed along the seabed pipeline axis to the starting position of each construction section under the synergistic effect of the four-point tension composite balance positioning system, providing an accurate centering foundation for subsequent trenching operations.

[0105] For example, the preset target speed is 2 meters per minute. By adjusting the ratio of the retrieval speed to the release speed in real time, the movement speed of the transport barge is maintained within the range of 1 to 3 meters per minute. When moving towards the sea, the initial ratio of the first retrieval speed to the second release speed is set to 1.0, i.e., the first retrieval speed is 1.5 meters per minute and the second release speed is 1.5 meters per minute. When the movement speed is below 1 meter per minute, the ratio is increased to 1.5, i.e., the first retrieval speed is adjusted to 2.0 meters per minute and the second release speed remains at 1.5 meters per minute; when the movement speed is above 3 meters per minute, the ratio is adjusted to 1.0, i.e., both the first retrieval speed and the second release speed are 1.8 meters per minute. When moving towards the shore, the initial ratio of the third retrieval speed to the fourth release speed is set to 1.0, i.e., the third retrieval speed is 1.5 meters per minute and the fourth release speed is 1.5 meters per minute. When the moving speed is higher than 3 meters per minute, the ratio is reduced to 0.7, that is, the third cable retrieval speed is adjusted to 1.0 meters per minute, and the fourth cable release speed is kept at 1.5 meters per minute; when the moving speed is lower than 1 meter per minute, the ratio is restored to 1.0, that is, the third cable retrieval speed and the fourth cable release speed are both 1.4 meters per minute.

[0106] In addition, the preset tension difference can be set to 8 tons. Taking movement towards the sea as an example, when the difference between the sum of the tension of the anchor cables on the sea side and the sum of the tension of the anchor cables on the shore side reaches 8 tons, the transport barge moves at a constant speed of 2 meters per minute. When changes in sea state cause the tension difference to increase to 12 tons, the tension difference is restored to 8 tons by adjusting the release and retrieval speed. When the shore anchor cable is released to a length of 100 meters, the benchmark release and retrieval speed is set to 1.8 meters per minute; when the shore anchor cable is released to a length of 300 meters, the benchmark release and retrieval speed is set to 1.2 meters per minute.

[0107] like Figure 1 and Figure 2 As shown, the following provides a specific embodiment to illustrate the implementation process of the present invention.

[0108] The shallow-water post-dredging system of the present invention includes: a pipeline transport barge 1, serving as a shallow-water operation platform; a crawler crane, fixedly mounted on the deck of the pipeline transport barge 1; a post-dredging device 3, lowered to the seabed 7 directly above the subsea pipeline 6 via the crawler crane's slings; an umbilical cable 4, connecting the post-dredging device 3 to the pipeline transport barge 1, providing power and control signals to the post-dredging device 3; a towing rope 5, connecting the post-dredging device 3 to the pipeline transport barge 1, used to traction the post-dredging device 3 to advance along the axis of the subsea pipeline 6; and the subsea pipeline 6, laid on the seabed. 7. Above; Sea level 8, located above seabed 7; Shoreside anchor 9 is located on the shore and connected to the near shore side of the transport barge 1 via shoreside anchor cable 14; Shoreside anchor 10 is located on the shore and connected to the near shore side of the transport barge 1 via shoreside anchor cable 13; Pipe-laying vessel 11 is located at sea and connected to the far shore side of the transport barge 1 via pipe-laying vessel anchor cable 15; Auxiliary tugboat 12 is located at sea and connected to the far shore side of the transport barge 1 via auxiliary tugboat anchor cable 16; Land pipeline 17 is connected to the landing section of submarine pipeline 6.

[0109] This embodiment includes the following steps: S1, near-shore section pipeline towing completed. The welding of the subsea pipeline 6 was completed by the pipelaying vessel 11, and the pipeline was towed ashore by a linear winch on shore. The pipelaying vessel 11 was then positioned to its lowest draft.

[0110] S2. Deploy shallow water operation platforms The pipe transport barge 1 (which can be a flat barge) used during pipe laying is used as a shallow water trenching operation platform. A lifting device 2 (which can be a crawler crane) is fixed on the deck of the pipe transport barge. The crawler crane has a rated lifting capacity of not less than 25 tons.

[0111] S3. Establish a four-point tension composite positioning system: Two shore-side anchor winches are installed in the nearshore area. The two shore-side anchor cables, 13 and 14, are each 0.5-1.5 km long and can cover a post-dug trench area with a shallow water section of about 1 km in length. The longer shore-side anchor cables can form a natural catenary structure, and the angle of inclination of the shore-side anchor cables into the water is controlled between 20° and 35°.

[0112] The force exerted by the anchor cable on the shore side is calculated using the following formula: F = 0.5 × ρ × Cd × A × V² Where: ρ is the seawater density (approximately 1025 kg / m³), Cd is the drag coefficient (taken as 1.0-1.2), A is the waterline projected area (approximately 800-1200 m²), and V is the flow velocity (≤1.5 m / s). Conservatively estimated, fluid resistance is typically in the range of 10-15 tons. Considering the wave superposition effect and a safety factor of 1.5-2.0, the rated pulling force of the anchor winch is preferably no less than 20 tons to ensure a safety margin even under extreme conditions.

[0113] The shore-side anchor cable of the shore-side anchor winch is connected to both sides of the pipelaying barge. The pipelaying vessel 11 is connected to one side of the pipelaying barge via the pipelaying vessel anchor cable 15, and the auxiliary tugboat 12 is connected to the other side of the pipelaying barge via the auxiliary tugboat anchor cable 16, forming a four-point positioning and balance system. If the shore-side anchor winch tightens anchor, the pipelaying vessel and the auxiliary tugboat should appropriately loosen the anchor cable; if the pipelaying vessel and the auxiliary tugboat tighten anchor cables, the shore-side anchor winch should release them accordingly. Through coordination between the two parties, the axial movement of the pipelaying barge is made smooth, avoiding instantaneous tension peaks.

[0114] S4. Trenching equipment after deployment: The post-dredging equipment 3 was lowered to a position directly above the submarine pipeline using a crawler crane. For shallow water post-dredging, a water jet trencher should be selected.

[0115] S5. Traction control after trenching: The trenching equipment is axially pulled by steel wire ropes to advance it along the pipeline axis.

[0116] S6. Segmented Cyclic Operation: A new four-point tension balance positioning system enables segmented cyclical advancement. This cyclical movement relies on multi-directional tension coordination formed by the shore-side anchor winch, the pipelaying vessel's anchor cable, and auxiliary tugboats. After completing a single section of trenching, the platform moves forward by adjusting the positions of the anchor cable and tugboats, repeating the operation. The method is as follows: Anchor winches on the shore tighten or release anchor cables; the pipe-laying vessel and auxiliary tugboats adjust the anchor cable tension appropriately; the pipe transport barge moves in a controlled manner along the pipeline axis. During the movement, tension changes in any direction are monitored in real time to ensure that the tension does not exceed the system's safe value.

[0117] S7, Positioning and Monitoring To ensure platform positioning accuracy and operational safety, the transport and management barge is equipped with the following: a DGPS or RTK high-precision positioning system to monitor the barge's position in real time, controlling the barge's lateral deviation within ±0.5m and longitudinal deviation within ±1.0m; a tension monitoring system to collect real-time tension data of each anchor cable; a crane swing monitoring system; real-time wave and current velocity monitoring equipment; and an attitude monitoring system (heel and trim).

[0118] S8, Operation Window for Wind, Waves and Current Velocity This invention should be constructed during the non-monsoon season. Meteorological and sea conditions are as follows: significant wave height Hs ≤ 1.2m; peak period Tp ≤ 6s; near-bottom current velocity ≤ 1.5m / s; average wind speed ≤ 12m / s. When the wave height exceeds the above thresholds, trenching operations should be suspended, and only the positioning status should be maintained. The above parameters can be adjusted appropriately based on meteorological statistics of the sea area where the project is located.

[0119] S9, Traction and Operation Control Parameters The axial traction tension of the post-ditching equipment should preferably be controlled within 80% of the equipment's rated capacity to avoid wire rope fatigue, transient impact, and damage to the equipment structure. The deviation in post-ditching depth should preferably be controlled within 50cm.

[0120] S10. Safety control logic during operation The shore-side anchor winch must maintain a stressed state on at least one side, and movement must cease immediately if any anchor cable tension becomes abnormal. The heel angle of the transport barge must not exceed 3°, and the crane swing amplitude must not exceed the design safety angle. Through the above safety control mechanisms, the system remains controllable even under dynamic sea conditions.

[0121] Compared with the deployment of dedicated shallow-water construction vessels, the present invention has the following advantages: no need to deploy additional vessels; saves mobilization costs of approximately 30%-40%; reduces the construction preparation period; and avoids multiple re-anchoring.

[0122] Compared with pre-ditching methods, the present invention has the following advantages: avoids siltation and repeated ditch clearing; has higher ditch stability; has controllable burial depth; and shortens the overall construction period by about 25%-35%.

[0123] The following provides another specific embodiment to illustrate the implementation process of the present invention.

[0124] Taking a certain overseas subsea natural gas pipeline project as an example, the total length of the project is approximately 60km. The pipe material is API 5LX65 MS / MO acid-resistant steel pipe with a diameter of 24in and a wall thickness of 15.88mm. The pipe is covered with a 125mm thick concrete counterweight layer. The nearshore landing section is approximately 1km long. Due to limitations in the length of the intertidal zone, water depth variations, and construction capabilities, a zoned construction scheme was adopted.

[0125] Based on the actual measured water depth and intertidal zone width, the nearshore section was divided into three construction zones.

[0126] The first construction zone: the intertidal zone and very shallow water area (0–2m water depth), this section is approximately 110m long. The following problems exist: significant wave disturbance in the intertidal zone; insufficient water depth for stable operation of floating equipment; and the availability of land-based equipment for construction. Therefore, pre-ditching is adopted for this section.

[0127] The specific measures are as follows: temporary construction access roads will be constructed in the land and intertidal areas; steel sheet piles will be driven into both sides of the trench for support; land-based long-arm excavators will be used for excavation; and the excavation depth will meet the design burial depth requirements. The water depth at the end of the pre-excavated trench will be controlled at approximately 2 meters, serving as the interface between land-based construction and floating construction.

[0128] The second construction zone is the shallow water transition zone (2m–6m water depth). When the water depth exceeds 2m, the operational capacity of land-based equipment is limited; the construction cost of temporary channels increases significantly; and long-arm excavators cannot meet the requirements for trench depth and stability. Simultaneously, when the water depth is less than 6m, DP-positioned deep-water trenching vessels cannot enter; and the draft of large trenching equipment is limited. Therefore, the shallow-water trenching construction method of this invention is adopted in the 2m–6m water depth section.

[0129] The construction method for this section is as follows: after the overall pipeline laying is completed; a pipeline transport barge is converted into a shallow water operation platform; a four-point composite tension balance positioning system is constructed; trenching is carried out in stages and cycles; and the pipeline is gradually buried to the designed depth. This section belongs to a construction capacity gap zone and is the main applicable area for the method of this invention.

[0130] The third construction zone: deep water area (>6m depth). When the water depth exceeds 6m, the draft requirements of the DP-positioned deep-water dredging vessel are met; the floating dredging equipment can operate stably; and the impact of waves on the stability of the vessel is reduced. Therefore, this section adopts the conventional deep-water dredging construction method, with the DP-positioned deep-water dredging vessel working in conjunction with the dredging equipment to complete the burial operation.

[0131] In the second construction zone (i.e., the shallow water transition zone with a water depth of 2m–6m), based on the resource allocation of the pipelaying vessel group for this project and the on-site hydrodynamic conditions, the following shallow water post-ditching construction system will be adopted.

[0132] First, a 3,000-ton pipeline transport barge was selected as the support vessel for shallow-water post-dredging. This pipeline transport barge has a large deck area and a small draft, which can meet the operating conditions in shallow water areas, while also having sufficient stability to support lifting and towing equipment.

[0133] A crawler crane with a rated lifting capacity of 25 tons is installed on the deck of the transport and management barge for the lowering and retrieval of the post-dredging equipment. The crawler crane is secured to the deck of the transport and management barge by a base reinforcement and limiting device to ensure stability under wave and off-center load conditions.

[0134] Two anchor winches with a rated pulling force of 25 tons are installed on the shore. Based on the total length of the shallow water trench excavation in this section of approximately 1 km and the need for cyclical movement, the anchor cable length is preferably 1.5 km to ensure that frequent re-anchoring is not required during the segmented advancement process, while forming a sufficient catenary buffer structure.

[0135] According to the formula for calculating water flow resistance: F = 0.5 × ρ × Cd × A × V² Based on the on-site flow velocity (not exceeding 1.5 m / s), waterline area, and safety factor, the required anti-drift pulling force for the transport barge is approximately 18 tons. Considering safety margins, an anchor winch with a rated pulling force of 20 tons or more is selected to ensure a safety margin even under extreme working conditions.

[0136] On the sea side, the anchored pipelaying vessel serves as a support vessel on one side, with its existing anchor cable system participating in tension balancing; simultaneously, the auxiliary tugboat of the pipelaying vessel serves as a support unit on the other side. Through the combined action of the two anchor winches on the shore, the anchor cables of the pipelaying vessel, and the auxiliary tugboat, a four-point tension balancing positioning system is formed.

[0137] During the cyclical operation: the shore-side anchor winch tightens or releases the anchor cables; the pipe-laying vessel and auxiliary tugboats coordinate to adjust the tension; the pipe transport barge moves in a controlled manner along the pipeline axis; thus realizing segmented cyclical trenching operations. Under the conditions of this project, the average daily operating length of the shallow-water trenching section can reach approximately 300m. The effective excavation depth of a single trenching operation can reach 2m, meeting the design burial depth requirements of this project.

[0138] This construction method eliminates the need for additional dedicated shallow-water construction vessels. It utilizes existing pipelaying vessel resources to convert construction capabilities, thereby improving construction efficiency and reducing costs while meeting safety and design requirements.

[0139] Although embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. It can be applied to various fields suitable for the present invention. Other modifications can be readily made by those skilled in the art. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details and examples shown and described herein.

Claims

1. A method for constructing a trench in shallow waters near the shore landing section of a submarine pipeline, characterized in that, Includes the following steps: Step S1: Use the pipe transport barge that remains at the construction site after the pipe laying operation is completed as a shallow water trenching operation platform, and fix the hoisting equipment on the deck of the pipe transport barge. Step S2: Establish a four-point tension composite balance positioning system in the shallow water area of ​​the nearshore landing section. The system includes two shore-side anchor winches set on the shore side and a pipelaying vessel and an auxiliary tugboat located at sea. Each shore-side anchor winch is connected to the nearshore side of the pipe transport barge through a shore-side anchor cable. The pipelaying vessel and the auxiliary tugboat are connected to the sea side of the pipe transport barge through the anchor cables of the pipelaying vessel and the auxiliary tugboat, respectively. Step S3: Divide the post-ditching operation area along the seabed pipeline axis into multiple construction sections. Starting from the first construction section, perform the following operation process cyclically: The pipeline transport barge is towed to the starting position of the current construction section using two shore-side anchor winches, a pipelaying vessel, and auxiliary tugboats. While maintaining the barge in this position, the post-dredging equipment is lowered directly above the subsea pipeline using hoisting equipment. The post-dredging equipment is then towed along the subsea pipeline axis using the hoisting equipment to perform post-dredging operations for the current construction section. After completing the post-dredging operations for the current construction section, the pipeline transport barge is towed along the subsea pipeline axis to the starting position of the next construction section using two shore-side anchor winches, a pipelaying vessel, and auxiliary tugboats. This process is repeated until all construction sections are completed.

2. The method for constructing a shallow-water trench for the near-shore landing section of a submarine pipeline as described in claim 1, characterized in that, The inclination angle of the anchor cable on the shore is controlled between 20° and 35°.

3. The method for constructing a shallow-water trench for the nearshore landing section of a submarine pipeline as described in claim 1, characterized in that, The transport and management barge is equipped with a tension monitoring system; The tension monitoring system is used to monitor the tension of the two shore anchor cables, the pipelaying vessel anchor cable, and the auxiliary tugboat anchor cable in real time. When the tension of any of the two shore anchor cables, pipelaying vessel anchor cables, or auxiliary tugboat anchor cables exceeds the preset safety value, adjust the tension of the anchor cable exceeding the preset safety value until the tension of the anchor cable is no greater than the preset safety value.

4. The method for constructing a shallow-water trench for the near-shore landing section of a submarine pipeline as described in claim 3, characterized in that, At least one of the two shore-side anchor winches shall have its shore-side anchor cable under tension.

5. The method for constructing a trench in the shallow water area of ​​the nearshore landing section of a submarine pipeline as described in claim 1, characterized in that, The transport and management barge is equipped with a positioning and monitoring system; The positioning and monitoring system is used to monitor the position of the transport and management barge in real time, and to determine the longitudinal offset of the transport and management barge along the axis of the seabed pipeline and the lateral offset perpendicular to the axis of the seabed pipeline based on the monitored position. When the lateral offset exceeds ±0.5m or the longitudinal offset exceeds ±1.0m, the two shore-side anchor winches, the pipelaying vessel, and the auxiliary tugboats will work to reduce the lateral and longitudinal offsets until the lateral offset is no greater than ±0.5m and the longitudinal offset is no greater than ±1.0m.

6. The method for post-ditch excavation in shallow waters for nearshore landing sections of submarine pipelines as described in claim 1, characterized in that, The transport and management barge is equipped with an attitude monitoring system; The attitude monitoring system is used to monitor the heel angle of the transport and management barge in real time; When the heel angle exceeds 3°, the two shore-side anchor winches, the pipelaying vessel, and the auxiliary tugboat will work to restore the heel angle of the pipe transport barge to no more than 3°.

7. The method for constructing a shallow-water trench for the near-shore landing section of a submarine pipeline as described in claim 1, characterized in that, The hoisting equipment is equipped with a swing amplitude monitoring system; The swing amplitude monitoring system is used to monitor the swing amplitude of the hoisting equipment in real time; when the swing amplitude exceeds the preset safety angle, the trenching operation of the trenching equipment is stopped.

8. The method for constructing a trench in the shallow water area of ​​the nearshore landing section of a submarine pipeline as described in claim 1, characterized in that, The transport and management barge is equipped with real-time wave and current velocity monitoring equipment; the real-time wave and current velocity monitoring equipment is used to monitor the significant wave height, peak period and near-bottom current velocity in real time; Construction shall be carried out under the following sea conditions: significant wave height not greater than 1.2m, peak period not greater than 6s, near-bottom current velocity not greater than 1.5m / s, and average wind speed not greater than 12m / s. When any of the parameters of significant wave height, peak period, near-bottom current velocity, or average wind speed does not meet the sea state conditions, the post-dredging operation of the post-dredging equipment shall be stopped, and the pipe transport barge shall be maintained in its current position by two shore-side anchor winches, a pipe-laying vessel, and auxiliary tugboats.

9. The method for constructing a trench in the shallow water area of ​​the nearshore landing section of a submarine pipeline as described in any one of claims 1 to 8, characterized in that, The water depth in the shallow water area near the shore landing section is 2m-6m.

10. The method for constructing a trench in shallow waters near the shore landing section of a submarine pipeline as described in claim 1, characterized in that, During the process of towing the transport barge to the starting position of the current construction section, movement is controlled using the following methods: When moving towards the sea side, the pipelaying vessel and the auxiliary tugboat tighten their respective anchor cables at the first cable-reeling speed, and the two shore-side anchor winches release their respective anchor cables at the second cable-laying speed. The ratio of the first cable-reeling speed to the second cable-laying speed is adjusted in real time according to the current moving speed of the pipe-carrying barge, so that the pipe-carrying barge moves at a constant speed along the axis of the seabed pipeline. As the vessel moves toward the shore, the two shore-side anchor winches tighten their respective anchor cables at a third retrieval speed, while the pipe-laying vessel and auxiliary tugboat release their respective anchor cables at a fourth release speed. The ratio of the third retrieval speed to the fourth release speed is adjusted in real time according to the current moving speed of the pipe transport barge, so that the pipe transport barge moves at a constant speed along the axis of the seabed pipeline.

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