A method of laying a marine outfall pipeline

Abstract

The application discloses a kind of laying methods of sea tail water pipeline, including the trench excavation process of sequentially carrying out, the pipeline laying process of far shore section, the pipeline laying process of near shore section and the trench backfill process;Far shore section is far shore section starting point to far shore;Near shore section is far shore section starting point to pipeline starting shore;When carrying out trench excavation process, underwater slope excavation mode is used;When carrying out the pipeline laying process of far shore section, it includes the following steps: construction preparation step, main line roller is set on the deck of pipe-laying barge along the longitudinal center line of ship body, and eight stations are set on main line roller;Pipe-laying barge construction step;Pipe abandonment step;Pipe picking step;When carrying out the pipeline laying process of near shore section, it includes the following steps: pipe-laying barge construction step;Floating tow method construction step;Pipeline water injection sinking forms sea tail water pipeline whole step;When carrying out trench backfill process, hierarchical backfill is used.The application can ensure that construction safety and quality meet requirements.

Description

Technical Field

[0001] This invention relates to a method for laying a tailrace pipeline in a marine area. Background Technology

[0002] The laying of marine tailwater pipelines falls under the category of offshore discharge projects for wastewater treatment plant tailwater. Marine pipeline laying methods are generally divided into pipelaying vessel methods and towing methods, with the pipelaying vessel method being the most commonly used. The pipelaying vessel method is further divided into three types: S-Lay, J-Lay, and Reel-Lay. The S-Lay method is suitable for laying pipelines in shallow water and at depths up to 300 meters; the J-Lay and Reel-Lay methods are suitable for laying deep-water pipelines at depths of hundreds of meters and thousands of meters, with the Reel-Lay method specifically designed for laying flexible pipelines with small diameters and low strength.

[0003] The S-shaped pipelaying method typically involves installing a circular support frame at the stern of the pipelaying vessel. Under the combined forces of gravity, buoyancy, the support force of the support frame, the tension of the tensioner, and the winch pull, the pipe naturally bends, forming an "S"-shaped curve between the pipelaying vessel and the seabed. The majority of the pipe in the S-shaped section is suspended in the water by the support force of the support frame, the winch pull, and buoyancy. Depending on the stress state of the pipe, the suspended pipe is usually divided into three sections: the section extending downwards from the tensioner of the pipelaying vessel to the point where the pipe leaves the support frame is called the upper bend; the section from the point of inflection to the bottom is called the lower bend; and the section between the upper bend and the lower section is called the middle section, which is generally shorter and can be incorporated into the upper bend. The lower bend section is entirely underwater, and its length is related to the water depth at the pipeline laying location. The shallower the water, the shorter the lower bend section, and vice versa. The longer the lower bend section, the more susceptible the pipeline and support structure are to interference or even damage from external factors such as water flow and waves. Therefore, the risks of pipelaying operations gradually increase with water depth. The S-type pipelaying vessel method has advantages such as a wide range of pipe diameters adaptable, fast laying speed, and ease of operation. This method is the most mature and widely used shallow-water pipelaying technology in subsea pipeline laying projects. Currently, the vast majority of subsea pipelines worldwide are laid using the S-type pipelaying vessel method, which is also particularly suitable for tailwater discharge projects.

[0004] Pipeline towing has evolved to four methods based on the degree of submersion of the pipe sections: floating towing (floating on the water surface), submerged towing (floating without leaving the water), bottom towing (suspended on the seabed), and bottom towing (towing on the seabed). Pipeline towing is suitable for pipeline landing and construction in near-shore and shallow waters. It is commonly used when laying submarine pipelines in near-shore shallow waters and tidal flats where tidal influences limit the draft of vessels. Pipe sections are typically assembled on an engineering vessel or on land in open, sheltered waters. The sections are assembled to a specified length, and both ends are sealed with sealing plates equipped with venting and draining valves. Then, a lifting device moves the sections to a launching track, and with the help of buoys and their own buoyancy, tugboats tow the sections into the water. Following the designed route, the sections are transported to their designated positions, submerged, and finally, the sections are joined together to complete the pipeline submersion process.

[0005] Due to the complex and changeable marine environment, wind, waves, currents, and tides have a significant impact on key aspects of pipeline construction, such as alignment, welding, positioning, sinking, abandonment, and retrieval. Furthermore, the construction of marine pipelines is further complicated by multiple adverse factors, including land acquisition and resettlement, typhoons, and monsoons. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for laying tailrace pipelines in the sea. It adopts a combination of S-type pipelaying vessel method and pipe towing method to ensure construction safety and quality meet the requirements.

[0007] The objective of this invention is achieved as follows: a method for laying a tailrace pipeline in a marine area, comprising a trench excavation process, a pipeline laying process for the offshore section, a pipeline laying process for the nearshore section, and a trench backfilling process; wherein the offshore section is from the starting point of the offshore section to the offshore shore; and the nearshore section is from the starting point of the offshore section to the starting shore of the pipeline.

[0008] When carrying out the trench excavation process, the underwater slope excavation method is used to excavate the trench along the pipe laying route;

[0009] The pipeline laying process for offshore sections includes the following steps:

[0010] Construction preparation steps: On the deck of the pipelaying vessel, a main roller conveyor is set up along the longitudinal centerline of the hull, and eight workstations are set up on the main roller conveyor, namely, the pretreatment workstation, workstation A, workstation B, workstation C, workstation D, tensioner, workstation E and support frame are set up in sequence from front to back along the main roller conveyor.

[0011] The pipelaying vessel construction steps are as follows: After the pipelaying vessel is positioned at the starting point of the offshore section, the first traction head is sealed and welded to the first end of the first pipe section on the main line roller conveyor. The traction head is equipped with a water inlet valve. Then, the traction head is connected to the positioning anchor located on the extension line of the pipelaying route. Next, during the low tide period, the first pipe section is slowly sunk into the pipe trench on the seabed through the support frame. After each section of pipe is sunk, the pipelaying vessel is moved towards the offshore shore once by the anchor cable. After the pipe touches the bottom, divers conduct underwater exploration to check the landing condition of the first pipe section and confirm the position of the first end of the pipe through sea sweeping measurement. After confirming that there is no error, the pipe is continuously welded on the pipe along the main line roller conveyor on the pipelaying vessel, and the pipe is then submerged by filling it with water.

[0012] The procedure for abandoning pipelines is as follows: When encountering severe sea conditions or other unforeseen circumstances during pipeline laying that affect the normal laying of the pipeline, temporary abandonment of the pipeline must be implemented. The temporary abandonment of pipelines shall be carried out in accordance with the following procedures:

[0013] Step 1: Weld a temporary sealing plate to the end of the pipeline and keep the water inlet valve on the temporary sealing plate closed. Inflate the pipeline to 0.65 MPa through the air inlet valve on the temporary sealing plate, maintain the pressure for 15 minutes to confirm the pipeline's airtightness, then release the pressure to standard atmospheric pressure. Open the water inlet valve on the temporary sealing plate and close the air inlet valve. The diver opens the water inlet valve on the towing head underwater and injects water into the pipeline using water pressure. Observe the change in the angle between the upper bend of the pipeline and the horizontal plane, and then loosen the tensioner.

[0014] Step 2: Move the pipe-laying vessel with anchor cables to gradually detach the tail end of the pipe from the support frame of the main roller conveyor; when the upper bend of the pipe is basically submerged in the water and the angle between the tail end of the pipe and the horizontal plane reaches 10°, install a float at the tail end of the pipe, and then move the pipe-laying vessel with anchor cables to move the tail end of the pipe back to the rear end of the support frame.

[0015] Step 3: Continue to inject water into the pipe, adjust the angle of the support frame, and then discard the pipe into the water.

[0016] Step four: After the entire pipeline is submerged in water, observe the position of the float to determine if the pipeline has been accurately positioned. Once the diver confirms underwater that the end of the pipeline has been completely and stably submerged, close the inlet valve on the temporary sealing plate and the inlet valve on the traction head to complete the pipeline abandonment.

[0017] The pipe retrieval process involves retrieving the pipe when the weather and sea conditions improve and the pipelaying vessel resumes welding and laying. This retrieval is also known as pipe picking. The pipe picking operation is carried out according to the following procedures:

[0018] Step 1: The pipelaying vessel, guided by GPS, moves to the original abandoned pipe location and anchors in place;

[0019] Step two: The diver opens the water inlet valve on the towing head underwater; connects the lifting rope to the tail end of the pipeline, and slowly lifts the tail end of the pipeline from the seabed mud surface to a certain height using the lifting equipment on the pipelaying vessel, then suspends it, connects the air compressor pipe to the air inflation valve on the temporary sealing plate, and begins to slowly inflate the pipeline with air.

[0020] Step 3: Observe the tension indicator of the lifting equipment. During the inflation process, if the reading of the tension indicator gradually decreases as the inflation volume increases, continue inflation and continue to lift the end of the pipe. If the reading of the tension indicator does not change much, stop inflation immediately and lower the end of the pipe to the seabed mud surface.

[0021] Step 4: Continue inflating until the end of the pipe floats to the surface. Observe the angle between the upper bend of the pipe and the horizontal plane. When the angle between the upper bend of the pipe and the horizontal plane is less than 5°, lower the support frame and move the pipe-laying vessel to pick up the end of the pipe and put it back on the support frame.

[0022] Step 5: Continuously pressurize the pipe with air until the end of the pipe is retracted to the inside of the tensioner of the main line roller conveyor.

[0023] Step 5: Close the water inlet valve on the traction head, open the air valve on the temporary sealing plate to release pressure and air, so that the pipeline naturally forms an S-shape. When the pressure inside the pipeline drops to the standard atmospheric pressure, cut off the temporary sealing plate at the end of the pipeline. This completes the pipeline picking operation.

[0024] The pipeline laying process for the near-shore section includes the following steps:

[0025] Step 1: The pipelaying vessel turns around from near the far shore to the starting point of the far shore section and lays the pipe from the starting point of the far shore section to the starting shore of the pipeline until the pipelaying vessel is limited by the water depth.

[0026] Step 2: First, connect the remaining pipeline in the nearshore section to the pipeline originating on the shore. Then, float the remaining pipeline in the nearshore section to the position of the pipelaying vessel. Use two sets of side cranes on the side of the pipelaying vessel to lift the tail of the floated remaining pipeline in the nearshore section and the head of the already laid pipeline to the same horizontal plane. Complete the butt welding of the tail end of the remaining pipeline in the nearshore section and the head end of the already laid pipeline on the side of the pipelaying vessel.

[0027] Step 3: Inject water into the pipeline from the starting bank, causing the section of the remaining pipeline near the shore to sink to the bottom at the junction with the beginning of the already laid pipeline.

[0028] When carrying out the trench backfilling process, a staged backfilling method is adopted. The backfilling requirements from the bottom of the trench upwards are as follows:

[0029] (1) Crushed stone backfill, with a thickness of 500mm;

[0030] (2) C40 geotextile concrete pipe with a thickness of 1800mm; graded crushed stone backfill is used on both sides;

[0031] (3) Graded crushed stone backfill, 500mm thick;

[0032] (4) 200 to 250 kg of riprap, with a thickness of 800 mm;

[0033] (5) Backfill the original soil to the original seabed elevation.

[0034] The above-mentioned method for laying marine tailrace pipelines includes the following steps during construction preparation: The pretreatment station is used for cleaning the inside of the pipeline, removing rust from the beveling, and hoisting the pipe sections onto the support frame; station A is used for preheating the pipe ends, assembling the joints, and performing the first root pass welding; station B is used for the second root pass welding and the first fill pass welding; station C is used for the second fill pass welding and the first cap pass welding; station D is used for the second cap pass welding, non-destructive testing, and repair welding; the tensioner is used to prevent slippage and loosening during pipeline movement, maintaining the tension of the pipe sections within the allowable range, and mitigating the stress transmitted to the pipeline by waves through the ship's hull, thus ensuring pipeline stability and accurate laying route; station E is used to coat the inside and outside of the pipeline with anti-corrosion layers; and the support frame is used to guide the pipeline into the water and for water injection and sinking.

[0035] The above-mentioned method for laying marine tailwater pipelines involves using a grab bucket vessel to excavate the trench during the trench excavation step. After the excavation is completed, rocks are dumped at the bottom of the trench to remove silt and a bedding layer is laid.

[0036] In the above-mentioned method for laying marine tailwater pipelines, during the pipe-laying vessel construction step, a pair of limiting steel piles are driven every 30m along both sides of the pipe trench to assist in pipeline positioning.

[0037] The method for laying marine tailrace pipelines according to the present invention has the following characteristics:

[0038] 1. Based on the actual site conditions, a construction method combining the S-type pipelaying vessel method and the towing method was adopted. Due to the impact of aquaculture area relocation near the pipeline launch point, the pipeline in the nearshore section (580 meters from the starting shore to the far shore) was laid using a combination of the towing method and the pipelaying vessel method. In the farshore section (outside the nearshore section), the pipeline was laid using the pipelaying vessel method. Specifically, the pipeline in the farshore section was laid first using the S-type pipelaying vessel method, starting from the starting point of the farshore section and laying one pipe at a time until completion. Then, the vessel was turned around to start laying the pipeline in the nearshore section, starting from the starting point of the farshore section and laying pipes towards the starting shore, until the pipelaying vessel was limited by the draft. The remaining pipeline in the nearshore section (280 meters) was first connected to the pipeline on the starting shore, and then the towing method was used to float it at sea and connect it to the pipeline in the completed farshore section, ultimately forming a complete tailrace pipeline in the sea area.

[0039] 2. When using the S-type pipelaying vessel method, in the absence of linear winches and hydraulic tensioners, the abandonment and retrieval of pipes are achieved by making reasonable use of buoyancy, and the pipeline laying in the sea area is finally completed. Attached Figure Description

[0040] Figure 1 This is a side view of the pipelaying vessel used in the method for laying the tailrace pipeline in the sea area according to the present invention.

[0041] Figure 2a This is a first state diagram of the abandoned pipe step in the pipeline laying process of the offshore section of the marine tailwater pipeline laying method of the present invention.

[0042] Figure 2b This is a second state diagram of the abandoned pipe step in the pipeline laying process of the offshore section of the marine tailwater pipeline laying method of the present invention.

[0043] Figure 3a This is a first state diagram of the pipe-picking step in the pipeline laying process of the offshore section of the marine tailwater pipeline laying method of the present invention.

[0044] Figure 3b This is a second state diagram of the pipe-picking step in the pipeline laying process of the offshore section of the marine tailwater pipeline laying method of the present invention. Detailed Implementation

[0045] The invention will now be further described with reference to the accompanying drawings.

[0046] The following describes the laying method of the marine tailrace pipeline of this invention, taking a pipeline in a certain sea area that runs approximately east-west, with a total length of about 3667m in the sea area, including two diffuser sections each 150m long, a burial depth of about 3.60-6.17m, a designed pipe bottom elevation of -25.6--2.3m, and a slope of 0.52-27.9%. Due to the impact of aquaculture area relocation near the pipeline's starting shore, the nearshore section (580 meters from the starting shore to the far shore) is laid using a combination of pipe towing and pipelaying vessel methods, while the farshore section (outside the nearshore section) is still laid using the pipelaying vessel method. The offshore section is constructed first using the S-type pipelaying vessel method, starting from chainage KH1+600 and laying pipes one by one towards the offshore shore until completion. Then, the vessel is turned around to begin construction of the nearshore section, laying pipes from chainage KH1+600 towards the pipeline's starting shore until the pipelaying vessel is limited by its draft (chainage KH1+300). The remaining 280 meters of pipe in the nearshore section is first connected to the onshore pipeline, and then floated at sea using the pipe-towing method to connect with the completed pipeline, ultimately forming the entire subsea pipeline.

[0047] The method for laying a marine tailrace pipeline of the present invention includes a trench excavation process, a pipeline laying process for the offshore section, a pipeline laying process for the nearshore section, and a trench backfilling process, which are carried out sequentially; the offshore section is from the starting point of the offshore section to the offshore shore; the nearshore section is from the starting point of the offshore section to the starting shore of the pipeline.

[0048] When carrying out the trench excavation process, the underwater slope excavation method is used to excavate the trench along the pipeline laying route; the grab bucket boat is used for trench excavation, and after the excavation is completed, rocks are dumped and silt is squeezed out and a bedding layer is laid at the bottom of the trench.

[0049] The pipeline laying process for offshore sections includes the following steps:

[0050] Construction preparation steps: A mainline roller conveyor is installed along the longitudinal centerline of the hull on the deck of the pipelaying vessel. Eight workstations are set up on the mainline roller conveyor, namely, pretreatment workstation 10, workstation A20, workstation B30, workstation C40, workstation D50, tensioner 60, workstation E70, and support frame 80 (see...) Figure 1 Pre-treatment station 10 is used for internal pipe cleaning, beveling and rust removal, and hoisting pipe sections onto the support frame; station A20 is used for pipe end preheating, joint assembly, and the first root pass welding; station B30 is used for the second root pass welding and the first fill pass welding; station C40 is used for the second fill pass welding and the first cap pass welding; station D50 is used for the second cap pass welding, non-destructive testing, and repair welding; tensioner 60 is used to prevent slippage and loosening of the pipe during movement, maintain the tension of the pipe section within the allowable range, and alleviate the stress transmitted to the pipe by waves through the hull, so as to ensure pipeline stability and accurate laying route; station E70 is used to coat the inside and outside of the pipe with anti-corrosion coating; support frame 80 is used to guide the pipeline into the water and for water injection and sinking;

[0051] The eight workstations on the pipelaying vessel completed the pipeline connection work:

[0052] a. Pre-processing station

[0053] ① Lift the pipe section onto the auxiliary roller, remove the pipe bevel cover and clean the pipe ends to remove rust, and carry out preliminary preparations;

[0054] ②After the pipeline pretreatment is completed, it is hoisted onto the main welding line roller conveyor;

[0055] b. Workstation A

[0056] ①The main roller conveyor delivers the first pipe section to station A;

[0057] ②The main roller conveyor sends the second pipe section into station A;

[0058] ③ Use an internal alignment tool to adjust the mating ends of the first and second pipe sections to a suitable gap and position, and preheat them;

[0059] ④ Use self-shielding flux-cored welding wire for semi-automatic butt welding of the butt ends downwards for root pass;

[0060] ⑤ Move the pipe 12 meters towards the stern so that the weld joint at the docking end reaches station B; weld the third pipe section at station A according to the above process.

[0061] c. Workstation B

[0062] This workstation mainly performs filling welding operations at the pipe joint ends, using self-shielded flux-cored welding wire in a semi-automatic manner. After welding twice, the pipe is moved back 12 meters, and the original process is repeated.

[0063] d. Workstation C

[0064] This workstation mainly performs pipe filling welding operations, using self-shielded flux-cored welding wire for semi-automatic welding. After welding twice, the pipe is moved back 12 meters, and the original process is repeated.

[0065] e. Workstation D

[0066] The main task is to perform pipe cover welding, using self-shielded flux-cored welding wire in a semi-automatic process; after welding, the pipe is moved back 12 meters and the original process is repeated.

[0067] f. Workstation E;

[0068] An X-ray inspection crawler is used to enter the pipeline to perform non-destructive testing on the weld joints. If the weld joints pass the inspection, the next process will proceed; otherwise, they will be reworked at the next work station.

[0069] The pipelaying vessel construction steps are as follows: After the pipelaying vessel is positioned at the starting point of the offshore section, the first step is to seal and weld the traction head 100A at the beginning of the first pipe section on the main line roller conveyor. The traction head 100A is a hemispherical sealing plate and is equipped with a water inlet valve. Next, the traction head is connected to the positioning anchor located on the extension line of the pipelaying route. Then, during the low tide period, the first pipe section is slowly sunk into the pipe trench on the seabed through the support frame. After each section of pipe is sunk, the pipelaying vessel is moved towards the offshore shore once by the anchor cable. During the laying, a pair of limiting steel piles are driven every 30m along both sides of the pipe trench to assist in the positioning of the pipe. After the pipe touches the bottom, divers conduct underwater exploration to check the landing condition of the first pipe section and confirm the position of the pipe end by sweeping the seabed. After confirming that there is no error, the pipe is continuously welded on the main line roller conveyor on the pipelaying vessel and the pipe is then submerged by injecting water.

[0070] The procedure for abandoning pipelines is as follows: If, during pipeline laying operations, severe sea conditions such as excessively strong winds (above force 6), high waves (over 1 meter), or prolonged swell periods (greater than 8 seconds) threaten the safety of the pipeline and its equipment, or if other unforeseen circumstances affect the normal laying of the pipeline, temporary abandonment must be implemented within 48 hours. This involves filling the pipeline with water and completely submerging it on the seabed for safety. The temporary abandonment procedure is carried out according to the following steps:

[0071] Step 1: Weld a temporary sealing plate 100B to the tail end of pipe 100, and keep the water inlet valve on the temporary sealing plate 100B closed. Pressurize pipe 100 to 0.65 MPa using the air inflation valve on the temporary sealing plate 100B (see...). Figure 2a After holding the pressure for 15 minutes to confirm the airtightness of the pipeline, release the pressure to the standard atmospheric pressure, open the water inlet valve on the temporary sealing plate 100B, and close the air inflation valve on the temporary sealing plate 100B; the diver opens the water inlet valve on the towing head 100A underwater and injects water into the pipeline using water pressure; observe the change in the angle between the upper bend section 100C of the pipeline and the horizontal plane, and loosen the tensioner;

[0072] Step 2: Move the pipe-laying vessel with anchor cables to gradually detach the tail end of the pipe from the support frame of the main roller conveyor; when the upper bend of the pipe is basically submerged in the water and the angle between the tail end of the pipe and the horizontal plane reaches 10°, install a float at the tail end of the pipe, and then move the pipe-laying vessel with anchor cables to move the tail end of the pipe back to the rear end of the support frame.

[0073] Step 3: Continue to inject water into the pipe, adjust the angle of the support frame, and then discard the pipe into the water.

[0074] Step four: After the entire pipeline is submerged, observe the position of the buoy to determine if the pipeline has been accurately positioned. Once the diver confirms underwater that the pipeline's tail end has been completely and stably submerged, close the inlet valves on the temporary sealing plate 100B and the traction head 100A to complete the pipeline abandonment process (see...). Figure 2b );

[0075] The pipe retrieval process involves retrieving the pipe when the weather and sea conditions improve and the pipelaying vessel resumes welding and laying. This retrieval is also known as pipe picking. The pipe picking operation is carried out according to the following procedures:

[0076] Step 1: The pipelaying vessel, guided by GPS, moves to the original abandoned pipe location and anchors in place;

[0077] Step two: The diver opens the inlet valve on the towing head 100A underwater; connects the lifting rope to the tail end of the pipe, and uses the lifting equipment on the pipelaying vessel to slowly lift the tail end of the pipe from the seabed mud surface to a certain height and suspend it. Then, connects the air compressor pipe to the inflation valve on the temporary sealing plate 100B and begins to slowly inflate the pipe 100 (see...). Figure 3a );

[0078] Step 3: Observe the tension indicator of the lifting equipment. During the inflation process, if the reading of the tension indicator gradually decreases as the inflation volume increases, continue inflation and continue to lift the end of the pipe. If the reading of the tension indicator does not change much, stop inflation immediately and lower the end of the pipe to the seabed mud surface.

[0079] Step four: Continue inflating until the end of the pipe floats to the surface. Observe the angle between the upper bend (100C) of the pipe and the horizontal plane. When the angle between the upper bend (100C) of the pipe and the horizontal plane is less than 5°, lower the support frame and move the pipelaying vessel to pick up the end of the pipe and place it back onto the support frame (see...). Figure 3b );

[0080] Step 5: Continuously pressurize the pipe with air until the end of the pipe is retracted to the inside of the tensioner of the main line roller conveyor.

[0081] Step 5: Close the water inlet valve on the traction head 100A, open the air valve on the temporary sealing plate 100B to release pressure and air, so that the pipeline naturally forms an S-shape. When the pressure inside the pipeline drops to the standard atmospheric pressure, cut off the temporary sealing plate 100B at the end of the pipeline. This completes the pipeline picking operation.

[0082] The pipeline laying process for the near-shore section includes the following steps:

[0083] Step 1: The pipelaying vessel turns around from near the far shore to the starting point of the far shore section and lays the pipe from the starting point of the far shore section to the starting shore of the pipeline until the pipelaying vessel is limited by the water depth.

[0084] Step 2: First, connect the remaining pipeline in the nearshore section to the pipeline originating on the shore. Then, float the remaining pipeline in the nearshore section to the position of the pipelaying vessel. Use two sets of side cranes on the side of the pipelaying vessel to lift the tail of the floated remaining pipeline in the nearshore section and the head of the already laid pipeline to the same horizontal plane. Complete the butt welding of the tail end of the remaining pipeline in the nearshore section and the head end of the already laid pipeline on the side of the pipelaying vessel.

[0085] Step 3: Water is injected into the pipeline from the starting shore, causing the section at the junction of the remaining end of the nearshore section and the beginning of the already laid pipeline to sink to the bottom of the water, forming a complete tailwater pipeline in the sea area.

[0086] When carrying out the trench backfilling process, a staged backfilling method is adopted. The backfilling requirements from the bottom of the trench upwards are as follows:

[0087] (1) Crushed stone backfill, with a thickness of 500mm;

[0088] (2) C40 geotextile concrete pipe with a thickness of 1800mm; graded crushed stone backfill is used on both sides;

[0089] (3) Graded crushed stone backfill, 500mm thick;

[0090] (4) 200 to 250 kg of riprap, with a thickness of 800 mm;

[0091] (5) Backfill the original soil to the original seabed elevation.

[0092] The above embodiments are for illustrative purposes only and are not intended to limit the invention. Those skilled in the art can make various changes or modifications without departing from the spirit and scope of the invention. Therefore, all equivalent technical solutions should also fall within the scope of the invention and should be defined by the claims.

Claims

1. A method for laying a tailrace pipeline in a marine area, comprising a trench excavation process, a pipeline laying process for the offshore section, a pipeline laying process for the nearshore section, and a trench backfilling process; wherein the offshore section is from the offshore section starting point to the offshore shore; and the nearshore section is from the offshore section starting point to the pipeline originating shore; characterized in that, When carrying out the trench excavation process, the underwater slope excavation method is used to excavate the trench along the pipe laying route; The pipeline laying process for offshore sections includes the following steps: The construction preparation steps involve setting up a main line roller conveyor along the longitudinal centerline of the hull on the deck of the pipelaying vessel. Eight workstations are set up on the main line roller conveyor, namely, a pretreatment workstation, workstation A, workstation B, workstation C, workstation D, a tensioner, workstation E, and a support frame, arranged sequentially from front to back along the main line roller conveyor. The pretreatment workstation is used for internal pipe cleaning, beveling and rust removal, and hoisting the pipe sections onto the support frame. Workstation A is used for pipe end preheating, joint assembly, and the first root pass welding. Workstation B is used for the second root pass welding and the first filler weld. Workstation C is used for the second filler weld and the first cap weld. Workstation D is used for the second cap weld, non-destructive testing, and repair welding. The tensioner is used to prevent slippage and loosening of the pipe during movement, maintaining the tension of the pipe sections within the allowable range and mitigating the stress transmitted to the pipe by waves through the hull, thus ensuring pipeline stability and accurate laying route. Workstation E is used for applying anti-corrosion coatings to the inside and outside of the pipe. The support frame is used to guide the pipeline into the water and for water injection and sinking. The pipelaying vessel construction steps are as follows: After the pipelaying vessel is positioned at the starting point of the offshore section, a traction head is first sealed and welded to the first end of the first pipe section on the main line roller conveyor. This traction head is equipped with a water inlet valve. Next, the traction head is connected to a positioning anchor located on the extension line of the pipelaying route. Then, during the low tide period, the first pipe section is slowly sunk into the pipe trench on the seabed through a support frame. After each section of pipe is sunk, the pipelaying vessel is moved towards the offshore shore once by anchor cables. During the laying process, a pair of limiting steel piles are driven every 30 meters along both sides of the pipe trench to assist in pipe positioning. After the pipe touches the bottom, divers conduct underwater exploration to assess the landing condition of the first pipe section and confirm the position of the first end of the pipe through sea sweeping measurements. Once confirmed, the pipe is continuously welded on the pipe along the main line roller conveyor on the pipelaying vessel, and the pipe is then submerged by injecting water. The procedure for abandoning pipelines is as follows: If, during pipeline laying operations, severe sea conditions such as winds exceeding force 6, waves exceeding 1 meter in height, and swell periods greater than 8 seconds are encountered, threatening the safety of the pipeline and ship equipment, or if other unforeseen circumstances affect the normal laying of the pipeline, temporary abandonment of pipelines must be implemented within 48 hours. The temporary abandonment procedure is carried out according to the following steps: Step 1: Weld a temporary sealing plate to the end of the pipeline and keep the water inlet valve on the temporary sealing plate closed. Inflate the pipeline to 0.65 MPa through the air inlet valve on the temporary sealing plate, maintain the pressure for 15 minutes to confirm the pipeline's airtightness, then release the pressure to standard atmospheric pressure. Open the water inlet valve on the temporary sealing plate and close the air inlet valve. The diver opens the water inlet valve on the towing head underwater and injects water into the pipeline using water pressure. Observe the change in the angle between the upper bend of the pipeline and the horizontal plane, and then loosen the tensioner. Step 2: Move the pipe-laying vessel with anchor cables to gradually detach the tail end of the pipe from the support frame of the main roller conveyor; when the upper bend of the pipe is basically submerged in the water and the angle between the tail end of the pipe and the horizontal plane reaches 10°, install a float at the tail end of the pipe, and then move the pipe-laying vessel with anchor cables to move the tail end of the pipe back to the rear end of the support frame. Step 3: Continue to inject water into the pipe, adjust the angle of the support frame, and then discard the pipe into the water. Step four: After the entire pipeline is submerged in water, observe the position of the float to determine if the pipeline has been accurately positioned. Once the diver confirms underwater that the end of the pipeline has been completely and stably submerged, close the inlet valve on the temporary sealing plate and the inlet valve on the traction head to complete the pipeline abandonment. The pipe retrieval process involves retrieving the pipe when the weather and sea conditions improve and the pipelaying vessel resumes welding and laying. This retrieval is also known as pipe picking. The pipe picking operation is carried out according to the following procedures: Step 1: The pipelaying vessel, guided by GPS, moves to the original abandoned pipe location and anchors in place; Step two: The diver opens the water inlet valve on the towing head underwater; connects the lifting rope to the tail end of the pipeline, and slowly lifts the tail end of the pipeline from the seabed mud surface to a certain height using the lifting equipment on the pipelaying vessel, then suspends it, connects the air compressor pipe to the air inflation valve on the temporary sealing plate, and begins to slowly inflate the pipeline with air. Step 3: Observe the tension indicator of the lifting equipment. During the inflation process, if the reading of the tension indicator gradually decreases as the inflation volume increases, continue inflation and continue to lift the end of the pipe. If the reading of the tension indicator does not change much, stop inflation immediately and lower the end of the pipe to the seabed mud surface. Step 4: Continue inflating until the end of the pipe floats to the surface. Observe the angle between the upper bend of the pipe and the horizontal plane. When the angle between the upper bend of the pipe and the horizontal plane is less than 5°, lower the support frame and move the pipe-laying vessel to pick up the end of the pipe and put it back on the support frame. Step 5: Continuously pressurize the pipe with air until the end of the pipe is retracted to the inside of the tensioner of the main line roller conveyor. Step 6: Close the water inlet valve on the traction head, open the air valve on the temporary sealing plate to release pressure and air, so that the pipeline naturally forms an S-shape. When the pressure inside the pipeline drops to the standard atmospheric pressure, cut off the temporary sealing plate at the end of the pipeline. This completes the pipeline picking operation. The pipeline laying process for the near-shore section includes the following steps: Step 1: The pipelaying vessel turns around from near the far shore to the starting point of the far shore section and lays the pipe from the starting point of the far shore section to the starting shore of the pipeline until the pipelaying vessel is limited by the water depth. Step 2: First, connect the remaining pipeline in the nearshore section to the pipeline originating on the shore. Then, float the remaining pipeline in the nearshore section to the position of the pipelaying vessel. Use two sets of side cranes on the side of the pipelaying vessel to lift the tail of the floated remaining pipeline in the nearshore section and the head of the already laid pipeline to the same horizontal plane. Complete the butt welding of the tail end of the remaining pipeline in the nearshore section and the head end of the already laid pipeline on the side of the pipelaying vessel. Step 3: Inject water into the pipeline from the starting bank, causing the section of the remaining pipeline near the shore to sink to the bottom at the junction with the beginning of the already laid pipeline. When carrying out the trench backfilling process, a staged backfilling method is adopted. The backfilling requirements from the bottom of the trench upwards are as follows: (1) Crushed stone backfill, with a thickness of 500mm; (2) C40 geotextile concrete pipe with a thickness of 1800mm; graded crushed stone backfill is used on both sides; (3) Graded crushed stone backfill, 500mm thick; (4) 200 to 250 kg of riprap, with a thickness of 800 mm; (5) Backfill the original soil to the original seabed elevation.

2. The method for laying a marine tailrace pipeline according to claim 1, characterized in that, When excavating pipe trenches, a grab bucket boat is used for excavation. After excavation, rocks are dumped and silt is squeezed out at the bottom of the pipe trench, and a bedding layer is laid.

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