A method for constructing a preliminary trench for a super-deep back-dig trench in a seabed channel region

Abstract

The application discloses a kind of seabed channel area super deep post-digging ditch's preliminary ditch construction method, according to super deep bury design plan, and seabed pipeline bears bending, effective axial force, calculate the length of reserved seabed pipeline, according to the length of reserved seabed pipeline, channel partition and seabed geological condition, design preliminary ditch pipe gallery length and shape, carry out seabed channel area super deep post-digging ditch's preliminary ditch construction operation, while monitoring the operation process, obtain the detection data in the operation process, judge and analyze whether seabed pipeline free span and fatigue stress exceed preset value according to detection data, to determine the correction ditch or supplementary digging scheme. To this end, it is ensured that the length of seabed pipeline falling into the ditch before super deep post-digging meets the requirements of super deep post-digging, and at the same time, it is ensured that the length of seabed pipeline reserved can be completely submerged in the pipe gallery after super deep post-digging and can complete self-lowering to a certain depth.

Description

Technical Field

[0001] This invention relates to the field of construction technology for submarine pipelines crossing waterways, specifically to a method for constructing preparatory trenches for ultra-deep post-dug trenches in submarine waterway areas. Background Technology

[0002] Before excavating ultra-deep trenches, it is necessary to ensure that the length of the subsea pipeline entering the trench meets the requirements for ultra-deep trenching. This ensures that the reserved length of the subsea pipeline can be completely submerged in the utility tunnel after the ultra-deep trenching and can complete a certain depth of self-descent. In normal trenching operations, there is no need to excavate a preparatory trench. This is because most trenching depths only need to reach 1-3 meters, and the length of the subsea pipeline is sufficient, preventing the subsea pipeline from being suspended in the air due to trenching. However, when the trenching depth reaches 7 meters or more, it is necessary to construct a preparatory trench to ensure that the length of the pipeline sinking to the seabed in the ultra-deep trench meets the requirements for complete submersion in the utility tunnel.

[0003] Regarding protective trenching on the seabed, existing methods can be mainly categorized into three types: pre-laying trenching, trenching during pipeline laying, and post-laying trenching. Pre-laying trenching involves excavating a trench within 5 meters to the left and right of the planned pipeline laying location before the pipeline is laid. Based on original survey photographs, the original seabed surface is often uneven, with undulating terrain and significant elevation differences. If the seabed rock and soil are relatively firm, pre-treatment is performed to prevent large overhangs after the pipeline is laid. To protect the safety of the pipeline after laying, in addition to pre-treatment, the trench is further deepened to ensure the pipeline can be buried to a certain depth. This is achieved through seabed rock and soil, or by later-laid protective mats, or by dumped rocks and sand, providing an extra layer of protection for the pipeline. Due to frequent ship activity in waterways, situations such as anchoring, accidental anchoring, accidental anchor shifting, ship capsizing, and bottoming out can occur, causing external forces to dent, bend, or damage the subsea pipeline. Therefore, it is essential to pre-calculate and analyze the thickness of the subsea pipeline's cover soil and protective layer to determine the trenching plan. The advantage of pre-laying trenching (pre-drilling) is that it is carried out before the pipelaying vessel lays the subsea pipeline, eliminating the need to consider the safety of already laid pipelines during the operation. The construction method is relatively easier than trenching during or after pipelaying. Currently known pre-drilling machines include jet-powered skid trenchers; for deeper pipe gallery requirements, trailing suction hopper dredgers can be used; and for shallower water, a combination of barges and excavators can be used, greatly improving trenching efficiency. The disadvantage of trenching before pipe laying (pre-drilling) is that pipe laying must be carried out as soon as possible after trenching to prevent the pipe gallery from collapsing and being reburied due to the influence of seawater flow and geological activity, thus failing to achieve the protective function. At the same time, the disadvantage of pre-drilling is that the pre-positioning and the actual pipe laying position are inaccurate, which may result in the submarine pipeline not being able to be laid in the trench.

[0004] Preparatory trench construction shares similarities with pre-laying trenching (pre-excavation) in some respects. Both preparatory trenches are constructed before the submarine pipeline is laid, thus sharing the same advantages and disadvantages. Both aim to protect the safety of the laid submarine pipeline, but the purpose of preparatory trenching is not solely for protective purposes; it is more about preparing for ultra-deep post-laying trenching. Before construction, the length of submarine pipeline to be reserved for ultra-deep post-laying trenching is determined. To reserve this length, the length, width, and shape of the spare trench are designed, thereby determining the construction plan and methods.

[0005] Existing methods cannot solve the problem of constructing preparatory trenches for ultra-deep post-dredging in submarine shipping lane areas. Summary of the Invention

[0006] In view of the shortcomings of the existing technology, the purpose of this invention is to provide a method for constructing a preparatory trench for ultra-deep post-dredging trenches in submarine waterway areas.

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] A method for constructing a preparatory trench for ultra-deep post-dug trenches in a submarine channel area includes the following steps:

[0009] S1. Based on the ultra-deep burial plan and the bending and effective axial forces that the subsea pipeline can withstand, the total length of the reserved subsea pipeline is calculated and determined.

[0010] S2, based on the length of the reserved submarine pipeline, the navigation channel division and the submarine geological conditions, and to facilitate both ultra-deep post-dredging and initial trenching, designs the length and shape of the pre-dredging pipe gallery;

[0011] S3, based on the calculated length of the reserved submarine pipeline and the designed length and shape of the pre-dug trench, carry out the preparatory trench construction work for the ultra-deep post-dredging of the submarine channel area, and at the same time monitor the trenching operation process to obtain the detection data during the operation.

[0012] S4. Based on the detection data during the trenching operation, analyze whether the submarine pipeline's span data, vertical gradient data, local stress, allowable eccentricity, and fatigue information exceed the preset values, thereby determining the correct trenching or re-dredging scheme.

[0013] S5, conduct a final survey of the reserved submarine pipeline and utility tunnel conditions, and compile the data for preservation as survey data for ultra-deep post-ditching and initial trenching.

[0014] In this invention, preferably, in step S1, the total length of the submarine pipeline between the target locations is calculated according to the pre-buried plan. The total length is the sum of the parallel submarine pipeline and the slope connecting pipeline. At the same time, the pre-drilling depth and transition length before laying the submarine pipeline are calculated.

[0015] In this invention, preferably, in step S2, the design of the length and shape of the pre-dug trench pipe gallery is as follows:

[0016] S21, Design formula for pipe components subjected to bending, effective axial force, and external overpressure:

[0017]

[0018] in,

[0019]

[0020] S22, Design Load Formula:

[0021] M Sd =M F ·γ F ·γ C +M E ·γ E +M I ·γ F ·γ C +M A ·γ A ·γ C

[0022] ε Sd =ε F ·γ F ·γ C +ε E ·γ E +ε I ·γ F ·γ C +ε A ·γ A ·γ C

[0023] S Sd =S F ·γ F ·γ C +S E ·γ E +S I ·γ F ·γ C +S A ·γ A ·γ C

[0024] In the formula, D is the pipe diameter, t2 is the steel pipe wall thickness, M is the bending moment, P is the external pressure, ε is the strain, S is the stress, and γ and α are the influencing factors under various working conditions and loads.

[0025] In this invention, preferably, the verification is performed in units of 25-meter-long subsea pipelines and 1-1.5-meter-deep trenches. The specific steps are as follows:

[0026] S31 uses the bottom elevation data of the subsea pipeline every 25 meters, combined with the position monitoring of the subsea pipeline by the 3D sonar real-time imaging system, to obtain pipeline span data and vertical gradient data.

[0027] S32, Calculate the local stress in the subsea pipeline:

[0028] σ local =SCFσ no min al

[0029] in,

[0030]

[0031]

[0032] S33, Calculate the allowable eccentricity:

[0033]

[0034] S34, Calculate fatigue information:

[0035] The design SN curve for the subsea pipeline is as follows:

[0036]

[0037] In the formula, σ is the pipe stress, SCF is the stress concentration factor, D is the pipe outer diameter, t is the pipe wall thickness, L is the pipe length, δ is the deviation caused by wall thickness and ellipticity, and N is the fatigue limit number of cycles.

[0038] The fatigue limit N for constructing the submarine pipeline is calculated and compared with the actual number of deep trenching passes. If the number of trenching passes is less than N, the submarine pipeline is in a safe state.

[0039] In this invention, preferably, during the process of lowering the submarine pipeline in deep trenching operations, a 3D sonar real-time imaging system and a ship positioning system are used to locate the trenching machine and monitor the construction process in real time.

[0040] In this invention, preferably, the deep trenching operation is carried out using a water jet trenching machine.

[0041] In this invention, preferably, before the next trenching operation begins, the bottom elevation data of the subsea pipeline is obtained using a multibeam bathymetry method, and the trenching machine determines the target height for the next trenching operation based on the bottom elevation data.

[0042] In this invention, preferably, in step S1, after laying the subsea pipeline, water is injected into the subsea pipeline to increase its weight.

[0043] Compared with the prior art, the beneficial effects of the present invention are:

[0044] 1. The method of the present invention extracts and calculates the length of the reserved submarine pipeline and the length and shape of the trenched pipe gallery to ensure that the length of the submarine pipeline falling into the pipe trench meets the requirements of the trenching after the ultra-deep trenching, and at the same time ensures that the reserved length of the submarine pipeline can be completely submerged in the pipe gallery after the trenching after the ultra-deep trenching and can complete a certain depth of self-descent.

[0045] 2. The adoption of a 3D sonar real-time imaging monitoring system eliminates the need for divers to perform auxiliary operations, reducing costs, saving time, and lowering risks;

[0046] 3. After each trenching operation, the location data of the subsea pipeline bottom is analyzed to promptly identify and correct overhang issues, avoiding the risk of pipeline damage that may occur if trenching continues, and improving the quality of deep trenching construction. Attached Figure Description

[0047] Figure 1 This is a flowchart illustrating a method for constructing ultra-deep preparatory trenches in a submarine waterway area, as described in this invention. Detailed Implementation

[0048] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0049] It should be noted that when a component is described as "fixed to" another component, it can be directly on the other component or may have a component in between. When a component is considered "connected to" another component, it can be directly connected to the other component or may have a component in between. When a component is considered "set on" another component, it can be directly set on the other component or may have a component in between. The terms "vertical," "horizontal," "left," "right," and similar expressions used in this document are for illustrative purposes only.

[0050] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0051] Please see Figure 1 A preferred embodiment of this invention provides a preparatory trench construction method for ultra-deep post-dredging in submarine channel areas. The main objective is to overcome the problems existing in the prior art. This method employs a preparatory trench construction approach before ultra-deep post-dredging to prepare for ultra-deep burial of submarine pipelines. Before laying, the length of the reserved submarine pipeline and the length and shape of the trenched pipe gallery are calculated. During deep trenching, 3D sonar real-time imaging technology and ship positioning technology are used for quality control of trenching machine positioning and trenching operations. This solves the problems of large positional deviations when using sonar positioning and high safety risks during diver-assisted positioning. Simultaneously, after each trenching and lowering process, multibeam echocardiography is conducted to determine the submarine pipeline bottom data. Based on the obtained bottom data, the stress and safety of the submarine pipeline in place are checked, and adjustments are made to submarine pipelines exceeding the standard range to ensure the integrity of the submarine pipeline after final deep burial.

[0052] Specifically, the implementation steps are as follows:

[0053] S1. Based on the ultra-deep burial plan and the bending and effective axial forces that the subsea pipeline can withstand, the total length of the reserved subsea pipeline is calculated and determined.

[0054] Specifically, before constructing the subsea pipeline, a plan for laying ultra-deep subsea pipelines is determined. This plan includes the starting and ending points of the subsea pipeline, the overall pipeline laying method, and the pre-buried depth of the subsea pipeline. Based on the pre-buried plan, the total length of the subsea pipeline between the target locations is calculated. The total length is the sum of the parallel subsea pipeline and the slope connecting pipeline. The parallel subsea pipeline and the slope connecting pipeline are determined based on the actual laying area and the pre-buried depth. At the same time, the pre-excavation depth and transition length before laying the subsea pipeline are calculated. The pre-excavation trench is carried out according to the total length of the subsea pipeline to be laid. The length of the pre-excavation trench should be greater than the total length of the subsea pipeline to leave enough length for the deep burial of the subsea pipeline after the pipeline is laid.

[0055] S2, based on the length of the reserved submarine pipeline, the navigation channel division and the submarine geological conditions, and to facilitate both ultra-deep post-dredging and initial trenching, designs the length and shape of the pre-dredging pipe gallery;

[0056] S21, Design formula for pipe components subjected to bending, effective axial force, and external overpressure:

[0057]

[0058] in,

[0059]

[0060] S22, Design Load Formula:

[0061] M Sd =M F ·γ F ·γ C +M E ·γ E +M I ·γ F ·γ C +M A ·γ A ·γ C

[0062] ε Sd =ε F ·γ F ·γ C +ε E ·γ E +ε I ·γ F ·γ C +ε A ·γ A ·γ C

[0063] S Sd =S F ·γ F ·γ C +S E ·γ E +S I ·γ F ·γ C +S A ·γ A ·γ C

[0064] In the formula, D is the pipe diameter, t2 is the steel pipe wall thickness, M is the bending moment, P is the external pressure, ε is the strain, S is the stress, and γ and α are the influencing factors under various working conditions and loads.

[0065] S3, based on the calculated length of the reserved submarine pipeline and the designed length and shape of the pre-dug trench, carry out the preparatory trench construction work for the ultra-deep post-dredging of the submarine channel area, and at the same time monitor the trenching operation process to obtain the detection data during the operation.

[0066] Specifically, the preparatory trenching operation utilizes a water-jet trenching machine. This machine removes soil and gravel from beneath the subsea pipeline to a more distant location, allowing the pipeline to be lowered into the seabed sediment. A 3D sonar real-time imaging system and a ship positioning system are used for real-time monitoring of the trenching machine's location and the construction process. The 3D sonar system detects the real-time position and depth of the subsea pipeline, while the ship positioning system is a GPS system mounted on the trenching machine. By monitoring the relative position of the subsea pipeline and the trenching machine in real time, collisions between the two are avoided during trenching. The trenching depth is determined based on the subsea pipeline's position, and the ratio of trenching depth to trenching time is calculated and compared with a predetermined unit trenching depth range to assess the quality of the trenching operation. If the depth is outside the predetermined range, trenching is stopped, and the location of the trenching machine is investigated for potential malfunctions, trench wall collapses, or other unexpected situations, thus preventing failures in the deep burial operation and avoiding safety hazards.

[0067] Specifically, after one deep trenching operation, before the next trenching operation begins, the multibeam survey method is used to obtain the bottom elevation data of the submarine pipeline. The trenching machine determines the target height for the next trenching operation based on the bottom elevation data.

[0068] Simultaneously, the verification was conducted in units of 25-meter-long subsea pipelines and trench depths of 1-1.5 meters each time. The specific steps are as follows:

[0069] S31 uses the bottom elevation data of the subsea pipeline every 25 meters, combined with the position monitoring of the subsea pipeline by the 3D sonar real-time imaging system, to obtain pipeline span data and vertical gradient data, and to correct subsea pipelines that exceed the preset value.

[0070] S32, Calculate the local stress in the subsea pipeline:

[0071] σ local =SCFσ no min al

[0072] in,

[0073]

[0074]

[0075] In the formula, σ local SCF represents the localized stress generated during the construction of the subsea pipeline, where SCF is the stress concentration factor and σ is the stress concentration factor. no min alThe nominal stress is considered safe; local stress is considered safe if it is less than the design allowable stress of the subsea pipeline material.

[0076] S33, Calculate the allowable eccentricity:

[0077]

[0078] The eccentricity must be less than the set allowable eccentricity required for the subsea pipeline.

[0079] S34, Calculate fatigue information:

[0080] The design SN curve for the subsea pipeline is as follows:

[0081]

[0082] In the formula, σ is the pipe stress, SCF is the stress concentration factor, D is the pipe outer diameter, t is the pipe wall thickness, L is the pipe length, δ is the deviation caused by wall thickness and ellipticity, and N is the fatigue limit number of cycles.

[0083] The fatigue limit N for constructing the submarine pipeline is calculated and compared with the actual number of deep trenching passes. If the number of trenching passes is less than N, the submarine pipeline is in a safe state.

[0084] S4. Continue trenching and deep burial operations, repeating steps S2-S3 until all the subsea pipelines crossing the waterway are lowered to the designed depth. At the same time, analyze the detection data during the trenching operation to determine whether the subsea pipeline span data, vertical gradient data, local stress, allowable eccentricity and fatigue information exceed the preset values, thereby determining the correct trenching or supplementary trenching plan.

[0085] Specifically, based on the determined target trenching height, the trenching and burial operations will continue. Simultaneously, a 3D sonar real-time imaging system and a ship positioning system will be used for real-time monitoring of the trenching machine's location and the construction process. Verification will be conducted after each trenching operation to ensure that the construction proceeds smoothly and safely according to the design, avoiding situations such as cracking or deformation of the subsea pipeline due to improper construction or specific location conditions. Within 24 hours, corrective trenching or supplementary trenching will be carried out, and the operation process will be monitored. Steps S3-S4 will be repeated until the length of the reserved subsea pipeline and the shape of the reserved pipe gallery meet the requirements for trenching at ultra-deep depths.

[0086] S5, conduct a final survey of the reserved submarine pipeline and utility tunnel conditions, and compile the data for preservation as survey data for ultra-deep post-ditching and initial trenching;

[0087] Specifically, multibeam survey technology can be used to obtain the laying depth, geographical location, and span data of subsea pipelines, and to conduct regular patrols and maintenance of pre-dug trench areas, and to save the data of initial trenching and ultra-deep post-drilling.

[0088] The above description is a detailed description of the preferred embodiments of the present invention. However, the embodiments are not intended to limit the scope of the patent application of the present invention. All equivalent changes or modifications made under the technical spirit of the present invention should fall within the patent scope covered by the present invention.

Claims

1. A method of constructing a preliminary trench for an ultra-deep back-dredging trench in a seabed fairway area, characterized by, Includes the following steps: S1. Based on the ultra-deep burial plan, the total length of the reserved submarine pipeline is calculated and determined; S2, based on the length of the reserved submarine pipeline, the navigation channel division and the submarine geological conditions, and to facilitate both ultra-deep post-dredging and initial trenching, designs the length and shape of the pre-dredging pipe gallery; The design of the length and shape of the pre-excavated pipe gallery is as follows: S21, Design formula for pipe components subjected to bending, effective axial force, and external overpressure: in, S22, Design Load Formula: In the formula, D is the pipe diameter, t2 is the steel pipe wall thickness, M is the bending moment, P is the external pressure, ε is the strain, S is the stress, and γ and α are the influence factors under various load conditions. Subscript F indicates functional operating condition, E indicates environmental operating condition, A indicates accidental operating condition, C indicates combined operating condition, i indicates internal, e indicates external, and b indicates standard conditions. S3, based on the calculated length of the reserved submarine pipeline and the designed length and shape of the pre-dug trench, carry out the preparatory trench construction work for the ultra-deep post-dredging of the submarine channel area, and at the same time monitor the trenching operation process to obtain the detection data during the operation. The verification is conducted in units of 25-meter-long subsea pipelines, with each trench excavation depth ranging from 1 to 1.5 meters. The specific steps are as follows: S31 uses the bottom elevation data of the subsea pipeline every 25 meters, combined with the position monitoring of the subsea pipeline by the 3D sonar real-time imaging system, to obtain pipeline span data and vertical gradient data. S32, Calculate the local stress in the subsea pipeline: wherein is the local stress in the pipe during construction, is the stress concentration factor, is the nominal stress, in, S33, Calculate the allowable eccentricity: S34, Calculate fatigue information: The design SN curve for the subsea pipeline is as follows: In the formula, σ represents the pipe stress, SCF is the stress concentration factor, D is the pipe outer diameter, t is the pipe wall thickness, L is the pipe length, δ is the deviation caused by wall thickness and ellipticity, and N is the fatigue limit number of cycles. The fatigue limit number N for constructing the submarine pipeline is calculated and compared with the actual number of deep trenching passes. If the number of trenching passes is less than N, the submarine pipeline is in a safe state. S4. Based on the detection data during the trenching operation, analyze whether the submarine pipeline's span data, vertical gradient data, local stress, allowable eccentricity, and fatigue information exceed the preset values, thereby determining the correct trenching or re-dredging scheme. S5. Conduct a final survey of the reserved submarine pipeline and utility tunnel conditions, and compile the data for preservation as survey data for ultra-deep post-ditching and initial trenching.

2. The method of claim 1, wherein, In step S1, based on the ultra-deep burial plan, the total length of the submarine pipeline between the target locations is calculated. The total length is the sum of the parallel submarine pipeline and the slope connecting pipeline. At the same time, the pre-drilling depth and transition length before laying the submarine pipeline are calculated.

3. A method of constructing a preliminary trench for a superdeep backdredge trench in a seabed fairway area according to claim 2, wherein During the deep trenching and burial of submarine pipelines, a 3D sonar real-time imaging system and a ship positioning system are used to locate the trenching machine and monitor the construction process in real time.

4. The method of claim 2, wherein the method is a method of preparing a trench for a super deep back-dredging trench in a sea route area, characterized by, This deep trenching operation was carried out using a water jet trencher.

5. The method of claim 2, wherein the method is a method of preparing a trench for a super deep back-dredging trench in a sea route area, characterized by, Before the next trenching operation begins, the bottom elevation data of the subsea pipeline is obtained using the multibeam survey method. The trenching machine then determines the target height for the next trenching operation based on the bottom elevation data.

6. The method of claim 1, wherein, In step S1, after the submarine pipeline is laid, water is injected into the submarine pipeline to increase its weight.

Images