Submarine pipeline construction method under the condition of near-shore landing section reef sandy soil composite stratum

Abstract

The present application relates to a kind of offshore landing section reef sandy soil composite stratum conditions under the method for constructing submarine pipeline, belong to the field of ocean engineering technology.The method is aimed at the problem of large excavation quantity, high construction risk and high construction cost in reef area when uniform depth design is used in offshore landing section, by geological survey, the route is divided into sandy soil area and reef area, and first and second predetermined depth values are set respectively, transition zone is set in the interval and longitudinal curvature radius of pipeline is controlled to realize smooth transition of depth, according to which broken excavation, laying cushion layer and pipeline are carried out, and finally different backfilling methods are used according to actual depth.The method is used to construct submarine pipeline under composite stratum conditions, which can reduce reef excavation quantity, reduce construction risk and cost.

Description

Technical Field

[0001] This invention relates to the field of marine engineering technology. More specifically, this invention relates to a method for constructing subsea pipelines under nearshore landing conditions involving a composite stratum of reefs and sand. Background Technology

[0002] The nearshore landing section of a submarine pipeline is typically located in the wave zone or shallow waters, and its design must simultaneously meet multiple requirements, including seabed erosion, foundation liquefaction, and pipeline structural safety. Due to limitations such as the location of onshore facilities, navigation conditions, and construction windows, the pipeline route adjustment space for the nearshore landing section is limited, and in engineering practice, it is often difficult to completely avoid reef areas.

[0003] In existing technologies, the design of near-shore landing sections of subsea pipelines is generally based on unfavorable operating conditions such as seabed scour depth and the depth affected by foundation liquefaction, and a uniform burial depth design principle is adopted along the pipeline route. This uniform burial depth design method has created a technological inertia, that is, it is assumed by default that: in order to ensure the long-term operational safety of the pipeline, a consistent burial depth control standard must be adopted regardless of differences in strata.

[0004] However, when the pipeline route traverses a complex geological formation consisting of alternating reefs and sand, continuing to apply the aforementioned uniform burial depth design principle will inevitably lead to over-excavation in the reef area. Due to the high strength and poor breakability of the reef strata, over-excavation usually requires methods such as blasting or heavy machinery crushing, which not only significantly increases construction risks but also noticeably prolongs the construction period and substantially raises project costs. Furthermore, it may adversely affect construction organization, safety management, and environmental protection.

[0005] Furthermore, due to changes in geological conditions and design burial depth between the reef and sandy areas, stress concentration can easily occur during pipeline dragging construction and operation if reasonable transition control measures are lacking. This can affect the integrity of the pipeline's anti-corrosion layer and counterweight layer, thereby reducing the pipeline's long-term operational safety. Therefore, existing technologies lack a systematic design and construction method that can overcome the inertia of uniform burial depth design and is applicable to the composite reef-sand strata conditions of nearshore landing sections. Summary of the Invention

[0006] One object of the present invention is to provide a method for constructing a submarine pipeline under the condition of a composite stratum of reef and sand in the nearshore landing section, so as to at least solve the above-mentioned problems.

[0007] To achieve the objectives and other advantages of this invention, a method for constructing a submarine pipeline under composite strata of reefs and sand in a nearshore landing section is provided, comprising the following steps: S1, conducting geological surveys along the nearshore landing section, and dividing the pipeline route into sandy soil areas and reef areas based on strata distribution; in the sandy soil area, designing the pipeline burial depth to a first predetermined burial depth value; in the reef area, designing the pipeline burial depth to a second predetermined burial depth value, the second predetermined burial depth value being less than the first predetermined burial depth value; S2, setting a transition zone between the sandy soil area and the reef area, controlling the longitudinal radius of curvature of the pipeline within the transition zone to be 2500D-3500D, where D is the outer diameter of the pipeline, so that the pipeline burial depth is lower than the first predetermined burial depth value; S3. Smoothly transition from the first predetermined burial depth to the second predetermined burial depth; S4. Based on the first predetermined burial depth, the second predetermined burial depth, and the longitudinal radius of curvature, break and excavate the seabed surface to form a pipe trench; S5. Lay a buffer layer at the bottom of the pipe trench, and then carry out pipe laying operations; S6. After the pipe laying is completed, first lay a layer of crushed stone buffer on top of the pipe, and then carry out different backfilling treatments according to the actual burial depth of the pipe. When the actual burial depth of the pipe in a local location in the transition zone is less than the second predetermined burial depth, backfill with boulders to protrude from the seabed surface to form a facing boulders layer. The slope of the facing boulders layer is not less than 3:1, and the boulders are selected with an apparent density in air of not less than 2600 kg / m³. 3 The graded stone material was used, and the remaining trench was backfilled with excavated reefs until it was level with the seabed.

[0008] Preferably, in step S1, the determination of the first predetermined burial depth and the second predetermined burial depth specifically includes: S11, Differentiation analysis and benchmark value acquisition: For sandy soil areas, using the standard penetration test blow count obtained from geological exploration as the key input parameter, liquefaction stability analysis is adopted to calculate and obtain the critical burial depth H1 that meets the preset liquefaction safety factor Fs≥1.5 requirement; For rocky areas, taking into account the saturated uniaxial compressive strength of the rock mass and joint development status obtained from geological exploration, scour stability analysis is adopted to calculate and obtain the critical burial depth H2 that meets the preset scour safety factor Fsc≥2.0 requirement; S12, Collaborative design adjustment based on composite strata characteristics: The critical burial depth H2 is directly determined as the second predetermined burial depth; Based on the critical burial depth H1, a safety margin Δ is added to determine the first predetermined burial depth, the first predetermined burial depth = H1 + Δ; Wherein, the value range of the safety margin Δ is 0.5-1.0m, so that the first predetermined burial depth is always greater than the second predetermined burial depth.

[0009] Preferably, in step S3, in the reef area, the excavation method is selected according to the water depth conditions of the reef area: in areas with a water depth of less than or equal to 2m, a temporary construction channel is constructed and land-based machinery is used for breaking and excavation; in areas with a water depth greater than 2m, an offshore reef-breaking vessel is used for breaking and excavation.

[0010] Preferably, in step S3, the width of the temporary construction passage is 3-5m, and the number of temporary construction passages is determined according to the pipe diameter: a single construction passage is set when the pipe diameter is DN400 or less, and a double construction passage is set when the pipe diameter is DN400 or more.

[0011] Preferably, in step S4, the buffer pad is a fine sand layer with a thickness of 200-300 mm.

[0012] Preferably, in step S4, when laying the buffer layer, fine sand is first filled into biodegradable geotextile bags to form sandbags, and then multiple sandbags are laid at the bottom of the trench and spliced ​​together to form a continuous buffer layer.

[0013] Preferably, in step S5, the thickness of the crushed stone buffer layer is not less than 300 mm, and the particle size of the crushed stone is 20-60 mm.

[0014] Preferably, the weight of the stones in the facing stone layer is determined by the following formula:

[0015]

[0016] In the formula, W D The value is the weight of a single facing stone block, in kg; V is the velocity of the current under the combined action of waves and water flow, in m / s; g is the acceleration due to gravity, taken as 9.81 m / s². 2 ;γ w The density of water is taken as 1025 kg / m³. 3 ;γ r The apparent density of the stone is expressed in kg / m³. 3 ;θ B φ is the angle between the slope surface and the horizontal plane, i.e., the slope gradient; r y is the angle of repose of the stone, i.e., the angle of natural accumulation; y is a constant, dimensionless, taken as 0.86 for unburied stones and 1.20 for buried stones;

[0017] The dimensions of the stones in the facing stone layer are determined by the following formula:

[0018] ,

[0019] In the formula, D 50min This represents the minimum diameter for which 50% of the stones have a diameter greater than this value, expressed in mm.

[0020] Preferably, the particle size distribution of the stones in the facing stone layer meets the following requirements:

[0021] ,

[0022] In the formula, D 50maxThis represents the maximum diameter where 50% of the stones have a diameter smaller than this value, in mm; D 15min This represents the minimum diameter, in mm, for which 15% of the stones have a diameter greater than this value; D 15max This represents the maximum diameter of 15% of the stones, expressed in mm; D 100min This refers to all stones with a diameter greater than this value, i.e., the minimum stone diameter, in mm; D 100max This value represents the diameter of all stones smaller than this value, i.e., the maximum stone diameter, in mm.

[0023] Preferably, the backfill thickness of the facing rubble layer is determined by the following formula:

[0024] ,

[0025] In the formula, r is the minimum backfill thickness of the facing stone layer, in mm.

[0026] The present invention has at least the following beneficial effects:

[0027] This invention proposes a collaborative method for the design and construction of subsea pipelines based on geological differences. While ensuring the safety of the pipeline structure and the reliability of long-term operation, it minimizes excavation in reef areas and reduces construction risks and shortens the construction cycle through differentiated burial depth control, smooth transition of burial depth changes, and local functional equivalence compensation. Specifically, the burial depth control design based on geological differences breaks through the technical inertia of uniform burial depth design for near-shore landing sections, avoiding unnecessary deep excavation in reef areas; by setting a smooth longitudinal curvature transition zone, the adverse effects of burial depth changes on pipeline dragging construction and stress distribution during operation are effectively reduced, ensuring the integrity of the pipeline structure and anti-corrosion system; and through a local burial depth equivalence compensation mechanism, the pipeline can still achieve stability and protective performance equivalent to the target burial depth even under actual burial depth constraints.

[0028] 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

[0029] Figure 1 This is a schematic diagram of the design scheme for submarine pipelines under different nearshore landing sections with composite strata of reefs and sand.

[0030] Figure 2 This is a longitudinal section diagram of pipeline laying in a rocky area using a uniform burial depth, as described in existing technologies.

[0031] Figure 3 This is a longitudinal section view of the pipeline laying in the transition compensation backfill area of ​​the present invention.

[0032] In the figure, 1 is the seabed surface, 2 is the cross-section of pipeline construction in the reef area with a uniform burial depth in the prior art, 3 is the cross-section of pipeline construction in the reef area in this invention, 4 is the reef area, 5 is the transition compensation backfill area, 6 is the temporary construction passage, 7 is the buffer layer, 8 is the pipeline, 9 is the crushed stone buffer layer, 10 is the backfilled reef, and 11 is the facing stone layer. Detailed Implementation

[0033] The present invention will now be described in further detail with reference to the embodiments and accompanying drawings, so that those skilled in the art can implement it based on the description.

[0034] It should be understood that terms such as “having,” “comprising,” and “including” as used herein do not exclude the presence or addition of one or more other elements or combinations thereof.

[0035] It should be noted that, unless otherwise specified, the experimental methods described in the following embodiments are all conventional methods, and the reagents and materials described are all commercially available unless otherwise specified. In the description of this invention, the terms "lateral", "longitudinal", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", and "outer" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0036] like Figures 1 to 3 As shown, this invention provides a method for constructing a subsea pipeline under nearshore landing conditions involving a composite stratum of reef and sand, comprising:

[0037] S1. Conduct geological surveys along the nearshore landing section and divide the pipeline route into sandy soil area and rocky area 4 based on strata distribution. In the sandy soil area, design the pipeline burial depth as a first predetermined depth value, and in the rocky area 4, design the pipeline burial depth as a second predetermined depth value, which is less than the first predetermined depth value. Geological surveys can employ a combination of drilling, in-situ testing, and geophysical exploration methods. The spacing between survey points can be set at 50-100m, and the survey depth should preferably exceed the predetermined burial depth value by 10-15m. The criteria for identifying the sandy soil area can be a standard penetration test (SPT) blow count of less than 30 blows and particle size analysis showing that it is predominantly sand. The criteria for identifying the rocky area 4 can be a core recovery rate greater than 85% and a saturated uniaxial compressive strength greater than 20 MPa. The range of the first predetermined burial depth value can be 2.0-4.0m, and the range of the second predetermined burial depth value can be 1.0-2.0m, with the second predetermined burial depth value being less than the first predetermined burial depth value. The specific burial depth can be determined through stability analysis. For sandy soil areas, the main focus is on liquefaction resistance analysis, while for rocky areas, the main focus is on erosion resistance analysis.

[0038] S2. A transition zone is established between the sandy area and the rocky area 4, and the longitudinal radius of curvature of the pipeline within the transition zone is controlled to be 2500D-3500D, where D is the outer diameter of the pipeline, so that the pipeline burial depth smoothly transitions from the first predetermined burial depth value to the second predetermined burial depth value. The length of the transition zone can be calculated based on the burial depth difference and the radius of curvature. For example, when the burial depth difference is 1.0m, the outer diameter of the pipeline is 0.6m, and the radius of curvature is 3000D, the length of the transition zone is approximately 120m. Within the transition zone, the elevation of the pipeline axis changes along the length direction in an arc curve. The elevation of each control point can be determined by measurement and layout to guide the trench excavation. To ensure a smooth transition, an elevation control section can be set every 10m for verification.

[0039] S3. Based on the first predetermined burial depth, the second predetermined burial depth, and the longitudinal radius of curvature, the seabed surface is broken and excavated to form a trench. In sandy areas, dredgers or long-arm excavators can be used for excavation. In rocky areas, the excavation method must be selected according to the water depth: in areas with a water depth of less than or equal to 2m, a temporary construction channel 6 with a width of 3-5m can be constructed first, followed by breaking and excavation using land-based machinery such as hydraulic breakers; in areas with a water depth greater than 2m, offshore reef-breaking vessels equipped with rock hammers or blasting equipment are used for breaking and excavation. After the trench is excavated, its bottom elevation, width, and slope must meet the design requirements and can be inspected by multibeam echo sounding or divers.

[0040] S4. After laying a buffer layer 7 at the bottom of the trench, the pipe 8 is laid. The buffer layer 7 can be a 200-300mm thick layer of fine sand, with a mud content of less than 5%. During laying, the fine sand can be directly poured into the trench using an excavator to form a continuous layer at the bottom. The pipe 8 is towed into position using a linear winch, employing a semi-floating towing method during construction. Floating buoys are tied to the pipe 8 at intervals to maintain a suspension height of approximately 1-2 meters above the seabed during towing, thus avoiding direct contact with the seabed or reefs. Simultaneously, the towing tension is monitored and controlled in real time to ensure a safe, stable, and controllable towing process.

[0041] S5. After the pipeline is laid, a crushed stone buffer layer 9 is first laid on top of the pipe, and then different backfilling treatments are carried out according to the actual burial depth of the pipeline. The thickness of the crushed stone buffer layer 9 is not less than 300mm, and the particle size of the crushed stone is 20-60mm. During backfilling, for sections in the transition zone where the actual burial depth of the pipeline is less than the second predetermined burial depth, riprap backfilling is used to protect the area that is locally higher than the seabed surface 1, forming a protective riprap layer 11 with a slope of not less than 3:1. The riprap is selected with an apparent density of not less than 2600kg / m³ in air. 3The graded stone material, the height and size of the riprap backfill should be calculated for stability to ensure that it does not scour and become unstable under the action of waves and currents, and to avoid pipeline exposure; the remaining trench section is backfilled with the excavated backfill reef 10 to be level with the seabed surface 1. The facing riprap layer 11 is not simply used to increase external protection, but to functionally compensate for the local soil cover deficiency introduced by the differentiated burial depth design through riprap backfilling and other methods, so that the pipeline achieves the same technical effect as under the target burial depth conditions in terms of key control indicators such as anti-buoyancy stability, anti-scour stability and external load capacity.

[0042] In this embodiment, by setting the burial depth differently for different zones, a shallower burial depth is adopted in the reef area 4, and a transition zone with a smooth longitudinal curvature is set. Combined with targeted excavation, laying and backfilling processes, the amount of excavation work in the reef area 4 can be effectively reduced while meeting the requirements for pipeline stability and protection. This reduces the construction difficulty and risk under complex geological conditions and helps to control construction costs and schedule.

[0043] According to another embodiment of the present invention, in step S1, the determination of the first predetermined burial depth value and the second predetermined burial depth value specifically includes: S11, performing differential analysis and obtaining benchmark values ​​for the sandy soil area and the rocky area 4 respectively. For the sandy soil area, the standard penetration test blow count obtained from geological exploration is used as the key input parameter. The standard penetration test can be conducted at one exploration point every 50m, and the blow count can be the average value of multiple hammer blows at that point. Based on this blow count, the anti-liquefaction stability analysis method considering seismic action is adopted to calculate the anti-liquefaction safety factor Fs of the pipeline at different burial depths. The minimum burial depth that meets the preset safety factor Fs≥1.5 requirement is obtained through iterative calculation, which is the critical burial depth H1. This analysis can be performed using the simplified method recommended by the standard or finite element numerical simulation. For the rocky area 4, the saturated uniaxial compressive strength measured by the rock core sample obtained from geological exploration is evaluated by combining the joint development status described on site. The saturated uniaxial compressive strength of the rock mass can be obtained through laboratory testing, and the sample diameter should preferably be 50mm and the height 100mm. Joint development can be classified into four levels—undeveloped, moderately developed, developed, and highly developed—based on the number of joints per unit length. Based on rock mass strength and joint condition, an anti-scouring stability analysis considering wave and water flow scouring is employed to calculate the anti-scouring safety factor Fsc for the pipeline at different burial depths. The minimum burial depth required to meet the preset safety factor Fsc≥2.0 is then calculated, which is the critical burial depth H2.

[0044] S12. Based on the characteristics of the composite strata, the above benchmark values ​​are collaboratively adjusted to determine the final predetermined burial depth. For reef area 4, since its critical burial depth H2 is mainly controlled by scour resistance and the strata themselves have high strength, the calculated critical burial depth H2 can be directly determined as the second predetermined burial depth value for construction. For sandy soil area, a safety margin Δ is added to the calculated critical burial depth H1 to determine the first predetermined burial depth value, i.e., first predetermined burial depth value = H1 + Δ. The safety margin Δ ranges from 0.5 to 1.0 m, and the specific value can be selected according to the uniformity of the sandy soil layer, the reliability of the survey data, and the importance level of the project. For example, a higher value can be taken for sections with large soil layer variations or high importance levels. Through this adjustment, it is ensured that the first predetermined burial depth value is always greater than the second predetermined burial depth value, providing clear input conditions for the subsequent transition zone design.

[0045] In this embodiment, by conducting specific stability analyses for the two main failure modes of sand liquefaction and reef erosion, the critical burial depth benchmark value based on the mechanical properties of the strata itself is obtained. Then, reasonable adjustments are made in combination with engineering experience and safety considerations. This provides a clear, operable and rational method for determining the differentiated design of pipeline burial depth in sandy areas and reef areas, which helps to achieve the rationality and economy of the design while ensuring pipeline safety.

[0046] According to another embodiment of the present invention, different excavation methods are selected based on water depth conditions. First, a water depth threshold of 2m is set as the dividing line for selecting excavation methods. This value takes into account the safe operating water depth limits of common land-based engineering machinery, the safety risks of construction personnel working in water, and the minimum water intake and operational efficiency of offshore reef-breaking vessels. Before excavation, detailed water depth measurements are required along the reef area 4, with measurement point spacing of 20-30m, to accurately define the areas with water depths less than or equal to 2m and greater than 2m. Measurements can be performed using a single-beam echo sounder or RTK combined with a sounding rod. Then, two specific breaking and excavation methods are implemented for different water depth areas. For areas with a measured water depth less than or equal to 2m, a temporary construction channel 6 is constructed, and land-based machinery is used for breaking and excavation. The temporary construction channel 6 can be formed by filling with sand and gravel or laying steel sheet piles; its top surface needs to be approximately 0.5m above the water surface, and its width is 3-5m to provide a stable platform for machinery operation. For land-based machinery, long-arm excavators equipped with hydraulic breakers can be used. During construction, the breakers break the rocks sequentially from the center line of the trench outwards, ensuring the particle size of the broken rocks is below 300mm for easy cleaning. For areas with a water depth greater than 2m, offshore reef-breaking vessels are used for breaking and excavation. The reef-breaking vessel can be equipped with rock drills or perform drilling and blasting depending on the strength of the rocks. Precise positioning is required during construction, and breaking operations must be carried out along the designed trench axis. The impact energy, frequency, and swing amplitude of the rock drills can be adjusted according to the on-site rock properties.

[0047] In this embodiment, by using a clear water depth threshold as a standard, a suitable crushing and excavation scheme is provided for different operating environments in the reef area 4. This allows for the use of efficient land-based machinery in shallow water areas and the utilization of the advantages of specialized vessels in deep water areas, thus taking into account the overall feasibility of construction, operational safety and economy.

[0048] According to another embodiment of the present invention, the specific setting parameters of the temporary construction passage 6 are: width 3-5m. The determination of this width range needs to comprehensively consider the body width and turning radius of the land-based machinery used (such as long-arm excavators or crawler cranes), as well as the passage requirements of construction material transport vehicles. For example, the outer width of the tracks of a medium-sized hydraulic excavator is approximately 2.8m, requiring a certain amount of operating and swinging space during operation. The temporary construction passage 6 is usually filled with well-graded crushed stone or gravel. The filling material can be sourced locally or transported from elsewhere, and its compaction degree should not be less than 93% to ensure the load-bearing stability and deformation resistance of the passage. During construction, filling is first carried out according to the layout position, with a filling thickness of 0.8m to 1.2m, layered compaction, and finally leveling to form a construction platform with the required width. The number of temporary construction passages 6 is based on a nominal diameter of DN400, which is determined according to GB / T1047-2019 "Definition and Selection of Nominal Sizes of Pipeline Components". When the diameter of pipe 8 is less than DN400, a temporary construction passage 6 is set up. This single passage is usually arranged on one side of the pipe trench for mechanical access, crushing operations, and removal of excavated soil. When the diameter of pipe 8 is DN400 or larger, two temporary construction passages 6 are set up. These two passages can be arranged in parallel on both sides of the pipe trench to form a dual-passage working surface.

[0049] In this embodiment, by specifying the width range of the temporary construction channel 6 and setting the number of channels according to the differences in pipe diameter, specific construction standards for land-based mechanical construction in shallow water reef areas are provided, which helps to achieve reasonable allocation of construction resources and improve work efficiency while ensuring sufficient construction work surface and smooth logistics.

[0050] According to another embodiment of the present invention, the buffer layer 7 is made of fine sand, and its laying thickness is limited to 200-300 mm. The fine sand can be natural river sand or qualified manufactured sand, with uniform particle size distribution, a particle size range mainly between 0.075-2 mm, and a mud content (particle size less than 0.075 mm) of less than 5%. Before laying, the moisture content of the fine sand should be tested to ensure it is close to the optimum moisture content, which is beneficial for compaction. The laying operation can be carried out after the trench excavation is accepted, using manual labor and small machinery to evenly lay the fine sand at the bottom of the trench. Thickness control is required during the laying process, which can be checked using a ruler or a pre-set elevation stake to ensure that the thickness of the cushion layer is within the design range of 200-300 mm, and the thickness deviation should not exceed ±20 mm. After laying, the surface of the cushion layer can be moderately leveled and compacted using a plate compactor or other suitable methods to form a flat and dense support base.

[0051] In this embodiment, by specifying the material and thickness parameters of the buffer pad, a uniform and flexible foundation support is provided for the pipeline, which helps to distribute the pipeline load and reduce the risk of direct contact between the pipeline and hard protrusions at the bottom of the trench, thereby providing a certain mechanical protection for the pipeline's anti-corrosion layer.

[0052] According to another embodiment of the present invention, biodegradable geotextile bags can be used as filling containers for fine sand when laying the buffer layer 7. The geotextile bags are usually made of polylactic acid (PLA) or other polymer materials that can be degraded by hydrolysis or microbial action in a marine environment over a certain period of time, and their initial tensile strength must meet the stress requirements during handling and laying. The size of the bag can be designed according to the convenience of construction, for example, 1.0m long and 0.5m wide, with a reserved sand filling opening. The fine sand filling amount is about 70% to 80% of the bag volume to ensure that the sandbag has sufficient flexibility and is easy to seal and splice. During laying, multiple filled sandbags are laid and spliced ​​at the bottom of the trench to form a continuous buffer layer 7. Specifically, the sandbags are laid flat and adjacent sandbags are closely packed together. To ensure the integrity and continuity of the layer, the longitudinal joints between the sandbags can be laid in a staggered manner, that is, the joints of the upper and lower layers of sandbags are staggered by a certain distance, for example, not less than 0.3m. During the laying process, the flatness of the sandbags needs to be checked to avoid local bulges or depressions. This method of bagging and splicing can effectively construct and fix the buffer layer underwater or in complex terrain conditions, preventing the layer material from being washed away or disturbed by water flow before the pipeline is laid.

[0053] In this embodiment, by adopting a construction method of assembling biodegradable sandbags, the loose fine sand material can be shaped and positioned, which facilitates precise laying and quality control in the underwater environment. The resulting bedding layer has better integrity, and after fulfilling its pipeline support function, the bag material can gradually degrade, reducing the long-term impact on the environment.

[0054] According to another embodiment of the present invention, the weight W of the blocks in the facing stone layer 11 is... D Determined by the following formula:

[0055]

[0056] The parameters in the formula are determined as follows: The current velocity V under the combined action of waves and current can be obtained from hydrological observation data of the project area, or the near-bottom current velocity at the design return period can be obtained through numerical simulation calculations, and its value may be 1.5-3.0 m / s. The gravitational acceleration g is taken as 9.81 m / s². 2 The density of water γ w Take 1025 kg / m 3 γ represents the density of seawater. The apparent density γ of the rock is... r Not less than 2600 kg / m 3 The value is determined based on the apparent density of the actual boulders. The angle θ between the slope surface and the horizontal plane... B Determined by the design slope, when the slope is not less than 3:1, the corresponding angle is approximately 18.4°. The angle of repose φ of the boulders. r The value can be determined through on-site stacking tests, and is usually 35°-40° for angular boulders. The constant y is selected based on whether the boulders are buried. For the facing boulders layer 11 protruding from the seabed surface 1, it is usually considered unburied and is taken as 0.86, while for buried boulders it is taken as 1.20.

[0057] Based on the calculated weight W of the single block D Furthermore, the representative dimension D of the facing stones was determined using a formula. 50min .

[0058]

[0059] In the formula, the apparent density γ of the stone r The value of W is determined based on the apparent density of the actual stones. D With γ r Substituting into the formula and calculating, the obtained D 50min This indicates that 50% of the selected set of stones has a diameter greater than this value, which is in mm. It gives the minimum median grain size required to meet the erosion resistance stability.

[0060] In this embodiment, by providing clear formulas for calculating weight and size and methods for parameter values, a quantitative design basis is provided for the selection of material specifications for the facing stone layer, which helps to ensure that the selected stones have sufficient individual stability under specific hydrological conditions.

[0061] According to another embodiment of the present invention, the particle size distribution of the boulders in the facing stone layer 11 is determined by a series of processes related to D. 50minThe relevant formulas are used to define the gradation range of the stone group. Specifically,

[0062]

[0063] Among them, D 50max It was stipulated that 50% of the stones in the sample were smaller than this maximum diameter. (D) 15min and D 15max This together defines the upper and lower limits of particle size for the smaller particle portion (15% throughput) in the sample. 100min It is stipulated that all stones must be larger than this minimum diameter; that is, particles smaller than this size are not allowed. (D) 100max This stipulates that all stones must be smaller than this maximum diameter, meaning that oversized stones exceeding this particle size are not allowed. In actual engineering projects, specific gradation requirements can be proposed to the stone supplier based on the calculation results, or screening tests can be conducted on the mined stone to ensure that its particle size distribution curve falls within the envelope defined by the indicators calculated by the above formula.

[0064] In this embodiment, by quantitatively controlling the particle size distribution of the stones through multiple indicators, a good gradation of the protective layer material is ensured, which is conducive to the tight interlocking between the stones, forming a protective structure with stronger integrity, moderate water permeability and better erosion resistance.

[0065] According to another embodiment of the present invention, the backfill thickness of the facing stone layer 11 is determined by the following formula:

[0066] ,

[0067] The calculation process of this formula is as follows: First, calculate 3.2 multiplied by D. 50min Multiply this by (π / 6) to the power of 1 / 3 to obtain a calculated thickness value. Then compare this calculated value with 500mm, and take the smaller of the two as the final minimum backfill thickness r, in mm. This formula controls the thickness in two ways: the first part derives the thickness required to meet a certain number of coverage layers based on representative stone block sizes, while the 500mm upper limit set in the second part takes into account the operability and economy of actual construction, avoiding obtaining an excessively large theoretical thickness when the stone block size is small. During construction, the r value determined by this formula is used as the control minimum value, and the actual backfill thickness should not be less than this value. Backfilling should be carried out in layers, and the elevation should be measured and controlled during the process.

[0068] In this embodiment, by combining the size of the boulders with the thickness calculation formula based on the upper limit of experience, a clear thickness control standard is provided for the construction of the facing boulders layer, so as to ensure that the protective layer has the necessary coverage thickness and overall stability.

[0069] The following is a further explanation of the submarine pipeline construction method of the present invention using a specific application example.

[0070] A nearshore landing project for a natural gas pipeline is underway. The pipeline is 24 inches in diameter, made of API 5L x 65PSL2 steel, with a wall thickness of 15.88 mm. The corrosion protection layer is 3LPE, 3.2 mm thick, and includes a 125 mm thick counterweight layer. This pipeline is laid from deep water towards nearshore landing, transporting natural gas for downstream power plant generation. Figure 1 As shown, the nearshore landing section encountered a reef area 4 approximately 280m long (KP62.33-KP62.61), with the following geological conditions: KP62.42-KP62.61: shallow water area (<1m water depth), reef; KP62.33-KP62.42: relatively deep water area (>3m water depth), reef; KP62.33 and beyond: deep-sea clay.

[0071] Due to the presence of reefs in the nearshore section, traditional methods of excavating trenches using sheet piles and long-arm excavators are not feasible. Pipeline laying must take reef excavation into account, making underwater reef removal the biggest challenge of construction. Analysis shows that the wave-breaking zone in this project is approximately 2.5 km long (KP60.11-KP62.61). If traditional methods are used for the nearshore section design and construction, the trench excavation depth would reach 3 m, resulting in a massive excavation volume and an excessively long construction period. Therefore, this invention proposes a nearshore section design and construction method using composite strata to minimize reef removal. Calculations show that the first burial depth in the sandy section is 2 m, and the second burial depth in the reef area is 1 m. A transition zone is set between KP62.32 and KP62.43, gradually transitioning from 2 m to 1 m, with a transition curvature radius of 3000D. However, in the KP62.36–KP62.41 section (transitional compensation backfill area 5), ​​since the burial depth is less than 1m, it is necessary to use riprap backfill to ensure that the soil cover thickness is not less than 1m.

[0072] According to the feasibility analysis, the water depth in section KP62.42-KP62.61 is relatively shallow, allowing for the construction of two temporary construction channels, each 190m long. One hydraulic breaker (a 350mm excavator equipped with a dedicated hydraulic breaker) will be used for breaking the reef, and two 400mm excavators will be used to clear the reef. The broken reef material will be placed on both sides of the channels. However, in section KP62.33-KP62.42, due to the greater water depth, continuing with the temporary construction channel method would require a large amount of boulders, and the excavator boom length would limit the required excavation depth. Therefore, a reef-breaking vessel will be needed for offshore construction.

[0073] This plan employs six temporary construction channels on both sides, each 5m wide and 2m above the water surface, with a 1:1 slope on the channel sides. The offshore section will be operated by a reef-breaking vessel that meets the draft requirements. After excavation, the bottom of the channel will be backfilled with 300mm of fine sand, and linear winches will be installed. Subsequently, a pipe-laying vessel will complete the pipe welding, and the pipes will be towed using a semi-floating tow method, with a maximum towing force of approximately 100 tons, meeting the design requirements.

[0074] After the pipeline is pulled out, backfill with crushed stone within 300mm of the top of the pipe, and then backfill the trench with excavated reefs. In transitional compensation backfill area 5, it is calculated that approximately 0.8m diameter boulders are required, with a maximum backfill height of 1.3m.

[0075] The nearshore section design and construction scheme described in this invention can reduce the amount of reef excavation by about 50% compared with the traditional scheme, and the temporary construction channel 6 and the reef breaking construction period are about 2 months. If the traditional construction scheme is used, the construction period will be extended to 4 months, and the cost will also double.

[0076] The number of devices and processing scale described herein are for simplification of the invention. Applications, modifications, and variations of the subsea pipeline construction method under nearshore landing section reef-sand composite strata conditions of the present invention will be readily apparent to those skilled in the art.

[0077] Although embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. For those skilled in the art, other modifications can be easily made. 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 illustrations shown and described herein.

Claims

1. A method for constructing subsea pipelines in nearshore landing sections under composite strata of reefs and sand, characterized in that, Includes the following steps: S1. Conduct a geological survey along the nearshore landing section and divide the pipeline route into sandy soil area and rocky area according to the stratum distribution; in the sandy soil area, design the pipeline burial depth to the first predetermined burial depth value; in the rocky area, design the pipeline burial depth to the second predetermined burial depth value, which is less than the first predetermined burial depth value. S2. Set up a transition zone between the sandy soil area and the rocky area, and control the longitudinal curvature radius of the pipeline in the transition zone to be 2500D-3500D, where D is the outer diameter of the pipeline, so that the burial depth of the pipeline can smoothly transition from the first predetermined burial depth value to the second predetermined burial depth value. S3. Based on the first predetermined burial depth value, the second predetermined burial depth value, and the longitudinal radius of curvature, the seabed surface is broken and excavated to form a trench; S4. Lay a buffer layer at the bottom of the trench, and then carry out the pipeline laying operation; S5. After the pipeline is laid, first lay a layer of crushed stone buffer on top of the pipe. Then, backfill according to the actual burial depth of the pipeline. When the actual burial depth of the pipeline in a local area within the transition zone is less than the second predetermined burial depth, backfill with boulders up to the seabed surface to form a facing boulders layer. The slope of the facing boulders layer shall not be less than 3:1, and the boulders shall be selected with an apparent density of not less than 2600 kg / m³ in air. 3 The graded stone material was used, and the remaining trench was backfilled with excavated reefs until it was level with the seabed. In step S1, the determination of the first and second predetermined burial depths specifically includes: S11, Differentiation analysis and benchmark value acquisition: For sandy areas, using the standard penetration test (SPT) blow count obtained from geological surveys as the key input parameter, liquefaction stability analysis is used to calculate and obtain the critical burial depth H1 that meets the preset liquefaction safety factor Fs≥1.5; For rocky areas, considering the saturated uniaxial compressive strength of the rock mass and joint development status obtained from geological surveys, scour stability analysis is used to calculate and obtain the critical burial depth H2 that meets the preset scour safety factor Fsc≥2.0; S12, Collaborative design adjustment based on composite strata characteristics: The critical burial depth H2 is directly determined as the second predetermined burial depth; Based on the critical burial depth H1, a safety margin Δ is added to determine the first predetermined burial depth, where the first predetermined burial depth = H1 + Δ; The safety margin Δ ranges from 0.5 to 1.0 m to ensure that the first predetermined burial depth is always greater than the second predetermined burial depth.

2. The method for constructing a submarine pipeline under composite strata conditions of reefs and sand in the nearshore landing section as described in claim 1, characterized in that, In step S3, in the reef area, the excavation method is selected according to the water depth conditions of the reef area: in areas with a water depth of less than or equal to 2m, a temporary construction channel is constructed and land-based machinery is used for breaking and excavation; in areas with a water depth greater than 2m, an offshore reef-breaking vessel is used for breaking and excavation.

3. The method for constructing a submarine pipeline under composite strata of reefs and sand in the nearshore landing section as described in claim 2, characterized in that, In step S3, the width of the temporary construction passage is 3-5m, and the number of temporary construction passages is determined according to the pipe diameter: a single construction passage is set when the pipe diameter is below DN400, and a double construction passage is set when the pipe diameter is DN400 and above.

4. The method for constructing a submarine pipeline under composite strata conditions of reefs and sand in the nearshore landing section as described in claim 1, characterized in that, In step S4, the buffer pad is a fine sand layer with a thickness of 200-300mm.

5. The method for constructing a submarine pipeline under composite strata conditions of reefs and sand in the nearshore landing section as described in claim 4, characterized in that, In step S4, when laying the buffer layer, fine sand is first filled into biodegradable geotextile bags to form sandbags. Then, multiple sandbags are laid at the bottom of the trench and spliced ​​together to form a continuous buffer layer.

6. The method for constructing a submarine pipeline under composite strata of reefs and sand in the nearshore landing section as described in claim 1, characterized in that, In step S5, the thickness of the crushed stone buffer layer is not less than 300 mm, and the particle size of the crushed stone is 20-60 mm.

7. The method for constructing a submarine pipeline under composite strata of reefs and sand in the nearshore landing section as described in claim 1, characterized in that, The weight of the stones in the facing stone layer is determined by the following formula: In the formula, W D The value is the weight of a single facing stone block, in kg; V is the velocity of the current under the combined action of waves and water flow, in m / s; g is the acceleration due to gravity, taken as 9.81 m / s². 2 ;γ w The density of water is taken as 1025 kg / m³. 3 ;γ r The apparent density of the stone is expressed in kg / m³. 3 ;θ B φ is the angle between the slope surface and the horizontal plane, i.e., the slope gradient; r y is the angle of repose of the stone, i.e., the angle of natural accumulation; y is a constant, dimensionless, taken as 0.86 for unburied stones and 1.20 for buried stones; The dimensions of the stones in the facing stone layer are determined by the following formula: ， In the formula, D 50min This represents the minimum diameter for which 50% of the stones have a diameter greater than this value, expressed in mm.

8. The method for constructing a submarine pipeline under composite strata conditions of reefs and sand in the nearshore landing section as described in claim 7, characterized in that, The particle size distribution of the stones in the facing stone layer must meet the following requirements: ， In the formula, D 50max This represents the maximum diameter where 50% of the stones have a diameter smaller than this value, in mm; D 15min This represents the minimum diameter, in mm, for which 15% of the stones have a diameter greater than this value; D 15max This represents the maximum diameter of 15% of the stones, expressed in mm; D 100min This refers to all stones with a diameter greater than this value, i.e., the minimum stone diameter, in mm; D 100max This value represents the diameter of all stones smaller than this value, i.e., the maximum stone diameter, in mm.

9. The method for constructing a submarine pipeline under composite strata conditions of reefs and sand in the nearshore landing section as described in claim 7, characterized in that, The backfill thickness of the facing stone layer is determined by the following formula: ， In the formula, r is the minimum backfill thickness of the facing stone layer, in mm.

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