Submarine pipe cable three-dimensional local scour expansion rate forecasting method capable of overcoming scale effect problem of sediment physical experiment
By determining the transport rate and critical starting stress of the soil, combined with the seabed shear stress and sediment quality conservation, the problem of inability to consider the transport characteristics of clay soil in the three-dimensional local erosion forecast of subsea pipe cables is solved, and the seabed is achieved Accurate forecast of the pipe cable erosion expansion rate, serving the safety design and operation and maintenance of submarine cables.
Patent Information
- Application Number
- CN202411220313.8
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2024-09-02
- Publication Date
- 2024-12-24
AI Technical Summary
The existing three-dimensional local erosion forecast model of subsea pipe cables is based on loose sand without viscosity, and the transport characteristics of common clay soils in the project cannot be considered, resulting in the prediction of the pipe cable erosion expansion rate, which is too high, and the transport rate of clay soils cannot be accurately predicted. Critical starting stress has the problem of scale effect of silt and sand physics experiments.
By determining the transport rate and critical starting stress of soil, a quantitative relationship between soil transport rate and seabed shear stress is established, and combined with the law of conservation of sediment mass and stress amplification effect, the expansion rate of three-dimensional local erosion of submarine pipe cables is calculated. , overcome the scale effect of sediment physics experiments and provide accurate forecasting methods.
It realizes accurate forecasting of the three-dimensional local erosion expansion rate of submarine pipe cables, serves the safety design and operation and maintenance of submarine cables, effectively overcomes the problem of excessive prediction in the existing technology, and provides scientific basis and technical guarantees.
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Figure CN119180233A_ABST
Abstract
Description
Technical Field
[0001] The invention belongs to the research field of local scour and in-situ stability of submarine structures, and relates to a method for predicting the three-dimensional local scour expansion rate of submarine cables, which can overcome the scale effect problem of sediment physical experiments. Background Art
[0002] Submarine cable structures laid on the seabed will experience local scouring under the action of fluids. As time goes by, local scouring will continue to expand along the depth and axial direction of the cable, thus forming a suspended span with a certain length. Under the action of fluids, the cable in the suspended span will experience vortex-induced vibration and strong pipe-soil interaction, which will lead to fatigue fracture or wear damage, causing huge economic losses and even difficult-to-repair marine environmental disasters. Therefore, it is necessary to study the scouring expansion rate of the cable along the axial direction, and then clarify the temporal and spatial distribution law of the suspended span, so as to serve engineering design and cable operation and maintenance.
[0003] At present, a large number of scholars at home and abroad have carried out relevant research on the three-dimensional local scour of submarine pipelines and cables, and established a three-dimensional local scour prediction formula for pipelines and cables that can take into account different flow conditions. For details, see "Three-dimensional scour below offshore pipelines in steady currents." and "Fangjun Li. 3D scour below pipelines under waves and combined waves and currents." published by Liang et al. However, these empirical formulas are basically based on physical experiments of loose quartz sand (non-viscous). However, the actual seabed soil often shows obvious viscosity characteristics, and its critical starting stress is significantly higher than that of loose quartz sand. In addition, there is a large difference between the transport rate of cohesive soil and the transport rate of loose sand, which leads to the existing local scour expansion rate prediction formula of pipelines and cables based on loose sand often giving an over-high prediction result. This involves the problem of scale effect in sediment physics experiments. The fundamental reason is that sediment particles cannot be scaled according to the geometric scale of the structure. Otherwise, it will cause major changes in the physical and mechanical properties of the sediment, causing distortion of the physical experimental results. Therefore, it is necessary to develop physical experimental methods that can overcome the problem of scale effect in sediment physics experiments and achieve accurate prediction of the three-dimensional local scour expansion rate of submarine pipes and cables.
[0004] The present invention will start from the physical nature of local scour, that is, no matter what kind of soil or what kind of underwater structure, the physical mechanism of local scour is that the shear stress of the seabed near the structure is greater than the critical shear stress of the soil. Based on the above understanding, the present invention intends to establish a prediction method for the three-dimensional local scour expansion rate of pipes and cables that can overcome the problem of scale effect in sediment physical experiments. As mentioned above, there are often large differences in the transport rate and critical starting stress corresponding to different soil types. Due to the diversity of soil types, there is currently no unified prediction method that can predict the transport rate and critical starting stress of soil. Therefore, the core of the above prediction method is the critical shear stress and transport rate of different types of soil, and the stress amplification effect of the seabed under the pipe and cable. Summary of the invention
[0005] In order to solve the above problems existing in the prior art and provide a scientific basis and technical guarantee for the safe design and construction of submarine cables, the purpose of the present invention is to provide a method for predicting the three-dimensional local scour extension rate of submarine cables, which can overcome the difficulty of scale effect in sediment physical experiments, and overcome the major defect that the existing three-dimensional local scour prediction model of submarine cables is based on non-cohesive loose sand and cannot consider the transport characteristics of common cohesive soil in engineering, resulting in it often giving an excessively high cable scour extension rate, thereby providing a scientific basis and technical guarantee for the scour design and safe service of submarine cables.
[0006] The technical solution of the present invention:
[0007] A method for predicting the three-dimensional local scour expansion rate of submarine cables that can overcome the scale effect problem of sediment physical experiments includes the following steps:
[0008] (1) Quantitative relationship between sediment transport rate and seabed shear stress and determination of critical starting stress
[0009] For underwater structures, the fundamental reason for local scouring is the shear stress τ of the seabed around the structure. f Greater than the critical starting stress τ of the soil cr In addition, it is also related to the scouring transport rate of the soil Closely related; At present, for the soils with certain viscous effects commonly seen on engineering sites, a unified description method for describing their transport characteristics has not yet been established. Therefore, it is necessary to carry out scouring physical experiments on relevant soils to determine the transport rate and critical starting stress of the soils, and then establish the following quantitative relationship between the soil transport rate and the seabed shear stress:
[0010]
[0011] Formula (in which It represents the transport rate of soil, with the unit of m / s, reflecting the scouring depth of soil per unit time; τ represents the seabed shear stress caused by fluid movement; τcr It represents the critical starting stress of the soil, which reflects the soil's anti-scouring ability. When the seabed shear stress τ caused by the flow is greater than the critical starting stress τ of the soil cr , it means that scour effect will occur; A and B are constant coefficients, which need to be obtained through physical experiments on soil scour characteristics and fitting by the least square method;
[0012] The seabed shear stress τ caused by fluid movement can be determined by the distribution of horizontal flow velocity along the water depth satisfying the logarithmic rate function. The relevant formula is as follows:
[0013]
[0014] Where u(z) represents the horizontal velocity at depth z; κ≈0.42 is the Karman constant coefficient; k s =2.5d 50 represents the roughness height of the seabed, d 50 Indicates the median particle size of sediment, which can be obtained through the sediment grading curve; u f represents the bottom friction flow velocity; from formula (2), when the flow velocity at a certain point in space is known, the bottom friction flow velocity u can be calculated by formula (2): f , then the seabed shear stress caused by the flow conditions is τ=ρu f 2 , where ρ represents the density of the water body;
[0015] (2) Establishment of a three-dimensional local scour expansion rate method for submarine cables that can overcome the scale effect of physical experiments
[0016] For the three-dimensional local scouring of submarine cables, the scouring occurs in two dimensions: 1) Scouring along the depth direction of the cable, and the scouring extension rate is expressed as U V 2) Scour along the axial direction of the cable, the scour expansion rate is represented by U H express;
[0017] For the soil scouring characteristic experiment, the entire experimental soil sample is regarded as the control body. Assuming that the length of the experimental soil sample is L, according to the mass conservation law of sediment, the scouring rate of the soil sample is It can be determined using the following formula:
[0018]
[0019] Where ε represents the porosity of the sediment; q in and q out are the sediment transport rates into and out of the control volume, respectively; E and F are the erosion rate and sedimentation rate of the suspended sediment in the control volume; in the soil scour experiment, there is no sediment supply upstream, that is, q in = 0, then the sediment transport rate along the bed surface is qb =q out ; The sediment in the experiment is mainly transported by bedload, so the erosion rate E and sedimentation rate F of the suspended sediment are both zero; Based on the above understanding, combined with formula (1), formula (3) is further expressed as:
[0020]
[0021] When the cable structure is laid on the seabed, the sediment transport rate (mainly bed load) of the control body within the length of the cable diameter D also needs to satisfy the law of conservation of sediment mass, which can be expressed as follows:
[0022]
[0023] Among them, q p,in and q p,out They represent the sediment transport rate into and out of the control volume when there is a cable structure. When the cable is in the initial stage of scouring, the sediment transport rate is the largest and q p,out >>q p,in , then the sediment transport rate below the cable is q b =q p,out -q p,in =q p,out ;
[0024] At the time when the scouring of the cable starts, that is, t = 0, the sediment erosion rate under the cable is equal to the scouring extension rate in the depth direction, so:
[0025]
[0026] Among them, S(t) represents the change of the scour depth under the cable with time, and the value of its first-order derivative with respect to time at t = 0 is the scour expansion rate U along the cable depth direction. V , S 0 represents the equilibrium scour depth, T represents the scour time scale;
[0027] Due to the existence of the pipe cable, the seabed below it will produce a stress amplification effect. By introducing the stress amplification coefficient α, we can get:
[0028] q b,max | t＝0 =A(ατ-τ cr ) B (1-ε)L (7)
[0029] The physical meaning of the above formula is that when the shear stress of the seabed below the cable ατ>τ cr , local scouring will occur. Combining formulas (6) and (7), we have:
[0030]
[0031] In the initial stage of scouring, the expansion rate in the depth direction is the slope of the scouring depth over time, so the scouring expansion rate in the depth direction is U V The following formula can be used for calculation:
[0032]
[0033] When the local scour below the cable expands along the cable axis, a shoulder with a certain angle will be formed at the bottom of the cable. Assuming that the inclination angle of the shoulder is represented by φ, the scour expansion rate in the depth direction is U V The horizontal scour expansion rate U along the axial direction of the cable H Satisfies the following relationship:
[0034]
[0035] Combining formulas (9) and (10), the horizontal scour expansion rate U along the axial direction of the seabed cable is H It can be expressed as follows:
[0036]
[0037] From the above, it can be seen that the horizontal scour expansion rate U along the axial direction of the seabed cable H It is related to the scouring characteristics of the soil, which can be obtained through the soil scouring experiment in A; it is also related to the inclination angle φ at the shoulder and the shear stress amplification factor α under the pipe and cable.
[0038] (3) Determination of the shear stress amplification factor α of the seabed below the cable and the inclination angle φ at the shoulder
[0039] There is no unified formula to predict the specific values of the inclination angle φ at the shoulder and the shear stress amplification factor α of the seabed below the cable. Generally, they need to be determined through physical experiments. In the physical experiment, several ultrasonic Doppler probes (UDV) are arranged at equal intervals along the axial direction of the cable. They have the function of sounding and can directly measure the scour expansion rate U in the scour direction of the cable. V , then the shear stress amplification factor α of the seabed below the cable can be determined according to formula (9). Assuming the spacing between two adjacent UDV probes is ΔL, the scouring depth of the first UDV sensor is assumed to be S 1 , the flushing depth of adjacent sensors is S 2 , then tanφ=(S 1 -S 2 ) / ΔL.
[0040] Beneficial effects of the present invention:
[0041] 1) Through soil scouring experiments, the scouring rate and critical shear stress of the soil can be determined, thereby judging whether scouring will occur on the submarine cable;
[0042] 2) The three-dimensional local scour prediction method for submarine cables established by the present invention can effectively overcome the scale effect problem of sediment physics experiments, and accurately predict the three-dimensional local scour expansion rate of submarine cables, thereby serving the safe design and operation and maintenance of submarine cables. BRIEF DESCRIPTION OF THE DRAWINGS
[0043] Figure 1 Set up a physical experiment for soil scour characteristics;
[0044] Figure 2 is the quantitative relationship between the scouring rate of the soil and the shear stress of the seabed;
[0045] Figure 3 Set up a three-dimensional local scour physics experiment for submarine cables;
[0046] Figure 4 is the shoulder inclination angle at different span lengths along the axial direction of the cable;
[0047] Figure 5 is the stress amplification factor below the umbilical obtained by formula (9);
[0048] Figure 6 Comparison of the span length prediction results obtained by formula (11) with the experimental results. DETAILED DESCRIPTION
[0049] The specific implementation of the present invention is further described below in conjunction with the accompanying drawings and technical solutions.
[0050] First, physical experiments on soil scouring characteristics are needed to obtain the critical shear stress of the soil and the scouring rate of the soil sample. The quantitative relationship between τ and the seabed shear stress τ. The relevant physical experimental settings are as follows: Figure 1 As shown. 1 represents the experimental terrain, which can be cast by concrete; both ends of the experimental terrain are set with a slope of 1:10 2 to ensure that the flow of the fluid can slowly transition to the experimental terrain and form a mature boundary layer during the experiment; 3 is a soil sample box, which is used to carry the test soil, with a length of L = 20 cm (parallel to the incoming flow direction), and the depth and width can be set to 10 cm; 4 is a laser Doppler flow meter, located above the soil sample box, which is used to measure the flow velocity at a fixed point in space, and then the bottom friction velocity generated by the fluid flow is calculated by formula (2), then the seabed shear stress is τ = ρu f 25 is a three-dimensional laser topographic scanner, which can measure the scouring depth change of the soil in the soil sample box under a certain flow rate in real time. Assuming that the average depth change in the Δt time period measured by physical experiments is ΔH, the scouring rate of the soil sample is In physical experiments, different soil sample scouring rates can be obtained by loading different flow rates. and the corresponding seabed shear stress τ, the critical shear stress τ of the soil sample can be determined by the least squares method cr And the quantitative relationship between soil scouring rate and seabed shear stress as shown in formula (1). Related physical experimental results are as follows Figure 2 As shown, Figure 2 The points in the figure represent the results of physical experiment measurements, and the line represents the results of least squares fitting. Through least squares fitting, it can be determined that the constant coefficients A and B of formula (1) are 7.604×10 -6 and 0.782, when The corresponding horizontal axis is the critical shear stress of the soil, and its value is τ cr =0.42Pa.
[0051] On the basis of the above physical experiments, a three-dimensional local scouring physical experiment of submarine cables was carried out. Figure 3 As shown. 6 represents the experimental sand pit, which can be 6m in length in general to ensure sufficient sediment supply in the upstream direction, with a depth of not less than 40cm and a width consistent with the width of the experimental flume; 7 represents the model pipe cable, which can be D=5cm in diameter and has a length consistent with the width of the experimental flume. In the physical experiment, both ends of the model pipe cable need to be fixed to ensure that the model pipe cable does not move during the physical experiment; 8 represents the 1:10 transition slope at both ends of the experimental terrain to ensure that the incoming flow can slowly transition to the experimental section to form a stable flow. In the physical experiment, ultrasonic Doppler probes need to be arranged at equal intervals (no more than 15cm) inside the pipe cable. Assume that the spacing between two adjacent UDV probes is ΔL, and the scouring depth of the first UDV sensor is assumed to be S 1 , the flushing depth of adjacent sensors is S 2 The inclination angle of the waistband satisfies tanφ=(S 1 -S 2 ) / ΔL, the inclination angle φ at the waistband can be determined by this relationship. The relevant physical experimental results are as follows Figure 4 As shown in the figure, it can be seen that the average value of the inclination angle at the shoulder is about φ≈12°; the scour expansion rate U in the depth direction can be measured by the ultrasonic Doppler probe V , the shear stress amplification factor α of the seabed below the cable is further calculated by formula (9). The relevant physical experimental results are as follows: Figure 5As shown, the horizontal axis e / D represents the dimensionless initial buried depth of the cable. Based on the above physical quantities, the horizontal expansion rate U along the cable axis can be calculated by formula (11): H , and then multiply it by the time of the physical experiment to get the span length L H . Figure 6 The span length L predicted by formula (11) is given as H The circles in the figure represent the results of the physical experiment, and the dotted line represents the results predicted by formula (11). As can be seen from the figure, the predicted results are in good agreement with the experimental observations, with the maximum error within 5%, which fully proves the effectiveness of the method proposed in the present invention.
Claims
1. A method for predicting the three-dimensional local scour expansion rate of submarine cables that can overcome the scale effect problem of sediment physical experiments, characterized in that: The following steps are involved: (1) Quantitative relationship between sediment transport rate and seabed shear stress and determination of critical starting stress For underwater structures, local scouring occurs due to the shear stress τ of the seabed around the structure. f Greater than the critical starting stress τ of the soil cr In addition, it is also related to the scouring transport rate of the soil Closely related; physical experiments on the scouring characteristics of soil are carried out to determine the soil transport rate and critical starting stress, and then the following quantitative relationship between soil transport rate and seabed shear stress is established: In the formula, It represents the transport rate of soil, with the unit of m / s, reflecting the scouring depth of soil per unit time; τ represents the seabed shear stress caused by fluid movement; τ cr It represents the critical starting stress of the soil, which reflects the soil's anti-scouring ability. When the seabed shear stress τ caused by the flow is greater than the critical starting stress τ of the soil cr , which means that scour effect will occur; A and B are constant coefficients, obtained through physical experiments on soil scour characteristics and fitting by the least square method; The seabed shear stress τ caused by fluid movement is determined by the distribution of horizontal flow velocity along the water depth to satisfy the logarithmic rate function. The relevant formula is as follows: Where u(z) represents the horizontal velocity at depth z; κ = 0.42 is the Karman constant coefficient; k s =2.5d 50 represents the roughness height of the seabed, d 50 Indicates the median particle size of sediment, obtained through the sediment grading curve; u f represents the bottom friction velocity; from formula (2), when the velocity at a certain point in space is known, the bottom friction velocity u can be calculated by formula (2): f , then the seabed shear stress caused by the flow conditions is τ=ρu f 2 , where ρ represents the density of the water body; (2) Establishment of a three-dimensional local scour expansion rate method for submarine cables that can overcome the scale effect of physical experiments For the three-dimensional local scouring of submarine cables, the scouring occurs in two dimensions: 1) Scouring along the depth direction of the cable, and the scouring extension rate is expressed as U V 2) Scour along the axial direction of the cable, the scour expansion rate is represented by U H express; For the physical experiment of soil scouring characteristics, the entire experimental soil sample is regarded as the control body. Assuming that the length of the experimental soil sample is L, according to the law of conservation of mass of sediment, the scouring rate of the soil sample is Determined using the following formula: Where ε represents the porosity of the sediment; q in and q out represent the sediment transport rate into and out of the control volume, respectively; E and F represent the erosion rate and sedimentation rate of the suspended sediment inside the control volume; in the physical experiment of soil scouring characteristics, there is no sediment supply upstream, that is, q in = 0, then the sediment transport rate along the bed surface is q b =q out ; The sediment in the experiment is mainly transported by bedload, so the erosion rate E and sedimentation rate F of the suspended sediment are both zero; Based on the above understanding, combined with formula (1), formula (3) is further expressed as: When the cable structure is laid on the seabed, the sediment transport rate of the control body within the length of the cable diameter D must also satisfy the law of conservation of sediment mass, which is expressed as follows: Among them, q p,in and q p,out They represent the sediment transport rate into and out of the control volume when there is a cable-tube structure. When the cable-tube is in the initial stage of scouring, the sediment transport rate is the largest and q p,out >>q p,in , then the sediment transport rate below the cable is q b =q p,out -q p,in =q p,out ; At the time when the scouring of the cable starts, that is, t = 0, the sediment erosion rate under the cable is equal to the scouring extension rate in the depth direction, so: Among them, S(t) represents the change of the scour depth under the cable with time, and the value of its first-order derivative with respect to time at t = 0 is the scour expansion rate U along the cable depth direction. V , S0 represents the equilibrium scour depth, T represents the scour time scale; Due to the existence of the pipe cable, the seabed below it will produce a stress amplification effect. By introducing the stress amplification coefficient α, we can get: q b,max | t＝0 =A(at-t) cr ) B (1-e)L (7) The above formula is when the shear stress of the seabed below the cable ατ>τ cr , local scouring will occur; Combining formulas (6) and (7), we have: In the initial stage of scouring, the expansion rate in the depth direction is the slope of the scouring depth over time, so the scouring expansion rate in the depth direction U V The following formula is used for calculation: When the local scour below the cable expands along the cable axis, a shoulder with a certain angle will be formed at the bottom of the cable. Assuming that the inclination angle of the shoulder is represented by φ, the scour expansion rate in the depth direction is U V The horizontal scour expansion rate U along the axial direction of the cable H Satisfies the following relationship: Combining formulas (9) and (10), the horizontal scour expansion rate U along the axial direction of the seabed cable is H Expressed in the following form: From the above, it can be seen that the horizontal scour expansion rate U along the axial direction of the seabed cable H It is related to the scouring characteristics of the soil, obtained through physical experiments on the scouring characteristics of the soil; it is also related to the inclination angle φ at the shoulder and the shear stress amplification factor α under the pipe and cable; (3) Determination of the shear stress amplification factor α of the seabed below the cable and the inclination angle φ at the shoulder Several ultrasonic Doppler probes are arranged at equal intervals along the axial direction of the cable to directly measure the scouring extension rate U in the scouring direction of the cable. V , and then determine the seabed shear stress amplification factor α under the cable according to formula (9); assuming that the distance between two adjacent ultrasonic Doppler probes is ΔL, the scouring depth of the first ultrasonic Doppler probe is assumed to be S1, and the scouring depth of the adjacent sensor is S2, then tanφ=(S1-S2) / ΔL.