Submarine pipe cable stress assessment method capable of considering local scoured terrain influence effect

Abstract

The invention discloses a submarine pipe cable stress assessment method capable of considering local scoured terrain influence. Seabed pipe cables laid on a seabed can generate serious local scouring action under the action of fluid, and a scouring terrain is formed below the pipe cables. The existing research work shows that the local scoured terrain can obviously influence the stress of the submarine pipe cable and further influence the in-place stability of the submarine pipe cable. Aiming at the stress problem of the submarine pipe cable considering the influence of the local scoured terrain, related scholars mainly carry out research work on the hydrodynamic coefficient of the pipe cable under the specific parameter condition, but a parameterized description method of the stress of the pipe cable is not formed at present, so that the method is difficult to serve for actual engineering design. Through a large amount of theoretical analysis and research work, the relation between the hydrodynamic coefficient of the pipe cable and the thickness delta / D of the dimensionless incoming flow boundary layer and the Reynolds number Re is found, so that the parameterized description method of the hydrodynamic coefficient of the pipe cable is established, and the parameterized description method can be directly used for accurately forecasting the stress of the pipe cable in actual engineering.

Description

Technical Field

[0001] The invention belongs to the field of submarine cable stress and in-situ stability design, and relates to a submarine cable stress assessment method capable of taking into account the influence of local scouring terrain. Background Art

[0002] Under the action of water flow, the cables and pipes laid on the seabed will experience severe local scouring, thus forming a large-scale suspended span. For cables and pipes laid flat on the seabed, the flow field behind the cables and pipes is suppressed by the seabed, and vortex shedding is not easy to occur. As the scouring pit expands, the cables and pipes are suspended in the air, making vortex shedding more likely to occur. Therefore, before and after the scouring occurs, the hydrodynamic forces on the cables and pipes are significantly different, and the hydrodynamic coefficients also change accordingly, which may affect the in-situ stability of the cables and pipes. Accurately evaluating the impact of scouring pits on the hydrodynamic coefficients of cables and pipes is of great significance to the in-situ stability design of cables and pipes.

[0003] Relevant scholars have conducted research on the influence of scouring topography on the hydrodynamic characteristics of pipes and cables. Li et al. (2018) studied and compared the hydrodynamic coefficients of pipes and cables on flat seabeds and scouring balanced seabeds, and found that the average drag coefficient and lift coefficient of pipes and cables on flat seabeds were greater than the hydrodynamic coefficients of pipes and cables with scouring topography, which shows that local scouring topography will have a significant effect on the hydrodynamic coefficients of pipes and cables. In addition, Chen et al. (2020) studied the hydrodynamic characteristics of pipes and cables on scouring balanced terrain under different incoming flow boundary layer thickness conditions, and found that the boundary layer thickness will also have a significant effect on the flow structure and hydrodynamic coefficients around the pipes and cables. With the increase of the thickness of the incoming flow boundary layer, the vortex scale behind the pipes and cables continues to increase, but the corresponding vortex will decrease, which in turn affects the hydrodynamic coefficients. The research work of Chen et al. (2020) shows that the average drag coefficient, average lift, root mean square lift coefficient and Strouhal number decrease significantly with the increase of boundary layer thickness.

[0004] From the above analysis, it can be seen that after local scouring of the cable, the surrounding seabed topography will significantly affect the hydrodynamic coefficient of the cable. In addition, the boundary layer thickness of the incoming flow is also an important factor affecting the stress of the cable. However, the current research on the stress of the cable under the influence of local scouring terrain needs to be improved. The related research work mainly considers the influence of specific parameters on the stress of the cable, and has not yet formed a parameterized prediction method. Therefore, the present invention conducts numerical simulation research, deeply summarizes, summarizes and analyzes the relevant results, and forms a submarine cable stress assessment method that can consider the influence of local scouring terrain. This method can serve the design of submarine cable stress and in-situ stability, thereby providing a scientific basis and technical guarantee for the safe design and service of submarine cables. Summary of the invention

[0005] In order to solve the above-mentioned problems existing in the prior art and to provide a scientific basis and technical guarantee for the stress and in-situ stability assessment of submarine cables under the influence of scour pit terrain, the present invention provides a submarine cables stress assessment method which can take into account the influence of local scour terrain, overcoming the technical defect that only partial parametric research has been carried out in the prior art and a parametric prediction method for submarine cables stress assessment has not yet been formed.

[0006] The technical solution of the present invention:

[0007] A method for evaluating the stress of a submarine cable taking into account the influence of local scouring terrain comprises the following steps:

[0008] (1) Determination of boundary layer thickness under different flow conditions

[0009] The thickness of the incoming flow boundary layer will profoundly affect the development process of local scouring, and then affect the stress of the submarine cable. Therefore, it is necessary to first determine the boundary layer thickness δ under different incoming flow velocities, mainly focusing on the influence of local scouring topography on the stress of submarine cables under unidirectional flow conditions; under unidirectional flow conditions, the distribution of horizontal flow velocity along the water depth satisfies the logarithmic rate formula, which is as follows:

[0010]

[0011] Where u(z) represents the horizontal velocity at depth z; κ is the Karman constant coefficient, which is 0.42; z s =d 50 / 12 represents the roughness length of the seabed, d 50 Indicates the median particle size of the sandy seabed, obtained through the sediment gradation curve; u f represents the bottom friction velocity; for the marine hydrological survey carried out in actual engineering, the velocity u(z) at a point in the measurement space is substituted into formula (1) to calculate u f ; For unidirectional flow conditions, the velocity at the edge of the boundary layer is u = 0.99U, at which z = δ, and U represents the free stream velocity; Substituting u = 0.99U into equation (1), the water depth z = δ at the edge of the boundary layer is calculated, and then the boundary layer thickness δ is obtained;

[0012] (2) Evaluation method of submarine cable stress considering the influence of local scour terrain

[0013] Combining theoretical analysis and numerical simulation, the average drag coefficient C is obtained dm , average lift coefficient C lm and the lift coefficient root mean square C lrms The relationship between boundary layer thickness and Re number is studied. On this basis, a method for evaluating the stress of submarine cable considering the influence of local scouring terrain is established through least squares fitting. The stress of cable is calculated by the average drag coefficient C. dm, average lift coefficient C lm and the lift coefficient root mean square C lrms To characterize, the empirical prediction formula of the hydrodynamic coefficient of the cable obtained by least squares fitting is as follows:

[0014]

[0015] Wherein, δ represents the thickness of the incoming flow boundary layer, D represents the diameter of the cable, Re=UD / ν represents the Reynolds number, U represents the free stream velocity, and ν represents the kinematic viscosity coefficient of the fluid.

[0016] It can be seen from the above formula that after the thickness of the incoming flow boundary layer is determined, the proposed formula is used to evaluate the forces on the submarine cable under different Reynolds numbers.

[0017] The beneficial effects of the present invention are as follows: through the evaluation method of the present invention, the hydrodynamic coefficient of the submarine cable under the influence of local scouring terrain is accurately predicted, and the relevant empirical formula comprehensively considers the influence of the incoming flow boundary layer thickness and the Reynolds number on the hydrodynamic coefficient of the cable, so as to carry out the evaluation of the in-situ stability of the submarine cable, thereby overcoming the defect that the existing research work has not yet formed an empirical prediction formula for the hydrodynamic coefficient of the submarine cable considering the influence of local scouring terrain, and is difficult to serve engineering design and practice. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a schematic diagram of numerical simulation calculation of submarine cable forces under local scour pit conditions;

[0019] Figure 2 The comparison results of the average drag coefficient of submarine cable and the empirical prediction formula;

[0020] Figure 3 The comparison results of the average lift coefficient of submarine cable and the empirical prediction formula are shown in Figure 2.

[0021] Figure 4 Comparison results of the root mean square lift coefficient of submarine cable and the empirical prediction formula.

[0022] In the figure: 1 pipe cable; 2 seabed with local scour pits; 3 inlet; 4 outlet; 5 upper boundary of the numerical calculation domain. DETAILED DESCRIPTION

[0023] The specific implementation of the present invention is further described below in conjunction with the accompanying drawings and technical solutions.

[0024] Example

[0025] The present invention verifies the effectiveness of the proposed method for predicting the stress on the pipe and cable by means of numerical simulation.

[0026] The open source software OpenFOAM is used for numerical calculation. The governing equation of fluid flow is the Reynolds-averaged Navier-Stokes equation. In order to realize the influence of the force on the submarine cable under high Reynolds number conditions, the SST k-ω two-equation model is used for turbulence closure.

[0027] Related numerical calculation models such as Figure 1 As shown, 1 represents the cable, and the non-slip boundary condition is applied in the numerical calculation, that is, the horizontal flow velocity u and the vertical flow velocity v are both equal to zero, and the normal derivative of the pressure is 2 represents the seabed with local scour pits, and a non-slip boundary condition is applied in the numerical calculation; 3 represents the inlet, and a mature boundary layer is used as the velocity inlet, so that the numerical simulation of different boundary layer thicknesses can be realized; 4 represents the outflow, and a free outflow boundary condition is adopted; 5 represents the upper boundary of the numerical calculation domain, and a symmetric boundary condition is applied in the numerical model.

[0028] In order to verify the effectiveness of the numerical analysis model established by the present invention, Table 1 shows the average drag force coefficient C calculated by the numerical analysis model established by the present invention. dm , average lift coefficient C lm , lift coefficient root mean square C lrms and Strouhal number St with the physical experimental results of Jensen et al. (1990). In the experiment of Jensen et al. (1990), the submarine cable was placed on a flushable sandy seabed, and the variation of the above coefficients during the flushing process was studied. The present invention selects data at the time of flushing equilibrium to verify the numerical analysis model. From the comparison results in Table 1, it can be seen that the hydrodynamic coefficients calculated by the present invention are in good agreement with the experimental results, which verifies the reliability of the numerical analysis model established by the present invention. Table 1 is a verification of the effectiveness of the numerical analysis model, including the average drag coefficient, the average lift coefficient, the root mean square lift coefficient and the Strouhal number

[0029] <![CDATA[C dam ]]> <![CDATA[C lam ]]> <![CDATA[C larms ]]> St Re δ / D Experimental results (Jensen et al., 1990) 0.96 -0.21 0.5 0.17 10000 2.0 Numerical simulation results of the present invention 0.926 -0.189 0.65 0.174 10000 2.0

[0030] Figure 2 , Figure 3 and Figure 4 The numerical simulation results of the average drag coefficient, average lift coefficient and root mean square lift coefficient of the submarine cable are compared with the prediction results of the empirical formulas (2) to (5) established in the present invention. From the comparison results, it can be seen that the empirical prediction formula established in the present invention is in good agreement with the numerical simulation results, which verifies the reliability of the empirical prediction formula proposed in the present invention.

Claims

1. A method for evaluating the stress of submarine cables taking into account the influence of local scouring terrain, characterized in that: The following steps are involved: (1) Determination of boundary layer thickness under different flow conditions The thickness of the incoming flow boundary layer will profoundly affect the development process of local scouring, and then affect the stress of the submarine cable. Therefore, it is necessary to first determine the boundary layer thickness δ under different incoming flow velocities, mainly focusing on the influence of local scouring topography on the stress of submarine cables under unidirectional flow conditions; under unidirectional flow conditions, the distribution of horizontal flow velocity along the water depth satisfies the logarithmic rate formula, which is as follows: Where u(z) represents the horizontal velocity at depth z; κ is the Karman constant coefficient, which is 0.42; z s =d 50 / 12 represents the roughness length of the seabed, d 50 Indicates the median particle size of the sandy seabed, obtained through the sediment gradation curve; u f represents the bottom friction velocity; for the marine hydrological survey carried out in actual engineering, the velocity u(z) at a point in the measured space is substituted into formula (1) to calculate u f ; For unidirectional flow conditions, the velocity at the edge of the boundary layer is u = 0.99U, at which z = δ, and U represents the free stream velocity; Substituting u = 0.99U into equation (1), the water depth z = δ at the edge of the boundary layer is calculated, and then the boundary layer thickness δ is obtained; (2) Evaluation method of submarine cable stress considering the influence of local scour terrain Combining theoretical analysis and numerical simulation, the average drag coefficient C is obtained dm , average lift coefficient C lm and the lift coefficient root mean square C lrms The relationship between boundary layer thickness and Re number is studied. On this basis, a method for evaluating the stress of submarine cable considering the influence of local scouring terrain is established through least squares fitting. The stress of cable is calculated by the average drag coefficient C. dm , average lift coefficient C lm and the lift coefficient root mean square C lrms To characterize, the empirical prediction formula of the hydrodynamic coefficient of the cable obtained by least squares fitting is as follows: Wherein, δ represents the thickness of the incoming flow boundary layer, D represents the diameter of the cable, Re=UD / ν represents the Reynolds number, U represents the free stream velocity, and ν represents the kinematic viscosity coefficient of the fluid.

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