Experimental Study And Numerical Simulation Of Real Scale Inner Wall Erosion Of Marine Non-bonded Flexible Pipe

The non-bonded flexible pipe is a complex inner wall structure pipe made of 316 L stainless steel with an "S" shaped cross section spiral interlocking structure.It has been widely used in subsea oil and gas recovery manifold system because of its good compression and bending performance.The main reason for the failure of flexible pipe is that it is crushed by pressure in practice.However,recent pipe failure cases abroad show that the erosion and wear of sand particles in the pipe wall is also an important reason for the failure of flexible pipe.In this paper,the erosion of complex inner wall of flexible pipe by liquid-solid two-phase flow is studied by means of physical model test and numerical model.In terms of physical model experiment,the erosion test of flexible pipe with real scale was carried out on the platform by independently building a tubular flow test platform.According to the control variable method,the erosion rate and erosion distribution of flexible pipe skeleton layer were tested by using three different particle sizes of sand.In terms of numerical simulation research,unbonded flexible pipe unstructured mesh is realized in mesh software and imported into FLUENT software.Based on Generic erosion model,Realisable k-ε turbulence model is selected to consider the influence of sand diameter,velocity,mass content and bending diameter ratio on erosion rate of specific wall surface of flexible pipe.The erosion law of inner wall of flexible pipe is obtained.The results show that the maximum erosion position of the inner wall structure of the flexible pipe is at the junction of the circular section and smooth section of the water surface,and the erosion rate of the two sides decreases gradually.In the curved flexible pipe section,the maximum erosion rate occurs outside the elbow,and the maximum erosion occurs in the20°-35° area at the entrance of the flexible pipe.The relationship between erosion rate and particle size of flexible tube: erosion rate increases with the increase of particle size.Taking this test as an example,the erosion rate increases with the increase of particle size in the range of 275 μm to 550 μm is greater than that in the range of 550 μm to 950 μm.The ratio of the maximum erosion rate of curved flexible pipe section to that of straight flexible pipe section under the same condition is about 2.56 times.The erosion rate of flexible pipe has a linear relationship with the increase of particle mass content,an exponential relationship with the change of flow velocity,and an inverse relationship with the increase of bending to diameter ratio.Finally,based on the dimensional analysis method,based on the calculation formula of erosion of DNV specification welded round pipe,combined with the results of physical model test and numerical simulation,the prediction formula of erosion rate of flexible pipe complex inner wall based on the velocity of near-wall sand was constructed by using the method of parameter modification.The prediction formula was verified by the test data.The new prediction formula of erosion rate can better predict the erosion rate at different points in the inner wall of flexible pipe.