Study On Laying Steel Fiber Reinforced Concrete Sandwich Pipe With Variable Rigidity

The steel fiber reinforced concrete sandwich pipe has a good performance in bending property and heat preservation,and this makes it more suitable for deep water longdistance energy transportation.Due to the difference between the concrete core material layer and the steel tube layer in deformation load,this type of pipeline is prone to tensile cracking and compression cracking of the concrete core material when bending,so that its bending stiffness changes.In the S-laying of pipe operation,when the sandwich pipe is clamped by the tensioner,the arch bend on the stinger and the vertical bend after leaving the stinger,and finally laid into the seabed,the bending in both directions will definitely cause cumulative damage to the core material and change its bending stiffness.In order to accurately check the strength of the steel pipe layer during pipe laying,the abovementioned changes in pipe bending stiffness need to be considered in the pipe laying analysis.This makes the pipe laying analysis combined with the change of stiffness one of the key problems that need to be solved in the practical application of sandwich pipe.In this paper,the concrete damage plastic model is used to analyze the strength characteristics of the steel fiber reinforced concrete sandwich.The four-point bending loading method is used to simulate the inelastic behavior of the pure bending deformation of the sandwich tube.The nonlinear relationship between the bending curvature of steel fiber reinforced concrete sandwich pipe and its bending stiffness is established.Combined with the bending process of pipe laying operation,use logarithmic function and quadratic function to fit the relationship between stiffness and curvature in forward bending and reverse bending,respectively.For the analysis of sandwich pipe laying,the paper uses Orca Flex software to define the pipe-beam element for modeling and calculation.Under the same water depth and stiffener support conditions,the difference between the analysis results of variable stiffness and constant stiffness is compared,which fully demonstrates the necessity of variable stiffness research.On the basis of the above,a theoretical study on the analysis of the variable-stiffness laying of sandwich pipes is carried out.For the arc section of the forward bending on the stinger,referring to the multi-support continuous beam structure,the transfer matrix method is used to establish the variable stiffness control equations.The energy method is used to solve the supporting force of each idler,and then we can obtain the bending stiffness of the sandwich pipe on the holding frame and the bending moment distribution of the sandwich pipe between the idlers.In the vertical bending stage,based on the analysis of the stress of the micro-element tube section,the paper establishes a new control equation system with a nonlinear relationship between bending stiffness and curvature.On the basis of fitting the pipe stiffness with quadratic function,a perturbed differential equation with dimensionless small parameters is established.Then the external and internal solutions are solved analytically,and then the configuration of the overhanging section of the pipeline is obtained through iterative calculation.In this paper,when comparing the example of pipe laying at 500 m water depth,the analytical method proposed is similar to the Orca Flex simulation result,which verifies the effectiveness of the method.