Study On Torsional Instability And Reliability Of Marine Flexible Pipelines

Due to its unique multi-layer composite structure,marine flexible pipeline has many advantages,for example,good suppleness,easy installation and recycle,corrosion resistance and so on.It has been widely used in marine oil and gas exploration and development.Flexible pipelines are subjected to torque,tension and environmental loads during installation or operation,and are prone to torsional instability and kinking,resulting in failure.Research on the formation theory and preventive measures of kink is essential to ensure the safe operation of marine flexible pipelines.In this paper,theoretical mechanical analysis and finite element numerical calculation are used to systematically study the mechanical response characteristics and Instability mechanism of flexible pipeline under the combined action of torque and tension.The following work is carried out in this paper:(1)The buckling process of geometric beams or shafts supported by spherical hinges at both ends under torque and axial force is derived by theoretical mechanics analysis,obtaining the exact relationship between torque and axial force when the loop of flexible pipeline is formed by torsion.The flexible pipeline model of the catenary layout was established by Orca Flex,considering the environmental loads,such as waves,currents and seabed friction and so on,the process of forming a loop under the torque was simulated.(2)The formation of the loop under the combined action of torque and tension of flexible pipeline is studied from the perspective of torsional strain energy converting into bending strain energy,and the bending radius of loop and decreases under the action of tension,leading to the formation of kink.The process of forming a kink under the combined end torque and tension of the flexible pipeline is simulated by Orca Flex,and the numerical solution of the critical torque is compared with the torque prediction of Greenhill equation,and the reason for the error is analyzed.The results show that the relationship between torque and rotation angle is nonlinear and oscillation occurs;the touchdown zone is the active area where the flexible pipeline may form loop;the loop diameter of the flexible pipeline is related to the Bending stiffness,torque and tension;The loop radius will decrease when the end of the loop is subjected to tensile forces,causing the flexible pipeline to fail.(3)Introducing the concept of reliability and structural randomness,and analyzing the reliability of the marine flexible pipeline under the action of torque and tension,and obtaining the reliability parameters such as the probability of torsion failure and the reliability index.It has certain guiding significance for the design,manufacture and installation of marine flexible pipelines.