| Offshore slender flexible cylindrical structures such as marine risers and submarine pipelines are the key equipment in the process of oil and gas extraction and transportation.The fatigue damage or even fracture of such structures is easy to occur under the action of the external current.Due to the strong destructiveness of pipeline fatigue fracture and the idealization of the present research model,it is urgent to study the vortex-induced vibration fatigue characteristics of flexible cylindrical structures in accordance with engineering practice.Based on theoretical analysis and model experiments,the vortex-induced fatigue damage characteristics of submarine suspended pipelines under asymmetric boundary conditions and inclined cylindrical structures are studied in this paper.The main research contents and conclusion are as follows:Firstly,soil spring is used to simulate the pipe-soil interaction between the two ends of the pipeline.Based on this analysis,the effects of end asymmetric boundary conditions on vortex induced vibration fatigue damage of submarine suspended pipeline are analyzed.It is found that there is a critical region(10~6-10~8)for the torsional stiffness of soil at both ends of the suspended span.When the torsional stiffness changes in this area,the fatigue characteristics of the submarine suspending pipeline will change dramatically,and the end torsional spring stiffness has little effect on the fatigue of the structure outside the region.Secondly,based on the physical model experiments,the characteristics of vortex-induced fatigue damage of flexible cylindrical structures with inclination of 0°,15°,30°,45°and 60°are analyzed.The applicability of the independence principle in the field of vortex-induced fatigue damage of inclined flexible cylindrical structures is discussed.The following patterns were found:when the inclination angle is less than30°,the fatigue damage characteristics of vortex-induced vibration in the transverse flow direction of the structure are similar to those under the condition of vertical flow,and when the inclination angle reaches 45°and 60°,the independence principle is no longer applicable.For the inline flow direction of the structure,there is a significant deviation in predicting vortex-induced fatigue of inclined flexible cylinders by using the independence principle.The research results in this paper can provide necessary theoretical support and technical support for the structural design of slender flexible cylindrical structures such as submarine pipelines and marine risers,and have important theoretical value and practical engineering significance. |
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