Investigations On The Vibration And Fatigue Characteristics Of Piping Structures In Ocean Engineering

With the development of the offshore oil and gas industry,more and more oil and gas resources have been explored and discovered.No matter what kind of floating platform is used for offshore oilfield development,a large number of flexible pipe structures,such as production pipelines,submarine pipelines,offshore risers,need to be used to link the entire production of offshore oilfields closely.Therefore,the research on the flexible pipe structures in marine engineering has always been a hot spot in the investigation and development of marine mining equipment.The main dynamic loads that cause the dynamic response of the pipe are waves,ocean currents and top floating body motion.Under the action of ocean currents,the pipe is subjected to pulsating fluid forces in the cross-flow and in-line directions.Under the action of environmental loads such as waves,currents,and wind loads,the floating body will carry out complex movements together with the offshore pipe system,which will cause the pipe to vibrate and generate periodic stress,which in turn causes problems such as pipeline fatigue damage.In this paper,three types of pipes are selected as research objects,aiming to provide theoretical references for operation safety through the study of vibration response and fatigue characteristics of pipes in different marine environments.The main research work of the thesis is as follows:Based on the Euler-Bernoulli beam model,different forms of pipe structures are modeled.The effects of the gravity,top tension,internal fluid and environmental loads of the pipe are considered.Using Hamilton’s principle,the three-dimensional nonlinear dynamic equations are established in detail.Galerkin or Finite Element Method is used to discretize the dimensionless equations,and finally the fourth-order Runge-Kutta method is used to solve them.By comparing with the previous experimental results,the reliability of the proposed model and the accuracy of the numerical calculation method are verified.For steady current conditions,the van der Pol wake oscillator model is used to describe the force of the fluid on the structure due to the detachment of the vortex.The natural frequency,lock-in region of frequency,time domain response of vortex-induced vibration(VIV)of marine risers in uniform currents are calculated in detail,the influence of initial geometry and internal flow velocity is discussed,the critical internal flow velocity caused by buckling instability of offshore pipelines under different external conditions is derived in detail,and the regularity of natural frequency after buckling is studied.For the investigation of VIV in the oscillating ocean current,an improved wake oscillator model with oscillating velocity parameter and oscillating acceleration parameter is introduced.Through the analysis of the time-domain response curve,the "periodic" and "intermittent" characteristics of VIV in the oscillating flow are confirmed.The effects of Keulegan-Carpenter(KC)number,oscillating ocean current period,initial geometric amplitude,internal flow velocity and other parameters on the displacement envelope curve,lock-in region of frequency and amplitude response are discussed in detail.Therefore,the optimization suggestions of riser structure to suppress VIV are given,which provides a theoretical reference for improving the service life of risers in oscillating flow.For the curved pipe connected to the top floating body,based on wave spectrum and Response Amplitude Operator(RAO),the random motion of the top floating body is calculated,and on this basis,the motion of the floating body is used as the moving boundary to analyze the time domain response of the pipe.After obtaining the stress time domain curve of the offshore pipeline,the fatigue damage caused by VIV and floating body movement was calculated and compared by the rain flow counting method combined with the S-N curve.