Research On Mechanics Of Deepwater Drilling Risers And Vertical System And Vortex-induced Vibration Suppression

With the increase of deepwater oil and gas exploration and development activities,the drilling riser,the key offshore drilling engineering component with weak vulnerability,is in an increasingly harsh working condition.The mechanical behavior of the string is becoming more and more complex,which causes a variety of hazards and seriously affects the construction operation.Deepwater risers often produce large bending deformation under harsh conditions,which will cause damage to the riser and even cause safety accidents.At present,the geometric nonlinear deformation of the riser is rarely considered in the research on the mechanical properties of the riser.The bending deformation of the drilling riser can easily cause friction and wear with its internal drill string,which significantly affects the operation safety and efficiency.Adjusting the equipment parameters in the drilling riser system and using the drill pipe protector cannot fundamentally solve the problem.Vortexinduced vibration(VIV)is another major factor causing damage to drilling riser,but there are still some shortcomings in the existing VIV suppression devices.In this paper,a three-dimensional geometric nonlinear model of large deflection of deepwater drilling riser is established,and the relevant solving methods and the influence of geometric nonlinearity on the mechanical calculation of riser are studied.The model takes into account the axial linear elastic deformation,longitudinal vibration and velocity vector change of moving riser relative to the fluid particles in marine.The singular function is used to express the discontinuous on the risers,and the Galerkin finite element method is combined to make the calculation more direct and more convenient.Newmark-? method is used for time domain iteration,Newton-Raphson method is used to establish the incremental equation,and MATLAB calculation programs are written.The research shows that the geometric nonlinearity has obvious influence on the calculation results of the mechanical properties of the drilling riser.Based on the above calculation model and solution method,the mechanical properties of the drilling riser under complex marine conditions are studied.The modal analysis of the riser is carried out,and the static analysis of the riser under the action of sea current is carried out.The dynamic response of the riser under the action of linear wave,internal solitary wave and the combination of the two is analyzed.The VIV of the riser under the action of sea current is analyzed based on the independence principle(the VIV of the inclined cylinder is independent of the inflow component parallel to the axial direction).It is found that the buoyancy block,current velocity and the angle between different directions of ocean wave flow have great influence on the dynamic characteristics of the riser,and the displacement responses of the drilling risers in all directions have narrow-band characteristics in VIV.In order to reduce the bending deformation of the drilling riser string,deepwater drilling riser vertical system is researched,which can keep the riser string vertical in real time in the ocean condition,including structural design,simulation experiment,structural optimization calculation and mechanical analysis.Combined with the existing riser system and marine mooring equipment,the conceptual design and experimental simulation are carried out.The calculation model of the riser vertical system is established,the structure of the riser vertical system is optimized by particle swarm optimization(PSO)method,and the mechanical analysis is carried out.It is found that the deepwater drilling riser vertical system can effectively reduce the bending deformation and dynamic response amplitude under environmental loads in marine,and it meets the material strength requirements of the existing riser.In order to reduce the vortex-induced fatigue damage of the riser under marine conditions,a VIV suppression device is studied through the flow simulation experiment,which is mainly composed of two streamers with trailing edges.Based on the principle of bionics,the structure of VIV suppression device for drilling riser is studied,and the flow field near the suppression device model is analyzed by particle image velocimetry(PIV).It is found that the double-streamers structure can effectively inhibit the generation and development of the vortex behind the cylinder in the flow,and further suppress the VIV.