Study On Mechanical Behavior Of Suspension Mode Of Deepwater Drilling Riser

Deepwater marine riser is an important component that connects to offshore drilling device and subsea wellhead.The riser is subjected to complex forces that can have serious consequences in the event of an accident.Typhoon and other extreme weather is frequent occur in the deepwater.In order to protect the wellhead and the drilling device,the connection between the riser and the wellhead must be disconnected,and the riser is in hang off mode.When the typhoon is overtaken,it is necessary to implement the emergency evacuation,the state of the riser becomes more complicated.Therefore,it is very important to carry out the research on the mechanical behavior of the marine riser model,which is of great significance for the improvement of the safety control method of deepwater drilling.The main research results are as follows:(1)In view of the characteristics of the deepwater marine riser hanging off mode,considering the platform sinking movement,the compensation effect of the top tensioner,the seawater damping and the concentrated mass influence of the LMRP/BOP at the bottom of the riser,the riser mechanical model is established according to the basic principle of dynamics.And the finite difference method is used to discretize the differential equations.The equations are solved by combining the upper and lower boundary conditions of the model and the initial conditions.The results show that the displacement and stress of the axial vibration in the soft hang-off mode are obviously smaller than that hard hang-off mode.With the increase of water depth,wall thickness and seawater resistance coefficient,the axial displacement of the riser decreases gradually;the axial displacement of the riser increases gradually with the increase of the wave period;the BOP quality and the wall thickness of the riser are increased and the buoyancy is increased,which can reduce the axial stress of the riser;the influence of the wave period and seawater resistance coefficient on the axial stress of the riser is not obvious.(2)In view of the dynamic response of the marine environment to the lateral vibration of the riser,the transverse vibration mechanics model of the riser is established according to the basic principle of dynamics.Considering the influence of ocean current force,sea wave force and the weight of the riser,combined with the actual working condition of the hang off mode.The finite difference method is used to derive the difference scheme of the equations,and the equations are solved by FORTRAN programming.The results show that the platform drift,water depth and buoyancy block have a great influence on the lateral displacement of the riser.With the increase of the wall thickness,the BOP weight and the wave period,the lateral displacement of the riser decreases gradually.With the seawater resistance coefficient,The increase of the BOP quality,the wall thickness of the riser and the increase of the buoyant block can effectively reduce the stress of the riser;the influence of the wave period,seawater resistance coefficient and the velocity of the seas on the axial stress of the riser is not obvious.(3)In view of drilling fluid discharge when the riser is disengaged from the riser,the discharge model of the riser is established according to the basic principle of hydrodynamics.Considering the influence of the wall friction,the drag force of the liquid column impacting the seawater and the gravity of the liquid column,the equation is solved by combining the initial conditions of the model.The results show that with the increase of water depth and drilling fluid density,the drilling fluid discharge speed is accelerated,and the friction force,pressure loss and discharge acceleration are also increased.The diameter change of the riser has no obvious effect on the drilling fluid discharge.The control analyze combined axial vibration model and the drilling fluid discharge model,and the results show that reducing the top tension and using the small phase angle can reduce the time from the recoil to be locked by platform,and the amplitude of the platform sag has no significant effect on the recoil control.(4)In the view of the riser hang off work window,based on the axial vibration mechanics model and lateral vibration mechanics model of the riser,the operation window of the riser hang off mode is analyzed and calculated according to the safety standard of the riser window.The results show that the wind speed and sea surface velocity mainly affect the Mises equivalent stress at the top of the riser,and the operating window gradually decreases with the increase of the sea surface wind speed.The analysis of the emergency evacuation window by the riser shows that the current angle and speed of the current mainly affect the flexible corner of the riser,and the platform adopts the downstream evacuation,which allows the speed range to be large and can be evacuated to the safe area.The use of low-speed navigation,allowing a wide range of ocean currents,can be applied to the rapid changes in the marine environment weather conditions.