Recoil Analysis For Drilling Riser For Emergency Disconnection

Drilling riser is an important part of a drilling platform system,connecting wellhead to floating platform.When floating drilling platform is confronted with serious conditions or dynamic positioning failure,emergency disconnection of drilling riser is needed.The riser recoil after emergency disconnection is one of the technical problems of deep-water drilling operations.Therefore it is important to ensure the process of the riser disconnection and key points.This paper established riser recoil response model,which is of great significance for the guarantee of deep-water drilling riser system operation safety.First of all,the drilling fluid discharge model was studied.The calculation formula of drilling fluid discharge model is deduced for annulus model and the whole fluid column model is established,which is running in MATLAB software to calculate and analyze the influence of drilling fluid discharge for various factors.Results show that the longer drilling fluid column length,the larger discharge time,and the larger discharge friction.Drilling fluid density has great effect on drilling fluid discharge,the larger drilling fluid density,the larger discharge friction,the faster discharge velocity and the shorter discharge time.Riser recoil response coupling model is established in ANSYS APDL software,including tensioner,riser and the drilling fluid.Discharge friction is applied on riser each elements which simulated the influence of drilling fluid discharge on riser well.Comparison and analysis for this model with ChaoWei Li model calculation results,the results show that both have good consistency,which is proved that riser recoil model is correct.Comparative analysis with ANSYS coupling analysis model and whole fluid model,the results show that the maximum discharge of velocity coupling model is 7.44 m/s,maximum acceleration is 4.61 m/s~2.Meanwhile the effective tension and the spread of stress wave on riser are studied.Results show that the effective tension fluctuates after disconnection and tends to be stable.Hydrostatic pressure is applied on the bottom of riser after riser disconnection which spreads to tensioner after 0.291 seconds.Finally,considering the actual drilling platform movement in the process of drilling operation,the platform-tensioner-riser coupling model is established in ANSYS/AQWA software though using nonlinear springs to simulate tensioner and using ANSYS/External Force Calculation to apply discharge friction to riser.Riser recoil response has been carried on by different cases for various parameters,including different discharge friction,top tension force and the time of disconnection.Results show that the larger discharge friction,the smaller safe clearance of LMRP;the smaller top tension force,the smaller safe clearance of LMRP.The safe clearance of LMRP is 0.31m under 4.7MN top tension force.Disconnecting riser could lead to LMRP grounding in downward phase of platform heaving.Results also show that the riser disconnection operations need to consider the platform movement,top tension force and drilling fluid discharge friction to ensure the safety of the riser disconnection operations.