Study Of Kinetic Properties And Optimization Of Deep-sea Drilling Riser In Reentry

Great challenges are coming for the state of the art of offshore drilling techonogy,with the development of offshore oil & gas in deep water.Due to frequently suffering from rough sea states,drilling riser cannot avoid leaving the exploration area temporarily for a short time and then reentering the wellhead once sea conditions grow stable.As a typical engineering problem in the development of marine petroleum,the difficulties arise abruptly for drilling in reentry with an increasing work depth,in which two main stages are concerned,i.e.,horizontal movement and vertical launch stages.The drilling risers will exhibit complex nonlinear motions in reentry,including response delay,large lateral position deviation and oscillation,under various external excitations such as wave,current and vessel motion,which seriously affect the efficiency and accuracy of reentry operation.At present,few studies were undertaken on the kinetic properties and control optimization of riser in reentry,and imperfectly,most movement parameters of which choose empirically,resulting in many uncertainties.Moreover,under the single excitation of the vessel motion,only the horizontal movement stage of reentry was focused on,which cannot achieve the precise reentry operation of riser.Therefore,for an improvement of the efficiency and accuracy in reentry operation of deep-sea drilling riser,it is necessary to predict accurately strong nonlinear dynamic responses of the riser and formulate an optimized motion strategy.Since it is insufficient in previous study,the problem of drilling riser in reentry is investigated deeply,whose complex nonlinear kinetic properties are especially analyzed and optimized in vertical launch stage of reentry,under multiple compositions of excitations.The perspectives are provided as fllows.(1)In the investigation of horizontal movement,the dynamic responses of drilling riser,including shapes,position offsets and velocities at bottom under preset sea currents and top motions of horizontal movements,were analyzed,using a hydrodynamic model of free hanging riser with rigid micro-segment and a solver of improved Newton iteration algorithm.(2)In the investigation of vertical launch,with flexible discrete micro-segment instead of rigid one,the total length of riser with time dependent was achieved by fixing the quantity and changing the length of segment,and a hydrodynamic model of riser with variable length was proposed.In addition,an iterative solver was provided,with which dynamic responses of riser shapes and position offsets at bottom,under preset sea currents and top vertical motions of down-launching movements were studied.The complex responses of drilling riser were analyzed,under top compositions of excitations,with a small scale of horizontal movement combined with vertical motion.of down-launching movement.(3)In the investigation of optimization of composite motions in reentry,considering the kinetic properties of composite motions in reentry,the state transition probability,the restrain range and selection mechanism of optimization objectives,and the pheromone updating strategy involved in the ant colony algorithm,were effectively improved according to discrete constant duration.Two optimized straties of composite movement of riser,with top accelerations and top velocities as the optimal objectives,were performed to suppress the position deviation and oscillation of at bottom.The major conclusions were drawn as follows.(1)The hydrodynamic model of free bottom riser with six unknown variables,reduced effectively the order of nonlinear PDEs,which was convienent for numerical computations.In addition,the introduction of sub-relaxation factor in iterative procedure,helped to reduce the accumulative errors.(2)A significant hysteresis phenomenon was discovered at bottom during horizontal movement of riser in reentry,and the amplitude and durance time of oscillation were strongly affected by the top movement.The riser shape in the bilateral shear current presented a complex “S” shape,and the velocity of current had a significant effect on the position deviation and oscillation at bottom.(3)With flexible discrete micro-segments,a riser model of variable length was established by fixing the quantity and changing the length of segment,for an effective solution of adverse effect of segment length variation on the errors of numerical results,which were applied to simulate and calculate accurately the riser movements in the down-launching stage of reentry.(4)In vertical motion of down-launching movement of riser,the effect of the acceleration and deceleration phases on the shape and position deviation of the riser was obvious.Accelerated launch increased the deformation and lateral position deviation of the riser,and there was a clear inflection point in the shear current and the bilateral shear current.Under current and top vertical motion of down-launching movement,a large lateral position deviation occurred at bottom.(5)In the compositions of motions,combined with vertical motion of down-launching,the top horizontal movement played a major role in the shape change of riser,with lateral position deviation.The preset top composite motions could reduce the lateral position deviation at bottom,but the effect was required to be optimized,with many uncertain factors.(6)The improved ant colony algorithm could be applied more conveniently and effectively to the optimization of complex nonlinear motions of riser in the down-launching stage,and the optimized results were easily converged to the global optimal solution.The optimization strategies were shown in the form of top velocity or acceleration,which could be easily implemented in applications.(7)Two sets of independent top composite motions,with movement acceleration and velocity of optimization strategies,were developed,both of which could effectively reduce the position deviation and oscillation at bottom in the composite down-launching movement,and improve the efficiency and accuracy in reentry.However,considering the optimization effect and the degree of difficulty in the operations,the optimized results of top composite acceleration were recommended in priority in applications.This study provided a reference for not only marine drilling operations in reentry,the typical problem in the development of offshore oil & gas in deep water,but also further academic research.