Research On Overall Mechanical Properties Of Deepwater Drilling Riser-wellhead-conductor System

In deepwater drilling,the security and stability of the drilling riser – subsea well head – shallow conductors are the primary considerations for engineering design and field operation.Combining with the project of National Science Foundation of China,which named “Research on mechanical property of drilling riser-conductor casing and stability of subsea wellhead in deepwater”(Grant Number: 51574261),the following three modules of the drilling riser-subsea wellhead-shallow string has been sufficiently studied in this dissertation:(1)Mechanics property study of a drilling riser considering internal solitary wave(ISW)and vessel motion.ISW frequently occurs in South China Sea,but the method used to analyze the mechanical property of deepwater drilling riser under combined loads of ISW and other ones has not been sufficiently studied yet.In this dissertation,both quasi-static and dynamic analysis methods are constructed to analyze a drillilng riser under combined loads of non-uniform currents,surface waves,ISW and vessel motion.In the calculation,the flow field of ISW is simulated by the layered seawater model of two-layer flow and KdV theory;then,the maximum Von Mises stress of the pipe body is taken as the extremum judgment standard;furthermore,the linear system of equations is solved by the preconditioned GRMES method.Case calculation shows that ISW can exert sudden impact and shearing force on risers,also largely increase the force and deformation amplitude of the pipe body;additionally,the lateral deviation of the bottom riser which is in hang-off mode will expand by more than 50%.Besides,the larger the ISW amplitude and the greater the density differential between the two layers of seawater,the severer the impact of ISW on the stress and deformation of the drilling riser.In conclusion,the calculating methods and analyzing results could provide a basis for improving drilling riser analysis software and optimizing the design plan of drilling riser.(2)Drilling fluid discharge and recoil response analysis of deepwater drilling riser during emergency disconnection.In deepwater drilling,marine risers are required to perform emergency disconnection sequence in harsh environments or during loss of dynamic positioning control.After disconnecting the drilling riser emergently,drilling fluid in the riser would be discharged directly from the riser bottom,meanwhile,the riser recoil response would occur.Currently,the research on the emergency disconnection and recoil response of drilling riser is in the elementary stage in China.This dissertation systematically summarized the causes of emergent disconnection of drilling riser,the procedure of disconnection and the riser recoil response after disconnection.Two new calculating methods have been established to simulate the drilling fluid discharge from the riser after an emergent disconnection,also an useful MATLAB program was developed.These works provide a series of time-varying data,including flow velocity,frictional resistance,mass and pressure of liquid column.Besides,a necessary simulation analysis for riser recoil response after emergent disconnection was performed by “DeepRiser”,which is a famous commercial software.Case calculation shows,in the primary stage of discharge,the fluid column in the riser pipe is gravity-free,and it accelerates the recoil response of the drilling riser.In this case,the telescopic joint and outer cylinder of the tensioner will increase sharply,and exceed the stroke threshold,then lead to device damage,even force the top drilling riser to crash the drilling floor badly.Meanwhile,in the procedure of fluid discharge,the fluid column pressure of middle-deep pipe will sharply decrease to vacuum status and hold for 1 to 3 seconds,and this will increase the risk that the riser being crushed by seawater static pressure.The simulation indicates that,at the moment of riser emergent disconnection,the riser bottom will rise up rapidly,in contrary,the effective axial forces of tensioner ring and single riser tensioner will decrease drastically,the stroke of telescopic joint and tensioners will drop rapidly as well;and these parameters will remain oscillating in the subsequent recoil response stage.(3)Dynamic response research on deepwater shallow conductors considering the sand liquefactionCurrently,all of the researches about the stability of subsea wellhead – shallow conductors are mainly based on the finite element software simulation,but seldom consider the impact of sand liquefaction.In this dissertation,a non-linear differential equation of subsea shallow conductor lateral motion has been established,and the equation was discretized by finite element method.The lateral subgrade reaction of shallow conductor,which was caused by the liquefied sand,was calculated by the p-y curve method by considering the weak-hyperbolic form.The lateral deviation of shallow conductor was non-linear dynamic simulated in time domain by a self-developed MATLAB program which uses the Newton downhill method and Newmark-?.Case calculation indicates the lateral deviation amplitude of shallow string in completely liquefied sand soil is over 100% than that of the sand strata which fails to consider the liquefied effect.Particularly,the lower the sand strength,the weaker support effect will be caused by the liquefied sand.From the point of increasing subgrade reaction,selecting a big conductor(e.g.36 ")could effectively improve the stability of subsea wellhead;a special subsea template to limit horizontal deflection of shallow conductors would be another suggestion for small conductor(e.g.30").