The Format Criterion Of Doctoral Bachelor

The deepwater subsea wellhead system is an important component that carries the blowout preventer,the Christmas tree and the casing,and its stability determines whether the deepwater drilling and completion operations can be carried out smoothly.The upper part of the riser is connected to the floating platform,and the lower part is connected to the underwater wellhead,which is always subjected to complex forces such as waves,ocean currents,top tension,drilling string,and platform vibration and deflection.The dynamic load on the riser will be transmitted to the subsea wellhead through the bottom assembly,which will have a greater impact on its stability.Therefore,in order to ensure the smooth progress of drilling operations and the safety and stability of the subsea wellhead equipment,it is of great significance to study the influence of the riser system on the stability of the subsea wellhead.Based on the structural design and installation process of the riser and the subsea wellhead system,combined with the deep water environmental parameters,and considering the special structures of the riser buoyancy block,the upper and lower flexible joints,and the tensioner,the stability of the riser and the subsea wellhead under different working conditions is established.The finite element analysis model was used to analyze the influence of seawater velocity,tension at the top of the riser,and platform offset on the stability of the wellhead,and obtained the distribution law of the stress,displacement and soil stress of the underwater wellhead under different load factors.Based on the wellhead mechanics analysis model,a subsea wellhead stability analysis experiment was designed to verify the stress and displacement change laws of the subsea wellhead system under different lateral loads,and to analyze the easy yield position of the subsea wellhead system.Based on the S-N curve fatigue analysis method and the stress time curve,according to the rainflow counting method,the fatigue damage law of the subsea wellhead under different factors was analyzed,and the wellhead fatigue life under the drilling and completion conditions was predicted.Based on the marine environmental parameters,soil parameters and operating data of a gas field in the South China Sea,the maximum equivalent stress and maximum bending moment of the high and low pressure wellhead and surface conduits under different platform displacements were calculated,and the maximum allowable displacement of the drilling platform was recommended.Through numerical simulation and laboratory experimental analysis,it is obtained that the riser offset has the greatest influence on the stability of the subsea wellhead;the maximum position of the bending moment and stress of the subsea wellhead is 7-9m below the mudline;the subsea wellhead under drilling and completion conditions The wellhead fatigue life is about 10 years.The research results have been applied in a gas field in the South China Sea,providing certain guidance for field operations.