Research On Operation Analysis And Decision For Deepwater Drilling Riser And Subsea Wellhead

This dissertation is supported by the National Major Science and Technology Project“Research on Deepwater Drilling Riser Operation Management and Safety EvaluationTechnology” and the National Natural Science Foundation Project “Research on single jointdesign and operation management of ultra deepwater drilling riser in South China Sea”.According to the harsh environment of South China Sea, systematic study on technology ofoperation analysis and decision for deepwater drilling riser and subsea wellhead has beencarried out. The research covers some hot issues of deepwater drilling riser and wellheadincluding coupled analysis models research, riser configuration and top tension design,operability envelopes determination, wellhead and conductor pre-drilling configuration andemergency technique research, which acquires major progress and technological innovation.The research findings provide effective technical supports for the operation technologydevelopment of deepwater drilling and completion riser and subsea wellhead.1Research on analysis models for deepwater drilling riser and subsea wellhead systemAiming at important operation modes of deepwater drilling riser and subsea wellheadsystem, both static and dynamic analysis models for connected riser, axial dynamic analysismodel for hang-off riser and analysis model for wellhead and conductor are established.Based on theoretical models above, the problem of the riser-wellhead-conductor coupledanalysis is solved. Through comparison of analysis results between decoupling model andcoupled model, it can be seen that the results of riser-conductor-soil coupled model are mostaccurate and are in coincidence with field practice. If soil profile parameters have beenobtained before drilling, the riser-conductor-soil model should be adopted to analyze the riserand wellhead response. The analysis results of the riser-conductor model are close to that ofthe riser-conductor-soil model. If soil profile parameters could not be obtained, the riser-conductor model is recommended to acquire reliable riser and wellhead response data.2System configuration and top tension design for deepwater drilling riserBased on the determination of riser length, the checking of riser wall thickness, thelectotype of buoyancy modules and the arrangement of riser single joint, the configurationmethod of deepwater drilling riser system is developed. The riser system configurationconcept for1500m water depth target well is proposed to conduct the basic data for operationanalysis and decision of the riser and wellhead system. The stability method, the methodbased on the residual tension at bottom of the riser system and the method based on themaximal hook load during running procedure of the riser system for top tension determinationare studied. The determined configuration concept is adopted to perform top tensionapplicability analysis. The study indicates that the stability method can consider the influenceof multi-factors with large application range and complicated calculation. The method basedon the residual tension at bottom is simple with poor applicability. The method based on the maximal hook load during the running procedure is simple and applicable, which isrecommended as the priority method for determining the top tension.3Research on the determination method of operability envelopes for deepwater drillingriserAs the core parts of operation analysis and decision of the riser, the riser operabilityenvelopes play an important role in guiding the field operation of the riser. Through study onthe theoretical analysis model of the riser and wellhead system, the riser system configurationmethod and top tension design method, the riser and wellhead system coupled analysis modelis established. The combination limit criteria for different operation modes are proposed. Thecombination parameters of drilling platform offset, wave height and period, and surfacecurrent speed are used to determine the extreme environment loads allowing the riserdeployment and retrieval, drilling, completion, and hang-off operations. The study on theanalysis method of operability envelopes for the riser key operation modes has acquiredbreakthrough, which determines the most dangerous operation mode of the riser deploymentand retrieval and the soft hang-off mode for survival operation in typhoon. Based on analysisof the real time adjustment method of drilling envelope and influence factors of completionenvelope, the measures are proposed to improve the riser drilling and completion envelopes.4Research on pre-drilling configuration for subsea wellhead and conductorThe determination of wellhead stick-out and the design of conductor length belowmudline are important parts of pre-drilling configuration of the subsea wellhead system. Onthe basis of three algorithms for wellhead stick-out, the most applicable method in pre-drillingdesign is proposed. Based on analysis of initial capacity, real time capacity and extremecapacity of the soil, the relationship between undrained shear strength and average shearstrength and the recovery process of soil capacity with time are defined. The design methodsof the conductor minimum and maximum lengths below mudline are proposed. The minimumsetting lengths are calculated in several dangerous operation modes, such as conductor jetting,surface casing cement and BOP installation, the same as the maximum setting lengths in thecontinuous jetting and conductor stroking. The effect of different factors on maximum andminimum setting lengths is analyzed. Based on operation analysis of the wellhead andconductor, the determination method of the wellhead bending strength, the conductor bearingcapacity and the jetting parameters of wellhead and conductor are studied, which canguarantee the continuity of drilling operation and the wellhead system safety.5Research on emergency technique for deepwater drilling riser and subsea wellheadsystemThe emergency technique for deepwater drilling riser and wellhead can effectivelyreduce the risk of operation and prevent severe accidents. For the purpose of riser rapidevacuation from typhoon, the operating principle and basic structure of near surfacedisconnection package and air can are studied, the riser near surface disconnection concept isdesigned, and the setting points for emergency disconnect system(EDS) and for the riser nearsurface disconnection are analyzed. The wellhead emergency strategy for well blowout is studied to analyze the bearing capacity of the wellhead and conductor for LMRP cap methodand simple BOP method. Based on the wellhead and conductor running decision, the riserdrilling operation decision by monitoring, the equipment failure decision and the decision ofthe riser emergency disconnection and evacuation from typhoon, the emergency decisionmethod and process of the riser and wellhead operation under different conditions isestablished to provide guidance for real time and fast adjustment of field operation.