Research On Safety Assessment Approach And Its Application For Deepwater Hydrocarbon String In Service

This dissertation is supported by the National Major Science and Technology Project “Research on Deepwater Drilling Riser Operation Management and Safety Evaluation Technology” and the Research Project of CNOOC Zhanjiang Branch “Operation Safety Analysis on Deepwater Test String for LS17-2 Appraisal Program”.According to the site actual operational demand of HYSY981 platform,systematic research on safety assessment approach and its application for deepwater hydrocarbon string,taking test string as the major research object,has been carried out based on natural environments of the South China Sea.The research covers nonlinear simulation technology,self-induced vibration mechanism and fatigue damage,structural vulnerability analysis method,the operation early-warning boundary and strategic decision for deepwater hydrocarbon string,which acquires major progress and technological innovation.Furthermore,the life management and risk assessment method for deepwater hydrocarbon string are formed,providing effective technical supports for the safety operation of deepwater hydrocarbon string in the South China Sea.1 Research on analysis model and method for deepwater hydrocarbon stringConsidering soil nonlinear resistance and geometric nonlinearity of deepwater riserconductor-soil coupled system,the mathematical analysis model and finite element analysis model of deepwater hydrocarbon string are established.Then,the research on the determination method of stiffness matrix for soil resistance,mesh generation technique,nonlinear solution engine and result processing method has been carried out to develop simulation program for deepwater riser system subjected to the complex marine environment and soil resistance independently.Learned from design method of orthogonal experiments,accuracy and robustness of the self-designed program are verified under different water depths,top tensions and platform offsets,and research on drilling operation envelope of deepwater riser-conductor-soil coupled system is conducted based on the program.The results show that the self-designed program meets the requirement of calculation accuracy and application range,and it can provide guidance for operation management of deepwater hydrocarbon string in the South China Sea.2 Research on the nonlinear self-induced vibration mechanisms for deepwater hydrocarbon stringIn view of the destabilization of fluid mass,pressure and flowrate,an inflow-structure interaction model is established for deepwater hydrocarbon string in which oil and gas flow rapidly during production operation.The nonlinear self-induced vibration of deepwater hydrocarbon string is studied from three aspects,including inherent vibration characteristics and responses under start-up & shut-in mode and production disturbance mode.Given the annular clearance and centralizer space,the analysis procedure of inflow-induced vibration operation envelope for deepwater hydrocarbon string is proposed based on reliability theory.Meanwhile,the influence of tension ratio and centralizer space on start-up & shut-in envelope and production disturbance envelope is analyzed.According to the linear accumulated damage criterion,the self-induced fatigue damage assessment mothed is proposed and the influence of production and centralizer space is studied as well.At last,the design concept of centralizer is proposed in combination of offshore operation factors and inflow-induced vibration operation envelope.3 Research on the structural vulnerability analysis and assessment methods for deepwater hydrocarbon stringThe failure of any single joint of deepwater hydrocarbon string in service would interrupt the offshore operation.As a result,the structural vulnerability analysis is an important part in the design and evaluation process of the deepwater hydrocarbon string.In terms of pipe-in-pipe structure,a contact nonlinear coupling analysis model of deepwater hydrocarbon string is established based on interaction between the inner and outer string.Combined with the local finite element analysis method,the systematic and complete deepwater hydrocarbon string vulnerability analysis and assessment methods are formed.Taking the deepwater test string system as an example,strength vulnerability analysis,contact vulnerability analysis,fatigue vulnerability analysis and seal vulnerability analysis are conducted on the basis of the research of temperature and pressure loads,process load and marine environmental load.The mechanical properties and damage law of deepwater test string in typical operation conditions are studied,and operation management measures are proposed correspondingly.4 Research on operation analysis and decision-making for deepwater hydrocarbon stringAccording to emergency disconnect sequence and the standard for deepwater test string,criterion of the early-warning boundary is established and the magnetic search algorithm is designed based on the dominant limiting factor independently.Taking the deepwater test string system as an example,the warning control boundaries and the influence on the shape and size of them from top tension,SOLITON current and wellhead tilt angle are studied.The adjustment principle of top tension and platform offset are further researched to form the real-time alarm and decision-making program based on monitoring parameters.A multi-stage dynamic decision-making method is proposed on the basis of the reliability of monitoring data and expected loss of decision-making.The typhoon-avoidance strategy in emergency is drawn up from the aspects of platform heading,evacuation direction and evacuation speed.The research findings will improve the ability of dealing with typhoon in the South China Sea.5 Research on the life management and risk assessment for deepwater hydrocarbon string in serviceConsidering the damage pattern of the deepwater hydrocarbon string in service,the life management method of deepwater hydrocarbon string is established.The inversion technique using VIV monitoring data,calculation of fatigue crack growth,maintenance technologies and methods are studied in the paper to form life-cycle management system for deepwater hydrocarbon string.Baesd on the method above,the structure damage of deepwater hydrocarbon string can be identified,evaluated,predicted and updated sustainedly.Considering the prominent periodicity of deepwater hydrocarbon string operation,as well as the certain general characters of risk factor considered in every single well operation,risk assessment process of well operation for deepwater hydrocarbon string is developed.The process mainly consists of data collection,risk assessment and strategic planning,and covers environmental risk assessment,structure risk assessment and operation risk assessment.And it will provide the basis for the guidance of single-well operation risk assessment and management for deepwater hydrocarbon string in the South China Sea.