| Gas Hydrate is considered to be the most potential replacement energy in twenty-first century,which has the largest reserves untapped.Because of the huge risks in operation,how to safely and efficiently tap into gas hydrates has become an urgent problem.Dual gradient drilling maintains dual density drilling fluid system by effectively controlling annular pressure and bottom-hole pressure,which offers targeted solutions to solve the problem of small gap between the formation pressure and fracture pressure during drilling process,well leakage and blowout accidents that are easy to occur,its essence is a Managed Pressure Drilling(MPD).As a scheme of dual gradient drilling technology,the key of subsea mud lift drilling(SMD)technology is to solve the problem of pressure control at the inlet of the seabed pump.This thesis supported by the project of Gas Hydrate Drilling in Ocean Water and Safety Monitoring Technology(2016YFC0304005),mainly focuses on the research of pump control strategy and monitoring system design for SMD.On the analysis of the basic structure,working principle and operation condition of the SMD system,this thesis points out that one of the key technologies of subsea mud lifting drilling is to take certain measures to ensure the inlet pressure of the lifting pump is equal to the static pressure of seawater.Based on this,a control object model composed of frequency converter,motor,lift pump and mud circulation pipeline is built.Because the control object is a large inertia and nonlinear system,and multiple interference factors exist in the control process,a dual closed loop control strategy of pressure and speed based on adaptive fuzzy PID regulator is adopted.With the help of Fuzzy toolbox provided by Matlab,the design of fuzzy PID regulator is completed,and a simulation model of SMD system is built.By comparing with traditional PID controller,it is proved that fuzzy PID controller has the advantages of smaller overshoot,faster response speed and stronger anti-interference ability,which can completely meet the requirements of system control and achieve better control effect.The subsea lifting pump module is composed of multiple pumps in series and parallel connection,and each group of parallel pumps is equipped with a frequency converter.In order to meet the requirements of pressure control,lifting pump should be flexibly in and out of service.To solve the problem of great impact current of high-power three-phase asynchronous motor in SMD system when switching from variable frequency to power frequency,a three-phase asynchronous motor synchronous switching control strategy based on grid voltage space vector orientation is adopted.A mathematical model of three phase voltage source PWM inverter with LC filter in the two-phase synchronous rotating coordinate system is established,and a voltage and current double closed loop controlling method is put forward,in order to make the inverter output voltage respond to the load quickly and indiscriminately track grid voltage.Finally,a seamless switching is achieved.A lift pump control test platform with DSP,PWM inverter and three phase asynchronous motor is constructed,and the designs of software modules and peripheral hardware circuits are introduced.Based on this platform,the intelligent control of lifting pump has been realized.Host computer software development is entirely based on LabVIEW platform,on which block diagram program,front panel have been designed and SMD platform monitoring system has been built.The communication between DSP and LabVIEW has been realized by Ethernet. |
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