Research And Development Of Mobile Nitrogen System On Offshore Platform

Nitrogen is an inert gas, and its chemical properties are not live wave, so nitrogen is frequently used in routine maintenance, repair, reconstruction and other projects of offshore platforms. At present, nitrogen cylinders featuring small flow are used to supply gas on most offshore platforms, which leads to low construction efficiency and failure to meet construction requirements. Therefore, it is of great significance to develop a movable nitrogen production system for offshore platform. This paper mainly consists of the following parts:1. Selecting a nitrogen production method that applies to the movable nitrogen production system for offshore platform.There are three common methods for nitrogen production using air separation process, including deep cooling method, pressure swing adsorption (PSA) on a carbon molecular sieve, and membrane separation method. The membrane separation method has such advantages as low energy consumption, high reliability, long service life, small volume, light weight, short nitrogen production time and fewer valves; therefore, it can meet the requirements of the planned movable nitrogen production system.2. Designing a movable nitrogen production system for offshore platform.According to the demands for nitrogen in pipeline purge, pressure test and other construction projects on offshore platforms, technical indicators of the nitrogen production system were presented, and the nitrogen production process of this system was designed, and further, devices and instruments that satisfy the requirements for designing the nitrogen production system were chosen to complete the design of hardware and control system of this system.3. Determining parameters influencing the separation performance of hollow fiber membrane, and selecting a reasonable control algorithm to control the parameters.By an experimental analysis of the influence of membrane-entry pressure and membrane-entry temperature on the separation performance of hollow fiber membrane, it was judged that constant temperature had positive effect on the separation performance of hollower membrane. For the temperature control system, a mathematical model for air heater was established by the experiments; also, a conventional PID temperature control model and an improved PID temperature control model were established in Simulink of MATLAB. By analyzing the simulation curves of the two control algorithms, it was found that the improved PID model had such advantages as small overshoot, short adjustment time and high stability; therefore, it can better satisfy the requirements of the nitrogen production system.4. Designing the control program of the movable nitrogen production system for offshore platform.The control program of the movable nitrogen production system was designed according to the technical indicators and control requirements. Programs of the nitrogen production system include PLC control program and touch screen program. The PLC control program consists of main program for system control, sensor signal processing program, desiccant air dryer control program, output control program, alarm program, control program for low-pressure nitrogen outlet pressure, improved PID temperature control program for heater, PID control program for low-pressure nitrogen outlet pressure. As for the touch screen program, the industrial configuration software SIMATIC WinCC flexible2008was used for touch screen configuration which consists of connection between PLC and touch screen, variable configuration, image object configuration, and alarm configuration.5. Finishing field debugging of the movable nitrogen production system for offshore platform.The nitrogen production system has the following technical indicators:(1) nitrogen production capacity:200Nm3/h;(2) nitrogen purity:>97%;(3) temperature of outlet nitrogen:20～30℃;(4) switchable nitrogen output:nitrogen purge mode (0.6～1MPa), inflation mode (15MPa);(5) membrane-entry pressure:0.8～1.3MPa, membrane-entry feed temperature:45℃;(6) the system features automatic operation, monitoring, alarm, shutoff, etc..The movable nitrogen production system for offshore platform plays a non-negligible role in ensuring the safety and stability of the entire offshore platform. In case of failure of the fixed nitrogen production unit on the platform, this system can serve as an emergency substitute unit. For small and medium-scale projects using nitrogen cylinders to supply gas, this unit can be used to refill the cylinders, thus to achieve the goals of cost-saving, efficiency enhancement and shorter construction period.