LNG Carrier Cargo Safety System Designing Based On IEC61511

Natural gas, which is cleaner, cheaper and more efficient than oil and coal, is becoming the important part of world energy structure. Nowadays in China, the requirement of natural gas in the south-west coastal areas is large. LNG transportation by LNG Carriers is an important way to satisfy the requirement. LNG Carrier is the specialized ship which transports LNG in -163℃. Mastering the independence technology and intellectual property of LNG Carrier design and construction is the significant step of China National Energy strategy with great meaning.LNG Carrier is the critical component of LNG Carrier. The hazard character of LNG, such as hyper low temperature and the gasification rate of 1:600, will lead to the high risk of LNG Carrier cargo system. It should configure a high reliable safety system for LNG Carrier cargo system to avoid the risk or reduce the consequence of risk. Thus, I did the following work for the design of LNG Carrier Cargo Safety System.I. Introduce the composing and risk of LNG Carrie. Describe the necessarity of configuration related safety system for LNG Carrier Cargo System. Illustrate the concept and design method of safety system and propose the standardization requirement of LNG Carrier Cargo Safety System Designing. Follow the Function Safety Standard IEC61511, which is widely used in process industry, to design LNG Carrier Cargo Safety System.II. Introduce the development and application of IEC615111, especially its two important concepts, Safety Integrity Level (SIL) and Safety Lifecycle. This will be the theoretical basis of LNG Carrier Cargo Safety System Design.III. Introduce two hazard and risk analysis methods, HAZOP (Hazard and Operation Analysis) and ETA (Event Tree Analysis) and use these two methods together. Apply HAZOP to acquire the hazard events and their risk consequence and apply ETA to calculate the probability of risk. Engage the hazard and risk analysis of three factors, the level and pressure in cargo tanks, and the leakage in cargo pipes.IV. Design the LNG Carrier Cargo Tank Level Safety System regulated to IEC61511. The Safety Function is that when the level in cargo tank is too high (equal to or more than 98.7% of the whole volume of the cargo tank), start the two cargo pumps in the bottom of two discharging pump towers to discharge LNG. The expected Safety Integrity Level is SIL 3.V. Validate the designed safety system by Reliability Block Graph. Propose three improvement schemes, which are redundancy of final elements, redundancy of sensors and redundancy of both sensors and final elements. The Safety System with redundancy of both sensors and final elements satisfies the design requirement.According to the process and result of LNG Carrier Cargo Safety System designing, the application of IEC61511 in the LNG Carrier Cargo Safety System is successful. This helps the reference of LNG Carrier Cargo Safety System and the similar system designing using IEC61511.