Reliability Analysis System For Risk Management Of LNG Receiving Terminals And Piping Network

Daisuke TakaginTokyo Gas Co.,LtdOTokyo Kazuo KoyamaOTokyo Gas Co.,LtdOTokyo Akinori IshizukaOTokyo Gas Co.,LtdOTokyo Katsunori KawaiOMitsubishi Heavy Industries,LtdO Yokohama

Key Words:Reliability analysis,Risk management,Fault tree analysis,LNG, receiving terminalPiping network

SUMMARY & CONCLUSIONS

As a support tool of risk management, an approach to the reliability analysis system for the complex technical systems and its application to LNG (Liquefied Natural Gas) receiving terminals are described. We have developed a tool capable of quantitative evaluation of the reliability of LNG facilities based on FTA (Fault Tree Analysis) method and have implemented it in Gensym’s G2. A key feature of the system is it constructs a fault tree automatically from diagrams such as process flow diagrams, and sequence block diagrams. The system has actually been used in the evaluation of existing LNG receiving terminals, and has also contributed to cost reduction in constructing a new terminal.

1 .INTRODUCTION

In order to supply gas steadily at a fair price over an extended period of time, it is necessary for gas companies to assemble optimal production and supply facilities which are able to handle an increase in demand, while at the same time offer an adequate level of reliability. Risk management of such facilities is absolutely indispensable to rational decision making. To meet this need, Tokyo Gas Co., Ltd. and Mitsubishi Heavy Industries Ltd. have jointly developed a tool capable of quantitative evaluation of the reliability of LNG (Liquefied Natural Gas) facilities based on FTA (Fault Tree Analysis) method, and have implemented it in Gensym’s G2. The system calculates such reliability indices as the availability and reliability of the system, as well as the importance of each component. This system can also calculate the lowest Unavailability gas send-out pattem which satisfies the required flow and, pressure at gas govemor stations located in main piping network. This system has actually been used to analyze the reliability of the existing LNG receiving terminals of Tokyo Gas, and also used in the designing of the new

Tokyo Gas Ohgishima Terminal started operation in Oct. 1998. This paper describes an approach to the reliability analysis system for the complex technical systems and its application to LNG receiving terminals.

2.METHODOLOGY

Current technical systems requiring high reliability usually involve multiple components and have complex relationships among the components from the viewpoint of system reliability. This approach to the reliability analysis of such systems aims at supporting engineers who are domain experts but not necessarily familiar with reliability analysis. It can reduce a great deal of man-hours required if it’s performed by engineers manually. Accordingly, the method of FTA is adopted in this approach to evaluate system reliability because its results are easy to understand and it is one of the most commonly used methods for reliability analysis of industrial systems. In addition, process flow diagrams, main one line diagrams, and sequence block diagrams are adopted to represent hydraulic systems, electric power supply systems, and control systems, respectively. Each component constituting the system is represented as an object in the diagrams and its relationship to other components is represented by graphical connections. These diagrams are analyzed to construct a fault tree using knowledge to interpret the diagrams. Once the fault tree is constructed, the minimal cut sets are obtained from the fault tree using Boolian operations and used to evaluate the system reliability and the importance of the components. An application utilizing this new approach has been developed using Gensym’s object-oriented development environment, G2, which has been used to successfully develop a variety of expert systems.

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2. I. PLANT DESCRIPTION

In order for end-users to represent easily the LNG terminal to be analyzed, the formats of the diagrams described above are modified as follows. Process flow diagrams are used to represent both the main

systems that process LNG and LPG (Liquefied Petroleum Gas) and auxiliary systems supporting the main systems. It is tedious and unmanageable to draw those systems in the LNG terminal directly using their constituent components, so they are instead represented hierarchically using two levels of process flow diagrams. The diagram at the first level is used to represent the LNG terminal using their process units which are a group of components working together functionally, and the diagram at the second level is used to represent each process unit.

A diagram representing a main LNG process facility is shown in Figure 1 as an example of the diagram at the first level. This diagram is connected to other process flow diagrams at the same level through connection-posts, which are represented as triangle icons in Figure I . As a type of process unit, pipe units are also defined to represent pipes and their attached components such as valves. Capacity is assigned as an attribute to each process unit instead of to each component. As the success criteria for the process diagram shown in Figure I , gas supply rates tu utility and gas pipeline networks are specified using special objects. Figure 2 shows the process flow diagram of the process unit named Vaporizer-HI shown in Figure 1. This process flow diagram represents the configuration of components around the LNG-vaporizer. In addition, support systems for each component are specified on this diagram using special indicators.

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Figure 1 An example of process flow diagram at the first level

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to-Vaporizer-H1 from-Vaporizer-H1

Figure 2 An example of process flow diagram at the second level

2.2. FAULT TREE CONSTRUCTION

The fault tree for an LNG terminal is automatically constructed by synthesizing the fault trees constructed by analyzing diagrams of all systems in the terminal. The top level fault tree is constructed by analyzing the main LNG process flow diagram as shown in Figure 1. Relevant process flow diagrams to this diagram are also taken into consideration in this fault tree construction. Overall configuration of the fault tree is illustrated in Figure 3. The top level fault tree is a logical combination of the fault trees for process units. The fault tree for each process unit is also a logical combination of the fault trees for its constituent components and support systems such as the electric power supply system, instrument air

supply system, and control system. The fault tree at a component level is constructed by its appropriate failure modes.

2.3. RELIABILITYANALYSIS

Reliability analysis for an LNG terminal can be performed using the fault tree thus CO~struCted. In order to evaluate the system availability, reliability, shutdown time, and important combinations of failure, the minimal cut sets are obtained from the fault tree by Boolean operations. The importance of components can be also calculated using the minimal cut sets.

Figure 3 Overall configuration of the fault tree

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3.EVALUATION OF LNG RECEIVING TERMINALS AND PIPING NETWORK

Reliability analysis of LNG receiving terminals of Tokyo Gas was performed. Figure 4 illustrates the input flow diagram of the company’s Terminal A whose nominal LNG processing capacity is 700th, while two other terminals have the capacities of 400th (B) and 500th (C). Each terminal has redundancy of facilities to maintain high reliability, with the nominal capacity of each terminal ranging from 70% to 90% of full capacity. The difference between Terminal A and others is that A is located near a city whose gas demand is quite large. Therefore A has more redundancy of facilities, such as in the emergency gas send-out system. The database used in these studies was build by making use of reliable public documents (IEEE std 500-1984 (1983), for example), as well as data deduced from Tokyo Gas’ own experience with maintenance of LNG facilities over a period of some thirty years.

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Figure 5 shows the calculated Unavailability of each terminal according to the plant load, where Unavailability(Uav) is defined as follows.

(1) Uav = 1.0 -Availability Availability = MTBF / (MTBF + M’lTR) (2)

While, MTBF Mean Time Between Failures, M T I R Mean Time To Repair. Results of analysis show that, as the plant load increases, Uav rises nearly exponentially. The Uav of Terminal A is lower than B&C especially in high load. It is also demonstrated that Terminal A has high reliability owing to an emergency gas send-out system. Figure 6 shows the calculated Uav of each terminal according to the yearly gas send-out pattem.

Figure 4 Input flow diagram of Terminal A at the first level

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20 40 60 80 100

Load (%)

Figure 5 Unavailability according to the load

Load Terminal A Terminal B Maximum load 80% 100% Medium load 60% Min.

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1 2 3 4 5 6 7 8 9 1 0 1 1 1 2

Month

Figure 6 Unavailability according to the yearly send-out pattern

Terminal C 60% 40%

The reason the Uav of Terminal C is higher than that of the others is that C is operated at a higher load. But, why does C have high Uav in the summer season (from July to September) in spite of small gas demand? The answer is that main facilities of C have scheduled maintenance during these three months. Less redundancy during summer season makes Uav higher. This shows the effectiveness for facilities maintenance planning.

Table. 1 shows the calculated optimum gas send-out pattern of each terminal at maximum, medium and minimum piping network load. These results tell us that Terminal A, the lowest Uav is not required 100% load at maximum load because of piping network restriction. This analysis helped us optimize each terminal's load planning. Figure 7 illustrates the total piping network diagram of Tokyo gas in order to calculate gas flow and pressure required at the gas governor stations.

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4.CONCLUSION

Figure0 total piping network diagram of Tokyo gas

A new approach to the reliability analysis for the complex technical systems is proposed. FTA is adopted as the basic method. A key feature of the approach is it constructs a fault tree automatically from diagrams such as process flow diagrams, main one line diagrams, and sequence block diagrams, with which domain experts are familiar. This approach has been applied to the reliability analysis system for LNG terminals. It shows that this approach is useful to assist engineers in improving the reliability of the current system, designing the reasonable system configuration, and planning effective operation of the system because of easy description of complex plant configuration, and minimal need for knowledge of reliability analysis. The following results were obtained in developing the system. (1)Capability to perform a quantitative analysis of reliability for an entire LNG receiving terminal, including the power supply system, and instrumentation and control

system, which in the past had to be evaluated qualitatively. (2)Capability to determine reliability according to the yearly gas send-out pattern. This method also shows effectiveness for facilities maintenance planning. (3)Contribution to the cost reduction in building a new LNG receiving terminal, as well as improving the reliability of existing LNG terminals. (4)Capability to evaluate the optimum gas send-out pattern of LNG receiving terminals including the gas flow analysis of the total piping network. Our future development plan is summarized as follows. (1)Reliability analysis for the entire LNG chain including the marine transportation system and consumer facilities, etc. (2)Reliability analysis in the event of natural disasters, such as strong earthquakes. (3)Optimization of maintenance scheduling. (4)Optimization of plant operation. (5)Safety analysis related to gas leakage, etc.

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BIOGRAPHIES

Daisuke Takagi Production Engneering Department,Tokyo gas Co,.Ltd 1 -5-20,Kaigan,Minato-ku,Tokyo,Japan

e-mail:d-takagi '@ tokyo-gas.co.jp

Daisuke Takagi has 17 years experience in planning,design,construction and maintenance of LNG receiving termina1,especially in plant control system(DCS) and instrumentation.

Kazuo Koyama Production Engineering Department,Tokyo gas Co,.Ltd 1 -5-20,Kaigan,Minato-ku,Tokyo,Japan

e-mailkz-koyama@ tokyo-gas.co.jp

Kazuo Koyama has 28 years experience in system engineering for LNG terminals and Gas facil ties.Especially, he has the specialty in numerical analysis, engineering simulation and system development.

Akinori Ishizuk'a Production Engneering Department,Tokyo gas Co..Ltd 1 -5-20,Kaigan,Minato-ku,Tokyo,Japan

e-mail:ishizuka@ tokyo-gas.co.jp

Akinori lshizuka has 7 years experience in design,construction and maintenance of LNG receiving termina1,especially in construction of information system.

Katsunori Kawai Risk Analysis Engineering Section, MITSUBISHI HEAVY INDUSTRIES, LTD 3-1 Minatomirai 3-chome,Nishi-ku,Yokohama,Japan

e-mail:kkawai@atom.hq.mhi.co.jp

Katsunori Kawai has 17 years experience in probabilistic risk assessment ,especially in nuclear power plant and LNG receiving terminal.

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