Discharging Part of LNG Liquid Cargo Handling Simulator

With the development of the national economy, it is important to develop LNG (Liquefied Natural Gas) liquid cargo handling simulator. In this paper, the discharging part of Liquid Cargo Handling Simulator of LNG Carriers is developed by Visual Basic. The system includes 5 major forms and 3 mathematical models such as cargo pump model, the pressure and temperature model of the LNG vapour in the cargo tank and the liquid level model. The system can complete the unloading task. The system has become a preliminary model of LNG liquid cargo handlings simulator training.


INTRODUCTION
According to the latest report of the world energy, with the booming of the world economy, the request of energy is rapidly growing, and the request of natural gas is increasing even faster (BP Statistical Review of World Energy, 2012).Meanwhile China had put forward clearly that it would import much more LNG around the world.So China will need lots of LNG carriers and the crew of LNG carriers (SIGTTO, 2008).
While the LNG carriers as the high technology, high challenge and high value-added, have a very high demand for the crews.It is an urgent to train the crew for the LNG carriers (Zhiyong, 2012).In order to meet the requirement, Dalian Maritime University imports the Liquid cargo Handling Simulator of LNG Carriers from Kongsberg of Norway in 2009, and decides to digest and absorb it.At last, liquid cargo handling simulator can be developed with proprietary intellectual property rights (Guoqing et al., 2012).The discharging simulation as one of the constituent part of Liquid Cargo Handling Simulator is also very important.
On the other hand the development of Liquid Cargo Handling Simulator meets the requirement of STCW (Maritime Safety Administration of the People's Republic of China, 2010).Regulation I/12 of STCW Code, use of simulation.
The performance standards and other provisions set forth in section A-I/12 and such other requirements as are prescribed in part A of the STCW code for any certificate concerned shall be complied with in respect of: • All mandatory simulator-based training The discharge part of the simulator is an indispensable part of Liquid Cargo Handling Simulator, which meets with the mandatory requirement of the Convention.This paper aims at introducing the simulation of unloading operation of the LNG carriers.

INTERFACES DESIGN
The system is designed with Visual Basic, and it consist of 5 major forms and auxiliary forms which are used for logging in, managing system time, speeding up and calling models.
It is Cargo Tank in Fig. 1, Deck Line in Fig. 2, Manifolds in Fig. 3 Ship/Shore Connection in Fig. 4, and Shore Tank in Fig. 5.
The entire interface is referred to the Liquid Cargo Handling Simulator of Kongsberg of Norway (Steffen Harstad Jensen, 2008).

Pressure and temperature model of LNG vapour :
When it begins to unload the LNG, the pressure of cargo tank would decrease.In order to maintain the pressure of cargo tank in the right range, it is necessary to supply LNG vapour to ship from the shore station.So it is imperative to build the pressure and temperature model of LNG vapour in the cargo tank.In this study the responding model (Xianku et al., 2008) was built.In the model, there are four inputs and two outputs.The four inputs are x 1 for the discharge rate, x 2 for the gas-return speed, x 3 tor the temperature of return-gas, x 4 for the temperature of liquid cargo in the cargo tank.And the two outputs are y T for the temperature of LNG vapour, y P for the pressure of LNG vapour in the cargo tank.The model is built according to one of the unloading process.
At the beginning, the temperature of LNG vapour is -126.45°C, the pressure is 11.4kPa (G); at the end, the temperature is -158.74°C, the pressure is 12.8kPa (G).And the whole process is lasting about 18 hours (CLSICO, 2008).So the formula of transfer function is as follows: (1) In the formula (1) K 1 + K 2 + K 3 + K 4 = 1, K 5 + K 6 + K 7 + K 8 = 1; in the system the temperature would decrease as the proceeding of unloading, until completing.So here takes T 1 = T 2 = T 3 = T 4 = T T , the value is about one thirds of the unloading time; while the pressure is different, it shakes during the unloading process.The accommodation time is relative short, here takes it as 3 hours, so T 5 + T 6 + T 7 + T 8 = T P ; C T ; and C P is the initial value, in order to make the desperation and pressure reach the expectation effect.And the formula of transfer function comes as follows: In order to make sure the model is correct, the Simulink boxes of MATLAB are used for simulation.In Fig. 6, the simulation block diagram is as follows: The result of the simulation is shown as follows: According to the result of simulation, the temperature shown in Fig. 7 is decreased during the discharge period, and at last it is close to the temperature of return-gas.While the pressure shown in Fig. 8 is falling down quickly with relative less returngas at the beginning, it will back to the normal level (between 8 to 10 kPa) with the flow of return-gas increased.
The liquid level model: The shape of cargo tank of Dapeng Sun is octagon, which is the top and bottom side relative narrow and the middle is relative wide, which is shown in Fig. 9.
The included angle of each adjacent edge is 135°.Suppose the height of top and bottom to the middle is h 1 and h 2 , the height of middle is h 0 ; the wide of top is B 1 , the wide of bottom is B 2 , the wide of middle is B 0 ; h is the total height of cargo tank, L is the length of cargo tank.The particular of each cargo tank is shown Table 1.
According to Table 1, the liquid level model can be built as the formula (3).The cargo pump model: The cargo pump of Dapeng Sun is designed by Ebara International Corporation, its type is 12EC-24, fixed vertical.(CLSICO, 2008).
Table 2 is the particular of cargo pump in Dapeng Sun ship.In Fig. 10, the cargo pump const speed characteristic curve is shown.
According to the constitution mechanism, the formula of cargo pump const speed characteristic curve is a quadratic polynomial (Qian, 2008).So when building the model of cargo pump, the fitting of a polynomial can be introduced.The formula can be expressed as follows: In the formula, Q is the flow of cargo pump, H is the head of cargo pump.The coefficient of the formula is α 1 = -1.6308× 10 -5 , α 2 = -1.7001× 10 -5 , α 3 = -179.243.
In order to make the cargo pump work well, the pipeline characteristic curve should also be built.According to the actual operation condition point analysis of the cargo pump, the formula of pipeline characteristic can be obtained (Changzhen, 2008).
2 pipe 100 0.0000225 In the formula ( 5), H pipe represents the resistance of the pipeline.
While there are 8 cargo pumps in the LNG carriers, when they work together, the head of the cargo pump and the resistance of pipeline will be changed, as shown in Fig. 11.
In the Fig. 11, the first curve is the cargo pump characteristic curve, and the second curve is the pipeline characteristic curve.The value of the curves will follow as the formula (6).
In the formula (6) the value of a 1 , a 2 , a 2 is the same as formula (4).

DESIGN OF THE PROGRAM
The program flow chart: The program is designed and developed with Visual Basic.First, the unloading process of LNG carrier should be well understood, then the flow chart of program should be drawn (Yong, 2011).
In Fig. 12, the program flow chart is drawn, which is useful to project and programme (Steffen Harstad Jensen, 2008;CLSICO, 2008).
In order to make the programming more clearly, more flow charts need to be drawn at the same time, which is the process of cargo tank unloading, cargo tank pressure controlling and cargo tank liquid level monitoring.
In Fig. 13, the calling procedure of multi-model of cargo tank is shown, which can ensure the program work better.The working principle of cargo tank pressure control system is shown in Fig. 14.In the process of unloading, the pressure of cargo tank should maintain between 8 to 10 kPa, and when the pressure is less than 6.5kPa, the ship must inform the shore station to increase the rate of return gas which means calling gas-return model 2 in Fig. 14 (Steffen Harstad Jensen, 2008;CLSICO, 2008).
Figure 15 shows how the program monitors the cargo tank level.During the process of unloading, the crew must pay attention to the liquid level.According to the different liquid level, the trainer should take appropriate measures.If the trainers don't take the right measures, the system will sound alarm shown as in Fig. 15.
In order to make the program perfect, several other operations will be needed, such as the process of pipeline pre-cool and the cargo pump start-up.Due to the limit of space, they will not be shown in this study.

The detail of program:
In order to make the program more artistic and practical, the designer has done plenty of work.
In the program, there are at least 4 modules that are the system time management module which is used to keep the time synchronization, the cargo tank module which is used for calling the models of cargo tank, the pipeline module which is used for calling and managing the model of pipeline, and some other modules which can help make the interface more artistic.There are 10 custom components used in the program, which is in favor of programming and can make the interface more user-friendly.

RESULTS
In this study, the LNG liquid cargo operation simulator is introduced.The interface is designed as Fig. 1-5, and the program is developed in according with the several flow diagrams from Fig. 12-15, and the three models is added.At last, the discharge simulation as an integral part of the simulator is realize, which can complete the LNG discharge task and work well.The program can help the trainers be familiar with the process of LNG discharging.

CONCLUSION
According to the introduce of discharge simulation on the interface design, models building and programming realization, this paper can help realize the liquid cargo handling simulator of LNG carriers.Combining with the evaluation system of that (Hongqian, 2012), the Liquid Cargo Handling Simulator of LNG Carriers will be achieved in the future.
While there are still some problems which must be solved in the later study, such as replace the responding models with the mechanism models.Beside the unloading part, there are some other simulation modules need to be developed.They are the loading, insulation space inerting, CT(Cargo tank) drying, CT inerting, CT gassing up, CT cooling-down, laden voyage, ballast voyage, stripping, CT warm-up, CT inerting, CT aeration (Junquan, 2003;Guanfang, 2007;Jianjun, 2008).

Fig. 7 :
Fig. 7: The temperature variation curve of LNG vapour in cargo tank

Fig. 10 :
Fig. 10: The cargo pump const speed characteristic curve Fig. 12: Diagram flow chart of the unloading operation of LNG carrier