Risk Assessment of LNG Ship Navigation in Inland Water Based on FAHP-cloud Model

In order to understand the navigation safety risks of LNG ships in inland waterways, the paper combines the general accident risk assessment and the specific navigation environment of inland rivers[1], and analyzes the probability of collision accidents[9]. Combined with the characteristics of the specific navigation environment, to construct a hierarchical model of the risk factors for the navigation safety of inland LNG ships. Set the entry channel in the Dongsha operation area of Zhang jiagang Port area as the research scene, use the FAHP (fuzzy analytic hierarchy) process to determine the weight value of each navigation risk factor, use the cloud model to quantify the conversion of qualitative risk factors[13], quantitatively evaluate the risk level of risk factor indicators, and combine qualitative analysis to analyze the risk level of the approach channel in the Dongsha operation area from a quantitative perspective. The result objectively demonstrated the risk level of the channel, and verified that the evaluation result conforms to reality.


Introduction
In the "Outline of the Yangtze River Delta Regional Integrated Development Plan", it emphasizes the status of LNG receiving stations as important deployment facilities, to strengthen the construction of oil and gas facilities, improve the layout of regional oil and gas facilities, and promote the coordination and synchronization of cross-regional energy receiving facilities. In view of the flammable and explosive dangerous nature of LNG [1], LNG ships are always accompanied by certain risks during navigation. In the future, LNG ships will enter inland rivers, and ensuring the safety of ships' navigation is the foundation of inland river traffic safety and economic benefits of inland rivers [2]. In order to maintain the navigation order of inland waterways and fully guarantee the safety of water traffic, it is very necessary to conduct research on the navigation risks of inland LNG ships [3].

Method introduction
This paper combines FAHP and cloud model to evaluate the navigation safety risks of inland LNG ships. FAHP determines the weight value of each risk factor [4] [5]. In order to avoid the ambiguity caused by the artificial scoring process, the cloud model [6] is used for quantitative calculation, and the risk level of each influence factor of the channel is evaluated. IOP Conf. Series  LNG ship navigation safety risk factors have the characteristics of complexity and ambiguity. After using FAHP to determine the weights of different risk factors, the cloud model is used to quantitatively evaluate the qualitative indicators, and then the risk equivalent of the channel is obtained. The realization process of the risk model in this paper is shown in Figure 1.
(1) Establish a risk indicator system for navigation safety of inland LNG ships; (2) Make a questionnaire and invite experts to compare the risk indicators in pairs to obtain a judgment matrix. Calculate the weight values of the first and second level risk factors of the index system by FAHP [7]; (3) According to the calculation method of the cloud model [8], introduce the index weight value in (2) to obtain the final parameter value of the target channel. Compare the standard risk and the cloud model diagram of the calculated risk, and get the final risk distribution level of the channel.

Identification of risk factors for navigation safety of inland water LNG ships
This chapter adopts the method of general risk assessment and combines the environmental factors of inland navigation to identify the navigation safety risk factors of LNG ships [9].

Screening of risk factors
It is common to conduct risk assessment based on the probability and consequences of accidents [10], as shown in formula (1): In the formula, R represents navigation risk; p represents probability of accident; c represents accident consequences. For LNG ships, the navigation risk level is mainly dominated by collision accidents [11]. Through the IWRAP model, combined with the ship distribution function and its properties in the model, the probability of a collision event is shown in formula (2): Among them, V is ship speed; B is ship speed; Q is ship traffic; L is channel length; x is other secondary factors. Most of the consequences of collision accidents include damage to the hull, and there is exchange of kinetic energy in this process [12], thus showing the phenomenon of damage. The consequences of an LNG ship collision accident can be described by formula (3) By studying the engineering data of the entry channel in the Dongsha operation area and consulting related documents, supplementing the "other parameters x, y". Wind e41

Hierarchical model of navigation safety risk factors
Visibility e42 current e43 Figure 2. Hierarchical diagram of risk factors for navigation safety of inland water LNG ships Channel parameters：The connotation of channel parameters is channel dimensions. Based on the above analysis, the length of the channel affects the collision of LNG ships, and determines the ship's navigation time. In the case of different channel widths, LNG ships will choose exclusive or non-exclusive navigation [13]. Ship parameters ： Ship parameters include two indicators of LNG ship's seaworthiness and maneuverability. LNG ships of different sizes have different degrees of restriction on maneuvering, and the navigation safety of ships during the navigation process will also be different.
The weight vector of this judgment matrix is followed. The judgment matrix is calculated by consistency test, and the results are shown in Table 1: ... Comprehensively obtain the initial weight of the first-level indicators: Then calculate the degree of importance of triangular fuzzy number availability:

Cloud model computing
According to the safety research of water transportation navigation, the safety level of ship navigation can be divided into five levels "lower, low, normal, high and higher", among which the evaluation criteria of the risk degree of each factor are shown in Table 3: Experts are invited to score the degree of danger of the first-level indicators in the navigation risk factor model. The scoring standards are shown in Table 4  Risk is forbidden and must be avoided According to the experts' scoring of the security level of the waterway, combined with the weight value of the secondary index, using the cloud model calculation calculated by formula (4), the first level risk factor cloud model parameters are shown in Table 5: The comprehensive cloud algorithm in the virtual cloud is used to calculate the cloud model parameters of the target layer [14], as shown in formula (5) , 0 3422 En = .
, 05 He = . .That is, the cloud model parameters of the navigation safety evaluation of the approaching channel in the Dongsha operation area are 4 5174 0 3422 0 5 ( , , ) . , . . . Ex En He ( ) = . Through MATLAB software calculation, the risk assessment cloud model of this channel is obtained, as shown in Figure 3. It can be seen that the navigation risk of LNG ships entering the port channel in the Dongsha operation area is at the "normal risk" level, which is basically consistent with the actual conditions of the channel. IOP Conf. Series: Earth and Environmental Science 809 (2021)

Conclusion
Combining the specific inland navigation environment, establish an inland LNG ship navigation safety risk factor hierarchical model, and use the fuzzy analytic hierarchy process to quantitatively obtain the weight value of each risk factor, in order to solve the ambiguity of the evaluation process in the fuzzy analytic hierarchy process and the randomness of expert scoring. Combining the characteristics of the cloud model that can achieve qualitative and quantitative concept conversion, comprehensively conduct an objective risk assessment of the entry channel in the Dongsha operation area, and the final assessment result of the channel risk level is "general risk" [15].The weight of the risk factor presents the hazard distribution of LNG ship navigation, and gives ideas for maintaining the navigation safety of inland LNG ships.