Yu Cao ,Qiao-jiao Jia ,Shi-ming Wang ,Yong Jiang ,Yong Bai

a College of Engineering,Shanghai Ocean University,Shanghai 201306,China

b College of Civil Engineering and Architecture,Zhejiang University,Hangzhou 310058,China

c Shanghai Engineering Research Center of Marine Renewable Energy,Shanghai 201306,China

d School of Civil Engineering & Transportation,South China University of Technology,Guangzhou,510641,China

Keywords:CFD Concentration field Leakage and diffusion LNG powered ships Temperature field

ABSTRACT Liquefied natural gas (LNG) is a kind of clean energy,and many LNG-powered vessels have been designed and built around the world.However,LNG introduces the potential risk of leakage and explosion accidents;thus,it is necessary to conduct safety assessments on the layout of typical facilities during the initial design stage.In this paper,an on-deck LNG fuel area model is established by employing the computational fluid dynamics (CFD) method according to an LNG-powered Aframax oil tanker.The distribution of potential dangerous areas on the concentration field and temperature field is presented under leakage and diffusion accident conditions with vent masts of different heights.The results show the following: the gas concentration rises rapidly at the initial stage of leakage and tends to stabilize after a certain period;the leakage and diffusion trend of light gas is basically the same as that of heavy gas,and the lowest heavy gas and light gas temperature on the deck between the storage tank and the upper building is-7.9 C and 4.7 °C,respectively,when the venting mast leakage point is at the height of 6 m;and the higher the mast leakage point is,the more safe the ship design.For safety considerations,the height of the venting mast designed in this paper should not be lower than the height of the LNG storage tank,which is 10 m.

1.Introduction

LNG could alleviate the increasing global energy consumption and reduce discharges of various pollutants and greenhouse gas owing to its environmental-friendly characteristics.Compared with traditional type of marine fuel,LNG could decrease CO 2 emissions by at least 20% [1].With the regulations of the International Maritime Organization (IMO) on ship emissions becoming increasingly strict,it is demanded that the sulfur content of fuel oil used by all ships around the world should not exceed 0.5% since 2020 [2].Therefore,an increasing number of shipping companies have chosen LNG power as the preferred type of auxiliary power.In view of the simulation forecast "2050 ocean forecast" released by DNV GL[3],LNG will become the main energy source of fuel consumption(40-80% in 2050).Since Norway put the world’s first LNG-powered ferry Glutra into use in 2000,there have been more than 200 LNG-powered ships in operation or under construction all over the world [4,5].The growing number of LNG-powered ships may lead to an increase in maritime hazards and accidents [6].Compared with processing facilities onshore,LNG-powered ships are generally more prone to accidents because of limited topside space and rough environment [7].Leakage and explosion accidents of LNGpowered ships will cause serious economic and human life loss owing to the cryogenic,flammable and explosive characteristics of LNG.When the accumulated volume fraction of the leaked gas exceeds 5%,it will introduce fire and explosion hazards in the case of ignition sources [8].Thus,safety assessments of LNG powered ships should be completed at the beginning of the basic design phase,which is mainly focused on the height of the venting mast and the arrangement of topside buildings [9].

Many researchers have made important progress in the analysis of LNG leakage and explosion disaster analysis technology and have focused on the consequences of cascading disasters [10,11].Several experimental analyses have been carried out and show that the distribution of the risk source area is an important factor affecting the gas diffusion characteristics [12].In order to form the evolution theory of LNG pool leakage,Doulas et al.[13]analyzed and resolved the diffusion equation that became the basic theory of evaporation and diffusion.With the development of computer technology,several simulation models have been developed for the safety analysis of LNG disasters,such as semiempirical models,integral models,zone models and CFD models [14,15].Zhou et al.[16]used semiempirical models to numerically simulate the leakage and diffusion of a dangerous and heavy combustible gas in a tank area under different conditions.Shekhar et al.[17]analysed the influencing factors of flame heat flux in detail through integral models.Because CFD models have been widely used for risk analysis and accident modeling under various conditions,CFD methods could be used to simulate leakage and dispersion from LNGpowered ships for the prediction of combustion gas dispersion,smoke emission,and explosion progression and visibility under complex geometries [18].For instance,Anay [19]presented several boundary conditions for an LNG leakage simulation.Kim and Lee[20]studied the leakage characteristics of LNG based on Mark IIItype composite secondary barriers.Cormier et al.[21]conducted a scene analysis of LNG leakage through CFD simulation and studied the applicability of different turbulence models.Dadashzadeh et al.(2013) demonstrated that the dispersion of combustion substances produced after LNG vapor fire and explosion also causes a serious hazard to humans and surrounding structures [22].Baalisampang et al.[23]proposed a methodology composed of release scenarios,credible leak sizes,simulations,and comparisons of congestion levels and flammable vapor masses which can model the dispersion of a small LNG leak and its vapor in the way of typical layout through the CFD approach.Zhu et al.[24]simulated the transient process of LNG gasification in order to clarify the diffusion concentration rules and compared them with experimental results.

In recent decades,considerable effort s have been made towards studying flammable gas plumes resulting from LNG tank release,but there are few studies on the dispersion behavior of low temperature and flammable gas after LNG leaks out and spreads to the topside in the air can be found in the literature.In this paper,a model simulating the on-deck LNG fuel area has been established by the computational fluid dynamics (CFD) method according to an LNG powered Aframax oil tanker.The distribution of possible dangerous areas based on the concentration field and temperature field is presented separately with leakage and diffusion accidents of vent masts for safety assessments in engineering practice.

2.Methodology

The leakage and diffusion of combustible gas could be assumed to be of the continuous leakage mode,and the leakage rate could be described by the Bernoulli equation,as shown in Eq.(1) [24].

Based on the finite volume method,the LNG diffusion continuity equation,momentum conservation equation,energy equation,gas state equation and transport equation are used in the FLACS simulation.Among these equations,momentum conservation equation and standardK-?model transport equation are shown in Eq.(2-4) [25].

Momentum conservation equation:

StandardK-?model transport equation:

3.Simulation technology

The model of the on-deck LNG fuel area was established with the computational fluid dynamics software FLACS according to an LNG-powered Aframax oil tanker.In this study,the leakage simulation is performed with FLACS and to identify the integrated consequences is one of the primary aims in the case of low-temperature gas diffusion.FLACS,as a CFD tool,which was designed to simulate the behaviours of both leakage and diffusion,is adopted here to calculate the gas concentration and temperature field distribution.The simulation results are validated with experimental data [20].A review and validation test of LNG dispersion numerical models can also be found in literature [26,27].

To provide data support for gas diffusion simulation,wind tunnel experiments at the Beijing Institute of Technology are cited according to the corresponding reference (Xing J et al.,2013 [28]).The experiment setup consisted of two walls and a roof,the length between the two walls was built as 6.4 m,and the height was 5.2 m.Acoustic anemometers,thermometers,and concentration sensors were arranged to monitor the wind speed,ambient temperature,and gas concentration at different locations.The leakage rates were set as 8.8 m/s,10.6 m/s and 13.3 m/s,and the gas release direction was set vertically upwards above the ground.Then,the wind speed is set between 0 and 0.5 m/s to obtain stable laminar flow,and the wind direction is oriented along the direction of the wind tunnel (Table 1).

Table 1 Flow coefficient under the different leak hole shapes.

Table 2 The gas concentration results of the simulation and experiments (Unit: m3 /m3).

Fig.1.Prototype of Aframax dual powered tanker (from internet).

The FLACS simulation model is similar to the experimental model,and the proportion is 1:1.The three-level grid division method is used for grid division,and the grid is locally encrypted to represent the X,Y,and Z directions around the leak location to improve the calculation accuracy.The boundary of the inlet wind direction is set to the WIND condition,and the boundaries in other directions are set to the NOZZLE condition.As the wind speed is 0.6 m/s,the gas concentration results at different monitor points(M1,M2 and M3 are at the same height as the leakage point and are 8 m,10 m and 12 m away from the leakage point in the downwind direction,respectively) are given in Table 2.

The comparative research shows that FLACS software could be used to simulate gas diffusion in that the deviation of results does not exceed 20%;thus,the effectiveness of the modeling method,boundary condition setting and meshing method in this paper are verified.

4.Case study of an LNG powered ship leakage accident

4.1.Numeral model

Fig.2.Numerical model.

Fig.3.Schematic diagram of the grids division.

The case study evaluated here is based on an LNG-powered Aframax oil tanker,as shown in Fig.1.The length and breadth of the ship are 86 m and 45 m,respectively.According to the IGF code,the allowable height of the vent mast is not typically less than B/3 or 6 m (B refers to the breadth of the LNG powered ship),whichever is the greater.The vent mast height could be limited to a lower value according to special consideration by the authority,but safety analysis of gas leakage and diffusion potential is necessary to confirm whether such modifications are safe or not.As shown in Fig.2,the diameter and length of LNG Tanks A and B are 10 m and 26.72 m,respectively;the height,length and width of Building A are 16 m,26.24 m and 15.32 m,respectively;the height,length and width of Building B are 3 m,11.48 m and 4.8 m,respectively;and the height,length and width of Building C are 5 m,10 m and 8.8 m,respectively.Forty-three monitoring points are set and mainly arranged near Doom Platforms A and B in LNG Tanks A and B,which are located near Buildings A,B,and C.The vent mast is located between LNG Tanks A and B.As shown in Fig.3,the volume of the entire calculation domain is 920,000 m3,which is 230 m in the X direction,100 m in the Y direction,and 40 m in the Z direction.Structural cartesian grids are used in FLACS software,and the grids near the leak hole are partially encrypted.

4.2.Parameters setting

Here,the design height of the venting mast is set to 6 m.When the flammable gas leaks and disperses from the vent mast into the atmosphere under the action of wind and the ship’s velocity,a vapor cloud with a certain volume is formed due to the continuous release of gas,which will have an adverse impact on the operational safety of the main deck and its topsides.A typical leakage accident of a vent mast is selected as the standard case;the leak hole area is set as 0.16 m2,the release pressure is 0.4 MPa and the gas initial temperature is-131 °C.According to Eq.(1),the leakage rate of the LNG vent mast is 20.774 kg/s.Here,the ship velocity and wind velocity are set as 7.7 m/s and 10 m/s,respectively,and the direction is opposite along the X direction.In order to investigate which kind of gas has more influence on the deck,the gas composition was chosen based on buoyancy considerations and gas reactivity,and the compositions of the light and heavy gas volume fractions are given in Table 3.

Fig.4.Diffusion process of the heavy component LNG gas.

Table 3 Composition of light and heavy gas volume fractions.

5.Analysis results

5.1.Vapor cloud distribution

The simulation of LNG vapor cloud diffusion is carried out;the vapor cloud becomes steady 17 s after the leakage accident of the vent mast in the power zone.The heavy gas diffusion distribution cloud diagrams are shown in Fig.4.

Fig.4 shows that the gas volume gradually increases during the early period,and the gas does not sink down near the leakage point but rather moves towards the downwind direction.Then,the gas exhibits a downward diffusion trend under the influence of gravity,slowly sinking and spreading forward,forming an oblique downward diffusion cloud.When the vapor cloud touches the main deck,it continues to spread around,and the gas diffusion is blocked by the LNG tank,but there is no gas diffusion and accumulation phenomenon on the sides of the ship.According to the vapor cloud distribution,Doom Platforms A and B on the LNG tank are safe.Furthermore,the vapor cloud was blocked by stern Building A and B,and the forward diffusion trend was stopped and began to diffuse towards the side area.Specifically,vapor clouds continued to diffuse towards the stern on the port side in that there was no building blockage.The vapor cloud distribution of light gas is basically the same as that of heavy gas.It should be noted that the vapor cloud distribution does not represent the coverage of hazardous areas,which is also determined by the gas concentration and temperature.

5.2.Concentration field distribution

The monitoring point between LNG storage tanks A and B that obtains the maximum gas concentration is selected from 43 monitoring points,and the time history curves are shown in Fig.5 for heavy and light component LNG gas.

As shown in Fig.5,the gas concentration rises rapidly during the initial period and stabilizes when it reaches the peak value for both heavy and light component LNG gas.The gas concentration in the dangerous area of the deck basically reaches a stable value after a certain period.

The concentration distribution diagrams of heavy component LNG gas on the main deck at different times are given in Fig.6.

As shown in Fig.6,the concentration of heavy gas reached 8% at 9.5 s,and the concentration area continued to accumulate and gradually diffused towards the downwind direction.Because the combustible concentration range of LNG is within 5%-15%,the shadowed,core dangerous area in Fig.6 (a) is formed.Because Building C is located on the starboard side,gas accumulates on the corner,and dangerous areas are formed.After 12 s,the dangerous area on the main deck was basically formed,and the maximum concentration value changed from 6.31% in 4.5 s to 8.11% in 40 s.

The different concentration distribution profiles of light component LNG gas in the height direction are given in Fig.7.

As shown in Fig.7,light gas does not readily accumulate,and there is no dangerous area on the main deck formed between LNG Tanks A and B.Buildings A and B on the stern affect the diffusion path of light gasses along the original direction;thus,light gas accumulates on the corner,and a dangerous area is formed between Buildings A and C on the starboard side.

5.3.Temperature field distribution

Due to the ultra-cryogenic characteristics of LNG,the gas from the vent mast will exchange heat with the air during the leakage process,which will rapidly cool the temperature of the surrounding environment and cause frostbite to personnel and equipment.The different tem perature cloud profiles of heavy and light gasses in the height direction are shown in Figs.8 and 9.

The lowest temperature of heavy and light gasses in the area near the vent mast leak hole reached an initial temperature of 142.15 K,which is-131 °C at the profile of 5 m.The lowest heavy gas and light gas temperature near LNG Tank B reached-14 °C and-2.6 °C,respectively,and the low temperature field is distributed as a long strip at the profile of 4 m.The lowest heavy gas and light gas temperature at LNG Tank B is-10.5 °C and 1.3 °C,respectively at the profile of 2 m.The lowest heavy gas and light gas temperature in the area between the LNG tank and the stern Building C is-7.9 °C,and 4.7 °C,respectively,and the temperature distribution area of light gas is nearly the same as that of the heavy gas on the main deck.

5.The influence of venting mast height on LNG leakage safety

To improve the height design reliability of venting masts on the main deck of LNG powered ships and ensure the safety of main deck facilities and personnel in the case of vent mast leakage,a sensitivity study was carried out.Through safety analysis and a comparison of several reference values of the effect of the venting mast height on the concentration and temperature fields,the sensitivity characteristics of the vent mast height are obtained,the influence of dangerous area distributions with different height settings regarding the consequences of a leakage disaster is examined.Here,the height of the vent mast is set to 6 m,8 m,10 m,and 15 m above the main deck,and the other leakage parameters are kept the same for comparison.Based on the above research,leakage from heavy gas composition is more dangerous to the deck.Therefore,heavy gas composition is taken as the input data to determine the minimum safe height of the mast.The concentration distribution profiles on the main deck of heavy gas leakage from 6 m,8 m,10 m,and 15 m vent mast heights at 40 s are shown in Fig.10.

Fig.5.Time history of gas concentration at a certain monitoring point between Tank A and B.

Fig.6.Concentration distribution diagrams on the main deck at different times.

The highest concentration on the main deck of combustible gas forms in the red area in Fig.10.In detail,Fig.10 (a) shows that the dangerous area on the main deck is between LNG Tank A and Tank B for the leakage condition of 6 m vent mast height;Fig.10 (bd) shows that the dangerous area on the main deck forms on the same position among Building A,B and C for the leakage condition of 8 m,10 m and 15 m vent mast height.According to the Fig.10,the maximum gas concentration on the main deck is 0.08,0.05,0.034 and 0.02 when the height of the vent mast is set to 6 m,8 m,10 m and 15 m,respectively.Because the combustible concentration range of LNG is within 5%-15%,the reference design value of the vent mast height should not be less than 10 m in this paper from the consideration of gas concentration.

Fig.7.Concentration distribution curve of light gas with different heights at 24 s.

Fig.8.Temperature distribution profiles of light gas with different heights at 40 s.

Fig.9.Temperature distribution profiles of heavy gas with different heights at 40 s.

Fig.10.Concentration profile on the main deck of heavy gas leakage from different vent mast heights at 40 s.

Fig.11.Temperature distribution profiles on the main deck of heavy gas leakage from different vent mast heights at 40 s.

The temperature distribution profiles on the main deck of heavy gas leakage from 6 m,8 m,10 m,and 15 m vent mast heights are shown in Fig.11.

Fig.11 shows that the lowest temperatures on the main deck are-7.9 °C,2.6 °C,7.2 °C and 11 °C when the height of the vent mast is set to 6 m,8 m,10 m and 15 m,respectively.The lowest temperature is located in the deep blue area.Because personnel will suffer from frostbite injury,if the gas temperature is lower than-20 °C,the design values of the height of the venting mast are all safe from the perspective of the temperature field [29].

7.Conclusion

In this paper,a safety analysis of on-deck LNG fuel area model has been studied by employing the CFD method according to an LNG powered Aframax oil tanker.The distribution of potential dangerous areas on the concentration field and temperature field has been presented under leakage and diffusion accident conditions with vent masts of different heights.The gas concentration under different leakage rates which obtained from the simulations is in good agreement with the data from experiment,illustrating the accuracy and reliability of this proposed CFD method.The distribution of potential dangerous areas on the gas concentration field and temperature field is presented under leakage and diffusion accident conditions with vent masts of different heights.Useful conclusions can be drawn as follows:

(1) The numerical CFD simulation results are intuitive and could be used to effectively analyze the leakage and diffusion process of combustible gas and determine the distribution of the gas concentration and temperature field.

(2) The vapor cloud distribution of light gas is basically the same as that of heavy gas,and the gas concentration rises rapidly at the initial stage of leakage,and it basically reaches a stable value in the dangerous area of the main deck after a certain period,and the lowest temperature of the heavy gas and light gas in the area between the LNG tank and the stern building is-7.9 °C,and 4.7 °C,respectively,and the temperature distribution area is nearly the same on the main deck if the design height of the venting mast is set to 6 m.

(3) The higher the number of vent exits included,the better the safety will be.The height of the vent mast should not be lower than 10 m for the ship studied in this paper.Finally,the analysis of the vent mast leakage accident scenarios of the LNG powered ships could provide references for the layout of equipment modules in the deck power zone,optimization of leakage and dispersion detection systems,emergency route settings,and emergency escape response during the design phase.

As the primitive work that explores the safety analysis of the distribution of potential dangerous areas on the gas concentration and temperature field,the set of results presented here can give a global reference of height of the venting mast design.Also,the proposed CFD method and its reliable results could provide some suggestions for the layout of equipment modules on the deck of LNG storage zone,optimization of leakage and dispersion detection systems,settings of emergency route and emergency escape response during the design phase.

Declaration of Competing Interest

We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work,there is no professional or other personal interest of any nature or kind in any product,service and/or company that could be construed as influencing the position presented in,or the review of,the manuscript entitled,“Safety design analysis of a vent mast on an LNG powered ship during a low-temperature combustible gas leakage accident”..

Acknowledgement

The authors express their gratitude to the National Natural Science Foundation of China (Grant No.41976194)and the Shanghai Engineering Research Center of Marine Renewable Energy (Grant No.19DZ2254800) of China for their financial support of this study.