Predict the performance

In the Asia Pacific region, LNG is transferred by specialised LNG vessels with cryogenic tanks intended for long-distance transportation. A significant fraction of the cargo LNG volume evaporates during the voyage as boil-off gas (BOG), particularly during spray operation. It is challenging to use generated BOG for LNG vessel operation efficiently, and especially to minimise (optimise) the LNG heel stock during a ballast voyage. These LNG vessels can increase both fuel efficiency and delivery capacity by optimising spray schedules for storage tanks.

The article’s authors developed a dynamic simulation system to estimate LNG heel stock in the LNG cargo tanks of a Moss LNG cargo containment system, thus determining the required minimum LNG heel stock for a ballast voyage.

The simulation model, built in CHEMCAD NXT, considers the effect of several process factors such as LNG compositions, ambient temperature, pressure, sloshing factor (ship motion), spray operation schedule, and LNG vessel voyage plan. Based on the dynamic simulation, the optimum LNG heel stock can be determined for long-distance LNG transportation. The dynamic simulation model can be applied for: � LNG ballast handling (including spray schedule) during the voyage. � Thermo-economical voyage simulation of marine energy systems of LNG vessels.

Kenichi Matsuoka, Azbil Corp., Japan, Daichi Watanabe, MTI Co., Ltd, Japan, and David Hill, Carla Lara, and Aaron Herrick, Chemstations, USA, detail the development of LNG cargo tank simulation technology for ballast voyage.

Scope of process and process model

The most important operation during the ballast voyage is to cool down the LNG cargo tank wall, including at the end of the ballast voyage before reloading. This must be done while also satisfying the required fuel demand during the entire voyage. The process flow model is composed of the following equipment: LNG cargo tank, LNG forcing vaporiser (F/V), spray pump, piping system, control system, and fuel demand system (to engine or boiler for the vessel’s propulsion system).

Figure 1. Dynamic process modelling schematic for cargo tank system. Cargo tank model

LNG producing country JAPAN Loading Unloading

Laden voyage Ballast voyage

Before the ballast voyage, the heel stock for LNG tanks must be decided, taking into consideration the energy required for engine and spray operation. This decision is made by HQ on land.

At the end of the ballast voyage, the equator temperature for Moss LNG tanks must be less than -120˚C. Liquid spray into the vapour space lowers the vapour and wall temperature. The timing of this spray operation is decided onboard during the voyage.

Figure 2. Ballast voyage plan. Figure 3 shows the simplified tank model system of BOG generation from the LNG cargo tanks. The LNG cargo tank model is composed of the following sub-process models: � Heat input from LNG cargo tank wall. � Heat input by spray pump. � Heat accumulation on LNG cargo tank wall. � Heat accumulation on LNG liquid phase. � BOG generation from the interface between liquid and vapour. � Heat accumulation on LNG vapour phase. � LNG mist mass and heat balances in tank vapour phase by spray nozzle. � Spray efficiency.

This article describes the rigorous and realistic dynamic simulation of BOG, considering cargo tank spray operation and vessel energy requirements.

Vapour phase model in the tank

The gas phase model is described as follows: � Gas mass balance:

Outline of heel stock estimation simulator for ballast voyage

Before LNG is loaded into the LNG vessel, the tank wall temperature at the tank equator must be cooled down to at least -120˚C, to avoid metal brittle fractures.

In addition, fuel gas must be supplied to the engine system (or boiler system) to satisfy the fuel energy demand required by the LNG vessel.

Figure 1 shows the scope of processes to be conducted by dynamic process simulation. Each cargo tank has its own spray nozzle line to lower the temperature of the gas and the metal wall temperature.

This process simulator can be used to estimate the required LNG heel stock in the following scenarios: � Planning heel stock for ballast voyage. � Controlling spray operation in real-time during ballast voyage. � Mist mass balance:

� Energy balance:

 Where Wg (t); Gas weight in tank (kg).  FBOG (t); BOG rate from leaving from liquid phase (kg/h).  Fvms (t); Vaporised gas from mist in tank (kg/h).  Fo (t); Suction pure gas flow at tank (w/o mist flow) (kg/h).  Fm (t); Mist flowrate by spray (kg/h).  F(s,mist) (t); Entrained mist flowrate in suction gas flow (kg/h).  Hg (Tg); Gas enthalpy at Tg (˚C) (vapour phase temperature).

 Tg; Gas phase temperature (˚C).  Qin,g (t); Heat input to tank vapour phase (kcal/h).

Tank wall heat balance model in the tank

Figure 4 shows the process behaviour diagram of the tank wall in the cargo tank. Heat transfer between ambient and vapour phase (through tank wall) are shown as follows: � Heat balance for tank wall:

� Heat transfer coefficient between ambient and wall:

 Where Cpw; Gas specific heat capacity [kcal/(kg.˚C)].  Ww; Gas weight in tank (kg).  h(o,ins); Effective heat transfer coefficient between ambient and wall, including insulation [kcal/(m2.h.˚C)].  hi; Heat transfer coefficient between wall and gas phase [kcal/(m2.h.˚C)].

Liquid phase model in the tank

The liquid phase model is described as follows: � Mass balance:

� Energy balance:

 Where Vl (t); LNG volume in the tank (m3).  ρl; LNG density (kg/m3).  FBOG (t); BOG rate leaving from liquid phase (kg/h).  Wl; LNG weight in tank (kg).  Hl (Tl ); Liquid enthalpy at Tl (˚C) (liquid phase temperature).  Hg (Tl ); Gas enthalpy at Tl (˚C) (liquid phase temperature).  Q(in,l) (t); Heat input to tank liquid phase (kcal/h).  Tl; Liquid phase temperature (˚C).

Control logics

The dynamic simulation system has the following major control logics: � Tank pressure HH/LL logic:  Pressure HH: Excess BOG is to be vented to protect LNG cargo tank.  Pressure LL: Forced vaporisation is to start during fuel gas supply mode, and vaporised natural gas is to be supplied to the engine (or boiler). � Tank LNG level LL logic:  If spray pump for a tank with level LL is running, the pump stops operating to protect itself. When this occurs, a pump starts in another tank with adequate LNG inventory starts. � Fuel oil and fuel gas mode operation:  The model can be run either with fuel oil or with fuel oil and fuel gas. A priority combustion mode must be selected. If fuel oil mode is selected, forced vaporisation is not used.

System configuration

The model can be run as a standalone process control model, or it can be run with a fully integrated graphic user interface (GUI). The voyage input data for the dynamic simulation include:

Figure 3. Simplified cargo tank system.

Figure 4. Tank wall model.

Figure 5. TK#1 wall temperature at equator during ballast voyage.

Figure 6. TK#1 gas temperature during ballast voyage.

� Voyage plan with energy requirement for propulsion. � LNG cargo tank initial conditions. � Spray schedule. � Ambient conditions (temperature, pressure).

Based on the aforementioned input data, the calculation result (at every time step) is sent to the user interface to show the dynamic behaviour.

Dynamic process simulation for heel stock estimation

Process study 1: Dynamic model validation

The dynamic process model was validated based on actual ballast voyage data including spray operation.

The calculation results and the related actual voyage data are shown in Figures 5 and 6.

Process study 2: Effect of spray schedule on tank process behaviour

Sensitivity studies were designed and executed to find the optimum spray schedule to consume the lowest amount of spray flow and satisfy the fuel needs for the voyage. The primary focus for these studies was on: � The spray flowrate to satisfy the required minimum tank wall temperature (<-120˚C) at the end of the ballast voyage. � The net BOG generation rate from tanks and the change of LNG inventory during the entire voyage.

The ballast voyage simulation was performed for the following spray patterns: � BASE-CASE: Actual spray schedule. � CASE-1: Spray operation conducted throughout the entire voyage. � CASE-2: Spray operation conducted only during for the final part of the voyage.

As shown in Figure 8, the CASE-1 scenario is the longest duration of spray schedule to keep the cargo tank at lower temperature during the entire ballast voyage. This led to the highest BOG generation among the three cases. The CASE-2 scenario, which is the centralised spray operation at the end of ballast voyage, produced the lowest total BOG generation rate.

In terms of heat transfer rate into tanks, cargo tanks with lower vapour temperature had higher heat intake through the tank wall, which led to higher required spray flow amounts.

The optimum spray schedule uses the minimum amount of spray flow to cool down the tank wall. Other considerations must include voyage route and propulsion performance, both of which affect fuel usage.

Figure 7. TK#1 wall temperature, due to change of spray schedule.

Figure 8. Total LNG inventory in cargo tanks, due to change of spray schedule.

Summary

The dynamic process simulator with spray model for LNG cargo tank systems during ballast voyage was developed to estimate the required LNG heel stock and enable planning for optimum spray operation schedule during the ballast voyage. The dynamic model can be applied for LNG ballast handling (including spray schedule) during the voyage, and thermoeconomical voyage simulation of marine energy systems of LNG vessels.

The LNG cargo tank model is a dynamic process model for the evaporation of LNG during marine transportation. The advantage of this model is that it can simulate BOG rate and the quantity of LNG stock in the tanks, while obtaining a unified heat and material balance for the entire voyage.

This dynamic simulator, which can predict the behaviour of the LNG cargo tank process variables shown earlier, will be a part of the shipping voyage support system, with the aim of precisely understanding the fuel consumption and propulsion performance conditions of oceangoing vessels.

Note

The work is supported by Nippon Yusen Kabushiki Kaisha (NYK) and MTI Co. Ltd.

References

1. UMEYAMA, N., KAWASHIMA, M., and NAKAMURA, Y., ‘BOG control simulation during ballast voyage’, International

Conference on Liquefied Natural Gas’, 1995. 2. UMEYAMA, N., MATSUOKA, K., IWATA, A., and HILL, D.,

‘Development of dynamic process simulator on LNG vessel gas management system’, AICHE 15th Topical Conference on

Gas Utilization, 2015 . 3. LUYBEN, W.L., ‘Process modeling, simulation and control for

Chemical engineer’, second edition, McGraw Hill, USA.