Solutions

ASCENZ

Boil-Off Gas Management

Gain control over how Boil-Off Gas is managed on your LNG carrier.

The Boil-Off Gas Management application features a saturated vapour pressure calculator, LNG heel calculator, and provides visual insights on ship motion and natural boil-off.

LNG as the fastest-growing marine fuel

The Global Sulphur Cap 2020 adopted by the International Maritime Organisation (IMO) calls for ship owners to comply to the new sulphur oxide emissions of not more than 0.5% worldwide. Liquified Natural Gas (LNG) has already become an important clean burning fuel for the maritime industry. Compared to Heavy Fuel Oil (HFO), the use of LNG allows substantial reductions of more than 99% of Sulphur emissions, 80% for nitrogen oxides and 20% for carbon dioxide and is considered as the greenest fossil energy. With more and more ship owners turning to LNG propulsion system, as well as greater expected demand for the maritime transportation of LNG, this makes LNG the fastest-growing marine fuel.

However, because LNG is stored and transported in tanks as a cryogenic liquid (a liquid at a temperature below its boiling point), some amount of LNG evaporates at temperatures above its boiling point and generate boil-off gas (BOG). The amount of BOG depends on the design and operating condition of LNG carriers.

What are the main sources of Boil-Off Gas?

There are several sources responsible for Boil-Off Gas generation: (1) heat ingress source, (2) sloshing of cargo, (3) cooling down of tanks, (4) LNG loading and unloading conditions and (5) the cargo tanks pressure.

(1) Heat ingress

BOG, Heat Ingress The ingress of heat via the cargo tank insulation into cargo tanks is due to the difference between the temperature in the cargo tanks and the temperature of the environment surrounding it.

The liquid bulk receives the heat flux through the side and bottom walls of the tanks. A warm boundary layer is formed along the walls. The heat is evacuated by evaporation when the layer reaches the surface. Then, the cooled layer dives to the tank bottom.

Like the liquid area, the gaseous phase receives the heat flux by the side walls. A convection loop is formed which evacuates the evaporated flow from the liquid area and the warmed boundary layer by the gas dome.

As the cargo stays at its boiling point according to cargo tank pressure, any heat ingress causes evaporation (BOG).

(2) The sloshing of cargo: Liquid Motion

BOG, Sloshing The liquid movement inside the tanks (due to waves or navigation) brings mechanical energy to the system. It contributes to the evaporation (energy dissipation through turbulence), but it also acts as a spraying action and performs a partial cooling of the gas and the tank walls above the liquid level. Learn more about Sloshing Monitoring.

(3) The cooling-down of tanks

BOG, Cooling down of tanks

During the tank cooling-down, the sprayed liquid natural gas undergoes a sudden change of pressure which leads to its evaporation, a decrease of the tank temperature and generates BOG.

Additional heat ingress might also occur following completion of the tank insulation cooling-down during the first 48 hours of the laden voyage as the insulation and surrounding structure’s temperature normalises.

(4) LNG loading and unloading conditions

BOG, LNG Loading and Unloading Conditions During loading of an LNG tanker, differences in operating pressures between the ship’s and the terminal’s storage tank and the quality of the LNG can greatly influence BOG generation during the voyage. For instance, if the cargo is loaded at low pressure, the temperature of the LNG will decrease thus reducing BOG generated. If the cargo has a strong content of nitrogen, the BOG will be increased at the beginning of the voyage as the lighter nitrogen boils-off. Transfer of LNG (use of Pumps and valve manipulation) can be a further source of heat energy ingress to the LNG as it is transferred.

(5) Cargo tanks pressure decrease

BOG, Tank Pressure Decrease

Decreasing cargo tank pressure will modify the boiling point of the LNG. The LNG will then become superheated compared to this boiling point and will tend to evaporate at an increased rate until reaching the new boiling point.

The vessel loads its cargo at a given temperature imposed by the loading terminal and needs to deliver its cargo to a receiving terminal under specified conditions. The vessel has thus the “responsibility” to condition the cargo without having direct control on its characteristics.

To do so, the vessel is equipped to provide effective BOG management within the maximum pressure allowable inside the tanks. In the case that “warm” LNG is loaded the associated BOR will be high, reducing cargo tanks pressure management flexibility.

As a consequence, it may be necessary to manage excess BOG via the Gas Combustion Unit (GCU) instead of retaining the LNG for sale, valued as fuel or reliquefied.

The LNG composition is a significant parameter / variable and could in specific conditions even dictate the BOG generated as detailed hereafter:

LNG is composed of several components of varying volatility, each having its own pure component boiling point at any given pressure. The presence of other volatile components in a mixture affects the boiling temperature of all the components in the mixtures.

During a voyage the most volatile (lightest) components like nitrogen will evaporate first. This phenomenon is called “ageing” and has some importance on the BOG generated as the voyage progresses.

Effect of ageing of a cargo can differ greatly based on the cargo composition and the duration. The boiling temperature of the cargo can increase significantly due to the presence of heavy components such as propane and ethane.

The evaporation rate of LNG depends not only on the composition of the product but also on the temperature of LNG when the cargo is loaded at the loading terminal. If the temperature of the cargo is too high according to the cargo tanks pressure it will evaporate more on voyage.

Another imposed constraint during the voyage concerns the temperature limitations of the LNG at the receiving terminal. While the cargo is mixed with a significant amount of heavy components, there are few options:

Reliquefaction of BOG is one possibility but does consume energy
Ideally BOG is used for useful work
Last option consists in burning the gas by decreasing cargo tanks pressure to reach an acceptable temperature imposed by the receiving terminal

What could be done to minimise Boil-Off Gas?

It is clear that composition, ageing and operating constraints imposed by the terminals are of a major importance to BOG generation during the laden voyage.

There is an efficient way to limit the BOG generated which requires further cooling of the LNG during loading, or in the cargo tanks during the loading operation but this supplementary energy consumption can be important for the value chain.

Apart from loading LNG as cold as possible, there are also operational considerations which can be employed to improve and optimise the generation of Boil-Off Gas.

Reducing the pressure inside the tanks at loading as far as possible (use of HD compressor to send gas to shore)
The level of liquid motion inside the cargo tanks has an influence on the level of natural boil-off from the tanks
Define a route or heading to reduce ship motions
Anticipate slow or agitated passages by reducing cargo tank pressure when possible
Fuel Gas System set-up to avoid recirculation of warm fuel gas whilst maintaining a stable supply to consumers
Monitor cofferdam temperatures and maintain the temperature at the desired value
Wherever operationally possible, arrive with tanks cold and ready to load at the loading terminal

Why is Boil-off Gas a key issue for the LNG shipping industry?

As explained, the composition of LNG is a key element and “ageing” has some importance on the BOG generated as the voyage progresses. In the LNG trade, BOG has a significant impact as LNG is sold at the receiving terminal depending on its energy content. Since BOG reduces the quantity of cargo delivered and increases the heat value of LNG in the ship tank, the quantity of BOG is of economic evaluation, especially for ship charterers like oil & gas companies. Furthermore, given that BOG typically constitutes one-third of shipping cost for charterers, the ability to monitor and manage BOG on LNG carriers strongly points to potential savings in shipping costs.

Using Boil-off Gas Management as a digital solution

The Boil-Off Gas Management (also known as LNG Advisor™ from GTT) provides on-board crews and onshore teams with real time reliable data on the energy performance of the LNG carrier.

The system automatically monitors the engine and the gas combustion unit consumptions of boil-off gas, marine diesel oil and heavy fuel oil as well as the natural and forced BOG during navigation. The application features a saturated vapour pressure calculator, which provides the crew instantaneous calculations that determine the thermodynamic state of LNG cargo. The application also features an LNG heel calculator to reduce excessive LNG heels.

Boil-Off Gas Management helps the crew anticipate BOG and pressure spikes. The application also provides visual insights on ship motion and natural boil-off, giving crew members more confidence and flexibility in tank operations. Advanced online analytics provide KPIs regarding boil-off efficiency, propulsion, heel management, and environmental conditions.

By supplying the main parameters of boil-off to onboard crew in real-time for them to better manage BOG during the ship’s voyage, ship charterers and ship owners are given better visibility of the overall operational performance of the vessel.

LNG-Fuelled Ships (LFS)

Major industry players have taken the step to launch newbuild projects for LNG-fuelled ships (LFS) due to the competitiveness and reliance of LNG as a fuel source. Ascenz’s know-how on smart shipping solutions, together with GTT’s expertise in LNG, brings more value to customers by enhancing performance and efficiency of LNG-fuelled ships with LFS – find out more.