Advanced weather routing for LNG carriers

Route optimisation for Liquefied Natural Gas (LNG) carriers entails specific aspects resulting from their ability to use their cargo as propulsion fuel. This reinforces the potential gains from speed/power advice combined with route advice but also introduces additional complexities compared to other types of vessels.

The Syroco platform addresses this challenge by developing a model of the ship's power network in relation to her propulsion system and by including this model in the digital twin of the vessel. LNG power distribution models are therefore used for weather routing and voyage optimisation

Fuel consumption model

Engine fuel consumption is generally represented by a SFOC (Specific Fuel Oil Consumption) model which describes a nonlinear variation of fuel consumption with engine power. In the case of a LNG carrier with dual fuel, this propulsion model is extended to integrate Specific Gas Consumption (the gas burnt in the engine) and Specific Pilot Oil Consumption (pilot oil is used for ignition as natural gas is not self-combustible).

Mission-specific fuel mix

The selection of the fuel mix introduces a new optimisation variable and the need to redefine the target of the optimisation. In this case, a Syroco optimisation entails selecting the mission type that depends on the contract that applies to the voyage. This mission type may vary from one voyage to the other, and of course from one ship to the other in a given fleet, it is therefore part of the voyage parameters.

Typical LNG contracts include: 

• “No Gas”: boil-off LNG is fully reliquefied and not used for propulsion, vessel runs primarily on fuel oil (in some cases, extra boil-off can be consumed as “last resort”, if reliquefaction capacity is exceeded and to avoid safety release).
• “Full Gas”: only cargo LNG is used for propulsion, which may require forced boil off to complement natural boil-off.
• “Min Gas”: only natural boil-off LNG can be used for propulsion, fuel oil is used when natural boil off is not sufficient.

The optimal fuel selection therefore takes into account the ability (or not) to use gas as fuel, the natural or forced boil-off rate, and the ability of the vessel to reliquefy (and the power consumption of reliquefaction). The integration of hotel load and the use of shaft or diesel generators will also have an impact at the optimisation stage, increasing their global impact on total consumption compared to a single fuel model. 

Optimising speed and power selection

This optimal selection is done throughout the spectrum of ship speed and power (not limited to constant speed), allowing the tool to include in the assessment the potential impact of a safety release which may occur at reduced speeds (inability to consume all boil-off) or when oil is used as main fuel.

As a result the most optimised route will be calculated, even in cases of limited maximum speed (for example due to bad weather conditions, notably in ballast condition), and minimum velocity resulting from required time of arrival.

The Sankey diagrams below explain the power distribution models in two different types of contracts.