Integrating ship propulsion and energy models in the digital twin

The digital twin of the vessel used by Syroco Live computes the sum of all forces that apply in given conditions. Part of these forces originate in the propulsion systems. 

Putting aside the specific case of wind-propelled ships, we would like to take a closer look at these propulsion systems. To summarise, the propulsion chain of a vessel is composed of: 

• One or two propellers, each driven by a shaft. The propellers are operating in fixed pitch or variable pitch mode. 
• One or two main engines that drive the propeller shafts. These engines are either combustion engines or electric engines. 
• Any number of electric generators which produce the electricity needed on board the ship (hotel load and other specific uses), and the electricity used by the main engines when they are electric. Generators can be powered by a combustion fuel (diesel-electric, gas-electric, etc.), by a chemical reaction (fuel cell), or directly by the engine shaft. 

In addition to the propulsion, one needs to integrate to these models the energy consumers of the vessel. These include:

• Hotel load, which is the electricity needed to power the living quarters of the ship: ventilation and air conditioning, refrigerators & freezers, kitchens, water treatment, leisure installations, etc. Hotel load varies greatly between a cargo ship with a dozen crew members, and a cruise ship. 
• Reefers, which is the electricity needed to power the refrigerated cargo (reefer containers and refrigerated holds).
• Reliquefaction, which is specific to gas carriers and entails the reliquefaction of boil-off gas to avoid pressure built-up of gas that cannot be used for propulsion. 

Energy consumption of hotel load and reefers vary depending on the outside temperature, while reliquefaction (a huge power consumer) happens at specific times during a voyage and depends on the boil-off rate, which itself is influenced by the sea conditions (increased ship roll induces faster boil-off). 

This inventory would be incomplete without mentioning the boiler, which burns fuel to produce steam and therefore impacts the overall fuel consumption. 

At the end of the day, the reasons all these models are included in the digital twin of the vessel are twofold:

• Deliver accurate simulation of the fuel consumption in given circumstances, by taking into account all energy consumers and producers. 
• Ensure that the power available for propulsion in the routing simulations is realistic, given dependencies. For example: when a LNG carrier is running reliquefaction, power available for propulsion is significantly reduced and achievable speed is lower.  

Ultimately, a vessel’s propulsion and energy chain can range from relatively simple (for example: a combustion engine driving a fixed pitch propeller with an inline generator for hotel load) to quite complex (for example: several dual-fuel diesel & LNG generators powering electric engines and reliquefaction units). It is key for digital twins to take into account all the characteristics of these systems to properly simulate and analyse the performance of the voyage.