LNG is a clean and competitive fuel already chosen by major industry players, who have launched significant newbuild projects for LNG-fuelled vessels. These vessels can be carefully monitored to ensure a successful transition in an optimised manner, while taking into account the specific properties of LNG.
The digital solution LFS combines GTT’s expertise in LNG with Ascenz’s know-how on smart shipping solutions. The ambition is to support on-board and on-shore teams to ease operation of the fuel tank by gaining awareness from advanced LNG modeling capabilities.
The development of dual-fuel engines and suitable LNG containment technology has initiated the market for LNG-fuelled ships. The environmental benefits of LNG as fuel play a major role in its adoption by the maritime industry.
To bring added value decision-making tools to the industry, an advanced understanding of LNG behaviour is necessary. These are a few examples of the specific characteristics of LNG:
The composition of LNG is a key element. In fact, LNG comprises different molecules such as methane, nitrogen, ethane and heavier hydrocarbons. As each molecule has its own evaporation rate, the composition of LNG varies over time. This phenomenon is called LNG Ageing.
The indicator used to describe the quality of LNG as fuel is the Methane Number (MN). The higher the content in methane in the LNG, the higher the MN will be. If the Methane Number goes outside the range recommended by the Engine Maker, the engine can experience knocking and operate in a suboptimal manner. Thus, monitoring the Methane Number and predicting LNG Ageing is very important, especially for voyages far longer than those of LNG Carriers (up to 80 days).
Boil-Off Gas (BOG) is initiated via the passage of external heat through the tank wall, as well as from internal heat sources such as fuel gas pumps. This Boil-Off Gas will be sent to the engines. However, the natural BOG will not always be sufficient to meet the engine energy requirements when at sea. There is therefore a requirement for forced Boil-Off. Hence, a holistic vessel energy management provides a clear operational breakdown between natural BOG and forced BOG. When, the ship is at berth for a long period, the Natural Boil-Off Gas leads to a continuous increase in tank pressure. When the design maximum pressure limitation is reached, the extra Boil-Off Gas is sent to the Gas Combustion Unit to reduce the pressure. The maximum time a ship can wait at berth with a closed tank is called the Holding Time. It is also important to be able to predict this parameter. It depends on the tank design and the quantity and quality of the LNG.
LNG Bunkering can take place at different places. Also, due to LNG Ageing, the density of the LNG in the tank can change overtime. Thus, during the bunkering operation, the density of the bunkered LNG and the LNG inside the tank can be very different. In this situation, the liquid inside the tank can be stratified, and if not dealt with properly can lead to a liquid rollover. The main risk of a rollover accident is the rapid release of large amounts of vapour leading to over-pressurisation of the tank.
The digitalisation of LFS is based on three main data sources: navigation, consumers and tanks.
A Data Acquisition System (DAS) will gather the data from several systems (AIS, IAS, sometimes with a direct connection to sensors), and different protocols (mostly NMEA and Modbus). Afterwards, this information can be used immediately on-board for decision-making systems, and/or sent securely on-shore for remote performance and security monitoring.
Precise LNG ageing measurement and simulation are key, as it affects BOG generation, Methane Number and energy content of the gas sent to the engines.
It is highly recommended to pre-equip LNG-fuelled ships with the following sensors:
It helps measure the composition, recalibrate the prediction algorithms when required and brings transparency regarding the quality of the bunkered LNG.
They allow a precise measurement of the quantity of the LNG or NG taken from the tanks.
Real time acquisition of data enables prediction of the bunker status thanks to advanced LNG modeling techniques developed by GTT.
First, the system offers useful insights into operational limits (tank pressure filling levels, bunker temperature and inventory).
Second, the systems predict the bunker thermodynamic evolution. For instance, the evolution of the Methane Number over time during the voyage to ensure it remains within the engine maker’s recommendation, the pressure profile regarding engine demand for the next leg, or density prediction before a bunkering operation in order to prevent a possible rollover inside the tank.
Third, the system can measure and verify the quality of the bunker to make sure it fits the customer requirements. A deviation from the quality requirements has a direct impact on the operations and the economics.
As many maritime players are adopting LNG as a fuel, dedicated smart decision-making tools can help smooth this transition. In addition to the above operational and economic benefits, these systems provide a real help for the crew and the fleet managers, particularly if there are new to LNG.
Example of prediction capabilities of LFS