# Sensor-Data-Driven Prognosis Approach of Liquefied Natural Gas Satellite Plant
## 3 Problem Description  
A regasification LNG satellite plant (Figure 1) is composed of the following systems for its operation (Figure 2):  
1. [label=)]
2. LNG discharge: allows to carry out LNG transfer between tanker trucks and storage tanks. 3. Storage: integrated by one or more double containment tanks, used for storing LNG at cryogenic temperatures. * LNG vaporizer: an uncomplicated heat exchanger that vaporizes liquefied gas by using heat absorbed from ambient air. * Regulation system: reduces and establishes gas pressure for the supply through the distribution network. * Control system and plant operation: maintains the control of the system in all its operating parameters, with emergency stops that allow it to immediately cut the supply in case of an emergency. In LNG storage tanks, gas is generated such that, if not extracted, it accumulates in the tank causing pressure to rise. The main causes of this gas generation are due to:  
Figure 1: Satellite liquid natural gas (LNG) plant. Figure 2: Scheme of LNG satellite plant. * Vaporizing the LNG contained in the tank due to the exchange of heat with the outside through the walls and roof of the tank. Storage temperature is \(-162\,^{\circ}\)C, and, although the tank is cryogenic with two containers and important insulation in the chamber between them, there is a certain level of heat exchange, causing an increase in the tank temperature and moving the equilibrium point towards a higher pressure. * Compressing the gas in equilibrium with the stored LNG during tank filling by trucks. The gas generated in excess regarding the amount of gas in equilibrium with LNG, known as boil-off gas (BOG), has to be evacuated from the tanks through vent stacks (see Figure 2) to maintain stable pressure in the tanks. The BOG is recovered by a reliquifier and is mainly reintroduced in liquid phase into the tank to avoid product loss at the plant exit and unnecessary gas emissions into the atmosphere, avoiding a negative impact to the environment. The reliquifier is pressurized equipment in which gases coming from the boil-off come in contact with the subcooled LNG coming from the storage tank for its condensation. The regasification system heats the LNG from the tank. Then, the outflow of liquid from the tank decreases the level inside and increases the volume occupied by the gas phase at the top. Since the tank is thermally insulated, there is no vaporization of liquid to gas, so pressure of the gas phase decreases by increasing the volume it occupies. To maintain gas pressure and prevent its lowering, there is a regulator called rapid pressure setting (PPR) that controls the regasification of the extracted liquid gas from the tank in a closed circuit. The operation of the system is controlled by a programmable logic controller (PLC) that can acquire sensor values and command actuators. Sensor data are collected in PLCs and transmitted to a SCADA system, and data are stored in a database system based on a mechanism that combines a time-triggered period of one hour with an event-triggered approach. In turn, this means that each stored variable has its own mean sampling time. Figure 3 shows the time evolution of pressure (PT) and level (NVT) inside the tank, NG consumption, and ambient temperature of one year in one of the analyzed satellite plants. Pressure is controlled around 3-4 bar, and LNG level decreases by regasification consumption and refilling by LNG transfer between tanker trucks and storage tanks. In the middle of the year (mainly summer with a higher temperature), level drops are smoother because of a reduction in consumption. Figure 3: Pressure (PT) and level (NVT) in satellite-plant tank, NG consumption (QIC), and ambient temperature (\({}^{\circ}\)C). Two problems are addressed in this paper. First, validating raw sensor data and, in case that the data are not consistent, try to provide estimations that allow to reconstruct the complete database.