Method and system for determining and managing boil-off rate

1. A method for determining a momentary boil-off rate for a storage tank for a natural gas in liquid phase and gas phase, the method comprising:

measuring a mass flow from the gas phase of the natural gas at a discharge pipeline of the gas phase of the storage tank;

determining a volume of the gas phase of the natural gas and a volume of the liquid phase of the natural gas in the storage tank;

measuring a temperature and a pressure of the gas phase of the natural gas in the storage tank;

determining a density of the gas phase of the natural gas in the storage tank using a thermodynamic equation of state, the temperature in the storage tank, the pressure of the gas phase of the natural gas in the storage tank and a molar mass of the natural gas;

determining a mass of the gas phase of the natural gas in the storage tank by using the determined density of the gas phase of the natural gas and the determined volume of the gas phase of the natural gas;

determining a mass of the liquid phase of the natural gas in the storage tank by using a mass density of the liquid phase of the natural gas and the determined volume of the liquid phase of the natural gas and

applying a dynamical model to the measured and determined values to determine the momentary boil-off rate in the storage tank.

2. A method according to claim 1 , further comprising determining a mass flow from the liquid phase of the natural gas at a discharge pipeline of the liquid phase of the storage tank.

3. A method according to claim 1 , further comprising determining a mass flow into the liquid phase of the natural gas at a loading pipeline of the liquid phase of the storage tank.

4. A method according to claim 1 , wherein the determining of the volume of the gas phase of the natural gas and the volume of the liquid phase of the natural gas in the storage tank is performed using a liquid level sensor.

5. A method according to claim 1 , wherein the temperature of the gas phase of the natural gas in the storage tank is measured using at least one temperature sensor disposed above the surface of the liquid phase of the natural gas.

6. A method according to claim 1 , wherein the dynamical model is selected from a group of statistical state-space models.

7. A method according to claim 1 , wherein the boil-off rate is determined in real-time.

8. A method for controlling a storage tank for a natural gas in liquid phase and gas phase, wherein a real-time boil-off rate is determined according to claim 7 and the determined boil-off rate is used as input to a control unit controlling operation of an at least one valve or at least one compressor related to the storage tank.

9. A method according to claim 8 , wherein at least one valve is selected from a group consisting of valves controlling the discharge pipeline of the gas phase, the loading pipeline of the liquid phase and the discharge pipeline of the liquid phase of the storage tank.

10. A method according to claim 8 , wherein the determined boil-off rate is used as input to a control unit controlling at least one operation relating to controlling the pressure of the storage tank, modifying a course of a vessel carrying the storage tank and activating a sprayer to reduce the temperature of the storage tank.

11. A method according to claim 10 , wherein controlling the pressure of the storage tank includes controlling of opening and closing of at least one valve selected from a group consisting of valves controlling the discharge pipeline of the gas phase, the loading pipeline of the liquid phase and the discharge pipeline of the liquid phase of the storage tank.

12. A method according to claim 10 , wherein the gas phase of the natural gas obtained from the discharge pipeline of the gas phase is further processed and re-liquefied so as to be stored again within the storage tank.

13. A system for controlling a vessel comprising a storage tank for a natural gas in liquid phase and gas phase, the system comprising:

a storage tank for a natural gas in liquid phase and gas phase and comprising at least one discharge pipeline for the gas phase;

means for measuring a mass flow from gas phase of the natural gas at a discharge pipeline of the gas phase of the storage tank,

means for measuring a temperature and a pressure of the gas phase of the natural gas in the storage tank;

means for determining a volume of the gas phase of the natural gas in the storage tank, a volume of the liquid phase of the natural gas in the storage tank, a density of the gas phase of the natural gas in the storage tank, a mass of the gas phase of the natural gas in the storage tank and a mass of the liquid phase of the natural gas in the storage tank;

a boil-off rate measuring unit configured to apply a dynamical model to the measured values to determine real-time boil-off rate in the storage tank; and

a control unit configured to use the determined real-time boil-off rate in the storage tank to control at least one operation related to the vessel.

14. A system according to claim 13 , further comprising at least one discharge pipeline for the liquid phase and means for determining a mass flow into liquid phase of the natural gas at at least one of the discharge pipelines of the liquid phase of the storage tank.

15. A system according to claim 13 , further comprising at least one loading pipeline for the liquid phase and means for determining a mass flow into liquid phase of the natural gas at at least one of the loading pipelines of the liquid phase of the storage tank.