Method of starting gas delivery from liquefied gas fuel system to gas operated engine and liquefied gas fuel system for gas operated engine

The invention relates method of starting gas delivery from a liquefied gas fuel system (10) in which a fuel tank (12) is filled with liquefied gas by a filling procedure and the filling procedure results in that the pressure in the tank (12) is at least at operational pressure the required by the engine, in which method pressure in the tank (12) is controlled by controlling the operation of a pressure build up system (22) comprising the pressure build up evaporator (22.2) which is arranged in a conduit (22.1) leading from the bottom section of the tank (12) to the upper section of the tank (12). The pressure build up system (22) is controlled by controlling the flow rate of the evaporated gas in the conduit (22.1) while the heat transfer medium is allowed to flow unconstrained through the pressure build up evaporator (22.2). Further during refueling the ship's tank, its pressure is at a first stage lowered by introducing liquefied fuel in the gas phase for allowing transfer and before terminating the transfer at a second stage, liquefied fuel is introduced in the liquefied gas fuel, for allowing pressure to increase, thus meeting engine pressure requirements.