Abstract: For gas liquefaction processes, thermodynamic expansion is optimized by use of a hydraulic turbine expander for liquefied gases which can be adjusted to different flow rates and differential pressures by varying the rotational speed and/or guide vane position. The turbine expander is disposed in-line between an upstream system for gas liquefaction and a downstream system for liquefied gas handling including a terminal vessel for storage or phase separation. The invention further includes a device to measure the terminal pressure at an inlet pipe of and/or inside the terminal vessel, and a device to control the turbine expander in response to pressure measurements. The turbine controller sets the rotational speed and/or the guide vane position of the expander depending on changes in the terminal pressure such that the pressure inside the terminal vessel remains constant at a certain target value for different thermodynamic, hydraulic or chemical conditions of the gas liquefaction process.