The invention relates to a regasification unit, for example dedicated to supply natural gas to land facilities from a liquefied natural gas carrier vessel.

- BACKGROUND OF THE INVENTION --

Natural gas is conveniently stored and transported in liquid phase, whereas it is generally used in gaseous phase. Therefore very large volumes of natural gas are to be converted from liquid phase to gaseous phase, or to a superheated fluid phase when the pressure of the natural gas delivered is above the critical pressure value effective for natural gas. In the frame of this invention, gaseous phase is assumed to encompass both actual gaseous phase with pressure below the critical pressure value and superheated fluid phase with pressure above the critical pressure value, unless indicated otherwise.

In particular, natural gas is commonly transported using liquefied natural gas carrier vessels, and regasification is to be carried out upon delivering the natural gas to a gas conveying land pipe network. Regasification can be carried out either on board the carrier vessel or in land regasification units, or also in a floating storage and regasification unit.

Regasification requires heat supply, and therefore it is an issue to minimize the heat amount which is used for a fixed gas quantity converted into gaseous phase. Commonly used heat sources are sea water, ambient air and water steam, this latter being commonly called steam and being available on board a vessel or a floating storage and regasification unit, and also in land regasification units. Then, it is also well-known to use sea water as heat source when the sea water is warm enough, and to combine sea water with steam when the sea water temperature is between a first threshold and a second threshold less than the first one. When the sea water temperature is below the second threshold, then steam is used as sole heat source.

In addition, it is also known, for example from KR 101571364, to implement an intermediate closed loop circuit for transferring heat from the heat source to the natural gas to be vaporized, in order to get rid of the issues with direct contact between liquefied natural gas and sea water, like typically corrosion. Finally, gas is usually to be supplied to land facilities, for example to a gas conveying land pipe network, with meeting requirements about gas temperature, gas pressure and gas flow. In particular, the temperature of the gas to be delivered may be required to be in the range of 0°C to 5°C, but in any case above 0°C. For meeting this latter requirement, regasification units are operated up to now so as to output gas with a sufficient temperature margin above 0°C.

Starting from this situation, one object of the present invention consists in providing a new regasification unit which allows heat savings when vaporizing and delivering a fixed quantity of gas from a liquid phase gas tank.

-- SUMMARY OF THE INVENTION --

For meeting this object or another one, a first aspect of the present invention proposes a regasification unit for converting a gas to be delivered from liquid phase to gaseous phase, this regasification unit comprising:

- an intermediate closed loop circuit which implements a heat transfer fluid, this intermediate closed loop circuit comprising at least one first heat exchanger which is arranged for transferring heat from the heat transfer fluid to the gas to be delivered, so that the gas to be delivered is vaporized, and at least one second heat exchanger which is arranged for transferring heat from at least one external source to the heat transfer fluid; and

- a temperature sensor which is arranged for measuring a temperature of the gas to be delivered after output of the at least one first heat exchanger.

The invention regasification unit further comprises: - an adjustable valve set which is arranged for adjusting a flow of the heat transfer fluid within part of the intermediate closed loop circuit; and

- a controller which is arranged for controlling the adjustable valve set, based on at least one temperature measurement result which is supplied by the temperature sensor, in a feedback manner so that the temperature of the gas to be delivered after the output of the at least one first heat exchanger remains close to a setpoint value.

Thus, according to the invention, the operation of the regasification unit is adjusted in real time so as to maintain a desired value for the temperature of the gas delivered in a controlled manner. To this end, a temperature measurement of the gas delivered is implemented continually during gas delivery. In this way, the temperature of the gas delivered can be set close to a minimum value requested, thereby allowing heat savings by avoiding that the gas delivered is warmed unnecessarily. In preferred implementations of the invention, said at least first heat exchanger may comprise a vaporizer which is arranged for vaporizing the gas to be delivered, and a gas heater which is arranged in series of the vaporizer for increasing a temperature of the vaporized gas originating from the vaporizer. Both vaporizer and gas heater are fed in parallel with the heat transfer fluid, within the intermediate closed loop circuit. Then, the adjustable valve set may comprise a first adjustable valve arranged for adjusting a first part of the heat transfer fluid which is fed into the gas heater, separately from a second part of the heat transfer fluid which is fed into the vaporizer. In this way, adjustment of the temperature of the gas delivered can be performed in a simple, flexible and reliable manner using the gas heater. In addition for such implementations, the adjustable valve set may be further arranged for adjusting the second part of the heat transfer fluid which is fed into the vaporizer, in case the first part of the heat transfer fluid which is fed into the gas heater is zero. This ensures correct operation of the intermediate closed loop circuit, with suitable flow of the heat transfer fluid within the vaporizer for completely vaporizing the gas before it enters the gas heater, while avoiding that the temperature of the gas delivered is above the setpoint value. Advantageously when said at least first heat exchanger comprises a vaporizer and a gas heater arranged in series of the vaporizer, a security monitoring may be implemented based on the gas pressure and temperature as existing between the vaporizer and the gas heater. Knowing the gas pressure and temperature values at this location and the gas composition, it is possible to assess a quantity of liquid which has not been vaporized within the vaporizer. If this liquid quantity is too important for being vaporized by the gas heater, then the flow of gas delivered may be reduced for allowing the proportion of liquid remaining at the exit of the vaporizer to decrease. Such regasification unit may be suitable for delivering vaporized gas to land facilities from a liquefied gas carrier vessel. In particular, the regasification unit may be on board the liquefied gas carrier vessel, or may be on board a floating storage and regasification unit which is separate from the liquefied gas carrier vessel. The land facilities which are gas-supplied in this way may be a gas conveying land pipe network.

According to an improvement of the invention, the regasification unit may be further adapted to perform the following steps, while the controller is controlling the adjustable valve set:

IM detecting a change in at least one parameter among a temperature which relates to the external source, a pressure of the gas being currently delivered and a flow of this gas being currently delivered;

121 assessing a corrected setting of the adjustable valve set, which is suitable for the temperature of the gas delivered being close to the setpoint value despite the parameter change which has been detected; and

131 applying the corrected setting as a priority to the adjustable valve set.

Then the controller goes on controlling the adjustable valve set from the corrected setting so that the temperature of the gas delivered remains close to the setpoint value. Such instant control of the adjustable valve set in a feed- forward manner avoids or lessens a reaction delay of the feedback control after the parameter change has occurred. The regasification unit of the invention may be adapted for operating in a first mode where the at least one external source is comprised of sea water, when a temperature of the sea water is above a first threshold. Then, it may be advantageous, for saving additional heat source like steam, that sea water is still used as sole heat source when the sea water is not much below the first threshold. In this case, the desired temperature for the gas delivered may be maintained although the sea water is not warm enough, by reducing the flow of the gas delivered. Put another way, the regasification unit may be further adapted for operating according to the first mode where the at least one external source is comprised of sea water, also when the sea water temperature is comprised between the first threshold and the same first threshold minus an offset. To this end, the regasification unit may further comprise:

- another adjustable valve set which is arranged for adjusting a flow of the gas to be delivered; and

- another controller which is effective when the sea water temperature is comprised between the first threshold and this first threshold minus the offset, and which is arranged for controlling the another adjustable valve set based on the at least one temperature measurement result supplied by the temperature sensor, in a feedback manner so that the temperature of the gas to be delivered after the output of the at least one first heat exchanger remains close to the setpoint value.

In particular, the so-called another adjustable valve set may be arranged upstream the at least one first heat exchanger, for limiting a liquid flow of the gas to be delivered which is fed into the at least one first heat exchanger.

Alternatively, when the at least one first heat exchanger comprises a vaporizer and gas heater, the another adjustable valve set may be arranged on a duct which connects a gas output of the vaporizer to the gas heater, for limiting a flow of the gas to be delivered which is fed into the gas heater.

Advantageously, again when the sea water temperature is comprised between the first threshold and this first threshold minus the offset, the so- called another controller may control the so-called another adjustable valve set in a feed-forward manner for compensating changes in the temperature of the gas delivered.

In addition, the regasification unit may be further adapted for operating in a second mode where the at least one external source is comprised of sea water and steam regenerated within a steam closed loop, when the sea water temperature is below the first threshold and above a second threshold. To this purpose, the at least one second heat exchanger may comprise two second heat exchangers for transferring heat to the heat transfer fluid, from the sea water and from the steam respectively. Then, the steam closed loop may be controlled in such second operation mode so that a temperature of the heat transfer fluid which is fed into the at least one first heat exchanger is maintained close to another setpoint value relating to this heat transfer fluid.

Furthermore, the regasification unit may also be adapted for operating in a third mode where the least one external source is comprised of the sole steam regenerated within the steam closed loop, without sea water, when the sea water temperature is below the second threshold. To this end, the at least one second heat exchanger may be arranged for transferring heat to the heat transfer fluid from the steam but not from the sea water. Then, the steam closed loop may be controlled in such third operation mode again so that the temperature of the heat transfer fluid which is fed into the at least one first heat exchanger is maintained close to the so-called another setpoint value relating to the heat transfer fluid.

According to another possible improvement of the invention when the second and third operation modes are involved, the regasification unit may be further adapted to perform the following steps upon detecting a change in at least one parameter among the pressure and the flow of the gas being currently delivered, while the regasification unit is running in the second or third mode: /5/ assessing a corrected value for the setpoint which relates to the heat transfer fluid, suitable for the temperature of the gas delivered being close to the setpoint value despite the parameter change which has been detected; and

/6/ controlling the steam closed loop so that the temperature of the heat transfer fluid which is fed into said at least one first heat exchanger is maintained close to said corrected setpoint value which relates to said heat transfer fluid.

In this way, the operation of the steam closed loop can accommodate to the variations of the flow of the heat transfer fluid which are controlled in at least part of the closed loop circuit according to the invention. Advantageously, the regasification unit may be adapted to switch between different modes automatically, based on measurement results for the sea water temperature.

Generally for the invention, the gas to be delivered may comprise natural gas. Also generally for the invention, the heat transfer fluid may comprise one among water, glycol-water, propane, butane, ethylene glycol, propylene glycol, a mixed refrigerant and any low freezing temperature heat transfer fluid.

A second aspect of the invention relates to a process for operating a regasification unit, in order to convert a gas to be delivered from liquid phase to gaseous phase. In this invention process, the regasification unit comprises an intermediate closed loop circuit which implements a heat transfer fluid. The intermediate closed loop circuit comprises at least one first heat exchanger which is arranged for transferring heat from the heat transfer fluid to the gas to be delivered, so that this gas to be delivered is vaporized, and the intermediate closed loop circuit also comprises at least one second heat exchanger which is arranged for transferring heat from at least one external source to the heat transfer fluid. According to the invention, a flow of the heat transfer fluid within part of the intermediate closed loop circuit is adjusted based on a temperature of the gas to be delivered after output of the at least one first heat exchanger, in a feedback manner so that the temperature of the gas to be delivered after the output of the at least one first heat exchanger remains close to a setpoint value. Such invention process may be implemented using a regasification unit which is in accordance with the first invention aspect, including the above-cited optional features and improvements.

Such process may be implemented for delivering vaporized gas to land facilities from a liquefied gas carrier vessel.

In particular, the process parameters may be selected in the following way:

- the setpoint value which relates to the temperature of the gas to be delivered after the output of the at least one first heat exchanger, may be comprised between 3°C and 7°C, preferably between 4°C and 6°C;

- when the regasification unit is adapted for implementing the first operation mode, the first threshold for the sea water temperature may be comprised between 1 1 °C and 15°C, preferably between 12°C and 14°C;

- when the regasification unit is adapted for implementing the first operation mode also between the first threshold and the same minus an offset, this offset may be comprised between 0.5°C and 3°C, preferably between 0.5°C and 1 .5°C; and

- when the regasification unit is suitable for implementing the second operation mode, the second threshold for the sea water temperature may be comprised between 4°C and 8°C, preferably between 5°C and 7°C.

These and other features of the invention will be now described with reference to the appended figures, which relate to preferred but not-limiting embodiments of the invention.

- BRIEF DESCRIPTION OF THE DRAWINGS -- Figures 1 a and 1 b joined together through marks A and B form a general diagram for a first implementation of the invention.

Figures 2 and 3 correspond to Figure 1 b for second and third implementations of the invention.

Also, same reference numbers and labels which are indicated in different ones of these figures denote identical elements of elements with identical function.

-- DETAILED DESCRIPTION OF THE INVENTION -

The invention regasification unit vaporizes gas by transferring heat from an external heat source to the gas to be delivered, through a closed loop circuit. Such design is called intermediate fluid vaporizer in the jargon of the Man skilled in the art.

The external heat source may be sea water or steam, alternatively or in combination of both, as this will be explained later in this description. When sea water is used, it is pumped from the sea and fed into a heat exchanger 2a, also noted EXCH.1 in Figure 1 a, and thereafter discharged back into the sea. Steam may be produced by a dedicated steam system, also called steam closed loop, in a manner well-known in the art, in particular on board ships. The operation of the steam system may be controlled based on a feedback parameter, for example a temperature of a fluid which is increased through a heat exchanger fed with the steam. Such heat exchanger which is fed with steam is referenced 2b in Figure 1 a and noted EXCH.2.

The closed loop circuit comprises a vaporizer 1 a (Figure 1 b), a gas heater 1 b, adjustable valves 3a and 3b, and the heat exchangers 2a and 2b. Within this closed loop circuit, the heat exchangers 2a and 2b may be connected serially so that a heat transfer fluid which produces the heat transfer from the external heat source(s) to the gas to be delivered, first flows through the heat exchanger 2a and then through the heat exchanger 2b. Again within the closed loop circuit, the vaporizer 1 a and the gas heater 1 b may be connected in parallel. In this way, a first flow of the heat transfer fluid, noted FLOW1 in Figure 1 b, is lead from the heat exchangers 2a and 2b into the gas heater 1 b without going through the vaporizer 1 a, and a second flow of the heat transfer fluid, noted FLOW2, is lead also from the heat exchangers 2a and 2b but into the vaporizer 1 a without going through the gas heater 1 b. FLOW1 and FLOW2 have the same temperature and may be adjusted using the valves 3a and 3b. For example, the valve 3a is arranged serially with the gas heater 1 b, for controlling FLOW1 through this latter. The valve 3b may be arranged on a main duct of the closed loop circuit outside the branches which are dedicated separately to the vaporizer 1 a and the gas heater 1 b, for controlling a sum of FLOW1 and FLOW2. In an alternative embodiment of the closed loop circuit as shown in Figure 2, the valve 3b is suppressed but a valve 3c is added serially with the vaporizer 1 a for controlling FLOW2 independently from FLOW1 . The heat transfer fluid may be for example a mixture of glycol and water, commonly denoted glycol-water. The pump 7, denoted HTF PUMP for heat transfer fluid pump, propels the heat transfer fluid in the whole closed loop circuit. Possibly, the pump 7 may have a minimum flow value for the heat transfer fluid in the closed loop circuit, which is necessary for correct operation of the pump 7 itself. For this reason, the valves 3a and 3b or 3c may be set for ensuring this minimum flow value for the total flow of the heat transfer fluid, although the temperature of the gaseous natural gas which is delivered may increase above a temperature setpoint value. With reference to the general part of this description, the vaporizer 1 a and the gas heater 1 b form together the so-called at least one first heat exchanger, the heat exchangers 2a and 2b form together the so-called at least one second heat exchanger, and the valves 3a and 3b (Figure 1 b), or 3a and 3c (Figure 2) form the adjustable valve set. The Man skilled in the art will understand that both embodiments of Figures 1 b and 2 are equivalent with respect to the principle of the invention, although appropriate valve capacities are to be selected in each case separately.

The gas to be delivered in gaseous phase may be natural gas. It is pumped in liquid phase by the pump 4 to a high pressure, with the natural gas coming from a tank of liquefied natural gas, commonly denoted LNG. For example, the LNG tank may pertain to a liquefied natural gas carrier vessel, or to a floating storage and regasification unit. The LNG tank is commonly equipped with a low pressure in-tank pump, for feeding the high pressure pump 4 with LNG typically at about 5 to 10 bars. The liquid natural gas is then fed into the vaporizer 1 a for being transformed into gaseous natural gas, and the gaseous natural gas so-obtained is fed thereafter into the gas heater 1 b, before it is delivered to external gas handling, gas conveying or gas consuming facilities. For example, such external gas conveying facilities may be a national land pipe network, denoted NG for national ground. Pressure requirement for the gaseous natural gas which is delivered to NG may be 50 bars, for example. The flow of natural gas within the circuit from the LNG tank to the national ground NG may be controlled using a valve 5 which may be serially connected between the pump 4 and the vaporizer 1 a. The valve 5 thus acts on the liquid flow of natural gas. In an alternative embodiment illustrated by Figure 3, the valve 5 is suppressed, but a valve 6 may be added serially between the vaporizer 1 a and the gas heater 1 b, so as to be effective again on the gaseous flow of natural gas which is delivered to NG. The Man skilled in the art will understand that both embodiments of Figures 1 b and 3 are equivalent with respect to the principle of the invention, although appropriate valve types are to be selected in each case separately. But when the embodiment of Figure 3 is implemented, and the pump 4 is running at its maximum output pressure, for example 100 bars for LNG, then the vaporization is performed within the vaporizer 1 a in supercritical mode, which is similar to heating a dense fluid, leading to natural gas in superheated fluid phase. Such design, which also tends to relocate heat duty from the heat exchanger 1 b to the heat exchanger 1 a, may facilitate the calculation and design of the heat exchanger 1 a. Then, based on the composition of the natural gas, and the natural gas temperature and pressure existing at the output of the heat exchanger 1 a, a liquid percentage value can be assessed for the natural gas at this location. If this liquid percentage value is too high for being handled physically by the gas heater 1 b, then the flow of gaseous natural gas which is delivered to NG may be forced to decrease by reducing valve 6 (Figure 3), based on the measurement results for the natural gas temperature between the heat exchangers 1 a and 1 b, and also the measurement results for the natural gas temperature downstream heat exchanger 1 b. Typically, such security operation should occur only if the temperature of the natural gas between the vaporizer 1 a and the gas heater 1 b becomes below -25°C, corresponding to a true limit of -27°C but using an offset of 2°C. Then the controller CTRL3 controls a reduction of the valve 6 when the gas temperature is below -25°C between the heat exchangers 1 a and 1 b. This reduction is determined based on the liquid percentage value assessed. Temperature sensors are implemented in the following manner, for operating the regasifi cation unit just described:

- sensor denoted TC1 in Figure 1 b, for measuring the temperature of the gaseous natural gas before it is delivered to NG, or as delivered. Thus the sensor TC1 is located between the natural gas output of the gas heater 1 b and NG;

- sensor denoted TCO in Figure 1 b, which is optional and intended for measuring the temperature of the natural gas on the travel between the vaporizer 1 a and the gas heater 1 b; - sensor denoted TC2 in Figure 1 a, which is optional and intended for measuring the temperature of the sea water as pumped from the sea; and

- sensor denoted TC3 in Figure 1 a, which is optional and intended for measuring the temperature of the heat transfer liquid on the travel between the heat exchangers 2a and 2b on one hand, and the parallel- arrangement of the vaporizer 1 a and the heat exchanger 1 b on the other hand.

Optionally, additional sensors may comprise a pressure sensor PC which is arranged for measuring the pressure of the gaseous natural gas as delivered. Thus the sensor PC may be located between the natural gas output of the gas heater 1 b and NG. A flow sensor FC may also be located between the natural gas output of the gas heater 1 b and NG, for measuring the quantity of gaseous natural gas which is currently delivered to NG.

Commonly, a target value is provided for the flow of gaseous natural gas which is delivered to NG, as measured by the flow sensor FC. This target flow value is achieved by acting on the valve 5 (Figure 1 b or 2) or valve 6 (Figure 3) using a dedicated controller which is denoted CTRL3 in the figures. The controller CTRL3 controls opening or reducing of the valve 5 or 6 so that the flow measured by the flow sensor FC is close to the target flow value. Instead of fixing a flow value as a target, a pressure target value may be fixed. The system will then behave exactly in the same way since the flow value depends on the circuit head pressure, also commonly called back pressure, in particular when the LNG flow originates from a centrifugal pump.

Additionally, the controller CTRL3 has a security function, for avoiding that the temperature of the natural gas delivered as measured by the sensor TC1 decreases below a security threshold relating to NG. Such security threshold may be 3°C for example. When the sensor TC1 detects that the natural gas temperature reaches this security threshold, then the controller CTRL3 may control a reduction of the valve 5 or 6, thereby causing the temperature of the natural gas delivered to rise.

The controller 3 may have another security function, for avoiding that too much liquid is sent into the gas heater 1 b in case of incomplete vaporization occurring within the vaporizer 1 a. An additional pressure sensor (not represented) which is located between the vaporizer 1 a and the gas heater 1 b, together with the temperature value measured by the sensor TCO and the knowledge of the composition of the natural gas, allows assessing the proportion of natural gas which has not been converted into gaseous phase by the vaporizer 1 a. If this proportion is too high for being vaporized by the gas heater 1 b and maintaining the desired temperature value as measured by the sensor TC1 , then the controller 3 may control a reduction of the valve 5 or 6, thereby causing the liquid quantity at the exit of the vaporizer 1 a to decrease. According to a general feature of the invention when applied to the currently described embodiment, FLOW1 of the heat transfer fluid may be adjusted in real time as a function of temperature measurement results which are produced by the sensor TC1 . Such adjustment may be performed by a controller noted CTRL1 so as to maintain the temperature of the gaseous natural gas delivered close to a setpoint value. For example, this setpoint value may be 5°C. This avoids that the gaseous natural gas which is delivered is unnecessarily heated in the heat exchanger 1 b. In this way, the total flow of the heat transfer fluid within the closed loop circuit can be minimum or almost minimum but complying with a requirement of a minimum operating flow for the pump 7, while ensuring that the gaseous natural gas which is delivered meets the minimum temperature requirement. According to such invention feedback operation, an increase in the temperature of the gaseous natural gas which is currently delivered will be compensated by a decrease in FLOW1 controlled by the controller CTRL1 and produced by the valve 3a. Conversely, a decrease in the temperature of the gaseous natural gas which is currently delivered will be compensated by an increase in FLOW1 . Practically, the variations of the temperature of the gaseous natural gas which is currently delivered may be caused by uncontrolled variations of the NG capacity. Mainly, without the invention feedback operation, an increase in the NG capacity would cause the temperature of the natural gas delivered to decrease, and a decrease in the NG capacity would cause the temperature of the natural gas as delivered to increase.

In case FLOW1 cancels upon being controlled in this way, then FLOW2 of the heat transfer fluid may be controlled in turn as a function of the temperature measurement results which are produced by the sensor TC1 , using the same setpoint value as before for the temperature of the natural gas delivered. Such adjustment of FLOW2 may be performed by a controller noted CTRL2 acting on the valve 3b (Figures 1 b or 3) or 3c (Figure 2). Such control may also avoid that the vaporizer 1 a produces by its own gaseous natural gas with temperature as measured by the sensor TCO above the setpoint value, while the gas heater 1 b is off. Put another way, the controller CTRL2 together with the valve 3b or 3c takes over from the controller CTRL1 with the valve 3a for avoiding the temperature of the gaseous natural gas to be higher than the setpoint value.

Such control of the heat transfer fluid flows within the closed loop circuit, on the side of heat transfer to the natural gas, may be combined with several operation modes for providing the necessary heat amount to the heat transfer fluid.

A first operation mode may correspond to the whole heat amount which is consumed in the regasification unit being supplied by sea water. Such first operation mode may be used as long as the temperature of the sea water as measured by the sensor TC2 is higher than a first threshold, noted TH1 and for example equal to 13°C. Preferably, sea water is pumped and fed into the heat exchanger 2a with fixed water flow value, this latter depending on features relating to the elements of the regasification unit which are dedicated to sea water handling. For this reason, such first operation mode may be called open loop mode. For such first operation mode, the heat exchanger 2b may be bypassed by the heat transfer fluid through a duct (not represented in Figure 1 a) which is arranged in parallel with the heat exchanger 2b.

A second operation mode may correspond to the whole heat amount which is consumed in the regasification unit being supplied by sea water and steam in combination. Such second mode applies when the temperature of the sea water as measured by the sensor TC2 is not high enough for operation according to the first mode. Put another way, the sea water at the fixed water flow value is insufficient for providing the whole heat amount which is necessary for the flow of gaseous natural gas delivered to NG. Because of this, both heat exchangers 2a and 2b are used in the second operation mode, and the heat transfer fluid flows through these latter after one another, for example the heat exchanger 2a at first and then the heat exchanger 2b. The flow of sea water into the heat exchanger 2a is fixed again, and the operation of the steam system is adjusted for supplying a heat amount supplementary to that provided by the sea water, with respect to the total heat amount consumed for delivering the gaseous natural gas to NG. Possibly, the steam system may be controlled so as to maintain the temperature of the heat transfer fluid as measured by the sensor TC3 at a prescribed setpoint value, so-called another setpoint value in the general part of this description. This setpoint value relating to the heat transfer fluid may be 1 1 °C for example. Such second operation mode may be used when the sea water as measured by the sensor TC2 is comprised between the first threshold TH1 and a second threshold TH2 less than TH1 . For example, the second threshold TH2 may equal 6°C. Control of the steam system based on the measurement results produced by the sensor TC3 is well- known in the art. Such second operation mode may be called semi-open loop mode, since it combines open-loop operation for sea water and closed-loop operation for the steam system.

In the first and second operation modes, an increase in the temperature of the sea water would cause an increase to occur in the temperature of the gaseous natural gas which is currently delivered to NG. But thanks to the feedback operation introduced by the invention, the controller CTRL1 will reduce FLOW1 so as to damp or inhibit the increase in the temperature of the gaseous natural gas delivered. Conversely, a decrease in the temperature of the sea water would cause a decrease to occur in the temperature of the gaseous natural gas currently delivered to NG. But the controller CTRL1 will increase FLOW1 so as to damp or inhibit such increase.

A third operation mode applies when the sea water as measured by the sensor TC2 is below the second threshold TH2. Indeed using sea water as a heat source may lead to discharging the sea water at a temperature below a regulation-prescribed lower limit. Consequently, the whole heat amount which is consumed in the regasification unit is supplied in the third operation mode by the steam system only. This latter may be operated again based on the temperature of the heat transfer fluid as measured by the sensor TC3, in particular for maintaining again this temperature at the prescribed setpoint value relating to the heat transfer fluid. For this reason, the third operation mode may be called closed-loop mode. In such third operation mode, the heat exchanger 2a may be bypassed by the heat transfer fluid through another duct (not represented in Figure 1 a) which is arranged in parallel with the heat exchanger 2a. Advantageously, one among the first, second and third operation modes may be automatically selected by the regasification unit based on the value which is obtained by the temperature sensor TC2 for the sea water. Then the Man skilled in the art will be able to design appropriate control, in particular of sea water pumps, suitable steam valves and valves effective for the heat transfer fluid, for switching from one operation mode to another one.

However, it may be advantageous to run in the first operation mode even if the sea water temperature is below the first threshold TH1 , as long as the sea water temperature is not too low, in order to delay as much as possible start-up of expensive steam consumption. Such situation may be called first operation mode override, and may be implemented as long as the sea water temperature is above the first threshold TH1 minus a predetermined offset. The first threshold TH1 may equal 13°C again, and the offset may equal 1 °C. Thus, override is implemented for the first operation mode when the sea water temperature is between 12°C and 13°C. Within this temperature range, sea water is thus again the sole heat source which is used for vaporizing and conditioning the gaseous natural gas which is delivered, but a combined parameter control is then implemented for the natural gas delivered. The controller CTRL1 still adjusts the valve 3a for ensuring that the temperature of the gaseous natural gas which is delivered remains close to the desired setpoint value, but the controller CTRL3 simultaneously adjusts the valve 5 in the implementation of Figure 1 b, or the valve 6 in the implementation of Figure 3, for helping in maintaining the temperature of the gaseous natural gas delivered close to the setpoint value. The control of the valve 5 or 6 by the controller CTRL3 may be of feedback type based on the temperature measured by the sensor TC1 , or a combination of both feedback and feed-forward control types, based again on the temperature measured by the sensor TC1 . Actually, because the sea water temperature is then below the first threshold TH1 , the heat amount which is supplied by the sea water may be insufficient with respect to the flow value which is prescribed for the gaseous natural gas delivered to NG. In such case, the controller CTRL3 adjusts the valve 5 or 6 so as to reduce the flow of gaseous natural gas delivered below the prescribed flow value. Advantageously, selection of such first operation mode override may be implemented automatically, based again on the value which is obtained by the temperature sensor TC2 for the sea water.

Also generally for the invention, the feedback control of the operation of the regasification unit, based on the natural gas temperature as measured by the sensor TC1 , may be completed for allowing more rapid reaction after an external change has occurred. Such external change may relate to the pressure of the gaseous natural gas currently delivered, its flow value to NG, and also the temperature of the sea water for the operation modes which uses sea water as heat source, namely first and second operation modes as described before. To this end, the sensors PC, FC and TC2 may also be connected to dedicated inputs of the controller CTRL1 . Then, when the controller CTRL1 detects a sudden variation in at least one among the pressure value and the flow value of the gaseous natural gas which is currently delivered, and also possibly in the temperature value of the sea water for the first and second operation modes, it may force a corrected setting onto at least one of the valves 3a and 3b for the embodiment of Figure 1 b, or 3a and 3c for the embodiment of Figure 2. This corrected setting may be calculated or read out from a stored look-up table, based on the parameter values available, and applied with minimum delay to the valves 3a and 3b or 3c. Such operation is called feed-forward by the Man skilled in control implementations. Then, the controller CTRL1 resumes the feedback operation based on the temperature measurement results which are supplied by the sensor TC1 , from the corrected valve setting. Such feed-forward operation saves reaction time so that the temperature of the gaseous natural gas which is currently delivered sticks to the setpoint value in a closer extent, whatever the uncontrolled external changes.

For example, an increase in the pressure of the gaseous natural gas which is currently delivered to NG, or a decrease in its flow, due to a decrease in the NG capacity, would cause an increase to occur temporarily in the temperature values as continually supplied by the sensor TC1 . Then, an appropriate reduction in the value of FLOW1 , which is controlled by the controller CTRL1 acting in a feed-forward manner on the valve 3a, will immediately compensate at least partially for the natural gas pressure increase or flow decrease. Thus, the natural gas temperature increase as measured by the sensor TC1 is damped or inhibited. The time delay for recovering a temperature value of the gaseous natural gas delivered which is close to the setpoint value is thus shortened. The controller CTRL2 may adjust the valve 3b or 3c simultaneously in a suitable manner, possibly also in a feed-forward manner.

Another possible example of feed-forward operation relates to sea water temperature variations which may occur during the first and second operation modes. An increase in the sea water temperature would cause an increase to occur in the temperature of the gaseous natural gas delivered. Then, an appropriate feed-forward reduction in the values of FLOW1 and FLOW2 will immediately compensate at least partially for the sea water temperature increase, so that the natural gas temperature increase as measured by the sensor TC1 is damped more rapidly or inhibited.

In the second and third operation modes, a feed-forward control of the steam system may also be implemented, for decreasing the reaction delay after a change in the NG capacity has occurred. To this purpose, the setpoint value which relates to the temperature of the heat transfer fluid at the location of the sensor TC3 may be adjusted as a function of the pressure and flow of the gaseous natural gas delivered, as measured by the sensors PC and FC respectively. For example, an increase in the pressure measured for the gaseous natural gas delivered, and/or a decrease in the flow delivered, may be compensated by a suitable decrease in the value of the setpoint which relates to the temperature of the heat transfer fluid at the location of the sensor TC3. Conversely, a decrease in the pressure and/or an increase in the flow measured for the gaseous natural gas delivered may be compensated by a suitable increase in the value of the setpoint which relates to the temperature of the heat transfer fluid at the location of the sensor TC3. The corrected setpoint value relating to the temperature of the heat transfer fluid may be calculated or read out from a dedicated look-up table.

One will understand that the invention can be implemented while modifying or adapting secondary aspects with respect to the detailed description which has just been provided with reference to the appended figures. In particular, the invention may be implemented with gases other than natural gas, for example ammonia of hydrogen. Also practical embodiments for the first and second heat exchangers, for transferring heat from the heat transfer fluid to the gas to be delivered, and from the external heat source to the heat transfer fluid, may be implemented although different from those of the figures. However, the invention may still be applied to such modified embodiments, by designing and locating appropriately a valve within the closed loop circuit, which is to be adjusted based on the temperature of the gas delivered.