Liquefied natural gas reqasification plant and method with heat recovery

Field of the invention The present invention relates to a liquefied natural gas (LNG) regasification plant on a vessel, such as for example an LNG transport ship, an LNG storage barge or an offshore regasification unit or the like, using a heat transfer fluid, such as for example sea water or river water, as heat source for regasification. The present invention further relates to a method for regasifying LNG by means of such an LNG regasification plant on a vessel.

Background of the invention

A known LNG regasification plant on a vessel is shown in figures 1 and 2. This LNG regasification plant comprises a number of regasification trains for supplying and regasifying LNG from cargo tanks into a natural gas discharge line and a heating water circuit. Each regasification train comprises an LNG vaporizer having a heating water inlet and a heating water outlet. The heating water circuit comprises a first circuit part which is connected to the heating water inlet of each LNG vaporizer and a second circuit part which is connected to the heating water outlet of each

LNG vaporizer. As shown in figure 2, the plant can function in sea water heated mode and in steam heated mode. In sea water heated mode, sea water is supplied from the environment and, after heat is extracted from it in the LNG vaporizers, discharged back into the environment. In steam

heated mode, the sea water circulates in a closed loop between the LNG vaporizers and one or more steam heaters, by which the sea water is reheated to the desired temperature at the inlet of the LNG vaporizers. The steam for the steam heaters is condensed in the heating water heaters. On the other hand steam is also supplied to the steam turbines of the electrical generating plant onboard the vessel. The steam coming out of these turbines is condensed in the main condenser, which uses large volumes of sea water as cooling water.

The known LNG regasification plant has the disadvantage that in both modes, heat is exchanged with sea water from the environment, so that the plant cannot be used in areas where this is not permitted in view of environmental restrictions.

Disclosure of the invention It is an aim of the invention to provide an LNG regasification plant with which the heat exchange with water from the environment can be reduced or even eliminated.

This aim is achieved with the LNG regasification plant showing the technical features of claim 1. It is further an aim of the invention to provide a method for regasifying LNG with which the heat exchange with water form the environment can be reduced or even eliminated.

This aim is achieved with the method showing the technical steps of claim 8. The LNG regasification plant of the invention comprises at least one regasification train for supplying and regasifying LNG from one or more LNG cargo tanks into a natural gas discharge line. Each regasification train comprises an LNG vaporizer having a heating fluid inlet and a heating fluid outlet. The plant further comprises a heating fluid circuit, of which a first circuit part is connected to the heating fluid inlet of

each LNG vaporizer and a second circuit part is connected to the heating fluid outlet of each LNG vaporizer, and a cooling fluid circuit for supplying cooling fluid to an additional heat exchanger forming part of an additional thermodynamic circuit, also located on the vessel. The cooling fluid circuit comprises a third circuit part, which is connected between the second circuit part of the heating fluid circuit and a cooling fluid inlet of the additional heat exchanger, and a fourth circuit part, which is connected between a cooling fluid outlet of the additional heat exchanger and the first circuit part. In the LNG regasification plant of the invention at least part of the cooled heating fluid coming out of the one or more LNG vaporizers is used as a cold source for the additional thermodynamic circuit. In this additional thermodynamic circuit, the cooled heating fluid is reheated, after which it is fed back to the inlet of the one or more LNG vaporizers. As a result, at least part of the cold medium which comes out of the one or more LNG vaporizers is recovered, so that the amount of heat exchanged with the environment can be reduced.

For example, upon applying the invention to the known plant discussed above, the following applies. In sea water heated mode, the volume of sea water which has to be supplied from the environment as heating fluid and is discharged back into the environment after heat is extracted from it, is reduced by the volume which is used as cold source in the additional thermodynamic circuit. In steam heated mode, less steam needs to be produced in the main boiler, since part of the cooled heating fluid is reheated via the cooling circuit, so that the volume of sea water which needs to be supplied to the main condenser as cooling fluid can also be reduced. So in steam heated mode, there is also the advantage that the overall efficiency of the plant can be improved.

Preferably in the LNG regasification plant of the invention, the first circuit part is connected to an inlet via which water from the environment

can be supplied to the heating fluid circuit and the second circuit part is connected to an outlet via which heating fluid coming from the LNG vaporizers can be discharged towards the environment. The first and second circuit parts are preferably connected to each other via one or more steam heaters for reheating the heating fluid. The inlet and outlet towards the environment are preferably equipped with valves, so that a choice can be made between operation in sea water heated mode and steam heated mode.

Preferably in the LNG regasification plant of the invention, the one or more steam heaters form part of a steam circuit comprising at least one boiler for generating steam and a main condenser for condensing steam, the main condenser forming part of the additional thermodynamic circuit. This has the advantage that not only heat is recovered in the heating fluid circuit of the LNG vaporizers, but also in the main condenser cooling fluid. The need to use external sea water for cooling the main condenser can be eliminated and the heat otherwise lost into the sea through the main condenser cooling water can entirely be recovered. In other words, the cooling fluid of the main condenser can now be circulated in a closed loop as well. In one embodiment of the invention, the cooling fluid flowing through the cooling fluid circuit, which is connected to the heating fluid circuit of the LNG vaporizers, is used for cooling the main condenser cooling fluid in a recovery heat exchanger. In an alternative embodiment, the cooling fluid coming from the heating fluid circuit of the LNG vaporizers is used as main condenser cooling fluid, which means that the main condenser itself forms the additional heat exchanger.

In view of environmental issues, it is preferred to use natural water such as for example sea water or river water as heating/cooling fluid in the plant of the invention. However, any other heating/cooling fluid known to the person skilled in the art may also be used.

The above disclosure also applies for the method of the invention, so that further explanation is considered unnecessary.

Brief description of the drawings The invention will be further elucidated by means of the following description and the appended drawings.

Figure 1 shows a schematic drawing of a prior art LNG regasification plant on an LNG transport vessel.

Figure 2 shows a schematic drawing of another prior art LNG regasification plant on an LNG transport vessel.

Figure 3 shows a schematic drawing of an LNG regasification plant on an LNG transport vessel according to the invention.

Figure 4 shows a possible piping arrangement for an LNG regasification plant according to figure 3.

Modes for carrying out the invention

The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not necessarily correspond to actual reductions to practice of the invention.

Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. The terms are interchangeable under appropriate circumstances and the embodiments of the invention can operate in other sequences than described or illustrated herein.

Moreover, the terms top, bottom, over, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. The terms so used are interchangeable under appropriate circumstances and the embodiments of the invention described herein can operate in other orientations than described or illustrated herein.

The term "comprising", used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It needs to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression "a device comprising means A and B" should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.

For reasons of simplicity it is assumed in the description below that the heating/cooling fluid is sea water. However any other heating/cooling fluid may also be used in the plant. Fig. 1 shows a schematic drawing of a prior art LNG regasification system on an LNG transport vessel. LNG/NG side:

LNG is supplied from the cargo tanks 30 to the regasification plant through dedicated LNG feed pumps 31 installed in the main cargo tanks 30. The LNG is first fed to a suction drum 32, which functions as a buffer, and is from there distributed to the different regasification trains. Each regasification train comprises a high pressure LNG pump 33 and an LNG vaporizer 10.

From the LNG vaporizers 10 the NG is fed into a common discharge line fitted with a gas metering unit 34 and a pressure control

valve 35 (to control the pressure in the vaporizers). It is then fed either to a subsea pipeline or to a shore pipeline through a dedicated high pressure ship side manifold flex hose 36 which can be located at either side of the ship. In Fig. 1 , a subsea pipeline is shown which comprises a swivel 37 to which the hose 36 is connected and an STL mooring buoy/riser 38.

Heatinp water side:

The primary heating source for the vaporizers 10 is sea water. In one mode of operation, herein called "open loop" or "sea water heated" mode (see also Fig. 2 top), the sea water is fed from a sea water suction inlet 16 by the heating water/ballast pumps 25, located in the ship's engine room, to the heating water booster pumps 23 located in a pump room in the ship's forward area, from there fed to the LNG vaporizers 10 and finally to a sea water overboard discharge 17 located in the forward area of the ship.

Steam heaters 18 are also provided in the forward pump room which allow, depending on the prevailing environmental conditions or regulatory restrictions, additional heating of the sea water before sending it to the vaporizers 10, or circulation of the heating water in a fully closed system, herein called "closed loop" or 'steam heated" mode (see also Fig. 2 bottom). This way of doing can completely avoid the need to pump up external sea water. Steam is in this case fed from the ship's main boilers (not shown) located in the engine room.

In order to prevent freezing of the water in the vaporizers 10 the heating water outlet temperature is at all times to be maintained above approx. 4°C.

Main Condenser:

The propulsion system and power generating plant on LNG ships traditionally comprises steam driven turbines (not shown). Superheated

steam is generated in boilers having the possibility to burn heavy fuel oil as well as natural gas (including boil off gas from the main cargo tanks).

The steam coming from the main and auxiliary steam turbines is condensed in a vacuum condenser 20 ("The Main Condenser"). This condenser is operated under vacuum conditions, in order to obtain an acceptable efficiency of the steam cycle. In order to maintain the vacuum condition in the Main Condenser, large volumes of sea water (typically between 7,000 and 14,000 m ³ /h for a standard LNGC) are circulated over the condenser 20 to maintain the sea water outlet temperature as low as possible (typically δt < 4-5°C).

In existing systems, known from the prior art, the following limitations exist: Sea water heated mode:

- Minimum sea water inlet temperature is required (approx 14.5°C) to prevent freezing of the sea water in the vaporizer system.

- Very high volumes of sea water are circulated through the ship systems: appr. 2,000 m ³ /h per 100 MMscfd (Million standard cubic feet per day) regas capacity (600 MMscfd => 12,000 rrvYh) plus appr. 7,000 to 14,000 m ³ /h of seawater circulated over the Main Condenser, required for the electrical power generating plant.

- Many areas have environmental restrictions on the use of sea water

- Operation inside harbour precludes the use of harbour water, (muddy water, polluted water, erosion damage to quay)

Steam heated mode - No more restrictions on sea water temperature

- No circulation of external sea water through the regasification plant But:

- Still substantial volumes of external sea water are circulated through the main condenser, so environmental restrictions remain an issue and harbour operation remains questionable.

- Very high steam demand for the regasification system limits the overall regasification capacity (on the existing vessel the maximum regasification capacity in steam heated mode is limited to 450 MMscfd compared to 600 MMscfd in sea water heated mode).

The invention.

Fig. 3 shows the adaptations to such an existing system according to the invention. In addition to the components shown in Fig. 3, the plant further comprises the components shown in Fig. 1 and 2, namely the cargo tanks 30, the LNG feed pumps 31 , the suction drum 32, high pressure LNG pumps 33, an NG discharge line with a metering unit 34, a pressure control valve 35 and a flex hose 36 for coupling to a swivel 37 and an STL mooring bouy/riser 38 or any subsea or shore pipeline.

In the plant shown in Fig. 3, the cold heating water coming out of the LNG vaporizers 10 can be used for cooling of the main condenser 20, thus eliminating the need to use external sea water for the main condenser 20 and entirely recovering the heat otherwise lost into the sea through the main condenser cooling water. In other words, the cooling water of the main condenser 20 can now be used in a closed loop as well. In order to do this a recovery heat exchanger 15 can be provided with on one side the cooling water circulating through the main condenser 20 using a circulating pump 27 and on the other side part of the LNG vaporizer heating water which is branched off from the LNG vaporizer common heating water outlet, also using a circulating pump 25. In practice the recovery heat exchanger 15 can be located in the engine room aft, in the vicinity of the main condenser 20. The heating water can be circulated to the engine room and back forward using the existing water ballast lines 21 , 22 of the vessel which are running through a pipe duct along the ship's length. The heating water can be circulated

and the flow through the branch line can be controlled using one of the ship's ballast pumps 25 (at reduced speed if required).

Note that ballasting of the tanks during discharge operations will be performed through a separate filling line. The proposed system has following benefits:

- The use of external sea water for the main condenser 20 and LNG vaporizers 10 can be eliminated. Thus: o no restrictions on sea water temperature, o no environmental issues with respect to the use of sea water, o no restrictions with respect to quay side operation.

- The regasification capacity is substantially improved to the amount of heat that is recovered entirely from the main condenser resulting in a substantially reduced discharge time (an improvement of 15 to 20% of the regasification rate can typically be achieved with the same fuel consumption).

- Alternatively, the fuel efficiency of the regasification process is improved with 15 to 20%, resulting in reduced exhaust gas emissions and a more economical operation at constant regasification rate.

The same principles can also be used in case other propulsion and power generation systems are used for the vessel, such as Dual Fuel burning diesel electric systems. In this case heat can be recovered from the diesel/generators cooling system in about the same way as from the main condenser system.

Fig. 4 shows a possible piping arrangement based on an LNGC. On the right hand side, the regasification circuit is shown, comprising one or more heating water circulation pumps 23, one or more steam heaters 18 and one or more regas vaporizers 10. Note that the valves towards overboard outlets 17 are closed, meaning that the regasification circuit

functions in closed loop mode. The circuit further comprises an expansion tank 24 for pressurising the system. The heating water circulates through a first part 1 via which it is supplied to the inlets 11 of the vaporizers 10 and a second part 2 via which the cooled heating water is discharged from the outlets 12 of the vaporizers. These parts are also indicated in Fig. 3.

In the middle, it is shown how a part of the heating water on the output side of the vaporizers 10 is branched off and circulated via the ship's ballast lines 21 , 22 through a recovery exchanger 15 by means of the ship's water ballast pumps 25 (at reduced speed if required). Note that the valves at the sea water inlets 16 are closed, which again shows the circuit is operated in closed loop, although this is not essential. This cooling water circuit comprises a third part 3 (including ballast line 21 ) connecting the second part 2 of the regasification circuit to the inlet 13 of the recovery exchanger 15 and a fourth part 4 (including ballast line 22) connecting the outlet 14 of the recovery exchanger 15 to the first part 1 of the regasification circuit. These parts are also indicated in Fig. 3.

The recovery exchanger 15 forms part of the main condenser cooling water circuit along with a circulating pump 27, a sea water inlet 28 with supply pump 26 and a sea water outlet 29. Here, the heat absorbed by the cooling water in the main condenser 20 is exchanged with the (colder) heating water of the regasification circuit. As a result of this, the condenser cooling water is cooled to the temperature needed at the inlet of the main condenser 20, so that the cooling water circuit can also function in closed loop mode. Note that the main valves to the open sea inlet 28 and outlet 29 are closed as well. The heating water of the regasification circuit is heated, so that less heat needs to be added in the steam heater and energy consumption can be reduced.