OBJECT OF THE INVENTION

The object of the invention refers to a method for conversion of LNG (Liquefied Natural Gas) carriers steam turbine propulsion or hybrid steam and Diesel or hybrid steam and gas turbine propulsion installations.

Specially the object of the invention is referred to steam turbine propulsion installations that are obliged to operate at low loads by recent IMO legislation and hybrid propulsion installations which use marine boilers and steam systems including vessels where hybrid propulsion is retrofitted for reducing emissions while the speed of vessels is maintained or reaches operationally optimal levels of speed, taking in account the risk of corrosion by sulphur dioxide at low exhaust gas temperatures, modifying the steam cycle in order to recover its efficiency and saving a substantial amount of steam and, consequently, fuel in an economical and efficient way when the vessel steam plant operates at low loads.

BACKGROUND OF THE INVENTION

The type of propulsion used in LNG (Liquefied Natural Gas) carriers has changed substantially several times in the last fifteen years.

Since the first LNG carriers where built, about fifty-five years ago, until about fifteen years ago, practically all LNG carriers have used steam turbine propulsion.

This is a very reliable propulsion system, but its efficiency is substantially lower than diesel engines-based propulsion systems. In fact, only LNG carriers are using today steam turbine propulsion.

The main reason probably is the full ability of marine boilers to use the boil-off gas (BOG) produced by the LNG cargo evaporation as fuel. Liquid fuels, such as high sulphur heavy fuel oil (HSHFO), were also jointly used in any proportion, according to trip requirements and convenience.

The more frequently used main steam boilers of the type normally used in the propulsion on steam turbine LNG carriers includes boilers which integrate the following main parts:

- One steam drum.

- One water drum.

- Two superheated steam drums.

- A significant number of superheated steam tubes, normally vertically fitted, U shaped.

- Several furnace screen water tubes, located between furnace and superheater in one or more rows.

- A significant number of water tubes connecting the water drum with the steam drum arranged in several rows all together named as main generating bank, located after the superheater tubes.

- One high pressure feed water economizer, located above in the exhaust gas outlet.

- Several water wall bottom headers, located on the lower part of the furnace.

- One furnace, limited in all its walls by water wall tubes save one wall where screen water tubes are fitted.

- One combustion air chamber and its air inlet, located on top of the furnace, protected by water wall tubes in its lower part.

- Several burners, most frequently three, located on the upper part of the furnace entering through the combustion air chamber.

- One economizer upper drum feedwater inlet.

- One economizer lower drum feed water outlet.

- One exhaust gas duct outlet, located above economizer upper part.

- A significant number of water wall tubes going from water wall bottom header to water wall top headers.

- Some of those water wall tubes are going from the front water wall bottom header to the front water wall top header.

- Several water wall top headers. In the last years, the use of HSHFO has been progressively limited and finally banned by IMO (International Marine Organization). HSHFO has been substituted by low sulphur heavy fuel oil (LSHFO) and/or marine diesel oil (MDO) or even ultralow sulphur marine diesel oil (ULSMDO). At the same time and for the same reasons the share of BOG being used has been increasing continuously.

At present, about one third of the existing LNG carriers fleet is still steam turbine driven. Approximately there are now about 224 steam turbine driven LNG carriers in operation, a very substantial number of Vessels.

After 2007, propulsion trends start to change substantially and the dominant propulsion type was dual fuel Diesel electric, based on 3 to 5 dual fuel diesel generators plus 2 electric propulsion motors. Steam turbine was declining rapidly, but still some Vessels were built, mainly in Japan.

Again, the type of dominant propulsion changed only 10/12 years later, around at 2017/2018, and two stroke slow speed directly coupled dual fuel diesel engines are presently the preferred option.

This kind of rapid and accelerated changes have been very infrequent in marine propulsion. In fact, the changes are partly consequence of the increasing importance of Green House Gas (GHG) emissions and consequent requirement for increased efficiency.

The result of this rapid change is that steam turbine LNG carriers are being considered more and more as marginal Vessels. However, many of this Vessels are still at the middle of her economic life. Owners are pressed to try to find a kind of conversion able to improve the ability of the Vessels to survive in the present market. However, so far, there have been no conversions of the propulsion installation.

To do the things more complicated, the International Marine Organization (IMO) is taking regulatory steps to impose severe reductions in the gas emissions of about 30% reduction in GHG (Green House Gas emissions). A preliminary decision has been made last November by the Marine Environment Protection Committee (MEPC) and said decision has been confirmed and developed in June 2021 during the next MEPC meeting.

Compliance will be required after January 1 ^st . 2023 at the date of the corresponding survey. Additionally, some type of classification of the Vessels is to be established depending on the efficiency of her propulsion installation.

In this way, Vessels would be classified as A, B, C, D and E, A class Vessels being the most efficient, and E class Vessels being the less efficient. It is likely that IMO, under Carbon Intensity Index (CH) will oblige D and E class Vessels to improve its efficiency. Probably, most, if not all, of steam turbine LNG carriers will be ranked D or E.

Now, after IMO new rules, the situation of the fleet of steam turbine LNG carriers fleet is that about 224 of existing steam turbine Vessels, which are at the mid of her life, are supposed to reduce emissions by 30% in 2023.

It should be mentioned that final details of new ruling are still pending, and regulation may have a different requirement, but, in any case, there will appear new stringent regulations regarding GHG reduction of emissions.

The conventional steam turbine marine power plants are designed in such a way that if power decreases, the power plant efficiency decreases at an increasing rate. For example, when reducing the shaft horsepower by 50%, power plant efficiency decreases about a 25%.

This sharp decrease in efficiency is not inherent to the steam turbine by itself but is more the consequence of the way in which the complete cycle has been designed. The steam cycle has been designed specifically for high power operation and the slow steaming operation has been, basically, neglected. The result of new regulations and the way that existing steam cycle has been designed is that existing steam turbine driven LNG carriers are obliged to operate at about below 50% of her design shaft horsepower unless the propulsion plant is modified.

As previously mentioned, a few years ago, the use of HSHFO has been forbidden. Presently fuel is mainly boil-off gas and to a much lesser extent LSHFO or ULSMDO.

Both changes, fuel quality and GHG emissions limitations, open the door to modify the existing installations in several ways.

Under the present situation, owners of steam turbine LNG carriers are obliged to take decisions on the way they made their Vessels compliant with new coming IMO regulations.

However, some problems appear when they try to change the steam turbine propulsion installation of LNG Carriers. One of them is the efficiency lost due to operate in low load regimes for reducing emissions because conventional propulsion installations aren't designed for working in those regimes.

Another problem which appears when emissions must be reduced is that owners of LNG Carriers reduce the vessels shaft horsepower (SHP), so the vessels speed will be reduced significantly too, and that Vessels will not be compliant with the speed requirements established by TCH party (Time Charter).

DESCRIPTION OF THE INVENTION

The present invention refers to a method for conversion of Liquefied Natural Gas carriers steam or hybrid propulsion installations to reduce the required emissions of exhaust gases by IMO while maintaining the efficiency of the vessels or, at least, reducing the efficiency to acceptable levels and decreasing fuel consumption of steam or hybrid propulsion installations.

The method which is the subject of the invention attempts to solve the problems described above, by conversion of LNG carriers steam or hybrid propulsion installations. This method comprises a series of phases that could be applied in a new installation or in a conventional installation. Applying the method of the invention, emissions are reduced to the required standards of the IMO while carriers maintain the speed of the vessels, limit the reduction of vessel's speed or reach optimal speed according to the TCH party requirements.

Additionally, the method of the invention has two different targets:

One of them is keeping the Vessels compliant with the new IMO requirements regarding EEXI and CH for a given period (because the progressive character of IMO requirements).

This target implies to reach a given value of the EEXI as defined and required by IMO. To reach the required value one or several of the proposed efficiency improvements may be applied, until the attained value of the EEXI of a given Vessel meets IMO requirement. Same applies to CH, with the additional requirement of progressive improvement over the years.

Second target is making the Vessels more attractive commercially by reducing its energy consumption.

The first aspect of the invention comprises a method which incorporates a low- pressure economizer in marine boilers of the type used in the steam or hybrid propulsion installation driven existing in LNG carriers, so, propulsion installations can operate at low loads getting levels of efficiency closer those of the propulsion installation when it works at high loads.

The exhaust gases from the conventional boiler of this type of ship propulsion plant leave the boiler through the existent exhaust gas duct.

An high-pressure economiser exists in the exhaust gas duct, which uses the heat from the exhaust gases to increase the temperature of the boiler feed water, thereby reducing the fuel used and increasing the efficiency of the installation. The first aspect of the method is characterised by modifying the exhaust gas duct and placing a low-pressure economiser after the main economiser. The water supply to this low pressure is made from the main condenser extraction water circuit where the water temperature is low, making the heat exchange in the low-pressure economizer more efficient.

The introduction of the low-pressure economiser into the installation is carried out as follows: installing the low-pressure economiser either in line or in by-pass with the existing ducting. In this case, there are two options:

To install the economiser as a completely prefabricated module outside the existing engine housing or installing the economiser inside the engine casing at the top.

Again, taking advantage of the heat of the exhaust gases leaving the main economiser at a temperature between 150 and 175 degrees, the boiler feed water circulating through the low-pressure economiser increases its temperature by heat exchange with the exhaust gases, so that the exhaust gases are expelled from the boiler at a final temperature belowl 00 ^e C degree.

The design and arrangement of the low-pressure economizer will lower exhaust gas temperature recovering in this way heat. Depending on the design, it will be possible to condensate part of the water vapour content existing in the exhaust gas. In this case the amount of recovered heat will increase substantially. The low-pressure economizer is to be designed in such a way that the condensed water is collected and sent outside exhaust gas stream. This additional heat recovery is possible because the low temperature of the Main condenser extraction water that is circulating inside the low-pressure economizer. Indeed, the materials and design of the low-pressure economizer shall be adequate to resist the potential corrosion at low temperature in case of some sulphur content in the exhaust gas.

Finally, the low-pressure economizer can be installed in both boilers or only in one boiler. Almost in all the steam turbine LNG carriers two main boilers are installed. However, the retrofit of the second economizer in only one of the two existing boilers could be a very good option, reducing costs and installation time, because as explained before, the second economizer installation is linked with the operation of the steam power plant at slow steaming. In this case, one possibility is to operate the steam plant with only one boiler.

In case that a new power generator (Gas turbine or DF Diesel Generator) is also retrofitted as part of the compliance with IMO emissions reduction, then the steam plant will operate most of the time at slow steaming and only one boiler firing is required. Also, in case of operation at around 50% power by reason of IMO limitation in emissions, one boiler only operation is possible.

The second aspect of the invention comprises a method which is based on the conversion of one or both existing boilers in a heat recovery steam generator (HRSG).

The second aspect of the method is characterised by modify one of the water tube walls of the existing boiler furnace creating an opening in the selected water tube wall by cutting a section of water tubes in order to allow the inlet into the furnace of the boiler of the exhaust gas of a Gas Turbine power generator or a Diesel Engine power generator in order to that the existing marine steam boiler will operate as Heat Recovery Steam Generator (HRSG) but keeping, when it is required, the capacity to operate as a dual fired boiler and been also able to operate simultaneously in both ways.

The water tubes that have been cut in order to create the exhaust gas opening for exhaust gas inlet will be substituted by new especially bended pipes in a different plan that are welded on both ends to the previous cuts in the water pipes, maintaining the continuity of the water circulation in all and any pipes of the water wall of the boiler furnace.

The shape of the bends of new water pipes section will be in three dimensions and designed to: guarantying the water circulation in all the pipes, maintaining the smoke tightness of the water pipe walls, and creating an overlapping of the new sections of water pipes with the remaining water pipes thus creating the opening for exhaust gas inlet and/or creating a duct for the power generator exhaust gas guiding such gas flow in the required direction.

In this way one or both existing steam boilers may operate as fired boilers and/or as HRSG and or in both ways simultaneously.

Another option of modifying one of the existing boilers is introducing the gas turbine or diesel engine exhaust gas in the combustion air ducts after the forced draft fan and before combustion air entry in the furnace in such a way that the exhaust gas will enter in the boiler furnace through the top part existing openings in way of burners openings in the roof water pipes panel so that exhaust gas will flow inside the existing boiler in the same way that the combustion gases before conversion in a U shape flow. Ducts are to be provided with all the customary fittings and arrangements.

The advantage of the second aspect of the invention is that existing boilers can be used as HRSG while keeping its ability to be used as MPMB or be used as fired and HRSG simultaneously.

In this way the cost of new HRSG as well as the cost and time of its retrofit are saved. The MPMB will perform adequately as HRSG provided that its heat exchange capacity is adequate to the amount and conditions of the Gas Turbine (GT) exhaust gas.

This heat exchange capacity is to be checked by adequate calculations. Indeed, the steam production will correspond to the amount and temperature of the exhaust gas.

On the other hand, since GT fuel should in all normal conditions be boil-off gas or ULSGO both with no sulphur content, there is no risk of corrosion by low temperature exhaust gas.

The second aspect of the invention also method is supplemented by a third aspect of the operation consisting of retrofitting an additional steam superheater located in the newly retrofitted flue gas duct before the gas enters the furnace of the existing boiler through the created opening.

The new steam superheater will be connected to the existing superheated steam main pipe with adequate valves. Its inlet will be connected to the outlet of the existing superheater and its outlet will be connected to the boiler superheated steam main before the connection to the other boiler superheated steam main.

This connection then supplies superheated steam to the main turbines and the other main superheated steam users.

In this way the temperature of the superheated steam will be higher than the temperature that can be achieved in the existing superheater of the existing main boiler, reaching a temperature similar or closer to temperature of the superheated steam in the existing dual gas and liquid fuel steam boiler.

The new steam superheater will be built with similar materials to those used in the existing superheater (high temperature steel alloys) and will consist mainly of superheating steam pipes and superheated steam collectors as well as required valves and controls.

In the case that the boiler is operated as fired boiler, the new superheater module could be isolated if necessary. In this case a small steam circulation, using an adequate device, may be maintained to keep the adequate temperature in the new superheater tubes within acceptable limits when exposed to the residual level of radiation and convection without main steam circulation.

Also, a fourth aspect of the invention, which is based on integrating both existing and new auxiliary steam and water systems introducing in the selected piping points steam and/or heated water, as required, generated in a low-pressure exhaust boiler of conventional installation when works as exhaust gas steam generator (HRSG), which could be connected with the existing deaerator of the LNG Carrier-or said low pressure exhaust boiler can works independently. The steam which circulates inside the low-pressure exhaust boiler is heated by the exhaust gas of new dual fuel power generator normally a Diesel Generator (DFDG), installed as part of the conversion to create a hybrid propulsion.

The relatively low energy steam and heated water produced in the HRSG are used to one or several of the following uses: heating main condenser extraction water before entering the deaerator, producing steam to be injected into mentioned deaerator, heating main boiler combustion air in the existing air heater and injecting steam in the crossover after high pressure turbine (HPT) and before low pressure turbine (LPT) as well as other miscellaneous auxiliary steam uses.

In general, all miscellaneous uses requiring low pressure saturated or superheated auxiliary steam HRSG can be performed by the steam generated in the low pressure HRSG boiler.

The savings derived of such integration came from:

- The reduction of the high requirement before conversion of live steam produced in the main boilers used after one or more pressure reductions for several heating requirements derived from the closing steam of extraction systems of main turbines when operated at low load to reduce gas emissions.

- The availability of substantial amounts heat at moderate temperature in the exhaust gas of the new power generator without requiring any additional fuel or gas consumption.

- The availability of substantial amounts of hot water, up to 120 ^e C in the newly installed diesel engine in the jacket cooling water system, adequate to heat condenser extraction water at lower temperature.

The integration system is relatively simple and cheap by using most of the existing piping systems and heat exchangers being now feed by the new steam and/or hot water available from HRSG and/or diesel generator retrofitting as part of the new hybrid propulsion. In case of using the deaerator as steam/water separator in the new HRSG, the main such advantages are:

- Being an existent part, its fabrication and installation cost is cero.

- The deaerator is already connected with main condenser extraction pumps discharge. Such discharge piping would be modified including the water circulation through the new HRSG economizer and discharge to the deaerator. In this way, the condenser extraction water is heated, to, or close to, the required temperature.

- The HRSG forced water circulation pumps can easily aspire water from the lower part of the deaerator creating the circulation through the evaporator section of the HRSG and discharging to the deaerator, where the mixture of water and steam will separate.

- The superheater section of the HRSG will be feed from the upper part of the deaerator (or an adequate existing steam pipe already directly connected to the steam part of the deaerator). The superheated steam HRSG outlet will be connected to the different steam users where superheated steam is preferred, for example through a new connection to the crossover between HPT and LPT, where will be injected.

- The deaerator is already connected with many steam systems. In this way the connection is made in a very simple way.

In addition, any water and steam drum require level and pressure meters and controls. The deaerator is already fitted with such systems in a fully integrated way with existing installation.

In any case, the integration of the steam and hot water produced in the new HRSG can be performed using the deaerator or installing a new drum to separate steam and water as part of the retrofitted low pressure HRSG.

An additional application of the aspect is using the system as described as part of a combined cycle steam and Gas Turbine power system in the case of the two- pressure steam system. The low-pressure system will use the deaerator as steam drum. The low-pressure steam generated in the low-pressure evaporation section of the HRSG, will be separated in the deaerator the superheated in the low-pressure superheater of the HRSG and then injected in the low-pressure turbine (LPT) through a connection in the existing crossover between HPT and LPT.

This way the execution of the two steam pressures combined cycle has the advantage of using existing elements such as the deaerator, the LPT and the crossover fully integrating existing and new parts in a simple and economical way.

The invention could be complemented by a fifth aspect that includes implementing an automatic switching steam extractions system when the Vessel is operated at low loads in such a way that steam from main high-pressure turbine first extraction is switched automatically at certain loads range to the second extraction steam system.

In this way the different steam users of second extraction are fed by the first extraction steam instead of being fed directly by desuperheater live steam coming directly from the main boilers after passing one or more automatic pressure reduction valves.

This system is integrated installing several automatic valves and a branching pipe connecting first extraction steam piping system to second extraction steam piping system.

The existing and new fitted automatic valves will close and/or open sequentially in a programmed system driven by a parameter directly linked to the main turbines load.

The necessary valve interlocks will be included to avoid simultaneous entry of steam from two different automatic valves as well as pressure control valves if required.

In a similar way, this aspect could be applied to switching steam from the second extraction steam system feed from the cross over between HPT and LPT to the third extraction steam system feed from LPT. The switch will be activated at a selected load where the pressure at a given steam extraction is too low to feed that extraction steam extraction system but is adequate to feed the next lower pressure steam extraction system. At lower loads where the pressure is again too low to feed the lower pressure steam system, the extraction system will be closed.

The switching system will normally be applied simultaneously with the first aspect of the invention with the low-pressure economizer to complement it or in fully independent way. The system may be applied to switch one pair of extractions (first to second extraction) or two pairs of extractions (the previous and second to third extraction) as considered convenient.

The first aspect of the invention is directly applicable to steam propulsion installations and will improve efficiency at any load but specially at low loads in order to compensate the loss of efficiency due to low load operation.

The second aspect of the invention is directly applicable to hybrid propulsion installation, when a GT (Gas Turbine) or a DFDG (Dual Fuel Diesel Generator) is retrofitted and will improve steam cycle and overall efficiency. While keeping the Vessel compliant with IMO EEXI regulations, will contribute to increase the power and speed at which the vessel can operate.

Both aspects can be applied independently but the first aspect of the invention can also complement the second aspect to reach the optimal performance to meet the requirements in terms of exhaust emissions.

In turn, the second aspect can be complemented by the third aspect of the invention to achieve also optimal working requirements in terms of exhaust emissions.

In addition, the fourth aspect, that is applicable to hybrid propulsion installations when a DFDG or GT is retrofitted, could be applied alternatively to the second and third aspects of the invention, using a low-pressure exhaust boiler of conventional installations which is connected with the LNG carrier existing steam system through the deaerator or a new steam/water drum

Finally, the fifth aspect of the invention that is directly applicable to steam turbine propulsion installations can be applied independently or together with the other aspects of the invention to support the aspects mentioned before, specifically the first aspect of the invention, to complement the improvement of the steam power plant efficiency at low loads.

DESCRIPTION OF THE DRAWINGS

To complement the description being made and in order to aid towards a better understanding of the characteristics of the invention, in accordance with a preferred example of practical embodiment thereof, a set of drawings is attached as an integral part of said description wherein, with illustrative and non-limiting character, the following has been represented:

Figure 1.- Shows a modification of an existing exhaust gas outlet of LNG carrier boiler.

Figure 2.- Shows a modification of the tubes inside the combustion chamber of LNG carrier boiler.

Figure 3.- Shows a section view of the tubes inside the combustion chamber of LNG carrier boiler.

Figure 4.- Shows an integration of a superheater before the LNG carrier boiler.

Figure 5.- Shows an integration of an auxiliary steam and water system in the LNG carrier propulsion installation.

Figure 6.- Shows an integration of an automatic switching extraction system in the LNG carrier propulsion installation.

PREFERRED EMBODIMENT OF THE INVENTION

A preferred embodiment of conversion method of LNG carrier steam or hybrid propulsion installations is described below with the aid of Figures 1 to 6. Figure 1 shows a modification of a LNG carrier steam or hybrid propulsion installation which comprises a main condenser (5), a LNG carrier boiler (1) and a deaerator (53) connected to the main condenser (5) and to a LNG carrier boiler (1 ). In addition, the LNG carrier boiler (1 ) comprises a combustion chamber (10), an exhaust gas outlet duct (18) connected to the combustion chamber (10), a high- pressure economiser (7) located inside the exhaust gas outlet duct (18) and a first steam superheater (4) located inside the combustion chamber (10).

The modification is made by connecting (18) after the high-pressure economiser (7), which is located inside the existing gas outlet (18), a supplementary exhaust gas duct (23) and integrating a low-pressure economiser (22) inside the supplementary exhaust gas duct (23).

The low-pressure economiser (22) is configured to use the exhaust gases which circulate inside the exhaust gas outlets (18, 23) to heat the water going from the main condenser (5) to the deaerator (53), where said water is driven by an extraction pump (55) which is connected to the main condenser (5), when the steam or hybrid propulsion installation operates at low loads The quantity of live steam which the deaerator (53) require to reach its design temperature will be reduced accordingly.

Figure 2 shows a modification of the tubes which are inside the LNG carrier boiler (1) creating an opening (50) in the combustion furnace (11) by cutting a section of several existing waterwall tubes (47) and substituting that section by a new curved pipes (48) section overlapping remaining said existing tubes (47) and creating said opening (50) in one wall of the combustion chamber (11) to allow the inlet of exhaust gases from a Gas Turbine Power Generator (GTPG) (30).

In addition, the LNG carrier boiler (1 ) uses said exhaust gases to maintain the efficiency at low operative loads generating steam and increasing its operational temperature when the hybrid propulsion installation operates at low loads, like it worked at high operative loads. Another advantage is the decrease of use of fuel because said exhaust gases do not escape to the atmosphere, and also the LNG carrier boiler (1) can work as HRSG (Heat Recovery Steam Generator) or as dual fired boiler.

Complementary to Figure 2, Figure 3 shows a section view of the modification of the tubes which are inside the LNG carrier boiler (1) where an upper view of the disposition of the existing tubes (47) and the new curved pipes (48).

Figure 4 shows an integration of a second steam superheater (35) inside a GTPG (30) exhaust duct (8) which connects with LNG carrier boiler (1 ) and leads into the opening (50). The second steam superheater (35) is integrated before the opening (50) created in the LNG carrier boiler (1 ) in order to increase the temperature exhaust gases that came from the GTPG (30) so that, the operational temperature of the LNG carrier boiler (1 ) when the hybrid propulsion installation operates at low loads.

On the other hand, Figure 5 shows an integration of a retrofitted system (9), formed by a first set of water pipes (26), a second set of steam/water pipes (27) and a third set of steam pipes (28), and a steam/water separator drum (67).

Also, Figure 5 shows the components of a hybrid propulsion installations, where said components are a main condenser (5), an extraction pump (55) connected to the main condenser (5), a first step water heater (65) connected to the extraction pump (55), a deaerator (53) connected to the first step water heater (65), a LNG carrier boiler (1) connected to the deaerator (53), a High-Pressure Turbine (69) connected to the LNG carrier boiler (1), a Low-Pressure Turbine (80) connected to the High-Pressure Turbine (69) by a crossover pipe (77), a low-pressure exhaust boiler (59) which comprises a second low-pressure economiser (60), a saturated steam generator (61), and a low-pressure superheater (63), a DFDG (54) connected to the low-pressure exhaust boiler (59), and an auxiliary set of steam consumers (58).

The method comprises the integration of the retrofitted system (9) are based on connecting the main condenser (5) extraction water going from first step water heater (65) to the second low-pressure economiser (60) and from the second low- pressure economiser (60) to the deaerator (53) directly or through the steam/water separator drum (67) by the first steam pipes (26) in order to heat the water coming from the main condenser (5) before entering inside the deaerator (53).

Next step includes connecting the saturated steam generator (61) to the deaerator (53) directly or through the steam/water separator drum by the second steam-water pipes (27) in order to heat the water of the deaerator (53) which feeds the LNG carrier boiler (1).

Finally, the deaerator (53) is connected to the low-pressure superheater (63) and the low-pressure superheater (63) to the crossover pipe (77) and the first steam consumers (58) by the third steam pipes (28) in order to supply the steam which leads out the low-pressure superheater (63) to the crossover pipe (77) and the first steam consumers (58), substituting in this way the live steam supplied from the LNG carrier boiler (1) by steam coming said heating steam from the deaerator (53) or steam/water separator drum (67).

The retrofitted system (9) is integrated in selected piping points of the installation with the new steam system and use the heated water and steam generated in the low-pressure exhaust boiler (59) heated by the exhaust gas of the DFDG (54), to heat the water of that came from a main condenser (5) before the water enters into the deaerator (53), to heat combustion air of the LNG carrier boiler(1) using the existing steam/air heater, injecting steam in a crossover (77) before Low Pressure Turbine (LPT) (80) of the installation and, in general, substituting the auxiliary steam coming from the LNG carrier boiler (1).

Finally, Figure 6 shows the integration in the existing extraction steam system of an automatic switching extractions system (40) which comprises a first extraction steam pipe (32) connecting first extraction steam system coming from HPT (69) with a second extraction steam pipe (33) coming from the crossover (77) and a third extraction steam pipe (34) connecting second extraction steam system coming from the crossover (77) with third extraction steam system coming from LPT (80). The system also comprises several automatic valves (70, 71 , 72, 76, 78, 79, 81 ). Valves (70, 72) are opened after existing first and second extraction automatic valves (71 , 78) are closed. In this way, a set of second extraction steam consumers (74) are supplied with steam coming from first extraction.

Two other automatic valves (76, 79) are opened after automatic valves (78, 81 ) are closed. In this way third extraction consumers (75) are supplied with steam coming from second extraction. The automatic switching extractions system (40) is configured to control a steam extraction system of the steam or hybrid propulsion installation at low load operations when the pressure at a given steam extraction is too low to feed the corresponding system but is still able to feed next lower pressure system. The automatic switching extractions system (40) is connected to an automatic extraction controller (41 ) that controls the automatic switching extractions system (40).