GB2143589A - Propulsion plant for steam turbine driven ship - Google Patents

Abstract

The plant has at least one main boiler 1 with a superheater, high and low pressure steam turbines 3,4, a main condenser 5, a turbogenerator 6a, a feed water turbine driven pump 7a, a reduction gear 8, shafting and propeller 9, and a gas turbine 15 which feeds the plant with energy, partly thermal and partly mechanical. <IMAGE>

Description

SPECIFICATION Propulsion plant for steam turbine driven ship The improvements to which this patent application refer are aimed to reduce the specific fuel oil consumption of the propulsion plants of steam turbine driven ships in newly built vessels, and, specially, by means of adequate transformations using the above improvements, to reduce specific fuel oil consumption of the propulsion plants of existing steam turbine driven ships.

The above improvements are based on a completely new idea of the steam cycle of the steam turbine marine plants by introducing in the cycle a gas turbine which has the following tasks: to produce electricity for ship's use; to supply energy to the propeller shaft through a pre-reduction gear or an electric motor fed by a generator driven by the gas turbine; to produce exhaust gases which can be used as combustion air in the main boilers and to reheat main steam in an external reheater (exhaust gas boiler), or to reheat main steam in an external reheater only. Also included in the improvements of this patent application, as and when the propulsion plant has two boilers, is the utilization of the superheater of one boiler as main steam reheater, this being logically applicable to the transformation of an existing plant only.

The above improvements, or part thereof, are applicable, as mentioned earlier, to a newly built plant, but they are specially suited for its application to steam turbine plants of existing ships, which due to slow steaming at which they are forced to operate, specially the large ones, firstly because of the enormous increase of the fuel oil price, and, secondly, because of the low level of freights (this specially affecting large tankers), it is needed to reduce the propulsive power as against the original design power, whereby the specific fuel oil consumption at that reduced power notably increases and, consequently, the reduction of costs derived from slow steaming in steam turbine driven ships notably decreases due to that increase in specific fuel oil consumption.

By means of the improvements contained in this patent application it is possible in newly built ships to achieve a reduction in specific fuel oil consumption as against that of the conventional plants of about 30%: and in existing steam turbine driven ships operating at today's slow steaming it is also possible to achieve reduction in specific fuel oil consumption of about 30%, which for a 300,000 dwt tanker under today's operational conditions means a daily saving of about 40 tons of heavy fuel oil or about 2.0 U.S.

million dollars a year at today's heavy fuel oil prices.

In addition, the increase in specific fuel oil consumption in both cases for variations of the design power is reduced which in the case of existing plants modified and re-adapted to a reduced power by means of the improvements contained in this patent application, allows increases of said reduced power of the modified design of about 15 to 20%, with increases of the specific fuel oil consumption of about 4%, which improves the flexibility of the plant to adapt itself to a requirement of higher power if needed.

All this has a double interest: because of its intrinsic character and because until the so-called energy crisis and, in some cases, until some years later, steam propulsion co-existed with diesel propulsion in high powered ships, a co-existence which was deeply affected by said energy crisis.

Ship's propulsion plants are either diesel engines or steam turbines fed by steam boilers. In some exceptional cases and in some war ships propulsion plants consist of gas turbines.

Till the end of 1973 when the first oil crisis occurred, there was an equilibrium between diesel propulsion and steam turbine propulsion in ships with high power requirements. The advantage of steam was its greater reliability and better adaptability to large, high powered ships, namely, to large tankers, which offset its higher fuel oil consumption as against that of the diesel.

In 1973 specific fuel oil consumption of a conventional steam turbine propulsion plant was around 200210 g/SHP x h., whereas that of the diesel was around 150-160 g/bhp x h. It is imperative to add that both consumptions are not homogenous for the following reasons: The specific fuel oil consumption of a steam turbine propulsion plant is always given on heavy fuel oil, or Bunker C, which have a lower calorific value of 9,700-9,800 Kcal/Kg., whereas that of a diesel engine is given on gas oil having a low calorific value of 10,200-10,250 Kcal/Kg, which means a difference of about 5%.On the other hand, the specific fuel oil consumption of a steam plant always includes the consumptions of the plant's auxiliaries and also that of a turbogenerator which supplies the electric energy required on board, something which was not usual in diesel propelled ships and which could mean an additional difference of 4-6%. Therefore, when comparing actual consumption, the specific fuel oil consumption of the diesel engine plant shall be increased by about 10%-11% plus the amount corresponding to the consumption of cylinder oil which is of the order of 5 gbhp x h. of heavy fuel oil equivalent.

In spite of this, in 1973 there was a difference in "homogenous" fuel oil consumption of about 15% in favour of the diesel which was partly compensated by the better efficiency of the propeller in the steam turbine ships (lower propeller revolutions); and by less weight and less need of volume of the steam plant; and lesser maintenance costs and shorter off-hire of the steam turbine driven ships. And, naturally, by the very low cost of oil fuel which for small differences in consumption had only a marginal value.

In 1973/74 the first oil price rise took place which, referred to the oil price fob PG in 1970 represented an eleven fold increase. By the end of 1979 the second oil price rise occurred, which put the oil price 28 times higher, referring always to the 1970 fob PG basis. Oil prices went slightly down at the beginning of 1983, but the strength of the dollar offset in many cases the reduction in cost which would otherwise have followed.

On the engine building side it is to be noticed that main diesel engine builders started in 1975/76 a race to reduce specific fuel oil consumption of diesel engines, which after several reductions is now in the region of 118 to 125 g/bhp x h. At the same time, two stroke diesel engines rpm came down as the ratio stroke/bore increased, thus eliminating in large engines the advantage in propeller rpm of the geared steam turbine.

There finally is a very important feature which differentiates both types of propulsion consisting in that when for one or another reason the speed and, therefore, the power have to be reduced, diesel engines perform better because their specific fuel oil consumption varies very little with varying power, whereas in steam turbine plants specific fuel oil consumption considerably increases when reducing power in such a way that a steam turbine plant with a specific fuel oil consumption of 210 g/SHP x h. at its normal service power, which usually is of the order of 90% of its MCR (maximum continuous rating), may reach specific fuel oil consumptions of 240 and 250 g/SHP x h. for ratings of 70% to 50% of MCR.

The great weight the fuel bill has today in ship's operating costs together with the persistent low freight level in large tankers, urged as a general measure, a reduction in speed and, consequently, power, in order to optimize earnings or minimize losses. Thus, steam turbine propelled ships are at a considerable disadvantage compared with those propelled by diesel engines, due to the considerably higher specific fuel oil consumption of the former specially at reduced power. This fact, and the lesser flexibility of the steam turbine plant to operate at varying ratings insofar as specific fuel oil consumption is concerned have practically eliminated steam turbine propulsion in merchant ship building.

However, given the large number of ships propelled by high power steam turbine plants, the above situation led to the so-called re-engining in quite a number of cases, meaning by re-engining the substitution of the steam turbine plant by a diesel engine plant, generally with smaller power. This kind of conversion, depending on the engine generation, (specific fuel oil consumption), may achieve savings of up to 40% at slow steaming if compared with the previous consumption of the plant. Such conversion however implies a very important investment and a very long off-hire to carry it out, and is therefore very expensive.

In a parallel way, but on a smaller scale, there have been developed a series of conversions of the type steam to steam, consisting in an improvement of the steam cycle and in a restoring of the design conditions of the plant to the reduced power at which the ship is being operated. These kind of conversions give as a result a specific fuel oil consumption of the converted plant, at the new reduced power, which is in the region of 195 to 205 g/SHP x h. These conversions are cheaper than re-engining and also the required off-hire is shorter, but saving in fuel oil compared with re-engining is considerably less.

In both cases, either steam to diesel or steam to steam, the ship is left with a reduced power rating as a percentage of the original power which is variable, and there is no possibility in a practical way to come back to the original power, specially in re-enginging versions.

In the following and as a reference, there is described the conventional plant of a steam turbine driven ship. This plant which is schematically shown in Figure 1 is made up of the following parts which are identified as follows; Main boilers (one or two) Superheater 2 High pressure steam turbine 3 Low pressure steam turbine 4 Main condenser 5 Turbogenerator 6 Turbine driven feed pump 7 Reduction gear 8 Shafting and propeller 9 As typical values in marine plants for pressure and temperature the following figures are quoted: Steam pressure at high pressure turbine inlet 60 kg/sq.cm; corresponding temperature 505"C.

On the other hand, and in order to clarify and define some terms and expressions which will be commonly used reference should be made to the following: Superheating: Heating of the saturated steam which is normally performed by the same boiler producing such steam and before its use. Therefrom the words: "superheater" and 'superheated steam'.

Reheating: Reheating consists of heating up the steam again after it has been used. Usually, reheating is done in a reheater arranged within the same boiler. Therefrom the words: "reheater" and 'reheated steam'.

External Reheating: This is a reheating process performed into a reheater which is arranged outside the boiler producing the steam to be reheated, contrary to the practice established until now consisting of reheating the steam into a reheater arranged within the boiler producing the steam. When it is not stated that reheating is external, it will be assumed that it is performed into a reheater arranged within the boiler producing steam.

In order to increase the efficiency of the steam cycle, it is necessary to increase the drop of the enthalpy of the steam through the turbines, and for achieving this, the possible ways are the following: - To increase steam pressure - To increase steam temperature - To increase condenser vacuum - To use a cycle with intermediate reheating And whereas in a plant of new design it would be possible to take every measure outlined above, this would be practically impossible in an already existing plant or clearly uneconomic.

In some cases, there have been developed and utilized propulsion plants which include a cycle with intermediate reheating. These plants can be schematically represented as shown in Figure 2 where each part is identified as follows: Main boiler 1 Superheater 2 High pressure steam turbine 3 Medium pressure steam turbine 3a Low pressure steam turbine 4 Main condenser 5 Turbogenerator 6 Turbine driven feeding pump 7 Reduction gear 8 Shafting and propeller 9 Reheater 10 In some of these plants with intermediate reheating the turbogenerator and the turbine driven feeding pump have been substituted by a shaft driven generator respectively by an electrically driven feeding pump, thus achieving an additional improvement of the overall efficiency.

To adopt this cycle in existing plants is very difficult and expensive since it would imply either a considerable modification of main boilers to fit a reheater, or to arrange an external reheater with its corresponding burner, achieving in both cases a moderate improvement of the efficiency.

In both Figures 1 and 2 as well as in the remainder which follow, continuous lines represent steam ducts. Dashed lines with equal length represent electric lines. Dash and dot lines represent ducts of feed water or condensate.

The improvements envisaged by the present invention take into account the new situation created by the new price of oil and by the need of slow steaming, as well as, to a certain degree, the need for flexibility of the plant regarding power variation against new design power, and the specific fuel oil consumption variation when varying power; and are specially conceived for the conversion of existing steam turbine driven ships in whose propelling plants it is possible, through the invention to achieve specific fuel oil consumptions of the order of 160 to 170 g/SHP x h, as against 240 to 250 g/SHP x h, which nowadays is usual in these vessels when slow steaming. The improvements are described in principle for the conversion of the propulsion plant of an existing ship, but they are also applicable to a newly designed plant of a new ship.The difference consists in that whereas, if we make exception of LNG-carriers, there is absolutely no demand for steam propulsion plants for merchant ship buildings, there exists a rather important market for the conversion of steam propulsion plants of existing merchant ships, excepting LNG-carriers.

The special feature of LNG-carriers, or ships for the transport of liquified natural gas, consists in that they use as fuel a mixture of heavy fuel oil and boil-off gas originating from the cargo which otherwise would be lost in the atmosphere.

The proportion of the mixture depends on the rate of evaporation of the cargo which is of the order of 0.20 to 0.25- per day and on the consumption of the propulsion plant, and varies therefore with a ship's size and propulsion power, the proportion of gas being in general greater than 50-.

The use of that mixture as fuel is the reason why steam turbines are still used in LNG-carriers since problems in the burning of natural gas in diesel engines have not yet been solved.

On the other hand, and due to the large capital costs of the LNG-carriers and to their peculiar form of operation, always in long term charters, the influence of these vessels of the increased costs of the oil fuel have been much smaller than in other types of ship and, therefore, service speed and corresponding propulsion power have been subject to only slight variations.

Because of the reasons outlined above, until now there has not been any demand for converting the propulsion plant of existing LNG-carriers.

The application of the improvements of the invention to the propulsion plant of a new ship, can lead to the specific fuel oil consumption becoming as low as 145 to 135 g/SHP x h, values which compare well, after correction for homogenization described earlier, with those obtainable today in diesel engine plants.

It will be understood that the lesser consumption of the new steam plant as against that of the converted plant is due to the fact that it is a new plant.

In accordance with the principles underlying the improvements according to the present invention, and when the plant has two main boilers, a first important goal can be achieved by using the superheater of one of the boilers as reheater for a cycle with intermediate reheating. A plant like this, which derives from that shown in Figure 1 is schematically represented in Figure 3 where the different items are identified as follows:: Main boilers 1 Superheater 2 High pressure steam turbine 3 Medium pressure steam turbine 3a Low pressure steam turbine 4 Main condenser 5 Turbogenerator 6 Turbine driven feeding pump 7 Reduction gear 8 Shafting and propeller 9 Reheater 10 By means of a transformation with very simple modifications in the steam circuit, and the substitution of the high pressure steam turbine by one high pressure-medium pressure steam turbine, a new cycle can be obtained with intermediate reheating at a reasonable cost, and improve efficiency.With this, an adequate reduction of the design power, and the substitution of the turbogenerator and of the steam driven feeding pump respectively by a reduction gear driven generator and an electrically driven feeding pump, the specific fuel oil consumption can be lowered to 195-200 g/SHP x h as against 240/250 g/SHP x h. of the parent plant at reduced power. (It is assumed that the modified plant will have a design power approximately equal to the reduced power of the original plant).

Another way to improve the cycle efficiency which can be combined or not with that described above, which is embraced by the present invention and by means of which can be achieved a still lower specific fuel oil consumption, consists in the installation as a further component of the whole cycle, of a gas turbine which drives an electric generator, and to use the exhaust gases of the gas turbine, which contain a high amount of energy, to make one or several, (in general one or two), intermediate reheatings which will have a very high efficiency and that, by its own nature, would be of the type we have called 'external".The gas turbine driven electric generator will supply electric energy to the ship as well as to an electric motor which would deliver power to the propeller shaft via a reduction gear. (As an alternative described later, the gas turbine delivers power directly to the reduction gear and not via an electric motor).

In Figure 4, which includes the reheating arrangements of Figure 3, there is schematically represented a cycle with two intermediate reheats, the first one in the superheater of the second boiler, and the second one in a reheater arranged into the gas turbine exhaust gases recovery boiler, the gas turbine being also included with the functions described earlier.

The items schematically shown in Figure 4 are identified as follows; Main boilers 1 Superheater 2 High pressure steam turbine 3 Medium pressure steam turbine 3a Low pressure steam turbine 4 Main condenser 5 Gas turbine driven electric generator 6a Electrically driven feed pump 7a Reduction gear 8 Shafting and propeller 9 First reheater 10 Gas turbine exhaust gases recovery boiler 11 Second reheater (external) 12 Low pressure vaporizer in the recovery boiler 13 Gas turbine-gas generator 14 Gas turbine-power turbine 15 Electric motor connected to the reduction gear 16 Main switchboard 17 With the cycle shown in Figure 4 the specific fuel oil consumption at reduced power which is in the original plant of the order of 240-250 g/SHP x h. can be lowered to a figure of the order of 160-165 g/SHP x h for same reduced power. The figures given above correspond to a steam cycle and taking into ac count what has been said before about homogenization, 160-165 g/SHP x h in a steam plant are equivalent to 135-140 g/CV x h in a diesel plant, i.e. smaller than those of the diesel plants of 1974 and earlier, and immediately after 1974, which are those which co-exist today with the steam plants in large tankers.

Figure 5 schematically represents a cycle with fully external intermediate reheat and a gas turbine with the functions described above which more or less is an alternative arrangement to the cycle schematically shown in Figure 4, has a specific fuel oil consumption slightly higher, although not greater than 170 g/SHP x h, and is simpler, less expensive, and easier to install in an existing plant, since it is not necessary to modify main boilers and there are no problems of regulation. Moreover, it can be used in plants where there is only one boiler.

This cycle is schematically represented in Figure 5 where the different items are identified as follows: Main boiler 1 Superheater 2 High pressure steam turbine 3 Medium pressure steam turbine 3a Low pressure steam turbine 4 Main condenser 5 Gas turbine driven electric generator 6a Electrically driven feed pump 7a Reduction gear 8 Shafting and propeller 9 First reheater (external) 10a Gas turbine exhaust gases recovery boiler 11 Second reheater (external) 12 Low pressure vaporizer in the recovery boiler 13 Gas turbine-gas generator 14 Gas turbine-power turbine 15 Electric motor connected to the reduction gear 16 Main switchboard 17 The arrangement described before and shown in Figure 4 schematically, embodies four characteristics which are unique and new and are listed below as follows: - The use of a gas turbine in a steam cycle for ship propulsion.

- The use of the exhaust gases of the gas turbine in the main boilers for production of steam and superheating.

- The use of the superheater of one boiler as a reheater.

- The use of a second external reheater which utilizes for its function the energy of the gas turbine exhaust gases in one recovery boiler.

Regarding the arrangement described hereinabove and shown schematically in Figure 5, it embodies four characteristics which are unique and new which are the following: - The use of a gas turbine in a steam cycle for ship propulsion.

- The arrangement of a first external reheater in the gas turbine exhaust gases recovery boiler which utilizes for its function the energy of said gases.

- The arrangement of a second external reheater, also in the gas turbine exhaust gases recovery boiler, in series with the first one, and which utilizes for its function the energy of said gases.

- The fact of not producing main steam in the gas turbine exhaust gases recovery boiler which is a characteristic of the land based combined steam cycles.

In the above-mentioned arrangements, the gas turbine drives an electric generator which feeds an electric motor which, in its turn, delivers power to the shafting via a reduction gear. Alternatively the gas turbine could supply power to the reduction gear directly.

This arrangement which is schematically shown in Figure 6 would be more advantageous to the point of view of the global efficiency and, although more complicated mechanically, in the case of conversion of existing plants, it is, as a whole, less expensive.

Notwithstanding the above, the essential characteristics of the improvements of this patent application will not change.

As an alternative to the arrangement shown in Figure 5, Figure 6 schematically shows the above mentioned alternative. In this Figure the different items are identified as follows: Main boiler 1 Superheater 2 High pressure steam turbine 3 Medium pressure steam turbine 3a Low pressure steam turbine 4 Main condenser 5 Electric generator 6b Electrically driven feed pump 7a Reduction gear 8 Shafting and propeller 9 First reheater (external) 10a Exhaust gas energy recovery boiler 11 Second reheater (external) 12 Low pressure vaporizer in recovery boiler 13 Gas turbine-gas generator 14 Gas turbine-power turbine 15 Main switchboard 17 Intermediate gear 18 Alternatively, and in a different mechanical configuration, the gas turbine, depending on the type of ship or plant to be converted, could be arranged in series with the high pressure-medium pressure steam turbine, and the electric generator be driven through an appropriate gear.

In some cases, and in order to simplify, the second intermediate reheater as well as other non-essential components, could be eliminated, or alternatively the number of intermediate reheaters increased to more than two.

In other cases, and depending on the slow steaming rate, it could be useful to arrange between the steam turbines and the reduction gear an additional step of reduction ofspeed in order to return the turbines to their design speed and thus reduce investment and cost.

All steam cycles described and shown hereinabove are schematized. In particular, there have been omitted the steam bleedings of the steam turbines as well as the corresponding feed-water heater, and also many other items which are accessory in relation to the invention.

Finally, the improvements which are the object of this patent application essentially differ from the well known utilization of combined cycles gas turbine-steam turbine in land based installations. This is due to the essential difference existing between the arrangements of marine plants as well as compared with the land based plants and it is a fact that until now a gas turbine in a combined cycle with a steam turbine has never been used on board a ship. On the other hand, the arrangement of the reheaters on the cycles and the no production of main steam in the gas turbine exhaust gas recovery boiler are completely new in marine plants as well as in land based installations.

Finally, it is possible to obtain additional improvements in some cases by means of additional heating of the exhaust gases of the gas turbine at their inlet in the recovery boiler, or by heating the gases before their inlet in the power turbine, by using in both cases a burner.

Both possibilities are schematically shown in Figures 7a, 7b, 7c and 7d which are referred to in Figures 4, 5 and 6. In said Figures 7a, 7b, 7c and 7d, the references and the items correspond with those already mentioned in Figures 4, 5 and 6, whereas the burner is identified with 19 when heating the exhaust gases at the inlet of the recovery boiler, and with 20 when heating the gases before their inlet in the power turbine.

In said Figures as well as in the previous and following ones, air or gases inlets or outlets as well as the corresponding ducts are represented by two parallel lines with arrows inside which indicate the direction of the flow.

In Figure 8, there is schematically shown a recovery of the exhaust gases of main boilers in the recovery boiler, which Figure 8 corresponds with the scheme shown in Figure 4, the principle of which, being an alternative, could also be applied to the schemes shown in Figures 5 and 6. Numbers in Figure 8 correspond with same numbers in Figure 4.

It finally has to be taken into account that age of ship and/or other considerations might lead to a solution which is not one among those with lowest specific consumptions but to one which adequately combines investment, specific consumption and the circumstances of that ship. Having this in mind, the installation shown in Figure 9 has been developed which entails an important simplification of those represented in Figures 4 and 5, whereby the recovery exhaust gas boiler is eliminated and there is no substitution of the high pressure steam turbine by a new high pressure-medium pressure steam turbine, but a modification of the existing one for the new design power.It can also be considered as a notable simplification of the installation shown in Figure 6, whereby in addition to the same simplifications as be fore, the pre-reduction gear is also eliminated and mechanical transmission is substituted by an electrical transmission like that represented in Figures 4 and 5.

Essentially, the installation schematically shown in Figure 9 consists of arranging the gas turbine as an electric generating set and injecting the exhaust gases of said turbine into the main boilers as combustion air. The generating set feeds the main switchboard and an electric motor which drives either directly or through a reduction gear the ship's reduction gear, the most useful likely solution being that such electric motor drives directly the second reduction pinion of the low pressure steam turbine. Alternatively, as andwhen possible, electric transmisson could be substituted by a mechanical transmission which has a better efficiency.

The items of the installation schematically represented in Figure 9 are identified as follows: Main boilers 1 Superheater 2 High pressure steam turbine 3 Low pressure steam turbine 4 Main condenser 5 Electric generator Electrically driven feed pump 7a Reduction gear 8 Shaft and Propeller 9 Gas turbine-gas generator 14 Gas turbine-power turbine 15 Electric motor 16 Main switchboard 17 With this solution, thanks to additional small modifications in the main boilers, (elimination of air preheaters; modification of the air boxes; installation of an additonal economizer in the exhaust duct of each boiler), the specific fuel oil consumption only increases up to 180 g/SHP x h with an investment much smaller than in the previous solutions and, in certain cases, more profitable.Furthermore, this arrangement has the advantage of its greater flexibility insofar as concerns volume needed and ease of installation.

With the improvements described through this Specification and the drawings attached thereto, a considerable reduction of the specific oil consumption is achieved in the propulsion plants of steam turbine driven ships, for the reduced propulsion power normally used today when steaming at sea, this being a consequence of the enormous increase of the fuel price and of the low level of the freight rates, going down from specific fuel oil consumptions of 240 to 250 g/SHP x h common today, to specific fuel oil consumptions of the plant modified in accordance with the present invention 160 to 170 g/SHP x h. by means of a moderate investment and a rather short off-hire to carry out the conversion, i.e: in an economical and profitable way.

In case of using the solution schematically shown in Figure 9, specific fuel oil consumption would go up to 180 g/SHP x h, but the required investment would be considerably less and off-hire shorter. Furthermore, it is more flexible and easy to install.

The new arrangement resulting from the improvements has the additional advantage that for variations of plus or minus 15% of the new reduced design power for which the plant is re-designed, which reduced power has to be fixed jointly by Owner and Designer, the increase of specific fuel oil consumption is only of the order of 4%. This substantially enhances the flexibility of the converted plant for variations in power insofar as specific fuel oil consumption is concerned. Another advantage related to flexibility is the possibilityof a de-conversion coming back to the original plant in case of a great increase of freight rates, by cancelling the new cycle and keeping some improvements with the consequential amelioration of the efficiency.

It should also be borne in mind that the majority, practically all in case of large tankers, of the ships to which these improvements are applicable are contemporary of those diesel propelled ships with high specific fuel oil consumption, so that steam vessels converted in accordance with the present invention will have, after homogenization, specific fuel oil consumption smaller than their diesel contemporaries thus inverting the present situation. In the particular case of LNG-carriers mentioned earlier, the improvements according to the invention have additonal advantages which are described as follows: By having natural gas as fuel, it can be burnt in the gas turbine, for which it is the most appropriate fuel, thus allowing said gas turbine to operate at higher temperatures and obtaining as a result higher power and higher efficiency.Furthermore, since the exhaust gas temperature is higher, it eases and improves the recovery of the energy of such gases in the recovery boiler.

Since power and, therefore, consumption of the gas turbine is small if compared with total propulsion power and consumption, said gas turbine will always burn natural gas and thus the overall efficiency of the plant will be better due to the two reasons mentioned above.

On the other hand, and due to the fact that gas turbine will in LNG-carriers always burn natural gas instead of Bunker C, said gas turbine can be of the so-called light type instead of the heavy duty type required for burning Bunker C, and thereby the cost of the gas turbine will be notably less and efficiency will be enhanced again due to the better efficiency of the light type gas turbine against the heavy duty type thus having a more profitable installation due to these facts.

Finally, and continuing with LNG-carriers, it emerges from the above considerations that the improvements according to the invention are applicable to existing ships, whether in service or in construction, and also to newly designed LNG-carriers, since for the time being, propulsion of LNG-carriers will continue to be steam turbine. In both cases,conversion or application to a new design of the said improvements will result in substantial reductions in fuel oil consumption as against the ship with the conventional plant.

Moreover, the ship after being converted in accordance with the improvements described hereinabove will have two propulsion plants, i.e: the steam turbine and the gas turbine so that in case of failure of either of them the ship may be brought into port without the need of a tug.

Claims (8)

1. A propulsion plant for a steam turbine driven ship comprising at least one main boiler, each provided with a superheater; high pressure and low pressure steam turbines; a main condenser; a turbogenerator; a feed water turbine driven pump; reduction gear, shafting, and propeller; characterized by the incorporation in the plant of a gas turbine which feeds the plant with energy, partly in thermal form, and partly in mechanical form.

2. A propulsion plant according to Claim 1 further characterized in that when the plant includes two main boilers, one of them is used as a reheater.

3. A propulsion plant according to Claim 1 or Claim 2 further characterized by the use of an external reheater which recovers the energy of the gas turbine exhaust gases.

4. A propulsion plant according to Claim 1, further characterized in that the gas turbine exhaust gases are injected into the main boilers and used as combustion air.

5. A propulsion plant according to Claim 1 further characterized in that the gas turbine exhaust gases are used to reheat the main steam only once, in one gas turbine exhaust gas recovery boiler, without producing main steam in said recovery boiler.

6. A propulsion plant according to Claim 1 or Claim 5, further characterized by the performance of two or more intermediate reheats in an external reheater using the gas turbine exhaust gases.

7. A propulsion plant according to any one of the preceding Claims, further characterized in that a burner is fitted in order to increase the temperature of the gas turbine exhaust gases, this being done either at the exhaust gas recovery boiler gas inlet, or at the inlet of the power turbine of the proper gas turbine.

8. A propulsion plant for a steam turbine driven ship substantially as hereinbefore described with reference to Figure 3, Figure 4, Figure 5, Figure 6, Figures 7a-7d, Figure 8 or Figure 9 of the accompanying drawings.