NO20093579A1 - Hot cargo loading method and system - Google Patents

Abstract

A method of handling hot LPG cargo in the least cargo tank (100,110,120) located aboard an LPG cargo, preferably during loading, including reconding gas vapor released from the cargo inside the at least one cargo tank (100,110,120) by means of at least one recondensing unit ( 130,140,150) including a capacitor (170); and returning recondensed gas vapor to the at least one cargo tank (100,110,120). The method further includes operating the at least one recondensation unit (130, 140, 150) and the condenser (170) in a non-cooling mode to compress and condense gas vapor only; and flowing hot condensate from the capacitor (170) into a tire tank (160). A matching system is also described.

Description

Method and system for loading hot cargo The invention relates to a method and system for reducing the loading time at a loading port, for ocean-going tankers carrying liquefied petroleum gases commonly known as LPG, hereinafter referred to as LPG carriers, and in particular when loading cargo at a temperature higher than the corresponding saturation temperature at cargo tank pressure. In addition, secondary effects are achieved including the abolition of forced evaporation during unloading and vertex trimming during the loaded sea voyage.

A loading port shall be understood as an LPG export terminal, where the export terminal is either located on the coast or at sea.

An unloading port shall be understood as an import terminal, where the import terminal is either located on the coast or at sea.

A cargo tank shall hereafter be understood as a liquid-tight container with the purpose of holding LPG and installed on board an LPG carrier. Cargo tanks can be of any type such as e.g. integral tanks, membrane tanks or independent tanks.

A storage tank shall henceforth be understood as a liquid-tight container with the purpose of holding LPG either at a loading port or an unloading port.

LPG is to be understood as a range of different classifications of petroleum gases stored and transported as liquid cargo. Among the various petroleum gases, propane and butane are the main examples; Propane typically includes any concentration of ethane from 0% to 5% by volume, and the butane content of propane can be anywhere from 0% to 20% by volume. This mixture consisting mainly of propane, typically between 70 - 98% by volume, is known as commercial propane, hereafter called propane.

Butane can be any mixture of normal-butane and iso-butane, with possible portions of unsaturated hydrocarbons, and hereafter called butane.

In addition to propane and butane, LPG should also include, as a minimum, the following classifications:

Ammonia,

butadiene,

Butane - Propane mixture (any mixture)

Butylene,

diethyl ether,

Propene,

Vinyl chloride.

LPG stored and transported at temperatures below ambient will naturally continuously emit a certain amount of gas vapour. The normal procedure to maintain pressure in the cargo tanks is to extract this gas vapor, condense it and return it to the cargo tanks as condensate. A recondensation unit shall hereafter be understood as a cooling unit, the task of which is to condense said gas vapour, and the prefix "re" points to the condensation of gas vapor from liquefied gas.

Condensate shall hereafter be understood as liquefied gas vapour, gas vapor shall hereafter be understood as the sum of gas vapor consisting of:

Displaced gas volume during loading

Off-gas from loaded cargo

Displaced gas volume from return of condensate from recondensation unit Off-coking gas from returned condensate

Evaporation due to heat absorption in the cargo tanks

Hot cargo shall be understood as LPG loaded at a temperature above its saturation temperature corresponding to the applicable cargo tank pressure.

LPG is transported in liquid form either at pressure higher than atmospheric or at temperatures lower than the surroundings, or a combination of both. The present invention relates to: (1) LPG carriers that transport liquid cargo (LPG) at temperatures lower than ambient, known as fully refrigerated LPG carriers, and (2) LPG carriers that transport liquid cargo (LPG) at pressures higher than atmospheric and temperatures lower than the surroundings. The latter are known as semi-refrigerated/semi-pressurized LPG carriers.

Common to the LPG trade is that LPG carriers can transport different classifications of LPG from voyage to voyage, and it is also typical that the loaded LPG received from the loading port is at a higher saturation point than the maximum permissible working pressure of the cargo tanks. This means that the LPG carrier must cool down the loaded cargo to meet the working pressure range of the cargo tanks. Such cooling is usually carried out by rapidly evaporating (flashing) the liquid down to cargo tank pressure and recondensing the following generated gas vapours. Depending on the saturation pressure, the loading time can take from slightly less than 24 hours to more than 4 days.

A reduction in loading time will reduce loading port costs, increase available sailing time and hence reduced emissions of carbon dioxide into the atmosphere as a result of reduced fuel consumption. As of today, there are no other solutions available than the obvious one of increasing the cooling capacity of the LPG carrier's recondensation unit. It is not considered viable to increase the cooling performance of the recondensing unit on board the LPG carriers. The minimum requirements for cooling performance are determined by international rules and regulations, and typical installed cooling performance exceeds these requirements. An obvious, but not acceptable, solution would be to release all the gas vapor into the atmosphere.

A VLGC (Very Large Gas Carrier) with a typical size of approximately 80,000 m<3> normally has four installed recondensing units and during loaded sea voyages it is not uncommon for only one or two units to run periodically to handle the natural heat leakage case . There is an imbalance between installed unit capacity and normal heat leakage which often prevents continuous operation during loaded sea voyages. As mentioned above, the minimum required cooling capacity is determined by international rules and regulations, but practice shows that the installed cooling capacity exceeds these requirements and the excess capacity is based on shipowners' additional requirements mainly prompted by operational aspects, e.g. maximum acceptable loading times. A further increase in capacity for recondensation units would be too expensive and therefore not a viable solution.

It is also common for the LPG trade that the LPG carriers are equipped with a deck tank capable of holding LPG at the saturation pressure corresponding to warm ambient air conditions, although this is not typical for all vessels, but for many. The purpose of the deck tank is to hold sufficient liquid to replace the gas vapor atmosphere in the cargo containment system prior to changing the categorization of cargo to be shipped or after port calls when the cargo tanks have been emptied of gas and vented. Any mixture of different cargo is undesirable. In some cases, however, mixing of very small amounts of propane and butane can be accepted, as some cross-contamination of these two cargoes has often occurred prior to loading.

Some LPG cargoes, e.g. propane, butadiene, raw material is used in the chemical industry. Contaminating such cargo with other categorizations of cargo may impair its value as raw material. Consequently, a thorough cleaning of the containment system when changing the gas vapor atmosphere is common.

Changing the gas vapor atmosphere is usually carried out by first replacing the original gas vapor atmosphere with a neutral gas, either from a neutral gas generator, e.g. exhaust gas, or from a nitrogen generator. It is the type of cargo categorization that governs which neutral gas can be used. After the entire containment system has been neutralized, the neutral gas can be replaced with gas vapor of the new cargo categorization to be loaded on board the LPG carrier. This is done by opening the valve 310 on line 12 and vaporizing the LPG in the cargo vaporizer 190 and having the gas vapor flowing through the liquid lines, which replaces the neutral gas in the entire cargo containment system, see figure 2.

A cargo containment system shall be understood as cargo tanks with all associated cables and equipment.

A further feature of the seaborne transport of LPG is that it is quite common for LPG cargo to be unloaded from LPG carriers at an unloading port without the LPG carrier receiving gas vapor return to replace the removed liquid volume. Liquefied gases will evaporate the moment the vapor pressure is reduced and thus compensate to a certain extent for the LPG that is pumped out of the cargo tanks. However, it is not obvious that the total pressure reduction in the cargo tanks during unloading of LPG will be within the working pressure range of the cargo tanks. To prevent pressure problems during LPG unloading, it is common to vaporize part of the displaced liquid in a dedicated vaporizer and return the gas vapor back to the cargo tank. Other means are also possible, e.g. heating the gas vapor atmosphere in the cargo tanks by using a cargo compressor. This is done by circulating gas vapor through the cargo compressor without any cooling and returning it back to the cargo tanks.

Figure 1 shows for reference purposes a typical recondensation unit according to known technology. Liquid cargo flows in a line 1 from loading dock storage tanks. Cargo valves 261, 262,263 regulate the amount of received cargo for each cargo tank. Gas vapor from the cargo tanks 100, 110, 120 flows via a gas vapor line 2 and enters the cargo compressor 200 in which the gas vapor is compressed to an intermediate pressure. The amount of gas vapor not handled by the recondensing unit shown in Figure 1 flows via a continuation of the gas vapor line 2 to parallel operating units, not shown.

The cargo compressor 200 is typically the first stage of a multi-stage compressor.

Gas vapor leaves the cargo compressor 200 via line 3 into the combined de-superheater/flash economizer 210 in which the water vapor is brought near its saturation temperature. The gas vapor then flows via a line 4 from the superheater/flash economizer 210 to the cargo compressor 220 where they are compressed to the gas saturation pressure corresponding to the achievable temperature in the cargo condenser 170.

The cargo compressor 220 is typically the second stage of a multi-stage compressor.

The compressed water vapor enters the load condenser 170 via a line 5 to be condensed against sea water, any other cooling medium is typically above sea water temperature. Seawater is by far the most commonly used heat conductor for the loading condenser 170, but a mixture of water and antifreeze is also possible. Antifreeze can be any suitable glycol.

Hot condensate leaves the load condenser 170 flowing via a line 7 to a line 6 branched from line 7, in which a small proportion flows via level control valve 230 and provides the necessary interstage cooling and post-cooling of the main part of the hot condensate.

The remaining hot condensate to be returned to the cargo tanks 100, 110, 120 flows via a condensate line 7' through coil 215 inside the desuperheater/flash economizer 210 and leaves coil 215 in a subcooled state. The now subcooled condensate is pressure reduced with pressure control valve 210 and the resulting two-phase flow is mixed with condensate and gas vapor flowing via a line 8 from other recondensing units in operation. That current flows via a line 9 back to the loading tanks 100,110,120.

Figure 2 shows a typical arrangement on an LPG carrier with three recondensation units and three cargo tanks that load cargo without gas vapor return on land.

As mentioned above, the LPG carrier can have any combination of the number of cargo tanks and recondensation units, as an example an LPG carrier with four cargo tanks can be equipped with two recondensation units according to NO patent application 20092477.

The LPG carrier's cargo tanks 100,110,120 are loaded with LPG via cargo line 1 from the loading port. The loading valves 261,262, 263 regulate the loading speeds and protect against overfilling. The larger LPG carriers will typically have more than three tanks, but the number will not be of relevance to the invention. A certain proportion of LPG flowing into the cargo tanks will rapidly evaporate into the vapor phase in an amount dependent on the pressure difference between the pressure in a loading port storage tank and the LPG carrier's cargo tank pressure and the total heat absorption from storage tanks to cargo tanks.

Gas vapor flows from the cargo tanks 100, 110, 120 via the gas vapor line 2 to the cooling units 130, 140, 150 in which the gas vapor is recondensed and returned back to the cargo tanks 100, 110, 120 via the condensate line 9 as condensate, or more correctly a mixture of condensate and gas vapor. Valves 264, 265, 266 allow flexibility in returning the condensate to one cargo tank, all cargo tanks, or a combination thereof.

As the loading of LPG obviously results in a certain amount of gas vapor formation which must be handled by the recondensing units installed on the LPG carrier, it is clear that the loading rate is controlled by the cooling performance of the recondensing units. The number of recondensation units typically depends on the size of the LPG carrier and will not be relevant to the invention.

The loading evaporator 190 shown in Figure 2 is not in operation during the loading operation. This normally also applies to the tire tank 160.

In use, the deck tank 160 is filled with LPG directly via a line 10 connected to the loading line 1. It is thus normal for the deck tank to be filled with cold cargo directly from the loading line 1 during loading, alternatively the deck tank 160 can be filled while unloading cargo. During unloading of cargo, cargo line 1 is used as an export line.

When the tire tank 160 is loaded, an isolation valve 320 is open. The tire tank has no thermal insulation and is allowed to heat up. When the tire tank 160 is emptied, a valve 310 is open and regulates the flow through the loading evaporator 190 which evaporates the liquid. The gas vapor product flows via a line 12 and is connected to the cargo liquid head, other connections are also common but not relevant to the description. The isolation valve 340 prevents backflow into the cargo evaporator 190 under normal operating conditions.

Displaced gas vapor from the cover tank 160 flows via a line 11 and is connected to the gas vapor head. The isolation valve 330 isolates the cover tank from the gas vapor head under normal operating conditions.

A typical medium LPG carrier with cargo tanks of 35,000 m<3>cargo cargo capacity is loaded with propane at a loading port with storage tanks that have a gas vapor pressure of 0.42 bar g. The temperature of the received LPG can be read to be - 38 °C from graph 1 below.

Graph 1

For this special case, it is desirable that the LPG carrier during loading should have a cargo tank pressure of 0.275 bar g. The load curve for this special LPG carrier and this special case is shown in graph 2 below.

Graph 2

From graph 2, the loading capacity can be found to be approximately 170 tonnes/hour. Fully loaded cargo mass will be 19,788 tonnes and a resulting loading time of 4.9 days.

There is no known technique that is environmentally friendly, i.e. does not vent gas vapor to the atmosphere to achieve a significant reduction in loading time.

The main purpose of the present invention is to remedy the drawbacks discussed above.

According to a first aspect of the present invention, this is achieved by a method for handling hot LPG cargo in at least one cargo tank located on board an LPG carrier, preferably during loading, including recondensing gas vapor released from the cargo inside at least one cargo tank using of the at least one recondensation unit including a condenser; and returning recondensed gas vapor to the at least one cargo tank. The method further includes operating the at least one recondensing unit and the condenser in a non-refrigeration mode to compress and condense gas vapor only; and to flow hot condensate from the condenser into a deck tank.

A second aspect of the present invention is to provide a system for handling hot LPG cargo in at least one cargo tank located on board an LPG carrier, preferably during loading, including that gas vapor released from the cargo inside the at least one cargo tank is recondensed by means of at least one recondensation unit including a condenser; and recondensed gas vapor is returned to the at least one cargo tank, wherein the at least one recondensing unit and the condenser are operated in a non-cooling mode to compress and condense gas vapor only; and hot condensate from the condenser flows into a cover tank.

To compress and flow hot condensate into the cover tank, gas vapor is compressed by means of a compressor device inside the at least one recondensing unit and then condensed by the condenser arranged in connection with the compressor.

However, compressed gas vapor at an intermediate pressure can be flowed through a combined de-superheater and flash economiser arranged in the at least one recondenser unit in front of the condenser, or if appropriate even bypassing it.

Gas vapor can be returned from the deck tank by means of one of the following i) route gas vapor back to the suction side of the cargo compressor; ii) routing gas vapor back to the discharge side of a first compression stage of the cargo compressor; iii) routing gas vapor back to the suction side of a third cargo compressor, in which three compressor stages are applicable; and iv) mixing gas vapor with the loaded LPG.

Among other things, to compensate for the pressure reduction during loading operations and to start unloading at more moderate tank pressures, the deck tank can be emptied into at least one of the cargo tanks during unloading, hot gas vapor is flowed under pressure through spray cooling nozzles arranged inside the at least one cargo tank.

In order to ensure an initial saturation pressure for condensate flowed into the cover tank below the latter's maximum working pressure, a part of the at least one cooling unit including both the combined desuperheater and the rapid evaporator economizer is in operation.

In addition, propane originating from hot condensate in the deck tank can be used as fuel for propulsion of LGP freighter engines using a low-pressure fuel pump.

Other advantageous embodiments are indicated in the independent patent claims and the detailed description below.

In short, the loading time in the loading port for ocean-going LPG-carrying tankers is reduced in this way, and in particular when cargo with a higher temperature than the corresponding saturation temperature at cargo tank pressure is loaded. As already mentioned above, secondary effects are also achieved, e.g. abolition of forced evaporation during unloading and peak pruning during loaded sea travel

Now the present invention will be described in greater detail based on the accompanying drawings, in which:

Figures 1 and 2 schematically show prior art recondensation units; and

Figures 3 to 11 schematically illustrate preferred embodiments of a system for transporting liquefied petroleum gases, and particularly, but not exclusively, for reducing LPG freighters' loading time when loading hot cargo.

The origin relates to a method and system for transporting liquefied petroleum gases, and in particular to reduce the loading time of LPG carriers when loading hot cargo. The invention typically uses existing equipment installed on LPG carriers but in other configurations than currently known.

Figure 3 shows a general schematic arrangement of the invention and is described as follows: The LPG carrier receives LPG from the loading port via a cargo liquid line 1 which runs to at least one cargo tank 100,110,120. The LPG carrier can have any number of cargo tanks, but typically between two and four.

Gas vapor flows from the cargo tanks 100,110,120 via a gas vapor line 2 to the recondensation units. Figure 3 shows a generalized recondensation unit 130 with all units present made up of a compressor device 400 and a condensate subcooling device 500 and the condenser 170. The compressor device will typically include at least one two-stage compressor while the condensate subcooling device can have different configurations with all of them for the purpose of reducing the temperature of the gas to be compressed and to subcool the condensate prior to reducing the condensate pressure down to cargo tank pressure in order to reduce the amount of coke gas in the cargo tank 160.

Gas vapor not handled by the recondensation unit 130 flows on via gas vapor line 2 also connected to the further parallel recondensation units, not shown.

Any number of recondensing units can be used, but typically between two and four units are common.

Gas vapor flowing from the cargo tanks 100, 110, 120 via the gas vapor line 2 enters a cargo compressor 200 where the gas vapor is compressed to an intermediate pressure, typically in the range from 3 bar g to 5 bar g. The compressed gas vapor leaves the cargo compressor 200 via a line 3 and enters the combined overheads/fast evaporator economizer 210. There is no liquid flow through a line 6 feeding the combined overheads/fast evaporator economizer 210 and gas vapor exits the combined overheads/fast evaporator economizer 210 via a line 4 in the same condition as at the inlet.

The gas vapor flows on to a cargo compressor 220 in which the gas vapor is compressed to a pressure corresponding to at least the saturation pressure based on the achievable temperature in the downstream condenser 170. The cooling medium used in the condenser 170 is either seawater or any water/glycol mixture, not shown in Figure 3. Gas vapor leaves the cargo compressor 220 via a line 5 and enters the condenser 170 to be condensed. Isolation valve 267 is closed and an isolation valve 268 is open which allows hot condensate to flow via a line 16 to the deck tank 160. A control valve 370 ensures sufficient back pressure for the cargo compressor 220. An isolation valve 380 is open.

Another operational configuration is to bypass the combined desuperheater/flash economizer. The compressed gas vapor leaves the cargo compressor 200 via line 3 but bypasses the combined desuperheater/fast evaporator economizer 210 via a line 3b, see figure 10. The bypass is supplied by closing the isolation valve 380 and opening an isolation valve 390. Line 3b is connected to line 4.

There is only gas vapor flow through the combined desuperheater/fast evaporator economizer 210. Thus, the respective recondensing unit is operated in a non-cooling mode in which the total gas vapor is only compressed and condensed.

Hot condensate from other parallel operated recondensation units enters line 16 via a line 13. The valve 380 isolates the cover tank 160 from the liquid lines and protects against overfilling. A valve 350 on a line 17 enables the filling of cold condensate. The line 17 branches off from a condensate return line 9 and connects back to a liquid line 10. Displaced gas vapor flows from the cover tank 160 via a line 14 back to the loading compressor part of the recondensation unit and gas vapor first to other parallel operated recondensation units. A valve 360 regulates the gas vapor pressure in the tire tank 160.

In solutions from known technology, the tire tank is filled with LPG, while for the present invention it is filled with condensate. The main difference between an LPG classification with propane with e.g. 5 mol % ethane means that the condensate is an equilibrium composition of the gas phase and typically has an ethane content of 26 mol %.

Gas vapor from the cargo tanks is made up of the elements stated above but has orders of magnitude as specified below:

Displaced gas volume during loading: 5 - 10%

Off-gas from loaded cargo: 35 - 36%

Displaced gas volume from condensate return: 0 - 1%

Off-gas from returned condensate: 15 - 30%

Evaporation due to heat absorption in the cargo tanks: 25 - 35%

Depending on cargo classification, content of volatile components in LPG, temperatures and operational aspects, the percentage distribution may differ from the above.

The gas vapor handling capacity is fixed for each LPG carrier as it is controlled by the capacity of the cargo compressor and the number of possible simultaneously parallel recondensation units in operation. By sending all the condensate to the cover tank 160, the gas vapor portion which is due to coking gas from condensate is removed from the collected gas vapor and thus an increased loading speed is made possible.

Gas vapor from the tire tank 160 can be handled by either becoming:

1. routed back to the suction side of the cargo compressor 200, see figure 3.

2. routed back to the discharge side of the cargo compressor 200. The gas vapor line

14 is connected to the discharge side of the loading compressor 200, see figure 4.

3. routed back to the suction side of the suction side of a third cargo compressor 225, in which three compression stages are applicable. The gas vapor line 14 is connected to the discharge side of the cargo compressor 220, see Figure 5. 4. mixed with the loaded LPG, see Figure 6. Displaced gas vapor from the deck tank 160 flows via the line 17 into the cargo liquid line 1, in which the gas vapor is completely or partially absorbed in the liquid flow.

The cargo compressor device shown in Figures 3 to 6 is typically a cargo piston compressor with normally two or three compression stages. Other cargo compressor types such as cargo screw compressors or cargo centrifugal compressors can also be used.

Emptying of the deck tank 160 is carried out via the liquid line 9 during unloading of the LPG carriers, and the LPG content in the deck tank 160 is flowed under pressure through spray cooling nozzles 50, 60 in the cargo tanks, see figure 7, which does not show suitable connections to the respective recondensation units. By sending the condensate stored in the deck tank 160 into the LPG cargo tanks, 100,110,120 during unloading, the following advantages are achieved: Rapid evaporation through the spray nozzles will compensate for the pressure reduction during unloading operations. Consequently, certain operations will not be necessary:

o Pressure replenishment with evaporation of pumped cargo; and

o Heating of steam gas rooms.

• Unloading can start at more moderate tank pressures, i.e. it is not necessary to ensure pressure build-up at the end of the sea voyage. • If LPG is transported, the cargo-carrying capacity is increased by the deck tank volume.

The evaporation rate from the cargo tanks is significantly higher during the first days of a sea voyage than during later parts of the sea voyage. Such an increase in the rate of evaporation is due to the fact that the cargo containment system has not reached a stable temperature for the duration of the cargo during loading.

The amount of gas vapor necessary to replace the pump-out volume during cargo unloading can be supplied by flash-evaporating hot condensate from the deck tank 160. However, depending on the loaded cargo temperature, the amount of condensate sent to the deck tank 160 may not be sufficient to meet the gas vapor requirements during unloading. So, by sending hot condensate to the deck tank also during the first days of sea voyage, the total available gas vapor by rapid evaporation of condensate back to the cargo tanks during unloading can at least be balanced by the total amount of LPG vaporized to maintain pressure in the cargo tanks 100 , 110, 120. That operational change will save pumping power for seawater used in the cargo evaporator 190 shown in Figure 2, for example, and save fuel consumption in the first days of the loaded sea voyage as the operational change will require fewer recondensing units in operation. The latter is based on the fact that approximately 20 - 35% of the condensate returned to the cargo tanks 100, 110, 120 is flashed to gas vapor and recycled back to the cargo compressor device.

Figures 3, 4, 5, 6 show a recondensation unit of known technology with the proposed connections to the tire tank according to the present invention.

During a loaded sea voyage, operation on board an LPG carrier often involves intermittent operation of the recondensation units in the sense that the cargo tank pressure is allowed to rise to a high level, i.e. not running any of the recondensation units, then running one or typically two of the recondensation units often during the day for to reduce cargo tank pressure. By using the new connections to the deck tank 160, the following operational advantages are achieved: During the day and especially during the hottest time of the day, two recondensation units often need to be in operation. Only one recondensing unit in operation will be sufficient for many journeys if parts of the hot condensate are sent via line 16 to the deck tank 160. This increases the total gas vapor handling capacity for the one recondensing unit in operation. In periods where the temperatures are more moderate, the condensate filled in the deck tank 160 can be sent back to the cargo tanks via the line 16 or alternatively the line 10. In this way, the deck tank 160 can be actively used to peaks that have the highest evaporation rates during the hottest hours and consequently reduce the number of required recondensation units in operation.

Example

A typical LPG carrier with 35,000 m<3> capacity is loaded with a light propane mixture at a temperature of -37.5 °C. The tank pressure during loading is 0.22 bar g and the corresponding saturation pressure of LPG is 0.45 bar g.

Gas vapor flowing from the cargo tanks during loading is made up of the following elements:

By removing all condensate in the hot state from the recondensation unit, the off-coke gas contribution is removed from the condensate and the gas vapor flow from the loading tank is reduced. This reduction in total gas vapor flow rate provides a potential to increase the loading rate to maintain the same initial gas vapor flow rate. For the same conditions as above, the percentage distribution of the individual gas vapor elements is:

From this example it is easy to conclude that a significantly increased loading speed is available through the present invention.

There may be cases where the LPG has a content of volatile components higher than the normal range and causes high saturation pressures in the tire tank 160 exceeding the design pressure. These high contributions of more volatile components can also cause unacceptable outlet temperatures. Also, some operational aspects cause saturation pressures to exceed the design pressure of the tire tank.

More volatile components may involve propane with a higher ethane content than is acceptable in commercial propane. Then the cargo compressor outlet temperature will rise and consequently the outlet temperature.

Operational aspects can be seawater temperatures exceeding design limits causing higher condensing temperatures.

For these cases, it will be necessary to control the portion of the recondensation units including both the desuperheater/flash economizer 210 to ensure that the initial saturation pressure of the condensate sent to the deck tank 160 is well below the maximum working pressure of the deck tank 160. Heat leakage into the deck tank 160 will slowly increase the pressure within and at a given time a gas vapor line 14 must be opened. This will normally occur after loading with the LPG carrier sails.

Figure 8 shows a principle diagram of how this will be arranged. Isolation valve 268 is closed and isolation valve 267 is opened ensuring that hot condensate flows towards the combined desuperheater/flash economizer 210 via line 7. A small portion of the condensate is routed directly through line 6 and ensures required condensate sub-cooling and inter-stage cooling. Isolation valves 264,265,266 and 320 are all closed. The isolation valve 350 is open and ensures flow of subcooled condensate via line 7 connected to line 10 which fills the cover tank. Subcooling temperatures are typically below 10 °C.

Some time after the end of filling, the temperature of the tire tank 160 reaches a level where the associated saturation pressure reaches the maximum permissible working pressure. At this point, the control valve 360 is opened and maintains this pressure.

Displaced gas vapor from the cover tank 160 is routed via the line 14 back to the compressor device 400 for recompression. Displaced gas vapor to other recondensation units is branched off from line 14.

To enable a more flexible arrangement of the tire tank, a transfer pump 160 is arranged on line 16 to counteract the frictional pressure losses in the lines running to the tire tank, cf. figure 11.

Although the shipping industry is by far the most carbon-efficient form of commercial transport, due to its size it has a significant global impact, approximately 3% of global carbon dioxide emissions.

The shipping industry has therefore expressed that targets for carbon reduction must be at least as ambitious as the future carbon reductions decided in any new United Nations Climate Change Convention.

An alternative that allows the shipping industry to meet its carbon targets is to gradually switch to fuel with less carbon impact. For LPG carriers, this will typically be implemented by installing alternative fuel engines and mainly running on propane.

Alternate fuel low speed diesel engines running on propane will require a fuel tank of suitable capacity. The deck tank 160 holds sufficient propane volume for most sea voyages and by combining the deck tank's functionality with also being a fuel roof, several advantages are achieved:

1. There is no need for an additional tank.

2. There is no need for additional fueling systems.

3. Fuel tanks can be filled during sea travel from the recondensation unit.

Emptying the cover tank 160 will typically be effected with a low-pressure pump which supplies a high-pressure pump which pushes up the condensate to sufficiently high pressures. Final supply pressure will typically be between 350 - 550 bar g. The low-pressure fuel pump 450 takes suction from the tire tank 160 via a line 20 connected to the line 16, see Figure 6. A valve 455 isolates the fuel system when the LGP carrier is not running on propane. The fuel supply pump 450 delivers condensate via a line 21 to a downstream high pressure fuel supply system, not shown.

As the natural evaporation of gas vapor during sea travel can be sent to the deck tank as hot condensate, the deck tank does not need to be sized to sail at speed on the longest sailing distances. The possibilities during cargo unloading can also be maintained. Figure 9 shows the combined assembly.

As mentioned above, the deck tank 160 is present on LPG carriers, but this does not exclude using one or more tanks as an alternative in addition to the traditional deck tank.

Claims (20)

1. Method for handling hot LPG cargo in at least one cargo tank (100,110,120) located on board an LPG carrier, preferably during loading, including: recondensing gas vapor released from the cargo inside the at least one cargo tank (100, 110, 120) using at least one recondensation unit (130,140,150) including a condenser (170); and returning recondensed gas vapor to the at least one loading tank (100, 110, 120), characterized in that the method further includes: operating the at least one recondensing unit (130, 140, 150) and the condenser (170) in a non-cooling mode for only to compress and condense gas vapour; and flowing hot condensate from the condenser (170) into a cover tank (160). 2. Method according to claim 1, characterized in that the method further includes: compressing gas vapor using a compressor device (400) inside the at least one recondensation unit (130, 140, 150), where the condenser (170) is arranged in connection with the compressor device to condense and flow hot condensate into the deck tank. 3. Method according to claim 1, characterized in that the method further includes: bypassing a combined desuperheater and rapid cooling economizer (210) when compressed gas vapor is introduced into the condenser (170). 4. Method according to at least one of the preceding claims, characterized in that the method further includes: returning gas vapor from the deck tank by doing at least one of the following: i. routing gas vapor back to the suction side of the cargo compressor (200); ii. routing gas vapor back to the discharge side of the cargo compressor (200); iii. routing gas vapor back to the suction side of the suction side of a third cargo compressor (225), wherein three stages of compression are applicable; and iv, mixing gas vapor with the loaded LPG. 5. Method according to at least one of the preceding claims, characterized in that the method further includes: emptying the deck tank (160) into at least one of the cargo tanks (1,2, 3) during unloading, hot gas vapor is flowed under pressure through spray cooling nozzles (50, 60) arranged inside at least one cargo tank. 6. A method according to at least one of the preceding claims, characterized in that the method further includes: controlling a portion of the at least one recondensation unit (130, 140, 150) including both the combined desuperheater and the flash economizer (210) to ensure that the initial saturation pressure of the condensate flowed into the tire tank under the maximum working pressure of the tire tank. 7. Method according to at least one of the preceding claims, characterized in that the method further includes: using LPG originating from hot condensate in the deck tank (160) as fuel for propulsion of the LPG transport engines by means of a low-pressure fuel pump (450). 8. System for handling hot LPG cargo in at least one cargo tank (100, 110, 120) located on board an LPG carrier, preferably during loading, including: gas vapor released from the cargo inside the at least one cargo tank (100, 110, 120) is recondensed using at least one recondensation unit (130,140,150) including a capacitor (170); and recondensed gas vapor is returned to the at least one loading tank (100, 110, 120), characterized in that the system further includes: the at least one recondensing unit (130, 140, 150) and the condenser (170) are operated in a non-cooling mode to compress only and condense gas vapour; and hot condensate flows from the condenser (170) into a cover tank (160). 9. System according to claim 8, characterized in that the system further includes: hot condensate from the condenser (170) flows into the cover tank (160) via a line (16) and isolation valves (370, 380). 10. System according to claims 8 and 9, characterized in that the system further includes: gas vapor is compressed by means of a compressor device (400) inside the at least one recondensation unit (130, 140, 150), where the condenser (170) is arranged in connection with the compressor device in order to condense and flow hot condensate into the deck tank (160). 11. System according to claim 10, characterized in that the system additionally includes: compressed gas vapor flows into a combined desuperheater and rapid cooling economizer (210) via a line (3) and continued from there via a line (4). 12. System according to claim 10, characterized in that the system additionally includes: a combined desuperheater and rapid cooling economizer (210) is bypassed when compressed gas vapor is fed into the condenser (170). 13. System according to claim 12, characterized in that the system additionally includes: the combined desuperheater and rapid cooling economizer (210) is bypassed by means of a line (3a) connected to the lines (3, 4), where the lines (3, 3b) include respective isolation valves (380, 390). 14. System according to at least one of claims 8 to 13, characterized in that the system further includes: gas vapor from the tire tank is returned by doing at least one of the following: i. gas vapor is routed back to the suction side of the cargo compressor (200); ii. gas vapor is routed back to the discharge side of the cargo compressor (200); iii. gas vapor is routed back to the suction side of the suction side of a third loading compressor (225), in which three stages of compression are applied; and iv. gas vapor is mixed with the loaded LPG. 15. System according to claim 14, characterized in that the system further includes: gas vapor is routed to the respective sides of the compressor device by means of a gas vapor line (14) including a control valve (360). 16. System according to claim 15, characterized in that the system further includes: gas vapor is mixed with the loaded LPG via a gas vapor line (17) including a control valve (360). 17. System according to at least one of claims 8 to 16, characterized in that the system further includes: the cover tank (160) is emptied into at least one of the loading tanks (1,2, 3) during unloading, hot gas vapor is flowed under pressure through spray cooling nozzles (50, 60) arranged inside the at least one cargo tank. 18. System according to at least one of claims 8 to 17, characterized in that the system further includes: a piece of the at least one recondensing unit (130,140,150) including both the combined desuperheater and the flash economizer (210) is controlled to ensure that the initial saturation pressure of the condensate flowed into the tire tank below the maximum working pressure of the tire tank. 19. System according to at least one of claims 8 to 18, characterized by the fact that the system further includes: LPG originating from hot condensate in the deck tank (160) is used as fuel for propulsion of LGP transport engines by means of a low-pressure fuel pump (450). 20. System according to claim 19, characterized in that the system further includes: the low-pressure fuel pump feeds a high-pressure pump included in a downstream high-pressure fuel system via a line (21).