WO2011078688A1

WO2011078688A1 - A method and system for handling warm lpg cargo

Info

Publication number: WO2011078688A1
Application number: PCT/NO2010/000475
Authority: WO
Inventors: Carl Jørgen RUMMELHOFF; Stein Thoresen
Original assignee: Hamworthy Oil & Gas Systems As
Priority date: 2009-12-22
Filing date: 2010-12-20
Publication date: 2011-06-30
Also published as: JP5448123B2; EP2516918A4; KR20120089377A; CN102713402B; NO20093579A1; JP2013514944A; CN102713402A; NO333898B1; KR101458580B1; EP2516918A1

Abstract

A method for the handling of warm LPG cargo in at least one cargo tank (100, 110, 120) situated on board a LPC carrier, preferentially during loading, comprising re liquefying vapour released from the cargo within the at least one cargo tank (100, 110, 120) by means of at least one re liquefaction unit (130, 140, 150) including a condenser (170); and returning re liquefied vapour into the at least one cargo tank (100, 110, 120). The method further comprises operating the at least one re liquefaction unit (130, 140, 150) and the condenser (170) in non-refrigeration modus as to only compress and condense vapour; and flowing warm condensate from the condenser (170) into a deck tank (160). A corresponding system has been disclosed.

Description

A method and system for handling warm LPG cargo

The invention relates to a method and system for reducing the loading time at loading port, for ocean going tanker vessels carrying liquefied petroleum gases normally known as LPG, hereinafter referred to as LPG carriers, and particularly when loading cargo at a temperature higher than the corresponding saturation temperature at cargo tank pressure. Additionally, secondary effects are achieved involving elimination of forced vaporisation during unloading and peak shaving during laden voyage.

A loading port is to be understood as the LPG export terminal, the export terminal either being located at shore or offshore.

A discharge port is to be understood as the import terminal, the import terminal either being located at shore or offshore.

A cargo tank is hereinafter to be understood as one liquid tight container with purpose to hold LPG and installed onboard an LPG carrier. Cargo tanks can be of any type as e.g. integral tanks, membrane tanks or independent tanks.

A storage tank is hereinafter to be understood as one liquid tight container with purpose to hold LPG either at loading port or discharge port.

LPG is to be understood as a range of different grades or products of petroleum gases stored and transported as liquid cargo. Among the various petroleum gases Propane and Butane are the principal examples, Propane typically including any concentration of Ethane from 0 % by volume up to 5 % by volume and Butane content in Propane can be anything from 0 % by volume up to 20 % by volume. This mixture consisting of mainly Propane, between typically 70 - 98 volume % is known as commercial Propane and hereinafter called Propane.

Butane can be any mixture of normal-Butane and iso-Butane with possible fractions of unsaturated hydrocarbons and hereinafter called Butane.

In addition to Propane and Butane LPG should as a minimum include the following grades:

Ammonia, Butadiene,

Butane - Propane mixture (any mixture),

Butylenes,

Diethyl ether,

Propylene,

Vinyl chloride.

LPG stored and transported at temperatures below ambient will naturally continuously release a certain amount of vapour. The normal procedure of maintaining the pressure in the cargo tanks is to extract this vapour, liquefy and return it back to the cargo tanks as condensate. A reliquefaction unit is hereinafter to be understood as a refrigeration unit which duty is to liquefy said vapour, and the prefix "re" points to liquefaction of vapour from liquefied gases.

Condensate is hereinafter to be understood as liquefied vapour, vapour hereinafter being understood as the product of vapours consisting of:

Displaced vapour volume during loading

Flash vapour from loaded cargo

Displaced vapour from return of condensate from reliquefaction unit

Flash from returned condensate

Evaporation due to heat ingress into the cargo tanks

Warm cargo is to be understood as LPG loaded at a temperature above its saturation temperature that corresponds to current cargo tank pressure.

LPG is transported in liquid form either at pressures greater than atmospheric or at temperatures below ambient, or a combination of both. The present invention relates to:

(1) LPG carriers transporting liquefied cargoes (LPG) at temperatures below ambient, known as fully refrigerated LPG carriers, and

(2) LPG carriers transporting liquefied cargoes (LPG) at pressures greater than atmospheric and temperatures below ambient. The latter is known as semi refrigerated/semi pressurised LPG carriers.

Common for the LPG trade is that the LPG carrier can carry different grades of LPG from voyage to voyage, and it is also typical that the loaded LPG received from loading port is at a greater saturation pressure than maximum allowable operational pressure of the cargo tanks. This implies that the LPG carrier will have to cool down the loaded cargo to meet the operational pressure range of the cargo tanks. Such a cooling is normally done by flashing the liquid down to cargo tank pressure and liquefying the resulting vapours generated. Depending on saturation pressure, loading time can take from somehow less than 24 hours to more than 4 days.

A reduction in loading time will reduce loading port costs, increase allowable sailing time and hence emission of carbon dioxide to atmosphere is reduced due to reduced consumption of fuel. As of today, no feature apart from the evident of increasing the refrigeration capacity of the LPG carriers' reliquefaction unit is available. It is not regarded as viable to increase the refrigerant duty of the reliquefaction unit onboard the LPG carriers. The minimum requirement to refrigerant duty is set forth by international rules and regulations and typically installed refrigerant duty is above these requirements. An obvious but not acceptable solution will be to vent all vapour to atmosphere.

A VLGC (Very Large Gas Carrier) with typical size of approximately 80 000m³ has normally four installed reliquefaction units and during laden voyage it is not uncommon that only one to two units are running intermittently to cope with the natural heat leakages. There is an unbalance with installed unit capacity and normal heat leakage often preventing continuous operation during laden voyages. As mentioned above, the minimum required refrigerant capacity is governed by international rules and regulations, but practice shows that the installed refrigerant capacity is in excess of these requirements and the excess capacity is based on ship owners additional requirements primarily caused by operational aspects, e.g. maximum acceptable loading times. A further capacity increase of the reliquefaction units will be too costly and hence not a viable solution.

It is also common for the LPG trade that the LPG carrier is equipped with a deck tank capable of holding LPG at saturation pressure corresponding to warm ambient air conditions, although not typical for all vessels but many. The purpose of the deck tank is to hold sufficient liquid to replace the vapour atmosphere in the cargo containment system prior to changing grade of cargo to be shipped or after docking when the cargo tanks have been gas freed and aerated. Any mixing of different cargoes is undesirable. In some cases, however mixing of very small quantities of Propane and Butane may be accepted, as some mutual contamination of these two cargoes very often have occurred already prior to loading. Some LPG cargoes, e.g. Propylene and Butadiene are used as feedstock in the chemical industry. Contaminating such cargoes with other grades of cargo may deteriorate its value as feed stock. Hence, strict cleaning of containment system with change of vapour atmosphere is common.

Change of vapour atmosphere is normally carried out by first replacing the original vapour atmosphere with an inert gas, either from an inert gas generator, e.g. exhaust gas, or from a Nitrogen generator. It is type of cargo grade that rules out what inert gas that can be used. After inerting the entire containment system, the inert gas is replaced with vapour of the new cargo grade to be loaded onboard the LPG carrier. This is done by opening the valve 310 on a line 12 and vaporise the LPG in the cargo vaporiser 190 and have the vapour flowing through the liquid lines replacing the inert gas in the entire cargo containment system, see figure 2.

A cargo containment system is to be understood as cargo tanks with all associated piping and equipments.

A further feature of the sea going transport of LPG is that it is quite common that LPG cargo is discharged from the LPG carriers at a discharge port without the LPG carrier is receiving vapour return to replace the removed liquid volume. Liquefied gases will evaporate as soon as the vapour pressure is reduced and thus to a certain degree compensate for the LPG being pumped out of the cargo tanks. It is however not evident that the total pressure reduction in the cargo tanks during discharge of LPG will be within the working pressure range of the cargo tanks. To prevent pressure problems during discharge of LPG, it is common to vaporise a portion of the discharged liquid in a dedicated vaporiser and returning the vapour back to the cargo tank. Other means are also possible, e.g. warming up the vapour atmosphere in the cargo tanks by using the cargo compressor. This is done by circulating vapour through the cargo compressor without any cooling and returning it back to the cargo tanks.

Figure 1 shows for reference a typical prior art reliquefaction unit. Liquid cargo flows in a line 1 from the loading ports storage tanks. Loading valves 261, 262, 263 regulate the amount of received cargo to each cargo tank. Vapour from the cargo tanks 100, 110, 120 flows via a vapour line 2 and enters the cargo compressor 200 in which the vapour is compressed to an intermediate pressure. The amount of vapour that is not handled by the reliquefaction unit shown in figure 1 flows via a continuation of the vapour line 2 to parallel operating units, not illustrated.

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

Vapour exiting the cargo compressor 200 via a line 3 enters the combined de- superheater/flash economiser 210 in which the vapour is brought close to its saturation temperature. The vapour then flows via a line 4 from the de-superheater/flash economiser 210 to the cargo compressor 220 in which it is compressed to bubble point pressure corresponding to the achievable temperature in the cargo condenser 170.

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

The compressed vapour enters the cargo condenser 170 via a line 5 to be condensed against seawater or any cooling medium typically above seawater temperatures. Sea- water is by far the most common used heat sink for the cargo condenser 170 but a mixture of water and an anti freeze agent is also possible. Anti freeze agents can be any suitable glycol.

Warm condensate leaving the cargo condenser 170 flows via a line 7 to a line 6 branched off from the line 7 in which a small portion flows via the level control valve 230 providing required interstage cooling and subcooling of the main portion of warm condensate.

The remaining warm condensate to be returned to the cargo tanks 100, 110, 120 flows further via a condensate line 7' through the coil 215 inside the de-superheater/flash economiser 210 and leaves the coil 215 at a subcooled state. The now subcooled condensate is reduced in pressure by the pressure control valve 240 and the resulting two phase flow is mixed with condensate and vapour flowing via a line 8 from other operating reliquefaction units. The resulting flow flows via a line 9 back to the cargo tanks 100, 110, 120.

Figure 2 shows a typical arrangement on an LPG carrier with three reliquefaction units and three cargo tanks loading cargo without vapour return on shore. As mentioned above, the LPG carrier can have any combination of number of cargo tanks and reliquefaction units, as an example a LPG carrier with four cargo tanks can be equipped with two reliquefaction units according to NO Patent Application 20092477.

The LPG carrier's cargo tanks 100, 110, 120 are loaded with LPG via the cargo loading line 1 from the loading port. The loading valves 261, 262, 263 regulate the loading rates and protect against overfilling. The larger LPG carriers will typically have more than three tanks but the number shall be of no relevance for the invention. A certain portion of the LPG flowing into the cargo tanks will flash into vapour phase in an amount dependent on the pressure difference between pressure within a loading port storage tank and LPG carriers cargo tank pressure and the total heat ingress from storage tanks to cargo tanks.

Vapour flows from the cargo tanks 100, 110, 120 via the vapour line 2 to the refrigeration units 130, 140, 150 in which the vapour is reliquefied 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 vapour. The valves 264, 265, 266 enable flexibility in returning the condensate either to one cargo tank, all cargo tanks or any combination thereof.

Since loading of LPG evidently results in a certain amount of vapour generation that must be handled by the reliquefaction units installed on the LPG carrier, it is clear that the loading rate is governed by the refrigerant duty of the refrigeration units. The number of reliquefaction units is typically dependent on the size of the LPG carrier and shall not be relevant for the invention.

The cargo vaporiser 190 shown in figure 2 is not in operation during the loading operation. Normally this also applies for the deck tank 160.

When 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 that the deck tank is filled with cold cargo directly from the loading line 1 during loading, alternatively the deck tank 160 can be filled during discharge of cargo. During discharge of cargo the loading line 1 is used as export line.

When loading the deck tank 160, an isolation valve 320 is open. The deck tank 160 does not have any thermal insulation and is allowed to warm up. When emptying the deck tank 160 a valve 310 opens and regulates the flow through the cargo vaporiser 190 va- pourising the liquid. The vapour product flows via a line 12 and connects to the cargo liquid header, other connections are also common but not relevant for the description. The isolation valve 340 prevents reversed flow into the cargo vaporiser 190 during normal loading operations.

Displaced vapour from the deck tank 160 flows via a line 11 and connects to the vapour header. The isolation valve 330 isolates the deck tank 160 from the vapour header during normal operations.

A typical medium sized LPG carrier with cargo tanks of 35 000 m³ cargo carrying capacity is loaded with Propane at loading port with storage tanks having a vapour pressure of 0.42 bar g. The temperature of the LPG received can be read to be -38°C from Graph 1 below.

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50

Vapor pressure, bar g

Graph 1

For this particular example it is desirable that the LPG carrier during loading shall have a cargo tank pressure of 0.275 bar g. The loading curve for this particular LPG carrier and this particular case is shown in Graph 2 below.

-41 -40 -39 -38 -37 -36 -35

Cargo inlet temperature, °C

Graph 2

From Graph 2 the loading capacity can be found to be approximately 170 tons/h. Total loaded mass of cargo will be 19788 tons and a loading time of 4,9 days results.

There is no prior art solutions that is environmentally friendly, i.e. not venting vapour to atmosphere to achieve reduction in loading time of any significance.

The main objective of the present invention is to remedy the disadvantage discussed above.

According to a first aspect, this is achieved by a method for the handling of warm LPG cargo in at least one cargo tank situated onboard a LPG carrier, preferentially during loading, comprising reliquefying vapour released from the cargo within the at least one cargo tank by means of at least one reliquefaction unit including a condenser; and returning reliquefied vapour into the at least one cargo tank. The method is further comprising operating the at least one reliquefaction unit and the condenser in non- refrigeration modus as to only compress and condense vapour; and flowing warm condensate from the condenser into a deck tank.

A second aspect of the invention provides a system for the handling of warm LPG cargo in at least one cargo tank situated onboard a LPG carrier, preferentially during loading, comprising vapour released from the cargo within the at least one cargo tank is reliquefied by means of at least one reliquefaction unit including a condenser; and reliquefied vapour is returned into the at least one cargo tank, wherein the at least one reliquefaction unit and the condenser are operated in non-refrigeration modus as to only compress and condense vapour; and warm condensate from the condenser is flowed into a deck tank.

To condense and flow warm condensate into the deck tank, vapour is compressed by means of a compressor arrangement within the at least one reliquefaction unit and then condensed by the condenser arranged in connection with the compressor.

However, compressed vapour at an intermediate pressure can be flow through a combined de-superheater and flash economiser arranged in the at least one reliquefaction unit in front of the condenser, or if appropriate even bypassed the same.

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

Inter alia to compensate for the pressure reduction during unloading operations and start unloading at more moderate tank pressures, the deck tank can be emptied into at least one of the cargo tanks during unloading, warm vapour being flowed by pressure through spray cooling nozzles arranged within the at least one cargo tank.

To ensure an initial saturation pressure of condensate flowed into the deck tank below its maximal operating pressure, a section of the at least one reliquefaction unit including both the combined de-superheater and flash economiser is operated.

Additionally, Propane originating from warm condensate in the deck tank can be used as fuel for propulsion of LPG carrier engines by means of a low pressure fuel pump.

Other favourable embodiments are indicated in dependent patent claims and the detailed discussion below

Briefly, the loading time at loading port is thus reduced for ocean going tanker vessels carrying LPG, and particularly when loading cargo at a temperature higher than the corresponding saturation temperature at cargo tank pressure. As already mentioned above additionally, secondary effects are achieved, e.g. elimination of forced vaporisation during unloading and peak shaving during laden voyage. Now, the present invention is to be discussed in more detail based on the accompanying drawings, in which:

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

Figures 3 to 11 illustrate schematically preferred embodiments of a system for transporting liquefied petroleum gases, and in particular but not exclusively to reduce the LPG carriers loading time when loading warm cargo.

The invention relates to a method and system for transporting liquefied petroleum gases, and in particular to reduce the LPG carriers loading time when loading warm cargo. The invention uses typically existing equipment installed on LPG carriers but in different configurations 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 running to at least one cargo tank 100, 110, 120. The LPG carrier can have any number of cargo tanks but typically between two to four.

Vapour flows from the cargo tanks 100, 110, 120 via a vapour line 2 to the reliquefaction units. Figure 3 shows one generalised reliquefaction unit 130 of all units being present built up by a compressor arrangement 400 and a condensate subcooling arrangement 500 and the condenser 170. The compressor arrangement will typically comprise at least a two stage compressor whilst the condensate sub-cooling arrangement can have different configurations but all with a purpose to reduce 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 amount of flash gas in the cargo tank 160.

Vapour not handled by the reliquefaction unit 130 flows further via the vapour line 2 that also connect to the additional parallel reliquefaction units, not illustrated. Any number of reliquefaction units can be used but typically between two to four units are common.

Vapour flowing from the cargo tanks 100, 110, 120 via the vapour line 2 enters a cargo compressor 200 in which the vapour is compressed to an intermediate pressure, typically in the range from 3 bar g to 5 bar g. The compressed vapour leaves the cargo compressor 200 via a line 3 and enters the combined de-superheater/flash economiser 210. There is no liquid flow through a line 6 feeding the combined de-superheater/flash economiser 210 and the vapour passes out of the combined de-superheater/flash economiser 210 via a line 4 as same state as entering.

The vapour flows further to a cargo compressor 220 in which the vapour 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. Vapour leaves the cargo compressor 220 via a line 5 and enters the condenser 170 to be condensed. An isolation valve 267 is closed and an isolation valve 268 is open enabling warm condensate to flow via a line 16 to the deck tank 160. A regulating valve 370 ensures sufficient backpressure for the cargo compressor 220. An isolation valve 380 is open.

Another operational configuration is to bypass the combined de-superheater/flash economiser. The compressed vapour leaves cargo compressor 200 via line 3 but bypasses combined de-superheater/flash economiser 210 via a line 3b, see figure 10. The bypass is catered for by closing the isolation valve 380 and opening an isolation valve 390. The line 3b connects to the line 4.

There is only vapour flow through the combined de-superheater/flash economiser 210. Thus, the respective reliquefaction unit operates in a non-refrigeration modus in which the total vapour is only compressed and condensed.

Warm condensate from other parallel operating reliquefaction units enters the line 16 via a line 13. The valve 380 isolates the deck tank 160 from the liquid lines and protects against overfilling. A valve 350 on a line 17 enables filling of cold condensate. The line 17 branches off from a condensate return line 9 and connects to a liquid line 10. Displaced vapour flows from the deck tank 160 via a line 14 back to the cargo compressor section of the reliquefaction unit and vapour is passed to other parallel operating reliquefaction units. A valve 360 regulates the vapour pressure in the deck tank 160.

For prior art solutions the deck tank is filled with LPG whilst for the present invention it is filled with condensate. The principal difference between an LPG grade of Propane with e.g. 5% by mole Ethane is that the condensate being the equilibrium composition of the vapour phase and has typically an Ethane content of 26 % by mole.

Vapour from the cargo tanks is built up by elements given above but having magnitudes as specified below:

Displaced vapour volume during loading : 5 - 10%

Flash vapour from loaded cargo: 35 - 65%

Displaced vapour from return of condensate: 0 - 1 %

Flash from returned condensate: 15 - 30%

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

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

The vapour handling capacity is fixed for each LPG carrier being governed by the capacity of the cargo compressor and number of possible simultaneous parallel operating reliquefaction units. By sending all condensate to the deck tank 160, the vapour portion created by condensate flash is removed from total vapour and thus an increased loading rate is made possible.

Vapour from the deck tank 160 can be handled by either being:

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 vapour line 14 connects to the discharge side of cargo 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 vapour line 14 connects to the discharge side of cargo compressor 220, see figure 5.

4. mixed with the loaded LPG, see figure 6. Displaced vapour from the deck tank 160 flows via the line 17 into the cargo liquid line 1 in which the vapour is either fully or partially absorbed into the liquid stream. The cargo compressor arrangement illustrated in figure 3 to 6 is typically a. reciprocating cargo compressor with normally two or three compression stages. Other cargo compressor types as screw cargo compressor or centrifugal cargo compressors may also be used.

Emptying of the deck tank 160 is effected via the liquid line 9 during unloading of the LPG carriers, and the LPG content in the deck tank 160 flows by pressure through spray cooling nozzles 50, 60 in the cargo tanks, see Figure 7 not illustrating suitable connections to the respective reliquefaction unit. By sending the condensate stored in the deck tank 160 into the LPG cargo tanks 100, 110, 120 during unloading, the following benefits are gained:

• Flashing through the spray nozzles will compensate for the pressure reduction during unloading operations. Hence, it will not be required operations as :

o Pressure make-up by vaporisation of pumped cargo and,

o Heating of vapour space.

• Unloading can start at more moderate tank pressures, i.e. not necessary to ensure pressure build up at end of voyage.

• The LPG carriers cargo carrying capacity is increased by the volume of the deck tank.

The rate of evaporation from the cargo tanks is significantly higher during the first few days of voyage than later parts of the voyage. Such an increased rate of evaporation is due to the fact that the cargo containment system has not reached steady temperature corresponding to the cargo during loading

The amount of vapour required to replace the pump out volume during cargo discharging can be supplied by flashing warm condensate from the deck tank 160. However, dependent on the loaded cargo temperature, the amount of condensate sent to the deck tank 160 might not meet the vapour requirements during discharging. Thus, by sending warm condensate to the deck tank 160 also during the first few days of voyage, the total available vapour by flashing condensate back to the cargo tanks during discharging can be at least balanced with the total amount of LPG vaporised to maintain the pressure in the cargo tanks 100, 110, 120. This change in operation will save pumping power for seawater used in the cargo vaporiser 190 shown on figure 2, for instance, and save fuel during the first days of laden voyage since the change in operation will require less reliquefaction units in operation. The latter is based on the fact that about 20 - 35% of the condensate returned back to the cargo tanks 100, 110, 120 flashes into vapour and is recycled back to the cargo compressor arrangement.

Figure 3, 4, 5, 6 shows a prior art reliquefaction unit with the proposed connections to the deck tank according to the present invention.

During laden voyage the operations onboard an LPG carrier often involves intermittent operations of the reliquefaction units meaning that the cargo tank pressure is allowed to raise to a high level i.e. not running any of the reliquefaction units, then running one or typically two of the reliquefaction units often during day time to reduce the cargo tank pressure. By using the new connections to the deck tank 160 the following operational advantages are gained :

During daytime and particularly during the warmest part of the day, often two reliquefaction units need to be in operation. Only one operating reliquefaction unit will be sufficient for many of the voyages if sending part of the warm condensate via the line 16 to the deck tank 160. Thereby, the total vapour handling capacity of the one operating reliquefaction unit is increased. During periods when the temperatures are more moderate, the condensate filled into the deck tank 160 can be sent back to the cargo tanks via the line 16 or alternatively the line 10. Thus, the deck tank 160 can actively be used to peaks having the high vapour rates during the warmest hours and hence reduce the number of required operating reliquefaction units.

Example

A typical LPG carrier of 35 000 m³ capacity is loaded with a light Propane mixture at a temperature of -37.5°C. The cargo tank pressure during loading is 0.22 bar g and the corresponding saturation pressure of the LPG is 0.45 bar g.

Vapour flow from the cargo tanks during loading is built up by the following elements:

1 - Displaced vapour volume during loading 6.6%

2 - Flash from reliquefaction 20.8%

3 - Displaced vapour volume from reliquefaction 0.4%

4 - Flash vapour from loaded cargo 42.0%

5 - Evaporation due to heat ingress 30.2%

Total 100% By removing all condensate in its warm state from the reliquefaction unit the flash contribution from the condensate is eliminated and vapour flow from the cargo tank reduces. This reduction in total vapour flow rate gives a potential to increase the loading rate to maintain the same initial vapour flow rate. For same conditions as above, the percentage distribution of the individual vapour elements are:

1 - Displaced vapour volume during loading 9.8%

2 - Flash from reliquefaction 0.0%

3 - Displaced vapour volume from reliquefaction 0.0%

4 - Flash vapour from loaded cargo 62.6%

5 - Evaporation due to heat ingress 27.6%

Total 100%

From this example it is simple to conclude that a significant increased loading rate is available through the present invention.

It could be cases in which the LPG has a content of volatile components above a normal range causing high saturation pressures in the deck tank 160 exceeding its design pressures. These high contributions of more volatile components can also cause unacceptable discharge temperatures. Moreover, some operational aspects causes saturation pressures exceeding the design pressure of the deck tank.

More volatile components can involve Propane with a higher Ethane content than acceptable in commercial Propane. Then, the cargo compressor discharge pressure will increase and accordingly the discharge temperature.

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

For these cases it will be required to operate the section of the reliquefaction units including both the de-superheater/flash economiser 210 to ensure that the initial saturation pressure of the condensate sent to the deck tank 160 is well below maximum operating pressure for the deck tank 160. Heat leakage into the deck tank 160 will slowly raise the pressure therein and at a given time a vapour line 14 must be opened. This will normally occur after loading when the LPG carrier is sailing. Figure 8 shows a schematic on how this will be arranged. The isolation valve 268 closes and the isolation valve 267 opens ensuring that warm condensate flows towards the combined de-superheater/flash economiser 210 via the line 7. A small portion of the condensate is directed through the line 6 ensuring required condensate subcooling and interstage cooling. The isolation valves 264, 265, 266 and 320 are all closed. The isolation valve 350 is open ensuring flow of subcooled condensate via the line 7 connecting to the line 10 filling the deck tank. Subcooling temperatures are typically below 10°C.

At some time after end filling the temperature of the deck tank 160 reaches a level at which its corresponding saturation pressure meets maximum allowable operational pressure. At this point the regulating valve 360 opens and maintains this pressure.

Displaced vapour from the deck tank 160 is routed via the line 14 back to the compressor arrangement 400 for recompression. Displaced vapour to other reliquefaction units branches off from the line 14.

To allow for a more flexible arrangement of the deck tank a transfer pump 460 is located on line 16 to counteract the frictional pressure losses in the lines running to the deck tank, cf figure 11.

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

The shipping industry therefore has signalled that their carbon reduction target should be at least just as ambiguous as the future carbon reduction agreed under any new United Nations Climate Change Convention.

One alternative enabling the shipping industry to meet their carbon target is a phase over to fuels of less carbon impact. For LPG carriers this could typically be effected by installing dual fuel engines and primarily running on Propane.

Dual fuel slow speed diesel engines running on propane will require a fuel tank of suitable capacity. The deck tank 160 holds sufficient volume of Propane for most voyages and by combining the deck tanks functionality with also being a fuel tank several benefits are gained: 1. There is no need for an additional tank

2. There is no need for additional fuel filling system

3. Fuel tank can be filled during voyage from the reliquefaction unit

Emptying of the deck tank 160 will typically be effected by a low pressure pump supplying a high pressure pumps boosting the condensate up 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 deck tank 160 via a line 20 connecting to the line 16, see figure 9. A valve 455 isolates the fuel system when the LPG 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 illustrated.

Since the natural boil off vapour during the voyage can be sent to the deck tank as warm condensate, the deck tank does not need to be sized for the sailing trades with longest sailing distances. Also the offers described during cargo unloading can be maintained. Figure 9 shows the combined arrangement.

As mentioned above, the deck tank 160 is present at the LPG carrier but this fact do not exclude using one or more tanks as an alternative or supplement to the traditional deck tank.

Claims

P a t e n t c l a i m s 1.

A method for the handling of warm LPG cargo in at least one cargo tank (100, 110, 120) situated onboard a LPG carrier, preferentially during loading, comprising:

reliquefying vapour released from the cargo within the at least one cargo tank (100, 110, 120) by means of at least one reliquefaction unit (130, 140, 150) including a condenser (170); and

returning reliquefied vapour into the at least one cargo tank (100, 110, 120), c h a r a c t e r i z e d i n that the method is further comprising: operating the at least one reliquefaction unit (130, 140, 150) and the condenser (170) in non-refrigeration modus as to only compress and condense vapour; and

flowing warm condensate from the condenser (170) into a deck tank (160).

2.

A method according to the claim 1, c h a r a c t e r i z e d i n that the method is further comprising:

compressing vapour by means of a compressor arrangement (400) within the at least one reliquefaction unit (130, 140, 150), the condenser (170) being arranged in connection with the compressor arrangement as to condense and flow warm condensate into the deck tank (160).

3.

A method according to the claim 2, c h a r a c t e r i z e d i n that the method is further comprising:

bypassing the combined de-superheater and flash economiser (210) when passing compressed vapour into the condenser (170).

4.

A method according to any of the preceding claims, c h a r a c t e r i z e d i n that the method is further comprising:

returning vapour from the deck tank by means of one of the following:

i) routing vapour back to the suction side of a cargo compressor

(200);

. ii) routing vapour back to the discharge side of the cargo compressor (200); iii) routing vapour back to the suction side of a third cargo compressor (225) in which three compression stages are applicable; and iv) mixing vapour with the loaded LPG.

5.

emptying the deck tank (160) into at least one of the cargo tanks (1, 2, 3) during unloading, warm vapour being flowed by pressure through spray cooling nozzles (50, 60) arranged within the at least one cargo tank.

6.

operating a section of the at least one reliquefaction unit (130, 140, 150) including both the combined de-superheater and flash economiser (210) as to ensure an initial saturation pressure of condensate flowed into the deck tank below its maximal operating pressure.

7.

using LPG originating from warm condensate in the deck tank (160) as fuel for propulsion of LPG carrier engines by means of a low pressure fuel pump (450).

8.

A system for the handling of warm LPG cargo in at least one cargo tank (100, 110, 120) situated onboard a LPG carrier, preferentially during loading, comprising:

vapour released from the cargo within the at least one cargo tank (100, 110, 120) is reliquefied by means of at least one reliquefaction unit (130, 140, 150) including a condenser (170); and

reliquefied vapour is returned into the at least one cargo tank (100, 110, 120), c h a r a c t e r i z e d i n that the system is further comprising: the at least one reliquefaction unit (130, 140, 150) and the condenser (170) are operated in non-refrigeration modus as to only compress and condense vapour; and warm condensate from the condenser (170) is flowed into a deck tank (160).

9.

A system according to the claim 8, c h a r a c t e r i z e d i n that the system is further comprising:

warm condensate from the condenser (170) is flowed into the deck tank via a line (16) including regulating and isolating valves (370, 380).

10.

A system according to the claim 8 and 9, c h a r a c t e r i z e d i n that the system is further comprising:

vapour is compressed by means of a compressor arrangement (400) within the at least one reliquefaction unit (130, 140, 150), the condenser (170) being arranged in connection with the compressor arrangement as to condense and flow warm condensate into the deck tank (160).

11.

A system according to the claim 10, c h a r a c t e r i z e d i n - that the system is further comprising:

compressed vapour is flowed into the combined de-superheater and flash economiser (210) via a line (3) and passed therefrom via a line (4).

12.

the combined de-superheater and flash economiser (210) is bypassed when passing compressed vapour into the condenser (170).

13.

A system according to the claim 12, c h a r a c t e r i z e d i n - that the system is further comprising:

the combined de-superheater and flash economiser (210) is bypassed by means of a line (3a) connecting the lines (3, 4), the lines (3, 3b) are including isolating valves (380, 390), respectively.

14.

A system according to any of the preceding claims 8 to 13, c h a r a c t e r i z e d i n that the system is further comprising:

vapour from the deck tank is returned by means of one of the following: i) vapour is routed back to the suction side of a cargo compressor (200);

ii) vapour is routed back to the discharge side of the cargo compressor (200);

iii) vapour is routed back to the suction side of a third cargo compressor (225) in which three compression stages are applicable; and iv) vapour is mixed with the loaded LPG.

15.

A system according to the claim 14, c h a r a c t e r i z e d i n - that the system is further comprising:

vapour is routed back to the respective side of compressor arrangement by means of a vapour line (14) including a regulating valve (360).

16.

A system according to the claim 15, c h a r a c t e r i z e d i n - that the system is further comprising:

vapour is mixed with the loaded LPG via a vapour line (17) line including a regulating valve (360).

17.

A system according to any of the preceding claims 8 to 16, c h a r a c t e r i z e d i n that the system is further comprising:

the deck tank (160) is emptied into at least one of the cargo tanks (1, 2, 3) during unloading, warm vapour being flowed by pressure through spray cooling nozzles (50, 60) arranged within the at least one cargo tank.

18.

A system according to any of the preceding claims 8 to 17, c h a r a c t e r i z e d i n that the system is further comprising:

a section of the at least one reliquefaction unit (130, 140, 150) including both the combined de-superheater and flash economiser (210) is operated as to ensure an initial saturation pressure of condensate flowed into the deck tank below its maximal operating pressure.

19.

A system according to any of the preceding claims 8 to 18, c h a r a c t e r i z e d i n that the system is further comprising:

LPG originating from warm condensate in the deck tank (160) is used as fuel for propulsion of LPG carrier engines by means of a low pressure fuel pump (450).

20.

A system according to the claim 19, c h a r a c t e r i z e d i n - that the system is further comprising:

the low pressure fuel pump is feeding a high pressure pump included in a downstream high pressure fuel system via a line (21).