Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B63—SHIPS OR OTHER WATERBORNE VESSELS; RELATED EQUIPMENT
  • B63B—SHIPS OR OTHER WATERBORNE VESSELS; EQUIPMENT FOR SHIPPING 
  • B63B25/00—Load-accommodating arrangements, e.g. stowing, trimming; Vessels characterised thereby
  • B63B25/02—Load-accommodating arrangements, e.g. stowing, trimming; Vessels characterised thereby for bulk goods
  • B63B25/08—Load-accommodating arrangements, e.g. stowing, trimming; Vessels characterised thereby for bulk goods fluid
  • B63B25/12—Load-accommodating arrangements, e.g. stowing, trimming; Vessels characterised thereby for bulk goods fluid closed
  • B63B25/16—Load-accommodating arrangements, e.g. stowing, trimming; Vessels characterised thereby for bulk goods fluid closed heat-insulated
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
  • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
  • F17C5/00—Methods or apparatus for filling containers with liquefied, solidified, or compressed gases under pressures
  • F17C5/02—Methods or apparatus for filling containers with liquefied, solidified, or compressed gases under pressures for filling with liquefied gases
  • F17C5/04—Methods or apparatus for filling containers with liquefied, solidified, or compressed gases under pressures for filling with liquefied gases requiring the use of refrigeration, e.g. filling with helium or hydrogen
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B63—SHIPS OR OTHER WATERBORNE VESSELS; RELATED EQUIPMENT
  • B63B—SHIPS OR OTHER WATERBORNE VESSELS; EQUIPMENT FOR SHIPPING 
  • B63B27/00—Arrangement of ship-based loading or unloading equipment for cargo or passengers
  • B63B27/24—Arrangement of ship-based loading or unloading equipment for cargo or passengers of pipe-lines
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
  • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
  • F17C1/00—Pressure vessels, e.g. gas cylinder, gas tank, replaceable cartridge
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
  • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
  • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
  • F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
  • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
  • F17C2201/00—Vessel construction, in particular geometry, arrangement or size
  • F17C2201/05—Size
  • F17C2201/052—Size large (>1000 m3)
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
  • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
  • F17C2221/00—Handled fluid, in particular type of fluid
  • F17C2221/03—Mixtures
  • F17C2221/032—Hydrocarbons
  • F17C2221/035—Propane butane, e.g. LPG, GPL
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
  • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
  • F17C2223/00—Handled fluid before transfer, i.e. state of fluid when stored in the vessel or before transfer from the vessel
  • F17C2223/01—Handled fluid before transfer, i.e. state of fluid when stored in the vessel or before transfer from the vessel characterised by the phase
  • F17C2223/0146—Two-phase
  • F17C2223/0153—Liquefied gas, e.g. LPG, GPL
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
  • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
  • F17C2223/00—Handled fluid before transfer, i.e. state of fluid when stored in the vessel or before transfer from the vessel
  • F17C2223/03—Handled fluid before transfer, i.e. state of fluid when stored in the vessel or before transfer from the vessel characterised by the pressure level
  • F17C2223/033—Small pressure, e.g. for liquefied gas
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
  • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
  • F17C2223/00—Handled fluid before transfer, i.e. state of fluid when stored in the vessel or before transfer from the vessel
  • F17C2223/04—Handled fluid before transfer, i.e. state of fluid when stored in the vessel or before transfer from the vessel characterised by other properties of handled fluid before transfer
  • F17C2223/042—Localisation of the removal point
  • F17C2223/046—Localisation of the removal point in the liquid
  • F17C2223/047—Localisation of the removal point in the liquid with a dip tube
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
  • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
  • F17C2225/00—Handled fluid after transfer, i.e. state of fluid after transfer from the vessel
  • F17C2225/01—Handled fluid after transfer, i.e. state of fluid after transfer from the vessel characterised by the phase
  • F17C2225/0146—Two-phase
  • F17C2225/0153—Liquefied gas, e.g. LPG, GPL
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
  • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
  • F17C2225/00—Handled fluid after transfer, i.e. state of fluid after transfer from the vessel
  • F17C2225/04—Handled fluid after transfer, i.e. state of fluid after transfer from the vessel characterised by other properties of handled fluid after transfer
  • F17C2225/042—Localisation of the filling point
  • F17C2225/043—Localisation of the filling point in the gas
  • F17C2225/044—Localisation of the filling point in the gas at several points, e.g. with a device for recondensing gas
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
  • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
  • F17C2227/00—Transfer of fluids, i.e. method or means for transferring the fluid; Heat exchange with the fluid
  • F17C2227/01—Propulsion of the fluid
  • F17C2227/0128—Propulsion of the fluid with pumps or compressors
  • F17C2227/0135—Pumps
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
  • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
  • F17C2227/00—Transfer of fluids, i.e. method or means for transferring the fluid; Heat exchange with the fluid
  • F17C2227/01—Propulsion of the fluid
  • F17C2227/0128—Propulsion of the fluid with pumps or compressors
  • F17C2227/0157—Compressors
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
  • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
  • F17C2227/00—Transfer of fluids, i.e. method or means for transferring the fluid; Heat exchange with the fluid
  • F17C2227/03—Heat exchange with the fluid
  • F17C2227/0337—Heat exchange with the fluid by cooling
  • F17C2227/0341—Heat exchange with the fluid by cooling using another fluid
  • F17C2227/0348—Water cooling
  • F17C2227/0351—Water cooling using seawater
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
  • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
  • F17C2227/00—Transfer of fluids, i.e. method or means for transferring the fluid; Heat exchange with the fluid
  • F17C2227/03—Heat exchange with the fluid
  • F17C2227/0367—Localisation of heat exchange
  • F17C2227/0388—Localisation of heat exchange separate
  • F17C2227/039—Localisation of heat exchange separate on the pipes
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
  • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
  • F17C2250/00—Accessories; Control means; Indicating, measuring or monitoring of parameters
  • F17C2250/06—Controlling or regulating of parameters as output values
  • F17C2250/0605—Parameters
  • F17C2250/0626—Pressure
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
  • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
  • F17C2260/00—Purposes of gas storage and gas handling
  • F17C2260/02—Improving properties related to fluid or fluid transfer
  • F17C2260/025—Reducing transfer time
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
  • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
  • F17C2265/00—Effects achieved by gas storage or gas handling
  • F17C2265/03—Treating the boil-off
  • F17C2265/032—Treating the boil-off by recovery
  • F17C2265/033—Treating the boil-off by recovery with cooling
  • F17C2265/034—Treating the boil-off by recovery with cooling with condensing the gas phase
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
  • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
  • F17C2265/00—Effects achieved by gas storage or gas handling
  • F17C2265/03—Treating the boil-off
  • F17C2265/032—Treating the boil-off by recovery
  • F17C2265/037—Treating the boil-off by recovery with pressurising
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
  • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
  • F17C2270/00—Applications
  • F17C2270/01—Applications for fluid transport or storage
  • F17C2270/0102—Applications for fluid transport or storage on or in the water
  • F17C2270/0105—Ships
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02T—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO TRANSPORTATION
  • Y02T70/00—Maritime or waterways transport

Definitions

• 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.
• LPG liquefied petroleum gases
• 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.
• 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.
• 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:
• 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:
• LPG carriers transporting liquefied cargoes (LPG) at temperatures below ambient, known as fully refrigerated LPG carriers, and
• LPG carriers transporting liquefied cargoes (LPG) at pressures greater than atmospheric and temperatures below ambient.
• LPG liquefied cargoes
• 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.
• 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
• a VLGC Very Large Gas Carrier
• a VLGC Very Large Gas Carrier
• a VLGC Very Large Gas Carrier
• a VLGC Very Large Gas Carrier
• a VLGC Very Large Gas Carrier
• 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.
• 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.
• an inert gas generator e.g. exhaust gas
• Nitrogen generator e.g. Nitrogen generator
• 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.
• FIG. 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.
• 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.
• 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.
• 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.
• an isolation valve 320 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.
• 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 m 3 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.
• 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.
• the main objective of the present invention is to remedy the disadvantage discussed above.
• 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.
• 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.
• 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.
• 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.
• a section of the at least one reliquefaction unit including both the combined de-superheater and flash economiser is operated.
• 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.
• 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.
• secondary effects are achieved, e.g. elimination of forced vaporisation during unloading and peak shaving during laden voyage.
• Figures 1 and 2 show schematically prior art reliquefaction units
• 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.
• FIG. 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.
• 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.
• 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.
• the respective reliquefaction unit operates in a non-refrigeration modus in which the total vapour is only compressed and condensed.
• the deck tank is filled with LPG whilst for the present invention it is filled with condensate.
• 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:
• 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.
• Vapour from the deck tank 160 can be handled by either being:
• vapour line 14 connects to the discharge side of cargo compressor 200, see figure 4.
• vapour line 14 connects to the discharge side of cargo compressor 220, see figure 5.
• 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.
• 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.
• the amount of condensate sent to the deck tank 160 might not meet the vapour requirements during discharging.
• 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.
• 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.
• 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.
• a typical LPG carrier of 35 000 m 3 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:
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.

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• 2009
  • 2009-12-22 NO NO20093579A patent/NO333898B1/no unknown
• 2010
  • 2010-12-20 EP EP10839844.7A patent/EP2516918A4/en not_active Withdrawn
  • 2010-12-20 CN CN201080057985.1A patent/CN102713402B/zh active Active
  • 2010-12-20 JP JP2012545887A patent/JP5448123B2/ja active Active
  • 2010-12-20 WO PCT/NO2010/000475 patent/WO2011078688A1/en active Application Filing
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