WO2007104078A1

WO2007104078A1 - Onboard regasification of lng

Info

Publication number: WO2007104078A1
Application number: PCT/AU2007/000226
Authority: WO
Inventors: Solomon Aladja Faka; Robert Hannan; David Wynne JONES
Original assignee: Woodside Energy Limited
Priority date: 2006-03-15
Filing date: 2007-02-28
Publication date: 2007-09-20
Also published as: AU2007224992B2; JP2013079069A; AU2007224992A1; EP1994328A1; EP1994328A4; JP2009529456A; KR20080111463A; JP5283514B2

Abstract

A method is provided for offshore regasification of liquid natural gas (LNG) for delivery onshore as a gas. The method includes offloading LNG from a delivery vessel to a receiving vessel at an ship-to-ship transfer location, the receiving vessel including an onboard regasification facility, transferring the receiving vessel from the ship-to-ship transfer location to a mooring location closer to shore, regasifying the LNG onboard the receiving vessel to form natural gas, and transferring the regasified natural gas to an onshore gas distribution facility for delivery to an end user.

Description

ONBOARD REGASIFICATION OF LNG

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from U.S. Provisional Patent Application Serial No. 60/782,282, entitled Onboard Regasification of LNG" and filed 15 March 2006 and

U. S. Complete Patent Application Serial No. 11/559,136 entitled Onboard

Regasification of LNG" filed 13 November 2006 and U.S. Provisional Patent

Application Serial No. 60/843,395, entitled "Boil Off Gas Management During Ship-

To-Ship Transfer of LNG" filed on 11 September 2006. The disclosure of each of the above-identified patent applications is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a method for onboard regasification of liquefied natural gas ("LNG"). The present invention further relates to a receiving vessel for onboard regasification of LNG.

BACKGROUND TO THE INVENTION

Natural gas is the cleanest burning fossil fuel as it produces less emissions and pollutants than either coal or oil. Natural gas ("NG") is routinely transported from one location to another location in its liquid state as "Liquefied Natural Gas ("LNG").

Liquefaction of the natural gas makes it more economical to transport as LNG occupies only about 1 /600^th of the volume that the same amount of natural gas does in its gaseous state. Transportation of LNG from one location to another is most commonly achieved using double-hulled ocean-going vessels with cryogenic storage capability referred to as "LNGCs".

LNG is typically stored in cryogenic storage tanks onboard the LNGC, the storage tanks being operated either at or slightly above atmospheric pressure. The majority of existing LNGCs have an LNG cargo storage capacity in the size range of 120,000 m³ to 150,000 m³, with some LNGCs having a storage capacity of up to 264,000 m³. LNG is normally regasified before distribution to end users through a pipeline or other distribution network at a temperature and pressure that meets the delivery requirements of the end users. Regasification of the LNG is most commonly achieved by raising the temperature of the LNG above the LNG boiling point for a given pressure. It is common for an LNGC to receive its cargo of LNG at an export terminal located in one country and then sail across the ocean to deliver its cargo to an import terminal located in another country. Upon arrival at the import terminal, the LNGC berths at a pier or jetty and offloads the LNG as a liquid to an onshore storage and regasification facility located at the import terminal The regasification facility typically comprises a plurality of heat exchangers or vaporisers, pumps and compressors. Such onshore storage and regasification facilities are typically large and the costs associated with building and operating such facilities are significant.

Recently, public concern over safety of onshore regasification facilities has led to the building of offshore regasification terminals which are removed from populated areas and onshore activities. Various offshore terminals with different configurations and combinations have been proposed.

For example, US Patent 6,089,022 described a system and a method for regasifying LNG aboard a carrier vessel before the re-vaporized natural gas is transferred to shore. Seawater taken from the body of water surrounding said vessel is flowed through the vaporizer to heat and vaporize the LNG back into natural gas before the natural gas is off-loaded to onshore facilities. Regasification takes place onboard an LNGC which has been modified so that the regasification facility travels with the LNGC all of the way from the export terminal to the import terminal. The LNG onboard the converted LNGC is regasified and delivered to shore through a sub-sea pipeline connected by risers to the mooring buoy. The use of seawater is problematic and expensive due its highly corrosive nature. The main concerns however are the presence of organisms in the seawater which may well be killed and the environmental impact of cooled seawater returned to the marine environment. In another example, an offshore regasification terminal is used which includes a non-propelled barge fitted with cryogenic storage tanks. The barge is permanently moored to and able to weathervane around a mooring buoy but cannot travel under its own steam. The barge is typically longer than an LNGC to assist in side-by-side berthing of the LNGC alongside the barge so that the LNG can be offloaded from the LNGC into the storage tanks onboard the permanently moored barge. The barge includes at least one regasification unit which is typically built adjacent to and forward of the storage tanks. Regasified natural gas flows from the barge to shore through a sub-sea pipeline which is connected to the barge through a marine riser connected to the mooring buoy.

Various mediums and various types of vaporisers have been used to regasify LNG.

US Patent 4,170,115 describes an apparatus for vaporizing liquefied natural gas using estuarine water. The apparatus comprises a series of heat exchangers of the indirect heating, intermediate fluid type. The LNG is vaporised using a heating medium being a refrigerant vaporized using estuarine water as a heat source and having a temperature not higher than the freezing point of the estuarine water. A multi-tubular concurrent heat exchanger is used to bring the low-temperature vaporized natural gas from the heat exchanger into concurrent contact with estuarine water serving as a heat source to heat the vaporized natural gas. US Patent 4,224,802 describes a variation on this type of apparatus that also uses estuarine water in a multi-tubular heat exchanger.

US Patent 4,331,129 describes the utilization of solar energy for LNG vaporization. Solar energy is used to heat an intermediate fluid such as water. The heated water is then used to regasify the LNG. The water contains an anti-freeze additive so as to prevent freezing of the water during the vaporization process.

US Patent 4,399,660 describes an atmospheric vaporizer suitable for vaporizing cryogenic liquids on a continuous basis. The vaporiser employs heat absorbed from the ambient air and comprises at least three substantially vertical passes are piped together. Each pass includes a centre tube with a plurality of fins substantially equally spaced around the tube.

US Patent 5,251 ,452 also describes an ambient air vaporizer and heater for cryogenic liquids. This apparatus utilizes a plurality of vertically mounted and parallel connected heat exchange tubes. Each tube has a plurality of external fins and a plurality of internal peripheral passageways symmetrically arranged in fluid communication with a central opening. A solid bar extends within the central opening for a predetermined length of each tube to increase the rate of heat transfer between the cryogenic fluid in its vapor phase and the ambient air. The fluid is raised from its boiling point at the bottom of the tubes to a temperature at the top suitable for manufacturing and other operations.

US Patent 6,622,492 describes apparatus and process for vaporizing liquefied natural gas including the extraction of heat from ambient air to heat circulating water. The heat exchange process includes a heat exchanger for the vaporization of liquefied natural gas, a circulating water system, and a water tower extracting heat from the ambient air to heat the circulating water. To make the process work throughout the year the process may be supplemented by a submerged fired heater connected to the water tower basin.

US Patent 6,644,041 describes a process for vaporizing liquefied natural gas including passing water into a water tower so as to elevate a temperature of the water, pumping the elevated temperature water through a first heat exchanger, passing a circulating fluid through the first heat exchanger so as to transfer heat from the elevated temperature water into the circulating fluid, passing the liquefied natural gas into a second heat exchanger, pumping the heated circulating fluid from the first heat exchanger into the second heat exchanger so as to transfer heat from the circulating fluid to the liquefied natural gas, and discharging vaporized natural gas from the second heat exchanger.

US Patent 5,819,542 describes a heat exchange device having a first heat exchanger for evaporation of LNG and a second heat exchanger for superheating of gaseous natural gas. The heat exchangers are arranged for heating these fluids by means of a heating medium and having an outlet which is connected to a mixing device for mixing the heated fluids with the corresponding unheated fluids. The heat exchangers comprise a common housing in which they are provided with separate passages for the fluids. The mixing device constitutes a unit together with the housing and has a single mixing chamber with one single fluid outlet. In separate passages, there are provided valves for the supply of LNG in the housing and in the mixing chamber.

Despite the progress achieved through the prior art, there remains a need to develop other methods for offshore regasification of LNG.

SUMMARY OF THE INVENTION According to one aspect of the present invention there is provided a method for offshore regasification of liquid natural gas (LNG) for delivery onshore as a gas, the method comprising: offloading LNG from a delivery vessel to a receiving vessel at a ship- to-ship transfer location; moving the receiving vessel under its own power from the ship-to-ship transfer location to a mooring location, the mooring location being closer to shore than the ship-to-ship transfer location; regasifying the LNG using a regasification facility onboard the receiving vessel to form natural gas at the mooring location; and, transferring the natural gas to an onshore gas distribution facility for delivery to an end user.

In one embodiment, the method further comprises the step of mooring the delivery vessel to the receiving vessel prior to the step of offloading LNG from the delivery vessel to the receiving vessel. When making the approach for mooring, the less manoeuvrable of the delivery vessel or the receiving vessel may be underway at a pre-agreed speed, with a heading selected such that the less manoeuvrable of the two vessels has the dominant swell on the aft port quarter and the heading is maintained until the delivery vessel has been moored to the receiving vessel. Alternatively, the less manoeuvrable of the delivery vessel or the receiving vessel is underway at a pre-agreed speed, with a heading selected such that the more manoeuvrable of the two vessels is heading directly into the prevailing weather and the heading is maintained until the delivery vessel has been moored to the receiving vessel.

In one embodiment, the delivery and receiving vessels are moored together and are underway whilst traveling side-by-side when LNG is offloaded from the delivery vessel to the receiving vessel. Alternatively, the delivery and receiving vessels are moored together and drifting side-by-side when LNG is offloaded from the delivery vessel to the receiving vessel. In yet another alternative, the delivery and receiving vessels are traveling in tandem when LNG is offloaded from the delivery vessel to the receiving vessel by tandem offloading.

To maintain stability of the vessel when the storage tanks are partially filled with LNG, the LNG may be stored in the hull of the receiving vessel in slosh tolerant tanks and the receiving vessel has an associated supporting hull structure.

In one embodiment, the receiving vessel has reduced engine capacity compared with the engine capacity of the delivery vessel. The receiving vessel has a propulsion system and the propulsion system may comprise dual fuel gas turbines, dual fuel diesel, or duel fuel diesel-electric systems. Advantageously, the power requirement for the propulsion system of the receiving vessel is shared with the power requirement for regasification onboard the receiving vessel. To improve maneuverability of the receiving vessel, the propulsion system of the receiving vessel is fitted with twin screw, fixed pitch propellers with transverse thrusters located both forward and aft that provide the receiving vessel with superior mooring and position capability compared with the delivery vessel.

In one embodiment of the present invention, the mooring location includes a submersible, disconnectable mooring buoy for mooring the receiving vessel at the mooring location, the mooring buoy being locatable within a recess disposed within the hull and towards the bow of the receiving vessel.

To reduce impact on the environment, the LNG may be regasified using ambient air as a source of heat to the onboard regasification facility. In one embodiment, the LNG is regasified through heat exchange with an intermediate fluid and the intermediate fluid is heated using ambient air as a source of heat. Preferably, heat exchange between the ambient air and the LNG or intermediate fluid is encouraged through use of forced draft fans

According to a second aspect of the present invention there is provided a receiving vessel for offshore regasification of liquid natural gas (LNG) for delivery onshore as a gas, the system comprising: a propulsion system for moving the receiving vessel under its own power to and from a ship-to-ship transfer location for receiving LNG from a delivery vessel, to a mooring location, the mooring location being closer to shore than the ship-to-ship transfer location; a regasification facility onboard the receiving vessel for regasifying the LNG to form natural gas; and, an onshore gas distribution facility for receiving the natural gas for delivery to an end user.

To improve hydrodynamic stability, the LNG may be stored in the hull of the receiving vessel in slosh tolerant tanks and the receiving vessel has an associated supporting hull structure.

In one embodiment, the receiving vessel has reduced engine capacity compared with the engine capacity of the delivery vessel. For greater efficiency and reduced environmental impact, the propulsion system of the receiving vessel may comprise dual fuel gas turbines, dual fuel diesel, or duel fuel diesel-electric systems. Advantageously, the power requirement for the propulsion system of the receiving vessel may be shared with the power requirement for regasification onboard the receiving vessel. To provide the receiving vessel with superior mooring and position capability compared with the delivery vessel, the propulsion system of the receiving vessel may be fitted with twin screw, fixed pitch propellers with transverse thrusters located both forward and aft.

In one embodiment, the receiving vessel includes a recess disposed within the hull and towards the bow of the receiving vessel for receiving a submersible, disconnectable mooring buoy for mooring the receiving vessel at the regasification location.

To reduce impact on the environment, the onboard regasification facility may use ambient air as a source of heat. In one embodiment, the LNG is regasified through heat exchange with an intermediate fluid and the intermediate fluid is heated using ambient air as a source of heat to the onboard regasification facility. Preferably, heat exchange between the ambient air and the LNG or between ambient air and an intermediate fluid may be encouraged through use of forced draft fans.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to facilitate a more detailed understanding of the nature of the invention several embodiments of the present invention will now be described in detail, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic plan view of a delivery vessel underway whilst a receiving vessel makes an approach;

Figure 2 is a schematic plan view of the delivery and receiving vessels of Figure 1 as the receiving vessel manoeuvres closer to the delivery vessel;

Figure 3 is a schematic plan view of a first embodiment of the present invention illustrating the delivery and receiving vessels positioned at an ship-to-ship transfer location during transfer of the LNG from the delivery vessel to the receiving vessel, the receiving vessel being fitted with an onboard regasification facility; Figure 4 is a schematic plan view of a regasification location and sub- sea pipeline for delivering gas to an onshore distribution facility;

Figure 5 is a schematic side view of the receiving vessel moored at a turret mooring buoy as the LNG is regasified onboard the receiving vessel and transferred through one or more marine riser(s) and associated sub-sea pipeline(s) to shore;

Figure 6 illustrates one possible flow diagram for the regasification facility onboard the receiving vessel;

Figure 7 is a schematic plan view of a vessel with an onboard regasification facility which is underway whilst an LNGC makes an approach;

Figure 8 is a schematic plan view of the vessels of Figure 7 as the LNGC vessel manoeuvres closer to the delivery vessel; and,

Figure 9 is a schematic plan view of a second embodiment of the present invention illustrating the vessels of Figure 8 positioned at a ship-to-ship transfer location during transfer of the LNG from the delivery vessel to the receiving vessel, the receiving vessel being fitted with the onboard regasification facility.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Particular embodiments of method for offshore regasification of LNG for delivery onshore as a gas of the present invention are now described. The terminology used herein is for the purpose of describing particular embodiments only, and is not intended to limit the scope of the present invention. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art to which this invention belongs.

The term "lightering" is used in the petroleum industry art to describe the process of ship-to-ship transfer of liquid cargoes, such as oil, ammonia and liquefied petroleum gas (LPG). Lightering is a process that is used to unload crude oil cargoes from very large crude carriers (VLCCs) or from ultra large crude carriers (ULCCs) to smaller vessels in situations where ports are not deep enough or not wide enough to allow berthing of tankers of this size. Whilst lightering is an established practice in relation to crude oil carriers, ship-to-ship transfers of LNG at cryogenic temperatures as a part of normal operations have never been attempted in the past. One of the reasons for this is that because onshore storage and regasification facilities at an import terminal are typically large and the costs associated with building and operating such facilities are significant, traditional LNG contracts have traditionally been based on long term (20 year) deals to justify the costs associated with the construction of the import terminal. Another reason that ship-to-ship transfers of LNG have not been used in the past is that it requires expensive modifications to be made to the receiving ship and there are large technical risks that need to be managed. The present invention was developed in part to overcome these technical challenges.

A first embodiment of the present invention is now described with reference to Figures 1 to 6. A delivery vessel 12 is loaded with a cargo of LNG at an onshore or offshore liquefaction facility associated with natural gas wells or pipelines. In this embodiment the delivery vessel 12 is a traditional LNG Carrier fitted with Moss-style tanks. The loaded delivery vessel 12 then travels towards a receiving terminal located typically in a different country to the country of origin where the liquefaction facility is located. Upon arrival, a receiving vessel 14 is dispatched to dock with the delivery vessel 12 at a ship-to-ship transfer location (generally designated by the reference numeral 16).

The ship-to-ship transfer location 16 can be any location where the delivery and receiving vessels 12 and 14 respectively, can be moored together to facilitate transfer of LNG from the delivery vessel 12 to the receiving vessel 14. The selection of the ship-to-ship transfer location depends on a number of relevant factors including water depth, sea conditions, prevailing winds, regulatory requirements and traffic. The distance of the ship-to-ship transfer location 16 from the coast may vary widely but is preferably outside the territorial sea of the delivery country, at a distance in the range of 12 to 200 nautical miles off the coast. When the ship-to-ship transfer occurs within the territorial sea of a given country, local regulations may require a government agency to provide control of each of the two vessels during mooring and transfer operations. In such a scenario, a support vessel 28 is used to deliver a Mooring Master to the delivery or receiving vessel with a Mooring Master Assistant stationed on the other vessel. Alternatively, ship-to-ship transfer may take place in international waters under the control of the captains of each of the vessels.

The receiving vessel 14 can be a modified ocean-going LNG vessel or a vessel that is custom built to include a regasification facility 30. To be economic, the delivery vessel 12 should travel the larger part of the distance between the liquefaction facility and the receiving facility, leaving the receiving vessel 14 to travel as short a distance as possible between the ship-to-ship transfer location 16 and a regasification location 20. However, it is to be understood that the receiving vessel 14 can be fitted with a power system that enables it to travel under its own power across an ocean or sea between an import terminal and an export terminal if required.

The current operating fleet of ocean-going LNG vessels rely on steam propulsion plants for power. Steam is used primarily due to the ease of burning the boil-off gas from the LNG storage tanks on the LNG vessel. A number of patents have been granted in relation to alternative types of power plants and turbine systems for an LNG vessel including US Patent 6,609,360; US Patent 6,598,401; US Patent 6,581,368; US Patent 6,374,591; US patent 6,367,258; US patent 5,457,951; US Patent 4,995,234; and US Patent Application Number 20050061002, all of which are incorporated herein by reference.

In one embodiment of the present invention, the receiving vessel 14 is provided with one or more dual fuelled engines, for example a combination of gas and diesel engines which are more suitable than steam turbines as they do not require the specialised operating and maintenance crew that is otherwise required for steam turbines. In another embodiment, dual fuelled diesel engines are direct coupled to generators, and electrical power so generated is directed to electric motors to drive the propeller shafts as well as to drive any electrically powered LNG pumps, fans or other equipment associated with an onboard regasification facility 30. Electricity is also used for the hotel load associated with an accommodation unit 38 onboard the receiving vessel 14. Use of dual fuelled engines enables the power to be directed to a propulsion system 18 onboard the receiving vessel 14, when the receiving vessel 14 is underway and to the regasification facility 30 when the receiving vessel 14 is positioned at the regasification location 20. It also simplifies the powering of manoeuvring devices such as transverse thrusters 48 forward and aft to facilitate mooring. The above described power sharing enables an overall reduction in installed power and all power generation by the most efficient engines available.

After the LNG is transferred from the delivery vessel 12 to the receiving vessel 14 at the ship-to-ship transfer location 16, the delivery vessel 12 can sail away back to the liquefaction facility, export terminal or another location for reloading. A key advantage of the use of a receiving vessel 14 over a permanently moored offshore storage structure is that the receiving vessel 14 can travel under its own power offshore or up and down a coastline to avoid extreme weather conditions or to avoid a threat of terrorism or to transit to a dockyard or to transit to another LNG import or export terminal. In this event, the receiving vessel 14 may do so with or without LNG stored onboard during this journey. Similarly, if demand for gas no longer exists at a particular location, the receiving vessel 14 can sail under its own power to another location where demand is higher.

The manner in which the two vessels approach and are moored is now described. If required, Mooring Masters and their assistants are stationed aboard the vessels in order to manage the approach, mooring and disconnect operations. In most situations, it is safest for the larger of the two vessels to be underway at a pre- agreed speed, with a heading selected such that the larger of the two vessels has the prevailing weather on the stern quarter opposite to the intended mooring side or with the larger of the two vessels heading directly into the prevailing weather. This heading is maintained until the two vessels have been moored to each other. In this way, the larger of the two vessels provides a lee for the smaller vessel and thus acts as a weather shield which makes it safer for the smaller of the two vessels to approach the larger one. The larger of the two vessels can be either the delivery vessel 12 or the receiving vessel 14.

In some circumstances, however, it is safest for the more manoeuvrable of the two vessels (irrespective of size) to make the approach with the less manoeuvrable vessel underway at a pre-agreed speed heading into the prevailing weather or with prevailing weather on the windward side.

In either scenario, the preferred speed during final approach and mooring is in the range of 3 to 6 knots. However, the speed is adjusted in each case to optimize the manoeuvrability of both vessels and to take into account such relevant factors as the engine idle speed of each vessel and the prevailing environmental conditions (winds, waves and currents). Either the deliver vessel 12 or the receiving vessel 14 may be more manoeuvrable. For example, when the receiving vessel 14 is fitted with an electric propulsion system and high manoeuvrability characteristics, it may idle at any speed including 0 knots.

In the embodiment illustrated in Figures 1 to 3, the delivery vessel 12 maintains a set course and is underway at a pre-agreed speed heading directly into the prevailing weather. It is equally possible to perform the approach with both vessels heading with the prevailing conditions. The receiving vessel 14 comes alongside the delivery vessel 12 by manoeuvring closer to the delivery vessel 12 until the course and speed of the receiving vessel 14 matches the course and speed of the delivery vessel 12. The approach angle between the two vessels generally decreases as the parallel distance between the two vessels decreases with the relative heading angle being nominally 2 to 5 degrees. Either vessel can then perform a heading change to bring the vessels parallel.

In another embodiment illustrated in Figures 7 to 9, the receiving vessel 14 maintains a set course with the dominant swell on the aft port quarter 22 and the delivery vessel 12 approaches the receiving vessel 14 from the leeward side 24. It is equally possible to perform the approach with both vessels heading into the prevailing conditions. In this embodiment, the delivery vessel 12 comes alongside the receiving vessel 14 by manoeuvring closer to the receiving vessel 14 until the course and speed of the delivery vessel 12 matches the course and speed of the receiving vessel 14. The approach angle between the two vessels generally decreases as the parallel distance between the two vessels decreases with the relative heading angle being nominally 2 to 5 degrees. In this embodiment, the propulsion system 18 of the receiving vessel 14 includes twin screw, fixed pitch propellers with transverse thrusters 48 located both forward and aft or just forward that provides the receiving vessel 14 with mooring and positioning capability. This high level of manoeuvrability comes is useful during the approach, mooring and unmooring operations or if the receiving vessel 14 transits to a dry dock (not shown).

The delivery or receiving vessel 12 or 14 is provided with fendering equipment 26 to absorb the forces associated with bringing the two vessels together. The fendering equipment is of a similar type to that used during oil lightering operations, for example a plurality of air-inflated rubber "cushions", covered with rubber tyres, so that the cushions float on water between the two vessels. As the two vessels come closer together, the vessels are manoeuvred in such a way that the load of the impact is shared as evenly as possible across the fendering equipment 26. The fendering equipment 26 may equally be provided on and travel with the delivery or receiving vessel 12 or 14, or be delivered to the ship-to-ship transfer location 16 using a separate support vessel 28 which assists in positioning the fendering equipment 26 on the delivery or receiving vessel 12 or 14 or between the two vessels.

After the two vessels have completed the approach, the delivery vessel 12 is moored to the receiving vessel 14 using a suitable arrangement of mooring lines 31. The arrangement includes spring lines 32, stern lines 34 and bow lines 36. Once moored together, the two vessels either continue underway or are allowed to drift together or are anchored, during the ship-to-ship transfer operation. In one preferred embodiment of the present invention, the vessels are allowed to drift after mooring, with the propulsion system of the smaller vessel turned off. The propulsion system of the larger vessel is kept on to make minor adjustments if needed to keep both vessels on course. In this scenario, the arrangement of mooring lines 31 includes sufficient mooring lines 31 to allow the larger vessel to effectively tow the smaller vessel around whilst drifting.

Transfer of LNG from the delivery vessel 12 to the receiving vessel 14 can be conducted while the two vessels are underway, drifting, or at anchor depending on such relevant factors as the weather, sea currents, the relative sizes of the vessels, the arrangement mooring lines and fenders, the type of manifold configuration that will allow the LNG transfer system to be connected, the motion characteristics in certain sea states and the manoeuvring characteristics. Ballasting devices (not shown) can be used to ensure that the freeboard of the receiving vessel 14 is maintained substantially the same as that of the delivery vessel 12 during LNG transfer operations if desired.

Ship-to-ship transfer of LNG from the delivery vessel 12 to the receiving vessel 14 is conducted using any suitable LNG transfer equipment 40, for example, the system for offshore transfer of LNG described in US Patent 6,637,479, which system comprises a coupling head mounted at one end of a flexible pipe means and arranged for attachment on a platform at one end of one vessel when it is not in use, and a connection unit mounted at one end of the other vessel and comprising a pull-in funnel shaped for guided pull-in of the coupling head to a locking position in which the pipe means can be connected to transfer pipes on the other vessel via a valve means arranged in the coupling head. The coupling head is provided with a guide means and is connected to at least one pull-in wire for guided pull-in of the coupling head into the connection unit by a winch means on the other vessel. The system of US Patent 6,637,479 is constructed and dimensioned for a normal rate of transfer of 10,000 m³ LNG per hour via a flexible pipe or other LNG transfer means mounted on the quarter-deck of the LNG vessel. Because of the fact that the primary couplers will be subjected to strong icing at the extremely low temperature (-163⁰C) in the cryogenic transfer, it is desirable to have an emergency disconnection system ensuring quick disconnection of in an emergency situation. The LNG transfer equipment 40 or hoses are provided on either the delivery or receiving vessels 12 or 14, respectively. The LNG transfer equipment 40 can equally be delivered to the ship-to-ship transfer location 16 using a support vessel, either the same support vessel as the support vessel that delivers the fendering equipment or another support vessel. During transfer of the LNG from the delivery vessel 12 to the receiving vessel 14, the transfer rate is initially set at a slow rate until it is confirmed that flow has been established without leaks, with the transfer rate then being increased to a higher rate. Depending on the length of the run and the rate of transfer of the LNG from the delivery vessel 12 to the receiving vessel 14, it may be necessary for the moored vessels to be turned around during the transfer operation. In any event, if a long enough run is not available at a particular ship-to-ship transfer location, the vessels can be caused to travel in circular pattern during the transfer operation.

LNG is stored aboard the delivery vessel 12 in one or more cryogenic storage tanks 42. The receiving vessel 14 is similarly provided with one or more cryogenic storage tank(s) 46 (best seen in Figure 5). After the receiving vessel 14 moors alongside delivery vessel 12, LNG from the storage tank(s) 42 aboard the delivery vessel 12 is transferred into the storage tank(s) 46 aboard the receiving vessel 14.

There are four principal types of LNG storage tanks designed for use on an LNG vessel and these are loosely categorized as self-supporting types or membrane types. The most common type of self-supporting tanks are the spherical aluminium tanks referred to in the art as a "Moss tank" and the prismatic self-supporting tanks developed by Ishikawajima-Harima Heavy Industries ("IHI"). The most well known types of membrane tank are the TGZ Mark III developed by Technigaz which includes a stainless steel membrane with 'waffles' to absorb the thermal contraction when the tank is cooled down, and the GT NO96 developed by Gaz Transport which consists of a primary and secondary thin membranes made of the iron-nickel alloy FeNi36, which has almost no thermal contraction. The insulation is constructed of plywood boxes filled with a lightweight insulating material such as perlite. Non- limiting examples of suitable storage systems include the cryogenic storage systems described in US Patent 6,378,722; US Patent 6,263,818; US Patent 5,727,492; US Patent 5,529,239; or US Patent 5,099,779, the contents of which are incorporated herein by reference.

In a preferred embodiment, the receiving vessel 14 has a supporting hull structure 44 capable of withstanding the loads imposed from intermediate filling levels when the receiving vessel 14 is subject to harsh, multi-directional environmental conditions. The storage tank(s) 46 onboard the receiving vessel 14 are robust to or reduce sloshing of the LNG when the storage tanks 46 are partly filled or when the receiving vessel 14 is riding out a storm whilst positioned at a regasification location 20 whilst transferring natural gas to the onshore distribution facility 62. To reduce the effects of sloshing, the storage tank(s) 46 can be provided with a plurality of internal baffles and/or a reinforced membrane with SPB type B membrane tanks being a preferred option. Self supporting spherical cryogenic storage tanks, for example Moss type tanks, are not considered to be suitable if the receiving vessel 14 is fitted with an onboard regasification facility 30, as Moss tanks reduce the deck area available to position the regasification facility 30 on the deck of the receiving vessel 14.

The holding capacity of the storage tank(s) 46 onboard the receiving vessel 14 may be the same, similar to or greater than the holding capacity of the storage tank(s) 42 aboard the delivery vessel 12, so that the entire payload of LNG onboard the delivery vessel 12 can be transferred to the receiving vessel 14. Whilst it is preferable to empty the delivery vessel 12 in a single ship-to-ship transfer operation, it is equally possible for the holding capacity of the storage tank(s) 46 onboard the receiving vessel 14 to be greater than the holding capacity of the storage tank(s) 42 aboard the delivery vessel 12. In this scenario, the receiving vessel 14 may receive LNG from more than one delivery vessel 12.

In one embodiment, the receiving vessel 14 is provided with 4 or 7 storage tanks, each storage tank having a gross storage capacity in the range of 30,000 to 50,000m³. Thus transfer volumes are in the range of 125,000 - 220,000m³ depending on the relative size of the storage tanks onboard the two vessels. There is no need for the storage tanks 42 to be designed to be robust to sloshing but this is considered advantageous to better withstand the forces generated as the level of LNG in the storage tank(s) 42 is reduced during the ship-to-ship transfer.

After offloading of the LNG from the delivery vessel 12 to the receiving vessel 14 is complete, the receiving vessel 14 is unmoored and unberthed from the delivery vessel 12 and travels under its own steam from the ship-to-ship transfer location 16 to a regasification location 20 closer to the shore 60. In the preferred embodiments of the present invention, the receiving vessel 14 is provided with an onboard regasification facility 30 and the LNG stored onboard the receiving vessel 14 is regasified onboard the receiving vessel 14 to form natural gas (NG) which is then transferred to an onshore gas distribution facility 62 as described in greater detail below.

With reference to the embodiment illustrated in Figure 5, the natural gas that is produced in the regasification facility 30 provided onboard the receiving vessel 14 is transferred via the gas delivery line 72 to the turret mooring buoy 64. When the receiving vessel 14 connects to the mooring buoy 64 it becomes moored at the regasification location 20. A flexible marine riser(s) 66 (best seen in Figure 5) is used to transfer natural gas from the regasification facility 30 to the onshore gas distribution facility 62. The marine riser(s) 66 is fluidly connected at its upper end to the turret mooring buoy 64 and is fluidly connected at its lower end to a sub-sea pipeline(s) 68 that travels across a beach crossing 70 to the onshore gas distribution facility 62.

With reference to the embodiment illustrated in Figure 5, the receiving vessel 14 is designed or retrofitted to include a recess or "moonpool" 74 to facilitate docking of the receiving vessel 14 with an internal turret mooring buoy 64. This is done in a manner that permits the receiving vessel 14 to weathervane around the turret mooring buoy 64. To facilitate weathervaning, the moonpool 74 is positioned towards the bow 58 of the receiving vessel 14. An example of a suitable type of turret mooring systems is described in US Patent 6,688,114, the contents of which are incorporated herein by reference.

The mooring buoy 64 is moored by anchor lines 76 to the seabed 78. The mooring buoy 64 is provided with one or more marine risers 66 which serve as conduits for the delivery of regasified natural gas through the mooring buoy 64 to the sub-sea pipeline(s) 68. Fluid connections are made between the inlet of the marine risers

66 and a gas delivery line 72 from the regasification facility 30 onboard the receiving vessel 14. A rigid arm connection over the bow of the receiving vessel to a riser turret mooring could equally be used, but is not preferred. To allow the receiving vessel 14 to pick up the mooring buoy 64 without assistance, the receiving vessel

14 is highly manoeuvrable.

Several companies have developed patented processes related to suitable mooring systems. These companies include the following: Single Buoy Moorings lnc (US Patent 6,811 ,355, US Patent 6,692,192; US Patent 6,623,043; US Patent 6,623,043; US Patent 6,517,290); Bluewater Terminal Systems N.V. (US Patent 6,354,376; US Patent 6,244,920; US Patent 6,109,830; US Patent 5,944,840; US Patent 5,584,607); SOFEC, lnc (US Patent 5,292,271 ; US Patent 5,240,466; US Patent 5129848; US Patent 5372531 ; US Patent 5356321; US Patent 5316509; US Patent 5306186); and FMC Technologies (US patent application number 20040094082, 20040025772 and 20030226487). All of these patents are incorporated herein by reference.

With reference to Figure 5, the regasification facility 30 is disposed on the deck 56 of the receiving vessel 14. A high pressure onboard piping system 33 is used to convey LNG from the storage tanks 46 to the regasification facility 30 via at least one cryogenic pump 80 per tank. Examples of suitable cryogenic pumps include a centrifugal pump, a positive-displacement pumps, a screw pump, a velocity-head pump, a rotary pump, a gear pump, a plunger pump, a piston pump, a vane pump, a radial-plunger pumps, a swash-plate pump, a smooth flow pump, a pulsating flow pump, or other pumps that meet the discharge head and flow rate requirements of the vaporizers. The capacity of the pump is selected based upon the type and quantity of vaporizers installed, the surface area and efficiency of the vaporizers and the degree of redundancy desired. They are also sized such that the receiving vessel 14 can discharge its cargo at a conventional import terminal at a rate of 10,00um7hr (nominal) with a peak of 12,000m³/hr.

With reference to Figure 6, the regasification facility 30 includes at least one vaporizer 82 for regasifying LNG to natural gas. LNG is regasified in the vaporiser 82 and exits the vaporiser 82 in the form of natural gas (NG). The heat necessary for onboard regasification can come from a number of possible sources. Vaporizers that use ambient air as the source of heat are preferred to reduce environmental impact and to keep emissions of nitrous oxide, sulphur dioxide, carbon dioxide and particulate matter to a minimum. To take advantage of the heating capacity of ambient air, the vaporiser 82 is preferably arranged on the deck 56 of the receiving vessel 14. Ambient air may be used as the primary or only source of heat for regasification of the LNG or can be used in combination with a secondary source of heat.

The secondary source of heat can be used either for regasification of the LNG during cold weather or to superheat the natural gas that has already been regasified. Suitable secondary sources of heat include waste heat recovery from the propulsion system, steam from a boiler or other source, a submerged combustion vaporizer, solar energy, electric water heaters using the excess electric generating capacity of the propulsion plant when the receiving vessel 14 is moored, exhaust gas heat exchangers fitted to the combustion exhausts of the diesel engines and gas turbines, or natural gas fired hot water or thermal oil heaters. The secondary source can equally be generated by direct firing of fuel gas when additional heat is needed. The secondary source of heat may equally be used to heat an intermediate fluid that exchanges heat with the LNG or the natural gas. Suitable intermediate fluids are glycol, propane, salt water or fresh water or any other fluid with an acceptable heat capacity and boiling point that is commonly known to a person skilled in the art. The vaporizer 82 can be arranged such that ambient air exchanges heat directly with the LNG or the ambient air can be used to heat an intermediate fluid in a separate heat exchanger 84, with the heated intermediate fluid then being pumped to the vaporizer where it exchanges heat with the LNG being regasified. In the embodiment illustrated in Figure 3, an intermediate fluid is circulated around a closed loop through the vaporizer 82 using a pump 86. Heat exchange between the intermediate fluid and the LNG results in regasification of the LNG to natural gas and significant cooling of the intermediate fluid. The intermediate fluid is reheated in a heat exchanger 84 using ambient air or another source of heat, for example any of the secondary sources of heat referred to above. This makes it easy to add heat into the intermediate fluid and so limit freezing of vaporizer 82. Heat exchange between the ambient air and the intermediate fluid can be encouraged using one or more forced draft fans 85.

The use of ambient air as a source of heat for onboard regasification has not previously been used or proposed. Existing onshore vaporizers are not necessarily suitable for onboard regasification duty. Modification is made to assure uniform distribution of LNG in the tubes, to remove condensation on the external surfaces of the vaporisers, to accommodate the differential thermal contraction between the LNG and the source of heat for regasification, to control fog that is generated around the vaporisers and to accommodate the added loads from shipboard motions. The materials used for the pumps, vaporizers and piping associated with the onboard regasification facility 30 should be selected to withstand the corrosive effects of seawater. A variety of materials that are suitable for use in marine environments are well known to persons skilled in the relevant art.

The vaporizers are designed to withstand the structural loads associated with being disposed on the deck of an LNG vessel that transits or is moored offshore. The fans for the forced system are similarly designed to withstand the loads associated with the vessel moving while vaporizing during a storm including the loads associated with motions and possibly green water loads. The size and surface area of the vaporizer 82 can vary widely, depending upon the volume and flow rate of LNG being regasified for delivery and the type of heat source being used. When ambient air is being used as a source of heat, the temperature of the ambient air can vary according to the seasons. To provide sufficient surface area for heat exchange, a plurality of vaporizers 82 can be arranged in a variety of configurations, for example in series or in banks. The type of vaporizer could equally be a shell and tube heat exchanger, a finned tube heat exchanger, a bent-tube fixed-tube-sheet exchanger, a spiral tube exchanger, a plate-type heat exchanger, an intermediate fluid vaporizer, a submerged combustion vaporizer or any other heat exchanger commonly known by those skilled in the art that meets the temperature, volumetric and heat absorption requirements for LNG to be regasified.

Now that several embodiments of the invention have been described in detail, it will be apparent to persons skilled in the relevant art that numerous variations and modifications can be made without departing from the basic inventive concepts. All such modifications and variations are considered to be within the scope of the present invention, the nature of which is to be determined from the foregoing description and the appended claims.

All of the patents cited in this specification, are herein incorporated by reference. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents forms part of the common general knowledge in the art, in Australia or in any other country. In the summary of the invention, the description and claims which follow, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

Claims

Claims defining the invention:

1. A method for offshore regasification of liquid natural gas (LNG) for delivery onshore as a gas, the method comprising: offloading LNG from a delivery vessel to a receiving vessel at a ship- to-ship transfer location; moving the receiving vessel under its own power from the ship-to-ship transfer location to a mooring location, the mooring location being closer to shore than the ship-to-ship transfer location; regasifying the LNG using a regasification facility onboard the receiving vessel to form natural gas at the mooring location; and, transferring the natural gas to an onshore gas distribution facility for delivery to an end user.

2. The method of claim 1 further comprising the step of mooring the delivery vessel to the receiving vessel prior to the step of offloading LNG from the delivery vessel to the receiving vessel,

3. The method of claim 2 wherein the less manoeuvrable of the delivery vessel or the receiving vessel is underway at a pre-agreed speed, with a heading selected such that the less manoeuvrable of the two vessels has the dominant swell on the aft port quarter and the heading is maintained until the delivery vessel has been moored to the receiving vessel.

4. The method of claim 2 wherein the less manoeuvrable of the delivery vessel or the receiving vessel is underway at a pre-agreed speed, with a heading selected such that the more manoeuvrable of the two vessels is heading directly into the prevailing weather and the heading is maintained until the delivery vessel has been moored to the receiving vessel.

5. The method of claim 1 wherein the delivery and receiving vessels are moored together and are underway whilst traveling side-by-side when LNG is offloaded from the delivery vessel to the receiving vessel.

6. The method of claim 1 wherein the delivery and receiving vessels are moored together and drifting side-by-side when LNG is offloaded from the delivery vessel to the receiving vessel.

7. The method of claim 1 wherein the delivery and receiving vessels are traveling in tandem when LNG is offloaded from the delivery vessel to the receiving vessel by tandem offloading.

8. The method of any one of the preceding claims wherein the LNG is stored in the hull of the receiving vessel in slosh tolerant tanks and the receiving vessel has an associated supporting hull structure.

9. The method of any one of the preceding claims wherein the receiving vessel has reduced engine capacity compared with the engine capacity of the delivery vessel.

10. The method of any one of the preceding claims wherein the receiving vessel has a propulsion system and the propulsion system comprises dual fuel gas turbines, dual fuel diesel, or duel fuel diesel-electric systems.

11. The method of any one of the preceding claims wherein the power requirement for the propulsion system of the receiving vessel is shared with the power requirement for regasification onboard the receiving vessel.

12. The method of any one of the preceding claims wherein the propulsion system of the receiving vessel is fitted with twin screw, fixed pitch propellers with transverse thrusters located both forward and aft that provide the receiving vessel with superior mooring and position capability compared with the delivery vessel.

13. The method of any one of the preceding claims wherein the mooring location includes a submersible, disconnectable mooring buoy for mooring the receiving vessel at the mooring location, the mooring buoy being locatable within a recess disposed within the hull and towards the bow of the receiving vessel.

14. The method of any one of the preceding claims wherein the LNG is regasified using ambient air as a source of heat to the onboard regasification facility.

15. The method of any one of the preceding claims wherein the LNG is regasified through heat exchange with an intermediate fluid and the intermediate fluid is heated using ambient air as a source of heat.

16. The method of any one of the preceding claims wherein heat exchange between the ambient air and the LNG or intermediate fluid is encouraged through use of forced draft fans

17. A receiving vessel for offshore regasification of liquid natural gas (LNG) for delivery onshore as a gas, the system comprising: a propulsion system for moving the receiving vessel under its own power to and from a ship-to-ship transfer location for receiving LNG from a delivery vessel, to a mooring location, the mooring location being closer to shore than the ship-to-ship transfer location; a regasification facility onboard the receiving vessel for regasifying the LNG to form natural gas; and, an onshore gas distribution facility for receiving the natural gas for delivery to an end user.

18. The vessel of claim 17 wherein the LNG is stored in the hull of the receiving vessel in slosh tolerant tanks and the receiving vessel has an associated supporting hull structure.

19. The vessel of claim 17 or 18 wherein the receiving vessel has reduced engine capacity compared with the engine capacity of the delivery vessel.

20. The vessel of any one of claims 17 to 19 wherein the propulsion system of the receiving vessel comprises dual fuel gas turbines, dual fuel diesel, or duel fuel diesel-electric systems.

21. The vessel of any one of claims 17 to 20 wherein the power requirement for the propulsion system of the receiving vessel is shared with the power requirement for regasification onboard the receiving vessel.

22. The vessel of any one of claims 17 to 20 wherein the propulsion system of the receiving vessel is fitted with twin screw, fixed pitch propellers with transverse thrusters located both forward and aft that provide the receiving vessel with superior mooring and position capability compared with the delivery vessel.

23. The vessel of any one of claims 17 to 22 wherein the receiving vessel includes a recess disposed within the hull and towards the bow of the receiving vessel for receiving a submersible, disconnectable mooring buoy for mooring the receiving vessel at the regasification location.

24. The vessel of any one of claims 17 to 23 wherein the onboard regasification facility uses ambient air as a source of heat.

25. The vessel of any one of claims 17 to 24 wherein the LNG is regasified through heat exchange with an intermediate fluid and the intermediate fluid is heated using ambient air as a source of heat to the onboard regasification facility.

26. The vessel of any one of claims 17 to 25 wherein heat exchange between the ambient air and the LNG or between ambient air and an intermediate fluid is encouraged through use of forced draft fans.