WO2016001115A1

WO2016001115A1 - System and method for off-shore storing and transporting a conditioned hydrocarbon liquid

Info

Publication number: WO2016001115A1
Application number: PCT/EP2015/064650
Authority: WO
Inventors: David John MELTON; Stefan Adrianus ROVERS
Original assignee: Shell Internationale Research Maatschappij B.V.; Shell Oil Company
Priority date: 2014-06-30
Filing date: 2015-06-29
Publication date: 2016-01-07

Abstract

A platform supply vessel is employed for off-shore storing and transporting the conditioned hydrocarbon liquid to an off-shore location such as an off-shore structure. Such platform supply vessel comprises a hull enclosing a hull space, and a cargo deck exposed upward facing open air whereby at least part of the hull space is vertically below the cargo deck, and a propulsion system. A fuel tank is configured within the hull space and is fluidly connected to the propulsion system and operable to provide a supply of fuel to the propulsion system. At least one cryogenic liquid cargo tank is configured within said hull space in addition to the fuel tank. The cryogenic liquid cargo tank has a space for holding the conditioned hydrocarbon liquid under vapour pressure. The cryogenic liquid cargo tank is functionally connected to a refrigeration system to control said vapour pressure.

Description

SYSTEM AND METHOD FOR OFF-SHORE STORING AND TRANSPORTING A CONDITIONED HYDROCARBON LIQUID

The present invention relates to system and method for off-shore storing and transporting a conditioned hydrocarbon liquid.

The storage, transport and transfer of cryogenic hydrocarbon fluids on and between off-shore vessels and structures present a variety of technical and logistical problems associated with one or more of the storage, the transport and the transfer of the fluids. For instance, the fluid itself may require special considerations, for example because it may be hazardous, due to its chemical nature, such as flammability, and/or it may be have particular handling requirements, for example if it is a cryogenic fluid. The off-shore structures may be those used in the production of hydrocarbons from natural hydrocarbon reservoirs, such as off-shore oil and natural gas reservoirs.

One important fluid to be stored and supplied, particularly to a floating structure, is a refrigerant substance, which can be used in a variety of cooling processes. Such a refrigerant substance may be a hydrocarbon fluid. Hydrocarbon refrigerant substances, particularly single component hydrocarbon refrigerant substances such as ethane and propane, are useful in the liquefaction of natural gas.

Natural gas is a useful fuel source, as well as being a source of various hydrocarbon compounds. It is not unusual to liquefy natural gas in a liquefied natural gas (LNG) plant at or near the source of a natural gas stream to form a liquefied natural gas stream. Liquefied natural gas can be stored in cryogenic tanks, and/or transported in carrier vessels, at atmospheric pressure and a temperature of about -162 °C.

Usually, natural gas, comprising predominantly methane, enters an LNG plant at elevated pressures and is pre-treated to produce a purified feed stream suitable for liquefaction at cryogenic temperatures. The purified gas is processed through a plurality of cooling stages where it is cooled against refrigerants in heat

exchangers to progressively reduce its temperature until liquefaction is achieved. The liquid natural gas is then further cooled and expanded to final atmospheric

pressure, suitable for storage and transportation.

Traditionally, the liquefaction of natural gas, together with any necessary pre-treatment, is carried out in an on-shore facility. The natural gas must be transported from the natural hydrocarbon reservoir where it extracted to the liquefaction and optionally pre- treatment facility. Hydrocarbon reservoirs producing natural gas may be found off-shore. An off-shore structure, preferably a floating structure, configured for processing of natural gas is advantageous because it provides an off-shore alternative to on-shore

liquefaction plants. A floating structure for the liquefaction of natural gas can be moored off-shore, close to or at a gas field, in waters deep enough to allow off-loading of the LNG product onto a carrier vessel .

Examples of such floating structures include a

Floating Liquefaction Storage Off-shore (FLSO) facility which combines the natural gas liquefaction process, storage tanks, loading systems and other infrastructure into a single floating structure. A Floating

Liquefaction of Natural Gas (FLNG) facility or a Floating Production, Liquefaction, Storage and Off-loading (FPLSO) facility further contains other processing capabilities necessary to prepare the natural gas being produced from the natural hydrocarbon reservoir for the liquefaction process .

The natural gas liquefaction process, including processes operated on-shore or off-shore (e.g. aboard a floating structure), often utilise a refrigerant to reduce the temperature of the natural gas to cryogenic temperatures. The refrigerant can consist of a single component hydrocarbon refrigerant substance or of a mixture of single component hydrocarbon refrigerant substances, such that the refrigerant contains a

plurality of refrigerant components. The natural gas itself may provide a source of such hydrocarbon

refrigerant substances, such that they can be produced as part of the liquefaction process. However, not all hydrocarbon reservoirs contain natural gas with

sufficient concentrations of heavier hydrocarbons, such as ethane and propane, to provide them in sufficient quantities for use as refrigerant components. Even if the natural gas does contain the necessary amounts of heavier hydrocarbons, it would require installation and operation of process equipment to extract the refrigerant substances from the natural gas and to prepare them for use as refrigerant . Available space on an off-shore and/or floating structure is limited. In either case, the installation and operation of process equipment to extract and process the refrigerant substances can be avoided by importing the refrigerant substances from an external location, thereby freeing up space which can be put to other uses. Consequently, a need exits for means to provide from a location external to the off-shore structure one or more hydrocarbons suitable for use as refrigerant substances in the refrigeration cycle of a liquefaction facility internal to the off-shore structure.

US Patent Application Publication No. 2011/0132033 discloses a process for conditioning an ethane-rich stream for storage and transportation. This document discloses that the conditioned ethane-rich stream may be transported using a pipeline, a truck, a rail car, or tanker ship. In embodiments, the pipeline, truck, rail car, or tanker ship may be one which is normally

configured to transport LNG, CNG, NGLs, or any other type of hydrocarbons. The vessels that contain the

conditioned ethane on the transport vehicles may be insulated to minimize heat ingress and ethane boil-off. In some cases, the transport vehicles may be equipped with recycle systems and cooling systems to re-liquefy the boiled-off ethane vapour.

For off-shore locations, truck and rail cars are inappropriate means of transportation, while laying an insulated pipeline for the purpose of supplying cryogenic fluids would be prohibitively expensive. Maritime

Economics, by Martin Stopford (Taylor & Francis 2009, 3^rd Edition), discusses in Chapter 12, section 4, liquefied gas commodities such as liquefied petroleum gas,

typically propane, ethane or butane and olefins such as ethylene and propylene and the transportation of such commodities in gas tankers. These will be referred to as hydrocarbon carrier vessels herein. Chapter 14, section

6 of Maritime Economics discloses that such hydrocarbon carrier vessels can carry their hydrocarbon cargo in refrigerated and/or pressurised tanks. Such vessels may be efficient for the transportation of large quantities of hydrocarbons, but are uneconomic for the transportation of smaller quantities of

hydrocarbons. According to "Maritime Economics", Chapter 14, section 6, hydrocarbon carrier vessels, such as ethylene carriers, which can be repurposed to carry ethane, typically have a minimum cargo size of

approximately 2000 m³. In contrast, the monthly average requirements of a hydrocarbon refrigerant substance for a liquefaction facility may be considerably less than the cargo carrying capacity of such a hydrocarbon carrier vessel, for instance 10% to 15% (and possibly even less) by volume. Thus, the use of hydrocarbon carrier vessels as the transporter may thus not be practical if

quantities of hydrocarbon less than that of a carrier vessel are to be supplied to a floating structure, for instance on a monthly basis.

A need exists to provide an alternative means to store and supply a hydrocarbon refrigerant fluid, more particularly a cryogenic hydrocarbon fluid, in quantities less than the cargo carrying capacity of a carrier vessel .

In accordance with a first aspect of the present invention, there is provided a system for off-shore storing and transporting a conditioned hydrocarbon liquid, comprising a platform supply vessel as for instance defined in Lloyd's Register Classification Society Rules and Regulations, said platform supply vessel comprising a hull enclosing a hull space, and a cargo deck exposed upward facing open air whereby at least part of the hull space is vertically below the cargo deck, and a propulsion system, wherein a fuel tank is configured within the hull space which is fluidly connected to the propulsion system and operable to provide a supply of fuel to the propulsion system, and wherein at least one cryogenic liquid cargo tank is configured within said hull space in addition to the fuel tank, wherein the cryogenic liquid cargo tank has a space for holding the conditioned hydrocarbon liquid under vapour pressure, and wherein cryogenic liquid cargo tank is functionally connected to a refrigeration system to control said vapour pressure.

In accordance with another aspect of the invention, such a system as provided in the first aspect may be used in a method of off-shore storing and transporting a conditioned hydrocarbon liquid, comprising:

- connecting the at least one cryogenic liquid cargo tank to a disconnectable hydrocarbon fluid supply system external to the platform supply vessel;

- transferring a conditioned hydrocarbon liquid stream from the hydrocarbon fluid supply system into the at least one cryogenic liquid cargo tank;

- sealing the cryogenic liquid cargo tank thereby creating the space;

- storing the conditioned hydrocarbon liquid within the space of the cryogenic liquid cargo tank under vapour pressure ;

- disconnecting the hydrocarbon fluid supply system from the platform supply vessel; and subsequently:

- fuelling the propulsion system with a supply of fuel from the fuel tank;

- propelling the platform supply vessel to an off-shore structure using the propulsion system; and

- removing heat from the space of the cryogenic liquid cargo tank during said propelling, with a refrigeration system, to compensate for heat ingress from ambient into the cryogenic liquid cargo tank thereby controlling the vapour pressure.

In accordance with an aspect of the present

invention, there is provided a method of off-shore storing and transporting a conditioned hydrocarbon liquid, comprising:

- providing a platform supply vessel as for instance defined in Lloyd's Register Classification Society Rules and Regulations, said platform supply vessel comprising a hull enclosing a hull space, and a cargo deck exposed upward facing open air whereby at least part of the hull space is vertically below the cargo deck, and a

propulsion system, wherein a fuel tank is configured within the hull space which is fluidly connected to the propulsion system and operable to provide a supply of fuel to the propulsion system, and wherein at least one cryogenic liquid cargo tank is configured within said hull space in addition to the fuel tank, wherein the cryogenic liquid cargo tank has a space for holding the conditioned hydrocarbon liquid, and wherein cryogenic liquid cargo tank is functionally connected to a

refrigeration system;

- sealing the cryogenic liquid cargo tank thereby creating the space;

- storing the conditioned hydrocarbon liquid within the space of the cryogenic liquid cargo tank under vapour pressure ; - disconnecting the hydrocarbon fluid supply system from the platform supply vessel; and subsequently:

- fuelling the propulsion system with a supply of fuel from the fuel tank;

The space for holding the conditioned hydrocarbon liquid under vapour pressure may also be referred to in this text as being an enclosed or fully enclosed space. The skilled person will understand that the term

"enclosed" or "fully enclosed" are used to indicate that the space is arranged for holding the conditioned hydrocarbon liquid under vapour pressure and doesn't exclude the cryogenic liquid cargo tank from being connected to the refrigeration system and doesn't exclude the option of transferring a conditioned hydrocarbon liquid stream from or into the at least one cryogenic liquid cargo tank.

The invention will be further illustrated hereinafter by way of example only, and with reference to the non- limiting drawing in which;

Fig. 1 schematically shows a cut-away side view of a platform supply vessel comprising at least one cryogenic liquid cargo tank inside the hull space;

Fig. 2 schematically shows a top view of the platform supply vessel of Fig. 1;

Fig. 3 schematically shows a cut-away top view of the platform supply vessel of Fig. 1; Fig. 4 schematically shows the platform supply vessel of Fig. 1 during a loading operation;

Fig. 5 schematically shows the platform supply vessel of Fig. 1 during an off-shore offloading operation.

For the purpose of this description, a single reference number will be assigned to a line as well as a stream carried in that line. Same reference numbers refer to similar components. The person skilled in the art will readily understand that, while the invention is illustrated making reference to one or more a specific combinations of features and measures, many of those features and measures are functionally independent from other features and measures such that they can be equally or similarly applied independently in other embodiments or combinations.

In the present disclosure it is proposed to employ a platform supply vessel as for instance defined in Lloyd's Register Classification Society Rules and Regulations for off-shore storing and transporting the conditioned hydrocarbon liquid to an off-shore location such as an off-shore structure. Such platform supply vessel comprises a hull enclosing a hull space, and a cargo deck exposed upward facing open air whereby at least part of the hull space is vertically below the cargo deck, and a propulsion system. A fuel tank is configured within the hull space and is fluidly connected to the propulsion system and operable to provide a supply of fuel to the propulsion system. At least one cryogenic liquid cargo tank is configured within said hull space in addition to the fuel tank. The cryogenic liquid cargo tank has a

(fully enclosed) space for holding the conditioned hydrocarbon liquid under vapour pressure. The cryogenic liquid cargo tank is functionally connected to a

refrigeration system to control said vapour pressure.

As indicated above, the space for holding the conditioned hydrocarbon liquid under vapour pressure maybe referred to as an enclosed space or fully enclosed space. The term "enclosed" or "fully enclosed" are used in this text to indicate that the space is capable of holding the conditioned hydrocarbon liquid under vapour pressure .

The platform supply vessel may be an existing platform supply vessel modified to comprise the at least one cryogenic liquid cargo tank and the associated refrigeration system. It may also be a platform supply vessel that has been specifically designed to include the at least one cryogenic liquid cargo tank and the

associated refrigeration system.

An advantage of the proposal is that the conditioned hydrocarbon liquid can be stored and transported at near atmospheric pressure (the vapour pressure under which the conditioned hydrocarbon liquid is stored can be less that

0.1 barg) , and at a quantity that is tailored to the demand .

Herewith it is possible to offload the conditioned hydrocarbon liquid at low pressure, without the need for pressure reduction at arrival. This is advantageous as pressure reduction typically requires subcooling of the hydrocarbon liquid to avoid the pressure reduction causing evaporation. Furthermore, the quantity can be tailored for demand, whereby partial offloading and transporting with half-filled cargo tanks can be avoided.

The proposed solution is also more cost effective.

It is a possibility to store flammable substances in the hull space of a platform supply vessel, as for instance the fuel for propulsion is typically stored in the hull space, but it is preferred to limit the volume of the conditioned hydrocarbon liquid stored in the at least one cryogenic liquid tank to less than 800 m^.

The conditioned hydrocarbon liquid generally is a cryogenic liquid. As used herein, the term "cryogenic" is intended to mean a fluid at a temperature of lower than -30 °C.

The pressure unit "barg" as used herein is identical to "bar guage". As used herein, the term "conditioned hydrocarbon liquid" is intended to represent a liquid at its bubble point under its own vapour pressure, or below said bubble point.

Platform supply vessel is a term recognized and defined in Lloyd's Register Classification Society Rules and Regulations as Sea-going ships specially designed and constructed for the carriage of specialized stores and cargoes to mobile offshore units and other offshore installations . Sometimes platform supply vessels are referred to as off-shore supply vessel; for the purpose of the present disclosure, off-shore supply vessel is equated to platform supply vessel. Platform supply vessel is often referred to by the three letter acronym PSV.

Typically a PSV has a length over all of in a range of from 70 m to 100 m, and a moulded beam (breath) in a range of from 15 m to 21 m. A PSV is usually equipped with a dynamic positioning system (DPS) . The cargo deck available for loading cargo has a space ("cargo deck space") typically in the range of from 600 m^ to 1275 m^, and/or more than half, such as between 50 and 80 %, of the gross deck space meaning the total area of the vessel's contours in vertical projection. The cargo deck is exposed upward facing open air.

Figure 1 shows a schematic side view of a platform supply vessel 1 adopted for use in the present invention. The platform supply vessel 1 comprises a hull 2 enclosing a hull space 4. The hull space 4 is made visible in this particular view. A cargo deck 3 is exposed upward facing open air. At least part of the hull space 4 is

vertically below the cargo deck 3. The cargo deck 3 is perhaps better viewed in Figure 2, which shows a top view of the platform supply vessel. The cargo deck 3 of the platform supply vessel 1 may be circumferenced by an upward extending part of the hull material such that it is recessed by a few meters, typically by between 4 m and 9 m, relative to an upper rim 6 of the hull steel. An optional railing 7 may be mounted on top of the upper rim 6. Typically crew quarters including a bridge or control station 8 in this type of vessel is arranged at the bow side of the cargo deck 3. Cargo handling systems such as a crane 9 may optionally be provided.

The platform supply vessel 1 further comprises a propulsion system 5. The propulsion system typically comprises a screw or the like driven by an engine or a motor. In the embodiment of Figure 1, the platform supply vessel 1 further comprises aft thrusters and bow thrusters 18, which may be used to position the platform supply vessel 1 with a dynamic positioning system.

At least one cryogenic liquid cargo tank, represented in the Figure as first cryogenic liquid cargo tank 10, is configured within the hull space 4 the first cryogenic liquid cargo tank 10 has a (fully enclosed) space 11 for holding conditioned hydrocarbon liquid under vapour pressure. Preferably the At least one cryogenic liquid cargo tank is firmly secured to the platform supply vessel 1. The first cryogenic liquid cargo tank 10 is functionally connected to a refrigeration system 12, to control the vapour pressure.

The refrigeration system 12 may be provided in any suitable form. For instance, it may comprise a closed refrigeration cycle driven by a compressor and using a sea water-cooled refrigerant condenser. The choice of the refrigerant depends on the composition of the conditioned hydrocarbon liquid, which determines the evaporation temperature of the refrigerant, and the maximum available pressure, which determines whether the refrigerant can be condensed by the sea water.

Alternatively a cascaded refrigeration scheme may be employed, or a vapour refrigeration cycle which does not require condensation. Suitably refrigeration systems exist within the LPG shipping industry.

Each cryogenic liquid cargo tank of the at least one cryogenic liquid cargo tank is suitably a small IMO type C containment system. IMO stands for International Maritime Organization. Such IMO type C containment system is a self-supported independent cargo tank of which the structural integrity does not rely on the ship's strength. Large IMO type C containment systems with cargo volume of well over 10,000 m^ are commercially in use on large LNG carriers. Smaller versions, for instance having a capacity of 500 m^ have been proposed for storing LNG fuel on large LNG powered ships such as a crude oil carrier.

If the at least one cryogenic liquid cargo tank is a pressure shell, regardless whether it is an IMO type C containment system or another type of pressure shell, for in-hull storage of the conditioned hydrocarbon liquid in a platform supply vessel it is preferred that the at least one cryogenic liquid cargo tank is provided with a pressure relief valve 14 having a set pressure in a range of from 0.2 barg to 0.4 barg to atmosphere. This is generally much lower than the maximum design pressure for a pressure shell, but it is the aim to avoid hazards associated with pressurized storage inside the hull space 4. The conditioned hydrocarbon fluid, is

preferably conditioned such that it is in the liquid state at a vapour pressure higher than 0.0 barg but lower than 0.1 barg. It will be apparent that conditioned hydrocarbons such as ethane or propane in the liquid state at a pressure in the range of between 0.0 barg and 0.1 barg are cryogenic fluids.

Figure 3 shows a top view of the platform supply vessel cut away to provide a view underneath the cargo deck 3. The at least one cryogenic liquid tank can consist of a plurality of cryogenic liquid cargo tanks, such as two. In the example of Figure 3 at least one cryogenic liquid cargo tank consists of two cryogenic liquid cargo tanks : the first cryogenic liquid cargo tank and a second cryogenic liquid cargo tank 10'. One cryogenic liquid cargo tank may have cargo capacity in a range of from 200 m^ to 500 m^ . All of the cryogenic liquid cargo tanks taken together may represent a total cargo capacity in the cryogenic liquid cargo tanks of in a range of from 400 m^ to 800 m^ .

The at least one cryogenic liquid cargo tank 10 is in fluid communication with an offloading system 17 that is configured on the platform supply vessel 1. Such offloading system 17 may comprise an offloading line 61 fluidly connected with a connection interface to connect to an external offloading conduit. Suitably the connection interface is a hose connection interface 3, which can be selectively coupled to or uncoupled from a flexible cryogenic hose, as will be discussed in more detail later below with reference to Fig. 5. The at least one cryogenic liquid cargo tank 10 preferably comprises a submerged pump 16 configured inside the (fully enclosed) space 11, to pump the conditioned hydrocarbon liquid out of the (fully enclosed) space into the offloading line 61. The submerged pump 16 is suitably connected to the offloading line 16.

In addition to the at least one cryogenic liquid cargo tank, a fuel tank 13 is configured within the hull space 4. The fuel tank 13 is fluidly connected to the propulsion system 5, and operable to provide a supply of fuel to the propulsion system 5.

Finally, the platform supply vessel 1 may be

configured with one or more associated systems including inert gas supply for gassing up, cooling down, gas freeing and possibilities to purge or aerate the at least one cryogenic liquid cargo tank 10,10'.

The conditioned hydrocarbon liquid may consist for at least 95 mol%, preferably for at least 98 mol%, of a single hydrocarbon component. Examples of single hydrocarbon components are ethane, ethylene, propane, propylene, butane/isobutene, pentane/isopentane . These may be suitable for use as a single component hydrocarbon refrigerant substance without further fractionation. As used herein, the single hydrocarbon component is the component that constitutes the highest proportion, by mol%, of the conditioned hydrocarbon liquid.

Further components, in addition to the single hydrocarbon components may be present in the liquid, such as other hydrocarbons, carbon dioxide, mercury and/or water. Preferably, the further components do not preclude the operation of the conditioned hydrocarbon liquid in a refrigerant at the temperature and pressure under which the refrigerant is intended to be used. In this respect, particularly when used in aluminium-based heat exchangers, it is preferred that the conditioned hydrocarbon liquid comprises less than 50 ng/m^,

preferably less than 10 ng/m^, and more preferably less than 5 ng/m^ mercury. Particularly, when used at temperatures below -120 °C to liquefy natural gas of which more than 80 mol% consists methane, it is preferred that the conditioned hydrocarbon liquid contains no more than 2 ppmmol of water, preferably no more than 1 ppmmol of water, and no more than 150 ppmmol, preferably no more than 50 ppmmol of carbon dioxide.

The platform supply vessel may be employed in a method of off-shore storing and transporting the

conditioned hydrocarbon liquid. First the at least one cryogenic liquid cargo tank 10,10' is to be loaded with a charge of the conditioned hydrocarbon liquid. To this end the at least one cryogenic liquid cargo tank 10,10' is connected to a disconnectable hydrocarbon fluid supply system 40 external to the platform supply vessel 1.

Suitably, the disconnectable hydrocarbon fluid supply system 40 is located on or adjacent to a mooring facility inside a service port or another appropriate location. The mooring facility may comprise a jetty 42. The disconnectable hydrocarbon fluid supply system 40 may suitably comprise a loading boom 44, holding loading conduits 46,46' such as flexible hoses or hard arms to transfer the conditioned hydrocarbon liquid from a source 48 to the at least one cryogenic liquid cargo tank

10,10'. Flexible hoses are preferred over hard arms. The conditioned hydrocarbon liquid stream may thus be safely transferred from the hydrocarbon fluid supply system 48 into the at least one cryogenic liquid cargo tank 10,10'. Preferably, each cryogenic liquid cargo tank 10,10' is connected to its own dedicated loading conduit 46,46' to avoid cross contamination between the respective

conditioned hydrocarbon liquids being transferred to each of the cryogenic liquid cargo tanks 10,10'. This is shown schematically in Fig. 4.

Preferably, flexible composite hoses are employed as loading conduit 46,46'. Such flexible composite hoses may meet international standards including one or more, preferably all of: EN 13766; EN 1474-2; EN 1474-3; IMO IGC code. Such flexible composite hoses are commercially available. As example, reference is made to Multi-LPG

White, a composite hose available from Gutteling B.V. designed for use with fully refrigerated conveyants on ship, barges and in marine terminals. This hose is suitable for a variety of conveyants, including is including: liquid ethane at -88°C, liquid ethylene at

-105 °C, liquid propane, liquid propylene, liquid butane, and liquid butylene .

An emergency shutdown valve (not shown) may be provided in each loading conduit 46,46' at the base 41 of the loading boom 44, as well as downstream of the loading manifold 47 on the platform supply vessel 1. Herewith the volume of unintentional hydrocarbon fluid spills may be limited in case of loss of containment. Loading pumps (not shown) may be employed to provide the driving force for the transport of the conditioned hydrocarbon liquid streams through the loading conduits 46,46'. Break away couplers may be applied to minimize damage to either side of the loading operation in case of excessive drift of the platform supply vessel 1 away from the mooring facility .

Boil-off gas created during the transfer of the conditioned hydrocarbon liquids may be re-condensed by means of the refrigeration system 12 on board of the platform supply vessel 1. This, however, requires a relatively large refrigeration system 12 which most of the time will not be fully utilized.

In alternative embodiments a boil-off gas disposal conduit 49,49' is provided on each of the at least one cryogenic liquid cargo tank 10,10', to dispose boil-off gas generated during loading mode of the at least one cryogenic liquid cargo tank 10,10'. The boil-off gas disposal conduit 49,49' may be fluidly connected to a boil-off gas disposal system, which is preferably arranged external to the platform supply vessel 1. The boil-off gas disposal system suitably comprises a flare system (not shown) for incinerating the boil-off gas, or any other disposal channel. Optionally, each boil-off gas disposal conduit 49,49' may be provided with a boil- off gas blower on the platform supply vessel 1. The blower may be provided in the form of a compressor. This allows for compressing the boil-off gas on the platform supply vessel prior to disposing. Suitably, the boil-off gas disposal conduit 49,49' extends across the loading boom 44 as a vapour return line. Such boil-off disposal conduit 49,49' may also be provided in embodiments that rely on the refrigeration system 12 for managing the boil-off gas. In such a case there is normally no flow through the boil-off disposal conduit 49,49' during the loading of the at least one cryogenic liquid cargo tank 10,10'. This provides a disposal path to fall back on in case there is a problem with the refrigeration system 12. When a sufficient volume has been transferred, the cryogenic liquid cargo tank 10,10' is suitably sealed, thereby creating the (fully enclosed) spaces 11,11'.

Then the hydrocarbon fluid supply system 40 may be disconnected from the platform supply vessel 1. The conditioned hydrocarbon liquid is stored within the (fully enclosed) space 11,11' of each cryogenic liquid cargo tank 10,10' under vapour pressure. The aim is to maintain a vapour pressure of the conditioned hydrocarbon liquid in the at least one cryogenic liquid cargo tank above 0.0 barg, and below a maximum of 0.1 barg,

preferably below 0.05 barg.

Once successfully disconnected, the platform supply vessel 1 may start to sail to its off-shore destination. The propulsion system 5 is fuelled with a supply of fuel from the fuel tank 13, and the platform supply vessel 1 is propelled to an off-shore structure using the

propulsion system 5. During said propelling, optionally already starting before and/or continuing after the propelling, heat is removed from the (fully enclosed) space 11,11' of each cryogenic liquid cargo tank 10,10', with the refrigeration system 12. In case of multiple cryogenic liquid cargo tanks, a plurality of

refrigeration systems may be configures whereby each of the refrigeration systems removes heat from exclusively one of the cryogenic liquid cargo tanks 10,10'. The refrigeration system 12 removes enough heat from the (fully enclosed) space 11 to compensate for heat ingress from ambient into the cryogenic liquid cargo tank 10. This way the vapour pressure inside the cryogenic liquid cargo tank 10 can be controlled. Preferably, the vapour pressure is controlled such that the vapour pressure is kept above 0.0 barg but never exceeds 0.1 barg, preferably never exceeds 0.05 barg. Thus the amount of heat removed per unit of time is preferably enough to keep the vapour pressure of the conditioned hydrocarbon liquid in the tank within the desired range of between 0.0 barg and 0.1 barg inclusive.

Figure 5 illustrates an offloading operation of the at least one cryogenic liquid cargo tank at the off-shore structure 50. In the embodiment as presented the offshore structure is a floating LNG barge, but the

principles explained apply for other types of off-shore structures in a similar way, preferably off-shore structures comprising a natural gas liquefaction

facility, such as a FLSO, FLNG or FPLSO or other similar structure requiring external supply of one or more refrigerant substances. During the entire offloading operation, the at least one cryogenic liquid cargo tank stays inside the hull space 4.

The platform supply vessel 1 is positioned adjacent to the off-shore structure 50. Depending on local circumstances, the platform supply vessel 1 may be moored to the off-shore structure 50, be anchored to the sea bed, or use its dynamic positioning system (DPS) to maintain a steady position. Suitably an optional data cable 55 is connected.

Then a flexible cryogenic hose 54 is deployed and connected to the hose connection interface 3 of the offloading system 17 available on the platform supply vessel 1. The offloading system 17 further comprises a breakaway coupling 52, provided with a spool side emergency disconnect valve 58 and a hose-side emergency disconnect valve 59. An optional flow regulating valve 60 may be provided as well. Preferably, the flexible cryogenic hose 54 is a floating cryogenic hose. As can be seen in Fig. 5, there is typically a large size difference between the offshore structure 50 and the platform supply vessel 1. The connection points at the off-shore structure 50 and the platform supply vessel could be at significantly

different heights above sea level. Maintaining the flexible cryogenic hose in an aerial hose configuration would result in a steeply angled hose, with relatively little slag due to the low freeboard of the platform supply vessel 1. Due to this little slag, the hose would tension regularly, leading to trips in offloading operation resulting from opening of the breakaway coupling 52. In the embodiment of Fig. 5, the floating cryogenic hose is suitably supported in downward concave bends by guide rails 63,64.

Flexible composite hoses suitable for off-shore tranferring of cryogenic liquids are available on the market, and improved versions are currently under development. For example Gutteling B.V. carries a heavy duty version of Multi-LPG White that is suitable for offshore use. Trelleborg Industrie SAS carries Cryoline LPG and Cryoline LNG hoses for off-shore use. The flexible cryogenic hoses preferably meet international standards, including one or more, preferably all of: EN 13766; EN

1474-2; EN 1474-3; IMO IGC code.

Suitably, the flexible cryogenic hose 54 is deployed from the off-shore structure 50. The flexible cryogenic hose 54 may for instance be deployed from a hose reel 56 arranged on the off-shore structure 50. The rotating hose reel is in fluid communication with fixed piping system and a cryogenic storage tank (not shown) on the off-shore structure 50 via a cryogenic swivel 57. After successful deployment, part of the flexible cryogenic hose 54 floats on the sea 65 between the platform supply vessel 1 and the off-shore structure 50. Then, by means of the submerged pump 16, the conditioned hydrocarbon liquid is transferred from the at least one cryogenic liquid cargo tank 10 through the offloading line 61 of the offloading system 17 and the hose

connection interface 3 into the flexible cryogenic hose 54, whereby pumping the conditioned hydrocarbon liquid out of the (fully enclosed) space 11 of the at least one cryogenic liquid cargo tank 10.

Preferably there is no vapour return line to bring back boil-off gas from the off-shore structure 50 to the platform supply vessel 1. Any boil-off gas generated during the offloading operation is preferably handled by a boil-off gas system provided on the off-shore structure 50. The cryogenic storage tank on the off-shore

structure to which the platform supply vessel 1 is offloading share the same vapour header as the LNG product tanks (in case the off-shore structure has an LNG plant) . Therefore there is a potential of LNG vapour blow back to the cryogenic liquid cargo tank on the platform supply vessel. This scenario is avoided by carrying out the offloading operation without vapour return to the platform supply vessel.

Simultaneous to pumping the conditioned hydrocarbon liquid out of the (fully enclosed) space 11 of the at least one cryogenic liquid cargo tank 10, an amount of the conditioned hydrocarbon liquid is vaporized in a vaporizer 22 that is provided in communication with the

(fully enclosed) space 11 of the at least one cryogenic liquid cargo tank 10. The vaporizer 22 is configured to discharge vapour into the (fully enclosed) space 11. Herewith the under pressure created in the (fully enclosed) space 11 during the offloading operation as a result of volume displacement when transferring the conditioned hydrocarbon liquid can be compensated. The vapour thus generated may be discharged from the

vaporizer 22 into the (fully enclosed) space 11, whereby maintaining a pressure within the enclosed space of higher than 0.0 barg and below 0.2 barg while the hydrocarbon liquid is being pumped out of the at least one cryogenic liquid cargo tank 10. The amount of the conditioned hydrocarbon liquid that needs to be vaporized per unit of time is proportional to the flow rate of the conditioned hydrocarbon liquid that flows through the submerged pump 16.

Preferably, each cryogenic liquid cargo tank is connected to its own dedicated offloading conduit and offloading system, to avoid cross contamination between the respective conditioned hydrocarbon liquids being offloaded .

In one example the platform supply vessel 1 as described above is employed to store and transport two distinct conditioned hydrocarbon liquids from an onshore supply base to the off-shore structure. The off-shore structure is a floating LNG plant. One of the

conditioned hydrocarbon liquids is ethane having the following specification: at least 95 mol% of ethanes (ethane and ethylene) whereby maximally 1.0 mol% of the total composition, preferably maximally 0.5 mol% of the total composition, is ethylene and the remainder of the at least 95 mol% is ethane; maximally 1 mol% of methane; maximally 5 mol% of propanes (propane, propylene, isopropane); maximally 150 ppmmol of carbon dioxide;

maximally 1 ppmmol of water; maximally 50 ng/Nm3 of mercury. The temperature of this conditioned

hydrocarbon liquid is typically between -90 °C and

-86 °C. The other one of the conditioned hydrocarbon liquids is propane having the following specification: at least 98 mol% of propanes (propane, propylene,

isopropane) whereby maximally 8 mol%, preferably

maximally 5 mol%, of the total composition is propylene and the remainder of the 98 mol% is propane and/or isopropane; maximally 1 mol% of methane; maximally 1 mol% of ethanes (ethane and/or ethylene), preferably whereby maximally 0.5 mol% of the total composition is ethylene; maximally 150 ppmmol of carbon dioxide; maximally

2 ppmmol of water; maximally 50 ng/Nm3 of mercury. The temperature of this conditioned hydrocarbon liquid is typically between -44 °C and -40 °C. Although a limited amount of ethylene and/or propylene may be tolerable, for off-shore applications these are preferably avoided.

Therefore, any content of ethylene and/or propylene in either of these conditioned hydrocarbon liquids is as low as reasonably possible.

These hydrocarbon liquids are employed on the offshore structure as refrigerant substances. They are classified as single component hydrocarbon refrigerant substances. Preferably, these single component

refrigerant substances used as ingredients to form mixtures of single component hydrocarbon refrigerant substances, such that the refrigerant contains a

plurality of refrigerant components . Other refrigerant components that may be added to the mixture comprise nitrogen and methane. These refrigerant components may be available on the off-shore structure from local resources such as the feed gas to the liquefied natural gas plant and/or the ambient air. An attractive option for the presently proposed use of the platform supply vessel 1 is presented if the platform supply vessel is fuelled by natural gas. In such a case the fuel tank is suitably a liquefied natural gas tank. The fuel tank may then advantageously be in fluid communication with the refrigeration system of the at least one cryogenic liquid cargo tank, to provide refrigeration to the refrigeration system. Any

additional boil off natural gas created this way may be used for propulsion of the platform supply vessel. In a preferred embodiment, the at least one cryogenic liquid cargo tank may be filled with liquefied natural gas to add additional fuel bunkering capacity when the at least one cryogenic liquid cargo tank is not in use to store and transport conditioned hydrocarbon liquids as cargo to an off-shore structure. This way the at least one cryogenic liquid cargo tank can be kept cold. In this preferred embodiment, a fluid connection between the at least one cryogenic liquid cargo tank and the propulsion system may be provided, either directly or indirectly via the fuel tank.

The person skilled in the art will understand that the present invention can be carried out in many various ways without departing from the scope of the appended claims .

Claims

SP0307 - 26 - C L A I M S

1. A system for off-shore storing and transporting a conditioned hydrocarbon liquid, comprising a platform supply vessel, said platform supply vessel comprising a hull enclosing a hull space, and a cargo deck exposed upward facing open air whereby at least part of the hull space is vertically below the cargo deck, and a

propulsion system, wherein a fuel tank is configured within the hull space which is fluidly connected to the propulsion system and operable to provide a supply of fuel to the propulsion system, and wherein at least one cryogenic liquid cargo tank is configured within said hull space in addition to the fuel tank, wherein the cryogenic liquid cargo tank has a space for holding the conditioned hydrocarbon liquid under vapour pressure, and wherein cryogenic liquid cargo tank is functionally connected to a refrigeration system to control said vapour pressure.

2. The system of claim 1, wherein one cryogenic liquid cargo tank has a cargo capacity in a range of from 200 m³ to 500 m³.

3. The system of claim 1 or 2, wherein at least one cryogenic liquid cargo tank consists of a plurality of cryogenic liquid cargo tanks, preferably two cryogenic liquid cargo tanks . 0

4. The system of claim 3, wherein a total cargo capacity of said plurality of cryogenic liquid cargo tanks is in a range of from 400 m³ to 800 m³.

5. The system of claim 3 or 4, further comprising one offloading system for each cryogenic liquid cargo tank of the plurality of cryogenic liquid cargo tanks, whereby each offloading system is completely segregated from each other offloading system.

6. The system of any one of claims 3 to 5, further comprising one loading system for each cryogenic liquid cargo tank of the plurality of cryogenic liquid cargo tanks, whereby each loading system is completely

segregated from each other loading system.

7. The system of any one of the preceding claims, wherein each cryogenic liquid cargo tank of the at least one cryogenic liquid cargo tank is an IMO type C

containment system.

8. The system of any one of the preceding claims, wherein at least one cryogenic liquid cargo tank

comprises a submerged pump configured inside the space to pump the conditioned hydrocarbon liquid out of the space of the at least one cryogenic liquid cargo tank during offloading .

9. The system of claim 8, wherein a vaporizer is provided in communication with the space of the at least one cryogenic liquid cargo tank during offloading to vaporize an amount of the conditioned hydrocarbon liquid, whereby the vaporizer is configured to discharge vapour into the space.

10. The system of claim 8 or 9, wherein the offloading system comprises an offloading line provided with a hose connection interface allowing selective coupling and uncoupling of a cryogenic hose to the offloading line, whereby the submerged pump is connected to the offloading line to discharge the conditioned hydrocarbon liquid through the hose connection interface into the cryogenic hose .

11. The system of any one of the preceding claims, further comprising a boil-off gas disposal conduit, optionally provided with a boil-off gas blower, to dispose boil-off gas generated during loading mode of the at least one cryogenic liquid cargo tank.

12. The system of any one of the preceding claims, wherein the vapour pressure of the conditioned

hydrocarbon liquid in the at least one cryogenic liquid cargo tank is above 0.0 barg, and below a maximum of 0.1 barg, preferably below 0.05 barg.

13. The system of any one of the preceding claims, wherein the at least one cryogenic liquid cargo tank comprises a pressure relief valve having a set pressure in a range of from 0.2 barg to 0.4 barg to atmosphere.

14. The system of any one of the preceding claims, wherein the platform supply vessel is powered by natural gas, wherein the fuel tank is a liquefied natural gas tank, and wherein the fuel tank is in fluid communication with the refrigeration system to provide refrigeration to the refrigeration system.

15. The system of any one of the preceding claims, wherein the platform supply vessel is a platform supply vessel as defined in Lloyd's Register Classification Society Rules and Regulations.

16. The system of any one of the preceding claims, wherein the platform supply vessel

- has a length over all of in a range of from 70 m to 100 m,

- has a moulded beam (breath) in a range of from 15 m to 21 m,

- is optionally equipped with a dynamic positioning system (DPS),

- has a cargo deck available for loading cargo having a cargo deck space in the range of from 600 m^ to 1275 m^, and

- the cargo deck being exposed upward facing open air.

17. A method of off-shore storing and transporting a conditioned hydrocarbon liquid, comprising:

- providing a platform supply vessel, said platform supply vessel comprising a hull enclosing a hull space, and a cargo deck exposed upward facing open air whereby at least part of the hull space is vertically below the cargo deck, and a propulsion system, wherein a fuel tank is configured within the hull space which is fluidly connected to the propulsion system and operable to provide a supply of fuel to the propulsion system, and wherein at least one cryogenic liquid cargo tank is configured within said hull space in addition to the fuel tank, wherein the cryogenic liquid cargo tank has a space for holding the conditioned hydrocarbon liquid, and wherein cryogenic liquid cargo tank is functionally connected to a refrigeration system; - connecting the at least one cryogenic liquid cargo tank to a disconnectable hydrocarbon fluid supply system external to the platform supply vessel;

- sealing the cryogenic liquid cargo tank thereby creating the space;

- fuelling the propulsion system with a supply of fuel from the fuel tank;

18. The method of claim 17, wherein the vapour pressure of the conditioned hydrocarbon liquid in the cryogenic liquid cargo tank is kept above 0.0 barg, and below a maximum of 0.1 barg, preferably below 0.05 barg, by said removing of heat from the space in the cryogenic liquid cargo tank.

19. The method of claim 17 or 18, further comprising collecting boil-off gas during said transferring of the hydrocarbon fluid stream and disposing the collected boil-off gas to a boil-off gas disposal system,

preferably arranged external to the platform supply vessel, said transferring optionally comprising

compressing the boil-off gas on the platform supply vessel prior to said disposing.

20. The method of any one of claims 17 to 19, further comprising :

- connecting a cryogenic hose to a hose connection interface of an offloading system available on the platform supply vessel;

- transferring the conditioned hydrocarbon liquid from the at least one cryogenic liquid cargo tank through an offloading line of the offloading system and the hose connection interface into the cryogenic hose, by means of a submerged pump configured inside the space whereby pumping the conditioned hydrocarbon liquid out of the space of the at least one cryogenic liquid cargo tank;

- vaporizing an amount of the hydrocarbon liquid in a vaporizer that is provided in communication with the space of the at least one cryogenic liquid cargo tank, and discharging vapour thus generated into the space whereby maintaining a pressure within the enclosed space of higher than 0.0 barg and below 0.2 barg while the hydrocarbon liquid is being pumped out of the at least one cryogenic liquid cargo tank.

21. The method of any one of the claims 17 - 20, wherein the platform supply vessel is a platform supply vessel as defined in Lloyd's Register Classification Society Rules and Regulations .

22. The method of any one of the claims 17 - 21, wherein the platform supply vessel

- has a length over all of in a range of from 70 m to 100 m,

- has a moulded beam (breath) in a range of from 15 m to

21 m,

- is optionally equipped with a dynamic positioning system (DPS),

- the cargo deck being exposed upward facing open air