DUAL-FUEL FEED CIRCUIT SYSTEM USING COMPRESSED NATURAL GAS FOR DUAL-FEED CONVERTED SHIP ENGINES, AND INTEGRATION THEREOF IN A CNG

MARINE TRANSPORTATION SYSTEM [0001] The present invention relates to a novel dual-fuel feed circuit system for compressed natural gas, otherwise known as CNG, for dual-feed converted marine engines, and also to integration of said circuit in a CNG marine transportation system.

Field of application

[0002] Fuel gas is conventionally transported by sea principally in the form of LNG, liquefied natural gas, or in the form of LPG, liquefied petroleum gas. The former is composed for the most part of methane in a liquid state and is conserved at a pressure close to atmospheric pressure and at a temperature close to -170°C; the latter is composed of butane, propane and other hydrocarbons and it is conserved at a moderate pressure and at a temperature close to -48°C. Transporation and storage as CNG, an acronym for compressed natural gas, was only introduced more recently. CNG's composition is similar to LNG but it is conserved in a gaseous state, albeit at high pressures: typically about 250 bar (or barg) at ambient temperatures, i.e. around 15°C, or at lower temperatures, e.g. around -30°C, thereby reducing the pressure to about 160 bar. CNG may contain a liquid fraction, yet still be referred to as CNG, rather than LNG.

[0003] In CNG transportation, as compared to LNG, a first great advantage derived from the compression and reduction of the volume of the gas from the in-use condition thereof, is a better efficiency of achieving the required transportation condition, yet still maintaining the increase of transported product compared to its in-use condition, and thus a decrease in overall transport costs.

[0004] The engines conventionally used in ships and barges intended for the transportation of gas are internal combustion engines of the "spark-ignited" or diesel type; for some time now it is known that companies operating in the marine transportation sector have been carrying research into replacement of conventional petroleum-derived fuels, such as diesel fuel and petrol, with less polluting and also more economical fuels such as, in particular, natural gas. It is in fact known that methane produces a lower quantity of polluting substances during combustion, being regarded as a "clean" fuel; for example, compared to diesel fuel, the reduction in the C0 ₂ emissions is generally quantified at about 20%, with a reduction of about 90% in the NOx emissions and also substantially negligible emissions of SOx and particulate. The new standards for marine transportation, such as the IMO Tier III Standards, to be implemented as of 2016, impose strict limitations on polluting substances and indicate the need for research into "clean" engines.

[0005] For years the companies operating in the sector of marine transportation of natural gas have been investigating solutions able to employ as fuel part of the actual gas transported. The first ship to use LNG also as a fuel, in particular its fraction known as "boil off gas", has been operative since 1964. In the particular case of CNG transportation, which is recent and not yet optimized, it appears that greater advantages can be achieved in terms of transportation costs by using that specific portion of residual gas - called "heel" - which is generally not removed from the containers during emptying because it is not advantageous from a cost point of view - due to the proximity of its pressure to the pipe-delivery pressure, it is too slow to offload, and thus too costly to recover fully. Further, it is appreciated that the possibility of using the transported gas as fuel in a conventional engine would result in greater autonomy and flexibility of use.

[0006] For technical reasons linked with combustion, the said use in conventional engines, i.e. the ones that are already installed in the ships, must be compatible with the fact that those engines also run off conventional fuel, i.e. diesel fuel or petrol. This mode of use, and thus also the engines therefor, is generally referred to as a "dual-fuel" or "bi-fuel" arrangement. Further, it envisages that at certain stages the said engine is fed simultaneously with both fuel types.

[0007] A bi-fuel engine, or a dual-fuel engine, is traditionally a normal petrol engine that has been upgraded or modified, e.g. via its ignition and fuel delivery systems, in order to perform combustion using natural gas combined with petrol. By contrast, the only-gas solutions would be by way of an engine specifically developed for burning natural gas. Therefore, while the first solution can be achieved by converting existing petrol engines mounted already on or within operating ships, the second solution can only be performed in the next generation of ships.

[0008] Reasons for switching over to dual-fuel systems are mainly: the economic benefits, flexibility of use and environmental aspects, the latter being by virtue of the fact that CNG is low in pollutants. This reason is particularly important in the context of the new environmental legislation which is expected to forbid the petrol-thrusted navigation in the docking areas.

[0009] In general, the use of CNG as fuel in conventional engines which have been converted into dual-fuel engines gives rise to known difficulties with regard to ignition and combustion, owing to the variability of the gaseous state and the pressure and, for this reason, it is required to maintain a partly dual-fuel feed system. In addition to this it should be remembered, in particular, that an optimized and integrated conversion to dual fuel engines designed for specific use of the uncommercial residual portion of the CNG load, otherwise known as "heel", is extremely complex from a technical point of view since it requires a plurality of combined adaptation and integration measures such as: dedicated feed circuits; specific systems for connection to the vessels with piping which is integrated but separate from the system for loading/unloading the product, including also dedicated sensors, valves and headers; systems for treatment of the gas conveyed to the engines, with optimum pressure and temperature control; electronic control systems for the management of the dual feed system as a function of the conditions and the residual quantity of the CNG.

[0010] Finally, it should be remembered that CNG marine transportation systems require specific application technologies and piping configurations which are in fact recent and not yet optimized. For example there are difficulties associated with the manufacture of high-pressure containers having diameters greater than one metre and also difficulties in the management of circuits or pipework having different pressures therein, which therefore necessarily have complex interconnections, and in particular the automatic and controlled management of the conditioning, filling, maintenance and emptying operations.

[0011] It is therefore desirable that companies operating in the sector of CNG marine transportation should carry out research into integrated and optimized systems for the conversion of conventional diesel fuel or petrol engines on-board ships or barges into dual-fuel systems which, at the same time, may use the gas transported in pressure vessel containers and, in particular, the residual unloaded gas or "heel".

State of the art

[0012] The systems most widely used for transportation of CNG by sea envisage a plurality of cylindrical containers, called pressure vessels or pipes depending on their shape, of varying diameter and length, mainly with a diameter of 1 m and length proportional to that of the ship, which are arranged alongside each other, usually in parallel, and typically either vertically or horizontally. They are usually made of steel or composite materials and are designed specifically to withstand high pressures and to be introduced inside the hull of ships designed for this purpose.

[0013] The process for filling said containers involves conditioning the gas at a given pressure and temperature range, or at specific values, depending on the composition of the storage gas itself. Typically this is at a pressure value of about 250 barg and a temperature of about 15°C, or a temperature so as to prevent or reduce the formation of condensates during storage and transportation. That conditioned gas is used to fill the containers, suitably designed for this purpose and connected to a specific piping system. The container emptying process, which may include scavenging processes as described in PCT/EP201 1/071792, PCT/EP201 1/071802 or PCT/EP201 1/071803, the whole contents of which are incorporated herein by way of reference, then usually involves expansion of the gas to a lower pressure, e.g. about 90 barg, this operation being a preparatory step for transfer of the CNG into a condition suitable for sale. During the intermediate stages the pressure and temperature values are sometimes modified in order to facilitate the operations, being then reset to the correct levels for delivery.

[0014] It is known that the temperature is a critical property in relation to modification of the state of the gas, it being variable and often dependent on the atmospheric conditions. In particular, conventional thinking is that it must not drop too low. This is in order to prevent or minimise liquefaction - if it is too cold, upon compressing the gas there can be excessive liquefaction, e.g. of impurities. Further, the temperature can directly determine the pressure of the contained gas and also it can limit or reduce the quantity of product which can actually be transported in a given vessel. Cooling devices and also devices suitable for preventing the change in state of the gas, such as heat exchangers and vertical separators, are therefore used. Systems suitable for controlling and modifying the pressures, such as rotary compressor and throttle valves, are also used.

[0015] In general, since CNG marine transportation systems are substantially recent, the leading companies in the sector are carrying out research into advanced transportation systems which also have optimized containment, piping and conditioning systems. The main difficulties include, for example, the high operating pressures and the extreme variability of the product conditions.

[0016] The use of natural gas as a fuel in the marine transportation systems, unlike the motor vehicle sector where using CNG as a fuel is widely known and standardized, is also still relatively uncommon despite the fact that the first LNG ship to use as a fuel a gas which has changed from a liquid state to a gaseous state during transportation - otherwise known as BOG or "Boil Off Gas" - has been operative since 1964. In particular, the conversion of conventional engines into "dual-fuel" CNG engines has not been extensively developed.

[0017] In general it is known that "dual-fuel" engines could represent a polluting emission reduction compared to state of art technology in the marine field. In fact LNG utilized as marine fuel is already cost competitive and has already been well proven in the marine market by certain leading manufacturers such as, for example, Wartsila (Helsinki, Finland), Man (Munich, Germany) and Caterpillar (Peoria ILL, USA). In particular Wartsila started producing dual-fuel engines in 1996. The engines designed are 32DF, 50DF and most recently 34 DF. In the gas mode, the engines run on gas with about 1 % diesel. The combustion of gas and air mixture follows an Otto cycle triggered by pilot diesel injection. Alternatively, in the diesel mode they simply run diesel, so that the mixture of diesel and air is burned through a diesel cycle.

[0018] Natural gas-petrol dual-fuel engines are also already available and are used for liquefied natural gas (LNG) ships as well as LNG storage systems. This solution has been developed for dual-fuel converted generic ships. However, this is a disadvantageous solution because of the need for complex and expensive chilling and re-gasification systems. In particular this solution requires high energy consumption specifically for maintaining the low temperature - it requires the use or maintenance of cryogenic liquids.

[0019] From the early 1980s a number of other ships - ones which are not intended for the transportation of gas but instead for generic transportation, or for car and passenger ferry services, have been powered by natural gas in the form of LNG or CNG. These include, for example, the cargo ship "Accolade II" (1982 Adelaide, Australia) and also the ferries "Katawa" and "Kelleet" (1985 - 1988 Vancouver, Canada). They have CNG converted dual-fuel engines. More recently, there are also the ferries "Elisabeth River I" (1995 Norfolk, Virginia, USA) operating with CNG, and "Glutra" (2000 Molde, Norway) and "Fjordl " (201 1 Molde, Norway) operating with LNG. Other known examples of ship which are powered by natural gas are used, for example, in Amsterdam, Holland, and in St. Petersburg, Russia.

[0020] Until now, however, very few ships use natural gas as a fuel. There have been some preliminary studies, and among them was a BC Ferries (Victoria, Canada) study, adopting CNG storage. Nevertheless, it has not shown to be a significantly viable option for that firm at that time, due to technical challenges.

[0021] It is also known that some of the leading companies operating in the CNG marine transportation sector are developing innovative solutions such as, for example, the ship "Coselle CNG" described in the website of Sea NG (www.coselle.com). It is equipped with dual-fuel engines which use as fuel part of the gas transported in the special "Coselle" spool containing systems.

[0022] Optimized solutions for converting conventional systems for transportation of CNG in pressure vessels into dual-mode systems, in such a way as to use part of the transported gas as fuel - with particular preference for recovery of the low-pressure residue known as "heel" which is unsuitable for sale - are not known.

[0023] It is known in fact that replacement of the engines and the feed systems already installed on large ships for marine transportation of CNG is very problematic from an economic and technological point of view and not cost-effective; it is decidedly more advantageous to convert the engine and the existing circuits into a dual-fuel design so as to maintain substantially the same transportation system, with a very significant reduction in terms of costs and time and also avoiding wastage of material and energy resources.

[0024] In order to perform a dual-fuel conversion, generic solutions for the engine unit, such as, for example, those proposed by the company Energy Conversions Inc. (Tacoma WA, USA) and intended for LNG-driven systems, are known, but solutions which are optimized - also with integrated devices and circuits, so as to overcome the various known problems arising from actual application to systems for marine transportation of CNG contained in pressure vessels, using the residual gas - are not known. These problems include, for example: correct combustion depending on the pressure and temperature variables of the residual gas; correct mixing with the conventional fuel and electronic control of the parameters; extraction, management and conditioning of the said residual gas; integration of the new feed circuit in the piping system intended for delivery of the product, using dedicated sensors, valves and headers incorporated in the system; electronic control of the entire system; safety problems associated with the high pressures; energy efficiency of the engines. The present invention looks to improve upon one or more of these issues.

[0025] A search carried out in patent literature of public databases has produced a number of prior art publications, the following of which are mentioned:

D1 : US7155918 (Shivers)

D2: US2002/0046547 (Bishop et al.)

D3: JP56081206 (Yokoyama et al.)

D4: EP0069717 (Kvamsdal)

D5: JP2207153 (Yamada Tomoo)

D6: JP6336193 (Taru et al.) D7: JP2010173483 (Numaguchi et al.)

D8: KR20090098387 (Seog et al.)

D9: JP2010201991 (Yuasa)

D10: US2005172880 (Laurilehto et al.)

D1 1 : WO2008000898 (Sipilae et al.)

D12: WO2008/10901 1 (White)

D1 describes a system for treatment and marine transportation of compressed natural gas including various product conditioning devices such as: a separator, a decontaminating unit, a dehydration unit, a cooling unit and also gas containing elements which are connected so as to allow use of part of the transported CNG, in the gaseous state, as fuel for the engine of the cargo ship itself.

D2 proposes a combined system for transportation on ships of CNG inside pressure vessels where it is also envisaged that part of the transported gas is used as fuel.

D3 to D8 describe dual-fuel systems which envisage use of LNG as well as circuits and devices for the dedicated feed system. D9 proposes a marine transportation system including a vapour generator with a dual- fuel feed system to be combined with a conventional engine so as to recover part of the energy which is not used, via a heat exchanger.

D10 describes a combined LNG-powered barge-tug system.

D11 describes a feed system for a gas-powered vessel including interconnected tanks where one tank is assigned for feed supply and contains the high-pressure liquefied gas, while the other tanks are assigned to hold the fuel and are subject to a hydrostatic pressure.

Prior art closest to the invention

D12 proposes a complex system for marine transportation of natural gas, also in the form of CNG, in a plurality of containers of the pressure vessel type which are connected in sequence at the bottom and also at the top so as to allow the treatment, filling, storage and unloading of the product under the required conditions. The invention indicates as an important cost-related factor, to be developed also in the future, the reduction of the known residual portion of unsold gas, known as "heel". In particular it cites the use of the said residual gas as a fuel for powering the ship's engine, being a solution which is able to reduce the impact of costs in connection with this problem.

[0026] It is therefore reasonable to consider as known a system for marine transportation of CNG or LNG which is formed by:

cylindrical containers, of the pressure vessel or pipe type, arranged in parallel and alongside each other inside the hull, also organized in groups, and with associated piping system including headers, valves, sensors and safety devices; devices for conditioning the gas, both during filling and during storage and also emptying, in particular for modifying the pressure and temperature parameters, such as compressors, refrigerators and also heat exchangers for transferring heat from a secondary circuit to a primary circuit and vice versa;

devices for controlling and modifying the phase state and also for purifying the gas, such as separators and decontaminating units;

electronic control systems for automatically opening and closing the valves, of the DCS or PLC type;

[0027] It is also reasonable to consider as known:

marine internal-combustion engines powered by liquefied natural gas (LNG), or compressed natural gas (CNG);

marine internal-combustion engines, both Otto cycle spark-ignition engines and diesel cycle engines, which have a double feed system for natural gas and petroleum derivatives, and where said engines are referred to as being "dual fuel" or bi-fuel" and where said natural gas may be LNG or CNG supplied from special tanks, including also conditioning of the gas so that it is suitable for combustion;

- systems for converting large conventional engines into "dual fuel" engines. Drawbacks

[0028] In general, the systems which use LNG, as in D3 to D10, although designed effectively for the same purposes, are affected by problems which are substantially different from those of the present invention, i.e. as experienced when using CNG. That is since the conditions of the transported product are very different. Likewise the treatment required for combustion is also different. For example, such conditions include high differentials in the variables associated with the state, pressure and temperature, and thus also the need to deal with safety-related aspects.

[0029] In the case of the systems which use the transported CNG as fuel, as described in D1 , D2 and D12, it is not indicated how, from a technical point of view, part of the transported gas is to be extracted, treated and then used as the fuel. In other words, it is not indicated how the known technical problems associated with the specific application to bi-fuel engines are solved. Those problems become even more complex in the particular case of dual-fuel conversion of systems for marine transportation of CNG contained in pressure vessels, using the residual gas or "heel".

[0030] In order to perform said dual-fuel conversion, in particular it is necessary to consider not only engine and feed system, but also the integration of the new circuits and devices in the existing piping and containment systems, and also the loading and unloading systems.

[0031] A second problem encountered in the said conversion process relates to the correct combustion as a function of the pressure and temperature variables of the residual gas.

[0032] A third problem encountered relates to correct mixing of the CNG with the conventional fuel during "dual mode" operation, and thus specifically the electronic control of said mixing process. A related problem concerns optimization of the engine efficiency and also reduction of the polluting emissions in accordance with marine Standards.

[0033] A fourth problem encountered relates to the extraction, management and conditioning of the residual gas so as to ensure it is suitable for the use as fuel on the ship.

[0034] A fifth problem encountered relates to the integration of the new feed circuit inside the piping system intended for delivery of the product, with dedicated sensors, valves and headers integrated in the system. A related problem consists in the electronic control of the entire system, including management of the load and unloading of the pressure vessels.

[0035] A sixth problem encountered is safety-related and concerns the need to manage with extreme precision the temperature and pressure conditions of the gas, using also integrated detection systems, so as to prevent any risk of failures or explosions.

[0036] A further problem encountered relates to control of the change in state, in particular to avoid or minimise liquefaction.

[0037] Considering all the above, it is reasonable to conclude that there exists the need for companies in the sector to find innovative solutions able to solve one or more of the aforementioned problems.

Summary of the invention

[0038] These and other problems or objects are considered or addressed by the present invention, as defined by the claims. According to the characteristic features described in the accompanying claims, the problems mentioned are at least in part solved by a first inventive aspect, namely an integrated dual-fuel feed circuit system for converted marine engines (80), using CNG gas extracted from the containers (21 ) together with also the residual part called "heel", said gas being conveyed inside a treatment and conditioning unit (10) of the skid-mounted type and being rendered utilisable as fuel. A skid mounted type product is one that is delivered off the shelf, ready to go, usually on skids, so that it can be sited on the workfloor/deck and pushed into its final position on the skids (or manipulated thereto with a forklift). The main operating steps are: a) "extraction" with flow along a single header (30); b) "cleaning"; c) "deviation" in multiline HP-LP piping (41 , 42) with the assistance of accessory compressed-air control line (40) which is in charge to manage the main valves (51 , 52, 53, 56a, 56b); d1 ) "heat exchange" in an HP header with a heater (60); d2) scavenging compression in an LP header through the compressor (61 ); e) "deviation" along a redundancy dual line (44a, 44b); f) "final measurement" (24) and safety discharge (45). The said operating steps are combined to ensure optimum use of the gas as fuel depending on the system variables with maximum safety and continuity of feeding. According to the present invention there can be provided a fuel feed system for a dual fuel marine engine, the system feeding compressed natural gas, otherwise known as CNG, as at least one of the fuels for the engine, the system being integrated into or onto a vessel for marine transportation of CNG, wherein or whereon the CNG is contained in interconnected pressure vessels, wherein the CNG used as at least one of the fuels is extracted directly from one or more of the pressure vessels, the CNG including at least a part which is a residual part of the stored or transported CNG, which part is known as the heel, and wherein the said CNG is conveyed into and through a treatment and conditioning unit formed as a module, in which module the gas undergoes a plurality of treatment and conditioning operations aimed at making it utilisable as the at least one of the fuels for the marine engine of the vessel, the said plurality of treatment and conditioning operations consisting of at least the following steps:

cleaning of the extracted CNG to remove undesired contaminates;

dividing the CNG into a multiline piping network, comprising at least one high pressure (HP) line and one low pressure (LP) line;

heat exchanging on an HP header using a heater;

scavenging compression on an LP header;

dividing the feed through a redundancy-providing dual feed and processing line in order to improve the feeding in term of reliability;

and

providing a measurement to check suitability for use as the at least one of the fuels;

the module additionally having a safety discharge point in a position located after the measurement;

the said plurality of treatment and conditioning operations being performed by means of interconnected devices combined together so as to ensure correct use of the CNG as a fuel so as to account for and compensate for storage and transport variables, including at least the temperature, pressure, flowrate and state of the extracted CNG. Preferably the engines are converted to dual feed, rather than initially being designed and set up for dual feed.

Preferably the module is of a skid-mounted type. Preferably the CNG is extracted from the interconnected pressure vessels and carried from a position downstream thereof along a single header (30) that delivers the CNG into the module.

Preferably the said accessory compressed-air control circuit (40) is integrated and functional for the said plurality of treatment and conditioning operations, and is calibrated so as to activate main valves and a compressor, as appropriate; and where the said accessory control circuit controls the flow of the CNG; and where the said accessory circuit controls the scavenging operation; and where the said accessory circuit is calibrated in accordance with gas conditioning parameters required by the specific mode of use of the engine.

Preferably in a dual-fuel mode of the engine, the said high-pressure line and low- pressure line have pressure values, in use, which are variable, but which range, respectively, between 250 and 6-7 bar and between 6-7 and 1 bar.

Preferably the engine is converted into a dual-fuel mode and is adapted such that the conventional pressure value for feeding fuel thereto is in the range of between 6 and 7 bar.

Preferably the accessory control circuit is adapted to open the L.P. line and close the H.P. line when the gas reaches a pressure value between 6 and 7 bar, thus starting the scavenging compression phase in order to help empty out the pressure vessels. Preferably the scavenging compression phase is intended to help empty out the pressure vessels down to pressure of 1 bar.

Preferably main valves and actuators (51 , 52, 53, 56a, 56b, 62) connected to the accessory control circuit are of an electropneumatic type.

Preferably the cleaning step includes one or more step performed by means of a device of the scrubber type.

Preferably the cleaning step includes one or more step performed by means of a filtering device.

Preferably the process steps and devices of the module are managed in an integrated manner by means of control logic units with dedicated processors and software. According to another aspect of the present invention there is provided a fuel feed system for a methane-fuelled marine engine, the system feeding compressed natural gas, otherwise known as CNG, as the fuel for the engine, the system being integrated into or onto a vessel for marine transportation of CNG, wherein or whereon the CNG is contained in interconnected pressure vessels, wherein the CNG used as the fuel is extracted directly from one or more of the pressure vessels, the CNG including at least a part which is a residual part of the stored or transported CNG, which part is known as the heel, and wherein the said CNG is conveyed into and through a treatment and conditioning unit formed as a module, in which module the gas undergoes a plurality of treatment and conditioning operations aimed at making it utilisable as the fuel for the marine engine of the vessel, the said plurality of treatment and conditioning operations consisting of at least the following steps:

cleaning of the extracted CNG to remove undesired contaminates;

dividing the CNG into a multiline piping network, comprising at least one high pressure (HP) line and one low pressure (LP) line;

heat exchanging on an HP header using a heater;

scavenging compression on an LP header;

dividing the feed through a redundancy-providing dual feed and processing line in order to improve the feeding in term of reliability;

and

providing a measurement to check suitability for use as the fuel;

the module additionally having a safety discharge point in a position located after the measurement;

the said plurality of treatment and conditioning operations being performed by means of interconnected devices combined together so as to ensure correct use of the CNG as a fuel so as to account for and compensate for storage and transport variables, including at least the temperature, pressure, flowrate and state of the extracted CNG.

Preferably the methane fuelled engine is designed to run best on a methane-only fuel.

Preferably the fuel is fed to the engine at a feed pressure value of about 25 bar.

Preferably the module is of a skid-mounted type.

Preferably the CNG is extracted from the interconnected pressure vessels and carried from a position downstream thereof along a single header (30) that delivers the CNG into the module. Preferably the said accessory compressed-air control circuit (40) is integrated and functional for the said plurality of treatment and conditioning operations, and is calibrated so as to activate main valves and a compressor, as appropriate; and where the said accessory control circuit controls the flow of the CNG; and where the said accessory circuit controls the scavenging operation; and where the said accessory circuit is calibrated in accordance with gas conditioning parameters required by the specific mode of use of the engine.

Preferably the said high-pressure line and low-pressure line have pressure values, in use, which are variable, but which range, respectively, between 250 and 25 bar and between 25 and 1 bar.

Preferably the accessory control circuit is adapted to open the L.P. line valve and close the H.P. line valve when the gas reaches a pressure of about 25 bar, thus starting a scavenging compression phase in order to help empty out the pressure vessels.

Preferably the scavenging compression phase is intended to help empty out the pressure vessels down to pressure of 1 bar. Preferably main valves and actuators (51 , 52, 53, 56a, 56b, 62) connected to the accessory control circuit are of an electropneumatic type.

Preferably the cleaning step includes one or more step performed by means of a device of the scrubber type.

Preferably the cleaning step includes one or more step performed by means of a filtering device.

Preferably the process steps and devices of the module are managed in an integrated manner by means of control logic units with dedicated processors and software.

Objects

[0039] In light of the present invention, there has been an immediate and significant technical progress, and various important objects are achieved. [0040] A first object consists in the "integrated conversion" of the propulsion system of ships from petrol fuel to hybrid/dual fuel, i.e. an engine that is able to burn natural gas too, the said natural gas being extracted from the CNG load, rather than dedicated fuel tanks. The invention also provides two new alternative feed systems: "dual-fuel" and also "methane-only" systems (i.e. just using CNG as the fuel-source). In particular, the integrated conversion solves a known problem of resolving the difference between the optimum conditions for the gas used as propellant in dual-fuel mode and methane-only mode. For example, the pressure is greater in the second, or methane only mode. The said conversion, moreover, involves minimum modifications to the existing engine and piping system, it being characterized by the advantageous integration of a dedicated feed circuit system which includes the devices for treating the gas for combustion purposes and also the control systems.

[0041] A second object consists in a substantial reduction of transportation costs with use of the dual-fuel mode, in particular making use of the residual portion of gas or "heel" - which is otherwise a waste-product or ballast.

[0042] A third object consists in a substantial reduction of polluting emissions by the use of the dual-fuel mode, which can keep within the emissions parameters stipulated by the IMO Standards applicable as of 2016.

[0043] A fourth object consists in a substantial increase in the flexibility of use of the transportation system.

[0044] A fifth object consists in providing an integrated conversion system, with gas treatment unit, of the multilevel type, which operates at different pressures and which is also extremely versatile.

[0045] A further object consists in providing an integrated conversion system, with gas treatment unit, which can be manufactured industrially in a simple manner, being compact and suitable for prefabrication, substantially comprising a centralized module of the skid-mounted type.

[0046] Another object consists in providing an integrated conversion system which reduces the risk of failures or explosions.

[0047] These and other features and advantages will be further described in the following detailed description of a preferred embodiment provided with reference to the accompanying drawings.

CNG loading and offloading procedures and facilities depend on several factors linked to the locations of gas sources and the composition of the gas concerned.

With respect to facilities for connecting to ships (buoys, platform, jetty, etc ..) it is desirable to increase flexibility and minimize infrastructure costs. Typically, the selection of which facility to use is made taking the following criteria into consideration:

safety;

reliability and regularity;

· bathymetric characteristics water depth and movement characteristics; and

ship operation: proximity and manoeuvring.

A typical platform comprises an infrastructure for collecting the gas which is connected with the seabed.

A jetty is another typical solution for connecting to ships (loading or offloading) which finds application when the gas source is onshore. From a treatment plant, where gas is treated and compressed to suitable loading pressure as CNG, a gas pipeline extends to the jetty and is used for loading and offloading operations. A mechanical arm extends from the jetty to a ship.

Jetties are a relatively well-established solution. However, building a new jetty is expensive and time-intensive. Jetties also require a significant amount of space and have a relatively high environmental impact, specifically in protected areas and for marine traffic.

Solutions utilizing buoys can be categorized as follows:

· CALM buoy;

STL system;

SLS system; and

SAL system.

The Catenary Anchor Leg Mooring (CALM) buoy is particularly suitable for shallow water. The system is based on having the ship moor to a buoy floating on the surface of the water. The main components of the system are: a buoy with an integrated turret, a swivel, piping, utilities, one or more hoses, hawsers for connecting to the ship, a mooring system including chains and anchors connecting to the seabed. The system also comprises a flexible riser connected to the seabed. This type of buoy requires the support of an auxiliary/service vessel for connecting the hawser and piping to the ship.

The Submerged Turret Loading System (STL) comprises a connection and disconnection device for rough sea conditions. The system is based on a floating buoy moored to the seabed (the buoy will float in an equilibrium position below the sea surface ready for the connection). When connecting to a ship, the buoy is pulled up and secured to a mating cone inside the ship. The connection allows free rotation of the ship hull around the buoy turret. The system also comprises a flexible riser connected to the seabed, but requires dedicated spaces inside the ship to allow the connection.

The Submerged Loading System (SLS) consists of a seabed mounted swivel system connected to a loading/offloading riser and acoustic transponders. The connection of the floating hose can be performed easily without a support vessel. By means of a pick up rope the flexible riser can be lifted and then connected to a corresponding connector on the ship.

The Single Anchor Loading (SAL) comprises a mooring and a fluid swivel with a single mooring line, a flexible riser for fluid transfer and a single anchor for anchoring to the seabed. A tanker is connected to the system by pulling the mooring line and the riser together from the seabed and up towards the vessel. Then the mooring line is secured and the riser is connected to the vessel.

Description of the drawings

Fig. 1a is a graph which shows the progression over time of the pressure within a pressure vessel during normal unloading processes involving a end-period scavenging process for lower pressure CNG retrieval;

Fig. 1 b is a graph which shows the energy usage for achieving the unloading operation according to Fig. 1 a;

Fig. 1c is a table indicating the number of ISO CNG pressure vessels needed to transport a given amount of power capacity over specified time durations;

Fig. 2 is a simplified P&ID (Piping and Instrumentation Diagram) relating to a proposed engine feed system;

Fig. 3 is a schematic, cut-away, longitudinal cross-section through a ship for transportation of CNG, showing pressure vessels, a converted propulsion unit and a skid-mounted unit of the integrated piping system according to Fig. 2;

Fig. 4 is a block diagram relating to the general configuration of the proposed system with the main process steps set out;

Practical embodiment of the invention

[0048] The present invention relates to a novel dual-fuel feed circuit system using compressed natural gas, otherwise known as CNG, for dual-feed converted marine engines, including integration of said circuit in a CNG marine transportation system, the said natural gas being extracted from the CNG load in such a way as to allow the use of two new alternative feed systems: "dual-fuel" and "methane-only". [0049] One of the main advantages of the proposed system consists in the use also, in dual-fuel mode, of the unused and unsold gas portion known as "heel" which usually remains inside the containers. Generally the unsold portion of CNG remains inside the containing system since it is not commercially worthwhile to unload the gas that remains at relatively low pressures, for example at less than 30 barg, since such removal would take a relatively long time, and would involve large amounts of energy (compare Figs. 1 a and 1 b). It is preferred instead, therefore, to use this residual gas as a fuel source for the ship, whereby it becomes possible to make a more efficient use of the resources on the ship - i.e. resources that would otherwise not be used, and which would thus otherwise be little more than waste or ballast.

[0050] Fig. 1 a shows a conventional diagram illustrating the offload progression over time in terms of the internal pressure of the pressure vessels both during an unpowered unloading step and then during any compressor-driven ongoing operation. The latter steps typically switch on when internal pressure values of PVs drops below the delivery pressure, that in this non-limitative example is considered about 90 bar, and switch off when the non-assisted off-load rate reaches a point at which the time-per-offload- volume (e.g. measured in scfs per minute) becomes economically difficult to justify, that in this non-limitative example it happens when the internal pressure into PV reaches about 30 bar. Basic data and assumptions in the offloading graph are: the contained fluid is clean methane; CNG ship storage capacity is about 800 MMSCF; standard transport storage pressure is about 250 bar; residual pressure is about 30 bar; storage temperature is about 15 °C; maximum unloading time is about 18 hours; maximum allowable gas velocity is about 20 m/s.

[0051] Fig. 1 b shows the diagram with the energy demands for a compressor extraction operation (scavenging). Contextual analysis on graphs reported in Fig. 1 a and Fig. 1 b showed that emptying PVs providing a forced pressure drop gradient via compressor units is not convenient in terms of both energy and time. In fact, from Fig. 1 b, it can be noticed the exponential trend of additional energy needed starting from about 90 bar to continue the offloading process via scavenging. Clearly, therefore, for the remaining residual gas after the 18 hour offload period and with 30 bar remaining into PVs (in this non-limitative example) there is a greater advantage in using that residual gas as fuel for propelling the ship, since the pressure value at which the gas has to be compressed in order to feed the engine (25 bar for methane only or 6/7 bar for dual-fuel) is much less than the delivery pressure (e.g. 90 bar).

[0052] The system of the present invention envisages using compressed natural gas (CNG) instead of liquefied natural gas (LNG) as that fuel source. This solution has some known positive implications, such as facilities simplification and consequential economical savings.

[0053] CNG system consists of a series of pressure vessels (PVs) containing CNG at 250 bar and ambient temperature. PVs can be placed in safe position on board of any kind of ships and connected with a dedicated piping system to a specific skid-mounted unit. This unit is used in order to manage the gas flow and to deliver it to the engine in the required condition.

[0054] The pressure vessels can be fitted within ISO-containers, so as to be removable, replaceable and easily transportable. Such an arrangement could be arranged such that the ISO containers with pressure vessels therein simply substitute the ISO-containers at a normal jetty equipped for the handling of such containers.

Alternatively it is possible to install the pressure vessels fixedly on the ship. In this case the refilling operation must necessarily be done in a gas compression/decompression station, such as one located either on-shore or on-board.

[0055] Pressure vessels suitable for the transportation and delivery of CNG can be made of various materials, and using a variety of production technologies. We can list below eight different categories of pressure vessel:

1 . All-steel pressure vessels (known as type 1 ), with the metal being used as the structure for the containment;

2. Composite Hoop-Wrapped steel tanks with structural steel heads (domes) and hybrid a hybrid material body (steel + fibre-reinforced polymer, the fibre- reinforcement being in hoop sections), the hybrid material being in a load sharing condition (known as type 2);

3. Metallic liner with non-metallic structural overwrap (known as type 3). The metal liner is only there for fluidic containment purposes. The non-metallic external structural overwrap is made out of, in the preferred arrangements, a fibre- reinforced polymer; other non-metallic overwraps are also possible.

4. Non-metallic liner with non-metallic structural overwrap (known as type 4). The non-metallic liner (such as a thermoplastic or a thermosetting polymer liner) is only there for fluidic containment purposes. The non-metallic external structural overwrap can again be made out of, in the preferred arrangements, a fibre- reinforced polymer.

5. A fully non-metallic structure (no separate liner), with the non-metallic structure having been built on a substrate that is removed after the manufacturing process (known as type 5).

6. Steel body section fitted with composite heads or domes (known as type 6). The pressure vessels have a structural steel body section and fibre-reinforced polymer heads or domes fitted thereto with a sealed joint;

7. Composite Hoop-Wrapped steel bodies, with composite heads or domes (known as type 7). The pressure vessels have hybrid steel + fibre-reinforced polymer hoop wrapped body section, with the materials in a load sharing condition and fibre-reinforced polymer heads or domes fitted thereto with a sealed joint.

8. Near-Sphere shaped pressure vessels formed from a non-metallic liner with a non-metallic structural overwrap (like the type 4 above, but with the specific near spherical shape). These pressure vessels have a non-metallic liner (such as a thermoplastic or a thermosetting polymer) which serves only for fluidic containment purposes. The non-metallic external structural overwrap is typically made out of, in the preferred arrangements, a fibre-reinforced polymer.

Prior applications describing preferred aspects of these vessels include PCT/EP201 1/071793, PCT/EP201 1/071797, PCT/EP201 1/071805,

PCT/EP201 1/071794, PCT/EP201 1/071789, PCT/EP201 1/071799,

PCT/EP201 1/071788, PCT/EP201 1/071786, PCT/EP201 1/071810, PCT/EP201 1/071809, PCT/EP201 1/071808, PCT/EP201 1/071800,

PCT/EP201 1/07181 1 , PCT/EP201 1/071812, PCT/EP201 1/071815,

PCT/EP201 1/071813, PCT/EP201 1/071814, PCT/EP201 1/071807,

PCT/EP201 1/071801 and PCT/EP201 1/071818, all of which are incorporated herein in full by way of reference. The features of the pressure vessels disclosed in those prior filings are relevant to the present invention in that they can provide the storage means for storing the fuel. As such, they can each either separately or collectively assist in differentiating the present invention over prior art arrangements.

[0056] Fibre-reinforced polymer, also known as fibre-reinforced plastic, is a composite material, consisting in a polymer matrix reinforced with fibres, which are usually fibreglass, aramid or carbon; the polymer is generally an epoxy, vinylester, polyester or another thermosetting polymer or mixture thereof.

[0057] The number of pressure vessels (PV) needed for a trip depends on the engine's specific fuel requirements, the rate of work of the engine and the trip distance and it can be calculated using the following formula:

n° PV=t ^* P ^* SFC/SV

where SFC is the specific fuel consumption, P is the median or average engine power, i.e. the average rate of work, t is the trip duration and SV is the standard volume of CNG storable in each PV.

[0058] Fig.1 c, for example, shows a table with the number of ISO containers of the pressure vessel type considering a ship with a SFC of 0.1 mmscfd/MW and SV of 0.318 mmscf, where mmscf is million standard cubic feet and mmscfd is million standard cubic feet per day. Million standard cubic feet is a standard term for quantifying a stored amount of useable CNG.

A standard cubic foot (abbreviated as scf) is a measure of quantity of gas, equal to a cubic foot of volume at 60 degrees Fahrenheit (15.6 degrees Celsius) and either 14.696 psi (1 atm or 101.325 kPa) or 14.73 psi (30 inHg or 101 .6 kPa) of pressure. A standard cubic foot is thus not a unit of volume but of quantity, and the conversion to normal cubic metres is not the same as converting cubic feet to cubic metres (multiplying by 0.0283...), since the standard temperature and pressure used are different. Assuming an ideal gas, a standard cubic foot using the convention of 14.73 psi represents 1 .19804 moles (0.0026412 pound moles), equivalent to 0.026853 normal cubic meters.

Common oilfield units of gas volumes include ccf (hundred cubic feet), Mcf (thousand cubic feet), MMcf (million cubic feet), Bcf (billion cubic feet), Tcf (trillion cubic feet), Qcf (quadrillion cubic feet), etc. The M refers to the Roman numeral for thousand. Two M's would be one thousand thousand, or one million. The s for "standard" is sometimes included, but often omitted and implied. We have used it above in the statements pertaining to the invention. Still referring to Figure 1 c, the numbers refer to the numbers of containers that are fully loaded at normal storage pressure (e.g. 250 bar).

The so called "ISO container" is a typical freight container manufactured according to specifications set down therefor by the ISO - the International Organization for Standardization. Such containers can be used as reusable transport and storage unit.

[0059] For safety reasons PVs should be placed in a zone of the ship where the necessary ventilation can generally always be guaranteed. This is to prevent or minimise the risk of the formation of an explosive environment.

To optimize piping arrangements, and the size thereof, it is useful to mount the skid- mounted unit close to the PVs.

From the skid-mounted unit, the gas will flow to the engine at the correct desired pressure and rate, thus allowing the characterization of that later piping system to be predetermined or standardised too.

[0060] Fig. 2 shows the preferential P&ID (Piping and Instrumentation Diagram) for the proposed integrated conversion system, including the connections functional for feeding both in dual-fuel mode and methane-only mode. In particular, the system for extracting, distributing and treating gas, including the interconnections between the process apparatus and also the instrumentation used to control the process itself, is illustrated.

[0061] With specific reference to the said P&ID according to Fig. 2 and also the diagrams in Figs. 3 and 4 which show it in context, the proposed circuit system with gas treatment unit (10) comprises a centralized module of the skid-mounted type which uses CNG gas extracted from containers (21 ), including the residual part not commercially suitable for sale, i.e. the "heel", so as to make that part utilisable as a fuel in a marine engine (80), that engine (80) having been converted or designed to operate in a dual- fuel mode.

The operation of the system can be summarised essentially by the following sequence of steps, referring to the path of said gas from the containers (21 ) to the engine (80): a) "extraction" of the gas from one or more container (21 ), that gas then typically being conveyed in a single header or pipe (30), by means of a deviation or diverter (22) located upstream of the pipes (70) for the gas to be sold, i.e. closer to the containers (21 ) than those pipes (70). The said extraction may be performed naturally by means of direct expansion of the gas at a higher pressure, or may be forced or assisted, for example by means of scavenging with a compressor (see step d2). The compressor may be separately powered, or it may be driven by the expansion of the high pressure gas stream. See, for example, PCT/EP201 1/071792, the contents of which are incorporated herein by way of reference;

b) "cleaning" of the extracted gas from the containers (21 ), preferably by means of a device of the scrubber type, or a filter (14), with discharge (32) of the liquid fraction and retention (31 ) of the gaseous part;

c) "deviation" of the gaseous part into a multiline, multi-pressure, piping arrangement consisting of a high pressure HP line or circuit (41 ) and low pressure LP line or circuit (42). This can be achieved using an accessory compressed-air header (40) (inside which compressed air flows at a pressure of 3 or 4 bar) used to activate precisely the main electropneumatic valves (51 , 52, 53, 56a, 56b) and the on/off switch (62) depending on the type of propulsion system required for the ship. A typical value of the optimum operating pressure in the case of a turbine driven (methane only) engine is about 25 bar, while the value of the combustion pressure of the marine engine converted into dual-fuel is typically in the range of 6-7 bar. Therefore, in the case of a turbine-driven (methane-only) ship engine, typical range value of the operating pressures is from 250 to 25 bar in the natural unloading HP circuit (41 ), where the pressure variation is between the design pressure and the combustion pressure of the turbine, and from 25 to 1 bar in the LP circuit (42), where the variation in pressure is between the combustion pressure of the turbine and atmospheric pressure. This is since it is usually necessary to compress the gas in order to reach the optimum combustion pressure.

In the case of a ship engine converted into dual-fuel mode, the pressure value at which a deviation of the flow from the HP circuit to the LP circuit occurs is equal to the value of the combustion pressure of the engine motor itself (optimum values 6-7 bar). Therefore the natural-unloading HP circuit is used for pressure values of between 250 and 7 bar while the LP circuit is used for pressure values ranging between 7 and 1 bar. d1 ) "heat exchange" with the gas in the HP header (41 ) by means of a heater (60). This is in order to avoid an excessive decreasing of temperature. d2) "compression or scavenging" in the LP header (42) by means of a compressor (61 ), for example of the turbomachinery type with electric motor (E), in order to increase the pressure, raising it to a level equal to or greater than the combustion pressure, and also in order to allow forced emptying of the containers (21 ) at the same time as the extraction step (a), if necessary until completely emptied. In particular, the pressure levels for activation of the compression system (56a, 56b, 61 , 62) are variable and set according to the optimum operating modes of the engine or the scavenging parameters; e) "deviation" along the redundancy dual line (44a, 44b) in the event of faults, in order to increase the reliability and prevent the interruption of feeding, being performed by means of a plurality of interconnected devices (54, 55, PC, PI, PT) for controlling the pressure and the flow. These devices (54, 55, PC, PI, PT) are arranged in the same way on both line (44a) and line (44 b) so as to obtain identical and parallel lines formed downstream of the union (44); f) "final measurement" of the parameters which characterize the gas ready for combustion in the converted engine (80), by means of a general measurement device (24) located downstream of the union (43), also followed by a safety discharge (45).

[0062] The said steps (a, b, c, d1 , d2, e, f) are combined in sequence so as to help to optimise the use of the gas as a fuel depending on the system variables and also to help to offer maximum safety and operating continuity. In particular, steps d1 and d2 are reciprocally exclusive.

[0063] Downstream of extraction (a) (Figs. 2 and 3), the gas is conveyed in a single header (30) to the scrubber/filter (14) from where the gas flow (31 ) is deviated into the two different headers, i.e. the high pressure header (41 ) and a lower pressure header (42), which are controlled by means of the main electropneumatic valves (51 , 52, 53, 56a, 56b), where these valves are opened/closed at a specific set of pressure values according to the requirement of the converted engine (80) and also the gas extracted from the containers (21 ). The main valves (51 , 52, 53, 56a, 56b), managed by header (40), are preferably of the electropneumatic type for reliable safety, efficiency and operating precision. The secondary valves (54, 55), not connected directly to the header (40), are instead preferably of the simple pneumatic type (54) or electromagnetic type (55). This can reduce costs.

[0064] As indicated previously, typical delivery gas pressure to engine varies from 1 to 25 bar. In particular a turbine engine for driving a ship in methane-only mode will usually work at optimum values of about 25 bar, while in dual-fuel mode it might instead work optimally at about 6-7 bar. Natural (i.e. not powered) emptying of the containers or pressure vessels (21 ) will usually be relied upon at pressures of between their design pressure (i.e. when full) - often 250 bar, and the gas delivery pressure - e.g. 90 bar.

[0065] For example, in the case of use of the system (10) in methane-only mode with the container or containers still full, or partly full (i.e. while relying still on natural emptying), the scrubber (14) cleans up the raw CNG. The stored methane at design pressure before the scavenging, and at delivery pressure afterwards, will be decompressed naturally, flowing through High Pressure lines (41 ) where a heating system (60) will avoid an excessive decreasing of temperature due to the so called Joule - Thomson effect. This effect, also known as Joule-Kelvin effect, represents an expansion or compression happening at constant entropy - an adiabatic transformation producing no work. When the gas (31 ) reaches 6-7 bar (for the dual fuel engines), a control system (40) opens the L.P. line valve (52, 56a, 56b) and closes the H.P. line valve (51 ), starting the scavenging compression phase (42, 61 , E, 31 , 30) in order to assist with the emptying out of the pressure vessel (21 ), e.g. potentially down to 1 bar of pressure. [0066] After the re-conjunction of the High Pressure and Low Pressure lines (44) there is a flow rate regulation system constituted by a series of valves and sensors (54, 55, PC, PI, PT). This system is doubled up (44a, 44b) for offering redundancy. This is in order to avoid a stop of the whole power system in the case of a failure of this important section.

After the regulation system there is a metering box (24) followed by an emergency discharging line (45) with the function of avoiding sending an excessively high-pressure gas to the engine (80) in the case of a malfunction of the unit (10). [0067] The entire procedure described above is managed electronically by means of a DCS or PLC system using dedicated processors and software which are widely available.

[0068] The optimized filling methods described above are in compliance with the existing regulations governing high-pressure compressed gas devices, such as ASME or API, for example, and the corresponding industrial standards.

Key (10) Treatment and conditioning unit with centralized module, of the skid-mounted type, where the gas extracted from the pressure vessels is rendered utilisable as fuel;

(14) cleaning device, for example a scrubber or a filter;

(20) unit for storing the CNG in interconnected pressure vessels 21 , for example transportable or ship hold modules;

(21 ) tanks or containers containing CNG, of the pressure vessel type; (22) feed gas deviation;

(24) metering;

(30) header for feed gas to be conditioned, usually comprising the unsold residual portion or "heel";

(31 ) header for gas after separation from the liquid part;

(32) line for liquid part;

(40) compressed-air "accessory" circuit;

(41 ) HP (High Pressure) Header;

(42) LP (Low Pressure) Header;

(43) header for treated fuel gas, to the engine;

(44) H.P. and L.P. united lines;

(44a) first parallel line for redundancy;

(44b) second parallel line for redundancy;

(45) emergency discharging line;

(51 ) High Pressure Header "main valve";

(52) Low Pressure Header "main valve";

(53) "main valve" on treated fuel gas header

(54) secondary valve of the simple pneumatic type;

(55) secondary valve of the electromagnetic type;

(56a) "main valve" downstream of the compressor;

(56b) "main valve" upstream of the compressor;

(60) heat exchanger;

(61 ) compressor;

(62) on/off switch;

(70) lines for sale gas;

(80) marine engine converted into dual-fuel mode or methane-only engine;

(E) engine of the compressor;

(PC) pressure controller;

(PI) pressure indicator;

(PT) pressure transmitter;

The pressure vessels described herein can carry a variety of gases, such as raw gas straight from a bore well, including raw natural gas, e.g. when compressed - raw CNG or RCNG, or H2, or processed natural gas (methane), or raw or part processed natural gas, e.g. with C02 allowances of up to 14% molar, H2S allowances of up to 1 ,000 ppm, or H2 and C02 gas impurities, or other impurities or corrosive species. The preferred use, however, is CNG, be that raw CNG, part processed CNG or clean CNG - processed to a standard deliverable to the end user, e.g. commercial, industrial or residential.

The CNG will typically be carried at a pressure in excess of 60 barg, and potentially in excess of 100 bar, 150 bar, 200 bar or 250 bar, and potentially peaking at 300 bar or 350 bar.

CNG can include various potential component parts in a variable mixture of ratios, some in their gas phase and others in a liquid phase, or a mix of both. Those component parts will typically comprise one or more of the following compounds: C2H6, C3H8, C4H10, C5H 12, C6H14, C7H16, C8H18, C9+ hydrocarbons, C02 and H2S, plus potentially toluene, diesel and octane in a liquid state, and other impurities/species.

The present invention has been described above purely by way of example. Modifications in detail may be made to the invention within the scope of the claims appended hereto.