The present invention relates to a system and method for boosting BOG (Boil Off Gas) in a LNG (Liquid Natural Gas) fuel system, in particular boosting the BOG using an ejector.

Several methods have been proposed on how to fuel an LNG carrier with gas instead of marine diesel oil or similar. Proposed methods distinguish between primarily two types of main engines:

-Medium speed engines requiring relatively low fuel gas pressure of about 500 kPa.

-Slow speed diesel engines requiring between 20 000 to 30 000 kPa fuel gas pressure.

One such method is disclosed in EP 1 990 272 Bl where LNG is pumped from the cargo tanks, heat exchanged with compressed boil off gas, and then further compressed to injection pressure. The compressed boil off gas is liquefied against the LNG and returned to the cargo tanks. When the main engine is running on marine diesel or running on low speed, in particular on laden voyage, no or little LNG is available to remove heat flowing into the cargo containment system.

NO20082158 and NO20093562 discloses systems and methods for gas supply to gas or duel- fuel engines, where the gas supply system is integrated with a reliquefaction plant to remove heat flowing into the cargo containment system.

GB1440318A discloses a system for system using an ejector to control the vapor pressure in LNG cargo tanks by pumping LNG, vaporizing it and route the resulting gas as the motive fluid to an ejector taking suction from the vapor space, thus eliminating the need for compressor operations in order to maintain the vapor pressure. The system of GB 1440318 A is intended for dual fuel boilers used for main propulsion machinery and do not discuss the technical problems encountered if the main propulsion machinery where to be slow speed diesel engines. Dual fuel boilers requires typically a fuel gas pressure less than 200 kPa, whilst a slow speed diesel engine requires fuel gas pressures between 20 000 and 30 000 kPa.

As slow speed diesel engines requires high fuel gas pressure a HP-pump (High Pressure) is usually arranged in the system to provide the required gas pressure. To avoid pump cavitations it is important to ensure that the HP pump has the required NPSH (Net Positive Suction Height) during all operating modes, and particularly during ballast voyage where it is normal to have a free flow circulation of the boil off gas in order to avoid boil of gass compressor recirculation in order not to enter into the surge area of the compressor. Boil off rates are significantly lower during ballast voyage than laden voyage, hence it may be more convenient to have the compressor not running and route the boil off gas directly to the reliquefaction plant using the principles of free flow (natural circulation). Particularly in rough sea conditions in combination with ballast voyage, experience shows that on prior art installations pump cavitations due to insufficient NPSH is a challenge.

This invention describes a system and a method to ensure that the HP pump has required NPSH (Net Positive Suction Height) during all operating modes, and particularly during ballast voyage where it is normal to have a free flow circulation of the boil off gas in order to avoid compressor recirculation. Particularly in rough sea conditions in combination with ballast voyage, experience shows that on equivalent installations pump cavitations due to insufficient NPSH is a challenge. It is therefore an object of the present invention to provide a system and a method to ensure that the HP pump has required NPSH (Net Positive Suction Height) during all operating modes.

To meet the problems above the present invention discloses a gas supply system for dual-fuel or gas engines, comprising at least one liquid natural gas (LNG) cargo tank 1 , a first pump 2, a compressor 30, a cryogenic heat exchanger 16 and a high pressure pump 10 delivering gas to the engines, wherein the system further comprises an ejector 4 and a suction drum 6; the ejector 4)is arranged to receive LNG motive fluid from the first pump, draw condensate from the cryogenic heat exchanger 16 and discharge a mixture of the motive fluid and condensate to the suction drum 6; and the suction drum 6 is arranged to supply the mixture of the LNG motive fluid and the condensate to the high pressure pump 10.

The present invention also discloses a method of supplying gas to dual-fuel or gas engines in a system comprising at least one liquid natural gas (LNG) cargo tank 1, a first pump 2, a compressor 30, a cryogenic heat exchanger 16 and a high pressure pump 10 delivering gas to the engines, wherein the method comprises providing a suction drum (6) upstream the high pressure pump (10); providing an ejector (4) between the first pump (2) and the suction drum (6); pumping, by the first pump (2), LNG motive fluid to the ejector (4); drawing, by LNG motive fluid received by the ejector (4), condensate from the cryogenic heat exchanger (16); discharging, by the ejector (4); a mixture of the motive fluid and condensate to the suction drum (6); and supplying from the suction drum (6) the mixture of the LNG motive fluid and the condensate to the high pressure pump (10). Other favorable embodiments of the present invention are to be understood by the dependent patent claims and the detailed description hereinafter, with reference to the amended figures; wherein:

Fig. 1 is a schematic overview of a reliquefaction unit of prior art;

Fig. 2 is a schematic overview of one embodiment of a system according to the present invention; and

Fig. 3 is a schematic overview of another embodiment of a system according to the present invention.

Figure 1 illustrates a typical prior art cargo containment system in a LNG carrier having at least one cargo tank 1. Cargo loading lines are not shown. Due to natural heat leakage in to the cargo containment system a certain amount of the cargo is evaporated, known as Boil Off Gas (BOG). The BOG flows via line 14 to a BOG compressor 30. BOG Compressor 30 can be of single or a multistage type. Compressed BOG leaves the BOG compressor via line 15 and enters cooler 31. Choice of cooling medium may depend on compressor discharge temperature. Different outlines are known to the skilled person and it is not further described herein. Cooled and compressed boil off gas leaves the BOG compressor after cooler 31 via line 32 and enters a cryogenic heat exchanger 16. The cryogenic heat exchanger is typically a multi stream exchanger where the streams are connected to a circulating refrigerant loop. This is not further illustrated herein as such systems in known to the person skilled in the art.

One operational mode of the LNG carrier is that the propulsion machinery and auxiliary engines (not shown) are running on marine diesel oil or equivalent. This mode illustrates current state of the art and is illustrated in figure 1. In this mode, condensate leaves the cryogenic heat exchanger 16 via line 17 to separator 18. Valve 25 is fully open and pump 8 in combination with valve 34 controls the liquid level in separator 18 by known control principles, and the liquid is pumped back to the at least one cargo tank 1 via line 33. In the case non-condensable gases entering separator 18, they are released via line 21 to safe location.

Condensate shall be understood as liquefied boil off gas, where boil off gas is vapor emitting from the cargo due to a constant heat leakage into the cargo tanks.

Fig. 2 is a schematic overview of a system according to the present invention wherein the LNG cargo system of fig. 1 is integrated with a LNG fuel gas supply system, the LNG fuel gas supply system being provided with a suction drum 6, an ejector 4, a HP-pump 10 and additional valves 12, 19, 26, 39, 40. The pump 2, hereinafter referred to as a motive fluid pump, pumps LNG via line 3 into the ejector 4, wherein the LNG as motive fluid draws the condensate via valve 19 into the ejector 4, whereupon the ejector 4 discharges a mixture of motive fluid (LNG) and condensate at a pressure above the condensing pressure in the heat exchanger 16 and is sent via line 5 to suction drum 6. As the pressure in 5 is above the condensing pressure in the heat exchanger 16 the suction drum 6 can be located at an elevation higher than separator 18 such that an increased static head is provided between the liquid surface and the inlet flange of HP pump 10, whereby ensuring that a sufficient NPSH is maintained at any time for the High Pressure (HP) pump 10. This arrangement thus eliminates operation of pump 8 and the problem of pump cavitations is eliminated.

The HP pump 10 then takes suction from the suction drum 6 via line 7 and lifts the liquid in pressure (liquid is the mixture of LNG and condensate) and compresses it to typically 30 000 kPa. Liquid flowing from the HP pump 10 then typically enters a series of heat exchangers in order to be heated and transferred to the propulsion machinery and /or auxiliary engines. This principle is well documented in e.g. NO20093562 and is thus omitted from this description.

In the exemplary operational mode of the LNG carrier described with reference to fig. 1 the propulsion machinery and auxiliary engines are running on marine diesel oil or equivalent. When the same operation mode is applied to the system of figure 2, the following applies: a three way valve 26 is open towards the separator 18. Liquid flows from separator 18 via line 20, through valve 25, then lifted in pressure by pump 8, then via line 33 back to the at least one cargo tank 1.

Again with reference to fig. 2, according to another exemplary embodiment of the present invention, as the rate of condensate supply or LNG supply might change due to operational issues, the suction drum 6 typically has a retention time sufficient to ensure stable calorific value of the combined mixture of LNG and condensed boil off gas. This implies that the high pressure 10 pump can feed the gas engine for a period of time without the motive fluid pump 2 being in operation.

The boil off rate during laden voyage depends on several factors as thermal insulation thickness, ambient temperatures, cargo composition and sea state. For situations where the boil off rate is in excess of the fuel demand, excess condensate is returned back to cargo tank 1. This mode is described in NO20093562. For modes where the fuel demand is in excess of the boil off rate, additional fuel is supplied from the cargo tank via motive fluid pump 2 through line 3 to ejector 4. Condensate flows either via line 20 or 27 to ejector 4. The amount of motive fluid supplied via line 3 is balanced to ensure that the resulting fluid discharged from the ejector 4 has sufficient head to enter suction drum 6. High pressure pump 10 takes suction from this tank via line 7, while any excessive liquid entering 6 flows via line 33 back to cargo tank 1.

During ballast voyage the amount of boil off gas is considerable less than during laden voyage and in order not to waste energy by recirculation of a portion of the BOG through the boil off gas compressor 30, free flow of BOG is accommodated for by opening recycle valve 37 fully such that the boil off gas can flow freely from the at least one cargo tank 1 to the cryogenic heat exchanger 16.

Free flow is not easily started and its rate of increase is governed by the rate of condensation at any instant in the cryogenic heat exchanger 16. By common experience, free flow can take several hours before required flow rate is achieved.

One possible method to start the free flow would be to open valve 22 and have the tank vapor flowing through the system and then to an oxidizer or to a vent mast. This is not a good solution since both oxidizer and vent mast involves putting greenhouse gases to the atmosphere.

According to an exemplary embodiment of the present invention, the free flow is kick started by pumping LNG with motive fluid pump 2 and send it via line 3 to the ejector 4 as the motive fluid. The ejector 4 generates a suction via lines 27,17 or via line 20 to receive inlet fluid from the separator 18 or the cold box 16, respectively. In both cases valve 25 is closed.

The separator 18 is typically used when there are large amounts of non condensable gases (nitrogen) present that need to be vented from the condensate. The flow path from the cryogenic heat exchanger 16 will then be on line 17 via the three way valve 26, now open to separator 18. Condensate is drawn from separator 18 via line 20 to the ejector 4. Valve 19 regulates the flow based on known principles. Prior and during kick start the control algorithm for level control of 18 is bypassed.

Bypassing the separator is a typical option when the amount of Nitrogen in the boil off gas is low so that complete liquefaction is achieved. Valve 26 now opens to line 27 routing condensate from the cryogenic heat exchanger 16 via lines 17, 27 and 20 to the ejector 4. Flow is controlled by valve 19. Bypassing the separator is a preferred kick start mode. When during startup the system has not reached sufficiently low temperatures, a certain degree of boiling will occur. This vapor is recycled via line 11 back to the inlet side of the BOG compressor 30. Another alternative embodiment of the present invention is described with reference to figure 3. Occasionally during laden voyage it is desirable to use LNG pumped from the cargo tank 1 instead of BOG as fuel, e.g. if the LNG has a calorific value higher than acceptable pipeline grid values at the reception side, such as a onshore plant. It is common to inject Nitrogen into the vaporized LNG at the reception side in order to reduce the calorific value. Thus to minimize the amount of required injected Nitrogen at the shore side it is desirable to recycle as much as possible of the evaporated Nitrogen back to the at least one cargo tank 1. Also, the installed reliquefaction unit is not necessarily capable of fully liquefying the boil off gas, thus it is not uncommon that a certain degree of Nitrogen will have to be released via line 21. There is also an unwanted Methane loss associated with the Nitrogen release via line 21.

In order to capture, recycle and utilize parts of the Nitrogen / Methane losses released in 21, the system of figure 2 is further provided with a line 49 including a control valve 50, a second ejector 46, and a three way selector valve 47. Alternatively, the functionality of the three way selector valve 47 could be provided by different means, such as two regular valves. Motive fluid for the second ejector 46 is delivered by the motive fluid pump 2 via line 3 through valve 47, then via line 48 to the ejector 46. Non condensed gases separated in 18 flows via line 21, valve 22 are closed and the gases flow via line 49 to ejector 46. Valve 50 ensures correct pressure in separator 18. The resulting mixture of motive fluid (LNG) and gas from line 49 is discharged from the ejector via line 45 and enters separator 6. Gases (if any) separated in separator 6 flows via line 51 and mixes with the liquefied boil off gas in stream 33 before returning back to the at least one cargo tank 1.

While the invention has been illustrated and described in detail in the drawings and forgoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive and is not intended to limit the invention to the disclosed embodiments. The mere fact that certain features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be used advantageously. Any reference signs in the claims should not be construed as limiting the scope of the invention.