Technical Field

The present invention relates to a system wherein vented vapours from the cargo tanks are compressed and used as complementary fuel to a vaporised LNG primary fuel stream and premixed in a mixer unit before supplied to a combustion engine comprising a fuel mix unit for maintaining the mixing ratio within engine required fuel quality (here Methane Number, MN).

During loading and transport of crude oil or any treated and / or refined oil cargo on oceangoing vessels volatile organic compounds (VOC) are released from the ship's cargo tanks to the atmosphere. VOC are organic chemicals that have a high vapour pressure and VOC emitted from crude oil and oil cargoes are normally built up by hydrocarbon gases such as methane, ethane, propane, butane and pentane. Fractions of heavier components may also be present in the emitted VOC gas.

On oil tankers the emission of VOC will increase the tank pressure. Since cargo tanks on oil tankers typically are designed for a tank pressure of only 120 to 200 mbar, the tank pressure must be relieved when it is approaching the maximum tank pressure. This is normally done by relieving the pressure through the common vent lines on deck, which lead to a vent mast riser. In the vent line leading to the vent mast riser, there is a closing valve, manual or automatic, which will be opened to relieve the pressure from the tanks. The tanks are also equipped with pressure / vacuum valves, which will relieve the pressure directly to atmosphere if the maximum design pressure for the tank is exceeded.

Opposed to shuttle tankers taking loading primarily at sea and recovering the VOC used during the relatively short loading interval (compared to voyage), a crude oil tanker will recover the VOC during the entire loaded voyage. The recovery system will be vastly different due to chemical composition of the VOC (e.g. low in inert gas content), and emission rates during sailing will typically be 1/10 ^th or less of the emission rates during loading.

Also opposed to the shuttle tankers, the LVOC fraction recovered, which typically is constituted by ethane, propane, butane and higher hydrocarbons, are normally routed back to the cargo tanks, and a stream of surplus VOC, referred to as SVOC, that will not liquefy will be used as supplement to fuel the VOC recovery apparatus or other purpose.

VOC consists of hydrocarbons which are considered green house gases, of which one is typically methane, which has a CO2 equivalent of 84 based on a 10 Year Cycle. VOC emission is also harmful for the environment in other aspects, such as e.g. the formation of ground near ozone which is recognised as a serious health issue as well as it is damaging to vegetation and the ecosystem in general. Ground level ozone is created by chemical reactions between NO _x and VOC in the presence of sunlight.

Although an oil tanker is often to be understood as a ship/vessel transporting treated and/or refined oil normally between onshore facilities, in relation to the present invention the term oil tanker shall not be limited to transport between onshore facilities only. The term oil tanker will be used also for crude oil shuttle tankers. In this text, tankers are also referred to as carriers.

Where output of the VOC recovery process is a liquefied VOC, LVOC, the VOC recovery process typically also outputs an amount of surplus VOC, SVOC, which is typically constituted by hydrocarbons that do not easily liquefy. The SVOC being obtained from the VOC recovery is typically vented to the environment, either directly or by abatement that could include flaring or other type of oxidation. In some cases, the SVOC is routed back for re-absorption into the hydrocarbon load, or it is delivered to a burner of e.g. a boiler to generate steam to be used for different on-board purposes, such as e.g. to drive an on-board steam turbine.

With an increasing environmental awareness, and the realisation of the inventors of the present invention that the SVOC represents a high value fuel that can be utilised for propulsion instead of being vented to the atmosphere as is the current solution, there is a need for new systems offering a solution where SVOC can be used as complementary fuel.

Marine engine technology has in recent years developed such that LNG can be utilised as fuel onboard oil tankers. The requirements for typical engines are Methane Number of 70 and an energy content in the fuel of no less than 28MJ/kg fuel. Methane number is defined as a measure of resistance of a gas fuel to knock, which is assigned to a test fuel based upon operation in a knock testing unit at the same standard knock intensity. It is assigned that pure methane is used as the knock resistant reference fuel, that is, methane number of pure methane is 100, and pure hydrogen is used as the knock sensitive reference fuel, methane number of pure hydrogen is 0.

However, the methane number, MN, of a typical SVOC composition will normally be too low for use as the sole fuel gas for a gas fueled ottomotor type high power ship propulsion engine, OT-SPE, since heavy hydrocarbons of the typical SVOC contribute to a low MN. Sending SVOC directly the OT-SPE could result in harmful engine knocking and possibly damage of the OT-SPE.

During a typical voyage of a VLCC, which is an industry known term for a super tanker (Very Large Crude Carrier) with an oil carrying capacity typically between 200 000 tons and 400 000 tons, VOC emissions from crude oil will be approx. 0.085% of the total cargo volume per week for a 320 000 dwt VLCC, which equates to a VOC emission of about 272 tons of VOC every week. As a comparison in respect of fuel consumption, a VLCC with installed propulsion machinery of 17 MW will consume around 400 tons of LNG each week.

Typical heating value for captured SVOC can be upto 45 - 48 MJ/kg, which is close to the heating value of LNG; i.e., with reference to the example above, the VOC emissions in energy content represent more than 60% of the fuel requirements. However, it is not straight forward to utilise this amount of SVOC as fuel of the high power ship propulsion engine.

A further challenge related to using SVOC as a secondary fuel for high power OT-SPE is firstly its varying quality, as a single fuel it does not meet the required fuel properties of modern gas fuelled high power propulsion engines. Furthermore, the consumption of SVOC rcovered during a journey should be as high as possible to ensure that the recovered volume of SVOC can be fully consumed before reaching the destination, as it otherwise has to be discharged in some way to the environment, alternatively be buffered back into the cargo tanks by absorbing parts or the entire amount of SVOC into the cargo. Generally, there will not be only a single curve for the SVOC to LNG primary fuel mixing ratio versus engine demand, due to the fact that both LNG and SVOC may/will vary in composition between each loading / bunkering. The composition of SVOC differs from different loading points, and is influenced by weather conditions and e.g. the chemical composition of the carried cargo, which also has to be accounted for to determine a correct mixing ratio of recovered SVOC into the engine fuel.

Marintek Report no. 239114.00.01 discusses the use of surplus VOC, SVOC, therein referred to as "un-recovered VOC", as supplementary fuel to on-board engines where the purpose is to reduce emissions of un-recovered VOC by consuming it in on-board engines for power production. More specifically, Marintek Report no. 239114.00.01 discloses the un-recovered gas downstream of a recovery plant is a vide variation of lean low calorific gases depending on e.g. the loaded crude oil ond the recovery technique applied. These gas qualities could be fed to a gas engine for power production. Gas engines in the range of 2-4MW is available (one, two or three units) but have to be adapted to the specific case and improved control system for handling variations in gas qualities.

There is a risk involved in using untreated, compressed VOC, since, when compressing untreated VOC, some of the heavier hydrocarbons can form as condensate, which could damage the engine if supplied to an engine as fuel. Due to the high content of heavy hydrocarbons of a typical VOC, only a limited amount of VOC hydrocarbons can be mixed with the LNG in order to keep the methane number above a minimum required level.

Summary of the Invention

It is therefore an object of the present invention to provide a system and method where SVOC can be utilised as a fuel in combination with vaporized LNG fuel for high power propulsion engines on board ocean going crude oil tankers and ocean going oil tankers.

Objects of the invention are reached with the tanker apparatus and method of the invention specified in the independent claims 1 and 8, respectively.

Objects of the invention are also reached with the method of the invention specified in the independent claim 14. Further embodiments of the tanker apparatus and methods of present invention are as set out in the dependent claims 2-7, 9-13, and 15-22.

Brief Description of the Drawings

Embodiments of the invention will now be described in further detail with reference to the following figures in order to exemplify its principles, operation and advantages.

Figure 1 shows a principal arrangement of a vent mast riser.

Figure 2 shows a principal arrangement of an oil tanker with a VOC recovery plant and vaporised LNG is used as fuel for propulsion.

Figure 3 shows a principal arrangement of an oil tanker with a VOC recovery unit and LVOC return to the load, and where SVOC from the VOC recovery unit is mixed with the vaporised LNG fuel and the mixture is used as fuel for the propulsion engine.

Figure 4A shows a principal arrangement of an oil tanker with a VOC recovery unit and LVOC stored in a LVOC tank, and where SVOC from the VOC recovery unit is mixed with the vaporised LNG fuel and the mixture is used as fuel for the propulsion engine.

Figure 4B shows a principal arrangement of an oil tanker with a VOC recovery unit and LVOC stored in a LVOC tank, and where SVOC from the VOC recovery unit and vaporised LVOC from the LVOC tank are mixed with the vaporised LNG fuel and the mixture is used as fuel for the propulsion engine.

Figure 5 shows a detailed flow diagram of an exemplary embodiment of the arrangement illustrated in figure 3.

Figure 6 shows a detailed flow diagram of an exemplary embodiment of the arrangement illustrated in figure 4.

Figure 7 shows a flow diagram for a fuel mixing controller. Figure 8 shows a flow diagram for a valve ratio controller. Figure 9 shows a flow diagram for a mixing ratio limiter.

Figure 10 gives further details of a mixing ratio limiter corresponding to the one shown in figure 9.

Figure 11 shows typical OT-SPE relative power output as a function of MN (derating curve). The curve will normally be engine dependent.

Detailed description of the Invention

Natural gas is stored on the ship in dedicated fuel tanks in liquid form and known as liquefied natural gas (LNG). LNG is pumped to correct pressure, vaporised and mixed with SVOC to form a one-gaseous fuel source to the main OT-SPE or an auxiliary engine. In order to ensure stable combustion, the OT-SPE may have a pilot fuel supply.

Such a ship may have one or two main engines propelling the ship but the invention shall be equally valid for ships with more than two engines propelling the ships as e.g. ships with three, four or more main engines in an engine / electric configuration.

By using SVOC output from the VOC recovery plant mixed directly with LNG to serve as fuel for the marine engine,is mitigated the risk involved in using untreated, compressed VOC.

A solution for recovering VOC from the load is to install a condensation plant for VOC. The heavier hydrocarbons are condensed and separated out together with water (if any) that are present in the emitted VOC and returned to the cargo tanks. Downstream of the recovery plant, un-recovered light hydrocarbons, herein referred to as SVOC, is allowed to flow as a gas to the fuel mixing unit where SVOC mixes with vaporised LNG. The resulting mixed gas then is allowed to flow as fuel to the propulsion engine.

Figure 3 shows a principal sketch of an oil tanker with a VOC recovery plant where VOC emitted from the cargo tanks are captured by the VOC recovery unit, LVOC is separated, and SVOC is mixed with vaporised LNG and the gas mixture is sent as fuel to the propulsion engine. Optionally, the part of captured VOC is processed into liquefied VOC, LVOC, and stored in an on-board LVOC storage tank. Un-recovered VOC, SVOC, and vaporised LNG are mixed in the fuel mixing unit. To simplify Figure 4, it does not show separation of water in the LVOC tank nor does it show the fuel mixing control. In detail, the VOC plant will consist of an inlet filter, a VOC compressor, and a seawater / VOC condenser, and a three phase separator separating the output from the condenser into water, liquid VOC (LVOC) and surplus VOC (SVOC). Depending on engine fuel gas pressure requirements an additional booster compressor might be added after the separation. Also depending on engine requirements for maximum water content in the fuel gas, an adsorption drier as e.g. a molecular-sieve bed drier can be added to remove excess water. Other types of available driers may also be used.

Figure 5 shows a more detailed view of the invention where SVOC is utilised as fuel. Emitted VOC from the cargo tanks during sailing flows firstly via a condensate separator where any liquids or solid particles are removed prior to being compressed in the VOC compressor. Compressor discharge pressure shall normally be sufficiently higher than the engine fuel gas pressure requirement. Typical engine fuel gas requirement ranges from 8 to 17 barg and compressor discharge pressure must be moderately above the requirement to accommodate for pressure losses and e.g. surges during sudden increase in fuel consumption. The VOC compressor can have one or more stages of compression with or without interstage cooling, this is common knowledge and thus not shown. Higher injection pressures are also possible and the invention shall not be limited with regard to range. The seawater cooled VOC cooler removes the heat of compression additionally to allowing for a variable rate of condensation controlled by a mixing ratio controller providing increased cooling if the methane number gets to low and less cooling if the methane number gets above a predefined value for safe operation from the point of harmful engine knocking.

Increased cooling condenses out the heavier components and by such raises the methane number of the mixture. The VOC Separator separates out the LVOC (if any) and routes it back on level control to the cargo tank for re-absorption. In situations where the engine load are suddenly reduced or stopped, the VOC separator pressure increases and a pressure indicator sends a signal to the level controller to drain the separator and allow SVOC flow via the LVOC line and back to the cargo tank bottom where the SVOC is absorbed into the cargo in an arrangement as e.g. described in N0331559 B 1. SVOC flows from the VOC separator via valve V4 under control by the mixing ratio controller. SVOC is mixed with vaporised LNG in a fuel mixing unit. A gas analysing device monitors the chemical composition of the SVOC and the mixed gas product (SVOC + vaporised LNG) and sends the result to the mixing ratio controller for it to calculate mixing ratio limits.

Typically, in a system based on the system illustrated in figure 5, there will also be flow rate controllers / elements on the fuel supply, however, this is not shown here.

Figure 6 shows a more detailed view of the invention where SVOC is captured and LVOC is stored in an on-board LVOC storage tank. The process route is at least in part similar to the one illustrated in figure 5, except that LVOC is not not routed back to the load, but is instead stored in a dedicated LVOC tank

When being returned to the load in cargo tanks, the LVOC becomes depressurized. The depressurized LVOC shall be returned through the drop line (cargo discharge line) into one or more tanks. An additional benefit of this is that emissions from cargo tank breathing are reduced, possibly to zero, and that the delivered cargo volume is increased by returning LVOC to the cargo tanks.

VOC emission rate is influenced by weather, especially temperature and ship movements as well as the characteristics of the oil cargo. If the amount of surplus VOC, SVOC, from the on-board VOC recovery plant is below allowable amounts that the engine can accept of added SVOC, condensing temperature can advantageously be increased to result in less condensation of LVOC and hence more SVOC to the engine. It is contemplated that this is done by installing a regulating valve on the seawater inlet to the VOC cooler. This valve will be governed by two parameters; namely by a methane number analysed by a gas chromatograph in the fuel line, and by an upper acceptable fuel temperature depending on engine manufacturers' specification and LNG fuel temperature. Seawater inlet to the VOC cooler can also be a glycol / water coolant flow or any other type of cooling medium used in industrial cooling applications.

On the other hand, there might be too much VOC caused by light oil cargo, by heavy ship movements and/or by an increase in temperature. In this case, the vent mast riser valve will need to open when necessary in order to keep the correct tank pressure.

Figure 1 shows a typical vent system. Typically, in use with a state of the art OT-SPE on laden voyages, the present invention allows SVOC to be mixed into the vaporised LNG fuel to constitute as much as or exceeding 50% of the fuel gas mixture. Assuming 21 weeks laden, and less fuel consumption during ballast voyage, a fuel saving of 25% can be expected. This may be applicable to "slow steaming" sailing. At "maximum speed" sailing, the invention allows a mixing of SVOC into vaporised LNG fuel gas of up to as much as or exceeding 30%) of the fuel gas mixture.

For some oil tankers it is desired also to utilise the LVOC, and hence not return the LVOC back to the cargo tanks.

In a preferred operation scheme, the SVOC will be consumed as fuel for the OT-SPE before the next loading, however, the invention is not limited to a complete

consumption of the SVOC as fuel for the OT-SPE before the next loading, also partial consumption of the SVOC as fuel for the OT-SPE before the next loading is within the scope of the invention.

The propulsion engines are gas engines fuelled by LNG. Due to its low methane number, SVOC does not possess the quality to be used as a single fuel source.

Therefore, a certain quantity of SVOC is mixed into the vaporised LNG fuel under control to keep harmful engine knocking from occuring. In order to prevent harmful engine knocking from occurring, the mixing ratio between vaporized LNG and SVOC is held such that the methane number of the mix of vaporized LNG and SVOC equals or are higher than minimum specified methane number for the OT-SPE. Also to be considered is that a presence of heavier hydrocarbon components in the SVOC reduces the methane number, whilst a presence of lighter hydrocarbon components, such as e.g. methane, increases the methane number.

Required methane number for a specific OT-SPE is not a fixed value. It varies with the engine load. A low load (higher de-rating) allows for a lower methane number, and hence more SVOC can be mixed into the LNG. During a voyage, energy demand will typically be less than the engines' maximum continuous rating using vaporized LNG only. Hence, a certain amount of SVOC can be added to the vaporized LNG fuel, and at slower vessel speeds (less power demand), even more SVOC can be added Figure 10 shows typical engine relative power output as a function of methane number, MN, of the fuel supplied to the engine. The curve will normally be engine dependent.

Use of measuring of the chemical composition of both the LNG and SVOC, e.g. by gas chromatography, GC, enables prediction of a range of mixing ratios for current LNG amount of LNG loaded and current amount of SVOC captured. Other means of analysis, such as as e.g. infrared detection, may be used as an alternative to GC.

To utilize as much recovered SVOC as possible, it is contemplated that the engine(s) is operated towards the engines knocking threshold, for best power and fuel economy.

Optionally as an additional safety level, the engine control system can be extended such that the engine control system issues a pre-warning before harmful knocking occurs and sends a feedback to the ratio controller that will change the mixture rate.

For purposes of simplicity and cost, in the case of mixing the SVOC into a supply of vaporized LNG fuel to a plurality of on-board engins, it is beneficial to use one common fuel mixing station for all engines, as engines will normally share the loads, however, individual mixing stations for each respective OT-SPE is also contemplated.

In the cases of a common SVOC and vaporized LNG fuel mixing station, the fuel mixing ratio controller ensures that the mixing ratio is always within its limit with a margin related to the highest loaded engine, such as e.g. in an arrangement of two or more engines operating in parallel. The margin assures that a sudden load increase will not cause a problem of harmful knocking problem for the engines.

In certain engine configurations with big variations in engine loads, the fuel mixing control can be adopted for individual engines control.

The pre-defined mixing ratio with margins, obtained e.g. from GC measurement and initial ratio calculations, allows fast response and minimum control fluctuations during any load demand changes.

In figure 7, 8, 9, and 10, which illustrate elements and functions of an exemplary embodiment of a fuel mixing controller, features are identified by the following reference numbers: 20 Change in FCV2 position to correct measured mixing ratio if it deviate from mixing ration set point (MRsp)

22 The calculated valve position of FCV2 by MRsp and valve characteristics

24 The corrected FCV2 position based on mass flow measurements

26,28,40 MRsp limit to valve ratio controller and Mixing trim controller

30,32,36 FCV1 position setpoint

34 Mixing ratio, manual input / engine load

35 FCV2 Stem position

38 Gas to engine pressure set point

FT1 Massflow of vaporised LNG sent to engine

FT2 Massflow of vaporized LVOC sent to engine, optionally combined with

SVOC if SVOC is used as fuel.

PT1 Pressure reading sensor

CP1 Connecting point to valve ratio controller

CP2 Connecting point to valve ratio controller

CP3 Connecting point to mixing ratio limiter

FCV1 Flow control valve, vaporised LNG

FCV2 Flow control valve, SVOC.

Figure 8 shows a flow diagram of an exemplary embodiment of a valve ratio controller with features identified by the following reference numbers:

Tl Temperature reading element

PT2 Pressure reading sensor

PT3 Pressure reading sensor

T2 Temperature reading element

PT4 Pressure reading sensor

PT5 Pressure reading sensor

CP1 Connecting point to valve ratio controller

CP2 Connecting point to valve ratio controller

FCV1 Flow control valve, vaporised LNG

FCV2 Flow control valve, SVOC.

Figure 9 shows a flow diagram of an exemplary embodiment of a mixing ratio limiter with features identified by the following definitions: 40 Mixing ratio limiter (MRlim) limits the fuel mixing value to lowest required methane number according to the following functions:

MRlim = MRman + k * dMN where MRman is mixing ratio manual setpoint, k is a constant. dMN = MNmix - (MNreq + MNmargine) where MNmix is fuel mixture MN, MNreq is required engine MN and MNmargin is a constant that can be set by an operator.

42 Mixing ratio manual setpoint (MRman)

46 Gas analyser data: SVOC composition.

48 Gas analyser data: Vaporised LNG composition.

50 Optional engine load measurement

52 Required engine MN (MNreq). The required methane number is a function of engine load where the engine load is found by total fuel flow to the engine and engine efficiency factor. Optionally by direct measurements.

54 Fuel mixture MN (MNmix), given by composition and mixture ratio. Gas composition is found by online GC or similar instruments, item 46 & 48 above.

FT1 Massflow of vaporised LNG sent to engine

FT2 Massflow of SVOC sent to engine

CP3 Connecting point to mixing ratio limiter

CP4 Connecting point to mixing ratio limiter

The SVOC and vaporized LNG fuel mixing system ensures correct mixed fuel supply pressure and correct MN at any load situations and is next described briefly by reference to Figures7, 8, 9 and 10.

1. At a given load the engine methane number calculation function calculates

minimum required MN, illustrated by the derating curve of figure 11. The calculation function receives input from operator manual fuel override (if requested) and fuel flow rates (FT1 and FT2). 2. The fuel MN calculation function calculates correct mixing ratio of the fuels. The calculation function receives input from GC measurement, fuel flow rates (FT1 and FT2) and engine MN calculation function.

3. The mixing ratio limiter applies a margin to the required engine MN and adjusts the fuel mixing ratio to account for this margin.

4. The mixing ratio limiter sends the mixing ratio to both the valve ratio controller and the mixing trim controller.

5. The mixing trim controller receives flow rate information from FT1 and FT2 and finetunes the mixing ratio and inputs data to FCV2 valve flow/opening function.

6. The valve ratio controller receives mix ratios from the mixing ratio limiter and calculates valve openings based on pressures, temperatures and Cv. Output is sent to the individual valve flow/opening functions for FCVl and FCV2.

7. The mixing point pressure controller ensures that the engine sees correct fuel gas pressure at any load situation.

The main function of the system is to control the supply pressure at the mixing point denoted as PTl in Figure 5, and to maintain at any time correct mixing ratios between the fuels ensuring that the MN of the fuel mixture meets the requirements set by the engine methane number calculation function in Figure 7.

Vaporised LNG is the main fuel source and valve FCVl maintains the pressure PTl at the mixing point, preferably just downstream of the physical mixer. The mixing point pressure controller is preferably an independent pressure control loop and will not be further described.

The valve ratio controller shown in Figure 8 controls the mixture ratio of SVOC and vaporised LNG. The mixing ratio is controlled by adjusting the valve positions of valves FCVl and FCV2, respectively.

By using valve characteristics, pressures readings, and temperature readings, the valve ratio controller gives valve positions in accordance with the mixing ratio requirement. It is suggested that pressures are read both upstream and downstream of the two valves, while temperature is read only upstream of said valves. The mixing ratio limiter (Figure 9) assures that the mixing ratio of the SVOC and vaporized LNG is always controlled such that the MN of the SVOC and vaporized LNG fuel gas mixture for delivery to the OT-SPE is within requirements that are continuously calculated by the engine methane number calculation function in Figure 9. Furthermore, the mixing trim controller receives measured flowrate values from FTl and FT2 and conducts a final finetuning of the mixing ratio.

Fine tuner means are arranged to finetune the mixing ratio of vaporised LNG/SVOC, wherein the two flows are measured by mass flowmeters, the actual mixing ratio is calculated and any deviation from the mixing ratio set point is then corrected by the SVOC valve, said correction being slow when increasing the mixing ratio and fast with an offset when reducing mixing ratio, to always assure the actual mixing ratio is below the mixing ratio set point.

The valve ratio controller (see Figure 8) computes the mass flow through each of the two valves FCV1 and FCV2 by using the following input values:

1. Valve position as a process value feedback from the control system with system stored valve Cv values. The Cv value is a universal known flow coefficient giving a relative measure of its efficiency at allowing fluid flow.

2. Pressures upstream and downstream of the two valves read by pressure

transducers PT2 & PT3 and PT4 & PT5.

3. Upstream temperatures of the two valves read by temperature transducers by Tl and T2.

4. Independent physical properties of the SVOC and vaporized LNG to be mixed, such as e.g. their respective specific heats and densities.

Densities of the SVOC and the vaporized LNG are calculated by the valve ratio controller or measured by using the two flow meters FTl and FT2.

The valve ratio controller then calculates the required mixing ratio, and based on the flow of the vaporised LNG, the SVOC flow and the corresponding valve position is determined. Since the valve ratio controller does not have mass flow value process feedback, it does do not know if the actual mixing ratio is correct or not, and a mixture trim controller is needed. The mixture trim controller adjusts the mixing ratio calculated by the valve ratio controller by actual flow rate readings via FTl and FT2. Compared to the response properties of the valve ratio controller, the mixture trim controller typically has a slower response, and can thus only be used for fine tuning of the mixing rate, typically fine tuning within +/- 20% of the FCV2 open signal (valve flow/opening function, see Figure 7). In order to get correct valve openings, the output of

flow/opening function of the valve must be converted to a linear relationship with the valve stem opening (VSP). Computed mass flows are based on actual valve flow coefficient, Cv, values for the two valves FCVl and FCV2, and the control output to the valves is converted by a linearization function using a linearization table; see table 1 below for typical examples. Table 1

Valve Flow/Opening Function Valve Flow/Opening Function

FCV2 FCVl

VSP Relative Cv VSP Relativ Cv

0 0 0 0

5 2.293292 5 0.45

10 4.444872 10 1.849402

15 6.525701 15 5.178793

20 8.606736 20 9.701079

25 10.75894 25 15.23943

30 13.05326 30 21.61703

35 15.56067 35 28.65703

40 18.35213 40 36.18262

45 21.49858 45 44.01697

50 25.071 50 51.98325

55 29.14034 55 59.90463

60 33.77755 60 67.60428

65 39.05361 65 74.90537

70 45.03946 70 81.63109

75 51.80606 75 87.60459

80 59.42438 80 92.64906

85 67.96538 85 96.58766

90 77.50001 90 99.24357

95 88.09923 95 99.8

100 100 100 100 The mixing ratio limiter shown in Figure 9 ensures that the MN of the fuel gas mixture of SVOC and vaporized LNG at any time meets the MN requirements set by the engine methane number calculation function. As an example, SVOC may for some cargoes have a MN in the range between 35 and 45, whilst vaporised LNG typically has a MN of 90. Engine MN requirements vary with engine load and its resistance to engine knocking; e.g., at low loads the MN requirements will allow more SVOC to be mixed in with vaporized LNG to be provided as fuel to the high power marine propulsion engine. The MN required by the high power marine OT-SPE is compared with the actual MN fuel gas mixture of SVOC and vaporized LNG, and the actual MN is adjusted with a specified margin to always give a comfortable margin from the point of harmful engine knocking. A reason for harmful engine knoking is one or more "pockets" of air/gas that detonate uncontrolled outside the area of the front of the intended combustion. It is contemplated that a gas analyser reads the chemical composition of the SVOC and the vaporized LNG (46 and 48, respectively), and on that basis are the individual MN calculated, and hence a mixing ratio is computed.

LVH is a" lower heating value", also refererd to as a "net calorific value", representing the amount of heat obtained by the complete combustion of a unit quantity of material.

The Cdensity parameter reflects the density of the gas at a given temperature and a given pressure. Based on mass flow measurements from FT1 and FT2, the value is adjustable for it to represent the actual gas flow, while the actual gas flow will have a degree of variation due to variations in the composition of the gas.

Conversion factor "k" is applied to convert the methane number margin, MN margin, between that of the engine requirement and that of the gas, to a "mix ratio margin".

The inventors of the present invention have found that due to the fuel transfer delay between the mixer and the engine, a certain compensation margin in the MN calculation should be included. The inventors of the present invention have found that in order to minimise this margin, the fuel mixer is advantageously located as close as possible to the engine. The mixing ratio can be manually set by a human operator, and can typically be based on loaded crude data or experiences from past voyages where typical SVOC data have been recorded. The mixing ratio limiter will override a mixing ratio value set manually by a human operator if the engine requires a higher MN than what has been manually specified. Optionally, the mixing ratio limiter is adaptive so as to increase the mixing ratio if a required MN becomes lower than the calculated MN. Hence, the mixing ratio set point is dynamically adapted to the engine load.

The valve ratio (FCVl : FCV2) is advantageously such that the mass flow ratios of the SVOC and the vaporized LNG to the mixer shall be constant for a given MN, and this linked relationship between the valves ensures stable fuel quality and with minimum pressure instability.

The status of the fuel mixing system (fuel supply system) is advantageously shown to a human operator by displaying the mixing ratio set point provided by the operator (see Figure 9), and the maximum mixing ratio calculated by fuel mixing system to protect the engine from harmful knocking. The information will typically be displayed on a visual display unit (VDU). The resulting actual mixing ratio of the SVOC and the vaporized LNG is advantageously displayed on the VDU as well as the actual MN, together with engine required MN. From the VDU, the operator can read/see that the MN of the fuel mixture has a higher value than the engine required MN, where this difference is the MN margin applied from minimum acceptable MN to avoid harmful engine knocking. This information of the MN margin size will allow the operator to set the engine load by also considering the amount of SVOC to be mixed into the vaporised LNG fuel supply. E.g. running three (3) engines on reduced load instead of two (2) engines on a higher load will allow a larger amount of SVOC to be mixed into the vaporised LNG.