Determining operational energy losses in a liquefied gas transport ship

Abstract

The invention relates to a method for determining (10) operational energy losses (Df, Ds), over a time interval, of a liquefied gas transport ship provided with a liquefaction system, comprising steps of: - calculating (12) an amount of energy consumed (D1) by an energy consumer of the ship; -calculating (14) an energy variation (D2) as a function of a temperature variation of the liquefied gas; - adding (16) the energy variation (D2) to the amount of energy consumed (D1), resulting in an actual operational energy loss (Df); and when the liquefaction system is activated over the time interval: - calculating (18) an energy saving (D3) by the liquefaction system; and - adding (20) the energy saving (D3) to the actual operational energy loss (Df), resulting in an estimated operational energy loss (Ds) that would have been observed without the activation of the liquefaction system.

Description

[0001] DESCRIPTION

[0002] Title of the invention: Determination of operational energy losses in a liquefied gas transport vessel

[0003] The present invention relates to the field of liquefied gas transport vessels, in particular LNG carriers, and to the field of equipment for monitoring energy consumption in these vessels. More specifically, the invention relates to a method for determining data representative of operational energy losses in such a vessel.

[0004] Such a ship has a hold designed to contain one or more liquid gas transport tanks, these tanks having a capacity of several thousand or even tens of thousands of cubic meters. When it comes to natural gas, it is kept in the tanks at approximately -163°C (degrees Celsius), at atmospheric pressure. The tanks are therefore watertight and thermally insulated by a double layer of insulation. However, since liquefied natural gas (LNG) tends to evaporate, due for example to a reduction in gas pressure in the tanks or a heat flow passing through the tank walls despite the double insulation, the upper part of each tank, called the tank gas headspace, is filled with gas in the vapor state. This is generally used as fuel for one or more of the ship's engines.The unused gas in vapor state that cannot be kept in the tanks is burned via a flare and then released into the atmosphere, or more rarely released as is, or reliquefied by a (re)liquefaction system on the ship, called a liquefaction system in this application, and returned at -163°C in the liquid state to the tanks. This liquefaction system therefore saves gas, and therefore energy, which would be lost without this system, while itself consuming energy to operate.

[0005] The energy balance of operational losses of boil-off gas during a ship's voyage, also known as a ship's operational BOR (Boil-Off Rate), is difficult to estimate. It depends in particular on the consumption of the ship's engines, which reduce the pressure in at least one of the tanks, weather conditions (temperature, swell), the thermal resistance of the tanks, and the efficiency of the liquefaction system. Difficult weather conditions can cause significant movements of the ship and therefore of the liquefied gas in the tanks, which generates greater evaporation of the liquefied gas than when the ship is sailing in calm waters.

[0006] However, the owners of such vessels are keen to know the precise performance of their vessels in terms of operational energy losses, particularly for reasons of contractual guarantees.

[0007] Currently, shipowners use the reduction in the volume of liquefied gas in the tanks to assess this operational performance, but this method is not precise, particularly due to variations in the movement of liquid in the tanks due to the movement of the ship itself. Furthermore, this reduction does not distinguish between energy losses due to gas consumption by the ship's engines and other losses.

[0008] Furthermore, operational performance of interest to shipowners should take into account the best conditions of use of the vessel, and in particular the vessel's liquefaction system. However, this liquefaction system, itself an energy consumer, is not always activated during the vessel's journey, and therefore the operational performance measured by the volume variation method is often not representative of the operational performance enabled by the vessel.

[0009] Predictive methods, using an estimated energy consumption of the vessel over a journey based on different usage contexts, can provide operational performance taking into account the best conditions of use of the vessel. However, these predictive methods are complex and ultimately imprecise, and result in a poor perception of the operational performance that the vessel allows, resulting in a lack of interest and confidence in the data provided by these methods.

[0010] There is therefore a need to provide an accurate operational performance of a liquefied gas transport vessel equipped with a liquefaction system, so as to enable an understanding of the differences in operational performance depending on the conditions of use of the vessel. The present invention aims to at least partially overcome the drawbacks of the prior art by providing a method for determining data representative of operational energy losses over a time interval, in a liquefied gas transport vessel, a corresponding determination device and system, as well as a computer program, which make it possible to accurately compare an active operational performance corresponding to a mode of use of the vessel in which the liquefaction system is active, with a passive operational performance corresponding to a mode of use of the vessel in which the liquefaction system is deactivated.

[0011] To this end, the invention proposes a method for determining data representative of operational energy losses over a time interval, of a liquefied gas transport vessel, this energy being initially stored in the form of liquefied gas and evaporated gas in at least one tank of the vessel, the vessel being equipped with a system for liquefying the evaporated gas, the determination method comprising steps of:

[0012] - calculation of a first data item representative of a quantity of energy consumed over the time interval, based on at least one measurement relating to the activity of at least one energy consumer of the ship,

[0013] - calculation of a second data item representative of a variation in the energy of the liquefied gas present in the tank, as a function of a variation in the temperature of the liquefied gas in the tank over the time interval,

[0014] - adding the second data to the first data resulting in data representative of an effective operational energy loss in the ship, the determination method being characterized in that it further comprises, when the liquefaction system is activated over the time interval:

[0015] - a step of calculating a third data item representative of a quantity of energy saved by the liquefaction system over the time interval, and

[0016] - a step of adding the third data item to the data item representing an actual operational energy loss, resulting in data item representing an estimated operational energy loss that would have been observed without the activation of the liquefaction system. In this determination method according to the invention, the first data item is a function of one or more measurements each linked to one or more consumers of the ship, for example several engines, so as to calculate the total energy consumed by the consumers of the ship. The measurements are provided by one or more sensors, for example a flow meter in a supply line of an engine of the ship.

[0017] Furthermore, when the ship has several tanks, the second data point corresponds to an energy variation linked to a temperature variation of the liquefied gas on all the tanks, which can be positive or negative. Indeed, when the ship consumes a lot of evaporated gas, the pressure of the liquefied gas is likely to drop, causing the temperature of the LNG to drop. In this case, the LNG provided energy, which corresponds to a second negative data point. On the contrary, when little evaporated gas is consumed, the pressure of the liquefied gas is likely to increase and therefore cause the temperature of the liquefied gas to rise, resulting in energy absorption by the LNG, which corresponds to a second positive data point. Finally, the third data point corresponds to the energy saved on all the tanks by the liquefaction system.

[0018] The time interval for which the first, second and third data are calculated corresponds, for example, to a day or an hour, this interval being able to be adapted according to the smallest time interval over which the liquefaction system is activated or deactivated. It should be noted in fact that the calculations of the third data and the estimated operational energy loss can also be carried out when the liquefaction system is deactivated, simply the third data will have the zero value and the estimated operational energy loss will be worth the actual operational energy loss.

[0019] Thanks to the determination method according to the invention, the data representative of the operational energy losses are more precise than in the prior art. In particular, the calculation of the first data representative of the energy consumed by consumers of the ship, uses measurements whose integration over the time interval makes it possible to know a real mass of gas escaping from the tank, unlike an estimate of mass loss of the prior art, calculated using a variation in volume, this estimate being very imprecise because of the movements of liquid which bias the volume and density data estimated mathematically.

[0020] In addition, the determination method makes it possible to compare an actual operational energy loss corresponding to an active operational performance and to a mode of use where the liquefaction system is activated, with an estimated operational energy loss corresponding to a passive operational performance and to a mode of use where the liquefaction system is deactivated. This comparison is based on the quantity of energy saved by the liquefaction system, the value of which is not estimated but calculated precisely from a measured mass, and is therefore very precise. Thus the passive operational performance is estimated very reliably, which encourages a user of the determination method according to the invention to assess the operational performance actually enabled by the ship, and to activate the liquefaction system more frequently and therefore to save energy on the ship's journey.This approach differs from the prior art in which operational performance under unfulfilled usage conditions is predicted from estimated energy savings or energy losses, and is therefore much less reliable. In particular, estimates of volume losses from liquid heights in the ship's tanks are very unreliable, particularly due to the ship's movements.

[0021] In one embodiment, the ship being equipped with a device for vaporizing liquefied gas, the method for determining data representative of operational energy losses further comprises a step of calculating a fifth data item representative of a quantity of energy of the liquefied gas pumped from the tank and intended to pass through the vaporization device over the time interval and a step of subtracting the fifth data item from the data item representative of the actual operational energy loss.

[0022] In another embodiment, the method for determining data representative of operational energy losses further comprises a step of calculating a sixth data item representative of a variation in energy of the gas phase of the liquefied gas present in the tank, as a function of a variation in pressure and temperature in the tank over the time interval, and a step of adding the sixth data item to the data item representative of the actual operational energy loss. In one embodiment of the invention, the first, second and third, fifth and sixth data items as well as the data item representative of an actual operational energy loss and the data item representative of an estimated operational energy loss each correspond to a ratio between, on the one hand, a quantity of energy lost over the time interval and, on the other hand, a reference quantity of energy chosen from:

[0023] - a quantity of energy calculated at the start of a period of use of the determination method, or

[0024] - a quantity of energy charged by the ship.

[0025] The quantity of energy loaded by the ship corresponds to the energy contained in the ship's tank(s) immediately after they are filled. Other reference energy quantities are of course usable, for example the chosen reference energy quantity is the quantity of energy that the liquefied gas loaded by the ship would represent if it were loaded to a predetermined percentage of its total cargo, for example 100%, 95%, or 90%.

[0026] The use of this ratio allows a user of the determination method according to the invention to better perceive the operational energy losses over the time interval.

[0027] Preferably, the determination method according to the invention comprises an additional step of restoring the first, second and third data as well as the data representative of an actual operational energy loss and the data representative of an estimated operational energy loss, on a human-machine interface. Such a restoration consists for example of a display on a screen of said interface.

[0028] The invention also relates to a method for monitoring operational data linked to at least one tank of a liquefied gas transport vessel, characterized in that it implements the method for determining data representative of operational energy losses according to the invention, and in that it comprises a step of calculating a fourth data item representative of a variation in the volume of liquefied gas in the tank over the time interval.

[0029] This latter calculation allows a user of the supervision method according to the invention to compare the estimate provided by the prior art with the operational energy loss estimated according to the invention. It shows the variability of this estimate of the prior art in comparison with the data representative of the estimated operational energy loss.

[0030] The invention also relates to a device for determining data representative of operational energy losses, comprising means for implementing the determination method according to the invention. The determination device implements the determination method according to the invention in hardware and / or software. It is capable of recovering measurement data from sensors allowing it to implement the different steps of the determination method according to the invention. It is therefore, for example, a computer on board the ship, or a computer connected to it, or a remote server capable of receiving this data.

[0031] The invention also relates to a system for determining data representative of operational energy losses over a time interval, of a liquefied gas transport vessel, this energy being initially stored in the form of liquefied gas and evaporated gas in at least one tank of the vessel, the vessel being equipped with a system for liquefying the evaporated gas, the determination system comprising:

[0032] - first means for calculating a first data item representative of a quantity of energy consumed over the time interval, as a function of at least one measurement relating to the activity of at least one energy consumer of the ship,

[0033] - second means for calculating a second data item representative of a variation in energy of the liquefied gas present in the tank, as a function of a variation in temperature of the liquefied gas in the tank over the time interval,

[0034] - means for adding the second data to the first data, delivering data representative of an effective operational energy loss in the ship, the determination system being characterized in that it further comprises:

[0035] - third means for calculating a third data item representative of a quantity of energy saved by the liquefaction system over the time interval, when the liquefaction system is activated over the time interval, and

[0036] - means for adding the third data item to the data item representing an actual operational energy loss, delivering, when the liquefaction system is activated over the time interval, data item representing an estimated operational energy loss, which would have been observed without the activation of the liquefaction system.

[0037] The determination system according to the invention is implemented in hardware and / or software. The means of the determination system according to the invention enable it to implement the determination method according to the invention. The determination system according to the invention is therefore, for example, identical to the determination device according to the invention, or comprises several remote physical entities, for example, it is formed of the ship's on-board computer and a remote server capable of receiving data from the ship's on-board computer, and possibly a terminal connected to this remote server. The remote server is possibly formed of several computers connected to each other by one or more communication networks. In another example, the determination system according to the invention comprises only the on-board computer and a terminal connected to this on-board computer.The determination system according to the invention may further comprise sensors such as liquefied gas temperature sensors or flow meters making it possible to measure the gas consumption of one or more consumers of the ship.

[0038] In one embodiment of the invention, the ship being equipped with a device for vaporizing the liquefied gas, the determination system further comprises fifth means for calculating a fifth data item representative of a quantity of energy of the liquefied gas pumped from the tank and intended to pass through the vaporization device over the time interval and additional means for subtracting the fifth data item from the data item representative of the actual operational energy loss.

[0039] In another embodiment of the invention, the determination system may further comprise sixth means for calculating a sixth data item representative of a variation in energy of the gaseous phase of the liquefied gas present in the tank, as a function of a variation in pressure and temperature in the tank over the time interval, and additional means for adding the sixth data item to the data item representative of the effective operational energy loss.

[0040] According to a preferred characteristic of the determination system according to the invention, the latter comprises a human-machine interface comprising display means capable of providing a representation of the first, second, third, fifth and sixth data, as well as data representative of the actual and estimated operational energy losses, in which the data representative of an actual operational energy loss is displayed in the form of a sum between at least two of the first, second, fifth and sixth data and, when the liquefaction system is activated over the time interval, the data representative of an estimated operational energy loss is displayed in the form of a cumulative total of the third data and the data representative of the actual operational energy loss.In other words, this human-machine interface is capable of implementing the additional step of rendering the determination method according to the invention, in visual form.

[0041] In one embodiment of the invention, said representation is a waterfall chart. Such a chart is well suited to rendering the mathematical relationships between the restored data in an immediately understandable manner. Of course, in an alternative embodiment, other graphic means such as tables of values, classic histograms or pie charts can be used.

[0042] In this embodiment of the invention, the display means are capable of representing the data representative of the actual operational energy loss in a first location of the waterfall graph when the liquefaction system is activated over the time interval, or in a second location of the waterfall graph, distinct from the first location, when the liquefaction system is deactivated over the time interval, the data representative of an estimated operational energy loss being represented by the display means at the second location when the liquefaction system is activated over the time interval. This embodiment makes it possible to always reserve the first location on the graph for the active operational performance, and the second location for the passive operational performance.In other words, this allows a user of the determination system according to the invention to easily correlate the operational losses of effective energy to a context of activation of the liquefaction system or to a context of deactivation of the liquefaction system.

[0043] In this embodiment, the restitution of the data is linked to a time interval or to an average over time intervals. In particular in this embodiment, the human-machine interface of the determination system according to the invention further comprises means for selecting at least one portion of a journey made by the ship, and means for selecting first time intervals during which the liquefaction system was activated on this portion of the journey, the display means being capable of representing when the first time intervals are selected by a user of the determination system:

[0044] - an average of each data item among the first, second, fifth and sixth data items delivered by the means for calculating said data item for each of the first time intervals,

[0045] - an average of data representative of operational losses of effective energy delivered by the addition means, for each of the first time intervals,

[0046] - an average of third data delivered by the third calculation means for each of the first time intervals, and

[0047] - an average of representative data of estimated operational energy losses delivered by the addition means for each of the first time intervals.

[0048] In this embodiment of the invention, the human-machine interface of the determination system according to the invention further comprises means for selecting second time intervals during which the liquefaction system was deactivated on this portion of the journey, the display means being capable of representing when the second time intervals are selected by a user of the determination system: - an average of each data item among the first, second, fifth and sixth data items delivered by the means for calculating said data item for each of the second time intervals, and

[0049] - an average of data representative of operational losses of effective energy delivered by the adding means for each of the second time intervals. The means for selecting the first or second intervals are, for example, buttons or graphic tabs on the graphic interface rendered by the display means.

[0050] Additional supervision data are returned in this embodiment, allowing a user of the determination system according to the invention to correlate more data in order to better understand the evolution of the operational energy losses returned by the human-machine interface. Thus in this embodiment, the human-machine interface further comprises means for selecting at least one tab linked to an operational data item relating to the liquefied, evaporated, reliquefied or consumed gas in the tanks, capable of causing the display means to display a graph of values ​​corresponding to the selected tab, the values ​​being those of the operational data item over one or more time intervals and being recorded in a memory of the determination system. The operational data item is taken for example from:

[0051] - a flow of evaporated gas sent to the liquefaction system over time,

[0052] - a cumulative mass of gas lost over time,

[0053] - a non-cumulative mass of gas lost over time,

[0054] - a density of the liquefied gas in the tank over time,

[0055] - a temperature of the liquefied gas in the tank over time,

[0056] - pressure of the gas evaporated in the tank over time,

[0057] - a volume of liquefied gas over time,

[0058] - a ship speed over time.

[0059] This data can be instantaneous or linked to a predefined time interval.

[0060] The invention also relates to a system for monitoring operational data linked to at least one tank of a liquefied gas transport vessel, characterized in that it comprises the system for determining data representative of operational energy losses according to the invention, and in that it comprises means for calculating a fourth data item representative of a variation in the volume of liquefied gas in the tank over the time interval. The display means of the determination system according to the invention are preferably identical to display means of the monitoring system according to the invention, and therefore also restore the fourth data item representative of an energy loss in the tank over the time interval considered.

[0061] The invention finally relates to a computer program comprising program code instructions for executing the steps of the determination method according to the invention, when said program is executed on one or more processors.

[0062] The computer program is executed on a single processor, for example when it is executed in the determination device according to the invention, equipped with this processor. It is executed on several processors, for example in the determination system according to the invention when the latter comprises several remote physical entities each equipped with one of these processors, in particular the calculation steps of the determination method according to the invention are implemented, for example, in a remote server or in the ship's on-board computer, and the restitution step is implemented in a terminal connected to the remote server or to the on-board computer.

[0063] It should also be noted that the determination method according to the invention is implemented dynamically during the ship's journey, or statically once the ship has arrived at its destination. When the calculation steps of the determination method according to the invention are implemented in a remote server of the determination system according to the invention, the remote server therefore comprises means of wireless communication with the ship's on-board computer, for example means of internet communication via satellite, to dynamically restore the data representative of the ship's operational energy losses. These wireless communication means can be replaced by a transfer of data via a physical medium, such as a data storage key, when the restoration is carried out only once the ship has arrived at its destination.The determination device according to the invention, the determination system according to the invention, the supervision system according to the invention and the computer program according to the invention, have advantages similar to those of the determination method according to the invention and the supervision method according to the invention.

[0064] Other characteristics and advantages of the invention will become apparent from the following description on the one hand, and from several examples of embodiment given for informational and non-limiting purposes with reference to the attached schematic drawings on the other hand, in which:

[0065] [fig 1] represents steps of a method of determining according to the invention, data representative of operational energy losses in a liquefied gas transport vessel, over a time interval, in an embodiment of the invention, and steps of a supervision method implementing this method of determining, in this embodiment of the invention,

[0066] [fig 2] represents a determination system according to the invention, in this embodiment of the invention, comprising means capable of implementing the determination method according to the invention,

[0067] [fig 3] represents a graphical interface allowing the restitution of the data determined by the methods of figure 1, in this embodiment of the invention, when a liquefaction system of the ship was activated over the time interval, and

[0068] [fig 4] represents the graphical interface of figure 3, when the ship's liquefaction system was deactivated over the time interval.

[0069] In an embodiment of the invention shown in Figures 1 to 4, a determination method 10 shown in Figure 1 makes it possible to deliver data representative of operational energy losses Df, Ds over a time interval, of a ship 40 shown in Figure 2. This determination method 10 is part of a more general method 11 for supervising operational data linked to the tanks 42 of the ship 40. The chosen or predetermined time interval is for example a day, an hour, a shorter or longer time interval. The time interval corresponds for example to a period of time necessary for the ship to complete a journey or a portion of a journey, selected by a user of the determination method 10. The ship 40 is a ship transporting liquefied gas, this gas being, in this embodiment of the invention, liquefied natural gas.However, in an alternative embodiment, this gas is, for example, liquid hydrogen or another type of gas.

[0070] The vessel 40 stores the liquefied gas at -163°C and at a pressure close to atmospheric pressure in sealed and thermally insulated tanks 42. Each tank 42 is filled with liquefied gas up to the liquid level referenced 46, the space in the tank above this liquid level 46, called the tank gas headspace, being filled with evaporated gas. The liquefied gas present in liquid and gaseous form in the tanks 42 of the ship 40 represents a quantity of energy transported by the ship 40. This quantity of energy decreases as the ship travels, due to the consumption of gas by the ship's consumers, gas losses through evaporation and energy losses linked to variations in pressure and temperature of the liquefied natural gas, these variations being able to be of various origins such as swell, meteorology, the thermal resistance of the tanks, the draft of gas by the ship's consumers 40, etc.The ship 40 nevertheless has a liquefaction system allowing part of the evaporated gas to be liquefied in the gaseous tank ceilings and thus saving gas loss.

[0071] The determination method 10 according to the invention makes it possible to determine:

[0072] - data representative of an effective operational energy loss Df, corresponding to an effective energy loss during the time interval, compared to a reference energy as explained below; this data representative of an effective operational energy loss Df is calculated, in this embodiment of the invention, whether the liquefaction system is active or not over the time interval. This time interval can be modified to correspond to a single active or deactivated state of the liquefaction system; and

[0073] - data representative of an estimated operational energy loss Ds, calculated only when the liquefaction system is active over the time interval, and corresponding to an energy loss during the time interval, which would have been observed if the liquefaction system had not been activated over the time interval, this energy loss also relating to a reference energy.

[0074] In this embodiment of the invention, the determination method 10 is implemented in hardware and / or software in a system 50 for determining data representative of operational energy losses Df, Ds, referenced in FIG. 2 and now described, the steps of the determination method 10 allowing the obtaining of data representative of operational energy losses Df, Ds being described next. The determination system 50 is part of a more general system for supervising operational data linked to the tanks of the ship, implementing in hardware and / or software the supervision method 11 according to the invention.

[0075] The determination system 50 comprises an on-board computer 52, which receives measurements from various sensors present in the ship 40, in particular temperature sensors 54 of the liquefied gas in the tanks 42 and at least one mass flow meter 56 measuring the mass of gas sent to the consumers of the ship during the time interval.

[0076] The determination system 50 also comprises a remote server 55 represented in the internet “cloud” 60, which dynamically receives measurements from the temperature sensors 54 and the mass flow meter 56, sent by the on-board computer 52 in an ml message. For sending these measurements, the on-board computer 52 comprises, for example, means for connecting to the internet via satellite. For receiving the measurements, the remote server 55 comprises, for example, a wired internet connection. The remote server 55 receives, for example, measurements from several gas transport vessels and stores them in a database 57. It comprises calculation, addition and addition means, making it possible to implement the calculation steps of the determination method 10 according to the invention and to determine the data representative of operational energy losses Df, Ds of the vessel 40 over the time interval, as described later.These means comprise at least one processor of the server 55, capable of reading data stored in the database 57. The determination system 50 also comprises a terminal 58, for example a portable personal computer or a multimedia mobile telephone, comprising means for displaying data representative of operational energy losses Df, Ds provided by the remote server 55, these display means comprising in particular a screen. This terminal 58 is, in this example of use of the invention, on the ship 40, but can also use the invention on land. It therefore uses means of connection to the internet via satellite to send a message m2 to the remote server 55, this message m2 containing a request to send data representative of operational energy losses Df, Ds over the time interval.Upon receipt of the message m2, the remote server 55 sends in a message m3, the data representative of operational energy losses Df, Ds to the terminal 58.

[0077] Returning to Figure 1, the determination method 10 comprises a first step 12 of calculating a first data item DI representative of a quantity of energy consumed over the time interval, as a function of a measurement provided by the mass flow meter 56. This mass flow meter 56 provides a total flow rate <p de consommation de gaz par les consommateurs 44 du navire, représentés sur la figure 2 et figurant notamment des moteurs principaux du navire 40, des moteurs auxiliaires du navire 40 et une torchère brûlant du gaz évaporé non récupéré, utilisée lorsque le système de liquéfaction est désactivé. En variante on utilise plusieurs débitmètres massiques, par exemple un par consommateur.

[0078] It should be noted that using consumer mass flow meters to estimate a consumed gas mass is more accurate than observing the liquid level difference over the time interval. Indeed, liquid level measurements can be very noisy due to liquid movements in the tanks 42.

[0079] In a case of use of the embodiment of the invention, the first data DI is a percentage of energy consumed over a day, calculated as follows, by first calculation means of the server 55:

[0080] With :

[0081] - * the multiplication operator

[0082] - 1 the duration of the time interval in days,

[0083] - tl the start of the time interval,

[0084] - 12 the end of the time interval,

[0085] - cp the flow rate supplied by the mass flow meter 56 in kilograms per day,

[0086] - Lhv an average latent heat of vaporization of the gas between the start and the end of the time interval, in kilojoules per kilogram, and

[0087] - N a reference energy in kilojoules.

[0088] The reference energy N is, for example, the quantity of energy represented by the liquefied gas in the tanks 42 at the start of a period of use of the determination method 10, identical to the start of the time interval or earlier:

[0089] N = dl * 1 * Ll where

[0090] - dl is the density in kilograms per cubic meter of the liquefied gas in the tanks 42 at the start of the period of use of the determination method 10,

[0091] - VI is the volume in cubic meters of liquefied gas in the tanks 42 at the start of the period of use of the determination method 10, and

[0092] - L1 is the latent heat of vaporization of the liquefied gas in the tanks 42 at the start of the period of use of the determination method 10, in kilojoules per kilogram.

[0093] Alternatively, the reference energy N is taken equal to the quantity of energy loaded into the tanks 42 immediately after the end of their filling:

[0094] N = d2 * V2 * L2 where

[0095] - d2 is the density in kilograms per cubic meter of the liquefied gas in the tanks 42 just after they are filled,

[0096] - V2 is the volume in cubic meters of liquefied gas in the tanks 42 just after they are filled, and

[0097] - L2 is the latent heat of vaporization of the liquefied gas in the tanks 42 just after they are filled, in kilojoules per kilogram.

[0098] In yet another variant, the reference energy N is taken equal to the quantity of energy loaded into the tanks 42 just after their filling, in the case where the liquefied gas is pure methane:

[0099] N = dm * 2 * Lm where

[0100] - dm is the density of pure methane in kilograms per cubic meter, and

[0101] - Lm is the latent heat of vaporization of pure methane, in kilojoules per kilogram.

[0102] It should be noted that in this embodiment, the term "pure methane" means that the gas is not formed from a mixture of methane with another gas such as propane for example, but is formed solely from methane, with the exception of impurities.

[0103] This latter variant has the advantage of not making the reference energy dependent on the measurement period but only on the volume of liquefied gas initially loaded into the ship.

[0104] A second step of the determination method 10 is a calculation step 14 of a second data item D2, representative of an energy variation linked to the temperature variation of the liquefied gas present in the tanks 42, as a function of temperature variations of the liquefied gas in the tanks 42 measured over the time interval.

[0105] This second data D2 reflects a variation in energy linked to changes in temperature in the tanks 42, and corresponds to a negative variation in energy if the temperature has dropped (for example due to a reduction in pressure) and therefore to a quantity of energy supplied by the LNG, or, if the temperature has increased, to a positive variation in energy and therefore to a quantity of energy absorbed by the LNG.

[0106] In the case of use of this embodiment of the invention, the second data D2 is a percentage of energy stored or taken on the day considered, calculated as follows, by second calculation means of the server 55: 100 with :

[0107] - df is the density in kilograms per cubic meter of the liquefied gas in tanks 42 at the end of the time interval,

[0108] - Vf is the volume in cubic meters of liquefied gas in tanks 42 at the end of the time interval,

[0109] - Cp is an average of the specific heat (also called specific heat capacity) of the liquefied gas between the start and the end of the time interval, in kilojoules per kilogram and per degree Celsius,

[0110] - T2 is an average of temperature measurements, in degrees Celsius, of the liquefied gas in tanks 42 at the end of the time interval, and

[0111] - Tl is an average of temperature measurements, in degrees Celsius, of the liquefied gas in tanks 42 at the start of the time interval.

[0112] Alternatively, the second data D2 is calculated by summing percentages of energy linked to the variation in temperature of the liquefied natural gas, therefore to an energy stored or lost over the time interval, these percentages being calculated individually on each of the tanks 42. The temperature measurements at the start of the time interval on the one hand, and at the end of the time interval on the other hand, are averaged for each tank 42 in order to have data representative of the temperature in the tank at the start or at the end of the time interval. The different temperature measurements in one of the tanks 42 at a given time, and which are averaged, correspond for example to measurements made by different temperature sensors spaced along a liquefied gas unloading mast, present in each tank 42.

[0113] A third step of the determination method 10 is a step 16 of adding the second data D2 to the first data D1 resulting in the data representative of an effective operational energy loss Df in the ship 40 over the time interval, the adding step being implemented by adding means of the server 55:

[0114] Df = Dl + D2

[0115] In the case of use of this embodiment of the invention, the data representative of an effective operational energy loss Df corresponds to a percentage of energy lost compared to a reference energy, on the day considered. It corresponds to an active operational performance if the liquefaction system was activated on this day, or to a passive operational performance if the liquefaction system was deactivated on this day.

[0116] A fourth step of the determination method 10 is a calculation step 18 of a third data item D3 representative of a quantity of energy saved by the liquefaction system over the time interval. It is assumed, in this case of use of the invention, that the liquefaction system was activated over the time interval, and therefore made it possible not to lose a mass of evaporated gas in the tanks 42 which was greater than the needs of the consumers 44, by reliquefying it. This mass would probably have been burned by the flare otherwise. The fourth step of the determination method 10 is implemented by third calculation means of the server 55.

[0117] The third data D3 is, in the case of use of this embodiment of the invention, a percentage of energy saved over the day considered: where 0 is the mass flow rate in kilograms per day of reliquefied gas using the liquefaction system. A flow meter connected to the liquefaction system provides this value. A fifth step of the determination method 10 is a step 20 of adding the third data D3 to the data representative of an effective operational energy loss Df, providing the data representative of an estimated operational energy loss Ds, the fifth step of the determination method 10 being implemented by addition means of the server 55:

[0118] Ds = D3 + Df

[0119] The data representing an estimated operational energy loss Ds therefore corresponds to an estimate of the operational energy loss that would have occurred during the day if the liquefaction system had not been activated. It therefore corresponds to a passive operational performance estimated a posteriori.

[0120] In a use case where the liquefaction system is deactivated, we have:

[0121] D3 = 0 => Ds = Df

[0122] In the latter case, the data representative of an estimated operational energy loss Ds therefore corresponds to the data representative of an effective operational energy loss Df, and no longer to an estimate.

[0123] In the case of a ship 40 equipped with a liquefied gas vaporization device, the method for determining data representative of operational energy losses may further comprise an optional step of calculating a fifth data item D5 representative of a quantity of energy of the liquefied gas pumped from the tank and intended to pass through the vaporization device over the time interval and a step of subtracting the fifth data item D5 from the data representative of the effective operational energy loss Df.

[0124] The fifth data D5 corresponds to the “forced BOR”. This is a quantity of energy linked to the mass of liquefied gas pumped from the tank and directed through the vaporization device, calculated as follows, by fifth calculation means of the server 55: 100

[0125] With: - * the multiplication operator,

[0126] - 1 the duration of the time interval in days,

[0127] - tl the start of the time interval,

[0128] - 12 the end of the time interval,

[0129] - the flow rate supplied by a mass flow meter downstream of the vaporization device in kilograms per day,

[0130] - Lhv_LG an average of the latent heat of vaporization of the liquefied gas between the start and the end of the time interval, in kilojoules per kilogram, and

[0131] - N a reference energy in kilojoules, already presented above.

[0132] The method for determining data representative of operational energy losses may also include an optional step of calculating a sixth data item D6 representative of a variation in energy of the gas phase of the liquefied gas present in the tank, as a function of a variation in pressure and temperature in the tank over the time interval, and a step of adding the sixth data item D6 to the data item representative of the effective operational energy loss Df.

[0133] The sixth data D6 corresponds to the “BOR ballast” or “BOR pressure”. This is a quantity of energy linked to the gas phase of the liquefied gas present in the tank, and which varies according to the pressure and temperature in the tank over the time interval, which can be, by way of non-limiting example, calculated as follows, by sixth calculation means of the server 55:

[0134] Amp representing the mass of evaporated gas contributing to the pressure rise in the tank and can be determined based on the ideal gas law:

[0135] With - * the multiplication operator,

[0136] - 1 the duration of the time interval in days,

[0137] - tl the start of the time interval,

[0138] - 12 the end of the time interval,

[0139] - d is the average density in kilograms per cubic meter of the liquefied gas in the tanks 42 over the time interval t,

[0140] - V is the average volume in cubic meters of liquefied gas in the tanks 42 over the time interval t,

[0141] - Lhv an average of the latent heat of vaporization of the gas between the beginning and the end of the time interval, in kilojoules per kilogram, (Lhv(tl), respectively Lhv(t2), corresponding to the latent heat of vaporization of the gas at time tl, respectively t2),

[0142] - R is the universal constant of ideal gases,

[0143] - P(t 1 ) , respectively P(t2), is the pressure in the tank at time tl, respectively t2,

[0144] - VBoc(tl), respectively VBOG (t2), is the volume of gas evaporated in the tank at time tl, respectively t2,

[0145] - TBOGCII), respectively TBOG (t2), is the temperature of the evaporated gas in the tank at time tl, respectively t2,

[0146] - MBOGCII), respectively MBOG (t2), is the molar mass of the gas evaporated in the tank at time tl, respectively t2.

[0147] The calculation of the sixth data item D6 contributes to a better determination of the operational energy losses over the chosen time interval. The sixth data item D6 takes into account the pressure variations in the tank which can reflect a potentially significant loss of mass, particularly during a ballast voyage during which the volume of gas in the tank is significant. In parallel with these first five steps and the optional steps of the determination method 10 according to the invention, the supervision method 11 according to the invention implements an additional step 22 of calculating a fourth data item D4 representative of an energy loss in the tanks 42 as a function of a variation in the volume of liquefied gas over the time interval.

[0148] In the case of use of the embodiment of the invention, D4 is a percentage of energy lost over the day considered, calculated as follows: where Vi is the volume in cubic meters of liquefied gas in tanks 42 at the start of the time interval.

[0149] Alternatively, D4 can be calculated by replacing Vi in the denominator with a volume of liquefied natural gas just after loading the ship, or with a volume of filling the tanks to a predetermined percentage, for example 95% or 98%.

[0150] This calculation gives an order of magnitude of the total energy loss in tanks 42 over the time interval. Indeed, this calculation is not precise because the quantity of liquefied gas evaporated depends heavily on the operations of the ship's crew. For example, if the crew decides to go at a high speed, the gas pressure in tanks 42 may drop, thus forcing the evaporation of the liquefied gas. In this way, the quantity of liquefied gas evaporated will be greater. Conversely, if the crew slows down, or if the ship is stopped, the gas needs of the various consumers will most likely be less than the gas emitted by tanks 42 by evaporation. Thus, the pressure will rise in tanks 42 and the quantity of gas evaporated will be lower.

[0151] This is why the invention considers the variation in temperature of the liquefied gas, because this variation in temperature is closely linked to the quantity of gas evaporated. If the latter is very significant, then the temperature of the liquefied gas will most likely drop. On the other hand, if the gas consumption is low, the temperature of the liquefied gas should rise as well as the gas pressure in the tanks 42. A sixth step of the determination method 10, common to the supervision method 11, is a step 24 of restitution of the data D1, D2, D3, D4, Ds, Df (and optionally D5, D6) previously calculated on a graphical interface displayed on a screen of the terminal 58. Of course, in other embodiments of the invention, this restitution can be done differently, for example in a textual or vocal manner on the terminal 58 or on another element of the determination system 50.

[0152] It should be noted that steps 12 to 24 are not necessarily executed in the order described, the first step 12, the second step 14, the fourth step 18 and the sixth step 22 being able in particular to be carried out independently and therefore in parallel with each other. Steps 12 to 22 are preferably repeated to provide data corresponding to several time intervals, for example they are executed at the end of each day of a journey made by the ship 40.

[0153] Finally, the determination method according to the invention may comprise a step of applying a corrective coefficient to one of the data delivered by the calculation means to take into account related physical phenomena that may impact the calculation of the data. For example, in the case of the third data D3 representative of a quantity of energy saved by the liquefaction system over the time interval, a corrective coefficient may be applied to take into account the fact that after liquefaction, the return of the gas to the tank may be accompanied by a re-evaporation of a fraction of liquefied gas. This re-evaporation contributes to mitigating the quantity of energy actually saved by the liquefaction system. The corrective coefficient makes it possible to take this effect into account. The impact of the re-evaporation may be determined by means of calculations known to the person skilled in the art.The corrective coefficient can be tabulated on the basis of calculations carried out previously according to the surrounding parameters of the tank or it can be calculated by the calculation means associated with the data by knowing the surrounding parameters of the tank.

[0154] Figure 3 shows a screenshot 30 of the terminal 58, showing the graphical interface divided into five zones 1 to 5. Zone 1 is a world map on which the route taken by the ship 40 is represented. A graphical button on this map allows a user of the determination method 10 to select all or a portion of the route.

[0155] Zone 2 displays fixed data related to the journey and to the vessel 40, in particular under the heading "id" the name of the boat ("Messi" in this example of use), under the heading "ref" a reference of the journey ("Voyage" in this example of use), GPS coordinates (from the English "Global Positioning System") of the starting point and the arrival point of the journey, the corresponding distance, a corresponding departure date and arrival date, the duration in hours of the journey, as well as the capacity of the vessel 40, its density and an indicator of a loaded voyage or a return using ballast, the indicator being at "L" if the vessel is on a loaded voyage.

[0156] Area 3 has user-selectable graphic tabs, allowing to view in area 3, when the corresponding graphic tab is selected, a graph representing:

[0157] - a flow rate vl of evaporated gas sent to the liquefaction system over time, in kilograms per hour,

[0158] - a cumulative mass of gas v2 lost over time, corresponding to data representative of an effective operational energy loss calculated on the selected portion of the journey in kilograms of gas rather than as a percentage,

[0159] - a mass flow rate of gas consumed v8 over time, corresponding for example to data representative of operational losses of effective energy calculated for each day of the selected portion of the journey in tonnes per day rather than in percentages,

[0160] - a density v7 of the liquefied gas in the tanks 42 over time,

[0161] - a temperature v5 of the liquefied gas in the tanks 42 over time,

[0162] - a pressure v6 of the gas evaporated in the tanks 42 over time,

[0163] - a volume v4 of liquefied gas in the tanks 42 over time, and

[0164] - a speed v3 of the ship 40 over time. Tl

[0165] In screenshot 30, the user has selected the tab corresponding to the flow rate vl of evaporated gas sent to the liquefaction system during the selected portion of the journey. On the graph corresponding to the flow rate vl, the periods corresponding to an activation of the liquefaction system, for which the gas flow rate sent to the liquefaction system is non-zero, are grayed out. This is due to the fact that in zone 4, the user has selected a graphic button B1 corresponding to the periods of activation of the liquefaction system on the selected portion of the journey.

[0166] Area 4 has a G1 waterfall chart representing:

[0167] - by a first histogram bar in a first location, an average of the first DI data calculated by the determination system 50 for each day of the selected journey portion, during which the liquefaction system was active, called “consumed BOR” or “mass BOR”,

[0168] - by a second histogram bar in a second location, an average of the second data D2 calculated by the determination system 50 for each day of the selected portion of the journey, during which the liquefaction system was active, called “BOR due to the variation in temperature of the LNG” or “rate due to the conditioning of the ship”,

[0169] - by a third histogram bar in a third location, the sum between the average represented in the first location and the average represented in the second location, called “active BOR”,

[0170] - by a fourth histogram bar in a fourth location, an average of the third data D3 calculated by the determination system 50 for each day of the selected journey portion, during which the liquefaction system was active, called “reliquefied BOR”,

[0171] - by a fifth histogram bar in a fifth location, the sum between the sum represented in the third location and the average represented in the fourth location, called “passive BOR”, and - by a sixth histogram bar in a sixth location, an average of the fourth D4 data calculated by the supervision system for each day of the selected journey portion, during which the liquefaction system was active, called “volumetric BOR”.

[0172] The waterfall chart therefore represents average percentages of energy losses or gains per day for the selected portion of the journey. Alternatively, it represents percentages of energy losses or gains over the entire portion of the selected journey, on a waterfall chart G2, or a cumulative sum of energy losses or gains over the entire portion of the selected journey, on a waterfall chart G3.

[0173] In the waterfall chart Gl, the first bar extends from a zero ordinate to a high end ordinate, the second bar extends from the high end ordinate of the first bar to a low end ordinate, and the third bar extends from the low end ordinate of the second bar to a zero ordinate. Thus the construction of the data representative of an effective operational energy loss Df as the sum between the first data DI and the second data D2 is visually represented. It should be noted that the high end ordinate of the first bar can become a low end ordinate of the second bar when the second data D2 is representative of an energy loss rather than an energy gain.

[0174] Similarly, the fourth bar extends from a low end ordinate equal to the low end ordinate of the second bar and also to a high end ordinate of the third bar, to a high end ordinate, and the fifth bar extends from a zero ordinate to the high end ordinate of the fourth bar. Thus the construction of the data representative of an estimated operational energy loss Ds as the sum between the third data D3 and the effective operational energy loss Df is visually represented.

[0175] Finally, zone 5 represents scales of values ​​ranging from green for low values ​​to red for high values, an arrow on each bar showing the measured value: - on a first scale El the speed of the ship is represented,

[0176] - on a second scale E2, the speed of the main engines of the ship 40 is represented, and

[0177] - on a third scale E3 the pressure in the tanks 42 is represented.

[0178] In Figure 4, a screenshot 32 of the graphical interface corresponds to the case where the user has selected in zone 3, the tab corresponding to the flow rate vl of gas sent to the liquefaction system during the selected portion of the journey, and where the user has selected in zone 4, a graphical button B2 corresponding to the periods of deactivation of the liquefaction system on the selected portion of the journey.

[0179] Therefore, on the graph corresponding to the flow rate vl, the periods for which the reliquefied gas flow rate is zero are grayed out. In addition, a new waterfall graph is shown in zone 4 and now only has four histogram bars:

[0180] - On the first location, a first histogram bar named “consumed BOR” or “mass BOR”, represents a new average of first DI data calculated by the determination system 50 for each day of the selected journey portion, during which the liquefaction system was deactivated,

[0181] - On the second location, a second bar named “BOR due to LNG temperature variation” or “rate due to ship conditioning”, represents a new average of second D2 data calculated by the determination system 50 for each day of the selected portion of the journey, during which the liquefaction system was deactivated,

[0182] - On the third location, no bar is present but the value 0% is displayed, this value being named “active BOR” on this third location,

[0183] - On the fourth location, no bar is present but the value 0% is displayed, corresponding to a zero percentage of energy saved by the liquefaction system, this value being named “reliquefied BOR” on this fourth location, - On the fifth location, a fifth histogram bar named “passive BOR” represents the sum between the new average represented on the first location and the new average represented on the second location, and therefore the data representative of an operational loss of energy Df, and

[0184] - On the sixth location, a sixth histogram bar named “volume BOR” represents a new average of fourth D4 data calculated by the supervision system for each day of the selected journey portion, during which the liquefaction system was deactivated.

[0185] In this new waterfall chart the first, second and fifth bars are arranged in a similar way to the waterfall chart of Figure 3, to visualize the fact that the fifth bar representing an effective operational energy loss is constructed as the sum between the first bar representing consumed energy and the second bar representing an energy variation linked to the change in temperature of the liquefied natural gas.

[0186] The data representative of an effective operational energy loss Df is therefore represented at the fifth location, where the data representative of an estimated operational energy loss Ds was represented in Figure 3. Indeed, this fifth location is dedicated to a passive operational performance, corresponding to an operating mode of the ship 40 in which the liquefaction system is deactivated. Similarly, the third location is dedicated to an active operational performance Dg, corresponding to an operating mode of the ship 40 in which the liquefaction system is active. As a result, this active operational performance Dg is equal to the data representative of the effective operational energy loss Df in Figure 3, but is equal to the zero value in Figure 4.

[0187] Finally, in zone 4, a graphic button B3 allows you to view a waterfall graph showing average percentages of energy lost or gained per day, over all the days of the selected section of the journey, whether the liquefaction system is active or not.On this waterfall graph, the third location of the graph dedicated to active operational performance will provide a first data item representative of an effective operational energy loss Df over all the time intervals for which the liquefaction system was active, and the fifth location of the graph dedicated to passive operational performance will provide a second data item representative of an effective operational energy loss Df over all the time intervals for which the liquefaction system was active and inactive, this second representative data item being obtained by adding the data item representative of the energy saved by reliquefaction over the entire portion of the journey, to the first data item representative of an effective operational energy loss Df.The same principle applies with the fifth and sixth data D5, D6 when the latter are taken into account in the method of determining the invention.

[0188] Of course, the invention is not limited to the examples which have just been described and numerous adjustments can be made to these examples without departing from the scope of the invention. In particular, the characteristics of the different embodiments or variants of the invention envisaged in this application can be combined to achieve the invention, insofar as these embodiments or variants are not incompatible with each other.

Claims

CLAIMS 1- Method for determining (10) data representative of operational energy losses (Df, Ds) over a time interval, of a ship (40) transporting liquefied gas, this energy being initially stored in the form of liquefied gas and evaporated gas in at least one tank (42) of the ship (40), the ship (40) being equipped with a system for liquefying the evaporated gas, the determination method (10) comprising steps of: - calculation (12) of a first data item (Dl) representative of a quantity of energy consumed over the time interval, as a function of at least one measurement relating to the activity of at least one consumer (44) of energy of the ship (40), - calculation (14) of a second data item (D2) representative of a variation in energy of the liquefied gas present in the tank, as a function of a variation in temperature of the liquefied gas in the tank (42) over the time interval, - addition (16) of the second data (D2) to the first data (Dl) resulting in data representative of an effective operational energy loss (Df) in the ship (40), the determination method (10) being characterized in that it further comprises, when the liquefaction system is activated over the time interval: - a step of calculating (18) a third data item (D3) representative of a quantity of energy saved by the liquefaction system over the time interval, and - a step of adding (20) the third data (D3) to the data representative of an effective operational energy loss (Df), resulting in data representative of an estimated operational energy loss (Ds) which would have been observed without the activation of the liquefaction system. 2- Method for determining (10) data representative of operational energy losses according to claim 1, the ship (40) being equipped with a device for vaporizing liquefied gas, the method further comprising a step of calculating a fifth data item (D5) representative of a quantity of energy of the liquefied gas pumped from the tank and intended to pass through the vaporization device on the time interval and a step of subtracting the fifth data (D5) from the data representing the effective operational energy loss (Df). 3- Method for determining (10) data representative of operational energy losses according to claim 1 or 2, further comprising a step of calculating a sixth data item (D6) representative of a variation in energy of the gas phase of the liquefied gas present in the tank, as a function of a variation in pressure and temperature in the tank over the time interval, and a step of adding (XX) the sixth data item (D6) to the data item representative of the effective operational energy loss (Df). 4- Method for determining (10) data representative of operational energy losses according to any one of claims 1 to 3, in which the first, second, third, fifth, sixth data (Dl, D2, D3, D5, D6) as well as the data representative of an effective operational energy loss (Df) and the data representative of an estimated operational energy loss (Ds) each correspond to a ratio between on the one hand a quantity of energy lost over the time interval and on the other hand a reference quantity of energy chosen from: - a quantity of energy calculated at the start of a period of use of the determination method (10), or - a quantity of energy charged by the ship (40). 5- Method for supervising (11) operational data linked to at least one tank (42) of a ship (40) for transporting liquefied gas, characterized in that it implements the method for determining (10) data representative of operational energy losses according to any one of claims 1 to 4, and in that it comprises a step of calculating (22) a fourth data item (D4) representative of a variation in the volume of liquefied gas in the tank (42) over the time interval. 6- Device for determining (55) data representative of operational energy losses, comprising means for implementing the determination method (10) according to any one of claims 1 to 4, or the supervision method (11) according to claim 5. 7- System for determining (50) data representative of operational energy losses over a time interval, of a ship (40) transporting liquefied gas, this energy being initially stored in the form of liquefied gas and evaporated gas in at least one tank (42) of the ship (40), the ship (40) being equipped with a system for liquefying the evaporated gas, the determination system (50) comprising: - first means for calculating a first data item (Dl) representative of a quantity of energy consumed over the time interval, as a function of at least one measurement relating to the activity of at least one consumer (44) of energy of the ship (40), - second means for calculating a second data item (D2) representative of a variation in energy of the liquefied gas present in the tank (42), as a function of a variation in temperature of the liquefied gas in the tank (42) over the time interval, - means for adding the second data (D2) to the first data (Dl), delivering data representative of an effective operational energy loss (Df) in the ship (40), the determination system (50) being characterized in that it further comprises: - third means for calculating a third data item (D3) representative of a quantity of energy saved by the liquefaction system over the time interval, when the liquefaction system is activated over the time interval, and - means for adding the third data (D3) to the data representative of an effective operational energy loss (Df), delivering, when the liquefaction system is activated over the time interval, data representative of an estimated operational energy loss (Ds) which would have been observed without the activation of the liquefaction system. 8- Determination system (50) according to claim 7, the ship (40) being equipped with a device for vaporizing the liquefied gas, further comprising fifth means for calculating a fifth data item (D5) representative of a quantity of energy of the liquefied gas pumped from the tank and intended to pass through the vaporization device over the time interval and additional means for subtracting the fifth data item (D5) from the data item representative of the effective operational energy loss (Df). 9- Determination system (50) according to claim 7 or 8, further comprising sixth means for calculating a sixth data item (D6) representative of a variation in energy of the gaseous phase of the liquefied gas present in the tank, as a function of a variation in pressure and temperature in the tank over the time interval, and additional means for adding the sixth data item (D6) to the data item representative of the effective operational energy loss (Df). 10- Determination system (50) according to any one of claims 7 to 9, comprising a human-machine interface comprising display means (58) capable of providing a representation of the first, second, third, fifth and sixth data (Dl, D2, D3, D5, D6), as well as data representative of the actual (Df) and estimated (Ds) operational energy losses, in which the data representative of an actual operational energy loss (Df) is displayed in the form of a sum between at least two of the first, second, fifth and sixth data (Dl, D2) and, when the liquefaction system is activated over the time interval, the data representative of an estimated operational energy loss (Ds) is displayed in the form of a cumulative total of the third data (D3) and the data representative of the actual operational energy loss (Df). 11- Determination system (50) according to the preceding claim, in which the representation is a waterfall graph (4). 12- Determination system (50) according to the preceding claim, in which the display means (58) are capable of representing the data representative of the effective operational energy loss (Df) in a first location of the waterfall graph (4) when the liquefaction system is activated over the time interval, or in a second location of the waterfall graph (4), distinct from the first location, when the liquefaction system is deactivated over the time interval, the data representative of an estimated operational energy loss (Ds) being represented by the display means (58) at the second location when the liquefaction system is activated over the time interval. 13- Determination system (50) according to any one of claims 10 to 12, in which the human-machine interface of the determination system (50) further comprises means (1) for selecting at least a portion of a journey made by the ship (40), and means (Bl) for selecting first time intervals during which the liquefaction system was activated on this portion of the journey, the display means (58) being capable of representing when the first time intervals are selected by a user of the determination system (50): - an average of each data item among the first, second, fifth and sixth data items (D1, D2, D5, D6) delivered by the means for calculating said data item for each of the first time intervals, - an average of data representative of operational losses of effective energy (Df) delivered by the addition means for each of the first time intervals, - an average of third data (D3) delivered by the third calculation means for each of the first time intervals, and - an average of data representative of estimated operational energy losses (Ds) delivered by the addition means for each of the first time intervals. 14- Determination system (50) according to the preceding claim, in which the human-machine interface of the determination system (50) further comprises means (B2) for selecting second time intervals during which the liquefaction system was deactivated on this portion of the journey, the display means (58) being capable of representing when the second time intervals are selected by a user of the determination system (50): - an average of each data item among the first, second, fifth and sixth data items (D1, D2, D5, D6) delivered by the means for calculating said data item for each of the second time intervals, and - an average of data representative of operational losses of effective energy (Df) delivered by the adding means for each of the second time intervals. 15- Determination system (50) according to any one of claims 10 to 14, in which the human-machine interface further comprises selection means of at least one tab linked to operational data relating to the liquefied, evaporated, reliquefied or consumed gas in the tanks (42), capable of displaying by the display means (58), a graph (3) of values ​​corresponding to the selected tab, the values ​​being those of the operational data over one or more time intervals and being recorded in a memory of the determination system (50). 16- Operational data monitoring system linked to at least one tank (42) of a liquefied gas transport vessel, characterized in that it comprises the system for determining (50) data representative of operational energy losses according to any one of claims 7 to 15, and in that it comprises means for calculating a fourth data item (D4) representative of a variation in the volume of liquefied gas in the tank (42) over the time interval. 17- Computer program comprising program code instructions for executing the steps of the determination method (10) according to any one of claims 1 to 4, when said program is executed on one or more processors.

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