RU2796234C2 - Method for managing the maintenance of the ship - Google Patents

Abstract

FIELD: ship maintenance.

SUBSTANCE: invention relates to a method for managing the maintenance of a ship containing a sealed and heat-insulating tank for transporting liquefied gas. The method includes the steps of determining (step 310) the current reservoir fill level, determining (step 320) the current ship movement state, determining (step 330) the current sloshing index Ibi based on the current reservoir fill level and the current ship motion state considering position and geometry of the reservoir, integrate (step 340) the determined current slosh rate IBi into a wear rate IUi that takes into account the history of slosh rates. The wear index is then compared to a threshold value to indicate whether an inspection of the tank is necessary, depending on the result of the comparison.

EFFECT: increase in the accuracy of wear assessment and a reduction in maintenance time.

15 cl, 5 dwg, 1 tbl

Description

FIELD OF TECHNOLOGY TO WHICH THE INVENTION RELATES

[001] The present invention relates to a method for managing the maintenance of a ship including one or more tanks. The present invention relates in particular to the inspection and maintenance of pressurized and thermally insulated tanks for the transport of liquefied gas. In this document, the expression "vessel" means a means of transporting liquefied gas between two points on the globe or a floating installation for the processing and / or storage of one or more types of liquefied gas.

BACKGROUND OF THE INVENTION

[002] Sealed and insulated tanks are commonly used for storage and/or transportation of liquefied gas at low temperature, for example, tanks for transporting liquefied petroleum gas (LPG) at temperatures, for example, from -50°C to 0°C inclusive, or for transportation of liquefied natural gas (LNG) at a temperature of approximately -162°C and atmospheric pressure. The tanks may be designed to transport liquefied gas and/or to receive liquefied gas serving as fuel for propulsion of the floating structure. Therefore, many types of liquefied gas are meant, in particular methane, ethane, propane, butane or ethylene.

[003] Vessel tanks can be single or double membrane tanks and allow transportation at atmospheric pressure. Sealing diaphragms are usually made of thin sheet stainless steel or Invar. The membrane is usually in direct contact with the liquefied gas.

[004] During transportation, the liquid contained in the tank is subjected to various movements. In particular, movements of the ship at sea, for example under the influence of climatic conditions, such as the state of the sea or wind, cause the liquid in the tank to vibrate. Vibration of the liquid, generally referred to as "sloshing", creates stresses in the walls of the tank, which can compromise the integrity of the tank.

[005] Splashing occurs on ships carrying natural gas, hereinafter referred to as LNG, or on methane tankers, as well as on anchored storage vessels known as FPSOs (floating oil production, storage and offloading units), for example, on production platform and liquefaction plant, commonly referred to as FLNG (Floating Natural Gas Liquefaction Units) or Floating Gas Regasification and Storage Unit (FSRU), i.e., more generally, a Floating Production, Storage and Transportation Unit . Splashing occurs both in rough seas and in calm seas, after the liquefied gas enters into resonance with the excitation created by waves affecting the ship, even with low amplitude. In the case of resonance, the sloshing can become very violent, in particular on impacts against vertical walls or corners, which can lead to damage to the LPG containment system or the insulation system located immediately after said containment system.

[006] However, the integrity of the tank is extremely important in the case of a liquefied gas storage tank, such as an LNG storage tank, due to the flammability or explosiveness of the transported liquid and the risk of cold spots forming on the steel hull of the floating structure in the event of a leak.

[007] US Pat. No. 8,643,509 describes a method for reducing the risks associated with LPG sloshing. In accordance with this document, the resonant frequency of the liquid in the tank is estimated depending on the tank and the fill level. During transportation, the frequency of movements of the vessel is estimated depending on the climatic and sea conditions and the speed of the vessel. The predicted frequency of movements along the ship's course is also estimated. If the frequency of movement approaches the resonant frequency of the liquid in the tank, a warning signal is given to change the course and/or change the speed of the ship to avoid a dangerous situation.

[008] Despite the adoption of measures to reduce sloshing, liquid sloshing in tanks, in particular due to resonance phenomena, can lead to occasional risks of deformation of the main sealing membrane, damage to underlying structures located in the main and / or auxiliary spaces, on which the main seal is supported, falling objects, in particular static equipment, which may damage the main seal in the short to medium term, or, more generally, deformation of the main seal in excess of design tolerances. Tank inspections are part of the preventive maintenance of ships to check that the tank is in good condition. However, in order to inspect the tank, it is necessary to completely empty the tank, for this the ship is stopped or the storage tanks of the floating installation are taken out of service for a certain period of time. Typically, on a ship with more than one tank, most or even all of the tanks are checked, and this downtime is very costly. However, in most cases, the inspection may not reveal any defects or malfunctions requiring repair, which means that the inspection was unnecessary, and the ship was taken out of service for no reason. In fact, the cost of taking the tank out of service, for example due to an inspection, has a negative impact on the cost effectiveness of the vessel; in other words, tank inspection is associated with significant decommissioning.

SUMMARY OF THE INVENTION

[009] The idea behind the invention is to determine the potential level of risk of damage to the vessel's tank, depending on what the vessel has been exposed to during its operating life, with the level of potential risk of damage to the tank comparable to the "wear index", reflecting the actual aging of the tank. In this regard, the index of liquid sloshing is determined depending on the movements of the vessel when the vessel is navigating between two points or when it is stationary. The liquid slosh index is stored and integrated over time to obtain a reservoir wear index. The wear indicator is then used to initiate an inspection of the tank and/or identify the potentially most damaged tank among all of the ship's tanks.

[010] According to a first embodiment, the invention provides a method for managing the maintenance of a ship including at least one sealed and insulated tank for transporting liquefied gas, the tank having a certain geometry and position on the ship. The method contains steps in which:

determine the current filling level of the tank,

determine the current state of the movement of the vessel,

determining the current slosh rate based on the current reservoir fill level and the current state of vessel movement, taking into account the position and geometry of the reservoir,

integrating a certain current slosh rate into a wear rate that takes into account the history of slosh rates,

comparing the wear index with the first threshold value to indicate whether it is necessary to inspect the tank, depending on the result of the comparison.

[011] Through the use of the first threshold value, a decision can be made on the need for an inspection of the tank based on an objective criterion representing the potential deterioration of the tank. The first threshold value may be set in various ways. In accordance with one embodiment, the first threshold value may be set in an absolute manner, indicating the level of wear above which the tank should be inspected. According to another embodiment, the first threshold value is a relative value indicating potentially more wear on one tank compared to another tank.

[012] The history of sloshing performance refers, for example, to a record of the current sloshing performance of liquids since the vessel was put into service or since the last partial or complete repair of a tank. Thus, the current fluid slosh rate increases the wear rate available at any time, which is determined based on previously measured/recorded current slosh rates.

[013] According to a preferred embodiment, the vessel includes a plurality of tanks, a wear rate is determined for each tank, and a first threshold corresponds to the wear rate of another tank to indicate that the tank having the highest wear rate should be inspected.

[014] In accordance with various embodiments, such a method may have one or more of the following features.

[015] After indicating the reservoir as a reservoir that has exceeded the first threshold value, an inspection of said reservoir can be carried out.

[016] After inspecting the tank corresponding to the maximum wear rate, and if the inspected tank is damaged, the next tank to be inspected is determined as the tank having the highest wear rate of the remaining tanks. Thus, tanks are inspected in order of decreasing wear indicators until the tank inspection reveals no damage requiring partial or complete repair.

[017] According to such a method, after the inspection of the tank, and if the tank is not damaged, the inspection of the tanks can be stopped.

[018] Comparison of the wear rate with the threshold value is performed during the maintenance operation of the vessel.

[019] According to an alternative embodiment, determining the current state of ship movement includes determining at least one of the following parameters: wave height, wave period, angle of incidence relative to ship orientation, ship speed, ship vibrations, measured ship movements .

[020] Measured vessel movements are understood to mean the measurement of one or more actual vessel movements. In accordance with this alternative embodiment, the determination of the measured movements of the vessel may include the measurement of the acceleration when moving and when turning around three mutually perpendicular axes.

[021] In accordance with another alternative embodiment, in determining the current sloshing index, at least one of the following additional parameters of the state of the structure is taken into account: the number of thermal cycles that the tank has undergone, the elongation of the vessel, the level of hull stiffness, the nature of the liquefied gas, the level of filling ballast tanks.

[022] Determining the current sloshing rate preferably consists in determining the probability of damage to the tank in the current state of the ship's movement.

[023] In accordance with one particular embodiment, the current slosh rate can be obtained by reading a value from a lookup table with multiple entries, where the entry corresponds to the filling of the tank, at least one entry corresponds to the current state of the movement of the vessel, and the read value represents the probability of damage reservoir.

[024] According to another embodiment, integrating the current slosh score corresponds to summing the current slosh scores, the scores being sequentially calculated over successive measurement periods.

[025] According to a second embodiment, the invention also provides a maintenance management system for a ship including at least one sealed and heat-insulated tank for transporting liquefied gas. The system includes at least one tank fill level sensor, a device for estimating vessel movements, a processing means, and a warning device. At least one tank fill level sensor is configured to measure the current fill level of at least one tank. The device for estimating the movements of the vessel is configured to determine the current state of the movement of the vessel. The processing means is configured to determine the current slosh rate based on the current tank fill level and the current state of the ship's movement, taking into account the position and geometry of the tank, integrate the determined current slosh rate into a wear rate that takes into account the history of slosh rates, and compare the wear rate with the first threshold value . The warning device indicates whether it is necessary to inspect at least one tank or each tank, depending on the result of the comparison.

[026] In accordance with one embodiment, a vessel movement estimation device includes a weather station that measures the height, frequency, and direction of waves in the vessel's environment.

[027] In accordance with another embodiment, a device for estimating vessel movements includes accelerometers located on the vessel.

[028] In a preferred example, the processing means is partially located at a monitoring station on land and communicates, for example, via radio or other means of communication, with the vessel to receive information related to the current filling level of at least one tank and the current status of the movement of the vessel.

[029] In another example, an LPG carrier includes a plurality of sealed and insulated tanks and a control system for each tank.

BRIEF DESCRIPTION OF THE DRAWINGS

[030] The present invention will become better understood, and other objects, details, features and advantages will become more apparent from the following description of specific embodiments of the invention, given solely as a non-limiting example with reference to the attached drawings.

[031] FIG. 1 is a schematic representation of a vessel for transporting liquefied gas.

[032] FIG. 2 illustrates the control system incorporated into the vessel shown in FIG. 1.

[033] FIG. 3 illustrates a control system in accordance with another embodiment.

[034] FIG. 4 is a flowchart illustrating a method for updating a wear rate.

[035] FIG. 5 is a flowchart illustrating a method for managing the maintenance of a ship's tanks.

DESCRIPTION OF EMBODIMENTS

[036] Embodiments are described below with reference to a vessel including a double hull forming a load-bearing structure in which a plurality of sealed and thermally insulated tanks are located. Tanks in a carrier structure of this type have, for example, a polyhedral geometry, for example a prismatic shape.

[037] Sealed and thermally insulated tanks are designed, for example, for the transport of liquefied gas. Liquefied gas is stored and transported in tanks at low temperatures, which makes it necessary to insulate the walls of the tank to maintain the desired temperature of the liquefied gas. Therefore, it is especially important to maintain the integrity of the tank walls, including the thermal insulation spaces located under the sealing membrane, on the one hand, to maintain the tightness of the tank and prevent leakage of liquefied gas from the tank, and, on the other hand, to prevent deterioration of the insulating characteristics of the tank to maintain gas in liquefied form.

[038] Sealed and thermally insulated tanks also include an insulating barrier attached to the double hull of the vessel and holding at least one sealed membrane. By way of example, the tanks may be manufactured using technologies marketed by the Applicant under the trademarks Mark III® and NO96®, or other technologies.

[039] FIG. 1 illustrates a vessel 1 including four sealed and insulated tanks 2. The four tanks 2 may have the same or different filling states. At sea, the vessel 1 is subjected to numerous movements associated with navigational conditions. The movements of the vessel 1 are transmitted to the liquid contained in the tanks 3, 4, 5, 6, which, as a result, moves in the tanks 3, 4, 5, 6. The movements of the liquid in the tanks 3, 4, 5, 6 lead to impacts on the walls of the tanks 3, 4, 5, 6 which can instantly damage tanks if they are too strong. In addition, repeated impacts on the walls of the tanks 3, 4, 5, 6 at high but non-destructive levels can lead to deterioration of the performance of said walls in terms of fatigue wear. Thus, it is important to maintain the integrity of the walls of tanks 3, 4, 5, 6 in order to maintain the tightness and insulating characteristics of tanks 3, 4, 5, 6.

[040] A well-known measure is the exclusion of critical navigational conditions to prevent fluid movements that can immediately damage the tank. With regard to wear-related damage to impacted materials, only regular inspections of the tank can reveal the damaged condition. To inspect the tank, it is necessary to completely empty the tank to ensure that it can enter the tank. Such an operation is relatively time consuming and results in a long decommissioning of the vessel. In order to optimize tank inspections, the present invention proposes to determine the wear rate of the tank to decide whether such an inspection is appropriate.

[041] The wear rate of the ship's tank should reflect the risk of damage to the tank depending on the impacts associated with the sloshing of liquefied gas. The risk of damage to a ship's cryogenic tank can be expressed by the following formula:

[42] [Math Expression 1]

, где

, Where

[043] nr(t) – tank filling level at time t,

mvt(t) is the displacement of the reservoir at time t,

Pres(dS,t) is the pressure exerted per unit dS of the tank surface area at time t,

Res(dS) – strength per unit dS of the tank surface area,

Prob(Pres(dS,t)>Res(dS)|nr(t), mvt(t)) - the probability that the pressure per unit area of the surface will be greater than its strength, depending on the filling level of the tank and the movement of the tank ,

T is the period during which the risk of damage is established.

[044] The tank wear rate can be obtained by applying the approximation of the above formula to the actual operating conditions of said tank. Accordingly, depending on what kind of impacts the tank is actually exposed to, the wear indicator will be a value representing the probability of damage.

[045] FIG. 2 illustrates an example of an onboard control system 100 of a vessel 1. The control system 100 includes a central processing unit 110 connected to a plurality of onboard sensors 120 that allow measurements of various parameters to be obtained. Thus, sensors 120 include, by way of non-limiting example, at least one fill level sensor 121 for each tank, various vessel motion sensors 122 (tachometer, heading sensor), and sea condition sensors 123. The control system 100 also includes a communication interface 130 allowing the central processing unit 110 to communicate with remote devices, for example, to obtain meteorological data, ship position data, and so on.

[046] The number and type of displacement sensors 122 and marine conditions sensors 123 depend on the choice of a person skilled in the art to determine the movements to which the vessel is subjected and any additional measurements. In a preferred embodiment, the sea sensor comprises a weather station that measures the height, frequency, and direction of waves in the ship's environment. Alternatively, sea conditions may not be measured, but inferred from weather information, depending on the exact position of the ship. However, it is preferable to have some data redundancy in case some sensors fail.

[047] The control system 100 further includes a human-machine interface 140. The human-machine interface 140 includes a display means 41. The display means 41 allows the operator to receive control information calculated by the system, or measurements taken by the sensors 120, or even the current state of the sloshing.

[048] The human-machine interface 140 further includes a data acquisition means 42 allowing an operator to manually transmit quantitative data to the central processing unit 110, typically to transmit to the central processing unit 110 data that cannot be received by the sensors because the vessel does not contain required sensor, or it is damaged. For example, in one embodiment, the data acquisition means allows an operator to enter wave height information based on visual observation.

[049] The control system 100 includes a database 150. The database 150 allows you to store measurements taken during navigation. According to a preferred embodiment, the database includes some quantitative data obtained in the laboratory or during onboard measurements at sea.

[050] FIG. 3 illustrates an example of a land-based control system 200 communicating with a ship 1. The ship includes a central processing unit 110, sensors 120, and a communication interface 130. The control system 200 includes a central processing unit 210, a communication interface 230, a human-machine interface 240, and a database 250. The operation of the control system 200 is similar to that of the control system 100 and differs only in sending the information received by the sensors 120 of the ship 1 to the land based control system 200 via the communication interfaces 130 and 230. As an example, the communication interfaces may use terrestrial or satellite RF data transmission.

[051] FIG. 4 illustrates a block diagram of an example of updating a wear rate. According to the first embodiment, the update is entirely performed in the central processing unit 110 constituting a single processing means. According to the second embodiment, the update is partly performed in the central processing unit 110 and partly in the central processing unit 210, the combination of which forms a common processing means. In a first step 310, the fill level sensor 121 of each ship's tank 3, 4, 5, 6 measures the liquid level in each tank to determine the current fill level of each tank i. Then, in the second step 320, the displacement sensors 122 and the sea conditions sensors 123 transmit information to the central processing unit 110 to determine the current state of the ship's movement.

[052] To simplify the calculations, it is preferable to take measurements per unit of time, during which the measurements are considered stable. As an example, the unit of time is one hour. Thus, measurements of the current fill level can represent the average measurements over one hour, which makes it possible not to take into account the influence of liquid movements on the measurement. As regards the current state of the ship's movement, either the average measurement or the worst measurement per unit of time can be considered. For example, if the sea sensors measured 2m high waves and 8m high waves in a unit of time, the saved measurement would correspond to 8m high waves. On the other hand, in the case of wave frequency, an average measurement may be preferable.

[053] After the measurements are taken, at step 330, the current slosh I Bi is calculated for each tank. The current sloshing index I Bi can be determined in various ways depending on the various measurements taken of the filling level of the tank and the current state of the movement of the vessel, taking into account the position and geometry of the tank.

[054] As an example, the first simplified embodiment takes into account the average filling level of the tank and the maximum wave height in one hour. According to the first embodiment, a lookup table stored in the database 150 or 250 is used to determine the corresponding sloshing index Ibi, simply by reading the table depending on the measurements taken. A lookup table can be compiled from measurements in a tank having a similar geometric profile, in which the number and strength of the impacts of the liquid against the walls of the tank are measured. The result of these measurements is entered into a correspondence table, for example, table 1 below:

[55] [Table 1]

Wave height (m) 0-4 4-8 8-12 12-16 16+ Tank level (%) 0-5 1 2 3 4 5 5-15 3 6 9 12 15 15-30 2 4 6 8 10 30-70 0 0 0 0 0 70-85 1 2 3 4 5 85-95 2 4 6 8 10 95-99 1 2 3 4 5

[056] Depending on the fill level and wave height, the current slosh value IB is directly read in relation to the tank geometry. Obviously, the larger the table, the more accurate the current IB slosh value. In addition, the table must correspond to each ship's tank, since liquid sloshing depends, among other things, on the shape of the tank and its position on the ship.

[057] The use of a displacement parameter alone, such as wave height, is insufficient to obtain an accurate determination of the effect of sloshing and provides only a rough indication. In fact, many other parameters can be used to determine the sloshing of a liquid, such as the frequency of the waves, the angle of incidence of the waves relative to the ship, the speed of the ship. In addition, auxiliary parameters can be added, such as the strength and direction of wind and currents. It may be envisaged to use a lookup table with a large number of entries. In addition, the data may be processed before being used in the table. For example, wave frequency, wave incidence and ship speed can be combined into the frequency of the waves acting on the ship, thus allowing the three parameters to be combined into one and hence reducing the size of the table, which still considers multiple parameters. Another possibility is to create multiple tables each providing a partial slosh score and combine these scores, for example by multiplying the partial scores.

[058] The use of the table also makes it possible to take into account the indirect causes of movement, for example, vibrations of the ship, barge or simply the tank, and the mentioned vibrations are the result of the work of, for example, the propulsion system or the blades of the ship's propellers. The amplitude and frequency of the vibrations can be tabulated as two entries obtained experimentally in a tank of the same size.

[059] Another possibility is to make actual measurements of the ship's wobble without regard to sea conditions. The sensors 120 determine the measured movements of the vessel, for example, by measuring the accelerations experienced by the vessel in three perpendicular axes during successive movement and rotation. An inertial measurement unit (IMU) consisting of one or more accelerometers and/or one or more gyroscopes, such as mechanical gyroscopes, and/or one or more magnetometers, can preferably be used to estimate the movements of the ship. Assuming that a plurality of measurement units (same type or two different types) are used, they are preferably distributed throughout the vessel to record an accurate measurement of the movement of the vessel. It should be noted that the IMU is sometimes referred to as the MRU (motion sensor). A lookup table may also be used, containing an entry for each dimension.

[060] Alternatively, an indicator representing the fluctuation of the liquid in the tank may also be calculated in another way. For example, depending on the level of the liquid and the geometry of the tank, the resonant frequency of the liquid in the tank is calculated. Then determine the average frequency of the ship, for example, by measuring the average frequency and the average amplitude of the ship using accelerometers. The current sloshing Ibi may be proportional to the ship's vibration amplitude and inversely proportional to the difference between the resonant frequency of the tank and the ship's vibration frequency.

[061] In order to determine the instantaneous slosh rate, additional parameters of the state of the structure can also be taken into account, which can influence the risk of damage to the tank. The reservoir experiences large thermal changes which cause the reservoir to elongate and contract. The thermal cycles that the tank is subjected to also affect wear, which weakens the tank. The multiplication factor of the instantaneous slosh index can take into account the number of thermal cycles, thereby reflecting the increased risk of damage associated with sloshing, depending on the number of thermal cycles. Thermal cycles can also have weight depending on the liquid content in the reservoir, the temperature of which can vary and therefore cause a change in the amplitude of the thermal cycle.

[062] Another additional parameter concerns the elongation of the vessel, in other words, the change in the length of the floating structure. Since the ship is made of metal, changes in length occur depending on the temperature of the sea and the sun. The length changes are independent of the expansion of the tank membranes related to the temperature of the LPG. The difference in expansion leads to additional stresses at the level of the membranes, which can weaken the membranes to a greater or lesser extent. The introduction of a multiplication factor allows these phenomena to be taken into account, for example, by increasing the weight of the instantaneous sloshing index by 10% if the elongation stresses exceed a predetermined threshold value.

[063] The level of rigidity of the hull or flexibility of the hull may also be considered, particularly when the tank or tanks are installed on a mobile vessel. When a boat is exposed to waves, it twists to a greater or lesser extent depending on its position relative to the waves. The resulting deformation leads to stresses in the tanks, which must be taken into account when determining the instantaneous sloshing index.

[64] The determination of the instantaneous slosh index can also be influenced by the nature of the cryogenic liquid being transported. The composition of the liquefied gas changes its density and fluidity, thereby changing the sloshing effects in a positive or negative way, or simply changing the resonant frequency of the liquid in the tank. If the tank is designed for more than one type of gas, an additional parameter must be taken into account to determine the instantaneous sloshing index, depending on the nature of the transported cryogenic liquid.

[065] Another additional parameter that can be taken into account when calculating the instantaneous slosh rate is the height of the liquid column in the ballast tanks. On some ships, ballast tanks may be in direct contact with the wall that supports the tank. Sea water in ballast tanks is also susceptible to sloshing. However, depending on the filling of the ballast tanks, sloshing in the tank and in the ballast tanks may increase or decrease. Only the design of the vessel makes it possible to determine the final effects.

[066] After determining the current sloshing index I Bi for each tank of the ship per unit of time at step 340, it must be integrated into the tank wear index IUi. Since the slosh rate is computed over a given unit of time, a simple update by summing the current slosh rate IBi into the pre-calculated wear rate IUi is sufficient to determine a wear rate IUi that takes into account the history of slosh rates, for example, since the ship was commissioned.

[067] The sloshing index I Bi and the wear index IUi do not have units because they reflect the probability of damage. In addition, the value of these indicators IBi and IUi depends on the accuracy of the calculation, which must be obtained. One skilled in the art will be able to determine which value to use with these metrics. For example, for a current sloshing IUi measured per hour, which can range from 0 to 15, the total wear IUi over 30 years of ship operation is between 0 and 4 million inclusive.

[068] According to the second embodiment, the flowchart shown in FIG. 4 is partially executed by the ground control system 200 communicating with the vessel. In accordance with the second embodiment, the ship 1 transmits all information from the sensors 120 to the ground station. Alternatively, the sloshing values I Bi are calculated and stored on the ship during the voyage, and the wear value IUi is calculated only after the ship arrives at the port after the sloshing values IBi are transmitted. calculated over the entire voyage of the ship.

[069] The calculated wear index IUi is used by the maintenance management system in accordance with the flowchart shown in Fig. 5, which is used when the ship arrives at the corresponding port. The flowchart includes steps performed by maintenance personnel and other steps performed by the control system 100 or 200. Depending on the level of maintenance automation, some steps can be performed either by maintenance personnel or by the system. At step 410, the wear rates IUi of all of the vessel's tanks are obtained. The first test 420 is carried out to determine if it is necessary to provide for an inspection of the tank, taking into account the current date and the number of days the ship has spent at sea.

[070] If inspection is not planned, test 430 compares the wear IUi with an inspection threshold SV corresponding to a high risk of damage, which is typically not reached prior to the inspection date. If one or more IUi values have reached the inspection threshold SV, it is required to indicate that an inspection of each tank that has reached the SV threshold value is required, for example, by activating a visual or audible warning signal at step 440. In this way, the crew will know that that, upon arrival at the port, an inspection of said tank or tanks should be carried out. If the SV threshold is not exceeded, or after step 440 is executed, the flowchart ends.

[071] If during the test 420 it is determined that a tank inspection is planned, then at step 450 it is determined which tank has the maximum wear rate. To do this, each wear index IUi is compared with the wear index IUi of all other tanks to determine the reservoir having the maximum wear index Max(IUi).

[072] After determining the maximum wear rate Max(IUi), the control system flags the corresponding tank at step 460, and only that tank is inspected.

[073] After inspecting the tank, a new test 470 is performed to determine if another tank needs to be inspected. This test 470 is to check if the inspected tank is damaged. If the tank is not damaged, there is no need to inspect other tanks, which, according to the IUi wear indicators, have a lower risk of damage. On the other hand, if the reservoir is damaged, step 480 must be performed.

[074] Step 480 is to remove the wear score corresponding to the damaged tank that was just inspected from the wear scores of the vessel before repeating step 450 to determine the next tank to be inspected, which is the tank having the highest wear score of the remaining tanks. If this reservoir is damaged, then the inspection of the reservoirs continues in descending order of wear IUi.

[075] If the inspection of the tank does not reveal any malfunctions or damage requiring at least repair, then there is no need to inspect other tanks. This method can significantly reduce the number of inspected tanks. The number of tanks to be inspected is reduced to the number of damaged tanks plus 1, eliminating the need to empty all tanks.

[076] As indicated, automation can be implemented to a greater or lesser extent. According to a relatively low automation embodiment, control systems 100 and 200 only store the wear values IUi and compare them with each other and with the inspection threshold SV. The control system then displays the wear values IUi in descending order with a visual (eg red/green) indicator that indicates whether the threshold value SV has been exceeded. In this configuration, only steps 410, 430, and 450 are performed by the control system 100 or 200, and the rest of the flowchart shown in FIG. 5 is performed by maintenance personnel.

[077] In accordance with the highly automated embodiment, only inspection steps 440 and 460 are performed by maintenance personnel, and the inspection result is recorded in the system.

[078] Alternatively, at 460, the wear index Max(IUi) can also be compared with a minimum wear threshold below which no inspection of the reservoir is necessary. This eliminates unnecessary tank inspections.

[079] Alternatively, test 430 and step 440 may be omitted. In fact, the inspection threshold SV must be relatively high to avoid tank inspections on each trip. If the threshold value is very high, this is not necessary.

[080] In accordance with another alternative implementation, it can be provided that it is not necessary to take into account the dates of the inspection, but, on the contrary, only the inspection threshold SV for initiating inspections should be considered. In this case, test 430 may show that several tanks have wear IUi above the threshold SV. To prevent inspection of multiple tanks, step 440 may be replaced by steps corresponding to steps 450 and 480 so that only one undamaged tank is inspected.

081] If the threshold value SV is reached and no tank is damaged, it is necessary to increase the threshold value SV to a new value, which is inputted by the data acquisition means 42 .

[82] If a tank inspection reveals damage to a tank, that tank must be repaired either immediately or at the next port stop, but the tank is considered decommissioned pending repair. The decommissioned tank may be ignored when executing the flowchart shown in FIG. 5.

[083] Until the repair of the tank is made, it is necessary to maintain a wear indicator representing the wear of the tank since the construction of the vessel. After a tank repair, this wear indicator may no longer reflect the wear condition of the tank and the indicator should be changed. In the event that the repair of the tank involves an almost complete restoration of the tank, the repaired tank can be considered as new, and therefore its wear index IUi should be set to zero using the data acquisition means 42 . On the other hand, if only a partial refurbishment of the tank has been carried out, and some signs of wear, which are considered minor, have not been removed, the wear rate may be reduced or may remain the same. In particular, if the repair affected the minimum area of the tank, the indicator remains unchanged.

[084] Although the invention has been described with reference to several specific embodiments, it is obvious that it is not limited to them in any way, and that it includes all technical equivalents and combinations of the described means, if they are within the scope of the invention.

[085] The use of the verb "comprise" or "comprise" and derivative forms does not exclude the presence of elements or steps other than those set forth in the claim.

[086] In the claims, no parenthetical references should be interpreted as limiting the claim.

Claims (38)

1. A method for managing the maintenance of a ship (1), including at least one sealed and heat-insulating tank for transporting liquefied gas, which has a certain geometry and position on the ship, the method comprising the steps of: determine (310) the current filling level of the tank, determine (320) the current state of the movement of the vessel, determining (330) the current sloshing index IBi based on the current tank filling level and the current state of the ship's movement, taking into account the position and geometry of the tank, integrating (340) the determined current sloshing rate I Bi into the wear rate IUi that takes into account the history of sloshing rates, comparing (430, 450) the wear rate with the first threshold value to indicate (440, 460) whether to inspect the tank, depending on the result of the comparison. 2. The method according to claim. 1, in which the vessel includes a plurality of tanks (3, 4, 5, 6), in which the wear index (IUi) is determined for each tank and in which the first threshold value corresponds to the wear index (IUi) of the other tank to indicate (460) that the tank having the maximum wear rate (Max(IUi)) needs to be inspected. 3. A method according to claim 1 or 2, wherein said tank is inspected. 4. The method according to paragraphs. 2 and 3, in which, after inspecting the tank corresponding to the maximum wear rate (Max(IUi)) if the inspected tank is damaged, the next tank to be checked is determined (480) as the tank having the maximum wear rate of the remaining tanks. 5. The method according to paragraphs. 2 and 3, in which after the inspection of the tank, if the tank is not damaged, the inspection of the tanks is stopped. 6. A method according to any one of the preceding claims, wherein the comparison (450) of the wear index (IUi) with a threshold value is performed during a maintenance operation of the ship. 7. The method according to any one of the preceding claims, wherein determining the current state of the ship's movement includes determining at least one of the following parameters: wave height, wave period, angle of incidence of waves relative to the orientation of the vessel, vessel speed, ship vibrations, measured movements of the vessel. 8. A method according to any one of the preceding claims, wherein at least one of the following additional structural state parameters is taken into account in determining the current sloshing index (IBi): the number of thermal cycles the tank has undergone, vessel lengthening, hull stiffness level, nature of liquefied gas, filling level of ballast tanks. 9. The method according to any one of the preceding claims, wherein determining the current sloshing index (IBi) is to determine the probability of damage to the tank in the current state of the ship's movement. 10. The method according to any one of the preceding claims, wherein the current sloshing index (IBi) is obtained by reading a value from a lookup table with multiple entries, where the entry corresponds to the filling of the tank, at least one entry corresponds to the current state of the movement of the vessel, and the read value represents the possibility of damage to the tank. 11. A method according to any one of the preceding claims, wherein integrating the current slosh rate (IBi) corresponds to summing the current slosh rates, the current slosh rates being sequentially calculated over successive measurement periods. 12. Vessel maintenance management system (100, 200), comprising at least one sealed and heat-insulating tank for transporting liquefied gas, the system including: at least one tank fill level sensor (121, 230) for measuring the current fill level of at least one tank, a device (122, 123, 130, 230) for estimating the movements of the vessel, configured to determine the current state of the movement of the vessel, processing means (110, 210) configured to: determining (330) the current sloshing index (IBi) based on the current reservoir fill level and the current state of vessel movement, taking into account the position and geometry of the reservoir, integrating (340) a determined current sloshing rate (IBi) into a wear rate (IUi) that takes into account the history of sloshing rates, comparing (430, 450) the wear rate with the first threshold value, a warning device (41) indicating (440, 460) whether it is necessary to inspect at least one tank, depending on the result of the comparison. 13. The system of claim. 12, in which the device (123) for estimating the movements of the ship includes a weather station that measures the height, frequency and direction of waves in the environment of the ship. 14. The system according to claim 12 or 13, wherein the processing means is partially located at the monitoring station (200) on land and is in radio communication with the vessel to receive information related to the current fill level of at least one tank and the current state of the movement of the vessel. 15. Vessel for transporting liquefied gas, including many sealed and insulating tanks, and the ship includes a system (100) control according to any one of paragraphs. 12-14 for each of the tanks.

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