Method and system for monitoring mooring system

Abstract

The invention relates to a method of monitoring a mooring system for a floating unit moored in a body of water, the mooring system comprising at least one mooring line coupled to the floating unit, the method comprising steps of: obtaining positional data of the floating unit; providing a coordinate system including at least one watch area corresponding to an expected range of positional data for the floating unit in case at least one of the mooring lines is broken; comparing the obtained positional data with the at least one watch area on the coordinate system to determine failure of any of the at least one mooring line if the obtained positional data lies within one of the at least one watch areas.

Description

[0001] METHOD AND SYSTEM FOR MONITORING MOORING SYSTEM

[0002] TECHNICAL FIELD

[0003] The present disclosure relates to a method and system of monitoring a mooring system for a moored floating unit. The present disclosure also relates to a computer program comprising instructions for performing said method, a computer readable storage medium having recorded thereon said computer program, and a system comprising a processor coupled to a memory which has recorded thereon said computer program.

[0004] TECHNICAL BACKGROUND

[0005] In the field of offshore oil and gas production, a floating unit or platform for loading / offloading and / or storage is moored to the seabed by a mooring system. The mooring system includes multiple mooring lines extending from a given side or corner of the floating unit to the seabed, that limit drift from an initial position of the floating unit and maintain it in a desired location.

[0006] It is important to monitor the mooring system to determine whether any of the mooring lines has been damaged or come loose from its anchoring point on the seabed, and whether there is any failure or defect to any of the mooring lines and / or the anchoring points. Such failure must be noticed and fixed early in order to avoid any further damage to a riser / export system which is crucial to the production and / or storage of the floating unit.

[0007] Some monitoring systems monitor the integrity of the mooring lines by measuring an angle of the mooring line at the mooring point to provide an indication of the tension in the mooring line. If the angle drops below a predetermined level, it is inferred that the mooring line has gone slack due to damage. Such monitoring systems employ a load cell or a compressive cell located either on the mooring line or on a guiding / supporting equipment or at a tensioning system, an inclinometer along the mooring line, or a sonar deployed beneath a floating unit. They are expensive, complicated to install, maintain and replace, and vulnerable to damage since they are installed underwater or close to water. In such monitoring system, the damage may not be immediately and precisely detected at the time of occurrence due to data transmission errors, and there is a risk of ignoring actual failure occurrence due to false alarms.

[0008] There is also an approach to detect mooring line failure based on a machine learning algorithm. Such approach requires enormous numeric simulations for training a model and is still inconvenient.

[0009] Document EP3325337 relates to a method and system of monitoring for changes in a mooring system for a floating vessel by monitoring the natural period of the vessel for a given geographical position and comparing it to the theoretical values for intact mooring system. This method is not reliable since it relies on the natural period of the vessel which is an indirect parameter.

[0010] Document EP2148806 relates to a method and system of monitoring for damage in the mooring system of a moored vessel having a mooring point to which a mooring line is connected, by calculating a geographical position of the mooring point by reference to a geographical position of a locating point remote from the mooring point and the heading of the vessel and comparing the calculated position of the mooring point to an expected, normal position of the intact mooring point. This method has limited accuracy, and even an anomaly in only one of variables would lead to false alarms.

[0011] Therefore, there is a need for providing an improved method and system of monitoring a mooring system for a moored floating unit.

[0012] SUMMARY OF THE INVENTION

[0013] There is provided a method of monitoring a mooring system for a floating unit moored in a body of water, the mooring system comprising at least one mooring line coupled to the floating unit, the method comprising steps of:

[0014] - obtaining positional data of the floating unit;

[0015] - providing a coordinate system including at least one watch area corresponding to an expected range of positional data for the floating unit in case at least one of the mooring lines is broken;

[0016] - comparing the obtained positional data with the at least one watch area on the coordinate system to determine failure of any of the at least one mooring line if the obtained positional data lies within one of the at least one watch areas.

[0017] According to some embodiments, the mooring system comprises a plurality of mooring lines, for each mooring line, the coordinate system includes a watch area associated with said mooring line, which corresponds to an expected range of positional data for the floating unit in case said mooring line is broken, the obtained positional data is compared with all watch areas of the coordinate system.

[0018] According to some embodiments, if the obtained positional data lies within a watch area associated with a mooring line, the failure of this mooring line is determined to have taken place.

[0019] According to some embodiments, the positional data of the floating unit comprises two geographical positional data, preferably a west-east positional value and a north-south positional value.

[0020] According to some embodiments, the positional data of the floating unit comprises a heading value.

[0021] According to some embodiments, the step of obtaining positional data of the floating unit comprises obtaining the positional data by means of a satellite positioning system and optionally by means of a gyrocompass.

[0022] According to some embodiments, the mooring system comprises two or more groups of mooring lines, each group comprising one or more mooring lines and corresponding to a coupling area of the floating unit, and mooring lines in the same group are coupled to the same coupling area of the floating unit.

[0023] According to some embodiments, the coordinate system includes two or more clusters of watch areas, each cluster being associated with a respective group of mooring lines, each watch area within a cluster corresponding to an expected range of positional data for the floating unit in case one of the mooring lines of the corresponding group is broken.

[0024] According to some embodiments, if the obtained positional data lies within a cluster of watch area(s), the failure of a mooring line within the group of mooring lines associated with said cluster is determined to have taken place.

[0025] According to some embodiments, the coordinate system includes at least one watch area corresponding to an expected range of positional data for the floating unit in case two or more of the mooring lines are broken; and, if the obtained positional data lies within said watch area, the failure of two mooring lines associated with said watch area is determined to have taken place.

[0026] According to some embodiments, the step of obtaining positional data of the floating unit comprises acquiring positional values over time and intermittently calculating average values from the acquired positional values, the positional data being a set of average values over time.

[0027] According to some embodiments, failure is determined if the obtained positional data lies within one watch area or within one cluster of watch areas for a minimum period of time, or if a minimum number of the obtained positional data lies within one watch area or within one cluster of watch areas, or if a minimum proportion of the obtained positional data lies within one watch area or within one cluster of watch areas for a predetermined period of time.

[0028] According to some embodiments, the method further comprises correcting the obtained positional data of the floating unit prior to comparing it with the at least one watch area, based on at least one of:

[0029] - meteorology conditions;

[0030] - oceanography conditions;

[0031] - site conditions of the body of water; and

[0032] - operation conditions of the floating unit.

[0033] According to some embodiments, the floating unit is one of a Floating Storage and Offloading unit (FSO), a Floating Production Unit (FPU), a Floating Production Storage and Offloading unit (FPSO), a Floating Storage and Regasification Unit (FSRU), a Floating Liquefied Natural Gas unit (FLNG), a Floating Offshore Wind Turbine (FOWT), Floating Storage Injection Unit (FSIU), and a Floating Drilling Production Storage and Offloading unit (FDPSO).

[0034] There is also provided a monitoring system for a mooring system of a floating unit moored in a body of water, the mooring system comprising at least one mooring line coupled to the floating unit, the monitoring system comprising a data processing system configured to perform the above-described method.

[0035] There is also provided a computer program comprising instructions for performing the above-described method.

[0036] There is also provided a computer readable storage medium having recorded thereon the above-described computer program.

[0037] There is also provided a system comprising a processor coupled to a memory, the memory having recorded thereon the above-described computer program.

[0038] The present disclosure enables to detect a damage or failure of any mooring line in the mooring system efficiently and effectively. Each watch area in the coordinate system represents a numerical model of the theoretically damaged floating unit, simulated with a damage or failure in each mooring line. When the actual measurement of the positional data of the floating unit lies within a certain watch area, it can be determined that the mooring line corresponding to that watch area is damaged.

[0039] The present disclosure provides a simpler and more robust method of monitoring a mooring system, which is based on direct and physical parameters of position of the floating unit. Since failure or damage detection is based on the positional data of the floating unit falling within the watch area for the theoretically damaged floating unit, any parameter error in positional data can be identified as an outlier on the coordinate system, thus false alarms can be effectively avoided. This is in contrast to a detection which would be simply based on positional data of the floating unit falling outside of an expected area for the undamaged floating unit. The method according to the disclosure requires minimum calculation effort to be adapted to any kind of floating unit.

[0040] BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Non-limiting examples will now be described in reference to the accompanying drawings, where:

[0042] FIG. 1A schematically shows an example of a spread-moored floating unit in an intact state.

[0043] FIG. 1 B schematically shows the example of a spread-moored floating unit of FIG. 1A in a damaged state.

[0044] FIG. 2A schematically shows an example of a turret-moored floating unit in an intact state.

[0045] FIG. 2B schematically shows the example of a turret-moored floating unit of FIG. 2A in a damaged state.

[0046] FIG. 3 schematically shows an example of coordinate system including watch areas.

[0047] FIG. 4A schematically shows projection in a plane of north-south and westeast coordinates of watch areas in case one mooring line is damaged.

[0048] FIG. 4B schematically shows projection in a plane of north-south and heading coordinates of watch areas in case one mooring line is damaged.

[0049] FIG. 5 schematically shows translation in a horizontal plane of watch areas in case two mooring lines are damaged.

[0050] FIG. 6 schematically shows an example of a spread-moored floating unit with possible external loads.

[0051] FIG. 7 schematically shows an example of a turret-moored floating unit with possible external loads.

[0052] FIG. 8 schematically shows an example of a system for computerimplementation of the method according to the disclosure.

[0053] DETAILED DESCRIPTION

[0054] The present disclosure will now be described in more detail without limitation in the following description.

[0055] It is provided a method of monitoring a mooring system for a floating unit moored in a body of water. The mooring system comprises at least one mooring line coupled to the floating unit. The method comprises steps of: - obtaining positional data of the floating unit;

[0056] - providing a coordinate system including at least one watch area corresponding to an expected range of positional data for the floating unit in case at least one of the mooring lines is broken;

[0057] - comparing the obtained positional data with the at least one watch area on the coordinate system to determine failure of any of the at least one mooring line if the obtained positional data lies within one of the at least one watch areas.

[0058] The method according to the present disclosure is applied to a floating unit moored in a body of water, for example a lake, a river, a sea and / or an ocean. For example, the floating unit is moored via at least one mooring line coupled to the floating unit. The at least one mooring line may be coupled between the floating unit and an anchor on a holding ground of the body of water. The holding ground of the body of water may be the bottom of the body of water, such as the seabed or sea floor, a shore of the body of water, or any structure connecting the bottom of the body of water.

[0059] In some embodiments, the floating unit is one of a Floating Storage and Offloading unit (FSO), a Floating Production Unit (FPU), a Floating Production Storage and Offloading unit (FPSO), a Floating Storage and Regasification Unit (FSRU), a Floating Liquefied Natural Gas unit (FLNG), a Floating Offshore Wind Turbine (FOWT), Floating Storage Injection Unit (FSIU), and a Floating Drilling Production Storage and Offloading unit (FDPSO).

[0060] Preferably, the coordinate system including at least one watch area is provided by calculation or computation using a numerical model of the mooring system. The calculation or computation is preferably a finite element method. The numerical model is preferably based on a set of equations based on the laws of physics, and on a number of parameters characterizing the floating unit, the mooring lines, and the body of water. The watch areas are not obtained by machine learning.

[0061] Each watch area can be obtained by simulating an expected range of positional data for the floating unit in the numerical model by deleting a respective mooring line in the numerical model.

[0062] Such numerical model and simulation comprise, for example Ariane marketed by Bureau Veritas, Orcaflex marketed by Orcina, DeepLines marketed by Principia, or SIMO marketed by DNV.

[0063] In some embodiments, the positional data of the floating unit comprises two geographical positional data. By “geographical positional data" is meant data defining the position of a point on a map, such as according to a longitude / latitude reference.

[0064] Preferably, the positional data of the floating unit comprises a west-east positional value and a north-south positional value. In the case of two geographical positional data, the coordinate system may be two-dimensional.

[0065] In some embodiments, the positional data of the floating unit comprises a heading value. Preferably, the positional data of the floating unit comprises two geographical positional data (e.g. a west-east positional value and a north-south positional value) and the heading value. In this case, the coordinate system may be three-dimensional.

[0066] By “heading" is meant the angular orientation of the floating unit, in a horizontal plane, relative to a fixed reference direction.

[0067] Preferably, each watch area may be a circle or ellipse on the two- dimensional coordinate system, or a sphere or ellipsoid on the three-dimensional coordinate system.

[0068] Preferably, the mooring system comprises a plurality of mooring lines. For each mooring line, the coordinate system includes a watch area associated with said mooring line, which corresponds to an expected range of positional data for the floating unit in case said mooring line is broken. The obtained positional data is compared with all watch areas of the coordinate system.

[0069] In some embodiments, if the obtained positional data lies within a watch area associated with a mooring line, the failure of this mooring line is determined to have taken place.

[0070] For example, the floating unit may be spread-moored in that a plurality (e.g. four) sets of mooring lines are each connected to respective coupling areas on a hull or body of the floating unit. Alternatively, the floating unit may be turret-moored via a mooring turret. The mooring turret includes coupling area(s) to which mooring lines are connected, and the floating unit can rotate (as a so-called “weather vane”) about the mooring turret. The mooring turret may be either an internal turret integrated in the hull or body of the floating unit, or an external turret outside the hull or body of the floating unit. For the turret-moored floating unit, the positional data of the floating unit may be provided with reference to positional data of the turret.

[0071] Preferably, the positional data of the floating unit represents the center of gravity of the floating unit. For the turret-moored floating unit, the positional data of the floating unit may represent the position (e.g., the center of gravity) of the turret. Alternatively, the positional data of the floating unit may represent an arbitrary point of the floating unit or the turret. For a spread-moored floating unit, preferably three positional data are used, namely two geographical positional data (e.g. west-east and north-south) and a heading.

[0072] For a turret-moored floating unit, preferably two positional data are used, namely two geographical positional data (e.g. west-east and north-south).

[0073] Preferably, the origin of the coordinate system represents the average position of the floating unit in an intact state, in which no mooring line is damaged. The coordinate system may comprise a predetermined area around the origin, within which the positional data of the floating unit should lie when there is no damage to any mooring line. The predetermined area may be considered as an expected range of positional data for the floating unit in the intact state. The position of the intact floating unit will fluctuate over time within this predetermined area, depending on weather conditions, body of water conditions, and other factors. Similarly, when a mooring line is damaged, the position of the floating unit will fluctuate over time within the watch area associated with the damaged mooring line.

[0074] Preferably, there is no overlap between any watch area and the predetermined area around the origin.

[0075] In some embodiments, the step of obtaining positional data of the floating unit comprises obtaining the positional data by means of a satellite positioning system, and optionally by means of a gyrocompass.

[0076] For example, the satellite positioning system may be the Global Positioning System (GPS) or Differential Global Positioning System (DGPS). Preferably, the geographical positional data such as the west-east positional value and the northsouth positional value, and / or the heading of the floating unit may be determined using GPS. For example, two spaced GPS antennas may be positioned at fixed locations on the floating unit in order that the difference in their positions may be used to determine the heading of the floating unit. A gyrocompass may also be used alternatively or additionally to the satellite positioning system. For example, a GPS antenna may be used for obtaining the two geographical positional values, and a gyrocompass may be used for obtaining the heading. Any other means to obtain positional data of the floating unit may also be contemplated.

[0077] In some embodiments, the mooring system comprises two or more groups of mooring lines. Each group comprises one or more of the mooring lines and corresponds to a coupling area of the floating unit. Mooring lines in the same group are coupled to the same coupling area of the floating unit.

[0078] In some embodiments, the coordinate system includes two or more clusters of watch areas. Each cluster is associated with a respective group of mooring lines. Each watch area within a cluster corresponds to an expected range of positional data for the floating unit in case one of the mooring lines of the corresponding group is broken.

[0079] In some embodiments, if the obtained positional data lies within a cluster of watch area(s), the failure of a mooring line within the group of mooring lines associated with said cluster is determined to have taken place.

[0080] Preferably, the respective clusters do not overlap. Two or more watch areas in the same cluster may or may not at least partly overlap with each other. Preferably, any cluster and the predetermined area around the origin do not overlap in that any cluster does not overlap and intersect with the predetermined area around the origin.

[0081] For example, the mooring system may comprise three or four groups of mooring lines, corresponding to three or four coupling areas (corners, for example) of the floating unit or the turret. Each group may comprise two, three, four, five or six mooring lines, each coupled to the same coupling area. However, the number of groups ( / .e., the number of coupling areas) and the number of mooring lines in each group are not limited.

[0082] In some embodiments, the step of obtaining positional data of the floating unit comprises acquiring positional values over time and intermittently calculating average values from the acquired positional values, and the positional data mentioned above corresponds to a set of average values over time.

[0083] For example, the positional data may be obtained once, twice or more per day, week, month or year. The positional data may be continuously obtained, for example every second, every minute, or every hour. For example, the positional values for the floating unit may be acquired continuously or intermittently several times during a day, week, month, or year, and an average value calculated from the acquired positional values may be provided as the positional data once per day, week, month or year. Alternatively or additionally, an individual set of positional values (e.g., north-south and west-east positional values, optionally with a heading value) acquired at a certain time may be directly provided as the positional data.

[0084] In some embodiments, a graphical representation of the obtained positional data on the coordinate system may be provided on a graphical user interface - although this is not mandatory. For example, the obtained positional data of the floating unit may be plotted or displayed on the coordinate system in substantially real-time (as they are obtained), and an alarm or alert may be issued when the obtained positional data of the floating unit lies within any watch area. Such an alarm or alert may be color indication (e.g., red, yellow etc.) of a point of the obtained positional data, a flashing or blinking sign, a text message and / or a warning sound, for example. For example, the obtained positional data may be indicated as a green point when it lies within the predetermined area around the origin, which implies that there is no damage to any mooring line, and may be indicated as a red point when it lies within any watch area, which implies that the mooring line corresponding to that watch area is damaged. Additionally, the obtained positional data may be indicated as a yellow point when it lies neither within the predetermined area around the origin nor within any watch area, which implies an outlier or a measurement error. The color indication may differ for each watch area and / or cluster.

[0085] For example, each watch area on the coordinate system is labelled with a reference of each mooring line.

[0086] FIG. 1A illustrates an example of floating unit which is spread-moored by a plurality of mooring lines, in an intact state. By way of example only, three mooring lines are coupled to each of four coupling areas (here, four corners) of the floating unit in FIG. 1A, but the number of mooring lines and the coupling areas are not limited. In the intact state in FIG. 1A, the center of the floating unit 10 (marked with “+”) is located in the initial position, which is taken as the origin of the coordinate system. There are four groups, each comprising three mooring lines, in this example. Specifically, the first group includes mooring lines S1 , S2, S3 coupled to a first corner of the floating unit 10, the second group includes mooring lines P1 , P2, P3 coupled to a second corner of the floating unit 10, the third group includes mooring lines S4, S5, S6 coupled to a third corner of the floating unit 10, and the fourth group includes mooring lines P4, P5, P6 coupled to a fourth corner of the floating unit 10.

[0087] FIG. 1 B illustrates the same example of floating unit of FIG. 1A, in a damaged state in which one mooring line (P4, in this example) at the top left corner is damaged. In this damaged state, the floating unit 10 is displaced from the initial position in the intact state (drawn with broken lines in FIG. 1 B). The displaced position of the center of the floating unit in the damaged state can be expressed with the west-east positional value, the north-south positional value and the heading value of the floating unit in the coordinate system with reference to the origin.

[0088] FIG. 2A illustrates an example of floating unit which is turret-moored, in an intact state. By way of example only, the floating unit 20 is moored via a turret 30 having three coupling areas, and three mooring lines are coupled to each of the coupling areas, but the number of mooring lines and coupling areas are not limited. In the intact state in FIG. 2A, the turret 30 (marked with “+”) is located in the initial position, which is taken as the origin of the coordinate system. There are three groups, each comprising three mooring lines, in this example. Specifically, the first group includes mooring lines Q1 , Q2, Q3 coupled to the first coupling area of the turret 30, the second group includes mooring lines R1 , R2, R3 coupled to the second coupling area of the turret 30, and the third group includes mooring lines S1 , S2, S3 coupled to the third coupling area of the turret 30.

[0089] FIG. 2B illustrates the same example of floating unit of FIG. 2A, in a damaged state in which one mooring line (S3, in this example) is damaged. Since the floating unit turns around the turret, displacement of the floating unit from the initial position in the intact state can be expressed without the heading value. The displaced position of the floating unit 20 in the damaged state can be expressed with the west-east positional value and the north-south positional value of the turret 30 in the coordinates system with reference to the origin.

[0090] FIG. 3 illustrates an example of coordinate system including watch areas. The coordinate system in FIG. 3 is based on a numerical model of a spread- moored floating unit moored via 12 mooring lines in total, as shown in FIG. 1A. Three mooring lines are respectively coupled to each of four coupling areas. The X, Y and Z axes in the coordinate system represent the north-south positional value, the west-east positional value, and the heading value of the floating unit, respectively. The origin (0, 0, 0) of the coordinate system represents the average position of the floating unit in an intact state. As seen in FIG. 3, the coordinate system includes four clusters (C1a, C2a, C3a, C4a), each cluster including three watch areas. For example, the top right cluster C1 a corresponds to the first coupling area to which a group of mooring lines S1 , S2, and S3 are coupled, and includes three watch areas corresponding to mooring lines S1 , S2, S3 respectively. Each watch area is obtained by simulating the positional data of the floating unit in case each mooring line S1 , S2, or S3 is damaged, by deleting each mooring line S1 , S2, or S3 in the numerical model of the floating unit. Similarly, the bottom right cluster C2a corresponds to the second coupling area to which a group of mooring lines P1 , P2 and P3 are coupled, and includes three watch areas corresponding to mooring lines P1 , P2 and P3 respectively. The bottom left cluster C3a corresponds to the third coupling area to which a group of mooring lines S4, S5 and S6 are coupled, and includes three watch areas corresponding to mooring lines S4, S5 and S6 respectively. The top left cluster C4a corresponds to the fourth coupling area to which a group of mooring lines P4, P5 and P6 are coupled, and includes three watch areas corresponding to mooring lines P4, P5 and P6 respectively. The actual positional data of the floating unit is obtained as mentioned above, and plotted as a point in the coordinate system. As seen in FIG. 3, when the point of the obtained positional data of the floating unit lies within the predetermined area around the origin, it is supposed that the floating unit is held in the right position by mooring lines and no mooring line has been damaged. On the other hand, when the point of the obtained positional data of the floating unit lies within one of the clusters, for example the top left cluster in FIG. 3, then it can be determined that any of mooring lines P4, P5 and P6 in this cluster has been damaged. More specifically, when the point of the obtained positional data of the floating unit lies within one of the watch areas, for example the watch area labelled P4 in FIG. 3, it can be determined that mooring line P4 has been damaged - if the watch areas within a cluster do not overlap, which is not the case in this example.

[0091] In this way, the coordinate system including the watch area according to the present disclosure provides a direct and noticeable indication of a damage or failure occurrence to any mooring line. As seen in FIG. 3, any point which does not lie within any watch area and which does not lie within the predetermined area around the origin is considered as an outlier. Since an alarm is issued when the obtained positional data lies within any of watch areas, a false alarm for an outlier can be effectively prevented.

[0092] FIG. 4A illustrates projection of the four clusters (C1a, C2a, C3a, C4a) each including three watch areas in a plane of north-south and west-east coordinates (x-y plane) for the example in FIG. 3. FIG. 4A shows the center of each watch area shifted from the origin. FIG. 4B illustrates projection of the four clusters (C1 a, C2a, C3a, C4a) each including three watch areas in a plane of north-south and heading coordinates (x-z plane) for the example in FIG. 3. FIG. 4B shows the center of each watch area shifted from the origin.

[0093] In some embodiments, the coordinate system includes at least one watch area corresponding to an expected range of positional data for the floating unit in case two or more of the mooring lines are broken. If the obtained positional data lies within said watch area, the failure of two mooring lines associated with said watch area is determined to have taken place.

[0094] Watch areas for all possible combinations of two mooring lines may be provided. Alternatively, only watch areas for possible combinations of two mooring lines belonging to the same group can be provided.

[0095] FIG. 5 provides an illustration of the latter variation, in one plane. The diagram shows four clusters (C1 a, C2a, C3a, C4a) comprising three watch areas each, wherein each watch area corresponds to one damaged mooring line. This is similar to what is described above in connection with FIG. 4A. Furthermore, the diagram shows four additional clusters (C1 b, C2b, C3b, C4b) of watch areas, wherein each watch area corresponds to two damaged mooring lines, within the same group of mooring lines. In this example, there are four groups of mooring lines, each group having three mooring lines. As a result, in the coordinate system, there are four initial clusters (C1 a, C2a, C3a, C4a), each comprising three watch areas, corresponding to the cases of failure of a single mooring line, and then four additional clusters (C1 b, C2b, C3b, C4b), each comprising three watch areas, corresponding to the cases of failure of two mooring lines within a group. By way of example only, with reference to FIG. 3 in combination with FIG. 5, cluster C1 a includes three watch areas, each corresponding to the case of failure of mooring line S1 , S2 or S3 in the same group, and cluster C1 b includes three watch areas, each corresponding to the case of failure of two mooring lines within the same group, i.e., mooring lines S1 and S2, S1 and S3, or S2 and S3.

[0096] Preferably, the additional clusters do not overlap with the initial clusters.

[0097] In some embodiments, failure is determined if the obtained positional data lies within one watch area or within one cluster of watch areas for a minimum period of time, or if a minimum number of the obtained positional data lies within one watch area or within one cluster of watch areas, or if a minimum proportion of the obtained positional data lies within one watch area or within one cluster of watch areas for a predetermined period of time.

[0098] For example, a damage or failure may be determined if the obtained positional data lies within one watch area or cluster for a period of time (such as several hours, one day or one week), or alternatively, when all (or at least two- thirds, or at least 90%, or at least 95%, etc.) of the positional data obtained during a period of time (such as several hours, one day or one week) lie within one watch area or cluster.

[0099] In some embodiments, the method further comprises correcting the obtained positional data of the floating unit prior to comparing it with the at least one watch area, based on at least one of:

[0100] - meteorology conditions;

[0101] - oceanography conditions;

[0102] - site conditions of the body of water; and

[0103] - operation conditions of the floating unit.

[0104] Preferably, the meteorology conditions comprise wind velocity and direction.

[0105] Preferably, the oceanography conditions comprise wave energy, wave spectrum, wave height, wave period, wave direction, current speed, and current direction. Preferably, the site conditions of the body of water comprise soil properties of seabed and water depth.

[0106] Preferably, the operation conditions of the floating unit comprise the action of a tanker on the floating unit and / or the action of one or several thrusters.

[0107] FIG. 6 illustrates an example of possible external loads that may be used to correct positional and heading values for a spread-moored floating unit.

[0108] FIG. 7 illustrates an example of possible external loads that may be used to correct positional values for a turret-moored floating unit.

[0109] Arrows in FIG. 6 and 7 illustrate environmental actions, respectively a current 40, a wave 50, and a wind 60. In FIG. 6 and 7, the mooring system is thruster assisted and the floating unit 20 is provided with thrusters 70 (eight thrusters in FIG. 6, and two thrusters in FIG.7, but not limited thereto). FIG. 7 shows a tandem-offloading system in which the floating unit 20 is connected via a hawser 80 to a shuttle tanker 90.

[0110] These environmental actions of the current, the wave and the wind, as well as the external efforts by the thrusters and / or the hawser may be considered as the external loads, and one or more of them may be used to correct positional and heading values for the floating unit. The positional and / or heading values may be corrected based on an additional measurement of, for example speed and / or direction of wind, tension in the hawser connecting the floating unit and the shuttle tanker, and / or the heading of the tanker. Such additional measurement makes it possible to calculate the external loads, which can be translated into displacement / rotation (positional vector) which is subtracted from the obtained positional data.

[0111] It is also provided a monitoring system for a mooring system of a floating unit moored in a body of water. The mooring system comprises at least one mooring line coupled to the floating unit. The monitoring system comprises a data processing system configured to perform the above-described method.

[0112] Preferably, the monitoring system comprises a display on which the abovedescribed coordinate system including at least one watch area is displayed.

[0113] It is also provided a computer program comprising instructions for performing the above-described method, and a computer readable storage medium having recorded thereon said computer program. It is also provided a system comprising a processor coupled to a memory, the memory having recorded thereon said computer program.

[0114] The above-described method may be computer-implemented. This means that steps (or substantially all the steps) of the method are executed by at least one computer, or any like system. Thus, steps of the method are performed by the computer, possibly fully automatically, or semi-automatically. In examples, the triggering of at least some of the steps of the method may be performed through user-computer interaction. The level of user-computer interaction required may depend on the level of automatism foreseen and put in balance with the need to implement user’s wishes. In examples, this level may be user-defined and / or predefined.

[0115] A typical example of computer-implementation of a method is to perform the method with a system adapted for this purpose. The system may comprise a processor coupled to a memory and a graphical user interface (GUI), the memory having recorded thereon a computer program comprising instructions for performing the method. The memory may also store a database. The memory is any hardware adapted for such storage, possibly comprising several physical distinct parts (e.g. one for the program, and possibly one for the database). For example, the system comprises a mobile phone, a laptop computer, a tablet computer, or any other portable electronic device.

[0116] FIG. 8 shows an example of a system for computer-implementation of the above-described method, wherein the system is a client computer system, e.g., a workstation of a user.

[0117] The client computer of the example comprises a central processing unit (CPU) 1010 connected to an internal communication BUS 1000, a random-access memory (RAM) 1070 also connected to the BUS. The client computer is further provided with a graphical processing unit (GPU) 1110 which is associated with a video random access memory 1100 connected to the BUS. Video RAM 1100 is also known in the art as frame buffer. A mass storage device controller 1020 manages accesses to a mass memory device, such as hard drive 1030. Mass memory devices suitable for tangibly embodying computer program instructions and data include all forms of nonvolatile memory, including by way of example semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks. Any of the foregoing may be supplemented by, or incorporated in, specially designed ASICs (application-specific integrated circuits). A network adapter 1050 manages accesses to a network 1060. The client computer may also include a haptic device 1090 such as cursor control device, a keyboard or the like. A cursor control device is used in the client computer to permit the user to selectively position a cursor at any desired location on display 1080. In addition, the cursor control device allows the user to select various commands, and input control signals. The cursor control device includes a number of signal generation devices for input control signals to system. Typically, a cursor control device may be a mouse, the button of the mouse being used to generate the signals. Alternatively or additionally, the client computer system may comprise a sensitive pad, and / or a sensitive screen.

[0118] The computer program may comprise instructions executable by a computer, the instructions comprising means for causing the above system to perform the method. The program may be recordable on any data storage medium, including the memory of the system. The program may for example be implemented in digital electronic circuitry, or in computer hardware, firmware, software, or in combinations of them. The program may be implemented as an apparatus, for example a product tangibly embodied in a machine-readable storage device for execution by a programmable processor. Method steps may be performed by a programmable processor executing a program of instructions to perform functions of the method by operating on input data and generating output. The processor may thus be programmable and coupled to receive data and instructions from, and to transmit data and instructions to, a data storage system, at least one input device, and at least one output device. The application program may be implemented in a high-level procedural or object-oriented programming language, or in assembly or machine language if desired. In any case, the language may be a compiled or interpreted language. The program may be a full installation program or an update program. Application of the program on the system results in any case in instructions for performing the method. The computer program may alternatively be stored and executed on a server of a cloud computing environment, the server being in communication across a network with one or more clients. In such a case a processing unit executes the instructions comprised by the program, thereby causing the method to be performed on the cloud computing environment.

[0119] More generally, all statements given for the method of monitoring a mooring system according to the disclosure are also applicable to the system of monitoring a mooring system and the computer program for performing said method, as well as the computer readable storage medium and the system related to said computer program.

Claims

CLAIMS1. A method of monitoring a mooring system for a floating unit moored in a body of water, the mooring system comprising at least one mooring line coupled to the floating unit, the method comprising steps of: obtaining positional data of the floating unit; providing a coordinate system including at least one watch area corresponding to an expected range of positional data for the floating unit in case at least one of the mooring lines is broken; comparing the obtained positional data with the at least one watch area on the coordinate system to determine failure of any of the at least one mooring line if the obtained positional data lies within one of the at least one watch areas.

2. The method according to claim 1 , wherein: the mooring system comprises a plurality of mooring lines, for each mooring line, the coordinate system includes a watch area associated with said mooring line, which corresponds to an expected range of positional data for the floating unit in case said mooring line is broken, the obtained positional data is compared with all watch areas of the coordinate system.

3. The method according to claim 2, wherein, if the obtained positional data lies within a watch area associated with a mooring line, the failure of this mooring line is determined to have taken place.

4. The method according to any one of claims 1 to 3, wherein the positional data of the floating unit comprises two geographical positional data, preferably a west-east positional value and a northsouth positional value.

5. The method according to claim 4, wherein the positional data of the floating unit comprises a heading value.

6. The method according to any one of claims 1 to 5, wherein the step of obtaining positional data of the floating unit comprises obtaining the positional data by means of a satellite positioning system and optionally by means of a gyrocompass.

7. The method according to any one of claims 1 to 6, wherein the mooring system comprises two or more groups of mooring lines, each group comprising one or more mooring lines and corresponding to a coupling area of the floating unit, and mooring lines in the same group are coupled to the same coupling area of the floating unit.

8. The method according to claim 7, wherein the coordinate system includes two or more clusters of watch areas, each cluster being associated with a respective group of mooring lines, each watch area within a cluster corresponding to an expected range of positional data for the floating unit in case one of the mooring lines of the corresponding group is broken.

9. The method according to claim 8, wherein, if the obtained positional data lies within a cluster of watch area(s), the failure of a mooring line within the group of mooring lines associated with said cluster is determined to have taken place.

10. The method according to any one of claims 1 to 9, wherein the coordinate system includes at least one watch area corresponding to an expected range of positional data for the floating unit in case two or more of the mooring lines are broken; and, if the obtained positional data lies within said watch area, the failure of two mooring lines associated with said watch area is determined to have taken place.

11. The method according to any one of claims 1 to 10, wherein the step of obtaining positional data of the floating unit comprises acquiring positional values over time and intermittently calculating average values from the acquired positional values, the positional data being a set of average values over time.

12. The method according to any one of claims 1 to 11 , wherein failure is determined if the obtained positional data lies within one watch area or within one cluster of watch areas for a minimum period of time, or if a minimum number of the obtained positional data lies within one watch area or within one cluster of watch areas, or if a minimum proportion of the obtained positional data lies within one watch area or within one cluster of watch areas for a predetermined period of time.

13. The method according to any one of claims 1 to 12, further comprising correcting the obtained positional data of the floating unit prior to comparing it with the at least one watch area, based on at least one of: meteorology conditions; oceanography conditions; site conditions of the body of water; and operation conditions of the floating unit.

14. The method according to any one of claims 1 to 13, wherein the floating unit is one of a Floating Storage and Offloading unit (FSO), a Floating Production Unit (FPU), a Floating Production Storage and Offloading unit (FPSO), a Floating Storage and Regasification Unit (FSRU), a Floating Liquefied Natural Gas unit (FLNG), a Floating Offshore Wind Turbine (FOWT), Floating Storage Injection Unit (FSIU), and a Floating Drilling Production Storage and Offloading unit (FDPSO).

15. A monitoring system for a mooring system of a floating unit moored in a body of water, the mooring system comprising at least one mooring line coupled to the floating unit, the monitoring system comprising a data processing system configured to perform the method according to any one of claims 1 to 14.

16. A computer program comprising instructions for performing the method according to any one of claims 1 to 14.

17. A computer readable storage medium having recorded thereon the computer program of claim 16.

18. A system comprising a processor coupled to a memory, the memory having recorded thereon the computer program of claim 16.

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