Patent Publication Number: US-2013239649-A1

Title: Method of determining the tension in a mooring line

Description:
METHOD OF DETERMINING THE TENSION IN A MOORING LINE 
     The present invention relates to the field of anchoring marine assets. More specifically, the present invention concerns methods of determining the tension in a mooring line employed to anchor a marine asset such as an oil rig, crane barge or offshore wind turbine. These methods also provide a means for calibrating a mooring line tension meter. 
     BACKGROUND TO THE INVENTION 
     There are an increasing number of marine assets that require anchoring (particularly as interest in deep water operations grows); for example oil rigs, crane barges and offshore wind turbines. Such assets are typically anchored to the seabed. The marine asset is generally attached to an anchor by a corresponding mooring line (also known as an anchor rode or anchor line). Mooring lines usually consist of a line, cable or chain. 
     Many of these marine assets, such as oil rigs, ships and offshore wind turbines, are anchored in areas where the seabed or land below the asset has pipelines or other hazards in the immediate vicinity. These marine assets have to maintain their position and elevation and, to do this, they normally have a number of mooring lines, typically eight or twelve, which are led from winches placed around the asset to anchors some distance away. In shallow waters (typically 25 m or less) the anchors are normally placed some ten times as far from the asset as the water is deep. However, in deeper waters (e.g. greater than 25 m) this can reduce to around a factor of three. A considerable area beneath the marine asset is therefore affected by the presence of these mooring lines. 
       FIG. 1  illustrates a marine asset, in this case an oil rig  1 , moored on the seabed  3  by way of several mooring lines  5  and corresponding anchors  7  (not all visible), and a pipeline  9  (indicated schematically) presenting a hazard in the vicinity. As a general rule, a mooring line  5  is several times longer than the vertical distance from the asset to the anchor, and as a result the mooring lines form a curve between the asset and the anchor (or the seabed where the mooring line makes contact). The naturally occurring curve improves anchor performance, particularly in the case of large assets in deep water, by producing a lower angle of pull on the anchor. However, the risk of contact with a hazard (such as the notional pipeline  9  indicated) is clearly increased. 
     In order to monitor the tension in a mooring line, for example to prevent it from contacting a pipeline or other hazard located below the marine asset, it is normal practice to attach a tension meter to each associated winch or asset attachment.  FIG. 2  illustrates a typical running line tensiometer  11  comprising three sheaves  13   a ,  13   b ,  13   c  disposed around a mooring line  15 . Tension in the mooring line  15  is measured by way of a load pin  17  on the uppermost sheave  13   a  and associated instrumentation which converts a displacement of the load pin into a measure of tension. 
     At various times during their operational lifetime, tension meters have to be calibrated. Typically this is achieved by boarding the asset and independently checking the tension at each winch by temporarily attaching a calibrated tension meter, for example a tension link that provides a measure of tension by which the permanent tension meter can be calibrated. The calibration process is time consuming and during these periods it is also necessary for all normal operations (e.g. drilling or production) on the marine asset to be suspended. Additional manpower is also required, or may need to be diverted from other operations, to connect the calibrated tension meter to the winch line and winch time to complete the calibration procedure. 
     The effective cost of calibration is further increased since cessation of operations during calibration testing results in a major loss of revenue. For example, the daily cost of hiring or operating a drilling platform may be in the region of several hundred thousand US$ per day, so it is essential to minimise downtime. Furthermore, small production rigs may produce thousands of barrels of oil equivalent per day (boepd) of oil and gas, while some of the world&#39;s largest production rigs produce several tens of thousands boepd of oil and gas. If one takes into account that there are approximately five hundred oil rigs stationed in waters around the world and that calibration tests have to be conducted approximately every two years, then the potential loss of revenue when production, or drilling, is temporarily ceased is extremely significant. 
     It is recognised in the present invention that considerable advantage is to be gained in the provision of a method of calibrating a tension meter attached to a mooring line of a marine asset which does not require the suspension of the normal operations of the marine asset. 
     Proposals have been made to use inclinometers on mooring lines or chains as an alternative to a running line tensiometer, for example as described in U.S. Pat. No. 3,722,268, U.S. Pat. No. 3,810,081 or WO 2007/079556. However, these methodologies have inherent limitations. Firstly, they all require provision of equipment on the line between the winch and the anchor, and usually at a location close to the winch. The additional equipment increases the capital expense of the system, and may require diver deployment, maintenance or retrieval. In addition, this equipment interferes with the ability of the master of the vessel to pay-in the line if the line needs to be re-tensioned, for example due to anchor drag. The methods are not suitable for use in bad weather. Furthermore, although the equipment of U.S. Pat. No. 3,722,268, U.S. Pat. No. 3,810,081 or WO 2007/079556 may be suitable for detecting large changes to the tension (for example due to the break of a line or anchor drag) they are insufficiently accurate to allow calibration of a tension meter. This problem exacerbated in deepwater and/or for large marine assets, conditions in which the tension measurement is highly sensitive to errors in the measured chain angle. At typical tensions for marine assets used in the hydrocarbon exploration and production industry (for example in excess of 25 tonnes and typically 40 to 60 tonnes) the systems of U.S. Pat. No. 3,722,268, U.S. Pat. No. 3,810,081 or WO 2007/079556 are ineffective. 
     Accordingly, it is an object of at least one aspect of the present invention to provide a method of determining the tension in a mooring line of a marine asset which may be performed during normal operations. 
     It is a further object of at least one aspect of the invention to provide a corresponding method of calibrating a tension meter attached to a mooring line of a marine asset. 
     Another object of at least one aspect of the invention to provide a method of calibrating a tension meter attached to a mooring line of a marine asset which is applicable to marine assets used in the oil and gas hydrocarbon exploration and production industry and/or in deepwater conditions. 
     Further aims and objects of the invention will become apparent from reading the following description. 
     SUMMARY OF INVENTION 
     According to a first aspect of the present invention there is provided a method of determining the tension in a mooring line of a marine asset, the method comprising determining at least one characteristic of a catenary trajectory formed by the mooring line and determining the tension in the mooring line based on the at least one characteristic. 
     The above method provides a method of determining the tension in a mooring line that does not require fitting additional equipment to the mooring line that would necessitate cessation of, for example, drilling or production operations or energy capture. Note that the skilled person may also use the term “catenary curve” to describe a catenary trajectory and the terms curve and trajectory may be used interchangeably. 
     This ability to measure the tension and calibrate the mooring line during normal operations provides significant savings for the operator of the marine asset. For example, a tension can be determined and/or a tension meter on the asset can be calibrated during one or more operations selected from the group consisting of: drilling; wellbore construction and/or completion of the well; well clean-up; well intervention and/or workover; well and/or reservoir stimulation; injection; well testing; and/or hydrocarbon production. 
     Preferably, determining at least one characteristic of a catenary trajectory formed by the mooring line comprises determining at least one characteristic of the catenary trajectory to generate characteristic data. 
     Preferably, determining the tension in the mooring line comprises receiving the characteristic data in a computer processor, processing the data to determine the tension in the mooring line, and outputting tension data from the computer processor. 
     Optionally determining at least one characteristic of the catenary trajectory formed by the mooring line comprises determining a shape of the catenary trajectory. The shape of the catenary trajectory formed by the mooring line is characteristic of the mooring line having a unique tension applied thereto. Optionally, determining at least one characteristic of the catenary trajectory comprises measuring the depth of the mooring line at a known distance from the asset. 
     Optionally, the method comprises generating a set of catenary trajectories for a range of values of tension. Preferably, determining the tension in the mooring line comprises identifying a generated catenary trajectory that corresponds with the at least one characteristic of the catenary trajectory formed by the mooring line. 
     Optionally, the method is repeated and a mean value of the tension in the mooring line determined. 
     Alternatively, or additionally, determining at least one characteristic of the catenary trajectory comprises measuring a first depth of the mooring line at a first known distance from the asset, measuring a second depth of the mooring line at a second known distance from the asset, and identifying a generated catenary trajectory that corresponds with the first and second depths of the catenary trajectory formed by the mooring line. 
     Optionally, determining at least one characteristic of the catenary trajectory comprises measuring a third depth of the mooring line at a third known distance from the asset. 
     Further alternatively, or additionally, determining at least one characteristic of the catenary trajectory comprises determining an elastic response of the mooring line. An elastic response of the mooring line corresponds directly to a particular tension being applied thereto. 
     Optionally, determining an elastic response of the mooring line comprises generating a set of elastic response profiles for a range of tension values, measuring an elastic response profile of the mooring line, and identifying a generated elastic response profile that corresponds with the measured elastic response profile. 
     Preferably, measuring an elastic response profile comprises determining a displacement between a connection point on the asset at one end of the mooring line and an anchor point at the opposite end of the mooring line, and determining the change in tension on the mooring line effected by the displacement. 
     Most preferably, determining a displacement comprises determining a horizontal displacement. Alternatively, or additionally, determining a displacement comprises determining a vertical displacement. 
     Optionally, the method is repeated periodically. Alternatively, the method is repeated continuously. 
     An alternative method of determining the catenary shape to derive tension comprises: capturing an image of a portion of a mooring line between a connection point on the marine asset and a surface of the water; 
     analysing the image in a computer processor to determine the angle subtended by the mooring line at the connection point;
 
determining the tension in the mooring line from the determined angle.
 
     The method may comprise determining the vertical height of the connection point of the mooring line; 
     determining the length of the mooring line between the connection point and the water surface; and
 
calculating the angle subtended by the mooring line at the connection point from the height and the length.
 
     The method may comprise determining the length of the mooring line between the connection point and the water surface by counting the number of chain links between the connection point and the water surface. Preferably, analysing the image in the computer processor comprises counting the number of chain links between the connection point and the water surface. 
     Alternatively, or in addition, the method may comprise analysing the foreshortening of the chain links in the computer processor to determine the angle subtended by the mooring line at the connection point. 
     According to a second aspect of the present invention there is provided a method of calibrating a tension meter arranged to measure the tension in a mooring line of a marine asset, the method comprising: 
     determining at least one characteristic of the catenary trajectory formed by the mooring line;
 
determining the tension in the mooring line based on the at least one characteristic; and
 
comparing the determined tension with a corresponding tension measurement from the tension meter.
 
     The above method provides an unobtrusive, contactless method for calibrating a tension meter. Importantly the method avoids the need for operation of the marine asset to be interrupted; e.g. there is no need to stop the drilling process on an oil rig or to halt energy capture on an offshore wind turbine so as to calibrate the mooring line tension meters. 
     The method may be carried out during normal operations of the marine asset. The method may be carried out during one or more operations selected from the group consisting of: drilling; wellbore construction and/or completion of a well; well clean-up; well intervention and/or workover; well stimulation and/or reservoir stimulation; wellbore and/or reservoir injection; well testing; and hydrocarbon production. 
     Optionally determining the characteristic of the catenary trajectory formed by the mooring line comprises determining a shape of the catenary trajectory. The shape of the catenary trajectory formed by the mooring line is characteristic of the mooring line having a unique tension applied thereto. 
     Determining the shape of the catenary trajectory may comprise: 
     calculating a set of catenary trajectories between a connection point of the asset and an anchor point of the mooring line for a particular seabed profile;
 
measuring a depth of the mooring line at a known distance from the connection point;
 
identifying a catenary trajectory corresponding to the known distance and measured depth.
 
     The connection point may, for example, be a fairlead point or the position of a winch to which the mooring line is attached. 
     Alternatively determining the shape of the catenary trajectory comprises: 
     measuring a first depth of the mooring line at a first known distance from a connection point;
 
measuring a second depth of the mooring line at a second known distance from the connection point; and
 
identifying a catenary trajectory corresponding to the first and second known distances and measured depths.
 
     Optionally determining the shape of the catenary trajectory further comprises measuring a third depth of the mooring line at a third known distance from the connection point. 
     Alternatively determining the characteristic of the catenary trajectory formed by the mooring line comprises determining an elastic response of the mooring line. The elastic response of the mooring line corresponds to the mooring line having a unique tension applied thereto. 
     Determining the elastic response of the mooring line may comprise: 
     calculating a set of elastic response profiles for the mooring line and a particular seabed profile for a range of mooring line tensions; and
 
measuring the elastic response profile induced upon the mooring line; and
 
comparing the measured elastic response profiles with the set of calculated elastic response profiles.
 
     Measuring the elastic response profile induced upon the mooring line may comprise: 
     measuring the relative displacement of a connection point of the mooring line from an anchor point;
 
measuring the changing tension induced by the relative movement.
 
     Preferably measuring the changing tension comprises observing the output of the tension meter. 
     Preferably the method of calibrating the tension meter further comprises: 
     changing the tension in the mooring line;
 
determining the characteristic of the catenary trajectory formed by the mooring line so as to identify the tension in the mooring line after the change; and
 
comparing the calculated tension with that observed from the tension meter.
 
     The method of calibrating the tension meter preferably comprises determining a scale factor correction for the tension meter. 
     The method of calibrating the tension meter preferably comprises determining a bias error for the tension meter. 
     Alternatively, determining the tension of the mooring line comprises comparing the at least one characteristic of the catenary trajectory with an appropriate look-up table. 
     Optionally, the method is repeated periodically. Alternatively, the method is repeated continuously. 
     The method may comprise determining the length of the mooring line between the connection point and the water surface by counting the number of chain links between the connection point and the water surface. Preferably, analysing the image in the computer processor comprises counting the number of chain links between the connection point and the water surface. 
     Alternatively, or in addition, the method may comprise analysing the foreshortening of the chain links in the computer processor to determine the angle subtended by the mooring line at the connection point. 
     Embodiments of the second aspect of the invention may comprise features to implement the preferred or optional features of the first aspect of the invention or vice versa. According to a third aspect of the present invention there is provide a method for determining the tension in a mooring line of a marine asset the method comprising: 
     generating a set of catenary trajectories between a connection point on the asset and an anchor point of the mooring line for a particular seabed profile and a plurality of tension values;
 
measuring a depth of the mooring line at a known distance from the connection point;
 
identifying a generated catenary trajectory corresponding to the known distance and measured depth.
 
     Embodiments of the third aspect of the invention may comprise features to implement the preferred or optional features of the first or second aspects of the invention or vice versa. 
     According to a fourth aspect of the present invention there is provided a method for measuring the tension in a mooring line of a marine asset the method comprising: 
     measuring a first depth of the mooring line at a first known distance from a connection point on the asset;
 
measuring a second depth of the mooring line at a second known distance from the connection point on the asset; and
 
identifying a catenary trajectory corresponding to the first and second known distances and measured depths.
 
     Optionally the method for measuring the tension in a mooring line further comprises measuring a third depth of the mooring line at a third known distance from the connection point. 
     Optionally, the method is repeated periodically. Alternatively, the method is repeated continuously. 
     Embodiments of the fourth aspect of the invention may comprise features to implement the preferred or optional features of the first, second or third aspects of the invention or vice versa. 
     According to a fifth aspect of the present invention there is provide a method for determining the tension in a mooring line of a marine asset the method comprising: 
     calculating a set of elastic response profiles for the mooring line and a particular seabed profile for a range of mooring line tensions; and
 
measuring the elastic response profile induced upon the mooring line; and
 
comparing the measured elastic response profiles with the set of calculated elastic response profiles.
 
     Optionally, the method is repeated periodically. Alternatively, the method is repeated continuously. 
     Embodiments of the fifth aspect of the invention may comprise features to implement the preferred or optional features of the first, second, third or fourth aspects of the invention or vice versa. 
     According to a sixth aspect of the present invention there is provided a method of determining the tension in a mooring line of a marine asset, the method comprising receiving at least one characteristic of a catenary trajectory formed by the mooring line and determining the tension in the mooring line based on the at least one received characteristic. 
     Embodiments of the sixth aspect of the invention may comprise features to implement the preferred or optional features of the first, second, third, fourth or fifth aspects of the invention or vice versa. 
     According to a seventh aspect of the present invention, there is provided a computer program for instructing a computer to perform the method of any of the first to fifth aspects of the invention when executed. 
     Optionally, the computer program comprises a generation module configured to generate a set of catenary trajectories, a measurement module configured to provide a measurement of at least one characteristic of the catenary trajectory formed by the mooring line, and a processing module adapted to determine which generated trajectory corresponds to the at least one characteristic and output a corresponding tension value. 
     Alternatively, the computer program comprises a measurement module configured to provide a measurement of at least two characteristics of the catenary trajectory formed by the mooring line, and a processing module adapted to identify a catenary trajectory that corresponds to the at least two characteristics and output a corresponding tension value. 
     Further alternatively, the computer program comprises a measurement module configured to provide a measurement of displacement of the asset and a measurement of a corresponding change in tension, a generation module configured to generate a set of elastic profiles, and a processing module configured to determine an elastic profile of the mooring line and to determine which generated elastic profile corresponds to the determined elastic profile and output a corresponding tension value. 
     Optionally, the computer software comprises a seabed profile module configured to provide information relating to a seabed profile. Optionally, the seabed profile module is comprised in the measurement module. 
     Preferably, the computer program further comprises a calibration module configured to calibrate a tension meter based on the tension value. 
     Embodiments of the seventh aspect of the invention may comprise features to implement the preferred or optional features of the first, second, third, fourth, fifth or sixth aspects of the invention or vice versa. 
     According to an eighth aspect of the present invention there is provided a method of determining the tension in a mooring line of a marine asset, the method comprising: 
     determining at least one characteristic of a catenary trajectory formed by the mooring line to generate catenary characteristic data;
 
receiving the catenary characteristic data in one or more computer processors; and
 
processing the catenary characteristic data using the one or more computer processors to determine the tension in the mooring line.
 
     The method may comprise outputting and/or displaying tension data from at least one of the one or more computer processors. 
     Optionally, processing the catenary characteristic data comprises generating a set of catenary trajectories for a range of values of tension and identifying a generated catenary trajectory that corresponds with the catenary characteristic data using at least one of the one or more computer processors. 
     Alternatively, processing the catenary characteristic data comprises generating a set of elastic response profiles for a range of values of tension and identifying a generated elastic response profile that corresponds with the catenary characteristic data using at least one of the one or more computer processors. 
     Optionally, the method is repeated periodically. Alternatively, the method is repeated continuously. 
     Embodiments of the eighth aspect of the invention may comprise features to implement the preferred or optional features of the first, second, third, fourth, fifth, sixth or seventh aspects of the invention or vice versa. 
     According to a ninth aspect of the present invention there is provided a method of calibrating a tension meter arranged to measure the tension in a mooring line of a marine asset, the method comprising: 
     determining at least one characteristic of a catenary trajectory formed by the mooring line to generate catenary characteristic data;
 
receiving the catenary characteristic data in one or more computer processors; processing the catenary characteristic data using the one or more computer processors to determine the tension in the mooring line; and
 
comparing the determined tension with corresponding tension measurement data from the tension meter to generate calibration data.
 
     The method may comprise outputting and/or displaying the calibration data from at least one of the one or more computer processors. 
     The method may be carried out during normal operations of the marine asset. The method may be carried out during one or more operations selected from the group consisting of: drilling; wellbore construction and/or completion of a well; well clean-up; well intervention and/or workover; well stimulation and/or reservoir stimulation; wellbore and/or reservoir injection; well testing; and hydrocarbon production. 
     Embodiments of the ninth aspect of the invention may comprise features to implement the preferred or optional features of the first, second, third, fourth, fifth, sixth, seventh or eighth aspects of the invention or vice versa. 
     According to a tenth aspect of the present invention there is provided a method for determining the position of a mooring line of a marine asset the method comprising: 
     calculating a set of elastic response profiles for the mooring line and a particular seabed profile for a range of mooring line tensions; and
 
measuring the elastic response profile induced upon the mooring line; and
 
comparing the measured elastic response profiles with the set of calculated elastic response profiles.
 
     The method may be carried out during normal operations of the marine asset. The method may be carried out during one or more operations selected from the group consisting of: drilling; wellbore construction and/or completion of a well; well clean-up; well intervention and/or workover; well stimulation and/or reservoir stimulation; wellbore and/or reservoir injection; well testing; and hydrocarbon production. 
     Embodiments of the tenth aspect of the invention may comprise features to implement the preferred or optional features of the first, second, third, fourth, fifth, sixth, seventh, eighth or ninth aspects of the invention or vice versa. 
     According to an eleventh aspect of the present invention there is provided a method of determining the tension in a mooring line of a marine asset, the method comprising: 
     determining an elastic response of the mooring line; and
 
determining the tension in the mooring line from the elastic response of the mooring line.
 
     The method may be carried out during normal operations of the marine asset. The method may be carried out during one or more operations selected from the group consisting of: drilling; wellbore construction and/or completion of a well; well clean-up; well intervention and/or workover; well stimulation and/or reservoir stimulation; wellbore and/or reservoir injection; well testing; and hydrocarbon production. 
     Embodiments of the eleventh aspect of the invention may comprise features to implement the preferred or optional features of the first, second, third, fourth, fifth, sixth, seventh, eighth, ninth or tenth aspects of the invention or vice versa. 
     According to a twelfth aspect of the present invention there is provided a method computer program for instructing a computer to perform the method of the eleventh aspect of the invention when executed. 
     The computer program may comprise a measurement module configured to provide a measurement of displacement of the asset and a measurement of a corresponding change in tension, and/or may comprise a generation module configured to generate a set of elastic response profiles. The computer program may comprise a processing module configured to determine an elastic response profile of the mooring line and to determine which generated elastic response profile corresponds to the determined elastic response profile and output a corresponding tension value. 
     The computer program may comprise a seabed profile module configured to provide information relating to a seabed profile. The seabed profile module may be comprised in the measurement module. 
     The computer program may comprise a calibration module configured to calibrate a tension meter based on the tension value. 
     Embodiments of the twelfth aspect of the invention may comprise features to implement the preferred or optional features of any of the first to eleventh aspects of the invention or vice versa. 
     According to a thirteenth aspect of the present invention there is provided a method of determining the tension in a mooring line of a marine asset, the method comprising: 
     determining at least one characteristic of a catenary trajectory formed by the mooring line to generate catenary characteristic data;
 
receiving the catenary characteristic data in one or more computer processors; and
 
processing the catenary characteristic data using the one or more computer processors to determine the tension in the mooring line
 
wherein processing the catenary characteristic data comprises generating a set of elastic response profiles for a range of values of tension and identifying a generated elastic response profile that corresponds with the catenary characteristic data using at least one of the one or more computer processors.
 
     The method may comprise outputting and/or displaying tension data from at least one of the one or more computer processors. 
     According to a thirteenth aspect of the present invention there is provided a method of calibrating a tension meter arranged to measure the tension in a mooring line of a marine asset, the method comprising: 
     determining at least one characteristic of a catenary trajectory formed by the mooring line to generate catenary characteristic data;
 
receiving the catenary characteristic data in one or more computer processors; processing the catenary characteristic data using the one or more computer processors to determine the tension in the mooring line; and
 
comparing the determined tension with corresponding tension measurement data from the tension meter to generate calibration data,
 
wherein processing the catenary characteristic data comprises generating a set of elastic response profiles for a range of values of tension and identifying a generated elastic response profile that corresponds with the catenary characteristic data using at least one of the one or more computer processors.
 
     The method may comprise outputting and/or displaying the calibration data from at least one of the one or more computer processors. 
     According to a fourteenth aspect of the present invention there is provided a method of determining the position of one or more mooring lines of a marine asset, the method comprising carrying out the method of any of any previous aspect of the invention, and deriving the position of the one or more mooring lines from a determined tension. 
     The method may comprise generating a display of the position of the one or more mooring lines. The method may be carried out during normal operations of the marine asset. The method may be carried out during one or more operations selected from the group consisting of: drilling; wellbore construction and/or completion of a well; well clean-up; well intervention and/or workover; well stimulation and/or reservoir stimulation; wellbore and/or reservoir injection; well testing; and hydrocarbon production. 
     According to a fifteenth aspect of the present invention there is provided a method of monitoring the position of one or more mooring lines of a marine asset, the method comprising: 
     calibrating a tension meter of the marine asset according to the methods of any previous aspect of the invention at a first time;
 
determining the position of the one or more mooring lines at the first time;
 
recalibrating the a tension meter of the marine asset according to the methods any previous aspect of the invention at a second later time; and
 
determining the position of the one or more mooring lines at the second later time.
 
     According to a sixteenth aspect of the present invention there is provided a method of producing a real-time representation of the position of one or more mooring lines of a marine asset, the method comprising repeating the method of the fifteenth aspect at a plurality of time intervals. 
     The method may be carried out during normal operations of the marine asset. The method may be carried out during one or more operations selected from the group consisting of: drilling; wellbore construction and/or completion of a well; well clean-up; well intervention and/or workover; well stimulation and/or reservoir stimulation; wellbore and/or reservoir injection; well testing; and hydrocarbon production. 
     Embodiments of the thirteenth to sixteenth aspects of the invention may comprise features to implement the preferred or optional features of any of the previous aspects of the invention or vice versa. 
     The methods of the various aspects of the invention and/or the critical steps thereof are preferably implemented in software, although it will be understood that the methods or steps thereof may also be implemented in firmware or hardware or in combinations of software, firmware or hardware. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Aspects and advantages of the present invention will become apparent upon reading the following detailed description and upon reference to the following drawings (like reference numerals referring to like features) in which: 
         FIG. 1  presents a schematic representation of a marine asset moored on the seabed by a number of mooring lines and anchors; 
         FIG. 2  presents a schematic representation of a running line tensiometer typical of the state of the art; 
         FIG. 3  presents a schematic representation of a method of measuring the tension in a mooring line in accordance with an embodiment of the present invention; 
         FIG. 4  presents a graph showing three different catenary trajectories and the forces acting thereon; 
         FIG. 5  provides a flow diagram illustrative of the method of determining the tension within the mooring line in accordance with the embodiment of the present invention presented in  FIG. 3 ; 
         FIG. 6  illustrates in schematic form a simplified process diagram representative of computer software configured to perform a method of determining the tension within the mooring line in accordance with the embodiment of the present invention presented in  FIG. 3  and  FIG. 5 ; 
         FIG. 7  presents a schematic representation of a method of measuring the tension in a mooring line in accordance with an alternative embodiment of the present invention; 
         FIG. 8  provides a flow diagram illustrative of the method of determining the tension within the mooring line in accordance with the alternative embodiment of the present invention presented in  FIG. 7 ; 
         FIG. 9  illustrates in schematic form a simplified process diagram representative of computer software configured to perform a method of determining the tension within the mooring line in accordance with the embodiment of the present invention presented in  FIG. 7  and  FIG. 8 ; 
         FIG. 10  provides a flow diagram illustrative of a method of calibrating one or more tension meters in accordance with an embodiment of the present invention; 
         FIG. 11  presents a schematic representation of a method of measuring the tension in a mooring line in accordance with a further alternative embodiment of the present invention; and 
         FIG. 12  provides a flow diagram illustrative of the method of determining the tension within the mooring line in accordance with the alternative embodiment of the present invention presented in  FIG. 11 . 
     
    
    
     DETAILED DESCRIPTION 
     There now follows a description of a first method of determining the tension in a mooring line in accordance with at least one aspect of the present invention.  FIG. 3  illustrates an exemplary implementation of said method.  FIG. 3  illustrates a marine asset  101 , in this example an oil rig, moored to the seabed  103  via connection point  102  (for example, a fairlead point) by a number of mooring lines  105 . 
     A depth measurement apparatus  121  is suspended from a ship  123  and provides a preferably non-contact means for determining the depth of a particular mooring line  105  at a predetermined distance x from the marine asset  101 . The depth measurement apparatus  121  may comprise a magnetometer, an echo sounder, a camera located on a wireline, or any other suitable means for determining the depth of the mooring line. 
     Mooring lines, which are usually chains or cables, take the form of a catenary trajectory between the asset and the point at which they touch the seabed (or connect to the anchor) and this trajectory and its grounding point are dependent upon the tension in the chain or cable. The shape of the catenary trajectory is dependent on a number of conditions such as tension, water depth, mooring weight, design, tidal flow, etc. As noted above, the skilled person may also use the term “catenary curve” to describe a catenary trajectory and the terms may be used interchangeably. 
       FIG. 4  presents a graph of a same mooring line exhibiting three different catenary trajectories  205   a ,  205   b  and  205   c  corresponding to the application of three different values of tension T. The touchdown points (i.e. the points at which the mooring line touches the seabed) for each of the catenary trajectories  205   a ,  205   b  and  205   c  is depicted by the reference numerals  206   a ,  206   b  and  206   c , respectively. It is noted that as the tension in the mooring line increases the distance between the touchdown point and the anchor position  207  decreases. 
     With continued reference to  FIG. 4 , for any given mooring line within a given water depth, the horizontal restraining force F is given by the following expression: 
         F=T−Hμ   (1)
 
     where
 
T—is the tension in the mooring line
 
H—is the connection point height above the seabed (see  FIG. 3 )
 
μ—is the weight per unit length of the mooring line
 
     The horizontal restraining force, F, is constant at any point along the length the mooring line so the tension T varies along the length s of the mooring line from its initial value T (at connection point  202 ) in accordance with the following equation: 
         T   s   2   =F   2   +V   s   2   (2)
 
     where V s —is the vertical load at a distance s along the mooring line.
 
At a distance s along the mooring line, the vertical load changes in accordance with the following expression
 
         V   s   =μsg   (3)
 
     where g—represents the gravitational field (i.e. 9.80665 N/kg). 
     Equations (2) and (3) thus allow the tension T s  and vertical load V s  to be calculated at any point s along the mooring line. 
     In addition to the above, a horizontal displacement of Δx can be calculated from the connection point  202  resulting from a change of ΔT=T 0 −T 1  in the mooring line  205  tension in accordance with the following expression: 
       Δ x=F  log (( T   0   +V   0 )/( T   1   +V   1 ))/μ  (4)
 
     where
 
T 0 —is the initial tension in the mooring line
 
V 0 —is the initial vertical load in the mooring line
 
T 1 —is the tension at a distance Δx
 
V 1 —is the vertical load at a distance Δx
 
     A corresponding change in the vertical displacement ΔH of the connection point  202  can also be calculated from the following equation: 
       Δ H =( T   0   −T   1 )/μ  (5)
 
     Equations (1) to (5) uniquely define the catenary trajectory formed by a mooring line  205  for a given tension. These equations can also be extended to include combinations of mooring lines  205  of different weights. Significantly however they always produce a unique catenary trajectory for a given tension T. 
     Referring back to  FIG. 3 , using the theory outlined above a unique set of catenary trajectories for the mooring line  105  can be calculated, each catenary trajectory corresponding to a unique tension T applied to the mooring line  105 . Thus a set of unique catenary trajectories for a particular mooring line  105  and seabed profile between the connection point  102  and the anchor point  107  can be accurately modelled. Accordingly, a single measurement of the depth of the mooring line  105  at distance x from the asset  101  is sufficient to identify the appropriate catenary trajectory and thereby determine the tension T within the mooring line  105 . 
     Note that information regarding the seabed may be obtained from charts or echo sounding techniques (for example, using on-board seabed profiling apparatus indicated schematically in  FIG. 3  by reference numeral  125 ). 
       FIG. 5  provides a flow diagram illustrative of the method of determining the tension within the mooring line in accordance with an embodiment of the present invention. The method can be seen to comprise a step of measuring a characteristic (or several characteristics) of the catenary trajectory  151 , for example by measuring the depth of the mooring line at a known distance from the asset. 
     The method can also be seen to comprise a step of generating a set of catenary trajectories  153 . As one can know the connection point height H and the weight per unit length of the mooring line (as well as information relating to the seabed), a plurality of trajectories can be generated for a range of tension values. 
     Based on the measured characteristic (or several measured characteristics) the particular generated catenary trajectory that corresponds to that measured characteristic, and hence the actual catenary trajectory, is identified from among all of the generated trajectories  155 . From this identified generated trajectory the corresponding tension value is extracted  157 . 
     It will be understood that the order in which the steps are performed may be changed dependent on the circumstances; for example, rather than generating a large set of catenary trajectories corresponding to a wide range of tension values, it may be desirable to generate a more limited set of catenary trajectories once a measurement (e.g. of depth) has already been made to reduce the computational burden of identifying the corresponding catenary trajectory. The process can also include the optional step of obtaining information relating to the profile of the seabed  159  for use in the generation of the set of catenary trajectories. 
     Another optional step is to repeat the process  161 , thus determining several tension values from which a mean tension value can be determined. 
     It should also be noted that the action of generating a set of catenary trajectories  153  may be carried out well in advance of performing a measurement on the mooring line; for example a comprehensive set of catenary trajectories for a range of parameters may be generated onshore to provide a look-up table that may be utilised offshore. This would reduce computational burden during the actual tension measuring process. 
       FIG. 6  illustrates, in schematic form, a simplified process diagram showing three modules that are comprised in a software implementation for determining the tension within a mooring line in accordance with an embodiment of the present invention. A generation module  253  is provided which is configured to generate a set of catenary trajectories for a range of tension values. A measurement module  251  is also provided which either measures or receives at least one measurement of a characteristic of the actual catenary trajectory formed by the mooring line, e.g. the depth of the mooring line at a particular distance from the asset. Finally, there is provided a processing module  255  which receives the at least one measurement from the measurement module, identifies the corresponding generated catenary trajectory, and provides a tension value as an output  257 . An optional calibration module  267  is provided to allow for calibration of one or more tension meters. 
     An alternative embodiment of the above method will now be described with reference to  FIG. 7 .  FIG. 7  again presents a ship  323  from which the depth measurement apparatus  321  is deployed. In this example the profile of seabed is unknown. 
     In order to identify the tension T in the mooring line  305  in the presently described example, the depth of the mooring line  305  is measured at two different distances x 1  and x 2  from the marine asset  301 . The depth d 2  at distance x 2  can be thought of as a stationary point in space (guyed in place by the rest of the mooring line  305 ). Although there exists a number of catenary trajectories that could potentially pass from the connection point  302  (0, d o ) to (x 2 , d 2 ) only one of these can also pass through the first point (x 1 , d 1 ). By employing this double measurement technique the appropriate catenary trajectory can be determined and hence the tension T identified without any knowledge of the seabed or touchdown point. 
     The accuracy of this method of identifying the tension T of the mooring line  305  may be further increased by taking several measurements along its length; by which a best fit catenary trajectory through all points (x n , d n ) can be determined by methods known to the skilled person. 
     Note that if the position of a connection point (e.g.  102 , 302 ) on an asset (e.g.  101 ,  301 ) is known, only two measurements are required in order to make a determination of the catenary trajectory. Conversely, if the position of a connection point (e.g.  102 , 302 ) on the asset (e.g.  101 , 301 ) is not known, the catenary trajectory can be determined if at least three measurements of position and depth are obtained. This may be helpful if the position of the measurements can be determined more accurately than the position of the connection point. 
       FIG. 8  provides a flow diagram illustrative of the alternative method of determining the tension within the mooring line illustrated in  FIG. 7 . The method can be seen to comprise a step of measuring a first depth of the mooring line at a first known distance from the asset  351   a , a step of measuring a second depth of the mooring line at a second known depth  351   b , and a step of identifying a catenary trajectory corresponding to the measured depth values  355 . Optionally, the measurement steps can be repeated n times to produce n sets of distance and depth values  361 , to which a more accurate catenary trajectory can be fitted. From the catenary trajectory, as discussed above, the corresponding tension value can be determined  357 . 
     This method may be less computationally intensive as it does not require the generation of sets of catenary trajectories; rather it makes use of curve-fitting algorithms that are well understood and may be readily implemented. However, it is foreseen that sets of catenary trajectories (either generated or in a look-up table) could be implemented for matching purposes in an alternative embodiment. 
     As an optional final step, the error in the tension value can be determined  363 , via methods well known to the skilled person. 
       FIG. 9  illustrates, in schematic form, a simplified process diagram showing two modules that are comprised in a software implementation for determining the tension within a mooring line in accordance with this alternative embodiment of the present invention. A measurement module  451  is provided which either measures or receives at least two measurements of the depth of the mooring line at known distances from the asset. A processing module  455  receives the at least two measurements from the measurement module  451 , performs curve-fitting algorithms on the measurements (as discussed above) to determine a catenary trajectory (or identifies a corresponding generated catenary trajectory), and provides a tension value as an output  457 . An optional calibration module  461  is provided to allow for calibration of one or more tension meters. 
     As indicated, the above described methods of identifying the tension T of a mooring line may be employed as a means to calibrate one or more tension meters located upon a marine asset, as illustrated schematically in  FIG. 10 . Firstly, a tension T calc  of the mooring line is calculated (or identified) by employing one of the depth measurement techniques described above  557 . Subsequently, an observed tension T obs  is read from the tension meter  560 . Thereafter, it is possible to calculate a tension error T err    563 , where T err =T obs −T calc . 
     To account for a scale factor and a bias error of the tension meter it is preferable for the above method to further comprise repeating for a number of alternative working tensions  562  and applying a linear equation fit technique to a graph of T obs  versus T err    564 . 
     The gradient of this line provides a scale factor correction for the readings of the tension meter while the intercept with T err  axis provide the bias error for the tension meter. 
     It is preferable for the above calibration process to be performed at a time of relatively calm weather. 
     Just as there is a unique catenary trajectory for a mooring line under a given tension T, there is also a unique elastic response exhibited by the mooring line. This allows for a further alternative method of identifying the tension T of a mooring line which is now described in further detail (and with reference to  FIG. 11 ). 
     This alternative method uses knowledge of the seabed profile in order to determine by how much the tension (T) changes for a given movement from the anchor point (e.g.  107 ). Thus the first stage involves establishing a seabed profile between a connection point and the anchor point  659 . A theoretical elastic response E theory  (e.g. a change in theoretical tension—ΔT theory —of a mooring line with this seabed profile for given movements of the connection point relative to the anchor point) is then calculated for a range of tension values  653  (e.g. 10 tonnes to 100 tonnes). 
     As the marine asset moves under the effects of heave, pitch, roll and yaw the position of the connection point relative to the anchor point changes. High accuracy positioning equipment located on the marine asset provides a means for measuring the vertical ΔH and horizontal Δx displacement of the connection point  651   a  (which may of course be determined via knowledge of the displacement of the asset). Suitable positioning equipment includes real-time kinematic GPS equipment, which is capable of measuring the position in 3D with accuracy in the order of millimetres; ring laser gyro altitude sensors which is capable of measuring pitch, roll and yaw to an accuracy in the order of 20 millidegrees or greater; GPS gyros equipment which is capable of measuring absolute heading to an accuracy in the order of 20 millidegrees or greater; or combinations of the above. This allows the movement of the fairlead point to be measured with high precision. 
     A tension meter can then be employed to measure the observed tension T obs , and calculate the corresponding changes in the observed tension (ΔT obs ) of the mooring line for a positional change. From the change in the observed tension (ΔT obs ) and the positional change, the corresponding elastic response (E obs ) can be calculated  651   b . The elastic response E obs  can then be used to derive a tension value. In the embodiment described, this is done by comparison of E obs  with the theoretical elastic responses E theory  in a look up table to output a tension value with a corresponding elastic response  657 . 
     It will be appreciated that the above-described method of determining the tension T of the mooring line may be used to calibrate the tension meter. This calculated tension value can be compared with the observed tension T obs  (read from the tension meter). A discrepancy between the observed tension T obs  and the tension calculated from ΔT obs  and the elastic response data indicates an incorrect calibration of the tension meter, which can be corrected. In other words, the tension meter can be recalibrated using the elastic response data. 
     To account for the scale factor and bias errors within the tension meter it is preferable for the above method to further comprise the action of carrying out, for example, a least squares analysis (e.g. a Monte Carlo simulation) between a number of observed results E obs  and the theoretical results E theory    664 . Scale factors and bias terms are then incorporated within the analysis until a linear fit is achieved between the observed tension changes (ΔT obs ) and those predicted by the theoretical models (ΔT theory ). 
     It will be appreciated that the above-described method can be varied within the scope of the invention, by for example calculating a tension value from the observed elastic response from theoretical relationships, rather than relying on a look-up table. 
     The invention may be implemented in a long term or permanent installation on a marine asset, in which case it may provide a periodic or real-time tension monitoring or calibrations system which periodically or continuously determines the tension in the mooring lines of the marine asset with a high level of accuracy. To effect a periodic, or indeed real-time, calibration method, the various actions may be repeated as indicated schematically by arrow  662  on  FIG. 11 . In this way, not only can a tension meter be calibrated without the need for any down-time or interruption of normal operations, but said calibration can take place regularly, or indeed continuously. This allows the system to provide a three-dimensional catenary map which records and/or displays the catenary position of the moorings in real-time with high positional accuracy. The natural movement of the asset allows periodic or continuous monitoring of the elastic response and periodic or continuous recalibration of the tension meter. It will be appreciated that although such a method is described a calibration, calibration data need not be output from the system. Rather, the calibration of the tension meter following one iteration may be used to correct the observed tension T obs  used in a subsequent iteration. It will be appreciated that the data from multiple iterations may be stacked or otherwise combined to improve the statistical accuracy of the method, and in fact the method may make use of real time data over a significant period of time (up to 3 days for example). 
       FIG. 12  illustrates, in schematic form, a simplified process diagram showing a number of modules that are comprised in a software implementation 700 for determining the tension within a mooring line in accordance with this further alternative embodiment of the present invention. A measurement module  751  is provided which either measures or receives measurements of the displacement of the asset as it moves, as well as measuring or receiving resulting changes in tension. A seabed profile module  759  (which may be comprised in the measurement module) either measures or receives information relating to a profile of the seabed. A generation module  753  generates a set of elastic responses corresponding to a range of tension values. 
     A processing module  755  receives the various measurements from the measurement module  751 , determines and compares the observed elastic response with the generated elastic responses, identifies the actual tension of the mooring line and provides the identified tension value as an output  757 . An optional calibration module  761  is provided to allow for calibration of the tension meter from which measurement of the tension changes are obtained, or to simply adjust the measured value of tension T obs  used in a subsequent iteration. The software implementation 700 may be configured in a long term or permanent installation on a marine asset, in which case it may provide a periodic or real-time tension monitoring or calibrations system. 
     Although several different methods for calibrating a tension meter have been described it will be appreciated that more than one of these techniques may be carried out simultaneously or sequentially. A higher degree of certainty and thus safety can therefore be established. In particular it may be desirable to verify the tension reading generated by one method by comparing with a measurement obtained by another method. In this regard, a further alternative embodiment and aspect of the invention is described with below. 
     The visual calibration method uses image processing to derive the tension in the mooring. For any catenary with a given tension applied, the vertical angle subtended by the chain at the fairlead will be unique. For the visual calibration, the angle subtended at the fairlead is measured. This can be done in several ways but the two preferred methods are described below. 
     In a first method, the chain is photographed from the side from a survey vessel. The photograph is processed using image processing software. The chain links exposed above the surface of the water are counted by the software and the vertical height of the lower edge of the fairlead above the water level is derived. The angle subtended by the fairlead is then the inverse cosine of the vertical height divided by the exposed chain length. Although the chain will have a slight curve, this will be negligible over the short length of chain exposed. 
     In an alternative method, a zoom lens is used to photograph the chain from above at a known point at deck level. High resolution expanded images can be analysed to determine the foreshortening of the known link dimensions by comparing with the observed link girth. The angle has to be corrected for the pitch and roll of the vessel but can be determined to a high degree of certainty. Several independent photographs can be similarly analysed to obtain statistical confidence in the result. At the same time as the photographs are taken, tension can be directly observed in the winch room or in the control room. The digital tension meter readings are displayed centrally as well as in the individual winch rooms. 
     The corrections between observed tensions in the winches and those derived by the visual methods described above can be reduced to derive scale factor terms and bias terms that can be subsequently used over the whole tension range. 
     Described above are a number of methods for identifying the tension within a mooring line based on determining a characteristic of the associated catenary trajectory. These methods are particularly useful since they provide a simple means for calibrating a tension meter, normally located on a marine asset, attached to the mooring line. This ensures that the tension in the associated mooring line is sufficient at all times to prevent it from touching any pipelines or other hazards located on the seabed. 
     A typical application will now be described in the context of the calibration of a tension meter on an offshore semi-submersible drilling rig. As an initial step, a suitably accurate positioning equipment is installed on the rig at a recognisable location. The preferred equipment includes a vector gyro with Differential Global Positioning capability and a pitch and roll sensor. An example of an easily identifiable origin is the centre of the aft end of the catwalk and horizontal offsets from this origin are then measured to the survey equipment and each of the Fairlead positions. 
     Similarly, in the vertical view the offsets will have a z component from the point of measurement and the sea level. A survey vessel tied alongside measures the water depth to allow for tidal variation. 
     In order to conduct a calibration, two processes are run in parallel with the rig operations. In this example the two methods are the visual calibration and the elastic calibration methods. 
     The visual calibration method, as described above, uses image processing to measure the angle subtended at the fairlead. At the same time as the photographs are taken, tension can be directly observed in the winch room or in the control room. The digital tension meter readings are displayed centrally as well as in the individual winch rooms. The corrections between observed tensions in the winches and those derived by the visual method are reduced to scale factor and bias terms that can be subsequently used over the whole tension range. 
     The second calibration method used in this example is the elastic response method. A typical process is as follows:
         a. For each heading pitch, roll and position the position of each fairlead is calculated;   b. The horizontal and vertical distances from fairlead to anchor are then determined. These need not be absolutely accurate but their relative movement from one observation to the next is important.   c. Changes in tension are compared with their corresponding changes in fairlead position relative to the anchor.   d. A mooring model is used to determine the expected tension increase for a given movement and the observed increase is compared with this.   e. If the observed increase is too small, the scale factor and bias in the tension meter are adjusted to the observed tensions until the elastic response is always consistent with the observed response.       

     By way of example, an increase in height above the anchor of 2.4 m and a shift in distance from the anchor of 1.8 m is expected to raise the tension by 1.4 tons if the original tension was 50 tons. The tension only increases by 1.1 tons so the actual tension in the line is somewhat less that the 50 observed. A software implementation of the above described method applies a variety of biases and scale factors to such observations until over the entire period with all variations in ballast height, tidal height, tidal current drag, thruster forces etc. over a period of a few days, the responses always match the predicted values. When calibrating an operational rig, the two methods will be used providing a check on the validity of the calibration result. 
     The principles of the invention may be implemented as part of a real-time positional monitoring system, which periodically or continuously determines the position of the mooring lines of a marine asset in three dimensional space with a high level of accuracy. This may be achieved because the generated catenary curve that is identified as matching the measured (or determined) characteristic is, by definition, the actual catenary curve formed by the mooring line. Accordingly, position may be determined without necessarily determining the tension. 
     The invention provides a method of calibrating a tension meter arranged to measure the tension in a mooring line of a marine asset is described. The method comprises the step of determining at least one characteristic of the catenary trajectory formed by the mooring line, and determining the tension in the mooring line based on the at least one characteristic. The characteristic may be, for example, shape of the catenary trajectory or elastic response of the mooring line. The determined tension is compared with a corresponding tension measurement from the tension meter to allow the tension meter to be calibrated. The method may be carried out during normal operations of the marine asset, such as drilling or hydrocarbon production. Computer implemented methods are described, as are methods of deriving a position map of the mooring lines. The invention may be implemented in a real-time monitoring system which forms a long-term or permanent feature of the marine asset. 
     The described methods also offer other significant advantages over those techniques known in the art. Since they may be considered to be non contact techniques they do not require an operator to make any contact with the mooring line, or for the operations of the marine asset to be suspended to allow measurements to be taken. This ability to measure the tension and calibrate the mooring line during normal operations provides significant savings for the operator of the marine asset e.g. an oil rig operator. 
     The methods described are suitable for use in bad weather, and are sufficiently accurate to allow calibration of a tension meter, even in deepwater and/or for large marine assets used in the hydrocarbon exploration and production industry where typical tensions are in excess of 25 tonnes and typically 40 to 60 tonnes). 
     The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed. The described embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilise the invention in various embodiments and with various modifications as are suited to the particular use contemplated. Therefore, further modifications or improvements may be incorporated without departing from the scope of the invention as herein intended.