Anchor monitoring system

Abstract

A system wherein a vessel ( 12 ) is moored by catenary mooring lines ( 14, 15, 16 ) that include polymer cable lengths ( 30 ) that can stretch under tension, which indicates the amount of polymer stretching. In the absence of polymer line stretching, a measured actual drift or excursion of the vessel from its initial position (as determined by a GPS system) would result in a predetermined nonstretch inclination (graph 50 ) of the top ( 42 ) of a first mooring line ( 14 ) from the horizontal. If the inclination is greater than the predicted nonstretch inclination, this indicates that the polymer line has stretched, with the amount of stretch indicated by the increase in inclination over the predicted nonstretch inclination.

1 . In a system for mooring a vessel that floats on a sea surface, that is connected by a riser to the sea floor, and that is moored by a plurality of mooring lines that extend from the vessel in primarily catenary curves and in different compass directions to the sea floor, wherein each mooring line includes a length of polymer cable that extends along a majority of the length of the mooring line, the improvement of a monitoring apparatus for monitoring the length of at least a first of said mooring lines, comprising:
 a positioning apparatus that indicates the actual position of a vessel location;   an inclinometer that measures the inclination of an upper end of said first mooring line;   means coupled to said positioning system and said inclinometer for indicating the stretch condition of said first mooring line.   
   
   
       2 . The improvement described in  claim 1  wherein:
 said means for indicating the stretch condition of said first mooring line, indicates the amount of drift of said actual position of said vessel location away from an initial position at which said vessel location lies in a quiescent sea with no stretch of any of said mooring lines, and determines a difference between the actual inclination of the first mooring line upper end and an expected inclination of the first mooring line upper end that would be expected at said actual position of said vessel location if there were no stretch of said first mooring line.   
   
   
       3 . The improvement described in  claim 2  wherein:
 said means for indicating indicates whether or not the actual inclination of the first mooring line upper end at said actual position of said vessel location is within or is outside a predetermined expected range of inclinations of the first mooring line upper end at said actual position, when said first line is not stretched.   
   
   
       4 . The improvement described in  claim 2  wherein:
 said means for indicating includes an electronic memory that stores a plurality of sets of data that each represents a graph of expected inclinations of the upper end of a mooring line in the absence of mooring line stretching, versus drift distance of said vessel location from said initial position, and also includes computing means for indicating an expected inclination at said actual position where said expected inclination lies between the inclination of two of said graphs at drift distances that are respectively greater and lesser than said actual position.   
   
   
       5 . The improvement described in  claim 1  wherein:
 said vessel has a turret with a chain table, said plurality of mooring lines extend downward from said chain table, and the horizontal vessel location lies on said turret.   
   
   
       6 . The improvement described in  claim 1  wherein:
 said means for indicating indicates when the length of said first mooring line has exceeded a predetermined limit; and including   means for raising an upper end of said first mooring line when the length of said first mooring line has exceeded said predetermined limit.   
   
   
       7 . A method for monitoring the condition of a system wherein a vessel floats on a sea surface and is connected by a riser to the sea floor and is moored by a plurality of mooring lines that extend from the vessel in primarily catenary curves and in different compass directions to the sea floor, wherein each mooring line includes a length of polymer cable that extends along a majority of the length of the mooring line, said method for monitoring the condition of the system comprising monitoring the length of at least a first of said mooring lines, comprising:
 monitoring changes in the horizontal position of a location on said vessel from an initial position;   monitoring changes in the inclination of an upper end of said first mooring line;   when the vessel has drifted from said initial position, comparing the actual change in inclination to the expected change in inclination that would be expected if the length of polymer cable of said first mooring line did not change.   
   
   
       8 . The method described in  claim 7 , wherein:
 said vessel has a chain table, and all of said mooring lines extend from said chain table, and said location on said vessel is vertically in line with said chain table.   
   
   
       9 . The method described in  claim 7  wherein:
 each of said mooring lines includes a set of mooring line elements that each extends within 10° of the direction of adjacent elements of the set, and wherein:   said step of monitoring changes in the inclination, includes monitoring changes in the inclination of the upper end of one of the mooring line elements of the set.   
   
   
       10 . The method described in  claim 7  wherein:
 said step of comparing the actual change in inclination to the expected change includes storing a plurality of sets of data in an electronic memory wherein each of said sets of data represents a graph of expected inclinations of the upper end of a mooring line versus drift distances in a predetermined direction of said vessel location from said initial position in the absence of mooring line stretching, and said step of comparing includes generating signals representing the expected inclinations where said expected inclinations lie between the expected inclinations represented by the data that represent two of said graphs.   
   
   
       11 . The method described in  claim 10  wherein:
 each of said sets of data that represents a graph represents expected inclinations at a predetermined vessel draft.