BACKGROUND OF THE INVENTION
A system for mooring a vessel in deep water (e.g. over 500 meters depth) can include mooring lines that extend in catenary curves and in different compass directions, from the vessel to the sea floor. The mooring lines limit vessel drift from an initial position to avoid damage to a riser that extends from the vessel down to the sea floor. The mooring lines may include long (over 250 meters length) polymer cable portions in addition to shorter steel chain portions, to limit the weight in water of the mooring line and the consequent line tension. For example, a cable of polyester has a density of about 1.3, which is close to that of water (density of 1.0), so the cable adds only moderate weight and consequent tension to the mooring line in water. However, one problem encountered in the use of polymer cables is that they commonly undergo permanent elongation (plastic deformation) when subjected to large tension, as during large storms.
It is important for a vessel operator to know whether and to what extent polymer mooring line portions have undergone permanent elongations. If the polymer portion of a mooring line has undergone significant elongation, then the vessel may drift away from the anchored end of that mooring line, further than predicted for given wind, wave and current conditions. The drift must be limited to prevent damage to the riser that extends down from the vessel to the sea floor.
SUMMARY OF THE INVENTION
In accordance with one embodiment of the invention, a method and system are provided for mooring a vessel using catenary mooring lines that include polymer cables, which indicates the amount of any stretching of a polymer cable. An inclinometer is attached to the upper end of the mooring line, and changes in the inclination from the horizontal are monitored. Drift of the vessel from its initial position is also monitored. For any measured actual vessel drift, a predetermined change in inclination is expected, if there is no cable stretching. If the inclination is greater than the amount expected for that drift, then this indicates that the polymer cable in the mooring line has been stretched. Such stretching can be compensated for by pulling up the top of the stretched mooring line by a limited amount.
If the inclination of a first line is greater than expected, which indicates stretching of its polymer cable portion, then the inclination of one or more mooring lines that extend opposite to the stretched line, should show a greater inclination from the horizontal than expected for the given vessel drift.
The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side elevation view of a vessel mooring system of the invention, with the vessel in its initial position.
FIG. 2 is a view similar to that of FIG. 1, but with the vessel having drifted from its initial position.
FIG. 3 is a plan view of the system of FIG. 1.
FIG. 4 is a graph showing inclination versus drift distance for one system that applicant has designed.
FIG. 5 is a partial sectional view of a chain table and of the upper end of a mooring line, showing an inclinometer attached to the mooring line and showing how the line length can be changed.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 illustrates a mooring system 10 that moors a vessel 12 (which is herein defined as any structure that floats at the sea surface) by the use of sets of mooring lines 14, 15, 16 (FIG. 3) that extend in catenary curves to anchors 17, 18, 19 at the sea floor. A riser 20 extends from a platform 22 on the sea floor 23 up to the vessel, and is useful to carry fluids such as hydrocarbons, or to carry tools, between them. The vessel has a turret 24 that allows the vessel to weathervane (the vessel can turn about the turret axis 26 without limit while the turret does not turn) and the mooring lines and riser connect to a chain table on the turret. The mooring lines are used to limit drift of the vessel from an initial position that the vessel assumes in a calm sea, as shown in FIG. 1, in order to avoid damage to the riser.
Each mooring line such as 14 includes a cable 30 of a polymer such as polyester, and lengths of steel chains 32, 34 at the upper and lower ends of the mooring line. While steel has a specific gravity of about 7.9 in water (which has a specific gravity of 1.0), polymers have much lower specific gravities. Polyesters have specific gravities of about 1.3 so their weight in water is small. For deep waters (at least 500 meters depth), it is preferred to use polymer cables that extend along a majority of the length of the mooring line. The lower steel chain 34 of a mooring line lays on the sea floor as the vessel drifts toward that mooring line, to slow vessel drift. One problem encountered in the use of polymer cables is that they permanently stretch (undergo permanent elongation) when elongated excessively. This may occur in large storms, and it results in the corresponding mooring line being longer than desired, so it allows excessive vessel drift away from that line. The present invention is directed to a method and system for indicating such stretching so the stretching can be compensated for as by shortening the mooring line that has undergone such stretching.
FIG. 2 illustrates the mooring system after the vessel (and specifically the location of the turret 24) has drifted by a distance B from its initial position. Such drift is easily measured by a GPS (global positioning system). One mooring line 14 has undergone an increase in tension while a partially opposite mooring line 15 has undergone a decrease in tension, all of which urges the vessel back toward its initial position. Applicant has also shown the mooring line at 14X which represents a line that previously had been elongated. As a result of the elongation, the angle of its upper end from the horizontal, has increased from the unstretched angle C to the stretched angle D.
The actual system of FIG. 1 is shown in FIG. 3, which shows that the system includes the three sets of mooring lines 14, 15, 16 that extend in compass directions angled 120° apart. Each mooring line set includes three mooring line elements such as 14A, 14B and 14C that are angled 5° apart. The three mooring line elements of a set include adjacent elements angled apart by no more than 10°, so they act like a single mooring element of greater strength and weight than any one of them. Accordingly, when discussing a mooring line such as 14, applicant refers to the three mooring line elements combined. A circle 36 indicates maximum allowed vessel drift that avoids damage to the riser.
FIG. 5 shows an inclinometer 40 attached to the upper end 42 of a mooring line 14A of a mooring line set 14 (or an inclinometer can be attached to each line). The inclinometer can be attached to a single one of the three mooring lines of the set. The inclinometer provides readings of the inclination angle A from the horizontal H of the upper end of the mooring line. It would be possible to use a load cell to indicate tension in a mooring line, instead of using the inclinometer, but a load cell is not practical to maintain. FIG. 4 includes a graph 50 that indicates the expected inclination of the inclinometer from the horizontal as a function of drift of the vessel in one direction (e.g. directly away from the anchor 17A of mooring line 14) from the initial position of FIG. 1, if the polymer cable of that mooring line has not undergone appreciable stretching. The graph 50 is valid for a predetermined vessel draft, which varies for a vessel that stores different amounts of hydrocarbons. The figure includes two other graphs 52, 54 that represent the acceptable tolerance around the nominal expected inclination represented by graph 50. If a point outside the limits of graphs 52, 54 is reached, this indicates that steps may have to be taken to shorten the stretched mooring line. The actual vessel location at the mooring line inclination therefore indicates the stretch condition, or amount of polymer line stretch from its initial length. The amount by which the actual mooring line upper end angle exceeds the acceptable limit, indicates the amount by which the mooring lines length should be changed (usually shortened).
FIG. 5 indicates that shortening of a mooring line can be accomplished by removing a holder 60 that holds the upper end of the mooring line element to the chain table 56 that lies at the bottom of the turret. Then, a winch 62 is turned to shorten the line, to compensate for the elongation. Graph 54 of FIG. 4 indicates a minimum expected inclination. If the inclination falls below the minimum inclination graph 54 then this indicates that the corresponding mooring line is too short, which can arise if the opposite mooring lines are too long. For any direction of drift of the vessel away from the initial position, corresponding graphs 50, 52 and 54 will appear on a screen to indicate the amount of inclination of a mooring line. Actually, corresponding graphs will appear for each of two mooring lines whose anchors the vessel is drifting away from and which therefore are under increasing tension.
As mentioned above, the present system includes a positioning apparatus 80 (FIG. 1) that determines the actual vessel position (in North-South and East-West directions, and at a measured vessel draft). The system determines the actual angle of a mooring line upper end from the horizontal direction (which is the equivalent to the angle to the vertical). The system then determines whether that line upper end angle is within or is outside a predicted range of angles for nonelongated lines. The amount by which the angle of a mooring line upper end lies beyond the predicted range, especially for a line extending away from the direction of vessel drift, indicates the degree of mooring line elongation.
The predicted range of angles for a nonelongated line can be determined in a plurality of different ways. One way is to create numerous graphs similar to those 50-54 of FIG. 4, for different directions of vessel drift relative to each mooring line, with different sets of graphs for different amounts of vessel draft. Such graphs and the output from the inclinometer and positioning apparatus, serve as means 56 for indicating the stretch condition of a mooring line. The graph can be stored in a computer (electronic memory) and downloaded as needed. Another way, which is more efficient, is to store data (in the electronic memory of a computer) that gives the predicted range of angles for one mooring line or each line, for each of numerous selected turret positions, and to use a computing means (in a computer) to calculate the predicted range for an actual turret position that lies between two of the selected turret positions. The calculation can be by linear averaging (e.g. a position one-third of the distance from a first selected position toward a second position has its predicted angle increased by one third of the difference of angles). The calculation can be made in more sophisticated ways. Applicant stores angle-excursion data (in addition to an angle-tension curve).
Thus, the invention provides a method and system for indicating when a mooring line that includes a polymer cable, has been stretched. The system uses a drift measuring apparatus such as a GPS (global positioning system) that indicates the amount and direction of drift, and an inclinometer that measures the inclination of the upper end of a mooring line(s) that the vessel has drifted away from. The difference between the actual mooring line inclination and the expected unstretched inclination (the expected inclination if the line has not been stretched since the last time any adjustment was made) is determined, to determine whether the actual inclination is outside the acceptable range. If the data indicate stretching, the effective mooring line length may be adjusted, as by shortening the line.
Although particular embodiments of the invention have been described and illustrated herein, it is recognized that modifications and variations may readily occur to those skilled in the art, and consequently, it is intended that the claims be interpreted to cover such modifications and equivalents.
Patent Application
20090084302
