BACKGROUND—FIELD OF THE INVENTION
This invention relates to a system and method for tensioning mooring lines, typically used to connect floating vessels and/or structures to an anchor on or in the sea floor.
As is known in the relevant art the various mooring lines in a mooring line installation must be brought to a desired deployed length and design pre-tension. Prior art systems and methods used tensioners mounted on the floating vessel. Such tensioners may be cost prohibitive and exhibit problematic maintenance routines and cost throughout the life of the asset.
Use of known vessel-mounted tensioners and related methods all present various issues, giving rise to a desire for an improved mooring line tensioning system and method that addresses these issues.
SUMMARY OF THE INVENTION
The off vessel tensioning system and method embodying the principles of the present invention comprises a method that utilizes existing mooring-related apparatus in a novel and non-obvious manner. The method(s) and related apparatus are described below.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1-10 illustrate the steps of connecting a pre-set mooring line to a floating vessel, and tensioning the mooring line to a desired deployed length and design pre-tension without the use of a vessel-mounted tensioning device. FIG. 10A shows an alternative placement of the underwater fairlead chain stopper (hereinafter usually referred to as UFCS 80).
FIGS. 11-20 illustrate the steps of replacing a mooring component segment in a mooring line assembly, similar to disconnecting a floating vessel or structure from its tensioned mooring line assembly, in substance nearly a reverse of the steps illustrated in FIGS. 1-10.
DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENT(S)
While various mooring line tensioning systems and methods can embody the principles of the present invention, with reference to the drawings some of the presently preferred embodiments can be described.
It is understood that the system and method herein described may be used in connection with any form of floating vessel or structure, including but not limited to offshore oil and gas drilling and/or production/processing facilities, floating wind turbines, or any other floating vessel or structure.
Installation and Tensioning of the Mooring Line(s)
Referring to FIG. 1: a pre-set mooring line assembly and anchor has been installed. The exemplary anchor and mooring line assembly comprises an anchor 10 (a suction pile anchor shown, but any type of anchor could be used), attached to a mooring line assembly 20 comprising a pile forerunner 22; a mooring line segment 28; and a top chain 30, also preferably comprising a tri-plate 40 and a shackle 42. As known in the art, anchor 10 and mooring line assembly 20 may be installed at a mooring location well before the floating vessel or structure 50 arrives on location, with mooring line assembly 20 run out along the ocean floor.
It is understood that the various segments of mooring line assembly 20 described above are by way of example only; the number of segments and the materials from which same are made may be varied to suit the particular installation.
An anchor handling tug supply (AHTS) vessel 60, referred to as AHTS 60, has picked up mooring line assembly 20 (typically by ROV assistance). In FIG. 1, AHTS 60 has moved to a position close to floating vessel or structure 50, for example 25 meters away or so.
FIG. 2: a line, namely an outer messenger line 70, has been transferred to AHTS 60 from floating vessel or structure 50.
FIG. 3A: additional detail of outer messenger line 70 deployed to AHTS 60 is shown. Note that outer messenger line 70 typically includes chain segment 72.
FIG. 3B: this is a top view of the setting shown in FIG. 3A. Outer messenger line 70 has been connected to top chain segment 30, thereby connecting floating vessel or structure 50 to the mooring line assembly 20. Abandonment and retrieval (“A&R”) rigging 74 connects a winch line on AHTS 60 to mooring line assembly 20, for example at the junction between chain segment 30 and outer messenger line 70.
FIG. 4: this illustrates the next step in the sequence of lowering mooring line assembly 20 from AHTS 60, via A&R rigging 74, and the weight of the mooring line has been transferred to the vessel and or structure.
FIG. 5: the inner messenger line 76 has been transferred to AHTS 60 by a crane or similar means. It is understood that inner messenger line 76 is a line typically separated from outer messenger line 70 by a chain segment 72, to allow the weight of the mooring line to be assumed by the vessel and or structure. FIG. 5 shows chain segment 72 passing through underwater fairlead chain stopper (hereinafter usually referred to as “UFCS”) 80 on floating vessel or structure 50. It is understood that UFCS 80 is positioned below the fairlead wheel, and may be locked (or alternatively has a ratchet type mechanism), to prevent movement of chain segment 72 outwardly therethrough, effectively locking chain segment 72 to floating vessel or structure 50.
It is understood that the system and method described herein, in a preferred embodiment, requires use of an underwater fairlead chain stopper or underwater stopper/hard point (padeye).
FIG. 6: AHTS 60 moves away from floating vessel or structure 50 a suitable distance, so as to place the stern of AHTS 60 substantially vertically over a connection point 100 in mooring line assembly 20, preferably over a connection point 100 in the top chain. A hold-back line 90 is lowered from AHTS 60, and connected to connection point 100, typically with ROV assistance. AHTS winch line 92, connected to inner messenger line 76, is paid out as required to position AHTS 60 as needed.
FIG. 7: hold-back line 90 is pulled in by a winch on AHTS 60. Inner messenger line 76 is also tensioned and/or pulled into AHTS 60 by a winch on AHTS 60, as needed. It is understood that hold back line 90 is tensioned minimally or not at all by bollard pull from AHTS 60, but predominantly by vessel buoyancy and a winch on AHTS 60.
FIG. 8: hold-back line 90 continues to be pulled in by a winch on AHTS 60. It is understood that generally some slack is thereby created in the section of the mooring line assembly between connection point 100 and floating vessel or structure 50. Outer messenger line 70 is then pulled into AHTS 60 by a winch, typically a sufficient amount that top chain 30 passes through UFCS 80. When the desired length of top chain 30 is pulled through UFCS 80 (which may be indicated by a marked link of top chain 30), UFCS 80 is then locked (it being understood that a ratcheting mechanism may serve to permit chain passage in one direction but lock in the other), so that top chain 30 is fixed to floating vessel or structure 50; effectively, a fixed length of mooring line assembly is thereby created between floating vessel or structure 50 and anchor 10, to yield the desired tension in mooring line assembly 20.
It is understood that the desired tension is predominantly placed in mooring line assembly 20 not by bollard pull by AHTS 60 on holdback line 90 (bollard pull force being necessarily limited in magnitude); but by vessel buoyancy and a winch on AHTS 60 pulling on hold-back line 90. In a preferred implementation of this system, bollard pull is limited to only the magnitude necessary to keep AHTS 60 in a desired position.
It is understood that mooring line tension calculations are performed to determine the appropriate tension to be placed on the mooring line assembly by hold-back line 90, and to determine the appropriate length of mooring line assembly 20 to be created from floating vessel or structure 50.
FIGS. 9A and 9B show additional detail of the step of FIG. 8.
FIG. 10: with top chain 30 locked to prevent outward chain movement through UFCS 80, AHTS 60 slacks off on hold-back line 90 to relieve tension therefrom; by ROV or similar means hold-back line 90 can then be disconnected from mooring line assembly 20, as shown in FIG. 10. Inner messenger line 76 is transferred back to floating vessel or structure 50 to serve as platform (floating vessel) hang off rigging. Alternatively, excess top chain is either cut/removed and/or prepared for hang-off on the vessel/structure, or deployed in the water to free hang.
FIG. 10A shows an alternative placement of UFCS 80. In this embodiment, UFCS 80 is not mounted on floating vessel or structure 50, but is positioned in-line in the mooring line assembly 20 and utilized in the same manner.
Replacement of Mooring Line Segment/Reversal of Tensioning Sequence
FIGS. 11-20 generally shows a reverse of the tensioning sequence described above. It is to be understood that the following exemplary description is directed toward de-tensioning to allow replacement of a mooring line segment, but that the same general steps would apply to simply decommissioning of a mooring line assembly from a floating vessel or structure.
FIG. 11: AHTS 60 moves into position proximal floating vessel or structure 50, so that the top chain hang-off rigging can be moved to AHTS 60 and attached to a disconnection messenger line 102. FIG. 11A shows detail of some of the rigging on the deck of AHTS 60. Preferably, disconnection messenger line 102 is either installed previously or installed by AHTS 60 to facilitate in disconnecting the mooring line.
FIG. 12: AHTS 60 moves to a position generally vertically above the holdback line 90 connection point 100, paying out disconnection messenger line 102 as needed. Holdback line 90 is connected to mooring line assembly 20 at connection point 100, typically by ROV assistance. UFCS 80 is opened or unlocked, permitting top chain 30 to pay out.
FIG. 13: AHTS 60 continues to pay out disconnection messenger line 102 until chain segment 72 is positioned in UFCS 80, where it is secured.
FIG. 14: AHTS 60 slackens disconnect messenger line 102 and ultimately disconnects same and passes it to floating vessel or structure 50 for hang off.
FIG. 15: AHTS 60 moves away from floating vessel or structure 50, positioning its stern generally vertically over a connection point 100 in mooring line assembly 20, and connects to same, usually with ROV assistance.
FIG. 16: A&R rigging 74 is winched in by AHTS 60, bringing top chain 30 and mooring component 28 on deck.
FIGS. 17-19: top chain 30 and old mooring component 28 are brought onto the deck of AHTS 60, and a new mooring component installed, if required.
FIG. 20 shows mooring line assembly being lowered by A&R rigging 74. It is understood that the tensioning method described above in connection with FIGS. 1-10 may be used to tension mooring line assembly.
If it is desired to decommission the mooring line assembly, it can be readily understood that a reversal of the tensioning method described above in connection with FIGS. 1-10, along with elements of the method described above in connection with FIGS. 11-20, may be employed.
CONCLUSION
While the preceding description contains many specificities, it is to be understood that same are presented only to describe some of the presently preferred embodiments of the invention, and not by way of limitation. Changes can be made to various aspects of the invention, without departing from the scope thereof.
Therefore, the scope of the invention is to be determined not by the illustrative examples set forth above, but by the appended claims and their legal equivalents.
Patent Application
20210078676
