Buoys – rafts – and aquatic devices – Buoy – For mooring a vessel
Reexamination Certificate
1998-09-01
2001-03-13
Sotelo, Jesus D. (Department: 3617)
Buoys, rafts, and aquatic devices
Buoy
For mooring a vessel
C114S230150, C114S293000, C405S169000
Reexamination Certificate
active
06200180
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a mooring system for tanker vessels that employs a vessel mounted turret in combination with a single anchor chain, three leg mooring arrangement to automatically accommodate movement of the vessel due to environmental forces, provide a removable connection to a subsea oil riser, and be easily adapted to varying water depths.
FPSOs are Floating Production, Storage and Offloading vessels which are employed to temporarily store oil received from offshore subsea and dry surface wells (or in the case of an FSO, pumped from offshore production facilities) until an oil tanker anives to receive the oil from the FPSO. Typically, an FPSO is itself an oil tanker that has been modified to receive the oil directly from one or more subsea risers (flexible pipelines), and be moored in a fixed position near the offshore oil wellheads or production facilities. The mooring systems employed to hold the FPSOs in place must be specially designed for a number of reasons. In particular, the mooring system must not interfere with one or more subsea oil risers that are employed to transfer oil from the offshore production facility to the FPSO. Additionally, the mooring system must be designed to withstand adverse environmental conditions, including rough seas and strong winds, since offshore production facilities are typically located in areas subject to such conditions.
The foregoing requirements have resulted in the design of a number of different mooring arrangements for FPSOs. The most popular design is known as a single point mooring system in which only the bow (or stern) of the FPSO is attached to the mooring, thus allowing the FPSO to pivot or “weathervane” about a single axis of rotation in response to wind or water currents. The most popular of the single point mooring systems is the turret mooring system. Previous turret mooring systems have consisted of a number of mooring lines that are attached to a rotatable turret. The risers enter the rotatable turret centrally inside a turret chain attachment-point radius. The turret mooring may be attached to the sea bed by 5 to 20 mooring lines in a radial or clustered pattern. A number of these turret mooring systems have been built so that they can be disconnected before severe storms approach. The turret mooring is then dropped or lowered into the sea from the FPSO while the turret turntable is still attached to the risers and the anchor chains. These systems are complex and expensive to build, and they are not built for multiple relocations at low cost. The weathervaning action also presents problems if the water and/or wind currents are such that the FPSO pivots more than 360 degrees about its starting position, thereby twisting the oil riser(s) and the mooring chains. Hence, the turret moorings are made such that they can rotate around a single point of rotation on bearings.
SUMMARY OF THE INVENTION
The present invention addresses the foregoing concerns through provision of a mooring system that employs a vessel mounted turret in combination with a single anchor chain, three leg mooring arrangement. The turret includes a mooring table which is mounted to the vessel by means of bearings that allow the table to rotate relative to the vessel. A chain stopper and a riser guide are provided on the mooring table for holding the anchor chain and the risers in a spaced relationship to one another. The single anchor chain simplifies the arrangement in this regard, and is connected a number of meters below the vessel to a group of three mooring leg chains, each of which is anchored to the seabed.
Mounted above the mooring table is a swivel stack comprising a plurality of swivel joints that are associated with water injection, well effluent and gas injection risers. A quick connect/disconnect connection is also provided to facilitate easy connection and disconnection of the swivel stack to and from the riser without having to disconnect the mooring table.
In a first embodiment of the invention, the swivel stack and a pulley sheave for an anchor chain pull-in line are both mounted on a movable frame to assist the mooring connection/disconnection process. More particularly, the frame is movable to a first position in which the pulley sheave is positioned directly over the chain stopper to allow the pull-in line and anchor chain to be retrieved from the water by a vessel mounted winch, and secured to the chain stopper. The frame is then moved to a second position in which the swivel stack is positioned in line with the mooring table's rotational axis, and an elbow pipe is in an engagement position with the quick connect/disconnect connection. In this manner, the movable frame facilitates quick and easy connection and disconnection of the anchor chain and risers to and from the vessel. In a second embodiment of the invention, a frame for the pulley sheave is mounted on a pivot which enables the sheave frame to rotate about a vertical axis between a first and second positions. In the first position, the pulley sheave is directly over the chain stopper as in the first embodiment to allow the pull-in line and anchor chain to be retrieved, while in the second position, the sheave frame is rotated clear of the mooring table so that the table and swivel stack can rotate freely without interference from the sheave frame.
REFERENCES:
patent: 5002433 (1991-03-01), Paasche et al.
patent: 5025742 (1991-06-01), Urdshals
patent: 5113778 (1992-05-01), Paasche et al.
patent: 5237948 (1993-08-01), Hvide
patent: 5515804 (1996-05-01), Pollack
patent: 5564957 (1996-10-01), brevik et al.
patent: 5944448 (1999-08-01), williams
patent: 0 399 719 (1993-02-01), None
patent: 2296904 (1996-07-01), None
Jones, Tullar & Cooper
Nortrans Offshore (S) Pte Ltd
Sotelo Jesus D.
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