Method and system for mooring

Details Method and system for mooring Method and system for mooring

2002-02-22

2003-11-25

Basinger, Sherman (Department: 3617)

Ships

Anchoring arrangement

C114S230100, C114S230230

Reexamination Certificate

active

06651580

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The present invention relates to an improved method of mooring floating offshore structures such as semi-submersible drilling rigs. Specifically, the invention relates to a method and apparatus for deploying additional mooring lines to secure a rig in anticipation of adverse weather conditions.
BACKGROUND OF THE INVENTION
Floating offshore structures, for example, semi-submersible drilling platforms, drill ships and the like, are used by the oil and gas industry in increasing water depths. Unlike platforms that sit on the seabed, these structures must have a means of keeping the structure within reasonable proximity of the well site; such a means can include a mooring system and/or a dynamic positioning system. While applicable to all types of floating offshore structures, the mooring system will be explained in terms of semi-submersible drilling platforms.
Semi-submersible drilling platforms are specialized vessels used to prospect for and produce oil and natural gas from the sea floor, and are especially useful when the water depth precludes the use of fixed structures on the ocean bottom. These rigs are very large and very expensive to construct, commonly costing hundreds of millions of dollars each.
As seen in the example of
FIG. 1A
, semi-submersible drilling platforms comprise three main parts: buoyant members
108
, work section
109
, and stability columns
111
, which elevate work section
109
above the water level. Work section
109
contains or supports the equipment that is necessary for drilling and other related activities, as well as the living quarters for the crew. The platform is towed or proceeds under its own power between locations with buoyant members
108
floating on the surface, but once on location, the buoyant members are filled with seawater until the platform is partially submerged with the operating water level approximately mid way up the stability columns. In this example, a drilling derrick
102
and cranes
106
for handling large items are mounted on upper deck
104
of the work section
109
. For the sake of simplicity, it will be understood that this drawing is not drawn to scale, nor does it begin to show the variety of equipment carried on such a platform. Mooring lines carried on the semi-submersible drilling platform are shown deployed. These mooring lines typically consist of a length of chain, which is attached at one end to the anchor and at the opposite end to a steel cable (also referred to as wire rope). The cable is attached to a traction winch, which provides necessary tension, and is stored on a remotely located storage reel. The chain in the mooring line provides weight near the anchor, so that force applied to the anchor is always in a horizontal direction. Cable, on the other hand, provides a better strength to weight ratio, so that the strength of the line is not all used in simply supporting the length of line in the water.
FIG. 1B
is a close-up of the onboard hardware for the mooring system. Chain for the anchor is carried in chain lockers
116
, while cable is carried on a spool
122
. A combination traction winch/windlass
120
is positioned on upper deck
104
to enable handling of the cable and chain. Traction winch/windlass
120
has three major components, some of which are better seen in
FIG. 1C
, which shows a side of the semi-submersible adjacent the side seen in FIG.
1
A. The three components are traction winch
120
A, which handles and tensions the wire rope
118
, windlass
120
B, which handles and tensions the chain
114
, and shaft
120
C, which, combined with an electric motor (not shown), provides the drive power to run the other two components. Anchors
110
are normally carried by the semi-submersible drilling platform already attached to chain
114
and positioned on structures known as anchor bolsters
124
. From its attachment to the anchor
110
, chain
114
is led under fairlead
112
, then over the windlass
120
B to the chain locker
116
, with the chain and windlass holding the anchors in place. Each rig will be equipped with enough chain and cable to handle a given depth of water. In this example, the semi-submersible drilling platform carries enough chain and cable for water depths up to approximately 5,000 feet, although this value can vary according to design. Platform
175
is located on the stability column below the traction winch and provides a location and facilities for connecting the chain to the wire when the chain has been run out and the anchor line is transferred from the windlass to the traction winch. These systems will be explained in greater detail, shortly.
Depending on the water depth and the operator's choice, the platform can be moored either by using the system that is carried onboard, or by utilizing pre-set anchors, such as suction pile anchors or deep embedded anchors connected to mooring lines comprising some combination of chain, wire rope and synthetic rope and with the free end supported by a floating buoy. In water depths greater than the platform is equipped for (5,000 feet in our example), pre-set anchors must be utilized, but in shallower water, either system can be used. For example, the operator may want to begin drilling as soon after the arrival of the rig as possible. In this instance, they may choose preset anchors, as much of the time necessary to set the anchors can take place before the rig arrives on site.
FIG. 2
shows an exemplary deployment of the mooring system of a drill platform, as seen from a birds-eye view. Drill rig
102
and cranes
106
are seen, as are traction winch/windlasses
120
. Each comer has two combination traction winch/windlasses
120
run from a single shaft through clutches, with their mooring lines generally placed 45 degrees from each other and arranged symmetrically around the platform. However, it should be understood that this is only one possible arrangement of mooring lines, and even this example may be biased to resist environmental forces, such as wind and waves, which may be stronger from one direction than from another.
Setting the onboard anchor is demonstrated with reference to
FIGS. 3A and 3B
. A chaser line
310
, attached to anchor/chain
110
/
114
, is given to an anchor handling boat
300
. This chaser line
310
encircles the anchor and chain, and as boat
300
moves away from platform
100
, anchor
110
is freed from its bolster
124
. The boat steams away from the rig until sufficient chain
114
has been deployed. The anchor chain
114
A is then separated at platform
175
from the chain
114
B in chain locker
116
and attached to the connector
176
on the free end of the wire rope running over the traction winch. One method of handling this change from chain to cable is shown in U.S. Pat. No. 3,967,572 to Santa Fe International Corporation, titled “Anchoring System and Chain Stopper Therefor”, which is hereby incorporated by reference. After the anchor chain
114
A is attached to wire rope
118
at connector
176
, boat
300
continues to steam away from the rig until the traction winch pays out the required amount of wire. The boat then lowers anchor
110
on chaser wire
310
until it reaches the seabed; then the rig tensions up the mooring line with the traction winch
120
A to set the anchor
110
. In
FIG. 3B
, the boat returns the chaser and chaser line
310
to the rig.
FIG. 4
demonstrates the conventional way of connecting to a preset anchor. Preset anchor
420
has been previously placed in position, with a mooring line
425
of some combination of chain, wire rope, and synthetic rope. Buoy
410
, is attached to the mooring line to mark the location of the anchor and to support the free end of the mooring line. When platform
100
is to be attached to preset anchors, onboard anchor
110
must first be removed from the chain
114
and stored and the chain removed from the fairlead
112
. Then wire rope
118
is led from traction winch
120
A through fairlead
112
and given to an anchor handling boat
300
. The boat runs the end of the wire out to

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