Wells – Submerged well – Submerged – buoyant wellhead or riser
Reexamination Certificate
1999-07-29
2001-06-12
Tsay, Frank (Department: 3672)
Wells
Submerged well
Submerged, buoyant wellhead or riser
C175S008000, C405S204000
Reexamination Certificate
active
06244347
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to apparatus for use in the drilling, completion and/or workover of a subsea well from a spar buoy at the sea level above the well. More particularly, it relates to apparatus of this type having an improved riser system which extends through a well in the spar buoy.
Spar buoys are caissons of such vertical length as to extend within relatively quiescent areas of the ocean. They have a well through which a riser extends from the platform at the upper end of the buoy for connection at their lower ends to the subsea well.
During a drilling process, a bit on the lower end of a string of drill pipe is lowered into and raised from the well through the riser, with drilling mud circulated downwardly through the bit being returned to the drill ship through the annulus between the drill string and the riser. During a completion or workover process, the drilling riser is replaced by a completion riser through which tubing and casing strings may be run for installation within the bore of the well.
DESCRIPTION OF RELATED ART
According to U.S. Pat. No. 4,702,321, it has been proposed to support the weight of the riser pipes by means of buoyancy units, usually air filled tanks which surround the riser to support it within the well of the spar buoy. In this way, the tanks as well as the upper portions of the risers enjoy the protection afforded within the well by the buoy passageway.
As shown in such patent, however, the tanks connect to the platform, so that, although the risers are maintained in tension, the tanks are in compression. This of course subjects the tanks to buckling, and for this reason, the riser system includes several intermediate decks within the well through which the tanks are guideably received to counter their tendency to buckle.
SUMMARY OF THE INVENTION
It is the object of this invention to provide apparatus of this type in which the riser system is of such construction as to minimize compression in the buoyancy tanks and thereby, among other things, alleviate the need for their lateral stability within the well of the spar buoy.
This and other objects are accomplished, in accordance with the illustrated embodiments of the invention, by subsea drilling, completion and/or workover apparatus which comprises a spar buoy having a well deck and; as in the Horton patent, a riser system which includes a riser which extends through the well of the buoy to the subsea well during drilling and/or completion of the well, and one or more buoyancy tanks formed between inner and outer walls disposed about the risers for supporting them. More particularly, the inner wall of the tank has a shoulder on which the riser is landed, and the riser is free to move vertically with respect to the well deck of the platform so that the tank is not placed in compression other than that due to its weight. As shown in the illustrated embodiments of the invention the shoulder may be adjacent the lower or upper end of the inner wall or positions between them. Also, a tubular extension on the upper end of the tank may surround a downward extension of the deck for guided vertical reciprocation with respect thereto.
For this purpose, and as shown in the preferred embodiments of the invention, the inner wall or the tank has an upwardly facing seat on which a downwardly facing shoulder about the riser is supported. More particularly, the riser shoulder is formed on an outward enlargement which is closely received within the inner wall to space the riser therefrom.
Supporting the stem of the riser on a shoulder near the upper end of the inner wall of the can will of course reduce the length of it which must be stabilized, and further facilitates access to its upper end for test purposes. On the other hand, if supported on a shoulder near the lower end of the inner wall of the tank, the riser stem is more accessible for repair or replacement. In either case, or even when the riser stem is more centrally supported, compression in the tank is minimized.
The spar buoy also has at least one deck across its well having an opening through which the tank is guideably received intermediate its upper and lower ends. As illustrated, the deck opening and tank preferably have vertically interfitting parts to prevent relative rotation between them.
As also illustrated, there may be a plurality of side by side risers within the well of the buoy, and a corresponding number of tanks, with each riser extending through and supported within each tank being freely moveable vertically with respect to the well deck so that, as previously described, compression in each is minimized.
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Browning & Bushman P.C.
Dril-Quip Inc.
Tsay Frank
LandOfFree
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