Wells – Submerged well – Riser
Reexamination Certificate
2001-01-26
2003-02-18
Pezzuto, Robert E. (Department: 3672)
Wells
Submerged well
Riser
C166S339000, C166S359000, C166S345000, C405S224400, C405S224200
Reexamination Certificate
active
06520262
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a riser connector for use in offshore well operations. In particular, the present invention relates to a riser connector for connecting a riser system to a subsea wellhead assembly. In addition, the present invention relates to a method of conducting offshore well operations using the riser connector.
BACKGROUND OF THE INVENTION
Offshore operations for the exploration and production of hydrocarbons involve the use of a wellhead assembly situated on the floor of the ocean and a platform from which the operations are conducted. In the case of hydrocarbon deposits situated beneath shallow water, it is common practice to provide stationary platforms secured to the ocean floor. However, in deeper water, the use of stationary platforms becomes impractical and floating platforms are required. In use, floating platforms are connected to the blowout preventer and wellhead assemblies situated on the ocean floor by means of risers.
Currently, offshore operations are being required to be conducted in ever deeper water, necessitating the need for longer riser assemblies to connect the floating platform or vessel to the wellhead assembly on the ocean floor. However, increasing the length of the drilling riser connecting a floating platform or vessel to the assemblies on the ocean floor gives rise to a number of major problems. First, the overall weight of the riser assembly increases as the length of the riser increases. In addition, the weight of the drilling fluids or “mud” contained in the riser also increases as the length of the riser increases. Further, in operation, tensioning force applied by the floating platform must be maintained on the riser in order to prevent the riser from buckling. Again, the task of keeping the necessary tension on the drilling riser assembly becomes increasingly problematic as the length, and hence the weight, of the riser increases. As a result of these problems, many existing floating platforms and vessels have a maximum working depth of water in which they can safely perform downhole operations. Typically, many of these platforms and vessels are limited to operating in water up to depths of about 5000 feet. The need to operate in deeper water, for example in depths up to 10,000 feet or greater, necessitates the construction of new, larger floating platforms and vessels capable of carrying out well operations accommodating the even greater weight of risers required to operate at such depths.
As a solution to the aforementioned problems, it has been proposed to employ risers of smaller diameter than conventionally employed. Typically, offshore operations have used risers having an outer diameter of 21 inches and a nominal inner diameter of 19 inches. The proposal has been made to convert operations to a small bore riser, typically having an outer diameter of 16 inches and a nominal inner diameter of 13.5 inches. It will be readily appreciated that the reduction in diameter of the riser from 21 inches to 16 inches will result in a significant reduction in the weight of the riser assembly. The volume of a riser increases with the square of any increase in the diameter of the riser. In addition, therefore, it will also be appreciated that the weight of fluid to be retained and supported by the riser will also be significantly reduced by converting operations to a smaller diameter riser. The use of the smaller bore risers reduce the overall weight of the riser assembly, leading to savings in the loading placed on the floating platforms and vessels, reducing storage requirements, and reducing the effort required to tension the riser when in use. In principle, therefore, the use of a small bore riser would increase the working depth of many existing vessels and platforms, allowing them to operate in significantly greater depths than have been possible employing the conventional 21 inch riser.
In practice, however, converting operations to using small bore risers produces a umber of practical problems, which need to be overcome before the use of small bore risers can become feasible. The blowout preventer (BOP) stack present on the top of most undersea wellhead assemblies has a nominal internal diameter of 18.75 inches. This has become a conventional size in the design of BOP stacks, able to accommodate all the tools and equipment necessary to be passed through the wellhead assembly during subsea drilling operations. Thus, conventional BOP stacks in combination with standard 21 inch diameter risers allow the use of conventional tools, such as drill bits, cementing tools and the like. In addition, this combination accommodates the installation of standard 16 inch casings and casing hangers used in the construction of the upper portion of a well. In contrast, the small bore risers mentioned before, having an internal diameter of just 13.5 inches, will not accommodate much of the aforementioned equipment. In particular, standard drilling and cementing tools will not pass through a small bore riser. Further, it is not possible to install a 16 inch casing and its associated hangers through a small bore riser.
One solution to this problem would be to reduce the size of the wellhead assembly, including the BOP stack, and the equipment used in downhole operations. However, in addition to requiring the replacement of much of the existing equipment on platforms and vessels currently in use, this reduction in size would ultimately limit the eventual completion size of the well and its eventual production capacity. Accordingly, there is a need for a system which solves the problem of the excessive weight of the risers arising out of deep water operation, but which still allows the use of conventional downhole equipment.
One solution to this problem that has been proposed involves the use of a small bore riser, which is disconnected from the wellhead assembly at an appropriate time in order to allow for the passage of equipment of standard size. Thus, U.S. Pat. No. 4,147,221 discloses a marine riser system for use in deep water drilling operations from a floating vessel, which allows the lower end of the riser to be detached from the wellhead. The lower end of the riser is set aside to a position clear of the wellhead. A support is provided to retain the riser in the set aside position. With the riser in the set aside position, casings and tools having diameters greater than that of the riser internal diameter can then be passed into the BOP and the wellhead assembly and inserted into the well. The system of U.S. Pat No. 4,147,221 allows the use of a small bore riser, while retaining the possibility of using larger diameter equipment without the need of returning the small bore riser to the surface. However, the system of U.S. Pat. No. 4,147,221 requires the presence of support mechanisms and guidance systems in order to move the riser from the wellhead to the set aside position. In addition, means and methods must be provided in order to adjust the rig-applied tension during the riser's movement between the wellhead and the set aside support position.
An alternative design of set aside system is disclosed in International patent publication number WO 00/34618. The system uses a reduced diameter drilling riser, in turn reducing the size and cost of the attendant floating platform or vessel. The system has means for disconnecting the riser from the BOP stack on the wellhead assembly and repositioning it in a set aside position. The system comprises a mud return assembly to which the riser is connected in the set aside position. Larger diameter tools and casings can be passed through the BOP and wellhead into the well and downhole operations conducted with drilling fluids being returned through the mud return assembly and the reduced diameter riser. The riser may be returned to the BOP and wellhead assembly and normal operations resumed, once the operations requiring the larger diameter equipment have been completed. While the system of WO 00/34618 allows the riser to be used as the return path for the dri
Barnett Richard C.
Kotrla Johnnie
Taylor William M.
Beach Thomas A.
Bielinski Peter A.
Cooper Cameron Corporation
Hartmann Michael P.
Pezzuto Robert E.
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