Riser method and apparatus

Wells – Submerged well – With provision for disassembly

Reexamination Certificate

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Details

C166S367000, C405S224200

Reexamination Certificate

active

06367554

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable.
TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to marine riser systems and more particularly to a riser system having a small diameter riser that can be shifted to one side to allow access into and out of a well with large diameter casing, casing hangers and seal assemblies. Still more particularly, the present invention relates to a shiftable riser connection that continuously provides fluid communication between the well and the surface throughout the shifting operation and while accessing the large diameter borehole below the small diameter riser.
BACKGROUND OF THE INVENTION
Drilling operations for the recovery of offshore deposits of crude oil and natural gas are taking place in deeper and deeper waters. Drilling operations in deeper waters are typically carried out from floating vessels rather than from stationary platforms resting on the ocean floor and commonly used in shallow water. According to conventional procedures, a drilling vessel is dynamically stationed, or moored, above a well site on the ocean floor. After a wellhead has been established, a blowout preventer (BOP) stack is mounted on the well head to control the pressure at the surface.
Subsea wells are typically drilled with multiple boreholes having decreasing diameters as the wellbore extends deeper into the earth. Each borehole is lined with a casing string that extends into the borehole from a wellhead and is cemented within the borehole. The drilling, casing installation and cementing is performed through one or more risers that extend from the wellhead to the surface, such as to a floating drilling vessel.
A riser pipe extends from the floating vessel to the wellhead equipment on the ocean floor to conduct downhole operations. The riser is attached to the wellhead equipment and is supported in tension at or near the water surface so as to prevent its collapse. In drilling the borehole for the well, a drill string is passed from the floating vessel down through the riser and wellhead equipment and into the borehole.
By way of example, a 21 inch riser usually extends from a blowout preventer (BOP) stack mounted on the wellhead on the sea floor to the drilling platform on the drilling vessel at the surface. Typically, the BOP stack has a 18-¾ inch nominal bore and is commonly used for rilling operations in almost any water depth from a floating vessel. The 21 inch riser typically has an outside diameter (OD) of nominally 21 inches and an inside diameter (ID) of nominally 19 inches. Thus operations are conducted through the 19 inch ID of the 21 inch riser and the bore of the BOP stack.
Generally the largest casing string installed in the wellbore is 16 inch OD casing and then after drilling the next borehole, a 13-⅜ inch OD casing is installed. Typically the next casing string to be installed is a 10-¾ inch OD casing or alternatively a 9-⅝ inch OD casing. The next casing string to be run is typically either a 7 or 7-⅝ inch OD casing.
When the casing extends to a depth where it will encounter substantial downhole pressures, the casing string must be run into the wellbore under well controlled conditions, i.e. through a drilling riser and BOP stack. The 13-⅜ inch casing typically reaches such depths and requires well control. Thus, the BOP stack must be large enough to accommodate the new casing string, such as a 13-⅜ inch casing, that is to be installed under well control. A 16 inch riser will accommodate casing, casing hangers and well tools having an OD of up to 13-½ inches and thus a 16 inch riser will allow the passage of a 10-¾ inch OD casing and smaller. Neither a 16 or 13-⅜ inch casing string will pass through a 16 inch OD riser so a 21 inch riser may be required.
Wells are being drilled in deeper water, such as to depths of 10,000 feet, causing difficulties in using 21 inch risers. Because of the current drag forces and the weight of a 21 inch riser which is several thousand feet long and full of drilling mud, the large diameter riser becomes very unwieldy particularly in an ocean environment. The riser is maintained in tension from the floating drilling vessel and thus where a large diameter riser is several thousand feet long, the amount of tension that must be applied to the riser requires a very high tension force at the top of the riser on the vessel. This necessitates that the riser have increased strength to handle the increased tension thereby requiring that the thickness of the wall of the riser be increased which in turn increases the weight of the riser. The more weight that is required, the greater the tension that is required. Thus, the problem becomes greater as the length and size of the riser increases.
The floating drilling vessel must accommodate the riser required for downhole operations. Thus, the vessel must be specially equipped to handle large diameter risers and their associated large tension loads in deep water.
The drilling operation must be conducted through a riser which is large enough to accommodate the drill bit, the casing hangers, the seal assemblies and also provide an annulus around the new casing which is large enough to set and cement the casing. Typically, the drill pipe is 5 inch or 5-½ inch OD pipe with the larger 5-½ inch OD drill pipe typically being used in deeper water. Although typically the first bit into the well is a 17-½ inch bit, an expanding bit, such as an underreamer, hole opener, or bi-center bit, maybe used where the bit has a smaller OD to pass through a small diameter riser. Once in the borehole, the bit will drill a larger diameter borehole.
Drilling mud is circulated down through the drill string and returned to the vessel through the annulus formed between the riser and the drill pipe. It is necessary for the 21 inch riser, extending several thousand feet, to handle all of the drilling mud needed for drilling the boreholes. Because of the difference in density between the drilling mud and sea water, the large pressure created by the fluid column in the large diameter riser must be contained within the riser. The column of drilling mud can be approximately twice as heavy as sea water such that for every foot of depth, there is about one-half psi of mud gradient weight whereby at a depth of 10,000 feet, there could be 5,000 psi inside the large diameter riser relative to the sea water around the riser.
The drilling fluids in the riser also form a fluid column placing a hydrostatic head on the well for well control purposes. Well control is established by maintaining the density of the drilling fluid, and thus the hydrostatic pressure exerted on the subsurface formations, at a level that is sufficient to prevent the production fluids under pressure in the formation from overcoming the hydrostatic head. If the hydrostatic head on the well is insufficient, the pressurized gas and other formation fluids may exceed the hydrostatic head leading to a blowout, sometimes resulting in damage to property, the pollution of the ocean and loss of life.
On the other hand, if the hydrostatic head is too great, the pressure may force drilling fluids into the formation causing the loss of drilling fluids into the formation or a reduction or lost in production. If too much drilling fluid is lost into the formation and the level of drilling fluid drops in the riser, the hydrostatic head can decrease below the pressure of the formation and cause a blowout. Furthermore, the hydrostatic head may increase to an amount so as to fracture the formation resulting in increased lost circulation.
According to conventional practice, choke and kill lines typically extend from the drilling vessel to the wellhead to provide fluid communication for well control and circulation. The choke line is in fluid communication with the borehole at the wellhead and bypasses the riser to vent gases or other formation fluids directly to the surface. According to conventional practice, a surface-mounted choke valve is

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