Abstract:
A method and apparatus for installing conductor casing for offshore oil wells. In the preferred embodiment of the present invention, a retrievable suction embedment chamber assembly drives a string of casing into the seafloor using hydrostatic pressure by releasably forming a seal around the casing to be driven and using a pump to lower the pressure inside the chamber. The assembly may then be flooded with seawater, released from the casing, then repositioned at a higher point on the casing, and the embedment process repeated. The assembly may also be used to embed several subsequent strings of casing in series.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     FIELD OF THE INVENTION 
     The present invention relates to operations involving the drilling of wells in an offshore environment. In particular, the present invention relates to improved methods and an apparatus for installing conductor casing for offshore wells. More specifically, the present invention relates to method and apparatus for driving conductor casing into the seafloor using a retrievable suction embedment chamber assembly. 
     BACKGROUND OF THE INVENTION 
     Hydrocarbon reservoirs are found in special formations of rock usually located far underground. These reservoirs are exploited by drilling wells into the ground to facilitate the extraction of the hydrocarbons. As wells are drilled, many different formations of rock are encountered. Some of these formations are very hard, while others are loose or sandy. Shallow formations often contain water. In order to protect the integrity of the well as it is drilled, lengths of pipe, known as casing, are placed, or set, into the well. The casing acts as a barrier to prevent the sides of the well from caving in, to prevent the movement of fluid, like water or hydrocarbons, from one formation to another, and to increase efficiency of the well if it used to produce hydrocarbons. 
     A length of a casing is known as a string. A typical well will contain several strings of casing, each with a different diameter. The largest diameter casing will be at the top of the well with each successive casing string having a smaller diameter so that it can be moved through the casing already in the well. Conductor casing, also known as conductor pipe, is the largest diameter casing and therefore the first tubular placed when drilling a well. The purpose of the conductor casing is to prevent the loose, shallow formations from falling into the wellbore. 
     The diameter of the casing strings is determined by several factors including depth of the well and the type of formation being drilled in. As each string of casing is set, cement is pumped into the hole and around the outside of the casing to lock the casing in place and seal off the surrounding formation. When the well reaches the formations containing hydrocarbons, it is in effect a continuous, sealed conduit to the surface. Therefore, any hydrocarbons produced can not migrate into other formations or into ground water. Large valves, or pressure control equipment such as a blowout preventer or a production tree, are attached to the casing at the top of the well, known as the wellhead, to control the flow of material out of the well. 
     The formations of rock that contain hydrocarbons are found all over the world. Many of the recent efforts to find and produce these hydrocarbons have focused on formations located under water. In subsea wells, the conductor casing extends through an initial layer of mud and silt and provides a solid foundation for the well. The conductor casing is usually large diameter pipe ranging from 6 to 60 inches in diameter and can be several hundred feet long. 
     Early offshore wells were drilled in relatively shallow water a only few hundred feet deep. For wells drilled at these shallow depths, the conductor casing typically extends to the surface of the water and is attached to a platform. Conductor casing for these shallow water wells can be driven using equipment at the surface. For wells in deeper water, the conductor casing terminates at or near the seafloor. Because the conductor casing does not extend to the surface of the water, these deep-water wells require that the casing driving mechanisms be below the surface and typically at the seafloor. There are wells being drilled today in water depths up to two miles. 
     Prior art methods for installing conductor casing offshore include driving or hammering the conductor casing into the seafloor with a pile driver, rotary drilling with a drill bit to create a hole in the seafloor for the casing, and using high pressure liquids to wash, or jet, out a cavity in the seafloor for the casing. Prior art methods of installation require the heavy hoisting, rotary, or pumping equipment that can only be provided by the drilling vessel. The present invention overcomes these and other drawbacks of the prior art. 
     SUMMARY OF THE PREFERRED EMBODIMENTS 
     The embodiments of the present invention provide methods and apparatus for setting conductor casing for subsea wells using hydrostatic pressure as the driving force. The present invention allows conductor casing to be set more efficiently, and from smaller vessels, than the prior art methods. In one embodiment, a string of conductor casing is attached to a large chamber that is open at the base and substantially closed at the top. The conductor casing is placed through an opening in the top of the chamber and is attached to the chamber by an attachment mechanism which may be a ratcheting mechanism. The chamber is fitted with a device, such as a pump, for evacuating or filling the chamber from the top. 
     This assembly is lowered to the seafloor using a crane or winch mounted on a vessel. Once on the seafloor, the chamber is allowed to settle into the seafloor under its own weight, where the mud and silt of the seafloor create a seal around the base of the chamber. Once the assembly has settled into the seafloor, the pump is activated, and material, including water and mud, is pumped out of the chamber. As the pressure inside the chamber drops, the hydrostatic pressure operating on the outside of the chamber forces the chamber and the attached conductor casing into the seafloor. 
     When the chamber has moved as far into the seafloor as desired, the attachment mechanism can be released and water pumped into the chamber, creating a differential pressure that pushes the chamber out of the seafloor and toward the surface. Because the conductor casing is released from the chamber, the surface friction between the casing and the mud will hold the casing in place. In one embodiment, the chamber is moved up the casing and the attachment mechanism is re-engaged. The pump is reversed, lowering the pressure inside the chamber, and the chamber is again forced into the seafloor, pulling the casing with it. This sequence can be repeated until the casing reaches the desired depth. Once the casing reaches the desired depth, the attachment mechanism can be disengaged and the chamber moved free of the seafloor, where it can be retrieved to the surface or used to place additional strings of conductor casing. 
     The only equipment required to operate the above described embodiment is a lifting device, such as a crane or a winch, and a power supply to the pump and attachment mechanism. This equipment is typically available on many vessels that service the offshore petroleum industry. This allows flexibility in choosing the most economical vessel to use to set conductor casing. 
     Thus, the present invention comprises a combination of features and advantages which enable it to overcome various problems of prior devices. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments of the invention and by referring to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more detailed description of the preferred embodiment of the present invention, reference will now be made to the accompanying drawings, wherein: 
     FIG. 1 is a partial sectional side view with section of a retrievable suction embedment chamber assembly; 
     FIG. 2A is a cross-sectional view of the chamber throat with single-action spring grip; 
     FIG. 2B is a cross-sectional view of the chamber throat with a double-action hydraulic grip; 
     FIG. 3A is a side view of the assembly and casing during positioning; 
     FIG. 3B is a side view of the assembly and casing during casing embedding operation; 
     FIG. 3C is a side view if the assembly and casing at the point of maximum seafloor penetration; 
     FIG. 3D is a side view of the assembly and casing during assembly retrieval; 
     FIG. 4A is a side view with section of the assembly and a first casing; 
     FIG. 4B is a side view with section of the assembly and an embedded first casing with a lowered subsequent casing; and 
     FIG. 4C is a side view with section of the assembly and an embedded first casing mated to a subsequent casing. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings, FIG. 1 represents a side view of one embodiment of an assembly  32  for installing conductor casing  10 , generally including a body comprising a suction embedment chamber  12 , an attachment mechanism  15  located in the assembly throat  14 , a pumping system  16 , and lift points  20 . Assembly  32  may optionally include chamber protection plating  18  or platforms and mounting assemblies (not shown) enabling the assembly to serve as a subsea template for drilling and production. The pumping system  16  may be placed more from chamber  12 , such as on a surface facility (not shown), with hydraulic lines to and from the chamber or attached directly to the chamber. Assembly  32  also comprises a control system (not shown) in communication with attachment mechanism  15  and pumping system  16 . The control system may be of any suitable type, including electrical, mechanical, or multiplexed, and is preferably located at the surface with control signals transmitted down to assembly  32 . 
     The suction embedment chamber  12  is a chamber open at the base and substantially closeable at the top. The top of chamber  12  is equipped with an opening  13  to accommodate conductor casing  10  and at least one other opening  17  to provide a conduit for a pump  16 . One preferred embodiment of the present invention includes a cylindrical chamber with a flat top, constructed so that it can withstand external pressure without any internal bracing. It is envisioned that a typical chamber  12  would be at least 12-30 feet in diameter and 25-100 feet tall. Chamber  12  is preferably constructed out of steel, but other materials, such as other metals or composites, can be used. One of ordinary skill in the art will recognize that any number of chamber designs may be suitable, including a three-chamber design for level-critical applications. 
     Attachment mechanism  15 , embodiments of which are shown in FIGS. 2A and 2B, is located in throat  14  of chamber  12  and comprises a releasable gripping mechanism  26  and seals  22  and  28  between assembly  32  and conductor casing  10 . Attachment mechanism  15  comprises a suction seal  22 , a pressure seal  28 , and a gripping mechanism  26 . Gripping mechanism  26  comprises a bar  21  with pivots  23  at either end, and a gripping head  27  attached to the pivot  23  closest to casing  10 , and either a single-acting grip positioner  24 , as shown in FIG. 2A, or a double-acting grip positioner  25 , as shown in FIG.  2 B. Critical parts or the entire attachment mechanism  15  may be replaceable. 
     Depending on the soil shear strength and the crush-resistant features of casing  10 , the appropriate applicable gripping mechanism  26  may be selected. Single-acting mechanism  24  is spring loaded, causing gripping head  27  to adhere to the surface of casing  10 . Gripping mechanism  26  will grip casing  10  due to spring pressure and an angular resistance to relative casing motion. Double-acting mechanism  25  is a sealed diaphragm wherein the internal pressure can be changed to adjust the grip pressure. 
     The method for installing the conductor casing  10  is detailed in FIGS. 3A-3D. As shown in FIG. 3A, the suction embedment chamber assembly  32  and conductor casing  10  are transported to the well site by a vessel  34 . Conductor casing  10  is affixed to the assembly  32  by attachment mechanism  15  so that conductor casing  10  protrudes from the top of assembly  32 , which is supported by any suitable lowering mechanism. For example, assembly  32  may be attached by a reelable cable  36  to a winch  38  located on vessel  34 . As casing  10  and assembly  32  are lowered to seafloor  30 , some air preferably is retained in chamber  12  so as to provide a degree of buoyancy and thereby offset the weight of assembly  32 . 
     When assembly  32  lands on seafloor  30 , as shown in FIG. 3B, any remaining air is released, and assembly  32  is allowed to sink into seafloor  30  under its own weight. The base of assembly  32  sinking into seafloor  30  creates a seal around the outside of chamber  12 . The pumping system (not shown) is then activated and removes water and particulate material from inside chamber  12 . This creates differential pressure acting on the outside of chamber  12 , which forces assembly  32  into the seafloor  30 . Attachment mechanism  15  maintains the physical connection between conductor casing  10  and chamber  12  so that casing  10  is driven into the seafloor  30  along with assembly  32 . When assembly  32  reaches maximum penetration into the seafloor  30 , as shown in FIG. 3C, attachment mechanism  15  releases casing  10 , and pumping system  16  is reversed. 
     Pumping water into chamber  12  raises assembly  32  while conductor casing  10  remains in place, as shown in FIG.  3 D. When assembly  32  rises relative to seafloor  30 , attachment mechanism  15  can be re-engaged and pumping system  16  activated to again lower the pressure inside chamber  12  and continue advancing conductor casing  10  deeper into seafloor  30 . These steps can be repeated until casing  10  reaches the intended depth. When the intended depth has been reached, attachment mechanism  15  is released and pumping system  16  is again reversed to raise assembly  32  out of the seafloor  30 , so that it can be recovered and used again. 
     Referring briefly back to FIG. 1, pumping system  16  generally comprises at least one conduit  17  to the interior of chamber  12 , at least one pump (not shown), and at least one conduit  19  to the sea. Preferably, pumping system  16  is reversible, so that it can be used for both the evacuation and filling of chamber  12 . A preferred embodiment of pumping system  16  is to mount a single, reversible pump on the chamber  12  and control its operation from the surface through hydraulic, electric, or pneumatic controls. Another embodiment would be to mount the pump at the surface and attach it to chamber  12  through a riser or flexible conduit. One of ordinary skill in the art would recognize other pumping mechanisms including systems with multiple pumps or conduits. 
     Assembly  32  may also include modular platforms (not shown) that can be attached to chamber  12 . It is envisioned that the platforms can be used to support other subsea well equipment such as the wellhead, guidance structures, mud tanks, mud pumps, and process and drilling equipment. When equipped with the platforms, chamber  12  would preferably be left in place on the seafloor to serve as reinforcement of the wellhead and as support for wellhead or other subsea equipment. 
     This installation method may depend on the bearing and shear properties of the soil. The soil usually found on seafloor  30 , especially in deep water, is of a thick, soupy consistency. The bearing strength of the soil is such that the weight of assembly  32  is sufficient to push the assembly  32  a certain distance into seafloor  30 . At that point, due to the nature of the soil, a seal is formed around the perimeter of chamber  12 . 
     The installation method works on the principle that the force available to drive conductor casing  10  and chamber  12  into seafloor  30  is greater than the force required to shear the soil. The driving force is the difference between the pressure on the outside of chamber  12  and the pressure inside of chamber  12  multiplied times the area of the top of chamber  12 . The resisting force is created by friction between the soil and the sides of chamber  12  and casing  10 . The resisting force is approximately the total soil shear force required to shear the inside and outside surface areas of chamber  12  and conductor casing  10 . Chamber  12  and pumping system  16  are designed so that an adequate pressure differential can be maintained so that the hydrostatic pressure will provide adequate force to drive casing  10  into seafloor  30 . 
     A key advantage of the present invention is that, unlike prior art suction embedment systems, a penetration is provided through suction chamber  12 , allowing conductor casing  10  to be driven into the seafloor  30 . As assembly  32  is able to attach to and release conductor casing  10  subsea, assembly  32  may be retrieved and reused. Attachment mechanism  15  also enables assembly  32  to be used with a string of casing much longer than assembly  32 , because it can be detached and reattached at a higher point on the casing string. 
     Also, if additional strings of conductor casing  10  are required, they can be lowered from the surface and attached inline by attachment mechanism  15  to assembly  32 , as shown in FIGS. 4A-4C. As shown in FIG. 4A, a first string of casing  10  is driven into seafloor  30  by assembly  32 . FIG. 4B shows casing string  10  driven deeper into seafloor  30 , in preparation for mating with a subsequent string of casing  10 . The subsequent string of casing  10  can be mated to the original string  10  using an in-place torquing device (not shown) or, in the case of relatively shallow water depths, torqued from the surface. Once mated, pumping system  16  may be reversed and attachment mechanism  15  released to raise assembly  32  up mated casing strings  10 . Once at the desired height, attachment mechanism  15  may grasp the mated casings  10  and pumping system  16  engaged in order to drive mated casings  10  down deeper into seafloor  30 . These steps can be repeated until casing  10  has reached the intended depth. After casing  10  is set, assembly  32  can be retrieved to the surface and reused. 
     The embodiments set forth herein are merely illustrative and do not limit the scope of the invention of the details therein. It will be appreciated that many other modifications and improvements to the disclosure herein may be made without departing from the scope of the invention of the inventive concepts disclosed herein. As many varying and different embodiments may be made within the scope of the inventive concept taught herein, including equivalent structures or materials hereafter thought of, and because many modifications may be made in the embodiments herein detailed in accordance with the descriptive requirements of the law, it is to be understood that the details herein are to be interpreted as illustrative and not in a limiting sense.