Subsea Well Stage Cementing System

Abstract

A system for cementing plural cement stages in subsea wells having a well casing string connected to drill pipe and suspended from a casing hanger in a well bore below a subsea well head, the casing including a float collar; a lower first stage cementing plug releasably mounted above the float collar and below a stage cementing valve collar incorporated in the casing string, and an upper second stage cementing plug releasably mounted within the casing string above the stage cementing valve collar with actuating members of successively larger diameters which are adapted to pass through the drill pipe into the casing string and to successively: (a) release the first stage cementing plug, (b) close off the casing string below the stage cementing valve collar and open the cementing ports of the collar, and (c) release the second stage cementing plug which is adapted to close the cementing ports of the collar. Further cement staging may be provided with the casing string above the foregoing and below the casing hanger by providing appropriate additional components of the kind disclosed.

Description

BACKGROUND OF THE INVENTION

This invention generally pertains to the cementing of casing in subterranean wells and more particularly to the stage cementing of subsea wells.

In application of subsea cementing equipment such as disclosed herein, the well casing string to be cemented within the well is lowered and suspended within the well bore from the subsea well head and connected with drill pipe from the well head upwardly to the drilling rig floor.

Well casing is generally of relatively thin wall construction and large diameter and equipped with threaded pipe type connecting couplings primarily designed for permanent connection when installed. In contrast, well drill pipe is of generally smaller diameter with sturdy wall construction for withstanding the torque and tension imposed during the drilling of a well. Drill pipe is equipped with machined tool joints having tapered machined threads for rapid and repeated connection and disconnection.

In offshore locations where drilling operations are performed from a floating vessel, it is very desirable to use drill pipe principally because it is always available on board the float. The distances between the subsea well head and the surface of the water may be as much as 2000 feet or more. If casing were used, as much as 2000 feet would have to be transported to this vessel which may be miles off shore; and when the job has been completed, this same amount would have to be transported back to shore, as there would be no further use for casing of that size and there is no room on board for superfluous material. Furthermore, due to the type of threads and thin wall construction of casing, it is not easily assembled and disassembled. Another important advantage of using drill pipe is that, even if the floating drilling vessel is usually well secured, it still is subject to much sea motion. Telescoping bumper subs or joints are readily available and are used to absorb this movement for drill pipe but are unavailable for well casing.

Stage cementing methods are presently in use for land based well installations in which the lower portion of the well only is cemented in one operation, and the upper portion is cemented in subsequent operation. This type of operation requires a special type of equipment, such as bypassing cementing plugs and shut-off plugs and a stage cementing collar that is usually placed above a lower zone to be cemented in a previous cementing stage. As known, the cementing plugs used in stage cementing operations differ from cementing plugs used in conventional cementing operations. Since a stage cementing collar is designed with restrictions in its bore, the cementing plugs must be very flexible to pass through these restrictions. The added flexibility of these plugs is detrimental to their wiping qualities. This invention pertains to incorporating subsea stage cementing apparatus to be used in combination with subsea equipment from drill pipe, and at the same time provide a system that permits the use of conventional type cementing plugs with such subsea stage cementing equipment.

DESCRIPTION OF THE PRIOR ART

Previous methods and apparatus available for subsea cementing operations are disclosed in U.S. Pats. No. 3,507,325 and No. 3,545,542. Well cementing plugs and floating equipment are disclosed in U.S. Pat. No. 3,159,219 and No. 3,437,137. Also, for example, floating and stage cementing equipment and methods are disclosed on pages 582-590 of the 1970-71 edition of the COMPOSITE CATALOG of Oil Field Equipment and Services, published by Gulf Publishing Company, Houston, Texas, U.S.A., hereinafter referred to as the COMPOSITE CATALOG.

SUMMARY OF THE INVENTION

The present invention provides means to cement a subsea well in stages through drill pipe where the subsea well head may be several thousand feet under water.

The present invention also provides means to reduce the time and expense of separate concentric strings of well casing being cemented in a subterranean well where stage cementing may be utilized in lieu thereof.

The present invention further provides utilization of a maximum adaptation of commercially available and proven components with minimum requirements for relatively expensive specially manufactured components.

The foregoing and other accomplishments and advantages are attained in a system for cementing plural cement stages in subsea wells including a well casing string to be cemented into position which is suspended below a subsurface well head and from a casing hanger mounted within the well head; the casing hanger is connected through a removable installation tool, which tool is in turn connected to the lower end of a string of drill pipe suspended from equipment at the water surface. The casing string may include a float collar, a float guide shoe and/or a guide shoe. The casing string will include a lower first stage cementing plug releasably mounted below a stage cementing collar, which cementing collar is incorporated at a predetermined position in the casing string, and an upper second stage top cementing plug releasably mounted at a predetermined position within the casing string above the stage cementing collar. Respective actuating members of successively larger diameters are adapted to pass through the small drill pipe into the larger casing string and to successively: (a) actuate the first stage cementing plug, (b) close off the casing string below the stage cementing collar and open the cementing ports of the stage cementing collar, and (c) actuate the second stage cementing plug, which in turn is adapted to close the ports of the stage cementing collar. Further cement staging may be provided for a casing string above the foregoing apparatus and below the casing hanger, by providing additional apparatus of the kind herein disclosed.

BRIEF DESCRIPTION OF THE DRAWING

Referring to the drawing:

FIG. 1 is an elevational and partly sectional view of the apparatus of the present invention as installed in suspension from a schematically shown subsea well head and connected through drill pipe to a floating drilling rig above (not shown);

FIG. 2 is a detailed sectional view of the lower first stage cementing plug, the stage cementing collar and the upper second stage cementing plug;

FIG. 3 is an elevational sectional view of the apparatus during the first stage cementing phase wherein the cement has been pumped to below the first stage plug and the first stage plug actuating member is seated;

FIG. 4 is an elevational and sectional view of the apparatus after the first stage of cement has been pumped out of the casing string and with the stage cementing collar actuating member shown at the completion of the opening of the stage collar cementing ports;

FIG. 5 is an elevational sectional view of the apparatus during the second stage cementing phase wherein the cement has been pumped to below the upper second stage cementing plug and the second stage plug actuating member is seated; and

FIG. 6 is an elevational sectional view of the apparatus after the second stage of cement has been pumped out of the casing string through the stage cementing ports, the upper second stage plug having completed the closure of the stage collar cementing ports.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1 there is shown a casing string combination 10 disposed in a well bore 12 which extends below a sea floor 14. CAsing string 10 is suspended from a hanger body 16 which in turn is supported by a larger hanger body 18. Hanger body 18 is connected to a larger and shorter casing string (not shown) which, for illustration, would be previously installed and cemented into position. Hanger body 18 is connected within a subsea well head assembly 20, schematically shown. Casing string 10 is connected upwardly through an installation tool 22 and a string of drill pipe 24 into appropriate hoisting and cementing equipment (not shown) located at the surface of the water. Supplemental information concerning subsea well head installations and prior art features of this invention are shown in U.S. Pat. No. 3,545,542, specifically incorporated herein by reference.

The casing string combination 10 is comprised of a conventional guide shoe 26 connected through an upwardly disposed casing 28 of selected length into connection with a float collar 30. Float collar 30 is connected through an upwardly disposed casing 32 of selected length into connection with a first stage cementing plug combination 34 which in turn is threadedly connected to the lower end of a stage cementing collar 36. Cementing collar 36 is connected through an upwardly disposed casing 38 of selected length into connection with hanger body 16 whereby the entire casing string combination 10 is suspended from the hanger body.

A second stage cementing plug assembly 40 is disposed at a selected position within upper casing 38 and threadedly connected through a swivel 42 and an operating mandrel 44 into the bottom of installation tool 22. Cementing plug assembly 40 is preferably provided to fit very tightly within casing 38 to ensure a good wiping action as later described and the principal purpose of swivel 42 is to permit threaded connection and assembly of the components as shown without rotation of the tightly fitted plug assembly 40 within casing 38. It is to be noted that installation tool 22 and hanger 16 are provided with cooperating threads to form a reverse or lefthand threaded connection 46. As later mentioned, connection 46 is disengaged when desired by right hand rotation of installation tool 22 by drill pipe 24 relative to hanger 16.

Float collar 30 may be of conventional and commercially available construction such as shown at page 588 of the 1970-71 COMPOSITE CATALOG, for example. For illustration, float collar 30 is shown in include a check valve ball 48 retained in a cage assembly 50 and adapted to close against a seat 52. Valve ball 48 is disposed to permit fluids to be pumped downwardly through casing 32 and 28 and to seat against seat 52 to prevent upward fluid flow into casing 32 as shown in FIGS. 3-6.

Referring to FIG. 2, the first stage cementing plug assembly 34 is shown to comprise a flanged support sleeve 54 mounted in threaded relation within a threaded casing collar 55. Casing collar 55 also serves to threadedly connect casing 32 to the lower end of stage cementing collar 36. Rotatably mounted within support sleeve 54 is a shouldered swivel sleeve 56 which together with the support sleeve comprise a swivel structure 57. A retainer bushing 58 is threadedly connected to the lower end of swivel sleeve 56 to retain the sleeve in rotatable relation within support sleeve 54. Bushing 58 is fitted in nested relation within a counterbore formed in a hollow cylindrical body 60 of a cementing plug 62 and releasably retained in position by holding means such as shear pin 64. The body 60 of cementing plug 62 is encased in a rubber wiper structure 66 having a plurality of sturdy annular wiper lips as is conventional with full casing size cementing plugs of this kind. Cementing plug 62 is preferably provided as an adaptation of the structure taught by U.S. Pat. No. 3,437,137. Cementing plug 62 is provided to fit very tightly within casing 32 to ensure a good wiping action and the swivel structure 57 is provided to permit rotating connection of the threaded members during assembly of the structure as shown without incurring the difficulty of rotating cementing plug 62 within casing 32. Body 60 is provided with a second counterbore which defines a retainer seat 68. The purpose of seat 68 is to retain an actuating member such as a ball 70 (shown in FIGS. 3-6) which is utilized in later described steps of operation of the invention.

Stage cementing collar 36 is best shown in FIG. 2. The collar 36 may be of conventional types available from several sources and, as shown for illustration, is a modification of a stage cementing collar such as shown on page 588 of the 1970-71 COMPOSITE CATALOG. Generally described, collar 36 is provided with a cylindrical body 72 adapted for threaded connection to collar 36 and casing 38. A plurality of cementing ports 74 are defined through and around the walls of body 72 and is selectively closed off by an opening sleeve 76 slidably mounted within the bore of body 72. Slidably mounted within the bore of body 72 above opening sleeve 76 is an initial closing sleeve 78. Sleeves 76 and 78 are retained in initial position as shown by holding means such as shear pins 80 and 82. Fitted in slidable relation around body 72 is a sealing sleeve 84 which is retained in the upper open position as shown by means such as shear pins 86. Sealing sleeve 84 serves to permanently close off cementing ports 74 when moved to a lower position. Sealing sleeve 84 is also equipped with an inner ratchet ring 88 which is adapted to engage accommodating ratchet grooves defined around body 72 and retain the sealing sleeve in the lower closed position. A protector sleeve 90 is mounted about sealing sleeve 84. Protector sleeve 90 is provided with ports which are in registry with ports 74. A port 92 in body 72 provides pressure communication from within body 72 to the chamber defined by the upper end of sealing sleeve 84, body 72 and protector sleeve 90.

To the extent described above, stage cementing collar 36 is conventional in design and commercially available. The modifications to collar 36 for its adaption to the present invention are the provision of a port opening retainer seat 94 mounted within opening sleeve 76 and a port closing retainer seat 96 mounted within closing sleeve 78.

The purpose of opening retainer seat 94 is to retain an actuating member such as an opening ball 98 (shown in FIGS. 4-6) which is utilized in later described steps of operation of the invention. The purpose of closing retainer seat 96 is to retain the second stage cementing plug 40 (shown in FIGS. 3-6) in the operation of the invention.

Second stage cementing plug structure 40 is best shown in FIG. 2. Structure 40 is seen to include a hollow cylindrical body member 100 encased by a rubber wiper structure 102 having a plurality of sturdy annular wiper lips. A threaded retainer bushing 104 is nested within a counterbore defined in the upper end of body 100 and releasably fixed therein by means such as a shear pin 106. A latching groove 108 is defined within the bore of body 100 to receive the latching snap ring of an actuating means such as a wiping launching dart 110 (shown in FIGS. 5 and 6). As with plug structure 34, plug structure 40 is preferably provided as an adaptation of the structure taught in U.S. Pat. No. 3,437,137. Each of the annular lips, except the bottom lip, of plug structure 40 is ported or slightly vented as shown in FIG. 2 to permit fluid pressure to be transmitted across the plug structure as later mentioned.

Retainer bushing 104 is threadedly connected through a ventilated nipple into swivel 42. As previously mentioned, plug structure 40 is tightly fitted into casing 38 and the purpose of swivel 42 is to permit rotation of the various threaded connections of hanger body 18, operating mandrel 44 and installation tool 22 to assemble the structure as shown without the attendant difficulty of rotating plug structure 40 within casing 38.

DESCRIPTION OF METHOD AND OPERATION

Casing string 10 is assembled from its lower end to its upper end as shown in FIG. 1, then suspended from drill pipe 24 and lowered through the water and into well bore 12 until hanger body 16 is supported by hanger body 18. After casing string 10 is suspended from hanger bodies 16 and 18 within the bore as shown, drill pipe 24 is connected at the surface through a conventional cementing manifold to a pump (not shown). Such cementing manifolds are for the purpose of introducing actuating elements such as ball 70, ball 98 and launching dart 110 into the pump discharge and thereon through drill pipe 24 into casing string 10 for purposes hereinafter described.

When positioned as shown in FIG. 1, fluid under pressure, generally drilling fluid or possibly seawater, is pumped through drill pipe 24, casing string 10, out through guide shoe 26, up through the annulus between casing string 10 and well bore 12, and out between hanger body 16 and hanger body 18 to establish fluid circulation through the system.

Referring now to FIGS. 3-6, a first stage of cement 112 is introduced into drill pipe 24 and displaced with pumped fluid through and out of casing string 10 into the annulus within well bore 12. The actuating ball 70 is introduced into drill pipe 24 at or near the juncture of cement 112 and the displacing fluid and follows cement 112 into casing string 10 as shown in FIG. 3. The respective size of ball 70 and retainer seats 68, 94 and 96 are such that the ball passes through seats 94 and 96 and seats into sealing engagement with seat 68, thereby blocking fluid flow through the first stage cement plug structure 34. Further application of pump pressure causes pin 64 to shear, releasing the plug from support assembly 57. Ball 70 is preferably provided of density comparable to the fluids being pumped, hard rubber or plastic for example, in order to be conveniently moved along with the cement 112. Plug 34 and the fluid above the plug thereon displace cement 112 out of casing string 10 up into the annulus of well bore 12. Such displacement is continued, leaving the casing 32 free of cement, until plug 34 lands in supporting abutment on float structure 30 as shown in FIG. 4. As shown in FIGS. 5 and 6, no further flow of first stage cement 112 or the displacing fluid is permitted in either direction by the opposing check valve action of balls 48 and 70. First stage cement 112 is then in position to set as shown in FIG. 5.

At some selected time before or after cement 112 has set or has been even fully displaced, the opening ball 98 is introduced into the drill pipe 24 and thereafter allowed to drop or sink through the fluid in the drill pipe, casing string 10 and closing retainer seat 96 into sealing engagement with opening retainer seat 94. Ball 98 is provided of a high density material, such as a lead alloy for example, to hasten its rate of descent. Application of fluid pump pressure against ball 98 and retainer seat 94 causes pin 80 to shear and opening sleeve 76 to slide to its lower position, permitting fluid passage through cementing ports 74 and preventing fluid flow to below casing collar 36 as shown in FIG. 4. Pumping is thereon continued to establish fluid circulation through the cementing ports 74 and upwardly through the annulus.

When cement 112 has had a sufficient time period to set (generally a minimum of four hours) second stage cement 114 is introduced into drill pipe 24 and thereon into casing string 10. Cement 114 is displaced through drill pipe 24 by a launching dart 110 followed by pumped fluids to a position in casing 38 as shown in FIG. 5, whereupon launching dart 110 enters plug structure 40 and latches into the grooves 108. Additional pumping causes pin 106 to shear and the dart 110 and plug structure 40 to move down jointly through casing 38, displacing cement 114. Dart 110 serves to wipe the drill pipe clean of cement during its passage through the drill pipe and plug structure 40 combined with dart 110 serves to wipe casing 38 clean of cement during passage down through casing 38. Continued pumping forces cement 114 down through casing 38, out through ports 74 and upwardly in the annulus between well bore 12 and casing string 10, as shown in FIGS. 5 and 6, until plug structure 40 engages closing retainer seat 96. The fluid pressure above plug structure 40 thereupon causes pin 82 to shear and closing sleeve 78 to move to its lower position to close off ports 74. Closure of ports 74 causes an increase in pump pressure at the surface, indicating such closure, and also causes pressure to be transmitted through the vented wiping lips of plug structure 40 and through port 92 to be imposed on sealing sleeve 84, causing the sealing sleeve to move down across ports 74 and to be locked into closed position by ratchet ring 88 as shown in FIG. 6. Installation tool 22 then may be rotated out of hanger 16 and the entire assembly attached to drill pipe 24 withdrawn to the surface.

Cement 112 and cement 114 are then given time to set properly as determined by the operator (several hours to several days) after which time drilling may be resumed through casing string 10 to remove plug structure 40, retainer seats 94 and 96, sleeves 76 and 78, plug structure 34, the drillable interior portions of float collar 30 and guide shoe 26 and any residual cement as shown, to leave the full interior diameter of casing string 10 open for further drilling or completion operations.

It is to be noted that, though the preferred embodiment and method thereof is disclosed above, the system is operable without utilizing guide shoe 26 or even floating means such as float collar 30 under certain conditions.

The foregoing description and drawing will suggest other embodiments and variations to those skilled in the art, all of which are intended to be included in the spirit of the invention as herein set forth.

Claims

That being claimed is:

1. A subsea well installation comprising:

a. a composite string of well casing suspended within a subsea well bore from a casing hanger supported by a subsea well head, said casing being adapted to be connected through an installation means and a string of drill pipe to hoisting and cementing equipment disposed at the water surface above the well head;

b. a stage cementing collar having cementing ports which can be opened and closed incorporated at a selected intermediate position within said casing;

c. a hollow first stage casing cementing plug means mounted in swiveled and releasable relation within said casing below said cementing collar;

d. a valved casing floating means mounted within said casing below said first stage cementing plug means, and

e. a hollow second stage casing cementing plug means mounted in swiveled and releasable relation within said casing above said cementing collar.

2. The subsea installation of claim 1 further comprising:

a. a first actuating means adapted to pass through drill pipe, said second stage cementing plug means and said cementing collar to engage said first stage cementing plug means and move said first stage cementing plug means to a lower portion of said casing in response to fluid flow through said drill pipe and said casing;

b. a second actuating means adapted to pass through drill pipe and said second stage cementing plug means to open said cementing ports and close off fluid flow through said casing to below said cementing collar, and

c. a third actuating means adapted to pass through drill pipe to engage said second stage cementing plug means and move said second stage cementing plug means downwardly in said casing to close said cementing ports.

3. The subsea well installation of claim 1 wherein one of said casing cementing plugs comprises:

a. a circular flanged swivel support member defining an axially disposed cylindrical bore and adapted for mounting within a casing collar to be included in a well casing;

b. a shouldered swivel sleeve rotatably mounted within and extending below said support member;

c. a cylindrical cementing plug means including a cylindrical plug body member defining an axially disposed cylindrical bore having retainer means therein adapted to receive an actuating member passing through drill pipe; said body member being encircled by a resilient sealing and wiping structure having annular wiping lips and adapted for a tight sealing fit within a well casing, and

d. releasable connection means releasably connecting said cementing plug means to the lower end of said swivel sleeve to be released from said swivel sleeve by force applied between said swivel sleeve and said plug body member.

4. The subsea installation of claim 2 further comprising:

a. a first actuating means adapted to pass through drill pipe, said second stage cementing plug means and said cementing collar to engage said first stage cementing plug means and move said first stage cementing plug means to a lower portion of said casing in response to fluid flow through said drill pipe and said casing;

b. a second actuating means adapted to pass through drill pipe and said second stage cementing plug means to open said cementing ports and close off fluid flow through said casing to below said cementing collar; and

c. a third actuating means adapted to pass through drill pipe to engage said second stage cementing plug means and move said second stage cementing plug means downwardly in said casing to close said cementing ports.

5. An improved cementing plug means comprising:

a. a circular flanged swivel support member defining an axially disposed cylindrical bore and adapted for mounting within a casing collar to be included in a well casing;

b. a shouldered swivel sleeve rotatably mounted within and extending below said support member;

c. a cylindrical cementing plug means including a cylindrical plug body member defining an axially disposed cylindrical bore having a retainer seat thereon adapted to receive an actuating member passing through drill pipe; said body member being encircled by a resilient sealing and wiping structure having annular wiping lips and adapted for a tight sealing fit within a well casing;

d. releasable connection means releasably connecting said cementing plug means to the lower end of said swivel sleeve to be released from said swivel sleeve by force applied between said swivel member and said plug body.

6. Apparatus adapted to be suspended from a subsea well head within a subsea well bore and cemented in stages within the well bore, comprising:

a. a composite string of well casing adapted to be suspended from a casing hanger supported by a subsea well head, said casing being adapted to be connected through an installation means and a string of drill pipe to hoisting and cementing equipment disposed at the water surface above the well head;

b. a stage cementing collar having cementing ports which can be opened and closed mounted at a selected intermediate position within said casing;

c. a hollow first stage casing cementing plug means mounted in releasable relation within said casing below said cementing collar; and

d. a second stage casing cementing plug means mounted in releasable relation within said casing above said cementing collar.

7. The apparatus of claim 6 wherein at least one of said casing cementing plug means comprises:

a. a circular flanged swivel support member defining an axially disposed cylindrical bore and adapted for mounting within a casing collar to be included in a well casing;

b. a shouldered swivel sleeve rotatably mounted within and extending below said support member;

c. a cylindrical cementing plug means including a cylindrical plug body member defining an axially disposed cylindrical bore having a retainer seat thereon adapted to receive an actuating member passing through drill pipe; said body member being encircled by a resilient sealing and wiping structure having annular wiping lips and adapted for a tight sealing fit within a well casing;

d. releasable connection means releasably connecting said cementing plug means to the lower end of said swivel sleeve to be released from said swivel sleeve by force applied between said swivel sleeve and said plug body member.

8. The apparatus of claim 7 further comprising:

a. a first actuating means adapted to pass through drill pipe, said second stage cementing plug means and said cementing collar to engage said first stage cementing plug means and force said first stage cementing plug means to a lower portion of said casing in response to fluid flow through said drill pipe and said casing;

b. a second actuating means adapted to pass through drill pipe and said second stage cementing plug means to open said cementing ports and close off fluid flow through said casing to below said cementing collar, and

c. a third actuating means adapted to pass through drill pipe to engage said second stage cementing plug means and force said second stage cementing plug means downwardly in said casing to close said cementing ports.

9. A method of cementing a composite string of well casing in stages within a subsea well bore, said casing string including a stage cementing collar having cementing ports which can be opened and closed and which is incorporated at an intermediate position within said casing, a hollow first stage casing cementing plug means mounted in swiveled and releasable relation within said casing below said cementing collar, and a hollow second stage casing cementing plug mounted in swiveled and releasable relation within said casing above said cementing collar, the steps comprising:

a. suspending said well casing within a well bore from a casing hanger supported by a subsea well head;

b. connecting said casing through an installation means and a string of drill pipe to cementing equipment at the water surface above the well head:

c. passing a first stage of cement followed by a first actuating means through said drill pipe, said second stage casing cementing plug means, said cementing collar and said first stage cementing plug means, whereupon said first actuating means engages and forces said first stage cementing plug to a lower portion of said casing to displace said first stage cement out of said casing into said well bore;

d. passing a second actuating means through said drill pipe and said second stage cementing plug means to open said cementing ports and to close off fluid flow through said casing to below said cementing collar, and

e. passing a second stage of cement followed by a third actuating means through said drill pipe and said second stage cementing plug whereupon said third actuating means engages and forces said second stage cementing plug down through said casing to displace said second stage cement out of said ports and into said well bore and to close said ports following displacement of said second stage cement.

10. A method of cementing a composite string of well casing in stages within a subsea well bore, said casing string including a stage cementing collar having cementing ports which can be opened and closed and which is incorporated at an intermediate position within said casing, a hollow first stage casing cementing plug means mounted in releasable relation within said casing below said cementing collar, a valve casing floating means mounted with said casing below said first stage cementing plug means, and a second stage casing cementing plug mounted in releasable relation within said casing above said cementing collar, the steps comprising:

a. suspending said well casing within a well bore from a casing hanger supported by a subsea well head;

b. connecting said casing through an installation means and a string of drill pipe to cementing equipment at the water surface above the well head;

c. passing a first stage of cement followed by a first actuating means through said drill pipe, said second stage casing cementing plug means, said cementing collar and said first stage cementing plug means, whereupon said first actuating means engages and forces said first stage cementing plug downwardly into engagement with said casing floating means to displace said first stage cement out of said casing into said well bore;

d. passing a second actuating means through said drill pipe and said second stage cementing plug means to open said cementing ports and to close off fluid flow through said casing to below said cementing collar, and

e. passing a second stage of cement followed by a third actuating means through said drill pipe and said second stage cementing plug means whereupon said third actuating means engages and forces said second stage cementing plug down through said casing to displace said second stage cement out of said ports and into said well bore and to close said ports following displacement of said second stage cement.

11. A method of cementing a composite string of well casing in stages within a subsea well bore, said casing string including a stage cementing collar having cementing ports which can be opened and closed and which is incorporated at an intermediate position within said casing, a hollow first stage casing cementing plug means mounted in releasable relation within said casing below said cementing collar, and hollow second stage casing cementing plug mounted in releasable relation within said casing above said cementing collar, the steps comprising:

a. suspending said well casing within a well bore from a casing hanger supported by a subsea well head;

b. connecting said casing through an installation means and a string of drill pipe to cementing equipment at the water surface above the well head;

c. passing a first stage of cement followed by a first actuating means through said drill pipe, said second stage casing cementing plug means, said cementing collar and said first stage cementing plug means, whereupon said first actuating means engages and forces said first stage cementing plug to a lower portion of said casing to displace said first stage cement out of said casing into said well bore;

d. passing a second actuating means through said drill pipe and said second stage cementing plug means to open said cementing ports and to close off fluid flow through said casing to below said cementing collar;

e. passing a second stage of cement followed by a third actuating means through said drill pipe and said second stage cementing plug means whereupon said third actuating means engages and forces said second stage cementing plug down through said casing to displace said second stage cement out of said ports and into said well bore and to close said ports following displacement of said second stage cement;

f. retaining said first stage of cement and said second stage of cement in displaced position until set, and

g. drilling out obstructions within said casing.