US3465817A

US3465817A - Riser pipe

Info

Publication number: US3465817A
Application number: US650314A
Authority: US
Inventors: Renic P Vincent
Original assignee: Pan American Petroleum Corp
Current assignee: Pan American Petroleum Corp
Priority date: 1967-06-30
Filing date: 1967-06-30
Publication date: 1969-09-09
Anticipated expiration: 1986-09-09

Description

Sept. 9, 1969 R. P. VINCENT 3,465,817

RISER PIPE Filed June 30, 1967 4 Sheets-Sheet 1 ATTORNEY Sept. 9, 1969 R. P. VINCENT 3,465,817

RISER PIPE Filed June 30, 1967 4 Sheets-Sheet 2 W/YO 76 Y x) A X 3 FIG. 2

v7 1! II 1111 RENIC P VINCENT INVENTOR.

ATTORNEY Sept. 9, 1969 Filed June 30, 1967 FEET OCEAN O FLOOR FIG 3A RISER PIPE 4 Sheets-Sheet.

TOTAL STRESS (KS!) AXIAL a BENDING a H6 35 FIG 3C FIG 3 RENIC P. VINCENT INVENTOR.

ATTORNEY Sept. 9, 1969 R. P. VINCENT 3,465,817

RISER PIPE I Filed June 30, 1967 4 Sheets-Sheet 4 TOP OF RISER O B E FIG 4 SLIP JOINT LEVEL COMPRESQON IIIT l 90 I? "I 34 I I T 96 i I i 9 FIG 5 4 32 TENSION REN l C P. V I N C E NT INVhN'I'OR.

ATTORNEY United States Patent RISER PIPE Renic P. Vincent, Tulsa, Okla., assignor to Pan American Petroleum Corporation, Tulsa, Okla., a corporation of Delaware Filed June 30, 1967, Ser. No. 650,314 Int. Cl. E21b 43/01, 7/12, 15/02 US. Cl. 166-.5 14 Claims ABSTRACT OF THE DISCLOSURE This invention concerns a riser pipe for use in drilling wells from a vessel floating on a body of water. The riser pipe extends from the ship to an anchored well head assembly on the ocean floor. Typical diameters for the riser pipe are 16", 20" and 24". A drill bit and drill string are guided down through the inside of the riser pipe and the anchored well head assembly to perform drilling operations in the ocean floor. The riser pipe of this invention is typically of a fairly large diameter, e.g., 20", and at the very lower end it is reduced down to have an extension of iesser diameter, e.g., 16". There is an annular shoulder between the large diameter pipe and the smaller diameter extension. Differential pressure caused by the weight of the drilling mud (which is heavier than the sea water) acts downward on this annular shoulder. The slip joint is at the lower end. In one embodiment, the extension is slideably and sealingly fitted within a lower joint of pipe which is connected through a flex joint to a well head assembly. The thickness of the wall of the extension of the riser pipe can be made greater than the thickness of the large diameter pipe so that the bending moment will be decreased. In another embodiment an annular shoulder is attached to the larger diameter riser pipe and a sliding and sealing engagement is made between the annular member and a stub of pipe extending up from the flex joint. This system does not require a shipboard tensioning device.

This invention relates to drilling in earth formations located beneath a body of water, such as in the Gulf of Mexico, and in which the drilling operations are conducted from a floating vessel. More specifically, this invention relates to a method and apparatus involving a riser pipe which extends from the ocean floor to the floating vessel located at the surface of the body of water. It relates to an especially designed riser pipe in which the total stress along the length of the riser is greatly reduced.

Background In recent years it has become desirable to use a floating vessel from which to drill wells in marine locations. In such operations the floating vessel is sometimes connected to a submerged well bore by a long tubular member through which drilling tools, drilling fluids, etc., pass between the vessel and the well bore. This long tubular member is commonly referred to as a riser pipe.

The submerged well head usually includes a blow-out preventer and other control equipment. In one embodiment the upper part of the well head assembly includes a ball connector which provides a flexible connection between the well head assembly and the riser pipe. The lower end of the riser pipe is connected to this ball joint and is free to pivot thereabout. This is commonly called a flex joint. Other types of flex joints are commercially available; however, the ball and socket joint is enjoying increasing popularity. Although a vessel is anchored, it can have vertical movement of from a few feet up to 20-30 feet or more. To compensate for this vertical move- 3,465,817 Patented Sept. 9, 1969 ment a slip or telescopic joint is provided in the riser pipe.

If the conventional riser pipe is supported solely at its lower end, its own effective weight, i.e., weight in water, causes it to be in a state of axial compression increasing from zero at the top to maximum at the bottom support. When drilling in deep water the compressive stress in the wall of the riser pipe from this source alone is sometimes suflicient to buckle the riser pipe. To counteract this buckling effect it has become a practice to apply a tensile force to the top of the riser pipe. Special tensioning devices are mounted on the ship and have their cables attached to the upper end of the riser pipe but below the slip joint. These tensioning devices are commonly referred to as constant tensioning devices so that they can maintain a constant tension on the riser pipe although the ship may rise and fall with respect to the riser pipe. These constant tensioning systems are helpful but are also costly and must be maintained. A failure of the ten sioning system can be catastropic.

Brief description of the invention Briefly, this invention concerns an improved riser pipe in which the slip joint is located near the lower end. The riser pipe is supported from the vessel preferably by flexible lines. The lower end of the upper section of the riser pipe thus supported is provided with an internal shoulder. Slip joint means and flexible joint means are provided between the annular shoulder and a subsea well head assembly which is anchored to the floor of the body of water. In a preferred embodiment, a lower section or extension of pipe extends downwardly from the annular shoulder and slidingly and sealingly fits within a joint or stub of pipe which is supported from the well head assembly. This stub is preferably mounted with a ball and socket type joint to the well head assembly so as to give a flexible connection between the riser pipe assembly and the well head assembly. As will be explained in greater detail hereinafter, when using a drilling fluid having greater density than that of the water surrounding the riser pipe, this configuration of the riser pipe assembly, particularly including the shoulder member, results in a much smaller overall stress of the riser pipe along its length, when suspended in water from a vessel, than the stress would be if the annular shoulder were not provided, i.e., if the riser pipe assembly Was substantially the same in diameter along its entire length.

It is an object of this invention to provide a novel type riser pipe system so that such riser pipe tensioning system mentioned above will not be necessary in ordinary drilling.

Various other objects and a better understanding of the invention can be had from the following description taken in conjunction with the drawings in which:

FIGURE 1 is a preferred embodiment of the riser pipe assembly of this invention;

FIGURE 2 is a modification of the embodiment of FIGURE 1;

FIGURE 3 includes FIGURE 3A which illustrates in simplified form one embodiment of this invention, FIG- URE 3B shows the curve (in solid line) of total stress along the length of a riser pipe according to this invention suspended in water and illustrated in FIGURE 3A; FIGURE 3C illustrates a conventional riser, and the dotted and broken lines in FIGURE 3B show stress distribution for the riser pipe of FIGURE 3C; and

FIGURE 4 has curve useful in explaining the phenomena of the improved stress characteristics of the riser pipe of this invention;

FIGURE 5 illustrates a modification of the slip joint of the embodiment of FIGURE 1.

Attention is first directed to FIGURE 1 which illustrates a preferred form of the invention. Shown thereon is a floating vessel supported by a body of water 12 which has a floor or bottom 14. The body of water may be the Gulf of Mexico or generally any open body of water sufficiently large and deep to support a floating drilling vessel or barge. The vessel 10 is anchored by means not shown above a well in the ocean floor which is cased with surface casing 16, for example. Surface casing 16 is attached to a pad or other anchor means 18. Surface casing 16 and pad 18 can be cemented or otherwise secured to the ocean floor so that in effect they form an anchor. Mounted at the top of surface casing 16 is a well head assembly 20 which includes blow-out preventers. Mounted on the upper end of well head assembly 20 is the lower portion of a ball and socket joint 22 which mates with the lower end of the riser pipe assembly and which will be discussed next.

Vessel 10 has a well or moonpool 24 in which is suspended riser pipe 26 by cable 28. No constant tensioning device is ordinarily needed in the system of this invention.

Riser pipe assembly 26 includes an upper section 30 which is supported from vessel 10 and moves vertically with the movement of vessel 10, and a lower section or stub 32 which has a socket assembly which mates with the ball assembly of swivel joint means 22. Stub 32 then can Pivot with respect to well head assembly 20 but cannot move longitudinally therewith.

The lower end of upper section 30 of the riser pipe is provided with an extension 34 which is of reduced diameter from the upper portion 36 thereof. Typically, extension 34 is 3 to 6" smaller in diameter than upper portion 36. An annular member 38 connects and closes the space between the upper end of extension 34 and the lower end of upper riser pipe portion 36. The upper side of annular member 38 forms a shoulder 40. Annular member 38 need not be flat. Extension 34 extends into lower section 32. Sealing means 42 are provided between lower section 32 and extension 34. The inside diameter of lower section 32 can be of a diameter to give a sliding fit between it and the Outside diameter of extension 34. Alternatively, the section 32 can be made about the same diameter of upper portion 36. In this case it is desirable, as shown in FIGURE 1, to provide centralizing means 44. Extension 34 is of sufficient length to accommodate the maximum expected vertical movement of the ship, e.g., 30 to feet. Stub 32 is of sufficient length to accommodate extension 34.

The resistance to bending of a joint of pipe suspended vertically in a body of water is dependent upon the diameter of that pipe and also upon its wall thickness. Generally speaking, the larger the diameter and the thicker the wall, the greater the resistance to bending. Therefore, it may be desirable to make pipe extension 34 and stub 32 of greater thickness than the thickness of portion 36 so that it can withstand essentially as much bending H stress as can portion 36.

We shall next consider briefly the modification in F IG- URE 5 which is used to give greater resistance to bending of the riser pipe system at the slip joint. The embodiment of FIGURE 1 is modified by having a stiffener exextension 90 extended from upper section 36 down over stub 32. The internal diameter of stiffener 90 is slightly larger than the external diameter of stub 32 so that stiffener extension 90 can have a sliding, telescopic relationship with stub 32. If desired, bearings 92 are provided to reduce the friction as the two slide with respect to each other. The upper end of stiffener extension 90 is welded at 94 or otherwise secured to the lower end of the upper section of the riser pipe 36. The annulus 96 between stiffener 90 and extension 34 is in free fluid communication with the exterior of the device through ports 98. Alternatively, bearings 92 can be intermittently circumferentially spaced in the annular space between stiffener extension 90 and stub 32 to permit fluid communication between annulus 96 and the exterior of the riser pipe assembly. The thickness of stiffener is made sufficient to greatly increase the resistance to bending of the riser pipe system in the vicinity of the slip joint.

We shall now briefly consider the mathematical approach for determining the stress of a riser pipe along its depth when suspended in water from a vessel. This approach is helpful in showing advantages of my riser pipe system over prior type riser assemblies. The standard beam stress equation is slightly modified to account for the axial load. The equation which is applicable to the riser pipe of this invention is given in Equation 1:

(l d y d (it!) where y defiection of the riser x=incremental length along the riser EI=the flexural rigidity or bending stiffness of the riser P=the axial load in the riser q=the lateral load which can be wave load, current load,

etc.

El, P and q can vary with x. E=Youngs modulus of the material used in the riser. I=bending moment of inertia.

When P, q and EI vary with x, a closed form mathematical solution is not available. To get an approximate solution of the equation, the diflerential equation has to be rewritten using finite differences to obtain a difference equation form. Techniques for rewriting are well known; for example, see Methods of Mathematical Analysis and Computation, J. G. Herriot, John Wiley and Sons, New York, l963. This difference equation can then be solved on a digital computer applying appropriate boundary conditions to come up with an approximate numerical solution. In applying this solution to the riser pipe of this in vention, the axial load (P) must account for the buoyancy of the riser pipe in the surrounding Water medium and also for the drilling mud inside the pipe. Refrences discussing the axial load (P) are Influence of Tension and Compression on Straightness and Buckling of Tubular Goods in Oil Wells, Proceedings, Thirty-first Annual Meeting, American Petroleum Institute, Section IV, Production, vol. 31 (1951) and The Influence of Pressure on Buckling and Straightness of Tubular Goods and Rods in Wells, both by Arthur Lubinski, and Helical Buckling of Tubing Sealed in Packer, by Arurthr Lubinski, W. S. Althouse and J. L. Logan, Transactions of AIME, vol. 225, p. 655.

As an example, let us consider a riser pipe illustrated in FIGURE 3A and plot curves illustrating the distribution of axial force in the riser and also the distribution of virtual, or effective, buckling force in the riser. If we modify the riser so that it has diameter a for its entire length, as shown in FIGURE 3C, the actual force in the riser itself is illustrated in FIGURE 4 by the line AB. The riser is supported at the top by the ship, and the force increases from zero at the bottom to the maximum at the top, OB, which is a force equal to the full weight of the riser less the weight of water displaced by the riser. There is another force, however, called the virtual buckling force. Due to the drilling fluid inside being heavier than the water, the virtual, or effective, buckling force is as indicated by the line FB. We can see, then, there is an effective compressive force in the riser at the slip joint depicted by the magnitude AF which is equal to the pressure difference between the drilling fluid inside and the water outside at the slip joint times the area depicted by the diameter (1.

Attention is still directed to FIGURE 4 now to see the improvement gained by the riser illustrated in FIGURE 1 and in FIGURE 3A where we have both the diameter a and diameter A. The actual tension in the riser is depicted by the curve ACDE. Point C is the level at which the diameter of the riser pipe changes from a to A. The jump in tension at level C to D is equal to the pressure difference between the drilling fluid inside and the water outside multiplied by the difference between the areas of diameter A and a at level C. But, again, due to the drilling fluid inside being heavier than the water outside, the curve of virtual, or eifective, buckling force is as shown by the curve FGE. The tension represented by OE at the surface results from the suspended weight of the pipe in water plus the tensile force resulting from the differential pressure (diflerence in pressure of drilling fluid and water outside riser system) acting downward on the differential area (A-a) or the area of annular shoulder 68. Efiectively, the change in tension does not occur in one spot, as depicted by the line CD. Instead, it occurs gradually over the length of the riser as the diiierential pressure of the drilling fluid in the riser and that of the sea water decreases from a maximum at the slip joint to zero at the surface. Thus, in increasing the diameter of the riser from a to A, I do not change the effective buckling force at the slip joint, but I do change the effective force from this diameter change upwards, such that a shorter length of the riser is under an effective compression load for bending as indicated at points H and I on line 0. AI in my system is much less than AH in the prior system. A second advantage of the increased diameter is the increased bending stifiness to resist the buckling and bending due to wave load.

The total effect of these two advantages is illustrated by the curves presented in FIGURE 3. Dotted curve 50 and broken line curve 55 show the total stress in a riser whose diameter is uniform for the entire length as illustrated in FIGURE 3C and curve 52 shows the stress for a riser of my invention with the two diameters. These curves were calculated by the method described above for the riser pipe in which a=16 and A=l9" for curve 52, a=16 for curve 50, and [1:19 for curve 55. In each case the material of the riser pipe was the same quality steel and had a thickness of /2". The drilling mud weight was 14 pounds per gallon. The same water wave period and height and the same water current data were used for each riser system represented; the q for the riser having A of 19" had a larger lateral force than in the example in which the riser pipe had a diameter a of 16" for its entire length but the same in which a=l9". It is seen, from FIGURE 33, that the stress in the lower portion of the riser is decreased by a factor greater than 2, by increasing the diameter over the upper portion of the riser from 16" to 19".

If the riser has a diameter which is 19" for its entire length as illustrated by diameter a in FIGURE 3C, the stresses take the shape of broken line curve 55. This is the same diameter as the diameter A of FIGURE 3A. However, in this variation of FIGURE 3C, there is no shoulder member as in FIGURE 3A. The curve 55 shows that there is considerably more stress on about the lower two-thirds of the riser pipe of FIGURE 3C over that of the embodiment of FIGURE 3A. It should be noted that for each curve in FIGURE 3B there is no outside tensioning applied at the top of the riser pipe at the ship.

Attention is next directed to FIGURE 2 which illustrates another embodiment of my invention, Shown thereon is vessel having well 24 and from which is suspended the main riser pipe section 60. The well head assembly is not shown in FIGURE 2 as it is illustrated-in FIG- URE 1. The slip joint in FIGURE 2 is provided between main riser pipe section 60 and lower section or stub 62. Main section 60 is sufliciently larger than lower section 62 so as to provide annular member 64 which is attached to section 60. The inside diameter of the annular member 64 is approximately the same size as the outer diameter of section 62. Sealing means 66 can be provided if deemed desirable. However, a small amount of leakage is not too harmful. The upper side of annular member 64 forms an annular shoulder 68. This shoulder 68 etfectively causes a compression force in the riser pipe 60 at the slip joint or at its lower end depicted by the magnitude AF (of FIGURE 4 which was previously discussed) which is equal to the pressure of the drilling fluid at the slip joint times the area of shoulder 68. It is seen that the embodiment of FIGURE 2, similarly as the embodiment of FIGURE 1, provides an improvement or reduction of total stresses, particularly at the lower end of section 60, from that in which the riser pipe has no annular shoulder 68, i.e., the inside diameter of the upper portion of the telescopic joint being approximately the same as the outer diameter of the lower joint.

In operation of the embodiments of this invention, a drill pipe 70 is suspended in the riser pipe, as illustrated in FIGURE 2, and drilling fluid circulates downwardly through the drill pipe and upward in the annular member. The upward traveling fluid carries with it cuttings of rock made by the bit. These cuttings may have a tendency to collect in the annular space 72 between stub section 62 and larger diameter riser pipe 60. This problem can be eliminated or greatly reduced by providing for the circulation of fluid through port 74 just above shoulder 68. This is conveniently accomplished by providing a longitudinal pipe 76 which may be tied to riser pipe 60 by tie or clamp. This pipe 76 goes to a pump 80 on ship 10. Drilling fluid is then circulated through line 76 to carry cuttings upwardly through and out of annulus 72. An annular baffle 82 provides for more even distribution of the axial fluid throughout the annulus 72. This auxiliary fluid is preferably the same type drilling fluid as that being circulated downwardly through drill string 70.

While the above embodiments have been described with a great deal of detail, it is possible to produce modifications thereof without departing from the spirit or scope of the invention.

I claim:

1. A riser pipe device for use in drilling a well from a rig floating on a body of water which comprises.

an upper section of pipe supported from the ship and having a diameter A;

a lower section of pipe having a diameter a which is less than A and supported at its lower end from the floor of said body of water, said lower pipe extending into said upper pipe;

an annular member having an outer diameter A and an inner diameter of about a; the circumference of said annular member sealingly fixed to the lower end of said upper pipe, the space above the upper side of said annular member being in fluid communication with the interior of said upper section of pipe, the interior opening of said annular member sealingly fitting the outer surface of said lower pipe in a sealing and sliding relationship.

2. A device as defined in claim 1 including drilling fluid within said device, the density of said drilling fluid being greater than the density of the water surrounding said system, and including means to circulate drilling fluid through the annular space between said upper section of pipe and said lower section above said annular member.

3. A device as defined in claim 1 in which the thickness of the wall of said lower section is greater than the thickness of the Wall of said upper section.

4. A device as defined in claim 1 including conduit means supported by said upper section of pipe and opening into said annulus between said lower section and said upper section of pipe adjacent said annular member; and

means to direct a fluid downwardly through said conduit means.

5. A device as defined in claim 4 in which said means are provided in said annulus in said lower section between said upper section of pipe for distributing fluid circurnferentially around said annulus above said annular member.

6. A riser pipe assembly for use with a drilling rig floating on a body of water which comprises:

an upper section of pipe having an upper portion of diameter A and an extension of diameter a in which a is less than A, there being formed an annular space between the lower end of said upper portion of said upper section of pipe and said extension;

an annular member enclosing the annular space between the upper end of said extension and the lower end of said upper portion, the upper side of said annular member being in fluid communication with the interior of said upper section of pipe;

a lower pipe stub supported from the floor of the body of water, said lower pipe stub being of greater diameter than said extension of said upper section for receiving said extension in telescopic relation;

sealing means between said extension and said pipe stub.

7. An apparatus as defined in claim 6 in which said assembly contains a drilling fluid having greater density than that of the surrounding body of water.

8. An assembly as defined in claim 6 in which said extension has a wall thickness greater than that of said upper portion.

9. An assembly as defined in claim 6 including a hollow stiffening extension connected to the lower end of said upper portion of said upper section of pipe, said stiflening extension being of about the same length as said extension of diameter a, said stiffening extension extending downwardly over said lower pipe stub in a telescoping relationship therewith, and further including means establishing fluid communication between the interior of said stifiener extension and the exterior thereof.

10. An assembly as defined in claim 9 including bearings between the interior of the lower end of said stiffening extension and the exterior wall of said pipe stub and in which said stiflening extension has ports in the wall thereof.

11. A riser pipe assembly for use in drilling through an anchored sub-sea well head from a floating platform on a body of water which comprises:

an elongated hollow member extending from the vessel to a point above the well head, said elongated member having an internal annular shoulder at its lower end, the upper side of said annular shoulder being in fluid communication with the full length of said hollow member above said annular shoulder;

slip joint and conduit means between said annular shoulder and said well head to compensate for vertical movement of said elongated member with respect to said wellhead While maintaining sealed fluid communication between the well head and the interior of said elongated hollow member.

12. A device in claim 11 including drilling fluid within said device, the density of said drilling fiud beng greater than the densty of the water surrounding said system, said drilling fluid and the weight of said elongated hollow member providing the only tension to said hollow member.

13. A device as defined in claim 11 in which said slip joint and conduit means include (a) a vertical extension, the upper end of which terminates at and is sealingly fixed to the inner bore of said internal annular shoulder and (b) a lower pipe stub supported from the floor of said body of water, said pipe stub and said vertical extension having a telescoping and sealing relationship with each other.

14. A riser pipe device for use in drilling a Well from a rig floating on a body of water which comprises:

an upper section of pipe supported from the ship and having a diameter A;

an annular member having an outer diameter A and an inner diameter of about a; the circumference of said annular member sealingly fixed to the lower end of said upper pipe;

a drilling fluid within said device acting downwardly on said annular shoulder, the density of said drilling fluid being greater than the density of the water surrounding said system.

References Cited UNITED STATES PATENTS 2,606,003 8/1952 McNeill -7 3,179,179 4/1965 Kofahl 166.5 3,195,639 7/1965 Pollard et a1 166.5 3,211,224 10/1965 Lacy 166.5 3,313,345 4/1967 Fischer 166.5 3,313,358 4/1967 Postlewaite 175-7 3,354,950 11/1967 Hyde 166.5 3,372,745 3/1968 Holmes 166.6

CHARLES E. OCONNELL, Primary Examiner RICHARD E. FAVREAU, Assistant Examiner US. Cl. X.R. 175-7