GB2365044A

GB2365044A - System for drilling a subsea well

Info

Publication number: GB2365044A
Application number: GB0117529A
Authority: GB
Inventors: John T Wall; Louis Darnell Slaughter; Albert Michael Regan
Original assignee: Stewart and Stevenson Inc
Current assignee: Stewart and Stevenson Inc
Priority date: 2000-07-18
Filing date: 2001-07-18
Publication date: 2002-02-13
Also published as: GB0117529D0

Abstract

A system for use in the drilling of a subsea well includes a drilling riser which extends from the drill ship to a lower marine package installed on a blowout preventer on the upper end of the subsea wellhead. The riser includes a riser pipe having a line ML carried by and extending along its side to return drilling fluid from the annulus about the drill string extending through the package to the drill ship, and another line NL also carried thereby and extending along its side to inject gas from a source at the drill ship into the drill fluid for commingling therewith as it is returned thereby lightening the head of drill mud in the well bore. The riser pipes making up the marine riser are connected end to end by bolted flanges to which the ends of the lines are connected the flanges having an outer side edge which is generally oval shaped to reduce its weight.

Description

<Desc/Clms Page number 1> SYSTEM FOR DRILLING A SUBSEA WELL This application relates to a system for drilling deep water subsea wells, especially using dual gradient drilling techniques, but also having advantages that can be applied to more conventional subsea wells drilling techniques. In one of its aspects, it relates to such a system having a improved mud return system which enables these techniques, and, in another of its aspects, an improved marine riser made up of end to end bottom riser pipes for use in such a system.

In the conventional drilling of a subsea well, the wellhead at the ocean floor is connected to the drilling vessel at the surface by a riser system, which consists of a blowout preventer stack, a lower marine riser package, a riser string made up of the individual riser joints, a telescopic joint and diverter. The purpose of the riser system is to control the well at the ocean floor, to guide the drill string and other tools into the wellhead, and to return the drilling mud and cuttings back to the drilling vessel. The weight of the riser string is usually supported by a combination of buoyancy modules at the individual riser joints and the tensioning equipment attaching the riser to the

drilling vessel. A component of the tension requirements is the weight of the mud column in the riser string, which runs through the drilling riser. As the well is drilled, drilling fluid is circulated through the drill string out the bit and carries the cutting up the annulus between the drill string and the riser back to the drilling vessel for processing.

The drilling fluid performs several functions. It cools, lubricates and cleans the drill bit and caries the drilling cutting to the surface. The mud's hydrostatic weight controls the well bore pressure. The mud is an engineered fluid, so that if the hydrostatic pressure is too high, the drilling mud may be lost to and damage the formation. If the hydrostatic pressure is too low, formation fluids and gases may blow into the well, where they are controlled by the BOP equipment.

In deep water wells, it is difficult to control the hydrostatic head of the drilling mud. Small changes in the density of the drilling mud can cause large differences in down-hole pressure. Procedures have been proposed to address this problem. In one such procedure, known as riserless drilling, the drilling mud is returned to the drilling vessel by either being pumped and/or gas lifted from the subsea BOP to the surface. This is an attempt to remove the effects of the hydrostatic head from the top of the wellhead. This is usually accomplished by having a second separate mud return riser in addition to the drilling riser in the water at the same time. Since most drilling vessels can only handle one riser at a time, this is a time consuming and difficult task.

A second procedure proposed is known as underbalanced drilling. In underbalanced drilling, nitrogen gas is added to the drilling fluid. The density of the aired up mud can then be controlled to give the desired hydrostatic head at the subsea wellhead. In this system, the mud returns to the drilling vessel in the annulus between

the drill string and the riser. A rotating BOP is required at the surface to contain the gas in the annulus. In true underbalanced drilling, the internal pressure is less than the formation pressure. Under these conditions, the rate of drill penetration is greatly increased.

A further technique, which is preferred by many skilled in the art, is dual gradient drilling, wherein the returning column of drilling fluid is lightened, as compared with the density of the drilling fluid in the drill string.

An object of the invention is to provide a drilling riser system having an improved procedure for handling the returning drilling mud; which, in accordance with one embodiment of the invention, involves dual gradient techniques, but without the shortcomings of the above described procedures, and, in accordance with both embodiments, also involves the use of a booster line carried by and extending along the side of the riser for injecting gas into the returning drilling mud line. In both embodiments, returning drilling fluid is also deverted from the annulus into a mud return line carried by and extending along the outside of the riser, thus reducing the hydrostatic head on the formation. Also, a pump is located in a conduit connecting the annulus with the mud return line.

This avoids the need for a second riser in riserless drilling and the obvious risks of collapse of the riser pipe due to underbalanced when drilling, and also reduces the flow area available to the return mud, which increases the velocity of the mud, thus removing the cuttings from the well bore with less risk of their dropping out. The smaller column of returning mud also reduces the tensioning requirements of the rig as well as reducing the quantity of drilling mud to be carried on the rig. The system is environmentally more friendly than conventional drilling.

In the case of an unintentional drive off, it reduces the amount of mud that would be lost to the ocean floor. In fact, a mud return line valve could be closed saving the mud in the line. In the case of a planned disconnect at the lower marine riser package, less time would be required to displace the mud in the mud return line than in conventional drilling. Further, in accordance with one aspect, the mud is returned through a novel system which is a part of and has lines connected end to end with the riser joint in the same manner as the other flow lines, such as choke and kill.

In the drawings wherein like reference characters are used throughout to designate like parts: Fig. 1 is a side view of a drilling system constructed for operation in accordance with a first embodiment of this invention, and including a drill ship supporting a rig from which the marine riser, including the mud return line, is suspended as their sections are made up and broken out for connection to and disconnection from the upper end of the LMRP as it is run or retrieved from the ocean floor for connection to the subsea wellhead on the ocean floor; Fig. 2 is an enlarged side view, partly in section, of the lower marine riser package of Fig. 1, which is broken away in part to show the drill string extending through a riser adapter connected to the lower end of the lowermost riser joint, and an isolation tool closed upon the drill string to divert the return drilling mud within the annulus into and through a conduit for extension upwardly to connect with the lower end of a port in the riser adapter leading to a mud return line, as well as to show a conduit connecting with the lower end of a another port in the riser adapter in order to inject nitrogen or other gas from a source on the drill ship into the mud return line; Fig. 3 is an enlarged vertical sectional view of the adapter.

Fig. 4 is a plan view of the lower end of the flange of the lower marine riser joint, as seen along 3-3 of Fig. 3.

Fig 5. Is a plan view of an alternative embodiment of the flange of the lower riser joint, also taken along line 3-3; Figs 6, 6A and 6B are side and end view of a joint of the marine riser; Fig. 7 is a side view similar to Fig. 2 of drilling system for operation in accordance with a second embodiment of the invention.

Fig. 8 is a view similar to Fig. 3, but showing details of the adapter of the second embodiment.

With reference now to the details of the above described drawings, a drill ship DS is shown in Fig. 1 to support a drilling rig including a derrick above an opening or moon pool in the vessel. As well known in the art, the rig includes suitable apparatus for use in running and retrieving the riser string to and from the ocean floor as the drilling riser joints DR are made up or broken out within the derrick. As previously mentioned, the lower end of the riser string is connected to a riser adapter RA at the upper end of the lower marine riser package LMRP. The LMRP connects to the BOP stack, which in turn is connected to the wellhead on the ocean floor F, thus completing a through passageway from the well bore to the drilling rig. The upper end of the riser string RS terminates through a telescopic joint TJ to the diverter D mounted on the drilling vessel. As shown, the weight of drilling riser joints DR is supported by buoyancy modules BM disposed about them. Additional tension to support the riser is applied by riser tensioners RT at the telescopic joint TJ.

As well known in the art, the drill string is made up of successive joints of drill pipe which, like the marine riser, are made up at the drill floor. As the drill string is

rotated within the well bore, drilling mud is circulated downwardly through the drill string and out the bit within the well bore, and upwardly within the annulus between the drill string DS and the well bore and, as will be described, into the annulus between the drill string and riser.

As best shown in Fig. 2, the lower marine riser package includes a flex joint (FJ) connected to the lower end of the riser adapter (RA), and an isolation tool (IT) connected to the lower end of the flex joint to provide a continuation of the bore through which the drill string extends. The isolation tool can be placed anywhere in the LMRP depending on system requirements. A blowout preventor (BOP), which may be of an annular type, is connected to the lower end of the isolation tool and above the LMRP connector (C) to the subsea wellhead. The blowout preventer may be of well known construction, and the isolation tool (IT) includes a housing having a bore therethrough and an packer (PA) mounted in a recess about the bore for movement laterally between positions opening the bore and an inward position closed about the drill string (DS), as shown in Fig. 2. The tool must pass the drill collars and may be a rotating BOP, for example, to permit the drill string to be rotated as the annular passageway about it is essentially closed.

Closure of the packing (PA), as shown in Fig. 2, diverts drilling fluid passing upwardly through the annulus in the LMRP into an outlet 10 from the bore of the housing beneath the packer, which in turn connects with a fitting 11 connected at its lower end to another conduit 12 extending downwardly into a pump P. A further conduit 13 connects the upper end of the pump to the lower end of a side outlet 14 in the body of the riser adapter (RA). Thus, as described, drilling mud is diverted upwardly through a lower extension of the mud return line (ML) stabbed into the flange on the lowermost

riser joint connecting to a port in the upper end of the riser adapter (RA).

In the event it is desired to convert the system to a conventional type, the packing is opened, and valves in the side outlet are closed to cause the drilling mud to return to the surface through the annulus in the riser string. However, for dual gradient drilling, another conduit 16 connects the fitting beneath the lower end of another port in the upper flange of the adapter body which is stabbed by a lower extension of a gas or nitrogen line (NL) in the flange of the riser joint above it. As previously described, this enables nitrogen gas, or other fluid medium substantially lighter than the drilling mud, to be circulated downwardly from a source on the drill ship for injection into the drilling mud diverted into conduit 12 beneath the isolation tool for commingling therewith. This conduit preferably occupies a position of the mud booster line in a conventional system, and may be used for that purpose in conventional drilling.

As previously described, the adjacent flanges of adjacent ends of the riser joints (RJ) and between the lowermost riser joint and upper end of the riser adapter (RA) are connected in end to end relation at the drilling rig by means of bolts B which extend through their aligned bolt holes in bolt circles in the flanges.

As shown in Fig. 3, the lower end of a joint of the mud return line ML is sealably stabbed into a hole in the upper flange of the riser adapter (RA) which connects with the mud return conduit 13. In addition, and as shown in Figs. 4 and 5, conventional choke and kill lines (CL and KL) are stabbed into holes about the bolt circle intermediate adjacent bolts on opposite sides of the mud return line (ML). The lower ends of these lines, as well as the lower end of the nitrogen line (NL), are adapted to be stabbed into the holes similarly designated in the riser adapter (RA) as the flanges are bolted together at the

drilling rig.

In addition, there are other lines for directing the hydraulic fluid downwardly to the well for various purposes such as booster lines (B), and having their lower ends stabbed into holes in the flange of the upper joint (RA) for connection to one another, often used in conventional drill techniques as additional lines for injecting further mud into the mud return line. Still further, there is a lifting eye (E) on the flange of the riser joint for handline purposes which for weight distribution purposes, is on a diameter extending through the centers of the mud line and bolt circle.

Ordinarily, and as shown, the mud return line is of substantially greater diameter than the auxiliary choke and kill lines which are disposed on their opposite sides. As a result, if the flange were of a circular configuration, as is conventional, there would be considerably more untapped area than required to accommodate the choke and kill lines. Thus, as previously described, and in accordance with another novel aspect of the invention, the flanges of each of Figs. 4 and 5 are of generally and oval or elongated shape with a substantially longer longitudinal or horizontal dimension than lateral or vertical dimension - i.e., the line 3-3 on a diameter DL extending through the axis of the mud line is greater than the axis of DV extending vertically thereof. As also previously described, this enables the weight of the flange, and thus the overall weight and size of the riser joint (RJ) to be substantially smaller than might otherwise be required. In addition, the lower weight of the riser reduces the buoyancy requirements, further reducing the weight and cross-section of the assembled riser joint (RJ). In this embodiment of Fig. 4, the choke and kill lines are on the left side of the vertical diameter with their centers equally angularly disposed with respect to the vertical diameter.

The nitrogen line to the right of DV, and the lifting eye (E) to the right diametrically opposite to the mud return line. As also shown, the inner side of the line MR extends inwardly of a filet which joins the riser pipe to the flange. This may also be true of the choke and kill lines which are larger than the bolt holes, and, to some extent, of the nitrogen line (NL). A lifting eye (E) is shown in a diametrically opposed position to the mud line, thus better distributing the weight on the flange and thus the riser joint, or, in this case on the riser joint.

The flange, which is of generally oval shape, may be formed from a conventional flange with circular shape, with portions top and bottom by cuts made on an axes spaced vertically from the one shoulder.

As compared to the flange illustrated in Fig. 4, the alternative flange of Fig. 5 has choke and kill lines located on the right side of a vertical diameter, but nevertheless spaced from one another at equal angles with respect to one another. In this case, with the mud return line and choke and kill lines, as well as the bolt holes and holes for other lines, being formed in a generally oval shaped flange accomplishes advantages noted above with respect to the flange of Fig. 4. In fact, the embodiment of Fig. 5 may be preferred in that it tends to better balance the weight of the mud return line with that of the choke and kill lines.

The second embodiment shown in Fig. 7 is similar to the first embodiment in that it includes a riser adapter RA' at the upper end of a lower marine riser package which connects to the BOP `, which in turn is connected to the wellhead at the ocean floor, thus completing a through passageway from the well bore to the drilling rig. As in the first embodiment, the upper end of the riser string DS' terminates

at a telescopic joint TS mounted to the diverter mounted on the drilling vessel. Also, the lower marine riser package includes a flex joint FS connected to the lower end of the riser adapter RA' with the BOP', and a spool S connecting the lower end of the flex joint the BOP' which may be of the annular type. The spool thus occupies generally the same position as the isolation tool of the first embodiment, for reasons which we apparent to follow.

In this second embodiment, a side outlet 10' connects the bore of the spool S which the drill string extends, and thus with the returning drilling fluid in the annulus about the drill string to a fitting 11' connecting with a conduit 12' leading to a pump P'. A further conduit 13' connects the upper end of the pump with the lower end of a side outlet 14' in the lower end of the riser adapter RA'.

Thus, returning drilling fluid may flow upwardly from the annulus about the drill string through the outlet 10' and conduits 12' and 13' into the side outlet 14' of the riser adapter RA' whereby returning drilling fluid within the annulus may flow to the surface through mud return line ML' carried by and to the side of the riser, similarly to that of the first embodiment. As in the first embodiment, the lower extension of the mud return line is stabbed into the flange on the lower most riser joint which connects to a port in the upper end of the riser adapter RA'. Thus, drilling fluid, whose hydrostatic head may be so great as to otherwise damage the formation in the well bore, is lightened as it passes from the wellhead to the surface through the mud line ML'.

Another conduit 16' connects the fitting in the lower end of another port in the upper flange of the riser adapter RA', and is stabbed by the lower extension of a gas or nitrogen line NL' in the flange of the riser joint above it. More particularly, the lower end of the

conduit in turn connects with a side passage 17' leading to the bore of the adapter and thus to the annulus about the drill string above the side outlet 10'. A valve V is installed in the conduit 16' above the outlet 17' to selectively permit the gas to be injected into the annulus when the valve is open, and thus into the returning drilling fluid about the drill string so as to lighten its hydrostatic head of the drilling fluid in the annulus, as desired.

There is also a valve 11' in the conduit 12' which, when open, causes otherwise diverted drilling fluid to flow upwardly within the annulus about the drill pipe. Upon closing of the valve V in the conduit 16' to the injection of nitrogen or other gas, the drilling fluid may be returned through the adapter and the mud line ML'. On the other hand, and as previously described, in order to lighten the hydrostatic head of the returning drilling mud, the valve 11' is open to permit flow from the annulus through the pump P' and into the annulus about the drill string.

As in the first embodiment, the adjacent flanges of the adjacent ends of the riser joint between the lower joint and upper end of the riser adapter RA' are connected by bolts which extend through line bolt holes in bolt circles in the flanges. The lower end of the mud return line is stabbed into a hole in the upper flange of the riser adapter RA , as in the first embodiment. Similarly, the conventional choke and kill lines of the alternative second embodiment of the system may be stabbed into holes in the bolt circle intermediate adjacent bolt in opposite sides of the mud return line, as described in connection with the first embodiment. This is also true of the lower end of the booster line or nitrogen line NL' whose lower ends are stabbed into holes in the flange in the upper joint for connection to one another.

As is also true of the first embodiment, the mud return line ML' is of substantially greater diameter than the auxiliary choke lines which enables the same advantages to the system as described in connection with the first embodiment.

As will be apparent from the drawings, the mud line Wand booster or nitrogen line NL' are carried by and along the side of the riser joint of the second embodiment for extension to the drill ship for purposes previously described. The booster or nitrogen line NL' may be arranged as shown in the drawings of the first embodiment. Thus, as will be apparent, the use of the novel flanges in the second system have the advantages described in connection with the first system.

Claims

CLAIMS 1. A marine riser joint for use in the drilling of a subsea well, comprising a riser pipe having a flange at each end with a circle of holes therein to receive bolts for connecting them in end to end relation with flanges of adjacent joints, and a hole of larger diameter therein adjacent a pair of bolt holes in each flange and axially aligned with the larger diameter hole of the other flange of the joint, a drilling mud return line having its ends received in the larger holes, and the centers of the larger holes being outside the centers of the bolt circle, the length of a diameter extending through the outer edges of the flanges and the center of the larger hole being greater than that between the outer edges along another diameter perpendicular thereto, and additional holes in each flange to receive the ends of other pipes of larger diameter than the bolt holes and having their centers axially aligned with one another and arranged generally within the bolt circle.

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2. As in claim 1, wherein the other lines include choke and kill lines of larger diameter than the bolt holes but smaller than that of the mud return line. 3. As in claim 2, wherein the centers of the choke and kill lines have radii forming generally equal angles with the long diameter on opposite sides thereof. 4. As in claim 1, wherein the outer edges of each flange are curved to essentially an oval shape. 5. As in claim 4, wherein the outer sides of each flange include opposite curved ends bowed about the long axis and interconnecting sides curved about the transverse axis.

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6. A marine riser joint for use in the drilling of a subsea well, comprising a riser pipe having a flange at each end with a circle of holes therein to receive bolts for connecting them in end to end relation with flanges of adjacent joints, and a hole of larger diameter adjacent a first pair of bolt holes in each flange and axially aligned with the larger hole of the other flange of the joint, a drilling mud return line having its ends received in the larger holes, and a lifting eye on the top side of the top flange of each joint adjacent a second pair of bolt holes, the centers of the larger holes being outside the centers of the bolt circle and disposed generally within a diameter of flange which extends through the center of the bolt circle, the distance between the outer edges of the flanges along said diameter being greater than that between the outer edges along another diameter perpendicular thereto, additional pairs of holes in each flange having their centers arranged generally within the bolt circle and between adjacent bolt holes with the centers of each pair axially aligned, and choke and kill lines extending between the flanges each to connect with aligned holes of one pair and with their centers on radii forming generally equal angles with the long diameter on opposite sides thereof.

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7. As in claim 6, wherein the pipe is connected to each flange by a concave filet, and the radially inner edges of the large hole and the holes for the ends of the choke and kill lines overlap the outer edge of the filet. 8. As in claim 6, wherein the flanges have further axially aligned holes each intermediate to other bolt holes to receive the ends of another fluid line. 9. As in claim 6, wherein the outer edges of each flange are curved to essentially an oval shape. 10. As in claim 6, wherein the choke and kill line are on the same side of the transverse axis.

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11. A system for use in the drilling of a subsea well, comprising a drilling riser adapted to extend downwardly from a drill ship, and including a riser pipe and first and second fluid lines mounted on and extending along the sides thereof, a lower marine riser package for connecting the lower end of the riser with a BOP on the upper end of the subsea wellhead, and including an adapter at the upper end of the package having a bore therethrough connecting with the bores of the lower end of the riser pipe and the portion of the package beneath the adapter leading to the subsea wellhead, and a pair of ports each connecting with the lower end of a fluid line, and a pair of conduits each connecting at its upper end to another port of the adapter, the other end of a first conduit connecting to the first fluid line so as to return drilling fluid upwardly from the annulus about the drill string to the drill ship ,and the other end of the second conduit connecting with the first conduit to commingle a fluid medium from the surface with the drill fluid for return with it to the surface.

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12. As in claim 11, wherein the second line connects with means at the surface which may be used in lifting the drilling fluid upwardly through the first conduit. 13. As in claim 12, including a subsea pump in the first conduit for lifting the returning fluid in the first line. 14. As in claim 11, wherein the second fluid conduit connects at its upper end with a source of medium of lesser density than that of the drilling fluid, so that the medium may be introduced through the second conduit in the drilling fluid. 15. As in claim 11, wherein the lower marine riser package includes a housing connected beneath the adapter and having a bore therethrough connecting the well bore with that of the _ adapter bore, and a side outlet connecting its bore with the second conduit and means on the housing for selectively moving between an outer position fully opening its bore and an inner position closing the bore about the drill string for diverting the well fluid through the side outlet. 16. As in claim 13, wherein

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the lower end of the second conduit is connected to the first conduit to enable commingling of the medium with the drilling fluid, and there is a pump in the first conduit for use in lifting the drill fluid within the first line. 17, As in claim 11, including a source of gas at the drill ship for supply to the second conduit and thus to the returning drill fluid. 18. As in claim 17, wherein the gas is nitrogen. 19. As in claim 11, wherein there are flanges on the ends of adjacent riser pipes, and on the ends of the lowermost riser and the adapter, and a circle of bolt holes for connecting adjacent ends of the flanges to one another with the ends of the riser pipes and lines received in holes intermediate the bolts.

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20. A system for use in the drilling of a subsea well, comprising a drilling riser adapted to extend downwardly from a drill ship and including a riser pipe and a fluid line mounted on and extending along the side thereof, a lower marine riser package for connecting the lower end of the riser with a BOP on the upper end of the subsea wellhead, and including an adapter at the upper end of the package having a bore therethrough connecting with the bores of the lower end of the riser pipe and the portion of the package beneath the adapter leading to the subsea wellhead, and a port connecting with the lower end of the fluid line, and a conduit connecting the bore of the adapter with the lower end of the fluid line and having a pump therein so as to return drilling fluid upwardly from the annulus about the drill string within the adapter bore to the drill ship.

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21. As in 20, including another fluid line mounted on and extending along the side of the riser pipe, said adapter having another port connecting the lower end of the other fluid line and another conduit connecting the other port with the bore of the adapter so as to permit gas from the drill ship to be introduced into the returning drilling fluid in the annulas about the drill string. 22. As in 21, including a valve in the other conduit for controlling flow therethrough. 23. A marine riser joint substantially as described herein and illustrated in the accompanying drawings. 24. A system for use in the drilling of a subsea well substantially as described herein and illustrated in the accompanying drawings.