GB2457497A

GB2457497A - Flow stop valve controlled by pressure difference

Info

Publication number: GB2457497A
Application number: GB0802856A
Authority: GB
Inventors: George Swietlik; Robert Large
Original assignee: Pilot Drilling Control Ltd
Current assignee: Pilot Drilling Control Ltd
Priority date: 2008-02-15
Filing date: 2008-02-15
Publication date: 2009-08-19
Anticipated expiration: 2028-02-15
Also published as: US20130043045A1; US8776887B2; EP2260174B1; AU2009213898A1; US20110036591A1; EP2260174A2; AP3384A; MX347243B; MY164386A; BR122018072232B1; GB0802856D0; MX2010008983A; WO2009101424A4; WO2009101424A2; EP2469013B1; US20130220634A1; AU2009213898B2; EP2469013A3; BRPI0905918A2; AP2010005381A0

Abstract

A flow stop valve (20) is positioned in a downhole tubular (Fig.1c, 6), wherein: (A) the flow stop valve 20 is in a closed position when a pressure difference between fluid outside the downhole tubular (6) and inside the downhole tubular (6) at the flow stop valve 20 is below a threshold value, thereby preventing flow through the downhole tubular; and (B) the flow stop valve 20 is in an open position when the pressure difference between fluid outside the downhole tubular (6) and inside the downhole tubular (6) at the flow stop valve 20 is above a threshold value, thereby permitting flow through the downhole tubular (6). Alternatively the pressure differential may be measured internally to the tubular on either side of the flow stop valve 20.

Description

Flow stop valve This invention relates to a flow stop valve which may be positioned in a downhole tubular, and particularly relates to a flow stop valve for use in dual density drilling fluid systems.

Background

When drilling a well bore, it is desirable for the pressure of the drilling fluid in the newly drilled well bore, where there is no casing, to be greater than the local pore pressure of the formation to avoid flow from, or collapse of, the well wall. Similarly, the pressure of the drilling fluid should be less than the fracture pressure of the well to avoid well fracture or excessive loss of drilling fluid into the formation. In conventional onshore (or shallow offshore) drilling applications, the density of the drilling fluid is selected to ensure that the pressure of the drilling fluid is between the local formation pore pressure and the fracture pressure limits over a wide range of depths. (The pressure of the drilling fluid largely comprises the hydrostatic pressure of the well bore fluid with an additional component due to the pumping and resultant flow of the fluid.) However, in deep sea drilling applications the pressure of the formation at the seabed SB is substantially the same as the hydrostatic pressure HP of the sea at the seabed and the subsequent rate of pressure increase with depth d is different from that in the sea, as shown in figure 1 a (in which P represents pressure and FM and FC denote formation pressure and fracture pressure respectively). This change in pressure gradient makes it difficult to ensure that the pressure of the drilling fluid is between the formation and fracture pressures over a range of depths, because a single density SD drilling fluid does not exhibit this same step change in the pressure gradient.

To overcome this difficulty, shorter sections of a well are currently drilled before the well wall is secured with a casing. Once a casing section is in place, the density of the drilling fluid can be altered to better suit the pore pressure of the next formation section to be drilled. This process is continued until the desired depth is reached. However, the depths of successive sections are severely limited by the different pressure gradients, as shown by the single density SD curve in figure la, and the time and cost to drill to a certain depth are significantly increased.

In view of these difficulties, dual density DD drilling fluid systems have been proposed (see US200610070772 and W02004/033845 for example). Typically, in these proposed systems, the density of the drilling fluid returning from the welibore is adjusted at or near the seabed to approximately match the density of the seawater.

This is achieved by pumping to the seabed a second fluid with a different density and mixing this fluid with the drilling fluid returning to the surface. Figure lb shows an example of such a system in which a first density fluid 1 is pumped down a drillstring 6 and through a drilling head 8. The first density fluid 1 and any cuttings from the drilling process then flow between the well wall and the drilistring. Once this fluid reaches the seabed, it is mixed with a second density fluid 2, which is pumped from the surface SF via pipe 10. This mixing process results in a third density fluid 3, which flows to the surface within a riser 4, but is also outside the drillstring 6. The fluids and any drilling cuttings are then separated at the surface and the first and second density fluids are reformed for use in the process.

In alternative proposed systems, a single mixture is pumped down the drillstring and when returning to the surface the mixture is separated into its constituent parts at the seabed. These separate components are then returned to the surface via the riser 4 and pipe 10, where the mixture is reformed for use in the process.

With either of the dual density arrangements, the density of the drilling fluid below the seabed is substantially at the same density as the fluid within the drillstring and the density of the first and second density fluids may be selected so that the pressure of the drilling fluid outside the drillstring and within the exposed well bore is between the formation and fracture pressures.

Such systems are desirable because they recreate the step change in the hydrostatic pressure gradient so that the pressure gradient of the drilling fluid below the seabed can more closely follow the formation and fracture pressures over a wider range of depths (as shown by the dual density DO curve in figure la). Therefore, with a dual density system, greater depths can be drilled before having to case the exposed well bore or adjust the density of the drilling fluid and significant savings can be made.

Furthermore, dual density systems potentially allow deeper depths to be reached and hence greater reserves may be exploited.

However, one problem with the proposed dual density systems is that when the flow of drilling fluid stops, there is an inherent hydrostatic pressure imbalance between the fluid in the drillstring and the fluid outside the drillstring, because the fluid within the drillstring is a single density fluid which has a different hydrostatic head to the dual density fluid outside the drilistring. There is therefore a tendency for the denser drilling fluid in the drillstring to redress this imbalance by displacing the less dense fluid outside the drillstring, in the same manner as a U-tube manometer. The same problem also applies when lowering casing sections into the well bore.

Despite there being a long felt need for dual density drilling, the above-mentioned problem has to-date prevented the successful exploitation of dual density systems and the present invention aims to address this issue, and to reduce greatly the cost of dual density drilling.

Statements of Invention

According to a first aspect of the present invention, there is provided a flow stop valve positioned in a downhole tubular, wherein: (A) the flow stop valve is in a closed position when a pressure difference between fluid outside the downhole tubular and inside the downhole tubular at the flow stop valve is below a threshold value, thereby preventing flow through the downhole tubular; and (B) the flow stop valve is in an open position when the pressure difference between fluid outside the downhole tubular and inside the downhole tubular at the flow stop valve is above a threshold value, thereby permitting flow through the downhole tubular.

The threshold value for the pressure difference between fluid outside the tubular and inside the downhole tubular at the flow stop valve may be variable.

The flow stop valve may comprise: a biasing means; and a valve; wherein the biasing means may act on the valve such that the biasing means may bias the valve towards the closed position; and wherein the pressure difference between fluid outside the downhole tubular and inside the tubular may also act on the valve and may bias the valve towards an open position, such that when the pressure difference exceeds the threshold value the valve may be in the open position and drilling fluid may be permitted to flow through the downhole tubular. The biasing means may comprise a spring.

The flow stop valve may further comprise a housing, and a hollow tubular section and a sleeve located within the housing, the sleeve may be provided around the hollow tubular section and the sleeve may be located within the housing, the housing may comprise first and second ends and the hollow tubular section may comprise first and second ends, the first end of the hollow tubular section corresponding to the first end of the housing, and the second end of the hollow tubular section corresponding to a second end of the housing.

The hollow tubular section may be slidably engaged within the housing. The sleeve may be slidably engaged about the hollow tubular section.

The hollow tubular section may comprise a port such that the port may be selectively blocked by movement of the hollow tubular section or sleeve, the port may form the valve such that in an open position a flow path may exist from a first end of the housing, through the port and the centre of the tubular section to a second end of the housing.

A third abutment surface may be provided at a first end of the hollow tubular section such that the third abutment surface may limit the travel of the sleeve in the direction toward the first end of the housing. A flange may be provided at the second end of the hollow tubular section. A second abutment surface may be provided at the second end of the housing such that the second abutment surface of the housing may abut the flange of the tubular section limiting the travel of the hollow tubular section in a second direction, the second direction being in a direction towards the second end of the housing.

A first abutment surface may be provided within the housing between the second abutment surface of the housing and the first end of the housing, such that the first abutment surface may abut the flange of the hollow tubular section limiting the travel of the hollow tubular section in a first direction, the first direction being in a direction towards the first end of the housing.

A spacer element of variable dimensions may be provided between the second abutment surface of the housing and the flange of the hollow tubular section, such that the limit on the travel of the hollow tubular section in the second direction may be varied.

The spring may be provided about the hollow tubular section and the spring may be positioned between the first abutment surface of the housing and the sleeve such that it may resist movement of the sleeve in the second direction.

A piston head may be provided at the first end of the hollow tubular section. Fluid pressure at the first end of the housing may act on the piston head and an end of the sleeve facing the first end of the housing. The projected area of the piston head exposed to the fluid at the first end of the housing may be greater than the projected area of the sleeve exposed to the fluid at the first end of the housing.

The sleeve, housing, hollow tubular section and first abutment surface may define a first chamber, such that when the valve is closed, the first chamber may not be in flow communication with the second end of the housing. A passage may be provided through the sleeve, the passage may provide a flow path from the first end of the housing to the first chamber. The projected area of the sleeve facing the fluid in the first end of the housing is greater than the projected area of the sleeve facing the fluid in the first chamber.

A second chamber may be provided between the sleeve and the housing, the chamber may be sealed from flow communication with the first end of the housing and the first chamber. A fourth abutment surface may be provided on an outer surface of the sleeve and a fifth abutment surface may be provided within the housing, such that the fourth and fifth abutment surfaces may define the second chamber and limit the movement of the sleeve in the direction toward the second end of the housing.

A vent may be provided in the housing wall, the vent may provide a flow path between the second chamber and outside the housing of the flow stop valve. The surface of the sleeve defined by the difference between: (A) the projected area of the sleeve facing the fluid in the first end of the housing; and (B) the projected area of the sleeve facing the fluid in the first chamber, may be exposed to the fluid outside the flow stop valve.

A pressure difference between fluid on a first side of the valve and on a second side of the valve may be substantially the same as the pressure difference between fluid outside the downhole tubular and inside the downhole tubular at the flow stop valve.

The flow stop valve may comprise: a biasing means; and a valve; wherein the biasing means may act on the valve such that the biasing means may bias the valve towards the closed position; and wherein the pressure difference between fluid on a first side of the valve and on a second side of the valve may also act on the valve and bias the valve towards an open position, such that when the pressure difference exceeds the threshold value the valve may be in the open position and drilling fluid is permitted to flow through the downhole tubular.

The flow stop valve may further comprise a housing, and a spindle, the spindle may be located within the housing, and may be slidably received in a first receiving portion at a first end of the housing and a second receiving portion at a second end of the housing, the housing may comprise a first abutment surface and the spindle may comprise a second abutment surface, such that the valve may be in a closed position when the second abutment surface of the spindle engages the first abutment surface of the housing.

The spindle may comprise first and second ends, the first end of the spindle corresponding to the first end of the housing, and the second end of the spindle corresponding to a second end of the housing.

The first end of the spindle and the first receiving portion may define a first chamber and the second end of the spindle and the second receiving portion may define a second chamber, the first and second chambers may not be in flow communication with first and second ends of the housing respectively. The biasing means may comprise a spring provided in the first chamber.

There may be provided a first passage through the spindle from the first end of housing to the second chamber and a second passage through the spindle from the second end of the housing to the first chamber, such that the first chamber may be in flow communication with the second end of the housing and the second chamber may be in flow communication with the first end of the housing.

The projected area of the first end of the spindle facing the fluid in the first chamber may be less than the projected area of the second end of the spindle facing the fluid in the second chamber.

The constituent parts of the flow stop valve may be manufactured from drillable materials. The constituent parts of the flow stop valve may be manufactured from a selection of materials including brass and aluminium.

The flow stop valve may be for use in offshore deep sea drilling applications and the downhole tubular may extend, at least partially, from the surface to the seabed. The downhole tubular may be, at least partially, located within a riser, the riser extending from the seabed to the surface. The threshold value may be greater than or equal to the pressure difference between the fluid outside the tubular and inside the downhole tubular at the seabed. The first end of the housing may be located above the second end of the housing, the first end of the housing may be connected to a drilistring section and the second end of the housing may be connected to another drillstring section or a drilling device.

The fluid in the downhole tubular may be at a first density. A fluid at a second density may be combined at the seabed with fluid returning to the surface, so that the resulting mixture between the riser and downhole tubular may be at a third density.

According to a third aspect of the present invention, there is provided a method for preventing flow in a downhole tubular, wherein when a pressure difference between fluid outside the dowrihole tubular and inside the downhole tubular at a flow stop valve is: (A) below a threshold value, the flow stop valve is in a closed position, preventing flow through the downhole tubular; and (B) above a threshold value, the flow stop valve is in an open position, permitting flow through the downhole tubular.

According to a third aspect of the present invention, there is provided a method for preventing flow in a downhole tubular, wherein when a pressure difference between fluid on a first side of a flow stop valve and on a second side of the flow stop valve is: (A) below a threshold value, the flow stop valve is in a closed position, preventing flow through the downhole tubular; and (B) above a threshold value, the flow stop valve is in an open position, permitting flow through the downhole tubular.

Brief Description of the Drawincjs

For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the following drawings, in which: Figure la is a graph showing the variation of the formation and fracture pressures beneath the seabed; Figure lb is a schematic diagram showing a proposed arrangement for a dual density drilling system; Figure ic is a schematic diagram showing the positional arrangement of the flow stop valve according to a first embodiment of the invention; Figure 2 is a sectional side-view of the flow stop valve according to a first embodiment of the invention; Figures 3a and 3b are sectional side-views showing the valve sleeve according to a first embodiment of the invention with figure 3b being an enlarged view of figure 3a; Figures 4a, 4b and 4c are sectional side-views of the flow stop valve in the closed, pre-loaded and open positions according to a first embodiment of the invention; Figures 5a, 5b, 5c, 5d, 5e and 5f are sectional side-views of the flow stop valve according to a second embodiment of the invention.

Figure 6 is a sectional side-view of the flow stop valve according to a third embodiment of the invention; and Figure 7 is a sectional side-view of the flow stop valve according to a fourth embodiment of the invention.

Detailed Description of the Preferred Embodiments

With reference to figure ic, a flow stop valve 20, according to a first embodiment of the invention, is located in a drillstring 6 such that, when a drilling head 8 is in position for drilling, the flow stop valve 20 is at any desired point in the drilistring between the seabed SB and the drilling head 8. The flow stop valve 20 ensures that before the flow of drilling fluid 1 is started, or when it is stopped, the drilling fluid within the drillstring 6 is not in flow communication with the fluid 1, 3 outside the drillstring, thereby preventing uncontrollable flow due to the hydrostatic pressure difference described above.

With reference to Figure 2, the flow stop valve 20, according to a first embodiment of the invention, comprises a tubular housing 22 within which there is disposed a hollow tubular section 24. The housing 22 comprises a socket 38 at a first end of the housing and a plug 40 at a second end of the housing. (NB, the first end of a component will hereafter refer to the rightmost end as shown in figures 2-4 and accordingly the second end will refer to the leftmost end.) The socket 38 and plug 40 allow engagement of the flow stop valve 20 with adjacent sections of a drillstring and may comprise conventional box and pin threaded connections, respectively.

A sleeve 26 is slidably disposed within the housing 22 about a first end of the hollow tubular section 24, such that the sleeve 26 can slide along the hollow tubular section 24 at its first end, and the sleeve 26 can also slide within the housing 22. A flange 28 is provided at a second end of the hollow tubular section 24 and a first abutment shoulder is provided within the housing 22 between the first and second ends of the hollow tubular section 24 such that the hollow tubular section 24 is slidably engaged within the innermost portion of the first abutment shoulder 30 and the motion of the hollow tubular section 24 in a first direction towards the first end of the housing is limited by the abutment of the flange 28 against the first abutment shoulder 30. (NB, the first direction is hereafter a direction towards the rightmost end shown in figures 2-4 and accordingly the second direction is towards the leftmost end.) A second abutment shoulder 32 is provided within the housing 22 and is placed opposite the first abutment shoulder 30, so that the flange 28 is between the first and second abutment shoulders 30, 32. Furthermore, a variable width spacer element 34 is placed between the second abutment shoulder 32 and the flange 28 and motion of the hollow tubular section 24 in a second direction towards the second end of the housing is limited by the abutment of the flange 28 against the spacer element 34 and the abutment of the spacer element 34 against the second abutment shoulder 32. The flange 28 and spacer element 34 both have central openings so that the flow of fluid is permitted from the centre of the hollow tubular section 24 to the second end of the flow stop valve 20.

The flow stop valve 20, according to the first embodiment of the invention, is also provided with a spring 36, which is located between the first abutment shoulder 30 and the sleeve 26. The spring 36 resists motion of the sleeve 26 in the second direction.

With reference to figures 3a and 3b, the hollow tubular section 24, according to the first embodiment of the invention, further comprises a cone shaped piston head 44 disposed at the first end of the hollow tubular section 24. The piston head 44 is provided with a third abutment shoulder 42, which abuts a first end of the sleeve 26 thereby limiting motion of the sleeve 26 relative to the hollow tubular section 24 in the first direction. The hollow tubular section 24 further comprises one or more ports 46, which are provided in a side-wall of the hollow tubular section 24 at the first end of the hollow tubular section 24. The ports 46 permit flow from the first end of the flow stop valve 20 into the centre of the hollow tubular section 24, through the openings in the flange 28 and spacer element 34 and subsequently to the second end of the flow stop valve 20. However, when the sleeve 26 abuts the third abutment shoulder 42 of the piston head 44, the sleeve 26 blocks the ports 46 and hence prevents flow from the first end of the flow stop valve 20 to the centre of the hollow tubular section 24.

The sleeve 26 further comprises a sleeve vent 48 which provides a flow passage from the first end of the sleeve 26 to the second end of the sleeve 26 and thence to a first chamber 52, which contains the spring 36 and is defined by the housing 22, the hollow tubular section 24, the first abutment shoulder 30 and the second end of the sleeve 26.

The sleeve vent 48 thus ensures that the pressures acting on the first and second ends of the sleeve 26 are equal. However, the projected area of the first end of the sleeve 26 is greater than the projected area of the second end of the sleeve 26 so that the force due to the pressure acting on the first end of the sleeve 26 is greater than the force due to the pressure acting on the second end of the sleeve 26. This area difference is achieved by virtue of a fourth abutment shoulder 54 in the sleeve 26 and a corresponding fifth abutment shoulder 56 in the housing 22. The fourth abutment shoulder 54 is arranged so that the diameter of the sleeve 26 at its first end is greater than that at its second end and furthermore, motion of the sleeve 26 in the second direction is limited when the fourth and fifth abutment shoulders 54, 56 abut. The fourth and fifth abutment shoulders 54, 56, together with the sleeve 26 and housing 22 define a second chamber 58 and a housing vent 50 is provided in the side-wall of the housing 22 so that the second chamber 58 is in flow communication with the fluid outside the flow stop valve 20. The net force acting on the sleeve 26 is therefore the product of (1) the difference between the pressure outside the flow stop valve 20 and at the first end of the flow stop valve 20, and (2) the area difference between the first and second ends of the sleeve.

Seals 60, 62 are provided at the first and second ends of the sleeve 26 respectively to ensure that the second chamber 58 is sealed from the first end of the flow stop valve and the first chamber 52 respectively. Furthermore, seals 64 are provided on the innermost portion of the first abutment shoulder 30 so that the first chamber 52 is sealed from the second end of the flow stop valve 20.

With reference to Figure 4a, 4b and 4c, operation of the flow stop valve 20, according to a first embodiment of the invention, will now be explained. The flow stop valve 20 is located in a drillstring with the first end above the second end and the flow stop valve is connected to adjacent drillstring sections via the sockets 38 and plug 40. Prior to lowering of the drillstring into the riser of an offshore drilling rig, there is a small preload in the spring 36 so that the sleeve 26 abuts the third abutment shoulder 42 of the piston head 44 and the ports 46 are closed, as shown in Figure 4a. In this position no drilling fluid may pass through the flow stop valve 20.

As the drilistring and hence flow stop valve 20 is lowered into the riser, the hydrostatic pressures inside and outside the drilistring and flow stop valve 20 begin to rise. With a dual density drilling fluid system the density of the fluid within the drillstring is higher than the density of the fluid outside the drilistring, and the hydrostatic pressures within the drilistring (and hence those acting on the piston head 44 and first and second ends of the sleeve 26) therefore increase at a greater rate than the pressures outside the drillstring. The difference between the pressures inside and outside the drillstring increases until the seabed is reached, beyond which point the fluids inside and outside the drillstring have the same density and the pressures inside and outside the drillstring increase at the same rate.

Before the flow stop valve 20 reaches the seabed, the increasing pressure difference between the inside and outside of the drillstring also acts on the hollow tubular section 24 because the top (first) end of the flow stop valve 20 is not in flow communication with the bottom (second) end of the flow stop valve 20. This pressure difference acts on the projected area of the piston head 44, which in a preferred embodiment has the same outer diameter as the hollow tubular section 24. The same pressure difference also acts on the difference in areas between the first and second ends of the sleeve, however, this area difference is much smaller than the projected area of the piston head 44. Therefore, as the flow stop valve 20 is lowered into the riser, the force acting on the hollow tubular section 24 is greater than the force acting on the sleeve 26.

Once the forces acting on the hollow tubular section 24 and sleeve 26 overcome the small preload in the spring 36, the hollow tubular section 24 is moved downwards (i.e. in the second direction) and because the force on the piston head 44 is greater than that on the sleeve 26, the sleeve 26 remains abutted against the third abutment shoulder 42 of the piston head 44. This movement of the hollow tubular section 24 continues until the flange 28 abuts the spacer element 34, at which point the flow stop valve 20 is fully preloaded, as shown in Figure 4b. The pressure difference at which this occurs, and the resulting force in the spring, can be varied by changing the thickness of the spacer element 34. A larger spacer element 34 will require the hollow tubular section 24 to travel a shorter distance before the flow stop valve 20 is preloaded and will result in a smaller spring force. The opposite applies for a smaller spacer element 34. (The size of the spacer element 34 can be selected before installing the flow stop valve 20 into the drillstring.) When the flow stop valve 20 has become fully preloaded, the hollow tubular section 24 cannot move any further. However, the force acting on the sleeve 26 is not yet sufficient to overcome the fully preloaded spring force, because the pressure difference acting on the sleeve 26 acts on a much smaller area. The sleeve 26 therefore remains in contact with the third abutment shoulder 42 and the ports 46 stay closed. The flow stop valve 20 has to be lowered further for the pressure difference acting on the sleeve 26 to increase. The spacer element 34 thickness can be selected so that once the flow stop valve 20 reaches the seabed, the pressure difference and hence force acting on the sleeve 26 at this depth is sufficient to almost balance the fully preloaded spring force. As the flow stop valve 20 is lowered below the seabed, the pressure difference does not increase any more (for the reasons explained above) and hence the ports 46 will remain closed. Once the drilistring is in place and the flow of drilling fluid is required, an additional "cracking" pressure can be applied by the drilling fluid pumps, which is sufficient to overcome the fully preloaded spring force, thereby moving the sleeve 26 downwards (in the second direction) and permitting flow through the ports 46 and the flow stop valve 20.

By preventing flow until the drilling fluid pumps provide the required "cracking" pressure, the flow stop valve 20 described above solves the aforementioned problem of the fluid in the drillstring displacing the fluid outside the drillstring due to the density differences and resulting hydrostatic pressure imbalances.

In an alternative embodiment, the flange 28 could be replaced with a tightening nut disposed about the second end of the hollow tubular section 24, so that the initial length of the spring 36, and hence the fully preloaded spring force, can be varied at the surface. With such an arrangement, the spacer element 34 would no longer be necessary.

With reference to Figures 5a-f, the flow stop valve 20, according to a second embodiment of the invention, further comprises a second spring 70 disposed between the flange 28 and spacer element 34. The second spring 70 fits within the housing 22 and the second spring 70 is sized to allow the free passage of fluid through the flow stop valve 20. For example, the inner diameter of the second spring 70 is greater than, or equal to, the inner diameter of the hollow tubular section 24 and/or the spacer element 34. Ii an uncompressed state, the second spring 70 does not contact the flange 28 when the hollow tubular section 24 is in its raised position (as shown in figure 5a). Alternatively, when in an uncompressed state the second spring 70 may at all times contact both the flange 28 and spacer element 34.

Operation of the second embodiment will now be explained with reference to figures 5a-f, which show the various stages of the flow stop valve. Figure 5a shows the flow stop valve 20 at the surface prior to lowering into the hole with the sleeve 26 and hollow tubular section 24 in their first-most directions. Figure 5b shows the flow stop valve 20 as it is lowered into the hole and the higher pressure acting at the first end of the flow stop valve 20 causes the spring 36 to compress. When the flow stop valve 20 is lowered further into the hole, as shown in figure 5c, the pressure differential acting across the sleeve 26 and hollow tubular section 24 increases. The spring 36 is further compressed by the hollow tubular section 24 being forced in the second direction and, as the flange 28 comes into contact with the second spring 70, the second spring 70 is also compressed.

Figure Sd shows the flow stop valve 20 at a depth below the seabed. Once the "cracking" pressure has been applied the sleeve 26 begins to move in the second direction and the ports 46 are opened permitting flow through the flow stop valve 20.

As the fluid begins to flow, the pressure difference acting across the hollow tubular section 24 will be reduced. The downward force acting on the hollow tubular section 24 will therefore be reduced and the second spring 36 will then be able to force the hollow tubular section 24 upwards, i.e. in the first direction, as shown in figure Se.

Movement of the hollow tubular section 24 in the first direction will also cause the ports 46 to open more quickly. This will serve to further reduce the pressure drop across the flow stop valve 20, which will in turn further raise the hollow tubular section 24. A faster acting, more responsive flow stop valve 20 is thus provided.

As shown in figure Sf, when the pumping of drilling fluid is stopped, the sleeve 26 returns to the first end of the hollow tubular section 24 closing the ports 46 and hence the flow stop valve 20.

The second spring 70 may be any form of resilient member and for example may be a coiled spring, disc spring or rubber spring. The combined thickness of the spacer element 34 and the second spring 70 in a compressed state determines the preloading in the spring 36 and hence the "cracking" pressure required to open the flow stop valve 20. Therefore, to obtain an appropriate cracking pressure for the required depth, the thickness of the spacer element 34 and/or second spring 70 in a compressed state are selected before installing the flow stop valve 20 into the drillstring.

In an alternative to the second embodiment, the second spring 70 may completely replace the spacer element 34 so that the second spring 70 is located between the second abutment shoulder 32 and the flange 28. In such an embodiment the preloading in the spring 36 is determined by the length of the second spring 70 in a compressed state.

A flow stop valve according to a third embodiment of the invention relates to the lowering of a casing section into a newly drilled and exposed portion of a well bore.

The flow stop valve is located in a casing string being lowered into a well bore, such that, when a casing section is in position for sealing against the exposed well wall, the flow stop valve is at any point in the casing string between the seabed and the bottom of the casing string. The flow stop valve ensures that before the flow of cementing fluid is started, or when it is stopped, the cementing fluid within the casing string is not in flow communication with the fluid outside the drillstring, thereby preventing the flow due to the hydrostatic pressure difference described above.

With reference to Figure 6, the flow stop valve 120, according to the third embodiment of the invention, comprises a housing 122 and a spindle 124. The spindle 124 is slidably received in both a first receiving portion 126 and a second receiving portion 128. The first receiving portion 126 is attached to a first end of the housing 122 and the second receiving portion 128 is attached to a second end of the housing 122. (NB, the first end of a component will hereafter refer to the topmost end as shown in figure 6 and accordingly the second end will refer to the bottommost end of the third embodiment) The attachments between the housing 122 and the first and second receiving portions 126, 128 are arranged such that a flow is permitted between the housing 122 and the first receiving portion 126 and the housing 122 and the second receiving portion 128.

The housing further comprises a first annular abutment surface 130, which is located on the inner sidewall of the housing and between the first and second receiving portions 126, 128. The spindle 124 also comprises a second annular abutment surface 132, the second annular abutment surface being provided between first and second ends of the spindle 124. The arrangement of the first and second annular abutment surfaces 130, 132 permits motion of the spindle 124 in a first direction but limits motion in a second direction. (NB, the first direction is hereafter a direction towards the topmost end shown in figure 6 and accordingly the second direction is towards the bottommost end of the third embodiment.) Furthermore, the second annular abutment surface 132 is shaped for engagement with the first annular abutment surface 130, such that when the first and second annular abutment surfaces abut, flow from first end of the flow stop valve 120 to the second end of the flow stop valve 120 is prevented.

The first receiving portion 126 and first end of the spindle 124 together define a first chamber 134. Seals 136 are provided about the first end of the spindle 124 to ensure that the first chamber 134 is not in flow communication with the first end of the flow stop valve 120. Similarly, the second receiving portion 128 and the second end of the spindle 124 together define a second chamber 138. Seals 140 are provided about the second end of the spindle 124 to ensure that the second chamber 138 is not in flow communication with the second end of the flow stop valve 120.

The projected area of the first and second ends of the spindle 124 in the first and second chambers 134, 138 are equal and the projected area of the second annular abutment surface 132 is less than the projected area of the first and second ends of the spindle 124. In other words, the area defined by the outermost perimeter of the second annular abutment surface 132 is less than twice the projected area of the first and second ends of the spindle 124.

A spring 142 is provided in the first chamber 134 with a first end of the spring 142 in contact with the first receiving portion 126 and a second end of the spring 142 in contact with the spindle 124. The spring 142 biases the spindle 124 in the second direction such that the first and second abutment surfaces 130, 132 abut. A spacer element (not shown) may be provided in the first chamber 134 between the spring 142 and spindle 124 or the spring 124 and first receiving portion 126. The spacer element acts to reduce the initial length of the spring 142 and hence the pretension in the spring.

The spindle 124 is also provided with a first passage 144 and a second passage 146.

The first passage 144 provides a flow path from the first end of the flow stop valve 120 to the second chamber 138, whilst the second passage 146 provides a flow path from the second end of the slow stop valve 120 to the first chamber 134. However, when the first annular abutment surface 130 abuts the second annular abutment surface 132, the first passage 144 is not in flow communication with the second passage 146.

The flow stop valve 120 is manufactured from Aluminium (or any other soft drillable material, for example brass) to allow the flow stop valve 120 to be drilled out once the cementing operation is complete. In addition, the spring 142 is a wave spring; this allows the use of a larger spring wire section whilst still keeping it drillable. To assist in the drilling operation the flow stop valve 120 is located eccentrically in an outer casing to allow it to be easily drilled out by a conventional drill bit. Furthermore, the flow stop valve 120 is shaped to assist the fluid flows as much as possible and so reduce the wear of the flow stop valve 120 through erosion.

In operation the pressure from the first and second ends of the flow stop valve 120 acts on the second and first chambers 138, 134 respectively via the first and second passages 1441 146 respectively. The projected area of the first and second ends of the spindle 124 in the first and second chambers 134, 138 are equal, but because the pressure in the first end of the flow stop valve 120 is higher than the pressure in the second end of the flow stop valve 120 (due to the dual density system explained above) the forces acting in the second chamber 138 are higher than those in the first chamber 134. Furthermore, as the projected area of the second annular abutment surface 132 is less than the projected area of the first and second ends of the spindle 124, the net effect of the pressure forces is to move the spindle 124 in a first direction.

However, the spring 142 acts on the spindle 124 to oppose this force and keeps the flow stop valve 120 in a closed position (i.e. with the first and second annular abutment surfaces 130, 132 in engagement). The spring 142 does not support the complete pressure force, because the area in the first and second chambers 134, 138 is greater than that around the centre of the spindle 124 and the net force acting on the first and second chambers 134, 138 is in the opposite direction to the force acting on the second annular abutment surface 132.

The opening of the flow stop valve 120 occurs when the pressure differential acting over the spindle 124 reaches the required ucracking pressure. At this pressure, the net force acting on the spindle 124 is enough to cause the spindle 124 to move in a first direction, thereby allowing cementing fluid to flow. The pressure difference at which this occurs can be varied by selecting an appropriate spacer element to adjust the pretension in the spring.

However, once fluid starts to flow through the flow stop valve 120, the pressure difference acting across the spindle 124 will diminish, although a small pressure difference will remain due to pressure losses caused by the flow of fluid through the valve. Therefore, in the absence of the pressure differences present when there is no flow, the spring 142 will act to close the valve. However, as the valve closes the pressure differences will again act on the spindle 124, thereby causing it to re-open.

This process will repeat itself and the spindle 124 will "chatter" during use. The oscillation between the open and closed positions assists in maintaining the flow of cementing fluid and these dynamic effects help to prevent blockage between the first and second annular abutment surfaces 130, 132.

With reference to Figure 7, the flow stop valve 120, according to a fourth embodiment of the invention is substantially similar to the third embodiment of the invention, except that the flow stop valve 120 is orientated in the opposite direction (i.e. the first end of the housing 122 is at the bottommost end and the second end of the housing 122 is at the topmost end). In addition, the fourth embodiment differs from the third embodiment in that the projected area of the second annular abutment surface 132 is greater than the projected area of the first and second ends of the spindle 124. In other words, the area defined by the perimeter of the second annular abutment surface 132 is more than twice the projected area of the first and second ends of the spindle 124. Aside from these differences the fourth embodiment is otherwise the same as the second embodiment and like parts have the same name and reference numeral.

During operation of the third embodiment, higher pressure fluid from above the flow stop valve 120 acts on the first chamber 134 by virtue of the second passage 146, and lower pressure fluid acts on the second chamber 138 by virtue of first passage 144.

The pressure forces on the first and second chambers 134, 138, together with the spring force, act to close the flow stop valve 120 (i.e. with the first and second annular abutment surfaces 130, 132 in engagement). However, as the projected area of the first annular abutment surface 130 is greater than the projected area of the first and second ends of the spindle 124, the net effect of the pressure forces is to move the spindle 124 into an open position. Therefore, once the pressure forces have reached a particular threshold sufficient to overcome the spring force, the flow stop valve 120 will open.

In alternative embodiments, the first and second ends of the spindle 124 may have different projected areas. For example, increasing the projected area of the first end of the spindle 124 for the second embodiment relative to the first end of the spindle 124, would further bias the valve into a closed position and hence increase the "cracking" pressure to open the valve. Other modifications to the projected areas could be made in order to change the bias of the valve, as would be understood by one skilled in the art.

Claims

Claims 1. A flow stop valve positioned in a downhole tubular, wherein: (A) the flow stop valve is in a closed position when a pressure difference between fluid outside the downhole tubular and inside the downhole tubular at the flow stop valve is below a threshold value, thereby preventing flow through the downhole tubular; and (B) the flow stop valve is in an open position when the pressure difference between fluid outside the downhole tubular and inside the downhole tubular at the flow stop valve is above a threshold value, thereby permitting flow through the downhole tubular.
2. The flow stop valve according to claim 1, wherein the threshold value for the pressure difference between fluid outside the tubular and inside the downhole tubular at the flow stop valve is variable.
3. The flow stop valve according to claim 1 or 2, wherein the flow stop valve comprises: a biasing means; and a valve; wherein the biasing means acts on the valve such that the biasing means biases the valve towards the closed position; and wherein the pressure difference between fluid outside the downhole tubular and inside the tubular also acts on the valve and biases the valve towards an open position, such that when the pressure difference exceeds the threshold value the valve is in the open position and drilling fluid is permitted to flow through the downhole tubular.
4. The flow stop valve according to claim 3, wherein the flow stop valve further comprises a housing, and a hollow tubular section and a sleeve located within the housing, the sleeve being provided around the hollow tubular section and the sleeve being located within the housing, the housing comprising first and second ends and the hollow tubular section comprising first and second ends, the first end of the hollow tubular section corresponding to the first end of the housing, and the second end of the hollow tubular section corresponding to a second end of the housing.
5. The flow stop valve according to claim 4, wherein the hollow tubular section is slidably engaged within the housing.
6. The flow stop valve according to claim 4 or 5, wherein the sleeve is slidably engaged about the hollow tubular section.
7. The flow stop valve according to claims 5 or 6, wherein the hollow tubular section comprises a port such that the port is selectively blocked by movement of the hollow tubular section or sleeve, the port forming the valve such that in an open position a flow path exists from a first end of the housing, through the port and the centre of the tubular section to a second end of the housing.
8. The flow stop valve according to claim 6, wherein a third abutment surface is provided at a first end of the hollow tubular section such that the third abutment surface limits the travel of the sleeve in the direction toward the first end of the housing.
9. The flow stop valve according to any one of claims 5 to 8, wherein a flange is provided at the second end of the hollow tubular section.
10. The flow stop valve according to claim 9, wherein a second abutment surface is provided at the second end of the housing such that the second abutment surface of the housing abuts the flange of the tubular section limiting the travel of the hollow tubular section in a second direction, the second direction being in a direction towards the second end of the housing.
11. The flow stop valve according to claim 10, wherein a first abutment surface is provided within the housing between the second abutment surface of the housing and the first end of the housing, such that the first abutment surface abuts the flange of the hollow tubular section limiting the travel of the hollow tubular section in a first direction, the first direction being in a direction towards the first end of the housing.
12. The flow stop valve according to claim 10, wherein a spacer element of variable dimensions is provided between the second abutment surface of the housing and the flange of the hollow tubular section, such that the lim4t on the travel of the hollow tubular section in the second direction can be varied.
13. The flow stop valve according to any of claims 2 to 12, wherein the biasing means comprises a spring.
14. The flow stop valve according to claim 13, when dependent on claim 11, wherein the spring is provided about the hollow tubular section and the spring is positioned between the first abutment surface of the housing and the sleeve such that it resists movement of the sleeve in the second direction.
15. The flow stop valve according to any of claims 4 to 14, when dependent on claim 4, wherein a piston head is provided at the first end of the hollow tubular section.
16. The flow stop valve according to claim 15, wherein fluid pressure at the first end of the housing acts on the piston head and an end of the sleeve facing the first end of the housing.
17. The flow stop valve according to claim 15 or 16, wherein the projected area of the piston head exposed to the fluid at the first end of the housing is greater than the projected area of the sleeve exposed to the fluid at the first end of the housing.
18. The flow stop valve according to any one of claims 11 to 17, when dependent on claim 11, wherein the sleeve, housing, hollow tubular section and first abutment surface define a first chamber, such that when the valve is closed, the first chamber is not in flow communication with the second end of the housing.
19. The flow stop valve according to claim 18, wherein a passage is provided through the sleeve, the passage providing a flow path from the first end of the housing to the first chamber.
20. The flow stop valve according to claim 18 or 19, wherein the projected area of the sleeve facing the fluid in the first end of the housing is greater than the projected area of the sleeve facing the fluid in the first chamber.
21. The flow stop valve according to any one of claims 18 to 20, wherein a second chamber is provided between the sleeve and the housing, the chamber being sealed from flow communication with the first end of the housing and the first chamber.
22. The flow stop valve according to claim 21, wherein a fourth abutment surface is provided on an outer surface of the sleeve and a fifth abutment surface is provided within the housing, such that the fourth and fifth abutment surfaces define the second chamber and limit the movement of the sleeve in the direction toward the second end of the housing.
23. The flow stop valve according to claims 21 or 22, wherein a vent is provided in the housing wall, the vent providing a flow path between the second chamber and outside the housing of the flow stop valve.
24. The flow stop valve according to any one of claims 20 to 23, wherein the surface of the sleeve defined by the difference between: (A) the projected area of the sleeve facing the fluid in the first end of the housing: and (B) the projected area of the sleeve facing the fluid in the first chamber, is exposed to the fluid outside the flow stop valve.
25. The flow stop valve according to claim 1 or 2, wherein a pressure difference between fluid on a first side of the valve and on a second side of the valve is substantially the same as the pressure difference between fluid outside the downhole tubular and inside the downhole tubular at the flow stop valve.
26. The flow stop valve according to claim 25, wherein the flow stop valve comprises: a biasing means; and a valve; wherein the biasing means acts on the valve such that the biasing means biases the valve towards the closed position; and wherein the pressure difference between fluid on a first side of the valve and on a second side of the valve also acts on the valve and biases the valve towards an open position, such that when the pressure difference exceeds the threshold value the valve is in the open position and drilling fluid is permitted to flow through the downhole tubular.
27. The flow stop valve according to claim 26, wherein the flow stop valve further comprises a housing, and a spindle, the spindle being located within the housing, and being slidably received in a first receiving portion at a first end of the housing and a second receiving portion at a second end of the housing, the housing comprising a first abutment surface and the spindle comprising a second abutment surface, such that the valve is in a closed position when the second abutment surface of the spindle engages the first abutment surface of the housing.
28. The flow stop valve according to claim 27, wherein the spindle comprising first and second ends, the first end of the spindle corresponding to the first end of the housing, and the second end of the spindle corresponding to a second end of the housing.
29. The flow stop valve according to claim 28, wherein the first end of the spindle and the first receiving portion define a first chamber and the second end of the spindle and the second receiving portion define a second chamber, the first and second chambers not being in flow communication with first and second ends of the housing respectively.
30. The flow stop valve according to claim 29, wherein there is provided a first passage through the spindle from the first end of housing to the second chamber and a second passage through the spindle from the second end of the housing to the first chamber, such that the first chamber is in flow communication with the second end of the housing and the second chamber is in flow communication with the first end of the housing.
31. The flow stop valve according to claim 30, wherein the projected area of the first end of the spindle facing the fluid in the first chamber is less than the projected area of the second end of the spindle facing the fluid in the second chamber.
32. The flow stop valve according to any one of claims 29 to 31, wherein the biasing means comprises a spring provided in the first chamber.
33. The flow stop valve according to any one of claims 25 to 32, wherein the constituent parts of the flow stop valve are manufactured from drillable materials.
34. The flow stop valve according to claim 33, wherein the constituent parts of the flow stop valve are manufactured from a selection of materials including brass and aluminium.
35. The flow stop valve according to any preceding claim, wherein the flow stop valve is for use in offshore deep sea drilling applications and the downhole tubular extends, at least partially, from the surface to the seabed.
36. The flow stop valve according to claim 35, wherein the downhole tubular is, at feast partially, located within a riser, the riser extending from the seabed to the surface.
37. The flow stop valve according to claim 35, wherein the threshold value is greater than or equal to the pressure difference between the fluid outside the tubular and inside the downhole tubular at the seabed.
38. The flow stop valve according to claim 35, wherein the fluid in the downhole tubular is at a first density.
39. The flow stop valve according to any one of claims 35 to 38, wherein a fluid at a second density is combined at the seabed with fluid returning to the surface, the resulting mixture between the riser and downhole tubular being at a third density.
40. The flow stop valve according to claim 4 or 26, wherein the first end of the housing is located above the second end of the housing, the first end of the housing being connected to a drilistring section and the second end of the housing being connected to another drilistring section or a drilling device.
41. A method for preventing flow in a downhole tubular, wherein when a pressure difference between fluid outside the downhole tubular and inside the downhole tubular at a flow stop valve is: (A) below a threshold value, the flow stop valve is in a closed position, preventing flow through the downhole tubular; and (B) above a threshold value, the flow stop valve is in an open position, permitting flow through the downhole tubular.
42. A method for preventing flow in a downhole tubular, wherein when a pressure difference between fluid on a first side of a flow stop valve and on a second side of the flow stop valve is: (A) below a threshold value, the flow stop valve is in a closed position, preventing flow through the downhole tubular; and (B) above a threshold value, the flow stop valve is in an open position, permitting flow through the downhole tubular.
43. A flow stop valve substantially as described herein with reference to and as shown in the accompanying drawings.
44. A method for preventing flow in a downhole tubular substantially as described herein with reference to and as shown in the accompanying drawings.