Gun brake device

This application claims the benefit of U.S. Provisional Application No. 60/349,159, filed Oct. 26, 2001.

FIELD OF THE INVENTION

The subject matter of the present invention relates to a gun brake system. More specifically, the subject matter of the present invention relates to a gun brake system adapted to protect a subsea safety valve from a dropped gun string.

BACKGROUND OF THE INVENTION

A subsea safety valve is typically positioned in the production tubing several hundred meters below the surface. On many existing completions, during a perforating workover operation, the subsea safety valve is the only pressure control device that is available when a perforating gun string is being introduced or removed from the wellbore while the gun string is above the subsea safety valve.

If the well starts “blowing out” during deployment of the perforating gun string, the guns are dropped into the well, and the blind/shear rams are closed. The dropped gun string can impact and potentially damage the subsea safety valve, causing the completion to have to be pulled at great expense and productivity damage to the producing formation.

There exists, therefore, a need for a system that protects the subsea safety valve from a dropped gun string.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1

is sketch of an embodiment of the gun brake system of the present invention.

FIGS. 2A-E

illustrates of an embodiment of the deployment and removal of an embodiment of the gun brake system from a well.

FIG. 3

is a cross-sectional view of an embodiment of the gun brake system shown prior to activation.

FIG. 4

is a cross-sectional view of an embodiment of the gun brake system shown in its actuated state.

FIG. 5

is a cross-sectional view of an embodiment of the gun brake system shown after the brake has been released from its actuated state.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1

provides a schematic illustration of one embodiment of the gun brake system, indicated generally as

1

. As illustrated, a perforating gun string

5

is being lowered on wireline

10

into production tubing

15

. A subsurface safety valve

20

is positioned within the production tubing

15

. Typically, the subsurface safety valve

20

is installed several hundred meters below the surface.

In this embodiment, the gun brake system

1

is principally comprised of a gun brake

25

and a flapper valve

30

. The gun brake

25

is installed above the safety valve

20

at a distance that will enable the brake

25

to safely slow the descent of a dropped gun string

5

to protect the safety valve

20

. Absent the gun brake

25

, a dropped gun string

5

will free fall until striking the safety valve

20

with substantial velocity and force. Such falls can result in severe and costly damage to the safety valve

20

.

At its upper end

35

, the gun brake

25

has an upper sloped surface

38

that acts to guide the gun string

5

into the gun brake

25

and ensures that the gun string

5

will remain substantially centered as it descends therethrough. Similarly, at its lower end

50

, the gun brake

25

has a lower sloped surface

55

that acts to guide the gun string

5

back into the gun brake

25

after the guns have been fired. The lower sloped surfaces

55

facilitate retrieval of the gun string

5

.

The sloped surfaces

38

,

55

terminate at the brake body

40

. The brake body

40

is a long and relatively snug fitting restriction. The length and inner diameter of the brake body

40

is dependent upon the length and outer diameter of the gun string

5

being lowered therethrough. The length of the brake body

40

is also dependent upon the relative location of the safety valve

20

. Along a portion of the brake body

40

are fluid channels

45

. The number and depth of the channels is dependent upon the weight of the gun string

5

and the relative location of the safety valve

20

.

The flapper valve

30

is installed below the gun brake

25

and above the safety valve

20

. In its closed state, the flapper valve

30

maintains a limited wellbore fluid volume. The flapper valve

30

impedes the free flow of wellbore fluid while the safety valve

20

is open, thus maintaining a limited wellbore fluid volume in the production tubing

15

above the flapper valve

30

. In other words, the wellbore fluid volume in the portion of the production tubing where the gun brake

25

is installed, remains substantially constant.

It should be noted that although the described embodiment of the gun brake system

1

uses a flapper valve

30

to maintain the wellbore fluid volume, any number of valves, including additional safety valves can be utilized to achieve the intended result.

In normal operation, the perforating gun string

5

is run downhole on the wireline

10

. The gun string

5

passes through the gun brake

25

and then must open the flapper valve

30

. In the embodiment shown, affixed to the bottom of the gun string

5

is a shifting tool

8

adapted to open the flapper valve

30

. After the firing of the guns, the gun string

5

is retrieved back through the gun brake

25

.

If the well starts “blowing out” during deployment of the perforating gun string

5

, the safety valve

20

must be closed and the gun string

5

must be dropped. With the gun brake

25

installed, the descent of the gun string

5

is slowed such that the gun string

5

does not strike the safety valve

20

with a velocity and force that can damage the safety valve

20

. The descent of the gun string

5

is slowed by the interaction of the gun string

5

, the gun brake

25

and the wellbore fluid.

After being dropped, the perforating gun string

5

descends through the gun brake

25

and travels therethrough the brake body

40

characterized as a snug fitting restriction. With a limited wellbore fluid volume maintained by the flapper

30

, the descent of the gun string

5

forces the wellbore fluid to be quickly channeled between the fluid channels

45

of the gun brake

25

and the gun string

5

. The resistance to the fluid flow acts to slow the velocity of the dropped gun string

5

. It should be noted that although the embodiment described uses wellbore fluid to slow the gun string

5

, any number of other fluids could be maintained in the production tubing

15

above the flapper valve

30

to achieve the same result.

FIGS. 2A-2E

illustrate the deployment and removal of an embodiment of the gun brake

25

into and out of a well. As shown in

FIG. 2A

, the gun brake

25

comprises an upper sloped surface

38

, a brake body

40

acting as a snug fitting restriction, a series of channels

45

running along a portion of the brake body

40

, and a lower sloped surface

55

. The gun brake

25

is lowered into the production tubing

15

with a running tool

60

conveyed by means such as wireline, tubing, or slickline

65

. The gun brake

25

is lowered to a depth above the safety valve (not shown) that will enable the descent of a dropped gun string

5

to be slowed to prevent striking the safety valve

20

with potential damaging velocity and force.

While at the appropriate depth, the gun brake

1

is installed, or set, using standard setting equipment such as that used for packers or bridge plugs.

FIG. 2B

illustrates the set gun brake

25

after having been released by the running tool

60

.

FIG. 2C

illustrates the gun string

5

being lowered through the production tubing

15

and into the gun brake

25

. The gun string S is guided into the gun brake

25

by the upper sloped surface

38

of the gun brake

25

. As illustrated, the brake body

40

is a snug fitting restriction having an inner diameter just larger than that of the gun string

5

. As such, dropping of the gun string

5

through the brake body

40

forces existing wellbore fluid into the channels

45

. The resistance to such fluid flow acts to slow the descent of the gun string

5

.

After the guns of the gun string

5

have been fired, the running tool

60

is lowered by means such as wireline, tubing or slickline

65

back into engagement with the gun brake

25

as shown in FIG.

2

D. The setting means is released and the gun brake

1

is removed from the production tubing

15

as shown in FIG.

2

E.

Another embodiment of the gun brake system

1

is shown in

FIGS. 3-5

. The illustrations of

FIGS. 3-5

are cross-sectional views wherein the left-hand side of the drawings represents the topside of the tool.

FIG. 3

illustrates this embodiment of the gun brake

25

shown prior to its activation.

FIG. 4

illustrates this embodiment of the gun brake

25

shown in its actuated state.

FIG. 5

illustrates this embodiment of the gun brake

25

shown after the brake has been released from its actuated state. Although not shown, it is understood that the gun brake

25

is attached to the lower end of a tool string carrying one or more perforating guns, for example.

In this embodiment, the gun brake

25

is generally comprised of a switch

70

, an actuation mechanism

100

, a braking mechanism

130

, and a release mechanism

150

. The switch

70

senses any undesirable downward motion, or threshold velocity, of the tool string to which it is attached and activates. Upon activation, energy is supplied to the actuation mechanism

100

that in turn energizes the braking mechanism

130

. The braking mechanism

130

engages the inner diameter of the completion (tubing or casing) to slow and eventually stop the tool string. As stated above, such braking acts to prevent the tool string from damaging devices below such as safety valves. When the tool string is ready to be retrieved, the release mechanism

150

is activated to release the brake

25

and free the string.

Referring to

FIG. 3

, the switch

70

has a switch piston

72

within a switch housing

74

. The switch piston

72

has a switch conduit

76

contained therein. Several switch seals

77

a

-

77

e

isolate the inlet and outlet of the switch conduit

76

.

The role of the switch seals

77

a

-

77

e

is as follows. Switch seal

77

b

isolates the switch conduit

76

from the energy conduit

78

housed within the activation shaft

80

. Switch seals

77

c

and

77

d

isolate the switch conduit

76

from the switch supply line

82

that is also housed within the activation shaft

80

. Switch seal

77

e

isolates the switch conduit

76

from the downhole environment. Likewise, switch seal

77

a

isolates the energy conduit

78

from the downhole environment.

Prior to activation of the switch

70

, the switch piston

72

is held in position by activation pins

83

. The overall strength of the activation pins

83

is greater than the force

84

acting on the switch piston

72

as the gun brake

25

travels at normal speed (i.e., lowering the tool string in a controlled fashion), but is lower than the force

84

acting on the switch piston

72

when the gun brake

25

is traveling at an undesirable speed (e.g., uncontrolled free fall). The undesirable speed is considered the threshold velocity of the gun brake

25

.

The force

84

acting on the switch piston

72

is generated by the so-called “piston-effect.” The piston-effect force on a flat surface increases when the speed of fluid hitting the flat surface increases. Thus, if the tool string is dropped and is free falling through the production tubing, the switch piston

72

will be subjected to substantially increased piston-effect forces generated by the increased velocity of the gun brake

25

travel through the wellbore fluids.

The switch piston

72

is not moved by the differential pressure across the gun brake

25

because of pressure balance openings

86

and

88

that act to balance out the pressure on both sides of the switch piston

72

. Thus, the only means to activate the switch piston

72

is going to be with the piston-effect force

84

.

Within the switch housing

74

is an energy chamber

90

defined by the housing

74

, the activation shaft

80

, and the lower adapter

92

. In one embodiment, the energy source contained within the energy chamber

90

is nitrogen gas. However, it should be noted that other gases and liquids can be used to advantage as the energy source. The nitrogen gas is pumped into the energy chamber

90

through the filling port

94

and the filling conduit

96

. The energy chamber

90

is pressure-sealed by energy seals

98

a,

98

b,

and

98

c.

The energy chamber

90

is connected to the inside diameter of the switch piston

72

by the energy conduit

78

. Prior to activation of the switch

70

, the energy conduit

78

is unable to communicate with the switch conduit

76

thereby leaving the pressurized nitrogen trapped inside the energy chamber

90

.

The actuation mechanism

100

is primarily comprised of the actuation housing

102

and the actuation piston

104

. An actuation chamber

106

is defined by the actuation housing

102

and the actuation piston

104

. The actuation chamber

106

is isolated from the outside environment by actuation seals

109

a,

109

b,

and

109

c.

Prior to activation, the pressure inside the actuation chamber

106

is atmospheric.

An actuation conduit

108

connects the actuation chamber

106

with the actuation supply line

110

that in turn connects to the upper brake supply line

112

.

A spring chamber

114

is defined by the actuation housing

102

, the actuation piston

104

, and the upper adapter

116

. The spring chamber

114

houses a retraction spring

118

and is isolated from the environment by actuation seal

109

b

and spring seals

120

a

and

120

b.

Prior to activation of the gun brake

25

, the pressure inside the spring chamber

114

remains atmospheric.

The actuation mechanism

100

is “pressure-balanced” from outside pressure as long as the cross-sectional area of the actuation chamber

106

is the same as the cross-sectional area of the spring chamber

114

. Thus, the force generated by the actuation mechanism

100

is not affected by the downhole pressure.

In the embodiment shown, the braking mechanism

130

utilizes the slip/wedge design. As such, the braking mechanism

130

is comprised of a brake housing

132

, an upper wedge

134

, a lower wedge

136

, and slips

138

.

The slips

138

ride on the top of the tapered surfaces of the upper wedge

134

, and the lower wedge

136

. In some embodiments, the slips

138

additionally comprise dovetails for engagement with each other. When the lower wedge

136

moves toward the upper wedge

134

, the slips

138

are forced outward. Conversely, when the lower wedge

136

moves away from the upper wedge

134

, the dovetails drag the slips

138

inward.

The braking mechanism

130

further comprises a brake chamber

140

defined by the upper wedge

134

and the lower wedge

136

. The brake chamber

140

is isolated from the outside environment by the brake seal

142

. The brake chamber

140

is connected to the actuation chamber

106

via the actuation conduit

108

and the actuation supply line

110

. Additionally, the brake chamber

140

is connected to the switch supply line

82

via the lower adapter supply line

144

.

The release mechanism

150

primarily comprises the upper adapter

116

and the release housing

152

. The upper adapter

116

and the release housing

152

are connected by the release pins

154

. The total strength of the release pins

154

is greater than the weight of the gun brake

25

and can sustain normal shocks during transportation downhole. The strength of the release pins

154

is, however, less than a pre-set value of a pulling force.

A release chamber

156

is defined by the upper adapter

116

and the release housing

152

. The release chamber

156

is isolated from the outside environment by the first release seal

158

. Prior to release of the tool, the release chamber

156

is isolated from the release conduit

160

by the second release seal

162

. The release conduit

160

is connected to the upper adapter supply line

164

. The release chamber

156

is always connected to the spring chamber

114

via the spring conduit

166

.

A release nut

168

is threaded to the upper adapter

116

. The release nut

168

prevents the complete separation of the upper adapter

116

from the release housing

152

after the release pins

154

have been sheared. Once the release pins

154

have been sheared, this design can also be used as a jar to provide a second means to retrieve the gun brake

25

in the event the brake (or slips) become jammed.

Activation of this embodiment of the gun brake

25

is best described with reference to

FIGS. 3 and 4

.

FIG. 3

illustrates the gun brake

25

prior to activation while

FIG. 4

illustrates the gun brake

25

in its activated state.

Once the piston-effect force

84

acting on the switch piston

72

becomes larger than the total shear strength of the activation pins

83

, the activation pins

83

will shear and the switch piston

72

will move upward. As discussed above, the piston-effect force

84

will increase beyond the total shear strength of the activation pins

83

when the gun string

25

is traveling above the threshold velocity. Such velocity may be reached upon release of the tool string during a “blow-out” situation, for example.

With the switch piston

72

in its uppermost position, the switch conduit

76

becomes aligned with the energy conduit

78

and the switch supply line

82

. Consequently, the pressurized nitrogen gas flows from the energy chamber

90

through the energy conduit

78

, through the switch conduit

76

, through the switch supply line

82

, through the lower adapter supply line

144

, through the upper brake supply line

112

, through the actuation supply line

110

, through the actuation conduit

108

, and into the actuation chamber

106

.

At this point, the nitrogen pressure is isolated from the release chamber

156

by operation of the second release seal

162

. Thus, the pressure inside spring chamber

114

, which is connected to the release chamber

156

by the spring conduit

166

, remains atmospheric. The net force F acting on the actuation housing

102

is,

F=P

1

A

1

−P

2

A

2

−F

s

  Equation (1)

Where P

1

is the gas pressure inside the actuation chamber

106

, P

2

is the atmospheric pressure inside the spring chamber

114

, A

1

is the cross-sectional area of the actuation chamber

106

, A

2

is the cross-sectional area of the spring chamber

114

, and F

s

is the spring force of the retraction spring

118

.

The atmospheric pressure P

2

is relatively small compared to P

1

. Therefore, the contribution of P

2

can be ignored from Equation 1. Additionally, as discussed above, the cross-sectional areas A

1

and A

2

are equivalent. Thus, Equation 1 can be simplified as follows,

F=P

1

A

1

−F

s

  Equation (2)

Because the net force F is greater than zero, the actuation housing

102

will move upward and compress the retraction spring

118

. As the actuation housing

102

moves upwards, it drags the brake housing

132

, the lower adapter

92

, and the lower wedge

136

upward.

While the lower wedge

136

moves upward, the upper wedge

134

remains relatively stationary. The upper wedge

134

is connected to the actuation piston

104

which is in turn connected to the upper adapter

116

, the release housing

152

, and the tool string adapter

170

, which all remain stationary with the rest of the tool string above. Thus, the relative movement of the lower wedge

136

forces the slips

138

to move outward into engagement with the completion (tubing or casing). As the slips

138

move outward, the tool string is slowed and eventually stopped.

Release of this embodiment of the gun brake

25

is best described with reference to

FIGS. 4 and 5

.

FIG. 4

illustrates the gun brake

25

in its activated state, while

FIG. 5

illustrates the gun brake

25

in its released state.

In typical operations, when a tool string is ready to be removed from the completion of a well, a fishing tool is conveyed by means such as wireline, coiled tubing, or slickline. The fishing tool is lowered into the well until it engages the top of the tool string. Once engaged, the tool string can be pulled.

In the present invention, when the pulling force of the fishing tool (not shown) is greater than the total strength of the release pins

154

, the release pins

154

are sheared and the release housing

152

is pulled away from the upper adapter

116

until the release housing

152

abuts the release nut

168

.

In this position, the release chamber

156

is connected to the actuation chamber

106

by the release conduit

160

, the upper adapter supply line

164

, and the actuation supply line

110

. Additionally, the spring chamber

114

is now connected all the way back to the energy chamber

90

. Consequently, the spring chamber

114

is filled nitrogen gas with the same pressure as the rest of the circuit. At this point, the net force F acting on the actuation housing

102

is,

F=P

1

A

1

−P

2

A

2

−F

s

  Equation (3)

Where P

1

is the gas pressure inside the actuation chamber

106

, P

2

is the atmospheric pressure inside the spring chamber

114

, A

1

is the cross-sectional area of the actuation chamber

106

, A

2

is the cross-sectional area of the spring chamber

114

, and F

s

is the spring force of the retraction spring

118

.

The pressure P

1

is now equal to P

2

. Thus, Equation 3 can be simplified as follows,

F=−F

s

  Equation (4)

As such, the retraction spring

118

pushes the upper adapter

116

, the actuation housing

102

, the brake housing

132

, the lower adapter

92

, and the lower wedge

136

back to their initial positions. When this happens, the lower wedge

136

moves downward and away from the upper wedge

134

and the dovetails (not shown) on the slips

138

help the lower wedge

136

pull the slips

138

inward. As a result, the slips

138

disengage the completion and the tool string and the gun brake

25

are free to be removed from the well.

The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention and are intended to fall within the scope of the following non-limiting claims: