Wells – Valves – closures or changeable restrictors – Fluid operated
Reexamination Certificate
1997-11-10
2001-10-16
Bagnell, David (Department: 3673)
Wells
Valves, closures or changeable restrictors
Fluid operated
C166S321000
Reexamination Certificate
active
06302210
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates a subsurface safety valve and, more particularly, to a subsurface safety valve having a tubular housing and an axially shiftable flow tube used to manipulate a valve closure member.
BACKGROUND OF THE INVENTION
Subsurface safety valves (SSSVs) are used within well bores to prevent the uncontrolled escape of well bore fluids, which if not controlled could directly lead to a catastrophic well blowout. Certain styles of safety valves are called flapper type valves because the valve closure member is in the form of a circular disc or in the form of a curved disc. These flappers can be opened by the application of hydraulic pressure to a piston and cylinder assembly to move an opening prong against the flapper. The opening prong is biased by a helical spring in a direction to allow the flapper to close in the event that hydraulic fluid pressure is reduced or lost.
FIGS. 1 and 2
illustrate a standard safety valve configuration
10
wherein a safety valve
14
is interposed in a tubing string
12
. A control line
16
is used to open the valve. The valve
14
includes a tubular valve housing
18
with an axial passage
20
. When hydraulic pressure is applied through port
22
, the pressure forces a piston
24
to engage an axially shiftable opening prong
30
. As the pressure forces the piston downward, the opening prong engages the closure member
32
and pushes the member into an open position. A spring
28
opposes the motion of the piston so that when the hydraulic pressure is released, the piston and opening prong are returned to a first position. The weight of the hydraulic fluid produces a “head” force against the piston, and thus is a factor in sizing the spring
28
. In general, the pressure required to close the valve
14
is given by:
Pressure
closing
=Force
spring
/Area
piston
Setting subsurface safety valves deeper is typically just a matter of ensuring sufficient closing pressure to offset the hydrostatic pressure acting to cause the valve to stay open. Increasing closing pressure is accomplished by increasing the Force
spring
or decreasing Area
piston
terms.
As the valve closing pressure increases, so does the valve opening pressure. The surface capacity to provide operating pressure is a combination of the pressure needed to open the valve and the internal well pressure:
Pressure
surface
=Pressure
opening
+Pressure
well
However, the available surface operating pressure can be limited by the umbilical line used to deliver the hydraulic pressure. It is not uncommon for that limit to be approximately 10,000 psi. Thus, if the surface pressure is fixed and the well pressure increases with depth, the opening pressure decreases with depth.
For this reason, designs which operate independent of well pressure are required. Two well known designs are the dome charges safety valves and balance lines safety valves. A balance line valve
40
having a piston
48
in a housing
42
is illustrated in FIG.
3
. Two hydraulic chambers are pressurized on opposite sides of the piston
48
. A control line is coupled to a first port
44
while the balance line is coupled to a second port
46
. Each hydraulic line is filled with the same type of fluid. Hydrostatic pressure from the well above and below the piston is equal. Thus, there is no downward force on the spring as a result of the hydrostatic pressure. The valve is operated by pressurizing the upper chamber
55
using the control line connected to the first port
44
. This increases the downward force F
1
, displacing fluid from the lower chamber
51
and compressing the spring
50
to open the valve. Well pressure only has access to the upper seal
54
.
Well pressure acts upwards on seal
52
and downwards on seal
54
. Therefore, the radius
49
of the upper end of the piston
48
is equal to the radius
53
of the lower end, and pressure has no upward or downward resulting force on the piston as long as the seals
52
,
54
remain intact. Control line pressure acts downward on surface area
56
while balance line pressure acts upward on surface area
58
. Thus, the hydrostatic pressures on opposite sides of the piston
48
are equalized. If seal
52
fails, well pressure enters the balance pressure chamber
57
, acting on surface area
58
, and increasing F
3
. If the well pressure is great, it may be impossible to supply sufficient surface pressure to port
44
to force the opening prong downward. Thus, the safety valve fails to a closed position. If seal
54
fails, well pressure would enter the control chamber
55
and act on surface area
56
increasing F
1
. Without applying control line pressure, the F
1
would be greater than F
2
+F
3
. This imbalance causes the valve to fail in an open position. The valve can be closed by pressuring up the balance line port
46
so that F
3
+F
2
is greater that the well assisted F
1
. This is only possible if sufficient balance line pressure can be applied. Another failure mode occurs when gas in the well fluid migrates into the balance line, reducing the hydrostatic pressure applied by the balance line, i.e. reducing F
3
.
Another style of balance line safety valve is illustrated in FIG.
4
. The valve
60
has a piston
64
captured within a housing
62
and three hydraulic chambers
68
,
70
, and
72
, two above and one below the valve piston. Two hydraulic lines are run to the surface. Well pressure acts on seals
74
,
80
. Since the radius
63
of the upper end and the radius
68
of the lower end of the piston are the same, well pressure has no influence on the pressure required to displace the piston. One of the two hydraulic lines is a control line and is connected to port
77
. The other hydraulic line is a balance line and is connected to the upper port
75
and the lower port
79
. Control line and balance line hydrostatic pressures act on identical piston surface areas
65
,
67
B-A′ and B-A″, so there is no net upward or downward force. If seal
74
leaks, well pressure accesses the balance line system. This pressure acts on surface area
67
, boosting force F
3
, which with spring force F
2
will overcome F
1
, to close the valve. If seal
76
leaks, communication between the control and balance lines will be established. F
1
will always equal F
3
. Thus, F
2
will be the only active force causing the valve to close. If seal
78
leaks, it has the same effect as seal
76
leaking. If seal
80
leaks, tubing pressure accesses the balance line system. This pressure acts to increase F
3
, overcoming F
1
and closing the valve. Thus, if sufficient control line pressure is available and tubing pressure is relatively low, it may be possible to open the valve if upper seal
74
and/or lower seal
80
leak. Control line force F
1
must be greater than the tubing assisted balance force F
3
plus the spring force F
2
. In all modes of failure for this valve, the valve fails to a closed position.
A dome charge safety valve uses a captured gas charge. The gas charge provides a heavy spring force to achieve an increased closing pressure. However, dome charge designs are complex and require specialized manufacturing and personnel. This increases the cost and decreases the reliability of the design because numerous seals are required. Also, industry standards favor metal-to-metal (MTM) sealing systems. Gas charges require the use of elastomeric seals.
A need exists for a safety valve suitable for subsea applications and which is well pressure insensitive. Thus, it should incorporate the benefits of a balance line SSSV while overcoming the difficulties associated with gas migration into the balance line. Such a valve should also utilize MTM sealing systems for increased reliability. Finally, the improved valve should allow for the application of hydraulic pressure to close the valve in the event of a valve failure in an open position.
SUMMARY OF THE INVENTION
The present invention relates to an improved safety valve that can be used in deep set applications by utilizing a simple pre
Crow Robert Wesley
LeBoeuf Gerald L.
Meaders Michael Wade
Vinzant Michael Burl
Bagnell David
Carstens David W.
Halliburton Energy Service,s Inc.
Imwalle William M.
Singh Sunil
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