Patent US4478285A successfully refreshed from Google Patents.
US4478285A
Expired / Lapsed
US
⏱️ 0.0 years remaining
Method and apparatus for removal of downhole well debris
Assignee: BULL DOG TOOL Inc 214 SOUTH LEECH HOBBS NM 88241 A CORP OF NM
•
Filing: 1982-01-25
•
Granted: 1984-10-23
•
Expires: 2002-01-25
Abstract
Abstract A method and apparatus for removing downhole well debris involves an elongate string operable to pump the debris from a well into the string. A pumping portion of the string includes a hollow piston with a hollow, splined piston rod fixed to the upper portion of the string, above the pumping portion, and keyed to the pumping portion to transmit rotary motion from the upper portion to the debris retaining portion below the pumping portion. A hollow piston head includes a valve, advantageously a ball valve, to prevent downward fluid flow. By reciprocating the upper portion, well debris is pumped into the debris retaining portion and solid debris is trapped therein by a one-way check valve. The liquid debris, such as heavy water, continues upwardly through the string passing through the interior of the piston and into the upper portion. The upper portion selectively receives either a ported sub, in which case the liquid is recirculated back into the well or an unported sub that prevents the return of the debris to the well. An additional check valve may be located in the upper portion to retain the liquid in the upper portion and to relieve the stress on the ball valve. The check valves, removably secured within removable subs, preferably are flap valves, including a torsion spring biased flap element with the torsion spring located out of the direct path of fluid movement to decrease valve fouling.
Inventors
- 👤 Donald B. Caldwell
Bibliographic Details
Application Number
—
Jurisdiction
US
Data Source
google_patents
Last Synchronized
2026-11-08 20:27
CPC Classifications
F; F04; F04B; F04B47/00; E; E21; E21B; E21B27/00
Claims Analysis
Total Claims: 47
Claim 1
Independent Claim
221 words
An apparatus for removing downhole well debris, comprising: an elongate string extendable into a well, said string connectable to a source of reciprocatory and rotary motion; a debris retaining portion, located in said string, including an outlet and an inlet capable of selective communication with said well, said inlet including a debris retaining check valve; a downhole pump portion located in said string in fluid communication with the outlet of said debris retaining portion, said pump portion including a cylinder, a piston and a piston rod, said piston retained for reciprocation within said cylinder and keyed to said cylinder; an upper portion of said string in fluid communication with said pump portion and connectable to said source of reciprocatory and rotary motion on one end and to said piston rod on the other end, said piston keyed to said cylinder to communicate the rotary motion of said under portion to said debris retaining portion; said piston rod and piston head including an internal passageway establishing fluid communication between said upper portion and said debris retaining portion through said pump portion; a check valve located at the end of said passageway atop said pump portion; and a check valve arranged to permit flow through said passageway from said pump portion to said upper portion and to prevent flow in the reverse direction.
Claim 2
Independent Claim
221 words
An apparatus for removing downhole well debris, comprising: an elongate string extendable into a well, said string connectable to a source of reciprocatory and rotary motion; a debris retaining portion, located in said string, including an outlet and an inlet capable of selective communication with said well, said inlet including a debris retaining check valve; a downhole pump portion located in said string in fluid communication with the outlet of said debris retaining portion, said pump portion including a cylinder, a piston and a piston rod, said piston retained for reciprocation within said cylinder and keyed to said cylinder; an upper portion of said string in fluid communication with said pump portion and connectable to said source of reciprocatory and rotary motion on one end and to said piston rod on the other end, said piston keyed to said cylinder to communicate the rotary motion of said under portion to said debris retaining portion; said piston rod and piston head including an internal passageway establishing fluid communication between said upper portion and said debris retaining portion through said pump portion; a check valve located at the end of said passageway atop said pump portion; and a check valve arranged to permit flow through said passageway from said pump portion to said upper portion and to prevent flow in the reverse direction.
Claim 3
Dependent Claim
30 words
The apparatus of claim 1 wherein said piston rod is a hollow kelly bar, said cylinder including an upper end conforming to the splined exterior configuration of said kelly bar.
Claim 4
Dependent Claim
44 words
The apparatus of claim 1 wherein at least one of said check valves is a flap valve, said flap valve including a valve seat, a spring and a flap element, said flap element being biased by said spring into a position engaging said seat.
Claim 5
Dependent Claim
56 words
The apparatus of claim 3 wherein said flap valve is arranged within a tubular sub, said flap valve having a fluid passageway of smaller size than the size of the internal passageway within said sub, said spring being a coil spring arranged over said seat out of the path of direct fluid movement through said sub.
Claim 6
Dependent Claim
15 words
The apparatus of claim 4 wherein said flap valve is removably locatable within said sub.
Claim 7
Dependent Claim
16 words
The apparatus of claim 1 wherein said piston includes an internal valve located in said passageway.
Claim 8
Dependent Claim
13 words
The apparatus of claim 6 wherein said internal valve is a ball valve.
Claim 9
Dependent Claim
22 words
The apparatus of claim 6 wherein said internal valve includes means for preventing the valve from seating closed during downward piston movement.
Claim 10
Dependent Claim
13 words
The apparatus of claim 6 including a seat removably threaded into said piston.
Claim 11
Dependent Claim
21 words
The apparatus of claim 1 including a fluid collecting chamber located atop said pumping chamber in fluid communication with said passageway.
Claim 12
Dependent Claim
20 words
The apparatus of claim 10 wherein said fluid collecting chamber is connected to the upper end of said piston rod.
Claim 13
Dependent Claim
15 words
The apparatus of claim 11 wherein said fluid collecting chamber is removable from said string.
Claim 14
Dependent Claim
24 words
The apparatus of claim 1 including a ported sub removably located atop said piston rod in fluid communication with said passageway through said piston.
Claim 15
Dependent Claim
34 words
The apparatus of claim 1 including a feathering sub, said sub including a reduced diameter passageway for supplying an upward force to said string in response to the passage of liquid through said sub.
Claim 16
Dependent Claim
19 words
The apparatus of claim 1 including a drilling tool connected to the lowermost end of said debris retaining portion.
Claim 17
Dependent Claim
19 words
The apparatus of claim 1 including a fishing tool attached to the lowermost end of the debris retaining portion.
Claim 18
Dependent Claim
43 words
The apparatus of claim 1 including a drive sub connectable to the upper end of said cylinder, said drive sub including a fishing neck on its exterior surface and a broached interior surface conforming to the splined exterior surface of said piston rod.
Claim 19
Dependent Claim
15 words
The apparatus of claim 1 including jarring means for jarring said string free of entanglement.
Claim 20
Independent Claim
181 words
A method for removing downhole well debris comprising the steps of: forming a string including a debris retaining portion, a pumping portion including a piston and cylinder, and an upper portion; lowering the string into the well with the pumping portion located above the debris retaining portion and below the upper portion, until the debris retaining portion is in proximity to the well debris; lifting the upper portion of said string with respect to a debris retaining portion, causing relative motion to occur between said piston and cylinder, to pump the debris into the debris retaining portion; collecting the solid component of the debris within said debris retaining portion; passing the liquid component of the debris through the piston head and piston rod of said piston, in isolation from the cylinder, and into the upper portion; turning the string to loosen the debris; securing a hollow chamber along said string fixed to the piston rod of said piston; and subsequently replacing said hollow chamber with a chamber ported to the exterior of said string to retain said liquid in the well.
Claim 21
Independent Claim
181 words
A method for removing downhole well debris comprising the steps of: forming a string including a debris retaining portion, a pumping portion including a piston and cylinder, and an upper portion; lowering the string into the well with the pumping portion located above the debris retaining portion and below the upper portion, until the debris retaining portion is in proximity to the well debris; lifting the upper portion of said string with respect to a debris retaining portion, causing relative motion to occur between said piston and cylinder, to pump the debris into the debris retaining portion; collecting the solid component of the debris within said debris retaining portion; passing the liquid component of the debris through the piston head and piston rod of said piston, in isolation from the cylinder, and into the upper portion; turning the string to loosen the debris; securing a hollow chamber along said string fixed to the piston rod of said piston; and subsequently replacing said hollow chamber with a chamber ported to the exterior of said string to retain said liquid in the well.
Claim 22
Dependent Claim
13 words
The method of claim 19 including the step of hydraulically jarring said string.
Claim 23
Dependent Claim
13 words
The method of claim 19 including the step of manually jarring said string.
Claim 24
Dependent Claim
17 words
The method of claim 19 including the step of pumping liquid from said chamber to the surface.
Claim 25
Dependent Claim
26 words
The method of claim 19 including the step of holding said liquid within said chamber and subsequently lifting said string and said liquid from the well.
Claim 26
Independent Claim
121 words
A method for removing solid and liquid downhole well debris including caustic liquid or heavy water comprising the steps of: lowering a string into a well until the inlet end of such string is in proximity to said debris; reciprocating a portion of said string to pump said debris into said string; separating the solid debris from the liquid debris; collecting said solid debris in a lower chamber; passing said liquid debris through the interior of a reciprocating piston having a piston head and rod; collecting said liquid in a chamber connected to the upper end of said piston rod; removing said debris from said well; and withdrawing said string from the well and replacing said chamber with a ported sub.
Claim 27
Independent Claim
121 words
A method for removing solid and liquid downhole well debris including caustic liquid or heavy water comprising the steps of: lowering a string into a well until the inlet end of such string is in proximity to said debris; reciprocating a portion of said string to pump said debris into said string; separating the solid debris from the liquid debris; collecting said solid debris in a lower chamber; passing said liquid debris through the interior of a reciprocating piston having a piston head and rod; collecting said liquid in a chamber connected to the upper end of said piston rod; removing said debris from said well; and withdrawing said string from the well and replacing said chamber with a ported sub.
Claim 28
Dependent Claim
15 words
The method of claim 24 including the step of pumping said liquid to the surface.
Claim 29
Dependent Claim
18 words
The method of claim 24 including the step of lifting said liquid to the surface with said string.
Claim 30
Dependent Claim
18 words
The method of claim 24 including the step of rotating said string to drill and loosen said debris.
Claim 31
Dependent Claim
24 words
The method of claim 24 including the step of adding water to the well to generate a sufficient hydraulic head to enable fluid pumping.
Claim 32
Dependent Claim
27 words
The method of claim 24 including the step of pumping said liquid through an internal passage extending through a piston rod and piston head of said piston.
Claim 33
Independent Claim
296 words
An apparatus for removing downhole well debris, comprising: an elongate string extendable into a well, said string connectable to a source of reciprocatory and rotary motion; a debris retaining portion, located in said string, including an outlet and an inlet capable of selective communication with said well, said inlet including a debris retaining check valve; a downhole pump portion located in said string in fluid communication with the outlet of said debris retaining portion, said pump portion including a cylinder, a piston and a piston rod, said piston retained for reciprocation within said cylinder and keyed to said cylinder; an upper portion of said string in fluid communication with said pump portion and connectable to said source of reciprocatory and rotary motion on one end and to said piston rod on the other end, said piston keyed to said cylinder to communicate the rotary motion of said upper portion to said debris retaining portion; said piston rod and piston head including an internal passageway establishing fluid communication between said upper portion and said debris retaining portion through said pump portion; a check valve arranged to permit flow through said passageway from said pump portion to said upper portion and to prevent fluid flow in the reverse direction; and at least one of said check valves being a flap valve, said flap valve including a valve seat, a spring and a flap element, said flap element biased by said spring into a position engaging said seat, said flap valve being arranged within a tubular sub, said flap valve having a fluid passageway of smaller size than the size of the internal passageway within said sub, said spring being a coil spring arranged over said seat and out of the path of direct fluid movement through said sub.
Claim 34
Independent Claim
296 words
An apparatus for removing downhole well debris, comprising: an elongate string extendable into a well, said string connectable to a source of reciprocatory and rotary motion; a debris retaining portion, located in said string, including an outlet and an inlet capable of selective communication with said well, said inlet including a debris retaining check valve; a downhole pump portion located in said string in fluid communication with the outlet of said debris retaining portion, said pump portion including a cylinder, a piston and a piston rod, said piston retained for reciprocation within said cylinder and keyed to said cylinder; an upper portion of said string in fluid communication with said pump portion and connectable to said source of reciprocatory and rotary motion on one end and to said piston rod on the other end, said piston keyed to said cylinder to communicate the rotary motion of said upper portion to said debris retaining portion; said piston rod and piston head including an internal passageway establishing fluid communication between said upper portion and said debris retaining portion through said pump portion; a check valve arranged to permit flow through said passageway from said pump portion to said upper portion and to prevent fluid flow in the reverse direction; and at least one of said check valves being a flap valve, said flap valve including a valve seat, a spring and a flap element, said flap element biased by said spring into a position engaging said seat, said flap valve being arranged within a tubular sub, said flap valve having a fluid passageway of smaller size than the size of the internal passageway within said sub, said spring being a coil spring arranged over said seat and out of the path of direct fluid movement through said sub.
Claim 35
Dependent Claim
15 words
The apparatus of claim 30 wherein said flap valve is removably locatable within said sub.
Claim 36
Independent Claim
227 words
An apparatus for removing downhole well debris, comprising: an elongate string extendable into a well, said string connectable to a source of reciprocatory and rotary motion; a debris retaining portion, located in said string, including an outlet and an inlet capable of selective communication with said well, said inlet including a debris retaining check valve; a downhole pump portion located in said string in fluid communication with the outlet of said debris retaining portion, said pump portion including a cylinder, a piston and a piston rod, said piston retained for reciprocation within said cylinder and keyed to said cylinder; an upper portion of said string in fluid communication with said pump portion and connectable to said source of reciprocatory and rotary motion on one end and to said piston rod on the other end, said piston keyed to said cylinder to communicate the rotary motion of said upper portion to said debris retaining portion; said piston rod and piston head including an internal passageway establishing fluid communication between said upper portion and said debris retaining portion through said pump portion; and a check valve arranged within said piston and located in said passageway, to permit flow through said passageway from said pump portion to said upper portion and to prevent fluid flow in the reverse direction, said check valve including a seat removably threaded into said piston.
Claim 37
Independent Claim
227 words
An apparatus for removing downhole well debris, comprising: an elongate string extendable into a well, said string connectable to a source of reciprocatory and rotary motion; a debris retaining portion, located in said string, including an outlet and an inlet capable of selective communication with said well, said inlet including a debris retaining check valve; a downhole pump portion located in said string in fluid communication with the outlet of said debris retaining portion, said pump portion including a cylinder, a piston and a piston rod, said piston retained for reciprocation within said cylinder and keyed to said cylinder; an upper portion of said string in fluid communication with said pump portion and connectable to said source of reciprocatory and rotary motion on one end and to said piston rod on the other end, said piston keyed to said cylinder to communicate the rotary motion of said upper portion to said debris retaining portion; said piston rod and piston head including an internal passageway establishing fluid communication between said upper portion and said debris retaining portion through said pump portion; and a check valve arranged within said piston and located in said passageway, to permit flow through said passageway from said pump portion to said upper portion and to prevent fluid flow in the reverse direction, said check valve including a seat removably threaded into said piston.
Claim 38
Independent Claim
226 words
An apparatus for removing downhole well debris comprising: an elongate string extendable into a well, said string connectable to a source of reciprocatory and rotary motion; a debris retaining portion, located in said string, including an outlet and an inlet capable of selective communication with said well, said inlet including a debris retaining check valve; a downhole pump portion located in said string in fluid communication with the outlet of said debris retaining portion, said pump portion including a cylinder, a piston and a piston rod, said piston retained for reciprocation within said cylinder and keyed to said cylinder; an upper portion of said string in fluid communication with said pump portion and connectable to said source of reciprocatory and rotary motion on one end and to said piston rod on the other end, said piston keyed to said cylinder to communicate the rotary motion of said upper portion to said debris retaining portion; said piston rod and piston head including an internal passageway establishing fluid communication between said upper portion and said debris retaining portion through said pump portion; a check valve arranged to permit flow through said passageway from said pump portion to said upper portion and to prevent fluid flow in the reverse direction; and a ported sub removably located atop said piston rod in fluid communication with said passageway through said piston.
Claim 39
Independent Claim
226 words
An apparatus for removing downhole well debris comprising: an elongate string extendable into a well, said string connectable to a source of reciprocatory and rotary motion; a debris retaining portion, located in said string, including an outlet and an inlet capable of selective communication with said well, said inlet including a debris retaining check valve; a downhole pump portion located in said string in fluid communication with the outlet of said debris retaining portion, said pump portion including a cylinder, a piston and a piston rod, said piston retained for reciprocation within said cylinder and keyed to said cylinder; an upper portion of said string in fluid communication with said pump portion and connectable to said source of reciprocatory and rotary motion on one end and to said piston rod on the other end, said piston keyed to said cylinder to communicate the rotary motion of said upper portion to said debris retaining portion; said piston rod and piston head including an internal passageway establishing fluid communication between said upper portion and said debris retaining portion through said pump portion; a check valve arranged to permit flow through said passageway from said pump portion to said upper portion and to prevent fluid flow in the reverse direction; and a ported sub removably located atop said piston rod in fluid communication with said passageway through said piston.
Claim 40
Independent Claim
236 words
An apparatus for removing downhole well debris comprising: an elongate string extendable into a well, said string connectable to a source of reciprocatory and rotary motion; a debris retaining portion, located in said string, including an outlet and an inlet capable of selective communication with said well, said inlet including a debris retaining check valve; a downhole pump portion located in said string in fluid communication with the outlet of said debris retaining portion, said pump portion including a cylinder, a piston and a piston rod, said piston retained for reciprocation within said cylinder and keyed to said cylinder; an upper portion of said string in fluid communication with said pump portion and connectable to said source of reciprocatory and rotary motion on one end and to said piston rod on the other end, said piston keyed to said cylinder to communicate the rotary motion of said upper portion to said debris retaining portion; said piston rod and piston head including an internal passageway establishing fluid communication between said upper portion and said debris retaining portion through said pump portion; a check valve arranged to permit flow through said passageway from said pump portion to said upper portion and to prevent fluid flow in the reverse direction; and a feathering sub, said sub including a reduced diameter passageway for supplying an upward force to said string in response to the passage of liquid through said sub.
Claim 41
Independent Claim
236 words
An apparatus for removing downhole well debris comprising: an elongate string extendable into a well, said string connectable to a source of reciprocatory and rotary motion; a debris retaining portion, located in said string, including an outlet and an inlet capable of selective communication with said well, said inlet including a debris retaining check valve; a downhole pump portion located in said string in fluid communication with the outlet of said debris retaining portion, said pump portion including a cylinder, a piston and a piston rod, said piston retained for reciprocation within said cylinder and keyed to said cylinder; an upper portion of said string in fluid communication with said pump portion and connectable to said source of reciprocatory and rotary motion on one end and to said piston rod on the other end, said piston keyed to said cylinder to communicate the rotary motion of said upper portion to said debris retaining portion; said piston rod and piston head including an internal passageway establishing fluid communication between said upper portion and said debris retaining portion through said pump portion; a check valve arranged to permit flow through said passageway from said pump portion to said upper portion and to prevent fluid flow in the reverse direction; and a feathering sub, said sub including a reduced diameter passageway for supplying an upward force to said string in response to the passage of liquid through said sub.
Claim 42
Independent Claim
222 words
An apparatus for removing downhole well debris comprising: an elongate string extendable into a well, said string connectable to a source of reciprocatory and rotary motion; a debris retaining portion, located in said string, including an outlet and an inlet capable of selective communication with said well, said inlet including a debris retaining check valve; a downhole pump portion located in said string in fluid communication with the outlet of said debris retaining portion, said pump portion including a cylinder, a piston and a piston rod, said piston retained for reciprocation within said cylinder and keyed to said cylinder; an upper portion of said string in fluid communication with said pump portion and connectable to said source of reciprocatory and rotary motion on one end and to said piston rod on the other end, said piston keyed to said cylinder to communicate the rotary motion of said upper portion to said debris retaining portion; said piston rod and piston head including an internal passageway establishing fluid communication between said upper portion and said debris retaining portion through said pump portion; a check valve arranged to permit flow through said passageway from said pump portion to said upper portion and to prevent fluid flow in the reverse direction; and a fishing tool attached to the lower most end of the debris retaining portion.
Claim 43
Independent Claim
222 words
An apparatus for removing downhole well debris comprising: an elongate string extendable into a well, said string connectable to a source of reciprocatory and rotary motion; a debris retaining portion, located in said string, including an outlet and an inlet capable of selective communication with said well, said inlet including a debris retaining check valve; a downhole pump portion located in said string in fluid communication with the outlet of said debris retaining portion, said pump portion including a cylinder, a piston and a piston rod, said piston retained for reciprocation within said cylinder and keyed to said cylinder; an upper portion of said string in fluid communication with said pump portion and connectable to said source of reciprocatory and rotary motion on one end and to said piston rod on the other end, said piston keyed to said cylinder to communicate the rotary motion of said upper portion to said debris retaining portion; said piston rod and piston head including an internal passageway establishing fluid communication between said upper portion and said debris retaining portion through said pump portion; a check valve arranged to permit flow through said passageway from said pump portion to said upper portion and to prevent fluid flow in the reverse direction; and a fishing tool attached to the lower most end of the debris retaining portion.
Claim 44
Independent Claim
253 words
An apparatus for removing downhole well debris comprising: an elongate string extendable into a well, said string connectable to a source of reciprocatory and rotary motion; a debris retaining portion, located in said string, including an outlet and an inlet capable of selective communication with said well, said inlet including a debris retaining check valve; a downhole pump portion located in said string in fluid communication with the outlet of said debris retaining portion, said pump portion including a cylinder, a piston and a piston rod, said piston retained for reciprocation within said cylinder and keyed to said cylinder; an upper portion of said string in fluid communication with said pump portion and connectable to said source of reciprocatory and rotary motion on one end and to said piston rod on the other end, said piston keyed to said cylinder to communicate the rotary motion of said upper portion to said debris retaining portion; said piston rod and piston head including an internal passageway establishing fluid communication between said upper portion and said debris retaining portion through said pump portion; a check valve arranged to permit flow through said passageway from said pump portion to said upper portion and to prevent fluid flow in the reverse direction; and a drive sub connectable to the upper end of said cylinder, said piston rod having a splined exterior surface, said drive sub including a fishing neck on its exterior surface and a broached interior surface conforming to the splined exterior surface of said piston rod.
Claim 45
Independent Claim
253 words
An apparatus for removing downhole well debris comprising: an elongate string extendable into a well, said string connectable to a source of reciprocatory and rotary motion; a debris retaining portion, located in said string, including an outlet and an inlet capable of selective communication with said well, said inlet including a debris retaining check valve; a downhole pump portion located in said string in fluid communication with the outlet of said debris retaining portion, said pump portion including a cylinder, a piston and a piston rod, said piston retained for reciprocation within said cylinder and keyed to said cylinder; an upper portion of said string in fluid communication with said pump portion and connectable to said source of reciprocatory and rotary motion on one end and to said piston rod on the other end, said piston keyed to said cylinder to communicate the rotary motion of said upper portion to said debris retaining portion; said piston rod and piston head including an internal passageway establishing fluid communication between said upper portion and said debris retaining portion through said pump portion; a check valve arranged to permit flow through said passageway from said pump portion to said upper portion and to prevent fluid flow in the reverse direction; and a drive sub connectable to the upper end of said cylinder, said piston rod having a splined exterior surface, said drive sub including a fishing neck on its exterior surface and a broached interior surface conforming to the splined exterior surface of said piston rod.
Claim 46
Independent Claim
163 words
A method for removing downhole well debris comprising the steps of: forming a string including a debris retaining portion, a pumping portion including a piston and cylinder and an upper portion; lowering the string into the well with the pumping portion located above the debris retaining portion and below the upper portion, until the debris retaining portion is in proximity to the well debris; lifting the upper portion of said string with respect to a debris retaining portion, causing relative motion to occur between said piston and cylinder, to pump the debris into the debris retaining portion; collecting the solid component of the debris within said debris retaining portion; passing a liquid component of a debris through the piston head and piston rod of said piston, in isolation from the cylinder, and into the upper chamber; turning the string to loosen the debris; and checking the reverse flow of the liquid from the string at two successive locations above the debris retaining portion.
Claim 47
Independent Claim
163 words
A method for removing downhole well debris comprising the steps of: forming a string including a debris retaining portion, a pumping portion including a piston and cylinder and an upper portion; lowering the string into the well with the pumping portion located above the debris retaining portion and below the upper portion, until the debris retaining portion is in proximity to the well debris; lifting the upper portion of said string with respect to a debris retaining portion, causing relative motion to occur between said piston and cylinder, to pump the debris into the debris retaining portion; collecting the solid component of the debris within said debris retaining portion; passing a liquid component of a debris through the piston head and piston rod of said piston, in isolation from the cylinder, and into the upper chamber; turning the string to loosen the debris; and checking the reverse flow of the liquid from the string at two successive locations above the debris retaining portion.
References Cited (Backward Citations)
USPTO Annuity & Maintenance Windows
| Annuity Stage | Window Opens | Payment Deadline | Surcharge Period Until | Estimated Fee | Status |
|---|---|---|---|---|---|
| 3.5 Years (Year 4) | 1987-10-23 | 1988-04-23 | 1988-10-23 | $2000.00 | Window Closed / Lapsed |
| 7.5 Years (Year 8) | 1991-10-23 | 1992-04-23 | 1992-10-23 | $3760.00 | Window Closed / Lapsed |
| 11.5 Years (Year 12) | 1995-10-23 | 1996-04-23 | 1996-10-23 | $7700.00 | Window Closed / Lapsed |