Wellhead drive brake system

A braking mechanism, for controlling the release of energy in a rod string (34) for a down-well rotary pump (33), incorporates a rotary member (16) positioned in the energy loop from a prime mover to the top end of the rod string, requiring that the rotary member rotate at a consistent speed ratio and direction with respect to the top end of the rod string. The rotary member drives a fluid pump through a slip clutch so that when the top end of the string rotates in the normal direction, the clutch slips and does not run the fluid pump. However when the top end of the rod string seeks to rotate in the opposite direction, for example on shut-down or power failure, the fluid pump is operated to pump fluid from a reservoir (20) and back to the reservoir in a closed loop which includes a mechanism for restricting fluid flow.

FIELD OF THE INVENTION 
This invention relates generally to the oil production industry, and has to 
do particularly with improving the safety of rotary downhole pumps, 
particularly upon shut down or power failure. 
BACKGROUND OF THE INVENTION 
In the past, many conventional oil wells were operated by a downhole pump 
at or close to the bottom of the well, the pump being of a conventional 
reciprocating kind actuated by a rod string, in turn reciprocated 
vertically by a pump jack. 
Many of these older reciprocating pumps have been recently replaced by 
rotary-drive progressive cavity pumps. Such rotary pumps are particularly 
suited for the production of crude oil laden with sand and water. 
However, because of the typical depth of an oil well, the torque applied at 
the top of the rod string, and the resistance of the pump at the bottom, 
can cause the rod string to wind up like a spring, thus storing the torque 
energy. Whenever there is a power failure or the system is shut down, this 
stored torque energy, along with the energy created by the fluid head on 
the pump, must release itself. Without any control on the rate of backspin 
of the rod string, serious problems have occurred. The problems tend to be 
as follows: 
the motor, connected to the rod string through a reducer and a sheave and 
pulley arrangement, may reach reverse speeds exceeding safe limits. These 
speeds tend to damage the motor, and can even cause it to explode. 
one or both of the sheaves can reach speeds exceeding their limits. 
on drive configurations in which the polish rod extrudes out the top of the 
drive, the projecting portion can bend and break, and the broken-off 
portion will then be flung away from the installation, due to centrifugal 
force. 
without some form of braking, the rod string could uncouple, with the 
result that the rod string and the pump would be lost down the hole. 
GENERAL DESCRIPTION OF THIS INVENTION 
In view of the foregoing disadvantages, it is an object of one aspect of 
this invention to provide a braking mechanism for use with a rotary 
pumping system. 
More particularly, this invention provides, for use with a pumping system 
in which a downhole pump has a rotor which is rotated by the bottom end of 
a rod string of which the top end is in turn rotated by torque energy 
derived from a prime mover, and in which twist energy is stored in the rod 
string during operation, 
a braking mechanism for avoiding a too sudden release of said twist energy 
in the rod string on shut down or power failure, the mechanism comprising: 
a) a rotary member mounted so that it rotates at a consistent speed ratio 
and direction with respect to the top end of the rod string, 
b) a fluid pump, 
c) a reservoir containing a fluid, 
d) an input conduit communicating the fluid in the reservoir with the 
intake of said fluid pump, 
e) an output conduit communicating the fluid in the reservoir with the 
output of said fluid pump, 
f) an adjustable flow-control valve located in one of said conduits, and 
g) an over-running clutch operatively associated with the fluid pump such 
that, when the top end of the rod string rotates in the direction 
corresponding to normal operating of the downhole pump, no pumping work is 
done by the fluid pump, but when the top end of the rod string rotates in 
the direction opposite that corresponding to normal operation of the 
downhole pump, the fluid pump does the work of pumping the fluid out of 
and then back to said reservoir against a resistance determined by the 
setting of the valve, 
whereby if the stored energy in the rod string is suddenly released, the 
energy is dissipated in a controlled manner. 
This invention further provides a pumping system comprising: 
a downhole pump which includes a stator and a rotor, 
a rod string having a top end and a bottom end, the bottom end being 
connected to, supporting and rotating said rotor, 
a prime mover providing torque energy for rotating said top end, whereby 
twist energy is stored in the rod string during operation, and 
a braking mechanism for avoiding a too sudden release of said twist energy 
in the rod string on shut down or power failure, the mechanism including: 
a) a rotary member inserted in the energy train between the prime mover and 
the top end of the rod string, such that the rotary member rotates at a 
consistent speed ratio and direction with respect to the top end of the 
rod string, 
b) a fluid pump, 
c) an over-running clutch between said rotary member and said fluid pump, 
connected such that when the top end of the rod string rotates in the 
direction corresponding to normal operation of the downhole pump, the 
clutch slips and does not run the fluid pump, but when the top end of the 
rod string rotates in the direction opposite that corresponding to normal 
operation of the downhole pump, the clutch powers the fluid pump, 
d) a reservoir containing a fluid, 
e) an input conduit communicating the fluid in the reservoir with the 
intake of said fluid pump, 
f) an output conduit communicating the fluid in the reservoir with the 
output of said fluid pump, and 
g) an adjustable flow-control valve located in one of said conduits, 
whereby the stored energy, when being released from the rod string, is made 
to do the work of pumping fluid around a closed circuit which includes a 
resistance in the form of said valve, thus dissipating said stored energy 
in a controlled manner.

DETAILED DESCRIPTION OF THE DRAWINGS 
Attention is first directed to FIG. 1, which is a somewhat schematic 
representation of the major components of the braking system to be 
described herein. In FIG. 1, a prime mover is constituted by a motor 10 
which has an upstanding shaft 12 carrying a sheave 14. 
To the left in FIG. 1, a braking mechanism is illustrated, including a 
rotary member 16, carrying at the top a sheave 18 in alignment with the 
sheave 14. The rotary member 16 is an elongate shaft parallel with the 
shaft 12, and extends through the interior of a reservoir 20 which is open 
to the atmosphere at the top 22 and includes of two side walls 24 (only 
one visible in FIG. 1) and two end walls 26. A bottom wall 28 is also a 
part of the reservoir 20, and the shaft 16 passes through the bottom wall 
28, but is sealed thereagainst to prevent leakage. 
Also projecting through the interior of the reservoir 20 is a main drive 
shaft 30 which is supported for rotation by a seal housing 32. The main 
drive shaft 30 is adapted to support the top end of a rod string 34 which 
extends down the well. The main drive shaft 30 passes through the bottom 
wall 28 of the reservoir 20, and is appropriately sealed to prevent 
leakage. 
In a preferred embodiment, the reservoir 20 is filled to about 2/3rds with 
hydraulic fluid 36. 
Within the interior of the reservoir 20, the shafts 16 and 30 are 
interconnected. In one variant, each of the shafts 16 and 30 carries a 
pinion gear, the two gears meshing in such a way that the ratio of 
rotation between the shafts 16 and 30 remains constant (with the shafts 
rotating in opposite directions). Another variant involves the provision 
of a sprocket on each of the shafts 16 and 30, along with a chain engaging 
both sprockets. In the second case, the shafts 16 and 30 would rotate in 
the same direction. 
Attention is now directed to both FIGS. 1 and 2, for a more detailed 
description of the braking mechanism. 
As best seen in FIG. 2, a hydraulic pump 40 communicates on the suction 
side with an intake manifold 42, and on the discharge side with a 
discharge manifold 44. Located in the discharge manifold 44 is a flow 
control valve 46 which can be manually adjusted in order to determine the 
resistance to flow through the discharge manifold 44. Both the manifold 42 
and 44 communicate with the interior of the reservoir 20, through sealed 
openings. 
FIG. 2 shows schematically that the shaft 16 is connected to an 
over-running clutch 48 which is in turn connected through flexible 
couplings 50 to the input power shaft 52 of the pump 40. 
The over-running clutch 48 is also called a "sprague" clutch, which 
transmits power only in one direction of rotation, but "slips" when it 
rotates in the opposite direction. In the present case, the over-running 
clutch sends power to the pump 40 only when the top end of the rod string 
34 rotates in the direction opposite that corresponding to normal 
operation, as it attempts to do upon power failure or shut down. However, 
when the rod string rotates in the direction corresponding to normal 
operation of the downhole pump, the clutch slips and fails to run the 
fluid pump 40. 
In operation, whenever the downhole pump is being operated normally, the 
direction of rotation of the shaft 16 is such that no rotation is 
transmitted through the over running clutch 48 to the pump 40, and 
therefore no hydraulic fluid is pumped in the loop circuit constituted by 
the reservoir 20 and the manifolds 42 and 44. 
However, when the entire pumping system shuts down for any reason, the rod 
string 34 will attempt to spin backwards, as the stored torque energy is 
released, This will cause rotation of the shaft 30, which in turn will 
rotate the shaft 16 through the meshing gears or the chain-locked 
sprockets. During this back-spin of the rod string 34, the rotational 
direction of the shaft 16 is such as to power the hydraulic pump 40 
through the over-running clutch 48, thus causing oil to be drawn from the 
reservoir 20 through the intake manifold 42, and discharging it through 
the flow control valve 46 and into the discharge manifold 44. 
The flow control valve 46 is selected such that, when substantially fully 
opened, the rod string 34 will be allowed to spin back at a relatively 
slow rate of rotation. Thus, in the case of backspin, oil from the 
reservoir 20 is continuously pumped in a closed loop by the pump 40, the 
closed loop containing an adjustable restriction in the form of the flow 
control valve 46. 
At bottom right in FIG. 1, the bottom end 33 of the casing of a drilled 
well contains the stator 62 and the rotor 64 of a downhole, positive 
displacement rotary pump and the bottom end 65 of the rod string 34. 
While our embodiment of this invention has been illustrated in the 
accompanying drawings and described hereinabove, it will be evident to 
those skilled in the art that changes and modifications may be made 
therein without departing from the essence of the invention, as set forth 
in the appended claims.