US3871622A

US3871622A - Method and apparatus for the control of a weight suspended from a floating vessel

Info

Publication number: US3871622A
Application number: US378963A
Authority: US
Inventors: Edward Larralde; Glen Robinson
Original assignee: Vetco Offshore Industries Inc
Current assignee: Vetco Inc; Vetco Gray LLC
Priority date: 1972-07-25
Filing date: 1973-07-13
Publication date: 1975-03-18
Anticipated expiration: 1992-03-18

Abstract

The invention concerns the control of the suspended weight supported from a piston of a pneumatichydraulic system on a vessel subject to wave and tidal action and particularly a vessel employed in connection with submarine drilling operation. The particular improvement concerns the modulation of the pressure in a portion only of the system to compensate for changes in pressure in another portion of the system so as to maintain the total force on the piston substantially constant during each portion of the cycle action of the heave. This may be accomplished by supporting the load from a piston system supported by two cylindrical elements and independently modulating the pressure in one of them to compensate for variations in pressure in another of the cylinder elements.

Description

United States Ptt Larralde et al.

[ METHOD AND APPARATUS FOR THE CONTROL OF A WEIGHT SUSPENDED FROM A FLOATING VESSEL [75] Inventors: Edward Larralde; Glen Robinson,

bothof Santa Barbara, Calif.

[73] Assignee: Vetco Offshore Industries, Inc.,

Ventura, Calif.

[22] Filed: July 13, 1973 [21] Appl. No.: 378,963

Related US. Application Data [63] Continuation-in-part of Ser. No. 373,968, June 27, 1973, which is a continuation-in-part of Ser. No. 274,880, July 25, 1972, Pat, No. 3,841,607.

[52] US. Cl 254/172, 175/5, 175/27,

[51] Int. Cl E2lb 15/02 [58] Field of Search 254/172, 173 R, 173 A,

[56] References Cited UNITED STATES PATENTS 3.653636 4/1972 Burrell 175/5 Mar. 18, 1975 3,718,316 2/1973 Larralde 254/173 3,746,329 7/1973 Galle ..267/125 Primary E.\'aminer-Robert B. Reeves Assistant Eraminer-Thomas E. Kocovsky [57] ABSTRACT The invention concerns the control of the suspended weight supported from a piston of a pneumatichydraulic system on a vessel subject to wave and tidal action and particularly a vessel employed in connection with submarine drilling operation. The particular improvement concerns the modulation of the pressure in a portion only of the system to compensate for changes in pressure in another portion of the system so as to maintain the total force on the piston substantially constant during each portion of the cycle action of the heave. This may be accomplished by supporting the load from a piston system supported by two cylindrical elements and independently modulating the pressure in one of them to compensate for variations in pressure in another of the cylinder elements.

2 Claims, 3 Drawing Figures M MW PATENTEDW1 8' SHEET 1 {If 2 PATENTEUHAR] 8|975 I 3,871 ,622

sum 2 OF 2 METHOD AND APPARATUS FOR THE CONTROL OF A WEIGHT SUSPENDED FROM A FLOATING VESSEL This application is a continuation-in-part of application Ser. No. 373,968 filed June 27, 1973 which in turn is a continuation-in-part of application Ser. No. 274,880 filed July 25, 1972 now U.S. Pat. No. 3,841,607.

BACKGROUND OF THE INVENTION This invention relates to improvements in methods and apparatus for the control of a suspended weight from a floating vessel and is particularly directed to the control of forces imposed on the drill string of a floating vessel employed in drilling, coring, running casing, reaming, cementing, testing, or other services in bore holes drilled in subaqueous environments where the vessel is subjected to wave or tidal action.

The particular and preferred object of this invention is to improve the operation of such systems in which the element constituting the load is suspended from a pneumatically controlled hydraulic system. As is well known in the petroleum industry, the drill string, due to the great length above the drill collar, is a very flexible member subjected to considerable stretch due to its length and due to its own weight. The weight on the bit is less than the total static weight of the drill string because of the tension in the drill string imposed by the drilling lines as is well understood by those skilled in this art. The practice during drilling is to keep the drill pipe above the drill collar in tension. The drill collar acts as a weight-producing element which exerts the load on the drill bit.

In the hydraulic-pneumatic systems of the prior art, this is accomplished by a gas pressure in an accumulator which pressurizes liquid in a hydraulic cylinder underneath the piston which supports the weight. The'cylinder may be positioned to support a crown block or connected to the traveling block conventional in drilling derricks.

Wave action imposes a vertical oscillatory motion on the vessel which is imposed on an hydraulic cylinder resulting in variations in the tension in the drill pipe and therefore in variation in the load imposed upon the drill bit, when this is employed or any other load connected to the piston rod. In the case of the hydraulic-pneumatic systems, the pressure on the liquid underneath the piston rod is maintained by gas pressure in an accumulator; such systems are shown in the Hanes et al., U.S. Pat. No. 3,714,995 and in the Larralde et al., U.S. Pat. No. 3,718,316.

Experience with such systems has shown that a variation of about i 2 to i 5% of the suspended weight may be experienced at each wave cycle, even when no drilling advance is maintained. With drill advancing during drilling, an additional weight variation may occur. The results of these motions are that the pressure in the cylinder fluctuates and the degree of fluctuation increases as the drilling progresses.

Floating vessels operating as drilling vessels in the open sea may experience vertical motions, i.e., heave due to wave action ranging, for example, from as low as 2 inches to feet or more trough to crest as, for example, has been experienced in drilling of the North Sea. However, under ordinary conditions, the ships are on station and drilling when the heave is not more than about 10 to 15 feet. The wave action imposes a vertical displacement of the drilling vessel at a sinusoidal frequency. The period of such cycles has been reported in the range of 8 to 16 seconds but may be either greater or less.

The demand on the string will vary, depending on the services which they are to provide. Thus, for landing casing or instrument survey, it is desirable to hold the piston fixed in space at the desired level in order that the casing or instrument is not subject to displacement.

There is also another circumstance where it may become important that the piston be maintained at relatively stationary position in space. Thus, when the well suddenly develops a high pressure due to the production of gas and it becomes necessary to close the blowout preventer, it becomes highly important that the drill string remain fixed in spaced and not oscillate in the closed blow-out preventer so as not to damage it.

During drilling, however, the drill is to be advanced at a controlled rate while maintaining a controlled weight on the bit. It is desirable to maintain a desired upper load limit on the bit in order that excessive stresses and torque are not developed which will be so large as to injure or even cause rupture of the drill pipe. On the other hand, it-is desirable that the load on the bit be not reduced excessively so that the rate of advance is unreasonably reduced. Since the cost of operation of the drilling operation is materially effected by the drilling rate, it is desirable that the drilling rate be maintained at as high a rate as is consistent with safety. This is established by the driller based on his experience and the performance of the drilling operation under consideration. The driller sets the load required for the drilling advance to make the advance be at a satisfactory rate consistent with safety.

In the prior art, it has been suggested that the hydraulic cylinder be mounted on the derrick to support the crown block or between the traveling block and the hook and that a force be applied to the piston sufficient to maintain the desired fraction of the total-load of the drill string during drilling operation so as to hold the desired load upon the drill bit.

STATEMENT OF OUR INVENTION In our preferred embodiment, the weight, which in our presently preferred application of our invention may be a drill string, is connected .as above to the piston of a hydraulic cylinder with the liquid under the piston in communication with a pneumatic accumulator under gas pressure. It is the purpose of our invention to maintain a desired force on the piston in the cylinder to be maintained during the complete wave cycle so that a predetermined load or a predetermined load variation on the drill can be maintained notwithstanding the amplitude or frequency or changes in the frequency or amplitude of the wave action. Where it is desired that the drill be advanced, ourinvention will provide a control for the advance of the bit under a controlled load during the advance of the drill.

In our preferred embodiment, we accomplish this objective to maintain a substantially constant force on the piston rod, exerted by separately applied forces by separate pneumatic accumulators by modulating the pressure in certain thereof, whereby pressure variations in one of them arising from the tendency of the piston to move relative in space and relative to the movement of the cylinder, due to wave action, is compensated for by adustment of the pressure in the other accumulator. We may control force on the piston to hold the piston relatively fixed in space as when it is not desired to advance the drill string, but we may also, while maintaining the force, so modulate the volume of the pressures in the hydraulic system cause an advance of the piston in space as where a downward motion of the piston as in drilling is to be accomplished. In our preferred embodiment of a pneumatic-hydraulic system which supports a weight and is subject to cyclic action, we modulate the force exerted in the hydraulic-pneumatic system by imposing said force by independently controlled force applying means and varying the force imposed by at least one of them to compensate for variations of force in another of them so as to maintain a substantially constant force on the piston.

It is another object of our invention, in a pneumaticsystem, which is subjected to cycles of vertical displacement and which supports a weight, to maintain a substantially constant force on said weight by trimming force changes by independently modulating a portion of said force so as to compensate for the said force changes and to maintain a constant force on the system.

We accomplish .our objective by obtaining a signal responsive to the force on the piston rod supporting said weight, comparing said signal with a signal corresponding to a standard which is responsive to a desired force on the piston rod and thus obtain an error signal. We independently add or decrease a portiononly of the force exerted on the piston rod to reduceor ideally cancel the error signal.

DETAILED STATEMENT This invention will be further understood by reference to the drawings of which:

FIG. 1 is a somewhat schematic showing of the arrangement of the relations of parts of the system of our invention.

FIG. 2 is a section of a conventional valve employed in our system.

FIG. 3 is a schematic diagram of a control system of our invention.

FIG. 1 shows the application of a control of our invention to an operation from a floating vessel 1 acting as the drilling platform. The conventional derrick 2 mounted on the vessel carries the split crown block 3 from which the sheaves 5 are suspended by the drilling lines. The sheaves 5 carry a cylinder 4, vented at 7, in which is positioned a piston 8 connected to a tubular rod 9 from which is suspended the conventional hook 10 which carries a swivel 11 and the kelly 12. The drill pipe 14 is connected to the kelly and to the drill collar which is connected to the bit 15. The casing 16 is composed of the conventional marine riser and the bore hole casing assembly together with the usual drilling equipment.

The hollow piston rod carries a piston head 13 connected to the cylinder head by a closed-end tube 17 which passes through a seal in the piston 8. The tubular member 17 is bored at 6 to provide a communication between the inlet 19 to member 17 and to annulus 18 between the tubular rod 9 and the tubular member 17. This cylinder and rod construction and its use as a weight control have been described in the aforesaid application, Ser. No. 274,880, which is herewith incorporated in this specification by this reference. The tubular member 17 is connected by a pipe 19 via the solenoid valve 20, to be more fully described below, and to the reservoir 21 through valves 22, pump 24, and valve 23.

Valves

22 and 23 are solenoid controlled as will be described below. The reservoir 21 is also in communication with the accumulator 34 through the valve 27. The pump may circulate via valve 29 and also through

lines

31 and 32 via valve 20.

FIG. 2 illustrates a proportional metering valve 20 by which flow proportional to the magnitude and sign of the electric signal which activates coils 49 and is obtained as will be described below.

A torquing armature 47 is supported by a flexure tube 48 in such a manner that energizing coil 49 or coil 50 will cause torquing armature 47 to move in a direction determined by the relative forces exerted by the solenoid coils 49 and 50, moving element 51 and deflecting spring 52 with reference to pin 53. The resultant movement of the valve spool 56 will determine the area of the ports 61 and 61' which are uncovered and determine the relative flow of fluid from port 57 to ports and 62 via

interconnected ports

58, 61, and 61. When the electromagnetic forces from the

solenoids

49 and 50 are equal due to equal voltages applied to the solenoid coils or when both are unenergized, the spool 56 is centered, thus cutting off ports 57 from both ports 60 and 62. Y

Feedback shaft 53 engages feedback spring 52 which, in turn, bears on element 51 attenuating the movement of element 51 so that movement of element 51 represents the summation of forces resulting from the relative elasticity of flexure tube 48, feedback spring 52, and the magnetic flux forces in

coil

49 or 50, thus assuring a displacement of valve spool 56 in such a manner to allow flow proportional to the difference in the electric signal to

coils

49 and 50.

The valve 20 described herein is a well-known valve, and no invention is claimed for the valve apart from its use in the combination and for the purpose of our invention. Other valves to regulate the direction and magnitude of flow which will function similarly in our invention may be used.

The pneumatic accumulator 30 is in communication with the cylinder 4 through the valve 26. The tubular member 17 is connected to the reservoir 21 through

valves

20 and 22 or via valve 20, line 32, and valve 23. The tubular member 17 may also be connected to the accumulator 30 via the by-pass line 40 and valve 41 and to the accumulator 30a which may be pressurized by the gas inlet through valve 30b.

The line 19 is connected to the port 62. The line 31 is connected to the port 57 and to the accumulator 34 via valve 27 and to the reservoir 21 through the valve 22, pump 24 or through the regulator valve 29 to the reservoir 21. The pump during the operation of the system continuously circulates fluid through the by-pass valve 29. The reservoir may be at any pressure desired, e.g., it may be at ambient pressure and the pressure of the pump 24 may be set by the regulator 29. Line 32 is connected to the port 60 and to the reservoir through valve 23 and to the accumulator 34 through valve 27. The by-pass 40 with valve 41 connects the line 31 to the accumulator 30 via the by-pass 40 with the manual valve 41. The by-pass with the manual valve 45 connects the accumulator 34 and line 32. The valve 23 is by-passed by a manual valve 43 and the valve 22 is bypassed by a manual valve 42.

The pressure in the accumulator 34 is sensed by a pressure sensor 36 to give a voltage of e responsive to the pressure in 34. A strain gauge of 35 is mounted on the piston rod above the hook to give an output e proportional to the stress in the piston rod.

The stress and pressure sensors are provided with readouts which produce a voltageproportional to the parameters to which they respond.

The schematic block function diagram, FIG. 3, illustrates the servo control of the volume of the liquid in the accumulator. All electrical elements used in the system are conventional, and their selection will be understood by those skilled in the art to which they pertain. Their assembly in combination with the system here described illustrates the preferred embodiment of the control assembly of our invention.

The output voltage e of the strain gauge 35 sensor is compared with the output voltage e, of the pressure sensor 36 in comparator 46 to give an output proportional to the difference between e, and e The comparator may be any conventional device to give a signal responsive to the difference of two volt ages, such as a summation resistance network or a differential amplifier or a bridge. With

switches

65, 66 and 71 open (see FIG. 2),

valves

22 and 23 are closed;

manual valves

42, 41, 19b, 45, 27 open. Liquid under pressure is available for and 300 from the pump or the pressure source 24 if used. 7 I I We prefer to employ a differential amplifier-rectifier 46a. The inputs to the differential amplifier are the outputs e and e and the outputs of the differential amplifier-rectifier are applied, one to the coil 49 and the other to the coil 50. The differential tractive effort of 49 and 50 is, therefore, proportional to the respective signals and magnitudes of e, and e The resultant displacement of the spool 56 is, therefore proportional to this difference. The orifices at ports 65 and 61 will depend on the aforesaid difference. The rate of addition or removal of fluid from the annulus is thus made proportional to the demand in order to establish the desired level of forces.

During the charging of the accumulators and the cylinders and the annulus, manual switches 66 and 65 are open. The

solenoids

49 and 50 are de-energized. With switch 71

open solenoids

71 and 73 are both deactivated, the

valves

22 and 23 are closed.

With switch 66 open, the

solenoids

49 and 50 are both deenergized; the spool 56 is centered;

ports

60, 62, and 57 are shut off from each other; and the line 19 is closed off from line 31.

The pump 24 circulates fluid through the pressure regulator valve 29 and via the manual valve 42, line 31, by-pass valve 41, valve 45, manual valve 27 to the accumulator 34 and to the accumulator 30 and via the

manual valves

45 and 43 back to the reservoir.

The pressure regulator 29 is set so that the pressure at 22 is above the highest pressure attained in the annulus during operation. If the pressure source is used during operation to supply pressure to the annulus as described herein, it is established at this higher pressure.

Fluid also passes from line 31 via the by-pass line 40, valve 41 to the cylinder 4 and via the by-pass line 19a and manual valve 191; to the annulus 18 via line 19.

Pressure in the

accumulators

30, 30a and 34 is adjusted by adjusting the valve 29 and gas pressure in the accumulators and the circulation continued until the strain sensor 35 gives an output 2 measured at the readout 35a (see FIG. 3) which establishes the stress required to support the fraction of the weight of the drill string as described above. The output 2, of the pressure gauge 36 at that stress is read at the readout 360 (see FIG. 3). This pressure which corresponds to that stress is exerted in this annulus l8 and in the cylinder 4 and in the accumulators. The gas pressure in 34, 30, and 30a is adjusted to hold the required pressure.

Instead of connecting the accumulator 34 to accumu lator 30 via valve 34, we may use a precharged accumulator containing gas at a selected pressure less than the minimum pressure to be attained in operation. Valve 27 may be, but need not be, used. A fixed orifice 27a, shown in dotted lines, may be positioned in the line connecting the accumulator 34 and line 31. The accumulator 34 is connected to the accumulator 30 via valve 34. When the initial pressures are established in 30 and 30a at the level to establish the desired stress in the piston rod, the same pressure will be established in 34. The orifice is of such character and of such time constant that for the period of the heave and the pressure differences which are effective across the orifice the pressure in the accumulator remain substantially constant.

The pressure in 34 and the output e is thus a reference for the forces on the piston which establishes the stress required to support the desired load as measured by the output e; of the sensor 35.

Assume that the system is mounted on a floating vessel and moves upwards as the heave starts. In the form shown in FIG. 1, the cylinder moves upward with respect to the piston; and the volume in'the cylinder 4 under the piston 8 starts to decrease; and the volume in the annulus starts to decrease. Liquid transfers from the cylinder 4 and from the annulus 18 to the accumulator 30, valve 26 being open, increasing the gas pressure in 30 and resulting in an increased pressure in the cylinder 4 and in the annulus. This creates an increase in the force exerted on the piston and an increase in the stress in the piston rod.

The stress sensor output and the voltage e become greater than e the output voltage of the fixed reference sensor 36. The tractive effort of solenoid 50 exceeds that of the solenoid 49, and the spool shifts so as to place the port 57 in communication with both ports 60 and 62. The annulus discharges through port 60.

Withe greater than 2, and with solenoids and 68 oppositely poled, solenoid 70 closes switch 68, activating the solenoid 73 to open valve 23. Switch 67 remains open, and valve 22 is closed.

The pressure in the annulus 18 is vented through 19, valve 20, line 32, and valve 23 to the reservoir 21, to compensate for the increase in pressure in the cylinder 4, until the sum of the forces in the annulus 18 and the cylinder 4 as reported by the sensor output e equals the output e,. The switch 67 opens and valve 22 closes, 23 remaining closed. The tractive effort of 50 equals that of 49.

When this occurs, the spool 56 moves to close port 57 from the ports 60 and 62.

When the cylinder has reached the crest of the wave and starts to descend, the volume in the cylinder 4 increases and the volume in the annulus 18 increases. Pressure starts to fall in the cylinder and in the accumulator 34, and the stress sensor reports a decrease in the force on the piston rod and e falls in voltage below the output e The tractive effort of solenoid 49 exceeds that from the solenoid 50, and the spool 56 shifts to connect the

ports

62, 57, and 60. The solenoid 70 holds the switch 68 in open position and valve 23 closed. The

switch 67 closes, energizing the solenoid 71 opening valve 22. Pressure is exerted via valve 22, through 31, port 62 to the annulus 18.

As soon as the sum of the forces exerted on the piston in the cylinder 4 and annulus creates a stress so that e, e valve 22 closes as does valve 20 as described above.

The system thus withdraws fluid from the annulus and accumulator 30a during the period of the heave from the trough to the crest and adds fluid to the system during the period of the heave from the crest to the trough in an amount and under a pressure to maintain the total force on the piston substantially constant. The withdrawal or the addition is interrupted when the force in the piston sensed as a stress in the piston rod has reached a predetermined force at which the piston is to be supported. This operation will occur even though the descent of the piston from the drilling operation occurs. The criterion for the additional withdrawal of liquid is the deviation of the force from a predetermined norm, which is the force desired to be maintained under the piston under the conditions which it is sought to maintain the piston.

Should it be desired to change the conditions to adjust the pressure, the gas pressure is adjusted to either increase or decrease the pressure in 30 and 30a as desired; and the system will automatically adjust itself to that pressure as will be evident from what has been described above.

As has been explained above, the motion of the piston with respect to space is a combined motion of the piston due to the heave of thevessel and the advance of the drill during drilling.

The vessel and the cylinder are subjected to substantially sinusoidal motions which may be out of phase with the piston which is at lesser or greater amplitude, depending on the structure and operation conditions of the system. This will appear from the following:

LET:

y The spacial amplitude of displacement of the cylinder at any angle of the sinusoidal motion.

x The spacial amplitude of displacement of the piston at said angle 6.

A The maximum displacement of the cylinder, i.el, the amplitude of the heave at the crest, which is /z of the heave.

A, The effective area of the piston, i.e., the sum of the effective area a of the piston 8 and b the effective area of the piston 13.

V Volume of the gas in the accumulator when the system is at rest, i.e., when 0 0.

P The pressure in the gas on the liquid at V,,.

F The force on the piston.

V, The volume of the gas in the accumulator at any angle 6 of the cycle.

P The'pressure in the accumulator when the volume is V p, The pressure in the cylinder 4 and 1 is the pressure in the annulus when p, is the pressure in the accumulator.

z The spacial advance of the bit into the earth per second.

p The period in seconds of the cyclic motion.

w The contribution to the spacial displacement of the piston due to the advance of the drill string into the earth as in drilling per degree of the cycle at any angle 0 of the cycle.

The piston motion is influenced by damping considerations and moves out of phase with the motion of the cylinder. The phase angle (b depends on the dynamics of the system.

It has been observed that the cylinder motion with respect to the space is sinusoidal. Being sinusoidal, the spacial displacement of the cylinder at any angle 0 of the cyclic motion may be expressed as y A sin 6 and the displacement of the piston per degree of the cycle at the angle 0 of the cylinder cycle due only to the cyclic action is x A f (6) wheref( 0) is a function of the damping and other conditions of the system which may vary from cycle to cycle and even during any cycle and on wave condition, i.e., frequency and amplitude.

The relative displacement of the cylinder and piston, assuming no advance of the drill occurs, is, keeping the direction signs of the motion in mind,

if the sign of y and x are the same. Thus if the sign of x is opposite to the sign of y, then The change in volume AV of the liquid and of the gas, the total liquid volume being constant per degree of the cycle at any angle 0 is If, however, the piston descends due to the advance of the drill string, the total advance at any angle 6,d per degree of the cycle is At the end of each quarter cycle, the total advance of the drill in each quarter cycle ao P Since at y =A, the net displacement of the piston and cylinder at the end of each quarter cycle is 90 A i am) The net displacement of the piston relative to the cylinder is A i zp/4 The net change in volume of the liquid of the cylinder per degree of the cycle at any angle 0 is Since an equal change in the same sense will occur in the gas, the change in volume of the gas AV is At the end of each quarter cycle, the net volume change V during the quarter cycle is For example, assume the pressure p; and p are the same and equal to P in the accumulator when 0 0. The pressure in the cylinder and accumulator at any other angle will be a function of the volume change, i.e.,

where n is the polytropic gas constant which in the systems under consideration may be taken to be 1 to 1.1 for practical purposes.

V is the volume of the gas at any angle 0. AV is the change in volume in the accumulator at any angle 0 of any quarter of a cylce. The consequent pressure on the gas and liquid is where p, is the pressure in the accumulator and the cylinder at V i AV.

The total force F p,a p b. In order to maintain F constant, any change in p resulting from any AV must be compensated for by an opposite proportional change in p It is an object of our invention to maintain F substantially constant and to do so we provide means to assure that the average pressure per square inch under the pis' ton remains substantially constant during the controlled operation.

We do this by adding or substracting from the pressure under the piston in the annulus 18 an amount to compensate for the change in pressure in the cylinder and accumulator 30.

We may use any hydraulic cylinder of design to permit the support of the load by piston areas positioned in two or more cylinders, at least one of which contains liquid under the piston pressurized by a pneumatic accumulator. In our preferred embodiment, described above, however, we employ a cylinder which is a duplex hydraulic cylinder which is the subject of the aforesaid copending application, which is hereby incorporated by this reference. In a cylinder of this character, the piston is supported by two concentrically mounted cylindrical elements which move together; and the total force on the piston is the sum of the forces in each of the cylindrical elements. The elements are so arranged that on displacement in space one of the cylindrical elements simultaneously increases and desreases the volume in both cylinders.

The total change in volume of the gas in the accumulator as a result of one complete cycle is the relative displacement of the piston and cylinder resulting from the advance of the drill into the earth during the cycle.

In order to compensate for the variation in cylinder pressure resulting from the volume changes in the cylinder and in the accumulator during heave, we modulate the pressure in the annulus to maintain a substantially constant force on the piston.

The pressure p in the annulus is modulated upon any change in pressure in so that the force remains con stant.

As the pressure in the cylinder falls due to the transfer of liquid from the accumulator to the cylinder 4, the pressure in the annulus is increased 50 compensate for the decrease in pressure in the cylinder so as to maintain a substantially constant force on the piston. When the pressure rises in the cylinder clue to transfer of liquid from the cylinder to the accumulator, the pressure in the annulus is reduced to compensate for the increase in pressure in the cylinder. The result of this operation is to maintain a substantially constant force on the piston. This is accomplished both when the piston is held at substantially constant position in space or is advanced with respect to space at a constant or modified rate into the earth as in drilling.

The following example is for the purpose of illustrating the principles of our invention and not to be understood to be any limitation thereof' The following data may be taken as representative of possible practical conditions: Assume that a 0.45 sq. ft. and b 0.05 sq. ft., i.e., A 0.5 Assume that the drill string weights 200,000 lbs. and that the desired weight on the drill is 20,000. This requires a force on the piston of 180,000. The required pressures F 0.45 x 144 .05 x 144 180,000

F/144 0.45 .05 1250 psig If at 0 0 in the first cycle 1 =p then to attain 1250 psig, pl p2 2500 psig.

Let V 100 cu. ft. and y at 6 i.e., A be taken as 5 ft. The heave is thus 10 feet. Assume a period of 10 seconds, and z .02 feet per second, i.e., a drilling rate of 72 feet per hour.

D A id 5 i [0.02 X 10/4] 5 0.05

In the first quarter cycle at the crest, i.e., 6 90, the piston has traveled for 2.5 seconds and has descended 0.05 feet. The cylinder has risen 5 feet. Therefore, the decrease in volume of the liquid in cylinder 4 as a result of the displacement of the piston in the cylinder:

AV 0.45 (5 0.05) 2.2725 cu. ft.

Since this is a decrease in volume of liquid in the cylinder, this is a volume which is transferred to the accumulator and the volume of the gas is decreased by this amount. During the second and third cycle when the cylinder is moving from the crest to trough and the piston is advancing during drilling for 0.05 feet, while the cylinder is moving from crest to trough, the piston advances 0.05 feet in each quarter cycle, i.e., during the second and third cycle will be:

+AV= 2 X .45 (5 .05) 4.455 cubic feet This volume is transferred from the accumulator to the cylinder 4. The volume of liquid in the accumulator 30 has decreased and the gas volume has increased. In the last quarter, in moving from the trough to the midpoint of the heave, the piston has descended another 0.05 feet; and the cylinder has moved up 5 feet in space; so the volume of the liquid in the cylinder has decreased as has the gas in the accumulator.

-AV 0.45 (5 .05 feet) 2.2725 cu. ft.

This volume is transferred to the accumulator 30, decreasing the volume of the gas.

In going from 0 0 to the crest, in the first quarter cycle, the volume of the liquid in the accumulator increases by 2.2725 cu. ft. at the crest and the volume of the gas decreases by the same amount, i.e., to 97.7275 cu. ft. The pressure in the accumulator and in the cylinder 4 in pounds per square inch is:

p 2500 [100/97.7275] =2558 psig In order to re-establish the average pressure of 2500 psig and force of 180,000 pounds, the pressure in the annulus must be reduced so as to establish:

4,455 cu. ft. to 102.1825 cu. ft. and the pressure drops to 2447 psig. In order to re-establish an average pressure of 2500 psig, the annulus pressure is raised to 2980 from the 1980 psig at the crest, i.e., to re-establish a force of 180,000 lbs.

1n going from the trough to the midpoint of the rise, i.e., in the last quarter of the first cycle, the volume of liquid in the cylinder 4 again decreases and the liquid on transfer to the accumulator decreases the volume of the gas by 2.2725 cu. ft. from 102.1825, resulting in a volume of 99.91 cu. ft. and a pressure of 2502.3 psig. The pressure in the annulus must be reduced from 2981 psig to 2502.3 psig.

lt will be recognized that the process of pressure variation is a continuous function of the cyclic movement. In order for the forces in the piston to be maintained substantially constant, the pressure in the annulus must be adjusted continuously, as the pressure in the cylinder varies cyclically during the cyclic action.

Furthermore, the progressive changes in the volume of the gas in the accumulator will progressively change the magnitude of the compensating pressure in the annulus.

From the foregoing, it will be seen that the compensating pressure p at any angle 6 of any cycle is given by the general formula where 1,0 p b F and m-l is the number of complete cycles traversed and 0 is the angle in degrees traversed in the last incomplete cycle. As will appear, the value of p is at a maximum at the trough when 0 270, i.e. 31r/2.

The pressure p in the accumulator 30a is at the maximum at the trough in the first cycle since as m-l increases, p, becomes greater and thus p diminishes. 1t drops to 0 when p becomes 2777.8 lbs. so as to by itself establish a force of 180,000 lbs. The volume of the gas in the accumulator 30 is 2777.8 2500 (VJ/V1 AND cl V 2500 X 100/2777.8 90 cu. ft.

resulting from a transfer of 10 cu. ft. of liquid to the accumulator 30. This transfer occurs when the net transfer of liquid from the cylinder 4 to the accumulator 30 per cycle as shown above is:

2 X 2.2725 4.455 .09 cu. ft.

The transfer of 90 cu. ft. will occur in 100 cycles, i.e., 1,000 seconds. Since the piston descends at the rate of 0.02 feet/seconds; the maximum stroke available is 20 feet.

The total volume of fluid replaced to the annulus under pressure is during the half cycle from thecrest to the trough in each cycle 2 X .05 (5 .05) .495 cu. ft.

In 100 cycles 49.5 cu. ft., at an average pressure drop of 500 pounds, being the average of 1000 lbs. differer tial in the 3rd quarter of the first cycle to 0 psig in the 100th cycle, represents the energy expended.

At the end of the 100th cycle, the drilling lines are again adjusted to adjust the piston in the cylinder. The pressures are again establishehd as described above to begin again the new 100-cycle phase. When, as is usual in drilling practice, weight is added to the drill string,

where additional pipe is added, the pressures reflect this added weight in order to maintain the desired weight on the bit.

1n the above description of our preferred embodiment, we have employed a stress transducer as the signal to report the integrated forces on the piston system which generates the force on the piston rod. Since this I force is proportional to the sum of the forces in the cylinder 4 and annulus 18, we may use any means for reporting the magnitude of this sum. For example, we may employe pressure transducers for the pressure on each cylinder, i.e., in 4 and in the annulus 18. The butput signal may be a voltage which may be added, each multiplied by a factor proportional in one case to a and in the other, proportional to b. The multiplied voltages may then be added in a summation network to give the signal e which is employed as above. These expedients, multiplying and adding or subtracting voltages, will be understood by those skilled in the art. No invention is claimed for such components except as employed in the apparatus and process of our invention.

We maintain a constant force on the drill string and a constant load on the drill, irrespective of the variation in pressure in the cylinder resulting from cyclic action and advance of the drill string by introducing a compensating pressure in an auxiliary cylinder and piston which creates force generating pressures to maintain a substantially constant force on the piston and a substantially constant stress in the piston rod.

By monitoring the changes in the stress in the piston rod, we obtain a signal which integrates the changes during the advance of the piston in drilling. By regulating the increase or decrease of the net pressures responsive to change in the force on the piston, for example, by changes in stress in the piston rod, we integrate all the parameters which affect this force.

We do this in our preferred embodiment, as described above, by obtaining a signal which is responsive to the force on the piston during the cyclic motion of the cylinder and the advance of the piston as in drilling, if such occurs. We modulate the pressure in the annulus so as to cancel, substantially, the variation in the force signal from that which is responsive to the stress desired to be maintained.

While we have described the servo control system for maintaining a constant force on the piston by employing a hydraulic-pneumatic system, we may also use a pneumatic system. The force on each of the cylinders may thus be a pneumatic force exerted from a source of gas pressure. The pressures in the annulus 18 and the cylinder 4 may then be modulated by increasing and decreasing them as described above to maintain a constant force on the piston and constant stress on the piston rod.

We claim:

1. In an apparatus adapted to be mounted on a vessel subject to heave due to wave action, which apparatus includes a hydraulic cylinder, a first piston and a first piston rod in said cylinder, means to connect a load to said first piston rod, a second hydraulic cylinder and a second piston in said second hydraulic cylinder; said second piston operatively connected to said first piston, separate pneumatic accumulator systems, one each connected to each one of said cylinders for control of the forces exerted on said piston rod, the improvement which comprises means separately to vary the pressure in a first of said accumulator systems, to modulate the pressure in said first accumulator system to compensate for change in pressure in a second accumulator system on imposition of a load on said first piston, whereby the forces on said piston rod are maintained substantially constant.

2. In the apparatus of claim 1, said means to modulate said pressure including a source of liquid under pressure and a reservoir at a pressure lower than in one of said accumulator systems, control means for selectively opening a first communication between the said one of said accumulator systems and said source and selectively closing a second communication between said one of said accumulator systems and said reservoir when said first communication is opened and control means for opening said second communication and closing said first communication.

3. In the apparatus of claim 2, said control means comprising a signal means responsive to the forces imposed on the first piston rod by said load when said apparatus is mounted on said vessel, during all portions of the heave, means to generate a signal responsive to the forces predetermined to be maintained on the first piston rod by said load, means to generate an error signal responsive to the differences between said first and second-mentioned signals and means selectively to open and selective to close the aforesaid communications responsive to said error signal,

4. In the apparatus of claim 2, said communications including pipe connections between each of the cylinders and each accumulator system, a gas connection between each of the accumulator systems and a source of gas under pressure, a pipe connected between one of said cylinders and the accumulator system connected thereto and connected to said source, and another pipe connected between said last-named accumulator system and said reservoir, said means to open and close said communications through said pipes including valves in each of said pipes, means to open a first valve in one of said first-mentioned pipes and means to close a second valve in the other of said pipes when the first valve is opened and means to open said second valve when said first valve is closed.

5. In the apparatus of claim 4, control means to open the first valve in said pipe connected to said first accumulator system and said source and to close a second valve in the pipe connecting said first accumulator system to said reservoir system, when The force imposed on said piston rod by said lead when said apparatus is mounted on a vessel, is substantially below a predetermined value and control means to open the second valve and close the first valve when the force exerted on said piston rod is substantially above said predetermined value.

6. In the apparatus of claim 5, said control means comprising means to generate a signal responsive to the forces imposed on the first piston rod in said first cylinder, during all portions of the heave, means to generate a signal responsive to the forces predetermined to be maintained on the first piston rod, means to generate an error signal responsive to the differences between said first and second-mentioned signals and means to selectively open and selectively close the aforesaid valves responsive to said error signal.

7. In combination with a drilling vessel, a derrick, drilling lines suspended from said derrick, a hydraulic cylinder and piston-pneumatic accumulator system for control of the forces in said piston rod by a load connected to the piston rod, the improvement which comprises a plurality of accumulators, a plurality of pistons connected to the same piston rod, means to establish pressures separately on said pistons, said means including a cylinder element for each piston, one of said pneumatic accumulators connected to each cylinder, a source of liquid under pressure, a reservoir, a source of gas pressure for each accumulator, a first pipe means to connect to a first one of said accumulators to said source, and a second pipe means to connect said first one of said accumulators to said reservoir, and means opening communication through said pipe means between said source and said first one of said accumulators and closing said communication through said second pipe means, during that portion only of the heave when the volume of the liquid in a second one of said accumulators tends to decrease and means to open communication through said second pipe means and closing communication through said first pipe means during that portion only of the heave when the volume of liquid in the said second one of said accumulators tends to increase.

8. In the apparatus of claim 7, said means to open and close communication through each of said pipe means, valves in each of said pipe means and means responsive to said error signal to close and open the valves in said pipe means.

9. In the apparatus of claim 7', said control means comprising a means to generate a signal responsive to the forces imposed on the piston rod during all portions of the heave, means to generate a signal responsive to a predetermined stress to be maintained in the piston rod, means to generate an error signal responsive to the differences between said first and second-mentioned signals, and means to selectively open and selectively close the aforesaid communications responsive to said error signal.

10. In the apparatus of claim 9, said control means including means selectively to open a first valve in said first pipe means and to close a second valve in said second pipe means when the said force is substantially below a predetermined value and means to close said first valveand open the second valve when the force exerted on said piston is substantially above said predetermined value.

11. A method of controlling the spacial displacement of a piston rod connected to a piston in a cylinder which is mounted for vertical displacement on a vessel subject to wave action, and in which said piston is loaded by a weight connected to said piston, which load is opposed by liquid under pressure in said cylinder under the said piston, communicated from separate bodies of liquid under gas pressure contained in separate pneumatic accumulators, the steps of withdrawing liquid from a first one of said cylinders to a first one of said plurality of pneumatic accumulators and from said first one of said accumulators to decrease the pressure in said first one of said accumulators, when the volume of liquid in a second one of said plurality of pneumatic accumulators is increasing and adding liquid to said first one of said pneumatic accumulators to increase the pressure in said first one of said pneumatic accumulators whereby the force on said piston rod is maintained at a substantially constant value.

12. The method of claim 11, the step of advancing the piston spacially downward during each portion of the heave cycle and withdrawing from said firstmentioned said first one of said accumulators to an extraneous point a quantity of liquid responsive to the said spacial advance.

13. In the process of claim 12, said withdrawal occurringonly during that portion of the heave when the volume under the pistons in the cylinders is decreasing.

14. In the process of claim 13, introducing into the first one of said accumulators a quantity of liquid less than the quantity withdrawn, said introduction occurring only during that portion of the heave when said volume in said cylinders is increasing.

15. In an apparatus adapted to be mounted on a vessel subject to heave due to wave action, which apparatus includes a first cylinder, a first piston, and a first piston rod in said first cylinder, a second cylinder and a second piston in said second cylinder, said second pis ton in said second cylinder operatively connected to said first piston rod, a fluid pressure source connected to one of said cylinders for control of the forces exerted on said piston rod, which comprises means separately to vary the pressure in one of said cylinders, to modulate the pressure in one of the cylinders to compensate for change in pressure in another of said cylinders, whereby the forces on said piston rod are maintained substantially constant.

16. In the apparatus of claim 15,.a control means comprising a signal means responsive to the forces imposed on the first piston rod, during all portions of the heave, means to generate a signal responsive to the forces predetermined to be maintained on the first piston rod, means to generate an error signal responsive to the differences between said first and secondmentioned signals and means to adjust the pressure in one of said cylinders responsive to said error signal.

17. In combination with a drilling vessel, a derrick, drilling lines suspended from said derrick, a cylinder, and a gas pressure system for control of the forces in said piston rod, a plurality of pistons connected to the same piston rod, means to establish separate pressures on said pistons, said means including a cylinder element for each piston, a source of pressure connected to each cylinder, means to increase the pressure in one of said cylinders during that portion only of the heave when the volume under the piston on the other of said cylinders tends to decrease and means to decrease the pressure in said one cylinder during that portion only of the heave when the volume under the piston in the other cylinder tends to increase.

18. In the apparatus of claim 17, said means comprising a means to generate a signal responsive to the forces imposed on the piston rod during all portions of the heave, means to generate a signal responsive to a predetermined stress to be maintained in the piston rod, means to generate an error signal responsive to the differences between said first and second-mentioned signals, and means to selectively increase and decrease said pressure responsive to said error signal.

19. A method of controlling the spacial displacement of a piston rod connected to a pair of pistons, one in each of a pair of cylinders which are mounted for vertical displacement on a vessel subject to wave action, and in which said piston rod is loaded by a weight connected to said piston rod, which load is opposed by fluid pressure in each of said-cylinders under the said pistons, the steps of reducing the fluid pressure in one of said cylinders when the pressure in the other of said cylinders is increasing and increasing the pressure in said one of said cylinders when the pressure in the other of said cylinders is decreasing whereby the force on said piston rod is maintained at a substantially constant value.

20. The method of claim 19, the step of advancing the piston rod spacially downward during each portion of the heave cycle.

, ECHO Patent No. 3,871,622 Dated Mar. 18, 1975 Inventor) EDWARD LARRALDE and GLEN ROBINSON It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the Abstract, lines 2 and. 3, change "pneumatichy-draulic" to pneumatic-hydraulic Column 3, lines 17 and 18, after "pneumatic" insert hydraulic Column 3, lines 50-51, insert piston before "rod.

Column 5, line 35, change "signals" to signs Column 5, line 45, after "open" insert Column 6, line 17, change "34" to 34' Column 6, line 64, change "Pressure" to pressure and insert Gas before "pressure."

Column 8, line 36, change the formula from i 66 V A (yix) to i v A (yi Column 9, line 13, change the formula P (v from p .p (V (V V) to p 2 1 o o 1.. A 1 (V1 1 A Wn Column 9, line 27, correct the spelling of "subtracting."

Column 10, line 14, correct the spelling of "weighs.

Column 10, line 22, change "pl p2" to p p Patent NO. Dated Mar. 18,

EDWARD LARRALDE and GLEN ROBINSON Inventor(s) It is certified that error appears in the above-identified patent that said Letters Patent are hereby corrected as shown below:

Column 11, line 44, change the formula to read:

I 1 2 144b Y o P i aAsinG z??? j Column 11, line 60, delete "cl" after "AND."

Column 12 line 1, change to Column 12, line 14, correct the spelling of "established,"

Column 12, line 27, correct the spelling of "employ."

Column 13, line 42, change "selective" to selectively Column 15, lines 14-19 claim 12 should read as follows:

12. The method of claim ll, the step of advancing the piston spacially downward during each portion of the heave cycle and withdrawing from said firstmentioned accumulator to an extraneous point a quantity of liquid responsive to the said spacial advance.

Signed and ficalcd this twenty-eight Day 0f October 1975 [SEAL] Arrest" RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner of Patents and Trademarks

Claims

3. In the apparatus of claim 2, said control means comprising a signal means responsive to the forces imposed on the first piston rod by said load when said apparatus is mounted on said vessel, during all portions of the heave, means to generate a signal responsive to the forces predetermined to be maintained on the first piston rod by said load, means to generate an error signal responsive to the differences between said first and second-mentioned signals and means selectively to open and selective to close the aforesaid communications responsive to said error signal.

5. In the apparatus of claim 4, control means to open the first valve in said pipe connected to said first accumulator system and said source and to close a second valve in the pipe connecting said first accumulator system to said reservoir system, when the force imposed on said piston rod by said load when said apparatus is mounted on a vessel, is substantially below a predetermined value and control means to open the second valve and close the first valve when the force exerted on said piston rod is substantially above said predetermined value.

9. In the apparatus of claim 7, said control means comprising a means to generate a signal responsive to the forces imposed on the piston rod during all portions of the heave, means to generate a signal responsive to a predetermined stress to be maintained in the piston rod, means to generate an error signal responsive to the differences between said first and second-mentioned signals, and means to selectively open and selectively close the aforesaid communications responsive to said error signal.

10. In the apparatus of claim 9, said control means including means selectively to open a first valve in said first pipe means and to close a second valve in said second pipe means when the said force is substantially below a predetermined value and means to close said first valve and open the second valve when the force exerted on said piston is substantially above said predetermined value.

12. The method of claim 11, the step of advancing the piston spacially downward during each portion of the heave cycle and withdrawing from said first-mentioned said first one of said accumulators to an extraneous point a quantity of liquid responsive to the said spacial advance.

13. In the process of claim 12, said withdrawal occurring only during that portion of the heave when the volume under the pistons in the cylinders is decreasing.

15. In an apparatus adapted to be mounted on a vessel subject to heave due to wave action, which apparatus includes a first cylinder, a first piston, and a first piston rod in said first cylinder, a second cylinder and a second piston in said second cylinder, said second piston in said second cylinder operatively connected to said first piston rod, a fluid pressure source connected to one of said cylinders for control of the forces exerted on said piston rod, which comprises means separately to vary the pressure in one of said cylinders, to modulate the pressure in one of the cylinders to compensate for change in pressure in another of said cylinders, whereby the forces on said piston rod are maintained substantially constant.

16. In the apparatus of claim 15, a control means comprising a signal means responsive to the forces imposed on the first piston rod, during all portions of the heave, means to generate a signal responsive to the forces predetermined to be maintained on the first piston rod, means to generate an error signal responsive to the differences between said first and second-mentioned signals and means to adjust the pressure in one of said cylinders responsive to said error signal.

19. A method of controlling the spacial displacement of a piston rod connected to a pair of pistons, one in each of a pair of cylinders which are mounted for vertical displacement on a vessel subject to wave action, and in which said piston rod is loaded by a weight connected to said piston rod, which load is opposed by fluid pressure in each of said cylinders under the said pistons, the steps of reducing the fluid pressure in one of said cylinders when the pressure in the other of said cylinders is increasing and increasing the pressure in said one of said cylinders when the pressure in the other of said cylinders is decreasing whereby the force on said piston rod is maintained at a substantially constant value.