The present invention relates to a device for resiliently applying an essentially constant force over a range of movement. It may have application in many situations where it is desirable to apply a constant force, such as tension or compression, between two bodies which undergo relative movement. Such a situation is presented in conducting offshore drilling operations from a floating drill ship. The drilling apparatus, which is suspended from the ship, should remain stationary with respect to the well bore even though it oscilates relative to the drill ship which heaves up and down responsive to wave action. In order to keep the drilling apparatus stationary, the supporting tension exerted on it from the ship should remain essentially constant throughout the cyclical relative movement between the drilling apparatus and ship. The present invention provides a device for exerting constant tension throughout cyclical relative movement between two bodies.
Two embodiments of the invention are shown in connection with offshore drilling operations, although the invention is not limited to offshore drilling applications nor to the specific embodiments shown. One embodiment is as a motion compensator for supporting the drill string from a floating drill ship. The other is as a tensioner for supporting the marine riser through which drilling operations are conducted.
When floating drill ships are used in offshore drilling operations, the action of waves and tides cause vertical movement of the drill ship and derrick relative to the well bore and impart similar movement to the drill stem suspended from the ship. This vertical motion of the drill stem caused by the ship's motion has in the past been compensated for by the use of bumper subs, or other lost-motion connections, in the lower part of the drill string. These subs are expensive and require costly and time-consuming repairs. Also, vertical movements of the drill string make the use of a ram-type blowout preventer almost impossible because of the problem of stripping tool joints through the pressure-holding element. Additional problems are presented when it is necessary to close a ram-type of blowout preventer on a drill string in motion.
For these and other reasons, it has for some time been recognized as very desirable to be able to stabilize the drill stem and keep it essentially free from vertical motion caused by the ship's heave, thus eliminating the need for bumper subs and precluding the possibility of damage to a blowout preventer by the drill pipe oscillating back and forth through it. One prior art type of motion compensator which has been utilized to dampen drill stem motion is the so-called "pneumatic spring" arrangement, wherein a plurality of pneumatic cylinders carried by the drill ship are utilized to support the drill string. The cylinders are connected to an accumulator and supplied with pressurizing fluid so that the force of pressurizing fluid acting on the cylinder pistons is sufficient to support the weight of the drill string. Then, as the ship heaves down, in the trough of a wave, the pressurizing gas in the accumulator expands and the pistons in the pneumatic cylinders move upward with regard to the drill ship to keep the drill string in its original position. Conversely, when the ship heaves upward relative to the drill string, the pistons move downward in the cylinders and gas is compressed in the accumulator.
A principal problem with this type of arrangement has been the fact that a fixed amount of gas is confined within the pressure system so that the changes in volume of the system upon expansion and retraction of the cylinders result in changes in pressure of the pressurizing gas. Thus, when the ship heaves up the cylinder pistons move relatively downward and the volume of the pressurizing system is reduced. This volume reduction results in some increase in pressure which automatically increases the tension on the drill string and thus tends to lift the drill string relative to the well bore, or at least reduce the weight on the drill bit which should preferably remain constant. Conversely, when the ship heaves downward relative to the drill string, the system volume is increased and pressure drops somewhat. This reduces tension on the drill string and may allow the string to move down slightly, increasing weight on the bit. This pressure fluctuation may be minimized by the use of a very large accumulator so that the changes in volume due to the expansion and retraction of the pneumatic cylinders are small in relation to the total volume of the system, and the concomitant pressure changes are similarly small. However, the use of large accumulators is not economically attractive and, in any event, only minimizes the problem of pressure variations. Moreover, large accumulators must generally be mounted on the deck of the ship and connected to the cylinders (which are carried by the drawworks) by long, high-pressure hoses which constitute a safety hazard.
Similar problems exist in tensioning the cylindrical marine riser which extends between the sea floor mounted blow-out preventer stack and in the drill ship. Drilling operations are conducted through the bore of the riser and the annulus between the outside diameter of the drill string and the inside diameter of the riser is the conduit for return mud flow. A telescopic joint is provided atop the riser to accomodate the ship's motion, with the upper part of the telescopic joint being movable with the drill ship. The lower part of the telescopic joint and the remainder of the marine riser therebelow are resiliently supported from the ship. The tension exerted on the riser by the ship should remain essentially constant despite vertical movement of the ship to properly maintain the riser's integrity as a column.
The prior art method of applying tension to the riser is to fasten a number of wire ropes, usually from two to six, through the riser, reeve each around multiple sheeves on opposite ends of a pneumatic or hydraulic cylinder and thence to a dead end aboard the vessel. The cylinder is connected to a pneumatic accumulator and the cylinder and accumulator are pressurized, forcing the piston to extend from the cylinder and thus spreading the sheeve sets apart and tightening the wires. As the ship heaves, the cylinders expand and retract to accomodate the motion of the riser relative to the ship.
This system, as with the drill string motion compensator discussed above, is subject to pressure variations resulting from the cyclical operations of the cylinders. To reduce these pressure variations to an acceptable level requires the use of very large accumulators which are expensive to fabricate and which occupy a large amount of space in the vicinity of the derrick. It is apparent that these and similar problems encountered in other areas could be significantly reduced by a constant force device which is much smaller than prior art devices and which reduces force variations upon cycling.
The present invention provides means for minimizing pressure variations due to volume changes in the pressurizing system for an expansible cylinder by automatically varying the amount of gas present in the system. This is accomplished by providing in the accumulator a fluid present in both in gaseous and liquid phases and held at a temperature within the saturation range of the particular fluid. Then, as the pressurizing system volume changes due to expansion and retraction of the cylinders, the fluid will automatically evaporate, or condense, as necessary to accomodate the volume changes with minimal pressure variations. Moreover, since a small amount of fluid will yeild a large volume of gas upon vaporization, the accumulator may be much smaller than for prior art device while keeping pressure variations acceptably low.
It is therefore the principal object of the present invention to provide a device for exerting an essentially constant force on a force-bearing member over a range of relative movement between the device and the force-bearing member.
Another object is to provide such a device which may reduce force variations to a lower level than prior art devices and which is significantly more compact than prior art devices.
Another object is to provide such a device which utilizes a pneumatic cylinder connected to a pressurizing system for generating a force in which device a two-phase fluid accomodates volume changes in the pressurizing system while maintaining essentially constant pressure and constant force.
Another object is to provide a motion compensator of the pneumatic-spring type which utilizes a sufficiently small accumulator that the accumulator may be mounted with the motion compensator, which motion compensator nevertheless reduces pressure and tension variatons upon cycling of the cylinders to an acceptably low level.
Another object is to provide a motion compensator which utilizes a two-phase gas-liquid fluid in the accumulator whereby the amount of gas present in the pressurizing system varies directly with changes in volume of the system to thereby minimize pressure variations.
Another object is to provide such a motion compensator which may be mounted on, or below, the traveling block so that motions caused by the vessel's heave do not cause pulling in or playing out of the drilling line.
Another object is to provide an improved marine riser tensioner which incorporates these same advantages and which is smaller and more efficient than prior art tensioners.
These and other objects and advantages of the invention will be apparent from the drawings, specification and claims. In the accompanying drawings, which illustrate the preferred embodiment of the present invention, and in which like numerals indicate like parts:
FIG. 1 is a diagrammatic illustration in elevation of a ship having the present invention incorporated in a motion compensator and in a marine riser tensioner;
FIG. 2 is a view in elevation, with a portion in section, of the preferred form of drill string motion compensator of the present invention;
FIG. 3 is a view in elevation, with a portion in section, of an alternate form of motion compensator wherein the cylinders and accumulators are incorporated into a single unit; and
FIG. 4 is an enlarged fragmentary view of the marine riser and riser tensioner of FIG. 1 showing the expansible cylinder and pressurizing system.
Referring now to FIG. 1, there is shown a drill ship 10 floating on a body of water 12. The drill ship has a central well 13 through which drilling operations are conducted and mounts a derrick 14 which includes a crown block 16 with a travelling block 18 being suspended below the crown block by rotary line 20. A motion compensator embodying the constant force device of the present invention, is indicated generally at 22. It is mounted on the travelling block and supports the hook 24 and drill string 26. The drill string 26 includes the usual swivel 28 and Kelly 30 extending through the rotary table 32 with the remainder of the string therebelow being made up of sections of drill pipe 34 terminating in the usual drill collars and drill bit (not shown). It is the function of the motion compensator 22 to keep the drill string 26 stationary with respect to the ocean floor despite vertical movements of the drill ship 10 due to wave and tide action. Since the traveling block 18 moves with the ship 10, the motion compensator 22 may be considered to be mounted on the ship 10.
Extending between the ship 10 and the usual blowout preventer sack on the ocean floor (not shown) is the cylindrical marine riser 33 having a central bore 35 through which the drill string 26 extends. Drillling mud is sent into the well through the bore (not shown) of drill string 26 and returns to the ship through the annulus formed by the outer diameter of the drill string of the inner diameter of the marine riser.
The telescopic joint 37 is provided at the upper end of the riser to accomodate the ship's motion. The telescopic joint is comprised of upper cylindrical section 39 of reduced diameter which is mounted from and movable with the ship, and a lower cylindrical section 41. The lower portion 41 of the telescopic joint and the remainder of the riser therebelow are resiliently supported from the drill ship by a plurality of wire ropes 43 connected to riser tensioners 45 on the deck of the ship. As explained more fully below, the riser tensioners are constructed in accordance with the present invention to exert a constant resilient force on the force bearing members or wire ropes 43 despite movement of the wire ropes relative to the ship and tensioners 45.
Referring now to FIG. 2, there is shown in greater detail the preferred embodiment of the motion compensator. At the center of the motion compensator is the traveling block 18 suspended from rotary line 20. Attached to the lower end of the traveling block 18 is a horizontal yoke 36. Extending upward from each end of the yoke 36 is a cylinder 38 which includes a cylindrical body portion 40, a piston 42 slidable in the bore of cylinder body 40, and a cylinder rod 44 extending downward from piston 42. Force bearing means are provided for supporting the drill string from the cylinders which, in the preferred embodiment, comprise a second horizontal yoke 46 suspended from the lower ends of cylinder rods 44. Yoke 46 in turn supports the hook 24 and drill string 26 therebelow. The force bearing member, or yoke 46 will move relative to the ship 10 and motion compensator 22 as the ship heaves responsive to wave action. The motion compensator functions to accomodate this motion by expansion and retraction of the cylinders and, by virtue of the present invention will exert an essentially constant force on the force bearing member 46 so that the supporting tension for the drill string 26 remains constant.
Pressurizing means are provided for supplying pressurizing fluid to the cylinders which means include an accumulator 48, also carried by traveling block 18 and in fluid communication with each of the cylinders 38 by fluid lines 50. The pressurizing system thus comprises the accumulator 48, fluid lines 50, and that portion of the bores of cylinder bodies 40 below pistons 42. Thus, the effective volume of the pressurizing system will, respectively, increase and decrease as the pistons 42 move up and down in the bores of cylinder bodies 40.
Within the accumulator is a two-phase fluid comprising a liquid phase 52 and gaseous phase 54. The fluid may be a pure compound, such as carbon dioxide, or a mixture of compounds, as desired. As shown, the gas 54 is present throughout the pressurizing system above the liquid 52; that is, in the bores of piston bodies 40, in the fluid lines 50, and in the upper portion of the accumulator 48. However, if desired, the two-phase fluid can be confined within a closed or bladder-type accumulator with a liquid, such as hydraulic oil, in communication between the accumulator and the cylinders. In this way, the trapped two-phase fluid will accommodate the neccessary volume changes, with the hydraulic fluid merely transmitting the volume changes between the cylinders and accumulator.
The principle of operation of the constant force device depends upon the vapor-liquid fluid in the accumulator being held within the saturation range of the particular fluid or fluid mixture being used. The temperature of the saturated media will depend upon the pressure required by the load that is to be supported. Thus, the amount of load placed on the compensator dictates a particular pressure range which must be maintained in the cylinders to offset the load. This pressure in turn dictates at what temperature the two-phase fluid should be maintained in order that the liquid and saturated vapor phases will be in equilibrium at the given pressure. Means are therefore provided, indicated schematically by the heat transfer element 56, for adding heat to, or removing heat from, the fluid.
A weight indicator 58 connected to one of the fluid lines 50 senses pressure in the system and is calibrated to indicate the weight being supported by the motion compensator.
In operation, the cylinder rods 44 are extended as the vessel heaves up relative to the pipe string, and are retracted as the vessel heaves down. As the cylinder rods are extended, the pistons 42 move downwardly in the bores of cylinder bodies 40, decreasing the net volume of the pressurizing system. As the volume decreases, a portion of the gas 54, which is at its saturation point, condenses into its liquid phase. Conversely, when the piston rods are retracted, pistons 42 move upwardly in cylinder bodies 40, increasing the volume of the pressurizing system and causing a portion of the liquid 52 to boil back into the vapor phase. Since the amount of gas present in the pressurizing system thus varies directly with the volume of the system, it is not necessary for the system pressure to fluctuate as with prior art compensators.
The use of the present system permits the cylinders to cycle responsive to vertical movements of the drill ship with minimal pressure variation and with the use of a smaller accumulator than in prior art systems. However, the system as shown will still result in some pressure variation due to the latent heat of condensation and vaporization of the fluid. As gas is being condensed from the gaseous to liquid phase, heat of condensation is released, resulting in a temperature rise in the fluid. As the temperature rises, the pressure at which the gas and liquid phases are in equilibrium will also increase slightly, resulting in an overall pressure increase in the pressurizing system. Conversely, as gas is being evaporated, heat of vaporization is absorbed, resulting in a temperature drop. A drop in temperature results in a slight drop in system pressure. The actual amount of temperature fluctuation will depend on the volume of the accumulator and the amount of liquid present as well as on the heat transfer coefficient between the accumulator and the surrounding environment. A larger accumulator with more liquid provides a greater heat sink so that the temperature variation for a given quantity of heat gain, or loss, will be smaller. In any event, the pressure change is only temporary since heat will move across the accumulator wall and return the system to its original equilibrium. Alternatively, if desired, heat could be added or removed by the heat transfer element 56 to return the system to equilibrium more rapidly. However, in the system as shown it is preferred that the heat transfer element 56 be used only for adding or removing heat to alter the system equilibrium to accomodate lighter or heavier loads on the motion compensator as where additional sections of drill pipe 34 are added during drilling operations.
Referring to FIG. 3, there is shown an alternative form for the motion compensator wherein the cylinder and accumulator are incorporated into a single unit. In this form, the cylinder body 40 is surrounded by an outer, concentric shell 60 with the accumulator comprising the annulus between cylinder body 40 and shell 60. Ports 62 are provided in the wall of cylinder body 40 to permit fluid communication between the bore of cylinder body 40 and the annulus. The two-phase fluid is provided with the liquid phase 52 being present in the annulus and in the lower end of the bore of cylinder body 40. The gaseous phase 54 is present above the liquid phase in both the cylinder bore and the annulus. As before, the gaseous and liquid phases are in equilibrium so that the fluid will condense, or vaporize, as necessary to accomodate volume changes in the system as the pistons 42 move up and down responsive to the ship's vertical movements. As before, heat transfer means (not shown) are provided for altering the system equilibrium to accomodate lighter or heavier loads on the compensator.
It would be possible to provide a system wherein the separate accumulator is dispensed with altogether and the two-phase fluid is simply present in the bore of the cylinder. However, as discussed above, the vaporization and the condensation of the gas cause temperature changes in the system which temporarily alter the equilibrium. This makes the use of a very small amount of fluid (as would be the case if the accumulator were dispensed with) less desirable, since it would provide a very small heat sink to absorb the heat gains and losses. Consequently, the temperature fluctuations would be correspondingly larger, as would the pressure variations. Therefore, it is generally preferable to provide some substantial quantity of fluid in the pressurizing system to reduce the transient effect of the heat gains and losses. However, in certain applications where larger pressure variations can be tolerated, the accumulator could be dispensed with. In the design of an actual system, a balance is struck between pressure variations which may be tolerated and the size of the accumulator to be used.
Referring now to FIGS. 1 and 4, there is shown a constant force device according to the present invention embodied in a riser tensioner for resiliently supporting a marine riser 33. The riser 33 is supported from the drill ship by a plurality of force bearing members which move relative to the riser tensioner as the ship heaves. In the preferred form, the force bearing members comprise four wire ropes 43 which extend upward from the lower section 41 of telescoping joint 37 in marine riser 33 through the central well 13 of the drill ship 10 and are rove over idler pulleys 70 on the drill ship, from which they extend to the riser tensioner apparatus 45. While only one riser tensioner is shown in FIG. 4, preferably four such tensioners would be provided, one for each wire rope 43.
The riser tensioner 45 comprises an expansible cylinder 72 together with pressurizing means including an accumulator 74 and connecting fluid line 76. The cylinder 72 includes a body portion 78, a piston 80 slidable in the bore of cylinder body 78 and a piston stem 82. A set of sheaves 84 are mounted on each end of the cylinder and the wire rope 43 is rove over the sheaves and dead ended to the ship as by attachment to eye bolt 86. If desired, the wire rope 43 may be reeved over the sheaves in multiple parts, in which case the stroke of the cylinder 72 needed to compensate for a given vertical movement of the ship is reduced by a factor equal to the number of parts, but the force which the cylinder must generate is multiplied by the same factor.
Pressurizing fluid is supplied to the cylinder 72 from the accumulator 74 through connecting flow line 76 which is in fluid communication with the bore of cylinder body 78 below the piston 80. The pressurizing system therefore comprises the accumulator 74, flow line 76 and that portion of the cylinder body bore which is below the piston 80. The effective volume of the pressurizing system therefore will change as the piston 80 strokes up and down.
As with the drill string motion compensator above, within the accumulator 74 of the riser tensioner 45 is a two-phase fluid comprising liquid phase 88 and gaseous phase 90 held at a temperature within its saturation range so that the two phases are in saturated equilibrium. As shown in FIG. 4, the gaseous phase is present throughout the pressurizing system above the level of liquid 88. However, if desired, the two phase fluid could be confined within a closed or bladder-type accumulator with a liquid such as hydraulic oil in communication between the accumulator and the cylinders.
Heat transfer means, represented schematically by coil 92 are provided in the accumulator 74 for altering the temperature of the two-phase fluid to establish the desired equilibrium point as discussed for the drill string motion compensator above.
In the operation of the tensioner, the force generated by pressurizing fluid supplied to the cylinder 72 from the pressurizing system tends to extend the cylinder and move the sheaves 84 apart, thus producing a force on the force bearing member (wire rope 43) which pulls the wire rope tight and supports the marine riser 33.
Motion of the riser relative to the ship caused by the ship's heave then alternatively pulls and relaxes the wire rope 43 which motion is compensated for by extension and retraction of the cylinder 72. When the cylinder 72 expands responsive to a downward movement of the ship, the volume of the pressurizing system increases and a portion of the liquid 88 in the accumulator will vaporize to increase the amount of gas 90 in the system and restore equilibrium between the two phases at essentially the same pressure as before. Conversely, retraction of the cylinder, resulting from the ship heaving upward relative to the marine riser 33 and wire rope 43 causes the volume of the pressurizing system to decrease and a portion of the gas 90 will recondense into the fluid phase 88. By use of the two phase fluid in the accumulator, the cylinder may stroke to compensate for movements of the wire rope 43 caused by the vessel's heave with smaller pressuring variations and with a smaller accumulator than in prior art devices.
The foregoing disclosure and description of the invention is illustrative and explanatory thereof, and various changes in the size, shape and materials, as well as in the details of the illustrated construction, may be made within the scope of the appended claims without departing from the spirit of the invention.