Motion Compensated Crown Block System

Abstract

A motion compensated crown block system in which a crown block sheave means is movable along a vertical path defined by a framework usually forming part of a drilling rig which is supported on a vessel. The crown block sheave assembly, which carries the hook load, is principally supported by inclined fluid pressure cylinder and piston means having one of their ends pivotally connected to the sheave means and their other ends pivotally connected to the framework in lateral spaced relation to the generally vertical pathway of the sheave means. Changing inclination of the cylinder and piston means during relative movement of the crown block sheave assembly and vessel provides variation in the piston reaction vertical force component which is caused to be nearly proportional to the change in fluid pressure in the cylinder and piston means. Load variation of less than 5 percent of the hook load is achievable over the full path of travel of the sheave means and is in the order of 2 percent over a major portion of said path in which the crown block sheave assembly travels. Means for sensing motion of the crown block sheave means relative to the sea bed are also provided so that the crown block sheave means will be virtually stabilized to maintain unchanged relation with respect to the sea bed during heaving of the vessel as caused by wind and wave action. Means are also provided for reducing to a minimum travel of the sheave cables over idle sheaves during travel of the crown block sheave means in its vertical path.

Description

BACKGROUND OF INVENTION

Drilling oil wells at subsea locations from a floating vessel presents difficult problems in maintaining a controlled position of the drilling string relative to the sea floor to provide uniform stable drilling bit pressure and substantially uniform tension in a drilling string because of the heaving motion of the vessel supporting a drilling rig to which the drill string is attached. It is desirable that the drill string be maintained at uniform tension and that variations in tension be minimized in order to carry on normal drilling and well completion operations, prevent undue stressing of the drill string, uneven drill bit pressure, and excessive wear on the drilling equipment, as well as to maintain a fixed in-hole drill string elevation for landing casing, tubing, setting packers, cementing, reaming and other operations requiring close elevation control.

Prior proposed systems of stabilizing or minimizing the relative motion between the vessel and the drilling string have included the use of bumper subs, compensating the line tension of crown block and traveling block sheave assemblies, compensating the traveling block sheave assembly, and compensating the crown block sheave assembly. Various compensating systems are shown in Kammerer U.S. Pat. Nos. 2,945,677, 3,151,686, 3,158,206, and 3,158,208. A compensating system positioned between the traveling block and swivel of a drill string is disclosed in Parks U.S. Pat. No. 3,208,728. Another type of compensating system is disclosed in Berne et al. U.S. Pat. No. 3,285,574 wherein pneumatic or oleopneumatic vertical jacks and jacks inclined to the path of a return pulley means over which passes a flexible conduit or tubing used in place of a drill string for turbine drilling.

A prior proposed motion compensated crown block is disclosed in Horton U.S. Pat. No. 3,469,820, owned by a common assignee, in which a system of pneumatic cylinder and piston means support a major portion of the load on a crown block sheave assembly and double acting hydraulic cylinder and piston means are employed to additionally support such load.

In such prior proposed systems mentioned above and in which pneumatic systems are employed to support the load, the volume of gas required in the pressure system is very great and space requirements for gas accumulators are very substantial. In addition, such prior pneumatic systems have used costly inert gases, such as nitrogen, since use of larger volumes of air at required high pressures was hazardous and dangerous with oleo systems. Moreover, the force-displacement relationship to accomplish the desired motion compensation by the use of prior proposed gas and hydraulic cylinder systems permitted motion compensation to be achieved at 10 percent or more of the total weight of the supported load including frictional loads. Thus while variation of drill bit pressure was previously held within a relatively narrow range, the amplification and enlargement of equipment to still further narrow the load limits was not justified by the additional material costs and space involved.

In addition, the vertical movement of the drill string causes the shifting of stresses in the drill collars where the point of neutral stress (change between compression and tension) may cross a drill collar or pipe joint causing undue stressing of such joint and possible failure. Such constantly changing stress at drill collar sections may cause deformation of the drill collars and produce unnecessary cutting of sides of a hole or hole deviation by the resultant dissipation of drilling energy into a lateral direction instead of a vertical direction.

SUMMARY OF INVENTION

The present invention contemplates a novel crown block motion compensating system particularly useful in performing well operations at sea including drillings, well completion, work-over and any other operation wherein motion of a floating installation must be compensated to provide close elevational control between elements fixed with respect to a reference point such as the sea bed and an element movable relative thereto. The present motion compensating system affords operational and economic advantages over prior proposed systems while avoiding their disadvantages. Generally speaking, the motion compensating crown block system of the present invention provides a plurality of fluid actuated cylinder and piston means so arranged with respect to a crown block sheave means as to vary the sheave supporting vertical force component exerted by the fluid cylinder and piston means on the sheave means during travel of the sheave means in a vertical path. As a result of this disposition of the fluid primary support cylinder and piston means and the correlation therewith of a selected gas pressure and accumulator volume for the cylinder and piston means, a relatively flat force-displacement curve (hook load change versus crown block vertical displacement relative to the vessel) is obtained and accumulator volume is minimized. The accumulator system of a volume capable of use with this invention may be mounted on top of a derrick immediately adjacent the crown block sheave assembly, thereby avoiding long lines and attendant frictional losses and maintenance. The invention further contemplates an idler sheave arrangement associated with the crown block sheave means which will reduce wear ("ton mileage") on the wire rope or cable used in the sheave means. The invention contemplates a sensing means for the heave motion of the vessel so that pressure in the fluid cylinders may be manually or automatically controlled in order to maintain stability of the crown block sheave means with respect to the sea bed, and thereby the drilling string with virtually uniform tension therein.

It is therefore a primary object of the present invention to disclose and provide a novel motion compensating system for a crown block sheave means used in various well operations as well as transfer and lifting operations performed at sea, the sheave means being carried by a vessel subject to wave motion.

An object of the present invention is to disclose and provide a resilient highly responsive support means for a crown block sheave means on a drilling rig wherein the arrangement of the resilient support means and its connection to the sheave means and the drilling rig framework enhances the achievement of a desirable hook load versus crown block displacement curve.

Another object of the present invention is to disclose and provide a motion compensated crown block sheave system wherein a plurality of fluid cylinder and piston means are especially arranged with respect to the crown block sheave means to permit reduction of accumulator volume space to a minimum.

A further object of the present invention is to disclose and provide a novel motion compensated crown block sheave system wherein idler sheaves are so arranged and supported with respect to the crown block sheave means that wear on the sheave lines is minimized.

A still further object of the present invention is to disclose and provide a crown block sheave system as mentioned above wherein the inboard and outboard path of movement of idle sheaves is related to movement of the sheave means in its vertical path to minimize wear on the sheave lines.

Still another object of the present invention is to disclose and provide a means for sensing the effect of wave motion on the vessel and transmitting such motion sensing to suitable means for controlling pressure exerted by the fluid inclined cylinder and piston means which support the crown block sheave means.

A still further object of the present invention is to disclose a motion compensating system for a sheave assembly utilizing inclined cylinder and piston means embodying fluid control means for cushioning movement of the sheave means at extremities of its normal path of travel and wherein a framework providing a slidable guide means for the sheave means is provided with shock or load absorbing means at the ends of the path of movement of the sheave means.

A still further object of the present invention is to disclose and provide main support cylinder and piston means constructed to function as a combined fluid support for the sheave means and also as a dashpot means when the sheave means approach ends of its path of travel.

A still further object of the present invention is to disclose and provide a crown block sheave means guidably supported for movement in a pathway relative to heaving motion of the vessel and wherein shock absorbing means are provided at opposite ends of the pathway of the crown block sheave means.

A still further object of the present invention is to disclose and provide a motion compensating system wherein main support cylinders and piston means actuated by an air-liquid system are provided with safety and protective means in the event unusual undesired loads are imposed upon the sheave means and the framework in which the sheave means is guided.

The invention further contemplates a motion compensating system for a sheave assembly movable in a vertical pathway wherein total load variation is minimized during movement of the sheave assembly in its pathway, wherein a novel idler sheave linkage cooperates with the sheave assembly to further reduce or eliminate the effect of changes in hook load, wherein fluid cylinder and piston means of hydraulic or hydro-pneumatic types are associated with said sheave asseembly to serve as dashpots therefor in the event of an unexpected, unwanted change in hook load, loss of air pressure in the primary support cylinder and piston means, or other failure in the system; wherein the basic system may be readily made positively active to virtually eliminate drill bit load variation by maintaining a constant distance between the sea bed and the crown block sheave assembly, and wherein cost and time savings are obtained by increased drilling efficiency and less equipment downtime.

Other objects and advantages of the present invention will be readily apparent from the following description of the drawings in which exemplary embodiments of the invention are shown.

In the drawings:

FIG. 1 is a fragmentary schematic view of a drilling rig on a floating vessel provided with a crown block sheave means embodying this invention.

FIG. 2 is a schematic view of the crown block and traveling block sheave means shown in FIG. 1.

FIG. 3 is an enlarged fragmentary elevational view of the top of a drilling rig equipped with a crown block sheave system as shown in FIG. 1, the sheave means being at the lower end of its vertical path, and corresponds to the position of the vessel at the top of a wave crest.

FIG. 4 is a fragmentary elevational view of the sheave system shown in FIG. 3 wherein the crown block sheave means is at a midpoint of its vertical path of travel, and corresponds to a vessel at the midpoint of its heave motion.

FIG. 5 is a fragmentary elevational view of the crown block sheave system shown in FIG. 3 wherein the sheave means is in its uppermost position in its vertical travel, and corresponds to the position of a vessel at the lowermost point in a wave trough.

FIG. 6 is an enlarged fragmentary sectional view taken from the horizontal plane indicated by line VI--VI of FIG. 5.

FIG. 7 is a fragmentary elevational view of a modification of the crown block motion compensating system of this invention.

FIG. 8 is a further modification of the crown block motion compensating system of this invention.

FIG. 9 is a schematic elevational view, partly in section, of a motion sensing means utilized in the present invention.

FIG. 10 is a fragmentary elevational view, partly in section, of a modification of such motion sensing means shown in FIG. 9.

FIG. 10a is a fragmentary schematic view of a still further modification of a motion sensing system.

FIG. 11 is a schematic view of a pneumatic pressure system utilized in the present invention.

FIG. 12 is a schematic view of a combined gas-liquid pressure system adapted for use with the motion compensating system of this invention.

FIG. 13 is a graph showing exemplary hook load crown block displacement curves, the abscissa showing length of stroke of the crown block, the ordinate indicating percent of hook load.

FIG. 14 is a schematic view of a further modification of inclined fluid cylinder and piston means utilized to support the sheave means and embodying a novel built-in dashpot system.

FIG. 15 is a schematic view with fragmentary cylinder and piston means partly in section and showing the combined dashpot and support system of FIG. 14.

FIG. 16 is a diagrammatic layout of an exemplary hydropneumatic actuating system used in the compensating system of this invention.

In FIG. 1 a floating vessel 20 is fragmentarily shown and may carry a drilling derrick or rig 21 constructed in well-known manner. At the top of derrick 21 a superstructure comprising a framework means 22 is provided. Framework means 22 includes a water table or platform 23 and an upstanding framework 24 which defines a vertical pathway for movement of a crown block sheave means 25. Crown block sheave means 25 is reaved with a wire rope 26 to a traveling block 27 which carries attachment means 28 for connecting the traveling block to the upper end of a drill string 29. The traveling block is prevented from rotation about its vertical axis by sidewardly extending arms 30 which may slidably engage spaced parallel vertical guide lines 31. Drill string 29 extends below vessel 20 through the water of the ocean to a well hole being drilled in the sea bed, not shown. At the bottom end of the drill string is a drill bit, not shown, upon which a desired predetermined pressure is imposed to provide optimum drilling. The load thus imposed upon the crown block sheave means 25 principally includes the weight of the drill string less the weight of the drill bit. In FIG. 2 the line 26 is passed over idler sheaves 32 supported in novel manner laterally of the axis of the crown block sheave means. The dead line portion 33 of the line 26 may be fixed to the vessel and the fast line portion 34 may be connected to draw works drum 35 suitably driven to take in and pay out line 34 as the length of the drill string is increased or decreased during drilling operations, (not motion compensating operations).

In the embodiment of the invention shown in FIGS. 1-6 inclusive, the superstructure framework means 22 at the top of derrick 21 includes a horizontal platform 23 from which the upstanding framework 24 extends and provides means for guiding said sheave means in said vertical pathway. Framework 24 may comprise a rectilinear arrangement of spaced square section vertical guide columns 40 providing opposed longitudinal side faces 41 engaged by rollers 42 (FIG. 6) carried on end plate means 43 of crown block sheave means 25. Inboard faces 44 of each column 40 provide guide surfaces for rollers 45 carried on side plates 46 of sheave means 25. The square section columns 40 thus provide vertical guide faces 41 and 44 generally perpendicular to each other and engagement thereof by rollers 42 and 45 guides vertical movement of sheave means 25 without askew or lateral displacement of the sheave means. The guide columns 40 may be strengthened and braced by inclined columns 48. Upper ends of columns 40 and 48 may be interconnected by top horizontal transverse members 49. Crown block sheave means 25 is thereby guidably movable in a vertical path between lowermost and uppermost positions as shown in FIGS. 3 and 5 respectively.

End plates 43 of the sheave means 25 support in bearing means 50 a crown block sheave shaft 51 which may extend beyond each of the end plates 43. A plurality of sheave members are carried by shaft 51 between end plates 43. Immediately below the crown block sheave means 25 traveling block means 27 also comprises a sheave shaft provided with a plurality of sheave members carried thereon. Line 26 may be reaved in wellknown manner about sheave members of the crown block and travelling block.

Means for resiliently supporting the crown block sheave means 25 in this example, comprises sets of fluid actuated cylinder and piston means 55 at opposite ends of crown block sheave means 25. Each cylinder and piston means 55 is provided at its upper end with a pivotal connection 56a to pivot shaft 56b which extends between and through end plates 43 (FIGS. 4 and 6). A pivotal connection 57 to framework means 22 is provided at the opposite end of each cylinder and piston means 55 in lateral spaced relation to the pathway of the sheave means and below the lower most position of the sheave means. In the lowermost position of crown block sheave means 25 (FIG. 3) the inclination of the axis of the cylinder and piston means 55 lies at a selected angle theta ".theta." to the vertical path of the crown block sheave means. A projection of said piston means axis may pass slightly below the axis of the sheave shaft 51. The angle theta of inclination of piston means 55 increases as sheave assembly 25 moves vertically upward to its uppermost position (FIG. 5). Vertical displacement of the sheave assembly 25 from lowermost to uppermost position may be in the order of from 1 to 30 feet dedepending upon design parameters.

As best seen in FIGS. 3 to 5 inclusive, piston rods 58 of cylinder and piston means 55 are projected and inclined upwardly during vertical movement, of the crown block sheave means. As schematically illustrated in FIG. 11, the magnitude of the vertical force component is a function of the sine of angle theta and may be expressed as

Fv=Fc sin theta

wherein Fv is the vertical force component, Fc is the resultant force exerted by the cylinder and piston means 25, and angle ".theta." is the angle of inclination of the axis of the cylinder and piston means 55 with respect to the horizontal. Thus a variable vertical force component is provided which is related to the varying angle of inclination of the cylinder and piston means 55 and to the position of the sheave means in its vertical pathway as later more fully described with relation to the construction and operation of the cylinder and piston means 55.

In one example of this invention, cylinder and piston means 55 may comprise fluid actuated means such as air or gas as shown in FIG. 11. Piston rod 58 may comprise a hollow tube 61 having an outer diameter slightly less than the inner diameter of cylinder member 62 to provide an annular space 63 therebetween of relatively small volume connected by ports 61a while providing a hollow inner chamber 64 within piston tube 61 of relatively large volume. A piston head 65 is provided at the inner end of piston tube 61 to slidably engage the internal surfaces of the cylinder member 62. At the lower end of cylinder member 62, a pressure fluid line 66 may be suitably connected at 67 in fluid communication with the interior of cylinder member 62. Pressure fluid line 66 is connected at its other end to an accumulator or pressure vessel 69. A valve 70 is provided in pressure fluid line 66 for control of pressure fluid between the accumulator and the cylindrical member 62. Accumulators 69 may be connected to a suitable source of pressure fluid such as a reservoir and air compressor, not shown.

Each accumulator 69 in this example may be located on or near platform 23 and is positioned close to the cylinder and piston means 55 with which it is associated. To provide uniformity of pressure in the two sets of cylinder and piston means 55 located at opposite ends of the crown block sheave assembly, the accumulators may be interconnected by suitable lines for equalization of pressure therein. Accumulators 69 are illustrated as a spherical pressure vessel; other suitable pressure vessel shapes may be used. Other arrangements of accumulator 69 may be provided depending upon available space at the superstructure framework, for example, accumulators 69 may be stacked vertically one upon the other, accumulators 69 may be arranged in a row, or arranged and located in other suitable fashion.

It should be noted that accumulators 69 are relatively small and that the total accumulator volume includes not only that of the pressure vessels 69 but also the volume of the chambers in the cylinder member 62 and the diametrically enlarged piston tube 61. Pressure in cylinder and piston means 55 is controlled to produce a resultant force which has a vertical force component of a magnitude sufficient to support the hook load less the weight on the drill bit and with the crown block sheave assembly at a midportion of its vertical stroke. As the vessel moves relative to the crown block sheave assembly because of wave action, the increase or decrease in pressure in the cylinder and piston means 55 balances the variation in vertical force component caused by the difference in inclination of the cylinder and piston means 55 as indicated by the change in angle theta so that variation in the hook load caused by heaving of the vessel is minimized as more fully described later.

The crown block sheave means 25 may also be connected to a dashpot means 72, in this example, comprising a vertically disposed hydraulic cylinder and piston means 73 having a piston rod 74 pivotally connected at 75 to end plate means 43 of the crown block sheave assembly. The lower end of cylinder means 73 may be connected by trunnions 76 to framework structure adjacent to platform 23. The dashpot means 72 may include a double-acting fluid arrangement utilizing a liquid or an oil wherein the fluid may flow freely between opposite sides of the piston head therein by the provision of suitable passageways. When the crown block sheave assembly reaches the end portion of its vertical pathway, the passageways in the dashpot means are so arranged that the motion of the crown block sheave assembly is rapidly dampened and controlled so that over-running of the crown block sheave means caused by some failure in the system will be controlled and the motion cushioned so that substantial damage to the structure may be avoided. Thus in the event of failure of the main pressure fluid support system, the dashpot means 72 will cushion the movement of the crown block sheave assembly into its lowermost position in its pathway and in such position the crown block sheave assembly may be utilized without motion compensation advantages. Dashpot means 72 may also provide a means for positive, active adjustment of the position of the crown block sheave assembly during operations which require precise vertical positioning thereof. It may also be noted that the hydraulic system for the dashpot means is separate from the pneumatic air system for primary support cylinder and piston means 55.

The present invention contemplates a novel arrangement for reducing wear or "ton mileage" on the sheave line 26. Each idler sheave 32 is supported for movement toward and away from the vertical pathway of the sheave assembly 25 by a support member 80 having a pivotal connection at its upper end to the idle shaft 81 of idler sheave 32 and having a pivotal connection at its lower end at 82 to a bracket 83 on platform 23. Idler sheave 32 is supported for arcuate movement relative to the pivotal connection at 82.

Lateral in and out motion of each idler sheave 32 with respect to the vertical pathway of the crown block sheave assembly 25 is controlled by a link member 85 pivotally connected at one end to idler sheave shaft 81 and pivotally connected at its other end to shaft 51 of the crown block sheave assembly. Thus as the crown block sheave assembly moves up and down (FIGS. 3-5) the length L of the cable portion extending between tangent points on idler sheaves 32 and the crown block sheaves remains unchanged. Thus linear motion of line 26 relative to the idle sheave is virtually eliminated because the line 26 merely partly wraps or unwraps from the circumference of the sheaves. Such elimination of relative linear motion of line 26 at sheave 32 essentially eliminates the "ton-mileage" on the drill line, reduces to a minimum wear or fatigue and during the stroke of the crown block sheave means, distributes vibration fatigue over the portion of a line which is being wrapped or unwrapped on the idler sheave 32.

When sheave means 25 is in its lowermost position, idler sheaves 32 are drawn inwardly toward the vertical pathway of the sheave means (FIG. 3). When the sheave means 25 is at the midportion of its vertical pathway, the idler sheaves 32 are spaced their maximum distance from the vertical pathway as determined by the length of the link members 85. At the uppermost position of sheave means 25, the idler sheaves 32 are again drawn inwardly toward the vertical pathway. Thus during heave motion of the vessel, the line 26 is alternately unwrapped from the crown sheaves and wrapped on the idler sheaves without any change in length of the line portion extending between tangent points on the sheaves. It will be understood that the link member 85 does not change normal stretching of line 26 that may occur during hook load changes.

Means for sensing vertical motion of the vessel relative to a fixed reference such as the sea bed is shown in FIGS. 9, 10 and 10a. In the embodiment shown in FIG. 9, a tension line 90 becketed at the water table is passed over a sheave at 91 mounted on the crown block sheave assembly 25. Line 90 is passed over idle pulleys 92 in any suitable location on the drilling rig 22 to provide a most direct, non-obstructed pathway to a drill opening 93 in the vessel through which line 90 extends for connection to a pulley 94 pivotally mounted on an anchor base 95 on the sea floor 96. Line 90 may be returned to a deck 97 on the vessel for connection to a tensioning means 98. Tensioning means 98 may include an air or hydraulic operated take-up reel which may be preset to a selected tension in line 90. A fixed length between the crown block sheave assembly and the sea bed is thus determined. The tension in line 90 varies as the vessel heaves. Such change may be sensed by a suitable tension sensing means 100 in engagement with a portion of the line 90 above the deck 97. Tension sensing means 100 includes motion sensing of the crown block sheave assembly 25 and such motion may be indicated upon a suitable readout instrument 101 provided at a control console used by a drilling operator.

Another example of the vertical motion sensing system is shown in FIG. 10 wherein line 90' is connected at 91' to the end plate means 43 of the crown block sheave assembly 25. Line 90' is also guided over idle pulleys 92', its tension may be sensed by a tension sensing means 100', and its lower end may be connected at 103 to a tension line 104 used with a riser pipe tensioning system indicated at 105. Riser tension line 104 may be suitably connected at 106 to riser pipe 107 which extends upwardly from the sub sea well head 108. A slip joint 109 at the top of riser pipe is provided for vessel heave and tide conditions. To control the length of line 90' and its tension during start up of operation or tidal changes, a tensioning means 98' is inserted in the line 90'. Tensioning means 98' may be a double drum winch with a differential drive and controls wherein one drum is connected to the upper line portion and the other drum to the lower line portion. The drums may have a common axis, the diameter of the drums being identical. It will be apparent that connection of line 90' to the riser pipe tension line 104, or riser pipe 107 is essentially the same as connecting line 90' to the sea floor in that a fixed reference is provided at a fixed distance from the sea floor.

In FIG. 10a a further modification of the vertical motion sensing system is schematically shown. Line 90" may be connected at its upper end to the crown block sheave means, as in the prior embodiments, and at its lower end to a guide line 110 which is used in the normal guide line system for lowering equipment to anchor base 95, the other guide line used in such systems not being shown for clarity. The connection of line 90" to guide line 110 may be made at any suitable location such as at 111. It will be apparent that the connection at 111 provides an essentially fixed reference point with respect to the sea bed. Operation of this modification is similar to the operation of the motion sensing means described with respect to FIG. 10.

Before describing operation of the motion compensated crown block means of this invention, a description of the modifications shown in FIGS. 7 and 8 will be made. In FIG. 7 a modification of the means for mounting the cylinder and piston means 55 on the framework means is illustrated. Cylinder and piston means 55' are inclined to the vertical pathway of the crown block sheave means 25' in accordance with the invention. Means for pivotally mounting each cylinder and piston means 55' may comprise a mounting member 112 at the central portion of the cylinder member 62', member 112 being pivoted at 113 to a framework bracket member 114 depending from platform 23 and at a pivot axis spaced from the vertical pathway of the sheave means 25' and below the lowermost position of the sheave means in its stroke. Thus as the sheave means 25' travels upwardly in the vertical guide means, the cylinder member 62' is permitted some rotation about the pivot axis 113. The upper end of the piston member 58 is connected to the outer end of the sheave shaft 51 so that the axis of the cylinder and piston means 55' approximately passes through the axis of the sheave shaft 51 during travel of the sheave means 25' in its vertical pathway.

Framework bracket member 114 may also support a holding means 116 to which may be fixed the cylinder member 73' of the dashpot means 72'.

It will thus be understood from the consideration of FIG. 7 that the cylinder and piston means 55' and the dashpot means 72' may be mounted on the framework means 23 in various ways so as to permit variation of the angle theta of the axis of the cylinder and piston means 55 during the travel of the crown block sheave means and also to afford a desired magnitude of vertical force component acting to resiliently support the crown block sheave means 25'.

In FIG. 8 there is shown a modification of the resilient support means for the crown block sheave means and also a modification of the support for the idle sheave means.

Resilient support means shown in FIG. 8 may include cylinder and piston means 55" pivotally connected at 120 to platform 23' and pivotally connected at 121 to a triangular end plate means 122 at one of the corners of said triangular plate. Crown block sheave means 25" may include a sheave shaft 51" supported on end wall 124 of a vertically slidably movable carriage 125. Carriage 125 may be guided in its vertical pathway by a plurality of upstanding guide members 126 suitably braced and defining the vertical pathway for carriage 125. Carriage 125 may be provided with suitable anti-friction means engaging upstanding guide means 126 in well-known manner. Triangular arrangement on plate 122 is disposed in inverted fashion and the lower corner of the plate is pivotally mounted at the axis of shaft 51".

Cylinder and piston means 55" may operate as described in the first embodiment of the invention. In this modification, however, auxiliary cylinder and piston means 128 may be provided with the piston rod 129 pivotally connected at 121 to the corner of the triangular plate 122 at which a rod 58" of the cylinder and piston means 55" is also connected. The opposite end of cylinder and piston means 128 may be pivotally connected at 130 to an outwardly extending member 131 fixed to upstanding guide members 126 at a selected portion of the vertical path defined thereby, in this example, above the central portion of the pathway. In such mounting of the auxiliary cylinder and piston means 128, it will be apparent that when the crown block sheave means 25" is at the lowermost position of its path of travel, the cylinder and piston means 128 are inclined downwardly and inwardly. At the uppermost position of the crown block sheave means 25", the auxiliary cylinder and piston means 128 is inclined upwardly.

Cylinder and piston means 128 may be single or double-acting. As the crown block sheave assembly means progresses to its lower-most position, the auxiliary cylinder and piston means 128" exerts a downward vertical force component against sheave assembly means 25". This force component compensates the increasing force developed in the main cylinder 55" because of the decreasing volume in the accumulator system. It will be apparent that when the sheave means 25" is at a midportion of its travel, in view of the height of plate 122, that the auxiliary cylinder and piston means 128 will be substantially horizontal and no vertical force component will be exerted on sheave means 25". As the sheave means progresses to its upper-most position, it will be apparent that the auxiliary cylinder and piston means 128 exerts an upward vertical force component which helps support the sheave assembly means 25" in a manner opposite to that described above with respect to movement to lower-most position. Thus the auxiliary cylinder and piston means 128 act as trim cylinders to more precisely provide the desired vertical force component to maintain the crown block sheave assembly in stable relation to the fixed reference point or sea bed.

It may be noted that in the several embodiments shown that the pivotal connections of the cylinder and piston means 55, 55' and 55" vary in their distance from the axis of the vertical pathway of the sheave means 25. The specific location of the fixed pivotal connection of the primary cylinder and piston means 55 and also of the auxiliary or trim cylinder and piston means 128 becomes quite critical and is dependent upon several factors which affect the optimum design, namely the expected variation in hook loads, that is the weight of the drilling bit and drilling string, the drilling string changing in length as the well hole is drilled, the desired pressure at which the cylinder and piston means may be operated, and the total volume of the accumulators, such total volume being held to a minimum. Thus the difference in lateral and vertical spacing of the fixed pivotal connection of the cylinder and piston means from the vertical pathway is dependent upon several factors as mentioned above.

FIG. 8 also illustrates a modification of the support means for permitting inward and outward movement of the sheave means 32 from the vertical pathway as the crown block sheave means reciprocally moves up and down therein. In this embodiment each idle sheave 32', is pivotally mounted on a carriage 135 having rollers or wheels 136 guidably movable along a horizontal track 137 which extends outwardly from the vertical guide frame members 126. As in the previous embodiment, the pivotal axis of each idle sheave 32' is pivotally connected with one end of a link member 138 which has a pivotal connection at its other end to the sheave shaft 51" at the lower corner of plate 122. Thus during the vertical movement of sheave means 25" the length of the line portion 139 of the line 26' as measured between tangent points on the idle sheave and the crown block sheave remains constant, and wear on the line 26 is reduced to a minimum by such inward and outward lateral movement of the idle sheaves 32.

In the above described embodiment of the invention the primary support means for the crown block assembly included air or gas actuated cylinder and piston means. Under some conditions it is desirable to use gas-liquid cylinder and piston means wherein the gas may be either air or some inert gas such as nitrogen and the liquid means may be an oil or preferably a water base liquid. Two examples of air-liquid systems are hereafter described and are part of this invention.

In the first example of such an air-liquid system, the cylinder and piston means supporting the crown block assembly may include the construction shown in FIG. 12 wherein each of cylinder and piston means generally indicated at 145 may include a cylinder member 146 and a piston member 147 of solid construction provided with a piston head 148. Bypass passageways 149 may be provided in the solid piston member 147 between opposite sides of the piston head 148. The upper end of piston member 147 may be pivotally connected to the crown block sheave assembly as in the prior embodiment and the lower portion of cylinder member 146 may be pivotally connected to the framework means as in the prior embodiment.

A suitable gas and liquid reservoir 150 may include a floating piston, bladder or membrane 151 at the interface between the gas and liquid to prevent entrapment of gas in the liquid during operation. The gas portion of the reservoir 150 may be connected by line 152 to accumulator 69' as diagramatically indicated in FIG. 12. A control valve 153 is provided in gas pressure line 152. The liquid portion of the gas-liquid means 150 is connected by a flow line 155 to the bottom portion of the cylinder member 146 and has a control valve 156 in line 155. Control valve 156 senses the flow of liquid through line 155 and at a preselected flow speed valve 156 will close and thus lock out the piston and cylinder means 145 to prevent travel of the crown block means beyond a certain point.

In a further embodiment of the present invention shown in FIGS. 14 and 15 generally only those portions of the modified construction differing from the prior embodiment of the invention will be described for purposes of clarity and brevity.

In FIG. 14 framework means 170 provides a vertical pathway for a crown block sheave means 171 in a manner similar to the prior embodiments. The crown block sheave means 171 includes end plates 172 which support therebetween a sheave shaft 173 which carries the sheave 174, and which support through shafts 175 which provide pivotal connection for upper ends 176 of piston rods 177 of the four inclined cylinder and piston means 178. Cylinder and piston means 178 are each provided a pivotal connection at 179 to a frame member 180 forming part of the drilling rig superstructure.

Sheave means 171 may move in its path to the upper end 181 of the framework means 170 and to the lower end 182 thereof. Normally the stroke of the sheave means 171 is limited to a portion of the pathway between the extreme ends of the framework 181 and 182.

In this embodiment of the invention, another modification of means for cushioning and slowing down movement of the crown block sheave means at and adjacent to ends of its path of travel are provided. In FIG. 15 each inclined cylinder and piston means 178 may comprise a cylinder member 184 having a cylinder chamber 185 therein. Within chamber 185 is reciprocally slidable a piston head 186 carried on the end of piston rod 177 provided with an internal chamber 187. A suitable seal means 188 is provided between the cylindrical member 184 and the piston rod 177 at the upper end of the cylindrical member 184.

Means are provided at piston head 186 for permitting flow of liquid from cylinder chamber 185 to annular portion 190 of chamber 185 on the upper side of piston 186 and from annular chamber portion 190 to the internal chamber 187 of the piston rod 177. In this example, such communication is provided by an axial port 191 in piston head 186 which leads to a valve chamber 192 in which a ball valve element 193 is biased to normally open position by a suitable coil spring 194. A tapered ball valve seat 196 surrounds the inner end of port 191. Between port 191 and valve seat 196 a counterbore 195 is provided to seat and contain spring 194 when the ball valve is closed. In closed position, the top of ball 193 lies below the inlets of passages 198 leading to annular chamber portion 190. The walls of piston rod 177 are provided with ports 199 for communication between annular chamber portion 190 and inner chamber 187 of the piston rod. When the ball valve 193 is closed, fluid in chamber 190 is virtually confined except for limited flow about the piston head. A dashpot function is thus incorporated into the inclined cylinder and piston means and is operable in the event of sudden loss of load on the cylinder and piston means.

In operation of the cylinder and piston means 178, cylindrical chamber 185 may be connected through port 200 with a fluid or liquid line 202 connected to a reservoir 203 containing a body of liquid fluid such as a water base liquid or a suitable oil. The upper portion of reservoir 203 includes a chamber 204 for a compressable gaseous fluid such as air which may be connected by pressure line 205 to an accumulator and a compressor, (not shown). At the interface between the air and water based liquid may be a bladder or a suitable free floating follower or piston 207 for preventing absorption of one fluid into the other to reduce corrosion, to prevent possible air saturation of the liquid, and to facilitate equalization of liquid flow to the several main inclined support cylinders. The reservoir 203 is preferably located near the top of rig structure and close to the support cylinders to minimize liquid flow losses.

In fluid line 202 emergency shut off valves 210 and 211 are provided. Valve 210 may be of quick closing type and is operable in the event of a sudden loss of fluid pressure in the hydropneumatic system which would result in a failure of the support system for the sheave means. Thus upon sudden loss of pressure in the air reservoir the shut off valve 210 would become operable to maintain sheave supporting pressure in the cylinders.

Valve 211 is located immediately adjacent each cylinder and piston means 178 and becomes operable in the event of loss of all or a portion of the drill string or if the drill pipe breaks. Under such conditions the sudden relief from load on the cylinder and piston means would cause the pistons to urge the sheave means upwardly against the top of the framework. Closing of shut off valve 211 stops flow of liquid into the cylinder and piston means so that in cooperation with the dashpot arrangement of the cylinder and piston means such reaction to a pipe break is minimized.

In the event the crown block sheave means and end plates which are being guided in the framework 170 reaches the extreme ends of their path of travel at 181 or 182, shock absorbing means 214 at the top of the framework and shock absorbing means 215 at the bottom of the framework are provided to cushion and stop the travel of the sheave means. Shock absorbing means 214 and 215 may comprise a selected number of shock absorbing units which may be of well known construction and manufacture and may essentially comprise a cylinder with liquid fluid therein and a piston ported to control passage of fluid from one side of the piston to the other so that upon contact of the unit by the sheave means 171 motion of the sheave means will be cushioned and stopped without excessive damage to the framework 170.

In operation of the crown block compensating means of this invention, it will be understood that at the top of the drilling rig the crown block sheave means is resiliently supported by inclined cylinder and piston means which are connected to working accumulators relatively closely adjacent to the associated cylinder and piston means and that a liquid actuated dashpot cylinder and piston means is arranged to freely circulate liquid during compensation and hydraulic power means for pumping or supplying liquid to the dashpot means are also closely located with respect thereto. At the deck of the drilling rig or below the deck is provided an air compressor and an initial air supply reservoir which generally comprises only a relatively small portion of the gas volume as for example about 10-15 percent.

At the deck a control console is provided for the drilling operator which may include the heave stroke position indicator, a hook load pressure gauge, a compensator lock out control button for control of the liquid lines, a pressure gauge for the air reservoir and an air pressure bleed off control button.

In FIG. 16 an exemplary operating or actuating system for the motion compensating apparatus shown in FIGS. 14 and 15 is illustrated. It will be understood that a generally similar actuating system may be employed with respect to the apparatus of the prior embodiments utilizing an air-liquid actuating means In the prior embodiment utilizing only air, an actuating system . similar to that described in Horton U.S. Pat. No. 3,469,820 may be employed.

In FIG. 16 the load supported by the inclined hydraulic cylinders 178 is exemplary and schematically illustrated as 440,000 pounds which would represent the load carried by the crown block sheave means 171. Each cylinder 178 is in communication through fluid conducting line 202 with the hydropneumatic reservoir 203. Automatic cylinder emergency shut off valves 211 are shown adjacent each cylinder 178 and each valve 211 is provided communication with the other valves 211 through connecting lines 220. Adjacent reservoir 203, a quick closing safety valve 210 is provided as above described. The air portion of reservoir 203 is connected by line 205 with a supply accumulator 221 containing air under pressure, which is supplied by an air compressor 222 having communication through line 223 with a make up air supply reservoir 224 from which air under pressure is applied through an air controller 225 to the accumulator for reservoir 203. Air controller 225 may comprise a suitable flow valve system for maintaining a selected pressure in accumulator 221 and reservoir 203. An air control system 226 is connected by a suitable line 227 to air control 225. Air control system 226 provides a means whereby an operator on the rig may change and regulate the pressure supplied to the reservoir in accordance with the conditions encountered during initial make-up and later changes in operation.

Generally speaking, maintenance of a fixed distance between the crown block sheave means and a fixed reference point such as the sea bed is desired and in addition, hook load variation must be minimized in order to provide optimum drilling bit weight. Therefore, as wave action causes the vessel to heave, relative movement occurs between the crown block sheave assembly and the framework means. Thus as the vessel moves upwardly, the crown block moves downwardly relative thereto and gas in the cylinder and piston means is compressed. Likewise as the vessel moves downwardly and the crown block moves upwardly relative thereto, the air pressure in the cylinder and piston means is reduced. When the drilling operator has balanced the hook load which includes the weight of the drilling string, less the desired bit pressure, with the air pressure in the cylinder and piston means and when the inclined cylinder and piston means have been so arranged that with respect to the relative movement of the crown block sheave means and framework means, the vertical force component exerted by the inclined cylinder and piston means during variation of the angle of inclination substantially balances the weight variation in hook load and therefore the crown block sheave assembly is maintained in virtually stable fixed relation with respect to the sea bed.

This action of the inclined cylinder and piston means supporting the crown block sheave assembly may be best understood by referring to FIG. 13 which shows several curves indicating the percentage change of hook load variation with respect to vertical displacement of the crown block sheave assembly. In FIG. 13 the abscissa represents a stroke of the crown block sheave assembly for a distance of 15 feet. At the center of the 15 foot stroke, is provided an ordinate showing the percent of hook load variation on down stroke and up stroke of the crown block sheave assembly. It will be understood that in setting up this system the drilling operator balances the hook load at approximately the center portion of travel of the sheave assembly. The exponential line indicated at A represents a characteristic curve of vertically positioned pneumatic cylinder and piston means wherein the slope of the curve is a function of the ratio of cylinder displacement volume to accumulator volume, a curve being determined by pressure-volume gas formula PV.sup.k =C wherein P is the gas pressure, V is the volume, K is the constant and C is the cylinder force which is to be constant. It will be noted that over the 15 foot stroke the variation of hook load is approximately 17 percent which is excessive hook load variation for standard drilling practices.

Curve B (no trim) represents a condition where the cylinder and piston means are inclined and have a constant gas pressure or force, that is, an infinite accumulator volume, to resist movement of the crown block sheave means. Thus the vertical force component on the sheave means decreases as the cylinder and piston means changes its angle of inclination as the crown block sheave assembly moves to the lower portion of its stroke. Curve C is representative of a compensating system utilizing the resilient support means described hereinabove. Thus as the sheave means moves from the midpoint of its stroke, downwardly toward the bottom of its stroke (when the vessel heaves upwardly) the percent of hook load variation is approximately only 0.80 percent. As the sheave means moves from the midpoint of its stroke upwardly (as when the heave motion of the vessel is downwardly) the percent variation of the hook load at the top of the stroke is only about 4 percent of the total load variation. Thus over the entire stroke range of 15 feet, the total load variation is slightly less than 5 percent. However, during the 10 foot stroking range as measured from the bottom of the stroke to a few feet above the midpoint of the stroke which may normally be the working range of the crown block sheave assembly, since the drilling bit moves downwardly during the drilling, the total load variation is less than 2 percent, that is plus 0.80 percent and minus 1.20 percent. Thus a narrow band of hook load variation is achieved and a nearly constant bit weight is maintained during drilling. As the crown block sheave assembly moves vertically, the total cylinder force may be represented by the formula Pl=Po (Vo/Vl .sup.k) , and the vertical component changes as Fv=Fc sin .theta.. By balancing these force changes including the design parameters of the size of the cylinder and piston means, the angular variation of the cylinder and piston means during the stroke of the sheave means and gas accumulator volume, a total change in the vertical force in the order of 5 percent of the hook load excluding frictional losses may be achieved. Moreover, the inclined gas cylinder and piston means which serves as the main support for the crown block sheave assembly has reduced the gas volume requirements for an example as given above, to only 100 cubic feet and thus it becomes practical to mount working accumulators adjacent the cylinder and piston means on the frame work means 22 at platform 23. Thus no large gas conducting lines must be run up the derrick legs and friction losses in the line are minimized.

The load displacement relationship is also effected by other parameters such as friction in the system and fluid line losses. In addition, the kinematics of the idler sheave mechanism also affects the load displacement relation and for the idler sheave support and linkage arrangement previously described, an exemplary load displacement curve is shown in curve D. In curve D total load variation over the full stroke of 15 feet is indicated at about 4 percent. For a specific drilling situation, the idler sheave linkage configuration may be modified to virtually eliminate any change in hook load by providing a relatively flat curve over a major portion of the stroke of a sheave assembly.

During operation as the drilling hole depth is increased, the crown block sheave assembly will stroke lower and lower in the vertical guide framework. The drilling operator may then pay out more of line 26 in order to bring the crown block stroke into the desired range and position with respect to the vertical guide framework.

In the event there should be a failure in the gas pressure system, the dashpot cylinder and piston means may be locked out, that is the control valves therefore, may be closed so that the liquid in the dashpot system will serve as a cushion and will prevent damage to the drilling equipment. Under such conditions the compensating system is inoperative and drilling may be continued as if no compensation were provided.

It will be understood that while in the examples of the invention shown in FIGS. 4-7, the dashpot means are shown in vertical position and directly beneath the sheave shaft of the crown block sheave assembly, it may be desirable to locate separate dashpot cylinder and piston means in a different position in order to permit, for example, vertical pipe racking which may require utilization of the space immediately adjacent to the vertical pathway of the crown block sheave assembly. Under such conditions the vertical guide rails or columns may be set further apart and the dashpot means arranged to act upon the carriage of the crown block sheave assembly. Of course in the example of the invention shown in FIGS. 14 and 15, the dashpot functions are provided by the construction of the main cylinder and piston means 178 and by use of the shock absorbing means 214 and 215.

The above-described crown block motion compensating system lends itself to an even further reduction of hook load variation verses relative displacement of the sheave assembly by first utilizing hydraulic cylinders to positively drive the crown block sheave assembly such as for example shown in FIG. 8. Since virtually all of the load is resiliently supported upon the air actuated cylinder and piston means, only the relatively small unbalanced load must be driven. Thus power requirements for such auxiliary hydraulic cylinders are relatively small. Such liquid cylinder and piston means may be driven and made responsive to the changes in tension or vertical motion as indicated by the vertical motion sensing means previously described.

When the positive mechanical link is provided between the crown block sheave assembly and the sea bed or riser pipe as described in FIGS. 9, 10, and 10a virtually no bit weight variation is permitted and thus drilling operational efficiencies are greatly increased. Such vertical sensing systems as described in FIGS. 9, 10, and 10a facilitate very precise operations such as placing the blowout preventer stack, logging, setting packers, or fishing because the crown block sheave assembly is essentially fixed relative to the sea floor so that such conditions virtually simulate those of offshore drill platforms of fixed leg type.

It will be understood that various changes and modifications may be made in the embodiment of the invention shown above and all such changes which come within the spirit of this invention and which come within the scope of the appended claims are embraced thereby.

Claims

We claim:

1. In a motion compensated crown block system for a drilling rig on a vessel floating above a sea bed, the combination of:

a sheave block assembly including a crown block sheave and a traveling block sheave interconnected by a sheave cable, said traveling block being adapted to be connected to a rotary drill string and drill bit;

a framework means on said rig;

guide means on said framework means for vertical movement of said crown block sheave along a path of selected length above said drill string to compensate for vertical movement of the vessel relative to the sea bed and drill string;

means for supporting said crown block sheave along said path to compensate for such relative movement of said vessel,

said supporting means comprising upwardly inclined cylinder and piston means on opposite sides of said path and having upper ends pivotally connected to said crown block sheave and lower portions pivotally connected to said framework means;

a pressure fluid supply source in communication with said cylinder and piston means to provide fluid pressure forces at said cylinder and piston means to support said crown block sheave and the load carried thereby;

said supply source including accumulator means for said pressure fluid to permit variations in pressure force;

the angle of inclination of said cylinder and piston means being variable between ends of the path of travel of the crown block sheave to provide change in vertical force components of the cylinder and piston means in proportion to the change in pressure fluid support forces of the accumulator means whereby said support force and vertical force component are compensated and substantially uniform loading is maintained at said crown block sheave;

and means for sensing motion of said crown block sheave relative to the sea bed and operably connected to the crown block sheave for regulating the fluid pressure of said pressure fluid supply source.

2. In a system as stated in claim 1 wherein said accumulator means is mounted at the water table and is reduced in volume because of said force compensation.

3. In a system as stated in claim 1 wherein said means for sensing motion of said crown block sheave relative to the sea bed comprises

a tension line means connected at one end to said water table, a pulley attached to said crown block sheave over which said tension line passes, a pulley at the sea floor through which said line passes, and the opposite end of said tension line being connected to a draw works on said vessel.

4. In a system as stated in claim 1 wherein said means for sensing motion of said crown block sheave relative to the sea bed includes

a tension line having one end connected to said crown block sheave,

the opposite end of said line having a connection to a riser pipe on the pipe string,

and means on the vessel for adjusting tension of the tension line means.

5. In a system as stated in claim 1 wherein said means for sensing motion of said crown block sheave relative to the sea bed includes

a tension line means connected to said crown block sheave,

said tension line means being also connected to a guide line connected to the sea floor,

and means on said vessel for adjusting the tension of said tension line means.

6. In a system as stated in claim 1 wherein each cylinder and piston means includes a cylinder member closed at one end,

and a solid piston member occupying a major portion of the volume of a fluid chamber on the same side of said piston head as said piston member.

7. In a system as stated in claim 1 wherein each cylinder and piston means includes

a cylinder member closed at one end,

a hollow piston member,

a piston head on said piston member including passageways in communication with opposite sides of said piston head,

and valve means in said piston head.

8. In an apparatus for minimizing variations in loading of a sheave block means where load variations occur through relative movement of a floating vessel and the sheave block means, the combination of:

a sheave block assembly including a crown block, a traveling block, and a sheave cable reaved therebetween and having a dead line fastened to said vessel and a fast line connected to draw works, said traveling block being adapted to be connected to a rotary drill string;

support means for said sheave block assembly including upwardly inclined cylinder and cooperable piston members mounted for pivotal movement from said vessel and pivotally connected to the sheave block assembly to impart an upwardly directed load supporting force to said sheave block assembly;

a fluid pressure supply source in fluid communication with said cylinder member for exerting fluid pressure on said piston member to provide said load supporting force;

said sheave block assembly being limited to movement along a stroke path of selected length;

said load supporting force changing as said cylinder member and piston member move relative to each other during relative motion of said sheave block assembly and said floating vessel;

the angle of inclination of said cylinder and piston members changing during relative motion of the sheave block assembly and floating vessel and causing a change in vertical force component exerted by said cylinder and piston members on the sheave block assembly,

the changes in vertical force components being virtually compensated by the change in load supporting forces of the fluid pressure supply source whereby over a path length of normal stroke the percentage of load variation in respect of hook load at the sheave block assembly is in the order of about 2 percent of the supported load.

9. In a system as stated in claim 8 wherein each cylinder and piston means includes a cylinder member closed at one end,

a hollow piston member having a chamber in communication with the cylinder member,

and port means in said hollow piston member for bypassing pressure fluid around the piston head of the piston member.

10. In a system as stated in claim 8 including

a pair of idle sheaves supported from said framework laterally of said sheave means;

a link means pivotally interconnecting said crown block sheave and each idle sheave,

a link member pivotally connected to said idle sheave and to said framework means,

said link means and member defining a virtually laterally horizontally directed path for each idle sheave with respect to movement of said crown block sheave means.

11. In a system as stated in claim 8 including

top and bottom dashpot means supported in said framework means at opposite ends of the path of travel of said sheave means.

12. In a system as stated in claim 8 including trim cylinder and piston means on said framework means for further minimizing load variations at the crown block sheave comprising

a trim cylinder and piston having one end connected to said crown block sheave at the pivotal connection of the upper end of the support cylinder and piston means and having its other end connected to said framework means at a location between the ends of the path of travel of the crown block sheave.

13. In an apparatus as stated in claim 8 wherein the load variation in respect of hook load of the load supported by the sheave block means is between about +0.80 and -1.20 percent.

14. In an apparatus as stated in claim 8 wherein over the path length of maximum stroke of the sheave block assembly the percentage change of load variation in respect of hook load at the sheave block means is in the order of 5 percent or less.

15. In an apparatus as stated in claim 8 wherein said fluid pressure supply source includes

accumulator means of minimal volume mounted adjacent the cylinder members.

16. In an apparatus as stated in claim 8 wherein said support means for said sheave block assembly also includes

trim cylinder and piston members connected with said sheave block assembly and operable between ends of the stroke path and movable into upwardly and downwardly inclined relation to said sheave block assembly.

17. In an apparatus as stated in claim 8 including

means for sensing motion of said sheave block assembly comprising a line of fixed length extending between said sheave block assembly and a reference connection point relatively immovable with respect to said floating vessel.

18. In an apparatus as stated in claim 8 wherein said fluid pressure supply source includes a line in fluid communication between said supply source and said cylinder member;

and valve means in said line for controlling the rate of movement of said piston member upon rapid change in loading of the sheave block means.

19. In an apparatus as stated in claim 8 including

dashpot means supported at opposite ends of the path of travel of said sheave block assembly for cushioning movement of the crown block sheave at ends of its path of travel.