US3392534A

US3392534A - Offshore drilling structure

Info

Publication number: US3392534A
Application number: US486095A
Authority: US
Inventors: Kenneth A Blenkarn
Original assignee: Pan American Petroleum Corp
Current assignee: Pan American Petroleum Corp
Priority date: 1965-09-09
Filing date: 1965-09-09
Publication date: 1968-07-16
Anticipated expiration: 1985-07-16

Description

y 6, 1968 K. A. BLENKARN 3,392,534

OFFSHORE DRILLING STRUCTURE Filed Sept. 9, 1965 4 Sheets-Sheet l Q\ A A A A A AVAYA A A A A AYA AWA AYAYD 4 Zfiflmuuumumm KENNETH A. BLENKARN INI 'ENTOR ATTORNEY 4 Sheets-Sheet 2 Filed Sept. 9, 1965 FIG.6

KENNETH A.BLENKARN INVENTOR.

ATTORNEY.

15, 1968 K. A. BLENKARN 3,392,534

OFFSHORE DRILLING STRUCTURE Filed Sept. 9, 1965 4 Sheets-Sheet 5 i I FIG. 8

FIG.7

KENNETH A.BLENKARN I INVENTOR.

BY/Ql ATTORNEY.

July 16, 1968 K. A. BLENKARN 3,392,534

OFFSHORE DRILLING STRUCTURE Filed Sept. 9, 1965 4 Sheets-Sheet 4 r |0A I20 FIG. IO

KENNETH A BLENKARN INVENTOR.

BY%A9.M

AT TORNE X United States Patent 3,392,534 OFFSHORE. DRILLING STRUCTURE Kenneth A. Blenkarn, Tulsa, Okla., assignor to Pan American Petroleum Corporation, Tulsa, Okla., a corporation of Delaware Filed Sept. 9, 1965, Ser. No. 486,095 11 Claims. (Cl. 61-465) ABSTRACT OF THE DISCLOSURE This invention concerns an improvement in an offshore platform structure of the type commonly referred to as a jack-up barge. This type barge includes a platform supported by a plurality of legs which extend from the platform to the floor of the body of water. These legs pass through leg-guide means which are supported from the platform by horizontal hinge pins. Tilting means are provided for tilting each leg about its hinge pin. Resilient means, such as a very strong spring, connects the tilt assembly to the platform. The resilient means has a resiliency sufiicient to provide limited restraint to movement of the leg about the hinge pin yet being sufiicient to resist the rotational force about said hinge pin caused by gravity. Thus, energy can be absorbed by the resilient means without generating too great a bending moment in the leg.

This invention relates generally to offshore structures from which wells are drilled. It relates especially to improvements in mobile sea platforms.

In recent years the search for petroleum has resulted in many wells being drilled in offshore locations such as on the continental shelf of the Gulf of Mexico. These offshore wells are usually drilled from either fixed platforms or floating vessels. The floating vessels can normally be used in deeper water and are perhaps more readily moved from one location to another. However such floating vessels must normally be equipped with elaborate positioning means so that the ship stays substantially in the same position. In this system the drilling structure and equipment is carried on the floating vessel and the drill pipe is lowered through an opening called a well in the ship down through the water to the ocean floor where the drilling commences.

A common type fixed platform is a structure or drilling deck which is supported from piles which have been driven deep into the ocean floor. If a sufiicient number of piles of sufficient strength have been driven, one indeed has a very stable or fixed platform. However these fixed platforms are not at all readily movable. It is thus seen that the floating vessel has the advantage of being easily moved from one location to the other and that the fixed platform has the desired stability of location. Many companies want a unit that combines the stability of a fixed platform and the mobility of floating vessels. This combination is particularly desirable where wildcatting or testing acreages is in water up to 250 feet deep as off the coast of Louisiana, for example.

To meet this desired requirement so-called jack-up platforms or jack-up barges have been developed. One type of these jack-up platforms is provided with three legs. Each leg is supported by elongated guide means which extends along the leg. Such guide means is connected by a horizontal hinge pin to a buoyant platform or barge proper. The leg guide means are connected above the hinge pin to the barge by a tilt assembly which will be discussed hereinafter.

Means are provided between the guide means and the legs so that the legs can be extended up or down the guide means with respect to the barge. When the jackup platform is being towed to a new location, the legs Ice are jacked up to where they clear the bottom of the body of water and are supported by the barge proper. When the barge reaches the location at which the well is to be drilled, the legs are jacked down until they not only contact the bottom of the ocean floor but are continued to be jacked down with respect to the platform until the platform is high above the water, say 25 to 50 feet. It has been found that when the bottoms of the legs are tilted outwardly from vertical away from the barge the bearing load on the soil is greatly reduced. therefore a tilt assembly has been provided between the guide means and the barge. Such tilt assembly normally includes a geared tilt bar which is attached at one end to the platform and engages a gear Wheel supported from the guide members of the leg. Alternatively one end of the tilt bar is connected to the leg guide means and the gear wheel is supported from the platform. Power means is provided for driving the gear wheel. By rotating the gear wheel, the desired tilt of the leg is effected. Tilt is thus attained by rotation about the horizontal hinge pin.

With most jack-up barges of the type just described, there is an inherent weakness in the leg system. While the legs are rigid and strong with respect to bending about a horizontal axis perependicular to the hinge pin axis, the leg arrangement is weak with respect to bending about a horizontal axis parallel to the hinge pin axis. The weakness of the barge legs sometimes leads to various difficulties both with the barge afloat and the legs partially extended into the water and sometimes with the barge jacked up out of the water with the legs resting on the bottom.

In some cases with the legs extending some distance into the water but not on bottom, the action of waves upon the barge and legs may result in application of bending moments to the legs in the weak direction as described above. Such bending moments arise from both static and dynamic phenomena. One countermeasure for avoiding such described difliculties is to release the tilt mechanism so that the leg is free to rotate about the horizontal hinge pin axis. With freedom of movement about the hinge pin axis unrestrained, there will arise no bending moments in the weak direction of the leg. This particular solution for this particular problem can only be employed within limited ranges of operating conditions. For the leg to be stable in its upright position with free rotation about the horizontal hinge pins, the buoyant weight of the portion of submerged leg must be sufficient to prevent over-turning of the entire leg due to the weight of the leg extending into the air. Such desirable conditions do not usually exist.

It is an object of this invention to provide means which offers limited restraint to rotation of the leg about the horizontal hinge pin so that bending moments in the leg are small but the leg is nevertheless stable and will not fall over. An additional object is to provide means whereby energy can be absorbed without generating too large a bending moment in the leg. Briefly in a preferred embodiment of this invention, the platform is connected to the leg guide means above the hinge pin by the tilt assembly and a resilient means. For example, one end of the tilt bar is released or disenaged from its rigid connection to the platform deck. Attached to such disengaged end is a resilient member such as one end of a torsion bar; the other end of such torsion bar being rigid- .ly attached to the deckof the platform. The torsion bar has a resiliency providing limited restraint to movement of the leg about the horizontal hinge pin but being sufficient to resist rotational force about the hinge pin caused by gravity acting upon the leg itself. Preferably, a dash pot or other damping means is connected in parallel to the resilient means to aid in dissipating energy and damping vibrations of the leg to effectively reduce the bending moments to a safe level where failure does not occur.

Various objects and a better understanding of the invention can be had from the following description taken in conjunction with the drawing in which:

FIGURE 1 illustrates schematically an over-all view of a three-legged jack-up barge;

FIGURE 2 illustrates in greater detail the connection of a leg through a vertical leg guide and horizontal hinge pin to the barge;

FIGURE 3 illustrates a resilient member connecting the platform deck to the tilt assembly means of one of the leg guide means;

FIGURE 4 illustrates the connection of a tilt bar through a parallel spring and dash pot to the platform structure;

FIGURE 5 illustrates a slightly different modification in which the motor drive means of the tilt assembly is connected through a resilient means to the deck of the platform;

FIGURE 6 is similar to FIGURE 5 except that the resilient connection is between the tilt bar of the tilt assembly and the leg guide means;

FIGURE 7 is similar to FIGURE 6 except that the resilient connection is between the leg guide means and the motor drive means of the tilt assembly;

FIGURE 8 illustrates another modification of the tilt assembly means and its connection to the deck of the platfonm which provides means for releasably locking the tilt bar to the deck;

FIGURE 9 illustrates an auxiliary means of guying the legs when raised above the deck of the platform;

FIGURE 10 illustrates a modification of the embodi ment using a torsion bar as the resilient means such as shown in FIGURE 3;

FIGURE 10A illustrates a section taken along the line 10A10A of FIGURE 10.

Turning to the drawing, and FIGURE 1 in particular, there is schematically illustrated a three-legged jack-up barge. Shown thereon is a platform or barge means 10 which is supported from legs 12 by leg guide means 14 having a horizontal pin 24. The barge 10 is shown as jacked-up above the body of water 18. Legs 12 have cans or pads 20 which rest upon the bottom 22 of the bod-y of water 18. Platform 10 has a large number of compartments which serve various purposes such as to give buoyancy when the legs are jacked up and also for storing supplies and compartments for operating personnel. Platform 10 also supports a drilling derrick and other equipment not shown.

Attention is now directed toward FIGURE 2 which shows in more detail a typical connection between barge 10 and legs 12. Shown thereon is leg guide 14 which can be up to 50 feet or more in length. Mounted on leg guide 14 is a horizontal hinge pin 24 which is supported from the barge by brackets 26. The hinge pin permits rotation of leg guide 14.

Still referring to FIGURE 2 attention is now directed toward a discussion of how leg guide means 14 is connected to leg 12. A vertical elongated member 28 is supported by and secured to leg 12 by member 30. Elongated member 28 slidably fits into

slots

32 and 34 on the interior of guide member 14. Thus the only movement of leg 12 with respect to guide 14 is longitudinal or usually essentially vertical. Fastcned to elongated member 28 is vertical elongated gear member 36. Supported within guide member 14 is a gear wheel 38 which meshes with gears on elongated gear member 36. Motor means 40 are provided for driving gears 38. It is thus seen that by energizing motor 40, gears 38 drive gear member 36 and the leg 12 longitudinally with respect to guide 14. With the arrangement in FIGURE 2 it is seen that the individual legs can be jacked up or down with respect to the barge. As will be seen they can also be rotated or tilted about the horizontal axis 24.

Attention is now directed toward FIGURE 3 which illustrates a tilt assembly for controlling the amount of tilt of the leg about the hinge pin and also illustrates resilient means connecting the tilt control drive means of such a tilt assembly with the deck of the barge 10. Shown on FIGURE 3 is a partial view of leg 12 and barge 10. A tilt bar 42 is pivotally connected by pin 44 to the top of leg guide 14. Tilt bar 42 is provided with gear train 48. A motor having gear Wheel 52 is mounted on tilt bar 42. Motor 50 is resiliently mounted from barge 10. This resilient means as illustrated includes a torsion bar 54 rigidly mounted at one end by frame 56 to barge 10. The other end of torsion bar 54 is rigidly connected by elongated member 58 to motor 50. The resiliency of this system is sufficient to provide limited restraint of movement of leg 12 about hinge pin 24. The restraint is sufficient to resist the rotational force about the hinge pin caused by gravity of the leg when for example the leg is in its highest extended position for which this protection may be sought. In other words the resilient means should be sufficiently soft so as to permit motion, and thereby if desired absorption of vibratory energy of the legs, but stiff enough so that when raised in a tilted position, the legs will not fall over.

In some cases ground swells, that is, waves with long periods such as six second, cause the legs to resonate. It was been observed that this has occurred when the period of the waves coincided the natural period of the resonant frequency of the legs. In other words the resilience of the means connecting the tilt drive mechanism to the barge should be sufficient that it substantially changes the natural resonance of the leg in the direction perpendicular to the hinge pin. Normally the torsion bar or other resilient means should preferably contribute sufiicient added flexibility to increase the natural period of vibration of the leg by a factor of 2. It is further preferred that damping means be provided in conjunction with the resilient means. A suitable modification of the embodiment of FIGURE 3 to provide such damping means is shown in FIGURE 10 for example.

Attention is now directed to FIGURE 4 which illustrates another means of adding the resilient feature and accompanying damping means. The resilient means there includes a spring 60 and a dash pot 62 in parallel which connects the tilt bar 42 to an upright member 61 extending upward from and rigidly attached to the deck of barge 10. Inthis arrangement tilt bar 42 is not directly connected to the guide member 14 but rather is connected to tilt control drive means 66 which is attached to the guide member. Rotation of gear 64 of the tilt control drive means moves the leg guide 14 along tilt bar 42 to change the tilt of leg 12 as desired. The stiffness of the spring 60 is selected to meet the same requirements as the criteria for determining the resiliency of torsion bar 54. A dash pot 62 is provided to act in conjunction with the spring 60 to dissipate energy and help damp out vibrations on the leg. The dash pot for example can be a liquid shock absorber type system, a dry-friction type system, or other mechanism for dissipation of energy. This damping thus provided should preferably be equivalent to about 10% of the critical dampening of the leg system.

The tilt assembly can take various forms. Common essential features include a geared tilt bar and a mating circular gear with controlled rotation such as by a motor. In accordance with my invention, the resilient means can take various forms and can be inserted in various manners between the tilt assembly and either the platform or the leg. For example in FIGURE 3 previously discussed, the resilient means is connected between the tilt assembly and platform; or more particularly between the drive motor and the platform. The tilt bar is pivotally connected to the leg guide. In another embodiment shown in FIG- URE 4, also previously discussed, the driving means is fixed to the leg guide and the tilt bar is connected by limited resilient means to the barge. In the embodiment of FIGURE 5, housing 70 for the circular gear 72 is connected to the barge through a resilient means such as spring 74. The tilt bar 76 is connected to guide 14 by pivot pin 78. It is to be understood that the principle is the same whether the tilt bar or the drive mechanism is connected to the barge or to the leg guide means through the limited resilient means.

As mentioned above, the resilient means can, if desired, be inserted between the tilt assembly and the leg guide means. One such embodiment is illustrated in FIG- URE 6. 'Shownthereon is drive means 110 rigidly supported by upright member 112 from deck 10. A tilt bar 114 is arranged to be driven by drive means 110 and has one end free and the other end connected to a resilient means such as spring 116. Spring 116 is connected through pivot 118 to leg guide means 14. Still another alternative of providing a resilient means between the tilt assembly and the leg guideis set forth in FIGURE 7. There the control drive means 120 is resiliently supported from leg guide 14 bysprings 122 which connect the base of drive means 120 with a clamp 124 which is rigidly connected to leg guide means 14. The resiliency of the spring is in the direction of rotation of the leg about the hinge pin.

a to leg guide means 14. A tilt bar 126 is rigidly connected at one end through upright member 128 to deck of the floating platform. Tilt bar 126 has gear teeth which are meshed with the gear or gears on drive means 120'. By rotating the drive gears of drive means 120 the tilt of the leg is varied. Spring 122 provides the protection required to change the natural frequency of the legs and tilt assembly similarly as described above for FIGURE 3.

To briefly summarize then, it is seen that my invention embodies several alternatives. In each of these, some type resilient means is used with a tilt assembly in connecting the guide leg to the platform. Such resilient means may be between the tilt assembly and the platform. Each of these alternatives has additional alternatives in that the tilt assembly may be connected through either a tilt bar or its drive assembly to the resilient means. In each of these alternatives, it is normally preferred to use a dampening means operating in conjunction with the resilient means.

Attention is directed to FIGURE 8 which illustrates another and an especially preferred embodiment of the invention. Shown thereon is a barge 10 connected by hinge pin 24 to leg guide 14 as described before. A housing 80 supports gear wheel 82 from leg guide 14. The lower side of tilt bar 84 is slidably connected along guide means 86 so that the teeth of the tilt bar meshes with the teeth of gear 82. In this embodiment tilt bar 84 has two connections to barge '10. One is a releasable rigid-connection and the other is a resilient connection. The rigid connection includes a bar member having an upper section 88A and a lower section 88B rigidly connected at its lower end to the deck of barge 10. The upper end of the bar 88A is provided with a clamping means 90 which is firmly clamped about tilt bar 84. A locking means 92 releasably attaches upper bar section 88A to lower bar section 88B to form a rigid connection between tilt bar 84 and the platform. Means 92 can be, for example, a magnetic clutch which permits quick locking or unlocking of bar 84 rigidly to the deck of barge 10. Also provided between tilt bar 84 and barge 10 is a resilient means which can include a torsion bar 94 rigidly connected at one end through support member 96 to the barge 10. The other end of torsion bar 94 is fixed or attached to tilt bar 84 by upright member 85. In normal operation the device in FIGURE 8 is normally set so that tilt bar 84 is rigidly connected to deck 10. This is done by securing clamping means 90 to bar 88 by locking means 92. However, even though this is done the torsion bar 94 is in place and available for instant use should the need arise. But so long as locking means 92 is locking the tilt bar rigidly to the deck of the platform the resilient means is ineffective. However, should the leg start vibrating, all

that is necessary to do is to release locking mechanism 92 so that the tilt bar 84 is no longer rigidly attached to the platform. At this time torsion bar 94 influences and changes the resonant frequency of the leg and aids in dissipating the resonant energy so that vibration is stopped before excessively large bending moments are developed in the leg. With this arrangement I have both the benefits of the rigid connection of the tilt bar 84 to the platform and the protection afforded by a limited resilient connection.

FIGURES 10 and 10A illustrate another means of applying a damping effect in conjunction with the resilient means which, in this illustration, is torsion bar which is similar to that of 'FIGURE 3. The torsion bar is supported at one end rigidly from the deck 10 by support 112. Vertical arm 114 connects tilt bar 116 to the torsion bar at a point opposite support 112. Means for applying a damping elfect to the torsion bar is provided and includes brake shoe 118 which is clamped about the end of torsion bar 110 opposite support 112 in a well known manner. The brake shoe is supported by adjusting means 120 and support member 122 from the deck 10. By tightening the adjusting means the brake shoe 118 can resist rotation of torsion bar 110. The amount ofresistance is varied by varying the force applied to the brake shoe. This modification can be used with the other embodiment shown using a torsion bar as the resilient means.

When the jack-up barge is being moved from one drilling location to another the legs of this offshore structure are raised essentially clear of the water and extend a considerable distance into the air. The legs are now supported by the platform which in essence has been lowered into the water and serves as a barge. On occasion during moving of such jacked up legs, sudden winds have arisen causing considerable damage to the legs extending above the barge. It is believed that such damage has occurred primarily as a result of bending in the weak direction of the leg system. It will be recalled that the weak direction is in a direction perpendicular to the hinge pin axis. FIGURE 9 illustrates a guy wire arrangement which further aids in reducing this problem. Shown thereon is a leg 12 extending high above barge 10 from which it is supported primarily by hinge pin 24. The tilt mechanism such as described in relation toFIGURE 3 has maintained the leg at a desired angle, for example say 5 with the vertical. The resiliency feature of this arrangement of FIGURE 3 will normally provide all the protection needed. However when the legs are being moved from one location to another they may extend as high as 300 feet above the deck of barge 10. In this event it is sometimes desired to have additional protection. This can be provided by a line 100 which is connected through a spring 102 and a dash pot 104 which are connected in parallel to the platform. Dash pot 104 and spring 102 work in conjunction with the tilt bar arrangement to provide additional protection. l

While there are described above but a limited number of embodiments of the present invention it is possible to produce still other embodiments without departing from the inventive concept of the invention. It is desired that the scope of the invention be limited only by the appended claims.

I claim:

1. In an offshore platform structure in which the platform is supported by a plurality of leg members supported by leg guide members from the platform by horizontal hinge pins for variable tilt positions, the improvement which comprises:

connecting means for connecting said leg guide means to said platform, said connecting means including: a tilt assembly for varying the tilt position of each said leg, and resilient means between each said tilt assembly and each said leg guide member, said resilient means having resiliency suflicient to provide limited resistant to movement of said leg member about said hinge pin but sufficient to resist rotational force about said hinge pin caused by gravity.

2. A structure as defined in claim 1 in which the resilient means has a resiliency sufficient to contribute added flexibility for increasing the period of vibration of the leg member by a factor of two.

3. An apparatus as defined in claim 1 including a line connected to the upper part of a leg member and resilient and dampening means connecting said line to said platform.

4. In an offshore platform structure in which the platform is supported by a plurality of leg members each supported from the platform by a horizontal hinge pin for variable tilt positions as directed by a tilt assembly means, the improvement which comprises:

a torsion bar having a first end rigidly connected to said platform;

means connecting the other end of said torsion bar to said tilt assembly.

5. A structure as defined in claim 4 including braking means positioned about said torsion bar, said braking means being supported from said platform.

6. An offshore platform structure which comprises:

a buoyant platform; I

a plurality of leg members for supporting said platform;

leg guide means for receiving said leg members;

means for controllably driving said leg members through said leg guide means;

a horizontal hinge pin means connecting each said leg guide means to said platform;

a tilt assembly for rotating each said guide means about said hinge pin;

a torsion bar having a first end rigidly connected to said platform and the other end connected to said tilt assembly, said torsion bar having a resiliency sufficient to provide limited restraint to movement of said leg member about said hinge pin but sufficient to resist rotational force about said hinge pin caused by gravity; and

connecting means for releasably locking said tilt assembly rigidly to said platform.

7. A structure as defined in claim 6 including braking means connected about said torsion bar near the end connected to said tilt assembly.

8. An offshore platform structure which comprises:

a platform;

a plurality of leg members for supporting said platform;

leg guide means for each of said leg members;

means for controllably driving said legs through said leg guide means;

a horizontal hinge pin connecting each said leg guide means to said platform;'

tilt assembly for rotating said guide means about said hinge pin;

a torsion bar means having a first end rigidly connected to said platform;

means connecting said tilt assembly means with the other end of said torsion bar; 1 releasable locking means for releasably rigidly attaching said tilt assembly to said platform whereby said tilt assembly means can be rigidly attached or. detached from said platform;

a line connected to the upper part of said leg; and

resilient means and dampening means connected in parallel and connecting said line to said platform.

9. In an offshore platform structure in which the platform is supported by a plurality of leg members supported by leg guide members from the platform by horizontal hinge pin for variable tilt positions, the improvement which comprises:

connecting means forconnecting said leg guide means to said platform, such connecting means including: a tilt assembly for varying the tilt position of each said leg, and resilient means between each said tilt assembly and said platform, said resilient means having a resiliency sufficient to provide limited restraint to movement of said leg member about said hinge pin but sufficient to resist rotational force about said hinge pin caused by gravity.

10. A structure as defined in claim 9 in which the resilient means has a resiliency sufficient to contribute added flexibility for increasing the period of vibration of the leg member by a factor of two.

11. A structure as defined in claim 9 including damping means between the tilt assembly and the platform.

References Cited UNITED STATES PATENTS 939,878 11/1909 Urie 6l46.5 X 2,600,761 6/1952 Halliburton 6l46.5 X 3,171,259 3/ 1965 Roussel.

JACOB SHAPIRO, Primary Examiner.