US3532162A

US3532162A - Offshore apparatus including tensioning means for a marine conductor

Info

Publication number: US3532162A
Application number: US776913A
Authority: US
Inventors: William Fischer
Original assignee: Chevron Research and Technology Co
Current assignee: Chevron USA Inc
Priority date: 1968-11-19
Filing date: 1968-11-19
Publication date: 1970-10-06
Anticipated expiration: 1987-10-06
Also published as: GB1285600A

Description

United States Patent [72] Inventor William Fischer Fullerton, California [21] Appl. No. 776,913 [22] Filed Nov. 19, 1968 [45] Patented Oct. 6, 1970 [73] Assignee Chevron Research Company San Francisco, California a corporation of Delaware [54] OFFSHORE APPARATUS INCLUDING TENSIONING MEANS FOR A MARINE CONDUCTOR 10 Claims, 8 Drawing Figs.

[52] US. Cl 166/.5, 175/7 [51] lnt.Cl E2lb7/l2 [50] Field ofSearch 166/5, .6; 175/7, 5

[56] References Cited UNITED STATES PATENTS 3,021,909 2/1962 Postlewaite 166/.5X 3,179,179 4/1965 Koiahl 166/.5

3,195,639 7/1965 P0113111 Cl 211 166/.5

3,211,224 10/1965 Lacy 166/.5

3,265,130 8/1966 Watkins 166/.6

3,313,345 4/1967 Fischer i 166/.5

3,313,347 4/1967 Crain l i 166/.5 3,313,353 4/1967 Postlewaite l66/- X Primary Examiner- Ernest R. Purser Assistant Examiner Richard E. Favrcau Attorneys-A. L. Snow, F. E. Johnston, R. L. Freeland, Jr and C. J. Gibeau ABSTRACT: The apparatus for offshore use includes an arrangement for axially and elastically pulling on the lower end ofa marine conductor pipe connected between a floating platform and a submerged wellhead The marine conductor is placed under sufficient tension to prevent it from buckling under internal or external forces imposed on it by the drilling operation or by the marine environment. In one modification of the invention an elastically loaded cable system is used as the pulling means and its inherent elasticity applies an adjustably controllable tension to the lower end portion of the marine conductor while permitting limited axial movement of it.

I ll 11 II Patented Oct. 6, 1.970

Sh eet 2 of3 Patented Oct. 6, 1 970 Sheet I32 F|G.7 INVENTOR WILL/AM FISCHER ATTRNES OFFSHORE APPARATUS INCLUDING TENSIONING MEANS FOR A MARINE CONDUCTOR STATEM ENT OF INVENTION This invention relates to apparatus for drilling and working in wells in submerged land from a floating platform through an underwater wellhead. More particularly, this invention is directed to apparatus for drilling and working in submerged wells without the use of divers wherein a marine conductor pipe, sometimes called a riser, is employed to guide a drill string and other well working apparatus and equipment from the floating platform into the submerged well. A principal function of the marine conductor pipe is to permit the circulation of a drilling fluid downwardly through the drill pipe and thence back to the floating platform through the annulus between the drill pipe and the inner wall of the marine conductor. The function of the drilling fluid is, of course, the same as that in on-land operations; however, as noted in my previous patent application Ser. No. 372,061, filed June 2, 1964, now US Pat. No. 3,313,345, issued April I1, 1967, entitled Method and Apparatus for Offshore Drilling and Well Completion, the use of a heavy drilling fluid in combination with a marine conductor of the type of interest in the present invention presents difficulties not experienced in on-land operation, In effect, the heavy drilling fluid imposes an addi tional load on the marine conductor pipe which under deep water conditions increases the possibility that it will buckle and fail unless measures are taken to prevent this occurrence. The present invention deals with such measures.

OBJECT OF THE INVENTION It is an object of the present invention to use an elastically functioning force applied to the bottom end of a marine conductor pipe, the top end of which is supported by a floating platform at the surface of the water, to apply axial tension to it ofa sufficient amount to prevent the pipe from buckling under directly or indirectly applied axial or lateral loads.

in the illustrated embodiment of the invention an elastically loaded cable system is used to apply the tensioning force to the marine conductor. A telescopic joint is located in' the marine conductor adjacent the submerged wellhead to which it is connected to provide for relative displacement between the fixed wellhead and the moving platform. The lower end of the cable system is rceved through a pulley system mounted on the submerged wellhead apparatus and is then connected to the lower end of the marine conductor at a location above the telescopic joint. A cable having particular characteristics of elasticity is selected of appropriate length as permitted by the depth of water at the well site or by a combination of the depth of water plus additional reeving of the cable aboard the floating platform so that there will be a sufficient amount of elasticity in the total length of the cable to apply a near-con stant predetermined axial force on the lower end of the major upper portion of the marine conductor as the latter moves toward and away from the wellhead during the normal operation of the apparatus.

Since the upper end of the marine conductor is attached to the floating platform, it moves both vertically and transversely with the platform as the latter is moved by the forces at the surface of the water. The axially directed component of movement of the upper end of the marine conductor is, of course, transmitted along its length to the bottom portion of it and the telescopic joint at this location permits such movement within design limits while retaining a continuous conduit connection between the floating platform and the submerged well. The inherent elasticity of the cable system makes it possible for the lower end of the marine conductor to move axially relative to the submerged wellhead while inducing in the marine conductor a predetermined substantially constant axial tension which prevents it from buckling under the imposed load of heavy drilling fluid or other drilling or well working equipment or components, or by the lateral loads imposed on it by underwater currents, or by the stresses induced in it by the changing position of the floating platform as influenced by the variable forces at the surface of the water.

It will be appreciated as the description of the invention proceeds hereinafter that the apparatus employed avoids the use on the deck of the platform of such tensioning equipment as powerful fluid operated piston-cylinder arrangements 1 together with the necessary power sources and controls, complex counter weight systems, or slipping torque converters, or clutches with continuous power requirements, all of which not only take up valuable space on the floating platform but require constant attention. In accordance with the present invention, the elastic cable tensioning members are pretensioned an amount necessary to provide the required tension in the marine conductor and the ends of the cables aboard the floating platform are then secured in a fixed position relative to the platform. The inherent elasticity of the members then provides a near-constant tensile force on the bottom portion of the marine conductor.

An additional advantage that follows from applying the tensile force to thebottom rather than the top portion of the marine conductor is. the requirement of a lesser tensioning force since this force does not include a component equal to the submerged weight of the marine conductor as is required when the tensioning force is applied to the top of the conductor at the floating platform. Also, with the tensioning force applied to the bottom of the marine conductor, if the tensioning system fails the weight of the marine conductor hanging from its platform support above will in itself act to some degree in reducing or precluding the possibility of a buckling failure, depending on the mud weight and/or prevailing sea conditions. The hanging marine conductor inherently provides a degree of fall safeness not present in a marine conductor system in which the tensioning means pulls upwardly at the top end of the conductor pipe. One of the benefits made possible by the present invention is the elimination of high-pressure, high stress mechanisms which constantly present a dangerous situation aboard an offshore platform. Thus, this invention provides a system for placing a controlled amount of tension in a marine conductor, which system is relatively uncomplicated both in construction and operations while providing additional safety advantages aboard the floating platform.

The modification of the invention using elastic cables accomplishes also the dual purpose of serving as a tensioning means for the marine conductor and as a guide line means at some stages of the well working operation. The concept of this invention can be applied also to other apparatus using guide lines where the environmental conditions of water depth, wave conditions, currents and wind forces cause the guide line systems of prior art devices to malfunction. The elastic cables, when serving as guide lines, preclude the need for separate guide line tensioning means and assure taut guide lines during the whole operation. Thus the operational difficulties which result from inadvertently slacked guide lines are eliminated.

In a preferred form of the invention as applied to a marine conductor one end of each of a plurality of elastic cables is firmly secured to the lower end portion of it. The cables are threaded over respective sheaves to dispose each cable centrally of and passing through a respective hollow guide post. The sheaves and guide posts are mounted on a stationary base platform affixed to the submerged wellhead with the guide posts extending vertically upwardly therefrom. The elastic cables continue upwardly from the guide posts to the floating platform where they are tensioned to a predetermined operating value and locked or clamped in that condition. The material and dimensions of the cable are selected to apply the predetermined tension to the marine conductor while the cable is stretched within its elastic limit and without undergoing a permanent deformation or set. The length of the cable has, of course, a significant effect for its proper functioning in this manner, and if the distance between the floating platform and the submerged wellhead is not sufficient to provide an appropriate length of cable to supply the substantially constant elastic tensile force at the bottom of the marine conductor, the

cable may be effectively lengthened by threading it through a multi-sheave pulley system on the platform until sufficient length is provided to give it the desired elastic characteristics.

In an alternate form of the invention a continuous length of elastic cable is passed downwardly from the floating platform through a hollow guide post on the submerged base platform, thence by a system of sheaves it is directed to the lower end portion of the marine conductor where it is threaded over a sheave mounted on said lower portion and thence continues by another system of sheaves to the base and thence to a second hollow guide post through which it is passed upwardly to be returned to the floating platform. The single continuous length of cable is stressed to the desired operating tension and both ends of it are anchored on the drilling vessel. This arrangement permits an elastic tensioning cable to be replaced without removing the marine conductor from its attachment to the submerged wellhead by attaching a length of new cable to one end of the old cableat the drilling platform and then using the old cable to pull the new cable into threaded engagement with the several sheaves and components of the underwater apparatus.

By either arrangement an axial tensile force is applied to the marine conductor at its lower end adjacent the submerged wellhead rather than at its upper end adjacent the floating platform. Thus the force required to place the marine conductor under a predetermined operating tension can be applied to the marine conductor without superimposing on this force the additional force equal to the weight of the marine conductor in water as would be required if the total force to sustain and tension the marine conductor were to be applied at the upper end of it by tensioning means on the floating platform.

PRIOR ART In my US. Pat. No. 3,313,345 there is discussed the conditions under which a marine conductor will collapse when used as a return conduit for a drilling fluid having a density substantially greater than that of the surrounding water. A marine conductor designed for use in deeper water has the characteristics of a long, slender column which has very little effective inherent stiffness. In its normal operating environment this column is subjected to varying external forces such as those produced by water currents and waves and lateral excur sions or other motions of the floating platform, and these forces can induce buckling of the column when it is loaded with a weighted drilling fluid or has imposed on it the frictional drag of well working tools. In accordance with the teachings of the above-noted patent, the marine conductor is placed under working tension by a hydraulically operated tensioning system incorporated in the top portion of it. Other systems use hydraulic or pneumatic cylinders or rams which are operatively connected to the top of the marine conductor by highly stressed wire cables. Both such types of systems have been placed in field use. However, such systems have the disadvantage of being relatively complex and of being burdened by the requirement that it support the very significant immersed weight of the marine conductor in addition to supplying the force for tensioning it to an operating level. The failure of this type of tensioning system would result in the weight of the riser and its contents being transferred to and supported by its bottom end, a condition which almost certainly would lead to a buckling failure. Even a partial loss of the tensioning force in these systems would result in some degree of sagging of the marine conductor and cause the bottom of it to be displaced angularly from the vertical where it is connected to the submerged wellhead. This could result in failure of the balland-socket or other connection between the bottom end of the marine conductor and the wellhead, or if this connection held, the angular displacement would cause drill pipe operating inside the marine conductor to undergo heavy wear in this region or to be overstressed and broken.

The prior art also proposes that an annular cylinder be placed at the bottom end ofa marine conductor with the latter having affixed to it a piston operating in the cylinder and that the drilling fluid entrained by the marine conductor be introduced into the cylinder so that the weight of the drilling fluid will provide a force on the piston to pull the marine conductor downwardly in tension against its support on the floating platform. Such a system is shown in US. Pat. No. 3,21 1,224 to Lacey issued Oct. 12, 1965. The arrangement of this latter apparatus makes it possible for sand or drill cuttings to settle out of the drilling fluid and accumulate in the annular cylinder and thus make the system inoperable. Also, since the tensioning force in this system is derived from the weight of the drilling fluid alone, the system lacks flexibility in adjusting the tension in the marine conductor independently of the drilling fluid being used to enable it safely to withstand the effects of different water currents or vessel motion when these latter forces may be the dominant ones affecting the tensioning requirements of the marine conductor. This system requires that drilling fluids having a density greater than that of seawater must be used to develop the tension and that correspondingly heavier or more weighted drilling fluids must be used to develop greater tensile forces as may be necessary to prevent damage to the marine conductor, even though such weighted drilling fluids are not required solely for the drilling operation and, in fact, may be detrimental to it.

DRAWINGS Further objects and advantages of the present invention will become apparent from the following detailed description of it taken in conjunction with the accompanying drawings, which form part of this application.

In the drawings:

FIG. I illustrates schematically and partly in section an elevational view of an offshore drilling operation performed from a floating drilling vessel and illustrates the disposition of two elastic cables used both as guide lines and as a tensioning means for the marine conductor.

FIG. 2 illustrates schematically and partly in section an elevational view of an assemblage similar to that of FIG. I but showing the use of more than two elastic cables as guide lines and as a tensioning means for the marine conductor while illustrating other similar portions of the apparatus in an enlarged view.

FIG. 3 is a schematic elevational view of a portion of the apparatus shown in FIG. 2 and illustrates wellhead control apparatus being lowered and guided into a position for connection to a submerged well opening.

FIG. 4 is an elevational view, partly in section and on an enlarged scale, of the apparatus of FIG. 3 lowered into engagement with and connected to the submerged well opening.

FIG. 5 is a plan view, partly in section and with portions broken away, taken along the line 5-5 of FIG. 4.

FIG. 6 is a plan view, partly in section, taken along the line 6-6 of FIG. 4.

FIG. 7 illustrates schematically a portion of the apparatus of an alternate embodiment of the invention and illustrates the manner of connecting a single length of elastic cable to the bottom end of a marine conductor through a pulley system which permits both end portions of the cable to be connected as separate guide and tensioning lines between a floating platform and a submerged wellhead and which permits replacement of the elastic cable without disconnecting the marine conductor from the wellhead.

FIG. 8 is a plan view taken along the line 8-8 of FIG. 7 and shows the disposition of two continuous lengths of elastic cable which are positioned to function as four tensioning lines.

DETAILED DESCRIPTION FIG. 1 illustrates a floating drilling vessel, barge or platform 10 positioned on the surface of a body of water 12 above a submerged well site. The vessel is represented as being held in position by anchor lines 14; however, it will be apparent that the vessel may be maintained in position by continuously maneuvering it to compensate for the effect of forces tending to displace it from the desired position. A well bore 16 is being drilled into the submerged earth 18 by conventional rotary drilling equipment in which a string of drill pipe 20 having a drill bit 22 connected to the bottom thereof extends between the floating drilling vessel and the well bore. The upper end portion of the drill string is supported in a derrick 24 aboard the drilling vessel and passes through a rotary table 26 by which latter means the drill string is rotated. Drilling fluid is pumped down through the hollow drill pipe through a rotary swivel 28 and returns to the drilling vessel through the annulus between the drill string and a marine conductor 30, also variously called a riser and riser pipe. The return drilling fluid is diverted from the marine conductor through a conduit 32 in communication with the top of it, which conduit directs the return drilling fluid to mud treating apparatus, not shown, where it is reconditioned for recycling back into the well bore.

The upper end portion 34, P10. 2, of the marine conductor 30 is connected to the vessel by a gimbal 36 or other means such as is illustrated and described in detail in my aforementioned US. Pat. No. 3,313,345. The gimbal is positioned below the rotary table 26 and may be supported by the deck 38 of the vessel. Thus the vessel supports the weight of the conductor and any additional tensile force imposed on it but the gimbal support prevents the angular rolling and pitching displacement of the vessel from being transmitted to the conductor.

The marine conductor extends from the gimbal 36 downwardly to the submerged wellhead assembly 40 to which it is connected in fluid communicating relationship to provide a conduit from the drilling vessel to the submerged well.

The wellhead assembly comprises, in the embodiment illustrated herein, a sub-base 42 which rests on the submerged earth and supports above it a main base 44 through an angularly adjustable support 46. The main base has a plurality of rigid guide posts 48 fixedly connected to an extending vertically upwardly from it to perform the function of positioning in coaxial alignment with the well casing 50 apparatus and equipment lowered from the drilling vessel as will be understood by those skilled in the art. The spherical support 46 permits the guide posts to assume a vertical position even though the sub-base 42 is inclined from the horizontal as it rests on the uneven surface of the submerged earth. The well casing 50 is cemented in the borehole 16 as indicated by the cement 52. A passage extends vertically from the well through the base structures 42 and 44 to an upper terminus at a connector portion 54 (FlG. 3) to which ultimately additional wellhead apparatus is connected.

During the principal well drilling phases of the operation this wellhead apparatus will comprise a plurality of series-com nected blowout preventers, as 56 and 58, which form a blowout preventer stack 60. The blowout preventers are attached to the connector portion 54 in a fluid-tight detachable connection by means of a remotely operable complementary connector portion 62 which is secured to the bottom of the blowout preventer stack. As the drilling progresses, a second string of casing 64 is inserted into the well bore and cemented to it and to casing 50 to form a secure anchor for the wellhead apparatus.

The lower end of the marine conductor 30 is connected to the top of the blowout preventer stack 60 by a remotely operable detachable connector 66 which enables the marine conductor to be attached to and detached from a tubular fitting 68 affixed to the wellhead apparatus either while the components are at the vessel or when the blowout preventers are connected to the submerged well.

The blowout prcventers 56 and 58 and the connectors 62 and 66 are individually remotely operable from aboard the drilling vessel. This function is illustrated schematically in FIGS. 2 to 6 of the drawings by the respective control lines 57, 59, 63 and 67, which are connected to the appropriate portions of the wellhead apparatus. These control lines are represented as being gathered into a bundle 69 between the submerged wellhead and the drilling vessel. However, this particular disposition of the control lines is not essential to the proper operation of the present invention, and the control lines can be otherwise disposed, as being placed along and attached to the marine conductor, or the wellhead can be operated and controlled by signals, as acoustic signals transmitted from the vessel through the water.

Means are provided which permit relative angular displacement between the vertically fixed wellhead apparatus and the bottom end portion of the marine conductor while maintaining an open conduit communicating between these structures. In the modification of the invention illustrated in FIGS. l4, this means is in the form of a ball-anld-socket joint 70. This specific construction is not, however, necessary for the proper functioning of the present invention, and other flexible connecting means known to the art may be used in place of it without departing from the inventive concept.

Since the top portion of the marine conductor is supported so that there is no relative vertical motion between it and the drilling vessel, it partakes of the vertical excursions of the vessel induced by wave motion. A means is provided to permit relative vertical motion between the upper portion of the marine conductor and the lower end of it, which latter is connected to the submerged wellhead apparatus. Thus a telescopic connection 72 is provided in the lower end portion of the marine conductor with one of the complementary portions 74, FIG. 4, of the telescopic connection being connected through the ball-and-socket arrangement and the connector 66 to the vertically fixed wellhead while the other portion 76 of the telescopic connection moves vertically with the upper portion of the marine conductor 30. A packer means 78 prevents leakage of fluids between the complementary portions of the telescopic section both to keep the entrained drilling fluids within the marine conductor and to prevent the ambient water from leaking into it.

The major portion of the marine conductor is made up of elongated tubular sections 80 connected end-to-end by couplings 82 to extend from the support on the vessel 10 to the telescopic section fixed to the submerged wellhead. The tubular sections may, for example, have an outside diameter in the range of 14 /4 inches with a wall thickness of to V2 inch, although this size is not critical to the invention and is mentioned only for demonstration purposes. Thus the marine conductor in use in a substantial water depth, say 200 feet or more, becomes a long, slender column with very little inherent lateral rigidity to withstand buckling forces imposed on it. As explained in the previously mentioned U.S. Pat. No. 3,3l3,345, a weighted drilling fluid entrained in the marine conductor has the same effect insofar as potential buckling is concerned as would a load applied to the end of the column. This buckling effect occurs even though the marine conductor is supported from the vessel at its top end because the weighted drilling fluid will have an effect similar to a force pushing against the ends of the conductor. A purpose of the present invention is to provide a particular system for pulling downwardly on the lower end portion of the marine conductor with a force greater than the buckling effect of the weighted drilling fluid or other buckling force imposed on the marine conductor during use so that buckling of it will not occur.

As illustrated in FIG. 4, the similar rigid guide posts 48 are tubular members having an axial passage 84 extending entirely through them to receive a respective cable 86. Passage 84 communicates at its lower end with a chamber 88 formed in the base 44. Within this chamber beneath and in operative alignment with each guide post 48 is mounted a respective pulley 90 positioned to receive on its peripheral groove the corresponding cable 86 passing centrally through the guide post. Each cable extends upwardly from the base 44 through a respective passage 92 to a corresponding second pulley 94 mounted on the guide arm 96 which is secured to and extends radially from the connector portion 66. Each cable 86 continues upwardly to a corresponding third pulley 98 which is mounted on a respective bracket 100. These brackets are rigidly affixed to and extend radially outwardly from the lowermost portion of the marine conductor 30, as for example the lower portion 74 of the telescopic connection 72. The cables 86 extend upwardly from the third pulley stage to the anchor plates 102, which are rigidly affixed to and extend radially from the lower portion of the marine conductor 30 immediately above the telescopic connection 72. The end of each cable 86 is pivotally connected to its respective anchor plate 102 as by the pin and clevis connection 104, FIG. 4.

The other end of each cable 86 extends upwardly from its corresponding guide post 48 to the vessel where it is wound on a respective winch 106 or connected to some other appropriate mechanism which will enable tension to be applied to the cable. The winch is used also to control the cable while running in or recovering the marine conductor and may be used to apply a downward force through the cable to the lower end of the marine conductor as it is being run in to pull it into axial alignment with the submerged wellhead against transverse forces acting upon it. A gauging device 108 such as a strain gauge is provided at the winches to show the tension in the cables 86 so that the cables may be pulled to a predetermined operating line tension. Provision is made for locking the winch drums or otherwise clamping the top end of the cable at the platform when the cables are at the desired tension.

It is within the concept of this invention that the cable 86 is made of a material which has sufficient inherent elasticity to permit the drilling vessel to rise and fall approximately 40 feet before the pretensioned cable approaches its elastic limit. Preferably the characteristics of the cable are such that it will function as a very long spring and a relatively small vertical displacement of the vessel will not significantly increase or decrease the spring force. Thus the tension induced in the marine conductor by the cable while the drilling vessel is operating under normal water conditions will remain substantially constant. The particular material used in the cable will therefore depend, among other parameters, on the force to be applied to the bottom end of the marine conductor and on the water depth in which the operation is taking place. In some water depths cables of nylon or other manmade fibers or rubber will be most practical, while in deeper water steel or other metallic cables of special lay may prove more desirable. Also cables of mixed materials, such as steel and nylon, may be selected as having the proper characteristics for a particular installation. The invention therefore is not limited to a particular cable or a cable of particular material but to a cable which has the functions and characteristics desirable for the practice of this invention.

In water depths which require a marine conductor of a length which has an inherent buckling problem but yet not of such depth as to provide a cable of sufficient length to have the inherent total stretch required for the purposes explained heretofore, the necessary additional effective length of cable is obtained by reeving the cable between pulley blocks aboard the drilling vessel before it is wound on the winch drum. FIG. 1 illustrates the arrangement where the water depth is sufficient to provide directly the length of cable desired for the purpose of this invention. In this arrangement each cable 86 is supported in the trunk 110 of the drilling vessel by a respective pulley I12 rotatably mounted on a bracket 114 and thence passes directly to and is wound on winch drum 106. However, if an additional length of cable is required to produce the inherent total stretch of cable within the elastic limit to accommodate the predicted amount of displacement between the vessel and the fixed well head, this is accomplished by running the cable in a longer path over pulleys H6 and I18 as shown in FIG. 2 before winding it on the winch drum 106. As mentioned heretofore, the winches I06 can be controlled to apply the desired working tension to the cable.

Under some operating conditions it is possible to use a single cable to apply the necessary tension to the bottom of the marine conductor. However, generally it is more desirable to use a plurality of symmetrically placed cables both to apply balanced forces to the lower end of the marine conductor and to relieve a single cable of the burden of applying the total stress to the apparatus. FIG. 1 illustrates the disposition of the equipment when two cables are used as tensioning cables, while FIGS. 2-6 illustrate the use of four cables. It will, of course, be recognized that other numbers of cables than those illustrated herein will also function satisfactorily in this invention.

During some of the phases of the drilling and well completion operation, the tensioning cables can function as guide lines to guide apparatus and equipment between the floating drilling vessel and the submerged well. In this latter circumstance a plurality of cables is helpful in assuring the proper orientation and alignment of the equipment being lowered to the well and assists also in controlling the position of the equipment being raised to the vessel.

For example, FIG. 3 illustrates the blowout preventer stack with the marine conductor attached to the top of it being lowered from the vessel 10 while the several tensioned cables 86 in cooperation with the guide structure attached to the stack guide it into engagement with the guide posts 48 which subsequently guide it into alignment with the top 54 of the well casing to which it will be connected. In this operation the weight of the equipment being lowered from the vessel is supported from the derrick 24 either by the string of the marine conductor 30 as it is made up and lowered or by a string of drill pipe 20, the lower end of which is clamped in the blowout preventers.

The guide structure 120 includes guide arms 122 which correspond in number to the guide cables 86 and are secured to and extend radially outwardly from the blowout preventer assembly to support guide sleeves 124. These guide sleeves are constructed to engage respective cables 86 and guide post 48 in sliding relationship to restrain and guide the wellhead equipment as it is being lowered from or raised to the drilling vessel. Another set of guide sleeves 126, FIGS. 4 and 5, are built into the guide structure in a location radially aligned with and inwardly from the guide sleeves 124 to receive in sliding relationship the portion of the respective cables 86 which extends between the base pulleys 90 and the end thereof attached to the marine conductor 30. This second set of guide sleeves 126 assists in keeping this portion of the cable in operating alignment with the complementary pulleys 90 and 94 and helps restrain and guide the wellhead equipment as the latter is being lowered or raised. The guide sleeve 126 freely passes the cable 86 to permit the latter to stretch and relax without restriction. Diagonal members 128 may be secured between the extremities of the guide arms to rigidify the guide structure.

FIGS. 7 and 8 illustrate a modification of the invention wherein a single length of cable is used to form a pair of tensioning lines 130 and 132. This arrangement permits the tensioning cable to be replaced by using the old cable to pull a new one into an operating position in the apparatus without disconnecting the latter or removing it from the water. Further, worn portions of the cable can be removed from operation by winding one end of the cable on its associated winch drum while paying out the other end from its corresponding winch.

As illustrated in FIG. 7, a pulley 134 is rotatably mounted in the structure 136 which is rigidly fixed to the lower end portion of the marine conductor 30 in place of the anchor plate I02 previously described. The tensioning cable is reeved over a set of the various pulleys 90, 94 and 98 at the wellhead as described heretofore for a single cable 86, but instead of being pivotally anchored to the lower end of the marine conductor, it is reeved over the pulley 134 and thence continues through another appropriate set of pulleys 90, 94 and 98. Both ends 130 and 132 of the single cable are threaded through respective hollow guide posts 48 and pass upwardly to the drilling vessel in a manner similar to that illustrated in FIGS. 1 to 4. Thus by reeling in one end of the tensioning cable aboard the drilling vessel while paying out the other end, the old length of cable can be replaced in whole or in partas is necessary without disconnecting the marine conductor from the submerged wellhead and without the necessity of raising it to the drilling vessel to make the anchor plate 102 accessible.

Since the weight of the marine conductor 30 is hanging from the drilling vessel and does not contribute to the forces tending to cause the conductor to buckle, the effective buckling forces on the conductor and hence the amount of tension required to be applied to it to keep it from buckling can be reduced by removing any drilling or well working tools from it and purging the conductor of any heavy drilling fluids. This will reduce the buckling forces to an amount where the tensioning cable can safely be manipulated and replaced. While the cable is being replaced, the winches 106 on the drilling vessel can be controlled as the cable is being paid off of one and wound onto the other to keep an appreciable amount of tension in the cable even though it may not be desirable to keep it stressed to its full working capacity.

FlGS.7 and 8 illustrate further an arrangement where two pairs of tensioning cables are used, each pair being formed of one continuous length of cable. Thus, as described, the pair of cables 130 and 132 are formed of one continuous length while the second pair of cables 138 and 140 are formed of another continuous length. Each of these four cable portions preferably is wound on a respective separate winch drum on the drilling vessel, and one pair of cables can be kept at full working tension while the second pair is being replaced with new cable. In the arrangement illustrated in FIGS, 7 and 8 the pulleys 134 are disposed diagonally between a pair of complementary pulleys 98 to place both cables of the pair on the same side of the marine conductor. However, it is within the concept of this invention to provide a pulley arrangement which will place each cable of a pair on opposite sides of the marine conductor to thus symmetrically dispose the cables of each pair in a manner similar to the two cables illustrated in FIG. 1.

When the elastic tensioning cables are used as guide lines between the floating vessel and the submerged wellhead the inherent elasticity of the cables will accommodate the displacement of the floating vessel relative to the fixed wellhead without requiring the additional use of counterweight systems or other means to hold the guide lines at the predetermined operating tension. 7

It is apparent that various modifications of the illustrated embodiments of the invention can be made without departing from the inventive concept. Therefore it is desired and intended that the concept include all equivalents within the scope of the appended claims.

lclaim:

l. Apparatus to facilitate drilling and working in offshore wells comprising:

a platform floating on the surface ofa body of water;

a wellhead assembly submerged in said body of water beneath said platform in fixed relationship to the submerged earth;

an elongated conduit means extending downwardly from said platform to said wellhead assembly;

means connecting the lower end of said conduit means to said wellhead assembly;

a telescoping section in the lower end portion of said conduit means;

means connecting the top end of said conduit means to said platform against vertical displacement in relation thereto;

adjustably controllable tensioning means connected to the lower end portion of said conduit means at a location above said telescoping section for applying a downwardly directed force to said lower end portion to induce a tensile stress in the major portion of said conduit means;

said tensioning means being constructed to function elastically to apply said force continuously in a substantially constant amount as the said lower portion of said conduit means elongates and contracts at said telescoping section; and

control means on said platform for adjustably controlling the operation of said tensioning means to apply said force in a preselected amount to said conduit means.

2. Apparatus in accordance with claim 1 wherein said conduit means is a marine conductor:

means on said platform for circulating drilling fluid between said platform and a submerged well to return said drilling fluid to said platform through said marine conductor; and

means on said platform for lowering and raising equipment and apparatus through said marine conductor between said platform and said submerged well.

3. Apparatus to facilitate drilling and working in offshore wells comprising:

a platform floating on the surface of a body of water;

means connecting the lower end of said conduit means to said wellhead assembly;

pulley means mounted on said wellhead assembly;

a cable means extending downwardly from said platform to said pulley means and being operatively mounted on said pulley means and thence extending upwardly to place a portion of said cable means at a location adjacent the lower end portion of said conduit means;

means connecting said portion of said cable means to said lower end portion of said conduit means; and

means on said platform for pulling said cable means in tension to apply through said cable means an elastically functioning downwardly directed force to the lower end portion of said conduit means to thereby induce a tensile stress in the major portion of said conduit means.

4. Apparatus in accordance with claim 3 including:

a telescoping section formed in the lower end portion of said conduit means;

said telescoping section having an upper portion and a lower portion connected together in axially relatively movable telescopic relationship;

means detachably connecting the said lower portion of said telescoping section to said wellhead assembly in substantially fixed relationship therewith;

means connecting said portion of said cable means to the lower end portion of said conduit means at a location thereon above said telescoping section; and

said telescoping section operating to permit relative vertical motion between said wellhead assembly and the said major portion of said conduit means.

5. Apparatus in accordance with claim 3 wherein said cable means comprises at least one pair of cables extending downwardly from said platform to said wellhead assembly:

said pair of cables being formed of a continuous length of cable having both ends located on said platform; and

means on said lower end portion of said conduit means operatively engaging said continuous length of cable to enable said cable to apply said downwardly directed force to said conduit means while permitting said cable to be moved longitudinally of itself and relative to said conduit means.

6. Apparatus in accordance with claim 3 including:

at least one rigid guide post fixed to said wellhead assembly in a vertically disposed position, and terminating at its upper end within said body of water, an axially disposed passage extending throughout the length of said guide post;

said passage being constructed to receive said cable means in substantially coaxial relationship;

said pulley means being mounted on said wellhead assembly below and in alignment with said passage to receive said cable means in operative relationship; and

said cable means and said guide post forming a guide means between said platform and said wellhead assembly. 7. Apparatus in accordance with claim 3 wherein said conduit means is a marine conductor:

8. Apparatus in accordance with claim 3 including:

means for increasing the operating length of said elastic cable means an amount in addition to the length of said elastic cable means an amount in addition to the length of said cable means extending between said platform and said wellhead.

9. Apparatus in accordance with claim 3 wherein said cable means comprises a plurality of separate elastic cables with each of said separate cables disposed in laterally spaced relationship to each other and extending downwardly from said platform to said wellhead assembly:

complementary pulley means for each of said separate cables mounted on said wellhead assembly and positioned in laterally spaced apart relationship to each other; and

each of said separate cables being mounted on a said complementary pulley means and thence connected to said lower end portion of said conduit means.

10. Apparatus in accordance with claim 9 including:

a plurality of separate rigid guide posts fixed to said wellhead assembly in laterally spaced apart relationship to each other and terminating at their respective upper ends within said body of water;

a respective axially aligned passage extending throughout the length of each of said guide posts;

said guide posts being disposed to receive a complementary one of said plurality of separate elastic cables in substantially coaxial relationship in and extending through said passage;

each of said complementary pulley means being mounted on said wellhead assembly below and in alignment with a respective one of said passages to receive a corresponding one of said separate elastic cables in operative relationship; and

said plurality of separate elastic cables and the corresponding said plurality of rigid guide posts forming a guide means between said platform and said wellhead assembly.

mg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,532,162 Dated October 6 1970 Inventor) WILLIAM FISCHER It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Claim 8 (Column 11) lines 13 and 1 4, delete the phrase "said elastic cable means an amount in addition to the lenp'th of".

This phrase is an inadvertent repetition of the identical phrase appearing in the preceding lines 12 and 13 of this claim.

Signed and sealed this 18th day of January 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Acting Commissioner of Patents