Laterally Reinforced Subterranean Conduit For Deep Waters

Abstract

The invention relates to a submarine conduit such as a well casing, mud riser or other conductor which extends in a substantially vertical direction between a subterranean well, and a drilling vessel at the water's surface. The conduit is suspended from the floating vessel at one end, and connected to a well head base. A plurality of flexible tension lines such as steel cables or the like, are stretched parallel to, and adjacent the conduit, which lines are subjected to a constant pulling force. The lines are further connected to the conduit by a series of brackets, spaced therebetween whereby to laterally support the conduit against displacement by water currents and other forces.

Description

BACKGROUND OF THE INVENTION

In the drilling of an offshore petroleum or gas well beneath a body of water, the usual practice is to provide a well head or base positioned at the floor of the body of water. Thereafter, a conduit comprising a continuous length of cylindrical pipe or tubing, herein referred to as a mud riser, is connected to the well head whereby to define a closed path between the water's surface and the well. Through this confined tubing, a drill string is lowered to be rotated and advanced to form the well.

In the drilling of such a well, the process is facilitated by use of an appropriate drilling mud which is forced downwardly under pressure through the drill string. The mud, upon being discharged from the drill bit, passes upwardly along the walls of the well and thence into the mud riser. The mud flow thereby not only carries away drill cuttings, but lubricates the operation as well.

In offshore wells, the submarine conduit or mud riser is subjected to certain forces which would tend to laterally displace the riser. The displacing forces are primarily wave forces and currents acting against the conduit also including, among others, the forces induced as a result of the movement of the drilling vessel or ship when the latter is being used, instead of a stationary platform.

In the instance of wells drilled in relatively deep water, the mud riser would usually be subjected to considerably greater lateral displacing forces than would be the instance in shallow water. Further, as the water depth increases these displacing forces are not always constant in magnitude, or direction.

The basic structure of mud risers results in their being unstable in compression and therefore must be maintained in tension. This is usually accomplished by exerting an upward pull on the top of the riser by devices on the drill vessel. Thus, the mud riser in deep water is commonly subjected to severe bending stresses as well as being placed under considerable tension.

As an engineering necessity, the mud riser must be maintained within known tolerances. These tolerances include angular rotation at the bottom of the riser, displacement at the top of the riser, and combined bending and tensile stresses throughout whereby to remain stable and to continue the drilling operation. To mitigate such complications, the mud riser is generally tensioned to a degree by automatically functioning apparatus carried on the drilling vessel, such tension application having the effect of minimizing curvature and angular rotation at the bottom of the riser.

It is appreciated however that with greater water depths, and with the number of deformations induced in the mud riser, the tension which must be induced into the conduit becomes considerable and can eventually exceed the physical limits of existing known tensioning devices. Such circumstances could cause fracturing or cracking of the mud riser, a circumstance that would constitute a serious detriment to the drilling operation, or could cause costly delays in drilling operations due to angular and displacement tolerances being exceeded.

Toward obviating this drilling problem in deep water offshore wells, the present invention provides means for effectively minimizing the amount of distortion and angularity in a mud riser without the necessity of devising new or added means for exerting additional tension on the riser. Such means includes a tensioning system preferably in the form of a plurality of flexible lines such as steel cables, which are disposed adjacent to the vertical riser. The flexible tension lines are connected at their lower ends to an anchor base which further incorporates the subterranean well and well head equipment. The tension lines extend upwardly through the water in a direction parallel to the riser, and are operably connected to a constant tensioning apparatus on the drilling vessel. The latter apparatus is adapted to operably engage the flexible lines and to induce a constant stress thereon regardless of the undulations or movement of the drilling vessel. A plurality of brackets spaced between the riser and the tension lines, then constitute a reinforcing means which is connected to the vertical conduit, and spaced longitudinally therealong.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevation view partially in cross section, showing the instant mud riser and tensioning system.

FIG. 2 is a cross section segmentary on an enlarged scale taken along line A--A in FIG. 1.

FIG. 3 is a cross sectional view on an enlarged scale taken along line B--B in FIG. 1 to illustrate an alternate embodiment.

FIG. 4 is a segmentary view on an enlarged scale showing an alternate embodiment of the bracket arm.

FIG. 5 is another embodiment of the support bracket member shown in partial cross section.

In the drawings, FIG. 1 illustrates a mud riser installation of the type contemplated wherein a drilling vessel 10 is floatably positioned at the water's surface and there maintained in a relatively stable condition. The positioning can be achieved in a number of ways including the conventional anchoring by a plurality of anchors disposed about and at a distance from the vessel. However, vessel positioning can also be achieved by stabilizing means incorporated in the vessel in the form of lateral thrusters which function as needed to maintain the vessel over a desired location.

A well 11 formed in the substrate beneath the body of water comprises normally, a series of elongated casing members 12 disposed coaxially one with the other and fixed to the ocean floor by cementing or other suitable means. Well head and pressure control equipment 13 carried at the upper end of casing 12 is further supported by an anchoring base 14 which comprises a raised metallic platform formed of structural members securely connected to the outermost well conductor.

Pad 14 is further provided with retainers for holding the lower end of the tensioned system which can comprise two or more flexible lines 16 and 17. The latter extend from base 14, upwardly through the water in a direction parallel one to the other. The upper end of the respective lines are operably connected to constant tension apparatus 15 carried on the vessel.

Drilling of well 11 is achieved by means of a derrick 18 carried on vessel 10. Also included on the vessel is equipment necessary for lowering and rotating a drill string and bit downwardly into well 11.

A vertically positioned submarine conduit 19 extends between vessel 10 and well head 13 being removably connected to the latter and engaging the vessel at a slip joint 30. The latter functions to compensate for movement of vessel 10 while maintaining the vertical integrity of the conduit. Said conduit 19 which will be hereinafter also referred to as a mud riser 19, is normally connected to vessel 10 by a support means 29. The latter is adapted to carry the weight of riser 19 as well as to adjust the upward pull exerted thereon in response to movement of the vessel. The upper end of the slip joint 30 is also suspended from the vessel to maintain the top of the riser at a constant elevation in relation to the vessel.

A plurality of support brackets 20 spaced along conduit 19, extend outwardly from the latter to engage the respective tension lines 16 and 17 whereby to transfer lateral forces induced into the conduit by waves, water currents and the like, to the respective lines.

Drilling vessel 10 as mentioned, is of the floating type essentially because of the great water depth. The vessel further embodies the necessary equipment for achieving a well drilling and/or producing operation. The constant tension apparatus 15 carried on vessel 10 can assume a number of embodiments within the skill of the art. However, it is functionally adapted to operably engage the tension lines 16 and 17 while moving up and down with the vessel and maintaining a constant tension on said lines.

Said tensioning apparatus can be of the hydraulic or mechanical type either of which is automatically responsive to movement of vessel 10 as well as to the other factors whereby to automatically respond to vessel movement and adjust to maintain an upward pull on lines 16 and 17.

Vessel 10 further incorporates a mud riser support system embodying one or more units 29 which are connected to, and actuatable to apply an upward force to riser 19. This form of tensioning device is also known in the art and may assume a general configuration and function of a dash pot arrangement or hydraulic tensioning device whereby movement of mud riser 19 is stabilized through a suitable self actuating hydraulic or mechanical system.

Anchoring base 14, as previously mentioned comprises in essence a raised structure for centralizing pressure controls and riser connectors over the well bore. Said base is fabricated and assembled at the water's surface and thereafter lowered to the ocean floor at the site of a proposed drilling operation. Said anchoring structure 14 can of course assume a number of foundation-like embodiments as mentioned, which are well known in the art.

Riser conduit 19 is formed generally of separate lengths of steel casing, tubing or pipe, 16 to 24 inches in diameter. The pipe lengths are sequentially made up and joined by special connectors welded to each joint of conduit as the conduit is lowered to the ocean floor. Following normal practice, the riser conduit 19 is provided at its lower end with a connector 19a actuatable from the drilling vessel whereby to permit remote coupling or uncoupling operations as needed.

Referring to FIG. 1, the upper end of riser conduit 19 is provided with connectors such as a series of peripherally spaced lugs 22 and 23 which are adapted to connect to the tensioning apparatus 29. The function of the latter as noted is merely to continually transmit the tension force from the tensioning apparatus 29 to the riser.

Referring to FIG. 3, with riser conduit 19 properly disposed substantially vertically in the water, the drill string or casing 26 positioned centrally thereof will define an annular passage 21 between the walls of the adjacent conduit and drill string or casing. Return mud flow through passage 21 is urged by mud pumps carried on the drilling vessel pumping liquid mud down through the drill pipe to the bit and thence back to the surface through the annulus 21.

A plurality of radially extending brackets 20 are carried on the riser conduit 19 at spaced intervals along the length thereof. Said brackets, as shown in FIG. 3, comprise in one embodiment, a pair of hinged segments 27 and 28. When brought together within retainers 24 and 25 on the conduit outer surface, said segments can be tightly fastened to the latter as to maintain a fixed position thereon. The longitudinal spacing of said brackets is such as to provide the necessary support to the conduit and can vary as desired compensating for the water current conditions at a particular drilling site.

The lateral bracket 20 as shown in FIG. 3 is provided with a plurality of radially extending arms 31 and 32 which in the instant arrangement, comprises four. However, it should be appreciated that said arms can vary in number to accommodate the drilling system being used.

The remote end of each arm is provided with means to slidably engage the respective cables 16 and 17. Said engaging means is such to permit the arm to slide along the cable as the mud riser is extended and installed, and yet fits sufficiently tightly about the cable to minimize movement thereof in a horizontal direction.

In one embodiment, the connecting means at the arm 31 extremity comprises a split collar 33 having a center opening which is readily slidable along the cable 16. Collar 33 can be readily fastened in place about said cable by bolting, latching or other suitable means at joint 34.

In an alternate embodiment as shown in FIG. 4, the bracket's outer cable engaging portion comprises a pair of horizontally spaced apart parallel rollers 36 and 37 disposed at each arm end. The latter are contoured along their respective faces to receive the cable 16 therebetween and to facilitate movement of bracket 20 while maintaining the vertical disposition of mud riser 19.

In the instance of the roller type construction, outboard roller 36 is mounted on a hinged element 40, pivotable about joint 45 in a manner to removably engage arm 31. Thus, as the mud riser 19 is lowered, the roller unit can be laterally engaged and bolted or latched into place to position the respective rollers 36 and 37 about cable 16. Thereafter, roller 36 is adjusted into place and fastened by latch 34 to rigidly position cable 16 itself and fixed roller 37 thus obviating relative movement between the cable and the bracket except in a vertical direction.

Under normal operating circumstances where the tidal or current flow of water in a particular area exerts lateral forces against the mud riser 19, the strains imposed on the latter will be transferred along the radial arms 31 and 32 of the bracket 20, to the respective cables 16 and 17. Thus, by maintaining a desired constant tension on the respective cables, the latter will tend to neutralize said lateral forces whereby to minimize deformation or displacement of the riser.

In an alternate embodiment of the conduit structure and as shown in FIG. 5, riser conduit sections 51 and 52 can be provided with opposed flanged ends 42 and 43. The latter structure will then permit coupling of the respective segments through the use of a suitable connecting ring 44 which functions similarly to bracket 20. As shown, ring 44 comprises a pair of semi-circular segments having central channel section 46, which segments correspond one to the other. The segments, when positioned about the mud riser 19, will urge the corresponding flanges 42 and 43 into fluid tight sealing engagement through a gasketing member 47. Said split segments are likewise provided with radially extending arms as in the instance of the previously described bracket 20 to engage the tensional cables. Thus, as the mud riser 19 is lowered, the removable bracket comprising segments 51 and 52 can be applied by positioning conduit flanges 42 and 43 therebetween in proper alignment, to form the channel section firmly about the flanges.

Other modifications and variations of the invention as hereinbefore set forth may be made without departing from the spirit and scope thereof, and therefore, only such limitations should be imposed as are indicated in the appended claims.

Claims

We claim:

1. A rigid subterranean conduit positioned in an offshore body of water and disposed in a substantially vertical direction, being connected to a well formed in the floor of said body of water, and with a vessel floating at the water's surface, the latter including constant tension apparatus thereon,

a. an anchoring base fastened at said floor and having a well head depending upwardly therefrom, the lower end of said subterranean conduit being detachably connected to said well head and in communication with said well,

b. constant tension means spaced from said conduit having one end communicated with said anchor base and the other end operably connected to said constant tension apparatus whereby to induce a tension force into said tension means, and

c. a plurality of spacer brackets fixed on said conduit and extending outwardly therefrom, being operably engaged to said constant tension apparatus whereby to brace said conductor against displacement thereof in response to water currents and other lateral forces acting thereagainst,

d. said spacer brackets being formed of semi-circular components hinged at joining faces of one side thereof and connected by a connector at the opposite joining faces whereby to secure said spacer brackets to said conduit.