US3601075A

US3601075A - Riser support structure

Info

Publication number: US3601075A
Application number: US838513A
Authority: US
Inventors: John M Deslierres
Original assignee: North American Rockwell Corp
Current assignee: Boeing North American Inc
Priority date: 1969-07-02
Filing date: 1969-07-02
Publication date: 1971-08-24
Anticipated expiration: 1988-08-24

Abstract

A ship is described which is useful for recovery of liquid minerals from the ocean floor. A rotatable plug through an aperture in the ship''s hull is moored to the ocean floor with a riser pipe from the ocean floor passing through the center of the plug and connected to a substantially vertically extending tower above the ship to permit the ship to ''''weather-vane.'''' The tower, supporting the upper end of the riser, rotates with the ship, and vertical translation of the upper end of the riser relative to the length of the tower is permitted. The tower is mounted on gimbals near its base for tilting relative to the deck of the ship to remain aligned with the riser end. A tube connected to the tower extends below the gimbals and surrounds the riser. Rollers at the lower end of the tube engage the riser to permit longitudinal motion of the tube relative to the riser, and transfer some of the tilting force between the riser and the tower. The tube is rotatable about its axis to remain fixed relative to the riser while the ship rotates.

Description

United States Patent [72] inventor John M. Deslierres Primary Examiner-Trygve M. Blix Rolling Hills, Calif.

Attorneys-William R. Lane, Allan Rothenberg and Richard ABSTRACT: A ship is described which is useful for recovery of liquid minerals from the ocean floor. A rotatable plug through an aperture in the ship's hull is moored to the ocean floor with a riser pipe from the ocean floor passing through the center of the plug and connected to a substantially vertically extending tower above the ship to permit the ship to weathervane. The tower, supporting the upper end of the riser, rotates with the ship, and vertical translation of the upper end of the riser relative to the length of the tower is permitted. The tower is mounted on gimbals near its base for tilting relative to the deck of the ship to remain aligned with the riser end. A

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7 58 WWNO O 7MI3 v 00 n 1. 456 G 13|DO5 u bl m O QZM 6 m H "M o 8 2 C W u n e m .w .m We m E m in R R m m n m mm NM M mm MB mm m 1 T 9 m T m 3 S mm 1 1 .4 2 .m 5. w n" m... BI 8 mm .6 30 mr. h; SJAN D m T W .H m 0 de S m" 0 C L a m. me.w m M num S! k AFPA R2 Um F ill] 1.. il. l. 253 4 2 0 757-47 S 55 5 [iii 1. ii i References Cited tube connected to the tower extends below the gimbals and surrounds the riser. Rollers at the lower end of the tube en- UNITED STATES PATENTS gage the riser to permit longitudinal motion of the tube rela- 7/1968 Bromell et al.

1 14/05 D tive to the riser, and transfer some of the tilting force between the riser and the tower. The tube is rotatable about its axis to remain fixed relative to the riser while the ship rotates.

XD D 5m 04 ll 41 1 1 o vM so 67 99 11 ll 22 i O 9 9 3 mi? 6 4 w lu Patented Aug. 24, 1971 3,601,075

2 Shoots-Shoot 1 lNVIz'Nl'OIC. JOHN M. DESL/ERRE'S Y m4 XIQAZJM ATTORNEY 2 Sheets-Sheet 2 INVIiN'I'OR. JOHN M. DESL IERRES ATTORNEY RISERSUPPORTSTRUCTURE BACKGROUND In the offshore production of oil and other subaqueous fluid minerals there is a practical depth limitation for bottom mounted towers that extend above the surface. This practical limit is in the order of about 600 feet and economic considerations may limit the utility of towers to even shallower depths. An alternative to fixed towers is toprovidea portion of the production facilities on the sea floor and provide a permanently moored floating facility for the balance. When this is done a substantially vertically extending conduit or riser must be provided .between the seafloor and the floating facility. in

relatively shallow depths 'and calm waters many of the SUMMARY OF THE INVENTION Thus there is provided in practice of this invention a fluidcarrying riser having its upper end connected to a ship by means of a tiltable tower. A riser guide aligned with the tower extends along a portion of the riser for minimizing the stresses on the riser. The riser guide is .mounted so as to be rotatable relative'to the tower about an axis extending along the riser.

DRAWINGS Objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein;

FIG. 1 illustrates in perspective cutaway a portion of a ship and underwater riser with a .tower for accommodating pitch, roll and heave of shipi FIG. 2 illustrates in partial longitudinal section a riser guide tube and its mounting; and I FIG. 3 illustrates an end view of thebottom of the guide tube. V

Throughout the drawings like reference numerals refer to like parts.

DESCRIPTION This invention relates to an aspect of an interconnection between an underwater riser and a floating ship. Other aspects of the interconnection between the riser and ship are described, illustrated and claimed in copending U.S. Pat. applications Ser. No. 838,489, filed 'July 2, I969 entitled, .Une

. bitrary position relative tothe deck of the ship. This may be derwater Riserand Ship Connection" by George W. Morgan;

Ser. No. 838,435, filed July 2, 1969 entitled Permanent Ship Mooring System" by Bruno R. Naczkowski, and Ser. No. 838,489,, filed July 2 I969 entitled Oil Production Vessel" by George W. Morgan and Bruno R. Naczkowski, all of which .werefiled on the same date as this application are assigned to 7 North American Rockwell Corporation, assignee of this appli- 'cationand the contents of which are hereby incorporated by reference for full force and effect as if set forth in full herein.

The general relation of a ship-and its parts and a riser incorporating the principles of this invention are illustrated in FIG. 1. As illustrated in this embodiment a ship 10 is moored to the ocean floor by a plurality of anchor chains 11 each of which is connected to a mooring plug 12 extending vertically through the ship. A mooring plug is mounted on bearings 15 so that the ship may rotate relative to the mooring plug which is substantially fixed in position relative to the ocean floor. Additional detailed description of the mooring plug and anchoring system is contained in the aforementioned copending patent: applica tions. T

A vertically extending riser 13 passes upwardly from the ocean floor through a central aperture'in the mooring plug 12 and is connected to a towerl4 abovethe deck of the ship. The tower 14 is mounted on a pair of orthogonal gimbals l6 and 17 so that the tower can tilt relative to the ship and remain aligned with the riser 13 during pitch and roll of the ship or displacement of the ship laterally from a position directly over the base of the riser. The outer gimbal ring 18 upon which the tower 14 is supported is mounted on the mooring plug on a bearing l9'so that the tower 14 can remain fixed in position relative to the ship as the ship rotates about the fixed mooring swivel 12. A spherical member 21 is mounted in the inner gimba] 17 and a tower base 22 is fixed to the spherical member 21. A pair of side rails 23 forming the principal structural members of the tower are mounted on the tower base 22 and extend upwardly, therefrom. A guide tube 24 described in greater detail hereinafter extends downwardly from the spherical member 21 surrounding the periphery of the riser 13 to provide stiffening thereof and reduced stresses on the riser as the ship pitches and rolls relative to the riser.

As the ship pitches and rolls with. the riser remaining in a substantially vertical position the tower l4'and guide tube 24, which are both connected to the spherical member 21 so asto remain in alignment, tilt relative to the ship about the gimbals l6 and 17. The extent of tilt of the tower is limited to about 15 by guy cables 26 each which'is connected to a conventional damper 27 which has a limited travel. in normal operations the angular position of the tower relative to the'deck of the ship follows that of the upper end of the riser andtherefore the dampers 27 serveto limit the angular excursions that the tower can follow and also serve to inhibit rapid or oscilliatory motions. If desiredthe dampers 27 can be'locked in a fixed position so that the tower is effectively supported in any ardone when the riser is disconnected either in transport of the ship or, as described "in the aforesaid pending applications, during storm conditions. 7

In addition to pitch and 'roll of the ship it may heave with wave action or with the tides, or it maydrift a short distance ofl the position directly over the base of the riser, either of which may cause the height of the end of the riser above the deck of the ship to change. Means are therefore provided for moving the upper end of the riser vertically along the side rails 23 of the tower to accommodate the heave of the ship. A cagelike draw bar 28 is mounted with end portions fitted within the opposite sides between the flanges of the l-beam-.

shaped side rails 23 of the tower.

Bearing wheels (not shown) on the ends of the draw bar 28 engage the web and flanges of the side rails 23 of the tower to permit vertical motion of the draw bar along the length of the tower. The draw bar is rigidly connected to a pair of vertical structural members 29 extending above and below the draw bar on each side thereof. ,At their lower ends the structural bars 29 are each connected to a ring 31 which supports the upper end of the riser by means of a conventional swivel bearing (not shown). The upper ends of the structural members 29 are connected by way of an open cage 33 to a clevis 34. The clevis is in turn supported by a hook 36 supported by a cable block 37.

In operation, a tension is applied to the upper end of the riser to support at least a portion of the weight thereof. The

tension load on the upper end of the riser is maintained con stant by a conventionalconstant tension winch 38 on the deck of the ship. A pair of cables 39 extend from the winch 38 to the tower base and thence up along the side rails 23 of the tower by way of a series of pulleys 41. The tension cables 39 are led through the pulley block 37 as many times asdesired to obtain a given mechanical advantage. Thus by way of the constant tension winch '38, cables 39, block 37, hook 36, clevis 34, cage 33, vertical structural members 29, ring 31 and swivel bearing, seriatim, a constant tension load is applied to the upper end of the riser 13. The nature of a constant tension winch is to apply a constant tension to the cable wound thereon irrespective of the cable position. Thus as the ship may rise or fall, for example and the upper end of the riser remains in a substantially fixed position, the draw bar 28 will ride up and down the side rails of the tower and more or less cable 39 is paid out or withdrawn, as required, by the constant tension winch 38 With this arrangement a constant tension is applied to the upper end of the riser throughout the extent of any excursion of the riser end along the length of the tower.

As mentioned hereinabove as the ship pitches or rolls the substantially fixed upper end of the riser causes the tower 23 to tilt relative to the ship. In order to minimize the stresses on the riser at the location of the tower gimbal mounts, that is, the spherical member 21 below the tower base, the riser guide tube. 24 is provided. The guide tube 2.4 is attached to the spherical member 21 and extends downwardly therefrom substantially to the lower end of the mooring plug or swivel 12. Thus both the tower l4 and the guide tube 24 are rigidly secured to the spherical member within the gimbal mounting so as to remain in alignment. That is, the tower and guide tube are essentially a single elongated rigid member pivotally mounted near its middle. Transverse loads or bending moments between the riser and ship are coupled therebetween at the lower end of the guide tube 24, at the spherical member 21, and at the upper end of the riser adjacent the draw bar 28. By so distributing the loads, extreme bending moments on the riser at the gimbal support are avoided.

. Additional structural details of the guide tube and its mounting to the spherical member 21 are illustrated in FIGS. 2 and 3. As the ship heaves the riser must travel along the length of a guide tube 24 and therefore contoured rubber rollers 47 and 48 (not shown in FlG. I) are rotatably mounted transverse to the tube axis at the upper and lower ends, respectively, of the guide tube. In the illustrated embodiment a set of four concave rollers 47 contoured to mate with the outside of the riser are mounted at the upper end of the guide tube in a multiple pillow block 49 which is bolted to a flange 51 on the upper end of the guide tube. A similar pillow block 52 fastened to the opposite lower end of the support tube journals the contoured rubber rollers 48 at that end. The guide rollers 47 which may be segmented, if desired, to minimize friction are contoured so as to closely engage the periphery of the cylindrical riser and thereby permit free relative motion of the riser along the length of the tube as the ship rises and falls. Transverse loads by the riser on the guide tube 24 are transmitted thereto only by way of the

rollers

47 and 48 at the ends of the guide tube.

It will also be apparent that if the guide tube 24 is fixed relative to the tower which rotates as the ship weathervanes, the riser must apparently rotate about an axis along the length of the guide tube. Friction of the contoured

rubber rollers

47 and 48 hearing on the riser effectively prevent relative rotation between the riser and the guide tube. Therefore a pair of conventional ball bearings 53 are provided between the guide tube and the spherical member 21 so that the entire guide tube can rotate about its axis relative to the spherical member 21 and the tower, and yet transmit a substantial transverse load to the spherical member. O-ring seals 55 are provided between the spherical member and a sleeve 56 which is interposed between the upper bearing 53 and a flange 57 on the upper end of the guide tube. The sleeve 56 extends along the length of the guide tube to a position just beyond the spherical member 21 where a ring 58 is bolted on so as to support the lower ball bearing 53. Additional O-ring seals 59 are also provided at the lower end of the sleeve to prevent water from entering the ball bearings.

The bearing support system between the spherical member 21 and the guide tube 24 assures that the guide tube is free to rotate with the riser relative to the spherical member about an axis along the length of the tube; however, any transverse loads are readily transmitted between the guide tube and the spherical member.

Referring again to FIG. 1 it will be apparent that as the ship rolls, for example, with the tower and riser in a substantially fixed position a bending moment is applied by the riser to the lower end of the guide tube 24 and to the tower at the draw bar 28 with both of these bending moments tending to pivot the combined tower and guide tube in a clockwise or counter clockwise direction about the tower support gimbals. Since the load tending to cause tilt of the tower is applied at points both above and below the tower gimbals the stress on the riser is minimized. In the illustrated arrangement of FIG. 1 the upper end of the riser is near its maximum vertical extent and the bending moment which would be applied to the tower alone in the absence of a guide tube would probably be sufficient to tilt the tower without creating undue stress in the riser at the spherical member 21. It will be apparent, however, that when the riser head and draw bar are at their lower extent within the tower, the moment arm between the top of the riser and the tower gimbals would be short and the stresses required for tower tilting would be significantly higher. It is in this lower position that the guide tube is of maximum significance.

It will be apparent that many modifications and variations of the present invention are possible in light of the above teachings; thus, for example, a structural framework rather than the guide tube could be employed between the spherical member and the riser at a point substantially below the spherical member. Other modifications and variations would readily suggest themselves to one skilled in the art.

What I claim is:

1. In combination:

a ship having a mooring plug extending vertically through the ship,

bearing means disposed between said plug and said ship to allow said plug to rotate relative to said ship,

said mooring plug having a vertically disposed opening,

a riser tower support bearing mounted to said plug within said opening to allow said support to rotate with respect to said plug,

a riser tower gimbaled to said tower support to allow said riser tower to nutate and rotate with respect to said plug means on'said plug adapted to receive anchor chains so that said ship may weather vane around said plug.

guide cable means extending from the deck of said ship to said tower to maintain said tower in a substantially vertical position, and substantially nonrotational with respect to the deck,

a riser extending up through said opening, and

swivel means for connecting the upper end of said riser to said tower so that said tower may rotate with respect to said riser.

2. ln the combination of claim 1 wherein:

a rigid tubular guide is rigidly dependently from said tower and disposed around said riser to limit the bending stresses in said riser.

Claims

1. IN COMBINATION: A SHIP HAVING A MOORING PLUG EXTENDING VERTICALLY THROUGH THE SHIP, BEARING MEANS DISPOSED BETWEEN SAID PLUG AND SAID SHIP TO ALLOW SAID PLUG TO ROTATE RELATIVE TO SAID SHIP, SAID MOORING PLUG HAVING A VERTICALLY DISPOSED OPENING, A RISER TOWER SUPPORT BEARING MOUNTED TO SAID PLUG WITHIN SAID OPENING TO ALLOW SAID SUPPORT TO ROTATE WITH RESPECT TO SAID PLUG, A RISER TOWER GIMBALED TO SAID TOWER SUPPORT TO ALLOW SAID RISER TOWER TO NUTATE AND ROTATE WITH RESPECT TO SAID PLUG MEANS ON SAID PLUG ADAPTED TO RECEIVE ANCHOR CHAINS SO THAT SAID SHIP MAY WEATHER VANE AROUND SAID PLUG. GUIDE CABLE MEANS EXTENDING FROM THE DECK OF SAID SHIP TO SAID TOWER TO MAINTAIN SAID TOWER IN A SUBSTANTIALLY VERTICAL POSITION, AND SUBSTANTIALLY NONROTATIONAL WITH RESPECT TO THE DECK, A RISER EXTENDING UP THROUGH SAID OPENING, AND SWIVEL MEANS FOR CONNECTING THE UPPER END OF SAID RISER TO SAID TOWER SO THAT SAID TOWER MAY ROTATE WITH RESPECT TO SAID RISER.

2. In the combination of claim 1 wherein: a rigid tubular guide is rigidly dependently from said tower and disposed around said riser to limit the bending stresses in said riser.