US3605413A

US3605413A - Riser with a rigidity varying lower portion

Info

Publication number: US3605413A
Application number: US869313A
Authority: US
Inventors: George W Morgan
Original assignee: North American Rockwell Corp
Current assignee: Boeing North American Inc
Priority date: 1969-10-24
Filing date: 1969-10-24
Publication date: 1971-09-20
Anticipated expiration: 1988-09-20

Abstract

A RISER, ARRANGED FOR COMMUNICATION BETWEEN A STRUCTURE AT THE OCEAN FLOOR AND EQUIPMENT ON THE SURFACE WHERE THE WATER DEPTH MAY BE, FOR EXAMPLE, MORE THAN 600 FEET, HAS A LOWER PORTION WITH A RIGIDITY OR STIFFNESS SUFFICIENT TO WITHSTAND THE WORST CONDITION BENDING MOMENT APPLIED THERETO. THE STIFFNESS OF THE UPPER PORTION IS DETERMINED BY THE DESIGN REQUIREMENTS FOR EFFICIENT AND ECONOMICAL PIPING AND COMMUNICATION BETWEEN THE OCEAN FLOOR AND THE OCEAN SURFACE. IN ADDITION, THE SECOND MODULUS OF THE LOWER PORTION DECREASES AT STATIONS DISPOSED FURTHER FROM THE OCEAN BOTTOM AND WHERE THE UPPER AND LOWER PORTIONS JOIN THE SECTION MODULUS ARE SUBSTANTIALLY THE SAME.

Description

Sept. '20, 1971 3, w, R N 3,605,413

RISER WITH A RIGIDITY VARYING LOWER PORTION Filed Oct. 24, 1969 3 Sheets-Sheet 1 INVENTOR. GEORGE W. MORGWN ATTORNEY Sept. 20, 1971 3, w MORGAN 3,605,413

RISER WITH A RIG'IDITY VARYING LOWER PORTION Filed Oct. 24, 1969 3 Sheets-Sheet 2 INVIZN'IUR. GEORGE W. MORGAN ATTORNEY Sept. 20, 1971 9, w, MORGAN 3,605,413

RISER WITH A RIGIDITY VARYING LOWER PORTION 3 Shoots-Shoat ,1 3

FilBd Oct. 24, 1969 IN VENTOR. GEORGE W. MORGAN ATTORNEY United States Patent 3,605,413 RISER WITH A RIGIDITY VARYING LOWER PORTION George W. Morgan, Anaheim, Calif., assignor to North American Rockwell Corporation Filed Oct. 24, 1969, Ser. No. 869,313 Int. Cl. E21b 17/00 US. Cl. 61--46 13 Claims ABSTRACT OF THE DISCLOSURE A riser, arranged for communication between a structure at the ocean floor and equipment on the surface where the water depth may be, for example, more than 600 feet, has a lower portion with a rigidity or stiffness sufficient to withstand the worst condition bending moment applied thereto. The stiffness of the upper portion is determined by the design requirements for eflicient and economical piping and communication between the ocean floor and the ocean surface. In addition, the section modulus of the lower portion decreases at stations disposed further from the ocean bottom and where the upper and lower portions join the section modulus are substantially the same.

FIELD OF THE INVENTION This invention relates to a riser for communication between a point on the ocean floor and a point on the surface, and more particular, to a riser that is subject to relatively large environmental lateral forces in any horizontal direction.

BACKGROUND In most long term operations under the surface of the water and in many relatively short term operations, communication of various types is required between the operating structures at or near the bottom and operating structures at the surface. Such communications may include transfer of information, material, power, etc. which will be referred'to herein as utilities. Thus, in an underwater oil well, casing, cables, and other pipes, which will be referred to as a riser hereinafter, must extend from anapparatus at the water surface to a structure at the bottom of the body of water.

Structural support for such a riser poses problems that are greatly magnified with increasing water depth. The various pipes and cables themselves collectively or individually can be readily designed to withstand the forces acting through the ocean movement, since the riser is structurally designed to withstand the dynamic forces associated with actual sea state, hydrostatic forces associated with current and sea depth, and environmental forces which result in corrosion, abrasion and degradation of structural material properties. Changes in the position of a floating surface vessel to which the upper end of the riser is connected will naturally move the upper end of the riser, and the pipes in the riser would be designed to also withstand this force. The external forces, being unsteady and varying in magnitude and in direction along the length of the riser, cause the riser to deflect greatly and an additional design problem is encountered since the bottom of the riser needs to withstand these excursions of the riser.

Attempts to solve or avoid these structural design problems have resulted in consideration of a variety of different types of riser structural concepts, such as a cantilevered structure such as a tower which is free standing and unencumbered by buoys and mooring devices. Structural size and weight in such an arrangement are inordinately great in deep water. For example, for a depth of 2,000 feet the bottom of a tower structure would have a 3,605,413 Patented Sept. 20, 1971 "ice diameter on the order of twenty feet with a required wall thickness of one and one-half inches. With such an arrangement overturning moment is resisted at the base so that major design problems exist, particularly in ocean bottom with deep deposits of mud. Another concept that has been considered is an articulated riser, articulated on one or more axes. This riser requires a hydrofoil shape and bearings to permit the required rotation of the riser and to provide what is normally referred to as a pinned connection at the ocean bottom. Such bearings are not reliable over a long period of time, resulting in excessive drag and bending moments after the segments become locked. In another concept, the riser can be made fully flexible to permit a maximum amount of compliance with the load environment, but risers that are highly flexible, such as rubber composites, are limited by current state of art to depths of about 800 feet.

Therefore an object of this invention is to provide a riser that is of minimum size, maximum strength, and long life with little or no maintenance.

Another object of this invention is to provide a riser having the advantages of a flexible riser and also the advantages of a cantilevered riser cantilevered at the ocean floor.

Other objects and many of the attendant advantages of the invention will become more apparent from the following detailed description when taken in connection with the accompanying drawings wherein:

FIG. 1 is a schematic illustration not to scale of a system utilizing a riser constructed in accordance with principles of this invention;

FIG. 2 is an elevation partly insection, of a portion of the lower portion of the riser of FIG. 1 compressed, for illustration purposes, more in the vertical direction than in the horizontal direction;

FIGS. 3 and 4 are cross-sections of the riser taken on lines 33 and 4-4 respectively, in FIG. 2 as vie-wed in the direction of the arrows;

FIG. 5 is a cross-section of the riser taken on line 5-5 in FIG. 6, as viewed in the direction of the arrow;

FIG. 6 is a side view of the riser illustrating the structural arrangement for transfer of tension from the upper portion of the lower portion; and

FIG. 7 is a section in elevation illustrating aspects of the connection of the bottom section of the riser to a bottom structure.

DESCRIPTION OF THE DRAWINGS As illustrated in FIG. 1 a riser constructed in accordance with the principles of this invention includes an upper portion 20 and a base or lower portion 10 fixedly secured to a suitable fixed structure 12 at an ocean bottom 13 and rising to the ocean surface illustrated at 14. The riser has an upper end thereof connected to a moored, floating surface facility 16 in a manner for example as disclosed in a copending US. patent application No. 838,513 filed on July 2, 1969, and assigned to the same assignee as this application. The base portion 10 of the riser has a non-uniform rigidity or section modulus that decreases from a maximum at its point of attachment to the structure 12 to a minimum value at a tension transfer section 18, as will be more particularly described below. At the tension transfer section 18 the flexural rigidity of the base portion has decreased to a value substantially equal to that of the upper portion 20 which is relatively flexible. The upper portion 20 is termed flexible since its length to radius ratio is relatively large, for example, the length may be on the order of 1500 feet or more and the radius would be on the order of six inches. Accordingly, the structure will buckle under its own weight and substantial tension is required to minimize the curvature and minimize bending stresses in the steel pipes making the upper portion 20. A flexible riser constructed in accordance with the principles of this invention having a length of, for example, 1900 feet, would have the lower portion of a length of about 400* feet and formed of pipe with a wall thickness of 2 /2 inches and a diameter at its base of 41 inches while the upper portion 20 may be 1500 feet long made of a 12 inch diameter steel pipe having a Wall thickness of about 1 inch formed of high-test line pipe, API Standard, SLX, X60.

In this embodiment, substantially all of the tension is applied at the upper end of the riser, by means of a spherical float member 22 which is secured to the upper end of the flexible upper portion 20 but preferably is located beneath the ocean surface 14 at a distance of about 100 feet, according to the site characteristics. Such a float will have a diametr of, for example, 30 feet. In some situations for the purposes of this invention the facility 16 may be moved and then the riser itself may be temporary terminated at the float 22 which accordingly would provide the entire tension load and, further, because the float is free floating, a substantial amount of the required angular freedom or pinning of the upper end of the rises is inherently provided.

Because of the large size and weight of the lower portion 10 of the riser, supplementary flotation is required during the transportation and installation phases of riser, as is well known in the art.

Floats

24 and 26 are secured to the lower portion 10 to provide a predetermined amount of positive buoyancy when needed. However, the floats may be flooded after installation to provide negative buoyancy and may be connected to the surface facility so that the water may be blown out, if need be, to provide buoyancy assistance in controlled rising in case of riser removal.

The riser is schematically illustrated in FIG. 1 in an exaggerated curvature condition but is shown with the general configuration that it will assume with normal excursion of the moored floating facility 16 subject to wave motion and water currents which, as is known, have a velocity profile that decreases with depth. The floating facility 16 is illustrated in one position of extreme excursion, these excursions being limited by suitable moorings (not shown). The horizontal distance from the vertical line directly through the axis of the lower portion 10 identifies the radius of excursion of the floating facility 16 which excursion may be in any horizontal direction depending upon water current. Note that minimum radius of curvature occurs in the flexible upper portion 20, whereas the radius of curvature gradually increases in the lower portion 10 as the bottom attach point is approached. By including a rigidity transition structure in the riser, in accordance with the present invention, there is no abrupt change in curvature as would be the case where a flexible member is connected to a more rigid member. Thus, there is no discontinuity in curvature in the riser and particularly at the transfer section 18. Furthermore the curvature of the base portion 10 varies somewhat, inversely with its flexural rigidity or section modulus, from substantially zero curvature (infinite radius of curvature) at the point of connection to the bottom structure to increasing curvatures (reduced radii of curvature at station further removed from the base 12.

' portion 10 are changed to provide the desired section As illustrated in detail in FIG. 2 a steel pipe 30 having a diameter, as mentioned above in the exemplary embodiment, of 12 inches and a wall thickness of 1 inch, extends for the entire length of the riser, from the floatr22 to and through the lower portion 10. As will be readily understood to those skilled in the art, the flexural rigidity, defined as the product of modulus of elasticity and moment of inertia of a structure, may be varied by varying the radius of a circular beam or the wall thickness thereof or both. Since the lower portion is made of steel having a modulus of elasticity of 30,000,000 pounds per square inch, both radius and wall thickness of the lower modulus or flexural rigidity transition. The portion 10 comprises a number of sections for example, three

sections

32, 34, and 36 although a larger number of sections such as 10 may beused. The sections include the base section 32, the intermediate section 34 and the upper section 36. The base section 32 includes an annular enlarged flange 38 and a lower connection portion 40 arranged to be fixedly secured to the fixed underwater structure 12, for example as shown in FIG. 7 the details of which form no part of this invention, but form part of another application which will be copending with this application.

The ten sections in portion 10 of the to-be-fabricated embodiment have the same inside diameters such as 36 inches, for example, with the uppermost section having an outside diameter of 36.75 inches and the lowermost section having an outside diameter of 41 inches. In such an arrangement intermediate sections have outside diameters 31.0, 37.25, 37.75, 38.0, 38.50, 39.0, 39.5, and 40.0, respectively, to provide a progressively increasing diameter and a progressively increasing pipe wall thickness from the uppermost section to the lowermost section. The several sections, for

example sections

32, 35 and 36, are each connected to the others in end to end relation by welding as indicated at 42. The top of each section is tapered such as at 43 to provide smooth transition from one section to the adjacent section. This taper 43 can be made to extend forexample from section 36 to section 32.

In order to provide for transfer of stress from the pipe 30, which carries the tension, to the upper section 36 and to provide rigidity transition therebetween, a plurality of

stress transfer members

44, 46, 48 and 50 (FIG. 3) are provided. The stress transfer members comprise elongated flat plates disposed radially and each welded at its outer edge to the inner surface of the upper section 36 and'each welded at its inner edge to the outer surface of the pipe 30. As most clearly illustrated in connection with the stress transfer member 44 the members have their inner edges longer than their outer edges. In the 1900 feet riser embodiment referred to above, the total length of each of the

transfer members

44, 46, 48 and 50 is fourteen feet, while the length of the outer edges welded to section 36 is less than 7 feet.

In order to facilitate and enhance the transfer of stresses, the upper end of the section 36 is provided with a plurality of upstanding tooth sections such as

tooth sections

52,54, and 56 more clearly illustrated in FIG. 6. As can be seen in this figure, each of the

transfer members

44, 46, 48 and 50 is connected to the outer pipe section 36 substantially at a midpoint of a respective tooth section For further stiffness and improved stress transfer there are provided for each of the transfer members 44 through 50, a group of triangulated

gussets

58, 60, 62 and 63 (FIG. 3). The gussets have their base portions welded to the upper edges of the respectivetooth sections extending to an apex and welded to the outer sloping edges of the respective stress transfer plates, 44, 48, 46 and 50.

As illustrated in FIGS. 3, 4, and 5 a plurality of additional pipes for example, eight pipes, such as illustrated by

pipes

61, 64, 65, 66, 70, 72, 74 and 76, are carried by the riser to provide the necessary transfer of utilities between the floating facility 16 and structure 12 at the bottom of the body of water. A copending application of G. W. Morgan for Multi-Conduit Underwater Line, Ser. No. 721,014, filed Apr. 11, 1968, now Pat. No. 3,526,- 086 describes the structural details of such pipes to form a riser. The various conduits are secured by suitable means (not shown) to the pipe 30 at intervals spaced longitudinally among the pipe 30 and are preferably helically wound as strands are wound in a wire rope whereby the various pipes may move relative to each other and to the pipe 30. This relatively loose attachment of the eight carried pipes minimizes their bending stresses whereby they contribute little, if at all, to the fiexural rigidity of the upper section of the riser. The eight pipes extend for the entire length of the riser along the main tension carrying pipe 30.

Referring to FIG. 7 a simplified attachment between the riser and the structure 12 is schematically shown to give a more complete understanding of the usefulness and problems solved by this invention. A more complete description of the novel features of this arrangement is disclosed and claimed in a copending U.S. patent application titled, Undersea Riser Structure, by Walter Brown, and filed on Oct. 24, 1969, Ser. No. 869,078. On the bottom of the lower section 32 is formed the sleeve 40 which slips over a post 84 on the structure 12. A pair of vertically spaced

annular bearing projections

80 and 82 extending inwardly thereof from sleeve 40 provide the required bearing surface between the two members and allow for easy assembly. The center post 84 has disposed therein a pipe 88 which is suitably coupled to pipe 30 by coupling 90. Pipe 88 extends laterally through an opening 86 formed in one side of post 84. The section 32 of the cantilevered beam section may be provided with a plurality of apertures of which one is indicated at 92 to allow passage of the auxiliary pipes, for example pipe 61 to be connected to pipe 4 by a coupling 96. Although only auxiliary pipe 61 is shown, the other auxiliary pipes, may be connected in a manner substantially identical to that illustrated for line 61.

There has been described an improved structural riser particularly adapted for use in waters of great depth, on the order of 1,000 to 2,000 feet or more, which achieves an optimum combination of stability, rigidity, durability, compactness and strength by providing a flexible element for a major portion of its length and a rigidity transition element from the flexible element to a fixed rigid connection at its bottom portion.

What is claimed is:

1. A riser comprising:

an elongated structural member having a lower end adapted to be secured under a body of water and an upper end adapted to be connected to a floating body;

said member having a first portion with relatively low rigidity and having a second portion with a controlled rigidity extending from said lower end to the point of attachment with said first portion, said controlled rigidity increasing from its value at the point of attachment with said first portion to a value at said first end to allow the developed internal stresses to be below a given value and to allow said second portion to assume a curvature that increases with distance from said lower end.

2. An elongated structural tension member having:

an upper section extending over a major portion of its length, said upper section having a stiffness that will not support its weight; and

a terminal section attached to said upper section and formed with a stiffness that progressively increases from the point of attachment to the end of said upper section. I

3. The member of claim 2 wherein:

said end of said terminal section is fixedly secured beneath a'body of water; and

the other end of said upper section is buoyantly carried above said end of said terminal section.

4. The member of claim 3 further including:

a main stress carrying element extending substantially the full length of the member; and

said terminal section including a secondary stress carrying element providing the progressively increasing stiffness.

5. The member of claim 4 wherein:

said secondary stress element is a tubular element extending around and radially spaced from said main element, said tubular element having its upper end 6 formed with a plurality of longitudinally extending teeth; and

a plurality of stress transfer elements, each comprising a plate radially connected between said main element and the upper end of said tubular element.

6. The member of claim 5 wherein:

said main stress carrying element comprises a main pipe encircled by a plurality of pipes;

said tubular element enclosing said plurality of pipes and comprising a plurality of tubular sections rigidly connected in end to end relation, all tubular sections having the same internal diameter and each section having a smaller external diameter than the next lower section.

7. The member of claim 4 further including:

a plurality of pipes carried by said main stress carrying element;

said secondary stress carrying element comprising a tube encircling said main stress carrying element to provide a space therebetween; and

means for transferring stress between said main stress carrying element and the upper end of said tube.

8. The member of claim 7 wherein:

said tube comprises a plurality of sections rigidly connected in end to end relation;

all of said sections having the same internal diameter; and

each of said sections having a smaller external diameter than the next lower section.

9. An underwater riser that may be fixedly connected at one end underwater and at the other end to a floating body, whereby the riser is subject to forces and displacements due to wave motion and water currents, comprlsmg:

first means at said one end that can be fixedly secured to an underwater structure;

second means at said other end that can be connected to said floating body for exerting tension upon the riser;

third means extending from said other end for a major portion of said riser and said third means having a relatively low rigidity so that it cannot support its own weight; and

fourth means disposed between said first means and third means and having a varying rigidity that varies from a minimum value at the point of attachment to said third means to a maximum value at said first means.

10. A riser comprising:

a flexible upper portion of relatively low rigidity;

a bottom portion of relatively great rigidity; and

an intermediate transition portion between the upper and bottom portions having a rigidity that varies from the value of said bottom portion to the value of said upper portion.

11. A riser comprising:

a tension member;

means for exerting tension on the member at one end thereof; and

means for securing the other end of the member to a fixed underwater structure, said securing means comprising a cantilevered beam fixed at one of its ends to said underwater structure and attached at the other end to the tension member, said beam having a rigidity that increases from a lower value at said other beam end to a substantially higher value at the end thereof connected to the underwater structure.

12. A tensioned riser comprising:

a lower portion fixed to a structure at the bottom of a body of water and constructed to act as a cantilevered beam of decreasing rigidity; and

an upper portion, continuous with the lower portion,

and constructed to act as a tensioned cable.

13. A riser comprising: a first tubular member extending the entire length of the riser; second tubular member concentric with the first member and encircling the first member for a substantial portion of the lower length of the first member, said second tubular member comprising a plurality of sections rigidly connected to each other in end to end relation wherein each of the sections has substantially the same inside diameter and each has a wall thickness that is substantially less than the wall thickness of the next lower section of the second tubular member;

the uppermost section of the second tubular member having the upper end thereof formed with a plurality of axially extending teeth;

a plurality of stress transfer members each comprising an elongated plate disposed radially and connected to the respective teeth of said uppermost section References Cited UNITED STATES PATENTS Rhodes et a1. 175- 7 Triplett 166-.5 Howard 166-.5 Joubert et a1. 166--.5 Morgan 166--.5X

J. KARL BELL, Primary Examiner US. Cl. X.R.