US3391666A

US3391666A - Variably stabilized floating platforms

Info

Publication number: US3391666A
Application number: US587177A
Authority: US
Inventors: Jr Robert E Schuller
Original assignee: SCHULLER AND ALLEN Inc
Current assignee: SCHULLER AND ALLEN Inc
Priority date: 1966-10-17
Filing date: 1966-10-17
Publication date: 1968-07-09
Anticipated expiration: 1985-07-09

Description

J y 1968 R. E. SCHULLER, JR 3,

VARIABLY STABILIZED FLOATING PLATFORMS Fil ed oct. 17, 1966 2 Sheets-Sheet 1 I NVENTOR:

ROBERT E. SCHULLERJR.

"WJM

July 9, 1968 r R. E. SCHULLER, JR 3,3

VARIABLY STABILIZED FLOATING PLATFORMS Filed Oct. 17, 1966 2 Sheets-Sheet Z V V V V V V V V I I 25 I 23 .l4 I. 33.... !.!.L'2L

Water Line I .m-la I rlfi as I Y I I I v v v v 28 ll '9? 1, a? 000000 F- l3b INVENTOR= United States Patent 3,391,666 VARIABLY STABILIZED FLOATING PLATFORMS Robert E. Schuller, Jr., Houston, Tex., assignor to Schuller & Allen, Inc., Houston, Tex., a corporation of Texas Filed Oct. 17, 1966, Ser. No. 587,177 1 Claim. (Cl. 114-.5)

ABSTRACT OF THE DISCLOSURE An offshore floating platform having a submerged hull with a set of hollow stabilizing columns extending upwardly from the hull and supporting a superstructure above sea level. The hollow columns provide buoyancy chambers which may be flooded with sea water or charged with air, for varying the stabilizing effect of the columns to avoid resonant motion of the platform under different sea wave patterns.

This invention relates to new and useful improvements in offshore platforms of the general type used by the oil industry in searching for and production of offshore oil reserves, and in particular the invention concerns itself with such platforms which float, as distinguished from those which are supported by legs or the like from the bottom of the sea.

Although the invention is oriented especially toward oil drilling platforms, its principles are applicable to floating platforms in general, including those used for offshore military installations, radar tracking stations, meterological outposts, navigational aids, oceanography, et cetera.

In floating platforms of the aforementioned general type, the use of a conventional ship type hull is hardly satisfactory because of excessive motion which occurs when patterns of sea waves pass over the platform location. Thus, after much experimentation and actual construction, the art has developed a special floating platform which comprises a submerged hull and an elevated superstructure which is connected to the hull by horizontally spaced vertical columns. These columns provide the waterplane area and inertia required to keep the platform floating in an upright position, and thus the platform is stabilized by its columns.

Also, column-stabilized platforms of this character are semi-submersible, inthat their displacement may be varied so that they are submerged to a substantially greater draft when drilling, than when being floated from one drilling location to another.

Resonant motion, that is, motion encountered when the natural periods of heave, roll and pitch correspond to the wave periods, is at its maximum in a surface floating vessel, and for this reason a column-stabilized platform is superior to a surface vessel in that the size and spacing of the columns may be arranged so as to increase the natural periods of heave, roll and pitch of the platform so that they are out of range of the expected wave periods.

However, as the natural periods of the platform are increased, the overall stability is decreased and accordingly, a lower limit must be placed on the stability so that it will retain sufficient righting aims to counteract unexpected sudden changes in its initial position. It would be most desirable to operate the platform in a condition of as high a stability as possible, yet far enough out of the range of wave periods to avoid the possibility of resonant motion. However, since the wave periods vary, it is not possible to know in advance of design and construction what wave periods will be encountered and accordingly, the platform in keeping with conventional practice must be built for a maximum wave height and a period that can be tolerated.

The principal object of the present invention is to provide a floating platform with optimum stability combined with ability to avoid resonant motion under various wave patterns and periods. This object is attained by the provision of means for varying the stability of the platform in accordance with sea conditions encountered, the arrangement being such that in any given condition the stability need be decreased only in proportion to whatever increase is necessary to place the natural periods of heave, roll and pitch of the platform out of range of the wave periods in that condition. Thus, Without the conventional necessity of allowing for maximum tolerances, the variably stabilized platform of the invention is capable of operat ing under optimum conditions in any given wave pattern, whereupon its otherwise fixed characteristics may be changed to compensate for wave pattern changes.

As another important feature, the invention provides for varying the stability of the platform by a variation in the waterplane area and, consequently, inertia of the platform columns, while maintaining the displacement of the platform substantially constant. This is of particular importance in that the superstructure of the platform is sustained at a substantially fixed elevation above the sea level, not only for safety and convenience of the working crew, but also to prevent the sea from coming too close to the superstructure deck.

With the foregoing more important object and features in view and such other objects and features as may become apparent as this specification proceeds, the invention will be understood from the following description taken in conjunction with the accompanying drawings, wherein like characters of reference are used to designate like parts, and wherein for illustrative purposes:

FIGURE 1 is an elevational view of a floating platform embodying variably stabilized columns of the invention;

FIGURE 2 is a horizontal sectional view, taken substantially in the plane of the line 2-2 in FIG. 1;

FIGURE 3 is an enlarged cross-sectional detail of one of the columns, taken substantially in the plane of the line 3-3 in FIG. 1;

FIGURE 4 is a fragmentary vertical sectional detail, taken substantially in the plane of the line 4-4 in FIG. 3;

FIGURES 5, 6, 7, 8 and 9 are diagramatic illustrations of various column arrangements.

Referring now to the accompanying drawings in detail, FIG. 1 shows an offshore floating platform designated generally by the reference numeral 10, the same being of the general type used by the oil industry in searching for and production of offshore oil reserves, although as already indicated, the principles of the invention are not limited to this particular environment.

The floating platform 10 comprises a submerged hull 12 having a plurality of hollow stabilizing columns 13 extending upwardly therefrom to above the sea level 14, the upper ends of the columns 13 carrying a superstruc ture 15 on which may be mounted a drilling rig and other related equipment indicated collectively at 16. To facilitate passage of the drill stem (not shown) downwardly from the rig to the sea bed, the hull 12 may be provided with a clearance opening or recess 17, as shown in FIG. 2.

The columns 13 provide the waterplane area and inertia required to keep the platform floating in an upright position, and thus the platform is stabilized by these columns. However, in the presence of sea waves, the platform is subjected to motion such as heaving rolling, pitching, or any combination thereof. In heaving, the center of gravity of the platform is translated along a vertical axis, so that the platform rises and falls as the waves pass beneath it. In rolling, the platform moves rotationally about a longitudinal axis, while in pitching it moves rotationally about a transverse axis. In the instance of a square platform as illustrated, there is no distinction between a roll and a pitch.

In order to avoid resonant motion in which the natural periods of heave, roll and pitch of the platform would correspond to the wave periods, the size and spacing of the columns is usually arranged so as to place the natural periods of the platform out of range of the expected wave periods, but since the wave periods vary, a conventional platform must be constructed so that its natural periods are increased for a maximum wave height and period which can be tolerated. This automatically results in a decrease in stability which prevails even though the expected maximum wave conditions do not obtain.

The invention eliminates this disadvantage of conventional platforms by the provision of means for varying the stabilizing effect of the columns 13 so that at any given time, the platform may have as high a stability as possible, yet have its natural periods sufliciently out of range of existing wave periods to avoid resonant motion. This is attained by varying the waterplane area and consequently the inertia of the stabilizing columns while maintaining the displacement of the platform substan tially constant, so that the superstructure of the platform remains at substantially the same elevation above water.

In accordance with the invention, any number or all of the columns 13 have the interior thereof divided into a set of individual buoyancy chambers, which may be flooded with sea water or charged with air as hereinafter described, so as to respectively decrease or increase the Waterplane area and consequently the inertia of the column. As illustrated in FIGS. 3 and 4,

such buoyancy chambers

18, 19, 20, 21 are concentrically arranged and separated by concentric

tubular partitions

22, 23', 24 within the column 13. These partitions extend vertically between upper and lower

transverse bulkheads

25, 26, respectively, provided in the column above and below the normal water line 14, as will be apparent from FIGS. 1 and 4, so that the buoyancy chambers exist only in the intermediate portion of the column where they are intersected by the waterplane. By thus limiting the vertical extent of the chambers, as compared to the overall length or height of the column, the flooding of the chambers with water or filling them with air does not materially affect the total buoyancy and displacement is thereby kept sub stantially constant. Yet, flooding or blowing out any one or more of the chambers is effective in varying the waterplane area and inertia of the column, as required for purposes of the invention.

As shown in FIG. 4, a manifold 27 at the underside of the bulkhead 26 communicates with the sea and is provided with a set of valves 28 in communication with the

respective chambers

18, 19, 20, 21, so that these chambers may be selectively and individually flooded with sea water. A similar manifold 29, equipped with a set of valves 30, is provided above the bulkhead 25 in communication with the atmosphere, so that the chambers may be vented to the atmosphere when flooding takes place. In addition, an air supply manifold 31 is provided on the bulkhead 25 and equipped with a set of valves 32 in communication with the respective chambers, the manifold 31 being suitably connected to a source of compressed air (not shown) on the drilling rig 16 so that when the valves 30 are closed and the

valves

28, 32 are open, air under pressure may be admitted into the chambers for blowing water therefrom into the sea through the manifold 27. If so desired, the manifold 31 may be selectively connected to the air supply or vented to the atmosphere, in which event the manifold 29 and valves 30 need not be provided. Obviously, any suitable means (not shown), may be utilized for remote control operation of the

valves

28, 30 and 32.

It may be noted at this time that although the plat- 4 form 10 in FIGS. 1 and 2 has been shown as being provided with four equally spaced columns 13 on a square hull 12, the hull may be elongated or circular and any suitable number of columns may be used. Also, the buoyancy chambers may exist in all the columns or only in a certain number of them. Moreover, the buoyancy chambers need not be concentric as shown in FIGS. 3 and 4 and also diagrammatically in FIG. 5. Thus, for example, the columns may be partitioned to provide sector-shaped chambers as in FIG. 6, or a combination of segment-shaped chambers around a central tubular chamber as in FIG. 7. Another variation is shown in FIG. 8, wherein the column has a rectangular rather than a circular cross-section. FIG. 9 diagrammatically shows another possible arrangement wherein an elongated hull 12a is provided with

multiple columns

13a, 13b, which may be of different diameters. The columns need not be partitioned into chambers at all, and a selected few of the several columns may be flooded entirely to produce the same result as the aforementioned flooding of individual chambers in a partitioned column, as far as variation in the combined waterplane area of all the columns is concerned.

Some mathematical examples relating to the operation of the invention may be given. First, in the instance of heave motion, the natural period of heaving is:

where:

T=period of heave in seconds D=displacement in tons tons/ft.=tons per foot of column immersion.

In order to change the period of heave, the invention retains substantially the same displacement and changes the tons per foot of column immersion which is directly proportional to the waterplane area, being determined by the formula:

Waterplane area in square ft. 35

T=period of roll (or pitch) in seconds- K=radius of gyration (about proper axis) GM=metacentric height (about proper axis) where:

The radius of gyration is governed by the masses of the weights in relation to the axis and is not easily changed. The metacentric height, however, is easily changeable. The metacentric height is the distance from the center of gravity of the entire platform to the metacentric, which is an imaginary point representing the center of rolling and pitching, such height being measured in feet. The distance may be obtained by first calculating the BM (or the distance from the center of buoyancy to the metacenter) and subtracting therefrom the BM (or the distance from the center of buoyancy to the center of gravity). The formula for obtaining BM is:

where BM =distance in feet from the center of buoyancy to the metacenter The rolling (or pitching) period is inversely proportional to the GM and since GM and BM are similar, that period is also inversely proportional to the BM. Thus, to increase the period, the BM must be decreased, which the invention accomplishes by decreasing the inertia of the waterplane. Since the moment of inertia is equal to the area of the waterplane multiplied by the square of the distance from the axis, a reduction of the area produces a reduction of inertia which increases the roll (or pitch) period.

As a practical example let it be assumed that there is a platform with four columns as shown in FIGS. 1-4 and that each column has an outside diameter of 34, with concentric inner partitions at 30, 26 and 22' diameters. Then:

Area of each column Tons/ft. immersion waterplane (sq. ft.)

of [our columns Moment of intertia of each column waterplane about its own axis (it!) 34 Dia.907. 9 100. 9 65, 597 30 Dian-706. 8 80. 8 39, 761 26' Dia.530. 9 60. 7 22, 432 22 Dim-380. 1 43. 4 499 Let it be assumed further that the column spacing is 150 feet and that the total displacement complete with drilling equipment and ballast necessary to float the platform at a desired operating level is 15,000 tons, representing 425,000 cubic feet of sea water, Then the heave period with the full 34' diameter columns is:

When the outermost buoyancy chambers 18 of the columns are opened to the sea, the effective column diameter is reduced to 30 and the heave period becomes:

or 13.51 seconds or 15.09 seconds Wave Conditions Causing Effective Period of Resonant Motion Column Heave Dia. (tt.) (seconds) Approx. Wave Associated Wind Length (11.) Velocity (knots),

approx.

Changing the effective column diameter changes the total moment of inertia of the waterplane and hence affects the rolling and pitching periods of the platform. With the square platform in the given example, the rolling and pitching periods are identical, but the same theory is applicable even when the platform is not symmetrical about both horizontal axes. It will be understood, of course, that when the flooding valves 28 are opened to the sea, they remain so open until the effective column diameter is again to be varied. For example, when the outermost chamber 18 of the column is flooded, it remains open to the sea so that the effective column diameter is reduced from 34' to 30 as if the outer column wall at the 34' diameter did not exist.

The total moment of inertia of the waterplane is equal to the area of the columnsmultiplied by the square of the distance from the axis plus the inertia of each of the columns about its own axis. In the example given, the distance from the axis (one-half of 150 feet, or feet) remains the same and the square of this distance is 5625. The inertia of each of the variously sized columns about its own axis has already been determined. Thus, the total waterplane inertia of the platform as the various chambers are opened to the sea becomes:

Inertia Total Column Dre. (ft.) Area about own Moment .1 4x706. 8X5, 625= 15, 903, 000+4X39, 761 16, 062, 048 4X530. 9X5, 625= 11, 945, 250+4X22, 432=l2, 034, 974 4X380. 1X5, 625= 8, 552, 250+4Xll, 499= 8, 598, 246

A radius of gyration (K) in the given example may be assumed at 65 feet, and the rolling and pitching periods may then be calculated as follows:

For 34 diameter columns:

/Toc 11.30 seconds Similarly, 13.21 seconds for 30' column diameter; 16,000 seconds for 26 diameter; and 20.63 seconds for 22' diameter.

The following table may be set up for comparison with properties of waves:

Wave Conditions Causing Column Period of Roll Resonant Motion Dia. (it) and Fitch (see) Approx. Wave Associated Wind Length (in) Velocity (Knots) It will be apparent from the foregoing that by virtue of the invention it is possible to lengthen the heave, roll and pitch periods so that the platform may operate with considerable stability most of the time when sea conditions are favorable, but may be changed when sea conditions worsen, so as to keep out of range Where resonant motions otherwise would occur.

It may be also noted that the partitioned columns in the waterline area provide a series of barriers to the sea, so that if the outer wall of the column should become broken, as in a collision with a tug or a work boat, the next inner partition would keep out the sea and thus permit the column to continue its stabilizing function.

While in the foregoing there have been described and shown the preferred embodiments of the invention, various modifications may become apparent to those skilled in the art to which the invention relates. Accordingly, it is not desired to limit the invention to this disclosure and various modifications and equivalents may be resorted to, falling within the spirit and scope of the invention as claimed.

What is claimed as new is:

1. In an offshore floating platform, the combination of a submerged hull, a plurality of hollow stabilizing columns extending upwardly from said hull to above sea level, a superstructure mounted at the upper ends of said columns, and means for varying the stabilizing effect of said columns whereby to avoid resonant motion of the platform under different sea wave patterns while maintaining displacement of the platform substantially constant, said stabilizing effect varying means comprising means for varying natural periods of heave, roll and pitch of said platform, at least one of said columns being provided with vertical partitions separating the interior thereof into a set of individual buoyancy chambers, said References Cited UNITED STATES PATENTS 2,889,795 6/1959 Parks 1140.5

3,224,401 12/ 1965 Kobus 1140.5 3,273,526 9/1966 Glosten 114-05 FERGUS S. MIDDLETON, Primary Examiner.

TRYGVE M. BLIX, Examiner.