US3443543A

US3443543A - Semi-submersible floating structure of high stability

Info

Publication number: US3443543A
Application number: US619985A
Authority: US
Inventors: Pierre Willm; Rene Delmas
Original assignee: IFP Energies Nouvelles IFPEN
Current assignee: IFP Energies Nouvelles IFPEN
Priority date: 1966-03-03
Filing date: 1967-03-02
Publication date: 1969-05-13
Anticipated expiration: 1986-05-13
Also published as: FR1519528A; NL6703150A; BE694867A; ES337554A1; NL157370B; GB1150509A; OA02574A

Description

May 13, 1969 P, W|L LM ET AL I 3,443,543

SEMI-SUBMERSIBLE FLOATING STRUCTURE oF HIGH STABILITY Filed March 2, 1967 A35/V @f2/V4.5 1

ATmRNEY United States Patent O 1 Inf. c1. Bssb 35/44 U.S. Cl. 114-.5 7 Claims ABSTRACT OF THE DISCLOSURE This structure, comprising at its upper part a working plaform supported by buoyancy elements each of which being constituted by a vertical column integral at its lower end with a caisson, the dilerent columns being interconnected by a cross bracing system, is distinctive in that the buoyancy elements are located at the respective apices of a substantially regular pentagon and the caissons are all elongated in a direction parallel to a symmetry axis of the pentagon, with a length of these caissons smaller than the` length of the side of the pentagon, and with a cross-bracing system adapted to limit -a disengagement space between two adjacent columns of the structure which are symmetrical with respect to the vertical plane containing said symmetry axis.

The present invention relates to a floating structure which is particularly suitable for offshore drilling.

This structure which is provided with buoyancy elements is adapted to remain by itself on the water with a high draught in a position of partial immersion by the sole ballasting of its buoyancy elements and is thus of the semi-submersible type.

This type of structure is therefore distinctive from that of partially immersed platforms having elements of positive buoyancy which are held in semi-immersed condition by attachment to heavy anchor blocks lying on the water bottom.

A lirst object of the present invention is to realize a rigid structure of the type set forth having a high stability on the water even in case one of the buoyancy element should be damaged, while at the same time having a low amplitude of movement under the -action of the swell.

Another object of the invention is to realize a structure of the type set forth which is capable of resting in a stable position on the sea-bed in its semi-immersed position.

A supple-mental object of the invention is to provide a structure of the type set forth on which may be installed devices such as a crane or a derrick capable of hoisting or supporting elements of a high weight, constituting, for example, a drill string, in alignment with a vertical axis passing near the gravity center of the structure, the position of which allows to easily disengage these elements with respect to the structure.

An essential object of the invention is to reailize a structure of the -above mentioned type which associates to the preceding qualities the ability to be easily towed to the site selected for its operation.

An important object of the invention is to realize a structure of the type set forth whose resistance to currents and other external actions is substantially the same in all directions (isotropy).

Another object of the invention is to realize a structure of the type set forth comprising several buoyancy elements interconnected by a cross-bracing system adapted to form with said buoyancy elements a system of limited weight having only a small number of immersed elements, so as to improve the hydrodynamic behaviour of the structure.

The floating structure of the semi-submersible type according to the invention comprises at its upper part a working platform supported by buoyancy elements of which each is constituted by a vertical column having at its lower end a caisson, the different caissons being all of elongated shaped with their elongation axes all parallel to a common horizontal direction and the columns being made integral with each other by a rigid cross-bracing system offering little resistance to currents and swell.

This structure is mainly distinctive in that the buoyancy elements are located at the respective apices of a substantially regular pentagon with the common direction of elongation of the caissons being parallel to a symmetry axis of this pentagon, the length of said caissons measured along the direction of elongation thereof not exceeding the length of a side of said pentagon and being preferably smaller than one half of said last mentioned length.y

This structure comprises ballasting means which may be constituted by valves for admission of water into the buoyancy means -and by a pumping unit capable of accelerating this admission of water or alternatively of expelling the water contained in the buoyancy elements so as to deballast the structure. These operations may also be realized by using compressed air.

These ballasti'ng means allow to realize the partial immersion of the structure when the latter is located at the site selected for its operation.

The deballasting of the structure is realized so as to allow the towage thereof wit the caissons in a floating position, emerging at the surface of the water. Comparative tests on models with other regular polygonal structures provided with caissons having an elongation axis parallel to a symmetry `axis of the structure have proved the superiority of the structure according to the invention.

In particular with a triangular structure a less good isotropy with respect to the action of the swell and of the currents has been observed. For some directions of propagation of the swell this triangular structure would be subjected to displacements of large `amplitude for a fixed value of the wave length and would become much less agitated but subjected to high stresses under the action of the swell for a second value of the wave length thereof, said second value being equal to one half of said rst value.

Moreover a triangular platform has not as good a stability as the structure according to the invention and its inertia with respect to the action of the swell is lower for the same weight of the structure.

A square or rectangular structure has the same drawbacks as a triangular structure consisting of the anisotropy of its response to swell and currents action, with the same risks of excessive stresses or movements for certain directions of propagation of the swell and certain values of its Wave length.

The structure of the invention has shown in the course of tests with models, a substantially isotropic response to the yactions of currents and swell as well as a high stability and a small amplitude of movements under the action of the swell, these qualities resulting from a good dispersion of the volumes and masses of the structure around the geometrical `axis thereof and from their remoteness from this axis.

Furthermore the structure of the invention is of simpler, lighter and cheaper construction than a polygonal structure with more than five buoyancy elements, which would not offer any substantial advantage with respect to the present structure as far as stability and inertia against the action of the swell are concerned.

The tests performed on models have also shown that the multiplication of the buoyancy elements is a disadvantageous factor resulting in an increase of the movements of the structure in the swell.

Moreover a structure with six buoyancy elements located at the respective apices of a regular hexagon and the caissons of which are elongated in a direction parallel to a symmetry axis ofthe hexagon, presents a resistance to towing which is higher than that of the structure of the invention for a similar size of the structures, which can be explained by the increase in the number of immersed elements of the structure (buoyancy elements and cross bracing members).

A heptagonal structure is also less favorable. Such a structure has in fact a greater number of immersed elements and its towing resistance can only be reduced by decreasing the width of the caissons, which correlatively obliges to increase their length, this being detrimental to the isotropy of the structure.

The structure according to the invention is illustrated by the attached drawings in which FIG. l is a perspective view of the structure in its semiimmersed operating position,

FIG. 2 shows a view from above of this structure.

As shown by FIGS. 1 and 2, the structure according to the invention comprises an upper working platform`1 which can support an equipment for offshore drilling, particularly a drill derrick located as near as possible the gravity center of the platform.

This platform is supported through beams by lbuoyancy elements of which each is constituted by

vertical columns

2, 3, 4, 5, `6, which is normally partially immersed and integral at its lower end with

caissons

7, 8, 9, 10, 11 which are completely immersed when the structure is in its working position (FIG. 1).

The partial immersion of the structure on the sites selected for its operation may be realized by admitting water into the buoyancy elements.

This admission may be obtained by opening inlet valves and accelerated by using a pumping unit which may be also used for deballasting the structure by expelling the water from the buoyancy elements.

These buoyancy elements are located at the respective apices of a substantially regular pentagon and the caissons are of elongated shape parellel to a symmetry axis of the pentagon.

The length l of these caissons in their direction of elongation must not exceed the length L of the side of the pentagon (FIG. 2) so as to obtain, according to the invention, a structure which, while being easy to tow, shows a resistance to currents and swell which is substantially identical in all directions (isotropy). In the illustrated embodiment the length l s-near 1./2.

The direction of elongation of the caissons will be the direction for towing the structure of the invention.

So as to allow this towing the structure will be deballasted as hereinabove indicated so as to make the caissons emerge at the surface of the Water.

On FIG. 2 is shown in dashed line on the caissons the level of the water line, during the towing of the structure.

Under these conditions the cross-bracing elements are then all located above the lioating line during the towing operation, which makes this towing operation easier.

The structure according to the invention has shown a good stability under all circumstances, even in case a buoyancy element is damaged and thus become overooded.

Mooring means of any appropriate type will be used to substantially keep the structure in the location selected for its operation and prevent its drift.

In particular a plurality of moorings (cables or chains) may be used for this purpose in association with anchors or anchor blocks lying on the water bottom. It

will be moreover possible by ballasting to make the structure of the invention rest in a stable position on the water bottom in a semi-immersed position if the nature of the water floor and the water depth make it possible. To this end, according to a preferred embodiment, the caissons will be tiattened at their lower part along a plane which is perpendicular to the axes of the columns of the structure.

A device such as a derrick or a crane may be located on the working platform 1 in the vicinity of the vertical geometrical axis of the structure passing through the point 0 (FIG. 2), for example at the location 12 indicated on FIG. 2, i.e. at a place which makes it possible to handle elements of high weight, such as the elements of a drill string, without prejudice to the stability of the structure on the water.

These elements will be easily disengaged from the structure if the latter must be rapidly moved away from its place of operation, the cross-bracing system of the structure according to the invention being adapted to permit this disengagement, since it does not comprise between the columns 2 and 6 any cross-bracing element which would hinder such a disengagement.

The structure of the invention is such that along a vertical plane of symmetry it comprises an open space allowing to move the platform away from the drill string, this plane of symmetry being the one the axes of elongation of the caissons are parallel to.

The disengagement space is limited at the upper end of the structure by the columns 2, 6 adjacent to the vertical symmetry plane and by a tirst pair A, B of upper cross-bracing knots or members between on one hand a first line of upper cross-bracing elements forming a convex broken line of three cross-bracing elements and connecting the two columns 3, 5 most remote from the symmetry plane and on the other hand two upper lines of cross-bracing elements each constituting a broken line of three cross-bracing elements which forms a flattened S connecting respectively the column 4 located on the symmetry plane to each pile 2, 6 limiting the disengagement space.

At the lower level of the structure the disengagement space is also limited by the two columns 2, 6 adjacent to the vertical plane of symmetry and Iby the two lower cross-bracing knots (E, F) between on one hand a transverse line of lower cross-bracing elements forming a convex broken line of three cross-bracing elements and connecting the two columns 3, 5 most remote from the symmetry plane and, on the other hand, two rectilinear lower lines each of two cross-bracing elements connecting respectively the column 4 located on the vertical plane of symmetry to each of the columns 2, 6 limiting the disengagement space.

The structure comprises furthermore a second convex broken line of three upper cross-bracing elements interconnecting the two piles 3, 5 most remote from the symmetry plane and, at the intersection of said second convex line with the two afore-mentioned upper lines of cross-bracing elements converging to the column 4 which is located on the symmetry plane, a second pair of upper cross-bracing knots C, D.

In the structure considered each of the upper crossbracing knots A, B, C, D is connected by a cross-bracing element with the two nearest columns at the lower level thereof. Moreover the two columns 3, 5 most remote from the symmetry plane :are each respectively connected to the two columns 2, 6 limiting the disengagement space by a cross-bracing element at the upper level and another cross-bracing element at the lower level.

The four upper cross-bracing knots substantially form a square ABCD such that in horizontal projection the symmetry axis thereof which is perpendicular to the vertical symmetry plane is parallel to the cross-bracing element interconnecting the two lower knots E, F and slightly olf-set toward the disengagement space with respect to this cross-bracing element.

Each of the lower knots E, F is connected by two cross-bracing elements to the two upper knots which are located on the same side with respect to the symmetry plane.

The isostaticity of the structure is realized by a supplemental cross-bracing element connecting one or other (C) of the upper knots of the second pair to the lower knot F which is located on the opposite side with respect to the symmetry plane, and by a horizontal cross-bracing constituting a diagonal AD of the square formed by the four upper knots.

This isostaticity could also be realized by any other suitable interconnection system.

This cross-'bracing structure has the advantage to reduce to a minimum the total length of the cross-'bracing elements and specially of the immersed part thereof and therefore to limit to a minimum the weight of the crossbracing elements, thus the weight of the floating structure, while maintaining a sufficient cohesion of the elements of this structure and permitting an easy disengagement of a drill string placed along the vertical geometrical axis of the structure.

We claim:

1. Semi-submersible floating structure for offshore operations, particularly offshore drilling operations, cornprising a working platform supported by buoyant elements located at the respective apices of a substantially regular pentagon, each of said buoyant elements being constituted of a vertical column integral at its lower end with a caisson, said columns being interconnected by a cross-bracing system, said caissons having a length in a direction parallel to a symmetry axis of the pentagon smaller than the length of the side of the pentagon, said cross-bracing system adapted to define a disengagement space between two adjacent columns (2,6) of the structure which are symmetrical with respect to the vertical plane containing said symmetry axis.

2. Semi-submersible lioating structure according to claim 1, wherein said caissons are adapted, in the absence of ballasting of the structure, to maintain said crossbracing system above the level of the water and are elongated in a direction parallel to a symmetry axis of the pentagon whereby is reduced the resistance of the structure during the towing thereof due to the elongation of said caissons.

3. Semi-submersible floating structure according to claim 1, wherein the lower parts of said caissons are flattened along a plane which is perpendicular to the axes of the columns, to thereby allow the structure to rest in a stable position on the water bottom.

4. A semi-submersible oating structure in accordance with claim 1, wherein said buoyant elements comprise two additional columns (3, 5) disposed on the apices of said pentagon most remote from said vertical plane, and a single column (4) disposed in the plane of said vertical plane, and wherein said cross-bracing system comprises:

a first line of upper cross-bracing elements connecting said two additional columns (3, 5);

a second and third line of upper cross-bracing elements intersecting said first line of upper cross-bracing elements and connecting said single column (4) to said two adjacent columns (2, 6);

said disengagement space being defined at the upper level of the structure by said two adjacent columns (2, 6) and by adjacent portions of said lines of upper cross-bracing elements;

a first line of lower cross-bracing elements connecting said two additional columns (3, 5), a second and third line of lower cross-bracing elements intersecting said first line of lower cross-bracing elements and connecting said single column (4) to said adjacent columns (2, 6);

said disengagement space being defined at the lower level of the structure by said two adjacent columns (2, 6) and by adjacent portions of said lines of lower cross-bracing elements;

a fourth line of upper cross-bracing elements interconnecting said two additional columns (3, 5) and intersecting said second and third lines of upper crossbracing elements;

iirst diagonal cross-bracing elements connecting the points of intersection of each of said lines of upper cross-bracing elements to the lower end of two of the columns disposed nearest thereto;

additional cross-bracing elements directly connecting said two additional columns (3, 5) to said two adjacent columns (2, 6) at the upper and lower ends thereof;

said lines of upper cross-bracing elements having intermediate portions (A-B, B-D, D-C, D-A) substantially forming a square;

the axis of symmetry of said square disposed perpendicular to said vertical plane in horizontal projection being disposed parallel to an intermediate portion (E-F) of said first line of lower cross-bracing elements and offset towards said disengagement space with respect thereto;

second diagonal cross-bracing elements connecting the points of intersection of said lines of lower crossbracing elements to the points of intersection of said lines of upper cross-bracing elements similarly located on the same side of said vertical plane;

a third diagonal cross-bracing element connecting an intermediate portion of a line of elements of the pair of lines of upper cross-bracing elements connecting said single column to said adjacent columns with an intermediate portion of a line of lower cross-bracing elements of the pair of lower cross-bracing elements connecting said single column to said adjacent columns located on the opposite side thereof with respect to said vertical plane and a third diagonal cross-bracing element (A-C) disposed in the square (A-B-C-D) formed by said upper lines of crossbracing elements to thereby provide isostaticity for the structure.

5. A seal-submersible oating structure in accordance with claim 1, wherein said vertical plane is a symmetry plane for the structure, and wherein said buoyant elements comprise two additional columns (3, 5) disposed on the apices of said pentagon most remote from said vertical plane and a single column (4) disposed on said vertical plane, and wherein said cross-bracing system comprises:

a first line of lower cross-bracing elements connecting said two additional columns (3, 5);

a second and third line of lower cross-bracing elements intersecting said first line of lower cross-bracing elements and connecting said single column (4) to said adjacent columns (2, 6), said disengagement space being defined at the lower level of the structure by said two adjacent columns (2, 6), and adjacent portions of said lines of lower cross-bracing elements.

6. A semi-submersible floating structure in accordance with claim 5, wherein:

a first line of upper cross-bracing elements connects said two additional columns (3, 5)

a second and third line of `upper cross-bracing elements intersects said first line of upper cross-bracing elements and connects said single column (4) to said two adjacent columns (2, 6);

said lines of upper cross-bracing elements being located in a common plane substantially perpendicular to the direction of said columns.

7. A semi-submersible floating structure in accordance with claim 6, wherein first diagonal cross-bracing elements connect the points of intersection of each of said lines of upper cross-bracing elements to the lower end of the two of said columns disposed nearest thereto:

second diagonal cross-bracing elements connecting the points of intersection of said lines of lower crossbracing elements to the points of intersection of said lines of upper cross-bracing elements similarly 1o- 7 8 cated on the same side of said vertical plane there- 3,246,476 4/ 1966 Wolf 61-46.5 with. 3,343,372 9/ 1967 Smulders 61-46.5

References Cited UNITED STATES PATENTS TRYGVE M. BLIX, Przmary Exammer.

3,011,467 12/1961 Le Tourneau 6146.5 X 5 U.S. Cl. X.R.

3,154,039 10/1964 Knapp. 61-465