Semi-Submersible Floating Structure of High Stability for Offshore Operations

Description

PATENT SPECIFICATION

DRAWINGS ATTACHED Inwentors: PIERRE WILLM and RENE DELMAS 1L 15O,509 Date of Application and filing Complete Specification: 3 March, 1967.

No.10193 /67.

Application made in France (No.51950) on 3 March, 1966.

Complete Specification Published: 30 April, 1969.

Crown Copyright 1969.

Index at acceptance: -El 1-1(11, 14) Int. C: -E 02 d 17/00 COMPLETE SPECIFICATION

Semi-Submersible Floating Structure of High Stability for Offshore Operations We, INSTITUT FRANCAIS DU PETROLE DES CARBURANTS ET LUBRIFIANTS, a French Body Corporate, of 1 & 4, avenue de Bois-Preau, Rueil-Malmaison, Seine et Oise, France, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: -

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

A structure of this kind is provided with buoyancy elements and is adapted to remain by itself in 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.

The main object of the present invention is to provide a rigid structure of the type set forth having a high stability in the water even if one of the buoyancy elements should be damaged, while at the same time having a low amplitude of movement under the action cif the swell, and which is capable of resting in a stable position on the sea-bed in its semi-immersed position or of being easily towed, while its resistance to currents and other external actions is substantially the same in all directions (isotropy), and on which devices may be installed such as a crane or a derrick capable cf hoisting or supporting elements of high weight, for example a drill string, in alignment with a vertical axis passing near the centre of gravity of the structure, [Pripe 4s. 6d.] the position of which permits these elements to be disengaged from the structure.

According to the present invention a floating structure for offshore operations comprises a working platform supported by buoyant elements located at the respective apices of a substantially regular pentagon, each of said buoyant elements being formed by a vertical column integral at its lower end with a caisson, the columns being interconnected by a cross-bracing system, the caissons having a length in a direction parallel to an axis of symmetry cif the pentagon shorter than the length of the side of the pentagon, said crossbracing system being arranged to define a disengagement space between two adjacent columns of the structure which are symmetrically disposed with respect to the vertical plane containing said axis of symmetry.

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

These ballasting means permit partial immersion of the structure when it is located at the site selected for its operation.

The deballasting of the structure is effected to permit the structure to be towed with 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 one symmetry axis of the structure have proved.

the superiority of the structure of the invention.

In particular with a triangular structure 0 Wd 1,150,509 a less good isotropy with respect to the actioi 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 ol the swell for a second value of the wave length thereof, said second value being equal to one half cf said first value.

Moreover a, triangular platform is not as stable as the structure of 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 some directions ef propagation of the swell and scene 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 actions of currents and swell as well as a high stability and 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 structure of the invention 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 one symmetry axis of the 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 favourable. 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 necessitates their increase in length which is detrimental to the isotrony of the structure.

In order that the invention may be more afully understood an embodiment in accordnance therewith will now be described by way eof example with reference to the accom1 panying drawings in which:Fig. 1 is a perspective view of the structure 70 i in its semi-immersed operating position; and Fig. 2 shows a top plan view of the structure of Fig. 1.

As shown by Figs. 1 and 2, the structure comprises an upper working platform 1 which 75 can support equipment for offshore drilling, particularly a drill derrick located as near as possible to the centre of gravity of the platform.

This platform is supported through beams 80 by buoyancy elements of which each is a vertical column 2, 3, 4, 5 or 6 normally partially immersed and integral at its lower end with a caisson 7, 8, 9, 10, 11 respectively which is completely immersed when the structure 85 is in its working position (Fig. 1).

The partial immersion of the structure on the sites selected for its operation may be effected by admitting water into the buoyancy elements, as by opening inlet valves and may 90 be accelerated as 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 95 the respective apices of a substantially regular pentagon and the caissons are of elongated shape parallel to an axis of symmetry of the pentagon.

The length 1 of these caissons in their 100 direction of elongation must not exceed the length L of the side of the pentagon (Fig. 2) so as to obtain a structure which, while being easy to tow, shows a resistance to currents and swell which is substantially identical in 105 all directions (isotropy). In the construction L in the drawings the length I is near -.

2 The direction of elongation of the caissons 110 is the direction for towing the structure of the invention.

To allow for towing the structure is deballasted as hereinabove indicated so as to make the caissons emerge at the surface of 115 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 120 elements are then all located above the floating line during the towing operation, which facilitates the towing operation.

The structure has shown good stability under all circumstances, even if a buoyancy 125 element is damaged and thus becomes overflooded.

Mooring means of any appropriate type may be used to keep the structure substan1,150,509 tially 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 is possible by ballasting to make the structure rest in a stable position on the water bottom in a semi-immersed position if the nature of the 10, water floor and the water depth make it possible. To this end the caissons are preferably flattened at their lower part along a plane which is perpendicular to the vertical 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 where elements of high weight can be handled, such as the elements of a drill string, without prejudice to the stability of the structure on the water.

These elements may 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 being adapted to permit this disengagement, there being no cross-bracing element between the columns 2 and 6 which would hinder such a disengagement.

The structure is such that along a vertical plane of symmetry XY it comprises an open space so that the platform may be moved away from the drill string, the plane of symmetry XY being the one to which the longitudinal axes of the caissons are parallel.

The disengagement space is bounded at the upper end of the structure by the columns 2, 6 adjacent to the vertical symmetry plane and by a first pair A, B of upper cross-bracing ties 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 XY hereinafter referred to as wing columns, and, on the other hand, two upper lines of crossbracing elements each constituting a broken line of three cross-bracing elements which forms a flattened S connecting respectively the column 4 on the symmetry plane XY hereinafter referred to as the leading column, to each column 2, 6.

At the lower level of the structure the disengagement space is also bounded by the two columns 2, 6 adjacent to the vertical plane of symmetry XY and by two lower crossbracing ties 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 wing columns 3, 5 most remote from the symmetry plane XY.and, on the other hand, two rectilinear lower lines each of two crossbracing elements connecting respectively the leading column 4 on the vertical plane of symmetry XY to each of the columns 2, 6.

The structure comprises furthermore a 70 second convex broken line of three upper cross-bracing elements interconnecting the two columns 3, 5 most remote from the symmetry plane XY and, at the intersection ofsaid second convex line with the two aforementioned upper lines of cross-bracing elements converging to the column 4 on the symmetry plane XY, a second pair of upper cross-bracing ties C, D.

In the drawings each of the upper crossbracing ties 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 XY are each respectively connected to the two columns 2, 6 bounding the disengagement space by a cross-bracing element at the upper level and another crossbracing element at the lower level.

The four upper cross-bracing ties substantially form a square ABCD such that in horizontal projection the symmetry axis thereof which is perpendicular to, the vertical symmetry. plane XY is parallel to the cross-bracing element interconnecting the two lower 95 ties E, F and slightly off-set toward the disengagement space with respect to this crossbracing element.

Each of the lower ties E,, F is connected by two cross-bracing elements to the two upper 100 ties which are on the same side with respect to the symmetry plane XY..

The isostaticity of the structure is obtained.

by a supplemental cross-bracing element connecting one or other C of the upper ties of 105 the second nair to the lower tie F which is on the opposite side with respect to the symmetry plane XY, and by a horizontal crossbracing constituting a diagonal AD of the square formed by the four upper ties. 110 This isostaticity could also be obtained by any other suitable interconnection system.

This cross-bracing structure has the advantage of reducing to a minimum the total length of the cross-bracing elements and specially of 115; the immersed part thereof and therefore to limit to a minimum the weight of the crossbracing elements and thus the weight of the floating structure, while maintaining sufficient cohesion of the elements of the structure and 120 permitting easy disengagement of a drill string placed along the vertical geometrical axis of the structure.