US3815536A

US3815536A - Floating installation

Info

Publication number: US3815536A
Application number: US00264469A
Authority: US
Inventors: X Duc
Original assignee: IFP Energies Nouvelles IFPEN
Current assignee: IFP Energies Nouvelles IFPEN
Priority date: 1971-06-29
Filing date: 1972-06-20
Publication date: 1974-06-11
Anticipated expiration: 1991-06-11
Also published as: FR2144075A5; IT956886B; NL7209001A; CA974409A; GB1353375A; SE373800B; DE2231721A1; NO134549C; NO134549B

Abstract

Floating installation having a platform, at least one element providing for the buoyancy of the installation and apparatus connecting the platform to the buoyant element. The element is constituted by at least one flexible, tight and deformable bag housing a determined mass of gas, which is sufficient for maintaining, during its operation, the platform and the connecting apparatus above the maximum level reached by the crest of the waves, thus reducing considerably the action of swell on the installation.

Description

United States Patent [191 Due June 11, 1974 1 FLOATING INSTALLATION 3,468,279 9/1969 Hawkins 114/.5 D [75] Inventor. Xuony Nguyen Due 3,673,975 7/1972 Strauss 114/.5 R x Rueil-Malmaison, France Institut Francais Du Petrole Des Carburants Et Lubrifiants, Rueil-Malmaison, France Filed: June 20, 1972 Appl. N0.: 264,469

Assignee:

Foreign Application Priority Data June 29, 1971 France 71.23841 References Cited UNITED STATES PATENTS 9/1958 Ford 61/1 R Primary Examiner-Albert .l. Makay Assistant Examiner-Donald W. Underwood Attorney, Agent, or FirmCraig & Antonelli [57] ABSTRACT Floating installation having a platform, at least one element providing for the buoyancy of the installation and apparatus connecting the platform to the buoyant element. The element is constituted by at least one flexible, tight and deformable bag housing a determined mass of gas, which is sufficient for maintaining, during its operation, the platform and the connecting apparatus above the maximum level reached by the crest of the waves, thus reducing considerably the action of swell on the installation.

7 Claims, 10 Drawing Figures PATENTEDJUM 1 1 I914 SHEET 2 OF 3 Zlo The present invention relates to a new floating installation and more particularly to a floating installation which is substantially protected from the action of the swell.

The swell exerts on the floating installation during their operation, actions which constitute a nuisance for the crew living and/or working on such installations.

As a matter of fact, under the action of swell, these installations are subjected to alternating movements, such as vertical movements, movements of rotation about two horizontal axes at right angles to each other passing substantially through the center of gravity of the floating installation (roll and pitch).

Processes or devices used up to now to counteract the effects of the swell, considerably increase the cost of the installations and do not eliminate or reduce simultaneously all the effects of swell.

For example, reduced roll and pitch are obtainable with ships having a high draught or ships provided with heavy masses of a high inertia relative to the movements of roll and pitch, but these ships are still subjected to vertical movements.

In the case where underwater operations are performed from a floating installation, it is generally preferred to make use of platforms provided with caissons which are submerged, so as to be as much as possible protected from the action of the swell, these caissons being connected to the platform by a rigid structure of cross-bracing members which exhibits some transparency relative to the swell, this platform being maintained above the sea-level.

However the navigation of these platforms is not easy and their movements are still too substantial to permit performance, from such platforms, or precise operations, such as wire-line operations on an underwater wellhead, or underwater drilling operations. Specially designed apparatuses must then be used, which make heavier and encumber the surface installation and which are often an additional cause of unbalance of this installation.

During the last years it has been proposed to provide between the floating installation and the water surface a layer of pressurized air, which in some cases has led to a reduction in the action of swell. Experience has shown, however, that such a process can only be used for small floating installations, since this process requires the use of very powerful air compressors.

Moreover the utilization of this process has appeared to be possible only for small swells.

The main object of the invention is accordingly to provide a new floating installation which does not suffer from the drawbacks of the floating installations now in use.

Another object of the invention is to provide a floating installation which is simultaneously protected from the different actions of swell as indicated hereinabove.

These and other objects of the invention are achieved with a floating installation comprising a platform, at least one element providing for the buoyancy of the installation and means connecting said platform to said buoyant element, wherein said element is constituted by at least one flexible, tight and deformable bag housing a determined mass of gas, which is sufficient for maintaining during its operation, said platform and said connecting means above the maximum level reached by the crest of the waves, thus considerably reducing the action of the swell on the installation.

The invention will be fully understood and the advantages thereof will appear from the following description of non-limitative embodiments thereof illustrated by the accompanying drawings wherein:

FIG. 1 diagrammatically illustrates a floating installation according to the invention;

FIGS. 2 A to 2 D diagrammatically illustrate the behaviour of the floating installation of FIG. 1 under the action of the swell;

FIG. 3 represents a network of curves showing the amplitudes of the vertical movements of the floating installation, as a function of size parameters of this installation;

FIG. 4 illustrates a particular embodiment of a floating platform according to the invention;

FIG. 5 shows another embodiment of a floating drilling installation according to the invention;

FIGS. 6 and 7 illustrate further embodiments of a floating installation according to the invention.

A floating installation according to the invention is diagrammatically illustrated by FIG. 1.

The installation is constituted by a rigid part 1, of a weight P, maintained above the water surface 2 by a caisson 3 to which the rigid part may be secured by any known connected means diagrammatically indicated by 4.

The caisson 3 is constituted by a tight, flexible and defonnable bag or envelope 5, housing a determined quantity of a gas, such as, for example, air.

p being the pressure of the gas in the caisson, p P/S, wherein P is the weight of the assembly supported by the float and S the contact surface between the float 3 and the rigid part 1.

In calm waters, i.e., in the absence of swell, the draught h of the installation is given by the formula:

P=Shp wherein P and S have the above-indicated respective meanings, h being the draught and p the specific gravity of water.

When the sea is tough, the waves whose direction of propagation is indicated by the arrow in FIG. 1, act on the bag 5 of the caisson.

The fore-front of the waves applied to this caisson a pressure p which is greater than the pressure p, of the gas inside the caisson, while on the rear side of the waves, the pressure p" is smaller than p Thus FIGS. 2 A to 2 D diagrammatically illustrate the behaviour of the caisson subjected to the action of swell, assuming that the rigid part of the installation is stationary with respect to the water bottom.

In these drawings, the reference L indicates the length of the caisson in the direction of propagation of the waves, shown by the arrow, the reference A indicates the crest of a wave which is propagated.

For sake of simplification the waves have been considered as having a sinusoidal profile with the amplitude l, and wave length A, i.e., defined by the mathematical relationship:

l= 1, sin (2 vr/Mx x being the abscissa in a system of stationary axes (x, 01).

The volume Vof the caisson is at any instant proportional to the area s which is limited by the envelope in the plane of FIGS. 2 A to 2 D.

The value of the area s is defined at any instant by the formula:

3 (M2 1r)l, cos(2 w/Mx] +K x, being the absci s sa of the vertical wall 5 a in the system of axes (0i, 0x) and K a constant.

It is easily apparent from the drawing that during the propagation of the waves, the volume of the caisson is subjected to variations which are proportional to the variations A s of the area s, these variations being identified by the hatched portions in FIGS. 2 A to 2 D.

In practice, since the rigid part of the installation is not stationary with respect to the water bottom, the volume variations are compensated by a vertical displacement A lof the caisson, according to the relationship:

A l= As/L.

The following table gives the values of Alas a function of the length L of the caisson in the direction of propagation of the waves, L being defined by the relationship:

wherein n is an integer and k is a number comprised betweenOand 1(0 k 1) n 0 n I n 5 n l0 L n:

n A+(A/I6) 1.98 I, 0.l2 I, 0.024 I, 0.0!2 l, n )H-(A/S) L92 I, 0.22 I, 0.044 I, 0.022 I, n A+(M4) l.8 I, 0.36 I, 0.086 I, 0.044 I, n lt+(3l\/8) 1.57 I, 0.43 I, 0.1 I, 0.052 I, n )t-HAIZ) 1.28 I, 0.427 I, 0.l 16 I, 0.06l I, n A+(5A/8) 0.94 I, 0.363 I, 0.096 I, 0.051 I, n lei-(3M4) 0.6 I, 0.257 I, 0.078 I, 0.042 I, n )cl-(7h/8) 0.274 I, 0128 I, 0.038 I, 0.021 I )1 A A 0 O 0 0 L=(2 m l) (M2) m being an integer equal to or greater than I.

As it appears from this network of curves, the amplitude of the vertical movements of the caisson are greatly reduced and remains much smaller than the vertical movements of the floating installation.

In practice the vertical movements observed for the caisson will be smaller than the above-indicated values, since the waves will loose-a part of their energy as a result of the friction on the wall of the caisson and will have consequently a smaller amplitude.

The quantity of gas housed in the deformable envelope or bag will be such that the rigid part or platform of the installation and the connecting means between the platform and the caisson be always above the crest of the waves.

It will be possible to adjust this quantity of gas as a function of the amplitude of the swell in order to reduce the resistance to the wind of the emergent part of the installation.

Such deformable caissons permit the construction of floating installations such as drilling platforms, floating tanks, floating islands, barges for laying underwater pipe-lines, pontoons, ships, floating wharfs, etc.

In particular, a floating installation according to the invention will be well adapted for constituting offshore drilling platforms, for which it is essential to prevent the drilling column from being subjected to vertical movements.

The vertical movements of the installation being thus greatly reduced, it will be possible to perform oflshore works under swells of higher amplitudes, by using a floating installation according to the invention in combination with the already known compensation devices for preserving the drill column from the vertical movements of a floating installation.

Moreover, the anchoring by means of cables will be easier since, on the one hand, the effect of the swell is, as already indicated, reduced to the friction of water on the wall of the caisson and, on the other hand, the ve rtical movements are of small amplitude and accordingly the variations in the tension of the cables will be small.

FIG. 4 is a diagrammatical and purely illustrative view of an embodiment of a floating installation according to the invention.

Since the direction of propagation of the swell relative to the installation might vary as a function of time, the caisson will be preferably chosen of a substantially cylindrical shape, so that it is unnecessary to give the installation a particular orientation.

In this embodiment, the installation is provided with a plurality of

caissons

3 a, 3 b, 3 c etc. housing at rest air under the same pressure.

The so-formed assembly is surrounded by a protecting grid 6 having a good transparency with respect to the swell.

The protecting grid is designed for preventing any deterioration of the bags from the different caissons through possible obstacles which may be encountered during the navigation.

FIG. 5 diagrammatically shows an embodiment of a drilling platform according to the invention.

According to this embodiment, the caisson has a generally annular shape allowing the provision of a free space in the center of the platform for the passage of the drill column 8 connecting the surface installation, which carries the derrick 7, to the well 9 in the water bottom 10.

As shown in this figure, the caisson is constituted by a plurality of piled up

buoyant elements

3 a, 3 b, 3 c

. made solid with one another so as to make easier the building of the caisson and to prevent any ovalization of this Caisson.

Only one annular assembly of caisson elements has been shown in this drawing, but it will obviously be possible, without departing from the scope of the present invention, to place coaxially several assemblies of caisson elements.

FIG. 6 illustrates another embodiment of a floatin platform according to the invention.

In this embodiment the caisson 3 is connected to the rigid part 1 through a plurality of resilient means, such as damping elements, diagrammatically illustrated and indicated by reference numeral 11, so as to allow a reduction of the resistance to wind of the portion of the installation above the water surface.

In the other embodiment illustrated by FIG. 7, the caisson 3 has been replaced by a plurality of

elementary caissons

3 a, 3 b interconnected, for example, through resilient means 12.

The wall of the caisson is a flexible deformable wall satisfactorily withstanding sea water.

What I claim is: 1

l. A floating installation comprising 'a platform, buoyant means for supporting the platform, and means connecting said platform to said buoyant means, the

buoyant means including at least one flexible, tight and deformable closed bag housing a determined mass of gas, which is sufficient for maintaining during its operation said platform and said connecting means above the 6 I maximum level reached by the crest of the waves, the buoyant means being responsive to the action of a swell for assuming in a vertical plane theprofile of a substantially sinusoidal curve for considerably reducing the action of the swell on the installation, the connecting means including first resilient means for enabling the buoyant means to assume in a vertical plane the profile of a substantially sinusoidal curve in response to the action of the swell, the first resilient means being located between the platform and the buoyant means.

2. A floating installation according to claim 1, wherein said gas is air.

3. A floating installation according to claim 1, wherein the length of said buoyant means in the direction of propagation of the swell has a value at least equal to one half of the wave-length of swell.

4. A floating installation according to claim 1, wherein said buoyant means has a substantially cylindrical shape.

5. A floating installation according to claim 1, wherein said buoyant means has a substantially annular shape.

6. A floating installation according to claim 1, wherein said resilient means are damping means.

7. A floating installation according to claim 1, wherein the buoyant means include a plurality of closed bags, and further including second resilient means disposed between adjacent bags for interconnecting the plurality of bags.

Claims

1. A floating installation comprising a platform, buoyant means for supporting the platform, and means connecting said platform to said buoyant means, the buoyant means including at least one flexible, tight and deformable closed bag housing a determined mass of gas, which is sufficient for maintaining during its operation said platform and said connecting means above the maximum level reached by the crest of the waves, the buoyant means being responsive to the action of a swell for assuming in a vertical plane the profile of a substantially sinusoidal curve for considerably reducing the action of the swell on the installation, the connecting means including first resilient means for enabling the buoyant means to assume in a vertical plane the profile of a substantially sinusoidal curve in response to the action of the swell, the first resilient means being located between the platform and the buoyant means.

2. A floating installation according to claim 1, wherein said gas is air.