FR2507146A1 - MARINE PLATFORM, ESPECIALLY DRILLING, AND RELATED PLACING METHOD - Google Patents

Abstract

The platform comprises a body (1) having a density less than one, said body (1) carries a loading (6, 7, 27) and is connected to anchoring means (9) at the sea bottom (11) by mechanical tension linking means (12) so that it is totally immersed when in operation. The upper structure comprising the body (1) and its loading (6, 7, 27) have a positive buoyancy even when the loading reaches its maximum rated mass. The linking means (12) are a single attachment or a plurality of parallel attachments. Taking into account foreseeable lateral efforts (L) and the water depth (h) on the site , the body (1) is dimensioned so that the upward thrust (T) is sufficient to prevent the lateral drift (d) from exceeding a value considered as the maximum admissible value. Application particularly to the drilling or collecting of various energies.

Description

 The present invention relates to a marine platform, which may be intended for oil drilling, or more generally for any drilling in order to search for coal or ores, or else to capture the energy of swells, currents or winds. Other uses of the invention are also conceivable.

 The present invention also relates, more specifically, to a drilling platform.

 The present invention also relates to a method of setting up the platform on the site of use.

 We know different types of sea platforms, among which we choose according to the depth of the water at the location of the drilling.

 Beyond 300 meters deep, fixed platforms resting on rigid piers are abandoned in favor of floating structures.

 In particular, a platform is known comprising a submerged float carrying the working installations by means of legs crossing the surface of the water. The float is dimensioned so as to maintain its buoyancy even if the loading in installations or others is maximum. It is connected, by a series of stretched cables, to massive caissons resting on the bottom of the sea.

 The float is submerged to a sufficient depth so that the swell can never discover it, so that the platform has a certain insensitivity to bad weather.

In an attempt to maintain the platform in a substantially fixed position despite lateral forces such as winds and currents, some of the cables are arranged obliquely between the platform and the caissons.

 This known platform has certain drawbacks.

Indeed, the insensitivity to bad weather is very imperfect because the swell immersed in a variable way the legs supporting the emerged installations. The variation in Archimedes' thrust which results therefrom causes, due to the elasticity of the cables which is not negligible over lengths close to 500 or 1,000 m, a heaving movement of the platform which is extremely harmful whatever the intended use for this platform. This is how in the case of oil drilling, this heaving causes the cutting force on the drilling tool to fluctuate considerably and can cause it to break.

 In addition, this platform is inevitably sensitive to lateral forces. Indeed, the elasticity of the oblique cables does not completely avoid the drift. The float also has the ability to tilt, allowing it to drift even more than would be expected from the simple elasticity of the cables.

 We therefore understand that in the known platform, we must admit a certain sensitivity to bad weather as well as a certain sensitivity to lateral forces. These sensitivities are difficult to assess during construction, and in any case have unacceptable values for certain applications.

 It will also be noted that the numerous cables necessary in the known embodiment have a considerable mass. This mass is carried by the float, to the detriment of the payload that the latter can receive in the form of installations in particular.

 The object of the invention is to remedy these drawbacks by providing a marine platform which makes it possible to easily adjust, during construction, the sensitivity to swell and to lateral forces, and to give these sensitivities acceptable values for the almost all applications.

 The invention thus relates to a marine platform comprising a body of density less than unity, this body carrying a load and being connected to means for anchoring to the bottom of the water by means of connection under tension so that it is in substantially submerged service. The body is dimensioned so that the upper structure, comprising said body and its load, has positive buoyancy even when the load reaches its maximum expected mass.

 According to the invention, the marine platform is characterized in that the connecting means have their own deformability allowing the upper structure to drift under one lateral force such that these connecting means come substantially in alignment with the result of the forces undergone by the upper structure.

 During the construction of the platform, the maximum drift that is considered acceptable for the platform is defined with respect to the vertical from the anchoring means. The proportion between this drift and the depth of the water at the location where the platform is to be installed is calculated. We then choose the vertical upward force applied to the upper structure so that the proportion between the lateral forces expected from currents, winds, etc., and said vertical force is equal to the aforementioned proportion between the maximum allowable drift and the depth of the water.

 Certainly, increasing the vertical force amounts to increasing the volume of the float, therefore increasing the lateral forces due to the currents.However, Archimedes' thrust increases like the volume of the float while the hydrodynamic forces only increase in proportion to its malte torque. which is a surface.

A solution can therefore be found for each problem.

 It will be noted that the invention proceeds from an approach contrary to that of the state of the art. According to the invention, a position of natural equilibrium of the body is sought under the effect of the lateral forces, and the tension of the connecting means is adjusted so that this balance is compatible with the intended use.

According to the state of the art, the cables are multiplied and their orientations are varied to prevent the float from taking the equilibrium position which is sought according to the invention. Thus, in the known platform, it is sufficient for the cables to be tensioned, but the value of the voltage does not play a role, and in practice, of course, a reduced value is chosen.

 On the contrary, according to the invention, in most cases it is necessary to choose a very high de-tension value, so that, for example, the aero or hydrodynamic forces remain low compared to this tension.

 Thus, the variations in Archimedes' thrust generated by the swell on the upper structure are very small compared to the tension on the connecting means, and the resulting heaving is negligible.

 According to another aspect of the invention, the marine platform intended for oil or mining drilling, comprising installations in particular for bringing down a drilling rod towards the bottom of the batten, a body of density less than 1 carrying these installations and dimensioned so as to have a positive buoyancy even with the maximum loading envisaged, this body being connected to anchoring means at the bottom of the water by means of connection under tension so that it is in service submerged substantially in whole, is characterized in that the connecting means comprise a watertight tube, inside which the drill pipe is guided.

 In this aspect, the invention teaches an embodiment which makes it possible to benefit from the advantages of the first object of the invention, and which is also particularly advantageous in the context of drilling techniques.

 Indeed, given the efforts involved, the waterproof connection tube behaves like a flexible clip even if a connection of the embedding type connects it to the anchoring means. Its flexibility is sufficient for this. In addition, the tube is particularly advantageous in its role of guiding the drill pipe. If, for example, the drilling tool needs to be changed, there is no longer any problem finding the drilling.

According to another object of the invention, the method for setting up a platform of the above kind is characterized by the following steps:
the body is floated freely at the location where it is desired to install the platform;
an attachment intended to form part of the connecting means is unwound from the body, until the free end of this attachment reaches the bottom of the water;
the tie is moored at the bottom of the water; and
the tie is pulled from the body, so as to push it into the water to the desired depth.

 Other features and advantages of the invention will also result from the description below.

In the appended drawings, given by way of nonlimiting examples:
- Figure 1 is a schematic side elevation view of a platform according to the invention;
- Figure 2 is a side elevational view and axial section of the platform being set up;
- Figure 3 is a schematic side elevational view in axial section of a drilling platform according to the invention;
- Figure 4 is an axial sectional view of the anchoring means; - Figures 5 and 6 are @@@ @@ showing two stages of the establishment of the platform of Figure 3;
- the partial @ @ @ @ in side elevation and section @@ variant embodiment;
- Figure 8 is a side elevational view of a platform carrying a wind turbine; and
- Figure 9 is a sectional view along line IX-IX of Figure 8.

 The platform shown schematically in Figure 1 can be applied to different uses such as drilling or capturing energy from swells, winds or currents.

 It comprises a body 1, of density in S re to 1, consisting of a hollow waterproof sphere, immersed so that the hollows of the swell cannot then discover it, even if the swell has its maximum foreseeable amplitude, considering the site.

 The body 1 carries a load essentially consisting of installations the nature of which is linked to the use intended for the platform.

 The load also includes a tray 6 intended to allow ships to dock or helicopters to land. The plate 6 is supported by the body 1 by means of a leg 7 which crosses the surface of the water 8.

 The body 1 is connected to means 9 for anchoring to the bottom of the water 11 by connection means 12.

Even when the load supported by the body 1 is maximum, the assembly constituted by the body 1 and its load (comprising in particular the installations, the plate 6, the jamb 7, etc.) has a buoyancy pO9ltiV g that is to say that it tends to go up towards the
sX have the connecting means 12, which in turn prevent this ascent of the body 1.

 In accordance with the invention, the connection means 12 have their own deformability allowing the platform to drift under lateral force such that these connection means come into alignment with the result of the forces applied to the upper assembly which includes the body 1 and its load.

 These particularities are explained below with reference to FIG. 2, in which the sphere 1 is subjected, for example on the part of sea currents, to a lateral force L.

 On the other hand, the sphere 1 undergoes an upward vertical thrust P equal to the difference between the Archimedes' thrust undergone by the upper structure and its weight.

 The water depth is h at the location of the platform and, under the effect of the force L, the float 1 undergoes a drift d.

 According to the invention, the connecting means 12 are such that L / P = d / h.

 In the example shown in FIG. 1, the connecting means 12 are a single flexible attachment, such as a cable, and the condition stated amounts to saying that the resultant R of the forces P and L is substantially aligned with the cable 12 .

 In the simplified representation of FIG. 1, the angle of inclination of the cable 12 relative to the vertical has been deliberately exaggerated.

 In practice, we can make this angle as small as we want by increasing the thrust P, that is to say by increasing the difference between the buoyancy on the body 1 on the one hand and the weight of the body 1 and its loading on the other hand.

 Preferably, for this purpose, the mass of water displaced by the body 1 is at least about twice the mass of the body 1 and of its maximum expected loading.

Also preferably, the push is given
P a value such that the ratio d / h is at most 5%, in the case of a substantially maximum lateral force L.

 Of course, the values chosen depend on the application envisaged and the subjugations that it implies.

 On the other hand, the practical consequences on construction which result from the choice of the ratio d / h depend on the maximum effort L expected, and this generally depends on the place where the platform is to be installed.

 By way of example, a body 1 constituted by a sphere 10 meters in diameter has a volume of 525 m3.

If its wall is 0.02 m thick, its weight is around 49 tonnes. By respecting the aforementioned condition that the weight of water displaced is twice the weight of the floating assembly, this float can support a load of approximately 210 tonnes. If as one can reasonably imagine, such a sphere undergoes from the currents a lateral force L of 3 tonnes, the ratio L / P = d / h is equal to 3 / (525- (210 + 49)) = 1.13%.

We will now describe, with reference to FIG. 2, a method of installing the platform on the site of use. The process steps are as follows:
- An anchor 9 is prepared at the bottom of the water 11 and the body 1 provided with a free float on the site provided with a chimney intended to serve later as a jamb 7. The chimney 7 is surmounted by a winch 10 on which the cable 12 is wound.

 - The cable 12 which enters the body 1 is unwound through the chimney 7, and leaves the body 1 through an orifice 15 provided with sealing means preventing any entry of water into the body 1.

 - When the cable reaches the bottom of the water 11, it is fixed to the anchor 9, for example using personnel traveling to the scene by means of an underwater vehicle.

 - The cable 12 is rewound using the winch 10 so as to bring the body 1 down to the desired depth. This is the step shown in Figure 2.

 Consequently, the cable 12 constitutes the final attachment of the body 1 to the anchor 9.

 In the example of FIGS. 3 and 4, which relates to a marine platform intended for oil drilling, the elements similar to those of FIG. 1 are not described again. The connection means 9 here comprise a single tube 13 rigidly fixed to the anchoring means 9 and to the sphere 1. Despite this rigid fixing, the tube 13 behaves like a flexible fastener making it possible to ensure that the condition L / P - d / h is met.

 The tube 13 is waterproof and the construction has been made to contain no water.

 In addition, a steel tube was chosen whose thickness e satisfies with the diameter D of this tube the relation e = D / 31.2. Thus, taking into account the density of the steel equal to 7.8, it is checked that the weight of the tube immersed in water is balanced by the buoyancy, in other words that the apparent weight of the tube is zero.

 The diameter D is chosen as a function of the stresses undergone by the tube, in particular in traction under the effect of forces such as P, and in radial compression under the effect of the water pressure.

 The anchoring means 9 which are shown in detail in FIG. 4, comprise a tubular pile 14, waterproof, sealed by a cement or a resin 16 in a well 17 dug vertically at the bottom of the water. This type of anchoring is described in detail in French patent application No. 78 19 897 in the names of the Applicants.

 The tube 13, the diameter of which is less than the internal diameter of the tubular pile 14 extends to the bottom of the well 17, coaxial with the latter. A concrete seal 18 is interposed between the tube 13 and the pile 14. The seal 18 is extended upwards by a corolla 19 intended to limit the radius of curvature in bending of the tube 13. View in straight section, the face 21 of the corolla 19 which faces the tube 13 is tangent to the tube 13 at the mouth of the well 17, then moves away from the tube 13 in an arc, the radius of this arc 21 is the maximum radius that the curvature of the tube 13 is accepted. The height of the corolla 19 is such that the angle at the top of the arc of a circle 21 is at least equal to the maximum angle of inclination allowed for the tube 13, this angle being defined by the ratio d / h.

 The wall of the tube 13 is thickened from the bottom of the well 17 to the top of the corolla 19 because it is in this area that the tube 13 works in bending, as will be seen below.

 As shown in FIGS. 3 and 4, the drilling rod 22 used for petroleum exploration is mounted in a guide tube 23 which in turn is installed in the tube 13. The drilling 24 is carried out from the bottom of the well 17. The tube 23 is used to cover the wall of the borehole 24 and to evacuate upwards the materials resulting from the drilling work. Preferably, a safety shutter device 26 (FIG. 3) is installed in the anchoring means 9 to prevent any sudden spurting of oil or gas.

 The drilling rigs, shown diagrammatically in FIG. 3 in the form of a derrick 27, are installed in the body 1 which is a hollow sphere. Platform 6 allows ships to dock and / or helicopters to land.

 We will now describe the operation of the platform.

Only the leg 7 crosses the surface of the water 8, and since it only carries the plate 6 and not all of the installations, the leg 7 is relatively thin. As a result, the influence of swell on the volume of water displaced by the platform is very reduced and hearing variations in thrust
P very weak which are quite negligible if we relate them to the value of the thrust P. The swell is therefore without influence on the platform.

 Given the length of the tube 13, the latter behaves like a cable, that is to say that under the effect of the resultant R it tends to align with this resultant as soon as it leaves the means of anchoring 9. Thanks to the corolla 19, the effective stress in the tube 13 is limited by the radius of curvature of the face 21. Thus, any curvature of the tube 13 takes place in abutment against a part or, in extreme cases, the the entire height of the face 21. Above, the tube is stretched in a rectilinear fashion under the effect of the resultant R.

 When the drill rod 22 has to be completely extracted to change the drill tool (not shown) which is at its lower end, there is then no problem finding the drill 24 since it suffices to re-engage the rod drilling 22 in the guide tube 23.

FIGS. 5 and 6 show two stages of the method of placing the platform of FIGS. 3 and 4 on the site. This method comprises the following stages:
- The pile 14 is sealed in the well 17 and the float 1 is allowed to float freely on the site.

 - The tube 13 is constructed by elements 28 inside the sphere 1, and it is brought out of the sphere 1 downwards, through an orifice 29 formed at the base of the sphere 1. This is the step shown in Figure 5. A technique for unwinding a tube from a sealed enclosure without risking the intrusion of water into the enclosure is described in French patent application No. 78 19 467 of June 29, 1978 in the names of the Applicants.

 - When the tube 13 reaches the bottom of the well 17, the concrete seal 18 is made between the tube 13 and the pile 14. To this end, personnel go to the anchor 9 using a vehicle advancing on the seabed 11.

 - Cylinders 31 are installed in the sphere 1, between the sphere (1) and the end of the tube 13, and using these cylinders, the sphere 1 is pushed into the water to the desired depth . This is the step shown in Figure 6.

 In the example shown in FIG. 7, the body 1 is of oblong horizontal shape and can pivot around the tube 13 thanks to a sealed pivot 28.

The derrick 27 is mounted on a shoulder 29 of the tube 13.

 This embodiment allows the float to orient itself in its direction of least resistance to sea currents.

 In the example shown in FIGS. 8 and 9, the platform is intended to support a wind turbine 35.

 To provide it with the necessary great stability, the platform comprises three cylindrical bodies 1, of interior density at 1, arranged according to an equilateral triangle diagram (FIG. 9).

 Each body 1 is connected to a respective anchor 9, for example of the kind with hollow pile such as 14 driven into a well such as 17, by connecting means 9 each time comprising a rod 34 articulated to the anchor 9 corresponding by a universal joint 36 acting as a ball joint.

 The anchors 9 are arranged according to the same diagram as the bodies 1, so that the rods 34, which are of the same length, are parallel to each other.

 A frame 37, which rests on the three bodies 1 by means of three feet 38 which cross the surface of the water 8, lteolian door 35. To reinforce the rigidity of the upper structure, the bodies 1 are connected together by three spacers 39 which materialize the equilateral triangle diagram of the assembly.

 The platform obeys the general conditions laid down according to the invention and explained with reference to FIG. 1.

 The thrust P is equal to the difference between the weight of the water displaced by the upper structure and the total weight of the upper structure comprising the wind turbine 35, the plate 37, the three legs 38 and the three bodies 1.

 The bars 34 are rigidly fixed to the bodies 1. In the event of lateral force on the upper structure, they can undergo a very significant bending moment. To limit this moment, corollas 41, of a design similar to the corolla 19 of FIG. 4, but directed downwards, surround the rods 34 at their junction with the bodies 1.

 It has been observed that the invention, despite the surprising simplification which it provides relative to the state of the art of floating platforms, advantageously lends itself to very numerous applications.

 Of course, the invention is not limited to the examples described and shown and numerous modifications can be made to these examples without departing from the scope of the invention.

 Thus, for example, the entire mooring system by tube 13, pile 14, sealing 18 and corolla 19 is perfectly applicable to a platform which is not intended for drilling.

 On the other hand, in the positioning process described with reference to FIG. 2, it is possible to provide that the chimney is extended to the bottom of the sphere 1, in which case it suffices for a static seal between the chimney 7 and the orifice 15, the water being able without inconvenience to enter the chimney 7,

Claims (21)

 1. Marine platform comprising a body of density-less than unity, this body carrying a load and being connected to anchoring means (9) at the bottom of the water (11) by means of connection under tension (12) so that it is in service submerged substantially entirely, the body (1) being dimensioned so that the upper structure comprising said body (1) and its load (6, 7, 27) have buoyancy positive even when the load (6, 7,? 7) reaches its maximum planned mass, characterized in that the connecting means (12) have their own deformability allowing the upper structure (1, 6, 7, 27) to drift (d) under the lateral force (L), such that these connecting means (12) come substantially in alignment with the resultant (R) of the forces (L, P) undergone by the upper structure.  2. Platform according to claim 1, characterized in that the ratio between the lateral force (L) and the tensile force (P) due to the connecting elements (12) is less than approximately 5%.  3. Platform according to any one of claims 1 or 2, characterized in that the maximum mass of the upper structure (1, 6, 7, 27) is less than about half of the mass of water displaced by the submerged part of the upper structure.  4. Platform according to one of claims 1 to 3, characterized in that the connecting means (12) comprise a single fastener (12, 13) or several substantially parallel fasteners (34).  5. Platform according to claim 4, characterized in that this attachment is a rod (34) connected to the anchoring means (9) by a ball joint (36).  6. Platform according to any one of re vendications 1 to 5, characterized in that the connecting means (12) comprise a tube (13) waterproof, and whose thickness (e) / diameter ratio (D) is such that its apparent weight is substantially zero when it is immersed in water.  7. Platform according to claim 6, characterized in that the tube (13) is thickened in the vicinity of the anchoring means (9).  8. Platform according to any one of claims 6 or 7, characterized in that one of the ends of the tube (13) is rigidly fixed to the anchoring means (9) and the other end is rigidly fixed to the body (1).  9. Platform according to claim 8, characterized in that the anchoring means (9) carry, surrounding the tube (13), a corolla (19) for limiting the radius of curvature in bending of the tube (13).  10. Platform according to one of claims 1 to 9, characterized in that the anchoring means (9) comprise a stake (14) sealed in a well (17) dug at the bottom of the water (11) .  11. Marine platform according to any one of claims 1 to 10, characterized in that it comprises a series of angles (9) arranged relative to each other according to a certain diagram, as many bodies (1) as anchors (9), a series of fasteners (34) each connecting an anchor (9) and a body (1), and a frame (37) resting by at least one foot (38) on each of the bodies ( 1).  12. Marine platform intended for oil or mining drilling, comprising installations (27) in particular for bringing down a drill rod (22) towards the bottom of the water (11), a body (1) of density less than the unit, carrying these installations (27) and dimensioned so as to have a positive buoyancy even with the maximum loading (1, 6, 7, 27) expected, this body (1) being connected to anchoring means (9) at the bottom of the water (11) by means of connection under tension (12) so that it is in service submerged substantially entirely, characterized in that the connection means (12) comprise a tube tight to the water (13), inside which is guided the drill pipe (22).  13. Platform according to claim 13, characterized in that the connecting tube (13) contains a guide tube (23) of the rod (22), this guide tube (23) also serving to evacuate upwards materials resulting from drilling.  14. Platform according to any one of claims 12 or 13, characterized in that the anchoring means (9) comprise a tubular pile (14) sealed substantially vertically, by its external surface, to the internal wall of a well (17) made at the bottom of the sea (11), this pile (14), to which the connecting tube (13) is rigidly fixed, also being waterproof and traversed by the drill rod ( 22).  15. Platform according to claim 14, characterized in that the tubular pile contains a safety shutter device (26).  16. Platform according to any one of claims 12 to 15, characterized in that at least part of the installations (27) is arranged in the body (1), the latter carrying, by means of a leg (7) crossing the surface of the water (8), a plate (6) allowing ships to dock and / or helicopters to land.  17. Platform according to claim 16, characterized in that the body (1) is substantially spherical.  18. Platform according to claim 16, characterized in that the body (1) is of oblong horizontal shape and can pivot around the axis of the tube (13) to orient itself in the direction of the current.  19. Method for setting up a platform according to any one of claims 1 to LI, characterized by the following steps:  - the body (1) is floated freely at the location where it is desired to install the platform;  - A fastener (12, 13) intended to form part of the connecting means (12) is unwound from the body (1), until the free end of this fastener reaches the bottom of the water (11 );  - The fastener (12, 13) is moored at the bottom of the water (11); and  - Pulling on the clip (12, 13) from the body (1) so as to push it into the water to the desired depth.  20. The method of claim 19, applied in the case where the fastener is a tube (13) characterized in that to unwind the tube (13), it is constructed for elements (28) inside the body (1 ), and it is brought out of the body (1) through an orifice (29) formed at the base of the body (1) and furnished with sealing means.  21. The method of claim 19, applied to a platform in which the connecting means comprise a cable (12) characterized in that quton arranges at the top of the body (1) a chimney (7) surmounted by a winch ( 10), this chimney (7) being long enough for at least its end to emerge even when the body (1) has reached its service depth, in that the cable (12) is unwound by this chimney (7) and by an orifice (15) formed at the base of the body (1) and furnished with sealing means around the cable, and in that to push the body (1) into the water, the cable (12) is pulled using the winch (10).