EP1838570B1 - Stabilized floating support - Google Patents

Description

The present invention relates to a floating support which comprises a working bridge supporting installations connected to the seabed, and flotation devices supporting the working bridge. Upe such platform may be, for example, an oil or gas exploitation platform.

A floating support at sea has a vertical movement under the effect of the swell. This vertical movement, commonly called heave movement, depends on the swell and is particularly important because it conditions the operation of facilities that are both supported by the floating support and connected to the seabed. These installations may be, for example, drill pipes or pipes for transporting oil or gas. When the floating support pounding, these installations have a relative vertical movement relative to the support, and therefore, it is necessary to equip these installations telescopic compensation systems allowing each moment to compensate for the heave of the floating support to allow interventions at the top of these facilities. These compensation systems are very expensive, especially since the motion compensation to be achieved is important, and, moreover, they have technological limits of compensation.

The heave movement is approximately proportional to the wave height and is conventionally characterized by the quotient of the heave by the wave height, this quotient being in first approximation an invariant as a function of the height of the swell. The heave movement also depends on the shape of the flotation organs, the action of the swell generating pressures on the walls of the latter whose cumulative effect on all the walls gives at each moment a vertical force excitatory movement. The heave movement also depends on the period of the swell since the distribution of pressures on a flotation device having a predetermined shape depends on the wave period and its wavelength (for this purpose, in depth). high water, the wavelength of the swell (in meters) corresponds approximately to the square of its period (in second) multiplied by 1,56). Finally, the heave also depends on the impact of the swell, ie the orientation of the floating support relative to the direction of propagation of the swell.

Therefore, conventionally, the hulling movement of the support at its geometric center (usually the point of connection with the installations connected to the sea bed) is characterized by a heave transfer function which is the representation of the evolution of the quotient heave / wave height depending on the period of the swell.

In order to minimize the heave movement, in the current semi-submersible platforms, each flotation member (typically formed by a submerged float, the immersed part of a column supported by the submerged float and supporting the working bridge, and half of each of the adjacent submerged connecting elements connecting the column-float assembly to the other column-float assemblies) is shaped so that the cumulative effects of the pressures generated by the swell it undergoes vanishes for a period of time. predetermined, conventionally called the balancing period. The transfer function of the heave of such a platform has a value close to 0 for small periods, regularly increases to reach a relative maximum which is approximately equal to 0.5, drops back to 0 for the balancing period, and rises rapidly and strongly then.

Thus, in the prior art, the limitation of the heave movement is carried out by correctly configuring each floating element of the floating support so that the balancing period associated therewith is greater than the periods of the swells usually encountered on the site. use of the platform. Therefore, for the usual swells on the site, the heave transfer function will be at most equal to 0.5.

However, this value of 0.5 is relatively large and involves the use of relatively large compensation systems. In addition, the heave transfer function is greater than 0.25 for a significant range of wave periods.

Moreover, we know, for example, according to US 3,490,406 , which is considered to be the closest state of the art, irrespective of the distance separating two vertical axes passing through the center of volume of the flotation devices, the heave is particularly reduced when the floating support is subjected to swell whose direction of propagation is that connecting the two vertical axes and whose period is equal to twice the distance separating these two axes.

The present invention aims at producing a floating support having a particularly low heave transfer function for the usual swells.

According to the invention, the spacing between the vertical axes passing through the volume center of the flotation devices is such that, for each direction of propagation of the swell, when the period of the swell is equal, within 20%, to the period of 100-year storm swell associated with the direction of propagation considered, the centennial storm swell being the swell whose annual probability of being encountered on the site where the support is intended to be installed is 1/100, the sum of the moments, taken with respect to the horizontal axis perpendicular to the direction of propagation considered and passing through the center of gravity of the support, vertical forces of excitation of the swell on the flotation devices situated on one side of the vertical plane passing through this horizontal axis is equal to the corresponding sum associated with the flotation devices located on the other side of this vertical plane.

Thus, according to the present invention, the floating support is shaped so that, for each direction of wave propagation, the sum of the moments taken with respect to the horizontal axis perpendicular to the direction of propagation considered and passing through the center of the gravity of the platform, vertical forces of excitation of the swell on the flotation devices located on one side of the vertical plane passing through this horizontal axis is equal to the corresponding sum associated with the flotation devices located on the other side of this vertical plane for a swell of predetermined period, hereinafter referred to as the extinction period. The transfer function of the heave of such a platform for the direction of propagation of the swell considered has a value close to 0 for the extinction period. The cancellation of the heave motion at the center of gravity for the extinction period following the direction of propagation of the swell is due to the fact that, with a predetermined spacing between the various vertical axes passing through the volume center of the flotation devices, the sum of the moments, taken with respect to the horizontal axis perpendicular to the direction of propagation considered and passing through the center of gravity of the support, vertical forces of excitation of the swell on the flotation devices situated on one side of the plane vertical passing through this horizontal axis is equal to the corresponding sum associated with the flotation devices located on the other side of this vertical plane, although each of the efforts on each flotation device taken separately is not zero.

The phenomenon can be easily understood by imagining a floating support comprising a working bridge and two flotation devices. When the swell has for half-wavelength the distance separating the two vertical axes passing through the center of volume of the buoyancy members and has for direction of propagation the direction of alignment of the two axes, these two buoyancy devices are subject to to vertical forces in phase opposition due to the excitation of the swell (one being at the right of one ridge when the other is at the right of a hollow, for example) and, consequently, the moment, taken with respect to the horizontal axis perpendicular to the direction of propagation considered and passing through the central point of the bridge (located halfway between the two flotation organs), vertical forces of excitation of the swell on one both flotation devices are equal to the corresponding moment associated with the other flotation device. The period of the wave corresponding to this half-wavelength is the extinction period of the support. In addition, when the direction of propagation of the swell is perpendicular to the alignment direction of the two vertical axes, there is no extinction period for this direction of propagation.

According to a particularly advantageous embodiment, each flotation member is dimensioned (in the usual way) so that the sum of the vertical excitation forces it supports is zero for a swell whose period is equal to 1.5 times. the period of the centennial storm swell. Thus, according to this embodiment, the balancing period is equal to 1.5 times the extinction period.

The transfer function of the heave at the center of gravity of such a platform is then particularly remarkable: it has a value close to 0 for small periods, regularly increases to reach a first relative maximum which is less than 0.1 ( approximately equal to 0.075), drops back to 0 for the extinction period, increases again regularly to reach a second relative maximum which is less than 0.15 (approximately equal to 0.125), drops back to 0 for the equilibration period, and goes up quickly and strongly thereafter. With such a platform, the compensation systems used may have a reduced compensation amplitude, the heave transfer function being at most equal to 0.15 for all the swells encountered on the site.

• La figure 1 est un schéma en coupe illustrant le principe de la présente invention pour une structure flottante ayant quatre organes de flottaison, la coupe étant faite selon un plan vertical passant par le centre du pont,
• La figure 2 est un schéma illustrant la valeur de la fonction de transfert du pilonnement au centre de gravité pour une plate-forme conçue conformément à la présente invention, et celle pour une plate-forme semi-submersible classique, et
• La figure 3 représente la partie immergée d'une plate-forme comportant trois organes de flottaison.

Other particularities will appear in the description of the present invention in connection with the drawings given by way of non-limiting examples.

• The figure 1 is a sectional diagram illustrating the principle of the present invention for a floating structure having four flotation members, the section being made in a vertical plane passing through the center of the bridge,
• The figure 2 is a diagram illustrating the value of the transfer function of heave at the center of gravity for a platform designed in accordance with this invention, and that for a conventional semi-submersible platform, and
• The figure 3 represents the submerged part of a platform with three flotation devices.

The floating support 1 (in this case the semi-submersible platform 1) illustrated in FIG. figure 1 comprises a working bridge 2 and four buoyancy members 3 supporting the bridge 2. Installations 4 (in this case pipes 4) which are connected to the seabed are supported and connected to the bridge 2 at its geometric center 5. Each flotation member 3 is formed of a submerged float 6, the immersed part of a column 7 which is supported by the submerged float 6 and which supports the working bridge 2, and half of each submerged connecting element 11 connecting this float-column assembly to other float-column assemblies.

In the representation illustrated in figure 1 , the four flotation members 3 are arranged so that the vertical axes Z passing through the center of their respective volume form a square and the distance L separating the two vertical axes Z delimiting the same side of the square is equal to the half-length of wave H of a swell whose direction of movement corresponds to the alignment direction D of these two vertical axes Z. Because of this and because of the swell, the four flotation members 3 are, two by two, subjected to vertical forces of excitation in opposition of phase, and consequently, the sum of the moments, taken with respect to the horizontal axis perpendicular to the direction of propagation considered and passing through the center of gravity of the support (generally close of the geometric center 5 of the bridge 2), vertical forces of excitation of the swell on the floating members 3 situated on one side of the plane vertical P passing through this horizontal axis is equal to the corresponding sum associated with the flotation members 3 located on the other side of this vertical plane P. The period of the swell corresponding to this half-wavelength is the period of extinction of the support 1, when the swell has for direction of propagation the alignment direction D of the two axes Z.

The sizing of the platforms 1 according to the present invention is carried out as follows:

As a first step, it is necessary to identify, for the exploitation site where the platform 1 is intended, for each direction of wave propagation, the period of the hundred-year storm swell which is the swell whose annual probability to be encountered on the site is 1/100, the period of this swell will be within 20% of the extinction period chosen for the platform 1 in the direction of propagation considered.

In a second step, for a floating support 1 having a particular geometry (for example having three flotation members 3 arranged so that their vertical axes Z passing through their respective centers of volume form an equilateral triangle as illustrated in FIG. figure 3 or having four flotation members 3 arranged so that their vertical axes Z passing through their respective centers of volume form a square such as that shown in FIG. figure 1 ), the theoretical spacing between the vertical axes Z is determined so that the sum of the moments, taken with respect to the horizontal axis perpendicular to the direction of propagation of the swell and passing through the geometric center 5 of the bridge 2 (in general near the center of gravity of the platform 1), vertical forces of excitation of the swell on the flotation devices 3 situated on one side of the vertical plane P passing through this horizontal axis is equal to the corresponding sum associated with the buoyancy members 3 situated on the other side of this vertical plane P, for the swell whose period corresponds to the period of extinction.

The determination of this theoretical spacing is performed for a range of wave propagation directions. Given the possible symmetries, for a platform 1 having three flotation devices 3 arranged in equilateral triangle, the direction of propagation of the swell can vary by 60 °, and for a platform 1 having four floating members 3 arranged in square, it can vary from 45 °. This determination for different propagation directions makes it possible to choose an optimum spacing with respect to the hulling behavior of platform 1 for centennial storm swells, which defines the shutdown period of platform 1 for the propagation direction concerned. A tolerance of 20% over the extinction period makes it possible to adapt the geometry of the platform without damaging its heaving behavior.

In the case of a platform 1 having four flotation members 3 arranged so that the vertical axes Z passing through their respective volume centers form a square, the extinction period, for the direction of propagation of the parallel swell to one side of the square, is obtained when the length of one side of the square corresponds to the half-wavelength of the centennial storm swell. In the case of a platform 1 having three flotation members 3 arranged so that the vertical axes Z passing through their centers of respective volumes form an equilateral triangle, the extinction period is obtained when the height of the triangle corresponds to the half-length wave of the centennial storm swell. Thus, for an extinction period of 12 s, the wavelength of the corresponding swell is 224 m, the height of the equilateral triangle formed by the three vertical axes Z is 112 m, and the spacing between each axis vertical Z is 130 m.

According to a particular embodiment, each flotation member 3 is dimensioned (in the usual way) so that the sum of the vertical excitation forces to which it is subjected is canceled for a swell whose period is greater than the period of time. extinction, that is to say each flotation member 3 is dimensioned so that the balancing period associated with it is greater than the extinction period.

It is particularly advantageous that the period of each flotation device 3 is equal to about 1.5 times the extinction period. Thus, for a platform 1 having an extinction period of 12 s, it is particularly advantageous that each flotation device 3 is sized to have a balancing period of 18 s.

The figure 3 represents a platform with three flotation members 3 arranged so that the vertical axes Z passing through their centers of respective volumes form an equilateral triangle, and having an extinction period of 12 s (the distance between the vertical axes Z is therefore 130 m). Each flotation member 3 is configured to have a balancing period of 18 seconds, the submerged float 6 having the shape of a cylinder 30 meters in diameter, and the column 7 having the shape of a cylinder of 18 meters in diameter, the draft on site is 44 meters. The mass of the platform 1, including the oil processing facilities it supports, is 65 000 tonnes.

The figure 2 is the representation of the transfer function of the heave of two platforms having both the same balancing period of 18 s and having three flotation members 3 arranged so that the vertical axes Z passing through their centers of respective volumes form an equilateral triangle.

The first curve FT1 corresponds to a conventional platform classically sized and adapted to severe waves of 12 s, the vertical axes Z being spaced from each other by about 70 meters: the transfer function of the heave has a value close to 0 for short periods (less than 6 s), increases steadily to reach a relative maximum of about 0.5 (for a period of about 13 s), decreases to 0 for the balancing period (18 s ), and goes up quickly and strongly thereafter.

The second curve FT2 corresponds to a platform sized according to the present invention, the spacing between the vertical axes Z being 130 m so as to have an extinction period of 12 s: the transfer function of the heave has a value close to 0 for small periods (less than 6 s), regularly increases to reach a first relative maximum which is approximately equal to 0.075 (for a period of about 10 s), decreases to 0 for the extinction period (12 s), increases again regularly to reach a second relative maximum which is approximately equal to 0.125 (for a period of about 15 s), drops back to 0 for the balancing period (18 s), and rises rapidly and strongly thereafter .

The offshore behavior of a platform 1 according to the present invention is particularly improved.

The ranges of variation on the value of the extinction and balancing periods make it possible to obtain this good behavior, while allowing a flexibility for the realization of the platform vis-à-vis other dimensioning parameters.

Given the dimensions of such platforms 1, as can be seen in figure 1 it is advantageous for the bridge 2 and the buoyancy members 3 to be rigidly connected to one another by means of additional structures 8. In addition, it is preferable that only the installations connected to the seabed (the pipes 4 or the drill pipes as well as structures that allow their guidance in the vicinity of the sea level) are located at the geometric center 5 of the bridge 2, the associated facilities 9 can be collected above the columns 7 to limit the efforts in the structures of the working bridge 2 .

Furthermore, the working bridge 2 may comprise volumes that can be rendered watertight, in order to ensure the safety of the floating support 1 in the event of damage to a flotation member 3 causing its invasion by the water sea.

In relation with the present invention which makes it possible to limit the vertical movement of the working bridge 2 at its connection with the installations 4 connected to the seabed, it is also possible to limit the mechanical stresses experienced by these installations at this connection. due to pitching, rolling, yawing and overturning movements of the platform 1.

For this purpose, the platform 1 is associated with a guide structure which is adapted to be supported by the platform and to guide, in the vicinity of the sea level, the installations 4 (for example the pipes 4) connected to the seabed. The guiding structure comprises a cage which extends in a longitudinal direction (which corresponds substantially to the vertical when the structure is connected to the platform) and a connecting member which is adapted to cooperate with a complementary linkage member. by the platform so as to form a ball joint between it and the cage. In this way, when the platform is subjected to the action of the swell, the ball joint makes the guiding structure less sensitive to the overall movement of the platform, which greatly reduces the contact forces between the pipes and the guide structure. Advantageously, the guide structure can support vertical tensioning systems of the pipes, well heads, a derrick ... The connecting member may be arranged longitudinally at one end of the cage and, transversely, either in the center of the cage (the organ is then a spherical pivot, or at the periphery of the cage (the organ is then a spherical crown).

According to a particular embodiment, the guide structure also comprises a ballast element which is disposed at a portion of the cage longitudinally remote from the connecting member (the ballast element is fixed to the longitudinal end of the cage opposite to the one where the linkage is arranged). While the marine currents tend to deflect the cage and pipes from the vertical due to the ball joint between the cage and the floating support, the ballast element tends to reduce this deflection and thus protects the pipes from mechanical stresses. consecutive to this deviation. Such ballast element has an immersed mass-to-volume ratio at least equal to twice (or even triple) that of the cage.

According to another particular embodiment, so as to reduce the vertical forces between the cage and the platform at the level of the ball joint, floats are connected to the upper part of the cage, and more specifically, at the level of the cage that is adapted to be close to the surface of the sea.

Claims (10)

1. Floating support (1) designed to support structures (4) which are designed to be connected to the sea floor of a given site, the floating support (1) including a working deck (2) and floatation members (3) which support the working deck (2), characterised in that the spacing between the vertical axes (Z) passing through the centre of buoyancy of the floatation members (3) is such that, for each swell propagation direction, when the swell period is equal, within 20%, to the 100-year storm surge period associated with the propagation direction under consideration, the 100-year storm surge being the swell of which the yearly probability of being encountered at the site where the support is intended to be installed is 1/100, the sum of the moments, taken in relation to the horizontal axis perpendicular to the propagation direction under consideration and passing through the centre of gravity of the support (1), of the vertical excitation forces of the swell on the floatation members (3) situated on one side of the vertical plane (P) passing through this horizontal axis, is equal to the corresponding sum associated with the floatation members (3) situated on the other side of this vertical plane, this period being referred to as the extinction period according to the swell propagation direction.
2. Floating support (1) of claim 1, characterised in that it includes three floatation members (3) arranged in relation to one another so that the vertical axes (Z) passing through their respective centres of buoyancy form an equilateral triangle the height of which corresponds, within 20%, to the half wavelength of the 100-year storm surge.
3. Floating support (1) of claim 1, characterised in that it includes four floatation members (3) arranged in relation to one another so that the vertical axes (Z) passing through their respective centres of buoyancy form a square of which the length of the sides corresponds, within 20%, to the half wavelength of the 100-year storm surge.
4. Floating support (1) as claimed in one of claims 1 to 3, characterised in that each floatation member (3) is dimensioned so that the sum of the vertical excitation forces that it undergoes is cancelled out for a swell the period of which is greater than the extinction period, this period being referred to as the balancing period.
5. Floating support (1) of claim 4, characterised in that the balancing period is equal to 1.5 times the extinction period.
6. Floating support (1) as claimed in one of claims 1 to 5, characterised in that the connecting point (5) of the structures (4) intended to be connected to the sea floor is the centre of gravity of the support (1).
7. Floating support (1) of claim 6, characterised in that the structures (4) intended to be connected to the sea floor are associated with attached structures (9) arranged above the floatation members (3).
8. Floating support (1) as claimed in one of claims 1 to 7, characterised in that each floating member (3) consists of a submerged float (6), the submerged portion of a column (7) supported by the submerged float (6) and supporting the working deck (2), and half of each of the adjacent submerged connection elements connecting this submerged column-float assembly to the other column-float assemblies.
9. Method for dimensioning a floating support (1) which is suitable for supporting structures (4) designed to be connected to the sea floor of a given site, and which includes a working deck (2) and floatation members (3) supporting the working deck (2), characterised in that it includes a step during which identification is made, for the site for which the support (1) is intended, for each swell propagation direction, of the 100-year storm surge period, which is the swell of which the yearly probability of being encountered on the site is 1/100, a step during which determination is made, for an entire range of swell propagation directions, of the theoretical spacing between the vertical axes (Z) passing through the centre of buoyancy of the floatation members (3), so that the sum of the moments, taken in relation to the horizontal axis perpendicular to the swell propagation direction and passing through the geometric centre (5) of the deck (2), of the vertical excitation forces of the swell on the floatation members (3) situated on one side of the vertical plane (P) passing through this horizontal axis, is equal to the corresponding sum associated with the floatation members (3) situated on the other side of this vertical plane (P), a step during which determination is made of an optimum spacing with regard to the heave behaviour of the support (1) for 100-year storm surges, which defines the extinction period of the support (1) for the propagation direction under consideration, and a possible step during which the geometry of the support (1) is adapted within the limits of a tolerance of 20% with respect to the extinction period.
10. Method for dimensioning a floating support (1) of claim 1, characterised in that each floating member (3) is dimensioned so that the sum of the vertical excitation forces that it undergoes is cancelled out for a swell of which the period is equal to 1.5 times the extinction period.

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