The present invention relates to a device for the off-shore exploitation of hydrocarbons which is suited to arctic conditions, and to an associated method for exploiting hydrocarbons. The conditions under which hydrocarbons are exploited off-shore in an arctic environment are extreme and subject the rigs to unusual stress loadings. The rigs, notably for almost six months of the year, are subjected to extreme weather conditions with the presence of ice, for example sheets of ice which may be up to approximately 2 m thick and travel at a speed that may be as high as several knots, waves which may reach heights from 1 to 3 m or even of around 20 m during storms, violent gusts of wind in the autumn and in the winter, intense snowfall, poor visibility and average temperatures that fluctuate between 18°C and -23 °C.

This means that most currently known rigs are used only for part of the year so as to prevent them from becoming damaged during the winter.

The present invention proposes to overcome the resultant technical problem and targets a device for the off-shore exploitation of hydrocarbons that is suited to arctic conditions and can operate in these extreme positions throughout the year.

In order to improve known devices for the off-shore exploitation of hydrocarbons which are suited to arctic conditions, the invention targets a device for the off-shore exploitation of hydrocarbons, which is suited to arctic conditions and comprises:

• a floating off-shore hydrocarbons exploitation platform comprising a substantially horizontal deck to receive one or more hydrocarbons drilling and/or extraction device(s) and a hull comprising an empty volume to give the floating platform buoyancy, and an external lateral wall connected around an upper contour to the deck of the floating platform and connected around a lower contour to a bottom, in which part of the external lateral wall has a shape the cross section of which narrows between the top of this part and the bottom of this part so as to make it able to withstand the forces likely to be applied by ice and in which this part the cross section of which narrows is partially submerged;

· a fully submerged gravity base comprising a lower surface intended to rest on a floor and an upper surface arranged under the bottom of the hull of the floating hydrocarbons exploitation platform;

• a connecting device connecting the floating hydrocarbons exploitation platform and the gravity base so as to transmit forces resulting from the action of the ice on the floating hydrocarbons exploitation platform to the gravity base.

In the context of the present invention, a "hydrocarbon" means any organic compound made up exclusively of carbon (C) atoms and hydrogen (H) atoms, particularly crude oil and natural gas.

According to the here above wording, "a floating off-shore hydrocarbons exploitation platform" is not only an off-shore hydrocarbons exploitation platform that may float, but is unambiguously an off-shore hydrocarbons exploitation platform which is floating when being part of the device for the off-shore exploitation of hydrocarbons.

Thus a floating off-shore hydrocarbons exploitation platform, according to the present invention, comprises a substantially horizontal deck to receive one or more hydrocarbons drilling and/or extraction device(s) and a hull comprising an empty volume to give the floating platform buoyancy, and an external lateral wall connected around an upper contour to the deck of the floating platform and connected around a lower contour to a bottom, in which part of the external lateral wall has a shape the cross section of which narrows between the top of this part and the bottom of this part so as to make it able to withstand the forces likely to be applied by ice and in which this part the cross section of which narrows is partially submerged and the floating off-shore hydrocarbons exploitation platform is floating when being part of the device for the off-shore exploitation of hydrocarbons. The off-shore hydrocarbons exploitation platform is thus not only floating when the platform is moved from a place to another place as for usual platforms, but it is floating when connected to the fully submerged gravity base, namely when the off-shore exploitation of hydrocarbons is working.

Thus and according to the present invention, a floating off-shore hydrocarbons exploitation platform is movable when connected to the fully submerged gravity base; floating of the floating off-shore hydrocarbons exploitation platform allows slightly drifting when the ice applies strains on the platform; this feature is highly advantageous because it allows adapting the device for the off-shore exploitation to ice movements.

Accordingly, the connecting device connecting the floating hydrocarbons exploitation platform and the gravity base cannot be a gravity based connecting device, where the floating hydrocarbons exploitation platform weight would be transferred to the gravity base, because the platform would not be able to float nor to move when connected to the gravity base.

Thus, the connecting device connects the floating hydrocarbons exploitation platform and the gravity base so as to transmit forces resulting from the action of the ice on the floating hydrocarbons exploitation platform to the gravity base so that the floating hydrocarbons exploitation platform is floating and is movable when connected to the fully submerged gravity base.

According to an embodiment, said connecting device is a contact free connecting device, where no contact directly occurs between the floating hydrocarbons exploitation platform and the gravity base. The present invention also targets a device for the off-shore exploitation of hydrocarbons that is suited to arctic conditions and further comprises the features listed in the following embodiments, which may be combined with one another:

- part of the external lateral wall of which the cross section narrows has a shape chosen from the list consisting of any cone frustum, a frustum of a cone of revolution, a frustum of a pyramid cone, a cone frustum superposed on a cylinder; the gravity base is of a shape chosen from the list consisting of a cylinder, a cone frustum, a parallelepiped, a superposition of the aforementioned shapes;

the gravity base rests on a floor chosen from the list consisting of an ocean floor, an artificial floor placed on the ocean floor;

the distance d between the bottom of the hull of the floating hydrocarbons exploitation platform and the top surface of the gravity base is comprised between 1 m and 5 m;

the connecting device providing the connection between the floating hydrocarbons exploitation platform and the gravity base comprises a plurality of lines attached at one end to the bottom of the hull of the floating hydrocarbons exploitation platform and/or to the part of the external lateral wall the cross section of which narrows and attached at the other end to the upper surface of the gravity base and/or near the top surface of the gravity base;

the connecting device providing the connection between the floating hydrocarbons exploitation platform and the gravity base comprises a plurality of articulated arms attached at one end to the bottom of the hull of the floating hydrocarbons exploitation platform and/or to the part of the external lateral wall the cross section of which narrows and attached at the other end to the top surface of the gravity base and/or near the upper surface of the gravity base;

the gravity base comprises in its upper part a cavity able partially to receive the part of the external lateral wall the cross section of which narrows.

The present invention also targets a method for exploiting hydrocarbons comprising the following successive steps:

· bringing a gravity base onto site and submerging it completely so that a lower surface of the gravity base rests on a floor;

• bringing onto site a floating off-shore hydrocarbons exploitation platform comprising a substantially horizontal deck to receive one or more hydrocarbons drilling and/or extraction device(s) and a hull comprising an empty volume to give the floating platform buoyancy, and an external lateral wall connected around an upper contour to the deck of the floating platform and connected around a lower contour to a bottom, in which part of the external lateral wall has a shape the cross section of which narrows between the top of this part and the bottom of this part so as to make it able to withstand the forces likely to be applied by ice;

• arranging the floating hydrocarbons exploitation platform in such a way that an upper surface of the gravity base is arranged under the bottom of the hull of the floating hydrocarbons exploitation platform and so that the part of the floating exploitation platform the cross section of which narrows is partially submerged;

• assembling the floating hydrocarbons exploitation platform and the gravity base using a connecting device so as to transmit forces resulting from the action of the ice on the floating hydrocarbons exploitation platform to the gravity base.

The present invention also targets a method for exploiting hydrocarbons, further comprising the following steps:

· bringing onto and arranging on site a floating off-shore hydrocarbons exploitation platform according to the above method, in which the floating hydrocarbons exploitation platform comprises a deck equipped with a drilling device;

• performing a drilling;

· disconnecting the floating hydrocarbons exploitation platform from the gravity base;

• moving the floating hydrocarbons exploitation platform away from the gravity base. The present invention also targets a method for exploiting hydrocarbons, further comprising the following steps:

• bringing onto site a floating off-shore hydrocarbons exploitation platform comprising a substantially horizontal deck equipped with a hydrocarbons extraction device and a hull comprising an empty volume to give the floating platform buoyancy, and an external lateral wall connected around an upper contour to the deck of the floating platform and connected around a lower contour to a bottom, in which part of the external lateral wall has a shape the cross section of which narrows between the top of this part and the bottom of this part so as to make it able to withstand the forces likely to be applied by ice;

• arranging the floating hydrocarbons exploitation platform in such a way that an upper surface of the gravity base is arranged under the bottom of the hull of the floating hydrocarbons exploitation platform and so that the part of the floating exploitation platform the cross section of which narrows is partially submerged;

• assembling the floating hydrocarbons exploitation platform and the gravity base using a connecting device so as to transmit forces resulting from the action of the ice on the floating hydrocarbons exploitation platform to the gravity base.

The present invention also targets a method for exploiting hydrocarbons further comprising the following step:

· repeating the successive steps of one of the above-mentioned methods for exploiting hydrocarbons with the floating hydrocarbons exploitation platform comprising a deck equipped with a drilling device so as to arrange said floating hydrocarbons exploitation platform on a gravity base submerged on another site.

The present invention also targets a method for exploiting hydrocarbons comprising the following steps:

• bringing onto and arranging on site a floating off-shore hydrocarbons exploitation platform according to the method of one of the above- mentioned methods for exploiting hydrocarbons, in which the floating hydrocarbons exploitation platform comprises a deck equipped with a drilling device;

• performing a drilling; • moving the drilling device on the deck of the floating hydrocarbons exploitation platform and performing another drilling.

The present invention also targets a method for exploiting hydrocarbons further comprising the following steps:

• bringing onto and arranging on a site a floating off-shore hydrocarbons exploitation platform according to the method of one of the above-mentioned methods for exploiting hydrocarbons;

• bringing a gravity base onto a site nearby to the previous site and submerging it completely so that a lower surface of said gravity base rests on a floor so that the gravity base of said nearby site can serve as a fall-back position to receive another floating hydrocarbons exploitation platform in the event of an incident. The invention will be more clearly understood if reference is made to the attached drawing in which figure 1 depicts one embodiment of a device for the off-shore exploitation of hydrocarbons that is suited to arctic conditions according to the invention.

It should be noted that the components of the device have not necessarily been drawn to scale and that the sole purpose of the figure is to make the present invention easier to understand.

Figure 1 refers to the surface of the sea under calm conditions in order to define a horizontal line.

A downward direction is defined along a line perpendicular to said horizontal where the top is situated above the floating hydrocarbons exploitation platform and the bottom is situated at the bottom of the sea.

A first component is considered to be situated above or to be higher than a second component if the first component is arranged nearer the top than the second component; likewise, a first component is situated underneath or lower down than a second component if the first component is arranged nearer the bottom than the second component.

The device 1 for off-shore exploitation of hydrocarbons is depicted in schematic section in an arctic environment in which a layer of ice 400 is situated at the surface of an expanse of water 500, notably an ocean. The thickness of ice is typically of the order of 1 m but may reach markedly higher values.

In general, the waters in arctic regions are not very deep and the depth of water H is, for example, between 50 and 150 meters.

The sea beds 600 generally consist of sedimentary rock, at least in part.

The device 1 for the off-shore exploitation of hydrocarbons that is suited to arctic conditions according to the invention comprises three main components:

· a floating off-shore hydrocarbons exploitation platform 100;

• a fully submerged gravity base 200;

• a connecting device 300 connecting the floating hydrocarbons exploitation platform and the gravity base. The floating off-shore hydrocarbons exploitation platform 100 comprises a substantially horizontal deck 1 10. This deck is designed to receive one or more hydrocarbons extraction and/or drilling device(s). In the case depicted, the deck is equipped with a drilling device 150. The floating off-shore hydrocarbons exploitation platform 100 also comprises a hull 120; this hull 120 comprises an empty volume 130 (in the form of one or more caisson(s)) to provide the floating platform with buoyancy; the hull 120 comprises an external lateral wall 121 , 122 connected along an upper contour 1 15 to the deck 1 10 of the floating platform and connected along a lower contour 125 to a bottom 123. Part 122 of the external lateral wall has a shape the cross section of which narrows between the top of this part and the bottom of this part so as to be able to withstand the forces likely to be applied by ice 400 and in which this part the cross section of which narrows is partially submerged; the floating hydrocarbons exploitation platform 100 comprises at least one cavity 140 passing through it from top to bottom for the passage of the drilling or extraction means from the deck 1 10 toward the sea 500. Such a cavity is generally cylindrical. In the example depicted, a drilling tool 160 has been depicted with one end connected to the deck 100 and the other end embedded in the sea bed 600. Instead of, or in addition to, the drilling tool, the floating hydrocarbons exploitation platform 100 may be equipped with hydrocarbons extraction means notably comprising extraction pipes arranged in the cavity 140. The floating hydrocarbons exploitation platform 100 may also comprise tanks for storing hydrocarbons.

Such a floating off-shore hydrocarbons exploitation platform is known per se and is generally used in arctic conditions only during periods in which the thickness of ice is small. It is not suited to use on its own over the winter.

The gravity base 200 is fully submerged; it comprises a lower surface 225 intended to rest on a floor 650 and an upper surface 215 arranged under the bottom of the hull of the floating hydrocarbons exploitation platform; the floor 650 may be a sea bed on which the gravity base 200 rests directly or an artificial floor constructed on the sea bed; such an artificial floor may be a precast concrete base. The floor 650 is generally substantially horizontal. The floor 650 needs to be strong enough to support the gravity base 200. Sea beds such as stiff clays or dense-to-very-dense sand and shingle or surface rock may be suitable for direct contact with the gravity base. Often the sea bed is prepared in the first instance. This may notably involve excavating loose ground from the surface in order to reach a load-bearing layer.

A gravity base comprises a high-inertia raft, generally made of reinforced concrete (or as an exception, made of steel). A gravity base is able to counter- balance lateral forces using its own weight. A gravity base is often of essentially circular cross section; it may be fitted with spades that penetrate the foundation floor.

The gravity base 200 of the example depicted is of a shape that corresponds to the superposition of a cone frustum 220 and of a cylinder 210. The lower surface 225 of the gravity base is formed by the base of the cone frustum 220 and rests on the floor 650. The outer wall 222 of the cone frustum 220 widens from the top downward. The lower face 21 3 of the cylinder 210 rests on the upper face 221 of the cone frustum 220. The cross section of the outer wall 21 1 of the cylinder 210 is circular. The upper surface 215 of the gravity base is formed by the upper surface of the cylinder 210. This upper surface is essentially horizontal and comprises a lip 214.

According to another embodiment, the upper surface 215 of the gravity base is horizontal. According to another embodiment, the upper surface 215 of the gravity base is horizontal in the central region and designed in a peripheral region in such a way that the gravity base in its upper part comprises a cavity designed partially to accept the part 122 of the external lateral wall the cross section of which narrows.

The gravity base 200 comprises a central hole 240, 223 passing from the top downward respectively through the cylinder 210 and the cone frustum 220; this central hole is for the passage of the drilling or extraction means from the floating off-shore hydrocarbons exploitation platform 100 to the sea bed 600.

The connecting device 300 connects the floating hydrocarbons exploitation platform and the gravity base in such a way as to transmit forces resulting from the action of ice on the floating hydrocarbons exploitation platform to the gravity base. It may comprise a plurality of lines attached at one end to the bottom 123 of the hull of the floating hydrocarbons exploitation platform and/or to the part 122 of the external lateral wall the cross section of which narrows and attached at the other end to the upper surface of the gravity base and/or near the top surface of the gravity base. It may comprise a plurality of articulated arms attached at one end to the bottom 123 of the hull of the floating hydrocarbons exploitation platform and/or to the part 122 of the external lateral wall the section of which narrows and attached at the other end to the top surface of the gravity base and/or near the upper surface of the gravity base. Such arms may be actuated by electromechanical motors or by hydraulic devices.

According to one example, the distance d between the bottom of the hull

123 of the floating hydrocarbons exploitation platform and the top surface 215 of the gravity base is 1 m.

A device 1 for the off-shore exploitation of hydrocarbons which is suited to arctic conditions as depicted advantageously allows operation under extreme arctic conditions throughout the year.

Moreover, such a device can easily be dismantled and the floating offshore hydrocarbons exploitation platform 100 detached from the gravity base 200. It is then possible for the floating platform 100 to be replaced by another floating platform of the same type, for example to substitute an exploitation platform for a drilling platform. It is also possible to move the floating platform 100 and install it on another gravity base installed previously.

This ease with which the detached floating off-shore hydrocarbons exploitation platform 100 and gravity base 200 can be assembled and disconnected notably allows the above-mentioned methods for the exploitation of hydrocarbons to be implemented.