METHOD FOR LOWERING A SUBSEA STRUCTURE HAVING A SUBSTANTIALLY FLAT SUPPORT BASE INTO THE WATER THROUGH THE SPLASH ZONE

Abstract

The present invention relates to a method for lowering a subsea structure to be installed on the seabed into the water through the splash zone, said subsea structure comprising subsea equipment arranged on a substantially flat support base for preventing the subsea equipment to sink into the seabed. The method according to the invention comprises: lifting the subsea structure into the air in a horizontal position in which the flat support base extends substantially parallel to the horizontal plane; tilting the subsea structure while suspended in the air from the horizontal position into a tilted position in which the flat support base is angled with respect to the horizontal plane; lowering the subsea structure into the water through the splash zone in the tilted position; and tilting the subsea structure while suspended in the water below the splash zone back into the horizontal position.

Claims

1. A method for lowering a subsea structure to be installed on the seabed into the water through the splash zone, said subsea structure comprising subsea equipment arranged on a substantially flat support base for preventing the subsea equipment to sink into the seabed, the method comprising: lifting the subsea structure into the air in a horizontal position in which the flat support base extends substantially parallel to the horizontal plane; reducing the area of the flat support base of the subsea structure that comes into contact with the water at the moment the support base comes into contact with the water by tilting the subsea structure while suspended in the air from the horizontal position into a tilted position in which the flat support base is angled with respect to the horizontal plane; lowering the subsea structure into the water through the splash zone in the tilted position; and tilting the subsea structure while suspended in the water below the splash zone back into the horizontal position.

2. The method according to claim 1, wherein the tilting of the subsea structure in the air is performed above the splash zone.

3. The method according to claim 1, wherein for lifting the subsea structure, the subsea structure is suspended from a substantially flat lifting frame extending substantially parallel to the flat support base of the subsea structure, wherein the lifting frame is suspended from a first hoisting cable and a second hoisting cable, each connected to the lifting frame such that with the subsea structure in the horizontal position, the vertical component of the respective lifting forces exerted by the first hoisting cable and the second hoisting cable on the lifting frame are offset from the combined center of mass of the lifting frame and the subsea structure on opposite sides of the said combined center of mass; and for tilting the subsea structure one of the first hoisting cable and second hoisting cable is drawn in or payed out.

4. The method according to claim 3, wherein in the tilted position, the lifting frame is suspended from the first hoisting cable and the second hoisting cable, wherein the vertical component of the respective lifting forces exerted by the first hoisting cable and the second hoisting cable on the lifting frame are offset from the combined center of mass of the lifting frame and the subsea structure on opposite sides of the said combined center of mass.

5. The method according to claim 1, wherein for lifting the subsea structure, the subsea structure is suspended from a first hoisting cable and a second hoisting cable, each connected to the subsea structure such that with the subsea structure in the horizontal position, the vertical component of the respective lifting forces exerted by the first hoisting cable and the second hoisting cable on the lifting frame are offset from the center of mass of the subsea structure on opposite sides of the said center of mass; and for tilting the subsea structure one of the first hoisting cable and second hoisting cable is drawn in or payed out.

6. The method according to claim 5, wherein in the tilted position, the lifting frame is suspended from the first hoisting cable and the second hoisting cable, wherein the vertical component of the respective lifting forces exerted by the first hoisting cable and the second hoisting cable on the lifting frame are offset from the combined center of mass of the lifting frame and the subsea structure on opposite sides of the said combined center of mass.

7. The method according to claim 3, wherein, with the subsea structure in the horizontal position, the vertical component of the lifting force exerted by the first hoisting cable is offset from said center of mass by a first distance; the vertical component of the lifting force exerted by the second hoisting cable offset from said center of mass by a second distance, wherein the first distance is smaller than the second distance.

8. The method according to claim 3, wherein the second hoisting cable is drawn in for tilting the subsea structure from its horizontal position into its tilted position and is payed out for tilting the subsea structure from its tilted position back into its horizontal position.

9. The method according to claim 8, wherein the first hoisting cable is suspended from a hoisting block; the second hoisting cable is led through the hoisting block; and a stop is arranged on the second hoisting cable on the side of the hoisting block away from the lifting frame; wherein in the horizontal position of the subsea structure, the stop is in contact with the hoisting block.

10. A system for lowering a subsea structure to be installed on the seabed into the water through the splash zone, said subsea structure comprising subsea equipment arranged on a substantially flat support base for preventing the subsea equipment to sink into the seabed, comprising: lifting means for lifting the subsea structure into the air and for lowering the subsea structure into the water through the splash zone; tilting means for tilting the subsea structure from a horizontal position in which the flat support base extends substantially parallel to the horizontal plane into a tilted position in which the flat support base is angled with respect to the horizontal plane; wherein the lifting means and the tilting means are configured for: lifting the subsea structure into the air in the horizontal position; reducing the area of the flat support base of the subsea structure that comes into contact with the water at the moment the support base comes into contact with the water by tilting the subsea structure while suspended in the air from the horizontal position into the tilted position; lowering the subsea structure into the water through the splash zone in the tilted position; and tilting the subsea structure while suspended in the water below the splash zone back into the horizontal position.

11. The system according to claim 10, comprising: a hoisting installation; a substantially flat lifting frame suspended from the hoisting installation and configured for suspending therefrom the subsea structure; wherein the lifting frame is suspended from the hoisting installation via a first hoisting cable and a second hoisting cable, each connected to the lifting frame such that with the lifting frame in a horizontal position in which the lifting frame extends substantially parallel to the horizontal plane, the vertical component of the respective lifting forces exerted by the first hoisting cable and the second hoisting cable on the lifting frame are offset from the center of mass of the lifting frame on opposite sides of said center of mass; the hoisting installation comprises a first winch for operating the first hoisting cable and a second winch for operating the second hoisting cable independently from the first hoisting cable.

12. The system according to claim 11, wherein the first hoisting cable is offset from the center of mass of the lifting frame by a first distance; the second hoisting cable is offset from the center of mass of the lifting frame by a second distance, wherein the first distance is smaller than the second distance.

13. The system according to claim 11 or 12, wherein the first hoisting cable is suspended from a hoisting block; the second hoisting cable is led through the hoisting block; and a stop is arranged on the second hoisting cable on the side of the pulley away from the lifting frame; wherein in the horizontal position of the lifting frame, the stop is in contact with the hoisting block.

14. System according to claim 11, wherein the hoisting installation is arranged on a ship or offshore platform.

15. A set, comprising: a subsea structure to be installed on the seabed into the water through the splash zone, said subsea structure comprising subsea equipment arranged on a substantially flat support base for preventing the subsea equipment to sink into the seabed; a substantially flat lifting frame; wherein the subsea structure is coupled to the lifting frame for suspension wherein the substantially flat support base extends parallel to the substantially flat lifting frame; a first hoisting cable and a second hoisting cable are connected to the lifting frame for lifting the lifting frame and the subsea structure suspended therefrom; wherein the first hoisting cable and the second hoisting cable are connected to the lifting frame such that with the subsea structure in the horizontal position in which the substantially flat support base extends parallel to the horizontal plane, the vertical component of the respective lifting forces exerted by the first hoisting cable and the second hoisting cable on the lifting frame are offset from the combined center of mass of the lifting frame and the subsea structure on opposite sides of the said combined center of mass; and wherein the second hoisting cable is independently operable from the first hoisting cable.

16. An assembly for lowering a subsea structure to be installed on the seabed into the water through the splash zone, said subsea structure comprising subsea equipment arranged on a substantially flat support base for preventing the subsea equipment to sink into the seabed, comprising: a substantially flat lifting frame which is configured for suspending therefrom the subsea structure; a first hoisting cable and a second hoisting cable for lifting the lifting frame, each connected to the lifting frame such that with the lifting frame in a horizontal position in which the lifting frame extends substantially parallel to the horizontal plane, the vertical component of the respective lifting forces exerted by the first hoisting cable and the second hoisting cable on the lifting frame are offset from the center of mass of the lifting frame on opposite sides of said center of mass, wherein the second hoisting cable is independently operable from the first hoisting cable.

17. The method according to claim 5, wherein, with the subsea structure in the horizontal position, the vertical component of the lifting force exerted by the first hoisting cable is offset from said center of mass by a first distance; the vertical component of the lifting force exerted by the second hoisting cable is offset from said center of mass by a second distance; wherein the first distance is smaller than the second distance.

18. The method according to claim 5, wherein the second hoisting cable is drawn in for tilting the subsea structure from its horizontal position into its tilted position and is payed out for tilting the subsea structure from its tilted position back into its horizontal position.

19. The method according to claim 18, wherein the first hoisting cable is suspended from a hoisting block; the second hoisting cable is led through the hoisting block; and a stop is arranged on the second hoisting cable on the side of the hoisting block away from the lifting frame; wherein in the horizontal position of the subsea structure, the stop is in contact with the hoisting block.

Description

[0052] The present invention is further elucidated in the following description with reference to the accompanying schematic figures, in which:

[0053] FIGS. 1 to 4 show in side view an embodiment of a system according to the invention in four subsequent moments in time during the performance of an embodiment of the method according to the invention.

[0054] In FIGS. 1 to 4 a hoisting installation 1 is shown which is arranged on a ship 3 of which the hull 5 is shown and thrusters 7. The hoisting installation 1 has a crane 9 with a boom 11 and a jib 13. The hoisting installation 1 is provided with a main hoisting cable 15 operated by means of a main winch 17 and an auxiliary hoisting cable 19 operated by means of an auxiliary winch 21.

[0055] Suspended from the hoisting installation 1 is a substantially flat lifting frame 23. The lifting frame 23 is suspended from the hoisting installation 1 via a first hoisting cable 25 and a second hoisting cable 27. The first hoisting cable 25 is connected at one end to the lifting frame 23 and at an opposite end to a hoisting block 29 that, in turn, is connected to the main hoisting cable 15. The lifting frame 23 extends in a plane perpendicular to the drawing plane of FIG. 1. In order to prevent the tilting of the lifting frame 23 about the line 1 where the horizontal plane in which the lifting frame extends intersects the plane of the drawing, the first hoisting cable 25 is split into two cables 25a, 25b. This is shown in FIG. 1A in which the lifting frame 23 is shown in side view in a vertical plane perpendicular to the drawing plane of FIG. 1. The second hoisting cable 27 is at one end to the lifting frame 23 and led through the hoisting block 29 over a pulley 31 provided in the hoisting block 29. A stop 33 is arranged on the second hoisting cable 15 on the side of the hoisting block 29 away from the lifting frame 23.

[0056] Suspended from the lifting frame 23 by means of cables 35 is a subsea structure 37 that is to be installed on the seabed. The subsea structure 37 has a substantially flat support base 39 and subsea equipment 41 arranged thereon. The cables 35 are arranged such that the flat support base 39 of the subsea structure 37 is suspended parallel to the lifting frame 23. In particular the lifting frame 23, the support base 39 and the cables 35 are arranged in a parallelogram configuration. For connecting the cables 35 the subsea structure 37 and the lifting frame 23 are provided with pad eyes at corners of the lifting frame and the support base.

[0057] In FIG. 1 the subsea structure 37 is suspended in its horizontal position in which the flat support base 39 extends substantially parallel to the horizontal plane. The horizontal plane extends perpendicular to the plane of the drawing. The first hoisting cable 25 is connected to the lifting frame 23 such that with the subsea structure 37 in the shown horizontal position, the vertical component F.sub.v1 of the lifting force exerted by the first hoisting cable 25 is offset from the combined center of mass M of the lifting frame 23 and the subsea structure 37 by a first offset distance d.sub.1. The second hoisting cable 27 is connected to the lifting frame 23 such that with the subsea structure 37 in the shown horizontal position, the vertical component F.sub.v2 of the lifting force exerted by the second hoisting cable 27 is offset from the combined center of mass M of the lifting frame 23 and the subsea structure 37 by a second offset distance d.sub.2. The first offset distance d.sub.1 is smaller than the second offset distance d.sub.2. As a result of gravity, the stop 33 is pulled against the hoisting block 29. The auxiliary hoisting cable 19 is slack, such that all weight is supported by the main hoisting cable 17.

[0058] In FIG. 1 the subsea structure 37 has been lifted of the deck 41 of the ship 3, where it was positioned on its support base 37 in its horizontal position during its transport to the location where it is to be installed, and has been lifted in the air above the splash zone S.

[0059] In FIG. 2 is shown that, from the situation shown in FIG. 2, by drawing in the second hoisting cable 27 by pulling the second hoisting cable 27 in direction of arrow A by means of auxiliary hoisting cable 19 and auxiliary winch 21, the subsea structure 37 has been tilted while suspended in the air from the horizontal position (shown in FIG. 1) into a tilted position (shown in FIG. 2) in which the flat support base 39 is angled with respect to the horizontal plane.

[0060] During the tilting of the subsea structure 37, the main hoisting cable 15 has remained stationary.

[0061] In FIG. 3 is shown that from the situation shown in FIG. 2 the subsea structure 37 has been lowered into the water W through the splash zone S in the tilted position by paying out both main hoisting cable 17 and auxiliary hoisting cable 19 in the direction of arrows B. As shown in FIG. 3 the subsea structure 37 has been lowered in its tilted position into a location in the water below the splash zone S.

[0062] In FIG. 4 is shown that, from the situation shown in FIG. 3, by paying out the second hoisting cable 27, in particular by paying out auxiliary hoisting cable 19 in the direction of arrow C while remaining the main hoisting cable 15 stationary, the subsea structure 37 has been tilted while suspended in the water W below the splash zone S back from its tilted position (shown in FIG. 3) back into its horizontal position (shown in FIG. 4). In FIG. 4 the auxiliary hoisting cable 19 is slack, such that the subsea structure 37 is fully supported by the main hoisting cable 15.

[0063] From the situation shown in FIG. 4 the subsea structure 37 is further lowered towards the seabed in the direction of arrow D by paying out the main hoisting cable 15 and the auxiliary hoisting cable 19 by means of main winch 17 and auxiliary winch 21. Since the distance to the seabed can be large, for instance more than 1000 meters, a long range hoisting cable 43 coupled to the hoisting block 29 and a long range winch 45 are provided that take over the lowering of the subsea structure 37 and lifting frame 23 towards the seabed after decoupling of the main hoisting cable 15 from the hoisting block 29 and decoupling of the auxiliary hoisting cable 19 from the second hoisting cable 25. As a result of gravity the stop 33 is pulled against the hoisting block 29, such that the subsea structure 37 remains in its horizontal position even though no hoisting cable is connected to the end of the second hoisting cable 25 away from the lifting frame 23. In more shallow water, the main hoisting cable 15 can be used to lower the subsea structure 37 all the way to the seabed.

[0064] The subsea structure 37 is lowered to the seabed in its horizontal position, where it is installed on the seabed.

[0065] While the principles of the invention have been set out above in connection with specific embodiments, it is to be understood that this description is merely made by way of example and not as a limitation of the scope of protection, which is determined by the appended claims.