Deepwater disconnectable turret system with improved riser configuration

Abstract

A system for transporting hydrocarbons from reserves located under the sea floor to a turret connected to a hydrocarbon production vessel floating at the sea surface, the hydrocarbons being transferred through at least one rigid catenary riser extending from the sea floor to a buoy, the system for transporting hydrocarbons includes an upper section of the at least one substantially rigid riser directly attached to the buoy and provided with fairings, a middle section of the rigid riser is provided with buoyancy modules so to give it a lazy wave shape and a lower section of the substantially rigid riser is in contact with the seafloor at a distance X from the buoy vertical axis that is smaller than a distance Y between the buoy vertical axis and the mooring lines anchoring elements.

Claims

1. A system (1) for transporting hydrocarbons from reserves located under the sea floor (2) to a turret (3) that is rotatably connected to a hydrocarbon production vessel that is floating at the sea surface, comprising at least one substantially rigid catenary riser (4) extending from the sea floor (2) for transport of said hydrocarbons to the turret, said system comprising: three or more groups of mooring lines equally spaced apart, each group of mooring lines containing at least two individual mooring lines with polyester rope parts, the mooring lines each comprising two chain parts at the end, a polyester part between the chain parts and a spring buoy, wherein the lower ends of the mooring lines are anchored to the sea floor, said groups of mooring lines having open sectors therebetween in which the at least one substantially rigid catenary riser (4) is located, wherein the substantially rigid catenary riser (4) and the grouped mooring lines have upper ends connected to and supported by one buoy (6) which is adapted to be connected to and disconnected from the lower part of the turret (3), wherein said buoy, when connected to the turret, is partially accommodated within the turret and held in position with respect to the turret by one or more structural connectors (14), the upper part of the buoy being provided with a fluid connector that is in fluid connection with the upper end of the substantially rigid catenary riser (4) connector, for attachment to the fluid transfer system of the turret (3) to allow said transfer of hydrocarbons from the seabed to the production vessel, the buoy having a buoyancy such that when disconnected from the turret (3), the buoy with attached substantially rigid riser (4) and grouped mooring lines floats below the wave active zone in the upper half part of the water-depth, wherein, groups of the substantially rigid risers are connected to the lower part of the buoy (6), an upper section of each of the substantially rigid risers (4) is directly attached to the buoy, a middle section of each substantially rigid riser (4) is provided with buoyancy modules (8) so to give each substantially rigid riser (4) a lazy wave shape, and a lower section of each of the substantially rigid risers (4) is in contact with the sea floor at a radial distance X from the buoy vertical axis that is smaller than the radial distance Y between the buoy vertical axis and the location wherein the lower ends of the mooring lines are anchored to the sea floor, and the lazy wave shape of the substantially rigid riser is located below sea level.

2. The system for transporting hydrocarbons according to claim 1, wherein the height of the lazy wave riser (4) is between 80% and 100% of the radial distance X.

3. The system for transporting hydrocarbons according to claim 1, wherein a lower part of the lazy wave riser (4) is placed horizontally on the sea floor and can be at one end lifted off from the sea floor while the other end stays connected to the sea floor.

4. The system for transporting hydrocarbons according to claim 1, wherein the fluid transfer system comprises at least one lazy wave production riser (4) for transfer of hydrocarbons from a reserve to the vessel, at least one lazy wave riser (4) for exporting the produced gas from the vessel via a subsea pipeline and at least one lazy wave riser (4) for injection of water into a sub sea floor hydrocarbon reserve.

5. The system for transporting hydrocarbons according to claim 1, wherein the departure angle at the buoy of the mooring line is less than 60 degrees, preferably less than 45 degrees, more preferably less than 30 degrees with the vertical when the buoy is connected to the vessel.

6. The system for transporting hydrocarbons according to claim 1, wherein the buoy with attached substantially rigid riser (4) and grouped mooring lines floats below the wave active zone in the upper quarter part of the water-depth.

7. The system for transporting hydrocarbons according to claim 1, wherein the height of the lazy wave riser (4) is between 100% and 300% of the radial distance X.

8. The system for transporting hydrocarbons according to claim 1, wherein, with the buoy connected to the vessel, the lazy wave risers (4) and mooring system combined allow the vessel a maximal offset of 8% of the water depth.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be further described below in connection with exemplary embodiments with reference to the accompanying drawings, wherein

(2) FIG. 1 shows an embodiment according to the present invention of an external turret connected to a BTM with lazy wave SCRs;

(3) FIG. 2 shows a BTM buoy that is used to interface with an internal turret, according to another embodiment of the present invention; and

(4) FIG. 3 shows the riser and umbilical system with FPSO and BTM mooring system with an internal turret.

DESCRIPTION OF FIGURES

(5) FIG. 1 shows an embodiment according to the present invention of an external turret connected to a BTM with lazy wave SCRs.

(6) In FIG. 1 there is shown a system 1 for transporting hydrocarbons in large water-depths. In the embodiment of FIG. 1, a production vessel 7 is moored to the seabed via an external turret 3 from which lower part a buoy 6 can be connected and disconnected. Groups of mooring lines 5 and risers 4, in a lazy wave configuration, are connected to the lower part of the buoy 6. As shown in FIG. 1 and FIG. 3, there is a radial distance between the buoy 6 vertical axis and the point where a riser 4 is in contact with the sea floor 2. As shown, the radial distance Y between the buoy 6 vertical axis and the mooring lines 5 anchoring means is bigger than the radial distance X.

(7) FIG. 2 shows a BTM buoy that is used to interface with an internal turret, according to another embodiment of the present invention The BTM turret is shown in FIG. 2 and consists of the following components: A BTM buoy 6, interfacing with the internal turret 12 via a cage and a set of structural connectors. One or more structural connectors 14 between the buoy 6 and the vessel. It could be a central connector or several connectors that are distributed along the circumference on top of the BTM buoy 6. Connectors and retractors for the production fluid, export gas, and umbilical flow paths. These connectors are located on top the buoy. A structural bearing system 13 that transfers the turret payload to the vessel. The weathervaning system made of multiple bogeys A swivel stack 11 supported by a gantry structure 10.

(8) The main limitation of the BTM concept in deepwater is related to the riser and mooring payload which drives the size of the BTM buoy, especially in deeper water. In order to limit the payload of risers, the solution is to keep the Lazy Wave location at a shallow depth below the sea level. In deeper waters, this approach leads to an increased demand for buoyancy (hence higher cost) and a much larger foot-print of the riser system on the seabed. As for reducing the payload of mooring lines, the proposed solution is using polyester lines with spring buoys (about 40 tons of net buoyancy per mooring line in this case).

(9) The I-tubes of the steel risers are inclined at the nominal riser departure angle to allow the riser pulling from the turret once the FPSO is on site and connected to the buoy. The

(10) I-tubes of the umbilicals are vertical since the flexible umbilicals can be pulled through their bend-stiffeners.

(11) Each flow path, either those of risers or umbilicals, has a dedicated connector and retractor system on top of the buoy. The connected/retractor is rated for the design pressure of the fluid path and for the maximum depth of the BTM buoy when disconnected (about 120 m). The system can be disconnected in sea states up to Hs 8.8 m, and the disconnection can be carried in sea states up to at least Hs 2 m. The disconnection can be made without assistance from other vessels. More details of the turret and buoy including the flow line connectors/retractors.

(12) FIG. 3 shows the riser and umbilical system with FPSO and BTM mooring system with an internal turret. In this embodiment, the BTM is comprised of an internal turret FPSO 7 supporting a disconnectable buoy 6. The buoy is designed to support the mooring lines 5 and risers/umbilicals 4 upon disconnect. Risers 4 have a lazy wave configuration by utilizing distributed buoyancy 8 in each riser and umbilical, hence decoupling the vessel motions from the riser touchdown point.

(13) From this figure it also appears clearly that the radial distance X between the riser touchdown point and the buoy vertical axis is smaller than the radial distance Y between the buoy vertical axis and the mooring lines anchoring means.

(14) Although particular embodiments of the invention have been described and illustrated herein, it is recognized that modifications and variations may readily occur to those skilled in the art, and consequently, it is intended that the claims be interpreted to cover such modifications and equivalents.

LIST OF REFERENCE NUMERALS

(15) 1. System for transporting

(16) 2. Sea floor

(17) 3. External turret

(18) 4. Riser

(19) 5. Anchoring means

(20) 6. Buoy

(21) 7. Production vessel

(22) 8. Distributed buoyancy modules

(23) 9. —

(24) 10. Overhead gantry structure

(25) 11. Swivel stack

(26) 12. Turret structure

(27) 13. Bearing system

(28) 14. Structural connector

(29) X=radial distance between the riser touchdown point and the buoy vertical axis

(30) Y=between the buoy vertical axis and the mooring lines anchoring means