AU2006341561A1 - Floating production, storage and off-loading system with column buoy - Google Patents

Description

WO 2007/117300 PCT/US2006/060385 FLOATING PRODUCTION, STORAGE AND OFF-LOADING SYSTEM WITH COLUMN BUOY CROSS-REFERENCE TO RELATED APPLICATION [0001] This application claims the benefit, under 35 U.S.C. @119(e), of US Provisional Application No. 60/731,736, filed 31 October 2005, the disclosure of which is incorporated herein by reference. FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT Not Applicable BACKGROUND OF THE INVENTION [0002] The floating production, storage and off-loading (FPSO) system has long been used in the offshore production, storage. and off-loading of petroleum in harsh environmental conditions, particularly in deep water. Typically, an FPSO system includes a vessel (e.g., a ship or barge) having a turret mounted in the hull, whereby the hull is allowed to rotate or "weathervane" about the turret. Except for this relative yaw motion of the hull with respect to the turret, the turret and the hull move together in all other degrees of freedom. In particular, the vertical movement of the turret is coupled to that of the hull, so that the vertical motions of the hull are transmitted to the mooring and riser systems that are connected to the turret. Due to the magnitude of typical FPSO hull motions, only wet tree riser systems are practical for use with FPSO systems. Also, when used with steel catenary risers (SCRs), the large magnitude of the hull movements limits the fatigue life of SCRs. SUMMARY OF THE INVENTION [0003] Broadly, the present invention is an FPSO system comprising a hull and a column buoy mounted in the hull., whereby the hull is free to both rotate about the axis of the column buoy and to move vertically parallel to the axis of the column buoy. In a preferred embodiment of the invention, column buoy preferably has a lower caisson, and it includes the riser and mooring systems. The vertical motion of the column buoy is decoupled from the vertical motion of the

I

WO 2007/117300 PCT/US2006/060385 hull, and it undergoes relatively small vertical ("heave") motions. Thus, the risers are subjected to reduced stress, and their fatigue life is thereby extended. [0004] In a specific preferred embodiment, an FPSO system according to one aspect of the present invention comprises a hull configured for flotation above a seabed and having a deck; and a column buoy mounted in the hull so as to have an axis substantially perpendicular to the deck; whereby, when the column buoy is fixed to the seabed, the hull is free to move relative to the column buoy both parallel to the axis and rotationally about the axis. The column buoy may either be mounted in a receptacle within the hull ("internal" installation), or it may be installed in a receptacle formed as yoke extending outwardly from the hull ("external" installation). In either case, the column buoy is mounted in contact bearings or friction pads that act as relatively low friction guides or bearings that facilitate the rotational and vertical movements of the hull relative to the column buoy. Because of the reduced vertical movement ("heave") of the column buoy, it can be moored either by a taut leg or catenary mooring system or a vertical tendon system, and it can support either SCRs or top-tensioned risers (TTRs). [0005] In another aspect, the present invention is a method of constructing an FPSO system, comprising: (a) providing a hull having a deck, a keel, and a column buoy receptacle; (b) providing a column buoy lower part and installing it in the receptacle, the column buoy lower part having a top and a bottom; (c) fixing an adjustable ballast caisson to the bottom of the column buoy lower part; and (d) providing a column buoy upper part, installing it in the receptacle, and fixing it to the top of the column buoy lower part to form a column buoy that is movable rotationally and axially in the receptacle relative to the hull. [0006] In the method of the invention, the column buoy receptacle may be either a column buoy well extending from the deck through the keel of the hull, or a column buoy yoke attached to an end of the hull. The hull may be provided in a launching truss, either on a shipway or in a drydock, and the steps of installing the column buoy lower and upper parts, fixing the caisson, and fixing the column buoy upper part to the top of the column buoy lower part are performed WO 2007/117300 PCT/US2006/060385 while the hull is in the truss. Alternatively, the steps of installing the column buoy lower and upper parts, fixing the caisson, and fixing the column buoy upper part to te top of the column buoy lower part are performed while the hull is afloat. [0007] As will be better appreciated from the detailed description that follows, the present invention provides an FPSO in which the vertical ("heave") motion of the hull is decoupled from the column buoy, thereby making the use of dry tree riser systems and top-tensioned risers with an FPSO feasible, and making the use of steel catenary risers with an FPSO more practical due to longer fatigue life. These characteristics greatly extend the range of practical applications of the FPSO, such as for drilling. Moreover, method of constructing an FPSO in accordance with the present invention offers a quicker, easier, and more economical construction method than has heretofore been available. BRIEF DESCRIPTION OF THE DRAWINGS [0008] Figures 1-4 are simplified cross-sectional views showing the steps of constructing an FPSO with an internally-installed column buoy, in accordance with a first preferred embodiment of the invention, wherein the construction is done in a shipway; [0009] Figure 5 is a simplified cross-sectional view of an FPSO in accordance with the first preferred embodiment of the invention, showing the FPSO after construction in a drydock; [0010] Figure 6 is a side elevational view of an FPSO in accordance with the first preferred embodiment of the invention; [0011] Figure 7 is an elevational view, partly in cross-section, showing a column buoy installed in the hull of an FPSO, in accordance with one preferred embodiment of the invention; [0012] Figure 8 is a top plan view of the column buoy of Fig. 7; [0013] Figure 9 is a detailed view of the portion of Fig. 7 enclosed within the dashed circle 9; WO 2007/117300 PCT/US2006/060385 [0014] Figure 10 is a detailed view of the portion if Fig. 7 enclosed within the dashed circle 10; [0015] Figure 11 is a top plan view, similar to that of Fig. 8, showing a first alternative configuration of the column buoy; [0016] Figure 12 is a top plan view, similar to that of Fig. 8, showing a second alternative configuration of the column buoy; [0017] Figure 13 is a top plan view, similar to that of Fig. 8, showing a third alternative configuration of the column buoy; [0018] Figure 14 is a side elevational view showing an FPSO, in accordance with the first preferred embodiment of the present invention, being towed to an offshore site; [0019] Figure 15 is a side elevational view showing the FPSO of Fig. 14 moored by a taut leg mooring system; [0020] Figure 16 is a side elevational view showing the FPSO of Fig. 14 moored by a vertical tendon system; [0021] Figures 17- 20 are simplified cross-sectional views showing the steps of constructing an FPSO with an externally-installed column buoy, in accordance with a second preferred embodiment of the invention, wherein the construction is performed in a shipway; [0022] Figure 21 is a simplified cross-sectional view of an FPSO in accordance with the second prefetred embodiment of the invention, showing the FPSO after construction in a drydock; [0023] Figure 22 is a side elevational view of an FPSO in accordance with a second preferred embodiment of the invention; 4 WO 2007/117300 PCT/US2006/060385 [0024] Figures 23-25 are simplified elevational view, partly in cross-section, showing the steps of installing an externally-installed column buoy in an FPSO hull using an underwater construction technique; [0025] Figure 26 is a side elevational view showing an FPSO in accordance with the second preferred embodiment of the invention being towed to an offshore site; [0026] Figure 27 is a side elevational view showing the FPSO of Fig. 26 moored by a taut leg or catenary mooring system; and [0027] Figure 28 is a side elevational view showing the FPSO of Fig. 26 moored by a vertical tendon system. DETAILED DESCRIPTION OF THE INVENTION [0028] Figures 1-4 illustrate the steps in the process of constructing an FPSO with an internally installed column buoy, in accordance with a first preferred embodiment of the invention. As shown, the FPSO may be constructed in a shipway 10 that includes a launching truss 12. The FPSO comprises a hull 14 constructed in the shipway 10 and supported in the truss 12. The hull 14 is constructed with an internal column buoy receptacle in the form of a column buoy well 16 extending from the keel 17 through the deck 18 of the hull 14. Upper and lower horizontal friction pads 20, 21, respectively, in the forn of annular bearing rings, are installed in the well 16. A cylindrical column buoy lower part 22a is fabricated in a workshop (not shown), and it is fitted with a plurality of lower vertical or axial friction pad segments 24a, which are in the form of vertical or axial bearing strips arranged around the periphery of the column buoy lower part 22a. As shown in Fig. 2, the column buoy lower part 22a is installed in the well 16, and then, as shown in Fig. 3, an adjustable ballast caisson 26 is fixed to the bottom of the column buoy lower part 22a. Finally, a cylindrical column buoy upper part 22b (also constructed in a workshop) is installed in the well 16 and fixed to the top of the column buoy lower part 22a, as shown in Fig. 4, to form a completed column buoy 22. The column buoy upper part 22b includes a plurality of upper vertical or axial friction pad segments 24b that preferably align with the lower vertical friction pad segments 24a of the column buoy lower part 22a. 5 WO 2007/117300 PCT/US2006/060385 [0029] The installation of the column buoy upper part 22b may be performed either while the FPSO is still in the shipway (as shown in Fig. 4). or it may be performed after the FPSO is launched and is afloat. Figure 5 shows that the above-described construction steps can be performed in a drydock 28 that includes a hull supporting launching truss 30. [0030] Figure 6 shows a completed FPSO 40, in accordance with a first preferred embodiment of the invention, after it has been launched. In this embodiment, the FPSO 40 includes a fully assembled, internally-installed column buoy 22 installed through the hull 14. in the manner described above in connection with Figures 1-5. The column buoy 22 is in its raised position, in which the caisson 26 is adjacent to the keel of the hull 14, which is the position in which the column buoy 22 is maintained while the FPSO 40 is moving to its desired offshore site. [0031] Figures 7-10 illustrate the details of the structure of the column buoy 22 and the mechanism for mounting the column buoy 22 in the hull 14 of the FPSO. As discussed above, the column buoy 22 is installed in a column buoy well 16 that extends through the hull 14 of the FPSO. Installed in the well 16 are the upper horizontal friction pad 20 and the lower horizontal friction pad 21. The horizontal friction pads 20, 21 provide annular bearing surfaces on the interior wall of the well 16. As shown in Figure 9. the upper horizontal friction pad 20 is secured between a first pair of annular brackets 42 that are secured (e.g., by bolts 44 or by welding) to the interior wall surface of the column buoy well 16. Similarly, as shown in Figure 10, the lower horizontal friction pad 21 is secured between a second pair of annular brackets 46 that are secured by similar means to the interior wall surface of the column buoy well 16. [00321 Referring again to Figures 7 and 8, secured to the column buoy 22 is a plurality of vertical or axial friction pads 24 that are arranged around the periphery of the column buoy 22. Each of the vertical friction pads 24 is formed from a lower vertical friction pad segment 24a on the column buoy lower part 22a, and an upper vertical friction pad segment 24b on the column buoy upper part 22b, as described above. The vertical friction pads 24 thus provide a plurality of continuous vertical bearing surfaces that engage against the horizontal friction pads 20, 21. 6 WO 2007/117300 PCT/US2006/060385 [0033] While the preferred configuration of the column buoy22 is cylindrical, as described above and as shown in Figures 1-10, Figures 11-13 illustrate alternative configurations for the column buoy. Specifically. Fig. 11 shows a square column buoy 22'; Fig. 12 shows an octagonal column buoy 22"; and Fig. 1 shows a hexagonal column buoy 22'. These alternative configurations are exemplary only, and other configurations may be contemplated. All of these alternative configurations require the use of upper and lower horizontal friction pads 20, 21 (although only the upper pad 20 is shown in the drawings). As shown, each of these alternative configurations may optionally include vertical friction pads 24 along the vertices of the respective column buoys 22', 22", 22"', but the vertices themselves may be made of a suitable wear-resistant, low friction bearing material, so that the installation of separate vertical friction pads may be omitted. Each of the column buoys 22', 22", 22'" may preferably have a caisson 26', 26", 26'", respectively, fixed to its lower end. The caisson may, but does not necessarily, have the same configuration as the column buoy to which it is attached. [0034] Figure 14 shows the FPSO 40, in accordance with the internally-installed colun buoy embodiment of Fig. 7, after completion (i.e., outfitted with a deck superstructure 50 and equipment 52), as it is being towed to a selected offshore site by a towing vessel 54. Once at the site, the column buoy 22 is lowered (by such conventional means as, for example, vertical mooring line force, risers, and the ballasting of the caisson 26) to its operational position, as shown in Figures 15 and 16. In Figure 15, the column buoy 22 is moored to the seabed (not shown) by a plurality of taut-leg or catenary mooring lines 56. A plurality of top-tensioned risers 58 has been installed between the column buoy 22 and sub-sea wellheads (not shown). A plurality of SCRs (not shown) may also be installed. In Figure 16, the column buoy 22 is moored by a plurality of tendons 60, and a plurality of top-tensioned risers 58 and a plurality of SCRs 62 (only one of which is shown) have been installed. [0035] The column buoy 22, when installed in the hull 14, has an axis that is substantially perpendicular to the deck 18. It will be appreciated that the engagement between the horizontal and vertical friction pads allows the hull 14 of the FPSO both to rotate about the axis of the column buoy 22 (as in conventional FPSOs with turrets), and to move vertically relative to the column buoy 22; that is, longitudinally or parallel to the column buoy axis. This is because the 7 WO 2007/117300 PCT/US2006/060385 vertical ("heave") motion of the hull 14 is decoupled from the column buoy 22. Thus, the column buoy 22 does not experience the full magnitude of the heave motion of the hull 14, thereby reducing stress in the risers and the mooring lines, resulting in longer fatigue life for these components. [0036] Figures 17-20 illustrate the steps in the process of constructing an FPSO with an externally-installed column buoy, in accordance with a second preferred embodiment of the invention. As shown, the FPSO may be constructed in a shipway 10' that includes a launching truss 12'. The FPSO comprises a hull 74 constructed in the shipway 10' and supported in the truss 12'. Attached to the bow of the hull 74 is an external column buoy receptacle in the form of a column buoy yoke 76 having a bore 78 dimensioned for the installation of a column buoy, as described below. Upper and lower horizontal friction pads 80, 81, respectively, in the form of annular bearing rings, are installed in the bore 78 of the yoke 76. A cylindrical column buoy lower part 82a is fabricated in a workshop (not shown), and it is fitted with a plurality of lower vertical friction pad segments 84a, which are in the form of vertical bearing strips arranged around the periphery of the column buoy lower part 82a. As shown in Fig. 18, the column buoy lower part 82a is installed in the bore 78 of the yoke 76, and then, as shown in Fig. 19, an adjustable ballast caisson 86 is fixed to the bottom of the column buoy lower part 82a. Finally, a cylindrical column buoy upper part 82b (also constructed in a workshop) is installed in the bore 78 of the yoke 76 and fixed to the top of the column buoy lower part 82a, as shown in Fig. 20, to form a completed column buoy 82. The column buoy upper part 82b includes a plurality of upper vertical friction pad segments 84b that preferably align with the lower vertical friction pad segments 84a of the column buoy lower part 82a. [0037] The installation of the column buoy upper part 82b may be performed either while the FPSO is still in the shipway 10' (as shown in Fig. 20), or it may be performed after the FPSO is launched. Figure 21 shows that the above-described construction steps can be performed in a drydock 28' that includes a hull supporting launching truss 30'. Figure 22 shows an FPSO 90 with an externally-installed column buoy 82, in accordance with the second preferred embodiment of the invention, after the launch of the FPSO 90 from either the shipway 10' or the drydock 28'. 8 WO 2007/117300 PCT/US2006/060385 [0038] Figures 23-25 illustrate the steps in constructing the FPSO 90. in accordance with the second preferred embodiment of the invention, using an underwater method of column buoy installation, whereby neither a shipway or a drydock is needed. Specifically, the hull 74 with the yoke 76 attached is floated on the surface at a construction site. The upper and lower horizontal friction pads 80, 81 are installed in the bore 78 of the yoke 76, as explained above in connection with the FPSO 40 having the internally-installed column buoy 22. A column buoy lower part 82a, with the lower vertical friction pad segments attached to it, is fabricated in a workshop, and the caisson 86 is attached to its lower end. The assembled column buoy lower part 82a and caisson 86 are transported to the hull and submerged beneath the bore 78 of the hull 76, as shown in Fig. 23, by ballasting the caisson. The caisson 86 is then de-ballasted to raise the column buoy lower part 82a through bore 78 of the yoke 76, as shown in Fig. 24. Finally, as shown in Fig. 25, the column buoy upper part 82b is installed on the upper end of the column buoy lower part 82a. [0039] Figure 26 shows the FPSO 90, in accordance with the externally-installed column buoy embodiment of Figures 22 and 25, after completion (i.e., outfitted with deck superstructure 92 and equipment 94), as it is being towed to a selected offshore site by a towing vessel 96. Once at the site, the column buoy 82 is lowered (by conventional means, including vertical line force, risers, and the ballasting of the caisson 26) to its operational position, as shown in Figures 27 and 28. In Figure 27, the column buoy 82 is moored to the seabed (not shown) by a plurality of taut leg or catenary mooring lines 98. A plurality of top-tensioned risers 100 has been installed between the column buoy 82 and sub-sea wellheads (not shown). A plurality of SCRs (not shown) may also be installed. In Figure 28, the column buoy 82 is moored by a plurality of tendons 102, and a plurality of top-tensioned risers 100 and at least one SCR 104 have been installed. [0040] It will be appreciated that the externally-installed column buoy 82 of the FPSO 90 may be made in any desired configuration, including the configurations shown in Figures 11-13. It will also be appreciated that FPSO 90 with the externally-installed column buoy 82 will exhibit the same advantages as those described above for the FPSO 40 with the internally-installed column buoy 22. 9 WO 2007/117300 PCT/US2006/060385 [0041] While several preferred embodiments of the invention, in thle aspects of both an FPSO and a method of constructing an FPSO, have been described above and shown in the accompanying drawings, it will be appreciated that a number of modifications and variations will suggest themselves to those skilled in the pertinent arts. For example, the caisson fixed to the bottom of the column buoy may be a non-ballasted trusswork, in which case the column buoy would be raised and lowered only by conventional mechanical means. Also, when the FPSO is constructed with the hull afloat (i.e., after the hull is launched), it may be desirable to fix the caisson to the column buoy lower part before the column buoy is installed in the receptacle. 10

Claims (25)

1. A floating production, storage, and offloading (FPSO) system, comprising: a hull configured for flotation above a seabed and having a deck; and a column buoy mounted in the hull so as to have an axis substantially perpendicular to the deck, whereby, when the column buoy is fixed to the seabed, the hull is free to move relative to the column buoy both parallel to the axis and rotationally about the axis.

2. The FPSO system of claim 1, wherein the column buoy is mounted in a column buoy well extending through the hull.

3. The FPSO system of claim 1. wherein the hull includes a column buoy yoke attached thereto, and wherein the column buoy is mounted in the column buoy yoke.

4. The FPSO system of claim 3, wherein the hull has a bow, and wherein the column buoy yoke is attached to the bow.

5. The FPSO system of claim 2, wherein the column buoy well includes a horizontal bearing pad and wherein the column buoy is provided with a plurality of vertical bearing pads that bear against the horizontal bearing pad.

6. The FPSO system of claim 3, wherein the column buoy yoke defines a bore in which is mounted a horizontal bearing pad, and wherein the column buoy is provided with a plurality of vertical bearing pads that bear against the horizontal bearing pad.

7. The FPSO system of claim 5, wherein the horizontal bearing pad is a first horizontal bearing pad, and wherein the column buoy well includes a second horizontal bearing pad vertically spaced from the first horizontal bearing pad. 11 WO 2007/117300 PCT/US2006/060385

8. The FPSO system of claim 6, wherein the horizontal bearing pad is a first horizontal bearing pad, and wherein a second horizontal bearing pad vertically spaced from the first horizontal bearing pad is mounted in the bore of the column buoy yoke.

9. The FPSO system of claim 1, wherein the hull includes a keel, wherein the column buoy has a bottom that extends below the keel, and wherein the FPSO further comprises an adjustable ballast caisson fixed to the bottom of the column buoy.

10. A floating production, storage, and offloading (FPSO) system, comprising: a hull configured for flotation above a seabed. the hull having a deck and a keel; and a column buoy defining a longitudinal axis perpendicular to the deck and mounted in the hull so as to allow relative movement between the column buoy and the hull in a direction parallel to the axis, and to allow relative movement between the column buoy and the hull rotationally about the axis; a column buoy movement mechanism., operatively associated with the column buoy, for moving the column buoy longitudinally along the axis between a raised position and a lowered position; and mooring elements fixing the column buoy to the seabed when the column buoy is moved to its lowered position.

11. The FPSO of claim 10, wherein the column buoy is mounted in a column buoy well extending through the hull from the deck through the keel.

12. The FPSO of claim 10, wherein the column buoy is mounted in a column buoy yoke fixed to one end of the hull.

13. The FPSO system of claim 11. wherein the column buoy well includes a horizontal bearing pad and wherein the column buoy is provided with a plurality of vertical bearing pads that bear against the horizontal bearing pad. 12 WO 2007/117300 PCT/US2006/060385

14. The FPSO system of claim 12, wherein the column buoy yoke defines a bore in which is mounted a horizontal bearing pad, and wherein the column buoy is provided with a plurality of vertical bearing pads that bear against the horizontal bearing pad.

15. The FPSO system of claim 13, wherein the horizontal bearing pad is a first horizontal bearing pad, and wherein the column buoy well includes a second horizontal bearing pad vertically spaced from the first horizontal bearing pad.

16. The FPSO system of claim 14, wherein the horizontal bearing pad is a first horizontal bearing pad, and wherein a second horizontal bearing pad vertically spaced from the first horizontal bearing pad is mounted in the bore of the column buoy yoke.

17. The FPSO system of claim 10, wherein the column buoy has a bottom that extends below the keel, and wherein the column buoy movement mechanism includes an adjustable ballast caisson fixed to the bottom of the column buoy.

18. The FPSO system of claim 10, wherein the mooring elements are selected from the group consisting of one or more of taut-leg mooring lines, catenary mooring lines, and tendons.

19. A method of constructing a floating production, storage, and offloading (FPSO) system, comprising: providing a hull having a deck, a keel, and a column buoy receptacle; providing a column buoy lower part and installing it in the receptacle, the column buoy lower part having a top and a bottom; fixing an adjustable ballast caisson to the bottom of the column buoy lower part; and providing a column buoy upper part, installing it in the receptacle, and fixing it to the top of the column buoy lower part to form a column buoy that is movable rotationally and axially in the receptacle relative to the hull.

20. The method of claim 1.9, wherein the column buoy receptacle is a column buoy well extending from the deck through the keel of the hull. 13 WO 2007/117300 PCT/US2006/060385

21. The method of claim 19, wherein the column buoy receptacle is a column buoy yoke attached to an end of the hull.

22. The method of claim 19, wherein the hull is provided in a launching truss, and wherein the steps of installing the column buoy lower and upper parts, fixing the caisson, and fixing the column buoy upper part to the top of the column buoy lower part are performed while the hull is in the truss.

23. The method of claim 22, wherein the launching truss is in a shipway.

24. The method of claim 22, wherein the launching truss is in a drydock.

25. The method of claim 19 wherein the steps of installing the column buoy lower and upper parts, fixing the caisson, and fixing the column buoy upper part to the top of the column buoy lower part are performed while the hull is afloat. 14