Offshore vessel for production and storage of hydrocarbon products

Abstract

The present invention relates to a spread moored vessel for production and/or storing of hydrocarbons. The vessel comprises a laterally extending main deck, a symmetrical mooring arrangement for mooring the vessel to a seabed when the vessel is floating in a body of water and a longitudinal hull. The longitudinal hull further comprises a bow, a midbody, a stern, and a motion suppressing element protruding out from the longitudinal hull, below the vessel's maximum draught. The ratio between a maximum length (L.sub.wl) and a maximum breadth (B.sub.wl) of the longitudinal hull, at the vessel's maximum draught, is between 1.1 and 1.5. The specific hull shape with the particular length/breadth ratio and the motion suppressing element allows for favorable and uniform motions regarding of wave direction in relation to vessel heading.

Claims

1. A spread moored vessel for production and storage of hydrocarbons, the vessel comprising a laterally extending main deck, a mooring arrangement for mooring the vessel to a seabed when the vessel is floating in a body of water and a longitudinal hull, wherein the ratio between a maximum length and a maximum breadth of the longitudinal hull, at the vessel's maximum draught, is between 1.1 and 1.5 and wherein the longitudinal length of the vessel is separated into a cargo zone comprising at least one cargo tank and at least one non-cargo zone, wherein the longitudinal hull comprises a motion suppressing element protruding out from the longitudinal hull, below the vessel's maximum draught, for suppression of heave, pitch and roll, a bow having a lateral cross section in the shape of a rounded triangle, a midbody and a stern, wherein the lateral cross section of the midbody and the stern at the vessel's maximum draught has a rectangular shape.

2. The vessel according to claim 1, wherein the ratio between the maximum length and the maximum breadth of the longitudinal hull, at the vessel's maximum draught, is between 1.2 and 1.4.

3. The vessel according to claim 1 or 2, wherein the motion suppressing element protrudes out from the bow, the midbody and the stern, below the vessel's maximum draught.

4. The vessel according to claim 1, wherein the motion suppressing element protrudes laterally from the hull along at least 70% of the hull's lateral extending circumference.

5. The vessel according to claim 1, wherein the motion suppressing element protrudes laterally from a lowermost part of the hull.

6. The vessel according to claim 1, wherein the lateral protrusion length of the motion suppressing element is between 5% and 30% of the hull's maximum breadth at the vessel's maximum draught.

7. The vessel according to claim 1, wherein the midbody comprises a port side portion and a starboard side portion, where at least 30% of the longitudinal length of the midbody are flat and oriented parallel to a center plane of the hull, the center plane being the plane intersecting the hull midway between the port and starboard side portions and aligned perpendicular to the laterally extending main deck.

8. The vessel according to claim 1, wherein the transition region between the bow and the midbody forms abrupt change of angle at the vessel's maximum draught, relative to the center plane, the center plane being the plane intersecting the hull midway between the port and starboard side portions (2a, 2b) and aligned perpendicular to the laterally extending main deck.

9. The vessel according to claim 8, wherein the angle is at least 20 degrees.

10. The vessel according to claim 1, wherein the longitudinal length of the bow at the vessel's maximum draught is at least 25% of the maximum length of the hull.

11. The vessel according to claim 1, wherein the mooring arrangement comprising a plurality of mooring lines, wherein at least one mooring line is moorable from a location at or near the center of the bow relative to the hull's breadth, at least one mooring line is moorable from a location adjacent the stern at the port hull side and at least one mooring line is moorable from a location adjacent the stern at the starboard hull side.

12. The vessel according to claim 11, wherein the motion suppressing element displays recesses at the lateral locations of the plurality of mooring lines when the vessel is moored to the seabed.

13. The vessel according to claim 1, wherein the longitudinal hull further displays at least one slop tank situated adjacent to the at least one cargo tank.

14. The vessel according to claim 13, wherein the at least one slop tank (100b) is arranged in or adjacent to the center plane of the hull, the center plane being the plane intersecting the hull midway between a port side portion and a starboard side portion constituting the midbody and aligned perpendicular to the laterally extending main deck.

15. The vessel according to claim 1, wherein at least one of the at least one non-cargo zone is located within the bow.

16. The vessel according to claim 1, wherein the longitudinal hull comprises at least two walls having a space therebetween, into which at least one ballast tank is located.

17. The vessel according to claim 1, wherein the vessel is configured to allow hang off of a multiple riser arrangement at least one of the midbody, the bow and the stern.

18. The vessel according to claim 1, wherein a plurality of riser guide pipes are arranged along at least part of the lateral circumference of the longitudinal hull, where each of the plurality of riser guide pipes is configured to allow at least one riser to be guided therethrough.

19. The vessel according claim 1, wherein the projected lateral surface area of the hull at the vertical position of the main deck is larger than the projected lateral surface area of the hull at the vertical position of the vessel's maximum draught.

20. The vessel according to claim 1, wherein the projected lateral surface area of the hull at the vertical position of the main deck is at least 10% larger than the projected lateral surface area of the hull at the vertical position of the vessel's maximum draught.

21. The vessel according to claim 19 or 20, wherein the onset of increase of the projected lateral surface area of the hull from the vertical position of the vessel's maximum draught to the vertical position of the main deck commences at or above the vessel's maximum draught.

22. The vessel according to claim 20, wherein the increase of the projected lateral surface area of the hull from the vertical position of the vessel's maximum draught to the vertical position of the main deck is constant.

23. The vessel according to claim 1, wherein the ratio between the maximum length of the longitudinal hull and a maximum depth of the longitudinal hull defined as a distance from the vertical position of the main deck to the lowermost part of the hull is between 2 and 6.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be described with reference to the attached drawings, wherein:

(2) FIG. 1 is a side view of a vessel according to an embodiment of the present invention,

(3) FIG. 2 is a top view of the vessel according to FIG. 1 showing the elevated deck and exemplary positions of cargo tanks, slop tanks and mooring winch arrangement,

(4) FIG. 3 shows a horizontal section through a vessel according to FIGS. 1 and 2, showing exemplary locations of cargo tanks, slop tanks, fuel-tanks and ballast tanks, the horizontal section being in or around the waterline, i.e. between the hull's suppressing elements and the hull's flared side shell,

(5) FIG. 4 shows a transverse cross section through the cargo zone of the vessel according to FIGS. 1-3,

(6) FIG. 5 is a longitudinal cross-section through a center plane of the vessel according to FIGS. 1-4,

(7) FIG. 6 is a longitudinal cross-section through a center plane of a vessel according to a second embodiment of the invention showing an alternative configuration where a safe area with living quarters are located towards the stern of the vessel,

(8) FIG. 7 is a view of the bottom of the vessel according to the invention, including mooring lines and local recesses in the suppressing elements,

(9) FIGS. 8 (a) to (d) show representative motion characteristics of a vessel according to the invention as compared to conventional ship-shaped designs of comparable storage capacity by plotting simulated response of the heave motion as function of the wave period for the inventive FPSO (a) and a conventional FPSO (b) and by plotting simulated data of the ration pitch/roll motion as function of the wave period for the inventive FPSO (c) and the conventional FPSO (d).

DETAILED DESCRIPTION OF THE INVENTION

(10) FIGS. 1-7 show a first embodiment of a longitudinal vessel 1 according to the present invention having a maximum length and breadth at the location of the vessel's maximum draught of L.sub.wl and B.sub.wl, respectively (see in particular FIG. 2). The vessel 1 comprises a bow 3, a stern 4, a hull 2 with a parallel midship 2a,2b and a deck structure 5. The latter further comprises a main deck D, a processing deck P, and living quarters A supported on a fore deck F. Below the vessel's 1 maximum draught or waterline (w(l)), the hull 2 is provided with a suppressing element or damping extrusions 6 protruding outwards from the hull 2, preferably around the entire periphery of the hull 2. The suppressing element 6 may extend 10-25% of the vessel 6 breadth, depending on required motion characteristics. The safe area of the FPSO, i.e. the area of the main deck D containing the living quarters A, is segregated from the processing area either by distance or by a blast wall. The area of the bow 3 and/or the area of the stern 4 may be raised to provide improved protection with respect to green sea. Further, as is more apparent in FIG. 2, the deck area behind the area of the bow 3 is preferably rectangular, thereby enabling simple and effective arrangement of topside modules. The maximum length of the vessel 1 (L.sub.wl) is preferably within 1.1 to 1.5 times the maximum breadth of the vessel 1 (B.sub.wl), for example 1.3 times the breadth (B.sub.wl). As best seen in FIGS. 1 and 4, the upper side of the hull 2, that is the height of the hull 2 situated above the water line (w(l)) or maximum draught, is flared out to provide a larger deck area.

(11) The flared region FR typically starts about 1 meter above waterline (w(l)), and extends to the process deck P or above depending on the required deck space. The standard flare angle of the flared region is typically 1:2 in terms of horizontal versus vertical increment, but may be increased for areas in which wave slamming is not an issue. The flare angle may thus be varied around the circumference of the vessel 1.

(12) The main deck elevation D in relation to the waterline w(l) is determined for each specific application, but is as a rule kept as low as possible within the limits given by international load line convention, stability and green sea. A distance (d) of the main deck elevation D of about 10-12 meters above the waterline w(l) is typical for harsh environment areas, and somewhat less in case of benign conditions. The process deck P is typically located 4-6 meters above the main deck D. For very severe wave conditions, the fore deck F, at which the living quarter and lifeboats will be located, may be raised another 4-6 meters.

(13) The suppressing element 6 provides additional added mass that inter alia influences heave, pitch and roll motions of the vessel 1 caused by external forces such as waves. By tuning the size of the suppressing element, the vessel shape, including length to breadth ratio and waterline area, and the total mass of the vessel including added mass, it is thus possible to achieve a natural frequency outside the range of the critical wave excitation frequency. In selecting the actual shape and design of the vessel, coupling effects between inertia, damping and buoyancy forces need to be considered as these effects have significant influence on the heave, roll and pitch motions. It is the combination of the increased natural period and the mentioned coupling effects that gives the favorable motion characteristics of the present invention. This motion behavior has been documented and verified through calculations and model testing.

(14) FIGS. 2 and 3 show top view sections at the main deck D and the waterline w(l), respectively and give an overview of the tank arrangement of the vessel. The vessel 1 is divided into non-cargo zones (NCZ) comprising a plurality of ballast tanks 101 and fuel/MDO (marine diesel oil) tanks 102 and a cargo zones (CZ) comprising a plurality of cargo tanks 100a and appurtenant slop tanks 100b.

(15) The double hull configuration with flared outer hull 2 gives a significant area around the circumference of the main deck D in which there are no hydrocarbon content underneath. With a double side of 3-4 meters, and the mentioned hull 2 with the flared region FR, the width of the outer deck area above ballast tanks will be more than 8 meters. FIGS. 2 and 3 also show the distinctive rectangular shape of the aft part 4 and midbody part 2a,2b, as well as the triangular bow 3 including curved forward part (the latter being in FIG. 3 confined within the safety area, that is, forward the safety division S).

(16) The midbody of the hull 2 comprises a port side portion 2a and a starboard side portion 2b oriented parallel to a center plane CP of the hull 2, the center plane CP being defined as the plane intersecting the hull 2 midway between the port side portion 2a and the starboard side portion 2b and aligned perpendicular to the main deck D (see stippled line in FIG. 7).

(17) The wave excitation forces are greatest in the waterline area, and hence the vessels 1 shape and dimensions in this area are decisive in achieving the favorable and wave-direction-independent responses. The bow part 3 shown in FIGS. 2 and 3 constitutes about 35% of the length in waterline w(l), that is, 35% of L.sub.wl, and forms a bow angle (BA) between 20 and 60 degrees from the parallel midship. With a bow angle of 40 degrees, and with a length to width ratio (L.sub.wl/B.sub.wl) in waterline w(l) of about 1.3, the length and breadth range of the inventive design will be L.sub.wl=50-140 meters and B.sub.wl=35-100 meters for storage capacities from 100,000 bbls2,000,000 bbls, as an example only.

(18) As an alternative, the distribution of pump rooms 103 and fuel tanks 102 may be located in the aft part 4 of the vessel 1.

(19) The arrangement of ballast tanks 101 around the circumference of the hull 2 provide protection of the ballast and slop tanks 100a,100b, the fuel tanks 102 and the pump room 103. A double bottom 10 as shown in FIGS. 4-6 provides further protection to the tanks 100a,100b,102,103, as well as being used as space for additional ballast tank(s) 101. This tank arrangement, combined with the wide breadth of the vessel 1, results in high vessel stability. High stability allows for applying large process apparatus/systems on vessel 1 such as FPSOs or FLNGs. If using the vessel 1 for natural gas, the ballast and slop tanks should be separated from tanks for fluid cooled natural gas.

(20) An example of a mooring arrangement M is shown in FIGS. 2 and 7. The mooring arrangement M comprises a plurality of mooring lines arranged at the fore Mb and on aft corners M.sub.sp (port), M.sub.sb (starboard) of the vessel 1 such that the entire mooring arrangement M mirrors the central longitudinal plane CP of the vessel 1. Such a spread mooring arrangement M ensures a fixed, non-rotatable vessel-position during hydrocarbon production, thereby avoiding the need for costly and complex turret assemblies and/or dynamic positioning systems (DP). In the particular example shown in FIGS. 2 and 7, the mooring lines are distributed within three symmetrically arranged recesses 7 carved into the circumventing suppressing element 6.

(21) FIG. 4 shows a cross section of the hull 2 in a plane oriented along the vessel's breadth and within the vessel's 1 midship. The particular view visualizes the example tank arrangement including five cargo tanks 100a abreast, protected by double side ballast tanks 101 and a double bottom. A slop tank 100b is illustrated above the mid cargo tank 100a. The double bottom may be used for confining both ballast tanks 101 and void tank(s) 104 as illustrated in FIG. 4. The required slop capacity is typically 3% of the cargo carrying capacity. Hence, the slop tanks 100b are small compared to the cargo tanks 100a. For venting, access and operational purposes it is beneficiary to have access to the slop tanks 100b from main deck D. The slop tanks 100b are therefore typically located towards deck within the volume of the center cargo tanks 100a.

(22) FIG. 5 shows a section view through the longitudinally directed center plane CP of the vessel 1, illustrating the non-cargo zones and the cargo zone, as well as the tank distribution, in the vessel's 1 longitudinal direction. The figure also clearly shows that the living quarter A is not located above any cargo or slop tanks 100a,100b.

(23) FIG. 6 shows a sectional view through the longitudinally directed center plane CP of a second embodiment of the inventive vessel 1. In this embodiment the living quarter A is located at the stern 4 of the vessel 1, an embodiment that may be preferable in case the prevailing wind direction is opposite the direction of maximum wave height to which the bow 3 is facing. From a safety point of view, it is generally a preference to have the living quarter A upwind from the processing plant and flare region FR. FIG. 6 also shows a design in which the suppressing element 6 at the keel is extended compared to the embodiment shown in FIGS. 1-5 to further increase the natural period and dampen the motions. As for FIG. 5, the locations of the non-cargo zones and the cargo zone is illustrated in the vessel's 1 longitudinal direction.

(24) With the above-mentioned design, and within the constrains of an existing/standard yard- and construction facility, the inventive FPSO may obtain a storage capacity in excess of 2,000,000 bbls.

(25) FIGS. 8 a) and c) shows calculated heave RAO's (Response Amplitude Operator) and roll and pitch RAO's for the present invention, respectively, while FIGS. 8 b) and d) show the corresponding calculated RAO's for a conventional ship-shaped FPSO design. The axis scale is the same for the two concepts to enable direct comparison. As seen in FIGS. 8 a) and c) the motion behavior in beam and head see is practically uniform for the present invention, as compared to the ship-shaped design in FIGS. 8 b) and d). A comparison between FIG. 8 a) and FIG. 8 b) also shows that the natural period in heave is significantly higher for the present invention (about 16.6 s) then for the conventional ship (about 11 s). Further, it is apparent from these figures that there will be close to no response for wave periods less than 10 seconds for the inventive vessel, while the conventional ship-shaped design will experience heave motion at waves starting from 5 seconds.

(26) As clearly seen by comparing FIG. 8 c) with FIG. 8 d), the difference is even greater when it comes to roll and pitch motions. At an example wave period of 12 seconds the conventional ship-shaped vessel will experience pitch angles in head seas that are more than 3 times those seen for the inventive vessel and roll angles in beam seas of more than 10 times that of the inventive vessel.

(27) The presented calculations are for a Suezmax tanker of about 1,000,000 bbl storage capacity, where 1 bbl equals about 159 litres. The following input values have been used in the calculations:

(28) TABLE-US-00001 Inventive Typical conventional Hull dimension vessel tanker Length, Lwl [m] 93 250 Breadth, Bwl [m] 68 45 Draught [m] 26.5 16 Displacement [ton] 155,000 155,000 Extension of surpressing 6 element (bilge box) [m]

(29) The calculations of the RAO curves are made for motion responses in regular waves using potential theory, including corrections for viscous forces using Morison elements. Computer program used for the analyses is WADAM from DNV-GL. Calculations for larger and smaller size vessels show the same behavioral pattern.

(30) For the inventive vessel 1, the pitch and roll motions (FIG. 8 (c)) are very small compared to the heave motions (FIG. 8 (a)). Hence, the vertical motion at any given point will be governed by heave motions. This gives a vessel 1 with almost uniform vertical motion and acceleration across the length and breadth, regardless of wave heading, which in turn gives flexibility in location and/or orientation of topside equipment and allows riser-hang-off at any position on the vessel 1. That is, riser hang-off forward, at side, aft or along the centerline of the vessel 1. The risers will typically be free hanging, e.g. from the main deck or pulled in through guide pipes 8 in the double side hull and hung off at main deck elevation D. FIG. 3 shows example location of the riser guide tubes 8 arranged at the aft and at the port side of the bow on port side. The number of riser guide tubes 8 shown in the figures is for example only. The present invention may allow use of up to 60 risers if deemed necessary.

(31) In the preceding description, various aspects of the vessel according to the invention have been described with reference to the illustrative embodiment. For purposes of explanation, specific numbers, systems and configurations were set forth in order to provide a thorough understanding of the vessel and its workings. However, this description is not intended to be construed in a limiting sense. Various modifications and variations of the illustrative embodiment, as well as other embodiments of the vessel, which are apparent to persons skilled in the art to which the disclosed subject matter pertains, are deemed to lie within the scope of the present invention.

LIST OF REFERENCE NUMERALS/LETTERS

(32) 1 vessel 2 hull 2a port side portion 2b starboard side portion 3 bow 4 stern 5 deck/deck structure 6 suppressing element/bilge box 7 recess 8 riser guide pipes 9 mooring winch 10 bottom of the hull 100a cargo tanks 100b slop tanks 101 ballast tanks 102 fuel tank/MDO tank 103 pump room 104 void tank A living quarters L.sub.wl maximum hull length in waterline B.sub.wl maximum hull breadth in waterline CP center plane D main deck F fore deck FR flared region from waterline to process deck BA bow angle M mooring arrangement P processing deck S safety division w(l) water level of the vessel at its maximum draught