Transfer structure, a transfer system and a method for transferring LNG and/or electric power
10532796 · 2020-01-14
Assignee
Inventors
- David Mikal Knutsen (Oslo, NO)
- Stian Tunestveit Magnusson (Bønes, NO)
- Magnus Eikens (Oslo, NO)
- Andreas Norberg (Vestskogen, NO)
- Kjetil Sjølie Strand (Lier, NO)
Cpc classification
International classification
Abstract
A transfer structure for transfer of a fluid between a floating structure and a floating or non-floating facility and/or transmission of electric power between the floating or non-floating facility and the floating structure is disclosed. The transfer structure includes at least one attachment means passive-movably mounted relative to the transfer structure for releasable attachment of the transfer structure to the floating structure. The transfer structure is semi-submersible and floating.
Claims
1. A transfer structure for transfer of a fluid, through at least one transfer line and at least one aerial transfer line, between a non-weathervaning floating structure and a facility or transmission of electric power between the facility and the floating structure, the transfer structure comprising: a connecting device to which the at least one aerial transfer line can be releasably connected, the connecting device being adapted for connection to the at least one transfer line; and at least one attachment means mounted directly to the transfer structure for releasable attachment of the transfer structure to the floating structure, wherein: the at least one transfer line extends between the transfer structure and the facility during transfer of the fluid and the at least one aerial transfer line extends between the transfer structure and the floating structure during transfer of the fluid; the transfer structure is semi-submersible and floating; the transfer structure is configured to absorb tensional forces in the at least one transfer line arising from environmental loads acting on the at least one transfer line and distribute the tensional forces through the at least one attachment means to a hull of the floating structure to which the at least one attachment means is attached during transfer of the fluid; and the at least one attachment means is passive-movably mounted relative to the transfer structure, the at least one attachment means is adapted to allow the transfer structure to move substantially freely vertically and to rotate substantially freely about a horizontal axis relative to the floating structure, and the at least one attachment means is further adapted to passively substantially restrain relative horizontal translation and relative rotation about a vertical axis between the floating structure and the transfer structure.
2. The transfer structure according to claim 1, wherein the transfer structure is adapted for relocation and positioning by an assistance vessel.
3. The transfer structure according to claim 1, wherein the transfer structure is provided with berth means and anchoring means for a vessel.
4. The transfer structure according to claim 1, further comprising a top-side deck and a plurality of surface piercing columns each having a diameter, or a characteristic diameter, the surface piercing columns being separated by a distance which is at least four times as large as the diameter or the characteristic diameter.
5. The transfer structure according to claim 1, further comprising a top-side deck and a plurality of surface piercing columns, the surface piercing columns being provided with respective telescopic elements at lower end portions of the surface piercing columns, each of the telescopic elements being movable between an upper position and a lower position such that respective longitudinal lengths of the surface piercing columns are adjustable.
6. The transfer structure according to claim 5, wherein at least one of the surface piercing columns is provided with a storage room for a fluid, the storage room being delimited by the at least one of the surface piercing columns and the telescoping element of the at least one of the surface piercing columns such that a volume of the storage room is variable and depends on a vertical position of the telescopic element of the at least one of the surface piercing columns relative to the at least one of the surface piercing columns.
7. The transfer structure according to claim 6, wherein the storage room is provided with at least one opening or valve to the surroundings such that water can flow in and out of the storage room.
8. The transfer structure according to claim 1, wherein the transfer structure has a maximum draft in still water which is less than 5 meters.
9. The transfer structure according to claim 1, wherein the connecting device is a manifold to which the at least one transfer line and the at least one aerial transfer line can be connected for the transfer of fluid between the floating structure and the facility, or the connecting device is an electrical coupling device to which the at least one transfer line and the at least one aerial transfer line can be connected for the transmission of electrical power between the facility and the floating structure.
10. The transfer structure according to claim 1, wherein the transfer structure is a shallow-water transfer structure.
11. The transfer structure according to claim 1, wherein the transfer structure is non-motorized.
12. The transfer structure according to claim 1, wherein the facility is floating.
13. The transfer structure according to claim 1, wherein the facility is non-floating.
14. The transfer structure according to claim 1, wherein: the at least one transfer line extends from a lower side of the transfer structure; the at least one aerial transfer line extends from an upper side of the transfer structure; and the lower side of the transfer structure is opposite to the upper side of the transfer structure.
15. A transfer system for transfer of a fluid between a floating structure and a facility or transmission of electric power between the facility and the floating structure, the transfer system comprising a transfer structure according to claim 1, at least one transfer line, and a storage means for storing the at least one transfer line when the transfer system is not in use, the at least one transfer line extending between the transfer structure and the storage means, and the at least one transfer line being connected to: a storage means for the fluid which has been transferred from the floating structure or which is being transferred to the floating structure, or a pipeline for the fluid which has been transferred from the floating structure or which is being transferred to the floating structure, or a source of electric power for the transmission of electric power to or from the floating structure.
16. The transfer system according to claim 15, further comprising a docking facility for storing the transfer structure when the transfer structure is not in use.
17. The transfer system according to claim 16, further comprising an assistance vessel for relocating the transfer structure between the docking facility and the floating structure and for control of the transfer structure during attachment to or detachment from the floating structure.
18. The transfer system according to claim 15, wherein the at least one transfer line is flexible and the storage means for storing the at least one transfer line comprises at least one reel or turntable or basket on which the at least one transfer line is wound when the transfer system is not in use.
19. The transfer system according to claim 15, wherein the storage means for storing the at least one transfer line comprises a plurality of rollers on which the at least one transfer line may be pulled back to a storage position without being wound when the transfer system is not in use.
20. The transfer system according to claim 15, wherein the storing means for storing the at least one transfer line is located onshore, on a non-floating structure or on a floating structure.
21. The transfer system according to claim 15, wherein the facility is floating.
22. The transfer system according to claim 15, wherein the facility is non-floating.
23. A method of transferring a fluid between a floating structure and a facility or transmitting electric power between the facility and the floating structure, the method comprising: mooring the floating structure to a multi-buoy mooring system such that the floating structure is non-weathervaning, relocating a transfer structure according to claim 1 from a docking facility to the moored floating structure, and subsequently or simultaneously paying out a transfer line through which the fluid is to be transferred or the electric power is to be transmitted, releasably attaching the transfer structure to an outer surface of the floating structure with passive-movable attachment means mounted on the transfer structure, providing at least one aerial transfer line between the floating structure and the transfer structure such that the fluid can be transferred between the floating structure and the facility or such that the electric power can be transmitted between the facility and the floating structure, and transferring the fluid or transmitting the electric power through the transfer line and the at least one aerial transfer line connecting the floating structure and the facility.
24. The method according to claim 23, wherein an assistance vessel is used to relocate the transfer structure between the docking facility and the moored floating structure and to position the transfer structure before attachment to or detachment from the floating structure.
25. The method according to claim 23, wherein the transfer line is stored on at least one reel or turntable or basket when the transfer structure is not in use.
26. The method according to claim 23, wherein the transfer structure is stored in or is made fast to the docking facility when the transfer structure is not in use.
27. The method according to claim 23, wherein the facility is floating.
28. The method according to claim 23, wherein the facility is non-floating.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) Various advantages of the invention will become apparent to those skilled in the art from the following detailed description of a non-limiting embodiment of the present invention, when read in light of the accompanying drawings, wherein:
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DETAILED DESCRIPTION OF THE INVENTION
(14) Reference is made to
(15) The system further comprises a floating, semi-submersible transfer structure 2 which is shown beside the moored floating structure 1 in
(16) At least one aerial transfer line 3 is provided between the transfer structure 2 and the floating structure 1. The aerial transfer line 3 may be stored on the transfer structure 2 between transfer operations and connected to the floating structure 1 when a transfer operation is to take place. Alternatively, the aerial transfer line 3 may be stored on the floating structure and connected to the transfer structure 2 when a transfer operation is to take place. After the transfer operation, the aerial transfer line 3 may be disconnected again and stored on the transfer structure 2 or the floating structure 1. The aerial transfer line 3 enables transfer of a fluid or electric power between the floating structure 1 and the transfer structure 2. In the figures, the facility is shown as an onshore facility for transfer of a fluid, for example, a cryogenic fluid like LNG, between the floating structure and the onshore facility 6. The facility may, however, be a floating facility as shown in
(17) The aerial hoses 3 will usually be held in an S shape, for example, by the utilization of a crane 13 on the floating structure 1, as illustrated in
(18) For transfer of fluid, the transfer system further comprises at least one transfer line 4 in form of a floating, flexible pipe for transfer of fluid between the transfer structure 2 and the floating or non-floating facility 6 shown in the figures. For transfer of electric power, the transfer line 4 and the aerial transfer line are made up of at least one electrical cable or at least comprise an electrical cable.
(19) As can be seen in
(20) As mentioned, the facilitation of fluid transfer between the transfer structure 2 and the storage facility 6 is preferably achieved through at least one floating pipeline 4. The length of the at least one floating pipeline 4 is sufficient to allow the dynamic motion of the floating structure 1 during transfer operation. The at least one floating pipeline 4 may conveniently be stored on a reel or turntable 5 on shore, or on the transfer structure 2 between loading operations, hence reducing the obstruction and potential risk of collision with local sea traffic, increasing fatigue life and simplifying inspection and control of the pipeline. The floating pipeline(s) 4 may be specifically designed for the transfer of temperate or cryogenic fluids or both, and may or may not comprise buoyancy elements, isolation, bending stiffeners 28 and/or opportunity for optical and/or electric transmission.
(21) The storage arrangement 5 may also be linearly or otherwise conveniently arranged, but is generally characterized in that a large fraction of the floating pipeline(s) 4 may be conveniently retrieved from the water and temporarily stored in a designated location. As shown in
(22) Since the transfer structure 2 is unmoored and connected to the floating structure 1 during transfer operations, the mooring system 42 for the floating structure 1 must be arranged in such a way that it restricts the lateral motions of the floating structure 1 within the limits of lateral reach of the floating pipelines 4. Single point mooring with weathervaning is therefore not an option. The transfer system therefore preferably comprises a multi-buoy mooring system 42 which will prevent weathervaning, and hence protect the integrity of the floating pipelines 4. The multi-buoy mooring system 42 may vary in configuration and complexity, depending on local environmental conditions, incident water depth, and the size range of floating structures to use the mooring system. The multi-buoy mooring system 42 will typically comprise appropriate anchors depending on seabed conditions, connected to surface buoys by chain or fiber rope or a combination of both.
(23) The transfer system is further provided with a connection between the floating structure 1 and the transfer structure 2 which comprises a mechanical connection arrangement with the capability of producing attractive forces to the hull of the floating structure. The capability of producing attractive forces may preferably be established by sub-atmospheric pressure, for example, by the use of vacuum pads. Other options for establishing the required attractive force may be by electromagnetic attraction, by hawsers, or by a combination of hawsers and fenders.
(24) Without the desire to be bound by theory, it is implicit that the design of the connection of two independent floating structures in any significant seaway must, in an arbitrary degree of freedom, either allow for relative motion between the two structures, or be able to cope with the forces and/or moments resulting from restricting or partially restricting the two structures from moving independently. Any reaction forces or moments must be sufficiently distributed such that the force concentrations do not compromise the structural integrity of the floating structure, the transfer structure, or the connection system itself. The motions of a moored floating structure may conveniently be separated in linear motions, governed by wave excitation, and nonlinear slow drift motions, typically governed by a combination of nonlinear wave excitation and linear wind and current excitation, where linearity and nonlinearity refer to the relationship between excitation frequency and motion frequency. While wave excited motions typically are characterized by small amplitudes and large accelerations, slow drift motions on the other hand are typically characterized by large amplitudes and small accelerations. Furthermore, wave excited motions are often dominated by vertical translation and rotation about a horizontal axis, while slow drift motions act translationally in the horizontal plane and rotationally about a vertical axis. Since the amplitude of the reaction forces and moments related to the restriction of relative motion will be proportional with the relative accelerations, the forces and moments will be largest along degrees of freedom dominated by wave excitation, and since the two floating structures may not drift apart during a transfer operation, the connection arrangement may conveniently substantially freely allow relative motions dominated by wave excitation, while substantially restraining degrees of freedom related to slow drift motions.
(25) In
(26) The connection arrangement may typically comprise at least two attachment units 18 placed on the transfer structure 2. Each of the attachment units comprises at least one attachment means, for example, an air or water vacuum pad 19 or electromagnetic pad, for releasable attachment to a substantially vertical side of the floating structure 1, for example, to the shipside if the floating structure 1 is a ship. The connection arrangement comprising the pads 19 is preferably directly attached to the transfer structure 2 with connection means which allows the required relative movements between the transfer structure and the floating structure 1. The pad or pads 19 are mechanically connected to the transfer structure 2 through a beneficial combination of ball and/or disk joints 22 and linear-motion bearings 21 with integrated spring elements and/or damping elements. Each of the pads has opportunity for motion in six degrees of freedom relative to the transfer structure, wherein motion in degrees of freedom X, Y and RZ, as indicated by reference numbers 30, 31 and 35, respectively, in
(27) The connection arrangement permits the relative motions in the degrees of freedom with larger relative accelerations between the floating structure 1 and the transfer structure 2 from linear wave excitation, while restricting the smaller accelerations in the lateral plane due to slow drift motions, hence reducing arising connection forces and moments to a manageable level. The free vertical relative motion also allows a certain draft change of the floating structure 1 in case it is loading or offloading cargo. The connection arrangement is permanently installed on the transfer structure 2. It should be emphasized that the connection arrangement described above entails the at least one attachment means being mounted to the transfer structure 2 passive-movably relative to the transfer structure 2 meaning that the at least one attachment means will only move in one or more of its allowed degrees of freedom when external forces and/or moments are acting on the at least one attachment means.
(28) As mentioned above, the transfer system also comprises a transfer structure 2. The transfer structure 2 preferably has a design of a floating structure with several advantageous properties related to the specific purpose of serving as a transfer structure. The following section will briefly discuss the preferred requirements related to the performance and properties of the transfer structure 2.
(29) The partially restricted connection of the two independent floating structures, the floating structure 1 and the transfer structure 2, becomes increasingly difficult with larger relative motions. Large relative motions will complicate the connection operation, contribute to increased fatigue to the end terminations of transfer ducts, and possibly reduce personnel safety and comfort. Since the motions of the floating structure are predefined and cannot be changed, it is important that the motions of the transfer structure are small in the design seaway, for example, less than 0.5 meter heave motion amplitude and less than 5 degree rotational motion amplitude. The transfer operation normally takes place in a reasonably sheltered location, with a significant wave height of, for example, less than 1 meter and a seaway energy spectral peak period of, for example, less than 5 seconds, wherein significant wave height means the statistical mean of the through to crest height of the highest one-third of the waves in the seaway. Since the transfer operation typically takes place near the shoreline, and since it might be beneficial to move the transfer structure closer to shore between transfer operations to reduce the obstruction of local sea traffic, the incident water depth will in most cases pose restrictions on the draft of the transfer structure. The transfer structure must furthermore have sufficient stability to withstand all foreseeable heeling moments. While connected to the floating structure, the transfer structure will encounter heeling moments due to the connection arrangement itself, water drag forces from mean relative water speeds, due to tension in the floating pipelines, in addition to personnel and equipment. The platform might also encounter heeling moments during transit from shore to ship. Hence the water resistance must be small, and the vertical distance from the point of attachment to the floating structure, the water resistance resultant force of the submerged structure, which is related to the draft of the structure, must be small. Additionally, from a cost perspective, keeping the weight to a minimum is important. Thus, the main objective with the design of the transfer structure is to provide a platform with minimal wave excited motions, keeping drag resistance minimal, without considerably compromising stability, low weight or small draft.
(30) From a hydrodynamic and physical point of view this is problematic, since previously mentioned parameters are profoundly dependent on each other. The water particle motion and dynamic pressure field under a wave declines exponentially downward in the water column, and hence the local wave excitation of a floating object also declines with depth. Thus, the wave-induced response of a floating structure generally decreases with increasing draft. From hydrodynamic theory, it is known that small linear wave induced motion response of a floating object is achieved by arranging submerged geometry, structure weight and distribution of weight, in such a way that its natural frequencies of motion lie well outside the interval of wave frequencies with dominating energy in the considered seaway. For a freely floating object, this may effectively be achieved in heave by reducing the waterplane area, and in roll/pitch by reducing transverse/longitudinal stability sufficiently. Other parameters fixed, all natural frequencies of a floating object decrease with decreasing weight. Hence, small first order motions are generally achieved at the expense of stability, weight, draft, or a combination of the above. The present design has been created for the sole purpose of optimally satisfying the above-mentioned criteria for the present purpose.
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(32) As shown in
(33) The transfer structure 2, comprising a top-side deck 15 and a plurality of surface piercing columns 16, may have columns 16 which are provided with respective telescopic elements, for example, extrusions, at their lower end portions, the telescopic elements being movable between an upper position and a lower position such that the columns' respective longitudinal lengths are adjustable. At least one submerged extrusion 17 is shown in
(34) The columns may be provided with a storage room for a fluid, for example, in the form of the void space of the submerged extrusion 17 of the transfer structure column, each storage room being delimited by their respective columns and telescopic elements such that the storage room's volume is variable and depends on the vertical position of the telescopic element relative to the column 16. As shown in
(35) The transfer structure 2 may or may not be motorized for expedient transit. Furthermore, the transfer structure 2 may be provided with berth means 46 and anchoring means 48, the berth means 46 including a truss structure 12 (see
(36) The particular design described herein has through extensive experimental tests proven superior properties with regard to all above-discussed requirements as compared to several previously known floating concepts.
(37) Reference numbers used in the figures: 1. First floating structure, such as an LNG carrier 2. Transfer platform 3. Aerial hose(s) 4. Floating pipeline(s) or transfer line(s) 5. Storage arrangement for transfer lines(s) 6. Floating or non-floating storage, receiving or export facility 7. Mooring buoys 8. Idle mooring system or docking facility for transfer platform 9. Guiding rollers for transfer line(s) 10. Assistance vessel 11. Hawsers for berthing of tug or workboat to the transfer platform 12. Truss structure 13. Crane to connect and support aerial hose(s) 14. First floating structure manifold 15. Transfer platform topside deck 16. Transfer platform columns 17. Submerged extrusion(s) 18. Attachment unit 19. Pads for shipside attachment 20. Spring and/or damper element 21. Linear motion bearings 22. Disk or ball joint 23. Cleats, bitts, or bollards 24. Drip tray 25. Valve 26. Flange 27. Transfer line tie in 28. Transfer line bending stiffener 29. Railing 30. X-direction of motion 31. Y-direction of motion 32. Z-direction of motion 33. Rotation about the X-axis 34. Rotation about the Y-axis 35. Rotation about the Z-axis 36. Ballast water 37. Opening or valve 38. Hydraulic rod 39. Ballast water pump 40. Rigid piping in connection with storage tanks 41. Storage arrangement for floating pipeline(s) on rollers 42. Multi-buoy mooring system 45. Water surface 46. Berth means 47. Fender(s) 48. Anchoring means 49. Connecting device