Patent Application
20070292243
The present application relates to apparatus suitable for effecting the loading and offloading of fluids, such as liquefied natural gas.
Various methods have been developed to make offshore loading and offloading of liquefied natural gas (LNG) a practical proposition. Such a method is known from OTC paper 15301 (prepared for presentation at the 2003 Offshore Technology Conference held in Houston, Tex., U.S.A., 5-8 May 2003, and entitled “Offshore Transfer Re-Gasification and Salt Dome Storage of LNG”). This paper describes a single-point mooring system suitable for the mooring of an LNG carrier vessel. The paper further describes apparatus for discharging the LNG through the existing mid-ships manifold. The discharging apparatus comprises a rotatably mounted rigid arm which carries a standard fluid transfer system. The fluid transfer system consists of three pipe-in-pipe (PIP) lines; two of the lines are dedicated to the transfer of LNG and the third line is dedicated to vapour return. The fluid transfer system is hingedly coupled to the rigid arm to allow the arm to weathervane and pitch to accommodate changes in movement between the vessel and platform.
When offloading from the mid-ship manifold of a vessel moored at a single-point mooring structure, the structure may surge backwards and forwards under the influence of wave and wind loads. To accommodate this movement, the rigid arm of the transfer apparatus described in the above-referenced OTC paper would require a large reach capable of covering the full range of surge motions of the vessel, including those which may occur after failure of a mooring hawser and any further movements which take place in the time it takes to shut transfer valves before a physical disconnect can be performed.
With certain types of loading equipments, such as steel loading arms or flexible hoses, these large surge motions result in large angular offsets between the extremities of such loading arms and hoses and this poses large strains on their constituent components.
Moreover, since physical spacing between loading arms and hoses is limited, a certain “shadowing” occurs between the arms and hoses and hence, during a disconnect operation, clashing between mechanical components becomes unavoidable.
The present application, at least in preferred embodiments, attempts to address at least some of the problems outlined above.
Viewed from a first aspect, the present application relates to an apparatus suitable for the offshore transfer of fluids, the apparatus comprising a plurality of rotatable members, and a fluid transfer system suitable for connecting the apparatus to a fluid source or a fluid store; the apparatus being mountable on a base and said fluid transfer system being mounted on or coupled to one of said rotatable members; wherein, in use, the rotation of the rotatable members relative to each other is controlled so as to maintain the fluid transfer system in a pre-determined orientation relative to the base.
The apparatus of the present invention advantageously creates a stable platform on which the fluid transfer system may be mounted. The fluid transfer system may comprise hard loading arms or flexible hoses capable of spanning the entire range of surge motions such that the loading arms or hoses themselves can be operated with virtually zero angular offset during large surge motions. Clearly, this arrangement provides improved functionality and allows the fluid transfer system to operate without clashes occurring during the critical phases of any disconnect operation.
The rotatable members are preferably each rotatable about a respective axis. The axes are preferably parallel and, in use, the fluid transfer system preferably undergoes translation in a plane substantially perpendicular to said axes.
The apparatus preferably comprises first, second and third rotatable members. The second rotatable member is preferably mounted on the first rotatable member; and the third rotatable member is preferably mounted on the second rotatable member. The first rotatable member is preferably rotatably mounted on the base which may in turn be fixedly mounted, for example on a single point mooring structure. The base may in fact form part of the single point mooring structure. The fluid transfer system is preferably mounted on the third rotatable member. Of course, the apparatus may comprise 2, 4, 5, 6 or more rotatable members in alternative embodiments.
Preferably, the first member is rotatable about a first axis, the second member rotatable about a second axis and the third member rotatable about a third axis. The first and second axes are preferably offset from each other; and the second and third axes are preferably also offset from each other.
The apparatus thereby allows the fluid transfer system to undergo translation relative to the base and may, therefore, accommodate relative movement between a vessel and the fluid transfer system at least in one plane.
The first, second and third axes are preferably all arranged substantially vertically to facilitate translation of the fluid transfer system in a substantially horizontal plane.
The apparatus is preferably configured such that when the first element is rotated about the first axis through an angle of α°, the second member is rotated about a second axis through an angle of 2α° (i.e. the second member undergoes angular rotation twice that of the first member). The first member is preferably rotated in the first direction and the second member in a second direction, the first and second directions being opposite to each other.
The apparatus is preferably further configured such that, when the first element is rotated about said first axis through an angle of α°, the third member is also rotated through an angle of α° (i.e. the angular rotation of the first and third members is the same). The first and third members preferably rotate in the same direction.
This configuration of the first, second and third rotatable members may allow the third member, and consequently the fluid transfer system, to be maintained in a pre-determined orientation relative to the base, even when the first and second members are rotated.
The fluid transfer system is preferably translated along a straight line whilst the pre-determined orientation thereof is maintained. The distance between the first axis and the second axis is preferably the same as the distance between the second axis and the third axis to enable the fluid transfer system readily to be translated along a straight line. It is, of course, possible to implement translation along a straight line in arrangements where these distances are different, provided different angular rotations of the first, second and third rotatable members are implemented. The present application is intended also to cover these alternative arrangements.
The first, second and/or third rotatable members may be rotated by first, second and/or third actuators (such as electric motors or hydraulic cylinders) respectively. Preferably, however, the first member is rotated by an actuator and first and second linkage assemblies affect rotation of the second and third rotatable members respectively.
The apparatus may further include means for determining the angular rotation of each rotatable member, to allow for control of the apparatus.
The fluid system transfer may comprise at least one conduit supported on first and second hard (i.e. non-flexible) support arms. Alternatively, the fluid transfer system may comprise at least one flexible hose.
A conduit is preferably provided which defines a fluid pathway for the transfer of fluids through said rotatable members. Fluids to be transferred using the apparatus of the present invention may then be passed from the fluid transfer system through the interior of the rotatable members. Alternatively, one or more conduits may be provided to the exterior of the rotatable members.
The base on which the first rotatable member is rotatably mounted may be, for example, a column. The column may be provided on a single point mooring structure, or on a LNG carrier vessel.
Two preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:
A single point mooring (SPM) structure 1 of the type described in OTC paper 15301 is shown by way of example in
As shown in
The first rotatable member 3 is rotated by an electric motor. It will, of course, be appreciated that other actuation means, such as a hydraulic cylinder, may be implemented to effect rotation of the first rotatable member 3. The transfer apparatus further comprises a linkage bar system 15 for effecting angular rotation of the second and third rotable members 4, 5. The implementation of a linkage bar system 15 advantageously removes the need to provide additional actuation means and may thereby reduce the cost of the transfer apparatus.
The first rotatable member 3 is rotated about a first axis A; the second rotatable member 4 is rotated about a second axis B; and the third rotatable member 5 is rotated about a third axis C. The first, second and third axis A, B, C are arranged substantially vertically such that the first, second and third rotatable members 3, 4, 5 each rotate in respective substantially horizontal planes. The distance between the first axis A and the second axis B is the same as the distance between the second axis B and the third axis C.
The linkage bar assembly 15 is such that when the first rotatable member 3 undergoes an angular rotation of α° relative to the column 2, the second rotatable member 4 undergoes an angular rotation of 2α° relative to the first rotatable member 3. For example, if the first rotatable member 3 is rotated through 30° relative to a reference plane, the second rotatable member 4 undergoes an angular rotation of 60° relative to the first rotatable member 3.
The linkage bar system 15 is arranged such that rotation of the first rotatable member 3 through an angle of α° results in a corresponding rotation of the third rotatable member 5 through an angle of α° relative to the second rotatable member 4.
The linkage bar assembly 15 is arranged such that the first and third rotatable members 3, 5 rotate in the same direction, whereas the second rotatable member 4 rotates in the opposite direction. Thus, in the arrangement shown in
The net result of the arrangement of the first, second and third rotatable members 3, 4, 5 is that the fluid transfer system 6 undergoes translations along a horizontal axis. Advantageously, the transfer apparatus is arranged such that the fluid transfer system may undergo translation along a longitudinal axis parallel to an adjacent edge of the SPM structure 1. Thus, the fluid transfer system 6 may translate in the surge direction of the vessel carrying the liquefied natural gas without getting closer or further away from the manifold provided on the vessel.
It will be appreciated that the arrangement of the transfer apparatus is such that the orientation of the fluid transfer system 6 relative to the SPM structure 1 is maintained constant as it undergoes translation. This functionality is especially desirable as the stresses applied to the supporting arm 11-14 may be reduced as the fluid transfer system 6 may be maintained in an orientation perpendicular to the vessel.
Moreover, the longitudinal position of the fluid transfer system 6 may be maintained constant relative to the LNG carrier vessel by effecting appropriate rotation of the first, second and third rotatable members 3, 4, 5. This may further reduce or minimise side-loads on the fluid transfer system 6.
The support arms 11-14 are provided with an elbow joint to accommodate transverse movement of the LNG carrier vessel relative to the SPM structure 1.
As shown in
In order to monitor the motions of the LNG carrier vessel carrying the liquefied natural gas, an optical or other tracking system may be provided. This tracking system preferably provides continuous information as to the location of the vessel relative to the SPM structure 1 and allows the angular orientation of the first, second and third rotatable members 3, 4, 5 to be controlled to maintain the longitudinal position of the fluid transfer system 6 constant relative to the vessel.
A second embodiment of the transfer apparatus is shown in
To allow the catenary hoses to be suspended under the transfer apparatus, the third rotatable member 5 is rotatably mounted on the underside of the second rotatable member 4, which in turn is mounted on the underside of the first rotatable member 3.
The third rotatable member 5 is also provided with a support mechanism 18 for carrying the fluid transfer system 6.
The angular control of the first, second and third rotatable members 3, 4, 5 is generally the same as for the first embodiment such that the fluid transfer system 6 may translate along an axis parallel to the longitudinal axis of the SPM structure 1.
The support member 18 advantageously allows for transverse movement of the vessel relative to the SPM structure 1 in the same way as this motion was accommodated by the hard loading arms 11-14 of the first embodiment.
It will be noted that in the second embodiment, the distance between the first and second axis A, B is not the same as the distance between the second and third axis B, C. The relative angular rotation of the first, second and third members 3, 4, 5 is modified accordingly to ensure that the fluid transfer system 6 is maintained in a pre-determined orientation as it undergoes translation.
In certain cases, the assembly of the first, second and third rotatable members 3, 4, 5 is more optimally located well above the water level, for example to allow sufficient space for catenary type hoses, as shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 0425654.1 | Nov 2004 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB05/03748 | 11/22/2005 | WO | 5/22/2007 |
