Transfer system and method for transferring a cryogenic fluid from an onshore unit to a ship by means of a buoy comprising a reel for a flexible hose and which level in the water can be changed

A cryogenic transfer system includes:    The transfer duct includes a first and a second duct, each having an end part at or near the loading and/or offloading structure, the flexible hose being with the first end connectable to the end part of at least the first or the second duct.The loading and/or offloading structure includes lifting elements.

The invention relates to a cryogenic transfer system comprising:a cryogenic fluid storage and/or processing structure,an off shore loading and/or offloading structure comprising a base and a reel means rotatable relative to said base around a vertical axis,a transfer duct extending from the fluid storage and/or processing structure to the loading and/or offloading structure,a flexible hose windable around the reel means, connectable with a first end to the duct, and with a second end connectable to a floating structure.

The invention also relates to a method of transferring a cryogenic fluid.

Such a transfer system is known from U.S. Pat. No. 5,431,589. In this patent a submersible buoy is described comprising a rotatable turntable carrying a reel with a flexible hose, and a mooring hawser. The buoy is connected to a pipeline supported on the sea bed via an articulated pipe, the pipeline extending for instance to an onshore storage and processing facility for liquefied natural gas (LNG).

The known transfer structure is used in ice-infested waters, the loading and/or offloading structure being ballasted and submerged below the water surface when not in use. By storing the hose under water when not in use, the known hose is subject to fatigue. Furthermore, after placing the buoy into its operative position above water level, the hose on the reel will have to be cooled down first before cryogenic fluids can be transported through the hose. This will take considerable time and reduce the throughput of the known transfer structure for cryogenic fluids. Furthermore, the thermally induced expansion and contraction caused by the cooling and heating up, results in a reduced service life of the cryogenic fluid ducts.

It is an object of the present invention to provide a cryogenic transfer structure and method of transfer wherein a flexible hose can be stored on the loading and/or offloading structure and can be deployed into its operative position while being subject to reduced fatigue. It is a further object of the present invention to provide a transfer structure and transfer method for cryogenic fluids which can be maintained in a cooled state when not being operative in transferring cryogenic fluids, hence resulting in an increased throughput.

Hereto a transfer system according to the present invention is characterised in thatthe transfer duct comprises a first and a second duct, each duct having an end part at or near the loading and/or offloading structure, the floating hose being with a first end connectable to the end part of at least the first or the second duct,in a cooling configuration, the flexible hose being wound on the reel means, the reel means being situated above water level and rotatable around a vertical axis, an interconnecting duct section extending between the end parts of the first and second ducts,in a transfer configuration the flexible hose being at least partly unwound from the reel means and being with a second end connectable to a floating structure,
the loading and/or offloading structure comprising lifting means for lowering the flexible hose towards water level for placing the hose in the transfer configuration and for raising the hose away from water level for placing the flexible hose in the cooling configuration.

By storing the flexible hose on the reel above water level, the hose is not subject to fatigue due to movements induced by the water and the hose can be inspected and be maintained in a dry environment. The horizontal storage configuration of the flexible hose allows for easy winding and unwinding of the flexible hose onto and from the reel.

The lifting means may comprise rollers along the circumference of the buoy, or other hose support devices. In a preferred embodiment, the reel is lowerable towards water level and raisable away from water level. During storage, the reel is raised away from water level to a dry position (for instance by deballasting in case the loading/offloading structure comprises a buoy). During winding and unwinding, the reel is close to water level (just above or below) such that the flexible hose, which preferably comprises a floating hose, is easily stored on the reel means and deployed and attached to a tanker. In case the loading/offloading structure comprises a buoy, the reel may be lowered by ballasting of the buoy with water. The length of the flexible hose may have a length of hundred of meters or more. For example, for midships LNG offloading of an LNG carrier a hose length of at least 200 meters is needed.

When the flexible hose is in its wound position on the reel and no fluids are transferred from or to the cryogenic processing and/or storage structure, the two duct sections are interconnected and cryogenic fluid is circulated from the processing and/or storage structure, via a first or main duct, to the interconnecting duct section and back through the second, or return duct, to the processing and/or storage structure. The processing and/or storage structure may be on offshore structure, but preferably is comprised of an on shore import/export facility.

The offshore loading and/or offloading structure may comprise a terminal, which in one embodiment is provided with a mooring means, such as a turntable, and attachment for mooring of a tanker via a hawser attached to the turntable.

The two ducts extending from the processing and/or storage structure to the loading and/or offloading structure, which may be a single point mooring loading/offloading terminal, may have a length of several kilometers and are preferably comprised of hard piping, having a diameter of at least 16 inches, preferably 24 inches. The ducts can be separate ducts or can be one duct placed within the other one (pipe in pipe configuration). The interconnecting duct extending between the two ducts at or near the offshore loading and/or offloading structure may be comprised of an interconnecting flexible or rigid line, but preferably is comprised of the wound up flexible hose, such that this hose remains cooled at cryogenic temperatures at all times when idle.

In one embodiment the loading and/or offloading structure comprises a ballastable buoy connected to the sea bed via anchor lines, such as a CALM buoy. Upon winding and unwinding of the hose, the buoy is ballasted such that the reel is located close to water level. In the wound position, the buoy is deballasted such that the reel is situated at a sufficient distance above water level. In another embodiment, the loading and/or offloading structure comprises a tower, resting on the sea bed, the reel being raised or lowered along the tower towards and away from sea level.

A CALM buoy having a reel rotatable around a vertical axis for storing of a flexible hydrocarbon transfer hose is known from U.S. Pat. No. 3,472,536 which is incorporated herein by reference. A method of transferring LNG to a storage tank via two transfer ducts and recirculating LNG through a closed loop consisting of the two LNG transfer ducts during idle times is known from U.S. Pat. No. 6,244,053 which is incorporated herein by reference.

The term “cryogenic temperatures” as is used herein is intended to comprise temperatures below minus 80° C.

FIG. 1shows a cryogenic transfer system1, comprising an on shore storage and/or processing station2, and an offshore terminal, in this case formed by a single point mooring buoy3. The buoy3is anchored to the sea bed5via catenary anchor legs4. A tanker6is moored to the buoy3via a hawser7, attached to a turntable8of the buoy. The turntable8is rotatable around a vertical axis10(with “vertical” as is used herein is meant a direction which includes an angle of at least 45 degrees with a horizontal direction). The tanker6is in fluid connection with the on shore station2via a flexible, floating hose12, which is attached to a first duct13, extending along the sea bed to the on shore structure2. A second duct14extends parallel to duct13, and is closed at its end part by a closure device15. A branching duct section16interconnects the ducts13,14.

During or offloading of cryogenic fluids from the tanker6, the cryogenic fluid is supplied via the flexible floating hose12to the duct13and via the branching duct16, to the duct14for transport to the on shore station2. When no cryogenic fluid is transported, the hose12is decoupled from the tanker6, and is wound on a reel means17of the buoy3, for instance by rotation of turntable8around the vertical axis10relative to the fixed base18of the buoy3.

After the hose12has been wound around the buoy3, the free end of the flexible hose12, which is detached from the tanker6may remain disconnected as shown inFIG. 2aor may be connected to the end part of the duct14, the closure device15being opened, as shown inFIG. 3a. Cryogenic fluid is then circulated under pressure (e.g. 10 bar) from the on shore station2, via return duct14, optionally through the hose12, and back via duct13to the on shore station2.

The on shore station2may comprise an LNG, LPG or nitrogen liquefaction plant, a processing plant (for water separation and purification), a power station, a storage facility or any other cryogenic structure. The cryogenic structure2may be placed on shore as is shown in the example ofFIG. 1, but may also be situated at an off shore location, resting on the sea bed on a column or tower, or floating, e.g. supported on a barge.

The main and return transfer ducts13,14may be comprised of flexible hoses but are preferably comprised of rigid ducts, provided with insulation for preventing heat transfer into the ducts. The ducts13,14may have a parallel configuration, but in order to improve their insulating properties a concentric configuration is preferred.

FIG. 2ashows a top view of a cryogenic transfer structure of similar type as shown inFIG. 1in which the same reference numerals are used to indicate similar parts. InFIG. 2athe flexible hose12is in its cooling, or idle configuration, and is wound several times around the reel means17. A first end20of the flexible hose12is connected to the end part22of the duct13. A second end23of the hose12is provided with a fluid coupling and can be attached to the tanker6. In the idle or cooling stage shown inFIG. 2a, cryogenic fluid, such as liquefied natural gas or liquefied nitrogen, is circulated from the storage and/or processing structure2, via duct14, through branching duct section16and back through the return duct13, to maintain the ducts13and14at cryogenic temperatures such as minus 160° C. at a pressure of 10 bar, at a relative low flow rate but such that any major gasification of the cryogenic fluid will not occur. The ducts13and14may have a length of between 50 m and several kilometers, and maintaining these ducts at cryogenic temperatures prevents long cooling times (e.g. 20 hours) prior to loading/offloading.

InFIG. 2bit is indicated that the flexible hose12is unwound from the reel means17by rotating the reel means around the vertical axis10in the direction of arrow A. Prior to unwinding, the hose12is lowered towards water level24, for instance by ballasting the buoy3. The second end23is coupled to piping on the tanker6. Cryogenic fluid is transferred to the hose12via the ducts13,14, or vice versa.

InFIG. 3a, in the cooling configuration, the hose12is wound on the reel17. The first end part20of the hose12is connected to the end part22of the return duct13, the second end part23of the hose12being connected to the end part22′ of the main duct14via a releasable coupling26,27. A valve28is provided in the branching duct16, which is closed in the cooling configuration shown inFIG. 3a, wherein cryogenic fluid is supplied through the main duct14, via flexible hose12wound on reel17and back via return duct13to the processing/storage structure2. The hose12is placed in the transfer configuration by releasing the couplings26,27. The part26of the coupling forms a closing end part of the duct14, which is sealed in a fluid tight manner. Valve28in the branching duct16is opened, and coupling part27is attached to tanker6. Cryogenic fluid is supplied from the structure2via the ducts13,14to the hose12into the tanker6, or vice versa.

In an alternative embodiment it is possible to omit the branching duct16shown inFIGS. 3aand3b, in which case only duct13is available for transfer of fluid between the structure2and the vessel6. When the branching duct16is omitted and only duct13is available for transfer of LNG, it is also possible to connect the coupling26of duct14with a separate hose directly to piping on the tanker6for transfer of boil-off gas to station2. Depending on the LNG loading and/or offloading capacities needed, it is possible to use multiple interconnected transfer ducts13,14and multiple flexible hoses12for the cryogenic transfer system, resulting in one or more closed loops in a cooling configuration.

InFIG. 4a ballastable buoy30is shown in the cooling configuration, in which the hose12is wound on the reel means17above water level24. The buoy30is ballastable. A chain table18is connected to the sea bed via anchor chains4, whereas an annular buoy body31can rotate around the vertical axis10relative to the chain table18, driven by motor drive32.

In the embodiment ofFIG. 5, a ballastable buoy30is shown, the hose12being wound around the reel means17which is connected to a rotatable turntable35. The turntable is rotated by the motor drive32with respect to the fixed buoy body18.

In the embodiment ofFIG. 6, the reel means17is fixedly attached to the buoy body18. The first and second ends20,23of the hose12are connected to turntable35which is driven in rotation by motor drive32.

In the embodiment ofFIG. 7positioning of the hose12above water level24is not achieved by deballasting of the buoy30, but by rotating the reel means17attached to turntable35. The hose is guided over a plurality of rollers36extending transversely along the buoy body, in an upward path extending from below water level24upwards to the reel means17. Upon rotation of the turntable35, the floating hose is pulled in around the reel17over the rollers36, which can freely rotate around their longitudinal axes.

In the embodiment ofFIG. 8, a tower40is shown in which the ducts13,14extend internally inside the column41, resting on the sea bed5. The reel means17and the hose12are supported on a support frame42extending around the column, which frame can be raised and lowered along the column41via lifting device43.

In the embodiment ofFIG. 9, the support frame42is provided with ballast tanks44which can be filled with water or emptied by pressurised air to lower or raise the support frame42.

FIG. 10shows a cross-sectional view of ballastable buoy30according to the invention, with the chain table18, on which a central core54is supported. Rotatable around the core54an annular body60is supported by axial-radial bearings53and axial bearings61. The ducts13,14extend through the central core54to a manifold55, from which ducts connect to radial conduits56,57. The Flexible hose12is supported in a number of concentric loops in a horizontal plane on the reel means17. Via a pump and valve assembly58, water can be introduced into ballast compartments59of the buoy30.FIG. 11shows the buoy30ofFIG. 10in the cooling position, in which no water is present in the ballast compartments59, and the hose12is supported in a dry position above water level, wound in a horizontal plane around the annular body60. Cryogenic fluid is circulated through the ducts13,14and through the hose12. Prior to unwinding the hose from the reel means17, the ballast tanks59are filled by operating pump and valve assembly58and by introducing water into the tanks59such that the hose12is submerged below water level24, as is shown inFIG. 12.FIG. 13finally shows the hose12being placed into its transfer configuration, by detaching the couplings26,27, the radial conduit56being closed by closure device26, and unwinding the hose12, the coupling27being attached to a tanker. Cryogenic fluid is supplied via duct13, radial conduit57and the unwound floating flexible hose12.