This invention relates to the loading and unloading of ocean-going vessels transporting liquefied natural gas (LNG).
Natural gas is a highly desirable fuel for domestic and industrial use because of the environmental advantage of its clean burning. The internal availability of natural gas resources frequently falls far short of demand in larger developed and developing industrial countries, while certain less developed countries have reserves of natural gas which far exceeds their internal requirements. This situation has created a significant demand for ocean shipment of natural gas from countries with excess reserves to countries with excess demand, particularly when these countries have access to deep water port facilities.
In order for natural gas to be economically shipped in oceangoing vessels, it typically needs to be liquefied from its naturally occurring gaseous state by refrigeration to a temperature at or near its atmospheric boiling point of approximately minus 260xc2x0 F. (minus 160xc2x0 C.). Such liquefied natural gas is commonly referred to as xe2x80x9cLNGxe2x80x9d. Liquifaction and storage facilities for LNG are traditionally land-based and near to a deep water loading port from which the LNG may be exported. In a similar way receiving facilities for LNG typically include land-based LNG storage and regasification facilities installed near to a deep water receiving port. LNG receiving facilities typically regasify the LNG on site in order that the resulting natural gas may be distributed or utilized directly as a gaseous fuel. Over time the use of such land-based facilities has become increasingly problematical for reasons relating to public safety and cost, and potential use of floating offshore facilities for serving some or all of these functions has received increased attention. Floating facilities for the transfer, storage, liquifaction and/or regasification of LNG present significant design, construction and operating challenges. Some of these challenges, such as LNG containment and piping and pumping systems, are comparable to those of oceangoing LNG vessels and can be resolved utilizing available technologies. Similarly, liquifaction and regasification equipment installed on floating facilities is comparable to that used in land-based facilities, and a floating facility primarily needs to assure that appropriate space is provided for such equipment.
However, the safe open seas transfer of LNG between oceangoing vessels and floating storage facilities presents a unique challenge. Wind and waves can create severe irregular relative motions while the two are moored to each other, and such motions can impede or even prevent the safe transfer of LNG.
Furthermore, a floating LNG storage facility by definition must provide for transfer of either LNG or natural gas (in the case of liquifaction or regasification capability on board the floating facility) to or from land-based storage and piping systems.
Finally compared to shore-based facilities, floating facilities entail a further requirement for ongoing underwater maintenance, including periodic dry docking. The present invention addresses these unique requirements in a highly economical manner and furthermore accommodates installation of a variety of LNG tank containment and pumping systems traditionally installed on oceangoing LNG vessels.
An aspect of this invention is a floating transfer facility for safe transfer of LNG to or from an LNG ship or other ocean-going vessel, such as a barge.
Another aspect of this invention is a floating transfer facility for the safe transfer of LNG or natural gas to or from a land-based piping system.
Another aspect of this invention is the installation of insulated LNG tanks for the temporary storage of LNG.
Another aspect of this invention is a modular floating transfer facility which can continue in operation during the temporary removal of one of its modules for maintenance and repair.
Another aspect of this invention is a modular floating transfer facility which can carry out below-waterline maintenance and repair of at least one module by temporarily removing it, placing it in a well formed by the remaining modules, and lifting it clear of the water for maintenance and repair.
The invention is a semi-submersible floating LNG transfer facility which has some or all of the following capabilities: to (1) load or discharge oceangoing LNG tank vessels; (2) retain LNG on board the facility within specially built and insulated LNG storage tanks; (3) transfer LNG from a ship to on-board tanks, from on-board tanks to a ship, from a ship directly to underwater piping systems connecting to shore-side tanks, and/or from shore side piping systems to a ship; (4) when fitted with liquifaction equipment to receive natural gas from shore, liquefy it into LNG and either transfer the LNG directly into an oceangoing LNG tank vessel or into onboard LNG storage tanks; and (5) when fitted with regasification equipment to regasify LNG from either an oceangoing LNG tank vessel or on-board LNG storage tanks and transfer the resultant natural gas to shore through underwater piping systems.
The invention is a transfer station that includes one or more similar U-shaped semi-submersible structures, typically of steel, that we refer to as xe2x80x9cpontoonsxe2x80x9d. In embodiments containing multiple pontoons, the transfer station is modular, and pontoons can be reversibly interlocked one to another to create an LNG transfer station structure. The station is submersible by addition of water ballast until it achieves sufficient sinkage to allow entry of an oceangoing LNG vessel into the center area of the interlocked U-shaped pontoon or pontoons, where pontoon sidewalls provide shelter from waves and wind. The structure of the invention can then be raised by deballasting to bring it into contact or near contact with the bottom of the LNG vessel without the need to assume any significant weight of said vessel. Embodiments of the invention may also includes mooring means to temporarily moor the vessel to the invention at approximately the level of the vessel""s upper deck. The combination of this contact between vessel and transfer station structure with physical shelter from wind and seas provided to the vessel by side walls of the invention and temporary mooring and fendering arrangements provided between the vessel and transfer station at approximately the level of the vessel""s upper deck acts to minimize relative motion between vessel and transfer station. Said minimization of relative motion serves to ensure a safe connection between piping systems on the LNG vessel and the transfer station, thereby facilitating safe transfer of LNG either from LNG vessel tanks to transfer station or tanks located therein, or vice versa.
Each pontoon of the transfer station is equipped with a ballast/deballast system, preferably a rapid-response system, to maintain the desired contact between transfer station structure and bottom of the LNG vessel as the weight of LNG is transferred from LNG vessel to transfer station and vice versa. Said ballast/deballast system may also include a passive stabilization feature, which reduces wind and wave induced motions by the transfer station. Said passive stabilization feature interconnects ballast tanks on both sides of the pontoon in a manner which acts to transfer ballast from side to side in opposition to wave action.
In addition to temporarily interlocked similar U-shaped pontoons, the transfer station may be fitted with a dissimilar bow pontoon or section temporarily or permanently interlocked to the forwardmost U-shaped pontoon and connected at its forwardmost point to a mooring system anchored to the sea bottom. The bow section would typically have no U-shaped center well, be the same width and extreme depth at its after end as the U-shaped pontoon to which it is attached, be rounded or pointed at its forward end, and serve the functions of improving the transfer station""s hydrodynamic performance while closing and sheltering the U-shaped well of the transfer station in the normal bow sea situation. Said bow section may also be equipped with personnel accommodation and work spaces; liquifaction or regasification systems; machinery needed to generate electricity or other power required by the LNG transfer station to perform its various functions; and other equipment associated with the on-board storage of LNG, the mooring of LNG vessels within the LNG transfer station, the preferable single point mooring of the LNG transfer station to the ocean bottom, the transfer of LNG between LNG vessel and LNG transfer station, and the transfer of LNG or natural gas between the transfer station and shoreside facilities.
Whether its LNG tanks are empty, full or partially full, each pontoon has the capability to be submerged by addition of water ballast to a water depth of its U-shaped well deeper than the extreme draft of the oceangoing LNG vessels for which the LNG transfer station is intended, and when so submerged said pontoon will have adequate reserve buoyancy to survive normally anticipated sea conditions.
Each U-shaped pontoon may be fitted with two or more LNG storage tanks, a self-contained ballast/deballast system optionally including passive roll stabilization features, and piping and pumping system associated with the transfer and containment of L NG which can be interconnected to comparable piping and pumping systems installed on other pontoons. At least one of the pontoons is further equipped with a means of connecting the interconnected transfer station LNG piping and pumping system with the LNG piping and pumping system of the oceangoing LNG vessel. At least one of the pontoons is further equipped with a land-based piping system for LNG, natural gas, or both.
The physical dimensions of the invention are sufficient for it to (1) serve as transfer station for oceangoing vessels of a size traditionally used for the transportation of LNG (2) contain within its LNG tanks at least all or part of the LNG anticipated to be transferred to it from an oceangoing vessel, (3) contain within its ballast tanks sufficient sea water capacity to submerge its U-shaped well to a water depth which exceeds the extreme draft of traditional oceangoing LNG vessels, and (4) have sufficient reserve buoyancy during any operating condition to ensure safe operation and survive normally anticipated sea conditions.
As indicated, the transfer station according to his invention may be of modular construction comprising multiple pontoons. The pontoons may be dimensioned so as to be associated with the nearest dry-dock, where individual pontoons may be towed for repair. Alternatively or in addition the pontoons may be dimensioned such that the transfer station itself can serve as a floating dry-dock. For this purpose the fore/aft pontoon length is dimensioned such that an individual pontoon can be removed from the assembly, deballasted, turned ninety degrees and inserted into the U-shaped well of the remaining pontoons, where it can be hoisted free of the water or raised free of the water by deballasting the remaining interlocked pontoons for underside repair work.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.