Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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TITLE OF THE INVENTION
NATURAL GAS TRANSPORTATION APPARATUS AND SYSTEM
BACKGROUND OF THE INVENTION
The transportation of natural gas from offshore gas reservoirs to a point of sale
5 is presently most commonly done by high pressure subsea pipeline. With this process, the gas
is partially processed at the offshore location and transported to a shore facility where the
liquids are separated and the gas is further conditioned to make it a pipeline specification gas.
From this point the gas can be transported to customers by pipeline or liquefied and transported
long distances to major malke~s in the form of liquid natural gas ("LNG").
To make this type of offshore production economically feasible requires a large
enough reservoir of gas to provide the supply and a corresponding demand to justify the large
capital cost which must be written off over many years in order to begin and m~int~in
production.
As natural gas becomes more and more popular and the larger more readily
15 accessible sources of gas become exhausted, explorers are and will be going into deeper and
less accessible ocean locations.
Shipping raw or partially processed gas from an ocean location to a shore
facility in the form of a compressed natural gas ("CNG"), instead of LNG is therefore
desirable from many perspectives. For example: a reservoir can be put into production earlier
2 0 without having to have the long term economics in place that are normally associated with large
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scale production and processing facilities; ships carr.,ving CNG can go to several unloading
locations depending on demand; and liquid constituents, ~;ullcllLly flared or reinjected at an
offshore oil production facility, can be recaptured and sent to market as a conservation measure
or to enhance the economics of the development.
By and large all of the present systems for transporting gas over water focus onthe transportation of dehydrated and fully processed gas. U.S. patent 4,846,088 to Fanse et
al. does disclose a system of transporting CNG over water from "shut in" wells (i.e. wells not
accessed by pipe lines). The CNG is stored in storage elements which consist of lengths of
sealed common pipeline which are stacked above the deck of a seagoing vessel. However, the
storage cylinders of this system are not vertically integrated and therefore if there is separation
of the CNG into its gas and liquid constituents, no benefit from it can be achieved. C~n~ n
patent 1,211,702 to Bresie et al discloses a system for llal~relling "raw" natural gas, and its
associated liquids, from shut in wells to a delivery point in cylinders which are carried by
barge. Each cylinder is fitted with a "dip tube" which allows the gas and liquid constituents
of each cylinder to separate by gravity and thus the liquid can be unloaded separately from the
cylinder prior to the gas. However, the separation in this type of system is not fully effective
and requires the use of high pressure separation equipment at the delivery point.
SUMMARY OF THE INVENTION
2 0 In view of the foregoing, it is an object of this invention to provide an over the
water transportation system which allows for the h~n~lling of natural gas along with its liquid
reservoir constituents (which include water, con~1en.C~te, oil and cont~min~ntc) at high pressure.
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It is a further object of this invention to provide a transportation system which
provides for the rapid extraction of natural gas and its liquid reservoir constituents from the
reservoir for transport to a shore facility where rapid depressurization of the system will
quickly unload the gas.
It is a further object of this invention to provide a natural gas transportationsystem having rapid loading and offloading to maximize the economics of the system by
reducing the turn around time of the vessel (or vessels) being used.
Accordingly, the invention comprises a compressed natural gas storage
apparatus, comprising a storage cell consisting of a plurality of vertically interconnected gas
storage cylinders; a first valve means, disposed at the bottom of said storage cell, for filling
the cell and evacuating its contents from the bottom of the cell; and a second valve means,
disposed at the top of the storage cell, for filling the cell and evacuating the contents from the
top of the cell.
The invention further comprises a system for transporting compressed natural
gas over water from a production reservoir to a shore based facility, comprising a seagoing
vessel; a storage cell consisting of a plurality of vertically interconn~cte~l gas storage cylinders;
a first valve means, disposed at the bottom of said storage cell, for filling the cell and
evacl~ting its contents from the bottom of the cell; and a second valve means, disposed at the
top of the storage cell, for filling the cell and ev~cl-~ting the contents from the top of the cell.
The invention still further comprises a method of transporting compressed
natural gas from a production reservoir to a shore based facility, comprising the steps of
transferring compressed natural gas to a storage cell consisting of a plurality of vertically
interconn~cte~l gas storage cylinders; transporting said gas during which period it settles into
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a top gas phase and a bottom liquid phase; ev~cll~ting the top gas phase from the top of the
cell; and ev~ ting the bottom liquid phase from the bottom of the cell.
BRIEF DESCRIPTION OF THE DRAWINGS
The following is a description by way of example of a preferred embodiment
of the present invention, lefelellce being made to the accolllpallyillg drawings in which:
FIGURE 1 is a sch~m~tic diagram of a storage cell of the present invention;
FIGURE 2 is a sectional side view of an embodiment of the present invention;
FIGURE 2B is a sectional top view of an embodiment of the present invention;
1 0 and
FIGURE 3 is a cross-sectional view, taken along the line X-X, of the
embodiment of the invention shown in Figure 2.
While the invention will be described in conjunction with the illustrated
embodiment, it will be understood that it is not intended to limit the invention to such
embodiment. On the contrary, it is intended to cover all alternatives, modifications, and
equivalents as may be included within the spirit and scope of the invention as defined by the
appended claims.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the drawings, similar features have been given similar reference numerals.
As illustrated in Figure l, the plhllal~y component of the invention is a storage cell 10
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consisting of a plurality of runs of steel pipe 12 which have been layered horizontally and
which are secured and supported on contoured supports 18. Each run of pipe terminates, at
either end, with a weld cap 14 to form a cylinder 15. The inheritor of each cylinder 15 of the
storage cell 10 is conn~cte~l vertically by fittings 16.
Each storage cell 10 is connPcte~l to a bottom first header array connection line
19 and a top second header array connection line 20 to form an array of horizontally aligned
storage cells 22, as best seen in Figure 2A. The bottom first header array connection 19 is
connected to a bottom first header isolation valve 24 and top second header array connection
20 is conn~ctecl to a top second header isolation valve 26. Each array 22 can be isolated from
1 0 the other arrays by closing the top and bottom header isolation valves 24 and 26.
The bottom and top header array connections 18 and 20 are connected, via
their respective header isolation valves 24 and 26, to first and second loading/unloading lines
28 and 30. Each loading/unloading line has, respectively, first and second loading valves 32
and 34.
1 5 The first loading/unloading line 28 has a liquid metering and pressure control
36 and a liquid pump 38.
The second loading/unloading line 30 has a pressure and flow control 40, an
indirectly fired heater 42, compressor 44 and chemical injector 46 connected to chemical
supply 47. According to the plefelled embodiment, five to 10 storage cell arrays 22 are
2 0 provided in the hold 48 of a vessel 50, as best seen in Figure 2. However, any geometry of
placement is possible, depending on hold specifications. Additionally, storage cell arrays in
certain circumstances may also be placed on the deck of the vessel. Each storage cell within
the array will preferably be configured to hold 25 to 50 MMSCF of natural gas.
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In operation, the storage cell arrays 22 will preferably be top loaded via the
top header array connections 20 via the second loading/unloading line 34, although loading by
the bottom header array connections 19 and first loading/unloading line 32 is also feasible.
Loading of the cells will normally be completed by pressure let down from the higher pressure
5 in the gas reservoir in contrast to the storage system, however, the compressor 44 may be used
to assist in loading as required. By conn~cting the cylinders 15 of the cells 10 from top to
bottom using the fittings 16, a single gravity separator for the gas is obtained. Once loaded
there is no gas flow so that during transit the gas and liquids will separate into gas and liquid
components 52 and 54, at level separation point 56. Accordingly, when it is time to offload
1 0 the gas 52 and liquid 54, they can be off loaded separately by ev~c~l~ting the liquid along the
bottom array connection 19 through first loading/unloading line 32 and the gas along the top
header array connection 20 along the second loading/unloading line 30. Unloading is again
by pressure equalization into a land based node such as pipe storage, pipeline or feedline to an
end use operation. When loading or unloading on the second loading/unloading line 34 the
15 compressor 44 again be used to assist in the process as required. The pressure flow and
control 40 records the volumes of gas being loaded to protect against overloading and
exceeding the normal operating pressure of the arrays. The indirectly fired heater is used for
hydrate suppression. Finally, the chemical injector 46 can be used for the injection of
hydrate suppression, corrosion inhibitors, gas conditioning chemicals or other additives during
2 0 either the loading or unloading phases.
On the first loading/unloading line the liquid metering and pressure control 36
is again used to meter the volumes passing through the line and liquid pump 38 is used to assist
liquid loading and loading. As with any type of hydrocarbon shipment, good shipment and
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management practices must be employed to prevent hydrates, mitig~te and monitor for
corrosion and detect and control leaks. Additionally, routine pressure checks and inspection
programs will ensure reliability and safety.
In addition to providing an onboard storage function, the cells of the present
5 invention also permit some onboard processing of the gas and liquids being transported. This
involves moving the gas and/or liquids between the arrays 22. For example by closing the bottom
isolation valve 24 of an array, liquids from the other arrays can be pumped from the bottom
(liquid) header 19 and loaded into that array to concentrate on board liquids into one array.
Further, the liquids can be stabili_ed lltili7.ing the onboard compressor 44. Additionally, the gas
10 can be circulated through, for example, an on board dehydration unit to m~nllf~rture dry gas.
In summary, the following operating scenario is possible using the preferred
embodiment of the present invention.
A storage vessel outfitted with a suitable number of arrays of storage cells arrives
at an offshore gas/oil field and is provided access to the reinjection gas stream or gas reservoir
15 via appropliate mooring. The gas well, preferably prolific and of a higher pressure than storage
design pressure, will quickly fill the storage requiring a Il li ni~ time on site. While the ship is
travelling to the unloading location the gas and liquid phases will separate. If the liquids are
desirable at the sales point they will be unloaded. If the liquid is not required they will stay in
storage and be unloaded where they are needed or disposed of when the lost volume becomes
2 0 problematic. Liquids will normally be unloaded after gas unloading to provide a more stable
liquid and to avoid losing sales gas volumes. At the unloading location it is desirable if there is
a pipe storage facility and a pipeline or a process plant which would allow for rapid unloading
of the gas. Additionally, using an on board or land based compressor to pull additional gas out
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of storage will be advantageous. End use applications such as industrial use, processing, co-
generation, etc. which can consume partially processed gas will be an advantage because of the
lack of upstream processing.
