CA2225758A1

CA2225758A1 - Natural gas transportation apparatus and system

Info

Publication number: CA2225758A1
Application number: CA002225758A
Authority: CA
Inventors: A. Patrick Agnew
Original assignee: Individual
Current assignee: Individual
Priority date: 1997-12-23
Filing date: 1997-12-23
Publication date: 1999-06-23

Abstract

A sea going transportation system for transporting compressed natural from a production reservoir to a delivery node is disclosed, comprising: a storage cell consisting of a plurality of interconnected and vertically aligned gas storage cylinders; a first means, disposed at the bottom of said storage cell, for filling the cell and withdrawing its contents from the bottom of the cell; a second means disposed at the top of said storage cell, for filling the cell and withdrawing its contents from the bottom of the cell; the arrangement being such that the storage cell is filled with compressed natural gas at a reservoir and, during transport, the compressed natural gas settles out into a top gas phase and a bottom liquid phase, said top gas phase being withdrawable from the top of the cell and the liquid phase being withdrawable from the bottom of the cell at the delivery node.

Description

CA 0222~7~8 1997-12-23 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 CA 0222~7~8 1997-12-23 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.

CA 0222~7~8 1997-12-23 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 CA 0222~7~8 1997-12-23 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 CA 0222~7~8 1997-12-23 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.

CA 0222~7~8 1997-12-23 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 CA 0222~7~8 1997-12-23 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 CA 0222~7~8 1997-12-23 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.

Claims

1. 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;
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.

2. A storage apparatus according to claim 1 wherein said gas storage cylinders a vertically aligned.

3. A compressed natural gas storage apparatus consisting of a plurality of storage cells according to claim 1.

4. A storage apparatus according to claim 3 wherein the storage cells are connected at the bottom to a first header line and at the top to a second header line, said header lines being used for filling and evacuating the storage cells.

5. 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 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;
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.

6. A system according to claim 5 wherein said system comprises a plurality of storage cells according to claim 1.

7. A system according to claim 6 wherein the storage cells are connected at the bottom to a first header line and at the top to a second header line, said header lines being used for filling and evacuating the storage cells.

8. A system according to anyone of claims 5, 6, or 7 where the storage cell is filled with compressed natural gas at the production reservoir by pressure let down from the reservoir, it settles into a top gas phase and a bottom liquid phase, the top gas phase being evacuated from the top of the cell and the bottom liquid phase being evacuated from the bottom of the cell by pressure equalization to a node at the shore based facility.

9. 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 interconnected gas storage cylinders;
transporting said gas during which period it settles into a top gas phase and a bottom liquid phase;
evacuating the top gas phase from the top of the cell; and evacuating the bottom liquid phase from the bottom of the cell.

10. The method of claim 9 wherein the compressed natural gas is transferred to the storage cell by pressure let down from a gas reservoir.

11. The method of claim 9 wherein the gas and liquid phases of the compressed natural gas are evacuated from the storage cell by pressure equalization.