Submerged combustion installation

Abstract

A submerged burner for the combustion of large daily volumes of hydrocarbon gases. The device comprises a combustible mixture supply system, including an inlet for gas from the low-pressure separator into the atmospheric air supply pipe, an injection inlet for gas from the high-pressure separator at the mixer head, a combustion chamber fitted with a burner allowing a high heat flow density to be achieved, and a system to collect and discharge burnt gas into the atmosphere. This device is particularly suitable for the combustion of the gases accompanying liquid hydrocarbons in oil fields at sea.

Description

This invention concerns a submerged combustion installation for the gases accompanying liquid hydrocarbons produced at sea or on land, and which have to be eliminated because of the lack of any market outlet.

Natural gas obtained at sea and on land during testing, and later during production, and which is separated from the oil at the well-head and in storage centres, is usually burned by means of a flare, where no market outlet exists.

The volumes of gas involved can amount to several hundred thousand cubic meters daily. Volumes of several tens of thousands of cubic meters can be burned by means of flares on the production platforms. For larger amounts, the heat discharged becomes intense, and variations in wind direction can endanger production installations and workers. The flare then has to be installed some distance from the production platform, which means that a support structure has to be built, usually another platform, the cost of which increases very quickly where the water is deep.

When the gas cannot be reused on the field, or collected and dispatched, it is at present generally injected into the sea, in the hope that it will be dispersed more effectively than by being simply discharged into the atmosphere, and also that it will be dissolved to some extent by the seawater. In fact, the process is a dangerous one, leading in calm weather to the possible formation of blankets of an explosive gaseous mixture; it also pollutes the sea.

Another method consists of diluting the gas in air, using a mixing appliance, so that the mixture finally discharged into the atmosphere is non-combustible, remaining below the level of flammability.

This method removes the risk of explosion, but it involves cumbersome equipment with large installed capacity, and the need to extend and strengthen production platforms, where a second platform is not built. Moreover, the risk of atmospheric pollution remains, particularly when the gas contains a significant proportion of heavy hydrocarbons. When the proportion of hydrocarbon gas in the air is less than 3 percent, the risk of explosion is practically nil, but the toxicity threshhold has to be taken into account : 0.2 percent for propane and 0.05 percent for pentane, hexane and heptane.

The submerged system described in this invention overcomes these difficulties, allowing large amounts of gas to be eliminated, regardless of the position of the production platform, without endangering plant or workers.

This new industrial combustion installation, used particularly to eliminate gas discharges from hydrocarbon production wells, with its ancillary oil and gas separation appliances, comprising air and gas supply pipes, ignition and combustion monitoring systems, and means of monitoring the composition of the supply mixture and burnt gas, is characterized by the fact that the submerged, watertight part of the installation comprises at least one burner consisting of at least one combustion chamber and at least one combustible mixture feed chamber, communicating with each other by means of apertures or slits, and at least one burner feed pipe, leading to the open air and comprising a mixing zone, consisting of a divergent passage possibly preceded by a convergent passage, in the inlet of which is placed one discharge passage leading to the open air, and means of adjusting the flow of combustible gas and air.

In one embodiment, the walls of the feed chamber, extending from the feed pipe, consist of assembled slabs of refractory material, the consecutive adjoining edges of which form the burner slits.

In another embodiment, the burner feed chamber is a tube extending from the feed pipe, and containing perforations which form the burner apertures. To ensure better cooling of the perforated tube, two methods may be adopted : in the first, the feed chamber is divided into two or more sections by metal partitions, starting from openings through which the sections communicate with one another, with a screen to prevent any flashback, near the burner openings; in the second system, pipes containing a fluid are fitted against the inside wall of the tubular burner, with a screen to prevent any flashback, near the burner openings.

In one recommended embodiment, the feed pipe opens into a feed chamber, the side walls of which consist of pipes containing circulating fluid, placed in a cylindrical form, close enough together for the spaces between adjoining pipes to form the burner slits.

In these last two embodiments, the pipes containing the circulating liquid are preferably circular in cross-section.

In these same embodiments, the cooling fluid is preferably the liquid in which the burner is submerged, and it circulates by convection in the pipes, which cross the combustion chamber from side to side and are open at the ends.

In the various embodiments, means of adjusting the air supply consist of remote-controlled lengthwise movement of the combustible gas pressure injector or injectors.

In these same embodiments, a passage conveying the low-pressure gas coming mainly from the low-pressure separators opens into the feed pipe, above the injector for gas from the high-pressure separators.

In the various embodiments, the injector or injectors of combustible gas from a pressure source are cylindrical in cross-section.

In embodiments specially adapted for larger individual flow rates, the injector or injectors of combustible gas coming from a pressure source are convergent-divergent in cross section.

It will be easier to understand the invention from the following description of some embodiments, as illustrated in the drawings, in which:

FIG. 1 is a diagrammatic elevational view showing an installation for burning off gas

FIG. 2 is an operating diagram showing the installation of FIG. 1, with the separators shown as if nearer the burner, and the burner shown in greater detail;

FIG. 3 is a diagrammatic vertical sectional view of an embodiment in which the feed chamber is separated from the combustion chamber by a wall of refractory material;

FIG. 3a is a horizontal cross-sectional view taken along the line A--A of FIG. 3;

FIG. 4 is a diagrammatic vertical sectional view taken through a burner in which the feed chamber is divided into sections by vertical partitions;

FIG. 4a is a horizontal sectional view taken along the line B--B of FIG. 4;

FIG. 5 is a diagrammatic vertical sectional view taken through a burner in which the feed chamber is separated from the combustion chamber by a cylinder associated with vertical tubes;

FIG. 5a is a horizontal sectional view taken along the line C--C of FIG. 5;

FIG. 6 is a diagrammatic vertical sectional view of a burner in which the feed chamber is separated from the combustion chamber by a ring of vertical tubes; and

FIG. 6a is a horizontal sectional view taken along the line D--D of FIG. 6.

FIG. 1 shows a hydrocarbon production platform 1 installed at sea or in a lake or pond. Beside the well-heads and collection installation (not shown here) is the processing equipment, comprising a high-pressure separator 2, with high-pressure gas outlet 3 and an outlet for incompletely degassed oil 4, and a low-pressure separator 5, with a low-pressure gas outlet 6, and a storage oil outlet 7.

The submerged combustion installation is fixed to one supporting leg 8 of the platform. There is an approximately vertical supply pipe 9, leading to the open air through an aperture 10.

A duct 11 feeding in air supplied by an auxiliary starting blower 12 opens into this pipe, a short distance below the aperture 10; further down is a passage 13 conveying gas from the low-pressure separator. At the lower end of this pipe is a mixing zone 14, comprising two passages, a converging one 15, followed by a diverging one 16. At the inlet, and aligned with the convergent passage, is a nozzle 17 to which the gas from the high-pressure separator is injected; the position of this injector can be adjusted sideways, depending on the axis of the pipe.

The feed pipe 9 opens at the bottom into a feed chamber 18 for the burner 19. This burner also comprises a combustion chamber 20, alongside the feed chamber, from which it is separated by a partition 21 containing a number of apertures 22.

The combustion chamber 20 is connected by a set of collection passages (not shown here) to a pipe 23 discharging burnt gas into the atmosphere.

FIG. 2 shows the operating diagram for the gas-burning installation used in testing a well 1 or set of wells, or during production. The same references are used as for the different parts of FIG. 1. In addition, there is a control and safety unit 24, receiving information from the combustion-monitoring instruments 25, and transmitting impulses to the separator valves 26, 27 and 28, auxiliary blower 12 and ignition system 29.

The following figures show different types of burners, for gases of different compositions and different operating conditions, depending on whether the water is fresh or saline, how calm it is, and whether it is naturally replaced. All these burners are variations on the one described in connection with FIG. 1, as consisting of a feed chamber and combustion chamber, communicating with each other by means of a number of slits or apertures.

FIGS. 3, 4, 5 and 6 show various embodiments in which the burner comprises an outside casing 30, usually in the form of a conical segment between two slightly curved ends. The upper end, which is larger in diameter than the lower one, comprises an axial opening 31 connected to the feed pipe, and a number of openings 32 through which the burnt gas leaves, arranged around a circular crown, and linked to the burnt gas collection system 23.

In FIG. 3, the separation between the feed and combustion chambers consists of a cylinder on the same axis as the outside casing of the burner, formed by an assembly of refractory slabs 27, the edges 33 of which are combined in pairs, to form the burner slits 34. These slabs are joined to the upper and lower ends of the casing.

In FIGS. 4 and 5, the separation between the feed and combustion chambers consists of a tube 27, on the same axis as the outside casing of the burner, and joined to its upper and lower ends. This tube contains distributed perforations 35, forming combustion apertures. A screen 36 to prevent flashback is placed near the burner openings.

In FIG. 4, the feed chamber is divided into several sections by metal partitions 37, containing openings 38 which provide communication between the sections.

In FIGS. 5 and 5a, showing lengthwise and cross sections, parallel pipes 39, distributed uniformly against the inside wall of the tube 27, pass through the end walls of the feed chamber, and are open at each end, outside the chamber.

In FIGS. 6 and 6a, again showing lengthwise and cross sections, the separation between the feed and combustion chambers is provided by a cylindrical cage 27, the bars of which consist of a number of parallel pipes 39, passing through the upper and lower ends of the outside casing 30 and open at each end. They are fixed hermetically to the outside casing. These pipes are close enough together for the spaces between them to provide combustion slits. This embodiment is particularly suitable for use in installations in which flow rates may vary considerably, since the width of the zone in which the flames form varies depending on the amount of gas. In the embodiment shown in FIG. 8, the cage is made up of pipes with circular cross sections, but other sections can be chosen, depending on the specific problem involved. Pipes with an oval cross section, or in which the cross section consists of the segment of a ring bounded by two connecting curves, preferably in the forms of arcs of a circle, have been used sucessfully where it was necessary to provide a reduction in the number of combustion slits. Other forms, comprising a number of pipes for cooling water circulation, but in which combustion slits alternate with spaces filled in with refractory material, have also been suggested as a way of reconciling the need to provide feed and combustion chambers of considerable volume with the opposing need to reduce the surface area of the burning flues, and in this particular case the number of slits. Such arrangements allow a ratio of maximum capacity to minimum capacity of more than 10.

In general, the combustion chamber or chambers occupy the space surrounding the feed chamber. The outer casing of the burner is the cover of the combustion chamber or chambers, and this outside casing is usually equipped with cooling fins (not shown here).

If desired, and depending on the composition of the gas to be burned and the properties of the water in which the installation is submerged, the combustion chamber can be fitted with radiating bars, not shown here. Such bars are particularly useful in the case of gases containing a high percentage of methane and a very small amount of heavy products.

If desired, the combustion chambers in the different types of burners described above can be fitted with cooling pipes through which the liquid in which the installation is submerged circulates by convection; these are not shown here.

Submersion of the gas-burning installation, as described above in its various embodiments, gives it an reduced apparent weight, which may be made slightly positive or negative, depending on the installation technique adopted.

Individual flow-rates of 100,000 cu.m per day of combustible gas with a heating capacity of 6 to 15 therms per cu.m. require the provision of burners 3 to 4 meters in diameter and 3 to 6 meters high. The depth of submersion can be up to 50 meters, depending on local conditions.

This new installation allows considerable volumes of hydrocarbon gases, which in their original state would be a source of pollution and involve the danger of explosions, into neutral gases discharged in the atmosphere at a temperature that can be less than 100.degree.C.

The device requires no extra energy while functioning, since the volumes of air needed for perfect combustion are sucked in by atmospheric induction.

An auxiliary blower, of 10 or so kilowatts, is provided, to allow combustion to be started with small amounts of high-pressure gas, since atmospheric induction is ineffective in these conditions. It also allows the installation to be cleared by any combustible mixture after combustion has stopped, and thus remove any risk of explosion. The installation can swept out either with air or with a neutral gas like carbon dioxide.

The capacity of any given size of burner can be increased, by replacing the standard cylindrical nozzle of the pressure gas injector at the mixing chamber inlet with a converging-diverging nozzle.

A heat-recovery device can be attached to the various installations involving the types of burners described above, or variants on them. This device is fixed to the burnt gas discharge pipe. Such energy can be used for heating or to supply industrial steam or fresh water, or it can be converted into electricity to supply the needs of the platform or other installations.

Away from oil fields, the various types of burners described above, and particularly the one illustrated in FIG. 8, can be used wherever a high heat-flow density is required, for example in steam boilers, particularly those used to produce electricity, for urban heating schemes, or in the metallurgical or ceramics industries.

Claims

What is claimed is:

1. A submerged combustion installation for eliminating the combustible gas under pressure which is discharged from oil and gas separating appliances associated with hydrocarbon production wells, said installation comprising:

at least one submerged and watertight burner comprising at least one feed chamber for a combustible gaseous fuel mixture, encircled by a combustion chamber provided with ignition means for initiating combustion within said combustion chamber, said feed chamber and combustion chamber communicating with each other by means of a plurality of apertures,

at least one burner feed pipe, one end of which leads to open air while its other end is connected to the feed chamber of the burner by a diverging passage,

at least one injector connected to receive combustible gas under pressure from a separating appliance and extending within the burner feed pipe upstream of the diverging passage,

at least one discharge passage connected to the combustion chamber of the burner and leading to the open air, and,

means for adjusting the flow of combustible gas fed through the injector to the burner feed pipe.

2. A submerged combustion installation, according to claim 1, wherein the diverging passage, connecting the burner feed pipe to the feed chamber of the burner, is preceded by a converging passage.

3. An installation as defined in claim 1, in which the walls of the feed chamber, extending from the feed pipe, consist of assembled slabs of refractory material, the consecutive adjoining edges of which form the burner apertures.

4. An installation as defined in claim 1, comprising remotely controlled means for the lengthwise movement of said at least one combustible gas pressure injector.

5. An installation as defined in claim 1, in which at least one passage conveying the low-pressure gas opens into the feed pipe, above an injector for high pressure gas.

6. An installation as defined in claim 1 in which said at least one injector is cylindrical in crosssection.

7. An installation as defined in claim 1 in which said at least one injector is convergent-divergent in cross section.

8. An installation as defined in claim 1, in which the burner feed chamber is a tube extending from the feed pipe, and containing perforations forming the burner apertures.

9. An installation as defined in claim 8, in which the feed chamber is divided into at least two sections by metal partitions, provided with openings through which the sections communicate with each other, and comprises a screen to prevent flashback, near the burner apertures.

10. An installation as defined in claim 6, in which pipes containing a fluid are fitted against the inside wall of the tubular burner, with a screen to prevent flashback, near the burner openings.

11. An installation as defined in claim 10, in which the pipes containing the circulating fluid are circular in cross section.

12. An installation as defined in claim 10, in which the cooling fluid is the liquid in which the burner is submerged, and is circulated by convection in the pipes, which cross the combustion chamber from side to side and are open at the ends.

13. An installation as defined in claim 1, in which the feed pipe opens into a feed chamber, the side walls of which consist of pipes containing circulating fluid, placed in a cylindrical arrangement close enough together for the spaces between adjoining pipes to form the burner apertures.