NO180470B - Process for the treatment and transport of an unstable hydrocarbon mixture - Google Patents

Description

The invention relates to a method for treating and transporting a hydrocarbon mixture that is produced on an offshore production platform or a production ship during the production of oil and gas from a reservoir, the mixture being an unstabilized natural gas fraction with a significant content of _JjPG, and where a pressurized stream of the mixture is transferred via a pipeline to a nearby tanker where the mixture is processed and then stored in tanks on the vessel.

When producing hydrocarbons on an offshore production platform, in addition to the produced oil and gas, an intermediate product is also produced that is too heavy to be mixed with gas for gas export from the platform, and too light to be mixed with stabilized oil for shipping. This product contains 10-15 mol% methane and ethane, and further a significant proportion of LPG (propane and butane), namely of the order of 70 mol%, while the remaining content is heavier hydrocarbons. The composition will vary from field to field. Characteristic of the product, however, is the high content of LPG, i.e. liquefied petroleum gas (Liguefied Petroleum Gas).

Possible consumers of a hydrocarbon mixture of a quality corresponding to that described above are power stations and gas processing plants capable of extracting LPG from the mixture.

As can be seen from the above, this hydrocarbon mixture is of such a nature that it is traditionally considered to be of little value to the field as an export product, and it is therefore re-injected into the reservoir. However, with a simple and cheap process to make this petroleum fraction transportable, it would immediately have a significant value. A solution where reinjection of this fraction is eliminated, however, also represents increased value for the field in the form of lower operating costs for reinjection.

It is thus an object of the invention to provide a method which enables an unstabilized hydrocarbon mixture to be transported by suitable ships.

Another purpose of the invention is to provide a simple and cheap solution for the transport of such a mixture to land using suitable tankers, so that one can utilize a stream that would otherwise be reinjected into the reservoir.

The above-mentioned purpose is achieved with a method of the type indicated at the outset which, according to the invention, is characterized by

that the mixture is subjected to an initial cooling and then to a pressure relief on the vessel, with resulting cooling,

that the mixture is separated into an LPG mixture and excess gas, and the LPG mixture is stored in tanks on the vessel at a pressure of 1-10 bar and at a temperature in the range -40 °C to -50 °C, that the vessel, when the tanks is filled to the desired degree, is disconnected from the pipeline, in order to transport the LPG mixture to a desired destination.

The invention shall be described in more detail in the following in connection with an exemplary embodiment with reference to the drawings, where fig. 1 is a principle sketch which i.a. shows a production platform which is connected to a vessel via transmission lines for hydrocarbon mixture and gas, fig. 2 is a process diagram showing the necessary modification on the platform, and fig. 3 is a process diagram showing the necessary processing equipment on the vessel.

In fig. 1 shows a production platform 1 with a foundation 2 that rests on the seabed, and where the platform is connected via a flow line 3 to a production well 4 from an oil/gas reservoir. The platform 1 is connected to a loading buoy 5 for oil via transfer lines 6 and 7. Furthermore, the platform is connected to an anchored tanker 8 via transfer lines 9 and 10 for hydrocarbon mixture and gas respectively. When practicing the method according to the invention, a pressurized stream of an unstabilized hydrocarbon mixture is transferred from the platform to the moored vessel, where the mixture is cooled by pressure relief. Surplus gas is returned to the platform, while the produced LPG mixture is stored in the vessel's tanks in a cooled state at the appropriate temperature and pressure determined by the operating conditions. The LPG mixture can e.g. stored at a temperature of -40 °C to -50 °C, and at a pressure of from 1.05 to 10 bar a. The vessel is disconnected from the pipelines when the tanks are full. During the time period when the vessel is not available, the hydrocarbon mixture is re-injected into the reservoir. During the transfer, the hydrocarbon mixture should be pressurized to a pressure of at least 10 bar, preferably at least 20 bar, partly to overcome the pressure loss during the transfer, and partly so that dissolved amounts of light components (methane and ethane) during evaporation should contribute to a desired cooling of The LPG fraction during the treatment process on board the tanker.

In practice, the pipelines 9 and 10 can conveniently be connected to a submerged loading/unloading buoy of the so-called STL (Submerged Turret Loading) type, where the buoy is anchored to the seabed and designed to be introduced into and releasably fixed in a submerged, downwardly open receiving space in the vessel, and is further arranged for connection to a pipe system leading to the vessel's treatment equipment.

Although in the example shown in fig. 1 shows a production platform, the method can just as well be used in connection with a production ship.

The various steps in the method according to the invention will now be described in more detail with reference to fig. 2 and 3. In these process diagrams, a group of symbols consisting of a "rounded rectangle" and a parallelogram is shown at a number of places along the cable routes. The numbers in the rounded rectangles indicate the pressure of the hydrocarbon mixture in bar (absolute pressure), while the numbers in the parallelograms indicate the temperature of the mixture in °C at the relevant location. The numerical values shown as examples have been derived from theoretical calculations in connection with a relevant oil/gas field.

As shown in fig. 2, an unstabilized hydrocarbon mixture from processing equipment (not shown) on the platform 1 is supplied to a seawater cooler 20 where the mixture is cooled, in the case shown from 46 °C to 25 °C. In the example shown, the mixture is then supplied via a line 21 with a shut-off valve 22 to a liquid dryer 23 where the mixture is dried using a suitable drying compound. The line section 21 is connected in parallel with a line section 24 which contains a further, normally closed valve 25. The aforementioned units, which are shown within the delimited area in the figure, represent new equipment on the platform.

A liquid drier is preferably used where a drying mass is used which is not regenerated on the platform, but which is replaced to be deposited or regenerated on land. In fig. 2, however, there is also schematically shown a valve arrangement 26, 27 which forms part of a possible regeneration system.

Although the above assumes that drying of the hydrocarbon mixture is necessary, this will not always be the case. Moreover, such drying can advantageously be carried out on the tanker, instead of on the platform/production vessel. Drying on the tanker has the advantage that the drying mass can be replaced when the vessel is at the quay in connection with unloading the LPG product.

After the possible drying of the hydrocarbon mixture, this is supplied to a pump 28 which ensures that the mixture is transferred via the pipeline 9 to processing equipment on the vessel 8.

As previously mentioned, the hydrocarbon mixture is re-injected into the reservoir when the tanker 8 is not available. In this case, valve 22 is closed, while valve 25 is open. The mixture is then fed via a line (not shown) to the process plant's reinjection equipment.

As can be seen from fig. 3, the hydrocarbon mixture is fed from the platform to a suitable cooler 40, in the case shown a seawater cooler, in which the mixture is cooled, in the example shown from 41°C to 15°C. The cooler can be arranged on the vessel, but it can also consist of a cooling loop arranged in the water.

Instead of using a separately arranged cooler or cooling loop, the hydrocarbon mixture can be subjected to the initial cooling by the mixture being cooled during the transfer through the pipeline 9 due to heat exchange with the surrounding seawater. By thus utilizing natural conditions, the mixture can be cooled to a temperature that approaches the ambient temperature/seawater temperature, e.g. a temperature below 10 °C. In order for such a cooling effect to be achieved, the pipeline must have a certain length, e.g. 1 km or more. Such a distance between the platform/production vessel and the tanker will normally still be required based on considerations of safety zones around the installations.

After the above-mentioned cooling, the flow of hydrocarbon mixture is subjected to a pressure relief by passing the flow through a set of pressure reduction valves, in the figure represented by a single valve 41. Instead of pressure reduction valves, a liquid turbine can be used, although this is less cost-effective, as the amount of energy generated in a liquid turbine is relatively small.

In the example shown, the pressure of the mixture stream after the pressure relief step 41 is reduced to 1.55 bar a, and the temperature is reduced to -18.5 °C. The degree of pressure reduction will depend on the tanker's storage specifications. In the present case, storage conditions given at a pressure of 1.05 bar a and a temperature of -48 °C have been used.

After the pressure reduction, the hydrocarbon mixture in the example shown is cooled using the vessel's own cooling system 42 to -48 °C, where the mixture is then fed to a phase separator 43. In the phase separator, the mixture is separated into a bottom product and gas. The bottom product, which is therefore an LPG mixture with a pressure of 1.05 bar a and a temperature of -48 °C, is led via a valve 44 to storage tanks 45 on the vessel. The valve 44 is arranged to be able to regulate the level in the phase separator.

Normally, one would prefer to carry out the pressure relief of the hydrocarbon mixture by means of one or more pressure reduction valves 41 before the further cooling in the cooler or the cooling system 42, mainly because the cooler 42 can then be dimensioned for a lower pressure (approximate atmospheric pressure). However, it is conceivable that one chooses to carry out all indirect cooling (i.e. cooling in heat exchangers) before the pressure relief. In such cases, the valve(s) 41 will be placed in close proximity to the inlet of the phase separator 43, and further cooling will then not be relevant.

The gas separated in the phase separator 43 is supplied in the preferred embodiment shown to a compressor 46 which is shown to emit a gas flow with a pressure of 63 bar a and a temperature of 210 °C. The compressed gas is supplied to a compressor aftercooler 47 from which gas with a pressure of 62.5 bar a and a temperature of 55 °C is transported back to the platform 1 via the pipeline 10. The amount of gas produced is so small that it can go straight into the platform's gas recompression system. The pressure level of the gas is determined based on which step in the platform's gas compression line it is most appropriate to introduce the gas. The excess gas can also have other uses than being transported back to the platform/production vessel. Thus, it can be used as fuel on the tanker, or it can be burned ("flared").

When it comes to the operation of the vessel's cooling system, it should be noted that its cooling circuit can use evaporation gas from the vessel's storage tanks as a cooling medium. Alternatively, part of the gas from the phase separator can be used in the cooling circuit.

Claims (10)

1. Procedure for the treatment and transport of a hydrocarbon mixture that is produced on an offshore production platform or a production ship during the production of oil and gas from a reservoir, the mixture being an unstabilized natural gas fraction with a significant content of LPG, and where a pressurized stream of the mixture is transferred via a pipeline to a nearby tanker where the mixture is processed and then stored in tanks on the vessel, CHARACTERIZED BY that the mixture is subjected to an initial cooling and then to a pressure relief on the vessel, with resulting cooling, that the mixture is separated into an LPG mixture and excess gas, and the LPG mixture is stored in tanks on the vessel at a pressure of 1-10 bar and at a temperature in the range -40 °C to -50 °C, and that the vessel, when the tanks are filled to the desired degree, is disconnected from the pipeline, in order to transport the LPG mixture to a desired destination. 2. Method according to claim 1, CHARACTERIZED IN THAT the hydrocarbon mixture during the transfer to the tanker is pressurized to a pressure of at least 10 bar, preferably at least 20 bar. 3. Method according to claim 1 or 2, CHARACTERIZED WHEN the hydrocarbon mixture is dried before it is transferred to the tanker. 4. Method according to claim 1 or 2, CHARACTERIZED IN THAT the hydrocarbon mixture is dried on the tanker before it is subjected to the pressure relief. 5. Method according to one of the preceding claims, CHARACTERIZED IN THAT the hydrocarbon mixture is subjected to at least part of the initial cooling by the mixture being cooled during the transfer through the pipeline due to heat exchange with the surrounding seawater. 6. Method according to one of claims 1-4, CHARACTERIZED IN THAT the initial cooling of the mixture takes place by means of a seawater cooler. 7. Method according to one of the preceding claims, CHARACTERIZED BY the fact that the hydrocarbon mixture is totally cooled by means of the pressure relief and evaporation of light components so that the LPG mixture reaches a temperature lower than -40 °C. 8. Method according to one of the preceding claims, CHARACTERIZED IN THAT the pressure of the hydrocarbon mixture during the pressure relief is reduced to a pressure lower than 10 bar, and preferably to a pressure close to atmospheric pressure (1.05 bar). 9. Method according to one of the preceding claims, CHARACTERIZED BY subjecting the excess gas to a pressure increase to e.g. about. 60 bar and then transported back to the platform, preferably for reinjection. 10. Method according to claim 1, CHARACTERIZED IN THAT the hydrocarbon mixture is kept in the reservoir when the vessel is not available.