5 The invention relates to a method of treatment and transport of a hydrocarbon mixture produced on an offshore production platform or a production vessel when producing oil and gas from a reservoir, the mixture being an unstabilized natural gas fraction having a substantial content of LPG.

10 When producing hydrocarbons on an offshore production platform there is, in addition to the produced oil and gas, also produced an intermediate product which is too heavy to be mixed with gas for gas export from the platform, and too light to be mixed with stabilized oil for shipment. This product contains 10- i5 15 mole% methane and ethane, and in addition a substantial part of LPG (propane and butane), viz. of the order of 70 mole%, whereas the remaining content is heavier hydrocarbons. The composition will vary from field to field. Characteristic for the product is, however, the high content of LPG, i.e. liquefied

2o petroleum gas.

Potential purchasers of a hydrocarbon mixture of a quality corresponding to that which is described above, are power plants and gas treatment plants which are able to extract LPG from the mixture. ⁵ As appears from the above, this hydrocarbon mixture is of such a nature that, traditionally, it is considered to be of little value for the field as an export product, and it is therefore reinjected into the reservoir. With a simple and cheap process for making this petroleum fraction transportable, it 0 would, however, immediately have a considerable value. A solution whereby reinjection of this fraction is eliminated, represents, however, also an increased value for the field in the form of reduced operational costs in reinjection.

Thus, it is an object of the invention to provide a 5 method enabling an unstabilized hydrocarbon mixture to be transported with suitable ships.

Another object of the invention is to provide a simple and cheap solution for transport of such a mixture to land by means of suitable tankers, so that one may utilize a flow which

otherwise would be reinjected into the reservoir.

The above-mentioned objects are achieved with a method of the introductorily stated type which, according to the invention, is characterized in that a pressurized flow of the mixture is transferred via a pipeline to a tanker in the vicinity of the platform, that the mixture is subjected to an initial cooling and thereafter to a pressure relief on the tanker, with a cooling resulting therefrom, that the mixture is separated to an LPG mixture and surplus gas, and that the LPG mixture is stored in tanks on the tanker up to a desired degree of filling of the tanks, whereafter the tanker is disconnected from the pipeline, in order to transport the LPG mixture to a desired place of destination.

The invention will be further described below in connection with an exemplary embodiment with reference to the drawings, wherein

Fig. 1 is a principle drawing which, i.a., shows a production platform which is connected to a vessel through transfer conduits for a hydrocarbon mixture and gas;

Fig. 2 is a process diagram showing necessary modifica¬ tion on the platform; and

Fig. 3 is a process diagram showing necessary proces- sing equipment on the vessel.

In Fig. 1 there is shown a production platform 1 having a foundation 2 resting on the sea bed, and where the platform through a flowline 3 communicates with a production well 4 from an oil/gas reservoir. The platform 1 is connected with a loading buoy 5 for oil through transfer conduits 6 and 7. Further, the platform is connected with an anchored tanker 8 through transfer conduits 9 and 10 for hydrocarbon mixture and gas, respectively. When practising the method according to the invention, a pressurized flow of an unstabilized hydrocarbon mixture is transferred from the platform to the anchored vessel, wherein the mixture is cooled by pressure relief. Surplus gas is returned to the platform, whereas the produced LPG mixture is stored in the tanks of the vessel in cooled condition at a suitable temperature and pressure determined by the operating conditions. For example,

the LPG mixture may be stored at a temperature of -30 ^β C to -100 ^β C, e.g. -40 °C to -50 ^β C, and at a pressure of from 1,05 to 10 bars a. The vessel is disconnected from the pipelines when the tanks are full. During the time period when the vessel is not s available, the hydrocarbon mixture is reinjected into the reservoir.

The hydrocarbon mixture should, in the transfer, be pressurized to a pressure of at least 10 bars, preferably at least 20 bars, partly to overcome the pressure loss during the o transfer, and partly in order that dissolved quantities of light components (methane and ethane) by evaporation shall contribute to a desired cooling of the LPG fraction during the treatment process on board the tanker.

In practice, the pipelines 9 and 10 advantageously may s be connected with a submerged loading/unloading buoy of the so- called STL type (Submerged Turret Loading), where the buoy is anchored to the sea bed and arranged to be introduced into and secured releasably in a submerged downwardly open receiving space in the vessel, and further is arranged for connection to a pipe 0 system leading to the processing equipment of the vessel.

Even if, in the illustrated example of Fig. 1, there is shown a production platform, the method could as well be used in connection with a production vessel.

The different steps of the method according to the invention will now be further described with reference to Figs. 2 and 3. In these process diagrams there is, at a number of places along the line stretches, shown a symbol group consisting of a "rounded rectangle" and a parallelogram. The numbers in the rounded rectangles indicate the pressure of the hydrocarbon mixture in bars (absolute pressure), whereas the numbers in the parallelograms indicate the temperature of the mixture in °C at the topical place. The values stated as examples, have appeared from theoretical calculations in connection with a topical 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 sea water cooler 20 wherein the mixture is cooled, in the illustrated case from 46 ^β C to 25 ^β C. In the illustrated example, the mixture is thereafter supplied via a line stretch

21 with a stop valve 22 to a liquid dehydrator 23 wherein the mixture is dried by means of a suitable drying agent. The line or conduit stretch 21 is connected in parallel with a line stretch 24 containing an additional, normally closed valve 25. s Said units, which are shown within the delimited area in the Figure, represent new equipment on the platform.

Preferably, there is used a liquid dehydrator wherein there is used a drying mass which is not regenerated on the platform, but which is exchanged to be deposited or regenerated o on land. In Fig. 2 there is, however, also schematically shown a valve arrangement 26, 27 forming part of a possible regenera¬ ting system.

Even if it is presupposed above that drying of the hydrocarbon mixture is necessary, this will not always be the s case. Besides, such drying profitably may be carried out on the tanker, instead of on the platform/production vessel. Drying on the tanker has the advantage that exchange of the drying mass may take place when the vessel is alongside quay in connection with unloading of the LPG product. 0 After the possible drying of the hydrocarbon mixture, this is supplied to a pump 28 providing that the mixture is transferred through the pipeline 9 to processing equipment on the vessel 8.

As mentioned above, the hydrocarbon mixture is rein- jected into the reservoir when the tanker 8 is not available. In this case the valve 22 is closed, whereas the valve 25 is open. The mixture then is carried via a conduit (not shown) to the reinjection equipment of the processing plant.

As appears from Fig. 3, the hydrocarbon mixture from the platform is supplied to a suitable cooler 40, in the illustrated case a sea water cooler, in which the mixture is cooled, in the illustrated example from 41 °C to 15 °C. The cooler may be arranged on the vessel, but it may also consist of a cooling loop arranged in the water. Instead of using a separately arranged cooler or cooling loop, the hydrocarbon mixture may be subjected to the initial cooling in that the mixture during the transfer through the pipeline 9 is cooled because of heat exchange with the surrounding sea water. By thus utilizing conditions given by

the surrounding temperature or sea water 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, for example 1 km or more. Such a distance between the platform/production s vessel and the tanker as a rule will still be required out of regard for security zones around the installations.

After the above-mentioned cooling the flow of hydro¬ carbon mixture is subjected to a pressure relief in that the flow is conducted through a set of pressure reduction valves, in the o Figure represented by a single valve 41. Instead of pressure reduction valves, there may be used a liquid turbine, even if this is less cost efficient, as the generated energy quantity in a liquid turbine is relatively small.

In the illustrated example, the pressure of the mixture s flow after the pressure relief step 41 is reduced to 1,55 bars a, and the temperature is reduced to -18,5 °C. The degree of pressure reduction will be dependent on the storage specifica¬ tions of the tanker. In the present case there is used storage conditions given by a pressure of 1,05 bars a and a temperature of -48 °C.

After the pressure reduction the hydrocarbon mixture in the illustrated example is cooled to -48 "C by means of the own cooling system 42 of the vessel, where the mixture thereafter is supplied to a phase separator 43. In the phase separator the mixture is separated to a bottom product and gas. The bottom product, which is then an LPG mixture with a pressure of 1,05 bars a and a temperature of -48 °C, is conducted via a valve 44 to storage tanks 45 on the vessel. The valve 44 is arranged in order to be able to control the level in the phase separator. Normally it is preferred to carry out the pressure relief of the hydrocarbon mixture by means of one or more pressure reduction valves 41 before the additional cooling in the cooler or cooling system 42, primarily as a result of the fact that one may then dimension the cooler 42 for a lower pressure (approximately atmospheric pressure). It is, however, conceivable that one chooses to carry out all indirect cooling (i.e. cooling in heat exchangers) before the pressure relief. The valve/valves 41 in such cases will be placed in close connection with the inlet of the phase separator 43, and an additional cooling then

will not be of interest.

The gas separated in the phase separator 43 in the illustrated preferred embodiment is supplied to a compressor 46 which is shown to deliver a gas flow having a pressure of 63 bars 5 a and a temperature of 210 °C. The compressed gas is supplied to a compressor after-cooler 47 from which gas having a pressure of 62,5 bars a and a temperature of 55 °C is transported back to the platform 1 via the pipeline 10. The produced gas quantity is so small that it may go directly into the gas compression system of o the platform. The pressure level of the gas is determined on the basis of which stage in the gas compression line of the platform is found most convenient to introduce the gas.

The surplus gas can also have other applications than being transported back to the platform/production vessel. Thus, s it may be used as fuel on the tanker, or it may be burned off.

As regards the operation of the cooling system of the vessel, it is to be noted that the cooling circuit thereof may use evaporation gas from the storage tanks of the vessel as a cooling medium. Alternatively, a part of the gas from the phase o separator may be used in the cooling circuit.

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