OA21633A - Subsea installation and process for treating gas from a subsea gas production field. - Google Patents

Abstract

The invention relates to an installation and a method for treating gas (2) from a subsea gas production field, comprising a cooling module (6) supplied with a gas/liquid mixture directly from at least one subsea gas field (4), and a gas/liquid separation module (8) delivering treated gas to a gas export line (14) and liquid to a liquid export line (16), the gas/liquid separation module comprising a gas/liquid separator (18) and a coalescence filter separator (20) connected in series, the gas/liquid separator having a gas outlet (18b) connected to an inlet (20a) of the coalescence filter separator and a liquid outlet (18c) connected to the liquid export line (16), the coalescence filter separator having a gas outlet (20b) connected to the gas export line and a liquid outlet (20c) connected to the liquid export line.

Description

Description

Title of the invention: Subsea installation and method for treating gas from a subsea gas production field

Technical Domain

[0001] The present invention relates to the general field of underwater gas production. It relates more specifically to the underwater processing of gas with a view to its export to offshore or onshore installations.

Previous technique

[0002] In current configurations, the export of gas from underwater production fields, and in particular from gas condensate fields, is carried out, among other things, by long pipelines connecting the production fields to surface installations.

[0003] Gas export installations used in practice generally do not include any treatment unit. The gas produced is typically exported through a pipeline without prior treatment (apart from the addition of chemicals to prevent solid deposits, limit corrosion, etc.). Publications WO 2015/181386, WO 2016/192813 and WO 2014/079515 are also known concerning gas export installations which include in particular a cooling module supplied with a gas/liquid mixture coming directly from the gas fields, and a gas/liquid separation module which makes it possible to separate the gas from the liquid in order to convey them to the surface installations via dedicated export lines.

[0004]This type of installation is subject to various problems that need to be addressed. One of the main problems encountered is the accumulation of liquid that can occur in the low points of the gas export line pipe. This problem occurs when the gas velocity becomes insufficient to entrain the liquid and causes plug flows in the gas export line. Another problem is to ensure that the pressure of the exported gas always remains sufficient to allow efficient export to the surface installations. It is also important to limit, or even avoid, the formation of solid deposits (hydrates, paraffins, etc.) in the gas export line.

Disclosure of the invention

[0005] The main object of the present invention is therefore to propose an underwater treatment installation which makes it possible to resolve the problems mentioned above.

[0006] This goal is achieved by means of a gas treatment plant originating from a subsea gas production field, comprising:

- a cooling module supplied with a gas/liquid mixture coming directly from at least one underwater gas field in order to cool the gas to a temperature allowing the heavy hydrocarbons and water present in the mixture to condense; and

- a gas/liquid separation module supplied with gas/liquid mixture from the cooling module and delivering treated gas to a gas export line and liquid to a liquid export line;

- and wherein, in accordance with the invention, the gas/liquid separation module comprises a gas/liquid separator and a coalescence filter separator connected in series, the gas/liquid separator having a gas outlet which is connected to an inlet of the coalescence filter separator and a liquid outlet which is connected to the liquid export line, the coalescence filter separator having a gas outlet which is connected to the gas export line and a liquid outlet which is connected to the liquid export line.

[0007] The gas treatment installation according to the invention is remarkable in particular in that its gas/liquid separation module comprises a gas/liquid separator mounted in series with a coalescence filter separator, which gives it a high separation performance. More generally, the installation according to the invention is based on lowering the dew point of the gas/liquid mixture by combining a cooling module and a high-performance gas/liquid separation module, which can allow passive export of the gas (i.e. without the need for a compressor). Such an installation thus finds a particularly advantageous application in gas fields located at great depth and far from the shore. Furthermore, the lowering of the dew point of the gas/liquid mixture is carried out at a level subsequently allowing export of the gas which remains totally (or almost totally) in the gas phase throughout the export.

[0008] More particularly, the cooling module of the installation according to the invention makes it possible to condense the heavy hydrocarbons and the water present in the gas/liquid mixture which would normally condense in the gas export line. The cooling is also carried out at a lower temperature than that reached by the gas in the gas export line.

[0009] The cooling module may comprise a passive heat exchanger or an active heat exchanger. In this case, it may further comprise a Joule-Thomson effect valve or a turbo-expander installed downstream of the heat exchanger.

[0010] The gas/liquid separator of the gas/liquid separation module may be a gravity separator or a cyclonic separator or a centrifugal separator.

[0011] Preferably, the gas/liquid separation module further comprises a gas pre-treatment unit disposed between the gas/liquid separator and the coalescing filter separator in order to remove solid particles which could be entrained with the gas and also to reduce the liquid content before the coalescing filter separator, in order to extend the service life of the coalescing filter separator.

[0012] The coalescence filter separator can be integrated with the gas/liquid separator.

[0013] The gas export line may advantageously be devoid of gas compression means.

[0014]As for the liquid export line, it can advantageously be provided with liquid pumping means.

[0015] Preferably, the gas/liquid separation module comprises two coalescence filter separators installed in parallel in order to ensure redundancy in the event of a problem on one of the separators and in order to ensure continuity of production during maintenance phases of the coalescence filters.

[0016] Also preferably, the installation further comprises means for injecting a hydrate formation inhibitor upstream of the cooling module and/or means for injecting a paraffin inhibitor upstream of the cooling module. These injection means make it possible to limit, or even avoid, the formation of solid deposits (hydrates, paraffins, etc.) in the installation as well as in the gas and liquid export lines.

[0017] Correlatively, the invention also relates to a method for underwater treatment of gas originating from an underwater gas production field, comprising successively:

- a step of cooling the gas/liquid mixture coming directly from at least one subsea gas production field so as to condense the heavy hydrocarbons and the water and to bring the gas/liquid mixture to a temperature lower than that of the gas in a gas export line; and

- a subsea gas/liquid separation step for delivering treated gas to a gas export line and liquid to a liquid export line; and wherein

- the gas/liquid separation step comprises a first gas/liquid separation step followed directly by a second gas/liquid separation step by coalescence in order to lower the dew point of the treated gas to a level allowing its export in single-phase form.

[0018] The invention also relates to an application of the method as defined above to a condensate gas field currently being exploited or to a condensate gas field not yet exploited.

Brief description of the drawings

[0019] [Fig. 1] Figure 1 is a schematic view of an underwater gas treatment installation according to the invention.

[0020] [Fig. 2] Figure 2 is a schematic view of an example of a coalescence filter separator of the installation according to the invention.

Description of the embodiments

[0021] The invention relates to the subsea treatment of gas from a subsea gas production field. It finds a preferred (but not limited) application to subsea gas condensate fields located at great depth and far from the shore.

[0022] Figure 1 shows an example of a subsea gas treatment installation 2 according to the invention supplied by one or more subsea gas production fields 4.

[0023]Typically, this gas treatment installation 2 comprises a cooling module 6 which is supplied with a gas/liquid mixture coming directly from the underwater gas field(s) 4, and a gas/liquid separation module 8 which is supplied with a gas/liquid mixture coming from the cooling module.

[0024] The cooling module 6 has the function in particular of cooling the gas/liquid mixture to bring it to a temperature allowing the heavy hydrocarbons and water present in the mixture to condense (and which would normally condense further downstream in the gas export line).

[0025] For this purpose, the cooling module 6 of the exemplary embodiment of FIG. 1 comprises a passive heat exchanger 10 whose inlet is directly connected to the outlet of the wellhead installations of the underwater gas field(s) 4, and a Joule-Thomson effect valve 12 positioned at the outlet of the passive heat exchanger 10.

[0026] The Joule-Thomson effect valve 12 is controlled so as to obtain a constant and predetermined pressure at the outlet of the gas export line (usually this type of valve is controlled in order to maintain a determined temperature at the valve outlet). By knowing the characteristics of the gas export line, it is thus possible to control the pressure downstream of the Joule-Thomson effect valve. Incidentally, by ensuring a certain level of gas pressure at the outlet of the gas export line, a maximum temperature is ensured at the outlet of the Joule-Thomson effect valve 12.

[0027]As an alternative to the passive heat exchanger, an active heat exchanger may be provided, possibly associated with a Joule-Thomson effect valve downstream. This active heat exchanger may be a liquid/gas exchanger with pumping of sea water, a gas/gas exchanger with reuse of the cold gas at the station outlet, an exchanger with heat transfer fluid and refrigeration loop, etc.

[0028] Alternatively to the Joule-Thomson effect valve, a turbo-expander may be provided installed downstream of the passive heat exchanger (or downstream of the active heat exchanger if applicable).

[0029] The gas/liquid separation module 8 receives as input the cooled gas/liquid mixture coming from the cooling module and delivers both treated gas to a gas export line 14 and liquid to a liquid export line 16.

[0030] According to the invention, the gas/liquid separation module 8 comprises a gas/liquid separator 18 and a coalescence filter separator 20 which are connected in series.

[0031] More specifically, the gas/liquid separator 18 has a gas/liquid mixture inlet 18a which is connected to the outlet of the cooling module 6, a gas outlet 18b which is connected to an inlet of the coalescence filter separator 20 and a liquid outlet 18c which is connected to the liquid export line 16.

[0032] As for the coalescence filter separator 20, it has an inlet 20a which is connected to the gas outlet 18b of the gas/liquid separator 18, a gas outlet 20b which is connected to the gas export line 14 and at least one liquid outlet 20c which is connected to the liquid export line 16.

[0033] The gas/liquid separator 18 of the gas/liquid separation module may be a gravity type separator or a cyclonic type separator or a centrifugal type separator. For example, a vertical separator with multiple pipes may be used.

[0034] In connection with FIG. 2, an example of the architecture of the coalescence filter separator 20 will now be described.

[0035] As is known, coalescence is the action by which the fine liquid droplets of the gas/liquid mixture created by the turbulence within the separator clump together to form larger droplets. This phenomenon allows for better gravitational separation of the liquid during the passage time in the separator.

[0036]In the exemplary embodiment of FIG. 2, the inlet 20a of the coalescence filter separator 20 is located in the lower part of the separator, the gas outlet 20b is located in the upper part of the separator, and two liquid outlets are provided, namely: an outlet in the lower part 20c-1, and an outlet in the intermediate part 20c-2.

[0037] The gas loaded with liquid droplets which enters the interior of the separator through the inlet 20a is subjected to a first phase of gravitational separation in the lower part 22 of the separator. A portion of the liquid resulting from this separation is evacuated through the liquid outlet in the lower part 20c-1.

[0038] The gas/liquid mixture then rises towards the upper part 24 of the separator by passing through one or more filter cartridges 26 within which the liquid droplets clump together to form larger droplets. The liquid droplets thus formed fall back and accumulate at the bottom of the upper part 24 of the separator to be evacuated towards the outlet in the intermediate part 20c-2. As for the gas freed from these liquid droplets, it is evacuated upwards via the gas outlet 20b.

[0039]It will be noted that the coalescence filter separator can be a separate device from the gas/liquid separator or it can be integrated into it.

[0040] It will also be noted that the gas/liquid separation module can advantageously comprise two coalescence filter separators installed in parallel in order to ensure redundancy in the event of a breakdown.

[0041] Preferably, the gas/liquid separation module 8 further comprises a gas pre-treatment unit 27 which is arranged between the gas/liquid separator 18 and the coalescence filter separator 20 in order to remove solid particles which could be entrained with the gas and reduce the liquid content before the coalescence filter separator.

[0042] Also preferably, the liquid export line 16 is provided with liquid pumping means, typically an underwater pump 29.

[0043]Conversely, the installation according to the invention can make it possible for the gas export line 14 to avoid having to resort to gas compression means.

[0044] More preferably, the installation further comprises means for injecting a hydrate formation inhibitor 28 upstream of the cooling module 6. Similarly, the installation can also comprise means for injecting paraffin inhibitor 30 upstream of the cooling module.

Claims (14)

Claims [Claim 1] Subsea installation (2) for processing gas from a subsea gas production field, comprising: - a cooling module (6) supplied with a gas/liquid mixture coming directly from at least one underwater gas field (4) in order to cool the gas to a temperature allowing the heavy hydrocarbons and water present in the mixture to condense; and - a gas/liquid separation module (8) supplied with gas/liquid mixture from the cooling module (6) and delivering treated gas to a gas export line (14) and liquid to a liquid export line (16); - characterized in that the gas/liquid separation module (8) comprises a gas/liquid separator (18) and a coalescence filter separator (20) connected in series, the gas/liquid separator having a gas outlet (18b) which is connected to an inlet (20a) of the coalescence filter separator and a liquid outlet (18c) which is connected to the liquid export line (16), the coalescence filter separator having a gas outlet (20b) which is connected to the gas export line and a liquid outlet (20c) which is connected to the liquid export line. [Claim 2] Installation according to claim 1, in which the cooling module (6) comprises a passive heat exchanger (10) or an active heat exchanger. [Claim 3] Installation according to claim 2, in which the cooling module further comprises a Joule-Thomson effect valve (12) or a turbo-expander installed downstream of the heat exchanger (10). [Claim 4] Installation according to any one of claims 1 to 3, in which the gas/liquid separator (18) of the gas/liquid separation module is a gravity separator or a cyclonic separator or a centrifugal separator. [Claim 5] An installation according to any one of claims 1 to 4, in which the gas/liquid separation module (8) further comprises a gas pre-treatment unit (27) arranged between the gas/liquid separator (18) and the coalescence filter separator (20) in order to remove solid particles which could be entrained with the gas and reduce the liquid content before the coalescence filter separator. [Claim 6] Installation according to any one of claims 1 to 6. 5, in which the coalescence filter separator is integrated with the gas/liquid separator. [Claim 7] Installation according to any one of claims 1 to 5. 6, in which the gas export line (14) is devoid of gas compression means. [Claim 8] Installation according to any one of claims 1 to 7, in which the liquid export line (16) is provided with liquid pumping means (29). [Claim 9] Installation according to any one of claims 1 to 8, in which the gas/liquid separation module comprises two coalescence filter separators installed in parallel. [Claim 10] Installation according to any one of claims 1 to 9, further comprising means for injecting a hydrate formation inhibitor (28) upstream of the cooling module (6). [Claim 11] Installation according to any one of claims 1 to 10, further comprising means for injecting paraffin inhibitor (30) upstream of the cooling module (6). [Claim 12] A method of subsea treatment of gas from a subsea gas production field, comprising successively: - a step of cooling the gas/liquid mixture coming directly from at least one underwater gas production field (4) so as to condense the heavy hydrocarbons and the water and to bring the gas/liquid mixture to a temperature lower than that of the gas in a gas export line; and - a subsea gas/liquid separation step for delivering treated gas to a gas export line and liquid to a liquid export line; - characterized in that the gas/liquid separation step comprises a 5 first gas/liquid separation step followed directly by a second gas/liquid separation step by coalescence in order to lower the dew point of the treated gas to a level allowing its export in single-phase form. [Claim 13] Application of the method according to claim 12 to a condensate gas field during operation. [Claim 14] Application of the method according to claim 12 to a condensate gas field not yet exploited.