Method and system for reducing liquid accumulation in a multiphase flow pipeline The invention relates to the production of natural gas from an offshore production plant, either subsea or on a platform. More specifically, it relates to a method and system for reducing liquid accumulation in a multiphase flow pipeline, in particular in connection with a reduction in production. To be more precise, the invention relates to a system for reducing liquid accumulation in a pipeline for conveying a multiphase flow between a production plant for gas and a processing plant for said gas, the processing plant comprising a separator for separation of liquid from the gas. The invention also relates to a method for reducing liquid accumulation in a pipeline for conveying a multiphase flow between a production plant for gas and a processing plant for said gas, comprising a separation of the liquid from the gas in said processing plant.

In recent years a number of gas fields have been found in deep water, and these are intended to be produced by allowing the gas to flow directly from the field, driven by the pressure in the reservoir. As a rule, the gas contains some liquid which may be condensed water, produced water, hydrate inhibitor or gas condensate. For this reason, the term'two-phase flow'or'multiphase flow'is used to refer to the conditions in the pipelines running ashore.

Figure 1 shows a typical arrangement for a gas field of this kind. Wellheads with Christmas trees and manifolds are arranged in a plant 4 on the seabed above the reservoir 2, with pipelines 10,12 running along the seabed to a terminal 6 onshore or on a platform in shallower water, where the gas is processed to a desired quality for further transport and sale. Figure 1 shows the bringing ashore of gas through at least two parallel pipelines.

In a two-phase flow, the liquid will, as a consequence of surface phenomena between the phases, be swept along by the gas (liquid sweeping). The higher the gas velocity is, the greater the sweeping effect will be. The liquid phase in a two-phase flow will flow at a lower velocity than the gas phase, and result in an accumulation of liquid in the pipeline until equilibrium occurs between the sweeping effect and the inertia of the liquid. This point of equilibrium is also highly dependent upon the profile of the pipeline. In the case of pipelines that rise between the field and the onshore terminal, the liquid velocity will be reduced as the liquid will tend to run backwards. However, the sweeping effect will also be present in this case, though the point of equilibrium will be reached at a higher liquid accumulation.

At a given gas velocity, the amount of liquid accumulated in the pipeline will be a direct function of the composition of the liquid, the liquid rate and the gradient of the pipeline.

Figure 1 shows an example of a steep gradient in the first part of the pipeline, followed by a gentler course towards the onshore terminal.

Figure 2 illustrates a calculation example of the conditions in a pipeline system as illustrated in Figure 1, and shows liquid accumulation as a function of gas rate. The example represents a 24"pipeline with a liquid rate (water and MEG (MonoEthyleneGlycol) ) of about 75 Sm3/million Sm3 gas. For the first 15 kilometres, the pipeline rises from an ocean depth of about 650 metres to 100 metres (Midwater line). In the next 30 kilometres, the pipeline rises from a depth of 100 metres to +10 metres above sea level (Trunkline). The pipeline is dimensioned for a capacity of 20 million Sm3/day (Design Flow).

As illustrated in Figure 2, calculations show an amount of accumulated liquid of about 100 mu in both pipe segments when the pipeline delivers full capacity. If the flow rate is reduced to 50%, the amount of accumulated liquid will almost double in both pipe segments. A reduction to 30% results in quantities of liquid of 800 m3 in the steep part and about 400 m3 in the less steep part, in total 1200 m3. Corresponding figures for a reduction to 20% flow are 800 in3 and 1550 m3, giving a total of 2350 m3.

For practical purposes, it should be noted that the simulation tool used in the calculations is inaccurate at low rates, and therefore calculated values in this range must be increased substantially before use in connection with the design of equipment.

When the capacity of a pipeline which has been operated at a low rate is to be increased to a higher rate, the difference in accumulated liquid volume between the two operating conditions will be evacuated from the pipeline system. For example, an increase (ramp- up) from 30% flow to full capacity will result in a discharge of about 1000 m3 of liquid in addition to the quantity of produced liquid. The discharge rate will be greatest when the flow increase starts; already at 50% flow, 800 m3 of a total of 1000 m3 will be out of the system.

At the point where the pipelines enter the onshore terminal, there must be liquid accumulators to receive such extra surge volumes, so-called"slug catchers". These are normally dimensioned on the basis of expected surge volume and desired time for ramp- up, and can easily become units of large dimensions.

Slug catcher dimensions will also be affected by the processing capacity of the onshore plant downstream of the slug catcher. However, it will be desirable to limit this capacity to a minimum above the design load of the plant.

It is desirable to reduce the accumulation of liquid in the pipeline system with correspondingly high discharge rates, which will reduce required dimensions for the slug catcher systems and limit required over-capacity in the liquid processing systems downstream of the slug catcher. A reduced liquid accumulation will mean a reduction in slug catcher size and pump-out rate, a reduction in capacity for the liquid separator (separation of condensate from water/MEG), a reduction of storage tanks for water/MEG and regenerated MEG, and a reduction in the need for MEG regeneration capacity.

It is therefore desirable to operate the pipeline system at high throughput so that good liquid sweeping is maintained.

However, it is not always possible to operate the system at such a high production rate as might be desirable. The export possibilities (from the processing plant) may be limited, or different circumstances may necessitate a shutdown of parts of or the whole of the production system.

In a dual pipeline transport system, flow rates below 50% of full capacity will normally mean that all the gas is routed through one of the pipelines in order to maintain good liquid sweeping and minimum liquid accumulation in the pipeline used. Therefore, when production rates are low, one pipeline will be"idle".

As shown in Figure 2, there is a distinct point at which the liquid accumulation rate suddenly increases with falling flow rate; in the example at about 50% of the pipeline capacity.

It is therefore desirable to maintain a gas flow through the pipeline well above this point, for example at least 60 to 70% of the pipeline capacity.

The present invention solves the problems described above and meets the desired objectives by means of a system for reducing liquid accumulation in a pipeline for conveying a multiphase flow between a production plant for gas and a processing plant

for said gas, the processing plant including a separator for separation of liquid from the gas. The system according to the invention is characterised by a recirculation line which at its first end may selectively be connected to the flow on the gas outlet side of the separator, and which at its second end may selectively be connected to the pipeline at the production plant, whereby a selected amount of gas under pressure may selectively be passed from the processing plant into the pipeline at the production plant so as to increase the gas rate in the pipeline and thus the sweeping effect between liquid and gas, with the object of reducing liquid accumulation in the pipeline.

Preferred features of the system according to the invention are set forth in attached claims 2-8.

The method for reducing liquid accumulation in a pipeline for conveying a multiphase flow between a production plant for gas and a processing plant for said gas, including a separation of the liquid from the gas in said processing plant, is characterised by selectively passing a selected volume flow of separated gas under pressure from the processing plant via lines into the pipeline at the production plant, so as to increase the gas rate in the pipeline and thus the sweeping effect between liquid and gas, with the object of reducing liquid accumulation in the pipeline.

Preferred features of the method according to the invention are set forth in attached claims 10-12.

The invention permits reduced accumulation of liquid in the pipeline system with correspondingly high discharge rates, which results in a reduction in the required dimensions for different systems as indicated above. The object of the invention is obtained by operating the pipeline system at high throughput so that good sweeping effect is maintained.

By recycling gas from the onshore terminal back to the starting point of the pipeline out in the field through the"idle"pipeline, it is possible to maintain a gas flow through the pipeline of well over 50% of the capacity of the line, for example, 60-70% of the line's capacity. Recycling is started when the amount of produced gas is reduced to a desired flow. Production is reduced further to desired export volume, whilst the recycle rate is increased to maintain the volume flow in the first pipeline.

Such recycling of gas back to the starting point of the pipelines offshore requires, in a system where the reservoir flow is driven ashore by nothing other than the reservoir pressure, that the recycle gas is passed into the recirculation line at a slightly higher pressure than the arrival pressure, either in that the recycle gas is compressed in a separate recycle compressor, or that optional export compressors are used. However, the power requirement is limited, as it is only the friction loss in the two pipelines that is to be compensated, and not the pressure loss as a result of a change in elevation. The friction loss is also smaller than at full flow rate, especially in the recirculation line.

A variant of the system could be used where a compressor station is installed as a part of the offshore pipeline system. Sufficient pressure for recycling is established at the compressor station, and at the onshore terminal the flow is split into produced pipeline gas and recycle gas.

An embodiment of the invention will now be described in further detail, with reference to the attached figures where like parts have been given like reference numerals.

Figure 1 is a schematic outline of a subsea gas field, field-to-shore pipelines and onshore facilities.

Figure 2 illustrates a calculation example and shows liquid accumulation as a function of gas rate.

Figure 3 is a schematic diagram of a first embodiment of the system according to the invention.

Figure 4 shows the system in Figure 3, and illustrates a state where both pipelines produce from the reservoir.

Figure 5 shows the system in Figure 3, and illustrates the method according to the invention.

Figure 6 is a schematic diagram of a second embodiment of the system according to the invention.

Figure 7 shows the system in Figure 6, and illustrates a state where both pipelines produce from the reservoir.

Figure 8 shows the system in Figure 6, and illustrates the method according to the invention.

Figure 9 is a schematic diagram of a third embodiment of the apparatus according to the invention.

Figure 10 is an example of a production system that utilises the system and method according to the invention.

Figure 1 is as mentioned above an illustration of a typical arrangement for a gas field and processing plant where the invention may be used. As mentioned, the invention requires at least two pipelines 10,12 between the processing plant 6 and the production plant 4. In a practical embodiment of the invention, it is assumed that gas from the individual wells is passed to manifolds on the seabed, which in turn are connected to the two field-to-shore pipelines. This is indicated in Figure 10, which will be described later.

Figure 3 is a schematic diagram of a first embodiment of the system according to the invention. The figure shows a subterranean reservoir 2 and a production plant 4 which in Figures 1 and 10 is shown as a subsea plant, but which may also be platform-based.

The two field-to-shore pipelines 10,12 are connected to respectively a first and second slug catcher and inlet separator 14,16 in the processing plant 6. The processing plant 6 is onshore or on a platform above the surface of the sea.

As mentioned, the gas is produced from reservoir 2 and passed via the production plant 4 in one or both pipelines 10,12 to the processing plant 6. In the processing plant, the liquid is separated from the gas in units 14,16 that are known per se and separated liquid is passed out through respective outlets 15,17 in a known way. The gas is then further processed in processing equipment 26 before being passed into an export compressor 28 for further transport through an export pipeline 34. The processing in the separators 14,16 and the processing unit 26 is prior art and is not included in the invention. Figure 1 also shows a number of valves, the function of which will be described in more detail below. It should also be noted that only components which are necessary or expedient to describe the invention are included here.

The two production pipelines 10,12 are connected together in the production plant 4 via an interconnecting line 22. This interconnection (between the pipelines 10 and 12) may be selectively opened or closed via the valve 24.

In the processing plant 6, a recirculation line 30 is connected between the outlet side of the export compressor 28 and one of the pipelines 10,12 before the respective inlet separator and slug catcher (14,16 respectively). Figure 3 shows the recirculation line 30 connected to the second pipeline 12, but the invention also includes the case where the recirculation line 30 is connected to the first pipeline 10. Thus, Figure 3 illustrates the inventive idea.

Referring now to Figure 4, which per se is identical to Figure 3, but which in addition illustrates a state in which both pipelines 10,12 produce from the reservoir 2. It can be seen that the valves 18, 20 in the production plant 4 are both open, and a multiphase flow thus flows through the pipelines 10,12 to the processing plant 6. The valves 36, 37 are also open so that the multiphase flow is passed into respectively the first 14 and second 16 slug catcher and inlet separator before the gas is further processed in the processing equipment 26 and compressed in the export compressor 28 for export via the line 34. The recirculation line 30 is shut off by the valves 32,33. Similarly, the valve 24 in the interconnecting line 22 is closed to prevent flow between the pipelines 10,12.

Figure 5 shows the apparatus in Figure 3, and illustrates the method according to the invention where a part of the pipeline 12 is used as a part of the recirculation line. The <BR> <BR> figure shows that the valve 20 is closed, i. e. , gas is not produced from the reservoir in the pipeline 12. Production from the reservoir 2 now takes place only via the pipeline 10, and it can be seen that the flow passes through the open valves 18,37 and into the inlet and processing equipment 14,26 before the gas is further compressed in the export compressor 28. The valves 20 and 36 in the pipeline 12 are, as mentioned, closed.

Valve 33 in the recirculation line 30 is open to allow a selected amount of gas to be passed from the outlet side of the export compressor through the recirculation line 30, through the open valve 32 and then through a part of the pipeline 12 to the offshore production plant 4. Since the valve 20 is closed and the valve 24 is open, the recycle gas is thus passed under pressure through the interconnecting line 22 and into the pipeline 10. In this way, the gas rate is increased in the pipeline 10, and with it the sweeping effect between liquid and gas so that the liquid accumulation in the pipeline 10 is reduced.

Figure 6 shows a second embodiment of the system according to the invention. Unlike Figure 3, the recirculation line 30'in this embodiment is connected before the processing system 26 and the export compressor 28. The figure shows that the recirculation line 30'is connected after the first slug catcher and the inlet separator unit 14, but, as mentioned, before the processing equipment and export compressor, and is then connected to the second pipeline 12 at basically the same point as in Figure 1.

However, this embodiment requires a separate recycle compressor 40, as shown in Figure 6. In other respects, the plant is as shown in Figure 3.

Figure 7 shows (in the same way as Figure 4) a state in which both pipelines produce from the reservoir. In this figure, the valves 24,32', 33'are closed whilst the other valves in the figures are open. Thus, a multiphase flow is produced from the reservoir through both pipelines 10,12.

Figure 8 shows, like Figure 5, the method according to the invention for this second embodiment. The valves 20 and 36 are closed, whilst the valves 22,32'and 33'are open thereby preventing a multiphase flow from being produced through the pipeline 12, but allowing a selected amount of (recycle) gas to be passed through the recirculation line 30', compressed by the recycle compressor 40 and passed back to the production plant 4 via the line 30', 12 and 22.

Figure 9 is a schematic diagram of a third embodiment of the system according to the invention, and shows a compressor 40'in the production pipeline 10. A configuration of this kind may be necessary if the reservoir pressure is insufficient. The embodiment shown in Figure 9 may of course also be combined with the embodiment shown in Figures 6,7 and 8.

As mentioned, the invention requires at least two pipelines 10,12 between the processing plant 6 and the production plant 4.

With regard to installations offshore, it is assumed that gas from the individual wells is passed to manifolds which in turn are connected to the two field-to-shore pipelines, and that it is possible to connect the two pipelines together. At low production rates, production from a small number wells is presumed as indicated in Figure 10.

In Figure 10 the onshore plant is shown with two pipelines which end in their respective shut-off valve and slug catcher. Downstream systems such as separator, dehydration,

optional hydrocarbon dew point control and export compression are indicated for one of the pipes only, but in practice both are connected to these systems.

The recycle compressor is shown with suction from the downstream inlet separator, and delivers to the recirculation line on"the outside"of the shut-off valve. A recycle flow based on the use of the export compressors is also indicated. In this case, the recycle gas passes through the whole process plant, which is not really necessary, though hydrate inhibition of the recycle gas is then not required.

Figure 10 shows a production scenario with production of just 10% of the capacity of <BR> <BR> the field. The recycle rate is 20% of the field's capacity, i. e. , the rate in the production pipeline is 60% of this line's capacity. With reference to Figure 2, the liquid accumulation will be reduced from 2350 m3 to just 350 m3.

If the field has been shut down, with liquid accumulation in the low points of the pipeline system, the recycle method can be used to circulate out most of the accumulated liquid before a new start-up of production. If the system in this case has gas under pressure, recycling can be started by starting the compressor, with recycling of all gas. If the system has reduced pressure, the pipeline system must be filled up with gas from the reservoir to the desired pressure before start-up.

Although the invention is illustrated in this application with reference to a subsea production plant (4), the skilled person will understand that the invention is just as suitable in the case of a production plant installed above the surface of the sea or at other locations. Although the invention is also illustrated with reference to a land-based processing plant (6), the skilled person will understand that the invention is not limited to such plants.