Method for co2 storage

Abstract

The present invention relates to a method for CO2 storage, the method being carried out in an installation comprising a main pipeline fluidically connected to a subsea buffer tank, the subsea buffer tank being a pipeline other than the main pipeline, lying on the seabed and extending along a main axis between a lower end and an upper end, the method comprising: - a filling step comprising: - flowing CO2 through the main pipeline; - flowing CO2 out of the main pipeline and into an inlet of the subsea buffer tank located at the lower end of the subsea buffer tank; an emptying step comprising: - flowing CO2 out of an outlet of the subsea buffer tank, wherein the outlet of the subsea buffer tank is located at the lower end of the subsea buffer tank.

Description

[0001] METHOD FOR CO2STORAGE

[0002] Technical field

[0003] The present invention relates to the field of CO2 storage, and more specifically to a method for CO2 storage. The invention also relates to an installation for CO2 storage.

[0004] Technical background

[0005] Carbon capture storage (CCS) projects rely on transport and storage of CO2 from their emitters to the final CO2 subsurface or subterranean storage reservoir. The transport can be by CO2 carrier ships picking up the CO2 at an emitter’s location and delivering it to a temporary storage facility prior to injection in the final storage reservoir. A buffer volume is generally included in the system on land so as to manage the intermittent supply of CO2 to a system with a continuous outlet for injection. However the presence of such a CO2 buffer system onshore can present safety issues, for example in the event of a leak. Further, the intermittent supply of CO2 is typically at cryogenic temperature which needs to be conditioned to around zero degree or above before injection into a CCS reservoir. Conditioning the CO2 therefore results in a costly and non-environmentally friendly process as multiple heaters are needed to heat the CO2 up to a temperature suitable for injection into a subterranean storage reservoir.

[0006] Within this context, there is still a need for an improved method and installation for CO2 storage in a safer, more cost-effective and environmentally friendly manner.

[0007] Summary of the invention

[0008] It is therefore the object of this invention to provide a method for CO2 storage, the method being carried out in an installation comprising a main pipeline fluidically connected to a subsea buffer tank, the subsea buffer tank being a pipeline other than the main pipeline, lying on the seabed and extending along a main axis between a lower end and an upper end, the method comprising:

[0009] - a filling step comprising: o flowing CO2 through the main pipeline; o flowing CO2 out of the main pipeline and into an inlet of the subsea buffer tank located at the lower end of the subsea buffer tank;

[0010] - an emptying step comprising: o flowing CO2 out of an outlet of the subsea buffer tank, wherein the outlet of the subsea buffer tank is located at the lower end of the subsea buffer tank.

[0011] According to some embodiments, during the emptying step, CO2 flows out of the subsea buffer tank back into the main pipeline.

[0012] According to some embodiments, the method comprises a bypass step, wherein CO2 flow through the main pipeline bypasses the subsea buffer tank and directly flows to an injection well.

[0013] According to some embodiments, the bypass step is carried out at least partly simultaneously with the filling step, such that a portion of the CO2 flowing through the main pipeline is fed to the subsea buffer tank and another portion the CO2 flowing through the main pipeline directly flows to the injection well.

[0014] According to some embodiments, the bypass step is carried out at least partly separately from the filling step, such that the CO2 flowing through the main pipeline directly flows to the injection well.

[0015] According to some embodiments, during the emptying step, CO2 flows from the subsea buffer tank to the injection well.

[0016] According to some embodiments, the filling step comprises flowing CO2 into the main pipeline at a flow rate ranging from 500 to 3000 t / h, for example from 1000 to 2500 t / h.

[0017] According to some embodiments, the CO2 in the main pipeline is at a pressure ranging from 3 MPa to 27 MPa, for example from 50 MPa to 250 MPa.

[0018] According to some embodiments, a pump fluidically connected to the outlet of the subsea buffer tank pumps the CO2 flowing out of the subsea buffer tank.

[0019] According to some embodiments, the pump pumps the CO2 at a rate ranging from 80 t / h to 1200 t / h, for example from 200 t / h to 1000 t / h.

[0020] According to some embodiments, the pump pumps the CO2 to a pressure ranging from 3 MPa to 27 MPa, for example ranging from 5 MPa to 25 MPa.

[0021] According to some embodiments, a control valve fluidically connected to the inlet of the subsea buffer tank controls the CO2 flow into the subsea buffer tank.

[0022] According to some embodiments, the control valve controls the flow rate of the CO2 to enter the subsea buffer tank at a flow rate ranging from 500 t / h to 3000 t / h, for example from 1000 t / h to 2500 t / h. According to some embodiments, the CO2 in the subsea buffer tank is at a pressure ranging from 3 MPa to 7 MPa, for example from 4 MPa to 6 MPa.

[0023] According to some embodiments, the CO2 entering the subsea buffer tank is at a temperature ranging from -28 °C to -24 °C, for example -26 °C.

[0024] According to some embodiments, some of the CO2 is in a vapor phase and some of the CO2 is in a liquid phase so as to form a CO2 liquid-vapor interface in the subsea buffer tank.

[0025] According to some embodiments, the liquid-vapor interface moves during the filling step and / or the emptying step.

[0026] According to some embodiments, the method comprises an additional filling step, wherein additional CO2 flows from a transfer line into another inlet of the subsea buffer tank, the other inlet being preferably located at the upper end of the subsea buffer tank.

[0027] According to some embodiments, the additional filling step is carried out at least partly simultaneously with the emptying step.

[0028] According to some embodiments, the additional CO2 flows to the transfer line from an offshore CO2 source, for example a ship, a single anchor offloading unit, a cryogenic offloading tower or a near shore jetty FSIll.

[0029] According to some embodiments, the liquid-vapor interface moves during the additional filling step.

[0030] According to some embodiments, the method comprises taking pressure measurements and / or temperature measurements at the upper end of the subsea buffer tank and at the lower end of the subsea buffer tank.

[0031] According to some embodiments, the seawater heats the CO2 inside the subsea buffer tank.

[0032] According to some embodiments, the CO2 is heated to a temperature ranging from 1 °C to 30 °C within the subsea buffer tank.

[0033] According to some embodiments, the CO2 flows from the subsea buffer tank back to the mainland by a return pipeline fluidically connected to the outlet of the subsea buffer tank.

[0034] According to some embodiments, the main pipeline is at least partly subsea.

[0035] According to some embodiments, the main pipeline is fluidically connected to an onshore CO2 source.

[0036] It is further provided an installation for CO2 storage, wherein the installation comprises: a main pipeline; a subsea buffer tank being a pipeline other than the main pipeline, lying on the seabed and extending along a main axis between a lower end and an upper end; wherein the main pipeline is fluidically connected to the subsea buffer tank, the installation being configured to make CO2 flow from the main pipeline to an inlet of the subsea buffer tank and to make CO2 flow from an outlet of the subsea buffer tank, said inlet and outlet being located at the lower end of the subsea buffer tank.

[0037] According to some embodiments, the installation comprises a pump fluidically connected to the outlet of the subsea buffer tank.

[0038] According to some embodiments, the installation comprises a control valve configured for controlling the flow rate of CO2 from the main pipeline to the subsea buffer tank and / or from the subsea buffer tank to the main pipeline.

[0039] According to some embodiments, the main pipeline is fluidically connected to the subsea buffer tank by a connecting flow line comprising one end fluidically connected to an outlet of the main pipeline at a position along the length of the main pipeline, and another end fluidically connected to the inlet of the subsea buffer tank.

[0040] According to some embodiments, the outlet of the subsea buffer tank is fluidically connected to the main pipeline, the main pipeline being fluidically connected to an injection well at one end of the main pipeline.

[0041] According to some embodiments, the installation comprises a transfer line fluidically connected to another inlet of the subsea buffer tank, the other inlet being preferably located at the upper end of the subsea buffer tank.

[0042] According to some embodiments, the transfer line is fluidically connected to an offshore CO2 source, for example a ship, a single anchor offloading unit, a cryogenic offloading tower or a near shore jetty FSIll.

[0043] According to some embodiments, at least one pressure sensor is located at the upper end of the buffer tank and at least one pressure sensor is located at the lower end of the buffer tank.

[0044] According to some embodiments, at least one temperature sensor is located at the upper end of the buffer tank and at least one temperature sensor is located at the lower end of the buffer tank.

[0045] According to some embodiments, the outlet of buffer tank is fluidically connected to a return pipeline fluidically connected to the mainland.

[0046] According to some embodiments, the main pipeline is at least partly subsea. According to some embodiments, the main pipeline is fluidically connected to an onshore CO2 source.

[0047] According to some embodiments, the inlet of the subsea buffer tank is the same as the outlet of the subsea buffer tank.

[0048] The present invention makes it possible to address the need mentioned above. In particular, the method is carried out in an installation comprising a main pipeline fluidically connected to a subsea buffer tank, the subsea buffer tank being a pipeline other than the main pipeline, lying on the seabed and extending along a main axis between a lower end and an upper end. The installation therefore allows for an offshore temporary storage space for CO2, and therefore a safer temporary storage space for CO2, in the form of the buffer tank. Flowing CO2 out of the main pipeline and into an inlet of the subsea buffer tank located at the lower end of the subsea buffer tank allows for offloading a volume of CO2 flowing through the pipeline. This can be particularly useful for accommodating variations in the influx of CO2 into the main pipeline (for example due to batches of CO2 delivery at a relatively high flow rate) and / or variations in the outflow of CO2 to an injection facility. The subsea buffer tank can also be advantageous if it is desired to condition the CO2 for injection. The seawater surrounding the buffer tank may heat the CO2 inside the buffer tank to a temperature suitable for injection. Therefore, not only can the buffer tank store a surplus supply of CO2 but it can condition the CO2 while storing it. This allows for a more cost-effective and environmentally friendly solution than an onshore temporary storage unit and the number of heaters used to condition the CO2 can be reduced.

[0049] Further, the method comprises an emptying step comprising flowing CO2 out of an outlet of the subsea buffer tank, wherein the outlet of the subsea buffer tank is located at the lower end of the subsea buffer tank. According to some embodiments, the emptying can allow for providing a reserve of CO2 back into the main pipeline and then delivering the CO2 through the main pipeline to the intended facility. Additionally or alternatively, this can allow for providing a reserve of CO2 directly to a facility (e.g. an offshore injection facility, such as an injection well, or a return pipeline to an onshore injection facility, or an onshore transport system for injection at an offshore injection facility). Again, this is useful for accommodating variations in the influx of CO2 into the main pipeline (for example due to batches of CO2 delivery at a relatively high flow rate) and / or variations in the outflow of CO2 to an injection facility.

[0050] The filling step and emptying step together allow for controlling the manner in which the CO2 is stored in the buffer tank, and consequently the supply of CO2 to its intended location / s (subterranean storage reservoir / s). The method may comprise adapting each step so as to optimize supply to the intended location / s. The steps can thereby enable an optimal (e.g. maximum or minimum) amount of CO2 to be supplied to the intended location / s, and at a desired frequency.

[0051] Moreover, by the buffer tank lying on the seabed and extending along a main axis between a lower end and an upper end and by flowing CO2 into the an inlet of the buffer tank located at the lower end of the tank and by flowing CO2 out of an outlet of the buffer tank located at the lower end of the tank, CO2 flowing from the main pipeline can easily spill into the buffer tank and CO2 flowing out of the buffer tank can flow naturally (by gravity) and in a liquid state that is ideal for injection into a carbon storage facility.

[0052] Brief description of the drawings

[0053] Non-limiting examples will now be described in reference to the accompanying drawings, where:

[0054] FIG. 1A schematically shows an example of a comparative installation for subsea CO2 storage not according to the invention.

[0055] FIG. 1 B, FIG. 2, FIG. 3 and FIG. 4 each schematically show an example of an installation for subsea CO2 storage according to the invention.

[0056] FIG. 5 schematically shows an enlarged portion of FIG. 4, as labelled by dashed box A in FIG. 4.

[0057] Like reference numerals have the same meaning in the different drawings unless otherwise specified.

[0058] Detailed description

[0059] The invention will now be described in detail without limitation in the following description.

[0060] It is provided a method for CO2 storage, preferably subterranean storage. The method is carried out in an installation comprising a main pipeline (i.e. a pipeline for transporting CO2) flu id ically connected to a subsea buffer tank (i.e. a tank for temporarily storing an overflow of CO2 from the main pipeline). The subsea buffer tank is a pipeline other than the main pipeline (i.e. the main pipeline and buffer tank are two separate entities). The subsea buffer tank lies on the seabed and extends along a main axis between a lower end and an upper end. The lower end is the end connected to the main pipeline and therefore generally closer to the main pipeline.

[0061] Preferably, the buffer tank lies along a slope. In other words, an elevation difference is created along the length of the buffer tank. In this case, the lower end is deeper than the upper end, i.e. the upper end of the buffer tank lies at a distance to the water surface which is less than the distance of the lower end of the buffer tank to the water surface.

[0062] The method comprises a filling step (i.e. of CO2). The filling step comprises flowing CO2 through the main pipeline (e.g. from an onshore CO2 source). The filling step comprises flowing CO2 out of the main pipeline (from an outlet thereof) and into an inlet (e.g. a port) of the subsea buffer tank located at the lower end of the subsea buffer tank. The method comprises an emptying step (i.e. of CO2). The emptying step comprises flowing CO2 out of an outlet (e.g. a port) of the subsea buffer tank, wherein the outlet of the subsea buffer tank is located at the lower end of the subsea buffer tank.

[0063] The main pipeline may be a subsea pipeline. The main pipeline may be at least partly subsea, for example substantially subsea. The main pipeline may be entirely subsea. Alternatively, the main pipeline may be partly onshore, for example for one or more portions of the main pipeline that may be connected to the mainland. One or more of these portions of the main pipeline may lie onshore (i.e. may lie on the surface of the mainland). Additionally or alternatively, one or more of these portions of the main pipeline may be buried onshore.

[0064] It should be understood that the inlet of the subsea buffer tank (at the lower end thereof) connected to the main pipeline and the outlet of the subsea buffer tank (at the lower end thereof) connected to the main pipeline may be one and the same port (in which case CO2 may flow in both directions through this port) or may be two different ports.

[0065] In some variations, during the emptying step, CO2 may flow from the outlet of the subsea buffer tank to an inlet of the main pipeline. In this case, the inlet of the main pipeline connected to the subsea buffer tank (at the lower end thereof) and the outlet of the main pipeline connected to the subsea buffer tank (at the lower end thereof) may be one and the same port (in which case CO2 may flow in both directions through this port) or may be two different ports.

[0066] FIG. 1 A provides an example of a comparative installation for CO2 storage not according to the invention. A volume of liquid CO2 on board a ship 102 may be transported onshore via an offloading arm 104 to an temporary onshore storage unit 108. The CO2 may pass through a receiving pump 106 prior to entering the temporary onshore storage unit 108. The CO2 may then exit the onshore storage unit 108 and pass through a transfer pump 110 and heat exchanger 112 before entering an onshore pipeline 114. The CO2 may flow into one end of a main pipeline 118. A flowmeter 116 may measure the flow rate of the CO2 before the CO2 enters the main pipeline 118. The CO2 may flow through the pipeline 118 and from the other end of the main pipeline 118 may enter an injection flow line 122. The CO2 may flow through the injection flow line 122 and from the injection flow line 122 into an injection well 126. The injection well 126 may be an offshore injection well 126. The injection well 126 may rise above sea level (such as a platform well). The injection well 126 may be supported by a rigid riser 128. A pump 124 may assist the injecting by pumping the CO2 into the injection well 126.

[0067] FIG. 1 B provides an example of an installation for CO2 storage according to the present invention. The differences relative to the example of FIG. 1A are the following. The main pipeline 118 (also referred to herein as the main pipeline) may be connected to an onshore CO2 source (or onshore terminal 138, as shown in FIG. 2) at one end of the pipeline 118. The onshore source (or terminal 138) may be an intermediate onshore source, for example CO2 may flow from a ship 102 to the pipeline 118 as described with regard to FIG. 1 A without flowing into a temporary onshore storage unit 108 prior to flowing into the pipeline 118. The main pipeline 118 may be connected at its other end (i.e. the end opposite to that connected to the CO2 source) to an injection well 126. Alternatively, the injection well 126 may be a subsea injection well (i.e. without the riser 128). Instead of a temporary onshore storage unit 108, in FIG. 1 B, the installation comprises the main pipeline 118 being fluidically connected to a subsea buffer tank 132. This simplifies the onshore source or onshore terminal 138 and provides for a safer installation for the temporary storage of CO2.

[0068] In other words, in preferred embodiments, no onshore temporary CO2 storage facility is used in the method / is present in the installation.

[0069] The subsea buffer tank 132 is a pipeline other than the main pipeline 118. Although this not clearly visible on FIG. 1 B and FIG. 2, the subsea buffer tank 132 preferably partially or fully lies on the seabed 100. The main pipeline 118 may also lie at least partially on the seabed 100. The main pipeline 118 may run along a sloped surface of the seabed 100. As shown in FIG. 2, the pipeline 118 may lie along an uneven sloped surface of the seabed 100. The buffer tank 132 may too lie along an uneven sloped surface of the seabed 100. The main pipeline 118 and the buffer tank 132 may have different geometries (for example, length, internal diameter, external diameter, wall thickness). The buffer tank 132 may have a larger (internal) diameter than the main pipeline. The buffer tank 132 may have an internal diameter ranging from 60.96 cm to 182.88 cm. The wall thickness of the buffer tank 132 may be a function of the internal diameter of the buffer tank 132 and of the pressure rating of the buffer tank 132. If the internal diameter of the buffer tank 132 is greater than that of the main pipeline 118, the wall thickness of the buffer tank 132 may be greater than the wall thickness of the main pipeline 118. The main pipeline 118 and the buffer tank 132 may have different material properties. Therefore, CO2 can efficiently flow through the main pipeline and can efficiently flow into the buffer tank (and for example be withheld within the buffer tank or flow simultaneously out of the buffer tank).

[0070] A filling step comprises flowing CO2 through the main pipeline 118 and flowing CO2 out of the main pipeline 118 into an inlet of the subsea buffer tank located at the lower end of the subsea buffer tank (not indicated in FIG. 1 B or FIG. 2). The installation may comprise one or more other flow lines like the injection flow line 122 to facilitate connection between different elements of the installation, for example between respective inlets and outlets of the buffer tank and corresponding inlets and outlets of the main pipeline. An outlet of the main pipeline 118 may be located at a position along the length of the main pipeline 118. The pipeline 118 may be fluidically connected to the inlet of the subsea buffer tank 132 by a first connecting flow line (not shown) comprising one end fluidically connected to the outlet of the main pipeline, and another end fluidically connected to the inlet of the subsea buffer tank 132. In other variations, the inlet of the subsea buffer tank can be directly connected to the outlet of the main pipeline without any intermediate flow line. The filling step can allow for excess CO2 in the pipeline to be temporarily stored in the buffer tank. Excess CO2 in the pipeline refers to an amount of CO2 in the pipeline which is larger than a predetermined threshold value. There can be excess CO2 in the pipeline, for example if there is (e.g., transiently) no injection into the injection well, while there is still an influx of CO2 into the pipeline from a CO2 source, or if the flow rate of the CO2 from one or more CO2 sources into the pipeline is (e.g., transiently) larger than the flow rate of the CO2 from the pipeline to the injection well.

[0071] The filling may be a continuous filling, which may be preferable for a continuous reduced quantity of CO2 delivered to the injection well by the main pipeline. This can allow for the buffer tank to steadily remove excess CO2 from the main pipeline or to keep the amount of CO2 in the main pipeline below a predetermined threshold value (for example substantially at a certain constant value). Alternatively, the filling may be intermittent, i.e. CO2 may fill the buffer tank in several batches.

[0072] The emptying step comprises flowing CO2 out of an outlet of the subsea buffer tank, wherein the outlet of the subsea buffer tank is located at the lower end of the subsea buffer tank (not shown in FIG 1 B). As shown in FIG. 2, the pipeline 118 may be fluidically connected to the outlet of the subsea buffer tank 132 by a second connecting flow line 154 comprising one end fluidically connected to the inlet of the main pipeline 118, and another end fluidically connected to the outlet of the subsea buffer tank 132. In other variations, the outlet of the subsea buffer tank can be directly connected to the inlet of the main pipeline 118 without any intermediate flow line.

[0073] The emptying may be a continuous emptying, which may be preferable for a continuous supply of CO2 to the main pipeline, and / or directly to an offshore injection well, and / or to a return pipeline fluidically connected to an onshore injection facility (e.g. onshore injection well); all three options are referred collectively to as a “future location / s”. This can allow for a steady supply of CO2 to its future location. The main pipeline 118 may be directly connected to the offshore injection facility, or an additional pipeline may link the main pipeline 118 to the offshore injection facility. Emptying may comprise flowing CO2 from the buffer tank 132 to the main pipeline 118 and from the main pipeline 118 to an offshore injection well 126, and also injecting CO2 directly from the buffer tank 132 to the same injection well 126. Alternatively, emptying may comprise flowing CO2 from the buffer tank 132 to the main pipeline 118 and from the main pipeline 118 to an injection well 126, and also injecting CO2 directly from the buffer tank 132 to a different injection well 126 (onshore or offshore). Simultaneous steady filling and steady emptying can allow for stalling the rate at which CO2 is supplied to its future location / s. Alternatively, the emptying may be intermittent. This may be particularly advantageous when a sudden, readily available supply of CO2 is needed at the future location. A pump 134 fluidically connected to the outlet of the subsea buffer tank 132 may pump the CO2 flowing out of the subsea buffer tank 132. The pump 134 can thereby facilitate emptying the buffer tank 132 after the flowing of CO2 into the buffer tank 132.

[0074] In the example of FIG. 1 B, during the emptying step, CO2 may flow along the injection flow line 122 and into the offshore injection well 126, supported by rigid riser 128. CO2 may flow to the injection well 126 in a liquid phase. An additional pump 124 may assist the injecting of the CO2 into the injection well 126. The CO2 may be two-phase CO2 during injection into the injection well 126. Instead of the injection well 126 associated with the riser 128 as shown in FIG. 1 B, a fully subsea injection well can also be used (i.e. with no riser 128).

[0075] Additionally or alternatively, and as shown in the example of FIG. 2, during the emptying step, CO2 may flow back into the main pipeline 118. CO2 may flow back into the main pipeline via the second connecting flow line 154 described above. A pump 134 placed on the second connecting flow line 154 may assist in displacing the CO2 through this second connecting flow line 154. The CO2 may then flow from the pipeline 118 to the injection well 126. The injection well 126 may be supported by a rigid riser 128 above sea level (as shown) or may be a fully subsea injection well. The CO2 may flow from the pipeline 118 to the injection well 126 via the injection flowline 122 described above in connection with FIG. 1A and FIG. 1 B. A valve 129 for controlling the injection of CO2 into the injection well 126 may be located along the injection flow line 122. CO2 may flow through the injection well 126 into a subterranean storage reservoir 127.

[0076] Additionally or alternatively, CO2 may flow from the buffer tank 132 to a return pipeline fluidically connected to the outlet of the subsea buffer tank. The return pipeline, being connected to the buffer tank 132 at one end, may be fluidically connected to the mainland at the other end.

[0077] The CO2 flowing through the main pipeline 118 may be at a temperature ranging from 1 °C to 32 °C, for example from 2 °C to 30 °C, for example from 5 °C to 25 °C. The CO2 may flow from the main pipeline 118 into the buffer tank 132 at any one of these temperature ranges. The main pipeline 118 may be designed to operate at temperatures ranging from -10 °C to 60 °C, for example from -5 °C to 40 °C. The seawater 120 may heat the CO2 inside the buffer tank 132. This can allow for temporarily storing a volume of CO2 while also heating the CO2 to a temperature suitable for injection into an injection well without need for additional heaters to condition the CO2. The seawater 120 may condition the CO2 to above 0 °C. If the CO2 flows from the main pipeline 118 into the buffer tank 132 at a temperature that is already suitable for injecting the CO2 into an injection well, the seawater 120 can heat the CO2 so as to maintain the CO2 at a temperature suitable for injection.

[0078] The method may comprise filling the buffer tank 132 before emptying the buffer tank 132, so as to provide a sufficient amount of time for the seawater 120 to effectively heat the CO2. Alternatively, the filling and emptying may be continuous, with the emptying occurring at a reduced rate (e.g. by reducing the rate at which the pump 134 pumps the CO2) so as to provide a sufficient amount of time for the seawater 120 to effectively heat the CO2.

[0079] A step of continuous filling of the buffer tank 132 and a simultaneous step of continuous emptying of the buffer tank 132 may allow for providing a steady stream of conditioned CO2 to its future location. A step of non-continuous emptying may allow for providing, to its future location / s, a predetermined batch of CO2 that is conditioned.

[0080] The seabed 100 or soil upon which the buffer tank 132 lies may too heat the CO2 in the buffer tank 132. The seabed may be at a temperature ranging from 0 °C to 5 °C.

[0081] As previously mentioned, the CO2 in the pipeline 118 may be at a pressure ranging from 3 MPa to 27 MPa, for example from 5 MPa to 25 MPa. The CO2 in the subsea buffer tank 132 may be at a pressure ranging from 3 MPa to 7 MPa, for example from 4 MPa to 6 MPa. The buffer tank 132 may have a design pressure ranging from 10 MPa to 20 MPa, for example 15 MPa. The method may comprise operating the buffer tank 132 at saturation pressure, and thereby at a pressure lower than the pressure of the main pipeline 118. According to some embodiments, the design pressure of the buffer tank 132 may be the same as that of the main pipeline 1 18 for safety purposes. The buffer tank 132 may comprise dimensions selected for withholding a predetermined offload volume (batch) of CO2. The buffer tank 132 may be a large diameter pipeline. The buffer tank 132 may have an internal diameter ranging from 60 cm to 200 cm, for example ranging from 76.2 cm to 160 cm. The buffer tank 132 may have a length ranging from 5 km to 70 km, for example from 20 km to 50 km. The buffer tank 132 may have an internal volume ranging from 10 m3to 50,000 m3. A 2D map of the seabed area may be formed prior to designing the buffer tank 132. This can allow for designing the buffer tank 132 to have dimensions (e.g. taking into account the slope of the seabed 100) suited to the intended location on the seabed 100 for placing the buffer tank 132.

[0082] FIG. 3 provides an example of the flow of CO2 in and out of the buffer tank 132. CO2 may flow through the pipeline 118 (e.g., from a source supplied by an onshore terminal, as indicated by arrow 190a) to the buffer tank 132, as indicated by arrow 190b. The filling step may comprise flowing CO2 into the main pipeline 118 at a flow rate ranging from 500 to 3000 t / h, for example from 1000 to 2500 t / hr. The CO2 in the main pipeline 118 may be at a pressure ranging from 3 MPa to 27 MPa, for example from 5 MPa to 25 MPa. The buffer tank may act as an expansion tank to the main pipeline 118 for pressure increases experienced by the main pipeline 118. The buffer tank may therefore act as a saturation point for the pipeline by allowing CO2 in a vapor form to flow into the pipeline. This may result in the main pipeline carrying only (or at least mostly) liquid CO2.

[0083] A control valve 194 connected to the inlet of the buffer tank 132 may control the flow of CO2 flowing into the buffer tank 132. The valve 194 may provide a batch flow to the buffer tank 132. In other words, the control valve 194 may open intermittently so as to provide a predetermined volume of CO2 into the buffer tank 132. Additionally or alternatively, the same or another control valve 194 may be connected to the outlet of the buffer tank 132. The control valve 194 may control the flow of CO2 leaving the buffer tank 132. In this way, the valve 194 can control predetermined volumes of CO2 that can be released for flow to a future location / s, such as flowing back into the main pipeline 118 (and through the main pipeline 118 to an injection well 126, for example directly from the main pipeline 118 to the injection well, or for example via an additional pipeline linking the main pipeline to the injection facility), flowing directly to an injection well 126 or flowing back to the mainland via a return pipeline. Alternatively, the inlet and outlet of the buffer tank 132 may be the same, i.e. may be the same port providing the same flow path for the CO2. In this case where the inlet of the buffer tank 132 and the outlet of the buffer tank 132 are the same, the control valve 194 may control both the influx of CO2 into the buffer tank 132 and the outflow of CO2 from the buffer tank 132. If the inlet and outlet of the buffer tank are the same, the outlet of the main pipeline 118 for flowing CO2 to the buffer tank 132 and the inlet to the main pipeline 118 for flowing CO2 from the buffer tank 132 may also be the same. In this case, the first connecting flow line and the second connecting flow line 154 may be the same flow line.

[0084] As mentioned above, a pump 134 may be positioned at the outlet of the buffer tank 132 to facilitate emptying of the buffer tank 132. The control valve 194 may be fluidically connected to the main pipeline 118, for example the control valve may be connected directly to the pipeline 118 or by means of a flowline. The control valve 194 may open at a predetermined pressure to allow CO2 to flow into the buffer tank 132. The buffer tank 132 can therefore act as an overspill or overflow unit to the main pipeline 118. The buffer tank may comprise a vapor portion 132a and a liquid portion 132b, forming a CO2 vapor / liquid interface (interface level) within the buffer tank. As an alternative to the control valve opening at a predetermined pressure to allow CO2 to flow into the buffer tank 132, the control valve 194 may be open or closed depending on the interface level within the buffer tank 132.

[0085] The control valve 194 may control the flow rate of the CO2 to enter the buffer tank at a flow rate ranging from 500 t / h to 3000 t / h, for example from 1000 t / h to 2500 t / h. The buffer tank 132 may therefore be able to receive CO2 at the same rate (or at a rate similar) to the rate at which CO2 flows through the main pipeline 118. This can allow for efficient operation of the installation and an easy removal of a volume of CO2 from the main pipeline 118. The main pipeline may comprise a first part 118a and a second part 118b. CO2 may leave the main pipeline 118 from the first part of the pipeline 118a and enter the buffer tank 132. At the outlet of the buffer tank 132, and as demonstrated by arrow 190d, CO2 may flow out of the buffer tank 132 and the pump 134 may pump the CO2 to back into the pipeline 118, such as at the second part of the pipeline 1 18b. Additionally or alternatively, the pump 134 may pump the CO2 to another location / s. The buffer tank can therefore absorb the swing between batch influx and injection flow and may comprise a varying ratio of vapor CO2 132a to liquid CO2 132b of CO2. The vapor volume in the buffer tank may be considered to be the free storage space volume, or the buffer volume, in the buffer tank 132. The pump 134 may pump the CO2 out of the buffer tank 132 at a rate lower than that at which the CO2 flows into the buffer tank 132. The pump 134 may pump the CO2 out of the buffer tank 132 at a rate ranging from 80 t / h to 1200 t / h, for example from 200 t / h to 1000 t / h. The pump 134 may pump the CO2 to a pressure ranging from 3 MPa to 27 MPa, for example ranging from 5 MPa to 25 MPa. The CO2 may flow on to an injection well

[0086] 126 for injection into a subterranean reservoir 127 (e.g. a well or injection point) at a rate ranging from 80 t / h to 1200 t / h, for example from 200 t / h to 1000 t / h. The CO2 may flow on to the injection well 126 for injection into a subterranean reservoir

[0087] 127 at a pressure ranging from 3 MPa to 27 MPa, for example ranging from 5 MPa to 25 MPa.

[0088] The control valve 194 may open at the inlet to the buffer tank 132 when a predetermined volume of liquid CO2, or when no liquid CO2, is inside the buffer tank 132. The buffer tank 132 may be less than or equal to 95% full, or for example less than or equal to 75% full, or for example less than or equal to 50% full, or for example less than or equal to 25% full, or for example empty, or substantially empty. The control valve 194 may close at the inlet to the buffer tank 132 when a predetermined volume of liquid CO2 has filled the buffer tank 132. The buffer tank 132 may be full of liquid CO2, or substantially full of liquid CO2, or for example more than or equal to 75% full, or for example more than or equal to 50% full, or for example more than or equal to 25% full. Calculating the liquid level inside the buffer tank 132, for example by pressure and / or temperature sensors 152a, 152b, of FIG. 4 and FIG. 5 (discussed in further detail in relation to these figures) may trigger the opening and / or closing of the control valve 194 at the inlet when a predetermined volume of liquid CO2 is detected within the buffer tank 132. The control valve 194 may open at the outlet to the buffer tank 132 when a predetermined volume of liquid CO2 is inside the buffer tank 132. The buffer tank 132 may be full of liquid CO2, or substantially full of liquid CO2, or for example more than or equal to 75% full, or for example more than or equal to 50% full, or for example more than or equal to 25% full. The control valve 194 may close at the outlet to the buffer tank 132 when a predetermined volume of liquid CO2 has left the buffer tank 132. The buffer tank 132 may be less than or equal to 95% full, or for example less than or equal to 75% full, or for example less than or equal to 50% full, or for example less than or equal to 25% full, or for example empty, or substantially empty. Calculating the liquid level inside the buffer tank 132, for example by the pressure and / or temperature sensors 152a, 152b, may trigger the opening and / or closing of the control valve 194 at the outlet when a predetermined volume of liquid CO2 is detected within the buffer tank 132.

[0089] Controlling the flow of liquid CO2 in and out of the buffer tank 132 in such a manner can be particularly advantageous when the CO2 is supplied to the main pipeline 118 by a CO2 source other than a control plant. The main pipeline 118 can therefore accept a full batch of CO2 from a source that cannot necessarily stagger its CO2 supply into the main pipeline 118 as a function of actual injection well capacity. For example, as shown in FIG. 1 B and FIG. 2, a ship 102 may need to offload its total supply of CO2 through the onshore terminal 138 to the main pipeline 118 in one delivery. The buffer tank 132 enables the main pipeline 118 to take on this load of CO2 regardless as to whether it is currently possible to transfer the CO2 on to an injection well 126 or return pipeline or not; and regardless as to whether it is possible or not to transfer the CO2 on to an injection well 126 or return pipeline at the same flow rate as the incoming flow rate of CO2.

[0090] FIG. 4 also displays an example of the installation. Some of the CO2 may be in a vapor phase 132a and some of the CO2 may be in a liquid phase 132b so as to form a CO2 liquid-vapor interface in the subsea buffer tank 132. The liquidvapor interface may move during the filling step and / or the emptying step. CO2 may enter the main pipeline 118 from an onshore source of liquid CO2 166. CO2 may flow from the main pipeline 118 into the inlet of the buffer tank located at the lower end of the buffer tank. CO2 may flow out of the buffer tank 132 through an outlet located at the lower end of the buffer tank 132. The flow of CO2 from the main pipeline 118 into the buffer tank 132 and the flow of CO2 out of the buffer 132 tank will be described in more detail in relation to FIG. 5. The liquid-vapor interface in the buffer tank 132 may depend on the rate of CO2 flowing into the buffer tank and the rate of CO2 flowing out of the buffer tank. For example, if the rate of flowing CO2 into the buffer tank 132 (influx rate) is higher than the rate of flowing CO2 out of the buffer tank 132 (e.g. injection rate), or there is no CO2 simultaneously flowing out of the buffer tank 132, the liquid / vapor interface may move to a higher point of the buffer tank 132, i.e. the interface of liquid CO2 132b and vapor 132a may move upstream (closer to shore). Likewise, if the rate of flowing CO2 out the buffer tank 132 (e.g. injection rate) is higher than the rate of flowing CO2 out of the buffer tank 132 (i.e. influx rate), or there is no CO2 simultaneously flowing into the buffer tank 132, the liquid / vapor interface may move to a lower point of the buffer tank 132, i.e. the interface of liquid CO2 132b and vapor 132a may move downstream (further from shore).

[0091] The method may comprise taking pressure measurements and / or temperature measurements at the upper end of the buffer tank 132 and at the lower end of the buffer tank 132. The installation may comprise a temperature sensor 152a at the upper end of the buffer tank 132 and a temperature sensor 152b at the lower end of the buffer tank 132 to take the temperature measurements. The installation may comprise a pressure sensor 152a at the upper end of the buffer tank 132 and a pressure sensor 152b at the lower end of the buffer tank 132 to take the pressure measurements. The installation may comprise a pipeline end module 155a (PLEM) located at the upper end of the buffer tank and another PLEM 155b located at the lower end of the buffer tank 132. Each PLEM may provide a simplified connection (and therefore easier access) to the buffer tank 132. The sensor / s at the upper end of the buffer tank 132 may be located on the PLEM 155a of the upper end of the buffer tank 132. The sensor / s at the lower end of the buffer tank 132 may be located on the PLEM 155b of the lower end of the buffer tank 132. The lower PLEM 155b may comprise the pump 134 described above. The lower PLEM 155b may comprise the control valve 194 described above. The temperature and pressure measurements may together provide the change in pressure along the buffer tank 132 so as to determine the saturation pressure and hence the location of the vapor / liquid interface along the buffer tank 132. This can therefore act as a control for the liquid level in the buffer tank 132.

[0092] The positioning of the buffer tank 132 may be substantially uniformly sloped along its main axis. In other words, the buffer tank may retain a linear geometry so as not to comprise an pockets, caused for example by bumps or curves in the seabed. Therefore, the bathymetry of the buffer tank 132 allows for the CO2 to be in liquid form towards the lower end of the buffer tank 132. Further, by the buffer tank 132 having a substantially uniform slope, determining the vapor-liquid level can be facilitated as it reduces error that may be introduced in the determining caused by presence of bumps or curves in the buffer tank 132. The buffer tank 132 may have a gentle slope. However, the buffer tank 132 may be sloped so as to form a head of liquid CO2 in the buffer tank 132 large enough for correct operation of the pump 134 (as shown in FIG. 1 B, FIG. 2, FIG. 3 and as shown in PLEM 155b in FIG. 4 and FIG. 5). The head may have a value greater than 1 .5 m, for example greater than 2 m.

[0093] As indicated in FIG. 4, the installation may be adapted to receive one or more pigs (devices for inspecting and / or cleaning the buffer tank). The installation may comprise a temporary subsea pig launcher 158. The pig launcher 158 may be connected to the lower end of the buffer tank 132. The pig launcher 158 may be connected to the lower end of the buffer tank via the lower PLEM 155b. The pig may enter the pig launcher 158 and may travel along a flowline 156 to the PLEM 155b. The pig may travel along a first portion 154a of the connecting flowline 154 (described in more detail in relation to FIG. 5) to enter the lower end of the buffer tank. Alternatively, the pig may travel along a different flow line to enter the lower end of the buffer tank 132. The pig may enter the buffer tank 132 through an inlet that is the same inlet and / or outlet through which the CO2 flows. Alternatively, the pig may enter the buffer tank 132 through an inlet that is different to the inlet and / or outlet through which the CO2 flows.

[0094] Once inside the buffer tank 132, the subsea pig may clean one or more areas of the walls of the buffer tank 132. The subsea pig may be an intelligent pig. The pig may comprise a scraper or an intelligent scraper. The subsea pig may comprise one or more sensors. Each of the one or more pigs may use the sensors to measure various parameters or activities inside the buffer tank 132. For example, the pig may use the sensors to measure debris in the buffer tank 132 or the thickness of the buffer tank walls. The method may comprise interpreting this information to determine for any corrosion in the buffer tank 132. Determining corrosion in the tank can also allow for the determination of any subsequent leaks or holes in the buffer tank 132. The pig may have a length of up to 3 m. The installation may comprise a temporary subsea pig receiver 142. The pig receiver 142 may be connected to the upper end of the buffer tank 132. The pig receiver 142 may be connected to the buffer tank 132 via the upper PLEM 155a. The pig may exit the upper end of the buffer tank 132 by an outlet located at the upper end of the buffer tank and may travel into an upper end flow line 151 along PLEM 155a. A valve 144c located in PLEM 155 a and along the upper end flow line 151 may open to allow the pig to enter the pig receiver 142. The pig receiver 142 may comprise a vapor CO2 valve 144a. The valve 144a and vapor CO2 vent 146 can allow for venting vapor CO2 in the event of some CO2 entering the pig receiver 142 during the opening of valve 144c.

[0095] FIG. 5 provides an enlarged view of a portion of FIG. 4 as identified by dashed box A. As previously mentioned, the first connecting flow line and the second connecting flow line 154 may be the same connecting flow line 154. The buffer tank 132 may be connected to the main pipeline 118 by a connecting flow line 154 as described above comprised of a first portion 154a and a second portion 154b. The first portion 154a connects the buffer tank 132 to an entry of the lower PLEM 155b, while the second portion 154b connects the main pipeline 118 to another entry of the lower PLEM 155b. CO2 may flow from the main pipeline 118, through the second portion 154b and into the PLEM 155b. The control valve 194 in the PLEM 155b may open to let the CO2 flow further through the PLEM 155b by passages 188, 178. CO2 may exit the PLEM 155b from passage 178 and may flow into the first portion 154a of the connecting flow line 154, and then to the inlet of the buffer tank 132. The inlet and outlet of the buffer tank 132 may be the same, i.e. the same port providing the same flow path for the CO2. This can allow for a more simplified installation. Further, this can allow for efficient execution of the filling and emptying steps if these steps are not simultaneous (i.e. consecutive or sequential). CO2 may flow out of the buffer tank 132 into the first portion 154a of the connecting flow line 154 and may enter the PLEM 155b and through passage 178. CO2 may flow through the passage 178 to the pump 134. Pump 134 may then pump the CO2 through a passage 182 and out of the PLEM 155b to the pipeline 118, for example via the second portion 154b of the connecting flow line 154. The pump 134 may (additionally or alternatively) pump CO2 from the buffer tank directly to an injection facility 172 by a dedicated flowline 174. The pump 134 may pump CO2 to the main pipeline 118 which may flow CO2 to the same injection facility 172, via additional pipeline 170. The use of additional pipeline 170 may be preferable if the buffer tank is placed closer to the shore than to the offshore injection well 126, or if the buffer tank is located along (or towards) the middle of the pipeline 118. Additionally or alternatively, the pump 134 may pump CO2 to a different injection facility than that to which the main pipeline 1 18 flows the CO2. If CO2 flows to more than one location, use of PLEM 155b can allow for providing multiple connections for CO2 out of the buffer tank 132 without having to provide multiple outlets at the lower end of the buffer tank 132.

[0096] Alternatively, CO2 may enter the buffer tank 132 at an inlet different to the outlet of the buffer tank 132. In other words, the inlet and outlet of the buffer tank 132 may be two separate ports providing separate distinct flow paths for the CO2 flowing into the buffer tank and for CO2 flowing out of the buffer tank 132. CO2 may therefore exit the PLEM 155b from a first flow line fluidically connected to the inlet of the buffer tank 132, and CO2 may enter the PLEM 155b from a second flow line fluidically connected to the outlet of the buffer tank 132. This can allow for efficient execution of the filling and emptying steps if the filling and emptying occur simultaneously. The inlet and outlet may be within close proximity to one another, both features being located at the lower end of the buffer tank 132.

[0097] As displayed in each of FIG. 1 B, FIG. 2, FIG. 4 and FIG. 5, the method may comprise a bypass step. CO2 flow through the main pipeline 118 may bypass the subsea buffer tank 132 (i.e. continue to flow through the main pipeline 118 instead of into the buffer tank 132) and may directly flow to an injection well 126. For example, in FIG. 3, CO2 may flow through the first part 118a of the main pipeline 118, may bypass the buffer tank 132 as indicated by arrow 190c, and may continue on through the second part 118b of the main pipeline 118. The bypass step may be carried out at least partly simultaneously with the filling step, such that a portion of the CO2 flowing through the main pipeline 118 is fed to the subsea buffer tank 132 and another portion of the CO2 flowing through the main pipeline 118 directly flows to the injection well. The filling of the buffer tank may occur throughout the injecting into the injection well 126 by the pipeline 118. Alternatively, the filling may occur before injection into the injection well 126 by the main pipeline 1 18 is completed. Executing the bypass step at least partly simultaneously with the filling step can also allow for providing CO2 to the injection well 126 from the main pipeline 118 while also conditioning a volume of CO2 by heat transfer with the seawater 120. Emptying CO2 from the buffer tank 132 may also occur at least partly simultaneously with the at least partly simultaneous bypassing and filling. This can allow for providing CO2 to the injection well 126 from the main pipeline 118 while also providing CO2 to another location from the buffer tank 132, such as another subterranean reservoir onshore or offshore.

[0098] The bypass step may be carried out at least partly separately from the filling step, such that all of the CO2 flowing through the main pipeline 118 directly flows to the injection well 126. This can allow for providing a maximum flow rate of CO2 to the subterranean reservoir 127. When the need arises, the filling may begin and part or all of the incoming CO2 may flow into the buffer tank 132, for example for conditioning by the surrounding seawater 120, and / or for emptying CO2 to a different location / s, such as another subterranean reservoir onshore or offshore. Emptying CO2 from the buffer tank 132 to the main pipeline 1 18 may also occur at least partly simultaneously with the at least partly separate bypassing and filling. This can allow for CO2 which may already be contained inside the buffer tank 132 to empty into the pipeline 118, the pipeline providing both the CO2 of the buffer tank 132 and the CO2 initially in the pipeline 118 to the subterranean reservoir 127.

[0099] As shown in each of FIG. 1 B, FIG. 2 and FIG. 4, the method may comprise an additional filling step, wherein additional CO2 may be provided to the buffer tank 132 from another CO2 source. Additional CO2 may flow from a transfer line 130 into another inlet of the subsea buffer tank 132 fluidically connected to the other inlet of the subsea buffer tank 132. This step may be carried out when there is a low level of liquid CO2 in the buffer tank 132, for example when the buffer tank is less than or equal to 50% full. The other inlet may be located at the upper end of the buffer tank 132. The other inlet may be the same (i.e. same port) as the additional outlet by which the pig may exit the upper end of the buffer tank 132. Alternatively, the other inlet may be different (i.e. different port) to the additional outlet. As shown in FIG. 4, the transfer line 130 may be connected to the buffer tank 132 by the upper PLEM 155a. The transfer line 130 may be fluidically connected to the PLEM 155a at one end of the transfer line 130 so that CO2 passes through the PLEM before entering the buffer tank 132. The PLEM 155a may comprise a valve 144b connected to the transfer line 130. Closing the valve 144b may prevent CO2 from entering the PLEM 155a. Opening the valve 144b may allow the CO2 to enter the PLEM 155a. CO2 may flow through the PLEM via the upper end flowline 151 , the same flow line 151 and inlet through which the pig may travel. Alternatively, CO2 may travel through a different flow line and inlet to that through which the pig may travel.

[0100] The additional filling step may be carried out at least partly simultaneously with the emptying step. The additional CO2 may be cryogenic CO2. The buffer tank 132 may be designed to operate at temperatures as low as -50 °C, or for example as low as -20 °C. The buffer tank 132 may therefore be able to accommodate the flow of cryogenic CO2 into the buffer tank 132. The buffer tank 132 may be designed to operate at any temperature at which the additional CO2 is supplied to the buffer tank 132. The buffer tank 132 may be able to operate at lower temperatures and pressures than the main pipeline 118. If CO2 flowing from the outlet of the buffer tank 132 flows back into the main pipeline 118, the buffer tank 132 can therefore be considered to provide a pre-conditioning step for additional cryogenic CO2 prior to its flowing inside the main pipeline 118. Additionally or alternatively, CO2 may flow from the buffer tank during the emptying step to one or more other injection wells onshore or offshore.

[0101] The CO2 entering the buffer tank 132 may be at a temperature ranging from -10 °C to -50 °C, for example from -20 °C to -30 °C, for example from -28 °C to - 24 °C, for example -26 °C. The seawater 120 may heat the CO2 to a temperature above 0 °C. The seawater 120 may heat the CO2 to a temperature ranging from 1 °C to 30 °C, i.e. to a condition suitable for its injection into an injection well 126.

[0102] The additional CO2 may flow to the transfer line 130 from another CO2 source such as an offshore CO2 source 136, for example a ship, or a cryogenic offloading tower or a near shore jetty floating storage injection unit (FSIU). The near shore jetty FSIU may be preferable as an onshore terminal substitute, in particular as no qualification is needed, it can accept a standard ship carrying CO2 and may be automatically operable (i.e. may not require manning). As shown in FIG. 1 B and FIG. 2, the transfer line 130 may be a flexible riser that may float in the seawater 120 with support from an offshore buoy 137.

[0103] The additional filling step may be carried out at least partly simultaneously with the filling step. In this way, both the main pipeline 118 and the other CO2 source (the other CO2 source in this case being a different source to the main pipeline 118) may both offload CO2 to the buffer tank 132. This can allow the buffer tank 132 to be a temporary storage unit for more than one CO2 source.

[0104] The filling of the buffer tank 132 with the additional CO2, may have an impact on the vapor-liquid CO2 interface inside the buffer tank 132. CO2 may flow into the buffer tank 132 from the offshore source 136 into the transfer line 130 (or pipeline or other) and into an inlet located at the upper end of the pipeline 118. The liquid-vapor interface may move during the additional filling step. The flowing of additional CO2 into the upper inlet and the filling of CO2 into the lower inlet and / or the emptying of CO2 from the lower outlet may be simultaneous. Alternatively, the flowing of additional CO2 into the upper inlet and flowing of CO2 into the lower inlet and / or the emptying of CO2 from the lower outlet may be consecutive. The flowing of additional CO2 into the upper inlet and flowing of CO2 into the lower inlet and / or the emptying of CO2 from the lower outlet may be sequential. Non-simultaneous flowing of additional CO2 into the buffer tank 132 and emptying of the buffer tank may be preferable if the additional CO2 is cryogenic CO2. Depending on the flow pattern of additional CO2 flowing into the upper inlet and CO2 flowing into the lower inlet (filling) and out of the lower outlet (emptying), the liquid-vapor interface may move closer to the upper end of the buffer tank or to the lower end of the buffer tank 132. This may also depend on the rate at which the additional CO2 flows into the buffer tank 132 and the rate at which the CO2 flows into the inlet of the buffer tank 132 (filling) and out of the outlet of the buffer tank 132 (emptying). The temperature and pressure sensors 152a, 152b may monitor the positioning of the liquid front during flowing of additional CO2 into the buffer tank 132.

Claims

Claims1. A method for CO2 storage, the method being carried out in an installation comprising a main pipeline (118) fluidically connected to a subsea buffer tank (132), the subsea buffer tank (132) being a pipeline other than the main pipeline (118), lying on the seabed (100) and extending along a main axis between a lower end and an upper end, the method comprising:- a filling step comprising: o flowing CO2 through the main pipeline (118); o flowing CO2 out of the main pipeline (118) and into an inlet of the subsea buffer tank (132) located at the lower end of the subsea buffer tank (132);- an emptying step comprising: o flowing CO2 out of an outlet of the subsea buffer tank (132), wherein the outlet of the subsea buffer tank (132) is located at the lower end of the subsea buffer tank (132).

2. The method according to claim 1 , wherein, during the emptying step, CO2 flows out of the subsea buffer tank (132) back into the main pipeline (118).

3. The method according to claims 1 or 2, comprising a bypass step, wherein CO2 flow through the main pipeline (118) bypasses the subsea buffer tank (132) and directly flows to an injection well (126).

4. The method of claim 3, wherein the bypass step is carried out at least partly simultaneously with the filling step, such that a portion of the CO2 flowing through the main pipeline (118) is fed to the subsea buffer tank (132) and another portion the CO2 flowing through the main pipeline (132) directly flows to the injection well (126).

5. The method of claim 3 or 4, wherein the bypass step is carried out at least partly separately from the filling step, such that the CO2 flowing through the main pipeline (118) directly flows to the injection well (126).

6. The method of any one of claims 1 to 5, wherein, during the emptying step, CO2 flows from the subsea buffer tank to the injection well (126).

7. The method according to any one of claims 1 to 6, wherein the filling step comprises flowing CO2 into the main pipeline (118) at a flow rate ranging from 500 to 3000 t / h, for example from 1000 to 2500 t / h.

8. The method according to any one of claims 1 to 7, wherein the CO2 in the main pipeline (118) is at a pressure ranging from 3 MPa to 27 MPa, for example from 50 MPa to 250 MPa.

9. The method according to any one of claims 1 to 8, wherein a pump (134) fluidically connected to the outlet of the subsea buffer tank (132) pumps the CO2 flowing out of the subsea buffer tank (132).

10. The method according to claim 9, wherein the pump (134) pumps the CO2 at a rate ranging from 80 t / h to 1200 t / h, for example from 200 t / h to 1000 t / h.

11. The method according to claim 9 or 10, wherein the pump (134) pumps the CO2 to a pressure ranging from 3 MPa to 27 MPa, for example ranging from 5 MPa to 25 MPa.

12. The method according to any one of claims 1 to 11 , wherein a control valve (194) fluidically connected to the inlet of the subsea buffer tank (132) controls the CO2 flow into the subsea buffer tank (132).

13. The method according to claim 12, wherein the control valve (194) controls the flow rate of the CO2 to enter the subsea buffer tank (132) at a flow rate ranging from 500 t / h to 3000 t / h, for example from 1000 t / h to 2500 t / h.

14. The method according to any one of claims 1 to 13, wherein the CO2 in the subsea buffer tank (132) is at a pressure ranging from 3 MPa to 7 MPa, for example from 4 MPa to 6 MPa.

15. The method according to any one of claims 1 to 14, wherein the CO2 entering the subsea buffer tank (132) is at a temperature ranging from -28 °C to -24 °C, for example -26 °C.

16. The method according to any one of claims 1 to 15, wherein some of the CO2 is in a vapor phase (132a) and some of the CO2 is in a liquid phase (132b) so as to form a CO2 liquid-vapor interface in the subsea buffer tank (132).

17. The method according to claim 16, wherein the liquid-vapor interface moves during the filling step and / or the emptying step.

18. The method according to any one of claims 1 to 17, comprising an additional filling step, wherein additional CO2 flows from a transfer line into another inlet of the subsea buffer tank (132), the other inlet being preferably located at the upper end of the subsea buffer tank (132).

19. The method of claim 18, wherein the additional filling step is carried out at least partly simultaneously with the emptying step.

20. The method according to claim 18 or 19, wherein the additional CO2 flows to the transfer line from an offshore CO2 source (136), for example a ship, a single anchor offloading unit, a cryogenic offloading tower or a near shore jetty FSIll.

21. The method according to any one of claims 16 to 20, wherein the liquidvapor interface moves during the additional filling step.

22. The method according to any one of claims 1 to 21 , comprising taking pressure measurements and / or temperature measurements at the upper end of the subsea buffer tank (132) and at the lower end of the subsea buffer tank (132).

23. The method according to any one of claims 1 to 22, wherein the seawater heats the CO2 inside the subsea buffer tank (132).

24. The method according to claim 23, wherein the CO2 is heated to a temperature ranging from 1 °C to 30 °C within the subsea buffer tank (132).

25. The method according to any one of claims 1 to 24, wherein the CO2 flows from the subsea buffer tank (132) back to the mainland by a returnpipeline fluidically connected to the outlet of the subsea buffer tank (132).

26. The method according to any one of claims 1 to 25, wherein the main pipeline is at least partly subsea.

27. The method according to any one of claims 1 to 26, wherein the main pipeline (118) is fluidically connected to an onshore CO2 source (138).

28. An installation for CO2 storage, wherein the installation comprises:- a main pipeline (118);- a subsea buffer tank (132) being a pipeline other than the main pipeline (118), lying on the seabed (100) and extending along a main axis between a lower end and an upper end; wherein the main pipeline (118) is fluidically connected to the subsea buffer tank (132), the installation being configured to make CO2 flow from the main pipeline (118) to an inlet of the subsea buffer tank (132) and to make CO2 flow from an outlet of the subsea buffer tank (132), said inlet and outlet being located at the lower end of the subsea buffer tank (132).

29. The installation according to claim 28, comprising a pump (134) fluidically connected to the outlet of the subsea buffer tank (132).

30. The installation according to claim 28 or 29, comprising a control valve (194) configured for controlling the flow rate of CO2 from the main pipeline (118) to the subsea buffer tank (132) and / or from the subsea buffer tank to the main pipeline (118).

31. The installation according to any one of claims 28 to 30, wherein the main pipeline (118) is fluidically connected to the subsea buffer tank (132) by a connecting flow line (154) comprising one end fluidically connected to an outlet of the main pipeline (118) at a position along the length of the main pipeline (118), and another end fluidically connected to the inlet of the subsea buffer tank (132).

32. The installation according to any one of claims 28 to 31 , wherein the outlet of the subsea buffer tank (132) is fluidically connected to the mainpipeline (118), the main pipeline (118) being fluidically connected to an injection well (126) at one end of the main pipeline (118).

33. The installation according to any one of claims 28 to 32, comprising a transfer line (130) fluidically connected to another inlet of the subsea buffer tank (132), the other inlet being preferably located at the upper end of the subsea buffer tank (132).

34. The installation according to claim 33, wherein the transfer line (130) is fluidically connected to an offshore CO2 source (136), for example a ship, a single anchor offloading unit, a cryogenic offloading tower or a near shore jetty FSIll.

35. The installation according to any one of claims 28 to 34, wherein at least one pressure sensor (152a) is located at the upper end of the buffer tank (132) and at least one pressure sensor (152b) is located at the lower end of the buffer tank (132).

36. The installation according to any one of claims 28 to 35, wherein at least one temperature sensor (152a) is located at the upper end of the buffer tank (132) and at least one temperature sensor (152b) is located at the lower end of the buffer tank (132).

37. The installation according to any one of claims 28 to 36, wherein the outlet of buffer tank (132) is fluidically connected to a return pipeline fluidically connected to the mainland.

38. The installation according to any one of claims 28 to 37, wherein the main pipeline is at least partly subsea.

39. The installation according to any one of claims 28 to 38, wherein the main pipeline (118) is fluidically connected to an onshore CO2 source (138).

40. The installation according to any one of claims 28 to 39, wherein the inlet of the subsea buffer tank (132) is the same as the outlet of the subsea buffer tank (132).

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