FLUID INJECTION SYSTEM AND RELATED METHODS

Abstract

Obstructing fluid plugs are removed from a base section of a riser, which base section extends between a receiving end connected to an upright section of the riser and an emitting end of the riser connected to a subsea template located on a seabed over a drill hole to a subterranean void into which fluid received via the riser is to be injected from the subsea template (120). The obstructing fluid plugs are removed either by injecting heated assisting from a storage container into an injection point in the base section and injecting heated assisting liquid from the vessel in the upright section of the riser, or a heating unit is controlled to heat the base section to a predetermined temperature, and thereafter maintain a temperature level here above or equal to the predetermined temperature.

Claims

1. A method of removing obstructing fluid plugs from a base section of a riser, which base section extends between a receiving end connected to an upright section of the riser and an emitting end of the riser connected to a subsea template located on a seabed, which subsea template is further connected to a wellhead for a drill hole to a subterranean void into which fluid received via the riser is to be injected from the subsea template, the method comprising: (A) heating at least one assisting liquid to a predetermined temperature, the heating being effected in a vessel, (B) forwarding at least one transport container holding the at least one heated assisting liquid from the vessel to a storage container in the subsea template (120), (C) injecting the at least one heated assisting liquid from the storage container into at least one injection point in the base section, (D) injecting, from the vessel at least one heated assisting liquid in the upright section of the riser, and (E) repeat steps (A) through (D) until any plugs in the riser have melted away.

2. The method according to claim 1, wherein the base section comprises: a low-point section between the receiving end and the emitting end of the base section, which low-point section constitutes a geometrically lowermost part of the base section, a container connector configured to receive an output nozzle of the storage container, which container connector represents one of the at least one injection point in the base section and is arranged upstream of the low-point section relative to a direction of a fluid flow into an injection valve tree for the wellhead; and the method comprising: feeding the at least one heated assisting liquid from the storage container via the low-point section into the injection valve tree.

3. The method according to claim 2, wherein the base section comprises: a first temperature sensor arranged downstream of the container connector and upstream of the low-point section relative to the direction of the fluid flow, which first temperature sensor is configured to register a first temperature signal, a second temperature sensor arranged downstream of the low-point section relative to the direction of the fluid flow, which second temperature sensor is configured to register a second temperature signal; and the method further comprising: determining if there is a fluid plug in the low-point section based on how the second temperature signal varies over time compared to how the first temperature signal varies over time in response to feeding the at least one heated assisting liquid from the storage container into the base section.

4. The method according to claim 3, wherein, if it is determined there is a fluid plug in the low-point section, the method further comprising: reducing, temporarily, a flow rate at which the at least one heated assisting liquid is fed from the storage container into the base section.

5. A method of removing obstructing fluid plugs from a base section of a riser, which base section extends between a receiving end connected to an upright section of the riser and an emitting end of the riser connected to a subsea template located on a seabed, which subsea template is further connected to a wellhead for a drill hole to a subterranean void into which fluid received via the riser is to be injected from the subsea template, the subsea template comprising a heating unit that is arranged to heat at least one portion of the base section the method comprising: controlling the heating unit to heat the at least one portion of the base section to a predetermined temperature, and controlling the heating unit to maintain a temperature level above or equal to the predetermined temperature in the at least one section of the base section during a heating period.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The invention is now to be explained more closely by means of preferred embodiments, which are disclosed as examples, and with reference to the attached drawings.

[0032] FIG. 1 schematically illustrates a system for long term storage of fluids in a subterranean void according to one embodiment of the invention;

[0033] FIG. 2 shows a buoy configured to connect a vessel to a fluid-transporting riser according to one embodiment of the invention;

[0034] FIGS. 3a-c illustrate how a riser is connected to a buoy according to one embodiment of the invention;

[0035] FIG. 4 schematically illustrates an interior of a subsea template according to one embodiment of the invention;

[0036] FIG. 5 illustrates a connector arrangement for connecting the riser to the buoy according to one embodiment of the invention;

[0037] FIG. 6 illustrates, by means of a flow diagram a method according to one embodiment of the invention for connecting a riser to a buoy;

[0038] FIG. 7 illustrates, by means of a flow diagram a method according to one embodiment of the invention for connecting a riser to a subsea template;

[0039] FIGS. 8-9 illustrate, by means of flow diagrams, methods according to first and second embodiments of the invention for removing obstructing fluid plugs in a riser; and

[0040] FIG. 10 schematically illustrates the base section of the riser according to one embodiment of the invention.

DETAILED DESCRIPTION

[0041] In FIG. 1, we see a schematic illustration of a system according to one embodiment of the invention for long term storage of fluids, e.g. carbon dioxide, in a subterranean void or other accommodation space 150, which typically is a subterranean aquifer. However, according to the invention, the subterranean void 150 may equally well be a reservoir containing gas and/or oil, a depleted gas and/or oil reservoir, a carbon dioxide storage/disposal reservoir, or a combination thereof. These subterranean accommodation spaces are typically located in porous or fractured rock formations, which for example may be sandstones, carbonates, or fractured shales, igneous or metamorphic rocks.

[0042] The system includes at least one offshore injection site 100, which is configured to receive fluid, e.g. in a liquid phase, from at least one fluid tank 115 of a vessel 110. The offshore injection site 100, in turn, contains a subsea template 120 arranged on a seabed/sea bottom 130. The subsea template 120 is located at a wellhead for a drill hole 140 to the subterranean void 150. The subsea template 140 may also contain a utility system configured to cause the fluid from the vessel 110 to be injected into the subterranean void 150 in response to control commands C.sub.cmd. In other words, the utility system is not located onshore, which is advantageous for logistic reasons. For example therefore, in contrast to the above-mentioned Snohvit site, there is no need for any umbilicals or similar kinds of conduits to provide supplies to the utility system.

[0043] The utility system in the subsea template 120 may contain at least one storage tank. The at least one storage tank holds at least one assisting liquid, which is configured to facilitate at least one function associated with injecting the fluid into the subterranean void 150. The at least one assisting liquid contains a de-hydrating liquid and/or an anti-freezing liquid.

[0044] In FIG. 1, a control site, generically identified as 160, is adapted to generate the control commands C.sub.cmd for controlling the flow of fluid from the vessel 110 and down into the subterranean void 150. For example, the control commands C.sub.cmd may relate to opening and closure of valves when the vessel 110 connects to and disconnects from the buoy 170. The control site 160 is positioned at a location geographically separated from the offshore injection site 100, for example in a control room onshore. However, additionally or alternatively, the control site 160 may be positioned at an offshore location geographically separated from the offshore injection site, for example at another offshore injection site. Consequently, a single control site 160 can control multiple offshore injection sites 100. There is also large room for varying which control site 160 controls which offshore injection site 100. Communications and controls are thus located remote from the offshore injection site 100. However, as will be discussed below, the offshore injection site 100 may be powered locally, remotely or both.

[0045] In order to enable remote control from the control site 160, the subsea template 120 preferably contains a communication interface 120c that is communicatively connected to the control site 160. The subsea template 120 is also configured to receive the control commands C.sub.cmd via the communication interface 120c.

[0046] Depending on the channel(s) used for forwarding the control commands C.sub.cmd between the control site 160 and the offshore injection site 100, the communication interface 120c may be configured to receive the control commands C.sub.cmd via a submerged fiber-optic and/or copper cable 165, a terrestrial radio link (not shown) and/or a satellite link (not shown). In the latter two cases, the communication interface 120c includes at least one antenna arranged above the water surface 111.

[0047] Preferably, the communicative connection between the control site 160 and the subsea template 120 is bi-directional, so that for example acknowledge messages C.sub.ack may be returned to the control site 160 from the subsea template 120.

[0048] According to the invention, the offshore injection site 100 includes a buoy 170, for instance of submerged turret loading (STL) type. When inactive, the buoy 170 may be submerged to 30-50 meters depth, and when the vessel 110 approaches the offshore injection site 100 to offload fluid, the buoy 170 and at least one injection riser 171 and 172 connected thereto are elevated to the water surface 111. After that the vessel 110 has been positioned over the buoy 170, this unit is configured to be connected to the vessel 110 and receive the fluid from the vessel's fluid tank(s) 115, for example via a swivel assembly in the buoy 170. The buoy 170 is preferably anchored to the seabed 130 via one or more hold-back clamps 181, 182, 183 and 184, which enable the buoy 170 to elevated and lowered in the water.

[0049] Each of the injection risers 171 and 172 respectively is configured to forward the fluid from the buoy 170 to the subsea template 120, which, in turn, is configured to pass the fluid on via the wellhead and the drill hole 140 down to the subterranean void 150.

[0050] According to one embodiment of the invention, the subsea template 120 contains a power input interface 120p, which is configured to receive electric energy P.sub.E for operating the utility system and/or operating various functions in the buoy 170. The power input interface 120p may be also configured to receive the electric energy P.sub.E to be used in connection with operating a well at the wellhead, a safety barrier element of the subsea template 120 and/or a remotely operated vehicle (ROV) sta-tioned on the seabed 130 at the subsea template 120.

[0051] FIG. 1 illustrates a generic power source 180, which is configured to supply the electric power P.sub.E to the power input interface 120p. It is generally advantageous if the electric power P.sub.E is supplied via a cable 185 from the power source 180 in the form of low-power direct current (DC) in the range of 200V-1000V, preferably around 400V. The power source 180 may either be co-located with the offshore injection site 100, for instance as a wind turbine, a solar panel and/or a wave energy con-verter; and/or be positioned at an onshore site and/or at another offshore site geographically separated from the offshore injection site 100. Thus, there is a good potential for flexibility and redundancy with respect to the energy supply for the offshore injection site 100.

[0052] The subsea template 120 contains a valve system that is configured to control the injection of the fluid into the subterranean void 150. The valve system, as such, may be operated by hydraulic means, electric means or a combination thereof. The subsea template 120 preferably also includes at least one battery configured to store electric energy for use by the valve system as a backup to the electric energy P.sub.E received directly via the power input interface 120p. More precisely, if the valve system is hydraulically operated, the subsea template 120 contains a hydraulic pressure unit (HPU) configured to supply pressurized hydraulic fluid for operation of the valve system. For example, the HPU may supply the pressurized hydraulic fluid through a hydraulic small-bore piping system. The at least one battery is here configured to store electric backup energy for use by the hydraulic power unit and the valve system.

[0053] Alternatively, or additionally, the valve operations may also be operated using an electrical wiring system and electrically controlled valve actuators. In such a case, the subsea template 120 contains an electrical wiring system configured to operate the valve system by means of electrical control signals. Here, the at least one battery is configured to store electric backup energy for use by the electrical wiring system and the valve system.

[0054] Consequently, the valve system may be operated also if there is a temporary outage in the electric power supply to the offshore injection site. This, in turn, increases the overall reliability of the system.

[0055] Locating the utility system at the subsea template 120 in combination with the proposed remote control from the control site 160 avoids the need for offshore floating installations as well as permanent offshore marine installations. The invention allows direct injection from relatively uncomplicated maritime vessels 110. These factors render the system according to the invention very cost efficient.

[0056] According to the invention, further cost savings can be made by avoiding the complex offshore legislation and regulations. Namely, a permanent offshore installation acting as a field center for an offshore field development is bound by offshore legislation and regulations. There are strict safety requirements related to well control especially. For instance, offshore Norway, it is stipulated that floating offshore installations, permanent or temporary, that control well barriers must satisfy the dynamic positioning level 3 (DP3) requirement. This involves extensive requirements in to ensure that the floater remains in position also during extreme events like engine room fires, etc. Nevertheless, the vessel 110 according to the invention does not need to provide any utilities, well or barrier control, for the injection system. Consequently, the vessel 110 may operate under maritime legislation and regulations, which are normally far less restrictive than the offshore legislation and regulations.

[0057] FIG. 2 shows a buoy 170 according to one embodiment of the invention that is configured to enable a vessel, e.g. 110 shown in FIG. 1, to connect to the fluid-transporting riser 171, which, in turn, is connected to the subsea template 120 in further fluid connection with the subterranean void 150.

[0058] Referring again to FIG. 1, we see a fluid injection system arranged to receive fluid, e.g. containing CO.sub.2, from the vessel 110. The fluid injection system contains the buoy 170 configured to be connected with the vessel 111 and receive the fluid therefrom. The system also contains the subsea template 120, which is located on the seabed 130 at the wellhead for the drill hole 140 to the subterranean void 150.

[0059] Moreover, the system includes at least one riser, here exemplified by 171 and 172 respectively, which interconnect the buoy 170 and the subsea template 120. Each of the at least one riser 171 and 172 is configured to transport the fluid from the buoy 170 to the subsea template 120. Specifically, each of the at least one riser 171 and 172 is detachably connected to a bottom surface of the buoy 170 by means of a connector arrangement 210. FIG. 5 illustrates the connector arrangement 210 according to one embodiment of the invention, which connector arrangement 210 is configured to connect the riser 171 to the buoy 170. Naturally, although not illustrated in FIG. 2, any additional risers attached to the buoy 170 will be connected in an analogous manner.

[0060] The connector arrangement 210 includes a buoy guide member 510 configured to automatically steer a connector member 570 towards the buoy guide member 510 when the connector member 570 is moved towards the buoy guide member 510. The connector member 570 is attached in a head end 300 of the riser 171 to be connected to the buoy 170. The connector arrangement 210 further includes a mating member 550, for example embodied as so-called fingers, configured to attach a first sealing surface S70 of the connector member 570 to a second sealing surface S10 of the buoy guide member 510 when said head end 300 has been moved such that the connector member 570 contacts the buoy guide member 510. Additionally, the connector arrangement 210 includes a locking member 560 configured to lock the first and second sealing surfaces S70 and S10 to one another when these surfaces are aligned with one another.

[0061] Preferably, the connector arrangement 210 contains one collet connector for each riser to be connected to the buoy 170. In addition to the elements mentioned above, the collet connector typically also includes a seal gasket 530, which is arranged between the first and second sealing surfaces S70 and S10 to further reduce the risk of leakages.

[0062] FIGS. 3a, 3b and 3c illustrate how a riser 171 is connected to a buoy 170 according to one embodiment of the invention.

[0063] Here, the head end 300 of the riser 171 to be connected contains a plug member 317 covering the first sealing surface S70. Thus, water is and prevented from entering into the riser 171 before the riser 171 has been connected to the buoy 170. In addition to that, the head end 300 of the riser 171 to be connected preferably includes a drag-eye member 305, which facilitates connecting a winch wire to the head end 300 and pulling the riser 171 up to the buoy 170 as described below.

[0064] As illustrated in FIG. 3c, according to one embodiment of the invention, the plug member 317 is configured to encircle the riser 171 to be connected to the buoy 170. After that the plug member 317 has been disconnected from the head end 300 of the riser 171, the plug member 317 is further configured to be transported by gravity G down along said riser 171 towards the subsea template 120.

[0065] Referring now to FIG. 3a, according to one embodiment of the invention, the fluid injection system contains a winch unit 330, which is arranged on the seabed 130. The winch unit 330 is configured to pull up the head end 300 of the riser 171 to be connected to the buoy 170 via a winch wire 320 connected between the head end 300 of the riser 171 and the winch unit 330. The which wire 320 runs via the buoy 170 to the winch unit 330. Preferably, the winch wire 320 is led through the buoy 170 and via at least one sheave wheel 325 on the buoy 170 as illustrated in FIGS. 3a and 3b.

[0066] Preferably, the fluid injection system includes an ROV 350 that is configured to be remote controlled to attach the winch wire 320 to the head end 300 of the riser 171. Further preferably, the ROV 350 is configured to disconnect the plug member 317 from the first sealing surface S70 of the connector member 570 in the head end 300 of the riser 171; and thereafter, connect the riser 171 to the buoy 170.

[0067] Referring now to the flow diagram of FIG. 6, we will describe a method for connecting the riser 171 to the buoy 170 by using the ROV 350 according to one embodiment of the invention.

[0068] In a first step 610, the ROV 350 is controlled to attach the winch wire 320 to the head end 300 of the riser 171.

[0069] Then, in a step 620, the ROV 350 is controlled to lead the winch wire 320 via the buoy 170 to the winch unit 330 on the seabed 130 below the buoy 170.

[0070] Subsequently, in a step 630, the winch unit 330 is controlled to pull up the head end 300 of the riser (171) to a bottom side of the buoy 170.

[0071] Finally, in a step 640 thereafter, the ROV 350 is controlled to connect the head end 300 of the riser 171 to the connector arrangement 210 in the bottom of the buoy 170.

[0072] FIG. 4 schematically illustrates an interior of a subsea template 220 according to one embodiment of the invention. Here, an exemplary riser 171 is shown, which has a base section 410 and an upright section 420. The upright section 420 constitutes an uppermost part, which is further connected to the buoy 170. The base section 410 constitutes a lowermost part of the riser 171, which, in a receiving end 411, is connected to the upright section 420; and in an emitting end 412, is connected to the subsea template 120.

[0073] As illustrated in FIG. 1, it is desirable if each of the risers 171 and 172 contains a holdback clamp 17C, which is configured to hold the base section 410 of the riser in a desired position via a restraining riser 17R attached to the seabed 130.

[0074] According to one embodiment of the invention, the subsea template 120 contains an injection valve tree 460, which is in fluid connection with the wellhead 470 for the drill hole 140. The subsea template 120 also contains a sleeve member 440 having penetration means 441, e.g. represented by a pipe-piece extending substantially orthogonally relative to an extension of the sleeve member 440, which penetration means 441 is configured to penetrate the riser 171 in the emitting end 412 of the base section 410. As a result, when the emitting end 412 of the base section 410 is inserted into the sleeve member 440 the penetration means 441 will create an opening in the riser 171. This opening, in turn, is connectable to the injection valve tree 460.

[0075] Preferably, a vertical connector extending from the penetration means 441 has a relatively large tolerance for deviation, say allowing up to 5-10 degrees misalignment. Namely, this allows for a useful flexibility when installing the riser 171 in the subsea template 120. Tolerance budgets are estimated based upon accuracy of fabrication, assembly and installation, and flexibility in the piping and misalignment acceptance in the connectors used.

[0076] It is preferable if the sleeve member 440 contains, or is associated with, at least one guide member, which is exemplified by 432 in FIG. 4. The guide member 440 is shaped and arranged relative to the penetration means 441 so as to steer the emitting end 412 of the base section 410 towards the penetration means 441 to allow the emitting end 412 of the base section 410 to land down at a certain speed and provide a finer and finer alignment with the penetration means 441. Thus, for example, the guide member 432 may have a general funnel shape converging towards the penetration means 441. Thereby, the guide member 432 is configured to steer the emitting end 412 of the base section 410 towards the sleeve member when the emitting end 412 of the base section 410 is brought towards the subsea template 120.

[0077] Referring now to the flow diagram of FIG. 7, we will describe a method for connecting the riser 171 to the subsea template 120 according to one embodiment of the invention by using the ROV 350.

[0078] In a first step 710, the ROV 350 is controlled to steer the emitting end 412 of the base section 410 of the riser 171 to the template guide member 432 on the subsea template 120.

[0079] Thereafter, in a step 720, the ROV 350 is controlled to feed the emitting end 412 of the base section 410 of the riser 171 via the template guide member 432 to the sleeve member 440, which has penetration means 441 configured to penetrate the riser 171. Consequently, when the second end 412 of the base section 410 is fed into the sleeve member 440, the penetration means 441 is caused to penetrate the riser 171 in the second end 412 and create an opening in the riser 171.

[0080] Finally, in a subsequent step 730, the ROV 350 is controlled to connect the sleeve member 440 to the injection valve tree 460 in the subsea template 120.

[0081] According to one embodiment of the invention, the subsea template 120 contains a jumper pipe 450 having a general U-shape, which is configured to establish a fluid connection between the opening in the riser 171 and the injection valve tree 460. An advantage with the jumper pipe 450 exclusively being a pipe element is that can be made flexible enough to meet the tolerance requirements for making successful connection.

[0082] However, the jumper pipe 450 may also act as a injection choke bridge. This means that the jumper pipe 450 includes a choke valve and instrumentation for controlling the injection of the fluid. The jumper pipe 450 is designed with such design tolerances that it is attachable both onto the vertical connector extending from the penetration means 441 and the valve tree 460. Preferably, this connection also includes a valve 445, e.g. of ball or gate type, such that a rate of the fluid flow into the injection valve tree 460 can be regulated, and shut off if needed. It is advantageous if the valve 445 is configured to be operable by the ROV 350.

[0083] It is further preferable if the subsea template 120 contains at least one heating unit. In FIG. 4, a generic heating unit 480 is illustrated, which is configured to heat the fluid received from the riser 171 before the fluid is being injected into the subterranean void 150. Thus, for example obstructing fluid plugs can be removed from the base section 410 of the riser 171 in a straightforward manner.

[0084] Referring now to the flow diagram of FIG. 9, we will describe such a method. As mentioned above, the base section 410 extends between the receiving end 411 and the emitting end 412 of the riser 171, where the receiving end 411 is connected to the upright section 420 of the riser 171 and the emitting end 412 of the riser 171 is connected to the subsea template 120. The subsea template 120 is further connected to the wellhead (470) for a drill hole 140 to the subterranean void 150 into which fluid received via the riser 171 is to be injected from the subsea template 120.

[0085] In a first step 910, the heating unit 480 is controlled to heat at least one portion of the base section 410. A subsequent step 920 checks if the least one portion of the base section 410 has reached a predetermined temperature. If so, a step 930 follows; and otherwise, the procedure loops back to step 910.

[0086] In step 930, the heating unit 480 is controlled to maintain a temperature level above or equal to the predetermined temperature in the at least one section of the base section.

[0087] Thereafter, a step checks if a heating period has expired. If so, the procedure ends; and otherwise, the procedure loops back to step 930.

[0088] Referring again to FIG. 4, according to one embodiment of the invention, the subsea template 120 contains a power interface 120p that is configured to receive electric power P.sub.E via an electric power line 185 on the seabed 130, for example from an onshore power source 180. It is also advantageous if the subsea template 120 contains at least one battery 490 configured to provide electric power to at least one unit in the subsea template 120, for instance the heating unit 480, the valve 445 and/or the injection valve tree 460.

[0089] Naturally, it is preferable if also the at least one battery 490 is configured to be charged by electric power P.sub.E received via the power interface 120p.

[0090] In addition to the tasks mentioned above, the ROV 350 is preferably configured to be controlled to effect at least one procedure in connection with controlling the valve 445 in the subsea template 120, controlling one or more valves in the buoy 170 and/or performing maintenance of the fluid injection system.

[0091] FIG. 8 illustrates, by means of a flow diagram, a method for removing obstructing fluid plugs in the riser 171, which is an alternative to the method described above with reference to FIG. 9.

[0092] In a first step 810, at least one assisting liquid is heated to a predetermined temperature in the vessel 110.

[0093] Thereafter, in a step 820, at least one container holding the at least one heated assisting liquid is/are forwarded from the vessel 110 to a storage container in the subsea template 120.

[0094] In a subsequent step 830, the at least one heated assisting liquid is/are injected from the storage container into at least one injection point in the base section 410 of the riser 171, and from the vessel 110 into at least one injection point in the upright section 420 of the riser 171.

[0095] Then, in a step 840, it is checked if the plugs in the riser 171 have melted away. If so, the procedure ends; and otherwise, the procedure loops back to step 810.

[0096] Referring now to FIG. 10, we see a schematic illustration of the base section 410 of the riser 171 in the subsea template 420 according to one embodiment of the invention.

[0097] Here, the base section 410, which may be represented by a pipeline or a so-called spool, is typically around 60 to 100 meters long, has a low-point section 1030 between the receiving end 411 and the emitting end 412. The low-point section 1030 constitutes a geometrically lowermost part of the base section 410. Thus, if the fluid being fed through the riser 171 into the subterranean void 150 contains CO.sub.2, any undesired CO.sub.2 hydrates formed in the riser 171 will gather in the low-point section 1030. The CO.sub.2 hydrates may form under certain conditions, for example at particular combinations of pressure and temperature, due to water content in the CO.sub.2 composition and/or due to impurities therein. Nevertheless, the concentration of the CO.sub.2 hydrates low-point section 1030 facilitates dissolution of these components before they are transformed into obstructing fluid plugs in the riser 171.

[0098] According to one embodiment of the invention, the above-mentioned container holding at least one heated assisting liquid is represented by a storage container 1040 with heated MEG (Mono-Ethylene Glycol) brought to the subsea template 120 from the vessel 110 by means of the ROV 350. Further, a container connector 1047 is provided on the base section 410. The container connector 1047 is configured to receive an output nozzle of the storage container 1040 so as to enable the at least one heated assisting liquid in the storage container 1040 to be fed into the base section 410, for instance via a valve 1045. Hence, the container connector 1047 represents one of the at least one injection point in the base section 410 referred to above. The container connector 1047 is arranged upstream of the low-point section 1030 relative to a direction F of a fluid flow into an injection valve tree 460 for the wellhead 470.

[0099] Moreover, the method specifically involves feeding the at least one heated assisting liquid from the storage container 1040 via the low-point section 1030 into the injection valve tree 460. Preferably, a pump 1050 and/or a valve 1055 is arranged between the container connector 1047 and the base section 410, such that a flow rate of heated assisting liquid being fed into the base section 410 may be controlled; and if needed, be shut off completely.

[0100] According to another embodiment of the invention, the base section 410 contains first and second temperature sensors 1010 and 1020 respectively. The first temperature sensor 1010 is arranged downstream of the container connector 1047 and upstream of the low-point section 1030 relative to the direction F of the fluid flow in the base section 410 of the riser 171. The first temperature sensor 1010 is configured to register a first temperature signal T1. The second temperature sensor 1020 is arranged downstream of the low-point section 1030 relative to the direction F of the fluid flow. The second temperature sensor 1020 is configured to register a second temperature signal T2.

[0101] Through thermal convection through the wall of the pipe of the base section 410 the presence of the heated flow of at least one heated assisting liquid from the storage container 1040 can traced. Whether or not there is an obstructing fluid plug 1035 in the low-point section 1030 this may be detected by studying the first and second temperature signals T1 and T2. If no obstructing plug is present, the second temperature signal T2 will follow the first temperature signal T1 relatively closely both with respect to temporal behavior and magnitude. If, however, there is an obstructing fluid plug 1035 in the low-point section 1030, the second temperature signal T2 will be much less similar to the first temperature signal T1. The method therefore preferably involves determining if there is a fluid plug 1035 in the low-point section 1030 based on how the second temperature signal T2 varies over time compared to how the first temperature signal T1 varies over time in response to feeding the at least one heated assisting liquid from the storage container 1040 into the base section 410 via the container connector 1047.

[0102] If the base section 410 of the riser 171 is obstructed, there is a risk of over pressuring causes damages. Consequently, if it is determined there is a fluid plug 1035 in the low-point section 1030, the method further involves reducing, at least temporarily, a flow rate at which the at least one heated assisting liquid is fed from the storage container 1040 into the base section 410 via the container connector 1047. Of course, this reduction may also include stopping the flow of the at least one heated assisting liquid to avoid the buildup of an excessive pressure.

[0103] Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

[0104] The term comprises/comprising when used in this specification is taken to specify the presence of stated features, integers, steps or components. The term does not preclude the presence or addition of one or more additional elements, features, integers, steps or components or groups thereof. The indefinite article a or an does not exclude a plurality. In the claims, the word or is not to be interpreted as an exclusive or (sometimes referred to as XOR). On the contrary, expressions such as A or B covers all the cases A and not B, B and not A and A and B, unless otherwise indicated. The mere fact that certain measures are reci-ted in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

[0105] It is also to be noted that features from the various embodiments described herein may freely be combined, unless it is explicitly stated that such a combination would be unsuitable.

[0106] The invention is not restricted to the described embodiments in the figures, but may be varied freely within the scope of the claims.