Underwater system for heating a two-phase liquid / gas flow circulating in an underwater housing

Description

Technical Field

[0001] The present invention relates to the general field of heating metal fluid transport pipes, and in particular underwater pipes lying on the seabed and providing a connection between underwater hydrocarbon production wells, in particular oil and gas, and a surface installation, for example a floating production, storage and unloading unit. Prior art

[0002] In a single offshore hydrocarbon production field, it is common to operate several wells that may be separated from each other by several kilometers, or even tens of kilometers. The two-phase effluents (liquid / gas mixture) from these different wells must be collected by subsea pipelines laid on the seabed and transferred by bottom / surface connecting pipelines to a surface facility, for example a ship or a land-based collection point, which will collect them for storage (and possibly treatment).

[0003] Furthermore, due to their extraction at great depth in the subsurface, two-phase effluents from underwater hydrocarbon production wells come out at a relatively high temperature at seabed level (typically around 70°C). Since seawater is generally cold, especially at great depths where it is typically 4°C, if no measures are taken to conserve the heat of the effluents leaving the production wells, they will gradually cool down as they travel through the kilometers of underwater pipes. However, these two-phase effluents contain various chemical compounds for which cooling causes phenomena that are detrimental to the maintenance of good circulation conditions.

[0004] Thus, gas molecules, particularly methane, combine with water molecules to form hydrate crystals at low temperatures. These crystals can stick to the walls, agglomerate there, and lead to the formation of plugs capable of blocking the underwater pipeline. Similarly, the solubility of high-molecular-weight compounds, such as paraffins or asphaltenes, in oil decreases as the temperature drops, giving rise to solid deposits that can also block the underwater pipeline.

[0005] A known solution to prevent blockages in subsea pipelines is to heat the subsea pipelines along their entire length using one or more electrical cables wrapped around the pipelines to heat them by the Joule effect. This solution, called "trace heating," allows two-phase effluents transported in subsea pipelines to be maintained at a temperature above a critical threshold throughout their entire journey from the production well to the surface facility.

[0006] This solution presents obvious problems related to the installation of such electric heating cables along the entire length of the underwater pipelines, with the significant costs this represents in terms of installation and maintenance. Furthermore, traced heating is based on the continuity of the installation along the entire length of the underwater pipelines. However, if this continuity were to be broken for one reason or another at one point in the pipelines, the entire installation would be out of service. This constraint therefore requires this type of heating to be considered only for the so-called preservation phases of the transported effluents, and not for the operational phases of operation.

[0007] Publication WO 2016 / 066968 discloses a local heating station that can be placed in several locations along the underwater pipes in order to ensure a sufficient temperature level of the transported effluents over a very long length of pipes. In practice, each heating station can comprise a number of horizontal conduit windings that is a multiple of the number of phases of the supply electric current, the conduit windings each being a section of conduit around which a solenoid is wound, the solenoids being electrically mounted so as to obtain a polyphase assembly. This type of architecture thus offers great ease of installation and great flexibility of use.

[0008] The design of the local heating station must allow for the injection of very high thermal power levels while minimizing the length of pipe to be heated and respecting a maximum pipe temperature (typically of the order of 150°C). However, although efficient, the heating station solution presented in publication WO 2016 / 066968 requires heating very long lengths of pipe, which considerably increases the weight and dimensions of the heating station. Statement of the invention

[0009] The main object of the present invention is therefore to propose a heating installation which does not have the aforementioned drawbacks.

[0010] According to the invention, this aim is achieved by means of an underwater installation for heating a two-phase liquid / gas effluent circulating inside an underwater envelope according to the characteristics of independent claim 1.

[0011] The heating installation according to the invention is remarkable in that the heated pipe sections are inclined relative to the horizontal. By "horizontal" is meant here any direction perpendicular to the direction of gravity. This inclination makes it possible to promote a distributed flow regime of the two-phase effluent circulating in the installation. A so-called "distributed" flow (and in particular a slug flow) is characterized by a structure having a succession of gas pockets and liquid slugs which contain small bubbles. This type of flow is opposed to a so-called "stratified" flow which is obtained by a two-phase effluent flow circulating in horizontal pipe sections and whose liquid / gas interface is in the form of a continuous surface separating the two phases (the liquid is located below the gas under the effect of gravity).However, it has been found that a distributed flow regime makes it possible to increase the heat exchange coefficient between the pipe section and the two-phase effluent (in particular compared to a stratified flow which is inhomogeneous and detrimental to the heat exchange coefficient). The inclination of the heated pipe sections thus makes it possible to obtain a distributed flow regime and significantly improve the heat exchange coefficient and therefore promote the transfer of very high thermal powers. In this way, for the same levels of injected power, it is possible to very significantly reduce the length of the heated pipe sections, and therefore reduce the size and weight of the heating installation.

[0012] It should be noted that favoring a distributed flow regime inside the underwater envelope goes completely against the prejudices of the person skilled in the art who, on the contrary, tends to favor stratified flow regimes. Indeed, distributed flow regimes have the disadvantage of generating (at least for a range of flow rates) liquid plugs inside the pipe sections which subject the pipe to vibrations, in particular at the connection parts and moreover generate higher pressure losses.

[0013] The heating coil may comprise at least six straight pipe sections connected to each other by at least five curved pipe sections, an inlet for connection to the underwater jacket and an outlet, the inlet and outlet preferably being vertically offset from each other.

[0014] The heated pipe sections are preferably substantially straight. Also preferably, each heated pipe section of the heating coil is inclined relative to the horizontal at an angle of 5°.

[0015] The heated pipe sections of the heating coil may each be upward flowing for the effluent, and the curved pipe sections are each downward flowing for the effluent.

[0016] More preferably, at least one of the curved pipe sections is provided with an induction heating system (even if the flow in these curved pipe sections may be less distributed than for the heated pipe sections).

[0017] The heating system of each heated pipe section of the heating coil may comprise at least one induction coil disposed around the heated pipe section over at least a portion of its length and powered by an electric current so as to generate an induced current in the heated pipe section capable of causing heating thereof.

[0018] The induction coils of the heating system can have an electrical power of between 1 and 100 kW per meter of coiled pipe.

[0019] The heated pipe sections can advantageously be covered with a layer of thermal insulation around which the induction coils are wound. Brief description of the drawings

[0020] [ Fig. 1 ] There figure 1 is a schematic side view of an underwater heating installation according to one embodiment of the invention. Fig. 2 ] There figure 2 is a top view of the installation of the figure 1 . [ Fig. 3 ] There figure 3 is a schematic side view of an underwater heating installation according to another embodiment of the invention. Description of the embodiments

[0021] The invention applies to any network of single or multiple envelope underwater pipes providing a connection between at least one underwater hydrocarbon production well and a surface installation.

[0022] The purpose of such a subsea pipeline network is to transport hydrocarbon effluents (two-phase mixture of oil and gas) from one or more subsea production wells to a surface facility, for example a floating production, storage and offloading unit (also called FPSO for "Floating Production Storage and Offloading" in English).

[0023] These networks generally comprise several single or multiple-casing underwater pipes which are laid on the seabed and in which two-phase effluents from the production wells circulate.

[0024] To maintain the effluents transported in these underwater pipes at a temperature above a critical threshold making it possible to avoid the formation of deposits inside said pipes, the invention provides for connecting the pipes to one or more removable underwater heating installations such as that shown in the figures 1 And 2 .

[0025] The heating installation 2 shown in these figures is removably connected to a single-jacket underwater pipe (not shown). It is controlled from the surface installation (not shown in the figure) depending in particular on the network operating mode (typically: normal operating phase, preservation phase or production restart phase).

[0026] Generally, the heating installation 2 according to the invention comprises a plurality of heated pipe sections 4 which are successively connected to each other by curved pipe sections 6 so as to form a single heating coil, the heated pipe sections 4 typically being made of a conductive metal such as steel. The pipe sections 4, 6 forming the heating coil are mounted on a frame 7 formed of an assembly of vertical tubes 7a and horizontal tubes 7b.

[0027] More precisely, on the example of realization of the figures 1 And 2 , the heating installation 2 comprises an inlet 8 which is intended to be connected to the single-jacket underwater pipe and which opens into a first heated pipe section 4-1.

[0028] The latter opens (at the end opposite the inlet 8) into a first curved pipe section 6-1 which itself opens at one end of a second heated pipe section 4-2 running in the opposite direction to the first.

[0029] The circuit is repeated according to this same pattern so as to form a heating coil comprising here six heated pipe sections 4-1 to 4-6 which are connected to each other by five curved pipe sections 6-1 to 6-5, an inlet 8 and an outlet 10, the inlet and outlet being offset vertically and horizontally from each other.

[0030] It should be noted that in this embodiment, the heated pipe sections 4-1 to 4-6 are straight. Of course, they could be curved.

[0031] Still according to the invention, the heated pipe sections 4-1 to 4-6 are each inclined relative to the horizontal (or relative to the horizontal tubes 7b of the frame 7) by the same angle α which is between 2 and 10°, and which is preferably equal to 5°.

[0032] Preferably, the heated pipe sections 4-1 to 4-6 are each ascending in the direction of effluent flow, and the curved pipe sections 6-1 to 6-5 are each descending in the direction of effluent flow.

[0033] So, with such an arrangement, as shown in the figure 2 , heated pipe sections 4-1, 4-3 and 4-5 are parallel to each other, as are heated pipe sections 4-2, 4-4 and 4-6.

[0034] The inclination of the heated pipe sections makes it possible to promote a so-called "distributed" flow regime, and more particularly a plug flow, of the two-phase effluent circulating in the installation.

[0035] A distributed flow has a structure that presents a succession of gas pockets and liquid plugs containing small bubbles. This type of flow is opposed to a so-called "stratified" flow which is obtained by a two-phase effluent flow circulating in horizontal pipe sections and whose liquid / gas interface is in the form of a continuous surface separating the two phases (the liquid is located below the gas under the effect of gravity).

[0036] Still according to the invention, at least the heated pipe sections 4-1 to 4-6 are each provided with an induction heating system.

[0037] Typically, the heating system for each heated pipe section includes at least one induction coil 16 (or solenoid) that is disposed around the heated pipe section 4-1 to 4-6 over at least a portion of its length.

[0038] These induction coils 16 are supplied from the surface installation with three-phase electric current. Since the heated pipe sections are made of steel, the power supply to the induction coils generates an induced current in them, which causes them to heat up. The heat is then transmitted by the heated pipe sections to the circulating two-phase fluid by convection.

[0039] For example, to raise the outside temperature of the heated pipe sections, the induction coils of the heating system have an electrical power of between 1 and 100 kW per meter of coiled pipe.

[0040] It will be noted that the heated pipe sections 4-1 to 4-6, as well as the curved pipe sections 6-1 to 6-5, may be covered with a layer of thermal insulation 18. In this case, the induction coils 16 are wound around this layer of thermal insulation.

[0041] Such an arrangement of the underwater heating system with heated pipe sections inclined relative to the horizontal makes it possible to significantly improve the heat transfer between the inner face of the heated pipe sections and the circulating two-phase effluent.

[0042] For example, compared to a horizontal underwater heating installation (such as that described in publication WO 2016 / 066968), it is possible, for equal electrical power, to shorten the length of the heated pipe sections by more than 40%.

[0043] It should be noted that on the method of realization of the figures 1 And 2, only the heated pipe sections 4-1 to 4-6 are equipped with an induction heating system. Of course, curved pipe sections could also be heated.

[0044] In connection with the figure 3 , we will now describe an underwater heating installation 2' according to another embodiment of the invention.

[0045] This embodiment differs from that previously described in particular by its heating coil architecture which comprises twelve rectilinear heated pipe sections 4'-1 to 4'-12 which are successively connected to each other by eleven curved pipe sections 6'-1 to 6'-11. The heated pipe sections 4'-1 to 4'-12 are each equipped with an induction heating system (on the figure 3 , for convenience, only the pipe section 4'-9 is shown with an induction coil 16).

[0046] Furthermore, still in this embodiment, the inlet 8' and the outlet 10' are aligned vertically and offset horizontally from each other.

Claims

1. A subsea installation for heating (2; 2') for a two-phase liquid / gas effluent circulating inside a subsea casing, comprising a plurality of heated pipe sections (4-1 to 4-6; 4'-1 to 4'-12) which are connected successively to one another by curved pipe sections (6-1 to 6-5; 6'-1 to 6'-11) to form a heating coil, each heated pipe section being inclined with respect to the horizontal at an angle (α) comprised between 2 and 10° to promote a distributed flow regime of the two-phase effluent and being provided with an induction heating system (16), the heating installation comprising a frame (7), the heating coil being assembled on the frame (7) which is formed by an assembly of vertical (7a) and horizontal (7b) tubes, each section of heated pipe being inclined relative to the horizontal tubes (7b) of the frame (7) by the angle (α) comprised between 2 and 10°.

2. The installation according to claim 1, wherein the heating coil comprises an inlet (8; 8') for connection to the subsea casing and opening into a heated pipe section which is ascending for the effluent flowing therein.

3. The installation according to claim 1 or claim 2, wherein the heated pipe sections are substantially straight.

4. The installation according to any one of claims 1 to 3, wherein the heated pipe sections of the heating coil are each ascending for the effluent, and the curved pipe sections are each descending for the effluent.

5. The installation (2) according to any one of claims 1 to 4, wherein the heating coil comprises at least six straight pipe sections (4-1 to 4-6) connected to each other by at least five curved pipe sections (6-1 to 6-5), an inlet (8) for connection to the subsea jacket and an outlet (10), the inlet and the outlet preferably being vertically offset from each other.

6. The installation as claimed in according to any one of claims 1 to 5, wherein each heated pipe section of the heating coil is inclined with respect to the horizontal at an angle (a) of 5°.

7. The installation according to any one of claims 1 to 6, wherein at least one of the curved pipe sections is provided with an induction heating system.

8. The installation according to any one of claims 1 to 7, wherein the heating system of each heated pipe section of the heating coil comprises at least one induction coil (16) disposed around the heated pipe section over at least a portion of its length and powered by an electric current so as to generate an induced current in the heated pipe section capable of causing heating thereof.

9. The installation according to claim 8, wherein the induction coils of the heating system have an electrical power comprised between 1 and 100 kW per meter of wound pipe.

10. The installation according to claim 8 or claim 9, wherein the heated pipe sections are covered with a layer of thermal insulation (18) around which the induction coils (16) are wound.