Method of making safe an undersea bottom-to-surface production pipe when production is stopped

Abstract

Installation for producing fluid such as crude oil having a floating support having at least two tanks, an undersea bottom-to-surface connection pipe having a first pipe portion on the sea bottom from a well head to the bottom end of a second pipe portion at the floating support, a first auxiliary pipe for transporting gas extending from the floating support to the bottom end of the second pipe, and a plurality of valves for isolating or putting into communication the first auxiliary pipe, for isolating or putting into communication the first production pipe portion and the second production pipe portion, end to end, and suitable for isolating or putting into communication the proximal end of the first production pipe portion and the bottom end either of a fourth auxiliary pipe rising directly to the surface, or a bottom portion of the first auxiliary pipe.

Claims

1. An installation for producing fluid, the installation comprises: a ship or floating support on the surface having at least two tanks; an undersea bottom-to-surface connection production pipe comprising a first pipe portion resting on the sea bottom from a well head to the bottom end of a second pipe portion rising to a ship or floating support on the surface; a first auxiliary pipe for transporting gas extending at least from the ship or floating support on the surface to the bottom end of said second pipe portion; and a plurality of valves comprising: a valve (V6) suitable for isolating or putting into communication said first auxiliary pipe for transporting gas and the bottom end of said second production pipe portion; a valve (V3) suitable for isolating or putting into communication said first production pipe portion and said second production pipe portion, end to end; a valve (V5) suitable for isolating or putting into communication the proximal end of said first production pipe portion and the bottom end either of a fourth auxiliary pipe rising directly to the surface, or else a bottom portion of said first auxiliary pipe connected via an isolating or communicating valve to a top portion of said first auxiliary pipe, said first portion of said first auxiliary pipe being connected to said valve (V6) suitable for isolating or putting into communication said first auxiliary pipe and the bottom end of said second production pipe portion; a second auxiliary pipe extending at least from a first or second tank containing an inert replacement fluid or a first separator gel reagent on board the ship or floating support on the surface to a first static mixer, said second auxiliary pipe being suitable for transferring said inert replacement fluid or first separator gel reagent into said first mixer; a third auxiliary pipe extending at least from a third tank containing a second separator gel reagent on board the ship or floating support on the surface to a first static mixer, said third auxiliary pipe being suitable for transferring said second separator gel reagent into said first mixer; and a first separator gel-forming chamber leading at its other end to the proximity of the end of the first pipe portion resting on the sea bottom that is closest to the well head.

2. The installation according to claim 1, further comprising a plurality of valves, comprising at least: respective valves suitable for isolating or putting into communication said first chamber and the end of said first production pipe portion that is closest to the well head; respective valves suitable for isolating or putting into communication said second and third auxiliary pipes with said first mixer.

3. The installation according to claim 2, further comprising a valve (V9) suitable for isolating or putting into communication said second and third auxiliary pipes immediately ahead of said first mixer.

4. The installation according to claim 1, further comprising a valve suitable for isolating or putting into communication said second auxiliary pipe and the bottom end of said second pipe portion.

5. The installation according to claim 1, further comprising a buffer tank connected to the bottom end of said second pipe portion.

6. The installation according to claim 5, further comprising a separator gel-forming chamber leading at its other end to the proximity of the distal end of the buffer pipe resting on the sea bottom.

7. The installation according to claim 6, further comprising: a first branch connection pipe for transporting gas extending from said first auxiliary pipe to the distal end of the buffer pipe; a second branch connection pipe extending from said second auxiliary pipe to a second static mixer situated on the sea bottom and leading per se to a second gel-forming chamber; a third branch connection pipe extending from a third auxiliary pipe to said second static mixer situated on the sea bottom and leading per se to said second gel-forming chamber; and said second chamber leading to the distal end of the buffer pipe.

8. The installation according to claim 6, further comprising a plurality of valves, comprising at least: valves suitable for isolating or putting into communication second and third auxiliary branch connection pipes respectively with said second mixer.

9. The installation according to claim 8, further comprising a valve suitable for isolating or putting into communication said second and third branch connection pipes immediately ahead of said second mixer.

10. The installation according to claim 6, wherein said second separator gel-forming chamber is formed by a segment of pipe situated on the sea bottom to one end of which a second static mixer leads.

11. The installation according to claim 5, further comprising a plurality of valves, comprising at least: a valve (V5′) suitable for isolating or putting into communication the proximal end of a buffer pipe and the bottom end of said production pipe portion; and a valve (V9′) suitable for isolating or putting into communication the distal end of the buffer pipe and the distal end of said first auxiliary branch connection pipe for transporting gas.

12. The installation according to claim 11, further comprising a valve (V8′) suitable for isolating or putting into communication the distal end of said first auxiliary pipe for transporting gas or the proximal end of said first branch connection pipe for transporting gas with the proximal end of the buffer pipe.

13. The installation according to claim 5, wherein said buffer tank is a buffer pipe extending on the sea bottom from the bottom end of said second pipe portion.

14. The installation according to claim 1, wherein said first separator gel-forming chamber is formed by a pipe segment situated on the sea bottom at an end to which said first mixer leads.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other characteristics and advantages of the present invention appear from the following description made with reference to the accompanying drawings, which show embodiments having no limiting character. In the figures:

(2) FIG. 1 is a diagrammatic view of an installation for preserving a production pipe when stopping production and restarting production using the prior art conventional or hybrid loop technique;

(3) FIGS. 1A to 1C are diagrammatic views of an installation for preserving a production pipe when stopping production and restarting production in a first implementation of the invention using Example 1;

(4) FIGS. 2A and 2B are diagrammatic views of an installation for preserving a production pipe when stopping production and restarting production in a second implementation of the invention using Example 2; and

(5) FIG. 3 plots curves showing operating conditions in terms of pressure P and temperature T relative to forming hydrates in said first pipe 1-1 resting on the sea bottom and filled with production fluid.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

(6) In the present description, the term “valve” is used to designate a valve that is suitable for isolating two pipes from each other or for putting them into communication with each other.

(7) FIG. 1 shows an installation for securing a bottom-to-surface connection production pipe 1 that is to be made secure when stopping and restarting production, in which, using the prior art technique, a loop is established via an auxiliary pipe 18 that is connected to the end of the production pipe 1 so as to form a loop suitable for replacing the production fluid with an inert replacement fluid in the entire bottom-to-surface connection pipe 1.

(8) FIG. 3 plots typical curves showing operating conditions in terms of pressure P and temperature T relating to the formation of hydrates in said first pipe 1-1 resting on the sea bottom and filled with production fluid, as follows:

(9) curve A corresponds to conditions for forming hydrate crystals;

(10) curve B corresponds to conditions for dissolving and for dissociating hydrate crystals;

(11) zone Z1 is the zone where hydrates form and zone Z2 is a zone where there is a risk of hydrate crystals forming. Zones Z1 and Z2 represent conditions that are to be avoided. Zone Z3 is the zone in which hydrates do not form and in which production of the undersea oil field is performed in standard manner at present; and

(12) depressurizing said first pipe 1-1 in accordance with the invention makes it possible to follow the downward variation plotted along curve C from C1 to C2.

(13) In the event of an unplanned stoppage of production, the valve V0 at the top of the second pipe portion 1-2 might be closed before the well head production valve V1 is closed. This results in the pressure in the production pipe 1 rising and potentially in a small rise in temperature due to the compression of the production gas, as can be seen in the first rising portion of curve C. Thereafter, the cooling that follows stopping production causes the curve to vary to the left of C1. If the production fluid were to be left untouched, then curve C would reach the zone Z1.

(14) The curve C shown in FIG. 3 plots the path representing the desired variation in the (pressure, temperature) pair in accordance with the present invention for the production fluid in said first pipe 1-1 resting on the sea bottom going from the normal production point C1 at conditions of pressure P1 and temperature T1, to the preserved state at point C2 where the final temperature T0 is the temperature of the sea bottom, i.e. about 4° C., and the final pressure P2 is lower than the pressure for forming hydrates at the sea bottom temperature T0. In addition to the pressure and temperature conditions shown in FIG. 3, hydrate formation requires the presence of gas molecules (hydrocarbon gases from methane to butane, acid gases CO.sub.2 or H.sub.2S, or nitrogen) together with free water.

(15) The present invention thus makes it possible to preserve the pipe 1 without replacing the fluid, thereby saving the generally observed operating time that is needed for replacing the production fluid with an inert fluid.

(16) In order to specify the relative positions of the ends or of intermediate positions in the various pipes or valves, the terms “proximal” or “ahead of” are used in the description below to designate positions that are closer to the ship or the floating support on the surface, while the terms “distal” or “behind” refer to positions that are further away from the ship or the floating support on the surface relative to some other point such as another valve or another pipe end, following along the path of a fluid flowing in the pipe at this position.

(17) In the two implementations of FIGS. 1A-1C or of FIGS. 2A-2B, as described below in examples 1 and 2, an undersea bottom-to-surface connection pipe 1 is made secure or preserved when stopping production and when restarting production, the pipe comprising a first production pipe portion 1-1 (also referred to below as the “first production pipe 1-1”) resting on the sea bottom 16 from a well head 17 to a valve V3 communicating with the bottom end 1-2a of a second pipe portion 1-2 (also referred to below as the “second production pipe 1-2”) that rises up to a ship or a floating support 10 on the surface 15. The second pipe portion 1-2 may be constituted by a riser that is substantially vertical up to the surface, or it may be constituted by a hybrid pipe made up of a rigid rising pipe column or riser 1-21 that is substantially vertical being tensioned at its top 1-2c by a subsurface float 1-3, followed by a flexible hose 1-22 in the form of a dipping catenary serving to connect the riser 1-21 to the ship or floating support 10.

(18) In both implementations, on stopping production, the entire production pipe 1 is subjected to first depressurization followed by additional depressurization of the first pipe portion 1-1 that is full of production fluid, while isolating the first pipe portion 1-1 from the second pipe portion 1-2, and replacing the production fluid in the second pipe portion 1-2 with a replacement gas or fluid.

(19) Preferably, in both implementations, all of the liquid in the second pipe portion 1-2 is emptied out before putting it back into communication with the first pipe 1-1 prior to restarting production.

(20) In both implementations, on restarting production, a gel segment is used to isolate the old production fluid that is cold and depressurized both physically and thermally from the new production fluid that is hot.

(21) In both implementations, the installation includes a first auxiliary pipe 2 for delivering or discharging gas that extends from the ship or floating support 10 on the surface to at least the bottom end 1-2a of the riser 1-21 with which it communicates via a valve V6. As explained below, this first auxiliary pipe 2 serves to encourage production fluid to rise within the second pipe 1-2 in a production stage, and also to enable the production fluid in the second pipe portion 1-2 to be replaced by an inert fluid in Example 1, or else to empty the inert replacement fluid from the second pipe portion 1-2 in Example 1 or to discharge gas in order to depressurize the first production pipe in Example 1, or indeed to empty the buffer pipe in Example 2 by injecting gas upstream from the separator gel at the distal end of the buffer pipe 1a-1.

(22) The well head 17 communicates with the distal end of the first pipe 1-1 resting on the sea bottom via a pipe segment 1-1a that is defined by a valve V1 beside the well head 17 and by a valve V2 at the other end leading to the distal end of the first production pipe 1-1.

(23) A second auxiliary pipe 3 for injecting liquid extends either from a first tank 11 on the ship or floating support that contains methanol or a water and methanol mixture (i.e. a hydrate formation inhibitor), or else from a second tank 12 on the ship or floating support 10 on the surface, to a valve V7 at its distal end on the sea bottom and leading to the pipe segment 1-1a.

(24) The second tank 12 contains a liquid constituted by a reagent compound B. The reagent compound B is preferably a hydrate formation inhibitor of the glycol or methanol and ethylene glycol (MEG) type, and it also suitable for forming a gelled liquid, referred to below as the “separator gel”, when it is mixed with a reagent compound A contained in a third tank 13, the reagent A being a gelling agent that may be a cross-linking agent or a polymer or a mixture of both, and that is generally a proprietary composition. Examples of gelling agent are borate, or a polymer such as hydroxypropyl guar (HPG). By way of example, it is possible to use the gelling agents having the following commercial references:

(25) glycol pipeline gel (GPG) with the associated GPG gelling agent sold by the supplier Alchemy Oilfield Services Ltd.;

(26) gelling agents such as E-gel sold by the supplier Weatherford;

(27) gels for applications such as de-oiling pipes, as sold by the supplier Intelligent Gels; and

(28) substances known as “gel pigs” (separator gels, scraper gels) that are rigid or semi-rigid and sold by the supplier Inpipe Products.

(29) The solid separator gel is used as a physical, chemical, and thermal separator barrier that is interposed between the hot production fluid and the cold degassed fluid contained in the first pipe 1-1, the hot fluid pushing the gel and the cold fluid to the surface without any risk of forming plugs. Specifically, the newly-produced production fluid is inhibited by methanol, but only for the associated quantity of water that is produced. The mixture of this gas-containing production fluid with the degassed production fluid that is cold and contains non-inhibited water, could in principle lead to hydrates forming. This is thus a situation that must be avoided in compliance with current operating rules.

(30) In both implementations, a pipe segment forming a first chamber 5a for forming separator gel is arranged in situ at the sea bottom leading to a valve V4 for communicating with the distal end of said first pipe 1-1 ahead of the valve V2 of the pipe segment 1-1a.

(31) A third auxiliary pipe 4 extends from a third tank 13 at least as far as a first static mixer 6a ahead of the pipe segment forming the first chamber 5a. This third auxiliary pipe 4 is intended mainly for feeding the first mixer with the reagent A stored in the third tank 13.

(32) The bottom end of the second auxiliary pipe 3 also communicates via a valve V8 with the first mixer 6a. The bottom end of the third auxiliary pipe 4 communicates with the first mixer via a valve V11. A valve V9 serves to put said second and third auxiliary pipes 3 and 4 into communication with each other ahead of the valves V7, V8 and V11.

(33) The first mixer 6a serves to feed the first chamber 5a with the reaction mixture of the two reagents A and B in order to form the separator gel within the first chamber 5a.

(34) In examples 1 and 2 described below, said first and second production pipes 1-1 and 1-2 and the buffer pipe 1a are conventionally pipes having diameters of 10 inches (″) to 14″. Said auxiliary pipes 3 and 4 and branch pipes 3a and 4a are of smaller diameters and are conventionally referred to as “umbilicals”. The umbilicals are bundles of small pipes or tubing, having expected diameters lying in the range 1″ to 3″ for the auxiliary pipes and for the branch pipes 3-3a and 4-4a. Said auxiliary pipe 2 and said auxiliary branch pipe 2a may be constituted by way of example by rigid pipes of intermediate diameter, typically in the range 4″ to 6″. Another possibility is that said auxiliary pipe 2 is associated with the second production pipe 1-2 in a coaxial pipe configuration in which the second production pipe 1-2 is the inner pipe and said auxiliary pipe 2 is the annulus formed by the two coaxial pipes. Finally, said auxiliary pipe 2a may be in the form of a bundle of umbilical tubing having diameters in the range 2″ to 3″.

Example 1: First Implementation of FIGS. 1A-1C

(35) In this first implementation, the first auxiliary pipe 2 for transporting gas communicates via a valve V6 with the bottom end of the second pipe 1-2 ahead of the valve V3 (it is closer to the surface than V3). The second auxiliary pipe 3 communicates with the bottom end of the second pipe 1-2 via a branch connection 3′a from the point 3-1 ahead of the valve V9, the branch pipe 3′a having a valve V10 leading to the second pipe 1-2 between the valves V3 and V6.

(36) In a first variant shown in FIG. 1A, the first auxiliary pipe 2 for transporting gas has a top portion 2-1 communicating at its bottom end firstly with the valve V6 and secondly with a valve V19 suitable for isolating a bottom portion 2-2 of said first auxiliary pipe 2 having its distal end including a valve V5 communicating with the proximal end of the first pipe 1-1 immediately behind the valve V3 (further away from the surface than V3).

(37) In a second variant, shown in FIG. 1B, the first auxiliary pipe 2 for transporting gas does not have a said bottom portion 2-2 nor does it have a valve V19 suitable for isolating a bottom portion 2-2, however there is a valve V5 communicating with the proximal end of the first pipe 1-1 immediately behind the valve V3, which is connected to a fourth auxiliary pipe 7 rising to the surface.

(38) The first variant represents the solution that is more optimized in that the first auxiliary pipe 2 is already present for injecting gas to provide gas lift at the bottom of said second production pipe portion 1-2 so that only the bottom portion of the first auxiliary pipe 2-1 needs to be added to the configuration.

(39) A) Production Stage

(40) During a stage of production, only the valves V0, V1, V2, V3, and V6 are open. All of the other valves are closed. The open valves V1, V2, and V3 allow the production fluid (crude oil) to rise to the surface via the bottom-to-surface connection pipe 1. Opening the valve V6 and injecting gas into the auxiliary pipe 2 from the surface at the bottom end 1-2a of the second pipe 1-2

(41) serves to facilitate raising the production fluid to the surface in the second pipe 1-2.

(42) At this stage, the second and third auxiliary pipes 3 and 4 and the first chamber 5a and the first mixer 6a are full of methanol for performing preservation and restarting measures in the event of production subsequently being stopped, as described below.

(43) B) Stopping Production

(44) In order to stop production, the valves V0, V1, and V6 are closed. Thereafter, the valve V7 is opened and methanol is injected via the second auxiliary pipe 3 into the well head 17 and towards the valve V2 until the production fluid has been replaced. The valve V2 is then closed.

(45) Thereafter, the valve V0 on the surface at the top end 1-2b of the second pipe 1-2 is opened so as to degas the production fluid contained in the two production pipes 1-1 and 1-2, thereby performing first depressurization of said production pipes 1-1 and 1-2 in full.

(46) The fluid contained in the first pipe 1-1 is at an average pressure that is higher than in the second pipe 1-2 because of the liquid column in the second pipe 1-2 between the bottom and the surface. That is why it is subsequently depressurized again after closing the valves V3 and V7 and opening the valves V5 and V19 in the variant of FIG. 1A so as to discharge the residual gas contained in the production fluid within the first pipe 1-1 and reduce the pressure in the first pipe 1-1 so as to further impede any formation of hydrate plugs. Alternatively, in the variant of FIG. 1B, the additional depressurization of the first pipe 1-1 may be performed via a dedicated umbilical, i.e. the fourth auxiliary pipe 7, by opening the valve V5.

(47) By way of illustration, at a depth of 1000 m, the pressure in the first pipe goes from a pressure of several tens of bars (i.e. generally above the pressure at which hydrates form at ambient temperature (Z1)) prior to additional depressurization, to less than about ten bars (i.e. in the hydrate-free zone Z3) after the additional depressurization.

(48) Thereafter, the production fluid in the second pipe 1-2 is replaced by injecting the replacement fluid into the pipe. For this purpose, the valves V6, V8, and V9 being closed by default (normal operation position), and V7 being closed during the preceding step, the valve V10 is opened and then methanol or a water/methanol mixture is injected from the tank 11 via the third auxiliary pipe 3 to the second production pipe 1-2 at its bottom end 1-2a while discharging production fluid from the top 1-2b of the second pipe 1-2 on the surface. Thereafter, once the second production pipe is full of methanol, V10 is reclosed.

(49) In practice, and by way of illustration, for the second pipe portion 1-2 having a length of 1000 m, that represents about 50 cubic meters (m3) of replacement fluid.

(50) Alternatively, the fluid in the second production pipe 1-2 may be replaced by injecting a replacement fluid, methanol or a water/methanol mixture, from the tank 11 via the first auxiliary pipe 2, also referred to as the gas lift injection line. With the installation of FIG. 1A, after the second depressurization of the first production pipe 1-1, this operation then requires the valve V19 at least to be closed initially and the valve V6 to be re-opened.

(51) More precisely, the replacement fluid can be injected into the top portion 2-1 of said second auxiliary pipe from the ship or the floating support 10 to go to the second pipe 1-2, thus replacing and pushing the production fluid towards the ship or floating support 10 after depressurizing said first pipe 1-1, closing the valve V19, and then opening the valve V6. Thus, the replacement fluid can be injected into the top portion of said second auxiliary pipe from the ship or floating support 10 to go to the second pipe 1-2 to replace and thus push the production fluid towards the ship or floating support 10.

(52) C) Preparation Prior to Starting Production

(53) Prior to restarting production, the separator gel is prepared and stored in the first chamber 5a, and then the second pipe 1-2 is preferably emptied, as follows.

(54) In order to prepare and store the separator gel in the first chamber 5a, the valves V8 and V11 are opened while the valve V9 is left closed, and then the first static mixer 6a is fed with the reagent B, e.g. of the MEG type, via the second auxiliary pipe 3, and is fed with the reagent A via the third auxiliary pipe 4 so as to feed the chamber 5a and form the separator gel therein. Since the pressure in the first chamber 5a is higher than the pressure at the distal end of the first pipe portion 1-1, the valve V4 is opened. This ensures that production fluid does not flow back into the first chamber 5a. The methanol initially contained in the auxiliary pipes 3 and 4 and also in the first mixer 6a and the first chamber 5a is thus evacuated via the valve V4 into the first production pipe 1-1. Thereafter, the valve V4 is closed when the first chamber 5a is completely full of separator gel reaction mixture (A+B) and time is allowed for the gel to form.

(55) Thereafter, in order to avoid the reagent A stagnating for too long in the third auxiliary pipe 4 and in order to restore its pre-existing methanol state, replacement is performed using methanol. To do this, the valve V9 is opened and the valves V8 and V11 are closed, and methanol is sent from the tank 11 into the second auxiliary pipe 3, which methanol discharges through the valve V9 into the third auxiliary pipe 4 and then to the top of the third auxiliary pipe 4. Thereafter, when said auxiliary pipes 3 and 4 are full of methanol, the valve V9 is closed. After discharging the separator gel from the first chamber 5a, it is also possible to purge the first mixer 6a by keeping the valve V9 closed and the valves V8 and V11 open while performing methanol replacement.

(56) Prior to restarting production, the second pipe 1-2 is preferably emptied by injecting inert gas therein, preferably the dehydrated gas for gas lift from its top end 1-2b on the surface, and the replacement fluid contained in the second production pipe 1-2 is discharged via the first auxiliary pipe 2 through the open valve V6, while the valves V3, V5, and V10 are closed. This has the advantage of reducing pressure in the first pipe 1-1 on restarting when Opening the valve V3, thereby preventing the pressure of the column of liquid contained in the second pipe 1-2 being transferred to the first pipe 1-1, which is depressurized to a safe pressure since that might lead to a sudden increase in pressure with the potential risk of causing hydrate plugs to be formed in the first pipe 1-1.

(57) D) Restarting Production

(58) In order to restart production, the valves V4 and V11 or V8 are opened and the separator gel is injected from the first chamber 5a into the first production pipe 1-1 by injecting methanol via the valves V11 or V8 into the first mixer 6a. An additional methanol plug may also be created upstream (ahead) of the separator gel after it has been introduced into the first production pipe portion 1-1.

(59) Thereafter, once a segment of separator gel has been introduced into the production pipe 1-1, that one of the valves V4 or V8 that was opened is closed, and the valves V1, V2, and V7 are opened. Hot production fluid from the well head 17 is sent in behind the separator gel segment, which isolates the hot production fluid from the cold and degassed production fluid contained in the first production pipe 1-1, and is then caused to rise in the second production pipe 1-2, the valve V3 being re-opened. For this purpose, with the valve V6 open, gas lift gas is injected from the top of the first auxiliary pipe 2 in order to facilitate raising the production fluid moving up the second production pipe 1-2.

(60) At the same time, the valve V7 is opened and the inhibitor, i.e. methanol, is delivered in order to inhibit hydrate formation in the production fluid at the well head in the first pipe 1-1.

Example 2: Second Implementation of FIGS. 2A-2B with Buffer Pipe

(61) In this second implementation, the installation has the following differences and additional elements compared with the installation used in the first implementation.

(62) Firstly, the installation has a “buffer” pipe 1a lying on the sea bottom and extending from the bottom end 1-2a of said second production pipe 1-2 to which it is connected at its proximal end via a valve V5′, said buffer pipe being closed at its distal end 1a-1. This buffer pipe has a volume that is substantially equal to the volume of the second pipe portion 1-2.

(63) A said first auxiliary pipe 2 for transporting gas has, at its bottom end: firstly the valve V6 communicating with the bottom end of the second pipe 1-2 ahead of the valve V3 (closer to the surface than V3); and secondly a branch connection pipe 2a. The branch connection pipe 2a for transporting gas communicates with the buffer pipe 1a at two levels, firstly at the proximal end of the buffer pipe immediately behind the valve V5′ via a valve V8′, and secondly at the distal end 1a-1 of the buffer pipe via a valve V9′.

(64) In contrast, said first auxiliary pipe 2 for transporting gas no longer has the valve V5 communicating with the proximal end of the first pipe 1-1 immediately behind the valve V3, as in the first implementation.

(65) The second auxiliary pipe 3 for transporting methanol or reagent B such as MEG respectively from the tanks 11 or 12, has a second branch connection pipe 3a that goes from a point 3-1 ahead of the valve V9 to a valve V13 at its distal end leading to a second static mixer 6b. Likewise, the third auxiliary pipe 4 for transporting reagent A includes a third branch connection pipe 4a going from a point 4-1 situated immediately in front of a valve V16 in front of the valve V9 of the third auxiliary pipe 4. The third branch connection pipe 4a has a valve V17 at its proximal end, i.e. immediately after the branch connection point 4-1 and extending to a valve V18 leading to the second static mixer 6b.

(66) The second static mixer 6b leads to a pipe segment forming a second separator gel-forming chamber 5b. The second mixer 6b serves to feed the second chamber 5b with the reaction mixture of the two reagents A and B in order to form the separator gel within the second chamber 5b.

(67) The second chamber 5b communicates with the distal end of the buffer pipe 2a via a valve V6′.

(68) The separator gel is used to enable the buffer pipe to be emptied, as described below.

(69) The second and third branch connection pipes 3a and 4a communicate with each other via a valve V14 situated ahead of the valves V13 and V18 (V14 is thus in a proximal position closer to the surface than V13 and V18).

(70) The third auxiliary pipe 4 has a valve V16 after the branch connection point 4-1 ahead of the valve V9, which valve V16 when open serves to feed the reagent A to the first mixer 6a.

(71) A) Production Stage

(72) During a stage of production, only the valves V0, V1, V2, V3, and V6 are open. All of the other valves are closed. The procedure is as in Example 1. Opening the valves V1, V2, and V3 enables the production fluid (crude oil) to rise to the surface via the bottom-to-surface connection pipe 1. Opening the valve V6 serves to facilitate raising the production fluid to the surface in the second pipe 1-2 by injecting gas into the first auxiliary pipe 2 from the surface.

(73) The second and third auxiliary pipes 3 and 4 and the second and third branch connection pipes 3a and 4a and also the first and second chambers 5a and 5b and the first and second mixers 6a and 6b are all full of methanol.

(74) B) Stopping Production

(75) In order to stop production, the valves V0, V1, and V6 are closed. Thereafter, the valve V7 is opened and methanol is injected via the second auxiliary pipe 3 into the well head 17 and towards the valve V2 until the production fluid has been replaced. The valve V2 is then closed.

(76) Thereafter, the valve V0 is opened on the surface at the top end of the second pipe 1-2, in order to enable the production fluid contained in the first and second production pipes 1-1 and 1-2 to be degassed so as to perform first depressurization of said pipes 1-1 and 1-2, as described in Example 1.

(77) In this second implementation, in order to preserve the bottom-to-surface connection pipe 1 as much as possible from any hydrate formation, the second production pipe 1-2 is emptied and the first pipe portion 1-1 is depressurized more completely by degassing the empty second pipe portion.

(78) For this purpose, the content of the second production pipe 1-2 is passively drained or emptied into the buffer pipe 1a, by closing the valve 3V and opening the valves V5′ and V8′. Opening V8′ serves to discharge gas from the buffer pipe while it is being filled via the valve V5′ with the production fluid from the pipe 1-2.

(79) Once the second pipe 1-2 has emptied into the buffer pipe 2a, the valves V5′ and V8′ are closed and the valve V3 is opened to discharge more thoroughly the residual gas contained in the production fluid inside the first pipe 1-1 to the empty second pipe 1-2, thereby performing additional depressurization thereof via the empty second pipe 1-2. Thereafter, the valve V3 is closed once more.

(80) In this second implementation, it is thus possible to leave the second production pipe 1-2 full of production gas without filling it with methanol. The entire production pipe is then preserved since it is at a pressure lower than the pressure for forming hydrates at ambient temperature.

(81) It should be observed that in Example 1 it is not possible to empty the production fluid from the second pipe 1-2 by sending inert gas into it, possibly the gas used for gas lift, from its top and discharging the gas via the first auxiliary pipe 2, since that would lead to the risk of hydrates forming in the first auxiliary pipe 2. Specifically, the first auxiliary pipe 2, or gas lift line, is generally a line of small diameter with little thermal inertia and thus only a short available cooling time (a few hours). By passing a production fluid containing gas and including water in this pipe, it is very likely that the low temperature and the high pressure due to the movement and to the hydrostatic column as created in this way would lead to hydrates forming, which could quickly block this small section line.

(82) C) Preparation Prior to Starting Production

(83) Prior to restarting production, the separator gel is prepared and stored in the first and second chambers 5a and 5b, as follows.

(84) In order to fill the first chamber, the valves V8, V11, and V16 are opened while the valves V7, V9, V17, V13, and V14 are left closed. Thereafter the first static mixer 6a is fed with reagent B, e.g. of the MEG type, via the second auxiliary pipe 3, and it is fed with reagent A via the third auxiliary pipe 4 so as to feed the first chamber 5a with separator gel, as in Example 1. Initially, the methanol contained in the auxiliary pipes 3 and 4 and in the first mixer 6a and in the first chamber 5a is discharged, as in Example 1.

(85) In order to avoid leaving the reagent A stagnate for too long in the third auxiliary pipe 4, it is filled with methanol, as is the second auxiliary pipe 3, as in Example 1.

(86) In order to fill the second chamber 5b, with valves V4 and V7 being closed by default, the valves V16, V8, and V9 are closed and the valves V13, V17, and V18 are opened. Thereafter, the second static mixer 6b is fed with MEG type reagent B via the second auxiliary pipe 3 and the second branch connection pipe 3a, and it is fed with reagent A via the third auxiliary pipe 4 and the third branch connection pipe 4a so as to feed the second chamber 5b with separator gel. Initially, the methanol contained in the auxiliary pipes 3 and 4 and in the branch connection pipes 3a and 4a, and also in the second mixer 6b and the second chamber 5b is discharged via the valve V6′ that is open in the buffer pipe 2a, the valve V5′ being opened beforehand. Since the pressure in the second and third auxiliary pipes 3 and 4 and in the second and third branch connections 3a and 4a is higher than the pressure at the distal end of the buffer pipe 1a-1, the production fluid does not flow back into the chamber 5b.

(87) Thereafter, the valve V6′ is closed once the chamber 5b is completely full of separator gel reaction mixture (A+B) and time is allowed for the gel to form.

(88) Once the chamber 5b is full of gel, and in order to avoid the reagent A stagnating for too long in the third auxiliary pipe 4 and the third branch connection pipe 4a, they are filled with methanol, as are the second auxiliary pipe 3 and the second branch connection pipe 3a. For this purpose, the valves V14 and V17 are opened, the valves V13, V18, and V16 are closed, and methanol is sent from the tank 11 into the third auxiliary pipe 4 and into the branch connection pipe 4a, which methanol is discharged through the valve V14 via the branch connection pipe 3a to the top of the second auxiliary pipe 3 (the valves V8, V13, V17, and V18 being closed). Thereafter, when said auxiliary pipes 3 and 4 and said auxiliary branch connections 3a and 4a are full of methanol, the valve V14 is closed. After discharging the separator gel from the second chamber 5b, it is also possible to purge the first mixer 6b by keeping the valve V14 closed and the valves V18 and V13 open during the time required for replacement with methanol.

(89) The separator gel contained in the second chamber 5b is used to empty the buffer pipe without any risk of hydrates forming prior to restarting production by sending the gel to the distal end of the buffer pipe and by discharging it to the top of the second production pipe 1-2 as follows.

(90) The valves V13 and V6′ are opened, while the valves V14, V8, V17, and V18 are closed, and methanol is sent via the second auxiliary pipe 3 and the second branch connection pipe 3a, which methanol pushes the gel from the chamber 5b into the buffer pipe 2a.

(91) Thereafter, the valve V6′ is closed and the valve V9′ is opened, and inert gas, preferably the gas lift gas, is sent to the distal end 2a-1 of the buffer pipe 2a from the top of the first auxiliary pipe 2, the valve V8′ being closed. Said gas thus pushes the gel and the content in the buffer pipe ahead of the gel towards the second pipe 1-2 in order to be discharged at its top 1-2b. Once the buffer pipe 2a and then the second pipe portion 1-2 have been emptied of their liquid content, the valves V9′ and V5′ are closed.

(92) It would not be possible to empty the buffer pipe via the riser 1-2 without the gel merely by injecting gas into the buffer pipe, since because of its large section that would require the pressure and the flow rate of the gas to be unrealistic. Furthermore, the production fluid in the buffer pipe contains degassed oil and water at low temperature. Mixing it with high-pressure gas would cause hydrates to form. In contrast, since the gel is solid it can be pushed by the gas while maintaining a physical separation interface, given its mechanical and chemical qualities.

(93) In contrast, in Example 1, it is possible to push the liquid from the riser 1-2 upwards in the auxiliary pipe 2 with inert gas sent from the top of the riser 1-2, since the auxiliary pipe 2 up which it rises is of smaller diameter. Furthermore, the gas is then pushing a replacement fluid, which itself is a hydrate inhibitor.

(94) D) Restarting Production

(95) In order to restart production, the valve V4 is opened and the separator gel is sent from the first chamber 5a into the first production pipe 1-1, and the procedure continues as in Example 1.

(96) Thus, while there have shown and described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.