EP3392452B1 - A method of making safe an undersea bottom-to-surface production pipe when production is stopped - Google Patents

Description

Background of the invention

The present invention relates to the general field of fluid transport pipes for the transfer of hydrocarbons, in particular crude oil containing mainly an oily phase of hydrocarbons, water and gas, from subsea production wells , hereinafter referred to as production fluid.

It concerns more precisely a method for managing the stopping and restarting of the production of an underwater bottom-surface connection pipe connecting the seabed to supports floating on the surface.

This invention applies more particularly to the development of deep-sea oil fields, that is to say oil installations installed in the open sea, in which the surface equipment is generally located on floating structures, the wellheads being at bottom of the sea. The pipes concerned by the present invention comprising more particularly the risers called bottom-surface connection pipes rising towards the surface, but also the pipes resting at the bottom of the sea connecting the well heads to said risers.

The main application of the invention concerns submerged, underwater or subaquatic pipes or pipelines, and more particularly at great depth, beyond 300 meters, and conveying hot petroleum products, excessive cooling of which would be problematic in the event of production stoppage. Deep sea developments are carried out in water depths currently reaching 1500 m. Future developments are envisaged by water depths up to 3000-4000 m and beyond.

It is known to those skilled in the art that for great depths, the injection at the bottom of the riser of dehydrated gas (in English "gas lift") is used to reduce the pressure due to the hydrostatic column, and therefore improve the productivity of production wells.

• une forte augmentation de la viscosité qui diminue alors le débit de la conduite,
• une précipitation de paraffine dissoute qui augmente alors la viscosité du produit et dont le dépôt peut diminuer le diamètre intérieur utile de la conduite,
• la floculation des asphaltènes induisant les mêmes problèmes,
• la formation soudaine, compacte et massive d'hydrates de gaz qui précipitent à forte pression et faible température, obstruant ainsi brusquement la conduite en formant des bouchons.

In this type of applications, many problems arise in particular in the event of a production stoppage when the temperature of the petroleum products decreases by a significant value compared to their production temperature which is often beyond 60 to 80 °C while the temperature of the surrounding water, especially at great depths, can be well below 10°C and reach 4°C. If petroleum products cool for example below 30° to 60°C for an initial temperature of 70 to 80°C we generally observe:

• a strong increase in viscosity which then reduces the flow rate of the pipe,
• a precipitation of dissolved paraffin which then increases the viscosity of the product and whose deposition can reduce the useful internal diameter of the pipe,
• the flocculation of asphaltenes inducing the same problems,
• the sudden, compact and massive formation of gas hydrates which precipitate at high pressure and low temperature, thus suddenly obstructing the pipe by forming blockages.

Paraffins and asphaltenes remain attached to the wall and therefore require cleaning by scraping the inside of the pipe; on the other hand, hydrates are even more difficult, and sometimes even impossible, to absorb.

Additionally, in risers, the gas mixed with crude oil and water tends to relax as it rises because the hydrostatic pressure drops. This expansion being quasi-adiabatic, the calories are taken from the polyphase fluid itself, and this results in a significant lowering of the internal temperature, the latter being able to reach 8 to 15°C over a difference in altitude of 1500m which can generate the formation of hydrate plugs.

The thermal insulation and reheating of such pipes makes it possible to delay the cooling of the petroleum effluents conveyed not only in established production regime, so that their temperature is for example at least 40°C when arriving at the surface, for a temperature of production at the entrance to the pipe from 70°C to 80°C, but also in the event of a reduction or even cessation of production, in order to to prevent the temperature of the effluents from falling, for example, below 30°C, in order to limit the above problems, or at least to make them reversible.

It is known to heat double-jacketed pipes over their entire length using a plurality of electrical cables which are wound around the external surface of the internal jacket of the pipes to heat it by the Joule effect. This heating solution, which is called “heat tracing” in English, makes it possible to maintain the hydrocarbon fluids transported in the underwater pipes at a temperature above a critical threshold throughout their journey from the production well to the surface installation, and thus avoid the formation of hydrate crystals or other solid deposits leading to the creation of plugs capable of blocking the underwater pipe. In particular, this traced heating allows the temperature of the production fluid to be maintained above this critical threshold during shutdown phases, thus allowing almost immediate preservation after its activation. This process is illustrated on the figure 1 .

In the event of a shutdown of several days or weeks, at the time of shutdown the conditions of high pressure and falling temperature, there is a risk of causing the formation of a hydrate plug. For this reason, the standard preservation method is to depressurize the line first. This measure is not sufficient to preserve the pipe at great depth, after closing the wellhead valve upstream of the pipe and depressurizing it by opening the valve at the top of the riser on the surface, circulation in loop of an inert substitute product, for example diesel or degassed crude oil (“dead crude oil”) is initiated. By “inert” we mean here that the fluid does not react to form hydrate crystals.

This so-called conventional or hybrid loop process is illustrated on the figure 1 . This process allows the pipe to drop in temperature to 4°C without the formation of hydrate plugs. And, when restarting, the same diesel is generally used to reheat the pipe by circulating it in a loop from the floating support where it is heated by passing it through boilers or heat exchangers, recovering calories in from gas turbines. It is only after this reheating phase with diesel circulation that the wellhead valves can be reopened and production resumed.

Indeed, if a premature restart of production was carried out before sufficient and prior heating of the line, during the progression of the crude oil towards the FPSO and after a journey of a few kilometers, or even only a few hundred meters, the oil , even leaving the well at a high temperature, for example 75°C, would see its temperature drop to the critical value at which dreaded phenomena of formation of hydrate or paraffin plugs can occur, which would result in a blockage of the flow of crude oil.

In WO 2009/042307 , a process is described in which after depressurization of the pipe following a case of production stoppage, the fluid it contains is replaced with an inert replacement fluid. And, to replace the production fluid present in the pipe at the time of shutdown, a mechanical scraper is used, previously stored near the entrance to the first pipe, in combination with a product inhibiting the formation of blockages or not being able to form a hydrate hereinafter referred to as a hydrate formation inhibitor product such as methanol, glycol or mono ethylene glycol (MEG for short) and a displacement fluid injected into the pipe, upstream of it at the bottom of the sea, to move and advance the inhibitor product and the mechanical scraper by pushing it in the pipe towards the surface. The displacement fluid is degassed diesel or crude oil combined with a hydrate inhibitor and acts as a replacement fluid in the line. The water injection line allows the scraper to be replaced at the storage site to ensure subsequent preservation. Due to the fact that the replacement fluid does not contain gas or water and/or contains a product that inhibits the formation of hydrates, upon restarting there is no risk of hydrate formation.

The disadvantage of the process described in WO2009/042307 like the so-called loop processes, is that they require a large quantity of replacement fluid to fill the entire pipe on the one hand and on the other hand, the sending from the surface of a mechanical scraper.

However, in the case of a long subsea production pipeline (several kilometers) with a part of the pipeline resting on the seabed extending from a wellhead to the seabed and the lower end of a pipe in the form of a riser or riser, the process can become expensive and time-consuming to implement.

Furthermore, the use of a mechanical scraper during the preservation phase carries an operational risk of blocking the scraper, which, if necessary, could lead to the formation of a hydrate plug. We also know the document WO 2016/066967 which discloses a fluid production installation according to the preamble of claim 21.

Object and summary of the invention

The main aim of the present invention is therefore to provide an improved method for preserving and securing a production pipe forming a bottom-surface connection pipe, when production is stopped and when production is restarted to avoid the formation of hydrates and so that after a prolonged shutdown, the restart phase is facilitated.

In accordance with the invention, this goal is achieved by providing a method of stopping production and securing an underwater production bottom-surface connection pipe according to claim 1. Other provisions of the invention are set forth in the attached set of claims.

This additional degassing of the production fluid contained in the first part of the pipe makes it possible to reduce the pressure of the first part of the pipe more significantly to a pressure level close to that on the surface and thus eliminate a risk of formation of hydrates in said first part of pipe resting at the bottom of the sea without having to carry out fluid replacement within it. Otherwise, the pressure at the level of the first part of the pipe and at the level of the wellhead would be linked to the hydrostatic column of the rising pipe of said second part of the pipe and the depressurization would not make it possible to exclude in certain cases the risk of hydrate formation.

More particularly, to stop production and carry out the first depressurization of the entire pipe, at least one valve V2 is closed at the end closest to the well head of the first part of the pipe resting at the bottom. from the sea and a valve V0 is opened at the top of the second part of the surface pipe.

• isolant la première partie de conduite par rapport à la deuxième partie de conduite, en fermant une première vanne V3 au niveau de la liaison entre l'extrémité de la première partie de conduite et l'extrémité inférieure de deuxième partie de conduite, la dite première vanne V3 ainsi fermée empêchant la communication de fluides entre la dite première partie de conduite et la dite deuxième partie de conduite, et
• ouvrant une deuxième vanne V5 ou V5' située à proximité de la dite première vanne V3, la dite deuxième vanne débouchant directement (V5) sur une conduite annexe de remontée de gaz remontant jusqu'au navire ou support flottant en surface directement ou via (V5') un réservoir tampon, de préférence une conduite tampon décrite ci-après permettant la vidange de la deuxième partie de conduite et autorisant ainsi la dépressurisation de la première partie de conduite.

More particularly, the complementary depressurization of the first part of production management is carried out by:

• isolating the first pipe part from the second pipe part, by closing a first valve V3 at the connection between the end of the first pipe part and the lower end of the second pipe part, said first valve V3 thus closed preventing the communication of fluids between said first pipe part and said second pipe part, and
• opening a second valve V5 or V5' located near said first valve V3, said second valve opening directly (V5) onto an additional gas lift pipe going up to the ship or floating support on the surface directly or via (V5 ') a buffer tank, preferably a buffer pipe described below allowing the emptying of the second part of the pipe and thus authorizing the depressurization of the first part of the pipe.

Preferably, in the case where the degassed and cold production liquid in the first part of the pipe is in a condition of hydrate formation (zone Z1 or Z2 described below) at the pressure resulting from the liquid column of the second part pipe, before restarting production, before putting the first part of the production pipe into communication with said second part of the production pipe, any liquid contained in said second part of the pipe is emptied.

Preferably, after or during the restart of production, the rise of the production fluid is encouraged in said second part of the production pipe, by sending gas from the ship or floating support on the surface into a first auxiliary pipe for transporting gas opening out at the lower end of the second part of the production pipe to which it is connected.

Emptying said second pipe part before restarting production, before putting the first production pipe part filled with production fluid into communication makes it possible to avoid a sudden rise in pressure of the fluid inside the pipe. the first part of the pipe resting at the bottom of the sea during the communication of the first part of the production pipe with the second part of the production pipe by opening the first valve V3 which could cause the formation of hydrates in the said first part of driving. The said first part of the pipe is thus maintained at a pressure which does not allow the formation of hydrates at sea bottom temperature, i.e. approximately 4°C.

• a1) à l'étape a), après avoir isolé la dite deuxième partie de conduite de la dite première partie de conduite, on remplace le fluide de production au sein de la dite deuxième partie de conduite en injectant un fluide inerte de remplacement dans une deuxième conduite annexe s'étendant depuis un premier réservoir sur le navire ou support flottant en surface jusqu'à l'extrémité inférieure de la deuxième partie de conduite isolée de la première partie de conduite, de préférence un fluide inerte comportant ou constituant en outre un produit inhibiteur de la formation d'hydrates ; et
• b1) à l'étape b), on réalise une dépressurisation complémentaire de la première partie de conduite de production isolée de la dite deuxième partie de conduite et remplie de fluide de production, en diminuant la pression dans la dite première partie de conduite et en évacuant plus complètement le gaz contenu dans le fluide de production qu'elle contient, vers une conduite annexe d'évacuation de gaz s'étendant depuis l'extrémité de la dite première partie de conduite de production la plus proche de l'extrémité inférieure de la dite deuxième partie de conduite de production jusqu'au navire ou support flottant en surface.

According to a first embodiment, the following steps are carried out:

• a1) in step a), after having isolated said second pipe part from said first pipe part, the production fluid is replaced within said second pipe part by injecting an inert replacement fluid into a second auxiliary pipe extending from a first tank on the ship or surface floating support to the lower end of the second pipe part isolated from the first pipe part, preferably an inert fluid further comprising or constituting a product inhibiting the formation of hydrates; And
• b1) in step b), a complementary depressurization is carried out of the first production pipe part isolated from said second pipe part and filled with production fluid, by reducing the pressure in said first pipe part and evacuating more completely the gas contained in the production fluid which it contains, towards an additional gas evacuation pipe extending from the end of said first part of production pipe closest to the lower end of the said second part of the production pipeline to the vessel or floating support on the surface.

More particularly, in step a1), the production fluid is replaced within said second pipe part by injecting an inert replacement fluid, preferably an inert fluid further comprising or constituting a product inhibiting the formation of 'hydrates, from a first tank on the ship or floating support in a first annex gas lift pipe or second annex pipe extending to the lower end of the second part of pipe which is previously isolated from the first part of pipe after depressurization of said first part of production pipe, said inert fluid replacing and thus pushing back the production fluid towards the vessel or floating support.

Even more particularly, before restarting production, before putting the first part of the pipe resting on the seabed back into communication with the second part of the pipe rising to the surface and sending production fluid there from the wellhead , said second pipe part is drained by injection of inert gas into the second pipe part from the top of the second pipe part and the inert replacement fluid is evacuated from the second pipe part towards the surface via a first pipe gas lift annex which extends from the surface to the lower end of said second pipe part to which it is connected. This operation is necessary in the case where the degassed and cold production liquid in the first part of the pipe is in a condition of hydrate formation at the pressure resulting from the liquid column of the second part of the pipe. In the contrary case, that is to say if the degassed production at sea bottom temperature was not capable of forming hydrates even at the pressure resulting from the communication of the first and second part of the pipe without draining of the latter, it is therefore not necessary to empty the second pipe part of its replacement fluid.

It is then necessary to depressurize the gas used to purge said second part of pipe and said first annex pipe before being able to open said first valve V3 for separation between said first and second parts of production pipe and send therein production fluid from the wellhead.

This makes it possible to avoid a sudden rise in pressure of the fluid inside the first part of the pipe resting at the bottom of the sea, which could cause the formation of hydrates in said first part of the pipe when it is put into communication. of the first pipe part with the second production pipe part since said first pipe part is thus maintained at a pressure corresponding to the atmospheric pressure on the surface.

• a2) à l'étape a), on laisse le fluide de production dans la dite première partie de conduite de production, et on vidange la dite deuxième partie de conduite isolée de la dite première partie de conduite, en transférant le fluide de production au sein de la dite deuxième partie de conduite dans un réservoir tampon relié à l'extrémité inférieure de la dite deuxième partie de conduite, le dit réservoir tampon étant de préférence une conduite tampon s'étendant au fond de la mer depuis l'extrémité inférieure de la dite deuxième partie de conduite, et
• b2) à l'étape b), on réalise une dépressurisation complémentaire de la première partie de conduite de production remplie de fluide de production, en la mettant en communication avec la dite deuxième partie de conduite et en évacuant plus complètement le gaz contenu dans le fluide de production de la première partie de conduite vers la dite deuxième partie de conduite de production préalablement vidée de tout liquide.

According to a second embodiment, the following steps are carried out:

• a2) in step a), the production fluid is left in said first production pipe part, and said second pipe part isolated from said first pipe part is drained, by transferring the production fluid to the within said second pipe part in a buffer tank connected to the lower end of said second pipe part, said buffer tank preferably being a buffer pipe extending to the bottom of the sea from the lower end of said second driving part, and
• b2) in step b), a complementary depressurization of the first part of the production pipe filled with production fluid is carried out, by putting it in communication with said second part of the pipe and by evacuating more completely the gas contained in the production fluid from the first pipe part to said second production pipe part previously emptied of all liquid.

It is understood that said buffer pipe forms a buffer tank in that it is connected to the lower end of said second pipe part on the side of its so-called proximal end, its distal end being closed.

More particularly in step a2), to transfer the production fluid from said second part of pipe to a buffer tank formed by a buffer pipe extending to the bottom of the sea from the lower end of said second part of pipe, the gas contained in the buffer pipe is concomitantly evacuated via a first additional gas lift pipe which is connected to it by valves located on the one hand at its end
and on the other hand at its distal end.

Even more particularly, before restarting production, said buffer tank, preferably said buffer pipe, is drained. This allows the buffer pipe to be available to drain the second part of the production pipe during a next production stoppage.

Even more particularly, to drain the buffer pipe, a separation gel is introduced at the distal end of the buffer pipe and it is pushed by gas injection so as to move it with the liquid content of the buffer pipe towards the the lower end of the second part of the production pipe then all along it to evacuate it at its top. It is understood that the separation gel forms a sufficiently strong and waterproof chemical scraper to be able to be pushed by the gas and physically separate it from the liquid contents of the buffer pipe and thus drain it. In the absence of separation gel, the direct injection of gas into the production fluid of the buffer pipe due to the required increase in pressure would cause hydrate formation. Furthermore, the absence of a separator gel between the gas and the remaining production in the buffer pipe would lead to inefficient draining of the production liquid.

• c) on forme un gel à partir de deux réactifs dans une deuxième chambre de formation de gel de séparation, au fond de la mer, la dite deuxième chambre communiquant avec l'extrémité distale de la conduite tampon (1a), la dite deuxième chambre étant de préférence formée d'un tronçon de conduite in situ au fond de la mer dont l'extrémité débouche à proximité de l'extrémité distale de la conduite tampon reposant au fond de la mer, et
• d) on envoie une quantité de dit gel de séparation dans la conduite tampon depuis la dite deuxième chambre formant un tronçon de gel de séparation poussant le fluide contenu dans la conduite tampon jusqu'au sommet de la dite deuxième partie de conduite de production, avant de fermer la dite deuxième chambre.

Even more particularly, before emptying the buffer pipe by introducing a separation gel, the following steps are carried out in which:

• c) a gel is formed from two reagents in a second separation gel forming chamber, at the bottom of the sea, said second chamber communicating with the distal end of the buffer pipe (1a), said second chamber preferably being formed of a section of pipe in situ at the bottom of the sea, the end of which opens near the distal end of the buffer pipe resting at the bottom of the sea, and
• d) a quantity of said separation gel is sent into the buffer pipe from said second chamber forming a section of separation gel pushing the fluid contained in the buffer pipe to the top of said second part of the production pipe, before to close said second bedroom.

More particularly in step d), once the separation gel is in the buffer pipe, gas is injected from the vessel or support floating on the surface via a first annex pipe and a diversion pipe opening at the distal end of the buffer pipe for pushing the separation gel and the production fluid downstream thereof to the top of said second part of the production pipe.

• c1) on envoie, de préférence depuis le navire ou support flottant en surface, un premier composé liquide réactif dans une dite deuxième conduite annexe puis une deuxième conduite de dérivation s'étendant jusqu'à un deuxième mélangeur statique situé au fond de la mer et débouchant dans la dite deuxième chambre, et
• c2) on envoie, de préférence depuis le navire ou support flottant en surface, un deuxième composé liquide réactif dans une troisième conduite annexe puis une troisième conduite de dérivation s'étendant jusqu'au dit deuxième mélangeur statique situé au fond de la mer et débouchant dans la dite deuxième chambre, et
• c3) on mélange les deux réactifs au sein du dit deuxième mélangeur statique et on laisse le gel de séparation se former par réaction des deux réactifs en mélange au sein de la dite deuxième chambre.

Even more particularly, to form the separation gel in step c), the steps are carried out in which:

• c1) we send, preferably from the ship or support floating on the surface, a first reactive liquid compound into a said second annex pipe then a second diversion pipe extending to a second static mixer located at the bottom of the sea and opening into said second bedroom, and
• c2) we send, preferably from the ship or floating support on the surface, a second reactive liquid compound into a third annex pipe then a third diversion pipe extending to said second static mixer located at the bottom of the sea and opening in said second bedroom, and
• c3) the two reagents are mixed within said second static mixer and the separation gel is allowed to form by reaction of the two reagents mixed within said second chamber.

Alternatively, in steps c1 and c2), said first and second reactive compounds can be stored in tanks at the bottom of the sea and therefore transferred from said tanks at the bottom of the sea to said second static mixer.

Even more particularly, after step d), the reagents contained in said second and third annex pipes are replaced. and said second and third bypass lines with an inert replacement fluid, preferably methanol.

This makes it possible to prevent said reagents from stagnating in said pipes, potentially degrading and, if necessary, leading to the subsequent formation of a gel unsuitable for ensuring the separation and movement functions referred to here.

Even more particularly, the reagents contained in said second and third auxiliary pipes and said second and third diversion pipes are replaced by an inert replacement fluid, preferably methanol, by sending said replacement fluid from the ship or floating support. on the surface, in said second annex pipe and by evacuating the contents of said second annex pipe towards the third annex pipe then towards the top of the third annex pipe at the level of the ship or floating support on the surface, the two called second and third auxiliary pipes being made capable of communicating with each other, preferably just before said second mixer. This can be achieved by evacuating said contents of the third annex pipe through an open valve V14 for communication between the two said second and third annex pipes just before said second mixer or on the contrary, through said second mixer by closing or keeping the communication valve V14 between the second and third auxiliary pipes closed, a communication valve V6' between the second chamber (5b) and the distal end of the buffer pipe being closed, and valves V13 and V18 for isolating the second and third auxiliary pipes with said second mixer being open.

Even more particularly, in step d), before closing said second chamber, an inert fluid such as methanol is sent from the ship or support floating on the surface, into a said second or third annex pipe and a said second or third bypass pipe which pushes said separation gel from said second chamber into said buffer pipe before being pushed towards the top of said second production pipe part by gas injection at the end of the buffer pipe .

Even more particularly, in step d) or after step d), the gel and the liquid are raised in the said buffer pipe then in the second part of the production pipe, by sending from the ship or floating support on the surface into said first annex pipe inert gas opening at the distal end of the buffer pipe.

• e1) on forme un gel à partir de deux réactifs, de préférence dans une première chambre de formation de gel de séparation au fond de la mer, la dite première chambre communiquant avec l'extrémité de la première partie de conduite la plus proche de la tête de puits, la dite première chambre étant de préférence formée d'un tronçon de conduite in situ au fond de la mer dont l'extrémité débouche à proximité de l'extrémité la plus proche de la tête de puits de la première partie de conduite reposant au fond de la mer, et
• e2) on envoie une quantité de dit gel dans la première partie de conduite, de préférence depuis la dite première chambre formant un tronçon de gel de séparation poussant le fluide de production froid contenu dans la première partie de conduite vers la deuxième partie de conduite, avant de fermer la dite première chambre, puis
• e3) on démarre la production en envoyant depuis la tête de puits du fluide de production dans la première partie de conduite en arrière du dit tronçon de gel de séparation, le dit fluide de production poussant le dit tronçon de gel dans la dite conduite de liaison fond - surface vers son sommet, le dit gel formant une séparation physique et isolation thermique entre d'une part, le fluide de production en arrière d'un tronçon du dit tronçon de gel au sein de la première partie de conduite et d'autre part, un fluide dégazé au moins partiellement en avant dudit tronçon de gel au sein de la dite première partie de conduite de production.

According to another aspect of the invention, the following restart steps are carried out in which:

• e1) a gel is formed from two reagents, preferably in a first separation gel formation chamber at the bottom of the sea, said first chamber communicating with the end of the first pipe part closest to the wellhead, said first chamber preferably being formed of a section of pipe in situ at the bottom of the sea, the end of which opens near the end closest to the wellhead of the first part of pipe resting on the bottom of the sea, and
• e2) a quantity of said gel is sent into the first pipe part, preferably from said first chamber forming a section of separation gel pushing the cold production fluid contained in the first pipe part towards the second pipe part, before closing said first bedroom, then
• e3) production is started by sending production fluid from the wellhead into the first pipe part behind said separation gel section, said production fluid pushing said gel section into said connecting pipe bottom - surface towards its top, said gel forming a physical separation and thermal insulation between on the one hand, the production fluid behind a section of said section of gel within the first pipe part and on the other on the other hand, a degassed fluid at least partially in front of said gel section within said first part of the production pipe.

In steps e1) and e2), the gel can alternatively be formed on the ship and sent into the first part of the pipe from an annex pipe.

It is understood that said gel is sufficiently viscous and in sufficient quantity to form a physical separation preventing contact and mixing between the fluids located on either side of the gel in said first part of the production line, namely a hot fluid. of production sent from the wellhead and a cold production fluid, preferably degassed, initially contained in the pipe from a production shutdown. This separation constitutes insulation preventing the formation of hydrates within the first part of the pipe.

This type of gel is known to those skilled in the art in particular under the name "pig gel" to be formed on the surface on the ship or floating support and then sent from the surface into a pipe at the bottom of the sea in the activities pre-conditioning of the pipe at the time of its initial commissioning. However, the mixing of the two reagents is carried out in these known applications either during injection from a support boat, or before installation of the system, for example a FLET (“Flowline End Termination”). There is no known application in which the reagents are injected from an oil processing vessel (FPSO) onto the subsea production site to create the pig gel in-situ. There is also no application in which pig gel is used systematically for any other production start-up than the initial start-up after installation of the lines.

According to the present invention, the gel is thus available much more easily and quickly to send it into the first part of the pipe when production is restarted on the one hand and, on the other hand, for emptying the buffer pipe. The gel here fulfills a new function in that it serves to separate two production fluids, one degassed and the other newly produced and containing gas, and allows the subsea field to be restarted.

• e1-1) on envoie, de préférence depuis le navire ou support flottant en surface, un premier composé liquide réactif dans une deuxième conduite annexe s'étendant jusque dans un premier mélangeur statique situé au fond de la mer et débouchant dans la dite première chambre, et
• e1-2) en parallèle de e1-1), on envoie, de préférence depuis le navire ou support flottant en surface, un deuxième composé liquide réactif dans une troisième conduite annexe s'étendant jusque dans le dit premier mélangeur statique situé au fond de la mer et débouchant dans la dite première chambre, et
• e1-3) on mélange les deux réactifs au sein du dit mélangeur statique et on laisse le gel de séparation se former par réaction des deux réactifs en mélange au sein de la dite première chambre.

Even more particularly, in step e1), we carry out the steps in which:

• e1-1) we send, preferably from the ship or support floating on the surface, a first reactive liquid compound into a second annex pipe extending into a first static mixer located at the bottom of the sea and opening into said first chamber , And
• e1-2) in parallel with e1-1), a second reactive liquid compound is sent, preferably from the ship or support floating on the surface, into a third annex pipe extending into said first static mixer located at the bottom of the sea and opening into said first chamber, and
• e1-3) the two reagents are mixed within said static mixer and the separation gel is allowed to form by reaction of the two reagents mixed within said first chamber.

Alternatively, in steps e1-1) and e1-2), said first and second reactive compounds can be stored in tanks at the bottom of the sea and therefore transferred from said tanks at the bottom of the sea to said first mixer static.

Even more particularly, after step e1), the reagents contained in said second and third auxiliary pipes are replaced by an inert replacement fluid, preferably methanol.

This makes it possible to prevent said reagents from stagnating in said pipes from potentially degrading and, if necessary, leading to the subsequent formation of a gel unsuitable for ensuring the separation and displacement functions referred to here.

Even more particularly, the reagents contained in said second and third auxiliary pipes are replaced by an inert replacement fluid, preferably methanol, by sending said replacement fluid from the ship or support floating on the surface, into said second pipe. annex and by evacuating the contents of said second annex pipe towards the third annex pipe then towards the top of the third annex pipe at the level of the ship or floating support, the two said second and third annex pipes being made capable of communicating with each other, preferably just before said first mixer. This can be achieved if the two said second and third annex pipes are made capable of communicating with each other just before said first mixer through an open communication valve V9, isolation valves of these second and third annex pipes with the first mixer respectively V8 and V11 being closed. Alternatively it is possible to replace the reagents up to the mixer by circulation of this same inert replacement fluid, preferably methanol by closing or keeping closed the communication valve V9 between the second and third auxiliary pipes, a communication valve V4 between the first bedroom and the distal end of the first part of the production pipe being closed, and the isolation valves of the second and third auxiliary pipes with the first mixer being open (V8 and V9 respectively).

Even more particularly, in step e2), an inert fluid such as methanol is sent from the ship or support floating on the surface, into a said second or third annex pipe which pushes the said separation gel from said first chamber towards the said first part of production management.

More particularly still in a known manner, in step e3), a compound inhibiting the formation of hydrate is sent, preferably methanol, from the vessel or support floating on the surface, in a said second or third annex pipe up to the end of the first production pipe part near the wellhead, in the production fluid sent into the first pipe part.

The present invention also provides a fluid production installation as defined by claims 21 to 24.

Brief description of the drawings

• la figure 1 est une vue schématique d'une installation de préservation d'une conduite de production lors de l'arrêt de la production et redémarrage de la production selon la technique antérieure à boucle conventionnelle ou hybride ;
• les figures 1A à 1C sont des vues schématiques d'une installation de préservation d'une conduite de production lors de l'arrêt de la production et redémarrage de la production selon un premier mode de réalisation de l'invention de l'exemple 1 ;

• les figures 2A et 2B sont des vues schématiques d'une installation de préservation d'une conduite de production lors de l'arrêt de la production et redémarrage de la production selon un deuxième mode de réalisation de l'invention de l'exemple 2 ; et
• la figure 3, représente des courbes illustrant des conditions opératoires en termes de pression P et température T vis-à-vis de la formation d'hydrates dans la dite première conduite 1-1 reposant au fond de la mer remplie de fluide de production.

Other characteristics and advantages of the present invention will emerge from the description given below, with reference to the appended drawings which illustrate examples of embodiment devoid of any limiting character. In the figures:

• there figure 1 is a schematic view of an installation for preserving a production line when stopping production and restarting production according to the prior technique with a conventional or hybrid loop;
• THE Figures 1A to 1C are schematic views of an installation for preserving a production line during the shutdown of production and restarting production according to a first embodiment of the invention of Example 1;

• THE figures 2A And 2B are schematic views of an installation for preserving a production line when stopping production and restarting production according to a second embodiment of the invention of Example 2; And
• there Figure 3 , represents curves illustrating operating conditions in terms of pressure P and temperature T with respect to the formation of hydrates in said first pipe 1-1 resting at the bottom of the sea filled with production fluid.

Detailed description of the invention

In the present description, the term “valve” means a valve capable of isolating or communicating two pipes with each other.

On the figure 1 we show an installation for securing a production bottom-surface connection pipe 1 for securing the pipe when stopping and restarting production in which according to the prior art a loop is made with an annex pipe 18 connected to the end of the production pipe 1 and forming a loop capable of replacing the production fluid with an inert replacement fluid in the entire bottom-surface connection pipe 1.

• La courbe A correspond aux conditions de formation de cristaux d'hydrates.
• La courbe B correspond aux conditions de dissolution/ dissociation des cristaux d'hydrate.
• La zone Z1 est la zone de formation d'hydrates, la zone Z2 est une zone de risque de formation de cristaux d'hydrates. Les zones Z1 et Z2 représentent les conditions que l'on souhaite éviter. La zone Z3 est la zone sans formation d'hydrates dans laquelle la production du champ pétrolier sous-marin est opérée de façon standard à ce jour.
• La dépressurisation de la dite première conduite 1-1 selon l'invention permet de suivre l'évolution descendante le long de la courbe C depuis C1 à C2.

On the Figure 3 , the typical curves illustrating operating conditions in terms of pressure P and temperature T with respect to the formation of hydrates in said first pipe 1-1 resting at the bottom of the sea filled with production fluid as follows:

• Curve A corresponds to the conditions for the formation of hydrate crystals.
• Curve B corresponds to the dissolution/dissociation conditions of the hydrate crystals.
• Zone Z1 is the hydrate formation zone, zone Z2 is a risk zone for the formation of hydrate crystals. Zones Z1 and Z2 represent the conditions that we want to avoid. The Z3 zone is the zone without hydrate formation in which the production of the subsea oil field is operated in a standard manner to date.
• The depressurization of said first pipe 1-1 according to the invention makes it possible to follow the downward evolution along the curve C from C1 to C2.

During an unplanned production shutdown, it is possible that the valve V0 at the top of the second part of pipe 1-2 is closed before the wellhead production valve V1 is closed. This results in a rise in pressure in the production line 1, and potentially a slight rise in temperature due to the compression of the production gas, visible in the first ascending part of curve C. Then the cooling after stopping production causes the leftward evolution of C1. If the production fluid were left as is, then curve C would reach zone Z1.

Curve C presented in Figure 3 illustrates the path representing the desired evolution of the torque (Pressure, Temperature) according to the present invention for the production fluid in said first pipe 1-1 resting at the bottom of the sea from the normal production point C1, at pressure conditions P1 and temperature T1, until the preserved state at point C2 at the final temperature T0 which is that of the sea bottom, i.e. approximately 4°C, and final pressure P2 which is lower than the hydrate formation pressure at the temperature of the sea floor T0. In addition to the pressure and temperature conditions illustrated in Figure 3 , hydrate formation requires the presence of gas molecules (hydrocarbon gases from methane to butane, acid gases CO ₂ or H ₂ S, or nitrogen) and free water.

The present invention therefore makes it possible to preserve the pipe 1 without fluid replacement, thus saving the operational time necessary for replacing the production fluid with an inert fluid generally observed.

To indicate the relative positions of the ends or intermediate positions of the different pipes or valves, in this description below, the terms “proximal” or “(in)front” refer to a position closer to the vessel or floating support on the surface and "distal" or "behind" or "after" refer to a position further away from the vessel or floating support on the surface relative to another point such as another valve or other pipe end following the path of a fluid flowing in the pipe at this position.

In the two embodiments of the figures 1A-1C on the one hand and figures 2A-2B on the other hand, described in examples 1 and 2 below, the security or preservation is carried out when production is stopped and production is restarted of an underwater bottom-surface connection pipe 1 comprising a first production pipe part 1-1 (also hereinafter referred to as "first production pipe 1-1") resting at the bottom of the sea 16 from a wellhead 17 to a valve V3 communicating with the lower end 1-2a of a second pipe part 1-2 (also hereinafter referred to as "second production pipe 1-2") rising to a ship or floating support 10 on the surface 15. The second part pipe 1-2 may consist of a substantially vertical riser up to the surface or constituted of a hybrid pipe composed of a rigid pipe riser or substantially vertical riser 1-21 tensioned at its top 1-2c by a float 1-3 in the subsurface and a flexible pipe 1-22 in the form of a double diving chain ensuring the connection of the riser 1-21 to the ship or floating support 10.

In both embodiments, when production stops, a first depressurization of the entire production line 1 is carried out, followed by a complementary depressurization of the first part of the line 1-1 filled with production fluid. , by isolating the first pipe part 1-1 from the second pipe part 1-2, and the production fluid of the second pipe part 1-2 is replaced by a gas or a replacement fluid.

Preferably in both embodiments, the second part of pipe 1-2 is drained of all liquid before putting it back into communication with the first pipe 1-1 before restarting production.

In both embodiments, when production restarts, a section of gel is used which physically and thermally isolates the old cold and depressurized production fluid from the new hot production fluid.

In both embodiments, the installation comprises a first annex pipe 2 for supplying or discharging gas extending from the ship or floating support 10 on the surface to at least the lower end 1-2a of the riser 1- 21 with whom she communicates via a V6 valve. This first annex pipe 2 will be used as explained below to promote the rise of the production fluid within the second pipe 1-2 in the production phase, but also to allow the replacement of the production fluid in the second part of pipe 1 -2 by an inert fluid in example 1, or the draining of the inert replacement fluid from the second part of pipe 1-2 in example 1 or the evacuation of gas to depressurize the first production pipe in example 1 or even for emptying the buffer pipe in example 2 by injection of gas upstream of the separation gel at the distal end of the buffer pipe 1a-1.

The wellhead 17 communicates with the distal end of the first pipe 1-1 resting at the bottom of the sea via a section of pipe 1-1a delimited by a valve V1 on the side of the wellhead 17 and a valve V2 of the other side opening onto the distal end of the first production pipe 1-1.

A second annex pipe 3 for liquid injection extends from a first tank 11 on the ship or floating support containing methanol or water/methanol mixture (i.e. a product inhibiting the formation of hydrates) or from a second tank 12 on the ship or floating support 10 on the surface to a valve V7 at its distal end at the bottom of the sea opening onto the pipe section 1-1a.

• le GPG (Glycol Pipeline Gel) avec le produit gélifiant associé GPG Gelling agent commercialisés par la société Alchemy Oilfield Services Ltd.,
• des agents gélifiants comme l'E-gel commercialisé par la société Weatherford,
• des gels pour des applications comme le déshuilage de conduites commercialisés par la société Intelligent gels, et
• des produits dénommés « gel pigs » (gels de séparation, de raclage) rigides ou semi-rigides commercialisés par la société Inpipe products.

The second reservoir 12 contains a liquid consisting of a reactive compound B. This reagent B is preferably a product inhibiting the formation of hydrates, of the glycol or MEG or methanol type, but which is also capable of forming a gelled liquid. -after called "separation gel", in mixture with a reactive compound A contained in a third reservoir 13, the reagent A being a gelling agent which can be a crosslinking agent or a polymer or a mixture of the two, generally of proprietary composition. Examples of gelling agents are borate, or a polymer such as HPG (HydroxyPropyl Guar). For example, the following commercial reference gelling products can be used:

• GPG (Glycol Pipeline Gel) with the associated gelling product GPG Gelling agent marketed by the company Alchemy Oilfield Services Ltd.,
• gelling agents such as E-gel marketed by the Weatherford company,
• gels for applications such as pipe de-oiling marketed by the company Intelligent gels, and
• products called “gel pigs” (separation, scraping gels) rigid or semi-rigid marketed by the company Inpipe products.

This solid separation gel will be used as a physical, chemical and thermal separation barrier 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 towards the surface without causing any risk of blockages forming. In fact, the newly produced production fluid is inhibited by methanol but only for the quantity of associated produced water. Mixing this gassed production fluid with the degassed production fluid, but cold and containing uninhibited water, could in principle lead to the formation of hydrates. This is therefore a situation that current operating rules require us to avoid.

In both embodiments, a section of pipe forming a first chamber 5a for forming separation gel is placed in situ at the bottom of the sea opening onto a communication valve V4 at the distal end of said first pipe 1-1 before valve V2 of pipe section 1-1a.

A third annex pipe 4 extends from a third tank 13 to at least a first static mixer 6a before the pipe section forming the first chamber 5a. This third annex pipe 4 is intended mainly to supply the first mixer with reagent A stored in the third tank 13.

The lower end of the second annex pipe 3 also communicates via a valve V8 with the first mixer 6a. The lower end of the third annex pipe 4 communicates with the first mixer via a valve V11. A valve V9 makes it possible to communicate with each other the said second and third annex pipes 3 and 4 before the valves V7, V8 and V11.

The first mixer 6a makes it possible to supply the first chamber 5a with a reaction mixture of the two reagents A and B to form the separation gel within the first chamber 5a.

In examples 1 and 2 below, said first and second production pipes 1-1 and 1-2 and the buffer pipe 1a are pipes conventionally with diameters of 10" to 14". The said annex pipes 3 and 4 and branch pipes 3a and 4a are of smaller diameters and conventionally called “umbilical”. The umbilicals are bundles of small pipes, or “tubings”, whose expected diameters would be 1" to 3" for the ancillary pipes and branches 3-3a and 4-4a. Said annex pipe 2 and annex pipe 2a are for example rigid pipes of intermediate diameter, typically from 4" to 6". Another possibility is that said annex pipe 2 is associated with the second production pipe 1-2 with a configuration of coaxial pipes in which the second production pipe 1-2 is the internal pipe, and said annex pipe 2 is the annular formed by the two coaxial pipes. Finally, said annex pipe 2a can be in the form of a bundle of umbilical tubing with diameters of 2" to 3".

Example 1 : First embodiment of Figures 1A-1C.

In this first embodiment, the first gas transport annex pipe 2 communicates via a valve V6 with the lower end of the second pipe 1-2 before the valve V3 (closer to the surface than V3). And the second annex pipe 3 communicates with the lower end of the second pipe 1-2, via a branch 3'a from point 3-1 before the valve V9, the branch pipe 3'a comprising a valve V10 opening on the second line 1-2 between valves V3 and V6.

In a first variant shown Figure 1A , the first annex pipe 2 for transporting gas comprises an upper part 2-1 communicating at its lower end, with on the one hand the valve V6, on the other hand with a valve V19 capable of isolating it from a lower part 2-2 of said first annex pipe 2, the distal end of which comprises a valve V5 communicating with the proximal end of the first pipe 1-1 just after the valve V3 (further from the surface than V3).

In a second variant shown Figure 1B , the first annex pipe 2 for transporting gas does not include said lower part 2-2 nor a valve V19 capable of isolating it from a lower part 2-2, but there is a valve V5 communicating with the proximal end of the first pipe 1-1 just after valve V3 which is connected to a fourth annex pipe 7 going up to the surface.

The first variant represents the most optimized solution in that the first annex pipe 2 is already present for gas injection (in English gas lift) at the foot of said second part of production pipe 1-2 so that only the lower part of the first annex pipe 2-1 must be added to the architecture.

A) Production phase

In the production phase, only valves V0, V1, V2, V3 and V6 are open. All other valves are closed. Opening valves V1, V2 and V3 allows the production fluid (crude oil) to rise to the surface via the bottom-surface connection pipe 1. Opening valve V6 and injecting gas into annex pipe 2 from the surface to the lower end 1-2a of the second pipe 1-2 makes it possible to facilitate the rise of the production fluid towards the surface in the second pipe 1-2.

At this stage, the second and third annex pipes 3 and 4 as well as the first chamber 5a and first mixer 6a are filled with methanol for preservation measures and restart in the event of a subsequent cessation of production as described below.

B) Stopping production

To stop production, valves V0, V1 and V6 are closed. Then, valve V7 is opened and methanol is injected via the second annex pipe 3 into wellhead 17 and towards valve V2 until the production fluid is replaced. Valve V2 is then closed.

Then, the valve V0 is opened on the surface at the upper end 1-2b of the second pipe 1-2, to allow the degassing of the production fluid contained in the two production pipes 1-1 and 1-2, and thus carry out a first depressurization of said entire production lines 1-1 and 1-2.

The fluid contained in the first line 1-1 is at a higher average pressure than in the second line 1-2 due to the column of liquid in the second line 1-2 between the bottom and the surface. This is why, then, a complementary depressurization is carried out after closing the valves V3 and V7, and opening the valves V5 and V19 according to the variant of the Figure 1A to allow evacuation of the residual gas contained in the production fluid within the first pipe 1-1 and reduce the pressure in the first pipe 1-1 in order to further prevent the formation of hydrate plugs. Alternatively, depending on the variant of the Figure 1B , the additional depressurization of the first pipe 1-1 can be carried out through a dedicated umbilical, namely the fourth annex pipe 7, by opening the valve V5.

As an illustration, at a depth of 1000m, the pressure in the first pipe increases from a pressure of a few tens of bars (generally above the hydrate formation pressure at room temperature (Z1)) before additional depressurization at less than ten bars (i.e. Z3 zone preserved from hydrates) after additional depressurization.

Then, the production fluid is replaced in the second line 1-2 by injecting replacement fluid. To do this, valves V6, V8 and V9 being closed by default (normal position in operation), V7 having been closed in the previous step, valve V10 is opened, then methanol or water/methanol mixture is injected from the tank 11 via the third annex pipe 3 towards the second production pipe 1-2 at its lower end 1-2a by evacuating the production fluid at the top 1-2b of the second pipe 1-2 on the surface. Then once the second production line is filled with methanol, V10 is closed.

In practice, as an illustration, for a length of 1000 m of second part of pipe 1-2 this represents approximately 50m ³ of replacement fluid.

Alternatively, the replacement of the fluid of the second production line 1-2 can be carried out by injecting a replacement fluid, methanol or water/methanol mixture from the tank 11, through the first annex line 2, also called line gas lift injection. With the installation of the Figure 1A , this operation then requires that, after the second depressurization of the first production line 1-1, the valve V19 at least be closed beforehand and the valve V6 re-opened.

More precisely, the replacement fluid can be injected into the upper part 2-1 of said second annex pipe from the ship or floating support 10 towards the second replacement pipe 1-2 and thus pushing the production fluid towards the ship or support floating 10, after depressurization of said first pipe 1-1, closing of valve V19 then opening of valve V6. Thus the replacement fluid can be injected into the upper part of said second auxiliary pipe from the ship or floating support 10 towards the second replacing pipe 1-2 and thus pushing the production towards the ship or floating support 10.

C) Preparation before starting production

Before restarting production, separation gel is prepared and stored in the first chamber 5a then the second line 1-2 is drained, as follows.

To prepare and store separation gel in the first chamber 5a, the valves V8 and V11 are opened and the valve V9 is left closed, then the first static mixer 6a is supplied with reagent B, of the MEG type for example, via the second annex pipe 3 and reagent A via the third annex pipe 4 so as to supply the first chamber 5a to form the separation gel. The pressure in the first chamber 5a being greater than that of the distal end of the first pipe part 1-1, the valve V4 is open. It is therefore ensured that the production fluid does not flow back into the first chamber 5a. Thus, the methanol initially contained in the annex pipes 3 and 4, as well as in the first mixer 6a and the first chamber 5a is evacuated via the valve V4 in the first production pipe 1-1. Then, we close the valve V4, when the first chamber 5a is entirely full of reaction mixture (A+B) of separation gel and we wait for the gel to form.

Then, to avoid letting reagent A stagnate for too long in the third pipe annex 4 as well as to restore the pre-existing state in methanol, a replacement is made with methanol. For this to do, we open the valve V9, we close the valves V8 and V11, and we send methanol from the tank 11 into the second annex pipe 3 which methanol is evacuated through the valve V9 in the third annex pipe 4 then towards the top of the third annex pipe 4. Then, when said annex pipes 3 and 4 are full of methanol, valve V9 is closed. It is also possible, after evacuation of the separation gel from the first chamber 5a, to purge the first mixer 6a by keeping the valve V9 closed and the valves V8 and V11 open during the replacement with methanol.

Before restarting production, the second pipe 1-2 is preferably drained by injecting inert gas, preferably dehydrated gas called "gas lift", from its upper end 1-2b on the surface and the fluid is evacuated from the surface. content replacement of the second production line 1-2 via the first annex line 2 through the open V6 valve, the V3, V5 and V10 valves being closed. The interest here is to reduce the pressure at the level of the first line 1-1 when restarting when the valve V3 opens and thus prevent the pressure of the column of liquid contained in the second line 1-2 from being transferred on the first line 1-1 which is depressurized to a safety pressure which would cause a sudden increase in pressure and potentially risk creating a formation of hydrate plugs in the first line 1-1.

D) Restart of production

To restart production, valves V4 and V11 or V8 are opened, and the separation gel is injected from the first chamber 5a into the first production line 1-1 by injecting methanol via valves V11 or V8 into the first mixer 6a. An additional plug of methanol can also be created in front of the separation gel after its introduction into the first part of the production line 1-1.

Then, when a section of separation gel has been introduced into the production line 1-1, valves V4 or V8 are closed, whichever of the two has been open, and valves V1, V2 and V7 are opened. And, hot production fluid coming from the wellhead 17 is sent behind the separation gel section which isolates the hot production fluid from the cold degassed production fluid contained in the first production line 1-1, then brings it back up into the second production line 1-2, valve V3 being reopened. To do this, the V6 valve being reopened, gas is injected, "called gas lift", from the top of the first annex pipe 2 to facilitate the rise of the production fluid rising in the second production pipe 1-2 .

At the same time, valve V7 is opened and inhibitor product, namely methanol, is sent to inhibit the formation of hydrates in the production fluid at the wellhead in the first line 1-1.

Example 2 : Second embodiment of Figures 2A-2B with buffer pipe.

In this second embodiment, the installation includes the following differences and additional elements compared to the installation of the first embodiment.

The installation comprises first of all, a so-called “buffer” pipe 1a resting at the bottom of the sea and which extends from the lower 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 represents a volume substantially equal to that of the second part of pipe 1-2.

Said first auxiliary gas transport pipe 2 comprises at its lower end, on the one hand the valve V6 communicating with the lower end of the second pipe 1-2 before the valve V3 (closer to the surface than the valve V3 ) and on the other hand a diversion pipe 2a. This gas transport bypass pipe 2a communicates with the buffer pipe 1a at two levels on the one hand at the proximal end of the buffer pipe just after the valve V5' via a valve V8' and on the other hand at the level of the distal end 1a-1 of the buffer pipe via a valve V9'.

On the other hand, said first auxiliary gas transport pipe 2 no longer includes a valve V5 communicating with the proximal end of the first pipe 1-1 just behind the valve V3 as in the first embodiment.

The second annex pipe 3 for transporting methanol or reagent B such as MEG from the tanks 11 or 12 respectively, comprises a second branch pipe 3a which leaves from a point 3-1 before the valve V9 to a valve V13 at its distal end opening onto a second static mixer 6b. Likewise, the third annex pipe 4 for transporting reagent A comprises a third branch pipe 4a which leaves from a point 4-1 located just before a valve V16 before the valve V9 of the third annex pipe 4. The third annex pipe 4 diversion 4a has a valve V17 at its proximal end, that is to say just after the diversion point 4-1 and extends to a valve V18 opening onto the second static mixer 6b.

The second static mixer 6b opens onto a section of pipe forming a second separation gel formation chamber 5b. The second mixer 6b makes it possible to supply the second chamber 5b with a reaction mixture of the two reagents A and B to form the separation gel within the second chamber 5b.

The second chamber 5b communicates with the distal end of the buffer pipe 2a via a valve V6'.

This separation gel will be useful to allow the buffer line to be drained as described below.

The second and third diversion pipes 3a and 4a communicate with each other via a valve V14 located before the valves V13 and V18 (V14 is therefore in a proximal position or closer to the surface than V13 and V18).

The third annex pipe 4 comprises a valve V16 after the diversion point 4-1 before the valve V9, which open valve V16 allows the supply of reactive product A to the first mixer 6a.

A) Production phase

In the production phase, only valves V0, V1, V2, V3 and V6 are open. All other valves are closed. We proceed as in example 1. Opening valves V1, V2 and V3 allows the production fluid (crude oil) to rise to the surface via the bottom-surface connection pipe 1. Opening valve V6 allows facilitate the rise of the production fluid towards the surface in the second pipe 1-2 by injection of gas into the first annex pipe 2 from the surface.

The second and third annex pipes 3 and 4 and second and third branch pipes 3a and 4a as well as the first and second chambers 5a and 5b and first and second mixers 6a and 6b are filled with methanol.

B) Production stoppage

To stop production, valves V0, V1 and V6 are closed. Then, valve V7 is opened and methanol is injected via the second annex pipe 3 into wellhead 17 and towards valve V2 until the production fluid is replaced. Valve V2 is then closed.

Then we open the valve V0, on the surface at the upper end of the second pipe 1-2, to allow the degassing of the production fluid contained in the two first and second production pipes 1-1 and 1-2, to achieve a first depressurization of said lines 1-1 and 1-2 as described in Example 1.

In this second embodiment, to preserve the bottom-surface connection pipe 1 as much as possible from any formation of hydrates, the second production pipe 1-2 is emptied and the first part of the pipe 1-1 is more completely depressurized by degassing in the second part of the empty pipe.

To do this, we first carry out passive drainage or emptying of the contents of the second production pipe 1-2 in the buffer pipe 1a, by closing the valve V3 and opening the valves V5' and V8'. Opening V8' allows gas to be evacuated from the buffer line while it is being filled via valve V5' with the production fluid from line 1-2.

Once the second pipe 1-2 is emptied into the buffer pipe 2a, the valves V5' and V8' are closed and the valve V3 is opened to allow greater evacuation of the residual gas contained in the production fluid within the first line 1-1 towards the second empty line 1-2 and thus carry out additional depressurization of the latter via the second empty line 1-2. Then, we close V3 again.

In this second embodiment, it is thus possible to leave the second production line 1-2 filled with production gas without filling it with methanol. The entire production line is then preserved because at a pressure lower than the pressure of hydrate formation at room temperature.

Note that - in example 1 - we could not empty the production fluid from the second pipe 1-2 by sending inert gas, possibly from the "gas lift", from its top and evacuating it by the first annex pipe 2 because this would create risks of hydrate formation in the first annex pipe 2. In fact the first annex pipe 2, or "gas lift" line, is generally a small diameter line with a low thermal inertia and therefore a short available cooling time (a few hours). By passing a production fluid containing gas and containing water through this pipe, it is very likely that the low temperature and the high pressure, due to the displacement and the hydrostatic column thus created, cause hydrate formation. which could quickly block this small section line.

C) Preparation before starting production

Before restarting production, separation gel is prepared and stored in the first and second chambers 5a and 5b as follows.

To fill the first chamber, valves V8, V11 and V16 are opened and valves V7, V9, V17 and V13 and V14 are left closed. Then, the first static mixer 6a is supplied with reagent B, of the MEG type for example, via the second annex pipe 3 and with reagent A via the third annex pipe 4 so as to supply separation gel to the first chamber 5a as in Example 1. At the beginning, the methanol contained in the annex pipes 3 and 4, as well as in the first mixer 6a and the first chamber 5a is evacuated as in example 1.

To avoid letting reagent A stagnate for too long in the third annex pipe 4, it is filled with methanol as well as the second annex pipe 3 as in example 1.

To fill the second chamber 5b, V4 and V7 being closed by default, valves V16, V8 and V9 are closed and valves V13, V17 and V18 are opened. Then, the second static mixer 6b is supplied with reagent B of the MEG type via the second annex pipe 3 and second diversion pipe 3a and with reagent A via the third annex pipe 4 and third diversion pipe 4a so as to supply separation gel to the second chamber 5b. At the beginning, the methanol contained in the annex pipes 3 and 4 and bypass pipes 3a and 4a, as well as in the second mixer 6b and the second chamber 5b is discharged via the open valve V6' into the buffer pipe 2a, the valve V5 ' having been opened beforehand. The pressure in the second and third annex pipes 3 and 4 and bypass pipes 3a and 4a being greater than that of the distal end of the buffer pipe 1a-1, the production fluid does not flow back into the chamber 5b.

Then, we close the valve V6', when the chamber 5b is entirely full of reaction mixture (A+B) of separation gel and we wait for the gel to form.

To avoid allowing reactive product A to stagnate for too long in the third annex pipe 4 and the third diversion pipe 4a, they are filled with methanol as well as the second annex pipe 3 and the diversion pipe 3a, once the chamber 5b is filled. of frost. To do this, we open the valves V14 and V17, we close the valves V13, V18 and V16, and we send methanol from the tank 11 into the third annex pipe 4 and diversion pipe 4a which methanol is evacuated through the valve V14 via diversion pipe 3a towards the top of the second annex pipe 3 (valves V8, V13, V17 and V18 being closed). Then, when said annex pipes 3 and 4 and branch pipes 3a and 4a are full of methanol, valve V14 is closed. It is also possible, after evacuation of the separation gel from the second chamber 5b, to purge the first mixer 6b by keeping the valve V14 closed and the valves V18 and V13 open during the replacement with methanol.

The separation gel contained in the second chamber 5b will be used to empty the buffer pipe without risk of hydrate formation, before restarting production, by sending the gel to the distal end of the buffer pipe and evacuating it at the top of the second production line 1-2 in the following manner.

We open V13 and V6', the valves V14, V8 and V17 and V18 being closed, we send methanol via the second pipe annex 3 and second bypass line 3a which methanol pushes the gel from chamber 5b into buffer line 2a.

Then, we close V6' and we open V9', and we send inert gas, preferably "gas lift", to the distal end 2a-1 of the buffer pipe 2a, from the top of the first annex pipe 2 , valve V8' being closed. Thus, said gas pushes the gel and the contents of the buffer pipe in front of the gel towards the second pipe 1-2 to evacuate it at its top 1-2b. Once the buffer pipe 2a and successively the second part of pipe 1-2 have been emptied of their liquid content, the valves V9' and V5' are closed.

It would not be possible to empty the buffer pipe via riser 1-2 without the gel using only gas injection into the buffer pipe because it being of large section this would require an unrealistic pressure and gas flow. Furthermore, the production fluid in the buffer pipe contains degassed oil and water at low temperature. Mixing it with high pressure gas would cause hydrate formation. The gel being solid, on the other hand, can be pushed by the gas while maintaining an interface and physical separation taking into account its mechanical and chemical qualities.

On the other hand, in example 1, we can push the liquid from riser 1-2 upwards in the annex pipe 2 with gas sent from the top of the riser 1-2 because the annex pipe 2 of rise is smaller diameter. Furthermore, the inert gas then pushes a replacement fluid which is itself a hydrate inhibitor.

D) Restart of production

To restart production, valve V4 is opened and the separation gel is sent from the first chamber 5a into the first production line 1-1 and we proceed as in example 1.

Claims (24)

1. A method of stopping production and making safe an undersea bottom-to-surface connection production pipe (1) comprising a first pipe portion (1-1) resting on the sea bottom (16) from a well head (17) to the bottom end (1-2a) of a second pipe portion (1-2) going up to a ship or floating support (10) on the surface, in which method, after stopping production, first depressurization of the entire undersea bottom-to-surface connection production pipe (1) is performed allowing a portion only of the gas contained in the production fluid contained in said production pipe (1) to be discharged on the surface via its top end (1-2b);
   characterized in that the following subsequent steps are then performed:

   a) isolating said first production pipe portion (1-1) from said second pipe portion (1-2), and leaving the production fluid in said first production pipe portion (1-1), but not in said second pipe portion (1-2), which is emptied; and

   b) additionally depressurizing the first production pipe portion (1-1) filled with production fluid by reducing the pressure in said first pipe portion (1-1) to a pressure level close to that on the surface and by discharging more completely the gas contained in the production fluid that it contains.
2. A method according to claim 1, characterized in that prior to restarting production, and prior to putting the first production pipe portion (1-1) into communication with said second production pipe portion (1-2), all of the liquid contained in said second pipe portion (1-2) is emptied out.
3. A method according to claim 1 or claim 2, characterized in that the following steps are performed:

   a1) in step a), after isolating said second pipe portion from said first pipe portion, replacing the production fluid within said second pipe portion (1-2) by injecting an inert replacement fluid into a second auxiliary pipe (3) extending from a first tank (11) on the ship or floating support (10) on the surface to the bottom end (1-2a) of the second pipe portion (1-2) isolated from the first pipe portion (1-1), preferably an inert fluid also including or constituting a hydrate formation inhibitor; and

   b1) in step b), performing additional depressurization of the first production pipe portion (1-1) isolated from said second pipe portion (1-2) and filled with production fluid, by reducing the pressure in said first pipe portion (1-1) and more completely discharging the gas contained in the production fluid it contains, to an auxiliary gas discharge pipe (2, 7) extending from the end of said first production pipe portion (1-1) closest to the bottom end (1-2a) of said second production pipe portion (1-2) to the ship or floating support (10) on the surface.
4. A method according to claim 3, characterized in that in step a1), the production fluid within said second pipe portion (1-2) is replaced by injecting an inert replacement fluid, preferably an inert fluid also including or constituting a hydrate formation inhibitor, from a first tank (11) on the ship or floating support (10) into a first auxiliary gas riser pipe (2) or a second auxiliary pipe (3) extending to the bottom end (1-2a) of the second pipe portion (1-2) that is isolated beforehand from the first pipe portion (1-1) after depressurizing said first pipe portion (1-1), said inert fluid thus replacing and pushing the production fluid back towards the ship or floating support.
5. A method according to claim 3 or claim 4, characterized in that, before the steps of restarting production in step e3), before putting the first pipe portion (1-1) resting on the sea bottom into communication with the second pipe portion (1-2) rising to the surface and sending the production fluid therein from the well head, said second pipe portion is emptied by injecting inert gas into the second pipe portion from the top (1-2b) of the second pipe portion and discharging the inert replacement fluid from the second pipe portion to the surface via a first auxiliary gas riser pipe (2) that extends from the surface up to the bottom end (1-2a) of said second pipe portion to which it is connected.
6. A method according to any one of claims 2 to 5, characterized in that the following steps are performed:

   a2) in step a), leaving the production fluid in said first production pipe portion (1-1), and emptying said second pipe portion (1-2) isolated from said first pipe portion (1-1) by transferring the production fluid within said second pipe portion (1-2) into a buffer tank connected to the bottom end of said second pipe portion, said buffer tank preferably being a buffer pipe (1a) extending on the sea bottom from the bottom end (1-2a) of the said second pipe portion (1-2); and

   b2) in step b), performing additional depressurization of the first production pipe portion (1-1) filled with production fluid by putting it into communication with said second pipe portion (1-2) and by more completely discharging the gases contained in the production fluid of the first pipe portion towards said second production pipe portion (1-2) that has previously been emptied of all liquid.
7. A method according to claim 6, characterized in that in step a2), in order to transfer the production fluid from said second pipe portion (1-2) to a buffer tank formed by a buffer pipe (1a) extending on the sea bottom from the bottom end (1-2a) of said second pipe portion (1-2), the gas contained in the buffer pipe is simultaneously discharged via a first auxiliary gas riser pipe (2) that is connected thereto via respective valves (V8', V9') situated firstly at its proximal end (1-2a) and secondly at its distal end (1a-1).
8. A method according to claim 6 or claim 7, characterized in that before restarting production, said buffer tank, preferably said buffer pipe (1a), is emptied.
9. A method according to claim 8, characterized in that in order to empty the buffer pipe (1a), a separator gel is inserted at the distal end (1a-1) of the buffer pipe and is pushed by injecting gas so as to cause it to move together with the liquid content of the buffer pipe towards the bottom end (1-2a) of the second production pipe portion (1-2), and then all along the second pipe portion in order to be evacuated at the top (1-2b) thereof.
10. A method according to any one of claims 6 to 9, characterized in that before emptying the buffer pipe (1a) by introducing a separator gel, the following steps are performed:

    c) forming a gel from two reagents, preferably in a second gel-forming chamber (5b) on the sea bottom, said second chamber (5b) communicating with the distal end (1a-1) of the buffer pipe (1a), said second chamber (5b) preferably being formed by an in situ pipe segment on the sea bottom having its end leading to the proximity of the distal end (1a-1) of the buffer pipe (1a) resting on the sea bottom; and

    d) sending a quantity of said gel into the buffer pipe (1a), preferably from said second chamber (5b) and forming a separator gel segment pushing the fluid contained in the buffer pipe (1a) to the top of said second production pipe portion (1-2), prior to closing said second chamber (5b).
11. A method according to claim 10, characterized in that in order to form the separator gel in step c), the following steps are performed:

    c1 sending, preferably from the ship or floating support (10) on the surface, a first reagent liquid compound in a said second auxiliary pipe (3) and then a second branch connection pipe (3a) extending to a second static mixer (6b) situated at the sea bottom and leading to said second chamber (5b); and

    c2) ending, preferably from the ship or floating support (10) on the surface, a second reagent liquid compound in a third auxiliary pipe (4) and then a third branch connection pipe (4a) extending to said second static mixer (6b) situated on the sea bottom and leading to said second chamber (5b); and

    c3) mixing the two reagents within said second static mixer (6b) and allowing the separator gel to form by reaction between the mixture of two reagents within said second chamber (5b).
12. A method according to claim 11, characterized in that after step d), the reagents contained in said second and third auxiliary pipes (3, 4) and said second and third branch connection pipes (3a, 4a) are replaced by an inert replacement fluid, preferably methanol.
13. A method according to claim 12, characterized in that the reagents contained in said second and third auxiliary pipes (3-3a, 4-4a) and said second and third branch connection pipes (3a, 4a) are replaced by an inert replacement fluid, preferably methanol, by sending said replacement fluid from the ship or floating support (10) on the surface into said second auxiliary pipe (3) and discharging the content of said second auxiliary pipe (3) to the third auxiliary pipe (4) and then to the top of the third auxiliary pipe at the ship or floating support, said second and third auxiliary pipes (3, 4) being made suitable for communicating with each other, preferably immediately ahead of said second mixer.
14. A method according to any one of claims 10 to 13, characterized in that in step d), before closing said second chamber (5b), an inert fluid such as methanol is sent from the ship or floating support (10) on the surface into a said second or third auxiliary pipe (3-3a, 4-4a) and said second or third branch connection pipes (3a, 4a), thereby pushing said separator gel from said second chamber (5b) into said buffer pipe (1a) prior to pushing it to the top of said second production pipe portion (1-2) by injecting gas into the end (1a-1) of the buffer pipe.
15. A method according to any one of claims 10 to 14, characterized in that in step d), or after step d), the gel and the liquid in said buffer pipe and then in the second production pipe portion (1-2) is raised by sending inert gas from the ship or floating support on the surface into said first auxiliary pipe (2) leading to the distal end (1a-1) of the buffer pipe.
16. A method according to any one of claims 1 to 15, characterized in that the following steps are performed:

    e1) forming a gel from two reagents, preferably in a first separator gel-forming chamber (5a) on the sea bottom, said first chamber (5a) communicating with the end (1-1a) of the first pipe portion (1-1) that is closest to the well head, said first chamber (5a) preferably being formed by a pipe segment in situ on the sea bottom, having its end leading to the proximity of the end of the first pipe portion (1-1) resting on the sea bottom that is closest to the well head; and

    e2) sending a quantity of said gel into the first pipe portion (1-1), preferably from said first chamber (5a) forming a separator gel segment that pushes the cold fluid contained in the first pipe portion (1-1) to the second pipe portion (1-2), prior to closing said first chamber (5a); and then

    e3) starting production by sending said production fluid from the production fluid well into the first pipe portion (1-1) behind said separator gel segment, said production fluid pushing said gel segment into said bottom-to-surface connection pipe towards its top (1-2b), said gel forming a physical separation and thermal isolation between firstly the production fluid behind said gel segment within the first pipe portion and secondly a fluid that has been at least partially degassed ahead of said gel segment within said first production pipe portion (1-1).
17. A method according to claim 16, characterized in that in step e1), the following steps are performed:

    e1-1) sending, preferably from the ship or floating support (10) on the surface, a first liquid reagent compound into a second auxiliary pipe (3) extending to a first static mixer (6a) situated on the sea bottom and leading into said first chamber (5a); and

    e1-2) sending, preferably from the ship or floating support (10) on the surface, a second liquid reagent compound in a third auxiliary pipe (4) extending to said first static mixer (6a) situated on the sea bottom and leading into said first chamber (5a); and

    e1-3) mixing the two reagents within said static mixer (5a) and allowing the separator gel to form by reaction between the mixture of two reagents within said first chamber (5a).
18. A method according to claim 17, characterized in that after step e1), the reagents contained in said second and third auxiliary pipes (3, 4) are replaced by an inert replacement fluid, preferably methanol.
19. A method according to claim 18, characterized in that the reagents contained in said second and third auxiliary pipes (3, 4) are replaced by an inert replacement fluid, preferably methanol, by sending said replacement fluid from the ship or floating support (10) on the surface into said second auxiliary pipe (3) and by discharging the content of said second auxiliary pipe (3) to the third auxiliary pipe (4) and then to the top of the third auxiliary pipe at the ship or floating support, said second and third auxiliary pipes (3, 4) being made suitable for communicating with each other, preferably immediately ahead of said first mixer.
20. A method according to any one of claims 16 to 19, characterized in that in step e2), an inert fluid such as methanol is sent from the ship or floating support (10) on the surface in a said second or third auxiliary pipe (3, 4), thereby pushing said separator gel from said first chamber (5a) towards said first production pipe portion (1-1).
21. An installation for producing fluid such as crude oil and suitable for performing the method according to any one of claims 1 to 20, comprising at least:

    · a ship or floating support (10) on the surface having at least two tanks (11, 12), and preferably at least three tanks (11, 12, 13); and

    · an undersea bottom-to-surface connection production pipe (1) comprising a first pipe portion (1-1) resting on the sea bottom (16) from a well head (17) to the bottom end (1-2a) of a second pipe portion (1-2) rising to the ship or floating support (10) on the surface; and

    · a first auxiliary pipe (2) for transporting gas extending at least from the ship or floating support (10) on the surface to the bottom end (1-2a) of said second pipe portion (1-2); and

    · a plurality of valves comprising at least:

    a valve (V6) suitable for isolating or putting into communication said first auxiliary pipe (2) for transporting gas and the bottom end (1-2a) of said second production pipe portion (1-2); and

    a valve (V3) suitable for isolating or putting into communication said first production pipe portion (1-1) and said second production pipe portion (1-2), end to end; and

    a valve (V5) suitable for isolating or putting into communication the proximal end of said first production pipe portion (1-1) and the bottom end of a bottom portion (2-2) of said first auxiliary pipe (2) connected via an isolating or communicating valve (V19) to a top portion (2-1) of said first auxiliary pipe (2), said top portion (2-1) of said first auxiliary pipe (2) being connected to said valve (V6) suitable for isolating or putting into communication said first auxiliary pipe (2) and the bottom end (1-2a) of said second production pipe portion (1-2);

    characterized in that it further comprises:

    · a second auxiliary pipe (3) extending at least from a first tank (11, 12) containing an inert replacement fluid or a first separator gel reagent on board the ship or floating support (10) on the surface to a first static mixer (6a), said first static mixer being situated on the sea bottom, said second auxiliary pipe being suitable for transferring said inert replacement fluid or a first separator gel reagent into said first static mixer (6a); and

    · a third auxiliary pipe (4) extending at least from a second tank (13) containing a second separator gel reagent on board the ship or floating support (10) on the surface to the first static mixer (6a), said third auxiliary pipe being suitable for transferring said second separator gel reagent into said first static mixer; and

    · a first separator gel-forming chamber (5a), preferably formed by a pipe segment situated on the sea bottom at an end to which said first mixer leads, said first chamber leading at its other end to the proximity of the end of the first pipe portion (1-1) resting on the sea bottom that is closest to the well head.
22. An installation for producing fluid such as crude oil and suitable for performing the method according to any one of claims 1 to 20, comprising at least:

    · a ship or floating support (10) on the surface having at least two tanks (11, 12), and preferably at least three tanks (11, 12, 13); and

    · an undersea bottom-to-surface connection production pipe (1) comprising a first pipe portion (1-1) resting on the sea bottom (16) from a well head (17) to the bottom end (1-2a) of a second pipe portion (1-2) rising to the ship or floating support (10) on the surface; and

    · a first auxiliary pipe (2) for transporting gas extending at least from the ship or floating support (10) on the surface to the bottom end (1-2a) of said second pipe portion (1-2); and

    · a plurality of valves comprising at least:

    a valve (V6) suitable for isolating or putting into communication said first auxiliary pipe (2) for transporting gas and the bottom end (1-2a) of said second production pipe portion (1-2); and

    a valve (V3) suitable for isolating or putting into communication said first production pipe portion (1-1) and said second production pipe portion (1-2), end to end; and

    a valve (V5) suitable for isolating or putting into communication the proximal end of said first production pipe portion (1-1) and the bottom end of a fourth auxiliary pipe (7) rising directly to the surface, said proximal end being close to the valve (V3),

    characterized in that it further comprises:

    · a second auxiliary pipe (3) extending at least from a first tank (11, 12) containing an inert replacement fluid or a first separator gel reagent on board the ship or floating support (10) on the surface to a first static mixer (6a), said first static mixer being situated on the sea bottom, said second auxiliary pipe being suitable for transferring said inert replacement fluid or a first separator gel reagent into said first static mixer (6a); and

    · a third auxiliary pipe (4) extending at least from a second tank (13) containing a second separator gel reagent on board the ship or floating support (10) on the surface to the first static mixer (6a), said third auxiliary pipe being suitable for transferring said second separator gel reagent into said first static mixer; and

    · a first separator gel-forming chamber (5a), preferably formed by a pipe segment situated on the sea bottom at an end to which said first static mixer leads, said first chamber leading at its other end to the proximity of the end of the first pipe portion (1-1) resting on the sea bottom that is closest to the well head.
23. An installation according to claim 21 or claim 22, characterized in that it has a plurality of valves, comprising at least:

    · a valve (V4) suitable for isolating or putting into communication said first chamber (5a) and the end of said first production pipe portion (1-1) that is closest to the well head; and

    · valves (V8, V11) suitable for isolating or putting into communication said second and third auxiliary pipes (3, 4), respectively, with said first mixer (6a);

    · preferably, a valve (V9) suitable for isolating or putting into communication said second and third auxiliary pipes (3, 4) immediately ahead of said first mixer.
24. An installation according to claim 21 or claim 22 or claim 23, characterized in that it has a valve (V10) suitable for isolating or putting into communication said second auxiliary pipe (3) and the bottom end of said second pipe portion (1-2a).

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