FR3081908A1 - UNDERWATER INSTALLATION AND METHOD FOR COOLING A FLUID IN A HEAT EXCHANGER BY CIRCULATION OF SEA WATER. - Google Patents

Abstract

The present invention relates to an underwater process and installation for cooling (10) production fluid comprising the implementation of a heat exchanger (1) at the bottom of the sea (12), connected to transfer pipes. of said production fluid (11a, 11b) comprising an external cooling circuit (2) capable of circulating cooling sea water, using a circulation pump (3), comprising: supply devices (4, 4a) and evacuation devices (5, 5a) of sea water from said external cooling circuit (2) and monitoring and control devices (3a, 6-6a, 7- 7a) of the variation of an identical flow (d2) of the evacuation and of the seawater supply.

Description

Subsea installation and process for cooling a fluid in a heat exchanger by circulation of seawater.

Field and background of the invention

The present invention relates to a process and an underwater installation for cooling a fluid in an underwater heat exchanger, in particular lying at great depth at the bottom of the sea, by circulation of a cooling liquid consisting of sea water. .

The present invention relates more particularly to the general field of cooling of hydrocarbon fluids in underwater pipes for transporting hydrocarbons, in particular gas or crude oil mainly containing an oily phase of hydrocarbons, water and gas from subsea production wells, hereinafter called production fluid. It relates more particularly to an underwater bottom-surface connection pipe connecting the sea bottom to floating supports on the surface but also to an entirely underwater connection connecting production well heads to well heads. injection and associated equipment for cooling the production fluid.

This invention applies more particularly to the development of gas or oil fields in the deep sea, that is to say oil installations installed in the open sea, in which the surface equipment is generally located on floating structures, the well heads being at the bottom of the sea. The pipes concerned by the present invention more particularly include the pipes lying at the bottom of the sea connecting the well heads to risers called bottom-surface connecting pipes rising towards the surface but also to entirely underwater pipes connecting production well heads to injection well heads.

The main application of the invention therefore relates to submerged, submarine or underwater pipes or conduits, and more particularly at great depth, beyond 300 m, up to 4000 m, and conveying fluids of very diverse productions and associated equipment for cooling the production fluid.

The production fluid may range from a gas containing more than 95 mol% of methane and a few percent of ethane, propane and butane, to a gas containing very variable proportions of inert gases such as carbon dioxide and nitrogen, in a proportion which can range from less than 1 mol% to more than 80 mol%, passing through a rich gas containing less than 70 mol% of methane, more than 15 mol% of ethane, more than 7 mol% of propane, more than 4% butane and a few percent of heavier hydrocarbons containing more than 5 carbon atoms.

The production fluid may also be a liquid or multiphase effluent from an oil field containing, in addition to the light hydrocarbons mentioned above ranging from methane to butane, a significant proportion, up to more than 40 mol%, of hydrocarbon compounds. heavier consisting of 5 to more than 30 carbon atoms.

These production fluids may also contain a variable proportion of production water, namely water including up to more than one hundred grams per liter of salts, ranging from 0% by weight of production water at the start of treatment. of a deposit at more than 80% by weight of water at the end of treatment.

The temperature of such production fluids will depend on the conditions of the upstream deposit and on the preceding treatment steps, it is typically for example of the order of 50 ° C. to more than 150 ° C. Likewise, the pressure of such production fluids to be cooled may also vary over a wide range, depending on the treatment cases and the variation of the pressure of the upstream reservoir during its exploitation, for example more than 250 bars at a residual pressure of only a few bars.

These gaseous, liquid or multiphase petroleum products initially produced hot, in particular from 50 ° C. to 200 ° C. in certain cases require cooling before transfer in a so-called submarine transfer pipe in order to obtain acceptable physical properties for the rest of the unit. transfer, in particular to guarantee a minimum density at the suction of a transfer pump in the case of gaseous fluid in particular a density of at least 400 kg / m ³ , in particular from 400 kg / m ³ to 800 kg / m ³ , or be in conditions conducive to the use of materials or pipes made of particular materials sensitive to temperature and this for all the multi-year operating times despite the changes in composition and flow rates of the production fluid to be cooled.

In great depths, the sea water temperature is relatively low (typically 4 ° C) and allows the production fluid to be cooled by heat exchange using heat exchangers located at the bottom of the submarine heat exchangers installed to date are said to be passive, that is to say they use the natural convection of sea water which rises by chimney effect along tubes carrying the fluid to be cooled which tubes are not confined in a closed enclosure.

So-called active subsea heat exchangers, i.e. forced convection heat exchangers, are being developed using the principle in which seawater is circulated in the heat exchanger by a circulation pump. which ensures its passage through the enclosure of the heat exchanger to bring it into contact and cool the tubes contained in the closed enclosure of the exchanger, tubes in which the production fluid to be cooled circulates. The seawater is directly sucked from the ambient marine environment to be circulated in said enclosure before being discharged into the ambient marine environment in the heated state following the transfer of thermal load between the production fluid and the water. sea cooling circulating in the enclosure. It is an open circuit for circulation of the coolant.

Underwater modules of heat exchangers of the type known as “tube and shell exchangers” are known with a closed enclosure containing parallel tubes intended to convey the coolant and the production fluid and in particular with a network of superimposed layers. connected parallel tubes. We know in particular so-called pin heat exchangers more compact and easy to install since all the connections are on the same side, comprising a grille and a network of tubes in two parallel rectilinear parts connected to each other on a longitudinal side by a part forming a loop of a U (hairpin) all of the connections being made on the free side opposite the, lb as shown in Figure IA. Such exchangers are in particular marketed by AIT-STEIN GROUP (FRANCE).

Underwater cooling processes and installations using heat exchangers are known in W02013 / 004277 and US 9,719,698 using sea water in forced convection in which the tubes containing the fluid to be cooled are not not thermally isolated from the ambient marine environment and the circulation of the cooling sea water works in an open circuit as described above.

Subsea methods and installations for cooling using known heat exchangers do not make it possible to control the temperature of the cooling liquid or of the production fluid to avoid the following drawbacks.

The methods and installations of W02013 / 004277 and US 9,719,698 aim to be able to regulate the amount of heat exchanged. However, the control of the quantity of heat exchanged is limited there and the control of the skin temperature of the tubes (temperature at the level of the contact between the tube and the production fluid or the coolant) of the exchangers is practically impossible. The operating temperatures obtained in the exchangers are subjected to and result from their geometry, the conditions of the process and the conditions of the ambient medium.

These methods and installations of W02013 / 004277 and US 9,719,698 do not make it possible to control the skin temperature of the tubes in order to maintain their skin temperature above a certain minimum value in order to avoid unwanted solid deposits of paraffin or hydrates in particular in the form of plugs for the production fluid as described below, as well as lime and / or salt deposits on the side of the cooling sea water.

These deposits of lime and / or sea salts are detrimental to the proper functioning of the exchanger over time and can occur at skin temperatures above 45 ° C. so that it is desirable to maintain the temperature of the coolant. evacuated after heat exchange at not more than 45 ° C or even not more than 40 ° C.

In addition, many problems arise with the production fluid based on hydrocarbons, especially in the event of production stoppage when the temperature of the petroleum products decreases very rapidly, leading to the formation of hydrate and / or solid deposits. paraffins in the form of plugs as explained below. In this type of application, when the temperature of petroleum products decreases by a significant significant value compared to their production temperature, if petroleum products cool for example below 30 ° C to 60 ° C for an initial temperature from 70 ° C to 80 ° C while the temperature of the surrounding water, especially at great depths, can be well below 10 ° C and reach 4 ° C, we generally observe:

- a strong increase in viscosity which then decreases the flow rate of the pipe,

- a precipitation of dissolved paraffin which then increases the viscosity of the product and the deposition of which can reduce the useful internal diameter of the pipe,

- flocculation of asphaltenes inducing the same problems,

- the sudden, compact and massive formation of gas hydrates which form at high pressure and low temperature, thus suddenly obstructing the pipe by forming solid plugs.

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

These solid deposits of paraffins and / or of formation of hydrates which can create plugs are detrimental to the proper functioning of the cooling unit by heat exchanger.

In practice, the temperatures for the formation of solid hydrate phases or of paraffin deposition depend on the composition of the process fluid and its pressure. These temperatures are generally from 20 ° C to 50 ° C. More particularly, the formation of hydrate may occur for temperatures below about 25 ° C to 30 ° C. The paraffin deposit may be more or less significant in a fairly wide temperature range, rarely going from more than 45 ° C to values more usually of the order of 30 ° C or less.

Subject and summary of the invention

The object of the present invention is therefore to remedy these drawbacks in underwater installations of heat exchangers using the principle of forced convection of seawater as coolant.

More particularly, an object of the present invention is to have sufficient time, in the event of production stopping, before the temperature of the production fluid falls below the threshold where hydrates appear and significant deposits of paraffins can cause traffic jams and shutdown for a long time. This time can be used by the operator to restart his unit or put it in safety by inhibiting the production fluid with inhibiting products such as methanol, glycol or mono ethylene glycol (MEG for short) or / and by moving it to areas without risk of blockage, using substitute chemicals and / or inert products, for example diesel or degassed crude oil (“dead crude oil”). Production can therefore be restarted according to a usual procedure without first having to use exceptional specific operations to clean the installation in order to eliminate the clogging points.

The invention aims to be used to cool all types of production fluids requiring control of the cooling temperature and keeping it above a determined threshold for a defined minimum time after stopping production.

To do this, the present invention provides an underwater heat exchange installation between a production fluid and sea water from the ambient environment, using an underwater heat exchanger of the type called “exchanger with tubes and grille ”or of the so-called pin type using a principle of forced convection, combined with a semi-open loop circuit making it possible to modulate the thermal load required for this heat exchange.

More precisely, the present invention provides an underwater installation for cooling production fluid comprising at least one heat exchanger capable of being operated and resting on the sea bottom, connected to pipes for transferring said production fluid respectively upstream and in downstream of said heat exchanger, characterized in that it further comprises at least one external cooling circuit able to circulate in a loop in at least one external cooling pipe at least part of the cooling sea water leaving the said exchanger between an outlet and an inlet of an internal coolant circuit of the exchanger, using a circulation pump, preferably a pump resting on the seabed, said external coolant circuit including:

supply devices capable of supplying ambient sea water to said external cooling circuit upstream of said circulation pump at a supply branch, and

evacuation devices capable of evacuating part of the sea water from said external cooling circuit at an evacuation branch, downstream of the heat exchanger and upstream of said supply branch, and

- devices for controlling and controlling the variation of an identical flow rate of the discharge and of the seawater supply by the said supply devices and respectively discharge devices capable of controlling the variation of the said identical flow rate evacuation and seawater supply by said supply devices and respectively evacuation devices as a function of a first set value of the temperature of the production fluid in the downstream transfer transfer line of the exchanger and / or of a second setpoint for the temperature of the cooling sea water downstream of the pump and upstream of said heat exchanger.

The underwater cooling installation according to the invention thus makes it possible to control the temperatures of the production fluid leaving the exchanger and the cooling seawater upstream and downstream of the exchanger, and thus allows to adapt the cooling to the thermal power also called thermal load exchanged by the installation according to the invention between the cooling fluid and the production fluid which thermal load can change over time if the flow rate and / or the temperature of the production fluid varies as will be explained later.

In a preferred embodiment, said devices for monitoring and controlling said identical flow rate for the discharge and the seawater supply, are capable of controlling the variation of said identical flow rate for the discharge and supply of seawater by said supply devices and respectively discharge devices as a function of the variation in the flow rate of the production fluid and / or the temperature of the production fluid at the level of the upstream fluid transfer pipe from production supplying said heat exchanger.

More particularly, the installation according to the present invention makes it possible to maintain the temperature of the cooling water above a floor value, in particular to maintain their skin temperature above a certain floor value for a certain time in the event of installation shutdown, imposing a minimum operating temperature of the cooling seawater.

More particularly, said pump is equipped with a speed variator and said devices for controlling and controlling said identical flow rate of the evacuation and of the seawater supply, include:

a first temperature measurement device able to measure the temperature of the cooling seawater downstream of the pump and upstream of said heat exchanger connected to said seawater discharge devices so to automatically control said flow rate of seawater discharged from said external cooling circuit downstream of the heat exchanger and upstream of said first branch as a function of the temperature of the cooling seawater downstream of the pump and upstream of said heat exchanger, and

a second temperature measuring device able to measure the temperature of the production fluid in the downstream transfer pipe at the outlet of the exchanger connected to a device for varying the speed of said pump so as to automatically control the flow rate cooling water at the outlet of said pump as a function of the temperature of the production fluid in the downstream transfer pipe at the outlet of the exchanger.

The circulation of seawater in the cooling circuit is ensured by a circulation pump. The pump is equipped with a variable speed drive which makes it possible to regulate the cooling temperature of the production fluid by adapting the flow rate of cooling water circulation to the cooling thermal load required by the operating conditions of the process or vice versa regulate the amount of heat exchanged by modulating the circulation rate of the cooling water.

More particularly still, said ambient seawater supply devices and said cooling circuit are capable of automatically sucking a flow of ambient seawater in said external cooling circuit at the level of a said supply branch. , as a function of the speed of the circulation pump to complete the flow of cooling water in said cooling circuit downstream of said supply branch and upstream of said pump, identical to the water flow of sea evacuated by the said evacuation devices at an evacuation bypass.

More particularly still, the present invention makes it possible to adapt the cooling to the thermal load over time by implementing a semi-open or semi-closed loop circuit of the circulation coolant inside the radiator grille. the heat exchanger according to the following characteristics:

- an initial warm-up of the cooling water above a floor value and a loop cooling sea water circuit outside the semi-open heat exchanger with thermal insulation of the loop circuit cooling compared to the colder ambient environment;

- the evacuation of part of the cooling sea water at the outlet of the exchanger and an equivalent cold water inlet upstream of the exchanger, as well as

- control of the temperature of the cooling water at the inlet of the exchanger, and control of the speed of the pump and the water flow rate of the cooling circuit.

Thus, in operation, it is possible to lower the temperature of the cooling water (warmer) leaving the exchanger by expelling a part of it to the sea through a discharge valve, and by providing an equivalent inlet of cold water through a suction strainer open to the ambient marine environment upstream of the inlet of the heat exchanger to conserve the volume of water in the cooling circuit on the one hand and the water temperature at the inlet of the exchanger on the other hand.

The initial cooling water temperature above said floor value can be done:

- by circulation of the coolant (sea water) in the said closed-loop circuit with the circulation pump before the production fluid production is put into service by progressive accumulation of the friction energy that the pump dissipates in the volume of seawater contained in the cooling circuit; and or

- By electric heating, in particular it is known to heat the pipes over all or part of their length thanks to a plurality of electric cables which are wound around the external surface of the pipes to heat it by Joule effect. This heating solution, which is called “heat tracing” (or “heat tracing” in English), makes it possible to maintain the fluids of hydrocarbons transported in the underwater pipes at a temperature above a critical threshold throughout their journey.

The entire cooling circuit is thermally insulated from the ambient environment by suitable insulation. This guarantees that the temperature is maintained for a certain time when the unit is stopped and also guarantees a reduced time for the initial initial heating of the cooling system when it starts up. Preferably, said external cooling pipes are thermally insulated, the insulation of the section of external cooling pipe between said pump and the inlet of said heat exchanger being greater than that of the other sections of pipes of said external circuit. cooling.

More particularly still, according to other characteristics of the installation according to the invention:

- Said devices for supplying the external ambient sea water cooling circuit at the level of said supply branch comprise a strainer preferably cooperating with a non-return valve and a filter capable of preventing the suction of solid objects , and

- Said seawater evacuation devices from said external cooling circuit at an evacuation bypass include an evacuation valve with adjustable flow rate, and a complementary device upstream of said evacuation valve capable of permitting permanent leakage of the reduced-flow cooling seawater to reduce the pressure when the expansion of the cooling seawater at the outlet of the exchanger caused by its heating creates an overpressure. This device for permanent leakage of cooling water to the ocean takes place in particular via an orifice or equivalent known device. The temperature of the sea water in the cooling circuit is regulated by adding cold sea water to the ambient environment via an inlet line and discharging hot sea water from the cooling circuit to the ocean through a so-called regulating valve.

More particularly still, the cooling circuit further comprises at least one of the following additional devices:

a device for calibrating the flow rate so that the cooling seawater flow rate in the cooling circuit downstream of the heat exchanger and / or of the said discharge bypass does not exceed a determined limit value, and

- A first pressure sensor downstream of said pump and upstream of said heat exchanger and a second pressure sensor upstream of said supply branch and downstream of said discharge branch.

The cooling water circulates in the shell of one or more shell and tube type heat exchangers (“shell & tube”) or preferably hairpin in series or in parallel to cool the fluid. of production circulating in the exchanger tubes. The type of hairpin heat exchanger is more particularly suited to the service and may be supplied by vendors commonly used in the industry such as ΑΓΓ STEIN GROUP for example.

Preferably therefore, said heat exchanger is a hairpin heat exchanger. The term “pin heat exchanger” is understood here to mean, in a manner known to those skilled in the art, an exchanger comprising a U-shaped grille enclosing U-shaped tubes so that the inlets and outlets of a coolant and coolant are all made on the same longitudinal side of said exchanger at the ends not connected to each other of the branches of the U.

The present invention also provides a method of cooling a production fluid using an underwater cooling installation according to the invention, in which there is said installation at the bottom of the sea, the said heat exchanger. heat being connected to production fluid transfer pipes upstream and downstream and to pipes of a said cooling circuit and the following steps are carried out in which:

a hot production fluid is sent at a temperature in said upstream transfer pipe to cool it by heat exchange in said heat exchanger at a temperature of said first setpoint determined in said downstream transfer pipe,

- by circulating in said external cooling circuit and in said heat exchanger, a seawater flow, using said circulation pump, said seawater entering said heat exchanger at an input temperature of said second setpoint value determined.

More particularly, an initial temperature setting of the cooling water is carried out before circulation of the production fluid in order to reach a said inlet temperature of said second determined set value of the cooling water in the downstream cooling circuit. of the said circulation pump and upstream of the said heat exchanger, after supplying and filling the said cooling circuit with ambient sea water then circulation of the cooling water in the said cooling circuit in a closed circuit, the devices d evacuation being closed, the rise in temperature is made by progressive accumulation of the friction energy which the pump dissipates in the volume of sea water contained in the cooling circuit and / or by electric heating of at least part of the pipes of said cooling circuit, over all or part of their length, preferably using a plurality of cables s electrically wound around the pipes to heat it by the Joule effect.

More particularly still, in operation, when said production fluid is in circulation in said transfer pipes and in said heat exchanger, in order to maintain a said inlet temperature of said second determined set value of the cooling in the cooling circuit downstream of said circulation pump and upstream of said heat exchanger, and / or to maintain a temperature of said first set value determined in said downstream transfer pipe, the steps are carried out in which :

a cooling sea water flow is discharged from said cooling circuit using said evacuation devices at an evacuation bypass, and

- concomitantly, an identical flow of sea water from the ambient medium is sucked into the cooling circuit using said supply devices at a supply bypass.

More particularly, the variation in flow rates in said coolant circuit and said exchanger is controlled and controlled using the so-called control and command devices as a function of a first set value of the temperature of the production fluid. in the downstream transfer pipe at the outlet of the exchanger and / or of a said second setpoint of the temperature of the cooling sea water downstream of the pump and upstream of said heat exchanger.

More particularly still, the variation of said identical flow rate of the discharge and of the seawater supply is controlled by said supply devices and discharge devices respectively as a function of the variation in the flow rate of the production fluid and / or the temperature of the production fluid at the level of the upstream production fluid transfer pipe supplying said heat exchanger.

More particularly still, the variation of said flow rates in said coolant circuit and said exchanger is automatically controlled using said control and command devices and the steps comprising:

measuring the temperature of the cooling seawater downstream of the pump and upstream of said heat exchanger with a first temperature measurement device connected to said devices for discharging seawater, so as to automatically control said flow rate of seawater discharged from said external cooling circuit downstream of the heat exchanger and upstream of said first branch as a function of the temperature of the cooling seawater downstream of the pump and upstream of said heat exchanger, and

- the measurement of the temperature of the production fluid in the downstream transfer pipe at the outlet of the exchanger with a second measurement device connected to a device for varying the speed of said pump so as to automatically control the flow of l cooling water at the outlet of said pump as a function of the temperature of the production fluid in the downstream transfer pipe at the outlet of the exchanger.

More particularly still, said production fluid is a hydrocarbon fluid comprising gas and / or crude oil originating from underwater production wells, said production fluid transfer pipe preferably being connected to a pipe under -bottom of bottom-surface connection connecting the bottom of the sea to a floating support on the surface and / or to an underwater pipe of connection with submarine injection wells.

More particularly still, the said production fluid consists of hydrocarbons including methane, ethane, propane and butane and heavier hydrocarbon compounds consisting of 5 to more than 30 carbon atoms, and inert gas such as carbon dioxide and / or nitrogen, as well as a variable proportion of produced water, namely water including up to more than a hundred grams per liter of salts, which can range from 0% by weight d production water at the start of treatment of a deposit at more than 80% by weight of water at the end of treatment, the temperature of said production fluid being between 50 ° C and 200 ° C.

More particularly still, said production fluid has a temperature being from 50 ° C to 150 ° C .; the temperature of the sea water in the ambient marine environment is from 4 ° C to 20 ° C, the so-called first set point of the temperature of the production fluid (Tl) in the downstream transfer pipe at the outlet of the exchanger being between 30 ° C and 60 ° C and said second setpoint of the temperature of the seawater cooling T2 downstream of the pump and upstream of said heat exchanger being between 25 ° C and 35 ° C. The seawater inlet temperature is a function of the water depth at which the cooling loop is installed and can therefore be included in a range of 4 ° C for a large water depth at higher temperatures. , for example 20 ° C for shallower water depths.

More particularly still, the temperature of the cooling water (T3) after heat exchange at the outlet of said heat exchanger is between 30 ° C and 50 ° C, preferably less than 45 ° C.

The temperatures of the first and second setpoint values (T1) and (T2) above are compatible with this value of the temperature of the cooling water after heat exchange at the outlet of said exchanger (T3) and make it possible to avoid:

- lime deposits and sea salts detrimental to the proper functioning of the exchanger over time, which lime deposits can occur at skin temperatures above 45 ° C, and / or

- The formation of hydrate and / or solid deposit of paraffins in the form of plugs in said transfer pipes, in particular in the event of production stoppage as explained above.

The seawater inlet temperature is a function of the water depth at which the cooling loop is installed and can therefore be included in a range of 4 ° C for a large water depth at higher temperatures. , for example 20 ° C for shallower water depths. The temperature of the water in the cooling loop will be chosen to satisfy both the target cooling temperature for the production fluid leaving the exchanger, for example between 30 ° C and 40 ° C for a production fluid. entering the exchanger at 60-80 ° C, in order to have a certain margin with regard to possible solid phase deposition as described above, and at the same time avoid an excessively high cooling water temperature, which could lead to risks of lime deposits, especially on the internal tubes of heat exchangers.

The operating conditions of the cooling water loop temperature must also result from the best compromise between the cooling water circulation rate in the loop, the size required for the heat exchangers and the thermal inertia available in case system shutdown. The setting of these temperatures can then be adjusted by the final choice of the setpoints of the temperature regulators of the cooling water circuit loop. For example, the cold cooling water temperature at the inlet of the heat exchangers may be between 25 ° C and 35 ° C, and the hot cooling water temperature at the outlet of the heat exchangers may be between 30 ° C and 50 ° C.

More particularly still, the thermal power (or thermal load) of the cooling installation is between 1 megawatt and 10 megawatts. Higher thermal loads can be envisaged by using the required number of heat exchangers and / or by using several cooling loops operating in parallel.

More particularly still, the flow rate of circulation of the cooling water in the cooling circuit at the outlet of the pump and in the heat exchanger in operation is between 100 m ³ / hr to 1000 m ³ / hr, preferably of 300 to 600 m ³ / hr and the overall volume of cooling water contained in said cooling installation is 3 to 10 m ³ .

The circulation rate and the volume of cooling water in the cooling circuit loop will depend on the thermal load to be removed and the operating temperature conditions selected for the sizing of the system.

Other characteristics and advantages of the present invention will emerge more clearly on reading the description which follows, given in an illustrative and nonlimiting manner, with reference to

- Figure IA showing a commercial hairpin heat exchanger, and

- Figure 1 showing a diagram of an underwater installation according to the invention.

A) Example of installation according to the invention.

The underwater cooling installation 10 represented in FIG. 1 comprises a heat exchanger 1 of the pin type from the company ΑΓΓ-STEIN GROUP (France) whose volume capacity of the radiator grille is 2 m 3 of the cooling liquid shown in Figure IA.

The subsea cooling installation according to the invention comprises an external cooling circuit 2 allowing the circulation of seawater for cooling in a loop between the outlet 1b and the inlet 1a of the exchanger 1 using d '' a circulation pump 3 with in series:

a first section 2-1 of the seawater cooling water circulation pipe at a flow dl between the outlet lb of the exchanger 1 and an evacuation bypass D2 towards a valve 5 for evacuating the water sea cooling of circuit 2 in the ambient marine environment, and

- A second section 2-2 of the cooling water circulation pipe at a flow rate dl-d2 between (a) the said discharge branch D2 towards the discharge valve 5 and (b) a supply branch Dl to a device 4 for supplying cold water at approximately 4 ° C. from the marine environment, said supply diversion Dl being upstream of the inlet of the pump 3, and

- a third section 2-3 of the cooling water circulation pipe at a flow rate dl between (a) said supply branch Dl to said supply device 4 and (b) the pump inlet 3, and

- A fourth section 2-4 of the seawater cooling circulation pipe at a flow rate dl between the outlet of the pump 3 and the inlet la of the exchanger 1.

The cooling seawater circulation pipe 2 comprises, at the level of said supply branch DI between said second pipe section 2-2 and said third pipe section 2-3, a supply device 4 consisting of a strainer and non-return valve capable of automatically sucking in cold water at around 4 ° C sucked from the marine environment after filtration in a filter 4a preventing the aspiration of various solid objects in circuit 2. This aspiration occurs due to the circulation flow controlled by the speed of the pump and to fill the quantity of cooling water d2 discharged at the outlet of the heat exchanger 1.

Said evacuation bypass D2 from the first section of pipe 2-1 to the evacuation valve 5 comprises upstream of said evacuation valve 5 a complementary leakage device 5a allowing permanent leakage of the cooling water to the ocean via an orifice or equivalent in order to avoid any overpressure caused by the expansion of the cooling seawater when it is heated in the exchanger by heat exchange with the production fluid.

The evacuation valve 5 of the heated seawater cooling at the outlet of the exchanger 1 is connected by a first connection cable 6a to a first temperature measuring device 6 capable of measuring the temperature of the water of cooling at the level of said fourth section of pipe 2-4 between the outlet of the pump 3 and the inlet of the exchanger 1. Thus, it is possible to automatically control the quantity of cooled sea water d2 to be discharged by the said discharge valve 5 as a function of the temperature of the cooling sea water T2 at the level of the said fourth pipe section 2-4 downstream of the pump 3 and upstream of the exchanger 1.

A second temperature measuring device 7 is capable of measuring the temperature of the production fluid cooled at the outlet of the exchanger

1. This second temperature measurement device 7 is connected by a second connection cable 7a to a speed variator 3a of the pump motor 3 making it possible to automatically control the circulation rate of the cooling sea water in the circuit in a loop 2 as a function of the temperature of the cooled production fluid Tl at the outlet of the exchanger 1.

Said pipe sections of the cooling circuit are thermally insulated with an insulating coating of 130 mm on the pipe for said fourth pipe section 2-4 between the outlet of the pump 3 and the inlet of the exchanger 1 and a 50mm insulating coating on the pipe for the other pipe sections 2-1,2-2 and 2-3. The insulation is more important in this section 2-4 because it is the section of pipe in which the water temperature is the lowest therefore the closest to the lower limit below which it must not drop . This reinforced insulation makes it possible to decrease the rate of temperature drop during long stops and / or for maintenance operations and thus avoid the formation of hydrates in the production fluid.

For the thermal insulation of the cooling circuit, all types of thermal insulation systems and materials can be used according to the temperature maintenance constraints to be respected, from conventional passive type insulation to the use of more elaborate materials like those with phase change for example.

The cooling circuit 2 can be equipped with a device for calibrating the flow rate 8 so that it does not exceed a limit value determined downstream of the exchanger 1 and the evacuation bypass D2 before said derivation Dl.

The cooling circuit 2 can be equipped with a pressure sensor 9a between the pump 3 and the exchanger 1 and another pressure sensor 9b downstream of the evacuation bypass D2 to detect for example a pressure differential signaling a fouling of the exchanger 1 and / or a deficiency in the leakage device 5a and / or in the flow limiter 8.

The production fluid transfer lines 11a, 11b and / or the cooling circuit 2 can be equipped with devices for injecting various substitution chemicals or hydrate formation inhibitors (solvent, MEG, etc.) which can be used for installation safety operations during long shutdowns and / or for maintenance operations.

B) Example of a method for implementing the installation of example A The underwater cooling installation 10 represented in FIG. 1 is implemented at the bottom of the sea at a depth of 2100 m in order to cool a multiphase fluid for producing an underwater deposit consisting of a multiphase hydrocarbon fluid rich in CO2 and in production water, the said hydrocarbons containing less than 40 mole of methane, more than 3 mole of ethane, more than 2% mole of propane, more than 1% of butane and 1 to 5% of heavier hydrocarbons containing more than 5 carbon atoms.

This multiphase production fluid is produced at a temperature T0 of around 85 ° C and feeds an underwater separation unit at a pressure of around 200 bars and at a density of the gas fraction of around 300 kg / m3.

In order to facilitate the surface transport from the bottom of the sea of the liquid fraction in a bottom-surface transfer pipe, and the underwater reinjection of the gas fraction, the installation for cooling the gas fraction according to the present invention is configured to cool the gas to a temperature Tl of around 40 ° C at the outlet of the exchangers (2 exchangers in series are installed in this case) which allows its density to be increased to around 400 kg / m ³ while avoiding the formation of hydrates or paraffins in the underwater pipe.

The heated cooling water temperature T3 at the outlet of the heat exchangers must be limited to around 45 ° C to avoid the risk of lime deposits, especially on the internal tubes of the heat exchangers.

With a capacity in volume of cooling water of approximately 3m ³ of the heat exchangers, the operating conditions of maximum limit of cooling water temperature T3 at the outlet of the exchangers and the minimum temperature Tl of the cooled production gas of 40 ° C, can be satisfied with:

a cooling water temperature at the outlet of the pump 3 and before entering the first exchanger Tl brought to 30 ° C,

- a volume of cooling circuit pipe 2 of approximately 2m ³ bringing the overall volume of cooling water contained in the cooling loop to approximately 5 m ³ , i.e. for pipes of 8.625 with a total length of approximately 60 m,

a flow rate dl of circulation of the cooling water at the outlet of the pump 3 and in the exchanger 1 of approximately 500 m ³ / hr in operation, and

- A flow rate d2 of evacuation of the cooling water at the outlet of the second changer and of the concomitant cold water supply downstream of the pump 3 of approximately 150m ³ / hr.

This configuration corresponds to a power or thermal load at the level of the heat exchangers of approximately 5 megawatts corresponding to the quantity of heat (or thermal transfer) exchanged by the installation according to the invention per unit of time which is found in operation by difference between the thermal energy of the cooling water discharged at the discharge valve 5 and the thermal energy of the ambient supply water 4.

In production, over time, if the flow rate dO and / or the temperature T0 of the production gas varies, in particular in practice decreases, the thermal power required to cool it will vary accordingly, in particular also decrease, which requires a variation, in particular reduction in the flow rate of the cooling water dl at the pump outlet and in the exchanger 1 and in the flow rate d2 for discharging the cooling water at the discharge valve 5.

In practice if the flow rate dO of the production gas sent to the exchanger decreases (at temperature T0 unchanged) as long as the discharge flow rate of the cooling water d2 has not been reduced, we will see a decrease in the temperature of the production gas leaving the exchanger. This decrease is detected by the second temperature sensor 7 which controls a reduction in the speed of the pump 3 and therefore in the cooling water flow dl at the pump outlet 3 and in the exchanger 1. Thus if the flow dO of the gas is divided by n, the required thermal power and the flow of cooling water circulating at the pump outlet and in the exchanger dl will be divided by n. Incidentally, it will also be necessary to adapt the rate of discharge of the cooling water d2 at the outlet of the exchanger in the same proportion.

If it is the temperature T0 of the production gas which varies over time (at unchanged gas flow); this will result in a variation in the temperature of the cooling water in the cooling circuit detected at the level of the first temperature sensor 6 which will control a variation in the discharge rate of the cooling water d2 at the level of the exhaust valve 5. Incidentally, a variation in the temperature of the production gas Tl at the outlet of the exchanger will be detected at the second sensor 7 which will control a variation in the speed of the engine to vary the water flow from cooling dl at the pump outlet in the exchanger 1 in order to restore an adequate outlet temperature of the production gas at the outlet of the exchanger.

The initial warming-up T2 of the sea water of the cooling circuit can be done by an electric heating installed on the cooling circuit or preferably by circulation of the cooling water in closed circuit with the circulation and accumulation pump. progressive friction energy it dissipates in the volume of seawater contained in the cooling circuit 2.

Claims (20)

1. Underwater cooling installation (10) of production fluid comprising at least one heat exchanger (1) capable of being operated and resting at the bottom of the sea, connected to pipes for transferring said production fluid (lia , 11b) respectively upstream (11a) and downstream (11b) of said heat exchanger (1), characterized in that it further comprises at least one external cooling circuit (2) capable of circulating in a loop in at at least one external cooling pipe (2-1, 2-2, 2-3, 2-4) at least part of the cooling seawater leaving the said exchanger between an outlet (lb) and an inlet (the ) an internal coolant circuit of the exchanger, using a circulation pump (3), preferably a pump resting on the seabed, said external cooling circuit (2) comprising : - supply devices (4,4a) capable of supplying ambient sea water to said external cooling circuit (2) upstream of said circulation pump (3) at a supply branch (Dl), and - evacuation devices (5,5a) capable of evacuating part of the sea water from said external cooling circuit (2) at the level of an evacuation bypass (D2), downstream of the heat exchanger heat (1) and upstream of said supply branch (Dl), and - control and command devices (3a, 6-6a, 7-7a) of the variation of an identical flow (d2) of the evacuation and of the seawater supply by the said supply devices ( 4, 4a) and respectively discharge devices (5, 5a) capable of controlling the variation of said identical flow (d2) of the discharge and of the seawater supply by said supply devices (4, 4a) and respectively evacuation devices (5, 5a) as a function of a first set value of the temperature of the production fluid (Tl) in the downstream transfer pipe (11b) at the outlet of the exchanger (1) and / or a second setpoint for the temperature of the cooling seawater (T2) downstream of the pump (3) and upstream of said heat exchanger (1). 2. Subsea cooling installation (10) according to claim 1, characterized in that the said control and command devices (3a, 6-6a, 7-7a) of the said identical flow (d2) of the evacuation and seawater supply, are able to control the variation of said identical flow (d2) of the evacuation and seawater supply by said supply devices (4, 4a) and devices respectively evacuation (5, 5a) as a function of the variation of the flow rate (dO) of the production fluid and / or of the temperature (T0) of the production fluid at the level of the upstream production fluid transfer pipe (lia) supplying said heat exchanger (1). 3. Underwater cooling installation (10) according to one of claims 1 or 2, characterized in that said pump (3) is equipped with a speed variator (3a) and said control and command devices (3a, 6-6a, 7-7a) of said identical flow (d2) of the evacuation and seawater supply, include: - a first temperature measuring device (6) capable of measuring the temperature of the cooling sea water (T2) downstream of the pump (3) and upstream of said connected heat exchanger (1) (6a ) said seawater discharge devices (5, 5a) so as to automatically control said flow rate (d2) of seawater discharged from said external cooling circuit (2) downstream of the exchanger heat (1) and upstream of said first bypass (Dl) as a function of the temperature of the cooling seawater (T2) downstream of the pump (3) and upstream of said heat exchanger (1) , and - a second temperature measuring device (7) capable of measuring the temperature of the production fluid (Tl) in the downstream transfer pipe (11b) at the outlet of the exchanger (1) connected (7a) to a device for variation (3a) of the speed of said pump so as to automatically control the flow rate (dl) of the cooling water at the outlet of said pump as a function of the temperature of the production fluid (Tl) in the downstream pipe of transfer (11b) at the outlet of the exchanger (1). 4. Underwater cooling installation (10) according to claim 3, characterized in that said supply devices (4, 4a) in ambient sea water of said cooling circuit are capable of automatically sucking in a flow (d2 ) of ambient sea water in said external cooling circuit (2) at the level of a said supply branch (Dl), as a function of the speed of the circulation pump to complete the water flow rate (dl) cooling in said cooling circuit downstream of said supply branch (Dl) and upstream of said pump (3), identical to the seawater flow (d2) discharged by said discharge devices (5,5a) at an evacuation branch (D2). 5. Underwater cooling installation (10) according to one of claims 1 to 4, characterized in that said external cooling pipes (2-1, 2-2, 2-3, 2-4) are isolated thermally, the insulation of the section of external cooling pipe (2-4) between said pump (3) and the inlet of said heat exchanger (1) being greater than that of the other sections of pipes (2-2 , 2-3, 2-4) of said external cooling circuit (2). 6. Underwater cooling installation (10) according to one of claims 1 to 5, characterized in that: - said supply devices (4, 4a) for the external cooling circuit (2) in ambient sea water at the level of said supply branch (Dl) comprise a strainer preferably cooperating with a non-return valve (4 ) and a filter (4a) capable of preventing the suction of solid objects, and - Said evacuation devices (5, 5a) of sea water from said external cooling circuit (2) at an evacuation branch (D2) include an evacuation valve with adjustable flow (5) , and an additional device upstream of the said discharge valve capable of permitting permanent leakage of the seawater cooling at a reduced rate to reduce the pressure when the expansion of the seawater cooling leaving the exchanger caused by its heating creates an overpressure. 7. Underwater cooling installation (10) according to one of claims 1 to 6, characterized in that the cooling circuit (2) further comprises at least one of the following additional devices: - a device for calibrating the flow (8) so that the cooling sea water flow in the cooling circuit downstream of the exchanger (1) and / or of the said evacuation bypass (D2) does not exceed a determined limit value (dl), and - a first pressure sensor (9a) downstream of said pump (3) and upstream of said heat exchanger (1) and a second pressure sensor (9b) upstream of said supply branch (Dl) and downstream of the said evacuation bypass (D2). 8. Underwater cooling installation (10) according to one of claims 1 to 7, characterized in that said heat exchanger is a pinned heat exchanger. 9. A method of cooling a production fluid using an underwater cooling installation (10) according to one of claims 1 to 8, in which there is said installation (10) at the bottom of sea (12), said heat exchanger (1) being connected to production fluid transfer pipes upstream (11a) and downstream (11b) and to pipes of a said external cooling circuit and performs the following steps in which: - A hot production fluid is sent at a temperature T0 in said upstream transfer pipe (lia) to cool it by heat exchange in said heat exchanger (1) to a temperature of first determined setpoint (Tl) in said downstream transfer pipe (11b), - by circulating in said external cooling circuit (2) and in said heat exchanger (1), a flow rate (dl) of sea water, using said circulation pump (3), the said seawater entering the said heat exchanger at a determined set inlet temperature (T2). 10. A cooling method according to claim 9, characterized in that an initial temperature setting of the cooling water is carried out before circulation of the production fluid to reach a said input temperature of determined second setpoint value (T2 ) cooling water in the cooling circuit downstream of said circulation pump (3) and upstream of said heat exchanger (1), after supplying and filling said cooling circuit (2) with water from ambient sea then circulation of the cooling water in said cooling circuit in a closed circuit, the evacuation devices (5, 5a) being closed, the temperature rise being made by progressive accumulation of the friction energy that the pump dissipates in the volume of seawater contained in the cooling circuit and / or by electric heating of at least part of the pipes (2-1 to 2-4) of said cooling circuit t, over all or part of their length, preferably using a plurality of electric cables wound around the pipes to heat it by the Joule effect. 11. A cooling method according to one of claims 9 or 10, characterized in that in operation, when said production fluid is circulating in said transfer pipes (11a, 11b) and in said heat exchanger (1), in order to maintain a said input temperature of the second determined setpoint (T2) of the cooling water in the cooling circuit downstream of said circulation pump (3) and upstream of said heat exchanger heat (1), and / or to maintain a temperature of said first determined setpoint value (Tl) in said downstream transfer pipe (11b), the steps are carried out in which: a flow (d2) of cooling sea water is discharged from said cooling circuit (2) using said discharge devices (5, 5a) at the level of a discharge branch (D2), and - concomitantly, an identical flow (d2) of seawater from the ambient medium in the cooling circuit (2) is sucked in using said supply devices (4, 4a) at a bypass of food (Dl). 12. The cooling method as claimed in claim 11, characterized in that the variation in flow rates in said coolant circuit and said exchanger (dl, d2) is controlled and controlled using said control and command devices. (3a, 6-6a, 7-7a) as a function of a first set value of the temperature of the production fluid (Tl) in the downstream transfer line (11b) at the outlet of the exchanger (1) and / or of a second set point of the cooling sea water temperature (T2) downstream of the pump (3) and upstream of said heat exchanger (1). 13. A cooling method according to one of claims 11 or 12, characterized in that one controls the variation of said identical flow (d2) of the evacuation and the seawater supply by said devices supply (4, 4a) and respectively evacuation devices (5, 5a) as a function of the variation of the temperature (T0) of the production fluid at the level of the upstream production fluid transfer pipe (lia) supplying the said heat exchanger (1). 14. The cooling method according to claim 13, characterized in that the variation of flow rates in said coolant circuit and said exchanger is automatically controlled using said control and command devices (3a, 6-6a , 7-7a) and the steps comprising: - the measurement of the temperature of the cooling sea water (T2) downstream of the pump (3) and upstream of said heat exchanger (1) with a first temperature measurement device (6) connected ( 6a) to said seawater discharge devices (5, 5a), so as to automatically control said flow rate (d2) of seawater discharged from said external cooling circuit (2) downstream of the heat exchanger (1) and upstream of said first branch (Dl) as a function of the temperature of the cooling sea water (T2) downstream of the pump (3) and upstream of said heat exchanger (1 ), and - the measurement of the temperature of the production fluid (Tl) in the downstream transfer pipe (11b) at the outlet of the exchanger (1) with a second measurement device connected (7a) to a variation device (3a) of the speed of said pump so as to automatically control the flow rate (dl) of the cooling water leaving said pump as a function of the temperature of the production fluid (Tl) in the downstream transfer pipe (11b) in exchanger outlet (1). 15. A cooling method according to one of claims 9 to 14, characterized in that said production fluid is a hydrocarbon fluid comprising gas and / or crude oil from subsea production wells, said conduit transfer of the production fluid preferably being connected to an underwater bottom-surface connection pipe connecting the seabed to a floating support on the surface and / or to entirely submarine pipes connecting production well heads to heads injection wells. 16. A cooling method according to one of claims 9 to 15, characterized in that said production fluid consists of hydrocarbons including methane, ethane, propane and butane and heavier hydrocarbon compounds consisting of 5 to more than 30 carbon atoms, and inert gas such as carbon dioxide and / or nitrogen, as well as a variable proportion of produced water, namely water including up to more than a hundred of gram per liter of salts, which can range from 0% by weight of production water at the start of treatment of a deposit to more than 80% weight of water at the end of treatment, the temperature of said production gas being between 50 ° C and 200 ° C. 17. A cooling method according to claim 16, characterized in that said production fluid has a temperature being from 50 ° C to 200 ° C; the temperature of the sea water in the ambient marine environment is 4 ° C. to 20 ° C., the said first set value of the temperature of the production fluid (Tl) in the downstream transfer pipe (11b) at the outlet of the exchanger (1) being between 30 ° C and 60 ° C and said second set point of the temperature of the cooling sea water (T2) downstream of the pump (3) and upstream of said heat exchanger (1) being between 25 ° C and 35 ° C. 18. A cooling method according to claim 17, characterized in that the temperature of the cooling water (T3) after heat exchange at the outlet of said heat exchanger is between 30 ° C and 50 ° C, preferably less than 45 ° C . 19. A cooling method according to one of claims 9 to 18, characterized in that the thermal power (or thermal load) of the cooling installation (10) is between 1 megawatt and 10 megawatts. 20. Cooling method according to one of claims 9 to 19, characterized in that the circulation rate of the cooling water in the cooling circuit in operation is between 100 m ³ / hr to 1000 m ³ / hr , preferably from 300 to 600 m ³ / hr and the overall volume of cooling water contained in said cooling installation (10) is 3 to 10 m ³ .