WO2002042182A1 - Method for separating two fluids and storage tank - Google Patents

Abstract

The invention concerns a method for separating two fluids stored in a tank by interposition of a membrane between the two fluids. The invention is characterised in that it comprises the following steps which consist in: making a stable emulsion from at least two immiscible liquids; placing said emulsion at the interface between the two fluids so as to constitute said membrane. The invention also concerns a tank comprising a separating membrane.

Description

METHOD FOR SEPARATING TWO FLUIDS AND STORAGE TANK

The present invention generally relates to the field of storage of

two products in one tank.

The present invention relates more particularly to the field of

storage of a product immiscible with fresh water or sea water.

It relates more particularly to the storage of petroleum at sea, i.e.

edge of a floating support installed on the surface, of an oil tanker or

chemical tanker, either within a fixed structure resting on the seabed.

As soon as the water depth becomes significant, the exploitation of

production fields, especially oil fields are generally carried out

from floating supports. This floating support generally comprises

anchoring means to stay in position despite the effects of currents,

winds and swell. It also generally comprises means of storage and

oil processing, as well as means of unloading to

transport tankers. The latter appear at regular intervals to

remove production. The name of these floating supports

of barge type is the Anglo-Saxon term of "Floating Production Storage

Offloading "(meaning" floating means of storage, production and

unloading ") hereinafter abbreviated by the usual acronym" FPSO ".

variants have been developed such as “SPARs” type buoys, long cigars vertical floats, held in position by catenary anchors, or

again the TLPs ("Tension Leg Platform"), platforms with anchor lines

taut, said lines being generally substantially vertical.

The well heads are often distributed over the entire field and the

production lines, laid on the sea floor, connect each head of

well at a fixed location, vertically above which the floating support is

positioned on the surface. The bottom-surface connection pipes are then

called "riser". Some wells are located vertical to the floating support and

the interior of the well is then accessible directly from the surface. In this

case the wellhead equipped with its "Christmas tree" can be installed in

surface, on board the floating support. We can then perform, from a

intervention device installed on said floating support, all operations

drilling, production and maintenance of the well throughout the duration of

well life. We then speak of a dry or aerial wellhead, and we add the

drilling function (floating drilling) and the FPSO becomes FDPSO.

We know the patent FR 2,754,011 describing a barge and a system of

guide for pipe connecting said barge to the bottom of the sea at head level

wells with direct access.

In the FDPSOs, the risers rise to the surface in a cavity

central floating support called drilling bay or "wellbay" or

"moon pool". The cavity crosses the hull vertically right through

height of about thirty meters, with a draft of about twenty meters, and is located generally close to the axis of the floating support, at equal

distance from its ends. In fact, this zone is where the amplitudes of the

movements and accelerations are lowest when the ship is

subject to roll, pitch and yaw phenomena.

In conventional FPSOs, the draft varies between a value

minimum corresponding to empty crude storage tanks, for example

right after a cargo transfer to an oil tanker, and a value

maximum corresponding to completely full tanks, for example just

before the arrival of the said oil tanker.

In a storage barge with surface well heads, we seek to

work at constant draft to minimize relative movements

between said barge and said wellheads on the surface, because the connections ensured

in general by hoses, must remain as compact as possible and therefore

have an optimal length. For this, the barge includes ballasts

which allow the buoyancy to be continuously adjusted

overall so that the draft remains substantially constant whatever the

loading level of crude oil tanks. This provision leads to a

considerable ballast congestion which requires an increase

significant size of the barge. Indeed, it takes substantially the same

ballast volume than crude storage, which leads to additional costs

considerable in the construction of these floating barges. For example, these

FDPSO barges can measure up to 350 m in length, 65m in width and can have a height at the level of the plating which can reach and

exceed 35 m. Such barges have a deadweight tonnage that can

reach and exceed 500,000 DWT.

We also know the gravity platforms installed in the North Sea,

which store crude oil mixed with water from their base

sea. The differences in fluid density are sufficient to create a plan

dissociation between seawater and crude oil, without mixing

emulsion type, because the support is fixed.

On the contrary, a floating support is subjected to movements which

create, especially in contact with the walls of the tanks, significant agitation which

can quickly transform the separation plan into a type emulsion

mayonnaise, whose thickness, uncontrollable, will not fail to create

major malfunctions when emptying the tanks as much as possible, or when

filling as much as possible. In the first case we risk sending

the emulsion to the oil tanker, in the second case, we can reject

sea, in addition to ballast water, a certain amount of crude oil which will create

unacceptable pollution.

Thus, the present invention makes it possible to carry out mixed storage of

two products, so that during the movement of fluids, during

filling and withdrawal phases, or when the structure is agitated

of the tank, mixing phenomena of the uncontrolled emulsion type are

reduced to a minimum, or even almost eliminated. The present invention relates to a method of separating two

fluids stored in a tank by the intercalation of a membrane between the

two fluids. In the method, the following steps are carried out:

- a stable emulsion is made from at least two non-liquid

miscible

- the emulsion is placed at the interface between the two fluids so as to

constitute the membrane.

The emulsion can be obtained from at least one of the stored fluids,

and preferably with both.

You can place the emulsion on the surface of the first smaller fluid

density, before filling the reservoir with the second fluid.

The invention also relates to a storage tank for two fluids

separated by a membrane, the membrane consisting of at least one

stable emulsion layer.

The emulsion can be obtained from at least one of the stored fluids.

The two stored fluids may be immiscible, such as oils and

the water.

The emulsion can be stabilized with at least one stabilizing product

specific.

The stabilizer can be chosen from the following group:

- polymers with macromolecular chains of hydrophilic mixed nature

/ hydrophobic; - natural heavy compounds such as resins, asphaltenes and

existing paraffins in crude oils;

- paraffinic compounds of defined chain length modified by

a hydrophilic compound,

- and their mixtures.

Alternatively, the stabilizer may be the result of the

polymerization of at least two monomers, each being soluble in a single

said stored fluids.

The tank according to the invention can be placed in a floating support

of oil exploitation.

The method according to the invention consists in creating, at the level of the plane of

separation of two fluids, a membrane intended to act as a barrier

between the two fluids. The method advantageously applies to two fluids

immiscible. By immiscible liquids, it should be understood that liquids

in question are mutually insoluble or partially soluble

only. By emulsion, it is necessary to understand a dispersion of a liquid in

another liquid. The membrane according to the invention consists of an emulsion

fine and stable, compatible with the two immiscible products, preferably

made from a combination of these two fluids and which will be qualified

in the following description of "pseudo-membrane". The stabilization of a such emulsion can be done mainly by the addition of tensio type additive

active.

The pseudo-membrane is preferably produced on site during the

start of production. For example, the tanks being filled three times

quarters, just before starting the transfer of crude oil, we realize in

a mixer an emulsion composed of crude oil combined with water

sea, and possibly stabilizing additives.

The mixture thus obtained is spread over the surface of the ballast water

so as to create a screen between the sea water and the gas constituting the rest of the

volume of the tank, which is preferably an inert gas, such as

combustion of fuel in the air. After setting up the pseudo

membrane, the tank is completely filled with crude oil.

To ensure a minimum of disturbance during the movements of

fluids, any change in the volume of crude oil to be offset by

the same volume of sea water, we will feed and draw advantageously

seawater in the tank by the so-called source method, i.e. from

a pipe opening near the bottom of the tank and fitted with a

device to minimize eddies. Likewise,

crude oil supplies and withdrawals will be near the ceiling

of the tank and the pipes will be fitted with similar devices

allowing to minimize the eddies. Thus, in successive filling and sampling cycles, the pseudo

membrane, having an intermediate density between seawater and

crude oil, will naturally remain at the junction between seawater and

crude oil and will avoid the formation of a mixture, for example of the emulsion type

on uncontrolled thicknesses.

Other features and advantages of the present invention

will appear on reading the detailed description of certain modes of

embodiment which follows, in no way limiting, illustrated by the figures below

appended, among which:

- Figure 1 is a sectional front view of a floating support FDPSO to

double plating, but without double bottom, equipped with risers with well heads in

surface and a drilling derrick, including port and mixed storage tanks

starboard are equipped with the pseudo membrane according to the invention,

- Figure 2 is a sectional front view of a terrestrial reservoir of

mixed storage equipped with the pseudo-membrane, means of filling and

withdrawal and measurement of the position of the interface between liquids.

Figure 1 shows in section in front view a FDPSO 1 equipped with a

derrick 2 and two risers with wellheads on the surface 3, guided in 4 at level

circulation floor 5 located in wellbay 6.

treatment 7 are installed on the main deck. The port tank 8a is completely full and has in its upper part 9 only

crude oil, and in its lower part only sea water 10, the pseudo

membrane 11 being located at the interface between the two fluids. The reservoir

starboard 8b is shown with a different filling level.

Figure 2 shows in section in front view a terrestrial tank 20

for mixed storage in the presence of water, a fluid with a density greater than 1.

The water 10 is then in the upper part of the tank and the fluid

heavy 21 is confined in the lower part, the pseudo-membrane 11 being located

the interface between the two fluids.

A filling and withdrawal line 22a, equipped at its end

of a device 23a limiting eddies during transfers of heavy fluid, is

located in the lower part of the tank. A similar device 22b-23b is installed

in the upper part and allows transfers of the light fluid, here described as being

fresh water.

A sensor 24, for example of the sonar or level radar type, is

installed at the top of the tank and allows the position of the interface to be measured.

Variations in density and absorption allow localization

successively the bottom of the tank, the lower and upper faces of the

pseudo membrane. Such devices being known to those skilled in the art in the

level measurement area will not be developed in more detail here.

According to the invention, it is proposed to create a "natural" barrier between

two fluids by forming an emulsified zone using the properties of the crude oil / seawater system, these properties can be

reinforced by the presence of stabilizing additives, or even emulsifiers.

The method consists, among other things, of setting up a known volume

of emulsion formed in specific means, said means making it possible to

control the nature of said emulsion. The principle is known and widely

used in many industries, such as cosmetics or food, and

consists in mixing determined volumes of oil and water phases in

applying known mechanical energy. In the case of crude oil

naturally containing surfactant-type products, under the effect of

agitation, there is formation of drops then droplets up to a size

critical corresponding to the end of the emulsification process. It is largely

known that the more small drops are desired, the more the agitation must

to be important. In this case, we mean that the finer the emulsion

the more stable it is, however, the critical droplet size is not the

the only determining parameter for stabilizing an emulsion.

There are commercially a large number of devices allowing

to make emulsions. For example, a device like

"homogenizer" submits an energy of high pressure to the liquid

mixing with the passage of a pump and valve. The principle is to create a

turbulent flow regime with high Reynolds number.

With the help of current know-how on petroleum emulsions,

little dependent on the nature of the oil insofar as there are products natural surfactants such as resins, asphaltenes, it is possible to

manufacture a stable emulsified system by bringing the mixture of

components sufficient energy, generally important. So we give the

emulsified system the stability sought so that the pseudo membrane

performs the separation function of the two products over time.

In the following example, two characteristic types of oil have been

mixed with seawater in a ratio of 1 using a

propeller rotating at 10,000 rpm. The nature, as well as the stability of

the emulsion formed, were analyzed. The whole is summarized in the table

next :

L'huile A est principalement composées de produits saturés et de résines

Oil A is mainly composed of saturated products and resins

and is defined as a paraffinic type oil. By comparison, oil B

contains asphaltenes and less saturated compounds, it is

this makes it more viscous.

The tendency to form stable emulsions, especially for oil

B, is characteristic of the presence of heavy compounds such as

asphaltenes in the rough.

Depending on the mixing energy supplied to the Gross / water system, even for

oils with low asphaltenes but containing resins such as oil A, a

stable emulsion exists thus creating a separation between the phases

hydrocarbon and water. In addition, in the case of this oil A, the phases

hydrocarbon and water are little or not "contaminated" by the opposite phase.

In addition, after or at the same time as the emulsion is formed, it is

necessary to stabilize it. For this it is better to add products from

stabilizer type which will fix the oil / water interfaces, i.e. stabilize the

droplets. Stabilizing products can be:

of the polymer type: macromolecular chains of mixed nature

hydrophilic / hydrophobic;

heavy natural compounds like resins, asphaltenes and

paraffins existing in crudes;

paraffinic compounds of modified defined chain length

with a hydrophilic compound; You can also stabilize the drops by creating in situ in the emulsion, a

polymer film around the drops. For this we inject into the oil phase

a partially soluble compound and in the water phase another compound

also partially soluble in its injection phase. Both products are

the active ingredients in the form of monomers of a rapid reaction of

polymerization which, due to the solubility of these products, will only take place

the interface of the oil and water phases. This type polymerization reaction

inter facial is known and used in so-called encapsulation techniques

emulsions. We can refer to the document FR 2 590 185 cited here in

reference.

The pseudo membrane has been described in detail, in the case of

mixed storage between water and crude oil, but we will not get out of

framework of the present invention when two non-stored products are stored

miscible with different density and which are created at the interface between

said fluids a pseudo membrane, said pseudo membrane may be

made from fluids other than stored fluids, insofar as these

products remain compatible with the higher density fluid and the

lower density.

The invention thus applies to all types of products, such as

rapeseed, trichlorethylene, dangerous products of all kinds, or any

other industrial, food or other product, for which mixed storage

might be required.

Claims

1. Method for separating two fluids stored in a tank by the intercalation of a membrane between the two fluids, characterized in what we do the following steps: - a stable emulsion is made from at least two liquids immiscible, - said emulsion is placed at the interface between the two fluids so as to constitute said membrane. 2. Method according to claim 1, wherein said emulsion is obtained from at least one of said stored fluids. 3. Method according to one of claims 1 or 2, in which one places said emulsion on the surface of the first fluid of smaller mass by volume, before filling said reservoir with the second fluid. 4. Storage tank for two fluids separated by a membrane, characterized in that said membrane consists of at least a stable emulsion layer. 5. Tank according to claim 4, wherein said emulsion is obtained from at least one of said stored fluids. 6. Tank according to one of claims 4 or 5, wherein said two fluids are immiscible, such as oils and water. 7. Tank according to one of claims 4 to 6, in which the emulsion is stabilized with at least one specific stabilizing product. 8. Tank according to claim 7, wherein said stabilizer is chosen from the following group: - polymers with macromolecular chains of mixed nature hydrophilic / hydrophobic; - natural heavy compounds such as resins, asphaltenes and existing paraffins in crude oils; - paraffinic compounds of modified defined chain length with a hydrophilic compound, - and their mixtures. 9. Tank according to claim 7, wherein said stabilizer is the result of the polymerization of at least two monomers, each being soluble in only one of said stored fluids. 10. Tank according to one of claims 4 to 9, arranged in a floating oil exploitation support. 11. Tank according to one of claims 4 to 9, arranged in a fixed oil exploitation support.