EP0088776B1 - System for preventing liquids from being driven to the flair stack tip - Google Patents

Abstract

The system according to the invention provides, in the gas flow chain, between the liquid driving source (1) and the flair or torch stack tip (5) or vent, at least one capacity, such as a vessel (6) at the foot of the flair or torch, such capacity being provided with an overflow column (10) opening under the level of the liquid, for example the sea water level, at a predetermined distance form its tapping (11) on said capacity. The invention applies particularly to sea production plants (offshore).

Description

The present invention relates to a safety system intended to eliminate any risk of entrainment of liquids at the torch nose, during flaring of the gases associated with the production of hydrocarbons, in particular at sea (off shore).

In general, it is known that entrainment of liquids in the nose of a torch, in particular as a result of clogging of the gas / oil or gas / condensate separators, constitutes a serious danger in the production and treatment of hydrocarbons and in particular in fixed or floating offshore production facilities.

Indeed, at the exit of the nose of the torch, the oil or the condensate entrained by the gas is ignited and falls back in flames on the installation or in the immediate vicinity of it, thus endangering the entire installation and the lives of all staff.

This danger is all the more important in offshore installations as the personnel risks being trapped on the floating platform or support on fire and, in addition, the oil or condensate floating on the sea can form a blanket of fire preventing any possibility of evacuation.

In an attempt to eliminate this risk, one of the best measures adopted to date is constituted by the interposition, between the drive source of liquid hydrocarbons and the nose of the torch, of three capacities, namely, a separator, a safety purifier and a torch foot balloon, mounted in series on the gas flow chain, these capacities being respectively equipped with three high level detection devices, which perform, in the event that the level of liquid exceeds a predetermined height, the shutdown of the installation's hydrocarbon supply.

In addition, in such an installation, the torch bottom flask has, or may have, a liquid retention capacity, such that it allocates sufficient time for the manual closing of the hydrocarbon supply valves of the installation. .

• - du fonctionnement des automatismes de détection et d'actions mécaniques qui sont toujours sujets dans le temps et selon les conditions de fonctionnement, à des avaries, et
• - en dernier recours, du temps de rétention de liquide dans le ballon de pied de torche, qui se trouve dimensionné en fonction d'une durée d'intervention humaine toujours discutable et dont le caractère aléatoire n'est pas en conformité avec une bonne logique de sécurité.

However it is clear that in any case, mainly in the case where the torch is vertical or subvertical on the production support, the safety of the personnel and the whole platform will depend:

• - the functioning of automatic detection and mechanical actions which are always subject over time and depending on operating conditions, to damage, and
• - as a last resort, the retention time of liquid in the torch foot balloon, which is sized according to a duration of human intervention always questionable and whose randomness is not in accordance with good logic of security.

In addition, it should also be noted that the torch foot balloon, which is generally placed in the lower part of the installation, risks causing considerable discomfort, if not unacceptable, by its dimensions.

By patent FR 2 157 733, a hydrocarbon production installation is known in which no element is interposed between the separation installations and the torch nose, so that in the event of disruption of said installations, all of the production of hydrocarbons, liquid or gaseous, goes through the torch. This then pours on or near the surface platform of the ignited hydrocarbons, thus endangering the personnel as well as the installations.

The same drawback is found in the installation according to patent FR 2 195 326 where there is no means for separating the liquids accidentally entrained by the gas at the outlet of the separation installations, nor means for deriving them from the gas jet . Here also, in the event of a disturbance in the installation, liquid hydrocarbons may be spilled on the surface platform.

The installation according to patent FR 2407 005 certainly includes a torch foot flask, but it does not include any means for detecting the accumulation of liquids in this flask, nor for discharging these liquids. A pipe fitted with a valve has been provided, but this valve, whether automatic or manual, is subject to blockages and irregularities, thus leading to overflow of liquid hydrocarbons.

The invention therefore aims to eliminate all these drawbacks.

It therefore proposes a safety system intended to eliminate any risk of entrainment of liquids at the torch nose or in the vent, during flaring or dispersion of the gases associated with the production or treatment of hydrocarbons on land and in sea, system of the type comprising a separator which receives the crude hydrocarbons by a supply line and in which the liquids and the gas separate, the liquids being evacuated by a recovery circuit and the gas flowing through at least one line , characterized in that in said gas flow pipe is mounted at least one capacity, such as a torch foot balloon, in which the gas and liquids which have been accidentally entrained by said gas separate, said capacity being provided with an overflow column which stings at a level corresponding to the predetermined maximum level of the liquid in the capacity and which opens below the level of a range of liquid, for example from the sea, at a depth predetermined inverter under its stitching on the capacity.

A particularly simple and reliable security system is therefore obtained which can supplement or even replace the usual security systems, and which has the advantage of not involving no detection device and no mechanical organ subject to damage.

• - qu'il faudrait un point chaud important dans la zone où le liquide atteindra la surface de la mer pour provoquer son ignition.
• - qu'en raison de la profondeur à laquelle il se trouve déversé, et des courants marins qui existent dans la plupart des sites, le liquide ne refera surface qu'à une certaine distance des installations.

Thus, in the event of a slow or sudden disruption of the installation, the excess liquid will be discharged into the sea with a very low probability of fire risk taking into account:

• - there should be a large hot spot in the area where the liquid will reach the surface of the sea to cause it to ignite.
• - that due to the depth to which it is spilled, and the sea currents which exist in most sites, the liquid will only resurface at a certain distance from the installations.

In addition, the overflow column, in some applications, will be equipped with a discharge device inside it, of various products having the main aims, but not limited to, delaying or preventing the ascent of the liquid hydrocarbons on the surface, reduce, delay or inhibit oil pollution.

Furthermore, in certain applications, the torch will be equipped with manual or automatic ignition and extinction means making it possible to initiate or smother the flame in different operating configurations or for safety or other reasons.

• La figure 1 est une représentation schématique d'une première installation de production équipée d'un dispositif de sécurité selon un premier exemple de réalisation de l'invention;
• La figure 2 est une représentation schématique d'une deuxième installation de plus faible encombrement et équipée de moyens anti-polluants.
• La figure 3 est une représentation schématique d'une troisième installation dans laquelle la fût de torche joue le rôle de ballon de pied de torche.

Embodiments of the invention will be described below, by way of nonlimiting examples, with reference to the appended drawings in which:

• Figure 1 is a schematic representation of a first production facility equipped with a safety device according to a first embodiment of the invention;
• Figure 2 is a schematic representation of a second installation of smaller footprint and equipped with anti-pollutant means.
• Figure 3 is a schematic representation of a third installation in which the torch barrel acts as a torch foot balloon.

With reference to FIG. 1, the installation comprises, first of all, a source of drive for liquid hydrocarbons constituted by a separator 1 receiving the crude oil or the gas by an inlet pipe 2. This separator is equipped conventionally of a circuit 3 for normal recovery of oil or condensates and a gas outlet connected to a gas flow chain 4 to the nose 5 of the torch.

This gas flow chain 4 comprises, between the separator 1 and the nose of the torch 5, a torch base balloon 6 equipped, in a conventional manner, with a drip recovery circuit 7 comprising or not a pump 8 .

The separator 1 and the torch bottom flask 6 are both equipped with a high level detection circuit, intended to shut off, in the event of an abnormally high liquid level, the supply of crude or gas to the installation. .

According to the invention, this installation comprises an overflow column 10 coming to be pricked (tapping 11) on the torch foot ball 6 at a location corresponding to a predetermined maximum level and opening below sea level 12 at a distance L _R below the tap 11 on said balloon 6.

This overflow column 10 is equipped with the above-mentioned spill device 14 ′ and means 13 ′ for taking up liquid hydrocarbons which have overflowed into the overflow column in order to reinsert them into the normal treatment circuits. These recovery means will consist, for example, of pumps of various types or liquid or gas ejectors (gas lift). They can be installed during the construction of the installation or, later, be removable or not.

Thus, in normal operation, the two level detection systems will inform operators of a malfunction, and will cut off the arrival of rough if the fault cannot be corrected.

In the event of a sudden disturbance and the non-functioning of the two level detection systems, the liquids will be discharged into the sea through the overflow column 10, the torch continuing to be supplied with gas, this until the correction of the fault or until manual or automatic closing of the crude oil arrival.

We therefore obtain a very high security circuit. However, due to the dimensions of the torch base balloon 6, it has a significant drawback (which can be eliminated by the arrangements which will be described in particular with regard to FIGS. 2 and 3).

• - éviter la montée des liquides dans le fût de torche 13, en cas d'engorgement;
• - éviter la sortie de gaz par l'extrémité inférieure 14 de la colonne de trop-plein 10 en service normal.

It should be specified that, in order to obtain acceptable operation, the overflow column 10 must satisfy determined dimensioning criteria meeting at least two main conditions, namely:

• - avoid the rise of liquids in the torch barrel 13, in the event of congestion;
• - Avoid the gas outlet through the lower end 14 of the overflow column 10 in normal service.

• A) Pour éviter l'engorgement du ballon de pied de torche 6 ou la montée des liquides dans le fût 13, il convient d'assurer l'écoulement des liquides à la mer, ce qui implique que l'équation (1) suivante soit au moins satisfaite, sans prendre en compte les différentes pertes de charge dans les conduites:
  

dans laquelle:

These conditions can be expressed as follows:

• A) To avoid engorgement of the torch foot balloon 6 or the rise of liquids in the barrel 13, it is necessary to ensure the flow of liquids overboard, which implies that the following equation (1) is at least satisfied, without taking into account the different pressure drops in the pipes:
  

in which:

The most unfavorable condition being reached when P ₁ = P _atm (case of stoppage after engorgement and total filling of the overflow column 10 with length L _R ), the relation (1) can be simplified to

Applications

• 1) For
  • d _w = 1020
  • d _o = 700 (imperfectly degassed oil),
  • L _R = 50 m
  the minimum height L ₁ is 15.7 m. (example 1)
• 2) Under the same conditions as above but with a better degassed oil of d _o = 800, the height L ₁ becomes 10.8 m. (example 2)
• 3) Under the same conditions as in Example 1 but with a shorter overflow column 10, L _R = 40 m, the minimum height L ₁ is 12.5 m. (example 3)
• 4) Under the same conditions as in Example 3 but with an oil density do = 800, the minimum height L ₁ becomes equal to 8.62 m. (example 4)

dans laquelle L₂ est la hauteur du piquage par rapport au niveau de la mer le plus bas (exprimée en m).B) To avoid the gas outlet from the lower end of the overflow column 10, the following relationship should be checked:

where L ₂ is the height of the connection above the lowest sea level (expressed in m).

Applications

• 1) with
  
  the pressure P ₁ must be less than 3.013 x 10 ⁵ P _a .
• 2) with
  
  the pressure P _l must be less than 2,013 x 10 ⁵ P _a .

On examining relations 1 and 3 above, it appears that the proposed safety system will not be applicable in all cases, and in particular in too shallow water depths.

If, for a given installation, relationships 1 and 3 are verified with reasonable safety coefficients, and if the dimensioning of the pipes is correct to take account of the different pressure drops, the risk of clogging the torch is very improbable.

However, the risk of entrainment of liquid at the torch nose 5 remains, unless the torch base balloon 6 is dimensioned as a two-phase separator operating at a very low level. This leads to the establishment of a balloon 6 whose dimensions and weight may be prohibitive.

Furthermore, the oil / gas separation being carried out without real control, in a balloon separator 6 where the internals are practically excluded, the risk of entrainment of liquid droplets remains great.

Examination of relationships 1 and 3 shows that increasing the dimensions Li and L _R goes in the direction of improving safety.

Pollution

In the event of congestion, whether or not the installation includes this safety system, the quantity of liquids put into the sea will be more or less the same.

However, during a blockage of limited duration, the volume of liquid "trapped" in the overflow column can be reassembled in the installations and evacuated.

Danger of inadvertent ignition of liquid hydrocarbons at sea

According to Figure 1, the liquids are put into the sea "at source", that is to say that a large hot spot will be needed in the area where the liquid hydrocarbons will reach the surface of the sea to cause their ignition.

However, given the sea currents which can exist on a good number of sites, liquid hydrocarbons will only resurface at a certain distance from the installations.

For example, for an overflow column 10 opening 50 m below the average water level and with an oil of density 850, this will only resurface around 60 m from the vertical of column 10 this for a current of 0.25 knots.

Wave effect

The water level inside the overflow column 10 will follow the water level outside with delay and damping. However, this point must be checked in order to avoid air entering the torch barrel 13, especially in the case of short overflow columns 10 and low gas flow rates. Generally, the longer the overflow column 10, the less the wave effect will be felt.

• Le ballon de pied de torche 6, généralement placé en point bas de l'installation, reprend, en service normal, les égouttures (circuits 7, 8). Ses dimensions et son poids redeviennent acceptables. Il est complété par un séparateur liquides/gaz 15 placé à la partie inférieure du fût de torche 13, celui-ci n'étant utile qu'en cas d'engorgement.

To take into account the points mentioned above, an arrangement (Figure 2) is developed as follows:

• The torch base balloon 6, generally placed at the low point of the installation, resumes, in normal service, the drips (circuits 7, 8). Its dimensions and weight become acceptable again. It is completed by a liquid / gas separator 15 placed at the bottom of the torch barrel 13, this being useful only in the event of waterlogging.

This separator 15 can be hidden in the tower 16 supporting the torch. It operates at very low level and low pressure.

From a certain depth, the overflow column 10 can optionally be expanded to form a retention capacity 17, thus avoiding any pollution for a limited period. The liquid hydrocarbons thus trapped can be reinserted later in the installations by means 13.

The lower part of column 10 may be screened laterally in order to better disperse the liquid hydrocarbons in the sea.

Finally, for certain applications, the embodiment shown in FIG. 3 presents a simplified solution.

The vertical torch barrel 20 is constituted by a tube of variable or non-variable cross-section which can be in certain applications self-resistant to external stresses, and of a diameter such that the rising speed of the gas is sufficiently low so that the gas / liquid separation takes place. The gas can be speeded up if necessary at the torch nose 5 by passing through a reduced tubular section 21 or by other means.

The lower end 22 of the torch barrel serves as a torch foot balloon in normal service, and is equipped with an overflow column 10 such as those previously described as well as a drip recovery circuit 7, 8 and 13.

An additional simplification will consist, for certain applications, in producing the overflow torch-column assembly, in a continuous tubing, of possibly variable section, the drip recovery circuit then being installed at a suitable altitude on the continuous tubing. In addition, in all the configurations of installations of this security system, the latter may use, for its production, parts of already existing tubing, made of steel or other materials, such as than concrete, and can fulfill other functions such as supporting installations. Its support can also be carried out using frames or supports required or not to fulfill other functions.

Claims (14)

1. Safety system for removing any risk of driving liquids to the flare stack tip (5) or vent, during burning or dispersion of the gases associated with the production of processing of hydrocarbons on land and at sea, system of the type comprising a separator (1) which receives the untreated hydrocarbons through a feed duct (2) and in which the liquids and the gases separate, the liquids being removed by a collection circuit (3) and the gas flowing through at least one duct (4), characterized in that in said gas flow duct (4) is mounted at least one capacity, such as a flare foot reservoir (6) in which the gas and liquids which have been accidentally driven along by said gas separate, said capacity being provided with an overflow column (10) which is connected at a level corresponding to the maximum predetermined level of the liquid in the capacity and which opens out below the level of a liqued extent, for example the sea, at a predetermined depth under its connection to said capacity.

2. System according to claim 1, characterized in that said flare foot reservoir (6) is equipped with a drip collecting duct (7) and possibly a pump (8) for continually recycling a liquid flow to the installation placed upstream of the flare foot reservoir.

3. System according to claim 2, characterized in that said flare foot reservoir (6) and the overflow column (10) are respectively equipped with high level detection means for closing the feed duct (2) should a liquid level be detected higher than the predetermined maximum level.

4. System according to one of the preceding claims, characterized in that the flare foot reservoir (6) is dimensioned so that the flow speed of the gas decreases sufficiently for the entrained liquid droplets to drop into the bottom of the reservoir.

5. System according to one of the preceding claims, characterized in that it comprises means for raising the volume of liquid hydrocarbons trapped in the overflow column (10).

6. System according to one of the preceding claims, characterized in that from a certain depth the overflow column (10) opens out so as to form a retention capacity and in that the lower part of said column may be fitted with a lateral perforated tube so as to better disperse the oil in the sea.

7. System according to one of the preceding claims, characterized in that the flare foot reservoir (6), placed preferably at a low point of the installation, is followed by an oil/gas separator (15) placed in the lower part of the flare pipe (13) preferably in the tower (16) supporting the flare.

8. System according to one of the preceding claims, characterized in that it is equipped with systems for avoiding or retarding pollution.

9. System according to one of the preceding claims, characterized in that it is equipped with systems preventing or retarding the liquid hydrocarbons from rising to the surface of the water.

10. System according to one of the preceding claims, characterized in that it is equipped with systems for automatic and/or manual extinction of the flame.

11. System according to one of the preceding claims, characterized in that the flare pipe (20) has a diameter such that the rising speed of the gas is sufficiently low for a gas/liquid separation to take place, in that the speeding up of the gas takes place if required at the flare tip (5) by passing through a reduced tubular section (21) and in that the lower end (22) of the flare pipe which serves as flare foot reservoir is equipped with an overflow column (10) of said type.

12. System according to one of the preceding claims, characterized in that the flare pipe (20) and the overflow column are the same piping of variable section.

13. System according to one of the preceding claims, characterized in that it is formed, partially or totally, not only for construction but also support thereof, by installation elements which may fulfil other functions.

14. System according to one of the preceding claims, characterized in that it is installed at land or at sea on any kind of fixed or floating support.

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