GB2279732A - A sump for an underground hydrocarbon storage cavity capable of simultaneous maintenance and use - Google Patents

Abstract

In an underground hydrocarbon storage facility comprising a cavity (2) formed in ground (1) that is impregnated with water that exerts sufficient hydrostatic pressure on the cavity (2) to keep the hydrocarbon contained in the cavity in the liquid state a sump (4) is formed to collect ground water that has seeped into the cavity. At least two sets of pumps, each comprising at least a water drainage pump (52, 62) and a hydrocarbon pump (51, 61), are provided to pump ground water and hydrocarbon from the sump (4). A partition wall (7) subdivides the sump (4) into compartments (41, 42), each having its own set of pumps. The partition wall (7) includes a top edge (71) that is situated at a level below the level of the raft (3) in the vicinity of the sump (4). At least one of the compartments (41, 42) may be provided at the raft with a low channelling wall (8) that surrounds said compartment and provides the latter with a raised top edge which serves to channel the ground water that has seeped into the cavity into an adjacent compartment. One of the compartments (42) is normally left in a safe i.e. water-filled condition, being put into operation when maintenance on the pumps in the other compartments (41) is required, enabling the facility to continue working. <IMAGE>

Description

A SUMP FOR AN UNDERGROUND HYDROCARBON STORAGE CAVITY CAPABLE OF SIMULTANEOUS MAINTENANCE AND USE The present invention relates to a type of underground storage facility for hydrocarbons in the liquid state such a LPG (liquefied petroleum gas), diesel oil, or even crude oil in an underground cavity formed in ground that is impregnated with water that exerts sufficient hydrostatic pressure on the cavity to keep the hydrocarbon in the liquid state. More particularly, the invention relates to a sump for an underground cavity for storing LPG in such a site.

In an impregnated cavity of the kind formed in a ground water table, there is a permanent flow of water seeping out from the rock into the cavity. The cavity is sealed and the hydrocarbon is put under pressure by said permanent flow of water which then collects in the bottom portion of the cavity. Such storage facilities are envisaged only for substances whose density, when in the liquid state, is less than that of water, and which are not miscible with water.

For example, in an LPG storage facility, three superposed phases are to be found in the cavity: at the bottom of the cavity there is a water phase; in an intermediate position above the water, there is LPG in the liquid phase; and in an upper position above the liquid surface there is LPG in the gas phase that fills all of the remaining upper portion of the cavity. To ensure sufficient hydrostatic pressure, the cavity must be situated at a depth which is determined as a function of the characteristics of the substance stored therein.

In general, such cavities include a foundation raft having a sump being formed therein. In order to enable water to collect under gravity, the raft slopes slightly towards the sump. The water-LPG interface is maintained at a substantially constant level in the sump by means of drainage pumps that come into operation whenever the water reaches a determined high level and that stop operating whenever the interface moves down to a determined low level. For this purpose, a water level measurement device is used which detects the high and low levels of the water.

The level of the liquid-gas equilibrium surface inside the cavity varies while the storage facility is in operation. To extract liquid substance, a suction trap is placed inside the cavity with its extraction orifice penetrating into the sump, but above the high level of the water.

Operation of such a facility requires a plurality of pumps to be used, some of which have a suction tube extending all the way down to the sump, in particular the drainage pumps and the hydrocarbon extraction pumps.

For storage facilities having a cavity that is particularly voluminous, it is common practice to use several sets of pumps as defined above. Under such circumstances, a plurality of drainage pump suction tubes and of hydrocarbon pump suction tubes all open out into the same sump which constitutes a single volume.

Until now, in the state of the art, if a hydrocarbon pump broke down or if it required maintenance, then the entire facility had to be stopped, i.e. made safe. All operation was therefore interrupted during the time required for repairs or maintenance. A known way of making such a site safe consists in allowing the water in the sump to rise above its maximum allowable level in operation. The suction tubes dipping into the sump are also allowed to fill up to an equilibrium height that is determined by the hydrostatic equilibrium pressure. That operation of making safe prevents any of the pumps being used and therefore reduces the productivity of the site.

Once maintenance has been completed, then it is necessary to begin by re-establishing the water level in the sump by bringing it below its maximum working level by means of the drainage pumps. Such pumps are generally low capacity pumps, having a capacity of about 10 m3 per hour, for example, so it takes a certain amount of time to bring the site back into operating conditions. That leads to additional loss of time and it cannot be shortened by using the hydrocarbon pump since it is not possible to guarantee the quality of the water extracted since it contains a non-negligible quantity of hydrocarbon.

The present invention seeks to remedy the above drawbacks by defining a special sump configuration enabling such a storage facility to be used continuously, even in the event of a pump breakdown.

The present invention provides an underground storage facility comprising a cavity designed to receive a hydrocarbon that is in liquid form at a temperature greater than 0 C, the density of the hydrocarbon in the liquid state being less than that of water and its miscibility with water being very limited, said cavity being formed in ground that is impregnated with water that exerts sufficient hydrostatic pressure in the cavity to keep the hydrocarbon contained in the cavity in the liquid state, the cavity having a foundation raft in which a sump is formed for collecting ground water that has seeped into the cavity, at least two sets of pumps each comprising at least a water drainage pump and a hydrocarbon pump provided for pumping the ground water and the hydrocarbon from the sump, the facility being characterized in that at least one partition wall subdivides the sump into compartments each containing a set of pumps.

Compartmentalizing the sump makes it possible to separate different sets of pumps so as to be able to make them safe independently, e.g. in order to perform maintenance while continuing operation via the other sets of pumps situated in adjacent compartments. In general, the sump will be divided into two (or more) compartments, one of which will be used as the sump in normal operation while the other compartment will constitute a spare sump for use only while maintenance is being performed on the normal sump. Thus, at the end of a maintenance operation, it is possible to have sufficient capacity in the adjacent compartment for collecting the water from the compartment made safe for maintenance.The procedure for getting back into operation then takes place in two stages: firstly the water is extracted from the normal operating compartment and is sent to the spare compartment that has previously been emptied of its water, with this extraction being performed by the hydrocarbon pump, until it begins to pump hydrocarbon, i.e. until the water level has fallen below its maximum operating level. Thereafter, normal water pumping by the drainage pumps of both compartments is put into operation. In a conventional storage cavity, it is not possible to extract water by means of the hydrocarbon pump because of the hydrocarbon content of the water.

However, with a sump as defined above this becomes possible since the hydrocarbon-containing water is dumped in the adjacent compartment and the water is subsequently pumped to the surface by means of its drainage pump.

This achieves a considerable saving in time, given that one set of pumps is continuously active while the other set of pumps is out of operation for a length of time that is much shorter than has been the case in the past.

The productivity of such a storage facility is therefore considerably improved.

In addition, by injecting hydrocarbon into the spare compartment, it is possible to avoid setting up turbulence that could cause water to rise to the inlets of the hydrocarbon pumps.

In one embodiment of the invention, the partition wall includes a top end situated at a level that is lower than the level of the raft in the vicinity of the sump.

Thus, the water that accumulates in one compartment during maintenance thereof can overflow into the adjacent other compartment without spreading throughout the cavity. It is undesirable to allow the water to spread in the cavity since it becomes more difficult to determine how much water there is in the cavity given that its area is much larger than that of the sump whose section is limited to a few meters. Preferably, the top of the partition wall is situated about 30 cm below the level of the raft in the vicinity of the sump.

Advantageously, at least one of said compartments is provided at the level of the raft with a low wall for channeling purposes that surrounds said compartment and that raises its top edge so as to channel ground water that has seeped into the cavity into an adjacent compartment.

During storage, the sump receives deposits conveyed by the water that accumulates in the bottom of the sump.

By raising a low wall around the compartment that is used as the spare sump, the deposits contained in the water are channeled and pour into the sump for normal operation which can be organized to receive such deposits. In this way, the spare compartment remains practically dry and free from deposits.

In a preferred embodiment, in which the cavity is elongate in shape having a lower level at one end at which the raft is located, the sump is formed close to said end and its compartment that is provided with a channeling low wall is situated adjacent to the end of the cavity.

It is conventional to install the sump where the raft is at its lowest. However, by placing the compartment provided with the channeling low wall adjacent to the end of the cavity, the amount of water that flows into the compartment is limited, thereby also limiting the amount of deposits. In addition, the water in said compartment can come only from the walls of the sump contiguous with said compartment. The work of the drainage pump is thus considerably eased and the compartment will remain clean throughout the lifetime of the storage facility.

The invention is described more fully with reference to the accompanying drawing showing a particular embodiment of the invention by way of non-limiting example.

In the drawings: Figure 1 is a diagrammatic section view through a storage cavity provided with a sump in accordance with the present invention; and Figure 2 is a cross-section of Figure 1 passing through the sump.

As mentioned above, the present invention applies to any type of hydrocarbon that is imiscible with water and whose density in the liquid state that is less than the density of water. For convenient's sake the description below uses LPG to illustrate the preferred embodiment of the invention.

In Figure 1, a cavity 2 is formed in waterimpregnated ground 1, e.g. within a ground water table.

In general, such a cavity is in the form of galleries cut out in the rock and communicating with the surface via a well 10. The cavity has a foundation raft 3 that slopes slightly towards the end of one of the galleries. The well 10 opens out into that gallery at the location where the raft 3 has its lowest level. Since the cavity 2 is formed in ground that is impregnated with water, a permanent flow of water seeps into the cavity 2 and flows towards the bottom portion of the cavity 2. A sump 4 is provided in line with the well 10 for the purpose of collecting the water. Figure 1 thus shows only a portion of the cavity 2 corresponding to the location where the well 10 opens out therein and is extended by the sump 4.

The hydrocarbon stored in such a cavity is kept in the liquid state by the pressure of the water flow being greater than the pressure of the liquid hydrocarbon.

Simultaneously, this flow of water serves to prevent hydrocarbons leaking from the cavity. In order to obtain sufficient pressure, it is necessary to form the cavity 2 at a depth that is situated below a determined height of water column.

Consequently, for storage of LPG, the cavity contains LPG in coexistence with its gas phase 2b which occupies the entire top portion of the cavity 2. This top portion is hermetically sealed by a plug 9 situated in a well 10 close to its inlet into the cavity 2. By injecting LPG, the liquid-gas interface can be caused to rise inside the cavity and some of the gas which now occupies a smaller volume condenses.

Since the density of water is greater than that of LPG, it accumulates in the sump 4 beneath the LPG. The approximate capacity of the sump 4 is several tens of m3, and it is subdivided into two compartments 41 and 42 by a partition wall 7. It is thus possible to cause the water levels to vary independently on either side of the wall 7.

A plurality of tubes located in the well 10 pass through the plug 9 and extend into the compartmentalized sump 4. In Figure 1, there can be seen four tubes, each comprising a delivery tube connected to a pump.

Two of the tubes dip into the compartment 41 and the other two tubes into the compartment 42. By way of example, in the compartment 41 (the same applies to the compartment 42), one of the two tubes penetrates into a trap 51a (61a in the other compartment) resting on the bottom of the sump 4 and including an LPG pump beneath its open end for extracting the hydrocarbon. The suction orifices of the pumps 51a and 61a are therefore situated close to the bottom of the sump 4. Each of the other two tubes contains a water drainage pump for delivering to the surface the water that accumulates in the sump 4.

Each compartment 41, 42 thus includes a set of pumps comprising a water drainage pump 52, 62 and an LPG pump 51, 61. When the storage facility is in operation, the water level in the sump 4 must not exceed the height of the trap 51a, 61a since otherwise the trap would fill with water and the hydrocarbon pump would start pumping water, which should be avoided. To make sure that this happens, the water drainage pump is activated as soon as the water in the sump 4 reaches a maximum level. To detect this maximum level, a water level measurer 53, 63 is provided in each compartment of the sump. Each set of pumps is therefore associated with a level measurer 53, 63 lowered down the well 10 and passing through the plug 9. One or more LPG feed tubes also pass through the plug 9 in order to feed LPG into the cavity 2.However these tubes have deliberately been omitted from Figure 1 since they do not contribute to the present invention.

In order to enable the different phases in the cavity 2 and in the sump 4 to be easily distinguished, the volumes that they occupy have been given different patterns: in particular LPG is represented by wavy line shading while water is represented by straighter-line shading.

In Figure 1, it can be seen that the level of water 2c in the compartment 41 is lower than in the compartment 42. The compartment 41 is in operation, whereas the compartment 42 has been made safe for maintenance. The water drainage pump 52 and the LPG pump 51 are in operation and they send water and LPG to the surface.

The adjacent compartment 42 is flooded with water up to the top of the wall 7. The water drainage pump 62 and the LPG pump 61 are both stopped and their respective tubes are filled with water up to a level that is determined by the hydrostatic equilibrium pressure. When the compartment 42 has been made safe in this way, it is possible to raise the pumps up their tubes in order to perform the maintenance operations that are required.

While this is taking place, the compartment 41 remains in operation. This enables the site to be operated continuously except for the exceptional circumstances of simultaneous breakdowns in both sets of pumps.

In the preferred implementation, the compartment 41 is operated continuously, whereas the compartment 42 remains in an off state, having been made safe. It is thus possible to refer to a normal operation compartment 41 and to a "spare" compartment 42 on standby ready for being put into operation whenever maintenance is required in the compartment 41. The water in the compartment 42 is not pumped so it overflows over the top of the wall 7 into the adjacent compartment 41. Advantageously, the top 71 of the wall 7 is situated at a level lower than the level of the raft 3, preferably about 30 cm lower than said level. Since the level of the raft is not accurately plane, given that it is implemented by extracting machinery, it is safer to ensure this overflow by having the top of the wall 7 at a level that is significantly lower than the level of the raft.Athough it is not damaging to the site if water should overflow into the cavity, it is difficult to measure the level of water in the cavity 2, given its area. The water that overflows into the compartment 42 is pumped by the drainage pump 52 from the continuously operating compartment 41.

When maintenance of the set of pumps in the compartment 42 has been terminated and operation of that compartment can be restarted, the water filling it is pumped into the compartment 41 by means of the LPG pump 61. The use of the LPG pump has previously been impossible because of the poor quality of the water it pumps, however it is made possible in the present case by the sump 4 being compartmentalized. The use of the LPG pump accelerates the operation of putting the compartment 42 back into operation. Once the level of the water reaches the top of the trap 61a, the water continues to be pumped by means of the water drainage pump 62 in order to maintain a substantially constant level in the compartment 42.The water delivered into the compartment 41 by the LPG pump 61 is subsequently pumped out by the drainage pump 52, thereby guaranteeing a minimum content of LPG in the water that is pumped up and disposed of at the surface. Once the trap 61 has been emptied of its water, operation of the compartment 41 can be recommenced.

Thus, maintenance can be performed on the set of pumps in the compartment 42 while keeping the compartment 41 in operation. As a result the site can be operated continuously without being subjected to any interruption.

In its preferred embodiment, the invention provides for the sump 4 to be subdivided into a compartment 42 that is designed to be left in a safe condition and that is situated adjacent to one end of the cavity, as shown in Figure 2. In this way, the water collected by the compartment 42 can come only from the walls constituting the end of the cavity 2 and from the portion of the sump 4 that forms the compartment 42. The quantity of water collected is therefore very small compared with the water collected in the compartment 41 which receives practically all of the water to be found in the cavity 2.

That water runs preferentially into the compartment 41 and therefore leaves the compartment 42 practically free of water.

In an embodiment of the invention, the compartment 42 is provided on the raft 3 with a low channeling wall 8 that surrounds said compartment 42 and that raises the level thereof so as to channel the water flowing into the cavity 2 into the adjacent compartment 41. In this way, the compartment 42 is filled only with water that comes from inside the compartment 42, i.e. from its bottom and side walls, to the exclusion of any water coming from the cavity 2. This has the result of further drying the compartment 42. Also, the compartment 42 receives so little ground water that it is not necessary to provide a settling tank for the LPG pumped from said compartment 42. When LPG penetrates into the cavity an emulsion of water and LPG is created which then runs the risk of being pumped by the LPG pump. The settling tank is intended specifically to allow water droplets of larger size to combine so as to enable them to fall to the bottom of the sump 4. However, the use of a settling tank is a source of danger since it is under high pressure and it needs to be installed on the surface.

By implementing means of simple design, the present invention makes it possible to improve the productivity of such a storage site.

Claims (1)

1/ An underground storage facility comprising a cavity designed to receive a hydrocarbon that is in liquid form at a temperature greater than 0 C, the density of the hydrocarbon in the liquid state being less than that of water and its miscibility with water being very limited, said cavity being formed in ground that is impregnated with water that exerts sufficient hydrostatic pressure in the cavity to keep the hydrocarbon contained in the cavity in the liquid state, the cavity having a foundation raft in which a sump is formed for collecting ground water that has seeped into the cavity, at least two sets of pumps each comprising at least a water drainage pump and a hydrocarbon pump provided for pumping the ground water and the hydrocarbon from the sump, the facility being characterized in that at least one partition wall subdivides the sump into compartments each containing a set of pumps.

2/ An underground storage facility according to claim 1, in which the partition wall includes a top end situated at a level thaty is lower than the level of the raft in the vicinity of the sump.

3/ An underground storage facility according to claim 2, in which the top of the partition wall is situated about 30 cm below the level of the raft in the vicinity of the sump.

4/ An underground storage facility according to claim 1, 2, 3 or 4, in which at least one of said compartments is provided at the level of the raft with a low wall for channeling purposes that surrounds said compartment and that raises its top edge so as to channel ground water that has seeped into the cavity into an adjacent compartment.

5/ An underground storage facility according to claim 4, in which the cavity is elongate in shape having a lower level at one end at whichvthe raft is located, the sump being formed close to said end and having its compartment that is provided with a channeling low wall situated adjacent to the end of the cavity.

6/ An underground storage facility substantially as hereinbefore described with reference to the accompanying drawings.