The invention relates to a skimming device for removing substances having a density less than water, in particular oil, from contaminated water produced in petroleum or natural gas production.

Water contaminated with small amounts of oil, suspended solids and/or other insoluble substances is often a by-product of industrial operations, in particular the production and refining of crude petroleum and natural gas. For example, in the offshore production of crude petroleum and natural gas, many wells not only produce oil or gas, but also a certain amount of water (produced water) which is reinjected down-hole or disposed back into the sea after the contaminants have been removed.

To remove residual oil, natural gas and its condensates as well as solids floatably suspended in the water, a skimming device is known from US 4 986 903 having a large-volume degassing vessel which provides for a residence time long enough to release gas and lower-density substances suspended in the water. The contaminated water enters the degassing vessel through an inlet port while the clarified water exits through an outlet port at a distance from the inlet port. The gas exits the vessel through an outlet port at a position above the water inlet port and the water outlet port.

Oil and lower-density substances float on the surface of the water and are skimmed off into a container by means of a skimming weir which defines a predetermined liquid overflow level within the vessel. In order to reduce overflow of water into the container, a liquid level control maintains the liquid level below the predetermined liquid overflow level defined by the skimming weir. The skimming device known from US 4 986 903 provides for a continuous skimming operation, but it is difficult to control the liquid level exactly to the predetermined liquid overflow level defined by the height of the skimming weir because the layer of oil to be skimmed is rather thin and the vessel tends to move, in particular at offshore locations. A further difficulty of controlling a continuous skimming operation is due to deviations in level measurements.

Similar skimming devices are known from EP 1 504 800 A2, US

Patents Nos. 5 543 043, 6 337 023 and 4 627 922 and US 2004/0256325 A1.

It is a main object of the invention to provide a skimming device having improved efficiency in removing substances having a density less than water, in particular oil, from contaminated water produced in petroleum or natural gas production.

The skimming device according to the invention comprises: a degassing vessel having a contaminated-water inlet, a clarified-water outlet at a distance from the water inlet, and a gas outlet at a position above the water inlet and the water outlet; a skimming weir within the degassing vessel defining a predetermined liquid overflow level; a container adapted to receive substances overflowing the skimming weir; and a liquid level control adapted to maintain the level of liquid within the degassing vessel below the predetermined liquid overflow level of the skimming weir.

The invention is characterized in that a monitoring device is provided which detects inflow of substances into the container, and the liquid level control is adapted to lower the level of produced water within the degassing vessel in response to the monitoring device having detected inflow of substances into the container.

The invention provides for a batch skimming process. During degassing of the produced water, the liquid level within the degassing vessel is well below the liquid overflow level of the skimming weir. Upon a decision to start skimming operation, the liquid level control raises the liquid level near to, but below the overflow level of the skimming weir such that there is no continuous overflow towards the container. If the monitoring device detects inflow of substances into the container, the liquid level control lowers the liquid level within the degassing vessel so that only a batch of substances passes the skimming weir. Thereafter, the liquid level control again raises the liquid level near to, but below the overflow level to skim another batch of substances. Since the batches are skimmed in a sequence, the difficulties encountered as a result of vessel movements and deviations in level measurements are decreased and the skimming efficiency is improved.

It is a problem to control the liquid level in the degassing vessel to the same level as the skimming weir. As could easily be understood, monitoring a change of level in the separate container can be used to identify when liquid overflows the skimming weir, e.g. the liquid has reached the weir height and to use this information to control the liquid level in the degassing vessel.

In a preferred embodiment, the liquid level control is adapted to maintain the liquid level on a first level during degassing operation and, for a skimming operation, is further adapted to raise the liquid level to a second level higher than the first level, but below the predetermined liquid overflow level mentioned above. The liquid level control is responsive to the monitoring device to lower the liquid level to the second level if the monitoring device detects inflow of substances into the container. The monitoring device can detect the inflow of substances directly on the skimming weir, but preferably comprises a level sensor responsive to the level of substances within the container to detect a change of level upon inflow of substances. The level sensor can be of any suitable construction including a mechanical float to sense the surface level, but is preferably a touch-free sensor, in particular a radar-type sensor.

In a preferred embodiment, the monitoring device is further used to control drainage of the container. The monitoring device is responsive to the level sensor to detect a predetermined minimum level and a predetermined maximum level of substances within the container and is adapted to control a substance outlet of the container such that an actual level of substances within the container is maintained between the minimum and the maximum level. The substance outlet is preferably provided at the bottom of the container. Preferably, the skimming weir is connected via a controllable valve to the container, and the substance outlet of the container is provided with a controllable drain valve. The controllable valves are responsive to the monitoring device, which alternately opens and closes the valves to change between skimming operation and draining operation.

To assist drainage of the container, the liquid level within the degassing vessel further may be lowered if an inflow to the container from the skimming weir is detected in case a predefined high level of substances within the container, for example the predetermined maximum level mentioned above is reached. In this way further inflow into the container is stopped until the drainage of the container has been performed down to the minimum level.

Preferably, the container is positioned approximately at the level of the degassing tank. To enhance inflow from the skimming weir by increasing the hydrostatic level of the containment of the skimming weir, the separate container or at least its bottom portion of course could be positioned at a lower level than the degassing tank.

In a preferred embodiment, the skimming weir is in the form of a weir bucket connected through a liquid conduit to a bottom portion of the container. If necessary, the liquid in the weir bucket can be drained out in a batch sequence via the separate container to a proper location, for example a closed drain or a flare. This might be necessary in order to detect overflow into the separate container at the next batch skimming operation.

Connecting the weir bucket to the bottom portion of the container provides for a gas-tight sealing between the weir bucket and the container. The skimming action automatically comes to a stop if the liquid level within the container reaches the overflow level of the weir bucket. The container can be integrated into the degassing vessel, but is preferably a tank which is positioned separately and externally of the degassing vessel so that a prior art degassing vessel can be used without reconstruction. Preferably, the tank is in the form of a vertically extending pipe.

Gas blow-by encountered with prior art skimming devices having a substance-receiving container integrated into the degassing vessel is a major problem of those devices since gas blow-by through the weir disturbs skimming. To prevent gas blow-by, in particular in an embodiment in which the container is positioned separately and externally of the degassing vessel, an upper portion of the container extends above the liquid overflow level of the skimming weir and is connected to an upper portion of the degassing tank or its gas outlet through a gas equalizing pipe. Gas collected in the upper part of the container can vent back to the gas space of the degassing tank above the liquid level thereof.

The invention will be described hereinafter in more detail and by way of example with reference to the accompanying drawing, in which:

Fig. 1 schematically shows a skimming device according to the invention.

Figure 1 schematically shows a skimming device for removing oil from contaminated water produced in petroleum or natural gas production. The device comprises a large-volume cylindrical vessel 1 having an inlet 3 for produced water contaminated with oil and/or solid particles, and a clarified- water outlet 5 at a distance from the inlet 3 in the bottom region of the horizontally extending degassing vessel Un a top region of the vessel 1 , above the water inlet 3 and the water outlet 5, a gas outlet 7 is provided. Gas entrained in the produced water and/or oil as well as solid particles suspended in the produced water are separated from water due to gravity and buoyancy. Gas freed from the produced water leaves the vessel 1 through the gas outlet 7, while clarified water exits the vessel 1 through the water outlet 5. A liquid level controller 9 which actuates a controllable valve 11 associated with the water outlet 5 maintains the liquid level within the vessel 1 during degassing operation at a predetermined level 13 in response to a level sensor 15 which detects the actual height of the liquid level. The level sensor 15 can be of any known construction including a floater-type sensor, but is preferably a radar-type sensor.

To skim oil and other substances that have a density less than water and thus are floating as a thin layer on the surface of the water, a weir bucket 17 is provided within the vessel 1. The weir bucket 17 is arranged above the predetermined degassing level 13, but below the top of the vessel 1. The weir height of the weir bucket 17 determines a predetermined liquid overflow level as indicated at 19. Externally and preferably separately from the degassing vessel 1 , there is provided a vertically extending pipe tank 21 such that a bottom 23 of the tank is positioned below the overflow level 19, and a top 25 of the tank 21 is situated above the overflow level 19. The weir bucket 17 is connected through a pipe or conduit 27 to a port 29 at the bottom 23 of the tank 21. Oil and substances overflowing the upper edge of the weir bucket 17 enter the tank 21 through the pipe 27 and are collected therein. The top 25 of the tank 21 is connected through a gas equalizing pipe 31 to the gas outlet 7 and allows venting of gas in the upper portion of the tank 21 to the gas outlet 7. Of course, the equalizing pipe 31 may also be connected to the upper region of the vessel 1 itself. Fig. 1 shows the tank 21 approximately at the level of the vessel 1. To enhance the inflow from the weir bucket 17 to the tank 21 , the bottom 23 of the tank 21 can be positioned at a lower level than the vessel 1 in order to increase the hydrostatic level of the weir bucket 17 relatively to the bottom 23 and to increase the capacity of the tank 21.

In order to maintain the liquid level of oil and substances collected in the tank 21 between a minimum level 33 and a maximum level 35, a controller 37, in response to a level sensor 39, controls a controllable inlet valve 41 arranged in the pipe 27 between the weir bucket 17 and the port 29 and further controls a controllable drain valve 43 at a drain pipe 45 branching off from the port 29. In the embodiment of Figure 1 , the pipes 27 and 45 are connected both to the same port 29. Of course, the pipes can be connected to separate ports at the bottom 23 of the tank 21. If the controller 37 detects that the liquid level in the tank 21 has reached a predetermined high level while further detecting an inflow into the tank 21 from the weir bucket 17, the liquid level controller 9 in response to the controller 37 lowers the liquid level within the vessel 1 to instantly stop the inflow into the tank 21. This assist the drainage of the tank 21.

It is a problem of prior art degassing and skimming vessels to control the liquid level in the vessel to the same level as the weir bucket. It is an advantage of the skimming device according to the invention that the level control of the tank 21 can be used to identify a liquid overflow at the weir bucket 17, e.g. when the liquid has reached the weir height 19.

During regular degassing operation, the controller 9 controls the valve 11 so that the liquid level within the vessel 1 is maintained at the liquid level 13 well below the overflow level 19 of the weir bucket 17. The liquid level 13 is chosen to provide sufficient space above the liquid level so that the vessel 1 can handle a flow slug or other flow events.

In order to initiate skimming operation, the controller 37 opens the inlet valve 41 and closes the drain valve 43. Further, the controller 9 raises the liquid level to a maximum level 47 somewhat below the predetermined overflow level 19 of the weir bucket 17. Preferably, the speed at which the liquid level is raised towards the maximum level 47 is decreased during the 5 final approach to the maximum level 47.

While the liquid level within the vessel 1 is below the overflow level 19, the liquid level in the tank 21 remains constant. If the layer of oil and floating substances rises above the overflow level 19, oil will flow into the tank 21 soo that the liquid level in the tank 21 starts to increase, which will be detected by the sensor 39. Upon this event, the controller 9, which is responsive to the controller 37 and thus the level sensor 39, lowers the liquid level in the vessel 1 below the overflow level 19 to the maximum level 47 to stop the inflow of the oil batch into the tank 21. The oil batches are skimmed in as sequence until the liquid level in the tank 21 reaches the maximum level 35, whereupon the controller 37, responsive to the level sensor 39, closes the inflow valve 41 and opens the drain valve 43 until the liquid level has lowered to the minimum level 33. Thereupon, the controller 37 closes the drain valve 43 and again opens the inflow valve 41 to repeat batchwise skimming, ifo necessary. The sequence of skimming batches of oil and draining the tank 21 will be repeated as many times as required. When skimming is terminated, the liquid level in the vessel 1 is reduced to the normal liquid level 13 for degassing operation. If necessary, the liquid in the weir bucket 17 can be drained out via the tank 21 to a proper location, for example a5 closed drain or a flare also in a batch sequence. This might be necessary in order to detect overflow into the tank 21 at the next batch skimming operation.

The maximum level 35 in the tank 21 is chosen to be somewhat belowo the overflow level 19. If, for any reasons, the liquid level in the tank 21 rises up to the overflow level 19, the skimming operation will automatically stop.

The pipe 27 forms a siphon-type gas seal between the vessel 1 and the tank 21. The gas equalizing pipe 31 vents the gas from above the liquid level in the tank 21 to the gas outlet 7 or the gas space within the vessel 1 and thus prevents gas blow-by through the weir bucket 17 so as to prevent degrading of skimming efficiency.