Inclined separation tank

A vessel designed to separate out clean water from produced oil is disclosed. Conventional free water knockout vessels (FWKO) are normally disposed horizontally, and have a large diameter. The large diameter maximizes the distances between the oil/water interface and the exit points on the vessel for the water and the emulsion. The larger the vertical distance from the oil/water interface, the greater the chance that the oil and water can be agitated by slugs of gas and fluids. Tilting the vessel and reducing the diameter minimizes the distance the fluid has to travel to reach the top and bottom walls of the vessel, maximizes the distances from the oil/water interface, utilizes more fluid space in the vessel, reduces agitation caused by gas slugs, and forces the water to travel a longer distance which provides more separation time. Emulsions in most pipelines stratify, and the inlet nozzle is designed in such a way that it encourages the stratification of oil and water to continue as it flows into the vessel. Baffles are also incorporated to maximize the retention time of clean water and emulsion in the vessel.

FIELD OF THE INVENTION 
This invention relates to pressure vessels, and in particular to vessels 
for use in separating oil well fluids, such as oil, water and gas. Such 
vessels are commonly referred to as a Free Water Knockout (FWKO). 
BACKGROUND OF THE INVENTION 
A large number of oil fields are underlain by large aquifers, which provide 
the drive mechanism to move the oil out of the reservoir. These fields 
continuously recycle water and some are waterflooded which introduces 
water from other formations. Viscosity differences between the gas, oil 
and water along with depletion greatly increase the amount of water that 
is produced with the oil. In older fields the amount of water produced may 
be 98% of the total fluids produced. In order to produce economically, the 
water has to be removed cheaply from the revenue generating fluids. Large 
volumes must be processed in order to make the process economical. 
Factors in the separation of water from oil are: retention time, heat, 
chemicals, centrifugal forces and gravitational forces. Heat and chemicals 
are areas where little improvement can be made, and in addition they add 
to the operating cost. For this reason, conventional pressure vessels or 
Free Water Knockout vessels function by allowing the emulsion to sit in a 
relatively non-turbulent environment, allowing gravity to disengage the 
fluids from one another, which stratify into gas, oil and water. The water 
falls freely to the lower pan or bottom of the vessel and is removed 
therefrom. The oil and gas are withdrawn from higher locations in the 
vessel. 
It will be appreciated that, the larger the pressure vessel, the more time 
is given to the emulsion to sit and stratify, and thus the more complete 
the separation becomes. If follows from this that, as the amount of 
emulsion to process becomes greater, larger and larger vessels must be 
used in order to allow the emulsion to sit for the stipulated amount of 
time before being withdrawn from the vessel. 
Although large diameter vessels can increase retention time, they tend to 
be expensive and are usually limited to pressures less than 100 psi. 
Conventional FWKO's utilize a complicated system to monitor the oil-water 
interface, in order to determine when to dump the water and oil. The 
instrumentation to accomplish this is usually expensive to operate and 
complex. 
Conventional pressure vessels of the kind under consideration fall into two 
broad categories: horizontal and vertical, each type having its own 
advantages. Vertical vessels are preferred for the separation of liquids, 
because the relatively small liquid interface within the vessel forces the 
liquids to stratify into deep, vertically separated bands from which the 
different liquids can be withdrawn. By comparison, a vessel that is 
oriented horizontally separates the fluids into shallow or narrower bands 
or layers which are more difficult to control. This fact might suggest 
that a vertically oriented vessel would be the best choice, however it 
turns out that the vertical separator is not a viable option, due to the 
fact that the height and weight of a unit capable of performing this 
operation would be enormous For example, a vertical vessel with the same 
capability as a typical vessel made in accordance with the present 
invention, would be about 65 feet high. For his reason, horizontal vessels 
tend to be used as FWKO vessels, with the various associated drawbacks 
mentioned earlier. 
PRIOR ART 
A prior patent of interest is Canadian patent 1,237,372, of May 31, 1988. 
This patent shows a conventional horizontal vessel construction which is 
utilized for separating oil and water received from a well, the vessel 
utilizing a skimming device to remove oil from which water has separated 
by gravity. 
Another example of the prior art is seen in U.S. Pat. No. 2,206,835, of 
Jul. 2, 1940. In the arrangement disclosed in his patent, two tanks are 
used: an inclined tank to initially receive the fluid, operating in 
communication with a larger, vertically oriented tank. While the latter 
patent relies upon a second, vertical vessel to be in communication with 
an inclined tank in order to accomplish the separation, the vessel 
according to the present invention is a stand-alone unit, without moving 
mechanical parts, and without areas where solids can become trapped. 
Another early Canadian patent is 197,859, of Mar. 9, 1992, which discloses 
a tank used for the separation of oil and water. The orientation of the 
tank is not fixed, and it moves to a certain extent against the tension of 
suspension springs. When water and oil accumulate in the tank in a 
quantity sufficient to over-balance the springs and weights, the free end 
of the tank moves downward so that the tank assumes a horizontal position. 
As a result, the actual separation process occurs with the vessel oriented 
horizontally. 
Other examples of the prior an may be found in the following Canadian 
Patents: 
1,035,290 Jul. 25, 1978 
1,042,819 Nov. 21, 1978 
Canadian Published applications: 
2,041,479 filed Apr. 30, 1991 
2,108,297 field Oct. 13, 1993. 
Other U.S. issued Patents that are representative of the prior art are U.S. 
Pat. Nos. 5,326,474; 4,939,817; 4,120,796 and 4,132,651. 
SUMMARY OF THE INVENTION 
The Free Water Knockout vessel according to the present invention 
incorporates an advance wherein the vessel, which is essentially an 
elongate cylinder, is disposed obliquely to the horizontal. This obliquity 
provides certain improvements by comparison with a conventional horizontal 
vessel. 
Firstly, the vessel in accordance with the present invention increases the 
effect of gravity on the separation process, forcing water into the lower 
portion of the vessel with greater force than occurs with a horizontal 
separator. 
Secondly, the oblique disposition of the vessel results in deeper or wider 
liquid bands being developed, allowing both clean water and pure oil to be 
drawn off sooner than would be the case with a conventional horizontal 
vessel. This in turn allows the vessel according to the present invention 
to be smaller in diameter than an equivalent horizontal vessel known in 
the prior art. The small size of the vessel translates into a substantial 
cost saving, since the cost of the materials and labour required to build 
a larger diameter vessel increase very quickly with increasing diameter. 
More particularly, this invention provides a free water knockout vessel for 
receiving and separating oil well fluids, said vessel comprising: 
an elongate structure having a longitudinal axis disposed at an oblique 
angle to the horizontal, the structure having a lower end and an upper 
end, 
a lower head closing said lower end and an upper head closing said upper 
end, 
a plurality of spaced-apart baffle plates within the vessel, dividing the 
vessel into a plurality of interior compartments, 
an elongate inlet stratifier pipe within said vessel, the stratifier pipe 
having a longitudinal axis disposed substantially horizontally and being 
adapted to receive oil well fluids from a feed conduit and transmit them 
into said vessel, and 
exit means for withdrawing separated fluids from said vessel for further 
processing; 
said plurality of baffle plates including: 
A. a water baffle adjacent the lower end of said vessel, the water baffle 
having a manway opening in the lower region thereof, a manway panel 
adapted to close said manway opening but defining a water opening such 
that water may flow through the water opening into the lower end of said 
vessel, the water baffle having a weir opening adjacent the top thereof to 
allow oil trapped below the water baffle to escape into the higher region 
of the vessel, and 
B. an oil baffle located adjacent the upper end of the vessel to provide a 
barrier to water, the oil baffle including a weir opening adjacent the top 
thereof, to allow oil to flow over the top of the oil baffle. 
In addition, this invention provides a method of separating oil well 
fluids, comprising: providing a free water knockout vessel for receiving 
and separating oil well fluids, the vessel being in the form of an 
elongate structure with a longitudinal axis disposed at an oblique angle 
to the horizontal, the structure having closed lower and upper ends, and a 
plurality of spaced-apart baffle plates within the vessel, dividing the 
vessel into a plurality of interior compartments, one of said baffle 
plates being a water baffle adjacent the lower end of said vessel, the 
water baffle having a manway opening in the lower region thereof, a manway 
panel adapted to close said manway opening but defining a water opening, 
the water baffle further having a weir opening adjacent the top thereof, 
another one of said baffle plates being an oil baffle located adjacent the 
upper end of the vessel to provide a barrier to water, the oil baffle 
including a weir opening adjacent the top thereof, 
transmitting oil well fluids into said vessel by passing them through a 
feed conduit connected to the upstream end of an elongate inlet stratifier 
pipe within said vessel, the stratifier pipe having a longitudinal axis 
disposed substantially horizontally and having spaced apart perforations 
through which said oil well fluids pass, and 
withdrawing each separated fluid from said vessel through its own exit 
coupling, for further processing; 
said step of withdrawing including allowing water to flow through the water 
opening into the lower end of said vessel, the step of withdrawing further 
including allowing oil trapped below the water baffle to escape into the 
higher region of the vessel by flowing over the weir opening of the water 
baffle, and in which said step of withdrawing includes allowing oil to 
flow over the top of the oil baffle.

DETAILED DESCRIPTION OF THE DRAWINGS 
The person skilled in the art would generally agree that the ideal FWKO 
will have the following characteristics: 
1) Cheap to construct, 
2) Provides water quality that can be directly reinjected. 
3) Eliminates agitation in the vessel from gas slugs. 
4) Minimizes the vessel volume used for gas. 
5) Operation should be simple and cheap. 
6) Operates at pressures up to at least 250 psi. 
7) Ability to be installed on a pipeline, thus elating costly line looping. 
8) Provides ample gas pressure so that gas compression can be eliminated. 
9) Utilizes existing pipeline stratification of the oil and water. 
10) Operates over a wide range of volumes and conditions with no design 
changes. 
11) Separates gas from the emulsion. 
12) Reduces H.sub.2 S concentrations in the gas stream. 
13) Reduces future downstream equipment costs. 
As seen in FIG. 1, a vessel constructed in accordance with one embodiment 
of the present invention (an FWKO vessel) is illustrated generally at 10 
and is configured to define an elongate structure 12 which is mounted on 
pilings 14 in such a way that its longitudinal axis is disposed at an 
oblique angle to the horizontal. Due to the sloping disposition, the 
vessel has a lower end 16 and an upper end 18, with rounded heads 20 and 
22 respectively closing the lower end 16 and upper end 18. In the specific 
vessel embodiment illustrated in FIG. 1, the inclination is approximately 
7.degree. from the horizontal, although departures from this particular 
angle are contemplated within the scope of this invention. 
The vessel includes a cylindrical shell which may typically be about 55 
inches in diameter and approximately 54 feet in length. These dimensions 
and angles are given only by way of typical example, and are not to be 
regarded as limiting in terms of the scope of the present invention. 
In this connection, the length of the vessel and the angle of elevation 
were selected in such away as to maximize the angle and still utilize a 
workable height at the highest part of the vessel. 
A further consideration in connection with the tilt or obliquity of the 
vessel is the volume occupied by gas. In order to minimize this volume, 
the vessel must be tilted or angled. The greater the angle formed between 
the vessel and the horizontal plane, the smaller the emulsion/gas 
interface would become. Another benefit of providing the tilt is that the 
distance from the liquid/gas interface to the gas outlet nozzle can be 
maximized to reduce fluid carryover and yet still minimize the volume set 
aside for gas containment. 
Generally, the shell of the vessel should be sized to keep the fluid moving 
in laminar flow at all times, as it moves toward the outlet. In laminar 
flow, the velocity of the fluid is at its maximum at the vessel axis, and 
decreases sharply to zero at the wall. As oil particles move away from the 
vessel axis, they travel into an area where the forces are so small that 
there is no tendency for the oil to recombine with the water. Once the oil 
reaches the top of the vessel, there are no forces moving the oil. 
However, tilting the vessel allows gravity to move the oil toward the 
emulsion outlet. Thus, the tilting of Me vessel prevents a build-up of oil 
from occurring, which keeps the oil further away from the higher 
velocities at the vessel axis. The simplest way to look at the effect of 
tilting the vessel is to visualize the top of the vessel as a trap. Once 
oil enters his trap, there is no effective force to get the oil back into 
the water. The tilt of the vessel allows gravity to carry the oil back to 
the emulsion outlet. 
Each design situation may change the configuration of the vessel. The 
length and diameter of the vessel, along with the tilt, can be optimized 
in order to reduce the overall cost. If the tilt is increased, it may 
allow a reduction in the diameter and length of the vessel, but it may 
increase the cost of supporting the vessel. 
Referring now to FIGS. 1, 4 and 5, the vessel further incorporates a water 
baffle 24, located closer to the lower end 16 than to the upper end 18 of 
the vessel 10. The water baffle 24 has a substantially rectangular manway 
opening 25 with a manway panel 26 adapted to close the opening 25, the 
panel 26 having a water opening 27 in the bottom region thereof, allowing 
water to flow through the water opening 27. The water baffle 24 further 
has a weir opening 30 defined between a horizontal top edge or weir 31 and 
the upper inside wall of the vessel 10. This weir opening 30 allows oil 
trapped below the water baffle 24 to escape into the higher region of the 
vessel 10. 
Also provided, as seen in FIG. 5, is an oil baffle 32 which is located 
closer to the upper end 18 of the vessel 10 than to the lower end 16 
thereof. The oil baffle 32 provides a barrier to water, and includes a 
weir opening 33 adjacent the top thereof, to allow oil to flow over the 
top of the oil baffle 32. The weir opening is defined between an upper 
weir edge 34 and the adjacent upper inside surface of the vessel 10. 
Provided within the upper half of the vessel 10 is an elongate inlet 
stratifier pipe 36, the stratifier pipe having a longitudinal axis 
disposed substantially horizontally and being adapted to transmit oil well 
fluids generally into the top third of the vessel 10. An inlet pipe 37 
feeds the emulsion of oil well fluids to the leftward (upstream) end of 
the stratifier pipe 36. The stratifier pipe 36 is provided with upper and 
lower perforations 38, the perforations being of a size and distribution 
so as to partially block the flow of oil well fluids into the vessel 10, 
thus allowing water to fall out of the lower perforations and oil to exit 
through the upper perforations, thus resulting in a reduction of 
subsequently required separation. Also, in the embodiment illustrated, the 
downstream end of the stratifier pipe 36 is partially closed by a 
semicircular panel 39 as seen in FIG. 7. 
The lower end 16 of the vessel 10 is provided with a water drain coupling 
40, while the upper end of the vessel 10 is provided with a gas exit 
coupling 42 and an oil exit coupling 44. 
The stratifier pipe provided by the present invention is designed to 
minimize the effect of gas slugs, because these enter the vessel 10 close 
to the top of the vessel in the already dewatered emulsion. 
Essentially, the illustrated stratifier pipe 36 is a pipe wit elongate 
slots 38 cut out along the top surface and bottom surface, with the total 
area of the slots being determined by the velocity of fluid movement 
required in the stratifier pipe. Thus, due stratifier pipe slot size is 
ideally designed specifically for each particular application. 
The structure just described allows high velocity gas slugs to go directly 
out the downstream end of the stratifier pipe, thus minimizing agitation 
of the other fluids. Gas emerging from the downstream end of the 
stratifier pipe has only a short distance to travel to reach the top of 
the vessel 10. If agitation does occur, it will be in the emulsion leaving 
the vessel. However, a slight water increase in the emulsion at this 
location is of no concern. The primary objective is to get clean water out 
of the water outlet pipe. 
The oil baffle 32 at the top of the vessel, functioning as a weir, skims 
off the cleanest emulsion from the highest liquid point in the vessel. 
Near this weir is a float control, represented schematically by the box 
52, which can operate to maintain the liquid/gas interface in the vessel 
at a desired level. 
The bottom two-thirds of the vessel 10 functions to clean the water up to 
the point of injection quality. It is considered that the secret of 
obtaining the required water quality is the inclination of the vessel 10, 
which maximizes the height between the emulsion/water interface and the 
water outlet. Any oil entrained in the water does not have to travel far 
before it reaches the top wall of the vessel. As the top wall and the 
bottom wall of the vessel tend to be the quietest areas in the vessel, 
fluid in these areas is not affected by any disturbance. 
The water baffle 24, which is located in the bottom third of the vessel 10, 
forces all the water to the bottom of the vessel and creates a dead spot 
in front of the baffle. This baffle functions primarily to increase the 
retention time of the fluid before entering the water outlet, thus forcing 
the cleanest water into the water outlet. As previously mentioned, an 
opening is provided between the water baffle 24 and the top of the vessel, 
to allow oil that may settle out below the water baffle to migrate back to 
the oil/emulsion outlet. 
OPTIONAL ACCESSORIES 
In the event that the emulsion outlet 44 becomes restricted or blocked, a 
capacitance probe can be installed just above the water outlet 40. Such a 
capacitance probe is represented in FIG. 1 by the box 50. The capacitance 
probe reacts to the presence of oil, whereby if oil is detected, the water 
disposal pump connected to the water outlet 40 can be shut down. 
Alternatively, a control valve (not illustrated) can be closed. 
A further option is the installation of a float control 52, already 
mentioned. In FIG. 6, a float 54 is located within a housing 56 which has 
a plurality of apertures 58 that restrain and slow down the entry and exit 
of fluid in and out of the housing. This prevents false signals due to gas 
slugs or other sources of turbulence. If gas conservation is required, the 
float 54 will control a valve on the gas outlet 42 (not shown). When the 
liquid level goes down, the float will open the valve and allow some gas 
to escape, which in turn raises the liquid level. When the proper level is 
reached, the valve will close. If gas conservation is not required, the 
gas can exit from the vessel along with emulsion, and no control valve or 
float mechanism will be required. The utilization of higher vessel 
pressures can elate the high cost of gas compression, and the gas can be 
put directly into a pipe line system. 
Various nozzles and flanges are positioned along the vessel for sampling 
purposes, or as advance provisions for further concepts not yet filly 
developed. 
The operation of the vessel is easy to learn. The only control is a switch 
for the disposal pump at the water outlet 40. This pump is either on or 
off. For example, a particular design might require that a vessel be able 
to accommodate 2500 M.sup.3 /D of fluid. Theory may reveal that the vessel 
can remove 1500 M.sup.3 /D of injectable quality water. In this situation, 
a pump designed to inject 1500 M.sup.3 /D of water would be installed at 
the water outlet line, and would run at this continuous rate. The excess 
1000 M.sup.3 /D of emulsion is carried out the emulsion outlet to a 
further oil treating step. A rate control valve can be substituted for the 
pump if the water is not required for direct reinjection. 
While one embodiment of this invention has been illustrated in the 
accompanying drawings and described hereinabove, it will be evident to 
those skilled in the art that changes and modifications may be made 
therein, without departing from the essence of this invention, as set 
forth in the appended claims.