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Description  |
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BACKGROUND OF THE INVENTION
The invention relates to the pipeline transportation of wax-containing oil
and more particularly to a method for reducing the pipeline pressure
necessary to facilitate starting and restarting the pipeline flow.
Certain oils, such as crude petroleum and shale oil, contain a sufficient
concentration of wax so that below a certain temperature, referred to as
the pour point temperature, the wax content of the oil causes it to become
viscous and the oil may even gel if permitted to stand for sufficient
period of time below its pour point. The pour point temperature of an oil
may vary widely depending upon the nature of the oil, its wax content and
composition, and other factors. Thus, the pour point will vary widely
depending upon the oil and can range from temperatures as high as
90.degree. F to as low as 0.degree. F. At temperatures above the pour
point, the oil can be transported by pipeline efficiently and
economically. At temperatures below the pour point, however, the wax
content of the oil can begin to congeal and raise the viscosity of the oil
causing a high pressure loss through the line and requiring excessive pump
output pressures to move the oil.
A related problem with high pour point oils is caused by the fact that when
the oil is allowed to come to rest, such as would occur during a pipeline
shutdown at temperatures near or below the pour point of the oil, the oil
will have a tendency to form a gel-like material having a high yield
strength. Depending upon the length of the column of oil to be moved, the
size and number of pumps, the diameter of the pipeline, the temperature
and the yield strength of the oil at that temperature, the energy
requirements to restart the oil flow may exceed the maximum allowable
operating pressure of the pipe line resulting in a line shutdown until the
pour point temperature is exceeded or the gel otherwise broken.
Depending upon the geographical location of the pipeline, the season of the
year, whether the pipeline is buried or laid upon the bottom of a body of
water or exposed to the atmosphere, the temperature of the oil in the
pipeline can fall below its pour point and begin to thicken or gel in the
line, particularly if flow should be interrupted for any reason.
Consequently, pour point additives have been developed for mixture with an
oil in order to lower its pour point and thus render more economical the
pumping of the oil even at temperatures below its pour point. In addition
these additives are designed to permit restarting of the line in the event
of a shutdown where the temperature is at or below the pour point of the
oil being pumped. The prior art is replete with various additives designed
to inhibit the viscosity increase of the oil and to effectively reduce its
pour point. These additives, however, must be added in sufficiently high
concentration to inhibit not only the viscosity increase of the flowing
oil when temperatures below its pour point are encountered but also in
sufficiently high concentrations to inhibit the formation of the high
yield strength gel in the event of a pipeline shutdown at temperatures
below the pour point of the oil.
These additives increase the expense of pipeline transport of oil,
particularly during the winter months in northern climates. Consequently a
need exists for a more economical method for the transportation of high
pour point temperature oils in which the restarting of oil flow in the
event of a pipeline shutdown can be accomplished even at temperatures at
or below the pour point temperatures of the oil being pumped.
SUMMARY OF THE INVENTION
The present invention resides in a method for transporting oil in pipelines
where the oil may be exposed to temperatures below its pour point. In
accordance with the invention fluid spacers are disposed in the column of
oil at intervals along the column to divide the column into shorter
segments and thus reduce for force necessary to restart the flow of oil in
the event of a pipeline shutdown. The length and number of segments are
selected so that the restart pressure is below the maximum operating
pressure for the pipeline. The fluid spacers are comprised of a material
which is fluid at the temperature of the stagnant oil column and has a low
yield strength at that temperature.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a simplified model of the mechanical properties of a
wax-containing oil which has pooled at a temperature below its pour point;
FIG. 2 is a schematic sectional view of a pipeline illustrating a stagnant
column of oil segmented in accordance with the invention, which has been
held below its pour point for sufficient time to result in a gel formation
in the oil; and
FIG. 3 is the sectional view shown in FIG. 2 with one segment of the oil
column being yielded.
DESCRIPTION OF THE INVENTION
When oil containing dissolved wax is allowed to cool below the
solidification point of the wax, solid wax precipitates are formed which
increase the viscosity of the oil resulting in a greater energy
requirement necessary to pump the oil. In addition, when the oil is
permitted to come to rest at temperatures at or below the solidification
point of the wax, the wax precipitates can form a gel which causes the oil
to be converted from a simple fluid to a non-Newtonian fluid which may
require more energy to reinitiate flow than is available from the pumping
system. The point at which the oil begins to change from a simple fluid to
a non-Newtonian fluid is known as the "pour point" while the energy
required to initiate flow of the oil when in the gel condition is referred
to as the "yield strength". The lower the temperature the more rapidly is
the oil gelled and, similarly, the lower the temperature the greater the
yield strength of a given oil.
The term "pour point" as employed herein means the lowest temperature at
which the oil is observed to flow under conditions prescribed by ASTM test
method D97-66 entitled "Standard Method of Test for Pour Point", ASTM
Standards, American Society for Testing Materials, Part 17, November,
1971, pages 58-61, which procedure is herein incorporated by reference.
The term "yield strength" is used interchangeably with the term "yield
stress" and, as employing herein, either of the terms mean the shearing
stress at the yield point i.e., the point that a gel will begin to flow
under applied pressure. Yield strength is dependent not only on the wax
content and chemical makeup of the oil but also upon the thermal history,
such as the rate of cooling, the temperature range through which it has
been cooled and the like, the rheological history, for example the amount
of shearing to which the oil has been subjected and the like.
Referring to FIG. 1 there is shown a simplified model which illustrates the
mechanical properties of a gelled oil as related to yielding. The vertical
axis of the drawing represents shear stress while the horizontal axis
represents fluid displacement. It can be seen that as a force is applied
to the gelled oil there is some displacement as a result of elastic
deformation. The displacement continues until reaching a shear stress
value equal to the yield strength (.tau..sub.y) of the gel. At this point
the gel structure yields and there is a substantial drop in shear stress
accompanied by an increase in displacement. Having once been yielded, if
flow is stopped and then restarted, the stress or force required to
restart the displacement remains a substantially constant level below the
.tau..sub.y for the oil. This level is referred to as the remanent yield
value (.tau..sub.R). As reported by Vershuur et al in a paper entitled
"The Effect of Thermal Shrinkage and Compressibility on the Yielding of
Gelled Waxy Crude Oils and Pipelines", J. Inst. Pet. V. 57, No. 555, May,
1971, pages 131-137, the simplified model of FIG. 1 represents the
behavior of a variety of waxy oils although the peak of the curve as
represented by the yield strength, .tau..sub.y, the slope of the curve
after yielding and the level of the remanent yield stress, .tau..sub.R, of
the oil will vary widely for differing oils. The profile of mechanical
properties for the particular oil being pumped, however, is readily
determined by laboratory tests or by tests in the actual pipeline under
the environmental conditions to which it is anticipated the oil in the
pipeline will be subjected to. In this connection, it is conventional
practice to predict the minimum temperature to which the oil can be
reasonably expected to be exposed. Typically this information is used to
estimate the amount of pour point reducing additive required to insure
pumpability of the oil. In this case, however, the minimum temperature is
used in the determination of the .tau..sub.y for the oil being pumped
using the method set out in the Vershuur et al article referred to above.
The relationship between the yield strength (.tau..sub.y) of a gelled oil
and the pressure (.DELTA.P) require d to restart a line containing the
gelled oil is represented by the formula:
##EQU1##
where .DELTA.P = pump pressure;
L = length of the line to be yielded;
.tau..sub.y = yield strength of the oil; and
D = internal diameter of the pipe.
Thus, when .DELTA.P exceeds the maximum operating pressure limitations (P
max) of the line, initiation of flow of the oil in the line cannot be
accomplished. P max is dependent upon a number of factors well-known in
the art, including, for example, pump output, the number of pumps,
pressure limitation of the pipe line itself or a combination of these
factors.
In accordance with the present invention, the .DELTA.P of the oil in the
pipeline is maintained at or below the maximum pressure (P max) for the
system by dividing the oil in the pipeline into segments which are
separated from adjacent segments by readily yieldable fluid spacer
elements. In this manner the length of each segment is such that the
.DELTA.P required to overcome the .tau..sub.y of the segments in order to
initiate movement or flow of the segments is not greater than P max.
The minimum number of segments into which the oil is divided in accordance
with the present invention is related to the maximum operating pressure, P
max, of the oil line system and can be represented by the expression:
##EQU2##
where: K is the number of segments;
.tau..sub.y is the yield strength of the oil;
.tau..sub.R is the remanent yield stress of the oil; and
P max is the maximum operating pressure of the pipeline system.
The process of the invention is practiced by disposing yieldable fluid
spacers in the oil column to define in the column a plurality of smaller
segments each of which will require a lower pump pressure to yield than
the undivided column. The manner in which the fluid spacers are disposed
in the oil column is not critical and the spacers can be introduced to a
flowing stream of oil or to a static column.
For example, in a line provided with a plurality of injection ports, the
spacers can be introduced after the flow of oil has stopped. This is
primarily useful for scheduled shutdowns of short pipeline runs.
Under typical operating conditions, the fluid spacer is preferably
introduced into the line at a convenient point while oil is flowing
therein. The spacer is then carried along with the oil through the
pipeline to the terminus. This is most conveniently accomplished by
introducing the fluid spacer composition in a series of timed increments
to the flowing oil stream. The time between increments is determined by
the flow rate in the pipeline and by the length and number of segments
desired in the line as determined by the relationships discussed above.
Having once been introduced into the pipeline, should the flow of oil be
interrupted for a sufficient period of time to permit the oil to
statically cool to a temperature of about its pour point or less, the
spacer element serves as a yieldable barrier between segments to permit
flow to be reestablished at a lower pressure than would be required to
initiate flow for an unsegmented oil column.
As is more clearly shown in FIGS. 2 and 3, the pipeline 10 contains an oil
column which has been divided into segments 12 and 12' by a fluid spacer
14. The column of oil has been permitted to statically cool to a
temperature below its pour point which results in a contraction of the
column of oil away from a portion of the walls of the pipeline 10 to form
a space 16 between the oil and the pipe.
A force is imposed on the segment 12 of sufficient strength to exceed its
.tau..sub.y which results in a breakdown of the gel structure and
initiation of flow of the segment 12. Initiation of the flow displaces the
fluid spacer 14 from its normal position between the segments 12 and 12'
into the space 16 between the segment 12' and the wall of the pipeline 10.
This allows for the displacement of the segment 12 into contact with the
segment 12'. Upon contact between the segment 12 and the still gelled
segment 12', the force is transmitted to the segment 12' and the process
is repeated.
As can be seen from the relationships 1 and 2 above, dividing of the oil
column into smaller segments results in a reduction in the total force
required to restart flow. The thickness of the spacing element 14 and the
resultant spacing between individual segments of wax-containing oil is a
matter of choice depending upon the profile model of the oil being
transported. For example, if the slope between .tau..sub.y and .tau..sub.R
is steep, as shown in FIG. 1, the amount of displacement required to lower
the yield value to the remanent yield strength for the oil can be at a
minimum. On the other hand, should the slope be shallow, then the fluid
spacer 14 will necessarily be thicker in order to provide sufficient
displacement room for the segment of oil being yielded.
The spacing material used to separate the oil segments is selected from a
material which is capable of yielding at pressures below P max, even at
temperatures at or below which the oil will gel if permitted to stand.
Thus, the yieldable spacer material is a fluid, liquid or gaseous, which,
through displacement or compression, will permit displacement of the
preceding segment of oil.
Since the carrying capacity of the pipeline is of particular concern to the
pipeline operator, it is highly preferred that the spacer material
comprise an oil which can be refined and utilized at the terminus of the
pipeline. It will be recognized that pipelines extend over long distances
and that the combined volume of spacer material can represent a
substantial portion of the capacity of the pipeline. Accordingly, it is
preferred that the spacer material comprise a product which is useable at
the terminus of the pipeline. For these reasons it is highly preferred
that the spacer material comprise oil which has been dewaxed or otherwise
has a low pour point. Thus, a small facility for dewaxing the oil can be
set up at the head of the pipeline to dewax oil and thus provide a low
pour point oil for use as the fluid spacer. The spacer material is then
utilized along with the high wax oil at the terminus of the pipeline and
there is substantially no loss in carrying capacity of the pipeline due to
the use of fluid spacers to divide the high wax oil segments.
In cases where dewaxing facilities are not available, a pour point reducing
material can be admixed with the wax-containing oil to provide an oil
mixture which has a low pour point and can thus be utilized as the spacer
material. Any of the conventionally used pour point reducing agents may be
utilized in sufficiently high proportions to insure that the oil/point
reducing agent mixture is yieldable at the temperatures and cooling rates
to which the oil can be expected to be exposed. For example, various
copolymers of ethylene and ethylenically unsaturated esters are effective
in reducing the pour point and yield stress of high pour point
wax-containing oils. Examples of these copolymers are the copolymer
ethylene/vinyl formate, copolymer ethylene/allyl formate, copolymer
ethylene/vinyl acetate, copolymer ethylene/ethyl methacrylate, copolymer
ethylene/methyl methacrylate, copolymer ethylene/stearyl methacrylate, and
the like. In addition monohydroxy phenols having molecular weights below
about 300 may be incorporated along with the aforementioned copolymers
with good results. The proportion of pour point reducing additive utilized
with the oil will depend upon the wax content of the oil, its pour point
and other similar factors well-known to those skilled in the art of
reducing oil pour point by the use of pour point reducing agents. Although
the additive may be employed in proportions on the order of 10,000 ppm or
more, it is preferred to employ the additive at concentrations between
about 5 and about 200 ppm.
In addition to the foregoing, other methods are also known in the art for
reducing the pour point of wax-containing oils. For example, the oil can
be supersaturated with a gas which acts to prevent the agglomeration of
the wax into a continuous gel structure. This gas-saturated oil can be
utilized as a spacer material. Similarly, light hydrocarbon solvents of
the class consisting of butane, propane, and mixtures thereof, can be
added in sufficient quantity to dissolve the wax in the oil but in
insufficient amounts to establish a vapor pressure as great as the
pressures prevailing in the pipeline conduit at the operating temperatures
so that the light hydrocarbon solvent remains a liquid. Under normal
circumstances, pressures in the pipeline will remain relatively stable
even in the event of a pipeline stoppage, assuming of course that there is
no line rupture or similar reason for pressure loss.
In the preferred practice of the present invention, the oil segments are
formed by adding the fluid spacer material to the flowing oil in timed
increments, preferably at the line head. The spacer material then flows
with the oil through the line to effect separation between the segments of
oil. In flowing through the line, a certain amount of interface mixing
will occur between the spacer material and the oil. It is preferred to
maintain such interface mixing at a minimum and in accordance with
pipeline design considerations, mechanical features such as streamlined
headers, elimination of pockets, or deadends, and the like, are known to
have a substantial effect on reduction of interface mixing of different
fluids in a conduit. In addition, the more turbulent the flow, the less
will be the interface mixing. Consequently it is preferred from the
standpoint of maintaining interface mixing at a minimum that the oil
flowing through the pipeline have a high Reynolds number. It should be
clear, however, that interface mixing cannot be completely avoided and
some mixing does occur, particularly over long runs. Consequently, due to
factors understood in the art, the axial length of a spacer element will
typically increase in direct relationship to the length of the run and the
rate of increase is dependent upon the Reynolds number of the flow of oil
through the line. Another important consideration with interface mixing is
that the concentration of the material in the spacer element will
gradually decrease due to interface mixing with the surrounding
wax-containing oil as the spacer element travels through the pipeline and
the yield strength of the spacer element can gradually increase.
Consequently the concentration of material forming the spacer element must
be sufficiently high in the increments introduced into the pipeline so
that the downstream spacer elements will continue to have the desired low
yield strength and yieldability necessary to the proper functioning of the
method of the present invention even though some mixing of the fluid
spacer material and the wax-containing oil has occurred.
EXAMPLE
The following is illustrative of the application of the method of the
present invention in the transportation of crude shale oil. The crude
shale oil is transported through a line having a 6 inch diameter (nominal)
and a length of 70,000 feet. The line is designed to transport on the
order of 8,000 barrels per day of crude shale oil and the pumps are
designed to provide a maximum working pressure (P max) of 2,000 psi (or
288,000 lb/ft.sup.2). The minimum operating temperature of the line is
predicted to be 35.degree. F.
The shale oil has a typical gravity of 34.9.degree. API at 60.degree. F and
a pour point of 80.degree. F. At or below the pour point temperature, the
yield value of the gelled shale oil (.tau..sub.y), is about 0.7
lb/ft.sup.2 while the remanent yield value after flow (.tau..sub.R) is
about 0.4 lb/ft.sup.2. Assuming the shale oil is at or below its pour
point temperature after interruption of flow, the pressure drop required
to reinitiate flow is calculated by the relationship
##EQU3##
where .DELTA.P is the pressure drop (lb/ft.sup.2) over the length (L) of
pipeline;
.tau..sub.y is the yield value, lb/ft.sup.2 ; and
D is the pipe diameter, feet.
Thus, in the pipeline operation described above where the entire length of
the 6 inch diameter line is filled with static oil cooled to a temperature
below its pour point and having a yield value of 0.7 lb/ft.sup.2, the
pressure drop required to initiate flow is
##EQU4##
.DELTA.P = 392,000 lb/ft.sup.2 (2,722 lb/in.sup.2)
It becomes apparent that the .DELTA.P to initiate flow through the line is
in excess of P max and consequently should flow be interrupted and the oil
allowed to statically cool to or below its pour point temperature, it will
be impossible to reinitiate flow through the line until such time as the
temperature of the shale oil is raised to a point where the .tau..sub.y is
significantly reduced.
Utilizing the method of the present invention, the column of oil flowing
through the line is divided into a multiplicity of smaller segments which
individually, because of their shorter lengths, require lower pressures to
initiate flow of each stagnate oil segment. The minimum number of segments
required to reinitiate flow is calculated according to formula number (2)
##EQU5##
set forth above and given the line parameters set out above the minimum
number of segments is
##EQU6##
K > 2.63
Thus the number of segments should be 3 or greater.
It is preferred practice to actually divide the pipeline into more than the
minimum number of segments so as to insure that the yield stress required
to reinitiate flow in the line is less than P max and it is highly
preferred to divide the line into twice the minimum number of segments.
The length of the segments is approximated by dividing the number of
segments into the length of the line. Accordingly, using 6 segments, the
length of each segment is 11,666 feet and the total pressure drop required
to restart flow is calculated to be 261,360 lb/ft.sup.2 (1814 lb/in.sup.2)
which is calculated by substituting K = 6 in Equation 2 and solving for P.
The spacing between each of the segments if accomplished by adding an oil
flow improver such as Exxon Chemical Company's ECA 4821X, a fumarate vinyl
acetate copolymer. The additive-containing segments will be short compared
to the overall length of the pipeline and due to the presence of the
additive will have a very low yield value at temperatures to 35.degree. F.
Based on a flow rate of 8,000 barrels/day or 233 gal/min it is calculated
that it will take one segment of shale oil 73 minutes to pass a given
point in the line. Accordingly every 73 minutes approximately a gallon
increment of the additive composition is injected into the line at the
point where the shale oil leaves the storage tank and enters the line. The
low yield value mixture serves as the spacer element dividing individual
segments of shale oil. Due to interface mixing it is estimated that at the
terminus of the line the spacer element will have grown to a length of
approximately 650 feet, or in other words the additive will have been
diluted into approximately 955 gallons of shale oil. The amount of
additive composition introduced at the head of the line is selected so
that at the terminus of the line the concentration of additive in the oil
is about 1,000 ppm, which is an effective concentration of the additive in
the oil at 35.degree. F.
Although various embodiments of this invention have been described, it will
be clear that further modifications will be apparent to those skilled in
the art. Such modifications are included within the scope of this
invention as defined by the following claims.
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