Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
. The invention relates to the pipeline transportation of
- wax-containing oil and more par~icularly to a method for reducing
the pipeline pressure necessary to facilitate starting and
res~arting the pipeline flow.
. Certain oils, such as crude petroleum and shale oil,
: contain a sufficient concentration of wax o that below a certain
temperature, referred to as the pour point temperature, the wax
content of the oil cause~ it to become viscous and the oil may
, even gel if permitted to ~tand for ~ufficient period of time
.~ 10 below its pour poin~. The pour point t~mperature of an oil may
vary widely depending upon the na~ure of ~he oil, i~ wax content
and composition, and other factors. Thus, the pour point will
vary widely depending upon the oil and can range from t~mperatures
as high as 90~ to as low as 0F. At temperatures abova the pour
:; point, the oil can be transported by pipeline efficien~ly and
economically. At temperatures below the pour point, however, the
:~ wax content of the oil can begin to congeal and raise th~
viscosity of the oil causing a high pressure loss through the
line and requiring excessive pump output pressures to move the
o~l.
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 ~he pour point of the oil, the oil will have a tendency to
form a ge~like material having a high yield strength. Depending
upon the length of the column of oil to be moved, the æi2e and
number of pumps, the diameter of the pipeline, the temperature
and the yield ~trength of the oil at that temperature, the energy
requirements to restart the oil flow may exceed the maximum allow- :
: 30 able operating pressure of the pipe line resulting in a line sh~t- :
: down until the pour point temperature is exceeded or the gel other-
wise broken.
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Depending upon the geographical location of the pipe-
line, the season of the year, whether the pipeline i8 bu~ied or
laid upon the bottom of a body of wa*er or exposed to the atmos-
.~ phere, ~he temp~ra~ure of the oil in ~he pipeline can fall below
its pour point and begin to thicken or gel in the line,iparti-
cularly if flow should be interrupted for any rea~on. Conse-
quently, 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 ~hese additives are designed
*o permit restarting of the line ;n the event of a shutdown where
the tempe~ature is a~ or be3ow the pour point of the oil being
pumped. The prior art i8 replete with various additives designed
to ~nhibit the viscosity increase of the oil and to effectively
reduce its pour point. These additives, however, must be added
. in sufficiently high eoncentration to inhibit not only the
'; visc081ty increase of the flowing oil when temperatures below .-
it~ 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 trans-
port of oil, particularly during the winter months in cold :
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 6hut-
down can be accomplished even at temperatures at or below the pour
point temperatures of the oil being pumped.
~riefly, in the transportation of a wax-containing oil
by pipeline, the pre~ent invention::provides a method for reducing
the force required to initiate ~he flow of a gelled static column
of the oil in the pipeline, the method comprising: forming a
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plurality of ~egments in the oolumn of oil, the number and lengthof said segments ~eing selected ~o that the pressure required to
initiate flow of ~he oil at any point in the pipeline is les~ than
the maximum operating pressure allowable at that point; and
separating each segment from adjacent segment~ by a fluid spacer
comprising a material which is fluid at the temperature of the
s~a~ic oolumn of oil and which has a low yield strength relative
; to said oil at that temperature.
Fig. 1 is a simplified model of the mechanical proper-
tie~ of a wax-containing oil which has aooled at a temperature
; below its pour po~nt;
Fig. 2 is a ~chematic sectional view of a pipeline
illustrating a stagnant column of oil segmented in aecordanee
with the invention, which has been held below it~ pour point for
~ufficient time to result in a gel formation in the oil; and
Fig. 3 is the sectional view shown in Fig. 2 with one
: segmen~ of the oil eolumn being yielded.
When oil conta~ning dissolved wax is allowed to cool ::
.` belo~ the solidification point of the wax, solid wax precipitates
.: 20 are formed which increase the viscosi1:y of the oil resulting in a
:, grea~er 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 sy6tem. The
poin~ at which the oil begins to change from a simple fluid to a
non-Newtonian fluid i8 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 streng*h". 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.
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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, Ameri-
can Society for Testing Materials, Part 17, November, 1971,
pages 58-61.
The term "yield strength" is used interchangeably with
j~
the term "yield stress" and, as employed herein, either of the
....
terms mean the shearing stress at the yield point, i.e., the
; 10 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
, 20 shear stress while ~he horizontal axis represents fluid displace-
~ .
ment. It can be seen that as a force is applied t~ the gelled
,` oil there is some displacement as a result of elastic deforma-
tion. The displacement continues until reaching a shear stress
value equal to the yield streng~h ( Ty ) 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 foroe required to restart the displa~ement remains a sub-
; stantially constant level below the Ty îor the oil. This level
~ 30 is referred to as the remanent yield value ( TR) . As reported by
; Vershuur et al in a paper entitled "The Effect of Thermal
Shrinkage and Compressibility on the Yielding of Gelled Waxy
"
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C~ude Oils and Pipelines", J. Inst. Pe~. 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 ~he:~ curve as represented by the yield ~trength, ~y~ the slope of the
curve after yielding and the level.~of ~he remanent yield stress,
: T~, of the oil w~11 vary widely for differing oils. The profile
of mechanical propertie~ for the particular oil being pumped,
however, i~ rea~ily determined by laboratory tests or by tes*s
ln the actual pipeline under the environmental conditions to
which it is anticipated the oil in the pipeline will be subjected
to. In ~hiæ connection, it is conven~ional practice to predict
the minimum temperature to whiah the oil ean be reasonably expected
to be exposed. Typioally this information is used to estimate
~: the amount of pour point reducing additive requir~d to insure
pumpability of the oil. In this case, however, the minimum
t:emperature i8 used in the de~ermination of the Ty for the oil
~eing pumped using the method set out in the Yershuur et al :
ar~icle referred to above.
The relationship between ~he yield s~rength (~y) of a
~elled oil and the pressure (~P) requ:ired to restart a line con-
taining the gelled oil i~ represent~d by the for~ula:
; ~p _ 4L _ T : : -
D - ' Y ~1)
where:
~P - pump pressure; ~:
L = length of the line to be yielded;
~ Iy ~ yield strength of ~he oil; and
: D - internal diameter of the pipe.
: Thus, when ~P e~ceeds the maximum operating pressure
limitations (P max) of the line, initiation of flow of the oil
in the line cannot be accomplished. P max i~ dependen~ upon a
number of factors well-known in the art, including; for example
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pump output, the number of pump~, pres~ure limitatlon8 of the
pipeline itself or a combination of these factors.
In accordance with th~ pr~3sent invention 7 the ~P of
the oil in the pipel;ne is maintained at or below th~ maximum
pressure (P max) for the sy~tem by dividing th~ oil in the pipe-
line into segments which are separated from adjacent segmants by
, . ~
.' readily yieldable fluid spacer elements. In thi~ manner the
-, length of each 6egment is such that the ~P required to overcome
!,'~ `' the Iy of the segments in order to initiate movement or flow of
.i~. 10 the gegmen~s is not gr~ater than P max.
.~. .
~` The minimum number of ~egments into which the oil is
div~ded in accordance with the presen* invention is related to
;~ the maximum operating presxure, P max, of the oil line system
and can be rep~esented by the expression: :
~ K > ~L(~y~TR) (2)
;~ DP -4^L TR
where: ~ :
.i~ K is the number of segments;
is the~.!yield streng~h of th~ oil;
TR is the remanent yield stress of the oil; and
P max i8 the maximum opera~ing pressure of the pipe-
~
'` line system.
The process of the invention is prac~iced by disposing
yieldable fluid spacers in the oil column to defin~ in the column
a plurality of smaller segments each of whi~h will require a
lower pump pressure to yield than the undivided oolumn. The
manner in which the fluid spacars are disposed in the oil column
`.` i8 not critical and the spa¢ers can be introduced to a flowing
str¢am of oil or to a~static column.
. 30 For example, in a line provided with a plurality of
injection ports, the ~pacers can b~ introduced after the flow of
oil has stopped. This is primarily u~eful for schedule shutdowns
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of shor~ pipeline runs.
Under typical operating conditions, the fluid spacer ispreferably ~n~rodu~ed into ~he line at a convenien~ point while
oil i8 flowing therein. The spacer i5 ~hen carried along with
~: the oil through the pipeline ~o ~he terminus. This i6 most con-
- veniently ~ccomplished by introducing the ~luid spac~r 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 ~he
line as determined by the relationship~ disoussed 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 $tatically cool to a temperature of about its pour point or
less, the spacer element serves as a yieldable barrier between :~
~3egments to permit flow to be reestablished at a lower pr~sure
; than ~ould be required to initiate flow for an unsegmen~ed oil .
column. ;
As is more clearly shown in Figs. 2 and 3, the pipeline
~: :L0 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
re~ults in a contraction~sof the column of oil away from a portion
of the walls of the pipeline 10 to form a space 16 ~etween the oil
, ~ and the pipe.
A force is imposed on the segment 12 of suffieient
`, strength to exceed its Ty which ~esults in a breakdown of the ~el
structure and initiation of flow of the segment 1~. Initiaion of
the flow displaces the fluid spacer 14 from its normalpposition
~ betwean the segments 12 and 12' into the sp~ce 16 between the
.~. 30 segment 12' and the wall of the pipeline 10. This allows for
.' the displacement of the ~egment 12 into contact with the segment
12 ' . Upon contact between the segment 12 and the still gelled
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segment 12', the force is transmitted *o ~he segment 12' and
the process i8 repeated.
As can be seen from the relationships 1 and 2 above,
dividing O~r th~ oil column into smaller ~egments results in a
reduction in the total force required to restart flow. The
thickness of the spacing element 14 and the resultant ~pacing
between individual segments of wax-containing oil i8 a matter of
choice depending upon the profile model of the oil being trans-
ported. For example, if the slope between ~y and ~R is steep,
as ~hown in Fig~ 1~ the amoun~ 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 ~pacing material u~ed tv separate the oil se~ent~
'. i8 selected from a material which is capable of yielding at
: pressur¢s below P max, even at ~empera~ures at or below which
the oil will gel if permitted to stand. Thus, the yieldable
- 20 ~pacer material i a fluid, liquid or gaseous, which, througrh
displacement or compression, will permit displacement of the
preceding segment o~ oil.
,- Since the carrying capacity of the pip~lmse is of
particular concern to the pipeline operator, it i~ highly pre-
ferred 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 thc combined volume of spacer material can rep~esent a
6ubstantial portion of the capacity of the pipeline. Accordingly,
it is preferred that the spacer material comprise a product
which i8 useable at the tarminus of the pipeline. For theæe
:
, r2ason8 it is highly preferred tha~ the spacer material comprise
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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 ~he terminus of
the pipeline and there is substantially no los6 in carrying
capacity of the pipeline due to the use of fluid spacers to
div~de the high wax oil ~egments.
In cases where dewaxing facilities are not available,
a pour point reducing material can be admixed with the wax-con-
taining oil to provide an oil mixture which has a low pour point
and can ~hus 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 temperatureæ and
cooling rates to which the oil can be expected to be exposed.
For example, variou~ copolymers of ethylene and ethylenically
unsaturated esters are effective in r,educing the pour point and
yield stress of high pour point wax-containing oils. Examples
of these copolymers are the copolymer ethylene/vinyl formate,
c.opolymer ethylene/allyl formate, copolymer ethylene/vinyl acetate,
copolymer ethylene/ethyl methaerylate, 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 aforemention~d
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 fact~s
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 e~ploy the additive at concen- ;
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tra~ions between about 5 and abou~ 200 ppm.
In addition to the foregoing~ other methods are al~o
known in the art for reducing the pour point of wax-containing
oils~ For ~xample, th¢ oil can be supersaturated with a gas
which acts to prevent the agglomeration of the wax into a continu-
OU8 gel tructure- Thi~ gas-saturated oil can be utilized as a
spacer material. Similarly, light hy~rocarbon solvents of the
cla~s consisting of butane~ propane, and mixtures thereof, can
be added in ~ufficient quantity to dissolve the wax in the oil
but in insufficient amounts to establish a vapor pressure as
8reat as ~he pre~sures prevailing in the pipeline condui~ at
the operating temperatures 30 that the light hydrocarbon solvent
r~mains a liquidO Under normal circumstance~, pressures in the
pipeline will remain re~atively stable even in the event of a
pipeline stopplage, a8~uming of course that there is no line
rupture or similar reaso~ for pre~sure 1088.
In the pre~erred practice of ~he present invention,
the oil ~egments are formed by adding the fluid spacer material
to the flowing oil in timed increments, preferably at the line
~lead. The gpacer material then flows with the oil through the
line to ~ffect separation between the segment$ of oil. In
flowing through the line, a oertain amount of interface mixlng
will occur between the spacer material and the oil. It is
. .
: preferred to maintain such interface mixing at a minimum and ;n
. .
accordance with pipeline design considerations, mechanical
featureis such a~ streamlined headers, elimination of pockets,
.. or deadend~, and the like, are known to have a ~ubstantial
`: effect on reduction of interface mixing of different fluids in
., a conduit. In ~ddition, the more turbulent the flow, the less
0 will be the interface mixing. Consequently it i8 preferred from
~he standpoint of maintaining interface mixing a~ a minimum that
the oil flowing through the pipeline have a high Reynolds
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number. It should be clear, however, that interface mixing
cannot be completely avoided and some mixing does occur, parti-
cularly over long runs. Consequently, due to factors understood
in the art, ~he axial length of a ~pacer element will typically
increase in direct relationship ~o ~he length of the run and the
rate of increase i~ dependent upon the Reynolds number of the
~low of oil through the line. Anoth~r important consideration
-~ with interface mixing i~ that the concentration of ~he material
in the spacer element will gradually decreasq due to interface
mixing w~th the surrounding wax-containing oil as the spacer
element travel~ through the pipeline and the y~eld s~rength of
the spacer element can gradually increase. ~onsequently the
concentration of material forming the spacer element must be
sufficie~tly high in the increments introduced in~o the pipeline
~;o that the downstream ~pacer elements will continue to have the
. desired low yleld ~trength and yieldability nece~ary to the
; proper functioning of the method o~ the present invention even
through some mixing of the fluid spacer mate~ial and the wax-
containing oil has occurred.
The following i8 illustrative of the application of the
method of the present ~nvention in the transportation of crude
shale oil. The crudQ shale oil i8 tranæported through a line
having a 6-inah diameter (nominal) and a length of 70,000 feet.
The line i8 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/
ft2), The minimum operating temperature of the line is predicted
;. to bc 35F.
The shale oil has a typical gravity of 34.9 API at
.~ 30 60F and a pour point of 80F. At or below the pour point
temperature, the yield value of the gelled shale oil (Ty) ~ iS
": :
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about 0:.7 lb/ft2 while the remanent yield value after flow (TR)
.: is about 0.4 lb~ft2. Assuming th~ shale oil is at or below it~
;; .
pour point temperature after interruption of flow, the pressure
- drop required to initiate flow i8 calculated by the relationship
,:
i 4L . ~ .
:~ ~p (4) ~~? ~0.7)
where: 6P i8 the pressure drop (lb/ft2) over the leng*h (L)
: of pipeline;
Ty is the yield value, lb/ft2; and
D is the pipe diameter, fee-t. -
Thus, in the pipeline operation described above where the entire
length of ~he 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~ft2, the pressure drop requi~ed to initiate flow
is
~4) ~70~)00) (0,7)
~P = 392,000 lb/ft2 (2,722 lb/in2)
;~ It becomes a~parent that the ~P to initiate flow through the line
,:
.. 20 i8 in excegs of P max and consequently should flow be interrupted
2md the oil allowed to ~tatically cool to or below it~ pour point
`~, temperature, it will be impossible to reinitiate flow through the
i~,, line until such time as the temperature of the shale oil is ra;~ed
.:
to a point where the Ty is significantly reduced.
. Utilizing the method of the present invention, the
.~ column of oil flowing through the line is divided into a multi~
plicity of ~maller segments which individually 9 because of their
~. ~
shorter lengths, require lower pressuras to initia~e flow of
each stagnant oil segment. The minimum number of segments
required to reinitiate flow is calculated according to formula
` number (2):
` -12-
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12~i6
.,
K 4L (~y~T~)
R
set forth above and given the line parameters set out above the
minimum number of segments i8
K > ~4) (70,000) )D.7 - 0.4)
(0.5)~ 8-,000~ 70,000) rO.~)
K > 2063
Thus the number of segments should be 3 or greater.
I~ is preferred prac~ice to actually divide the pipeline
into more than the minimum number of segments ~o as to insure that
the yield stress required to reinitiate flow in the line i8 le88
than P max and it ix highly preferred to divide the line into
twice the minimum numbar of segments. The length of the segments
i8 approximated by dividing the number of segments into the length
of the lins. Accordingly, using 6 ~egmen~s, the length of each
~egment i8 11, 666 feet and the total pres~ure drop required to
restar~ flow i~ calculated ~o ~e 261,360 lb/ft2 (1814 lb/in2)
which i8 calculated by substituting X ~ 6 in Equation 2 and
solving for P.
The spacing between each of the segments is accomplished
by adding an oil flow improver such a~ Exxon Chemical Company's - -
ECA 4821X~ a fumarate vinyl acetate copolymer. The additive^
containing 8egment8 will be ~hort oompared to the overall length
of the pipeline and due to the presenee of the additiv~ will have
a very low yi~ld value at temperatur~ to 35F. Based on a flow
~ ,:
rate of 8,000 barrel~/day or 233 gal/~n it iS calculated that
. it will take one segment of shale oil 73 minute~ to pa~ a given
i; point in the line. Accordingly every 73 minutes approximately a
gallon increment of the additive compo~ition is injected ~nto
the line at the point where the shale oil leaves the storage tank
30 and enters tha line~ The low yield value mixture serves as the :.
~ spacer element dividing individual segmen~s of shale oil. Due
.` to interface mixi~g it i8 estimated that at the terminus of the
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line the spacer element will have grown to a length of approxi-
mately 650 feet, or in other words *he 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 terminu~ of the line the concent~ation
of additive in the oil is about 1~000 ppm, whiQh is an effective
concentration of the additive in ~he oil at 35F.
Although various embodiments of this invention have
been descr~bed, it will be clear that further modification8 will
be apparent to those skilled in the art. Such modif;cations
are included within ~he scope of this invention a~ defined by
tho following claimE:.
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