CA1082256A - Method for transporting waxy oils by pipeline - Google Patents

Abstract

"ABSTRACT OF THE DISCLOSURE"

A process for reducing the force required to restart the flow of wax-containing oil which has statically cooled to its pour point or below by inserting a plurality of yieldable fluid spacers in an oil column to divide the column into shorter hydraulically isolated segments and thus reduce the force required to yield the gelled oil.

Description

. 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 , 2Z~ii6 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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~2ZS6 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.
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~` 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 : -6-, . . .
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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 :, ' '' '-. : ' -, ,, , , , - , .

2ZC~6 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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s;P~5;6 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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, -108Z"~d~516 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 '', .

B2~56 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~
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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 : ~ A Trade Mark ., .

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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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Claims (11)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In the transportation of a wax-containing oil by pipeline, a method for reducing the force required to initiate the flow of a gelled static column of the oil in the pipeline, said method comprising:
forming a plurality of segments in the column of oil, the number and length of said segments being selected so that the pressure required to initiate flow of the oil at any point in the pipeline is less than the maximum operating pressure allowable at that point; and separating each segment from adjacent seg-ments by a fluid spacer comprising a material which is fluid at the temperature of the static column of oil and which has a low yield strength relative to said oil at that temperature.

2. The method of claim 1 wherein the number of seg-ments is equal to K, and K is an integer selected to satisfy the relationship:

where:
L is the length of the column of oil, ?y is the yield stress of the oil, ?R is the remanent yield stress of the oil after flow, D is the diameter of the pipeline, and PmaX is the maximum operating pressure of the pipeline.

3. The method of claim 2 wherein the number of seg-ments is selected to be at least twice the minimum value of K.

4. The method of claim 1 wherein said fluid spacer comprises a mixture of said wax-containing oil and an effective amount of a pour point reducing agent to reduce the pour point of the mixture so that the mixture has a low yield value at the temperature of the stagnant column of oil.

5. The method of claim 1 wherein said fluid spacer comprises dewaxed oil.

6. The method of claim 1 wherein said fluid spacer is introduced in timed increments into a flowing stream of said wax-containing oil in said pipeline.

7. The method of claim 1 wherein said material remains fluid at temperatures as low as predicted minimum operating temperatures throughout the entire run of said pipeline.

8. The method of claim 6 further including the step of dewaxing a portion of said wax-containing oil and using said dewaxed oil to form said fluid spacer.

9. The method of claim 1 wherein said wax-containing oil is crude petroleum.

10. The method of claim 1 wherein said wax-containing oil is crude shale oil.

11. The method of claim 1 wherein said fluid spacer is introduced into said static column of oil through a plurality of injection ports in said pipeline.