CA2089698A1 - Process for re-starting the operation of a pipeline containing fluids with a high value of yield stress and devices for carrying it out - Google Patents

Abstract

"PROCESS FOR RE-STARTING THE OPERATION OF A PIPELINE
CONTAINING FLUIDS WITH A HIGH VALUE OF YIELD STRESS AND
DEVICES FOR CARRYING IT OUT"

Abstract Process for re-starting the operation of a transport pipeline containing fluids with high values of yield stress, by charging to a plurality of sites along said pipeline, an auxiliary re-starting liquid means.
Such an auxiliary liquid means is charged with a reduced flowrate and under a suitable pressure, in order to de-structure the fluid contained inside the pipeline and restore the normal fluidity thereof, thus resuming the normal operating flowrate.

Description

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PROCESS ;OR RE-STARTING THE OPERATIO~ OF A PIPEL;NE
CONTAINING FLUIDS WITH A HIGH VhLUE OF ~IELD STRESS ANG
DEVICES FOR CARR~ING IT OUT"
The present invention relates to the p;pelines -for long-distance -transport of Fluid products, ;n particular fluids showing structuring behaviours after lorlg-time stopsO Such a customary designation relates to phenomena occurring in the absence oF flow, due to surface forces o-f in-termolecular attraction, which tend to generate provisional bonds between molecules of said fluid, which cause such increases ;n viscosity and consistency of said fluid, which may even reach a completely structured condition, that said fluid may result difficult to be moved in order to resume the operat;on of the pipeline.
The~ subject-matter of the present invention is a process for re-starting the operat;on of transport pipel;nes used for such types of fluids. In these cases, when one tries to re-start the operation of the p;pel;ne, a considerable pressure has to be developed in order to break the intermolecular bonds which have formed while the flow was discont;nued, and d;s;ntegrate the intermolecular s-tructures formed, and favour the~ flow;ng of the flu;d unt;l the latter has recovered its or;ginal flow;ng character;st;cs.
Uslng rheoLogy ~terms, the behav;our of these flu;ds is characterized by a "yield stress'`, accord;ng to the ~current designation, and their state equation can be described, in first approx;mation, by equations of the folLowl~ng type: ~

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a) 1~ = ~o + '~ (Bingham's flu;d) b) ~= ~o + k~ n tHerschel-Buckley's flu-id) wherein:
`~ and rO are -the shear stress under movement and stationary conditions, respect;vely, as expressed as Pa, is the shear rate expressed as sec-1, k ;s the cons;stency ;ndex expressed as Pa.sn, l~ ;5 the plastic v;scosity expressed as Pa.s 1C and n is the T;OW index.
The shear stress under stationary cond;tions ~ o is currently caLled "the yield s-tress" and -is expressed as N/m2.
1; Add;tionally to the above, the re-start;ng of these flu;ds~ can be furth~r~ compl~lcated by ~
thixotropic behaviour,~i.e., depending on the t;me during which the flow is d;s~co~n:t~1nued.~in fac~t, as time~
goes on, the y;eld stress can reach values which may be more than~10-times~as~large as the in;it;al~;value.
Such beh~avi~ours~ are~typical;~for~crude oils~with a h;gh content of linear par~affinic ;components, viz., ~paraffinic w~axes, when~ the~y ~re~ C D O l e d ~ d O W II~ ~t~o~
temperatures Dl~O se ~ to thelr~ pour~point~ senerally close~ to ûC,~ or~f~or~high-coal; coa;l-water mixtures, ` when~ver~y~react~i~ve c~oaL g~r~ades are~used~
In~ th;e~case of~the~c~rud~e o;ils~with~ a~;~h~igh conte;nt of~ wax components,~ t~he~ drawbacks deriving -From these~
cha~racteri~stics~ ~O~T'~ sa;;d crude~oils can~be;obvia-ted or 3û re~du~ d~by~lo.~r~iny their y~;e~ld~.re~ v~Lu~, ~.9., ~Dy ~, .

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using suitable additives calles pour po;nt depressants , or by mixing the crude oil with lower-viscos;ty hydrocarbon components, or by heating the flu;d by heat-insulating and/or heat;ng the pipel;ne and per-forming the pumping at high temperatures. Such measures can aLso be combined with one anotner.
Tilese remed;es resuLt to be economically burdensome in terms of operating costs, on considering tne fLow rates at wh;ch o;l p;peLines are required to tO operate and, in the case o~ the heated pipelines, also as regards the requ;red investment costs. aesides the above, ~the solution which env;sages heating -the Tluid is furthermore exposed to faiLures or malfunctioning of the heating system. In any cases, after a ~Long-time stop, the s-tationary fluid inside the pipeLine must be dispLaced with a Newtonian fLuid (gas oiL,~ kerosene, and the Like) and the pipeline must be pre-neated, by causing a hot fluid to fLow inslde it, before charging `
the wax-conta;ning crude oiL
2û In the case of w~ater-coaL mixtures, ~these remed;es are not appL;cable, because the ~o of such m;xtures;;s poorly sens;ble~to temperature, ~relat;vely to the increase which ;t undergoes over ~t;me under stat;c condit~;ons.
~ The re-s~tartlng o~f operat;o;n of p;pel;nes w;th the structured fL~u;d ;ns;de their ;nter;or requires, ;n general, pressure vaLues more than 3 to 5 t;mes as large as ~ the;r normaL operat;on pressure. For ::
~exem~pl;fy;ng purposes, this means that, wh;le under normaL operat;ng~conditions, an ;ndustrial pipeline 2 ~ J3`i 4.

will operate with a del;very pressure of 8G bar (8.10&
Pa), the operat;ng re-star-ting pressure could poss;bly reach values compr;sed w;th;n the range of from 24G -to 400 bar (24 - 40.10r; Pa). From the above, it clearly appears that des;gning the overall transport system (;.e., the pump;ng stat;ons and the p;pel;ne) for operat;ng at its re-starting pressure ~ould not be technically and econom;cally -feasible.
in order to obviate the operating re-s-tarting di-fficult;es, it was also proposed ;n the past to provide the oil p;pelines, in cold reg;ons -- besides a considerable heat ;nsulat;on -- w;th an aux;liary re-start;ng system fed with natural gas. This system makes ;t possible the main pipeline to be subdivided into d;screte portions ("stretches"), and to lay, sideways to it, a service pipeline for transporting high-pressure natural gas. The removal of the plugs ins;de the ma;n duct ;s carr;ed out by push;ng,~ segment-by-~segment, the s-tructured flu;d w;th pressur;zed methane, start;ng from downstream pipeline~ segments and progressively treat;ny m;ore and~ more upst~resm p~peline segments. Th;s technical~solut;on results to be very burdensome ;n terms of~l~nvestment costs and Turthe~rmore~
requires the availab;lity of natural gas. The ,problem Z5 exists furthermore,~ that ~ne gets m;xed ph~ases inside the plpeline, ;~wh;ch result to be very d;ff;cult to be controlled, in ~p;articular at the terminals and at the pumping stations~instslL~sd along~the~pipeline.
The~ transport of crude oils w;th hiyh values of 30 ~y;eld str~ess requlres hence, sccordlng to~the present , . ' ' .
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state of the art, the adoption of the burdensome contr;vances described hereinabove.
The su~ject-matter of the present invention ;s a process and dev;ces for re-starting the operat;on of a pipeline through wh;ch h;gh-y;eld-stress flu;ds flow, w;thout incur;ng the above descr;oed dra~oacks, and resum;ng the normal operat;ng cond;tions of the p-ipel;ne w;th;n a fe~ days. The present ;nvent;on ;s d;sclosed by referring to the accompany;ng F;gure 1, ~wh;ch discloses a typical form of pract;cal embod;ment thereof, for exempl;fy;~ng, non-l;m;tative purposes.
~ The transport system ;s designed ~ith dimensions based on the pa~rameters relevant to the steady-state operating cond;t;ons, and ls co;ns-titute~d by the departure and~arr;val t~erminals, ~the pipel;ne, the control valves~and pres~sure~wa~ve damp;ng ~valves, and;
;ts~ ~pump~ng ~sta~-tlons fo~r part;al sect;;ons, ~which~;are~
prov;de~d`~;n an~amount and;~a~t~ pos;t;~ons establ;shed on cons;der';ng~ the~Length~a~nd~the~àltlm~e~t~ricai`~prof;l`e oF
the~p;pei~ne. Ac~cor~d~ng t~o~ tbe~pre~sent~;n~v~ntlon, alon;g~
the p~art;al~ st~ret~ches of~ p;pe~ ne~betwee;n adjacent~
pump;ng stat;ons~ a~ p~lur~a~ t~y~`of~r~e-start~;ng s;tat;ons~
are~lns~talled,~a~lranged~ac~cord;ng to shorter~stretches.
~Each~ re~-;st~a~r~ ng~stat;on, ~substant~;~ally con~st;tuted`by~
a pump~ and~a st~o~red amount O T ~a~ re-starting l;quid~
f~lul~d,~ o~pe~rates~u~n~d~er~f~Low~ra~te ~and p~ressure ~con~d~;ti~ons w~hl~ch ~ar'e ~reduced;~relat~lve;ly to those ~wh~lch wouLd~ be~
r~eq~u;~red~ 'n~ orde~r~t~o re-~sta~rt~ the~ oper~atl~on~ of~t~he~
whol~e~p~ pe;l~;n~e~ E~a~c~h~re-~start~ing station is ~so des;gned~
~0 ~nd ~dl~ n~ioned~ tG g~ e~ pr~.-r~ h~

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substantially equates the designed steady-state pressure of the pipeline, and which is capable of dispLacing and de-structuring the stationary fluid contained inside the pipel;ne and resuming the -flow alony the short p;peL;ne stretch ;t ;s des-t;ned to serve, between two re-start;ng stat;ons~
Ihe length of the pipeline stretch to be served by each re-starting stat10n is determined in such a way that the applied pressure ;s sligh-tly h;gher, generally 10 5-10/o higher, than the required pressure based on -the y;eld stress value. In fact, by way of experiments it ~as found that, in general, when they are submitted to shearing stresses hiyher than the y;eld stress ~value, the wax-contairling crude oils get again their flu;dity li within time intervals of a few hours,and also at~low shear rate values (e.g., at O.3 s 1) jf they are subJected to shear::stresses greater;.
than the value of the yield stress:. ~ Basing on : this cr;teria, ;t ~resul~ts ~hence :it possible~ the normal fluidity to~be~:ob-ta;ned~dgain within a few ;day ~t;me, along the~whole pip~e~line, by~startiny :from t~he end downstream stretc~h ~of the transport system, and;
proyressively ~mov~ing:upstream.:~ Th;s enables~ the~ ra-~ ;
starting t;me and costs to be;reduced relatively to the~
t:echn;que ~bas:ed~ on;the comp~le~t~e~d~;splaceme~nt~ o-f: th~e ~ ~ :
~:structured ~fLu;~d~ lns;~de ;-the :~p;peline, ~which would~: :
~req;ulre~ùch l;onger~times.~
T~he: fL~ow:rat~e: of~each~ ;ndiv;dual ;~re-startlng ~ ~;
:station : ca~n~be~:a: reduced f:raction of ~the ratcd ~operating flowra~te of~the p;pel;ne, and ;s selected ln~
rela:t;;on::with~;:the nece~ssary flowrate and the expected :
time:s for ;n;tia:ting:and:~d~eveloping the normal:flo~rate : ~ : : ~:: : :

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inside the main duct.
in fact, it was surpris;ngly Found that ;t ;s enouyh to apply the re-start;ng pressure to the stationary flu;d ;nside the p;pel;ne, structured after ; that the flow;ny thereof was discontinued, for the necessary t;me ;n order to accompl;sh the d;splacement of a small fract;on o-f its volume conta;ned ;ns;de the concerned stretc'n of the p;pel;ne, to tr;gger the de-structuring of the stationary -Flu;d and to begin 1û resuminy the transport ~;th the a-id of -the normal pump;ny stations.
Such a fraction to be displaced, after which the normal flow condltions are restored and the normal operation of the pipellne can be re-started, does not exceed, for indicative purposesj the value of 10-2G% of the~ voLum~e contalned~lns;de -the stretcn in question, and~depends on the nature of the~same fluid.
The l;quid ~luids~used~to re-start the operation oF pipelines ("re-starting fluids") can~simply consis~-t, ;n the case~of water-coal~m;~xtures, of water or water~
coal m;xtures,~ wh;le, ;n the case of the transpo~rt of heavy crude o;ls, the ~re-start;ng ~flu;d can be consti~tu~t~ed~ b~y~L;ghter o;l~f~ract1on~s,~ s~uch as,~ e.g., gas oll and ker~osene, ~which~, among;others, can~also ~be used ;to ~feed the D;iesel~engines wh~ich drive the re-~st~art~;ng pumps,~` thus~ render;ng energet;cally self-~suffi~c;en~t the~;re-s~tartin~g s~tations. When ~electrical power~ls~avaiLabl~e~in_sltu, thé re-start~;~ny stat;on cdn ~ :~
~ ~be fed w;th e;l~e~ctr;cal powe~r. The~re-start;ng fluids a~r~e ~;st~o;red ;nslde`s~t~orage ta~nks~for~ mach ;re-start~;ny~

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station, and can be transported, from time to time, as batches, by means of the same main p;pel;ne~ or separate;y, by tank trucks.
Accord;ng to the process of the ;nvent;on, the operation of the p;pel;ne is re-started, when emergency condit;ons, and the blocking of the transported fluid, have occurred along the whole pipeline, by moving back~ards, starting from the terminal arrival station.
in other terms~ the pipel;ne stretch comprised between the last re-starting station and the arr;val terminal is re-started first, then, in unlnterrupted sequence, the immediately upstream~stretch ;s re-started, and so on, more and more upstream, unt;l the departure terminal ;s reached.
~The onerousness of tbe re-s-tart;ng operat;on depends to a large~ extent on~the nature of ~the transported ;luid; ~ f~or~merely indicatlve purposes, the ~time ~ ;nterv~al ~ elaps;~ng~between~ the;~ re-star-t;ng operations per~formed~;n succession at~ adjacent re-startlny~stati~ons ~;s compr;sed with;n the~range of from~
1 0 t o 20~hours.~
~ W~hen the ~mov~ement~of th~e whole~volume ~of~ flu1d conta;ned ;ns;de the p;p~el;ne ;s resumed, even ;f;w;th;
~a llmlted;~dlsplac~ement, f~rom~th~e~re-5~ta~rtlng~s~tatl0n~s~
~wh~;ch ~operate~w;t~h~a~reduce~d f~lou~rate as~compared`~to th~e norm~al`~flowrate~of the p;pel;ne~ the substantial ~dest~r~uctur;~ng;~o~f~the~flu~id`is~ o;btained, and th~e normal flu~d-dyn~amlc~ behavio~u~r~; t~hereof ;s res;tored~. AFter unbloc~k;~ng;~ th`e~fir~st~stre~t~ches~ of the ~pipqline,~ t~he;
operat;on ~o~f the ma~;n pumping stations can be ~, - :
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progressively resumed as ~ell.
The process accord;ng to the present inven-tion y;elds considerable advantages over the prior art, and, among them, the follow;ng features deserve a part;cular S ment;on.
The cont;nuous use is not necessary of pour point depressant addit;ves or of fluidizing products, wh;ch, on cons;der;ng the flowrate values normally des;gned for ;ndustr;al p;pelines, requ;re that many thousand tons of such products are purchased, handled and metered each year.
The operat;ng costs and ;nvestmen-ts for heat-;nsulat;on and heat;ng -the fluid, and/or the pipelines, are not requ;red.
The re-start;ng st~at;ons are, as reyards their s;ze and structure, much smaller than the main pump;ng stations, and simpler to be built and operated:~ they const;tute a moderate increase in the necessary investment costs, and their operat;ng costs are negl;g;ble. In fact, ~they~ are used f~or~ short t;me per;ods, an~d~only o~n occasion of emergency occurrences, and ~remain ~inactive during the norwal operat~ion and ~requîre~ the only scheduled serv;cing. An interesti~ng feature ~of~the~invent;on cons;sts~in that no reserve un;ts~ are~neoessary, oecause~-- as the re-start;ng - stat;on ;s use~d only in e~mergency s;tuations and for a ~short~ t~lme~per~;od ~and~f;or~a few tlmes ~only -- the norrnal~schedu~led serv;cing~makes it poss;ble the pumps to be kept in~ no~r~mal working~cond;t;ons.
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The present invention is applied to ~urla-~han Shan crude oiL pipeline in Popular Republic of China, schematically depicted in Figure 1.
The main data relevant to th;s transport installation ;s the follow;ng.
- Type of fluid crude oil con-tain;ng 7% of waxesj and with a pour po;nt of -20C.
- Rated flowrate: 10 m;llion t~year, equivalent to 1,380 m3;hour.
- Diameter: 2O ;nches - Length 491 km. ~ ~
Alony the p;peline two main pump;ng stat;ons are ;nstalled, respect;~vely at km 0 and km 148.4, w;th the foLlowing characterist;c~s:
- Number of pumps: 2 operating pumps + 1 reserve pump, :: :
connected in parallel; ~ ~ ~
- Pump type: horizontal centrifugal pumps driven by a variable-revolution-speed electrical motor;
~ -~Hourly flowrate:~ o90 m3/hour per each~pump;; ~ ;
- De~Livery~pressur~e~: 12~ba~r~g~aug~e;l7~2~1o~s Pa), - Installed po~w`er: 2,200 kW/yump, and~o~oOO;~kW;stat1on.
Acco~rd;ng to the i~nvention, along the~pipeline ten ro-start;ng stations aré~inst~al~led,~at hm~0, 5O,~100,~
148.4; 18~5, 240, 332.8, 380, 425~;460~respec;tively~
~ Each~ r~e-s;ta~rt~lng~s~tatlon~ can~del~iver L~1quid flui~ds with~a~ lowrate~of 100~m3/;hour with delivery pressures compr1~se~d; ~w1thln th~e~range of ~from 40 to~105 ba~r gauge 4~ 0.10~6~ Pa~ and~comprises~
a~ buried~concrete tan~k~of~2,000 m3 of capac;ty, in 30~ o`r~der-~to st~ore~the re startin~y flu;d, e.g~, gas o;l.

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9 ~ ~ 8 Said gas oil is transported with the sams pipeline, and is used ;n order to feed the eng;nes installed in the stat;on. It can be observed hera that the whole amount of stored fluid available per each re-starting s-tation corresponds to the displacement o-f the rluid contained inside the pipeline, over a distance or o km only, by far shorter than the lenyth OT the individual pipeLine stretches serviced by each re-starting station;
10 - a Diesel-driven stored-fluid-intake ver~tical centriTuyal pump~ which intakes the re-starting fluid from the buried storage tank and feeds it to the re-s l a r l i ng pump;
- a main, Diesel-driven, re-starting reciprocating 15 pump to~ap~ply the re-starting p~ressure to the pipeline stre~tch in question, with an~;nstalled power or 30~0~kW.
~ The stat~i~on~is very s~lmpLe;~f~rom des~i~g~n~ viewpoi~nt~
`` ;s ~e~asy~to be~bu;Lt,~ and~ls~equipped with the norma~
20 ~ in;struments ~ for~ ~ operat;ng and con~t~rolling~ the ~mach~l~nery, ~and~ f~necessary, can~also~be ~h;and-o p ~ r ~ ~ ~ d .

Claims (7)

1. Process for re-starting the operation of a transport pipeline containing fluids with very high values of yield stress, characterized in that said operation re-starting consists in restoring the normal fluidity of such fluids, and that said re-starting is carried out by charging to the pipeline, by means of a plurality or re-starting stations distributed along the same duct, an auxiliary re-starting liquid fluid, with a reduced flowrate as compared to the normal operating flowrate of the pipeline, and substantially with a pressure comprised within the design limits of the pipeline pressure, in order to generate the movement of the stationary fluid for a length not longer than 20% of the length of the partial pipeline stretch services by each re-starting station.

2. Process for re-starting the operation of a transport pipeline containing fluids with very high values of yield stress, according to claim 1, characterized in that said re-starting is carried out by operating backwards, starting from the terminal arrival station, re-starting first the operation of the stretch comprised between the last re-starting station and the arrival terminal and then, in uninterrupted sequence, restarting the operation of more and more upstream stretches until the departure terminal is reached, with the normal operation of the pipeline being resumed when the movement has been restored of all the fluid inside the pipeline.

3. Process for re-starting the operation of a 13.

transport pipeline containing fluids with very high values of yield stress, according to claim 2, characterized in that the time interval elapsing between the starting-up, in succession, of adjacent re-starting stations is comprised within the range of from 10 to 20 hours.

4. Process for re-starting the operation of a transport pipeline containing a heavy oil fraction with very high values of yield stress, according to one or more of claims from 1 to 3, characterized in that said re-starting is carried out by using, as the auxiliary re-starting fluid, a light oil fraction, such as gas oil or kerosene.

5. Process for re-starting the operation of a transport pipeline containing a water-coal mixture with very high values of yield stress, according to one or more of claims from 1 to 3, characterized in that said re-starting is carried out by using water as the auxiliary re-starting fluid.

6. Process for re-starting the operation of a transport pipeline containing a water-coat mixture with very high values of yield stress, according to claims from 1 to 2, characterized in that said re-starting is carried out by using a water-coal mixture as the re-starting auxiliary fluid.

7. Pipeline for transporting fluids with very high values of yield stress in order to implement the process according to one or more of claims from 1 to 6, characterized in that said pipeline is subdivided into partial stretched between adjacent main pumping 14.

stations, and that a plurality of re-starting stations are installed so as to define shorter pipeline stretches, with each re-starting station being substantially constituted by a re-starting pump and a stored amount of an auxiliary re-starting liquid fluid, said stored amount corresponding to a volume equivalent to a fraction of the pipeline volume serviced by the re-starting station in question.