Instrumentation(Ch16)

8/13/2019 Instrumentation(Ch16)

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Chapter 16 - Instrumentation

• Types of control systems

Developments in Pipe Line Instrumentation •

Flow measurements• Proving devices

• Valves

• Acoustic line break detectors

• "Smart" pressure sensors

• Densitometers

• Pipe line samplers

• Pipe line monitoring systems

• Computer systems

• SCADA systems

• Catodic protection

• System design guidelines• Future trends

• Coosing te !igt Tecnology for ntegrated SCADA Communications

Types of Control Systems

#arly pipe line systems depended upon manual control and te use of tanks to maintain a pipeline system in balance$ %n long pipe lines tat re&uired multiple pump stations' a surge tank was

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used to catc e(cess flow or make up te deficiency$ #ngine speed was varied to maintain televel in te surge tank at an acceptable level$ Tis type of operation was fine as long as one gradeof product was being pumped' but was not satisfactory for batcing different products unlesstere was a tank available for eac grade of product andled$

t was obvious tat to build a tigt line system' one tat would not re&uire surge tankage' some

kind of automatic control system was needed to keep te booster stations in balance$ )efore tewidespread use of centrifugal pumps' engine speed *for reciprocating pumps+ was controlled tomaintain te suction and discarge pressure witin limits by sensing discarge and suction pressure and directing a control signal to eiter te engine governor or a control valve$ ,erecontrol valves are used wit reciprocating pumps' tey are connected to recycle fluid from tediscarge of te pump to te pump section$

,en centrifugal pumps came into use' te same type of control system was used e(cept tat tefinal control element was a control valve installed in te pump discarge line as sown in Figure-$ Te control system at origin pump stations normally controls discarge pressure or systemflow rate$ ntermediate booster stations re&uire a control system tat will control suction or

discarge pressure and in some cases' motor load$ An e(amination of te system line friction and pump ead curve will sow tat simultaneous control of suction and discarge pressure and igflow rate is not re&uired since te pipe line system caracteristics will not permit te coe(istenceof tese events e(cept in case of e(treme system upset$ As will be seen later' te control systemis configured so tat only one controller will be in control at a given time$

Tere are several modes of control available to te pipe line engineer$ .ode of control is temanner in wic control systems make corrections relative to te deviation of te controlledvariable$ Te modes of control commonly used by pipe liners include/

• Proportional position

•

Proportional plus reset• Proportional0speed floating

%ter modes suc as proportional plus reset plus derivative are more commonly used in te process industries$ Tis mode of control may' owever' be used in pipe line measurementstations were it is necessary to control te temperature of a eater used to eat te product being pumped or measured$

n te proportional0position mode' tere is a constant linear relation between te controlled valueand te position of te final control element$ ,it tis type of control system' te amount ofcorrection will be te same regardless of te amount of deviation of te controlled variable$ A

proportional0position control system will not necessarily return te controlled variable to tee(act value of te set point wen an upset occurs$ Tis may be satisfactory for some types ofcontrol systems' but not for pipe line pump station control$

Te offset deviation noted in te proportional0position system can be overcome by adding resetto te controller$ Te reset action will continue to correct valve position until no deviation occurs between te controlled variable and controller setpoint$



Proportional0speed floating causes te final control element to cange at a rate proportional tote amount of deviation between te setpoint and te measured variable$ Correction to te finalcontrol element will continue until tere is no deviation between setpoint and te measuredvariable$ Te amount of te correction signal is proportional to te amount of deviation betweensetpoint and te measured variable$ Tis mode of control will produce an e(act correction forany system load cange$

Proportional0position and proportional0speed floating are te control modes most commonlyused for pipe line control$

Figure - sows a typical control sceme for a single unit booster station$ A minimum controlsystem will include a suction pressure and discarge pressure controller$ %ter controllers sucas motor current controllers and a ramp generator may be connected in te control loop$

Figure 1. Typical control sceme for a pipe line booster station$

Te suction pressure controller is set to maintain te suction pressure above te low suction pressure sut down setting wile te discarge pressure controller limits te station discarge pressure to a value wic will not e(ceed te ma(imum allowable operating pressure$ Te outputfrom eac of te controllers is fed to a signal selector wic will select eiter te ig or lowsignal' depending upon te control action selected' and direct tis signal to te control valve$Some control systems are designed so tat a selector module may not be re&uired' but tey willinclude a means for selecting te proper signal$ n a properly designed system' te transfer ofcontrol from one controller to anoter will be smoot and bumpless$

A ramp generator is used for ramping te control valve to a new position$ Te control valve may be closed wen te station is at rest' and upon station startup te ramp generator will cause tevalve to open slowly until one of te controllers is in control of te system or until te pump is



"floating" 0 tat is' operating at a condition were neiter suction nor discarge pressure controlis re&uired$ t may also be used to ramp te valve partially closed prior to unit sut down tominimi1e system upset$ Tis feature is desirable in a multi0unit booster station since wen oneunit sut down' te remaining operating units will be operating momentarily at a no0flowcondition$ Tis condition will continue to e(ist until te system pressure decays to a value e&ualto te discarge pressure of te remaining units$ !amp speed sould be ad2ustable so tat it can

be fine0tuned to te system re&uirements$

Te control valve action' i$e$' valve opens on increasing signal or valve closes on decreasingsignal' will determine ow te controller action is set$ Te suction pressure controller will be setopposite of te discarge controller$

Altoug now sown' te control system is usually e&uipped wit a manual3automatic switc sotat te control valve may be controlled in case one of te controllers fails or during systemmaintenance$

Te controllers may also be e&uipped wit remote setpoint control so tat controller setpoint may

be varied from a remote location to accommodate a canged condition in te system$

Figure 4 sows a control system for a multi0unit booster station$ Te control systems for single0and multi0unit stations are identical e(cept tat in a multi0unit station te station pressuretransmitters are located so tat one control system will suffice regardless of te number of unitsinstalled$ t sould also be noted tat one control valve serves bot units$

Figure 2. .ulti0unit booster station$

Te control valve may be operated electrically' pneumatically' or ydraulically$ 5ydraulicsystems offer te advantage of being able to operate te valve at a ig rate of speed at virtuallyany system load$ .ost ydraulic operators are e&uipped wit dual cylinders' and may be



e&uipped wit &uad cylinders were te system loads are ig$ Te selection of control valves iscovered in Section 6$

Variable speed drives or controls are becoming an economically attractive coice for teoperation of pumps$ Te use of tese devices will virtually eliminate trottling e(cess ead andte attendant energy costs$ Few systems are operated at a constant rate and terefore some

trottling is inevitable$ Also' systems transporting multiple grades will re&uire trottling to keepte system in balance as te batces move troug te system$

.ost control systems used for pipe line control are made up of standard control system elements$Some applications may' owever' ave uni&ue caracteristics tat will re&uire a system tat is 2ust a little bit different from a standard control system$ .ost of tese types of systems can be put togeter using standard modules' or a programmable logic controller may be used$

n Section -7 0 8i&uid 5ydraulics' te discussion on ydraulic gradients illustrated a case werea ill re&uired maintaining back pressure at te end of te line$ t was sown tat te amount of back pressure re&uired varied wit te system flow rate$ A conventional back pressure controller

and control valve installed at te end of te line and set for te worst case *ma(imum back pressure+ would work9 owever' it would not be economical since more pressure tan re&uiredwould be maintained at te iger flow rates$ Te pressure at te ill could be telemetered to teend of te line were it could be used as te measured variable for te controller$ Te setpointwould be set to te amount of pressure re&uired at te ill wic in tis case is 4:: psig$ Tistype of control system would not penali1e system economics since it would always maintain tere&uired 4:: psig at te ill$ 5owever' it would re&uire te installation and maintenance of atelemetry link$

Anoter option is to use a relationsip controller wic would not re&uire telemetry of te ill pressure$ Te relationsip controller would use te system flow rate to drive te setpoint of a

pressure controller$ Te controller would need to be e&uipped wit limit stops so tat te setpointcould only be canged between te upper and lower limits$ 5ig system flow rates would drivete controller setpoint to te minimum allowable value of back pressure and low system flowrates would drive te controller to te allowable ma(imum limit$ ntermediate system flow rateswould drive te setpoint to an intermediate pressure setting$

,it today;s tecnology and ardware' te control engineer is limited only by is imagination insolving simple as well as comple( control problems$ Programmable logic controllers *P8C+ andself0tuning controllers are being used more and more in te control of pipe lines and pipe linestations$ Control systems are being incorporated as a part of te P8C$ ,it te addition of amodem and a suitable communication protocol' te P8C may also be used as te remote terminalunit *!T<+ for te supervisory control and data ac&uisition system *SCADA+$

Te following article' "Developments in Pipe 8ine nstrumentation'" by Dr$ !$ A$ Funess' is ane(cellent discussion on te state0of0te0art in pipe line instrumentation$

Developments in Pipe Line Instrumentation

R. A. Furness' Center for Flow .easurement' Department of Fluid #ngineering andnstrumentation' Cranfield nstitute of Tecnology



Abstract

Pipe lines are an integral part of te world;s economy and literally billions of lb wort of fluidsare moved eac year in pipe lines of varying lengts and diameters$ As te cost of some of tesefluids and te price of moving tem as increased' so te need to measure te flows moreaccurately and control and operate te line more effectively as arisen$ nstrumentation and

control e&uipment ave developed steadily in te past decade but not as fast as te computersand microprocessors tat are now a part of most large scale pipe line systems$ t is te interfacingof te new generation of digital and sometimes "intelligent" instrumentation wit smaller andmore powerful computers tat as led to a &uiet but rapid revolution in pipe line monitoring andcontrol$ Tis paper looks at te more significant developments from te many tat ave appearedin te past few years and attempts to pro2ect future trends in te industry for te ne(t decade$

Introuction

%ur need to transport fluids from te point of production to te area of end use as led in te pasttwenty0five years to a large increase in te number of pipe liens being designed and constructed$

.any of tese carry a1ardous or to(ic products' often troug areas of ig environmentalsensitivity or close to centers of population$ Te cost of oil production rose sarply to over =7> per barrel before te recent sudden fall' and te cost of manufactured cemicals and industrialgases as also risen in te same period$ ,it te need to safeguard te cost of te transportedfluid and also te population and te environment close to te pipe line' on0line monitoring andin some cases 4?0our surveillance is now becoming routine in many systems trougout teworld$

,en one sees maps of te pipe line network in some parts of te world' te number and lengtof pipe lines is staggering$ At te recent Aberdeen conference' one speaker gave 2ust two of temany statistics tat can be produced to sow ow vast te network is$ %ne line runs from central

Canada eastwards to te Toronto3.ontreal area$ Tis line alone as >? million s&uare meters of pipe surface to survey$ Also in -@? some B billion barrels of oil and ?$> billion barrels ofmanufactured li&uids were transported in pipe lines in te <SA$ Te actual world volume of pipeline fluids is many times tis amount and te value immense$

Pipe lines are not new' indeed almost >: of te world;s total network is 7: or more years old$Tese ave been sub2ect to corrosion and wear and some ave carried fluids for wic tey werenot designed and oters ave been operated outside design limits$ .onitoring and controlsystems terefore are not 2ust re&uired for measurement purposes but also for integritysurveillance and on0line fault diagnostics$ Some of te many developments tat can be applied inmodern day line operation are briefly reviewed and some design pointers for teir integrationinto complete systems are given$

Flow Measurements

Flow measurement is te most important process variable in te operation and control of pipelines$ Simple line flow balances are used to ceck for discrepancies on ourly' daily' or weekly bases$ Tis may identify tat a problem e(ists but flowmeters at te end of a line will not identifyte location of te problem$ For tis' pressure transducers along te pipe line will be re&uired



togeter wit te flowmeters to establis were te pressure gradient as deviated from tat predicted from te basic e&uations of fluid flow$ Tese e&uations use pressure3flow values atinlet and outlet to confirm te line is in balance$ Suc a simple system is described in more detaillater in tis paper$

.ost flowmeters sold are of te differential pressure type and of tese te predominant meter is

te sarp0edge orifice plate$ .any undreds if not tousands of tese are installed on pipe linesaround te world to measure gas' li&uid' and mi(ed fluid streams$ ,it te arrival of te modernflow computer tese types of flowmeter ave almost received a new lease on life$ %ne fact notappreciated toug is te total measurement uncertainty of suc devices$ Fre&uently we read performance claims of :$> and better for tese pipe line metering systems$ Tis is verydifficult to believe wen it must be remembered tere is in uncertainty of :$B on te data usedto derive te flow coefficient in te basic calculation$ ,en all te oter variables tat can affectte uncertainty of measurement are included' suc as fluid composition canges' ambient and process temperature and pressure variations' errors in temperature and pressure measurement'A3D conversion and computational errors' to name but a few' ten it is unreasonable to e(pectuncertainties muc better tan - FSD$ f wide flow rates are also e(perienced ten te

uncertainty of te low flow determination could be > FSD or worse$ Tis makes teir use asaccurate pipe line flow devices somewat &uestionable' altoug tey are very common in temetering of natural gas in tis country$

Turbine flowmeters also find wide0spread use in pipe line applications' particularlyydrocarbons$ Tey all suffer from bearing limitations' owever' and tis means tey re&uireregular calibration to ensure good performance$ !ecent developments ave included twin rotordesigns' were measurements of te speed of bot rotors simultaneously as led to te meter becoming self diagnostic$ Tese meters can now detect teir own bearing wear and alsodeviations from initial calibration due to blade damage$ Te same on0board microprocessors canalso ceck te integrity of te data generated by te turbine meter a well as providing alarmoutput for loss of signal and oter problems$

Two different designs ave been applied in pipe line systems wit good success$ Te first design'from Euantum Dynamics and sown in Figure -' as two rotors wit two independent sets of bearings$ Te saft and slave rotor ride on one set of bearings wit te measurement rotorupstream of te slave motor riding on a second set of bearings on te rotating saft$

!elative motion between te indicating turbine and te saft terefore remains close to 1ero$Tis plus a drag0free !F pick0off results in an almost frictionless flowmeter$ Sould te saft ormeasuring motor bearings being to war' tis will result in a cange in te ratio of te rotor speedsfor a fi(ed flow rate$ Tus bearing wear can be detected on0line and action taken$ Tis design ofmeter is also different from all oter turbines because of te slender center body and a(ially sort



blades$ )ecause friction is very low te fluid dies not ave to be accelerated past te rotor toobtain sufficient tor&ue to overcome retarding forces$ Tus pressure drop is also low and pressure and temperature measurements can be carried out at te point of volume flow ratedetermination$ Tey ave been sown to be outstanding meters for leak detection purposes ongas lines$

Te Auto Ad2ust turbo0meter from !ockwell nternational is also self0cecking but is in additionself0ad2usting$ Te design sown in Figure 4 is &uite different from te Euantum meter$ Terotors are ne(t to eac oter wit te measuring rotor downstream of te main rotor$ constant performance is obtained because te sensor rotor detects deviations in blade angle of fluidleaving te main rotor$ ,it a very small angle' te sense motor runs at a muc lower speed tante upstream rotor$ Performance is ten given by te difference in te speed of te two rotorsrater tan from a single rotor$ ,en bearings begin to wear' te main rotor will spin sligtlyslower and so te e(it angle will also cange$ Tis is sensed by te second rotor' wic ad2ustsspeed by an e&ual amount$ )earing wear is terefore automatically ad2usted out$ Tese metersave been installed in many gas transmission lines in ort America wit generally very good performance$ Te Euantum meter' on te oter and' is a newcomer to pipe line applications' but

personal first and e(periences in a gas pipe line were favorable$

Pipe lines are generally operated on material balances *mass+ but most flowmeters are volumetricdevices$ Tis means tat for pure fluids measurements of temperature and pressure plus anappropriate e&uation of state are re&uired to enable density to be determined$ For mi(tures of gasor li&uids or indeed as an alternative to te above metod on0line densitometers are used$ Te product of volume flow and density gives mass flow$ <ntil recently pipe line mass flowmeterswere not available but recently a number of Coriolis type direct mass meters ave appeared$Tese ave found slow but steady acceptance in industrial applications and are beginning to find

applications in pipe line monitoring$ %ne suc design is sown in Figure 7 recently four oterdesigns were launced at te T#!GA.A trade sow in ,est Hermany$ Tis type of meter isapplicable to bot li&uids and gases and some ave been used on li&uid3li&uid and gas3li&uidmi(tures but wit mi(ed success$ Tey are currently undergoing rigorous evaluation and analysisin many cemical' process' and small pipe line applications and indications are tat tey arestanding up well in some applications but not oters$ Accuracy is around :$> of reading or better and tey do not need additional measurements to determine flowing density$ ndeed bymonitoring te output signal tey can be used to indicate density as well as mass flow$ Te



output is in digital and3or analog form making tem compatible wit many types of flowcomputers$

Direct mass measurement as one main advantage over volume measurement' since anintegration of te signal over a period of time will indicate te pipe line fluid inventory directly$Te disadvantage for pipe line operators is te current si1e range tat is available' te largest being ->: mm bore$ As most ma2or pipe lines are in te range of >::04'::: mm bore' tesecoriolosi meters must be installed in parallel to meet te larger applications$ .ass metering isrelatively new and te ne(t decade sould see a steady increase in sales and acceptance of tesemeters$

Te ideal flowmeter is one wic is installed on te outside of te line but gives te performanceof te bet flowmeters installed inside te line$ <ltrasonic time0of0fligt meters are slowly

developing toward tese apparently conflicting but demanding criteria$ .ultiple beam systemsave been installed on many pipe lines' te most notable of wic is te Alaskan oil pipe line$Some 47 meters eac ? ft in diameter look for gross imbalances along sections of tis importantinstallation$ Tey permit te early detection of product spillage and also indicate te section ofte line giving te problem$ Tis system as been operating since te late -@6:s wit millions of barrels of fluid being sipped troug te line in tat time$ #(perience of tese meters as beenvery favorable in tis application$

<ltrasonic meters' like most of te modern types of meter' were initially oversold but te main problem as been to convince users tat te two types available' te Doppler and transit timemeters' are completely different animals' tat bot use ultrasonic to infer flow$ ,en ultrasonic

meters are mentioned' people tend to tink of doppler types$ .ost of tese are relatively crudeflow indicating devices$ Te modern clamp0on transit time meter and indicate flow to around4 of reading or better depending on design' compared to te average Doppler meter wic isa > FSD device$ Te advantage of ultrasonic metods is te negligible eadloss and teability to install oter portable or dedicated systems witout line sutdown$ t is surprising tatmore ave not been installed in pipe line systems' but as discussed later in tis paper' tis is ane(ample of te conservatism often sown in te application of new tecnology$



Flow measurement as really been transformed by te application of digital tecni&ues to on0linesignal analysis$ .uc more data tan 2ust flow rate is now available and wit te latest systems'operators are taking advantage of te new generation of micro and flow computers to control andrun teir pipe lines$ t is unusual tese days not to install a local flow computer wit eacflowmeter' wit temperature' pressure' and differential pressure signals being fed into temacine for local "number cruncing$"

Proving Devices

Te problem wit all flowmeters is tat tey are affected to some degree by te fluid tey aremetering$ For te least uncertainty of measurement' te flowmeter' watever type is cosen'sould be calibrated on0line under actual operating conditions$ n te past tis was performedwit large and e(pensive pieces of ardware called ball provers$ Tese ad to be large to provideade&uate volume witin te calibrated section of pipe to ensure sufficient resolution andaccuracy for te flowmeter tey were intended to calibrate$

Te latest generation of proving devices' called compact provers' are essentially time3volumecalibrators tat eliminate te - count error present on teir "big broters$" t is pysically mucsmaller but wit te same or better performance$ Tere are currently -: different versions on temarket' of wic five are available in te <G$ Te prover' one design of wic is sown inFigure ?' uses pneumatics to launc a piston' wic is ten driven by te fluid troug acalibrated camber' displaying a known volume$ Te key to teir e(cellent performance is tedata ac&uisition system illustrated in te lower part of Figure ?$ As te piston passes troug tecalibrated section' pulses from a ig fre&uency internal clock are counted to determine te time'A' to displace te calibrated volume D$ %nce te counting as started a second internal time isgated to respond to te first input pulse from te flowmeter$ Tis timer ten records te timeinterval ) to collect C wole flowmeter pulses$



Figure !. .odern compact meter prover *courtesy )rooks nstruments+

During te proving run tere were ten C ( A 3 ) meter pulses and te meter factor is found bydividing tis count by te volume displaced/

G I *A 3 )+ ( *C 3 D+

Some designs are unidirectional and some bi0directional$ n te unidirectional types pneumaticsor ydraulics launc and retrieve te piston9 in te bi0directional designs te process fluid is usedto drive te piston$ %ter version incorporate double0barreled liners to remove te need for pressure compensation and also reduce te effect of temperature variations$

n one proving run tere may be as few as -:: pulses collected' so timing error must beminimi1ed$ f te pulse output is fre&uency modulated' from a positive displacement bulk meterfor e(ample' ten tese few pulses may not represent a true statistical sample of te pulse trainand multiple passes may be re&uired$ Alternatively' pulse interpolation tecni&ues may bere&uired' several of wic are applicable to give te accuracy re&uired$ Tese are currently tesub2ect of intense activity in standards committees wo are producing a new general code of practice on meter provers$



Te compact prover is better suited to offsore platform applications and to field use onoperating pipe lines$ )eing portable is can be easily coupled in series wit te meter to be proved$ A suitable coice of materials allows proving on a vast range of fluids to be carried out$%perational e(perience as generally been favorable and several direct comparison tests wit ball provers ave sown te new generation of compact provers to be superior in most respects$Teir acceptance toug in custody transfer applications as been slower tan ad been

e(pected' even toug several countries ave certified some designs for ,eigts and .easuresapproval$ Some international calibration centers are now using tis tecnology for routinetraceable calibrations and confidence is growing as operational e(perience is accumulated$

)y far te most economic metod of measuring flows in pipe lines is to install pairs offlowmeters in custom0designed metering stations$ )y monitoring te ratio of te outputs' ameasure of ear or drift is obtained$ Provided te outputs of te meters remain witin preset tigtlimits' uncertainty of flow rate measurement is low$ Te disadvantage of tis on pipe lineoperation is tat tere are two meters to be maintained and cecked and sould faults develop te pipe line may ave to be sut down more fre&uently' unless by0pass loops are provided wen temetering station is designed$ Turbine meter and PD meters wit pulse outputs give te best

performance' particularly if tey are regularly proved wit eiter a dedicated or portable proverof te type described$

For large bore lines' particularly gas lines' te proving of flow meters is very e(pensive if it can be performed at all$ Te ma2ority of installations rely on orifice plates built to te standards$!ecently a new ig0pressure gas proving station was completed by )ritis Has at bisopAuckland$ 5ere turbine and oter meters can be calibrated in actual operating conditions onnatural gas$ Suc an installation as previously only been available in 5olland were gascalibration standards ave been developed to a ig level of confidence over many years of patient data collection$ ,it te new )ritis Has facility we sould see te ability to cross ceckour own standards against international standards$

Valves



Valves are an important part of any pipe line systems$ Tey come in many sapes and si1es and perform numerous functions$ Some act as primary flow regulators' some as ceck valves for flowdirection control' and some as pressure reducers for safety purposes$ Developments avecentered in two main areas' materials and actuators *bot simple and intelligent+$ %ne of tedevelopment sources is simple ad intelligent+$ %nce of te development sources is again te ort Sea' were te demand for better reliability' better in0field performance' and reduce cost

ave forced many manufacturers to look critically at designs$ As one of te most e(pensivecomponents in te valve is te body' effort as been concentrated in tis area' bot in terms ofnew materials to meet more demanding speculations and of &uality and price$ Tese last twoitems are critical to te survival of many companies and in a market sector tat is growing slowlyor even declining' as suggested by one leading instrument 2ournal' valve body cost savingsowever small may mean te difference between life and deat$

Slimline butterfly valves ave recently been introduced by a number of manufacturers to meette demand for lower cost reliable devices$ Tese can be installed between e(isting pipe lineflanges$ Tey can be manufactured in a wide range of materials wit special seal materialscosen for te most demanding of duties$ ,it suitable actuators even valves as large as B:0in$

diameter do not take up muc space$

%ter designs of control valve provide straigt troug flow patterns to minimi1e flowturbulence' vibration' and noise$ Sould a pipe line be fitted wit acoustic line break detectors*see section following+ line noise will be important in setting te sensitivity of te detectors andence te type of valve fitted will be important to system operation$ %ter operational problemssuc as "unting" in globe valves ave prompted te designers to work more closely wit teoperators to define full specifications$ Pipe line operators often run teir systems wit rapidtransients present$ Demand canges of bot a positive and negative value can sub2ect te controlvalves to very severe step canges in flow and pressure and initially many conventional valvescould not witstand suc treatment$

Tis were intelligent sensors on pressure relief valves ave come into teir own$ Tey are ableto give better speed of response and meet some stringent operating criteria$ Pressure may alsorise gradually due to a steady rise in line temperature as well as te raid transient caused by blockvalve closure or pump failure3surge$ Tese transients travel at sonic velocity and even tent ofseconds advantage in operating te valves can affect safety on te line$ .icroprocessor0baseddetectors fitted to te valves can be used to sense line conditions and provide suitable inputs tote valve$ n some advanced designs manufactured in te <SA it is possible to programintelligence into te unit to begin closure3opening based on measurements of an anticipatedcondition suc as pump surge furter down te line$ f te signals from one part of te line can be transmitted to oter sectors' corrective action can be taken before safety blowoff valves areactivated' releasing product into te environment$

A pipe line pressure relief system based on tis principle is sown in Figure >$ A strain gaugesensor fitted on te suction side of te compressor is monitored by a microprocessor wiclineari1es and conditions te signal$ Te microprocessor operates a pair of solenoid valves actingte pilot mode wit te line pressure acting as te actuating medium$ )y switcing solenoids terelief valve can be opened' closed' or programmed to act as a regulator at te pressure trapped inte valve body$ Suc a system can provide ade&uate response to step canges on te order of 6: bar in 7 milliseconds$ n tis system te status of te pipe line pumps is provided as an additional



occurs troug corrosion or tird party intervention' ten tere are instances were vibrations ine(cess of 4: k51 are produced$ Tese fre&uencies are in te ultrasonic range and can be detectedwit suitable transducers$ Suc devices ave been made portable so tat pipe line crews canclamp tem at any point along te line to ceck for noise$ )y noting te signal strengt' tesource of te problem can be pinpointed$ )ritis Has as used tis principle to develop one oftwo types of pipe line monitoring tool tat is described later$

A similar tecni&ue' toug based on a different principle' is te acoustic "wavealert" monitor'toug more correctly called a negative pressure wave detector$ Te central element is a pie1o0electric sensor tat gives an output wen stressed$ ,en a line rupture or leak occurs' tere is asudden drop in line pressure at te source of te problem followed by line repressuri1ation a fewmilliseconds later$ Te rarefaction wave generated by tese events moves away from te leak in bot directions at te speed of sound$ Te pipewalls act as wave guides enabling tis wave totravel for great distances' attentuating in amplitude as it does so$ Sensors placed at known pointsalong te line can detect te passage of tis pressure transient$ From te triggering of internaltiming circuits and a knowledge of te prevailing acoustic velocity for te line' te leak locationcan be calculated$

Figure 6. egative pressure wave "wavealert" monitor *courtesy Spectra0Tek+

Suc devices are particularly useful for detecting large breaks in lines very rapidly$ Te transientwave moves at - mile per second in li&uids and - mile in > seconds in gases$ f te detectors are

positioned at intervals of 4 to > miles in critical areas' ten te break is detected in a fewseconds$ f te output signal is fed into a computer' ten tis can be used to trigger block valvesto isolate te problem section of te line very easily$ Figure B sows one furter developmentwere te instrument can be made to cancel line noise and use te full measuring capacity of tesensor for signal detection$ Tere is' owever' te practical problem in setting te backgroundtresold correctly$ Pipe line noise is generated by bends' valves' pumps' regulators' etc$' and itis crucial to be able to remove tis background noise to avoid unnecessary line sutdowns wente line noise e(ceeds te detector setting$ Te !eynolds number of te flow as been sown to be te important parameter in installations bot in te <SA and te <G$ #(perience as also



applying new tecnology tis is one section of te instrumentation industry were microelectronics will ave a great future influence' particularly wen used wit te computer and realtime models$ Tis will give te pipe line operator more confidence and more information on botte condition of te instrument and te accuracy of te measurement$ nterfacing problemssould also reduce$

Anoter type of pressure transducer tat looks promising for future applications in pipe lines iste vibrating wire sensor$ %ne suc instrument from Fo(boro as sown considerable reductionin si1e and manufacturing costs from oter sensors$ A tungsten wire encapsulated inside a cell isvibrated at its natural fre&uency$ As pressure is applied to te silicon barrier diapragms' tiscanges te tension in te wire and so alters te resonant fre&uency$ Tis is sensed and amplifiedand gives te transducer e(cellent resolution caracteristics$ Pressure and temperaturecompensation is accomplised witin te cell' and altoug te instrument is not strictly a"smart" transducer' it does possess basic caracteristics wic sould be attractive to pipe lineapplications$

Densitometers

Continuous ig accuracy on0line density measurement as become increasingly important in pipe line operations$ Te composition of natural gas and ydrocarbon streams are different fromeac production well and assumptions about constant composition cannot be made$ n fiscalmeasurement and were te energy content of te stream is re&uired' a knowledge of density isessential' particularly for gases$

!ecently great strides in te development of rugged on0line densitometers ave been made witte introduction of te vibrating element transducer$ Tis type of instrument is fast becoming teindustry standard for pipe line applications$ %ne suc instrument is sown in Figure 6$ Te

operation is very simple' being based on te spring mass principle$ A flat plate' cylinder' or forkis maintained at resonance by an electronic circuit$ A fluid flowing over or troug te vibratingelement adds to te vibrating mass and causes a decrease in natural fre&uency$ Te iger tefluid density te lower te fre&uency and vice versa$ )y calibration using fluids of preciselyknown density' te relationsip between fre&uency and density for eac element can bedetermined very accurately$ Te caracteristic e&uation for most vibrating element densitometersis of te form/

p I At4 L )> L C

were A' )' and C are calibration constants' t is te period of oscillation' and p te fluid density$

%nce te constants ave been determined by te laboratory tests' te caracteristic e&uation can be programmed into te system computer for on0line calculation of density$ n some cases it may be possible to ceck te performance in situ by passing a fluid of known density troug tetransducer$ Tis will establis if tere as been any drift from te original manufacturer;scalibration and will elp to minimi1e te uncertainty of te operating system$ Te latestgeneration of instruments can approac accuracies of :$- of reading under ideal conditions'and are typically :$4 of reading under actual operating conditions$



Figure ". Pipeline densitometer *courtesy Solartron nst and Control and nstrumentation+$

,en used in con2unction wit orifice meters' for e(ample' te flow troug te densitometer ismaintained by taking fluid from a tapping downstream of te orifice and feeding tis back at tedownstream tapping of te plate$ Tis metod' known as pressure recovery' is very popular in pipe line applications' and a typical installation is sown in figure $ %ter metods including pumping a small percentage of te stream flow troug a flowmeter and densitometer beforefeeding back into te main stream$

Figure #. Standard installation for pipeline density measurement *courtesy P manual section B$4 ov -@7+$

Tere are oter types of fluid densitometer suc as nuclear or force balance but tese do not findte same widespread use on pipe lines as te vibrating element devices$ Te coice of transducerwill depend on te application since fluid properties' flow rate' sensor element design' andmaterials of construction all influence te type of densitometer used$ For e(ample' if te fluidcontains solid particles or is of a "dirty" nature' ten straigt pipe vibrating cylinder elements are



more likely to give better long term performance$ Te actual elements are manufactured in awide range of materials$ i0Span0C issued for ig accuracy metering and tis material also asa low temperature coefficient$ Stainless steel' on te oter and' as better corrosioncaracteristics and is ceaper to use$ .aterials suc as astelloy C046B offers e(cellent resistanceto te most corrosive of fluids$

t is probable tat te use of tese instruments will increase as general e(perience on pipe linesas been favorable$ Tey must' owever' be operated in single pase fluids as te presence ofeiter droplets in gas lines or vapor bubbles in li&uid lines will produce erroneous outputs ofunknown magnitudes$ Te straigt tube cylinder0type designs could be used in low second paseconcentration applications if te densitometer is installed vertically wit upward flow$ Tis willensure closer appro(imation to omogeneous flow$ #(perience as sown' owever' tat two0 pase flows are significantly more troublesome to andle and te output of te transducer sould be used wit some caution$

Pipe Line Samplers

%il lines' particularly crude oil lines' often contain significant amounts of water$ Tis as to bemonitored and allowed for wen te oil is being metered for custody transfer or ta(ation purposes$ t is a growing and important area of pipe line tecnology to install on0line samplersfor te measurement of water content' and work is also underway to use statistical metods toestimate ow many samples are representative of te composition of te flowing stream$ Tereare currently two different basic metods being used on pipe lines to monitor te water content$Tese are te capacitance metod and grab samplers$

Te first metod is based on te fact tat te difference in te relative permittivities of water andoil makes a capacitive transducer sensitive to te water content flowing between two electrodes$n te past tree years' commercial instruments using tis metod ave appeared and ave

attracted a great deal of interest' particularly in ort Sea applications were water content could be a problem$ Tey are basically also non0intrusive sensors wit te potential for in0lineinstallation$ Tere are two basic designs' by0pass and main line types$ )y placing electrodes in a bypass line form te main flow' a measure of water content in te main stream can be made$ Teaccuracy of tese meters' owever' is igly dependent on te e(tent to wic te flow in te by0 pass loop is representative of te main stream$ ,it te main line type tis source of error isreduced but te problem of flow regime effects still remains$ )ot types are also affected by tee(citation fre&uency' te salinity of te water content' te water droplet si1e' te tickness of teisolation material between electrode and fluid' and te omogeneity of te electric field$



Field e(perience as sown tat meaningful measurements can only be made if te flow regimeis constant and te best results are obtained wen te water is evenly dispersed and in dropletform in te li&uid$ Suc criteria often do not e(ist' wit slugs of water passing down te pipe lineat irregular intervals$ %ne suc commercial instrument is sown in Figure @$ Te importantfeature of te design is te sort distance between te electrodes wic results in ig sensitivity$Te electrodes are intrusive and terefore liable to eiter coating or blockage effects if te oil is

particularly dirty$

Figure $. Capacitance type water content transducer *courtesy #ndress and 5auser+$

Hrab samplers are devices wic take small volumes of fluid from te line at regular intervals to build up a representative sample of te mi(ture over a period of time$ An international standard

is currently being reassessed in te ligt of operating e(periences by many oil producers$ Tecurrent practice states tat -:'::: "grabs" of around - cc eac sould be taken and tere areguidelines load down for te sampling fre&uency and oter criteria to be considered$ ,ork atCranfield and elsewere as been evaluating tis standard and some of te limitations ave beenidentified$ As wit te capacitance devices above' tese may be installed eiter in te mainstream or in a by0pass loop$ %ften signals from flowmeters in te line are used to set tesampling fre&uency and te use of 2et mi(ers' bends' pumps' and oter devices tat produce amore omogeneous stream is desirable to improve performance and accuracy$

Anoter variation' te fast loop sampler' is used for continuous monitoring of water content andsuc a system is sown in Figure -:$ Te main advantage of te fast loop sampler is tat flow is

continuously e(tracted and te sampler itself seems to ave a smaller perturbing effect on testream caracteristics$ A great deal of work is currently underway to e(amine te effect tat te presence of te sampler probe as on te fluid dynamics of te stream$ #arly work as indicatedtat te flow streamlines around te probe ead do ave an influence on te droplet distributionand ence te oil3water ratio$ Tis is developing tecnology and we sould see increased activityin tis area as some of te older ort Sea fields begin to produce greater concentrations ofwater$



Figure 1%. Fast loop pipeline sampling system$

Pipe Line Monitoring Systems

%n0line inspection tecni&ues ave recently been developed to a very ig level ofsopistication$ )ritis Has as spent some -4 years and over -:: million pounds producingintelligent monitoring tools and te data analysis software to inspect line si1es from 0in$ to ?40in$ bore$ Tere is a demand worldwide for teir e(pertise and wit a potential - million miles plus of line to survey' tey sould be busy for years to comeM Tese inspection tools arecommonly called "pigs'" but sould be distinguised from te pipe line pigs used for cleaning'dewa(ing' batcing' filling' and de0watering of pipe lines$ feel te term monitoring tool sould be used because tese devices are very advanced indeed$

Two types of monitoring tool ave been developed' one based on magnetic sensors to detectmetal loss and te second based on ultrasonics to sense defects and cracks in pipe walls$ Te twoused togeter give a complete picture of te mecanical state of te pipe line$ Tey are notcontinuous monitors like pressure' temperature' and flow transducers but are useful insupplementing te on0line measurements$ f te lines are inspected at regular intervals' tencanges can be identified and possible trouble spots pinpointed$ Te data can provide a veryuseful istorical record to be used wen maintenance' cleaning or line repairs are needed$

Te magnetic pipe line inspection veicle operates by magneti1ing a sort section of line as ittravels down te pipe' driven by te process fluid$ f a section of pipe as suffered metal loss itwill cause a local distortion of te magnetic field$ Te data are recorded on -? cannel tape

systems and on0board software reduces te amount of data saved to only tat were significantsignals are noted$ Te information from te clean or undamaged pipe is not saved$ %nce teinspection run is complete' te data are analy1ed to identify te problem areas$ Te first off sorerun was completed in early -@B wen te !oug and .orecambe gas field lines were surveyed$Tis run took around > ours$ Te metod can be used to survey sections of line around 7:: kmlong altoug a more typical single pass would be around : km *>: miles+ * Monitor ' o$ 4'-@B+$



Te ultrasonic system is virtually identical e(cept for te transducers and detectors$ Te sensorsare ultrasonic operating in te megaert1 region$ Tey are oused in weels wic press againstte pipe walls$ Tey are pulsed from an on0board electronic control system and te reflectedwaves are stored in te same way as in te magnetic system$ )ot inspection veicles travel ataround tree meters per second$ f a line problem is detected by an imbalance betweenflowmeters for e(ample' te location could be confirmed by te acoustic detectors on board te

inspection veicle$ Tis could be arranged to alarm wen te detector output reaces a ma(imumand te precise location can be given by radio transmitters mounted on board$

%ter problems tat can be identified include corrosion' fatigue' ground movement *causing pipe buckling+ and tird party damage$ Te inspection center at Cramlington as already undertaken anumber of overseas surveys and as e(perience in gas' crude oil' kerosene and gasoline lines$Te impressive part of tese pieces of e&uipment is te engineering$ Tey ave to negotiate bends' valves' crossovers' etc$ and still protect te relatively fragile electronic recordinge&uipment from damage$ Tey ave been used bot in on0sore and off0sore lines wit e&ualsuccess$ Tis is one area of pipe line instrumentation development tat will be watcing overte ne(t two years as new on0line veicles are developed for pipe coat0less monitoring$

Computer Systems

t is in computer systems tat te greatest amount of data can be gatered' analy1ed and actedupon in te sortest amount of time$ .ost operators now ave some form of computer0basedmonitoring system employing eiter commercially available or custom developed software torun te system$ Programs can calculate te inventory of te line at any point in time and comparetis wit line measurements to provide independent cross cecking pipe line measurement data$Tese data can be transmitted by telemetry' radio' or telepone links to a central computer wiccontinuously monitors te "ealt" of te line$ Te central operations room is now more likely to

contain VD<s rater tan banks of process indictors and enables te engineers to ave rapid andaccurate updates of conditions along te entire line or network$ )y canging programs andsubroutines' a large number of tasks and functions can be performed easily' simultaneously andcost effectively$

Te many functions tat can be performed by computer based systems include/

• 8eak detection and location

• )atc tracking of fluids

• Pig tracking

• %n0line flow compensation

• !eal time pipe line modeling• nstrument data and malfunction cecking

• nventory and material balance accounting' etc$

Suc systems ave very rapid response' but are dependent on te accuracy of te input data to beeffective$ .ost computer based systems consist of two ma2or elements' namely/

-$ A supervisory computer plus associated software$



4$ A number of independent pipe line monitoring stations plus data transmission e&uipmentfor communication wit te computer$

Te comple(ity of te software' te number of input variables and te pipe line instrumentationselected' all influence te si1e and te coice of computer$ Some systems are capable of runningon te compact powerful micros suc as te ). AT or NT' wile te more comple( pipe line

networks will re&uire a main frame macine' possibly communicating wit satellite microsinstalled close to te measurement stations$ Te overall design time varies wit eac system butsould not be underestimated' especially if SCADA systems are being developed in0ouse *seesection following+$

At te most basic level te computer system can take flowmeter inputs to perform gross balancesalong te pipe line$ Suc a metod is very useful in identifying small but consistent loss of product from corrosion pits in te pipe wall$ Tis will arise if te integrated flowmeter outputsdiverge wit time wen te flow in te line is maintained constant$ f te line flow rate varieswit time ten imbalances are more different caracteristics or teir outputs may varynonlinearly wit flow rate$ A loss of product will be identified simply as te difference between

te steady state inventory of te system and te instantaneous inlet and outlet flows$.atematically' tis is/

were Delta*V+ is leakage volume' Vin and Vout are te metered inlet and outlet flows' and V- iste inventory of te pipe line$ Tis last term can be calculated as te average of te integratedflowmeter signals' but is dependent on variations in temperature' pressure' and many otervariables$ t is te use of computers tat enables tis last term to be calculated on0line and as afunction of pipe line elevation' pipe material caracteristics' process variations' pipe line fitting

loss data amongst te many variables$ Temperature and pressure variations cange bot tedensity of te fluid and te actual dimensions of te line$ )ot will give rise to sources or errorsin pipe line inventory if compensation is not included in te calculations$ Figure -- sows typicalinput data re&uirements for simple on0line inventory calculations$

Figure 11. nput data for computer inventory calculation$



At te oter e(treme' computer0based systems can control entire networks wit almost no needfor te operators to intervene unless serious problems arise$ Figure -4' taken from Oil and Gas Journal ' .arc 7' -@B' sows two e(amples from te many undreds tat e(ist of computer based monitoring$ Te benefits from te addition of real time pipe line models cannot beoverempasi1ed$ #ven wen operating transients occur' some commercial available programsenable problems to be detected$ An e(ample is sown in Figure -7$ 5ere a pipe line leak was

induced following pump failure and restart' and altoug te line was operating under iglytransient conditions' a material imbalance was detected$ !eal time modeling also allows density profiles to be calculated along te line$ t is fairly common practice to ave amounts of differentfluids in te same pipe line separated by a pig or spere$ Form te pressure drop3flowcaracteristics' te real time can track te progress of tese batces of fluids along te pipe$

Figure 12. #(amples of modern computer0based0pipeline systems *courtesy %il and Has Journal'-@B+$



Figure 1&. #(ample of transient real time modeling of pipelines *data from !ef @+$

SCADA Systems

!eal time computer modeling of pipe lines is a tecni&ue tat uses te full data gateringcapabilities of modern digital systems and te computational power of computers to giveaccurate "snapsots" of te line$ Tese enable te process operator to act &uickly wen problemsdevelop$ Te wole system is under te control of te SCADA *Supervisory Control and DataAc&uisition+ system$ Tis suite of programs polls te data stations on te line in turn' processeste data' controls te ruining of any pipe line model' and activates alarm and oter monitoringroutines$ .ost modern SCADA systems now include cecking routines to determine te validityof te data before any calculations are made$ t as been found tat te use of continuous &uality

and cross cecking of data improves te sensitivity of te system$ n addition to tese basicfunctions' some of te more advanced SCADA systems migt include predictive models toanaly1e "wat if" operating scenarios' include automatic startup and sutdown procedures' or provide optimi1ation routines for least0cost operating strategies$

Tere are a number of companies supplying standard SCADA packages' but te ma2ority ofsystems use custom written master routines to enable te commercially available programs to beused$ #ac pipe line is different' in line caracteristics' instrumentation installed' fluid properties'and need of te user to name but four re&uirements$ #(perience as sown tat te adaptation ofstandard software modules to specific needs takes muc longer tan is first estimated and alsogives rise to te greatest number of problems wen pipe line monitoring and control systems are

being commissioned$

t is very important for te software specialist to communicate directly wit te instrumentengineer wo specified te ardware' to ensure proper integration of te various parts of tesystem$ All too fre&uently tese two key members of te design team do not meet until te pro2ect is well underway$

Cat"o#ic Protection



Protecting a pipe line from corrosion is an important and e(pensive part of building and runninga pipe line system safely$ #very year' operators spend millions of lb on pipe coatings' gasdetectors' and sacrificial anodes to lengten te effective life of pipe lines by reducing te effectsof corrosion$ ,enever a steel structure of any type comes into contact wit te soil and3or watertere is almost certainly going to be corrosion$ Te structure' in our case te pipe line' dividesitself into anodes and catodes wit corrosion taking place at te anode$ Tis cemical process is

accompanied by te flow of electric current from one point on te line to anoter via te groundor water$ Tere are two basic preventative metods' sacrificial anodes and impressed currents*more commonly called catodic protection+$ Tis second metod is te more common and preferred metod for long pipe lines to identify te problem spots$

f a voltage is applied to a section of line' a profile of te current flow can be drawn up bymeasuring te current flow at intermediate points$ Tis gradient will sow areas were current isescaping' indicting a pipe defect' recently' tecni&ues using AC current ave been perfectedwere te magnitude of te current can be sensed inductively from above ground$

Tis metod can be used by pipe line crews walking te line and compact ligtweigt and0eld

units store data on te mecanical condition of te line in lengts as sort as five meters$ %n longcross0country lines te profile can be plotted at intervals of a few kilometers and from istoricalcross0comparisons' line canges can be identified$

Te ob2ect of catodic protection is to prevent current flow from te pipe$ Te operating principle is to apply a voltage in te opposite direction to te natural potential of te pipe$ Te pipe line voltage is made negative wit respect to eart so tat in teory te voltage and currentflowing from te catodic protection system balance te corrosion current tat would oterwiseflow$ !ecent developments include te use of solar panels and converters so tat energy from tesun is used to run te system$ %ne of te largest e(amples of tis new tecni&ue is te Sarir toTobruk in 8ibya were a 740in$ diameter' 7-@0mile pipe line is protected by suc an installation$

!emote station monitoring is easily accomplised wit te data being telemetered back to centralcontrol rooms many miles from te location of te actual installation$ Continuous automaticcontrol is often performed by means of tyristors$ Suc continuous monitoring is now routine onnew lines tat are being constructed and many older lines are being fitted wit some form ofcatodic protection system$ n most cases it is probably more important for te older lines to besurveyed tan te newer ones' especially were different fluids ave been transported over telifetime of te line$

System Design $ui#elines

Te aims and re&uirements of a pro2ect sould be specified as fully as possible as early as possible wen designing a pipe line monitoring system$ )ased on personal e(perience' tefollowing points sould be borne in mind$ Tey are not in any special order but do form a usefulcecklist wen starting system development$

-$ .ake te system as simple as is practical but do not underspecify te needs for te sakeof simplicity$



4$ Select instrumentation and pipe line e&uipment based on performance and not economicgrounds$ t is better to install fewer ig &uality pieces of e&uipment tan numerous poorones$

7$ #&uipment compatibility is important$ <se transducers' interface modules and oter pieces of ardware tat ave standard communications protocol$ Custom designedsystems invariably put u development costs and do lead to endless sources of trouble and

frustration$?$ ,en calibration and maintenance is due on te system do not put off$ ,erever possibleuse computer software to cross ceck data to aid in te identification of problems$ Tiswill elp to reduce operating and maintenance costs$

>$ ,ere system availability is important install devices tat are self cecking' selfdiagnosing or dual systems were appropriate$ Te initial cost may be iger but te benefits may be more economic in te longer term$

B$ Seek independent references' user e(perience or validation of te e&uipment cosen$.ost pieces of ardware perform differently in real applications to te specification publised under ideal conditions$

Future Tren#s

Future trends are in one respect easy to assess but it is very difficult to estimate were we will bein ten years time$ Tere is no doubt tat te application of computer based tecnology is going to precede at an ever increasing speed to bot te instrumentaiton and pipe line monitoring systems$Tere will be reliable and rugged intelligent flow' pressure' valve and oter ardwarecomponents appearing and once te initial over te application as occurred' te secondgeneration systems will give te desired performance' if previous istory is anyting to go on$

)y far te biggest problem facing te wole spectrum of control and instrumentation is to

develop standard protocols to facilitate communication between instruments$ Te situation at present means tat most if not all monitoring computers ave to ave a multitude of interface bo(es scattered troug te system to enable some elements of te system to "talk" to oters$ t issincerely oped tat manufacturers will develop microcomputers tat are able to communicatedirectly wit eac oter' wit full scale industrial process monitoring and control systems andwit te individual process sensors and associated controllers$ Te more elements tere are in tesystem' te greater te potential of line downtime$ Some moves are being made in tis area but itcould be some time before we see commonality of process protocol$

Te most rapid area of advancement as been in te microcip$ %il pipe line operators areamong te many people wo ave been watcing te development of 740bit macines$ Te ntel

:7B cip for e(ample' now contains over 46>'::: transistors in one piece of silicon$ Tisdevice as an operating speed of ? million instructions per second and as te capability toaddress over ? gigabytes of main memory$ %ne oter manufacturer is working on a computerwic will perform -: billion calculations per second' more tan -: times faster tan any otercomputer on te current market$ Suc macines will permit easier real time modeling of pipelines and will present te line operator wit many e(tra pieces of data on te condition of isline$ %f course complete new suites of software will be re&uired for tese fift generationmacines$



Conclusion

n suc a sort space' te paper cannot do 2ustice or even mention te vast number of recentinstrumentation developments tat ave ad an impact on pipe line operation and control$ Tetecnology of measurement and control is proceeding at an ever increasing pace and it is now&uite difficult to keep up wit developments in one area$ Pipe line instrumentation covers a very

wide spectrum from pressure and surge control to intelligent instrumentation and advancedsoftware$ Certainly our "friend" te microcip as totally transformed our tinking in te pastdecade and te poor pipe line operator now needs speciali1ed elp in specifying a system to meetis needs$ Tere are many options open' control using P8Cs at sources' control using software inte process computer' and manual override if re&uired$ ,it te development of e(pert systems'systems can not be programmed to almost preempt problems before tey arise and takeappropriate corrective action$ Te fle(ibility in te industry is vast but one ting is sure' youcannot control unless you measure accurately' and all too fre&uently te primary elements of asystem are under0specified compared to te rest of te system$ t is of very little use' fore(ample' to pressure and temperature compensate transducer outputs wen te basicmeasurement uncertainty is poor$

'ource

Measurement + Control ' Vol 4:' February -@6$

Re(erences

-$ Adams' !$ )$' ")uckeye moderni1es computer operations and SCADA systems'" Oil and

Gas Journal ' --' >>0>@' August -@B$4$ Anderson' C$ C$' "Te #kofisk pipe line control system'" Pipes and Pipelines

International ' 7:' o$ 7' 4-04>' -@>$

7$ )aker' !$ C$ and 5ayes' #$ !$' ".ultipase measurement problems and tecni&ues forcrude oil production systems'" Petroleum Review' -044' ovember -@>$?$ Cester' J$ !$ and Pillips' !$ D$' "Te best pressure relief valves/ simple or intelligent'"

InTech' >-0>>' January -@>$>$ Covington' .$ T$' "Transient models permit &uick leak identification'" Int Pipeline

Industry' August -@6@$B$ Davenport' S$ T$' "Forecast of ma2or international pipe line pro2ects for te -@B04:::

period'" Pipes and Pipelines International ' Vol$ 7-' o$ -' @0-?' -@B$6$ Furness' !$ A$' ".odern pipe line monitoring tecni&ues 0 Parts - and 4'" Pipes and

Pipelines International ' Vol$ 7:' o$ 7' 60--' and Vol$ 7:' o$ >' -?0-' .ay andSeptember -@>$

$ Gersaw' C$' "Pig tracking and location 0 Parts - and 4'" Pipes and Pipelines International ' Vol$ 7-' o$ 4' @0-> and Vol$ 7-' o$ 7' -70-B' -@B$

@$ Stewart' T$ 8$' "%perating e(perience using a computer model for pipe line leakdetector'" Journal of Pipelines' Vol$ 7' 4770476' -@7$

-:$ Taffe' P$' "ntelligent pigs crack pipe corrosion problems" Processing ' -@04:' September-@B$

--$ Tinam' )$' ".icros and sensors make smart transmitters'" Control and Instrumentation'>>0>6 September -@B$



-4$ ,illiams' !$ $' "Fundamentals of pipe line instrumentaiton' automation and supervisorycontrol'" I! Transactions' Vol$ 4-' o$ -' ?>0>?' -@4$

C"oosing t"e %ig"t Tec"nology for Integrate# SCADACommunications

Service was consolidated for both current and projected computer environments

)ernon *. 'terba' Director' System and Telecommunications' #nron Has Pipeline Hroup'Hroup Tecnical Services' 5ouston

Consolidation of overlapping telecommunications systems for #nron subsidiaries Florida HasTransmission' 5ouston Pipeline' ortern atural Has and Transwestern Pipeline eliminatedredundant facilities' and integrated information and communication re&uired for upcoming opentransportation services$

A wide0area communications study group *,AC+ was formed to define and documentoperations3SCADA communications re&uired for bot current and pro2ected computerenvironments$ A functional and tecnical design would be developed and documented' as well assupporting costs' an implementation timetable' and e&uipment specifications$

Te ,AC pro2ect was to meet te following re&uirements/

• Pipe line monitoring and control *SCADA+

• #lectronic flow measurement3custody transfer *#F.+

• !eal0time pipe line modeling and optimi1ation

• Compressor automation$

Since installing an advanced' integrated communication system re&uired a considerable capitalinvestment' te ,AC team cose not to take a "SCADA0only" view' since tis could missopportunities to effectively develop te e(isting communications network$

.ainframe data and voice communications were considered for consolidation$ Tis later becamea key factor in tecnical decision making$ Te pro2ect empasis was to identify were a cost0effective solution could simultaneously meet SCADA and oter voice and data re&uirements$

Te ,AC team consisted of #nron members representing bot organi1ational and tecnicale(perience$ n addition' a telecommunications specialist served as pro2ect leader and e(pediter

for formulating pro2ect metodology and imparting a "tird party" perspective to teamdiscussions$

+AC ,ethoolog

Te main se&uence of tasks was as follows/

• Define e(isting communications systems

• Define communication re&uirements



• Analy1e ade&uacy of e(isting systems

• dentify and evaluate tecnology alternatives

• #stablis communication "direction"

• Develop functional and tecnical design

• Develop pro2ect implementation scedule

Tis approac allowed te team to fulfill pro2ect ob2ectives in a sceduled time frame$ e(t' aninventory of communication circuits and facilities was entered into a PC database$ Te databaseincluded te latitude and longitude of all facilities' type and name of facility' communicationmedia' circuit number *were applicable+' and montly cost of leased communication$

Te database was used to develop communication system maps by interfacing wit a computer0automated design package$ Te maps identified redundant communication pats' duplication offacilities' ig cost communication routes' and oter details$ ,itout te maps; visual assistance'it is unlikely te team could ave &uickly developed an understanding of te e(tensive andcomple( communication system$

#(isting communication facilities included te following tecnology combinations/

• .ulti0drop and point0to0point leased lines$ Tese lines represented a ma2or cost

component9 many redundant circuits were identified and some surprising cost anomalieswere discovered$ For e(ample' te cost of a ?:0mi op from an !T< to te nearestmicrowave tower could cost more tan a 7::0mi op from te !T< to 5ouston$

• Company0owned 43B0H51 microwave system$ Te system provides voice and data

communications in Te(as inland and Hulf Coast areas' and incorporates voice circuits fortoll free calling at 7: field offices along te microwave system$

• !adio telemetry$ Tis system was used by ortern atural Has and Transwestern for

gatering SCADA data at master control units *.T<s+ or central data concentration

points in ma2or supply basins$ Te .T<s' in turn' relayed data to te SCADA master vialeased pone lines$• .eteor burst tecnology$ Te system "bounces" radio waves off te ioni1ed trails of

micro0meteorites entering te eart;s atmospere$ ortern atural Has used tistecnology' as a prototype' for nine !T< sites$

• VSAT tecnology$ Te VSATs *very small aperture terminals+ use satellites in orbit for

relaying radio signals$ Dis antennas of -$ to 4$?0meter diameter *B to ft+ werere&uired$ n -@B' Transwestern Pipeline installed -- satellite terminals to eliminate igconstruction costs involved wit leased line service at remote !T< sites$

Analsis o( echnolog

Sending data to te SCADA master re&uired bot sort and long0aul communication segments$Te segments could be establised by separate and independent tecnologies$

For e(ample' to aggregate data from several !T<s at a central ub' use radio telemetry or localleased pone lines' te long0aul tecnology *central ub to SCADA master+ could use eiter



leased pone line' microwave' or VSAT$ Tis re&uires a serial byte0oriented' SCADA protocolstandard' wic ad been in place since -@6$

To determine te best communication direction' various tecnology "mi(es" were appliedsystem0wide$ Tis provided comparative tecnology costs for te entire communication system$

Amorti1ed capital cost and %O. cost' including leased lines' were estimated for eac designcase$ Te cost analysis was conducted for bot current and pro2ect data density$ Te tecnologymi(es considered were/

Case 'hort-haul Long-haul

- radio leased line

4 radio microwave

7 radio VSAT

? leased line leased line

> leased line microwave

B leased line VSAT

6 VSAT VSAT

n bot current and future data density environments' te combination of radio telemetry andlong0aul leased lines *Case no$ -+ yielded te minimum montly cost$ Tis was followedclosely by radio telemetry and VSAT *Case no$ 7+$ ,en VSAT was used also as a mainframeconnection' Case no$ 7 became te minimum cost option$

n all cases' radio telemetry was te most economical tecnology for te sort0aulcommunication segment$ !adio systems bypass local telepone0company leased lines toeliminate escalating costs and service problems found in remote areas$ !adio is not alwaysavailable' due to fre&uency availability$

For e(isting facilities' te microwave network was a clear coice for long0aul systems' wit nocapital investment re&uired and a small incremental cost for adding circuits$ 5owever' wenconsidering microwave e(pansion' te situation canged significantly$ n te current data densityenvironment' increased microwave coverage *Cases no 4 and >+' did not 2ustify capital costsinvolved' due to insufficient data traffic$

n te pro2ected environment' iger data densities began to support te capital and %O. costsas microwave system capacity was fully utili1ed$ Tis moved microwave tecnology fromsevent to tird most favorable system' compared to oter tecnologies$

Henerali1ations sown in Table - apply to one system;s economics and topograpy' and are notintended to suggest te same options sould be cosen by oter gas pipe line companies$

able 1. Communication guidelines'hort-haul choices

-$ <se radio' if available' in a radio ub arrangement$4$ <se leased lines if economically available' connected to a ub or te ome office site$7$ <se VSAT only at remote sites wen te above options are infeasible$



Long-haul choices

-$ <se e(isting microwave' wen available$4$ <se VSAT for sites wit multiple applications$7$ <se single0application leased lines wen te montly cost is less tan =?>:9 oterwise

use VSAT$

For e(ample' te sort0aul radio option was not viable in some cases$ n one case studied' tecost of a ig radio tower e(ceeded te cost of a VSAT system$ Fre&uency availability was a problem as well$

n addition' te long0aul segment was affected by site0specific factors suc as data density'leased0line installation and operating costs' and VSAT line0of0sigt availability$ VSATs re&uirea clear view to te sout$

Te tecnical evaluation recommended combining sort0aul radio telemetry was e(isting

microwave or VSAT for te long0aul segment$ #conomics were especially favorable wen teVSAT could provide mainframe connections' a situation already found at a ma2ority of sites$

Anoter decision was weter to recommend C0band or Guband VSAT tecnology$ Tecompany was using C0band VSATs in te Transwestern Pipeline' but at te time ad noe(perience wit Gu0band VSATs$

C-ban )'A Avantages

• .ore cost effective for low data rates9 C0band ardware is generally 4>0>: less

e(pensive$•

Capabilities meet e(isting SCADA transmission speed re&uirements$• C0band SCADA communication services were available and ad been used since -@B' a

"known0&uantity" advantage$

C-ban )'A Disavantages

• ot amenable to moderate or ig0speed data transmission rates' voice or video$

• Sares te radio wave spectrum wit terrestrial microwave services' increasing te

possibility of interference' a rare occurrence in rural areas' but a ma2or concern inmetropolitan areas$

• A single vendor was te main provider of VSAT transponder capacity$

•

#(isting C0band satellites ave low utili1ation$• C0band is older tecnology$ Te e(pected life span of a satellite' bot C and Gu0band is

about seven years' usually set by fuel limitations for keeping te satellite on0station ingeostationary orbit$

• Pro2ected sortages of C0band satellite capacity were e(pected beyond -@@-' as e(isting

satellites reac life0span limits$ 5owever' te pro2ected sortage did not develop$• ,en conducting te ,AC pro2ect' tere were no launc plans for replacement C0band

satellites$



/u-ban Avantages

• Hrowt trend in satellite communications is toward Gu0band' wit future satellite

launces planned$• .ore suppliers are e(pected in te Gu0band marketplace$

• Gu0band VSATs can be used for ig0speed applications suc as voice' video' computer0

to0computer data communications' and SCADA and mainframe workstations$ 8ocalnetworks can be e(tended into wide0area networkers *,As+$• Altoug te value of satellite voice communications is limited by te inerent :$>0

second delay' receive0only video and data communications will become more valuable inte future' if costs decrease$

• Television receive0only *TV!%+ can be used to communicate wit a dispersed workforce

and provides cost0effective training wen compared to increasing airfare costs$• Computer0to0computer data communications will become necessary components as

microcomputer installations increase at field locations$

/u-ban Disavantages

• Cost usually e(ceeds C0band cost for one0application installation$

• Can be affected by eavy rainfall' suc as rainfall rates sufficient to restrict driving$ Tis

occurs because water droplets absorb te satellite fre&uency9 owever' tis can bemitigated by proper antenna design and oter e&uipment$ .ost users report actual outagerates less tan statistically predicted$

)'A Decisions

)ecause a distributed mainframe application was planned' te Gu0band scenario was cost0effective at a ma2ority sites$ A secondary decision was made not to add more C0band VSAT

sites$ Also' because of te economics mentioned earlier' radio became te vista coice for tesort0aul option$ Te combined radio3VSAT or radio3microwave became te "radio ub$"

%perating costs' for a Gu0band VSAT at eac SCADA site' was =76> to =?>: per mont$ Tee(act cost varied depending on vendor' data traffic costs' and capital amorti1ation period$ Tisestimated cost agreed wit oter publised analyses$-'4'7

%ne disadvantage of VSAT tecnology was te delay introduced by e&uipment and related store0and0forward message andling$ ,it one vendor' tis delay ran as long as -4 seconds' reducingte effective circuit speed$

Do0s an on0ts o( )'A technolog installation

• Do install bumper guards around VSTAs in compressor station yards$

• Do install a security fence around VSATs in or near public areas and be prepared to

install camoflauge' suc as srubbery' to ide te VSAT from public view$• Do allow e(tra time for installing VSATs mounted on building roofs' especially if te

building is not company0owned$• Do use dis eaters were ice storms are common$

• Do install antennas ig enoug to avoid snow drifts in nortern latitudes$



• Don;t install dis eaters in snowy regions were ice storms are fre&uent$

• Don;t install VSAT antennas 2ust outside office buildings9 tis may obstruct view or

reflect sunligt into windows$

)ecause of inerent VSAT delays' special features were mitigated into te SCADA system$ Areport0by0e(ception feature was implemented at data concentration *.T<+ sites$ Delay was

reduced also by using single0op rater tan double0op VSAT tecnology$

n a single0op system *Figure -+' te message pat is/

• %utbound poll 0 SCADA master to vendor master eart station *.#S+ via leased lines

• nbound response 0 VSAT at !T< to .#S via space segment9 .#S to SCADA master

via leased lines$

Te double0op system *Figure 4+' replaces te SCADA master to .#S leased0line link witanoter pair of space0segment ops' increasing time delay' but still cost0effective for smallersystems$

A dial0up backup to key radio ubs and oter critical VSAT sites was cosen$ Te SCADAsystem supports tis failure mode by automatically dialing up failed VSAT sites$

Figure 1. Single0op VSAT$



Figure 2. Double0ope VSAT$

Tis is cost0effective' since te business line;s montly cost is low relative to oter options$ Dial0up on0line costs are ig' but rarely needed$

Implementation

)ecause of very close bids and a ,AC team recommendation to use multiple VSAT vendors'two vendors were awarded contracts to install Gu0band VSAT;s across te nationwide 7':::0mi pipe line grid$ nitial construction was completed in -@@$

#nron currently as -4> VSAT sites trougout te <$S$ and as canged over to primarily Gu0 band tecnology$ ot all sites use te same VSAT options' as sown below/

/u-ban application area umber o( sites

SCADA -:?.ainframe B:

TV!% B7

%ter *ASC3N$4>+ -?

Total number of sites -4>

Te television receive0only *TV!%+ application currently is being installed$ Te N$4>application permits linkup between field and ome office systems' including local area network'



electronic mail' and remote access to centrali1ed D#C VAN applications witout any dial0upcost$ Since many sites are in remote areas' te dial0up costs are often dwarfed by te costs andfrustrations of poor &uality telepone connections$

Ac3no4legement

)ased on a paper' "VSAT Communications Tecnology for SCADA System Communications'" presented by te autor at te AHA Distribution Conference' asville' Tenn$' April -@@-$ Teautor received te Acker .edal Award -@@-0@4 for tis tecnical paper' sponsored by Central5udson Has and #lectric Corp$' in recognition of e(cellent contribution to te gas industry$

Literature Cite

-$ Sarma' !$' "VSAT etwork #conomics/ A Comparative Analysis'" ###communications' Feb$ -@@$

4$ Simo' #$' "VSATs and te Pipeline ndustry'" Satellite Communications' Aug$ -@$7$ Coulter' J$' "Design and nstallation of a Has Control SCADA etwork using Satellite'

.icrowave and !adio Systems'" #ntelec ews' Vol$ B-' no$ 4' Aug$ -@$

Instrumentation(Ch16)

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