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m.SOCiety of Petroleum Engineers

SPE 28481

Flowline Insulation Thermal Requirements for Deepwater Subsea Pipelines

M.T. Rubel and D.H. Broussard, Texaco Inc.

nCopFight 1994, Sccle!y of Petroleum E.gimws, 1...

T+, P.aPer was PrePa,ed for P,e$mailon al Gle SPE 69+h Annual Technical Cm fwmce .md ExhlblUon held [“ New Oriems, LA, U.SA., 25-28 SqXember 1994,

~1$ paper ..s s.IwM for pre$ent,at(m by m SPE Program Committee Mowing review of Information contained !. an absmcl s.bmimd by the author(s), Content, of the paper,as presented, ha.. not been revlmwd by !h. Society of I%ro[eum Engtn.ws an6 are subjec+ to camcll.n by the author(s), The material, as pres.nsed, does not necessarily reflecdany pwitlm o! She Society.1 Pe$roleum EnQine.am, i,. oHlcem. m members. Papers presented at SPE mee!lngs am s.bJacl w publication review by Editcd& C.amm!nees of the Society01 PeVde”m Engl,eer$, Permlsslon b cqy 1, reslr(cted to an abs!rad of “0! more than 303 wads, Illuslmtio”s may not be copied, The ibtrasl should cad” mn,Pic”o”% acknowledgmentof where ,.6 by whom the P,P,, 1$ Pmswted. VW,. IIYwlan. SPE, P,O. Box 833336. R(c+mrdso”. TX 7608%3836, USA Telex, 163245 SPEUT,

ABSTRACT

Current technology limits the distance over whichsubsea produced well fluids may be transported toapproximately 10-15 @es. ..ManY offshore leases existin water depths in the order of 4,000 feet that couldrequire transportation distances of 50-60 miles. Theprevention of hydrate formation in the flowline is ofconcern for this service. Injecting a chemical inhibitorhsa typically been considered as the primary method forpreventing hydrate formation. The cm-rent knowledgeof subsea insulation, particularly for deepwaterapplications, is very limited.

The purpose of this study is to develop data needed toevaluate the effectiveness of flowline insulating systemsin maintaining the fluid temperature above the hydrateformation temperature for a typical Gulf of Mexicoapplication.

INTRODUCTION

The current achievable distance over which producedwell fluids may be transported through subsea flowlinesis in the order of 10-15 miles. Many offshore leasesexist at water depths of about 4,000 feet. This wouldrequire transportation distnnces of 50-60 miles. One ofthe problems which must be overcome before thisextended reach capability can be achieved is thetendency for the hydrocarbon fluids to form hydratecrystals which in turn may block a production system.

The prevention of hydrate formation in flowlines hastypically been achieved injecting a chemical inhibitorinto the produced fluid. The current knowledge ofsubsea insulation, particularly for deepwateraPPhcatIOns, IS very limited. This paper will determinethe overall heat transfer coeftlcient required tomaintain the temperature of the produced fluids abovethe hydrate formation temperature over the length ofthe subsea flowline. The overall heat transfercoefficients will be reported for various flowlineinsulation systems. Bssed on these results, it is hopedthat a better understanding of whether flowlineinsulation may be a consideration in preventing theformation of hydrates for this application.

OBJECTIVE

The purpose of this study is to develop data needed toevaluate the effectiveness of flowline insulating systemsin maintaining the temperature of the produced fluidsabove the hydrate formation temperature over thelength of the subsea flowline.

SCOPE OF WORK

The project scope includes: 1) obtaining thermalproperties for various flowline insulating materials, 2)

References and illustrations at end of paper. 193

2 FLOWLfNE INSULATION THERiVLkL REQUIREMENTSFOR DEEPWATER SUBSEA PIPELINES

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SPE 28481

use PIPEPHASE~ to determine the overall heattransfer coefficient required t6 maintain thetemperature of the produced fluids above the hydrateformation temperature over the length of the subseaflowline for the anticipated operating conditions.

FLUID COMPOSITION AND PVT MATCH

For the hydraulic calculations, a compositional analysiswas used as opposed to black oil analysis.Compositional simulation performs ri”gorous energybalance calculations needed to accurately predict thetemperature profile along the fluid path in the wellboreand the pipeline. Accurate temperature measurementare the basis of calculating methanol requirements. Inaddition, vapor and liquid phases are needed for thedesign of pumps and compressors.

Aa a first step in this atudyj matching of the PVT datawas done. PVT data matching involves tuning anequation of state such as Soave-Redlich-Kwong (SRK) inorder to accurately predict the fluid phases at anyoperating temperature and preaeure. The compositionalfeature of PIPEPHASE was used in this tuningprocedure. The SRK equation of state was tuned tomatch the bubble point of the fluid, solution gas oil ratio(GOR), formation volume factor, fluid densities andcompositions given in Table 1. Accuracy of the tunedequation of state was within ten percent for the bubblepoint, eight percent for the GOR, one” perFint for theformation volume factor. The densities of the vapor andliquid were predicted within 2-3 percent compared tomeasured data.

The only drawback in using a compositional equation ofstate is that the viscosities at the various temperatureand pressures are poorly predicted. For this reason, thecompositional PVTGEN option in PIPEPHASE w“asused to generate fluid property tables over the desiredtemperature and pressure range. The viscositiesgenerated from a PIPEPHASE black oil PVTGEN runat the same temperature and pressure range were theninserted in place of the compositionally generatedviscosities, The viscosities generated by the black oilsystem are much closer to measured data than. thecompositionally generated viscosities. All other fluidproperties were generated compositionally.

Fluid compositions with various water cuts were

generated using the PRO-II@ simulator. The initialfluid composition wa8 blended with water cuts rangingfrom 10 to 90 percent. These water cuts were changing

@ pIpEPUE mid PRO II are regiattired trademarks ofSimulation Sciences Inc.

@ EQUIPHASE ia a registered trademark of D. B.Robinson & Associates.

during the life of the field. These fluid compositionswere used in the PIPEPHASE runs to

r$t ‘heflowline hydraulics and in the EQUIPHASE smmlatorto predict hydrate formation conditions.

DESIGN CRITERIA

Two production scenarios are investigated; a highflowrate high watercut case that represents fullproduction from a mature field, and a low flowrate nowatercut case which represents the anticipatedproduction during the early phase of the project. It isanticipated that the later case will prove to be the mostdiftlcult since a relatively low flowrate in a largeflowline will yield a low fluid velocity, and hence, arelatively high heat transfer rate. Table 2 shows therange of conditions tested.

OVEFULL HEAT TRANSFER COEFFICIENT

The overall heat transfer coeffkient,, U, is the sum of aIlthermal resistances in a system. Flgrn-e 1, for example,shows a schematic diagram of a flowline utilizing apipe-in-pipe insulating configuration. The thermalresistances for this system (RI - R5) would include: aninternal convective resistance resulting from theboundary layer on the inside of the pipe (RI), aconductive resistance due to the flowline pipe waII (R2),a conductive resistance due to the insulating material(R3), a conductive resistance due to the carrier pipe wsll(R4), and an external resistance due to the outsidesurrounding (R5).

For this study, the small internaf convective resistance(RI) is assumed to be negligible, i.e. the inside walltemperature is assumed equal to the produced fluid’stemperature. Furthermore, it is assumed that theflowline will be half buried; that is, the bottom half ofthe flowline will be buried in soil whereas the upper halfwill be surrounded by water. The external resistance(R5) is thus comprised of a conductive term(soil:PIPEHASE default = 0,8 BTU/hr-ft-”F) arid aconvective term (water). The resnfting external thermalresistance was determined using PIPEPHASE. Thewater depth, temperature, and velocity versus flowlinedistance information used for this investigation arerepresentative of a typical deepwater Gulf of Mexicowell, as shown in Figure 2.

Table 3 presents three categories of flowline insulatingsysteme that can be used in subsea applications. Thesearq double carrier, single carrier, and no carrier asshown in F@re 3. A carrier is simply a pipe-in-pipearrang–ement “With the annulus between the flowlinepipe and carrier pipe filled with an insulating material.In the severe environment associated with a deepwaterflowline, many conventional flowline insulations will not

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SPE 28481 M. T. RUBEL AND D.H. BROUSSARD 3

withstand the high external water pressures makingthe pipe-in-pipe arrangement necessary. In this table,the overall heat transfer coeffkients for each type arelisted both with and without the external resistanceterm. For thk study, the external resistance term hasbeen added, however, this thermal property data maybe needed for an application where the flowline is nothalf buried in soil.

RESULTS

PIPEPHASE Simulation

Four input data tiles were used to perform thePIPEPHASE runs: namely, low flowrate, low watercut,insulated; high flowrate, high watercut, insulate~ lowtlowrate, uninsulated; and high flowrate, uninsulated.In the first two runs, the overall heat tranafercoefficient was specified for each segment of flowline.The case study option was used to obtsin a temperatureprofile along the length of the flowline for a wide rangeof overall heat transfer coefticienta. Table 4 shows therange of overall heat transfer coefficients tested.

In the last two runs, the external thermal resistancewas specified rather than the overall heat transfercoefficient. This provided information required toproduce a temperature profile over the length of theflowline when the flowline was uninsulated. Theexternal resistance was determined by taking theaverage thermal resistance of an uninsulated flowlinecompletely buried in soil and a similar line completelysurrounded by water. The average was used since it isassumed the flowline will eventually settle half way inthe soil. The individual thermal resistances weredetermined by specifying PROPERTY.FULL ii theprint statement of the PIPEPHASE input file.

For each case study, PIPEPHASE generated a pressureversus temperature profile for the entire length of theflowline. Figure 4a shows the results of the lowflowrate, low watercut cases. Separate curves areshown for various values of flowline overall heattransfer coefficient from 0.03 BTU/hr.ft2-°F to the halfburied, bare (uninsulated) flowline. The resultingcurves show that, as anticipated, for any given value ofpressure, the fluid temperature decreases as the overallheat transfer coefficient increase& If, however, theoverall heat transfer coefficient 2 0.20 BTU/hr-fi2-°F,then the fluid temperature reaches the surroundingwater temperature and the reverse is true for anyfurther decrease in pressure. This is due to the fact thatas the fluid continues to flow, the water depth decreasesresulting in an increase in water temperature (recallFigure 2), The surrounding water temperature thusexceeds the produced fluid temperature. A low overallheat transfer coefficient reduces the amount of heat thatmay be transferred from the higher temperature

surrounding water to the lower temperature producedfluid temperature.

The hydrate formation curves for the produced fluidwith no chemical inhibitor and with 10 percentmethanol (as an example) are also shown. These figuresshow that the temperature will reach the hydrateformation temperature with no chemical inhibitor if theoverall heat transfer coeftlcient exceeds 0.10 BTU/hr-ft.2..F, ~s value of overall heat transfer cOefiicient iSextremely low. The low rate and relatively largeflowline diameter result in a low fluid velocity. This inturn results in a high heat transfer rate from the fluidto the surroundings.

For these runs, the flowrate and separator pressurewere held constant. An interesting observation fromthese figures is, as the value of overall heat transferdecreases (corresponds to an increasing level ofinsulation) the required flowline inlet pressure(wellhead) decreases. In fact, the difference betweenthe uninsulated flowline and an overall heat transfer

coefficient of 0.03 BTU/hr-ft2-OF, is 168 psi.

Figure 4b compares some insulating systems. Clearly,the vacuum pipe (single carrier) results in the lowestheat loss. The other systems shown do not maintain thefluid temperature above the hydrate formationtemperature during partial production. Although thescope of this project only included steady-state,operation, some discussion can be made regardingshutdowns. Simply, a lower overall heat transfercoefficient will result in a longer time required for thefluid temperature to reach the surrounding watertemperature. Therefore, the single carrier vacuum pipewill allow the longest shutdown before the temperatureof the fluid reaches the hydrate formation temperature.Since this is a transient probIem, PIPEPHASE can notpredict the pressure versus temperature curves as afunction of shutdown time. The double carrier, pipe-in-pipe system with hot water flowing in the inner amuluscould eliminate this problem, however, the capital costto install the facilities would be high.

Figure 5a shows the results of the high flowrate, highwatercut cases. Again, the hydrate formation curves forno chemical inhibitor and with 10 percent methanol areshown. These figures show that the temperature willreach the hydrate formation temperature if the overallheat trsnsfer coefficient is greater than 1.00 BTU/hr-ft2.°F. Figure 5b again compares some insulatingsystems. Dnring fuIl production, other systems such asa single carrier pipe-in-pipe system with a polyurethanefoam filled annulus, or the double carrier pipe-in-pipesystem with water in the inner annulus andpolyurethane foam filled outer annulus provideadeqnate insulation to maintain the fluid temperatureabove the hydrate formation temperature.

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4 FLOWLINE INSULATION THERMAL REQUIREMENTSFOR DEEPWATER SUBSEA PIPELINES

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SPE 28481

Insulating the flowline to maintain the fluidtemperature above the hydrate formation temperatureduring partial production requires an extremely lowvalue ofoverall heat transfer coefficient and may not beconsidered feasible. Insulating the flowline to maintainthe fluid temperature above the hydrate formationtemperature during full production (for example, with apipe-in-pipe single carrier whose annulus is tilled tithKerosene Gel) maysigniticantly reduce the amount ofchemical inhihitor required to maintain the fluidtemperature above the hydrate formation temperature.The potential savinga in chemical inhibitor cost mayhelp offset the capital expenditure required to installthe insulation system,

Flowline Heating Models

~STATOIL has developed aE’method for flowline heating which utilizes three 400 HzACpower cables mounted 120de~eesa partoutsideofan insulated pipeline, The power cablea inductivelyheat the flowline from outside of the insulation. Theinsulation must be sufficient to prevent hydrateformation during normal full production, therefore, thissystem is primarily intended for startup and shutdowns,

The overall heat transfer coefficient of the insulationwith the proposed system was found to be inadequate toprevent hydrate formation during full production for

this application (1.7 < U c 2.3 BTU/hr-ft2-”F, fullyburied).

Hot Wat. .

~. A method to reduce hydrateformation during startup and shutdowns is to circulatehot water over the flowline, A single carrier, pipe-in-pipe arrangement with the annulus used to carry thehot water would be a simple system of this type,although a return line would be required. Due to thehigh cost to heat the water, and surface facilitiesrequired to deliver the water, this system has beenconsidered for hydrate formation prevention duringstartup and shutdowns only. Here, the system isbasically 50-mile shell-and-tube heat exchanger. Aslong as the inlet water temperature and rate aresufficiently high enough, the produced fluidtemperature would remain above the hydrate formationtemperature.

During normal production, if the system were notoperational, the water would essentially sit motionlesswithin the annulus. Table 3 shows the overall heattransfer coefficient, not including the outersurroundings, is extremely high for this arrangement

(175.6 BTU/hr-ft2”-OF). This system would requireadditional insulation over the carrier pipe, Forexample, a double carrier, pipe-in-pipe arrangementwith the outer annulus tilled with polyurethane foamwould be one such arrangement. Table 3 shows that

this system isaufflcient to prevent hydrate formationduring normal production,

Although the cost to heat the water, and the surfacefacilities required to deliver the water would be high,the insulation system would only require an additionalcarrier pipe since the insulating material is water.Therefore, the cost of the system would not be muchhigher than asingle carrier pipe-in-pipe system. Thismay be a reasonable alternative if chemical inhibitorswere not required.

CONCLUSIONS

1.

2.

3.

4.

The temperature of the produced fluids will dropbelow the hydrate formation temperature for theanticipated operating conditions in an uninsulated,half buried flowline.

During the early phase of the project where only apartial production exists with very low watercut(approximately 7500 BPD) in a 16-inch nominaldiameter flowline, the overall heat transfercoefficient required to maintain the fluidtemperature above the hydrate formation

temperature is approximately 0,1 BTU/hr-ft2-OF.This overall heat transfer coefficient is”extremilylow and may only be achieved by one commerciallyavailable high performance flowline insulationsystem; aaingle carrier, pipe-in-pipe, vacuum pipe,The overall heat transfer coefficient for this systemis about 0.02 BTUihr-ft2-OF;

During full production with a very high watercut(approximately 40000 BPD) in a 16-inch nominaldiameter flowline, the overall heat transfercoefficient required to maintain the fluidtemperature above the hydrate formationtemperature is approximately 1.0 BTU/hr-ft2-°F.This required overall heat transfer coeftlcient maynearly be achieved by commercially available, highperformance flowline insulating materials, forexample, using a pipe-in-pipe single carrier whoseannulus is filled with Karosene Gel (U=l.3 BTU/lm.

ft2-oF).

Insulating the flowline to maintain the fluidtemperature above the hydrate formationtemperature during full production maysignificantly reduce the amount of chemicalinhibitor required to maintain the fluid temperatureabove the hydrate formation temperature duringpartial production, The potential savings duringthis phase of the project may help offset the capitalexpenditure required to install the insulationsystem.

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SPE 28481 M. T. RUBEL AND D.H. BROUSSARD 5

5. Insulating the flowline is not in itself sufficientenough to overcome all hydrate related problems.During extended shutdowns, the fluid temperaturewill eventually reach the surrounding watertemperature. Insulation, therefore, must provideSufficient time to allow for a reasonable shutdownwithout hydrate related problems.

6. A double carrier, pipe-in-pipe arrangement with hotwater circulating in the inner annulus duringstartup and shutdowns may be an effective methodto prevent hydrate formation. The high initial costmay be offset by eliminating the need for acontinuous demand for chemical inhibit ors.

Table 4 Overall Heat Transfer Coefficients Tested

Case Q W.c. Tin Di uNo. (BPD) (%) (F) (in) (BTu/llr-ft2-F)

1 7500 0 140 15 0.03-3.02 40000 70 157 15 0.03-3.0

7. Current electrical trace heating systems areineffective (U >2.0 BTU/hr-fi2-OF fully buried) forthis application.

Table 1 Reservoir Fluid PVT Properties

Component Mol %Carbon Dioxide 0.03Nitrogen 0.16Methane 56.34Ethane 6.75Propane 4.41Iso-Butane 1.18N-Butane 2.25Iso-Pentane 1.16N-Pentame 1.35Hexanes 2.85Heptanes Plus 23.47

Specific Gravity I0.8681Molecular Weight 268.5

HT. Prcuuuliw

Table 2 Range of Conditions Tested

Case Q W.c. GOR Tin DiNo. (BPD) (%) (F) (in)

1 7500 0 1126 140 152 40000 70 1126 157 15

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6 FLOWLINE INSULATION THERMAL REQUIREMENTSFOR DEEPWATER SUBSEA PIPELINES

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SPE 28481

Double Carrier

kWater filled annulusVacuum PipeRerosene Cd filled annulusMonoethylene Gell filled annulusPol rethane Foam filled annulusNo Carrier

Polychlomprene+ fillerTrade Name Vikotherm

Polypropylene foamTrade Nama Tbermotite

EPDM+glass miscrospheres?i-ade Name Syntactic@

1Notw AH values of U are based

Table 3 Thermal Property Data

Flowline Carrier-1 Carrier-2 U without uNOminaf I.D. Nominaf I.D Nominal I.D. Slu’rounding* haff buried

(inches) (inches) (inches) BTU/hr-ft2 - F BTU/br-ft2- F

16 20 22 1.55 1.06FIowIine Carrier Carrier U without u

Nominal I.D. Nmninal LD Thickness sun-ound~gs half buried(inches) (inches) (inches) BTU/hr-ft2- F BTU/lm-ft2- F

16 20 175.6 3.2716 18.5 0.03 0.0316 20 2.13 1.301616

4.44 1.90:: 0.44 0.39

Ffowline Insulation Thickness U without u*es) surmundimm haffburiedNomimd I.D. (Inc

(inches) I BTU/hr-ft2-> I BTU&ft2- FId 1 3.881 s.79

A1

1

1Flowline J

23

;31231

outer II2.16 1.311.55 1.065.07 2.012.82 1.532,03 L264.17 1,852.32 1.371.67 1.113.85 1.79

11Nominal I.D. Surroundings BTU&-ft2- F

(inches)16 Half Buried 2.98516 No Surroundings 29

n inside diameter of inner pipe

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SPE 28481 M. T. RUBEL AND D.H. BROUSSARD 7

O.tid. Surmuntin*

Frawi”e

%

Ri R2 R3 R, %

OverallHeatTransferCdrdent. u. 1R1+R2+R3+R4+R5

Figure 1. Schematic Diagram of Overall Heat Transfer Coeffkient

1000, ,80

“

.1030-

- 60

- 50

.3aao -

Wat.r Wlmity .0.33 W, do

.4000

-50000 10 20 30. 40 50 60

Figure 2. Flowfine Depth and Temperature versus Distance Profile

-CL*Doubt.Card.,

I!& F.&?. Ihd.* M.!.*

Slng!ecar,!., No Card.,

Figure 3. Categories of Ffowfine Insulating Systems

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8 FLOWLINE INSULATION THERMAL REQUIRE~NTSFOR DEEPWATER SUBSEil PIPELINES SPE 28481

3,000(BTU/lwft2 -F)

2,500t

Hydrate Formation Temperatures10% Methanol No Inhibitor mHalf Buried

Bare pipe

2 1,500zm

E 1,000I I

500 -

0 I I Io 50 100 150 200

Temperature (F)

Figure 4a. Pressure versus Temperature Profile; Low Flowrate, Low Watercut Case

u = 0.05—.—.

u =0.10

U=l.oo.- .- —

u = 5.00

3,000

2,500

g 1,500mu)g

L 1.000

500

Hydrate Formation Temperatures10% Methanol No Inhibitor

I

I

I

/ /

.------------------------””,/“,/\..

//“

/“/“”..-

(BTU~r-fF -F)

r

Half BuriedBare pipe

Single CarrierVacuum Pipe

—.-—

Single CarrierPolyurethane

---------

3-inch

w%?%%:. . . . --

2-inchNeoprene

1 I I. -.

01LJ w 100 150 200

Temperature (F)

F]gure 4b. Comparison of Various Insulation Systems; Low Flowrate, Low Watercut Case

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SPE 28481 M. T. RUBEL AND D.H, BROUSSARD

3,000

2,500

2,000

1,500

1,000

500

0

I (BTU/hr-ft2-F)

~E

Half BuriedHydrate Formation Temperatures Bare Pipe

107. Methanol No Inhibitor

\u = 0.03—.. —u = 0.05

/— .—.

U=o.lo-.. ------

IU=l.oo

/

. . . .. . --

u = 3.00. . -- ——--- .- --,- / -- --

I

,, -- -..,.:j

/ “““

,,’”//

~ /j/ :’”

~/;

/; ~;q,,”

0 50 100 150 200Temperature (F)

Figure 5a. Pressure versus Temperature Profile; High Flowrate, High Watercut Case

3,000

2,500

~ 2,000.-OYQ

g 1,500UYco

: 1,000

500

0

(BTUlhr-ft2-F)

HalfBuriedHydrate Formation Tempek?dures care Pipe

10% Methanol No InhibitorSingle CamkrVacuum Pipe

I —-. —

9

u 50 100 150 200Temperature (F)

Figure 5b. Comparison of Various Insulation Systems; High Flowrate, High Watercut Case

201