important aspects of gas temperature modeling in long subsea pipelines

7/29/2019 Important Aspects of Gas Temperature Modeling in Long Subsea Pipelines

1/17

PSIG 0901

Important Aspects of Gas Temperature Modeling in Long Subsea Pipelines

J oakim Ramsen, Polytec, Norway (1)Svein-Erik Losnegrd, Polytec, Norway (1)Leif Idar Langelandsvik, Gassco, Norway (2)Are J . Simonsen, Dynavec AS, Norway (3)Willy Postvoll, Gassco, Norway (4)

Copyright 2009, Pipeline Simulation Interest Group

This paper was prepared for presentation at the PSIG Annual Meeting held in Galveston,Texas, May 12 May 15 2009.

This paper was selected for presentation by the PSIG Board of Directors following review ofinformation contained in an abstract submitted by the author(s). The material, as presented,does not necessarily reflect any position of the Pipeline Simulation Interest Group, its officers,or members. Papers presented at PSIG meetings are subject to publication review by EditorialCommittees of the Pipeline Simulation Interest Group. Electronic reproduction, distribution, orstorage of any part of this paper for commercial purposes without the written consent of PSIGis prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300

words; illustrations may not be copied. The abstract must contain conspicuousacknowledgment of where and by whom the paper was presented. Write Librarian, PipelineSimulation Interest Group, P.O. Box 22625, Houston, TX 77227, U.S.A., fax 01-713-586-5955.

ABSTRACTGassco supplies Norwegian natural gas to the European

market through nearly 5,000 miles of large-diameter high-pressure subsea pipelines. In 2007 3106 MMSCF of gas were

exported from the Norwegian Continental Shelf (NCS).

During the winter, demand for gas usually exceeds theestimated transport capacity of the pipelines. More accurate

modeling of the flow can lead to improved use of available

network capacity.

The pipelines in Gasscos network are typically 200 560miles long, and the gas temperature is only measured at the

inlet and outlet. Consequently, the calculated gas temperature

along the pipeline depends on the accuracy of the assumedambient temperature and the estimated heat transfer. This

paper mainly focuses on heat transfer modeling, and how this

affects the estimated gas temperature. The importance of a

correct total heat transfer coefficient for different conditions

has been studied, and the most important parametersassociated with this coefficient have been identified. A

recommendation regarding which parameters to focus on

under different conditions such as different burial depths and

flow rates is given.

INTRODUCTIONThe Norwegian gas is transported in seven large diameter sub-

sea pipelines to United Kingdom and continental Europe,

covering around 15 % of the European natural gas

consumption. The transportation network is operated by thestate-owned company Gassco.

The Norwegian export pipelines are between 200 and 560

miles long, and have diameters up to 44 inches. Pressure

transmitters, flow meters and temperature measurements are

only located at the inlet and at the outlet. To know the state of

the gas between those two points one has to rely solely oncomputer models and simulators, which are very important in

order to obtain optimal operation of the pipelines. The

computer models are used for general monitoring of the gastransport, providing estimated arrival times for possibly

unwanted quality disturbances and pigs, predictive

simulations when the operational conditions changes and for

transport capacity calculations. The transport capacity is

usually made available to the shippers of the gas many yearsin advance, and accurate calculations early in a pipelines

lifetime are appreciated and valuable.

High accuracy in the transport capacity calculations isimportant to ensure optimal utilization of invested capital in

the pipeline infrastructure. The calculations need to be as close

to, but not higher than, the true capacity as possible. This wil

ensure optimal utilization of invested capital. As soon as apipeline is built, the true capacity is determined by the

diameter, length, available inlet compression, gas temperature

and other physical parameters. A lot of effort is put in to

estimate this capacity figure exactly.

In 2004 a research program was launched to optimize the gas

transport modeling involving high flow rates, high pressures

large diameters and very low roughnesses. The R&DFoundation Polytec and Gassco have worked together for

several years to optimize the gas transport modeling. In the

project several subtasks have been conducted or are ongoing

to improve the friction factor correlation, viscosity

measurements and implement new viscosity correlation, use of

more accurate ambient temperatures and more accuratemodeling of heat transfer. This paper will focus on work

performed related to heat transfer.

After realising that the simulation tool used in the project did

not model the heat transfer for partially buried pipelines, a

literature survey was conducted to identify an appropriate

model. It seems that very few published articles discuss thistopic [1, 2]. An analytical model [1] was identified, able to


2/17

2 J . RAMSEN, S-E. LOSNEGRD PSIG 090

calculate the heat transfer for partially buried pipelines. This

model, which has been evaluated by CFD simulations, hasbeen used to calculate the gas temperature in partially buried

pipelines.

This paper starts with a system description, followed by a

theory section about heat transfer including the most important

equations used in pipeline gas temperature modeling. Then,the parameters determining the total heat transfer coefficient

are studied. Also, the gas temperature responses for relevanttotal heat transfer coefficients are given. These responses are

used to discuss the accuracy of gas temperature modeling and

in particular the modeling of partially buried pipelines.

Mainly, a constant ambient temperature profile is applied, but

a scenario where the ambient temperature is increased over a

short distance is also looked into. Only gas temperaturemodeling in connection with steady-state simulation is

considered.

SYSTEM DESCRIPTION

Typical characteristics of the Gassco operated pipelines:Pressure range: 700 3,000 psi

Diameter: 30 45 inches

Composition: 80 95 % MethaneLength: 200 560 miles

Flow rate: 700 2,120 MMSCF/d

Roughness: ~104 inches, due to internal

coating

Location: Sea bed (Mostly partially buried)Sea depth: 150 100 ft

Metering: Pressure, temperature, flow and

composition at inlet and outlet

Pipe materials: Steel, Asphalt Enamel and

ConcreteTotal export 2007: 3106 MMSCF

Figure 1: Typical pipeline cross section consisting of a steelpipe coated with ashphalt enamel and concrete

Theory

The general heat transfer equation

The general heat transfer equation used by pipeline simulation

software is given by Equation 1.

( )gasamb TTAUQ =& Equation 1

Where

Q& Heat transferred between the gas and the

surrounding medium [Btu/h]

U Overall heat transfer coefficient [Btu/hftF]

A Surface area of the pipe [ft]

ambT Ambient temperature [F]

gasT Gas temperature [F]

The total heat transfer coefficient, U, describes the conductive

and convective heat transfer between the gas and the

surroundings.

Heat transfer with multiple wall layers

Long subsea pipelines are usually coated with asphalt ename

and concrete for corrosion protection and bouancystabilization. Thus, the thermal energy must pass through

several shells to enter/exit the pipe. Assuming pure radial

flow, these multiple resistances may be combined into a single

heat transfer coefficient as shown inEquation 2.

1

2 1

1ln

1

=

+

+=

N

n on

n

n

o

ii

o

hr

r

k

r

hr

rU Equation 2

Where

U Overall heat transfer coefficient of a pipe with

outer radius [Btu/hftF]or

ir Inner radius of pipe [ft]

or Outer radius of pipe [ft]

nr Outer radius of wall layer [ft]n

nk Conductivity of wall layer [Btu/hftF]n

ih Inside heat transfer coefficient [Btu/hftF]

oh Outside heat transfer coefficient [Btu/hftF]

The outer film coefficient has to be determined according to

the surroundings of the pipeline.


3/17

PSIG 0901 Important Aspects of Gas Temperature Modeling in Long Subsea Pipelines 3

Fully exposed pipelines

For pipelines fully exposed to water the convective heat

transfer is given by Equation 4, where the Nusselt number canbe obtained from Equation 3. The latter equation yields for

external flow across a cylinder for Reynolds numbers

exceeding 200. This corresponds to velocities greater than

310-4 ft/s, for typical sea water properties and pipeline

diameter.

3.06.0 PrRe26.0 =Nu Equation 3

Where

Nu Nusselt number

Re Reynolds number

Pr Prandtl number

The Nusselt number can then be used to calculate the outside

heat transfer coefficient.

o

seao

d

kNuh

= Equation 4

Where

Thermal conductivity of sea water [Btu/hftF]seak

Outer diameter [ft]od

Buried pipelines

For buried pipelines the outside heat transfer coefficient is

given by Equation 5.

( )1ln

2

2 +=

xx

dk

h osoil

o Equation 5

Where

soilk Thermal conductivity of the soil [Btu/hftF]

x od

H2=

H Distance from center of pipeline to sea floor [ft]

Partially buried pipelines

An analytical model has been used to model the heat transferin partially buried pipelines. This model was developed by

J. C. Morud [1]. It is described by Equation 6, from which Usea

is given by combining Equation 2 and Equation 4 and Uground

is given by Equation 7. Figure 6 illustrates how the burial

depth is defined.

groundb

seab

total UUU

+=

1 Equation 6

Where

Overall heat transfer coefficient [Btu/hftF]totalU

b

=

R

Harccos

R Outer radius of pipeline [ft]

seaU Combination of and convective heat

transfer to the sea water [Btu/hftF]

wallU

groundU Combination of and conductive heat

transfer to the soil [Btu/hftF]

wallU

( )

( )

0 for the base case.

The low conductivity case resulted in quite low U-values for

the whole burial span. Hence, for pipelines with a low U-value

the importance of using correct burial depths increases. Figure

12 shows the gas temperature profiles for this case. It is shown

that the gas temperature profile is more sensitive to changes inburial depth when the pipeline is more than half buried

(H/R>0).

Recovery length

Incorrect heat transfer modeling in the first part of the pipeline

can be disguised if a sufficient length of the last part iscorrectly modelled. In this paper that length is called the

recovery length, and a sensitivity study was carried out to see

how long this is for different U-values.

In the sensitivity study recovery length is the length required

to correct an error of 3.6 F in the calculated gas temperature

compared to the real gas temperature. It is assumed that Teq for

each U-value is equal to the correct gas temperature.

The gas temperature is set to be 3.6 F lower at distance 0.Then the recovery length is found when the gas temperature is

within -0.36 F from the real gas temperature (Teq), see Figure

13. Note that an isothermal ambient temperature profile is

used in the analysis.

The results from the analysis are shown in Table 8. It can be

seen that the recovery length decreases significantly withincreasing U-values. Thus, for pipelines with high U-values in

the last part, correct modeling of this part is more likely to

disguise errors in the heat transfer modeling of the first part

This can make it difficult to tune the model according to thetemperature reading at the outlet.

Table 8: Recovery length for different U-values at high flow

U-value[Btu/hftF]

Recovery Length[miles]

0.49 261

1.25 103

1.69 76

3.13 41

The U-values in a typical pipeline operated by Gassco are

plotted in Figure 14. It is shown that the U-values for the last

60 miles, range from 0.28 0.32 Btu/hftF. This is because

this part usually is fully buried. In zone A and B, however, thepipeline is partially buried or fully exposed. Hence, error

made in zone C will give a temperature deviation at the outlet

while errors made in zone A and B can be disguised.

TEMPERATURE TUNING

If there is a discrepancy between the modeled and measured

outlet temperature in a pipeline it is common to tune the

model, either by adjusting heat transfer or the ambient

temperature. From the observations made in this article the

following can be related to tuning.

For high flows (>16 ft/s):

If the U-values are high (>1.76 Btu/hftF) for themain part of the pipeline, the accuracy of calculatedgas temperature will depend strongly on the accuracy

of the ambient temperature. Thus, if the modeled

outlet temperature deviates significantly from the

measured temperature this is most likely due to anerror in the imposed ambient temperature profile.

For low U-values (


8/17


large pipelines leading to Europe. The U-values that were

tuned with these data, see Figure 15, are called Utuned, andrepresent the actual U-values of the pipeline.

Figure 5: The pig prior to launching at Krst gas processingplant. The pig launcher is seen in the background.

In todays model the pipeline will be assumed either fully

exposed or totally buried based on best available data. In this

analysis U is set to 0.7 Btu/hftF when H/R>0 and

correlation for fully exposed pipeline is used when H/R


9/17


error of 0.18 Btu/hftF can lead to an error of more

than 3.6 F in the calculated gas temperature.

Even though the calculated outlet temperature equalsthe measured one this does not necessarily verify the

model. If a part of the pipeline has a high U-value

(>1.76 Btu/hftF), a correct modeling of this part

may disguise errors in the model upstream.

The Morud model for partially buried pipelinesagrees well (within 10%) with CFD calculations. At

high Biot numbers (>102) the deviation increases,

especially for buried pipelines. Subsea pipelinesusually have a Biot number below 10.

REFERENCES1. J. C. Morud and A. Simonsen, Heat transfer from

partially buried pipes, 16th Australasian Fluid Mechanics

Conference December 2007

2. Alessandro Terenzi and Francesco Terra, External heatransfer coefficient of a partially sunken sealine, IntComm. Heat Mass Transfer. Vol. 28. No. 2, pp 171-179

2001

3. M. Mohitpur, H. Golshan and A. Murray, Pipeline Designand Construction A Practical Approach, p. 106-110

4. Leif Idar Langelandsvik, Modeling of natural gas

transport and friction factor for large scale pipelines Laboratory experiments and analysis of operational data

20085. Martin Mathiesen,North sea bottom temperature, Polytec

R&D Foundation June 2004

ACKNOWLEDGEMENTSThe Norwegian Research Council is acknowledged for the

funding provided for the PhD-work in this research project.


10/17


TABLES

Table 9: A sensitivity study on the properties of the pipeline and its surroundings. The sensitivities are tabulated as difference from thethe base case (sensitivity base case)

Value Utotal

Case DescriptionFrom To

FullyExpose H/R = -0.75 H/R = 0 H/R = 0.75

TotallyBuried

Base case Base case--- --- =3.1 =2.6 =1.9 =1.2 =0.5

Sensitivity 1 3.5 5.2 +1.1 +0.9 +0.6 +0.3 +0.0

Sensitivity 2

Change in concrete

conductivity3.5 1.7 -1.4 -1.1 -0.7 -0.4 -0.1

Sensitivity 3 1.2 1.7 +0.1 +0.1 +0.1 0.0 0.0

Sensitivity 4

Change in asphalt

conductivity1.2 0.7 -0.3 -0.2 -0.2 -0.1 0.0

Sensitivity 5 86.5 103.8 0.0 0.0 0.0 0.0 0.0

Sensitivity 6

Change in steel conductivity

86.5 69.2 0.0 0.0 0.0 0.0 0.0Sensitivity 7 4.2 5.4 -0.5 -0.4 -0.3 -0.2 0.0

Sensitivity 8

Change in concretethickness

4.2 2.9 +0.7 +0.6 +0.4 +0.2 0.0

Sensitivity 9 0.3 0.3 -0.1 -0.1 0.0 0.0 0.0

Sensitivity 10Change in asphalt thickness

0.3 0.2 +0.1 +0.1 0.0 0.0 0.0

Sensitivity 11 1.1 1.3 0.0 0.0 0.0 0.0 0.0

Sensitivity 12Change in steel thickness

1.1 1.0 0.0 0.0 0.0 0.0 0.0

Sensitivity 13 0.3 0.5 0.0 0.0 0.0 0.0 0.0

Sensitivity 14

Change in velocity of sea

current0.3 0.2 -0.1 -0.1 -0.1 0.0 0.0

Sensitivity 15 3.5 5.2 0.0 +0.1 +0.1 +0.1 +0.2

Sensitivity 16

Change in ground

conductivity3.5 1.7 0.0 -0.1 -0.1 -0.2 -0.2

Sensitivity 17 Ignoring hi 52.8 0.0 0.0 0.0 0.0 0.0 0.0


11/17


FIGURES

Figure 6: Definition of coordinates for partially buried pipelines

Figure 7: Qualitative description of gas temperature profiles


12/17


40.0

40.5

41.0

41.5

0 50 100 150 200 250 300

Distance [m i]

Tem

perature[F]

Tamb

Case 1

Case 2

Figure 8: Gas temperature responses for low flow and low pressure drop situations

30.0

35.0

40.0

45.0

0 50 100 150 200 250 300

Distance [m i]

Tem

perature[F]

Tamb

Case 3

Case 4

Figure 9: Gas temperature response for high flow and high pressure drop situations


13/17


30.0

35.0

40.0

45.0

0 50 100 150 200 250 300

Distance [m i]

Tem

perature[F

]

Tamb

Case 5

Case 6

Figure 10: Gas temperature response when the ambient temperature increase by 2 C over a short distance

0

2

4

6

8

10

12

14

16

18

0 0.5 1 1.5 2 2.5 3

U-value

Tamb-Teq

[F]

High Flow

Low Flow

Figure 11: Difference between Tamb and Teq versus U-value


14/17


25.0

30.0

35.0

40.0

45.0

0 50 100 150 200 250 300

Distance [m i]

Tem

perature[F]

Tamb

Fully exposed

H/R = -0.75

H/R = 0

H/R = 0.75

Totally buried

Figure 12: Gas temperature profiles including partially buried pipelines (Low conductivity case)

Figure 13: Illustration of recovery length

T = 0.36 F

At this point the simulatedgas temperature is 3.6 F

below the real gas

temperature.Recovery Length

Assumed that U is correct for this part

Temperature

Ambient temperature

Real gas temperature / Teq

Simulated gas temperatureGas temperature response

Distance

When gas temperature is 0.36 F from thereal gas temperature the recovery length is

found.


15/17


0

1

2

3

4

5

6

0 50 100 150 200 250 300 350 400 450

Length [mi]

U-value[Btu/ftFh]

Figure 14: U-values in a typical Gassco operated pipeline

0

1

2

3

4

5

6

10

50

90

130

170

210

250

mi

U-value[Btu/ftF

h]

U_tuned U_as_today U_pbur U_pbur_high_conductivity

Figure 15: Comparison of 4 sets of U-values


16/17


43.5

44.5

45.5

46.5

47.5

48.5

49.5

45 90 135 180 225 270

m i

Tem

perature[F]

U_ tuned Partial buried model As_ Today PBur high cond.

Figure 16: Temperature profiles based on simulation with different U-value sets.


17/17


Authors

Joakim Ramsen is a researcher at the R&D foundationPolytec located in Haugesund, Norway. He started his

professional career in 2003 and has worked mainly with

projects concerning gas transport modeling. Since 2005 he hasbeen the project leader for a project called optimized flow

modeling. In addition he has been involved in several CO2transport simulations projects conducted in Norway.

Svein-Erik Losnegrd is a researcher at Polytec R&DFoundation. He holds a MSc. Degree in Mechanical

engineering from NTNU (2006). His work has mainly been

concerned with gas transport modeling.

Leif Idar Langelandsvik is a principal engineer with Gassco,Technology Department. He is about to finish up a PhD

degree at the Norwegian University of Science and

Technology (NTNU) with focus on modeling of gas transport

in full scale pipelines at large Reynolds numbers. Particularfocus is on the wall friction at such conditions. The work at

Gassco is mainly focused on improving gas transport models,pipeline simulations, fluid dynamics and transport capacitycalculations. He received his MSc. degree in

cybernetics/control systems at NTNU in 1999.

Are Simonsen is currently project manager in Dynavec AS.He holds a PhD. in fluidmechanics from the University ofTrondheim, NTNU(2003). The last years he has worked with

fluidmechanics, heat-, and mass transfer related problems as a

researcher in SINTEF Materials&Chemistry.

Willy Postvoll is the Real Time Systems Advisor in GasscoAS. He holds a MSc degree in Petroleum and Reservoir

Engineering from the University of Stavanger (1985). Hestarted his professional career with Statoil in 1985 where he

worked as a Reservoir Engineer in the Oil and Gas Field

Development division. After spending 5 years as a Senior

Engineer providing technical support for reservoir simulation

he joined the Transport Division specializing in Real-TimeSystems, Transport Control and Supervision.

important aspects of gas temperature modeling in long subsea pipelines

Documents