1. PRESSURE DROP IN GAS PIPELINES AND WELLS Jón Steinar Guðmundsson February 2013
Importance of pressure drop and different pipes
Pressure drop in pipelines (depends on d5)
Equations for liquid and gas flow
North Sea gas pipelines
Friction factor and roughness
R&D on friction (roughness) and pressure drop
Summary

2. A: Wells, B: Flowlines, C: Risers, D: Process pipes, E: Off-Loading, F: Pipelines

3. Importance of pressure drop
Transport capacity, we want to be able to push as much gas as possible through existing pipelines to customers. Norwegian export pipelines >100 BCM annually.
Expensive gas compression (power and emissions) needed to give sufficient inlet pressure to overcome pressure drop. Gas turbines drive large centrifugal compressors offshore. Largest consumption of power offshore. Gas turbines and electrical motors on land.
Export pipelines have epoxy coating to make wall smoother to reduce wall friction and hence greater flow capacity.
Production capacity (subsea-to-beach), we want to maintain wellhead pressure as low as possible to sustain large production rate from gas fields with time. Eventually we need subsea compression.
Large diameter pipelines used to avoid compression platforms along export gas pipelines. On land, compressor stations along pipeline.

4. Natural Gas Pipelines
We have pipelines on land for long-distance transport and regional distribution.
We have buried pipelines for local distribution, to factories, businesses and homes.
We have pipes in processing plants, offshore and onshore.
We have subsea pipelines within field developments (flowlines).
We have large-diameter, long-distance subsea pipelines from natural gas provinces to market.
Pipelines are as important an infrastructure as roads, electricity masts, water pipelines, sewer pipelines etc.

6. Natural Gas Pipeline

7. Temperature in Pipelines

8. Temperature in Pipelines
T = Constant = Sea Temperature

9. Temperature and Distance

10. Temperature in Pipelines
Insulated pipeline on seafloor: 1 < U (W/m2.K) < 2
Non-insulated pipeline on seafloor: 15 < U (W/m2.K) < 25

11. Pressure and Temperature With Distance
Booster compressor duty: 15.5 MW (most likely roughness)
Aamodt (2006)

12. Effect of Roughness on Hydraulic Capacity and Outlet Pressure and Temperature
Aamodt (2006)

13. Pressure Drop in Pipelines
The total pressure drop in pipelines and wells consists of three terms
where g (gravitation), a and f stand for hydrostatic, acceleration and friction, respectively. The three terms can be expressed as
The angel α is measured from horizontal and the lenght is the pipe lenght, not height over/under the surface. The pressure drop due to friction is the Darcy-Weisbach equation.

14. Darcy-Weisbach Equation

15. Darcy-Weisbach Equation Liquid Flow and When Gas Average Density Used

16. Darcy-Weisbach Equation Force balance, steady-state pipe flow

17. North Sea Pipelines
Sletfjerding, E. (1999): Friction Factor in Smooth and Rough Gas Pipelines, Dr.Ing., Petroleum, NTNU.

18. North Sea Pipelines
Sletfjerding, E. (1999): Friction Factor in Smooth and Rough Gas Pipelines, Dr.Ing., Petroleum, NTNU.

19. North Sea Pipelines
Sletfjerding, E. (1999): Friction Factor in Smooth and Rough Gas Pipelines, Dr.Ing., Petroleum, NTNU.

20. Composition of Processed Gas
Molecule
Troll (1)
Norway
Sleipner (2)
Draugen (3)
Groningen (4)
Netherlands
Methane
Ethane
Propane
Iso-Butane
N-Butane
C5++
Nitrogen
Carbon-dioxide
93.070
3.720
0.582
0.346
0.083
0.203
1.657
0.319
83.465
8.653
3.004
0.250
0.327
0.105
0.745
3.429
44.659
13.64
22.825
4.875
9.466
3.078
0.738
0.720
81.29
2.87
0.38
0,15
0.04
0.06
14.32
0.89
100
After processing at Kollsnes (on-shore processing plant), average for November 2000.
After off-shore processing into off-shore pipelines, combination of Sleipner East and West, average November 2000.
After off-shore processing into pipeline Åsgard Transport to Kårstø (on–shore processing plant) for further processing, average for December 2000.
(4) Into onshore grid in The Netherlands.
Kilde: K. Jakobsen, A/S Norske Shell

21. North Sea Pipelines: Pressure Gradientp1 p2 L
(p1-p2)/L m
bar km
bar/100 km
kg/s A
108,42
85,59
812,4
2,81
205,50 B
166,26
145,59
303,5
6,81
383,50 C
107,97
94,16
619,0
2,23
185,40 D
65,22
63,64
48,5
3,26 E
129,85
86,8
6,95
334,10 F
72,03
67,45
9,44 G
136,3
112,1
227,0
10,66
167,60 H
146,7
95,5
812,8
6,30
348,40
6,06
Sletfjerding, E. (1999): Friction Factor in Smooth and Rough Gas Pipelines, Dr.Ing., Petroleum, NTNU.

22. Pressure Gradient in Gas Pipelines
Gradient (bar/100 km)
North Sea, Sletfjerding (1999)
Canada, Hughes (1993)*
6 (average 8 pipelines)
15-25
* Mokhatab o.a. (2006, s. 419)

23. Maximum Gas Velocity*
Sletfjerdings (1999) North Sea Pipelines A-H, uaverage (m/s), only % av NORSOK umaximum
*NORSOK P-001 (1999)

24. Pressure Drop Horizontal Gas Pipeline

25. Variable and Units d = Diameter [m] A = Cross sectional area [m2]
M = Molecular weight [kg/kmol]
f = Friction factor [-]
m = Mass flow rate [kg/s]
z = Compressibility factor [-]
R = Universal gas constant = 8314 [J/kmol.K]
T = Temperature [K]
p1 = Inlet pressure [Pa]
p2 = Outlet pressure [Pa]
L = Pipeline length [m]

26. Frictional Pressure Drop Gas Pipeline Horizontal Pipeline (Oil and Gas) Inclined Gas Well or Pipeline

27. Friction Factor in Pipelines

28. Nikuradse’s Sand-Grain Data

29. Moody Chart
Reference to fluid mechanics text book.

30. Blasius’ Equation Hydraulically Smooth Pipes
Re < 100,000

31. Haaland’s Equation

32. Wall Roughness in Pipes
Material
Average Absolut Roughness
(inch)
(µm)
Internally plastic coated pipeline
Honed bare carbon steel
Electropolished bare 13Cr
Cement lining
Bare carbon steel
Fiberglass lining
Bare 13Cr
0.200×10-3
0.492×10-3
1.18×10-3
1.30×10-3
1.38×10-3
1.50×10-3
2.10×10-3
5.1
12.5
30.0
33.0
35.1
38.1
53.3
Farshad og Rieke, JPT, oktober 2005, side

33. Blasius, Colebrook-White and Haaland
Haaland n=1 for liquids, same as Coolebrook-White
Haaland n=3 for gases, same as AGA data

34. Haaland Friction Factor Liquid n=1 and gas n=3, k/d=0.001
Gas 3.8 % lower than liquid at Re=106

35. Nikuradse’s Sand Grain and Real Roughness

36. Sletfjerding Ra = Arithmetic mean roughness
Rq = Root-mean-square roughness
Rz = Mean peak-to-valley roughness

38. Sand-grain roughness ks, Measured roughness Rq , Hurste exponent H
Pipes Used by Sletfjerding (Amsterdam 2001)
Sand-grain roughness ks, Measured roughness Rq , Hurste exponent H
4.5 < (ks/Rq) < 5.8
Sand-grain roughness estimate based on Nikuradse’s friction factor equation

39. Summary
Equation for pressure drop in horizontal gas pipelines; the natural logarithm term can often be neglected (because of gentle decrease in pressure in long pipelines)
Blasius’s equation used for smooth pipes and when Re<105 while Haaland’s equation is general and includes the effect of roughness (recommended).
Pressure drop in gas pipelines lower than in liquid pipelines. Indicates that semi-empirical correlations are not perfect.
Friction factor equations conservative, give 5-10 % higher friction factor and greater pressure drop than measured.
Friction correlations have come into focus after EOS (gas density) and gas viscosity correlations have improved.
Pressure drop in gas export pipelines (up to 1 m in diameter and km long) is of great economic importance for Norway as natural gas exporter.