pipeline lecture 2
TRANSCRIPT
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Submarine Pipeline Hydraulic Design,
Internal Pressure Design& Material Selection
Rod Pinna
Platform, Pipeline and Subsea Technology 403
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• Generally need to deliver oil or gas at a specified flow rate and pressure
• Hydraulic design required for preliminary selection of pipeline diameter
• Fluid must be kept above a minimum velocity– Minimise surging – Prevent build up of solids
• Fluid flow must be below a maximum velocity– Prevent erosion – Optimise pumping requirements
Hydraulic Design
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• Hydrocarbons for transport may be– Liquid (incompressible: straightforward to analyse)
– Gas (compressible & properties vary along pipe: more challenging to analyse)
– Multi-phase (e.g. gas & condensate) (highly complex)
Hydraulic Design
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• For liquid lines:
– Max velocity 4 m/sec
– Min velocity 1 m/sec
• For gas lines:
– Max velocity 18-25 m/sec
– Min velocity 4-5 m/sec
• Trade off between - CAPEX (Large pipe diameter) and - OPEX (Lower pumping costs)
Fluid Velocities
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• Pressure drop in liquid pipelines is principally due to
– Change in elevation (described by change in hydraulic head, or Pressure = gh )
– Friction loss
Pressure Drop
The remainder of the section on hydraulic design will be concerned with liquid pipelines
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There are two equations that may be used for calculating the friction loss
• Darcy-Weisbach
• Fanning
Friction Loss Calculation
2
2L DARCY
L Vh f
D g
æ öæ ö= ç ÷ç ÷è ø è ø
2
2L FANNING
L Vh f
D g
æ öæ ö= ç ÷ç ÷è ø è ø
Oil pipelines
Gas pipelines
So, fDARCY = 4fFANNING
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• For Laminar Flow
• For Turbulent Flow use the Moody Chart (Fig 2-3 in class notes)
Depends on pipe relative roughness
Friction Loss Calculation
64
ReDARCYf = For Re < 2300
For Re > 4000
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• The results of the Moody chart can also be described by the Colebrook Formula:
where: /D = pipe relative roughness
Friction Loss Calculation
( ) ( ){ }2
10
0.25
log / / 3.7 2.51/ ReDARCY
DARCY
f
D f=
é ù+ë û
* This is an implicit equation, so iterative solution required
* Also, may have to iterate for large changes in Pipe Diameter
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• Centrifugal pumps most commonly used for oil transport
• Pump Power:
Pump Power
ftin head total
gal/min in US rate flow
3960
UnitsImperial
=
=
=
h
Q
E
QhSW oil
hp
min head total
/secmin rate flow
9797
UnitsMetric
3
=
=
=
h
Q
E
QhSW oil
Watts
efficiency pump gravity specific Oil == ESoil
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• Gas compressors can be either centrifugal or reciprocating
• Compression is often carried out in stages with interstage cooling to maximize efficiency
• Compressor Power calculations can be complex and often need to refer to manufacturers data
Compressor Power
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• Pipe wall thickness is primarily driven by the need for pressure containment
• Design for internal pressure is based on consideration of hoop stress in pipe wall
Internal Pressure Design
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• Consider FBD of half of a thin-walled pipe
Calculation of Hoop Stress
F F
Pt
R
Rdf
f
df
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• For a small element Rdf vertical force due to pressure is: PRdf sin(f) (per unit length)
Calculation of Hoop Stress
F F
Pt
R
Rdf
f
df
PRdf sin(f)
Also note that P is really PINTERNAL - PEXTERNAL = P
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• Integrating over the circumference gives
Calculation of Hoop Stress
F F
Pt
R
Rdf
f
df
PR
dPRF
2
)sin(222
0
=
=
ff
PRF =
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• Stress (per unit length) in circumferential direction is:
or, rearranging:
Calculation of Hoop Stress
F F
Pt
R
Rdf
f
df
t
PD
t
PR
A
FHOOP 2
===
HOOP
PDt
2 =
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• Pipeline Design Codes usually have an equation for calculating the pipe wall thickness which is similar in form to this
• FD is a design factor (safety factor)
SMYS is the material Yield Stress
• Note there can be subtle differences as to the diameter used (OD or mean diameter)
Calculation of Hoop Stress
SMYSF
DPPt
D
EXTINT
=2
)(
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Pipe Wall Thickness
• Other factors which may add to wall thickness:
– Corrosion allowance
– Design against accidental damage
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Standard Sizes
• Pipeline sizes (diameters and wall thickness) have been standardized
• Relevant standard is American Petroleum Institute (API) Specification for Linepipe 5L, 42nd Ed., 2000
• Often quoted in nominal diameter (inches) & wall thickness (or SCHEDULE)– Nominal diameter not necessarily equal to OD
– Some w.t.’s more readily available (e.g. Schedule 40)
• See Appendix A of class notes for listing of standard pipe dimensions
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Linepipe Sizes
• Small diameter linepipe is usually seamless i.e. no longitudinal weld
• Larger diameter linepipe is rolled from plate and has a longitudinal weld
• Control of wall thickness is usually much better on welded linepipe
• For large linepipe orders (long pipelines) it is possible to order a specific wall thickness – may results in $$$ savings
• Standard pipe lengths are 12 or 18 metres
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Material Selection
• Selection of pipeline material can be important in determining overall cost
• Fundamental criteria include:– Corrosion resistance
– Strength
– Toughness
– Ductility
– Weldability
– Availability
– Cost
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Material Selection
• Key information for material selection includes:– Maximum operating pressure
– Pipe dimensions
– Maximum and minimum operating temperature
– Composition of fluids (including presence of water)
– Presence of H2S, CO2, Chlorides, etc.
– Design life of pipeline
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Material Selection
Important Material types include:
• Low alloy (Carbon-Manganese) steels X42 X65 X80
• Duplex Stainless Steel (e.g. 22% Cr 5% Ni)
• Austenitic Stainless Steel (e.g. 18% Cr 8% Ni)
• Titanium (catenary risers only)
SMYS = 80ksiLow Med High
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• Please read Sections 2.5 - 2.8 of notes (Including Free Span and Stability Analysis) over the next week
Homework