Submarine Pipeline Hydraulic Design,

Internal Pressure Design& Material Selection

Rod Pinna

Platform, Pipeline and Subsea Technology 403

• 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

• 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

• 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

• 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

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

• 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

• 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

• 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

• 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

• 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

• Consider FBD of half of a thin-walled pipe

Calculation of Hoop Stress

F F

Pt

R

Rdf

f

df

• 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

• Integrating over the circumference gives

Calculation of Hoop Stress

F F

Pt

R

Rdf

f

df

PR

dPRF

2

)sin(222

0

=

=

ff

PRF =

• 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 =

• 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

)(

Pipe Wall Thickness

• Other factors which may add to wall thickness:

– Corrosion allowance

– Design against accidental damage

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

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

Material Selection

• Selection of pipeline material can be important in determining overall cost

• Fundamental criteria include:– Corrosion resistance

– Strength

– Toughness

– Ductility

– Weldability

– Availability

– Cost

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

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

• Please read Sections 2.5 - 2.8 of notes (Including Free Span and Stability Analysis) over the next week

Homework