05-wall thickness design

Module on Pipeline Engineering

COURSE NOTES - 5

D-5: Wall Thickness Design

WALL THICKNESS DESIGN

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Module on Pipeline Engineering D-5: Wall Thickness Design

D-5: WALL THICKNESS DESIGN

TABLE OF CONTENTS

1 GENERAL CONSIDERATIONS

2 PRESSURE CONTAINMENT

2.1 Wall Thickness Calculation

2.1.1 THIN WALL EQUATION - BARLOW'S EQUATION

2.1.2 THICK WALL EQUATION

2.1.3 ACTUAL REQUIRED WALL THICKNESS

2.2 Wall Thickness Fabrication Tolerances

2.3 Design Factors

3 OTHER CHECKS FOR WALL THICKNESS

4 HOOP STRESS BASED ON INTERNAL AND EXTERNAL DIAMETERS

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1 GENERAL CONSIDERATIONS


D-5

WALL THICKNESS

DESIGN

The primary objective of pipeline design is for the pipeline to be strong enough to withstand

all the forces that are anticipated to act on the pipeline starting from construction to the end of

its operating life. The most important function of the pipeline is to carry the product at the

intended pressure without any structural loss of strength for a pre-determined period.

Therefore, a primary consideration in wall thickness design is the internal pressure produced

by the product. However, there are other forces that could impair the integrity of a pipeline

apart from the internal pressure. These other forces are caused by external pressure on

submerged or buried pipelines, construction activities, environmental forces, body forces,

thermal loads, accidental loads or loads caused by third party activities in its vicinity.

This section discusses the basic methods used in wall thickness design for pressure

containment and some further checks on the calculated wall thickness. It also identifies some

other checks that will be covered under other Course Notes. The conventional design

approach in this Course Note is based on application of design codes such as the BS 8010,

ANSI B3IA/31.8, DnV 1981.

It is pointed out that this Course Note does not include the design approach adopted by

DnV2000. Application of DnV2000 should be attempted after one has attained some

experience with the traditional design.

2 PRESSURE CONTAINMENT

2.1 Wall Thickness Calculation

2.1.1 THIN WALL EQUATION - BARLOW'S EQUATION

Wall thickness of a pipeline is primarily based on its capacity to contain the product pressure

by consideration of hoop stress. Most commonly used equation is the Barlow's formula for

hoop stress in thin walled cylinders subjected to internal and external pressure, Figure 5.1.

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©Dr. R. K. Jain. Highcrdcsigns Ltd.


(2.1 )

Where 0;, is hoop stress (N'mm"); Pi is internal design pressure (bar); P, is the external

pressure (bar); D" is the outside diameter of the pipe (mm) and t is wall thickness (mm). For

the subsea submerged pipelines,

(2.2)

where Pi is the density of seawater, g is the gravitational acceleration and h is the water depth.

Barlow's formula is used for cylinders with Dolt.? 50. Wall thickness t in (2.1) is chosen so

that

(2.3)

(2.4)

In the foregoing equation, 0; is the specified minimum yield strength and is determined from

API 5L code for the selected material grade; k is a design factor discussed further in Section

2.3. Combining Equations (2.1) and (2.3) gives

(P, - Po)Dot =-----'-2kcry

It is pointed out that the Barlow's formula implies that internal and external pressures are

both acting on the outer radius of the pipe. Barlow's formula is somewhat conservative but

most design codes and certification specify this method for wall thickness. A comparison of

stresses based on external and internal radii is discussed in Section 4.

2.1.2 THICK WALL EQUATION

If the calculated wall thickness from equation (2.4) is such that t/Ir.> 0.05, the wall thickness

should be recalculated using the following thick wall equation for hoop stress in cylinders.

This equation is known as the Lame's equation

(2.5)

Where, all terms are as defined above, and D, is the internal diameter of the pipe. Thus,

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t = (Do -D,)/2


(2.6)

Equation (2.5) is a corrected form of the equation given in BS 80 10.

2.1.3 ACTUAL REQUIRED WALL THICKNESS

Expected loss of wall thickness, tc' due to internal corrosion during pipeline life is added to

the calculated wall thickness. The method of predicting wall thickness loss is discussed in

Section 10. Thus the required thickness, t; is:

(2.7) .

According to a number of design codes, such as the OnV 198 I, ANSI and API codes, this is

the required nominal wall thickness that includes fabrication tolerances, Section 2.2, as per

specification, i.e.,

(2.8)

However, the wall thickness, t. is considered to be minimum required to contain pressure by

the British Standard BS80 10, and the ISO standard. Fabrication tolerances on wall thickness

are added to t. to determine the nominal thickness required. Denoting the positive and

negative fabrication tolerances byf andj", respectively, the required wall thickness will be

(2.9)

API 5L tabulates standard wall thickness for a range of pipe standard outside diameter. We

can use a standard API wall thickness that is equal to or greater than t"q, or we can use the

calculated required wall thickness. Note that most pipe mills are geared to making line pipe

to standard API specifications, therefore the latter approach is possible if large quantities of

pipe are ordered and are specially fabricated. The finally selected wall thickness is normally

referred to as the "nominal" wall thickness, tnom , thus

t ? tnom req (2.10).

For a given outside diameter, riser pipe computed wall thickness is greater than that for the

subsea section pipe leading to a smaller internal diameter and thus causing a step change on

the inside of the pipeline system. The step change may interfere with the passage of pigs

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during pigging operations. Therefore, it is generally recommended to select the riser pipe that

has the same internal diameter as for the subsea pipeline sections so that the step change will

occur on the outside surface. Barlow's formula can be used with some modifications and

iterations to determine riser pipe wall thickness.

The wall thickness computed to contain the internal pressure, as discussed above, must be

verified and increased, if necessary, by performing further analysis to ensure that the selected

wall thickness is adequate during installation, testing and operation. Some of these further

checks are discussed in the later sections.

2.2 Wall Thickness Fabrication Tolerances

The fabrication tolerances specified in API 5L for pipe grades, X 42 or higher, commonly

used in oil and gas industry are given in the following table:

Fabrication Tolerances - API 5L

Pipe Size (inches) Type Tolerances 0/0

::::;2.875 All +15.0, -12.5

>2.875 and <20 All +15.0, -12.5

;:::20 Welded +19.5, -8.0

>20 Seamless +17.5, -10.0

It is possible to procure the pipe with better tolerances than those of API 5L, particularly for

the seam welded pipes. It is quite common to specify lower negative tolerances. OnV

1996/2000 specifies generally lower fabrication tolerances as shown in the following table. A

number of leading pipe manufactures are able to meet OnV 2000 requirements.

Fabrication Tolerances - Dn V 2000

Wall Thickness Fabrication Tolerances

Welded Pipe Seamless Pipe

<15 mm ± 0.75 mm ±12.5%

;::: 15 mm ± 1.00 mm ±10%

2.3 Design Factors

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The design, or usage factor, is selected from the design code to be used. The design factor is

a safety factor to ensure that the calculated stress in the pipe shall not exceed a certain

specified percentage of the specified minimum yield strength of the pipe material. The values

for the design factor are based on the degree of hazard that the pipeline contents present to

personnel safety, property and environment pollution. For land pipelines, the design factors

are related to the population density close to the pipeline route. For subsea pipeline systems,

two values of the design factor are normally used, one for non-hazardous areas (subsea

sections away from the platforms and the shore) and one for hazardous areas (platforms and

near shore). The factors for different types of stresses and in different conditions are

specified by the codes. Following table summarises design factors from the commonly used

design codes.

CodeHoop Stress

Hazardous Non-Hazardous

BS 80101 0.6 0.72

DnV81 2 0.5 0.72

1S013623 0.6 0.77

Notes to Table:

I. Minimum wall thickness for hoop stress, other stresses use nominal wall thickness

2. DnV81: nominal wall thickness for all stresses; these factors for hoop stress also usedfor other functional

load stresses.

Other commonly used pipeline design codes, like ANSI B3 1.8/3 1.4 and API RPlll I, specify

similar factors as above for the hoop stress.

The above design factors are applicable to steel pipes operating at a maximum temperature of

1200 C. If the design temperature is in excess of 1200 C, then the design factors are reduced.

As a majority of pipelines operate within the above stated temperatures, we shall not consider

the reduction in design factors. The reduction factors may be found from ANSI B31.8, Table

84 I.l16A. Further reduction is applied in certain cases to account for the longitudinal weld

method. Again, we do not have to consider this effect since the pipes fabricated in

accordance with API 5L do not require this reduction in design factors except for furnace butt

welded pipe that is not used for transmission pipelines.

3 OTHER CHECKS FOR WALL THICKNESS

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. Apart from the internal pressure and temperature, a pipeline will be subjected to a variety of

other loads during its lifetime, these are

• Handling loads

• Installation loads

• Hydrotest loads

• Environmental loads due to wave, currents, wind, storms, earthquakes, etc.

• Soil pressure and friction loads

• Hydrostatic pressure on subsea pipelines

• Accidental loads from dropped objects, impact from fishing gear and anchors, etc.

Above identified loads are not necessarily affecting a pipeline in all cases. Different loads

have different probability of occurrence depending on the location, etc. For examples,

pipelines in the southern North Sea have higher probability of being hit by fishing gear than

most other places in the world. These various loads are identified at the start of a design by

hazard analysis studies usually carried out by specialist consultants.

At detailed design stage, stress analyses are performed using identified loads and checks

made to ensure that the selected pipeline has adequate strength. We shall discuss these other

loads in other Course Notes. A general treatment of stress analysis is discussed in Course

Note 8.

4 HOOP STRESS BASED ON INTERNAL AND EXTERNAL DIAMETERS

In the foregoing sections, we discussed the commonly used method of wall thickness

calculations. In this section, we briefly compare the hoop stress calculated from formulae

based on external and internal radii, and compare these with the hoop stress computed from

thick wall theory. For this discussion, we consider internal pressure only.

The hoop stress based on external diameter is given by equation (2.1); hoop stress based on

internal diameter is given by the equation

(J' = PID,h 2t

(2.11 )

The Lame's equation for hoop stress at any point in the pipe thickness is

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(2.12)

Hoop stress calculations have been performed for a 30-inch diameter pipeline for two wall

thickness values to illustrate the differences. The wall thicknesses considered for this

discussion are 25 mm and 35 mm.

The hoop stress variation through the pipe wall thickness for the two cases is shown in

Figures 5.2 and 5.3, respectively. A summary of results are tabulated hereunder:

Wall Thickness Lame's Equation Barlow's ID Based

(mm) Maximum Minimum Average (OD Based) Equation

25 225.8 210.5 218.2 233.2 217.9

35 159.3 144.0 151.6 166.5 151.3

The maximum hoop stress occurs at the inner radius and is greater than that computed from

equation (2.11). The average is calculated simply by considering a linear variation. If we

actually integrate the stress using equation (2.11) over the wall thickness and divide by the

wall thickness, the average is closer to the hoop stress based on equation (2.10). It is evident

that the Barlow's equation gives conservative values ofthe hoop stress.

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FIGURE 5.1 - PIPE SUBJECTED TO PRESSURES

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Hoop Stress Based From Lame's Equation

230

225

-NE 220E-z-~ 215e-(J)

0-o 210o

::J:

205

200356 358.5 361 363.5 366 368.5 371 373.5 376 378.5 381

Radial Coordinate (mm)

FIGURE 5.2: HOOP STRESS BASED ON LAME'S EQUATION

For Do = 762 mm, t = 25 mm, Pi = 15.3 N/mm2


©Dr R. K. Jain. Iligherdesigns Ltd.


Hoop Stress Based From Lame's Equation

165

160

-N< 155EE-z-I/) 150I/)

e-enc-o 1450J:

140

135

346 349.5 353 356.5 360 363.5 367 370.5 374 377.5 381

Radial Coordinate (mm)


©Dr. R. K Jain. Higherdesigns Ltd.


Waked C\t EXaf11JIe fa- EXercise

26" TRJNK UNE


5% negative tolerarx:e

A-oblem Description: A 26" diarreter pipeline and spool piece are to be designed with AR GradeX65 pipe.It is assumed that the OD is constant (usually 10is constant). Design factors for pipeline and spool are 0.72 and

0.6, respectively. aher data is as given below. Rease reproduce the wr cales.

Purpose is to illustrate effect of design factor and differences between DnVand BS8010

Pipeline Wall Ttickness DesiglHpeline Units Spoolpiece llit.es

Reference Size 26.00 in 26.00

Design A-essure(operation), pi 152.00 barg 152.00

Outside Diarrel er, Do 660.00 mm 660.00

Corrosion Allowance, ca 3.00 mm 3.00

~ative Mill Tolerance on vrt.. f 5.00 % 5.00

SMYS (AR 5L - X65) 448.00 Nlmm" 448.00

Water Depth, d 450.00 m 0.00 4

Water Density, rhew 1025.00 kglm' 1025.00

Contents Density, rhoc 0.00 kglm' 0.00

Wall Ttickness Caleuatioo

858010 DnV 1981A Ar '\ ( \

Hpeline Spoolpiece Hpeline Spoolpiece

Design Factor, fd 0.72 0.60 0.72 0.50Intemal A-essure, pi 15.20 NlI1'lI1i 15.20 15.20 NlI1'lI1i 15.20External Fressure, po 4.52 Nlmm" 0.00 4.52 NlI1'lI1i 0.00Minimum Wall Thickness,tmin 10.92 mm 18.66 1 10.92 mm 22.39

Minimum! Wall Thickness for corrosion, tminc 13.92 mm 21.66 2 13.92 mm 25.39

Requiredwall thickness, treq 14.65 mm 22.80 3 13.92 mm 25.39D:l/tmin Rltio 60.43 35.37

Equation for wal! thickness:

t min =(pj-Po)D o (I) Po = rhow.g.d2k.SMYS

flbtes:1. tmin corrputed by using equation (1)2. trrmc- tmin+ ca3. treq= tmincl(1-f/1 00) for BS 8010; treq= tminc for DnV 19814. Aser above water has no external pressure

flbte the differecnes betweenwei! thicknesses based on D1V 1981 andBS8010.1) pipeline wall thickness is conservative in BS 80102) Rser wall thickness is greater with DnV 1981

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Module on Pipeline Engineering D-S: Wall Thickness Design

I. An oil pipeline, which goes from the ground level down to a level of 100 metres. The

diameter of the pipeline is 24 inches, internal pressure at the ground level is 100 bar,

the product density is 870 kg/rn', corrosion allowance is 2 mm, negative fabrication

tolerance is 12.5 %, material grade is X-60 (SMYS: 413 MPa).

Compute wall thickness required at ground level and at the bottom using the design

codes BS 8010 and DnV 1981.

2. Determine wall thickness for a 30 inch diameter gas pipeline and the associated riser

operating at a pressure of 125 bar for a pipe of material grade X-52 (SMYS: 357

Mpa), no corrosion allowance and a negative fabrication tolerance of 12.5%. It is

assumed that the pipeline and the riser have constant internal diameters. The design

factors for the pipeline and the riser are 0.72 and 0.5, respectively.

3. Determine the maximum pressure for a pipeline that has the following design data:

00 = 30 inches; wall thickness = 20 mm, SMYS 448 Mpa,

corrosion allowance = 3 mm; design factor = 0.72,

negative fabrication tolerance = 8 %.


©Dr. R. K Jain. Higherdesigns Ltd.

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