INFLUENCE OF STEEL PIPELINE WALL MATERIAL LOSS DEFECTS … - Maciej_Witek.pdf · INFLUENCE OF STEEL...

1

INFLUENCE OF STEEL PIPELINE WALL MATERIAL LOSS DEFECTS ON GAS

NETWORK CAPACITY

Warsaw University of Technology, Heating and Gas Systems Department

MACIEJ WITEK, PhD Engineer

2

INTERNAL AND EXTERNAL WALL DEFECTS IN STEEL PIPELINES WITH A MAXIMUM OPERATING PRESSURE MOP>1.6 MPa

CAN BE CLASSIFIED AS FOLLOWS

material losses, resulting in reduction of wall thickness, for example as a consequence of electrochemical corrosion caused by oxidation, or by either direct or alternating current in locations of damaged insulation;

cracks caused, for example, by stress corrosion or fatigue;

geometric shape anomalies, caused by mechanical damage, such as dents or ovality;

other types of defect, emerging during pipe manufacture or due to welding imperfections, such as delamination or inadequate weld penetration in the welding joints.

In practice, more than a single type of defect can be found in a particular location on a steel pipeline; for example a dent may occur simultaneously with wall thinning (at the same point).


3

NOWADAYS, THE FOLLOWING TECHNOLOGIES CAN BE USED BY

A GAS NETWORK OPERATOR FOR THE ASSESSMENT OF THE TECHNICAL CONDITION OF HIGH-PRESSURE PIPELINES

internal geometry examination, with a geometry tool operating on the swing arm deflection principle, or with the use of eddy current-based contactless method;

inspection using the Magnetic Flux Leakage (MFL) method, either in Standard Resolution (SR) or High Resolution (HR);

inspection using guided waves-relying Electromagnetic Acoustic Transducers (EMAT), designed for the detection of gas pipeline wall cracks, especially Stress Corrosion Cracking (SCC) and fatigue cracks;

examination with eddy current-based Shallow Internal Corrosion (SIC) type tools, developed especially for the detection of internal gas pipeline wall defects up to 10 mm deep, which are detectable with a lower level of confidence through the use of MFL HR technology;

survey with the ultrasonic technology (US), for the detection of cracks in pipeline wall and welds, in particular Stress Corrosion Cracking (SCC) and fatigue cracks. The use of US technology in gas pipelines creates the need for the introduction of a liquid coupling medium between the sensors and the steel surface;


4

INSPECTION TECHNOLOGIES

examination with leak detection tools, enabling the detection and location of gas leaks by acoustic or pressure gradient measurement methods;

examination with XYZ Mapping tools, to determine three-dimensional geographical pipeline coordinates using internal navigation unit.

Among other possible inspection methods for pipelines with MOP>1.6MPa

used on industrial scale, inspections employing intelligent tools provide

the greatest amount of data, with the proviso that the above mentioned

technologies are able to detect various types of defects.

The selection by the network operator of the appropriate testing method

depends on the experience gained from previous inspections of each

pipeline, and particularly on the type of wall damage anticipated, based on

the available pipeline operational data. In some instances, it is justifiable

to combine two inspection technologies during a single tool run, for

example to apply a combination of magnetic and mapping technologies.


5

MATERIAL LOSS–TYPE DEFECTS

This type of flaws is connected with localized reduction in pipeline wall

thickness caused by various effects of the steel corossion process (the

photo on the left) and by the material loss occuring in gas pipe

manufacture (the photo on the right), which are detectable primarily with

the use of MFL technology.

In Polish gas network internal geometry survey tools and magnetic flux

leakage tools, detecting mainly gas pipeline wall material losses, have

been employed so far. The assessment of the long-term effect on the steel

gas pipeline operation parameters is the subject of analysis in this paper.


6

CALCULATION METHODS FOR DETERMINING THE PERMISSIBILITY OF THE WALL MATERIAL LOSS DEFECT IN A PIPELINE SUBJECTED TO

STATIC PRESSURE CAN BE CLASSIFIED ACCORDING TO THE FOLLOWING METHODS

analysis of limiting stress states, based on the permissible strains resulting from the wall material yield stress value;

use of the Finite Element Stress Analysis (FESA), for the nonlinear numerical computation of the components of permissible strains and stresses, which makes it possible to take into account complex load states as well as to analyse atypical shapes of wall material losses and put together defects of different types in the same location;

application of fracture mechanics methods using the failure assessment diagram (FAD), while regarding the wall material loss, in a model approach, as a notch.


7

THE RECENT ISSUES OF STANDARDS THAT PROVIDING

CALCULATION PROCEDURES FOR THE ACCEPTABILITY OF WALL

MATERIAL LOSS FLAWS OF STEEL PIPELINES ARE, AMONG

OTHERS, AS FOLLOWS

ASME B31G:2009 - the American Society of Mechanical Engineers

Standard, a Manual for Determining the Remaining Strength of

Corroded Pipelines: Supplement to ASME B 31:2009 Code for

Pressure Piping;

BSI BS 7910:2005 - the British Standard, a Guide to Methods for

Assessing the Acceptability of Flaws in Metallic Structures;

DNV-RP-F 101:2010 - Recommended Practice, Corroded Pipelines.


8

PLANAR SHAPE MODELS OF WALL MATERIAL LOSS DEFECTS TAKEN

FOR CALCULATIONS

t – design-rated wall thickness of a given pipe;

d – defect depth.

Cross-section of wall flaws of a gas

pipeline:

- parabolic;

- rectangular;


9

IN THE ASSESSMENT OF THE ACCEPTABILITY OF A SINGLE WALL

MATERIAL LOSS FLAW, TO DETERMINE THE CIRCUMFERENTIAL FAILURE

STRESSES ARE USED THE EQUATIONS (1,2,4) GIVEN IN ASME B31G:2009

)1(1.1

M3

21

3

21

1.1

M1.1

e

f

1e

f

1

e

f

t

d

R

t

dt

d

R

R

- for a defect in the entire wall cross-section, i.e. when d = t;

- for a partial gas pipeline wall loss defect, when: 0.1t < d < 0.8t

and M ≤ 4.12;

- for a partial wall loss defect, when: 0.1t < d < 0.8t

and M > 4.12.

2

Z

8.01M

tD

L

where:

σf - estimated circumferential stress level corresponding to the gas pipeline failure pressure, [MPa];

Re- specified yield strength at ambient conditions of given pipe steel, [MPa];

L - axial defect length, [mm];

d - depth of the metal loss, [mm];

DZ - the pipeline outer diameter, [mm];

t - design-rated wall thickness of a given gas pipeline, [mm].

(1)

(2)


(1)

10

With the estimated circumferential stress level σf of known, as determined

with the use of formulas (1, 2) corresponding to the failure in a gas pipeline

with a material loss defect, the static burst pressure Pf, can be determined

from the formula:

The pressure safety factor is defined by the relation (4):

ZD

tP

f

f

2

There are many modifications of the abovementioned limiting stress state

method, which enable the determination of material loss defect

acceptability, where the calculation procedures are based on identical

assumptions, and the formulas only differ in coefficients „M” and the

defect shape model.

(3)

MAOP

Pf (4)

RECENT ISSUES OF STANDARD DOCUMENTS


11

For a partial wall material loss defect, where:

0.1t < d < 0.8t and M ≤ 4.12,

we can plot a diagram of the allowable relative defect depth (d/t) as a

function of the so-called normalised defect length L/(Dz×t)0.5, where for a

gas pipeline length with a constant diameter and wall thickness, the

variable is L. We transform the formulas (1- 4) to the following form:

2

1

z

zfe

zfe

8,012

1,1

75,065,1

tD

L

t

DPR

t

DPR

t

d(5)

2

1

z

ze

ze

8,012

MOAP1,1

MAOP75,065,1

tD

L

t

DR

t

DR

t

d

kr

(6)



12



(7)

(8)

(9)

42

003375,06275,01

Dt

L

Dt

LM

3,3032,0

2

Dt

LM

Mtd

td

Re

f

/)/(85,01

)/(85,011,1

Numerous empirical and semi-empirical methods have been developed for

predicting the failure pressure of the pipeline with wall material loss defects

and the formulas only differ in coefficients and the defect shape model.

ASME Modified B31G:

For L2/Dt ≤ 50:

For L2/Dt > 50

(7)

(8)

(9)

13



(7)

(8)

(9)

Mtdy

tdy

tD

SMTStyMAOP

d

dm

/)/(1

)/(1

)(

2*

*

2

31.01M

tD

L

]/[)/(/*

tdStDtdtd dmeas

DNV-RP-F101 Recommended Practice, corroded pipes:

MOAP for a pipeline with corrosion defect is given by the acceptance equation with the partial safety factors:

ym – partial safety factor for model prediction;

yd – partial safety factor for corrosion depth;

Ɛd – factor for defining a fractile value for the corrosion depth;

D – nominal outside diameter.

The DNV-RP-F101 method can be applied to partial wall thinning with axial and bending loads.

(10)

(11)

(12)

14

Allowable operating pressure (MAOP) calculation results are provided for

several defects involving wall thickness reduction, which were obtained using

the standardised calculation method as per ASME B31G:2009.

For defects found as a result of the inspection of DN 700 gas pipeline no. 1

operating since year 1991, made of steel G-355 acc. to polish standard

PN-79/H74244 with Re = 355 MPa, calculations were made based on formulas

(1-3), whose results are summarised in the table below.


Defect no. Dz

[mm]

d

[mm]

t

[mm]

d/t

[-]

Dw

[mm]

L

[mm]

σf

[MPa]

Pf

[MPa]

D1 711 6.7 10.0 0.67 691 24 380.99 10.72

D2 711 4.0 10.0 0.40 691 15 388.74 10.94

D3 711 3.7 10.0 0.37 691 49 376.59 10.59

D4 711 3.6 10.0 0.36 691 37 382.15 10.75

D5 711 3.2 10.0 0.32 691 13 389.51 10.96

D6 711 3.2 10.0 0.32 691 22 387.75 10.91

where:

Dz –specified outside diameter of gas pipeline [mm];

The other designations are the same as in formulas (1-3).

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The diagram of the permissible relative defect depth (d/t) as a function of

L/(Dz×t)0.5 for the defects from Table 1 is shown in the Figure below, with the

pressure safety factor = 1.1, =1.25 and =1.39.


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A SUMMARY OF STEELS DESIGNED FOR MANUFACTURING PIPES ACCORDING TO CONTEMPORARY EUROPEAN STANDARDS


Standard

designation

Range of the required minimum yield strength in [MPa]

245÷450

290÷495

320÷525

360÷530

390÷545

415÷565

450÷600

485÷635

555÷705

API Spec 5L/

PN-EN ISO

3183

B

BN

BM

BQ

X42

X42N

X42M

X42Q

X46

X46N

X46M

X46Q

X52

X52N

X52M

X52Q

X56

X56N

X56M

X56Q

X60

X60N

X60M

X60Q

X65

X65N

X65M

X65Q

X70

X70N

X70M

X70Q

X80

X80N

X80M

X80Q

PN-EN

10208-2

L245NB

L245MB

L290NB

L290MB

–

L360NB

L360MB

L360QB

–

L415NB

L415MB

L415QB

L450MB

L450QB

L485MB

L485QB

L555MB

L555QB

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Assessment was presented for two example gas pipelines made of L485MB steel pipes conforming to PN-EN 10208-2:2011, having parameters as follows:

• Pipeline no. 2: Dz = 610 mm, MOP = 8.4 MPa, t = 11.3 mm (the pipes made

with no negative wall thickness deviation), Re0.5min= 485 MPa, with these

parameters indicating that the gas pipeline will operate at circumferential

stresses of up to 48% of the yield strength, which means the design factor

fo < 0.5, acc. to PN-EN 1594:2011.

• Pipeline no. 3: Dz = 914 mm, MOP = 8.4 MPa, t = 13.4 mm (the pipes made

with no negative wall thickness deviation), Re0.5min= 485 MPa, with these

parameters indicating that the gas pipeline will operate at circumferential

stresses of up to 61 % of the yield strength, which means the design factor

fo < 0.6.


CASE STUDY

18

Table 1. A summary of strength defect calculations for gas pipeline no. 2, Dz = 610 mm, MOP = 8.4 MPa

Defect

Re0.5min

[MPa]

DN d

[mm]

t

[mm]

d/t

[-]

L

[mm]

M

[-]

σf

[MPa]

Pf

[MPa]

MAOP

[MPa]

λ = 1,39

MAOP

[MPa]

λ = 1,65

MAOP

[MPa]

λ = 2,00

D1 485 600 4,10 11,3 0,35 210 2,4 450,54 16,69 12,01 10,12 8,35

D2 485 600 6,80 11,3 0,60 200 2,3 384,30 14,24 10,24 8,63 7,12

Table 2. A summary of strength defect calculations for gas pipeline no. 3, Dz = 914 mm, MOP = 8.4 MPa

Defect

Re0.5min

[MPa]

DN d

[mm]

t

[mm]

d/t

[-]

L

[mm]

M

[-]

σf

[MPa]

Pf

[MPa]

MAOP

[MPa]

λ = 1,10

MAOP

[MPa]

λ = 1,25

MAOP

[MPa]

λ = 1,39

D3 485 900 4,00 13,4 0,30 300 2,6 462,38 13,56 12,33 10,85 9,75

D4 485 900 6,00 13,4 0,45 300 2,6 422,25 12,38 11,26 9,90 8,91


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The values given in the last three columns of Tables 1 and 2 are calculated

with the use of formula (4) upon assuming the limiting values of the

pressure safety factor () in accordance with ASME B31.8S:2010 standard,

and are the values of the maximum allowable operating pressure possible

to be maintained in a given gas pipeline during its operation phase.

The above presented method, for analysing the acceptability of wall

material loss flaws depending on the maximum allowable operating

pressure is called the Fitness of Purpose Assessment (FPA) of a pipeline,

where the purpose is to transfer by the pipeline a certain amount of gas in a

time unit at the required pressure on delivery nodes.

The pressure values MAOP are input data as constraints at selected points

of the gas network for the calculation of the pipeline operational capacity,

which depends on many assumptions made by the operator.


20

For DN 600 gas pipeline no. 2 made of L485MB steel grade, operating at

circumferential stresses above 30% and not exceed 50% of the yield

strength, the diagram of the permissible relative defect depth (d/t) as a

function of L/(Dz×t)0.5, shown in the next figure, has been plotted for = 1.39;

= 1.65; = 2.00, taken from ASME B31.8S:2010:

= 1,39 – red curve refers to a re-inspection time interval of 5 years;

= 1,65 – yellow curve refers to a re-inspection time interval of 10 years;

= 2,00 – green curve refers to a re-inspection time interval of 15 years.

The defects of DN 600 gas pipeline no. 2, denoted by D1 and D2, have a

similar length, but significantly differ in depth, which constitutes,

respectively, 35% and 60% of the wall thickness. Considering the design

pressure safety factors for > 1.39 (see the red curve in the figure below),

neither the D1 defect nor the D2 defect requires immediate repair.


INSPECTION INTERVALS

21

For DN 600 gas pipeline no. 2, the diagram of the permissible relative

defect depth (d/t) as a function of L/(Dz×t)0.5, shown in the figure below.

D1

D2

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

0 0,5 1 1,5 2 2,5 3 3,5 4 4,5 5

d/t

L/(Dzt)0.5

λ=2,00

M≤4

,12

λ=1,65

λ=1,39


22

The next diagram of (d/t) as a function of L/(Dz×t)0.5, in the case of DN 900

diameter MOP = 8.4 MPa L485MB grade steel gas pipeline no. 3 operating at

circumferential stresses within 50-72% of the yield strength is presented for:

= 1,1 – red curve, refers to rapid direct assessment (DA);

= 1,25 – yellow curve, refers to a re-inspection time interval of 5 years;

= 1,39 – green curve, refers to a re-inspection time interval of 10 years.

ASME standard requires, if the diagnostic examination finds corrosion

defects in the wall of gas pipeline with a pressure safety factor of ≤ 1.1 or

in case of finding any metal losses in contact with factory longitudinal pipe

seams made by one of the following techniques:

• direct current welding;

• low frequency electric resistance welding;

• electric flash welding;

the operator shall be obliged to perform the direct assessment of each

defect within a time not exceeding 5 days.


23

The diagram of the permissible relative defect depth (d/t) as a function of

L/(Dz×t)0.5, in the examined case of DN 900 MOP = 8.4 MPa gas pipeline no. 3.


24

Estimation of the network capacity will be subject to certain variables and

constrains:

a) network topology: diameter, lenght, input and output nodes,

roughness factor, compressor unit data;

b) fluid specification: compositional range, density;

c) adopted model: Hydraulic equations used for simulations,

mathematical model of the network, model of the heat transfer;

d) border conditions: minimum and maximum pressures, flow rates;

e) demand characteristics: peak and average flow, demand curve,

contractual obligations;

f) operational constraints: reliability standards, maintenance practice.

Considering the above arguments, it seems reasonable to discuss only

the capacity limitations of concrete gas network.

NOMINAL TRANSMISSION CAPACITY


25

SCHEME OF SIMULATED GAS NETWORK

Operational capacity: the maximum aptitude of the system to transport

certain amount of gas, calculated assuming steady state condition and

isothermal gas flow.

In this papier capacity measure unit is the cubic meter in reference

conditions (273,15K, 101325 Pa) per hour (Qc) and in the case where the

simulated system operational capacity is the sum of flow rates:

Qc= Qz + Qp (13)


(13)

26

a) Pipeline no. 4: MOP 8,4 MPa, DN 1400 made of steel grade L485 MB,

acc. PN-EN 10208-2, inside diameter Dw=1384mm, roughness factor

k=2 µm.

b) Pipelines’ total lenght in the network is Lc= 685 km, segment distances

between compressor stations: LGr-T1= 2 km, LT1-T2= 126 km,

LT2-T3=118 km, LT3-T4= 121 km, LT4-T5= 178 km, LT5-Gr= 140 km.

c) Flow with the average gas temperature Tmean= 288 K.

d) For simulations the adopted model of turbulent steady state flow with

lambda=f(Re, Ɛ), commercial software was employed.

e) Lack of regulation valves, which means lack of local pressure drops.

f) Steady state of the heat transfer was assumed and constant value of

the heat-transfer coefficient between gas and the pipe gs=190 W/m2K.

g) Density of the transported gas ρ=0,75 kg/m3, fluid specification „E”,

acc. PN-C 04750, gas composition: XCH4= 97,6%, XCO2= 0,88%.

h) Range of regulated power of the compressor unit 5 MW ≤ N ≤ 25 MW.

i) Range o regulated pressure ratio of the compressors 1,10 ≤ Ɛ ≤ 1,45.

ASSUMPTIONS MADE BY THE CAPACITY CALCULATIONS


27

Relying on the equation (2) with constraint M ≤ 4,12, we calculate the

maximum defect lenght for the case of the studied pipeline Llim= 474 mm.

In the examination of DN 1400 MOP=8.4 MPa pipeline, made from steel

L485 MB, with = 1,39 the wall material loss flaw relative depth is

calculated from the equation (6): for L=300 mm → (d/t)kr= 0,56; for

L=400 mm →(d/t)kr= 0,52; for Llim= 474 mm, → (d/t)kr= 0,50.

In case of wall thinning lenght L>Llim, the MAOP value depends only on

relative defect depths obtained from equation (1), which gives the really

less calculated pressure, e.g. d/t= 0,60 → MAOP=4.1 MPa.

For the capacity analysis will be used material loss-type-flaw on the pipe

DN 1400, which corresponds to the MAOP=7.12MPa for the pipe segment

with a defect.

CAPACITY CALCULATIONS OF DEFECTED PIPES


28

a) In order to estimate the influence of the flaws’ presence on the system

capacity, several cases of defect locations on pipes between compressor

stations were made: LGr-T1= 2 km, LT1-T2= 126 km, LT2-T3=118 km,

LT3-T4= 121 km, LT4-T5= 178 km, LT5-Gr= 140 km.

b) The discharge pressure of the downstream compressor unit can’t exceed

the calculated MAOP value for the relevant pipeline segment.

c) The minimum input/ output pressure of the system on the east and west

metering points: OPin min= 6.1 MPa and OPout min= 6.1 MPa.

d) Constant gas demand conected to compressor station „T4”:

Qp=300 000 m3/h (standard cubic meter pro hour).

e) For the pipeline without the defects, we obtained calculated gas flowrate

Qz= 4 420 000 m3/h, which gives total capacity Qc= 4 720 000m3/h.

f) The maximum annual amount of gas transported through the system,

assuming 8760 h/year utilisation is 41 347 Mm3/year, .

CAPACITY CALCULATION RESULTS


29

RESULTS OF PRESSURES AND CAPACITIES

Node

No.

Pressure

[MPa]

Maximum

volumetric gas

flow rate

Qc [m3/h]

Maximum

volumetric gas

flow rate

Qz [m3/h]

1s 6,073

3 750 000 3 450 000

1t 8,400

2s 7,133

2t 8,400

3s 7,225

3t 8,400

4s 7,196

4t 7,196

5s 5,219

5t 7,613

Case no. 1 defect on the fourth pipeline segment. Case no. 2 defect on the fifth pipeline segment.

Node

No.

Pressure

[MPa]

Maximum

volumetric gas

flow rate

Qc [m3/h]

Maximum

volumetric gas

flow rate

Qz [m3/h]

1s 6,079

3 350 000 3 050 000

1t 8,400

2s 7,405

2t 8,400

3s 7,476

3t 8,400

4s 7,454

4t 8,400

5s 7,220

5t 7,220


s – suction; t - discharge.

30

PRESSURE DECREASE TO MAOP=7.12 MPa CAUSED BY DEFECT ON

EACH PIPILINE SEGMENT

a) LGr-T1= 2 km – no influence on pipeline capacity, cause OPwej min< MAOP;

b) LT1-T2= 126 km, Qz1 = 3 980 000 m3/h, it means decrease of - 10,0 %;

c) LT2-T3=117 km, Qz2 = 3 980 000 m3/h, it means decrease of - 10,0 %;

d) LT3-T4= 122 km, Qz3 = 3 710 000 m3/h, it means decrease of - 16,0 %;

e) LT4-T5= 177 km, Qz4 = 3 450 000 m3/h, it means decrease of - 21,9 %;

f) LT5-Gr= 139 km, Qz5 = 3 050 000 m3/h, it means decrease of - 31,0 %.

RELEVANT CONSTRAINTS IN THE ANALYSED SYSTEM

- case c) the power of compressor unit T2 is N=8.3 MW, with press.ratio Ɛ =1.06;

- case d) compressor station T3 must be shuted off;

- case e) compressor station T4 must be shuted off;

- case f) compressor station T5 must be shuted off, in spite of the pressure in

node OP5t = 7.22 MPa is higher than the limited value of MAOP=7.12 MPa.


31

INFLUENCE OF THE PRESSURE DECREASE ON EACH PIPILINE

SEGMENT ON THE TOTAL GAS SUPPLY SYSTEM CAPACITY

a) LGr-T1= 2.5 km – no influence on pipeline capacity, cause OPwej min< MAOP

b) LT1-T2= 126.5 km, Qc1 = 4 280 000 m3/h, it means a decrease of - 9,3 %;

c) LT2-T3=118 km, Qc2 = 4 280 000 m3/h, it means a decrease of - 9,3 %;

d) LT3-T4= 122 km, Qc3 = 4 010 000 m3/h, it means a decrease of - 15,0 %;

e) LT4-T5= 177 km, Qc4 = 3 750 000 m3/h, it means a decrease of - 20,2 %;

f) LT5-Gr= 139 km, Qc5 = 3 350 000 m3/h, it means a decrease of - 29,0 %.

The case study presented above shows that the magnitude of the decrease

in capacity depends strongly on the location of the defect in the gas

supply system.

In any pipeline operating around the world, the transmission capacity is a

controversial issue and requires a complete description of the framework

and assumptions made.


32

DETECTION OF THE WALL DEFECTS USING IN-LINE INSPECTION

TECHNOLOGIES AND EVALUATION OF THE INFLUENCE OF DEFECTS

ON THE MAXIMUM OPERATING PRESSURE ALLOW THE PIPELINE

OPERATOR TO MANAGE THE GAS NETWORK CAPACITY

AND TO EXTEND THE LIFETIME OF THE PIPELINE.

→


CONCLUSION

33 33


Thank you for your attention

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