Drop-Weight Tear Testing -- CSM -- 13 Nov 2013

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Drop Weight Tear Testing

General Issues

A. Fonzo


November 2013 Oil & Gas Business Area

Contents

Introduction

Materials selection for low temperature environment

DWTT specimens manufacturing

DWTT fracture appearance (regular or abnormal)

Historical basis of Battelle 85% Shear Area criterion

Current industry concerns

Open issues



Charpy V-Notched specimens may behave ductile but the pipe is brittle at the same temperature.

A full-thickness specimen (such as DWTT) predicts better the full scale pipe behavior.

DWTT, Drop Weight Tear Test (API RP 5L3) is used to investigate the conditions required for propagation of brittle/ductile fracture in line pipe (LP) and for measuring the LP material fracture appearance or fracture surface ductility.

The 85% shear area transition temperature Battelle criterion has been found to provide a good estimate of the full scale pipe’s FPTT.

The Submission to ISO TC67 SC2 WG16 is proposing an amendment of the ISO 3183, requiring that API PSL2 pipe be DWTT tested down to and including DN300 instead of the present cut-off of DN500.

This latter issue re-opened a series of issues to be investigated

Introduction



Outline

Introduction






Open issues



Problem:

Selection of suitable material for service in artic environment

Approach:

Fracture mechanics testing (CVN, DWT, DWTT, Wide Plate) at low temperature according to recognized Standards and Recommendations (ASTM E208, API RP 5L3, etc.).

Full scale burst testing (the so called West Jefferson test) on linepipe cooled down to -50°C.

Results:

Nil Ductility Temperature determination, Charpy-V and DWTT brittle-to-ductile transition temperature determination on laboratory (specimen) and full scale (pipe) condition.

Materials Selection for Low Temperature environment

DWTT lab test @ low temp West Jefferson test @ low temp

Ductile fracture in a West Jefferson test



Instrumented DWTT machine @ CSM

Typical test outcome

Materials Selection for Low Temperature environment

0

2

4

6

8

10

12

14

16

0

50

100

150

200

250

300

350

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0.000 0.005 0.010 0.015

En

erg

y [

kJ]

Lo

ad

[k

N]

Time [s]

DAL-C-CG-01

Load Energy

Initiation of fracture propagation

Initiation energy

Applied Load

Total absorbed energy



Outline

Introduction






Open issues



Pressed notch

Chevron notch

DWTT specimen manufacturing

Manufacturing quality is important for results repeatability



Gull-Winged specimens (mandatory for pipe geometry with OD/WT<40, acc. API RP 5L3)

Pressed notch Chevron notch





Gull-wing shaped specimen configuration

Straight flattened specimen configuration









ST-DWTT: standard pressed notchCN-DWTT: chevron notch, angle=60°

SCN-DWTT: slit chevron notch, angle=90


Influence of notch on Transition Temperature

Source:

T. Kubo, et Al. “Quality Evaluation of Line Pipes by Chevron Notched Drop Weight Tear Test”, ISIJ International, Vol.35, No.4, pp.426-432, 1995



Outline

Introduction






Open issues



Regular fracture of DWTT Differences in the Recommendations

According to API RP 5L3

According to DNV-OS-F101

ductile initiation under the notch is a matter of concern, especially for high upper shelf CVN energy steels



Regular fracture of DWTT

Notch

Brittle fracture

Shear fracture

Regular fracture – Valid tests



Inverse fracture Concern – Invalid tests

Notch

Brittle fracture

Shear fracture

Abnormal fracture of DWTT



Invalid tests according to API RP 5L3




Source:

R. Muraoka, et Al. “Evaluation for Abnormal Fracture Appearance in Drop Weight Tear Test with High Toughness

Linepipe”, OMAE2002-28183, Norway, 2002




Outline

Introduction






Open issues



Source:

Maxey, W.A., Kiefner, J.F. and Eiber, R.J. “Brittle Fracture Arrest in Gas Pipelines”, PRCI Report No. L51436, April 1983.

Brittle fracture propagation is prevented by ensuring that a DWTT specimen exhibits 85% shear area on fracture surface (this ensures that the steel is operating on the upper shelf).

ISO 3183 85% shear area criterion



DWTT

Full Scale

85% SA

…but curves are fitted on very scattered data

ISO 3183 85% shear area criterion

Source:




DWTT fitting Full scale burst test fitting

Source:


Data set is highly scattered: at the same temperature min and max Shear Area can differ by up to 0.5

Historical basis of 85% shear area criterion



Outline

Introduction






Open issues



If/How DWTT testing parameters can influence results and BDTT?

Influence of notch configuration on BDTT needs further investigation (the most representative notch choice seems to be pipe WT dependent)

DWTT specimen manufacturing problems (heavy-walled pipes, low OD/WT ratio)

Abnormal Fracture Appearance (AFA) in DWTT: ductile fracture initiation under the notch and inverse fracture invalidate QA tests

DWTT representativeness of full-scale results especially for modern steels (high Charpy-VN shelf energy)

Summary of current industry concerns



Outline

Introduction






Open issues



The influence of testing parameters should be better investigated and clarified (DWTT machine capacity, velocity at impact, etc).

Repeatability: the material fracture behavior in DWTT testing is affected by specimen preparation (flattening, notch type, etc) and geometry (thickness).

The type of specimen (straight flattened or gull-wing shaped) can affect the results especially on low OD/WT pipes.

Is it feasible to realize non-flattened specimens from heavy-walled pipes?

Ductile fracture initiation under the notch, especially for high toughness steels.

Reverse fracture is a DWTT test peculiarity (not present on line-pipes):

If inverse fracture occurs, the test is invalid

Influence on thickness and microstructure needs to be further investigated

How to account for reverse fracture remains an open issue:

Standards do not account for it (except DNV)

Many routes are currently followed while a unique criterion should be identified

Open Issues



PRCI (Pipeline Research Council International) research project MAT-8-1 Phase 1 Design Materials & Construction Technical Committee

Aim :

To develop methods that may allow the industry to better design safe gas pipelines made from modern steels

Outcome from Phase 1 (concluded) :

Criticalities of the DWTT and concerns with propagating cracks in gas pipelines made from modern steels with high upper-shelf Charpy Energy

Focus on sweet gas transmission pipelines with outer diameter over 12”, made from API X65, X70 and X80 steels

CSM Work for PRCI



Thank you for your attention!

Dr. Gianluca Mannucci Oil and Gas Business Area Director E-mail: [email protected] Phone +39 06 5055325

Dr. Andrea Fonzo Oil and Gas Business Manager E-mail: [email protected] Phone: +39 06 5055402

CONTACTS:

Drop-Weight Tear Testing -- CSM -- 13 Nov 2013

Documents

dwtt brittle

oil gas business area

low temp ductile fracture

low temp west jefferson

fracture surface ductility

scale pipe condition

scale pipe behavior

open issues