Deep wind´2014-11th Deep Sea offshore wind R&D Conference, 22th-24th January 2014, Trondheim, Norway

Assessment of environmental influence on fatigue crack growth in an Electron Beam (EB) welded flange connection

P. Nourya, M. Pavlovica, M. Möllera, M. Veljkovica

aLuleå University of Technology, Division of Structural and Construction Engineering, Luleå 97187, Sweden

Abstract. Fatigue assessment of the ring-bolted flange connection under variety of load levels and corresponding number of cycles has shown to be more critical than that of fracture

using Engineering Critical Assessment (ECA). The main objective of this paper is to determine the maximum acceptable flaw size of wind tower flanges which have a weld made by

electron beam welding. The lowest temperature of a construction site used in the case study is -40 C. Comparison is made between on-shore and off-shore conditions according to the

recommended Paris law parameters acc. to BS 7910 and ASME, Sect. XI. Fatigue crack growth for a range of flaw length/depth (aspect ratio) from 1 to 10 is considered for centric and

eccentric embedded flaw in a plate t=24mm (the shell segment closest to the ring flange), and internal and external circumferential surface flaw in the shell.

Is the determined

maximum allowable flaw

size smaller than the

inspected flaw size?

No

Inputs

Tower properties

(flange geometry)

Inspection

(flaw geometry)

Cross-sectional

forces

Material properties

& fatigue data

Engineering Critical Assessment (ECA)

Onshore Offshore

Fracture

analysis

Fatigue

analysis

Fracture

analysis

Fatigue

analysis

SCC

analysis

Assessment of maximum allowable flaw size

Calculate the set of possible maximum

allowable flaw sizes

Sensitivity

analysis

Finish

Propose a reparation action

Yes

METHODOLOGY CASE STUDY

MAIN RESULTS

• FEA using the commercial ABAQUS

finite element software have been used

to define the stress levels of 3374 mm

diameter EB welded ring flange

connection used in towers for wind

turbines.

• The fracture toughness of the flange

made of S355 at -50°𝐶 (the most sever

operational conditions) has been

determined using the Charpy values in

terms of T27J , and master curve

approach.

• Fatigue assessment using Paris law is

performed according to BS 7910 and

ASME.

• The maximum allowable size of flaws,

assuming the same loading conditions

both for on-shore and off-shore

conditions during wind tower’s lifespan

(25 years), have been determined for

the sake of comparison.

• Effect of flaw geometries on the fatigue

crack growth, internal and external

surface flaws in cylinder oriented

circumferentially and centric and

eccentric embedded flaws in plate have

been considered.

0

1

2

3

4

5

6

7

8

9

10

11

12

0 5 10 15 20 25 30 35 40 45 50

Fla

w D

ep

th (

2a, em

bed

ded

/a,

su

rfac

e )

, m

m

Flaw Length 2C, mm

Centric embedded flawin plate

Eccentric embedded flaw(e = 4 mm) in plate

Eccentric embedded flaw(e = 10 mm) in plate

Internal surface flaw incylinder orientedcircumferentially

External surface flaw incylinder orientedcircumferentially

2a

2c

𝒕/𝟐+𝟎

t

2a

2c

𝒕/𝟐+𝟒

t

2a

2c

𝒕/𝟐+𝟏𝟎

t

The set of acceptance flaws calculated for internal and external surface flaw

position in the shell (circumferential orientation) and centric and eccentric

embedded flaw in the shell for ferritic steels in the air environment (acc. to the

recommended Paris law parameters in ASME, Sect XI).

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

2.8

3

0 2 4 6 8 10 12 14 16 18 20

Fla

w D

ep

th 2

a, m

m

Flaw Length 2C, mm

Eccentric embedded flaw for fatigueof steel in air environment (BS7910)

Eccentric embedded flaw for fatigueof ferretic steel in air environment(ASME)

Eccentric embedded flaw for fatigueof steel in a marine environment withcathodic protection at -850 mV(BS7910)

Eccentric embedded flaw for fatigueof steel in a marine environment withcathodic protection at -1100 mV(BS7910)

2a

2c

𝒕/𝟐+𝟏𝟎

t

The acceptance criteria calculated for an eccentric embedded flaw (e = 10 mm) in a

plate for fatigue of steel in the air and marine environment (acc. to the

recommended Paris law parameters in BS 7910 and ASME, Sect XI).

CONCLUSIONS

• Eccentric embedded flaw (e = 10 mm) represents most critical acceptance criteria of those considered.

• ASME is predicted less conservative acceptance criteria in comparison to BS 7910 for fatigue of steel in air or other non-

aggressive environments.

• Eccentric embedded flaw (e = 10 mm) for fatigue of steel in a marine environment with cathodic protection at -1100 mV (according

to the recommended fatigue flaw growth parameters in BS 7910) represents most critical acceptance criteria.

• The validation study demonstrates that the ECA procedure can be used for failure assessment of wind turbine tower structures containing flaws.

Refrences:

• BS 7910:2005. Guide to methods for assessing

the acceptibility of flaws in metallic structures. The

British Standard, July 2005.

• P. Dillstrom et al. A combined deterministic and

probabilistic procedure for safty assessment of

components with cracks handbook. Swedish

Radiation Safety Authority, 2004.

EB Weld

Acknowledgment. The research leading to these results has received funding from the European Union's Seventh Framework

Programme managed by REA-Research Executive Agency http://ec.europa.eu/research/rea (FP7/2007-2013) under grant agreement

n° FP7-SME-2011-286603