Strutures, Safety and Reliability

Fatigue Testing and S-N Data for Fatigue Analysis of Piles

[+] Author and Article Information
Inge Lotsberg, Stig Wästberg, Hugo Ulle

 DNV Veritasveien 1, 1322 Høvik Norway

Per Haagensen

 NTNU, 7491 Tronolheim, Norway

Michael E. Hall

 ConocoPhillips Norge AS, Ekotiskveien 35, Tananger 4064 Stavanger, Norway

J. Offshore Mech. Arct. Eng 132(4), 041602 (Sep 23, 2010) (7 pages) doi:10.1115/1.4001417 History: Received September 20, 2008; Revised December 16, 2009; Published September 23, 2010; Online September 23, 2010

Design S-N curves in design codes are based on fatigue test data, where the stress cycle is under external tension load. It is observed that during pile driving most of the stress cycle is compressive and the design procedure used for fatigue analysis of piles might therefore be conservative. In order to investigate this further, it was proposed to perform laboratory fatigue testing of specimens that are representative for butt welds in piles under relevant loading conditions. In the present project 30 test specimens made from welded plates were fatigue tested at different loading conditions to assess effect of compressive stress cycles as compared with tensile stress cycles. In 2006, the Edda tripod in block 2/7 was taken ashore. This platform has been in service since 1976 and the piles are considered to be representative for the piles installed in the North Sea jacket structures during the 1970s. Therefore it was suggested to investigate the pile weld at the sea bed in detail to assess the stress due to fabrication and 30 years of in-service life and the residual fatigue life of the pile. Six test specimens made from the Edda pile were fatigue tested. The results from the assessment and the fatigue testing are presented in this paper.

Copyright © 2010 by American Society of Mechanical Engineers
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Figure 1

Shake-down in a ship structure

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Figure 2

Section of pile from Edda platform

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Figure 3

Macrograph of section Edda pile

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Figure 4

Section through weld

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Figure 5

Strain gauges on specimen

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Figure 6

Fatigue test data without including cycles from pile driving simulation

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Figure 7

Fatigue test data including number of cycles from pile driving as derived from Eq. 3

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Figure 8

Photo of fractured specimen 3–5

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Figure 9

Test data from the specimens from the Edda pile

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Figure 10

Calculated standard deviation as function of reduction factor on compressive stress

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Figure 11

Resulting S-N data for reduction factor α=0.8 in Eq. 6

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Figure 12

Stress cycling in actual pile

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Figure 13

Stress cycling in test specimen




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