Research Papers: Materials Technology

Fatigue Testing of Full Scale Girth Welded Pipes Under Variable Amplitude Loading

[+] Author and Article Information
Y.-H. Zhang

TWI Ltd.,
Cambridge CB21 6AL, UK
e-mail: yanhui.zhang@twi.co.uk

S. J. Maddox

TWI Ltd.,
Cambridge CB21 6AL, UK

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received January 23, 2013; final manuscript received October 15, 2013; published online January 17, 2014. Assoc. Editor: Xin Sun.

J. Offshore Mech. Arct. Eng 136(2), 021401 (Jan 17, 2014) (10 pages) Paper No: OMAE-13-1014; doi: 10.1115/1.4026025 History: Received January 23, 2013; Revised October 15, 2013

In the fatigue design of steel catenary risers, there are concerns regarding the fatigue damage to girth welds from low stresses, below the constant amplitude fatigue limit, in the loading spectrum and the validity of Miner's cumulative damage rule. These fundamental issues were addressed in a recent joint-industrial project (JIP). A key feature was development of the resonance fatigue testing rigs to enable them to test full-scale pipes under variable amplitude loading. Such tests were performed under a loading spectrum representative of that experienced by some risers, with many tests lasting over 100 million cycles to investigate the fatigue damage due to small stresses as well as the validity of Miner's rule. However, the resonance rigs are only capable of producing spectrum loading by gradually increasing or decreasing the applied load whereas more “spiky” random load sequences may be relevant in practice. Therefore, the program also included fatigue tests in conventional testing machines on strip specimens cut from pipes to compare the two types of loading sequence. This paper presents the results of these tests, conclusions drawn, and recommendations for changes to current fatigue design guidance for girth welded pipes regarding the definition of the fatigue limit, allowance for the damaging effect of low stresses, and the validity of Miner's rule.

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Fig. 1

Strip specimen dimensions and strain gauge locations

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Fig. 2

Resonance fatigue testing of the full-scale pipes

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Fig. 3

Stress distribution and definition for each subspectrum

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Fig. 4

An actual cyclic loading sequence (peak stresses in each loading and unloading cycle) that illustrates the good repeatability of the loading block in the VA testing

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Fig. 5

Loading patterns of the subspectrum VT-2, used for VA testing of the strip specimens: (a) sequential loading (same sequence as that used for the full scale pipes) and (b) random loading

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Fig. 6

Comparison of the constant amplitude fatigue performance of the full-scale and strip specimens with the class D mean curve

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Fig. 7

Comparison of variable amplitude test results for the full-scale pipes with bilinear versions of constant amplitude S-N curves, with VA data presented in terms of equivalent stress range calculated using a bilinear S-N curve with slope change from 3.0 to 5.0 at 5 × 107 cycles

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Fig. 8

Variable amplitude fatigue data obtained from full scale pipes and strip specimens illustrating effect of loading sequence (sequential or random) on fatigue performance

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Fig. 9

Macrosections of the two welds in pipe S12 after being tested under CA loading for 108 cycles: (a) weld W1, showing a 0.14 mm deep crack at the toe of the weld root bead and (b) weld W2, with no evidence of cracking in the region of poorest profile (WRBH = 0.47 mm). The definitions for hi-lo and WRBH are shown in the figure.

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Fig. 10

Macrosections of the two welds in pipe S15 after being tested under VA-5 loading (minimum stress range of 40 MPa) for 1.96 × 108 cycles, showing fatigue cracking at the toe of weld root bead: (a) W1 weld, 0.19 mm deep crack and (b) W2 weld, 93 μm deep crack that initiated in a region of poor profile (WRBH = 0.58 mm)

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Fig. 11

Dependence of the ratio of the calculated fatigue life based on a bilinear S-N curve to that based on a single slope S-N curve on the minimum stress range in the applied loading spectrum




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