ssc-367 - Ship Structure Committee

The DnV X-curve and the DEn Guidance Note Q-curve of 1977 were also
based on the original AWS test data and the recommended S-N curve.
Recent experimental work carried out in Europe has provided
additional data on fatigue strengthof tubular joints. Statistical
evaluation of these test results provided the basis for revision of
both the DnV (Reference 4.17) X-curve and the DEn Guidance Notes
(Reference1.6) Q-curve. As illustratedon Figure 4-5, the slopeof
the new T-curve is steeper and typically results in lower lives,
often necessitatingan increaseinwall thickness. The DEn Guidance
Notes reconunendedT-curve is identical to the DnV T-curve up to 10
million cycles for catholicallyprotected areas.
The basis for the revision of the S-N curves by both DnV and DEn is
primarily due to evaluation and assessmentof test data. While the
AWS data are based on some plate and some small-diameter thin-wall
sections,the Europeandata are obtainedmostly from largerdiameter
tubulars with 5/8 inch and 1-1/4 inch (16 mm and 32 mm) wall
thicknesses. It appears that an inverse log-log slope of 3.0
(versus4.38 for the API X-curve)was chosen for the T-curve because
of the scatter of data and to ensure consistency with the British
Standards BS5400. Basedon statisticalevaluationoftest data and
Gurney’s (Reference4.19) analyticalstudieson plate thickness,the
T-curve is adjusted due to changes in plate thickness.
Although the DnV (Reference4.17) document states that all tubular
joints are assumed to be of Class T, an X-curve is also considered
acceptable,providedweld profiling is carried out. The comparison
of the API X-curve and the T-curve (Figure 4-5) shows that the two
curves intersect at about 500,000 cycles and would yield similar
1ives for a plate thickness of 1-1/4 inch (32 mm). However, for
plate thicknesses greater than 1-1/4 inches the use of a T-curve in
the computation of fatigue lives will result in shorter lives.
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