Fatigue Assessment

1 · Input data

The permissible fatigue stress range of a welded connection, by the Simplified Fatigue Assessment Method of the ABS Guide for Fatigue Assessment of Offshore Structures (2003), Section 5 — as the office’s own Simplified fatigue.xls implements it. This page is that spreadsheet, made into a calculator: every cell number it prints is the sheet’s own, so the two can be read side by side, and the S-N curve table was read out of the sheet’s own cells J6:Q32. The method is a two-parameter Weibull distribution of stress range, a two-slope S-N curve, and the cumulative damage D in closed form through the incomplete gamma functions. D ≤ 1 passes.

Every field is required, and every one must be greater than zero. A field that is blank, not a number, zero or negative is refused by name in the pop-up and nothing is computed. The only things this page does not ask for are the ones the method cannot supply: the Weibull shape parameter g, the reference period YR, the stress response SR and the fatigue design factor DFF are the engineer’s, from the environmental data and the consequence category the Guide’s own tables name.

Two things the sheet’s curve table carries, which this page states rather than hides. The sheet’s DNV-(A)-D/T and DNV-(CP)-D/T rows each hold two curves with different thickness exponents (q = 0.2 for the D variant, 0.25 for the T variant), so they are offered here as four curves — one q each — and the wrong exponent cannot be applied. The sheet’s ABS-(FC)-F row holds C = 3 where the curve’s own m = r and NQ = 10⁸ make it single-slope, so C must equal A = 2.1×10¹¹; the table carries the corrected value. The sheet’s LR-curves row is not a curve — its cells are the text “same as ABS-(A)-curves”, and the row below says “multiply 0.5 on life based on (A)-curves and no slope change for N > NQ” — so the LR family is a rule over the (A) curves and is not offered; the sheet’s DNV-(A)-F row carries no numeric parameters and is not offered either.

2 · Solution