4.1.1 In general, any discontinuity in a stressed structure results in a local
increase in stress at the discontinuity. The ratio of the peak stress at the
discontinuity to the nominal average stress that would prevail in the absence of the
discontinuity is commonly referred to as the stress concentration factor (SCF). The
peak stress (i.e. nominal stress x SCF) is normally used in conjunction with an
appropriate S-N curve to derive the estimated fatigue life.
4.1.3 Stress concentration factors may be derived using a number of different
methods, such as finite element techniques, closed form analytical formula or from
model tests. For complex arrangements, a detailed finite element based analysis will
most likely be required.
4.1.4 For semi-submersible units, experience has shown that the areas of
minimum fatigue life are usually found at the joints, stiffener terminations,
penetrations in primary bracings and also at their junctions with hull, columns and
decks. For jack-up structures locations of minimum fatigue life are usually found on
the lattice legs and support structure. Other structures subjected to significant
cylic loading also require assessment.
4.1.5 Stress concentration factors for standard details may be determined from
LR‘s technical report prepared for the UK HSE, OTO 97-024 Geometric Stress
Concentration Factors for Classified Details, or an equivalent standard.
Stress concentration factors for tubular brace to chord connections may
be determined from LR‘s technical report prepared for the UK HSE, OTH 91-353
Stress Concentration Factors for Tubular Complex Joints, or an equivalent
standard.
4.1.6 Where finite element methods are used to determine local stress
distributions for fatigue assessment, the geometric hot spot stress should account
for the effect of structural discontinuities, excluding the presence of the weld.
Misalignment of structural members should be accounted for where applicable.
4.1.7 The geometric hot spot stress is to be determined at 0,5 x plate
thickness away from the point of interest (normally the discontinuity).
4.1.9 The maximum fabrication axial misalignment for fatigue prone locations
would normally be limited to the smaller of 0,1 x t or 3 mm.
where
t = thickness of thinner plate
For this guidance, it may be assumed that the effects of these maximum
fabrication misalignments are included within the S-N classification. Angular
misalignment is to be mutually agreed between the designer and the fabricator, and
is to be acceptable to LR.