Page 412 - Marine Structural Design

388 Part III Fatigue and Fracture

Specific Hotspot Analysis
Elements that do not pass the initial hotspot screening need to be reanalyzed using SCFs and
the associated S-N curves that are more appropriate for the actual structural detail and welding
procedure to be used. The calculated damages are reviewed and, at least, all elements with
fatigue lives less than the minimum required are summarized for further review and potential
redesign andor modification of welding procedures, and reanalysis.

Specific Hotspot Design
Structural details that do not pass the specific hotspot analysis are redesigned to improve their
fatigue strength. SCFs and associated S-N curves that are appropriate for the redesigned
structural details and welding procedures will be used in the fatigue reanalysis. All structural
details must meet the minimum fatigue requirements after their re-design and welding
procedures are finalized.
Detail Improvement
It is clear that the best time to improve the fatigue strength of welded structural details is
during the design stage. There are two factors, which need to be specially considered when
improving the fatigue strength of a structural detail:
Nominal stress level
The most efficient approach to improving fatigue strength is to increase the local scantling and
to configure the additional load path within the structure. This approach may reduce the
nominal stress level and hence the hotspot stress for a given structural detail.
Geometrical stress concentration
Adopting a good design of detail configuration by providing softer connections reduces the
geometrical stress concentration factor originally caused by the geometric discontinuity. It is
the most effective technique to improve fatigue strength. However, this technique usually
requires good workmanship since a soft toeiheel is used.

20.5.5 Fatigue Analysis and Design Checklist
Each item in the following checklist should be checked prior to the completion of fatigue
analysis:
Computer model topology - the model is plotted in sufficient views to validate model
connectivity.
Loading conditions - each applied loading condition is checked for accuracy.
Analysis and validation - Analysis results are checked step-by-step; discrepancies between
expected and obtained analysis results should be documented and explained.
Loading combinations - each applied loading combination should be summarized and
checked for accuracy.
Environmental conditions - wave scatter diagram and directional probability input should
be checked.
SCFs - SCFs used in the analysis should be confirmed for validity and applicability.
S-N curves - S-N curves used in the analysis should be confirmed for validity and
applicability.