Page 161 - Marine Structural Design

Chapter 7 Limit State Design of Offshore Structures 137

range is then defined through a short-term Rayleigh distribution within each sea state for
different wave directions.
Sefection of S-N Curve: For each critical location considered in the analysis, S-N curves are
assigned based on the structural geometry, applied loading, and welding quality.
Fatigue Analysis and Design: Several levels of fatigue analysis may be performed, including:
Fatigue screening
Detailed analysis
Reanalysis of welding improvements
Reanalysis of design improvements
Reanalysis of design and welding improvements
Fatigue Refiabifity: Each early step involves the considerable uncertainty. There are many
sources of complex interrelated uncertainties and variations. It is the primary purpose of a
fatigue reliability analysis to logically organize these sources, and then to quantitatively
evaluate them to determine what factors-of-safety (alternatively, levels of reliability) should
be employed in a given design-analysis framework.
Inspection, Maintenance, and Repair: Given the time dependent fatigue reliability analysis, a
rational riskheliability based inspection, maintenance, and repair plan should be developed to
minimize the life-cycle cost at the acceptable fatigue durability.
7.3.3 Fatigue Design
The fatigue resistance of critical structural details Cjoints) can be expressed in terms of S-N
curves. S-N curves are obtained hm laboratory testing in which a specimen is subjected to
cyclic loading until the occurrence of final fracture.
The spectral method is the most important fatigue design by analysis tool. The Weibull
method is a simplified fatigue analysis tool. These methods will be detailed in Part I11 of this
book.
Fatigue durability is a life-cycle problem. The fatigue durability can only be achieved if
Minimize stress-strain raisers (stress concentrations) and cyclic straining-stressing through
good engineering of the structural system and its details. This requires a high level of
engineering quality assurance (QA) at the concept-development-design stage.
Minimize flaws (misalignments, poor materials, porosity-voids, etc.) through good,
practical material and fabrication specifications and practices. This requires a high level
of QA during the development of plans and specifications and during construction
(involving material selection, fabrication, transportation, and installation). Furthermore, a
similar QA program is required during operations to properly maintain the system.
Minimize degradation at the local element by selecting good materials, fabrication
practices, and engineering designs (e.g. crack stoppers, damage localizers, and repairable
elements). This requires recognition that when fatigue degradation occurs, all reasonable
precautions are taken to restrict its development and effects. Note QA plays an essential
role, particularly during operations to disclose the presence of fatigue degradation (early
warning).