Page 159 - Marine Structural Design

Chapter 7 Limit State Design of qffshore Structures 135

primary aspects of design for fatigue reliability includes quality assurance and control
throughout the life cycle of the structure (inspection, maintenance, and repair).
In general, the design for fatigue reliability is concentrated on details of elements, and in
particular joints. This is the first line of fatigue "defense". It is in the local details and joints
that the significant or major stress-strain raisers are developed. However, given the very large
uncertainties associated with predictions of the cyclic strain histories and the fatigue strength,
high fatigue reliability of elements is rarely achieved.
Structure robustness or the ability of the structure system to tolerate defects without significant
reductions in its serviceability or ultimate limit state characteristics is the second line of
defense. Effective structure redundancy, ductility, and capacity must be mobilized.
The third line of defense is inspection, maintenance, and repair (IMR). Inspections help
disclose unanticipated flaws and defects, and confirm our design objectives. Maintenance is
intended to help preserve the structure so that it can fulfill its intended purposes. Repair
strategy is intended to draw the engineer's attention to the necessity for restoring the structure's
capacity after the future occurrence of damages and defects.
Present experience with the majority of marine structures indicates that although engineers
have adequately designed for fatigue failure, there are still notable exceptions, e.g. structures
in which certain types of loadings and stress raisers are ignored, high strength steels are used.
It should not be expected that the fatigue strength would increase proportionally to the yield
strength.
7.3.2 Fatigue Analysis
Fundamentally, the fatigue analysis approaches in engineering applications can be subdivided
into the following categories:
S-N based fatigue analysis approach
The local stress or strain approach where the calculation includes the local notch effects in
addition to the general stress concentration
The fracture mechanics approach which gives allowance for the effects of cracks in the
structure
These approaches have been well implemented in the fatigue design and assessment. However,
fatigue limit state design is still one of the most difficult topics in structural design, assessment
or reassessment. For marine structures, additional complications arise because of the
corrosive environment. The fundamental difficulties associated with fatigue problems are
related to:
Lack of understanding of some of the underlying phenomena at both the microscopic and
macroscopic levels
Lack of accurate information on the parameters affecting the fatigue life of a structure
The general explicit fatigue design by analysis of marine structures involves a complex
procedure. The dominant cause of the cyclic stresses within a marine structure is due to the
sea environment that it experiences. Therefore, a fatigue assessment requires a description of
the sea environment, or sequence of seastates, in which the structure is likely to meet over its
planned operational life. Vessel motions, wave pressures, stress transfer functions, and the
resulting fatigue stresses (generally expressed in terms of the number of cycles of various
stress ranges) at locations of potential crack sites (hotspot) are then calculated. In order to