Page 162 - Marine Structural Design

138 PART I Strucntral Design Principles

Minimize degradation at system level so that when local fatigue degradation occurs, there
are no significant effects on the system’s ability to perform satisfactorily. Here good
fatigue design requires system robustness (redundancy, ductility, capacity) and system QA.
Inspections and monitoring to disclose global system degradation are another strategy to
minimize potential fatigue effects.
Cyclic strains, material characteristics, engineering design, specifications, and life-cycle QA
(inspections, monitoring) are all parts of the fatigue equation. This is the engineering equation
of “fail safe design”--fatigue may occur, but the structure can continue to hction until the
fatigue symptoms are detected and repairs are made.
The alternative is “safe life design” -- no significant degradation will occur and no repairs will
be necessary. Safe life designs are difficult to realize in many long-life marine structures or
elements of these structures. This is because of the very large uncertainties that pervade in
fatigue design and analysis. Safe life design has been the traditional approach used in fatigue
design for most ocean systems. The problems that have been experienced with fatigue
cracking in marine structures and the extreme difficulties associated with inspections of all
types of marine structures, ensure that large factors of safety are needed to truly accomplish
safe life designs. For this reason, fail-safe design must be used whenever possible. Because of
the extreme difficulties associated with inspections of marine structures and the high
likelihood of undetected fatigue damages, it is not normally reasonable to expect that
inspections will provide the backup or defenses needed to assure fatigue durability.

7.4 References
1. NTS (1998), “NORSOK N-004, Design of Steel Structures”, Norwegian Technology
Standards Institution, (available from: www.nts.no/norsok).
2. API (2001), “API FV 2A WSD, Recommended Practice for Planning, Designing and
Constructing Fixed Offshore Platforms - Working Stress Design”, American Petroleum
Institute, Latest Edition.
3. API (1993), “API Rp 2A LRFD - Recommended Practice for Planning, Designing and
Constructing Fixed Offshore Platforms - Load and Resistance Factor design, First
Edition. 1993.
4. API (2001), “MI FW 2FPS, Recommended Practice for Planning, Designing and
Constructing Floating Production Systems”, First Edition.
5. API (1997), “API FV 2T - Recommended Practice for Planning, Designing and
Constructing Tension Leg Platforms”, Second Edition.
6. IS0 Codes for Design of Offshore Structures (being drafted).