Page 385 - Marine Structural Design

Chapter 18 Fatigue Loading and Stresses 361

and the number of cycles within the sea state of period is

T. 3x3600 -956
=
n. =I -
~
‘ 11.3
The stress range response may be assumed to be Rayleigh distributed within each sea state as

18.7 Concluding Remarks

In this Chapter, fatigue loads for ship and offshore structures have been discussed for
simplified fatigue assessment and spectral fatigue assessment.
For ship structures, key fatigue loads are the global wave loads local pressure and internal
loads. These fatigue loads are applied to a structural response model. The fatigue loads may be
applied using simplified fatigue assessment and spectral fatigue assessment, see Section 18.3
and Section 18.4. Areas that required future research include (Chen and Shin, 1995):
Calculation of the loads accounting for nonlinearities
Development of a theoretical method to combine high frequency and low frequency
response (e.g. ordinary wave-induced loads plus slam-induced whipping).
Development of hull-stress monitoring system that may link ship’s service experience with
0
anticipated fatigue failure
Quantification of uncertainty in load predictions including load combinations.
For offshore structures, key issues are the definition of scatter diagrams for random sea, swell,
wind and current loads for specific site offshore, and the estimation of vessel motions and
RAO based on the structural model, environmental conditions and loads. Areas that required
more research include:
Collection of reliable environmental data for specific sites
Fatigue load combinations for random sea, swell, wind and currents
Evaluation of vessel motion, RAO and low-frequency motions

18.8 References
1. Almar-Nss, A. (ed.) (1985), “Fatigue Handbook - qffshore Steel Structures”, Tapir
Press, Norway.
2. AF’I RP2A - WSD (2001), “Frecommended Practice for Planning, Designing and
Constructing Fixed Oflshore Platforms - Working Stress Design”, American Petrileum
Institute.
3. Baltrop, N. (1998) (editor), “Floating Structures: A Guide for Design and Analysis”
Oilfield Publications, Inc., Vol. 1.
4. BV, (1998), “Fatigue Strength of Welded Ship StructuresO, Bureau Veritas, July 1998