Page 371 - Marine Structural Design

Part I11

Fatigue and Fracture

Chapter 18 Fatigue Loading and Stresses

18.1 Introduction
Marine structures may be exposed to a variety of loads during their life cycle. The loads are
commonly classified as follows
Functional loads
- Deadloads
- LiveLoads
. Environmental Loads
- Sea Loads (waves and currents)
- Wind Loads
- Seismic Loads
Accidental loads
All loads that vary in magnitude and/or direction will cause stress variations in the structure,
which may lead to fatigue damage. Live loads and environmental loads are especially
important in this aspect. The environmental loads are dominating for the main part of marine
structures. The waves and currents are considered the most important sources of
environmental loads acting on marine structures. Moored floating structures are also sensitive
to wind loads.
Fatigue loading is one of the key parameters in the fatigue analysis. It is the long-term loading
during the fatigue damage process. Various studies have been conducted on fatigue loading on
marine structures, to characterize the sea environment, the structural response, and a statistical
description. The sea environment is generally characterized by the wave spectrum. The
structural response is determined using hydrodynamic theory and finite element analysis.
The objective of this Chapter is to present a general procedure for long-term fatigue stress
described using Weibull distribution function. Other methods of fatigue loading include design
wave approach and wave scatter diagram approach. The Weibull stress distribution function
has been used in the simplified fatigue assessment (see Chapter 19), while the wave scatter
diagram approach is applied in frequency-domain fatigue analysis and time-domain fatigue
analysis (See Chapter 20).
Some of the earlier research on fatigue loads has been summarized by Almar-Naess (1985).
Recent developments in this field may be found in Baltrop (1998) and papers such as Chen
and Shin (1995) and ISSC committee reports.