Page 387 - Marine Structural Design

Part I11

Fatigue and Fracture

Chapter 19 Simplified Fatigue Assessment

19.1 Introduction

Fatigue assessment of structural connections (tubular joints, plated connections, pipe welds
etc.) is one of the most critical issues in the design of marine structures such as ships, fixed
platforms, floating structures, pipelines, risers and mooring lines. The results of fatigue
assessment will influence costs and safety from several aspects:
Quality of the connection material
Quality of welding fabrication (such as welding, heat treatment, etc.)
Frequency of inspection and repairs
Consequence of potential fatigue failure and
Residual strength of partially damaged structural system
There are four key methodologies for the estimation of accumulated fatigue damages:
1. Deterministic Fatigue Analysis
2. Simplified Fatigue Assessment - assuming that the stress range follows a Weibull
distribution (discussed in this Chapter)
3. Spectral Fatigue Analysis (Chapter 20)
4. Time-Domain Fatigue Analysis (Chapter 20)
5. Fracture Mechanics-based Assessment of Fatigue Damages (Chapter 21)
The first four methodologies estimate fatigue damages using the S-N curve, while the last
methodology is based on fkacture mechanics (FM) approach.
Fatigue criteria in classifications rules (such as ABS (2002) Steel Vessel Rules) use a
simplified fatigue assessment based on empirical values for the Weibull shape parameters. The
simplified fatigue assessment is also supported by API RP 2A (2001) for some cases.
This Chapter describes a simplified procedure for fatigue assessment based on a two parameter
Weibull distribution. The Weibull shape parameter depends on the wave climate and the
character of the structural response, especially the possible influence of structural dynamics.
The fatigue evaluation result is very sensitive to the Weibull shape parameter. The advantage
of the simplified fatigue assessment is that closed form expression for fatigue damage may be
derived, and that the Webull shape parameter may be calibrated based on historical data of
fatigue cracks.