Page 397 - Marine Structural Design

Part I11

Fatigue and Fracture

Chapter 20 Spectral Fatigue Analysis and Design

20.1 Introduction
20.1.1 General

Recent offshore field development based on Tension Leg Platforms (TLP), semi-submersibles,
SPARS, FPSOs and other types of floating structures has clearly demonstrated that operators
are confident in deepwater technology and will continue the development of fields in ever
increasing water depths. Therefore, cost-effective floating structures will continue to be
developed for deepwater field development.
In the simplified fatigue assessment, the fatigue damage is estimated assuming that the stress
follows a Weibull distribution for long-term response. The simplified fatigue assessment has
been successfully applied to the ship fatigue design in which allowable stresses are pre-
calculated for different locations in a ship. Due to the excessive sensitivity of the estimated
fatigue damage to the Weibull parameters, a spectral fatigue assessment becomes more
popular for offshore structural analysis (Chen and Mavrakis, 1988).
Fatigue analysis and design include several steps of analysis:
Fatigue screening
Detailed structural analysis
Reanalysis of welding improvements
Reanalysis of design improvements
Reanalysis of design and welding improvements
This chapter describes a fatigue analysis of floating structures, such as:
. Spectral fatigue analysis, including computer modeling, load conditions, structural analysis
and validation, loading combinations, and fatigue damage assessment
Time-domain fatigue analysis
Fatigue design of local structural details
The spectral fatigue analysis may also be applied to ship structures provided that the wave
scatter diagram is adequately defined because the ships are designed for un-restricted services.
The frequently used codes and standards for fatigue analysis of the floating structures are API
Rp 2T (1997), API 2FPS (2001), AWS (1997) and UK DEn (1990) and guidance from
classification societies.