Page 160 - Marine Structural Design

136 PART I Structural Design Principles

describe the fatigue durability of joints of marine structures, experimental data based S-N
curves are selected or fiacture mechanics models are applied. This demand and capability
information is then used to calculate fatigue lives via a damage summation process (typically
via the Palmgren-Miner hypothesis) or critical crack size. This procedure is summarized as:
Characterization of the Sea Environment
Hydrodynamic Response Analysis
Structural Analysis
Stress Transfer Function
Stress Concentration Factor
Hotspot Stress Transfer Function
Long-term Stress Range
Selection of S-N Curves
Fatigue Analysis and Design
Fatigue Reliability Analysis
Inspection, Maintenance, and Repair Plan
Characterization of Sea Environment: The sea environment is represented by the number of
occurrences of various sea states, each defined by a set of spectra. A two-parameter
(significant wave height, zero up-crossing rate) wave-scatter diagram is used to characterize
the sea states. All sea state spectra are defined by e.g. the Pierson-Moskowitz relationship.
Wave direction probability is included in the sea environmental characterization.
Hydrodynamic Response Analysis: Once waves with appropriate frequencies, heights, and
directions are selected, the hydrodynamic response and the loading of the structure are
computed for each wave condition.
Structural Analysis: A global structural analysis is performed to determine the applied loading
for the local structure (load transfer fhnction per unit wave amplitude as a function of
frequency). The local structural analysis is carried out to determine the stress transfer function
per unit load at each hotspot in the structural detail.
Stress Transfer Function: The load transfer function per unit wave amplitude as a function of
wave frequency is multiplied by the stress transfer function per unit load.
Stress Concentration Factor: The geometric SCF is considered in the fatigue assessment. For
the fatigue screening analysis, an upper bound SCF is assumed 3.0. For the detailed fatigue
analysis, the SCF is determined using parametric equations or the fine mesh Finite Element
Analysis (FEA).
Hotspot Stress Transfer Function: The stress transfer function is multiplied by the stress
concentration factor to determine the hotspot stress transfer function.
Long-term Stress Range: Based on the wave spectrum, wave scatter diagram and hotspot
stress response per unit wave amplitude, the long-term stress range is determined. This was
done by multiplying the ordinate of the wave amplitude spectrum for each sea state by the
ordinate squared of the hotspot stress transfer function to determine the stress spectrum. The
stress range distribution is assumed to follow a Rayleigh distribution. The long-term stress