Page 379 - Marine Structural Design

Chapter I8 Fatigue Loading and Stresses 355

The dynamic interaction between the floating structure, mooring lines and risers should be
evaluated using a coupled analysis that provides a consistent modeling of the drag-induced
damping from mooring lines and risers. The coupled analysis may be based on a frequency
domain approach (Garret, et a1 ,2002) or a time-domain approach. In the coupled approaches,
the mooring lines and risers are included in the model together with the floating structure.
In return, the vessel motions impact fatigue of TLP tethers, mooring lines and risers. For
fatigue analysis of the tethers, mooring lines and risers, it is necessary to calculate vessel
motions such as:

Linear wave-induced motions and loads
e Second-order non-linear motions
The motion-induced fatigue is a key factor for selecting riser departure angle that's riser
dynamic response.

18.5 Fatigue Load Combinations

18.5.1 General
One of the fields that need research effort is perhaps load combinations for fatigue design.
Earlier research in this field has been summarized by Wen (1990) and Chakrabarti (1991). In
the determination of extreme loads for ultimate strength analysis, the aim is to select the
maximum anticipated load effect when the structure is subject to one of the design load sets.
However, for fatigue design, it is necessary to estimate the governing design load set and the
shape of the long-term stress range distribution at any structural location.
18.5.2 Fatigue Load Combinations for Ship Structures
One of the fields that need research effort is perhaps load combination. For ship structural
design, Munse et a1 (1983) identified the following cyclic fatigue load sources:
Low frequency wave-induced loads: lo7 - 108reversals during ship's life
High frequency wave-induced loads: 106reversals during ship's life
Still water loading: 300 - 500 cycles
Thermal loads: 7000 cycles
The amplitude of the fatigue loads is influenced by the wave statistics, change in the sourse,
speed and deadweight condition. Mansour and Thayamballi (1993) suggested to consider the
following loads and their combinations:
Fatigue loads resulting from hull girder bending
Fatigue loads resulting from local pressure oscillations
Cargo loading and unloading (low cycle effects)
Still water bending (mean level) effects
Of the loads listed in the above, the hull girder bending and local pressure fluctuation give far
more contribution to total fatigue damage. Depending on the location, one of these two loads