UFC 4-159-03
3 October 2005
A maximum load of 943 kilo-newtons (212 kips) occurs on Leg 1 at a wind
direction of 120 degrees. This provides a quasi-static factor of safety of approximately 4
to the breaking strength of 2.75-inch FM3 chain.
8-4.9
Dynamic Analysis. A dynamic analysis is performed on the mooring
system to evaluate peak mooring loads and vessel motions using a mooring analysis
program (W.S. Atkins Engineering Sciences, AQWA Reference Manual). The initial
location of the vessel nest is based on the equilibrium location of the vessel nest
determined in the quasi-static analysis. An Ochi-Shin wind spectrum is used to simulate
the design storm (Wind Turbulent Spectra for Design Consideration of Offshore
Structures, Ochi-Shin, 1988). This simulation is performed for a 60-minute duration at
the peak of the design storm.
Figure 8-14 shows that the four vessels in the nest are close together and
Figure 8-15 shows that the ships have a large ratio of ship draft to water depth. In this
case it is estimated that the ships will capture the water between them as the ships
move. Therefore, the nest of moored ships was modeled as a rectangular box having a
single mass with the dimensions of 161.2 meters (length of each ship at the waterline),
71.62 meters wide (four ship beams + 5 feet spacing between ships), and 6.5 meters
deep (average vessel draft). Added mass for sway and surge was computed as if the
nest was cylindrical in shape with a diameter equal to the average draft. Damping as a
function of frequency was estimated from a diffraction analysis (W.S. Atkins Engineering
Sciences, AQWA Reference Manual).
Dynamic analyses were performed for various combinations of wind and
current directions using a wind speed time history that simulated the design storm.
Results showing the instantaneous peak tensions for various wind directions in
conjunction with a flood tidal current of 0.6 knots (0.31 m/s) are shown on Table 8-6.
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