Mooring Loads Due to Parallel Passing Ships - State Lands ...

Figure 10. Sample data showing typical measured force and moment records
along with low pass filtering.
One feature shown in Figure 10 is the filtering of the raw data to remove various
high frequency oscillations. The interest in these tests is in the effects of the primary
pressure field which produces low frequency loads on the moored ship. Measurements,
however, also included effects of the secondary waves (diverging and transverse shipgenerated
waves). These were not too evident at the slowest speed tested but became
increasingly evident at higher speeds.
In addition, the moored ship exhibited motions at frequencies corresponding to
its natural frequencies in surge and sway. Tests were conducted to determine these
natural frequencies and it was found that the surge frequency was 3.2 Hz while the
sway frequency was about 6 Hz.
In order to remove these higher frequency effects, the raw data was filtered
using a low-pass filter with a 2 Hz cutoff frequency. As shown in Figure 10, this
effectively removed the high-frequency loads and isolated the low frequency loads of
interest. Similar filtering was done by Remery (1974), although it appears that the
filtering was done by simply hand-smoothing the data.
NFESC TR-6056-OCN
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