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WAVE MONITORING SYSTEM
Construction of Wave Monitoring System
The construction of the wave monitoring system is shown in Figure 10, including the navigation support system. Three cameras are connected to a video board of a personal computer. Instead of three accelerometers, an accelerometer and two angular velocimeters are equipped considering the increase of cameras because this combination of sensors can calculate vertical accelerations of any positions of the ship.
Figure 10. Construction of Wave Monitoring System
Figure 11 shows the time chart of the system. The data acquisition and video images processing continues ten minutes and the data is analyzed in five minutes. The video data is deleted just after detecting the relative wave motions because data saving needs much space of disk and time.
Figure 11. Time chart of the system
The camera casing and the arm are shown in Figures 12 and 13 respectively. The casing has a wiper to keep a clean view. The arm is made foldable because a permanent arm is against regulation.
At-Sea Experiment
The performance of the wave monitoring system was investigated using Shioji-maru, a training ship of Tokyo University of Mercantile Marine. The principal particulars of the ship, the installation of cameras and the course and velocity of the ship are shown in Table 1, Figures 14 and 15 respectively.
Figure 12. Camera casing
Figure 13. Camera arm
The main wave at the experiment was estimated by visual observation and hindcasting to be a swell coming from at 80 degrees (almost east) with the height of 1.0 m and the period of 7.0 seconds. The results by the proposed monitoring system are shown in Table 2. For comparison the results by the parametric method (PM) with the same input data are also shown in the table within parenthesis.
Table 1 Principal particulars
| Loa (m) |
49.9 |
| Lpp (m) |
46 |
| B (m) |
10 |
| D (m) |
6.1 |
| d (m) |
3 |
| Cb |
0.55 |
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Figure 14. Installation of cameras
Figure 15. Course and velocity of the ship
Table 2. Estimated results by the wave monitoring system
| |
H1/3 [m] |
T01 [sec.] |
χ[deg] |
| Case502 |
1.13 (1.11) |
7.09 (8.08) |
88 (121) |
| Case503 |
1.10 (1.15) |
4.87 (7.29) |
92 (160) |
| Case504 |
1.28 (1.55) |
6.18 (8.81) |
95 (130) |
| Case505 |
1.20 (1.43) |
9.43 (7.96) |
50 (90) |
| Case506 |
1.02 (1.24) |
6.19 (7.38) |
78 (0) |
| Case507 |
1.13 (0.78) |
6.28 (7.95) |
14 (85) |
| Case508 |
1.08 (0.78) |
6.79 (10.21) |
73 (185) |
| Case509 |
1.08 (1.11) |
7.69 (10.68) |
23 (115) |
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The agreement of the estimated results is fairly good for wave height. As for the wave period and direction, results somewhat depend on recorded time history and some are apart from the true value. However, the estimation is better than the parametric method and the averaged results are promising.
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