# Influencing Factors of Motion Responses for Large-Diameter Tripod Bucket Foundation
## 5 Results and Discussion
### Effect of Water Depth  
In ship structures, the vertical planar motion (heave and pitch) of a structure is enhanced in shallow water [12; 34; 35; 36]. The water depth from seabed to wave surface did not change in space and time during the transportation of LDTBF. During the towing of structures from deep water to shallow water, the wavelength becomes shorter, the wave velocity becomes slower, and the clearance between the bottoms of LDTBF to the seabed becomes smaller. The gravity center sinks due to the blocking effect of ocean fluid, equivalent to adding an increment to the draft of structure. During the towing of structure from shallow water to deep water, the wave exciting forces acting on the structure also change with a larger wavelength and a faster wave velocity. Figures 13-15 show the motion performances of Model 1, Model 2, and Model 3 with different water depths.  
The RAOs of heave acceleration and pitch angle for Model 1 with different water depths at 4.5 m draft are shown in Figure 13. The maximum amplitudes of heave acceleration and pitch angle, 0.537 m/s\({}^{2}\) and 11.67\({}^{\circ}\) at a 12.5 m water depth, 0.46 m/s\({}^{2}\) and 10.41\({}^{\circ}\) at a 11.25 m water depth, and 0.47 m/s\({}^{2}\) and 10.41\({}^{\circ}\) at a 10.0 m water depth, all occurred near the period of 6 s. It is shown that an increase in the water depth reduced the heave and pitch motion slightly with a shallower water depth of less than 11.25 m. The RAOs of heave acceleration increased with the increase in water depth with the period less than 6 s and larger than 13 s and decreased in the other periods. The RAOs of pitch angle show a similar decrease with increasing water depth at almost the entire range of the period. With a longer period, the RAOs of pitch angle were almost constant at 4.5\({}^{\circ}\).  
The RAOs of heave acceleration and pitch angle for Model 2 with different water depths at 4.5 m draft are shown in Figure 14. Only several periods significantly affect the RAOs of heave acceleration and pitch angle. They are 5 s, 6 s, and 15 s for heave motion and 5 s, 6 s, and 10 s for pitch motion. The maximum amplitudes of motion in heave and pitch first decreased and then increased with increasing water depth. The resonant period for heave motion decreased from 6 s to 5 s with a shallower water depth. This can be attributed to the added draft resulting from the blocking effect of ocean fluid. There is a positive influence on the mass involved in heaving vibration due to the added draft, decreasing the resonant period.  
The RAOs of heave acceleration and pitch angle for Model 3 with different water depths at 4.5 m draft are shown in Figure 15. In Figure 15a, the maximum amplitudes of heave acceleration, 0.461 m/s\({}^{2}\) at a 12.5 m water depth, 0.415 m/s\({}^{2}\) at a 11.25 m water depth, and 0.457 m/s\({}^{2}\) at a 10.0 m water depth, occurred near the period of 5 s, 5 s, and 6 s, respectively. The periods of maximum values of heave acceleration increased with increasing water depth. The amplitudes of heave acceleration at 11.25 m water depth are less than those of other water depths in most periods. In Figure 15b, a comparison of pitch RAOs with different water depths shows that an increase in the water depth slightly decreased the pitch angle. However, the maximum values of pitch RAO increased from 6.84\({}^{\circ}\) to 9.17\({}^{\circ}\) with the increase in water depth from 10.0 m to 12.5 m. A sharp fluctuation occurred around the resonant period. The reason for the fluctuation in pitch RAO can be attributed to the well-known shallow water effect: the shallower the water depth, the slower the heave and pitch motion.