Page 63 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

wavelength is the period (T). Because the period is the time required for the passing of one wave-
length, if either the wavelength or period of a wave is known, the other can be calculated, since 2
L (m) 5 1.56 (m/s)T .
Speed (S) 5 L/T.
For example: Speed (S) 5 L/T 5 156 m/10 s 5 15.6 m/s.
Another characteristic related to wavelength and speed is frequency. Frequency (f) is the number of wavelengths that pass a fixed point per unit of time and is equal to 1/T. If six wave- lengths pass a point in one minute, using the same wave system in the previous example, then:
Speed (S)5Lf5156 m36/min5936 m/min3(1 min/60s)515.6 m/s.
Because the speed and wavelength of ocean waves are such that less than one wavelength passes a point per second, the preferred unit of time for scientific measurements, period (rather than frequency), is the more practical measurement to use when calculating speed.
The complexities of air/wave as well as air/land and fixed structure interaction, while quite important to the operation of ROV equipment in a realistic environment, are beyond the scope of this text. Please refer to the bibliography for a more in-depth study of the subject.
A more practical system of gauging the overall sea state/wind combination was developed in 1806 by Admiral Sir Francis Beaufort of the British Navy. The scale runs from 0 to 12 (Table 2.5), calm to hurricane, with typical wave descriptions for each level of wind speed. The Beaufort scale reduces the wind/wave combination into category ranges based upon the energy of the combined forces acting upon the sea surface.
2.3.2 Effects of wave pattern upon ROV operation
There is a plethora of information on wave propagation and its effect upon vessel management. Please refer to the bibliography for more detailed information. This section will address the effect of waves upon ROV operations.
The energy to produce a sea wave comes principally from wind but can also be generated through some lesser factors, such as submarine earthquakes (which happen less often but are possibly devas- tating when they occur), volcanic eruptions, and, of course, the tide. The biggest concern for wave pattern management in ROV operations is during the launch and recovery of the vehicle. During ves- sel operations, the hull is subject to motions broken down into six components—pitch, roll, and yaw for rotational degrees of freedom, and heave, surge, and sway for translational motion. The distance from the vessel’s pivot point on any axis, combined with the translational motion from waves hitting the hull, will translate into the total swinging moment affecting the suspended weight—the ROV.
The closer the launch platform is located to the center of the vessel’s pivotal point, the smaller will be the arm for upsetting the suspended weight. The longer the line is from the hard point on the launch winch, the longer the arm for upsetting the suspended weight. Add a long transit dis- tance from the hard point to the water surface, with an over-the-side launch from a larger beam vessel, and the situation is one just waiting to get out of control (Figure 2.17).
Regardless of the sea state, it is best to complete this vulnerable launch operation (from the time the vehicle is lifted from the deck to the vehicle’s submergence) as quickly as possible, given operational and safety constraints. This concept will be addressed further in Chapter 9.
2.3 Ocean dynamics 51