Hydrodynamic analysis of underwater bodies for efficient station keeping in shallow waters with surface waves

To determine the effect of body shape on the response of underwater vehicles to
surface waves in shallow water, the wave radiation hydrodynamic forces are evaluated
for a family of (i) prolate spheroidal hull forms and (ii) cylindrical bodies with
hemispherical nose and conical tail sections by systematically varying the geometric
parameters but keeping displacement constant. The added-mass and wave damping
coefficients are determined using a frequency-domain, simple-source based boundary
integral method. Results are obtained for a range of wave frequencies and depths of
vehicle submergence all for a fixed water depth of 10 m. With the wave exciting force
and moment determined using the Froude-Krylov theory, the response transfer functions
for heave and pitch are then determined. The heave and pitch response spectra in actual
littoral seas are then determined with the sea state modeled using TMA spectral relations.
Results show that vehicle slenderness is a key factor affecting the hydrodynamic coefficients and response. The results show two characteristics that increase the radiation
hydrodynamic forces corresponding to heave and pitch motions: namely, vehicle length
and further-away from mid-vehicle location of the body shoulder. The opposite is true for
the oscillatory surge motion. By utilizing these observed characteristics, one can design
the lines for maximum radiation forces and consequently minimum hull response for the
critical modes of rigid-body motion in given waters and vehicle missions. In the studies
carried out in the thesis, a hull with a long parallel middle body with hemispherical nose
and conical tail sections has better heave and pitch response characteristics compared
prolate spheroid geometry of same volume. The methodology developed herein, which
is computationally efficient, can be used to determine optimal hull geometry for minimal
passive vehicle response in a given sea.