Abstract:
In this thesis consideration is given to a selection
of nonlinear dynamic problems in the field of offshore
engineering. Hydrodynamic loading on fixed horizontal and
vertical tubular members and the dynamic response of articulated
towers together with the distribution of shear force and
bending moment along the tower are investigated using various
wave theories. Effects of nonlinear convective acceleration
terms in the calculation of fluid inertia forces and moments
are examined and attention is given to integration of wave
forces up to the free surface for vertical members. Calculation
of fluid loading at the displaced position of the articulated
tower and any Mathieu type instabilities that may occur
have been considered. The dynamic analysis of a damaged
Single Anchor Leg Storage (SALS) system subject to loss
of buoyancy in the yoke chamber is studied. The equations
of motion of the yoke/riser system are derived assuming
large displacements and solved in the time domain. Time
histories of the response, variations of the riser tension,
velocities of riser top end and the time histories of pivot
reactions are given. Natural periods and mode shapes for
small displacements of the system are calculated. Two methods
of simulating random seas, both represented by a sum of
harmonic wave components, are used to simulate second order
low frequency (slow drift) force on a tanker in head seas
by Pinkster's time domain method. In one method the wave
amplitudes are generated randomly from a Rayleigh distribution
and in the other they are obtained deterministically via
the wave spectrum. Time histories of slow drift force and
response together with simulation results with various duration
lengths are presented and compared. Estimates of the extreme
vessel response and its relation to rms value are compared
with the result of a commonly used method of determining
peak/rms ratios. The results of these investigations highlight
the importance of accurately simulating nonlinear effects
in both fixed, floating and compliant offshore structures
from the point of view of safe design and operation of such-
systems.