Free vibration analysis and shape optimization of offshore tubular structures

The minimum natural frequency of a structure should be higher than the predominant excitation frequencies in order to prevent resonance. This may be achieved
by the use of structural shape optimization procedures in which shape or thickness of the
components of the structures are varied to achieve a specific objective satisfying certain
constraints. The offshore structures should experience minimal movement to provide a
stable workstation for operations. This paper presents applications in determining
frequencies, and mode shapes of different tubular offshore structures. The model is used
as a basis for optimizing separately natural frequencies of tubular towers. The
orientations of bracings are arranged in the form of isogrid configuration to optimize the
static and dynamic characteristics of tower assemblies. A series of detailed numerical
analysis of braced tubular structures subjected to ocean waves loading is performed for
different shapes and bracing arrangement using general purpose software (SAP 2000).
Finally an optimized tower shape with bracing arrangement is expressed in explicit form
based on the results of numerical investigations of the non-linear behavior of tubular
tower. The objective of this work is to develop and demonstrate the use of a robust and
inexpensive computational procedure for finding the shape and thickness variation of
optimal cross section of vibrating tubular structure for maximization of the fundamental
frequency.