The Response of an Oil Rig Model to Dynamic Excitation

Abstract

The purpose of the work described in this thesis is to examine the structural dynamics of a laboratory scale model of a fixed off-shore oil rig. This work forms part of a general study of condition monitoring. In the early stages, some preliminary theoretical investigations into the effect of structural system parameter variations on dynamic characteristics were carried out. The chosen structure was a simple cantilever which is subjected to a half-sine wave force impulse at its free end. Fourier analysis has been applied to the transient response at various locations on the cantilever. Theorectical investigations, which are based on the finite element technique and using the concept of lumping the masses of the elements at the nodes, were carried out to establish the vibration characteristics (natural frequencies and modal shapes) of the rig structure. The analytical investigations were repeated for the rig structure, in which some of the members were assumed to be partially damaged, with the aim of deducing the presence of any changes in the vibration characteristics. Computer programmes were developed to carry out the various computations involved in these analyses. Experimental work was undertaken to establish the lower mode shapes and corresponding natural frequencies and a satisfactory correlation was established between the experimental results and the theoretical prediction. The results of this work showed that the rigidity of the member joints was highly significant. To investigate this further static tests were designed to assess the conditions at the welded joints of the model rig. A tubular welded T-connection was used to acquire the necessary experimental data needed in establishing the actual end conditions. The results of the end condition analysis were used later in the higher modes analysis of the model rig using sub-structuring techniques.