The dynamics of flexible offshore structures in irregular seaways

Janardhanan, Kalyana Sundaram (1992) The dynamics of flexible offshore structures in irregular seaways. University of Southampton, Doctoral Thesis.

Abstract

In the design of offshore structures for deep water and harsh environmental conditions, flexibility of the structure assists in resisting a major part of the load. In this case, the knowledge of the dynamic behaviour of the structure is of paramount importance. For a flexible floating offshore structure with complicated underwater shape in the vicinity of free-surface, there exists a very significant fluid-structure interaction. The vibration of a floating flexible structure alters the local fluid pressure fields and hence the hydroelastic responses should be considered in the derivation of fluid forces. In a conventional design approach, however, the structure is considered as rigid and the hydrodynamic forces are derived quasi-statically. In the present thesis the full dynamic nature of the loads and the responses are accurately dealt with using three dimensional hydroelasticity theory and a comparison is made with the conventional quasi-state approach. A time domain approach is proposed for the solution of the generalised equations of motion where the non-linearities in the forcing terms are incorporated without any simplification. When the hydrodynamic coefficients exhibit significant dependency on the frequency, they are represented in the time-domain by means of impulse response functions. The generalised hydrodynamic damping data are used to derive the impulse response functions and the added mass coefficients were re-constructed from them. This method helped to eliminate spurious data points arising at irregular frequencies. An algorithm using Pade approximants was developed for the time domain solutions. The algorithm was validated against analytical and frequency domain results. A typical Northsea jackup rig was chosen to demonstrate the application of the proposed algorithm. The jackup rig was analysed in the `spudded' condition as well as during wet tow with partially submerged legs. For these cases a comparison was made between a quasi-static, spectral and non-linear time simulations. Non-linear statistical methods were employed to determine the most probable maxima from the time simulations. It is concluded that hydroelasticity is a viable method to study the dynamic responses of a flexible fixed or floating offshore structure providing a deeper insight into the problem than conventional approaches.

This record has no associated files available for download.

Contributors

Download statistics