Offshore Structural Systems Research Articles

Stochastic analysis for offshore structures with added mechanical dampers

Presented in this paper is a method to mitigate the vibration of an offshore structural system in the marine environment when subjected to in-line random wave forces. A stochastic approach by using an alternative complex analytical model for the viscoelastic material in the mechanical damper was used. The viscoelastic materials applied here were tested and verified that they have high energy absorption capacity. In the stochastic analysis, a random type of wave forces derived from the Morison equation for small bodies was applied. Results of the vibration responses for the system with added damping devices were presented and compared to the responses of structures with traditional design. It was observed that in terms of the power spectral density, the effect of the vibration mitigation and the dynamic performance of the offshore structural system were greatly improved when the new damping devices were applied to the offshore structural system.

Parametric uncertainties for dynamic response evaluation of offshore structures

The dynamic characteristics involved in the loading process in the ocean environment are extremely complicated and difficult for exact evaluation. In this paper, parametric studies are presented to assess the influence of uncertainties associated with various assumptions and approximations, involved in modelling the dynamic loading processes of ocean environment, for the evaluation of dynamic response of offshore structures. These uncertainties are represented by the inertia coefficient and the drag coefficient of the Morison equation which defines the wave forcing function, the mean height of the input sea waves and the shear wave velocity of the soil. The results show that the effects of uncertainties are closely related to the natural frequency of the structure and the predominant frequency of the wave force. These uncertainties, except the variation in the shear wave velocity in the soil, contribute significantly to the dynamic response and the first passage probability evaluation of the offshore structure systems.

Vibration mitigation of structures in the marine environment

The research described in this paper involves a method to mitigate the vibration of an offshore structural system in the marine environment when subjected to the in-plane wave forces. The method introduced include a practical application for the viscoelastic material to the offshore structural system, and the analytical technique to evaluate the structural responses when the system is incorporated with damping devices. The viscoelastic materials applied here were tested and verified that they have high energy absorption capacity. In the analysis, the applied wave forces are based on the fifth-order Stokes wave theory and Morrison equation for small body and the computation method is based on Newmark method for nonlinear system. Results of the vibration responses for the system with added damping devices are presented and compared to the responses of structures of the traditional design. It was observed that the effect of the vibration mitigation was significant and the dynamic performance of the offshore structural system were greatly improved when the new damping devices were applied.

Softsit-on-bottom type offshore structure and the applications

Softsit-on-bottom type offshore structure has been considered as one of the safest offshore structure systems against earthquake and settlement of ground. This system was developed in 1980 by the outhors and has been proposed for such purposes since then. In this paper, the structural charaeters of this structure system is described according to three casestudies of ocean city projects-Campus City in the Tokyo bay, Osaka bay marinecity and Makuhari messe island Project.

Offshore structural system reliability under changing load pattern

This paper considers structural system reliability analysis when the spatial force pattern changes as the load increases, specifically when the nodal forces on a steel offshore jacket structure change as the base shear increases under increasing wave height. This paper demonstrates the so-called “fragility approach” to systems analysis, wherein the reliability of the system is obtained for each of a set of increasing wave heights. These probabilities are conditional on the wave height. These results are multiplied by the (discrete) probability levels of these wave heights, and the products are summed to obtain the (total) system reliability. The method is applied to analyze a fixed offshore jacket in 140 feet of water and designed for a 100-year (63 foot) wave. The change of the relative importance of different member-failure sequences with wave loads corresponding to different wave heights is studied. In this case study, the inclusion of wave slamming forces on the deck at higher wave heights led to an increase of the failure probability by an order of magnitude. The use of an improved fixed wave-load pattern treatment is explored. A comparison of the member-level and system-level failure probabilities is presented in terms of factors measuring different system effects.

Analysis of tubular beam-columns and frames under reversed loading

The finite segment method, combined with the generalized stress-strain relationship, and the incremental stiffness approach and automatic load control techniques, is applied to trace the complete load-displacement and load-shortening relationship of some structural members and simple frames in offshore structural systems subjected to reversed loading. Extensive numerical results are obtained and compared with available experimental data as well as with the analytical results obtained previously by the influence coefficient method. Some simple frames are then solved using the computer model developed and also compared with the solutions using the simplified rigid plastic analysis. The structures considered in the present analysis are strut and portal types of beam-columns, simple moment frames with different structural systems, and simple single-diagonal braced frames.