A SIMPLE TIME DOMAIN STRUCTURAL REDUNDANCY ANALYSIS PROCEDURE FOR SEMI- SUBMERSIBLES

摘要

Offshore design codes like ABS and IMO require some level of redundancy in semi-submersible drilling vessels to withstand the loss of a slender bracing member without overall collapse of the structure, similar to fixed structures. Wave induced dynamic forces on semi-submersibles include hydrodynamic forces on 'large body', and inertia forces due to rigid body motions in six degrees of freedom. The amplitudes and phases of each component of the motion are important in defining the total force. Therefore, unlike static 'pushover' type analysis used in a relatively dynamically insensitive fixed jacket structure, semi-submersibles require nonlinear dynamic redundancy analysis in the time domain to determine the safety against collapse due to environmental loading. A simple time domain nonlinear analysis procedure is suggested in this study to capture the realistic behavior of the structure under wave loading. Dynamic loads are generated from hydrodynamic analysis of the floating body using a diffraction-radiation analysis program which assumes that the wave excitation is harmonic and so is the response. These loads are transferred to the structural analysis model. Each wave frequency is analyzed to produce a pair of loading conditions -'in-phase' and 'out of phase'. Combining these two components, a time history of the wave loading is created. In nonlinear structural analysis, first static loads are applied. Then wave load time history is applied for a few wave cycles in small increments. Results show that nonlinear analysis for one single cycle or two can usually predict the safety against collapse. If the analysis continues for a cycle or two, the structure passes the redundancy test. If it does not, the - structure has a deficiency that needs to be addressed.