Design of an experimental model for a semi-active vibration damping system on a jack-up platform

Abstract

Jack-up platforms are off-shore structure which are more often placed in deeper water and harsher weather conditions. The consequence is that the natural frequency of the platform coincides with the wave frequency and starts to resonate in its natural frequency. The problem becomes more complicated, due to the fact that the natural frequency of the platform is not constant and varies in time due to structural properties, variable deck loading and environmental conditions. These vibrations are undesired and have to be damped. The aim of this thesis is to design an experimental model for a damping system on a jack up platform, which can optimally damp the motion of the primary structure when it is excited in the time varying natural frequency. This problem is tackled by comparing passive, semi-active and active Tuned Mass Damper (TMD) systems, regarding feasibility and robustness for installation on a jack-up platform. Hereafter the optimal tuning frequency and damping ratio are obtained by studying the work of Tsai and Lin, Connor and Den Hartog. These tuning laws are then combined with a Self-Tuning Regulator (STR), which makes it possible to continuously tune the semi-active damping system to its optimal parameters by estimating the unknown structural parameter using a Recursive Least Square Estimator. The designed experimental model is a two degree of freedom model and incorporates crucial characteristics such as mass distribution, damping ratio and natural frequency. The semi-active TMD system is able to adapt its stiffness and damping coefficient, such that is it always optimally tuned when the structural parameters of the experimental model changes. This thesis concludes that a semi-active TMD system is the most appropriate type of damping system for a jack-up platform, regarding feasibility and robustness. A semi-active TMD system in combination with optimal tuning laws and a STR is able to optimally damp the vibrations of a jack-up platform, when it is excited in time varying natural frequency. This model can be used to test and validate the performance of the damping system and controller.