Reliability-Based Assessment of Vibrations on an Offshore Living-Quarters Platform in Mexico

Structural safety, integrity and reliability of offshore marine platforms under extreme loading is generally a widely studied topic [2, 4, 9]. However, ordinary environmental forces on particularly flexible components may be important sources of dynamic excitation and significant vibrations. For those cases where the structural system of the superstructure is totally different of the one used for the substructure, the non-uniform vertical distribution of the stiffness may impact the dynamic response by adding significant amplifications on the resulting motions of the more flexible structure. Significant motions were reported on a Mexican living-quarters platform. The platform was monitored and its dynamic characteristics were identified in order to prevent excessive displacements and accelerations. The platform was modeled as a single degree of freedom system with elasto-plastic behavior and the dynamic properties were calibrated from records obtained throughout a monitoring campaign of response displacements and accelerations on the platform. Fourier amplitude response spectra were generated and the fundamental period and damping percent were estimated. On the basis of Monte Carlo simulation of time history accelerations, the probability to exceed prescribed thresholds for lateral displacements, accelerations and lateral strength under several given wave heights were estimated. As these probabilities are conditional to the occurrence of the wave height exciting the platform, the unconditional probabilities were obtained by convolving the conditional probabilities over the probability of occurrence of the wave heights prescribed for the site of the platform.