A stochastic design methodology for nonlinear offshore structures based on a probabilistic approach encompassing criteria from existing (seemingly) independent deterministic and stochastic design procedures is demonstrated. Due to large displacements of compliant offshore structures, conventional deterministic design using a train of "100-year" periodic waves may not reflect the global nonlinear response behavior such as coexisting steady states. Results presented here show that the coexistence can be fully captured by a stochastic analysis counterpart using nearly periodic waves, which are superposition of the prescribed "100-year" waves and noise perturbations. The proposed stochastic methodology can be systematically applied to analyze stochastic responses of both nearly periodic and spectrum-specified random waves. Design values and system reliability can be estimated using a time-averaged probability density function. Numerical results indicate that purely random waves and the associated stochastic analysis provide more conservative design criteria and are recommended for detailed design considerations.
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