Stochastic fatigue analysis of FPSO topside structures with linear and nonlinear supports
Spanos, Pol D.
Doctor of Philosophy
Floating Production, Storage, and Offloading (FPSO) Systems are quite often subjected to stochastic sea wave loadings. In this thesis, a methodology is developed for estimating the fatigue life of topside structures. Proper Response Amplitude Operator (RAO) of the FPSO system, and the well-known Ochi-Hubble sea wave elevation spectrum are combined to provide the design spectrum at the deck level on topside FPSO. For ordinary Single-Degree-of-System (S-D-O-F) piece of equipment, the dynamic response is simulated by a time series model. A non-recursive Rainflow cycle counting method is applied to the equipment stress time history to identify significant cycles that produce fatigue damage in the time domain. The results of the Rainflow cycle counting method are supplemented by results from a power spectrum based, exclusively, approach. Further, pipe systems with/without limit stops on topside FPSO are modeled as Bernoulli-Euler beam. A Galerkin method is therefore employed in conjunction with the beam random vibration theory. Specifically, the statistical linearization technique is adopted to derive the equivalent linear system for the pipe example with nonlinear constraints. The applicability of the proposed approach is demonstrated by the analysis of both a simple S-D-O-F piece of equipment and by an illustrative example of pipeline conveying fluid with/without nonlinear constraints subject to sea wave loading. The proposed in this thesis integrated approach can be used for the stochastic fatigue analysis of structures on topsides FPSO during preliminary design when piping system responses must be estimated analytically. Further work can consider the coupling between the transverse and the longitudinal response of the pipelines on topside of FPSO and related issues.