A Study on the Strength and the Natural Vibration of Slender Offshore Structures

Abstract

This paper proposes a method for the probabilistic analysis on the strength and natural vibration of slender and composite offshore structures for large water depth. The composite structure is dealt as a simple tower model made by replacing its tower frameworks and platform etc. with the beam-columns with equivalent stiffness and strength. At first, the reliability assessment of the structures under the quasi-static extreme wave loads is performed by using the system developed for spatial frame structures based on ultimate strength analysis. Probabilistic properties of quasi-static extreme wave loads are estimated by using Stokes wave theory and Morison's formula. Collapse strength of structures under the notional extreme sea-state is evaluated by using a matrix method combined with a plastic node method. Probailistically dominant failure modes are selected by applying the so-called branch-and-bound method. Next, a study of the effect of structural parameter uncertainty on the natural vibration is also conveniently done by use of reliability methods. Finally, the applicability of the proposed method is demonstrated through the numerical example for 450m guyed tower.