Nonlinear Inverse Perturbation in Structural Dynamics Redesign of Risers

Abstract

Marine risers, and offshore structures in general, may have undesirable natural frequencies and/or mode shapes. Structural redesign is mandatory in such cases. An Inverse Perturbation Redesign (IPR) method, which uses only the finite element analysis of the baseline system and was developed in previous work for general structures, is extended in this work to handle systems with geometric stiffness matrices like marine risers. The IPR method is currently applicable to undamped structural systems or systems with Rayleigh damping and is valid for large or small changes which are frequently required to change modal characteristics of offshore structures. The vibratory characteristics of risers can be altered by modifying among others, the riser top tension, the drilling mud density and the geometry of the riser tubes. The effects of their change on the riser frequencies and modes are derived and applied to two typical riser redesign problems using the IPR method.