APPROXIMATE HARMONIC ANALYSIS OF MARINE RISERS
Master of Science
A model of a marine riser system which is appropriate for the study of its dynamic behavior in deep water conditions is developed. This model is used to estimate the steady-state riser response to harmonic excitation. In this regard, the finite element method yields a linear discrete multi-degree-of-freedom structural model; its stiffness matrix varies with time due to riser top tension fluctuations. Further, Morison's equation is used for estimating the hydrodynamic load on the riser. Due to the nonlinearity of the drag term appearing in this equation, the riser equation of motion becomes nonlinear. An approximate analysis procedure based on the concepts of equivalent linearization and of time averaging leads to efficient determination of the riser maximum stress. A continuous beam model under constant tension which is equal to the average of the top and the bottom riser tension is used to provide an estimate of its natural frequencies. Numerical results from a variety of parameter studies are reported.