Extreme Bending Moments on Long Catenary Risers Due to Heave Excitation

Abstract

This paper deals with the dynamic behaviour of catenary shaped risers under imposed motions applied at the top. Particular attention is paid to the heave component of motion which is of substantial importance for practical applications as it results to the amplification of large bending moments in the touch down region. In fact, the bending moment obtains its maximum value at the vicinity of the touch down point and very close to the location of its maximum static counterpart. This singular behaviour is discussed using the results from the solution of the eigenvalue problem. To this end the eigenfrequencies and the corresponding mode shapes are calculated using the WKB approximation making no assumption regarding the variation of the static components. In addition, the feature of the correlation between the axial component of the velocity of the excitation and the extreme bending moments at the lower part (Passano and Larsen, 2006) is further investigated through comparative numerical calculations of the problem using both frequency and time domain techniques.