On the dynamic compression of risers: an analytic expression for the critical load

A riser is anchored at the floating system in a quasi-vertical configuratio n, the angle between the tangent and the vertical line at the top end being , in general, small. As a consequence, the static tension at the touchdown point is also small and the riser usually becomes dynamically compressed wh en excited by a moderate sea state. In this paper, a physical argument, cou pled with a simple model for the quasi-steady buckling of a infinitely long curved beam, allows one to obtain a simple estimative for the critical loa d, namely, the maximum value of the compression permitted in a given situat ion. In this context, the total tension should follow nearly the harmonic r esult predicted by the algebraic expression derived in Aranha and Pinto [Dy namic tension in risers and mooring lines: an algebraic approximation for h armonic excitation (2001), submitted] but saturated, in the compressed part , at this critical load, a conclusion suggested by experimental results due to Andrade [EPSUP (1993)]. Comparison with numerical results, obtained fro m nonlinear time domain programs, indicate a fairly good agreement, in the sense that the numerically determined tensions tend, indeed, to 'saturate' in compression around the estimated critical load. (C) 2001 Elsevier Scienc e Ltd. All rights reserved.