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

Abstract

A riser is anchored at the floating system in a quasi-vertical configuration, 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 when excited by a moderate sea state. In this paper, a physical argument, coupled 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 load, namely, the maximum value of the compression permitted in a given situation. In this context, the total tension should follow nearly the harmonic result predicted by the algebraic expression derived in Aranha and Pinto [Dynamic tension in risers and mooring lines: an algebraic approximation for harmonic 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 from 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.