Three-dimensional static analysis of flexible risers by a lumped-mass method

Abstract

The present paper suggests a new lumped-mass method for a three-dimensional quasi-static analysis of flexible riser. Solution of the fully non-linear governing equations based on the Krichoff's beam theory is numerically approached. The considered nonlinear terms are the geometric non-linearity, the hydrodynamic non-linear drag forces and the nonlinear-effect of pipe flow. A continuous riser is discretized in a lumped-mass model, in which two neighboured lumped-mass points are connected with a straight massless beam element. The Dirac-dalta function is employed to solve the mathematically singular equations of the discrete numerical model. An incremental-iterative solving technique is chose, in which the nonlinear differential equations are linearized by means of the Newton-method in an incremental way. The riser/seafloor interaction is dealt in a new way, in which the normal reaction forces at contact points are treated as a unknown state variable. The effect of bottom friction is investigated with changing the loading speed of external forces on the riser.