Static Analysis of Marine Risers

Abstract

The static riser design problem is formulated and solved. A two dimensional, small slope, small deflection, linear model for bending of vertical circular tubular beams under tension, internal mud_static and external hydrostatic pressure is used to analyze the behavior of risers under external time invariant hydrodynamic loads. At the present state of the art, the prediction of hydrodynamic loads exerted on a circular cylinder moving in a viscous fluid, is possible only in certain cases that have been studied experimentally. To overcome this difficulty and study the design problem per se, a parametric approach has been adopted. The load profile along the riser is described by a general nth degree polynomial expression of the water depth and a series of loading parameters. &&The resulting static riser boundary value problem has been solved in terms of the Airy functions of the first and second kind. In addition to the exact solution, an approximate solution is derived. The approximate solution, called cable approximation to the riser static problem, violates two of the four riser boundary conditions. Two different methods have been developed to correct the cable approximation and make it satisfy all four boundary conditions. Each solution, the exact and the two corrected approximations, is applicable for different ranges of the design variables. The corrected cable approximations provide a satisfactory solution to the riser static boundary value problem and can be easily implemented numerically for values of the design variables for which the exact solution is numerically unstable.