The purpose of this paper is to present a variational approach for two-dimensional large strain static analysis of marine risers. The model formulation is developed in Cartesian coordinate system, in which deformation is described based on Lagrangian mechanics. The variational model formulation involves strain energy due to bending and axial stretching, and virtual work done by hydrostatic pressures and other external forces. The nonlinear static problem is numerically solved by using the finite element method. As an independent check for the validation of the variational approach, equilibrium equations based on the vectorial approach are developed and solved by the shooting method. Numerical examples are given to demonstrate the effect of large strain on the large displacement static behavior of marine risers.

The mathematical models for large deflection analysis of marine risers had been developed over the past several years. However, a number of research works is limited to the small strain analysis in which axial deformation may not be taken into account. Literature reviews on the mathematical models for large deflection analysis of marine risers are briefly mentioned herein. Felippa and Chung (1981) solved this nonlinear problem with an incremental procedure. Bernitsas (1982) proposed the equations of motion for three-dimensional nonlinear large deflection model for riser dynamics. Huang and Chucheepsakul (1985) introduced the energy functional for large displacement static analysis of a marine riser and solved the twodimensional problem using a hybrid method. McNamara, O'Brien and Gilrey (1986) used the interpolation functions of standard beam column element to solve the hybrid equation of nonlinear flexible riser. Bernitsas and Kokarakis (1988) presented a mathematical model for small strain and large deformation analysis of risers. Chung et al. (1994,1996) developed a three-dimensional model of deep-ocean pipe and demonstrated the effect of axial deformation on low flexible marine risers.