The ability to efficiently execute global nonlinear dynamic analysis of flexible risers with detailed cross section models has been an industry priority for some time. The reasons for this include the capturing of the complex riser motions due to coupled global/local nonlinear riser behavior and the ability to directly recover the resulting helical armour stresses. The most significant impediment to such an analysis is the required computational power for executing long duration nonlinear dynamic analyses with hundreds of millions of degrees of freedom. As such, current industry practice remains a decoupled approach: modeling the global system with equivalent line elements and recovering armour stresses with a local analysis of a detailed cross section model.
This paper presents an innovative approach wherein reduced order nonlinear dynamic substructures are formulated from the detailed finite element models and incorporated in highly efficient global nonlinear analyses. The global analysis captures both geometric as well as local nonlinear behavior. In addition, the method enables the formulation of a stress transformation matrix which enables the direct computation of armour stresses from the global analysis. To demonstrate the computational power of the method, both a 10 m and 500m flexible pipe finite element model, the latter totaling 48,000,000+ degrees of freedom, are simulated through two bending cycles in minutes on a single core computer.