Large Scale Nonlinear Dynamic Simulation of Flexible Risers with Detailed Finite Element Models - Part 1
Authors: Majed Arya , Cooper Philip 
Published: June 2014

Abstract
Flexible risers are in increasing demand for deeper water applications. Accurate large scale global simulations of flexible risers and prediction of helical armour stresses have become an industry priority. Standard riser dynamic analysis software packages utilize line element models that can not capture the complex three-dimensional behavior of flexible risers. Advanced finite element software can model the complex geometry and multi-layered behavior; however, the computational requirements of these solvers limit the models to just a few meters in length. An advanced methodology that affords very significant computational efficiencies is required to bridge the gap to large scale nonlinear dynamic simulations with detailed finite element models.
This paper demonstrates an advanced method of analysis that is capable of incorporating detailed finite element models into large scale fully nonlinear dynamic simulations while maintaining execution speeds of standard riser dynamic analysis software packages. Nonlinear Dynamic Substructuring (NDS) is utilized to efficiently execute a large scale nonlinear dynamic simulation of a 500 meter, 9 inch flexible riser system modeled with multi-layer shell finite element models. It is shown that the simulation captures coupled axial-bending-torsional response of the flexible riser. This complex interaction is due to the coupling of geometrically nonlinear global effects with the local winding/unwinding of the contra-wound helical armour layers. The simulation also models armour layer stick-slip friction behavior modeled via a generalized nonlinear bending hysteresis formulation. This bending hysteresis model couples to the geometrically nonlinear large deformation response of the riser.
In addition, NDS affords direct and efficient stress recoveries at any desired riser location/layer from the large scale simulation. This is done via an NDS Stress Transformation Matrix (STM) and will be demonstrated in follow-on paper. Effects such as carcass and zeta layers hoop stress compression due to armour layer windings are captured. Relative to computation times, this large scale NDS simulation of a 500m flexible riser, with the pre-NDS multi-layer finite element element models totaling 48,000,000+ degrees of freedom, was executed on a single core processor and in minutes.

Introduction
Flexible riser systems are in increased demand for deeper water applications necessitating the requirement for advanced, computationally efficient, large scale simulation methods. Prediction of flexible riser dynamic behavior is significantly complicated by the coupling of the geometrically nonlinear and multi-layer interaction effects. This coupling results in complex three-dimensional behavior which can not be modeled with standard riser global analysis software line element formulation. While advanced finite element analysis software can model the complex geometry and multi-layer interaction, the significant computation times required by the solver, even for very short riser lengths, is incongruent with large scale utilization (Ref 5). Clearly, efficient computation is one of the key challenges to large scale analysis of flexible risers with detailed models.