Strength Evaluation of Composite Risers
Authors: Rasheed H.A. , Tassoulas J.L. 
Published: October 2008

ABSTRACT
A numerical procedure is developed for strength analysis of composite risers. An isoparametric tube finite element is formulated which makes the analysis economical for use in design computations. Material and geometric nonlinearities are taken into account. Stress failure criteria are incorporated for in-plane and interface strength while an energy criterion is adopted for predicting the onset of delamination.

INTRODUCTION
The use of composite materials in deep-water applications such as risers and tendons is significantly advantageous. This is due to the superior characteristics of such materials over steel with regard to stiffness and strength-to-weight ratio as well as fatigue and corrosion resistance[1]. Composites are considered as an alternative to steel for use in depths of more than 5000ft. Uneconomical designs result from the use of steel for such depths due to high weight problems, as indicated by Steddum 1992[2] Thus, composite materials have recently attracted a lot of attention for use in the aforementioned applications. However, the behavior of composite tubes under combined loads is not sufficiently understood nor efficiently modeled at this time.
This study is intended to develop practical and reliable analytical techniques for use in design computations. This can be achieved through an economical finite element procedure, material modeling that incorporates appropriate failure criteria and a formulation that accounts for large changes in the tube geometry.
The procedure to be described herein is applicable to the analysis of fiber-reinforced epoxyresin tubes. It can be used to predict the strength of tubes subjected to internal pressure, external pressure, axial tension and bending moment. A check for possible delamination initiation is included but complete delamination buckling analysis will be considered in a future article.

MODELING AND FORMULATION MATERIAL MODELING

Composite Model
The present material model takes into account general orthotropic layers perfectly bonded together. Typically, the tube wall consists of fiber-reinforced epoxy-resin plies of different fiber orientations, fiber material, fiber-matrix properties and thicknesses. This implies distinct material characteristics in each layer. The model requires specification of all material properties of interest for each ply. In the present applications, the composite has (90°) plies, for which fibers run in the hoop direction, and (±?°)plies, for which fibers are wound at these two angles relative to the tube axis. The present model merges every pair of (±?°) plies into one layer to maintain cross section symmetry considered in the element formulation. This may be justified by the fact that ? ° is in the range of 0° to 20° making stress discontinuities in-between (±?°) plies negligible.
Linear-brittle behavior in both fiber and transverse directions and nonlinear in-plane shear behavior are adopted for each ply. This is justified by the phenomenological behavior of laminae types of interest[3,4], namely glass and carbon fiber-reinforced epoxy, and the laminate lay-ups of 90°/± (0-20°)[5] The exception may be the compressive behavior of glass fiber plies in the transverse direction which exhibits nonlinearity near cracking. This does not affect the strength evaluation of the tube since matrix cracking usually occurs at early stages of loading.