Mechanical behavior characterization of polyurethane used in bend stiffener

Abstract

Bend stiffeners are structures employed in the offshore oil and gas industry designed to restrict curvature of flexible lines and umbilical cables connected to floating production units. They are conically shaped and fabricated in polyurethane (PU), an elastomeric material whose properties can be largely modified by the multiplicity of resins available in the market. Therefore, detailed polyurethane characterization is important to define the properties of the materials as well as to evaluate its stability in marine environment. In this work, a bend stiffener grade polyurethane was exposed to synthetic sea water and to the weather for 6 months. The material was characterized by infrared spectroscopy, tensile and stress relaxation tests. It was found that the mechanical characteristics are affected by sea water with a drop in tensile strength when material is tested on the same day that the final ageing time is completed. Relaxation and stress-strain curve fittings were performed using hyperviscoelastic and hyperelastic models, respectively. Among several hyperelastic models used, Alexander, Polynomial and Yamashita-Kawabata were found to successfully describe the experimental behavior. The hyperviscoelasticity model was compared with hyperelastic models considering the same strain rate.