Modeling and parameter estimation in flexible riserbend stiffener top connection

Abstract

The flexible riser top connection is a critical region for fatigue lifetime (re)assessment due to large tension/curvature variation and modeling uncertainties. Both the bend stiffener effect on the curvature distribution and top tension time series could be its inherent uncertainties. In this work, a large deflection beam bending model is developed to represent the flexible riser-bend stiffener top connection with I-tube interface, considering the nonlinear bending behaviors of riser and bend stiffener subjected to a top tension with a rotated angle. The corresponding mathematical formulation with multipoint boundary conditions is numerically solved by an iterative procedure in Mathematica. Large deflection response of the top connection and parametric assessments of I-tube length and sleeve curved radius are analyzed in a case study, including the end-fitting curvature and contact forces of riser with bend stiffener and sleeve. A proposed monitoring approach composed by gyrometers installed along the riser/bend stiffener length combined with the Levenberg-Marquardt (L-M) algorithm, is investigated in riser/bend stiffener system to numerically estimate the unknown top tension and polyurethane elastic response. A preliminary feasibility investigation is implemented in a case study with the gyrometer data numerically generated by Monte Carlo simulations. The sensor number and arrangement, loading conditions and direct top connection model may significantly influence the estimations. Furthermore, a full-scale bendingtension test of riser/bend stiffener with the image-based measurements is presented with unknown material response.

The inverse analysis using L-M algorithm is employed in the bending-tension test system combined with a direct finite element model and configuration measurements to simultaneously estimate top tension and polyurethane material parameters, which indicates that the proposed approach is reliable for parameter estimation in the riser top connection.