Real-time trace of riser profile and stress with numerical inclinometers

Abstract

In this paper, the methodology for the real-time tracing of riser profile and bending moment by multiple inclinometers along the riser is presented with the assumption that its top (by GPS; global positioning system) and bottom (anchoring) points are known. The riser is modelled as a series of slender-beam segments and at each time step, the riser nodal points are progressively traced. For riser x-y-z displacement and bending-moment tracing, quadratic and cubic interpolation functions for each line element between two neighboring bi-axial inclinometers are employed with respect to the global and generalized coordinate systems. Analytical solutions are derived for the instantaneous riser profile and bending moment at each line's mid-point. To validate the developed riser-monitoring algorithms, a FPSO (floating production storage offloading) system with SCR (steel catenary riser) or SLWR (steel lazy wave riser) was simulated for 1-yr and 50-yr collinear wind-wave-current storm conditions with 45-deg heading and the corresponding numerical sensor signals were inputted to the algorithms for reproducing the real-time riser profiles and bending stresses. Based on the comparisons between the algorithm-predicted and actual values, the developed method is validated. The methodology can be applied to any kinds and shapes of lines and cables too. • The real-time tracing of riser profile and bending moment by multiple numerical inclinometers along the riser is presented. • Analytical solution using numerical inclinometer is derived for the instantaneous riser node displacement. • The analytic curved line length formula in 3D space is derived. • Simplified analytic formula for the bending moment at the mid-point of each riser segment is suggested.