An accurate and efficient method for static analysis of marine risers

Abstract

Risers are essential components of offshore oil and gas production systems since they are responsible for transport these fluids to/from the wells from/to the floating facilities. Thus, structural analysis of marine risers has been an active research field in the last decades. Currently, there are many reliable analysis programs for riser analysis based on the Finite Element Method (FEM). However, this approach incurs in high computational costs due to its complexity and alternatives that are more efficient have been sought. Risers are subjected to static and dynamic loads, but it is known that in the earlier steps of riser design it is very important to evaluate the riser behavior under static loads, as self-weight, buoyancy, hydrostatic pressure, currents and floater movements (static offset). This paper presents an efficient and accurate approach for riser static analysis based on the numerical integration of the differential equilibrium equations of a cable subjected to vertical and horizontal static loads. The riser is modeled as an inextensible cable without bending stiffness, subjected to effective weight, drag force and offset. The riser behavior is governed by a nonlinear system of ordinary differential equations. The resulting nonlinear Boundary Value Problem (BVP) is solved using the Newton-Raphson Method with line searches to guarantee global convergence and increase efficiency. Initial values are estimated in order to transform the BVP in an initial value problem. The fourth-order Runge-Kutta method is used in the numerical integration of the resulting initial value problem. A post-processing procedure is used to evaluate the bending moment along the riser. This approach is suitable for analyzing analyze different riser configurations, such as steel catenary risers and lazy-wave risers. The accuracy and efficiency of the proposed approach are assessed and the results are compared with the FEM for different riser configurations. The results show that the presented approach is not only much more efficient than FEM but also can be more accurate.