AN ACCURATE AND EFFICIENT METHOD FOR STATIC ANALYSIS OF MARINE RISERS

Resumo

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.