Dynamic Response and Fatigue Analysis of the Steel Catenary Riser Carrying Slug Flow Based on VFIFE Method

ABSTRACT: Steel catenary riser (SCR) is the critical equipment connecting floating platform and subsea pipelines for transferring oil and gas. The dynamics of SCR is a complex nonlinear problem, which includes the geometrically nonlinear beam model, platform motion, hydrodynamic load and pipe-soil contact. The vector form intrinsic finite element (VFIFE) method is used to model the SCR and the degradation P-y model is used to simulate the pipe-soil contact. The governing equation of SCR is solved by the finite difference method and the fatigue life of SCR is calculated. The accuracy of this model is validated by comparing with the published data. The results show that the slug flow induces an additional excitation force and reduces the fatigue life of SCR. The platform motion causes not only the change of SCR axial force, but also the drastic change of SCR bending moment, which significantly reduces the fatigue life. Furthermore, the effects of hydrodynamic load, current velocity and seabed strength on the dynamic response and fatigue life of SCR are discussed. This study has reference significance for the safe operation of SCR. 1. INTRODUCTION SCR is the equipment connecting the floating platform and subsea pipelines, and has been widely used in deepwater oil and gas production. SCR is under the effects of a complicated deepwater environment and requires long-time service. Accurate prediction of the dynamic response and fatigue life of SCR is of great significance to ensure safe and efficient production of deepwater oil and gas. Many scholars have analyzed the mechanical behavior of SCR through experiments and numerical simulations. Bridge (2005) conducted a systematic theoretical and experimental study of the mechanical behavior of SCR, observed the trench shape and tested the interaction between the SCR and seabed by a series of full-scale tests. Zhu et al. (2019, 2022) studied the dynamic response of SCR under the effects of external flow and slug flow through a series of experiments, and analyzed the dynamic response of the flexible riser and the details of the slug flow. In the study of pipe-soil contact, Aubeny et al. (2006) proposed the pipe-soil interaction model (AB model) based on experimental data. Randolph and Quiggin (2009) proposed a nonlinear hysteresis seabed model considering the strength degradation of seabed soil under cyclic loading. Jiao (2007) modified the AB model by using energy dissipation function to capture the seabed stiffness degradation effect. In the study of SCR dynamics, Shiri et al. (2019, 2020) used finite element method (FEM) to analyze the dynamic response and fatigue life of SCR, and conducted sensitivity analysis on soil parameters, trench profiles and slug flow. The FEM for solving SCR dynamics involves iterative solution. In order to reduce the computational complexity, Yu et al. (2021, 2022, 2023) established the dynamic model of SCR based on the vector form intrinsic finite element (VFIFE) method, this method has advantages in the calculation of geometric nonlinearity, material nonlinearity and structure fracture mechanics.