Novel Modeling Methodology of the Deep-water Flexible Riser with the Slug-flow

Abstract

. Slug flow, being the mixture of oil, gas and water, can increase the dynamics and structural response of a riser in internal fluid transportation due to the variation of slug flow's force caused by the time-space varying density. This paper presents a high-fidelity model of a flexible deep-water riser based on the absolute nodal coordinate formulation with slug flow in the arbitrary Lagrangian-Eulerian description. In the current paper, the Lagrangian and Eulerian description is introduced to describe the slug flow moving along the riser. Besides, a material coordinate is added together with the position and position gradient as the state variables. The riser is discretized into two types of elements, the constant-length and variable-length elements. The variable-length element is where the slug flow locates whose velocity of the material coordinates is equal to the slug flow speed, and its movement along the riser is simulated by the moving mesh technology. Considering the fact that the enormous ratio of the length to the riser's diameter, the Euler-Bernoulli beam theory is adopted to model the riser. In this paper, the equations of motion (EOM) of the riser subjected to the slug-flow and environmental loads are derived based on the generalized D'Alembert principle. The implicit time integration method is applied to solve the derived differential-algebraic equations. First, the proposed model and the slug flow method are validated. Second, Parametric studies are performed to quantitatively identify the design conditions most affected by the slug flow.