This study conducts a numerically investigation into the transient 3-D compressible gas and water-liquid flows through a subsea jumper under high pressure of 2MPa and 30°C. The CFD results reveal detailed gas-liquid flow patterns evolution within the jumper and the associated flow-induced forces acting on the bends walls. The study examines the effects of varying liquid flow rates in the mixture ratio, which ranged from 1 to 5 at the inlet. With increasing liquid mass flow rates, the bends altered the gas-liquid flow patterns. Across all flow conditions, the RMS void fraction monitored at the multiple bends increased in the following order: fourth, first, fifth, second, third and sixth bends, respectively. The RMS flow-induced forces increased from first to fourth bend before decreasing progressively to the sixth. The structural response of the jumper in terms of stresses and displacement were assessed with FEA technique. The stress cycling exhibited maximum energy distribution when the liquid phase is maximum of 5 in the gas-liquid mixture ratio. Rainflow counting technique per ASTM-E1049-85 and Palmgren Miner rule applied at stress concentration regions reveal accumulated damages. Ultimately, the subsea jumper operational life is analytically estimated and corroborated against S-N curve, offering a comprehensive insight into the fatigue life expectancy under various operating conditions.
In the Subsea Production System, the subsea jumper plays important role as pipe connector, facilitating alignment and reach between subsea structures such as trees and manifolds for transporting multiphase flows. The integration of multiple bends in the jumper design serves several purposes: to compensate for dynamic movement induced by ocean currents, temperature, and pressure variations; to assist with inspection and potential removal when necessary; and to navigate around obstacles on the seabed while maintaining the desired flow path. The complex geometric configuration of the jumper, coupled with the properties of the multiphase fluids transported, gives rise to a variety of flow patterns, including stratified-wavy, bubbly, slug, churn, and annular flows. These flow patterns are characterized by their distinct fluid distribution and behavior, with slug flow, for example, being defined by an alternating sequence of Taylor bubbles and liquid slugs traveling at high speeds. The rapid transit of intermittent slugs, carrying significant momentum forces, induces substantial cyclic loading on the walls of the bends.
Furthermore, the flow resistance caused by the bends promotes turbulence, marked by chaotic motion with fluctuations in pressure and velocity within the flow. This repeated turbulence and cyclic loading on the bend walls can lead to vibration in the jumper structure when the momentum forces fall within a specific frequency range. Thus, bends are identified as primary sources of vibration in piping (Nair et al., 2011). This fluid-structure interaction phenomenon, known as multiphase flow-induced vibration (FIV), commonly occurs at low frequencies, typically below 30Hz, and is a prevalent cause of cyclic fatigue failure in complex piping systems transporting multiphase flows and, by extension, in plant equipment. Prior studies have predominantly assumed flow conditions to be incompressible and at atmospheric conditions. Such assumptions are not pragmatically applicable to the design of piping systems for engineering plants, highlighting a significant gap in understanding both the evolution of flow patterns within the domain and the associated flow-induced forces arising from internal multiphase flows.
