  Two-phase flow with low liquid loads is common in high-pressure natural gas
offshore gathering and transmission pipelines. During gas production slowdowns
or shutdowns, an accumulation of liquid in the lower sections of subsea
pipelines may occur. This phenomenon is observed in jumpers that connect
different units in deep-water subsea gas production facilities. The
displacement of the accumulated liquid during production ramp-up induces
temporal variations in pressure drop across the jumper and forces on its
elbows, resulting in flow-induced vibrations (FIV) that pose potential risks to
the structural integrity of the jumper. To bridge the gap between laboratory
experiments and field conditions, transient 3D numerical simulations were
conducted using the OpenFOAM software. These simulations facilitated the
development of a mechanistic model to elucidate the factors contributing to
increased pressure and forces during the liquid purging process. The study
examined the influence of gas pressure level, pipe diameter, initially
accumulated liquid amount, liquid properties, and gas mass flow rate on the
transient pressure drop and the forces acting on the jumper's elbows. The
critical gas production rate required for complete liquid removal of the
accumulated liquid was determined, and scaling rules were proposed to predict
the effects of gas pressure and pipe diameter on this critical value. The
dominant frequencies of pressure and force fluctuations were identified, with
low-pressure systems exhibiting frequencies associated with two-phase flow
phenomena and high-pressure systems showing frequencies attributed to acoustic
waves.
