Handling Gas-In-Riser – Part II: Numerical Simulation of Riser Equilibrium Point (REP), Riser-Gas Tolerance (RGT) and Riser Unloading, and Validation of the 2023 IADC Deepwater RGH Guidelines Calculation Worksheet

Abstract Several tools can simulate gas-liquid flow under typical operational conditions. However, the rapid expansion of gas in the riser (GIR) as it approaches the surface represents a challenge for many of these tools, as flow conditions fall outside assumptions used in developing their numerical models. Tools able to simulate the full transient behavior of GIR require expert setup and extended run-times. This work uses these tools to run simulations to estimate REP, RGT and riser unloading. The simulation work shows that RGT is generally higher than what is perceived by the industry. This means risers can safely handle larger volumes of gas without exceeding their operational ratings, assuming the influx is circulated using the Fixed Choke, Constant Outflow (FCCO) method. The results also show that the riser equilibrium in WBM happens deeper in relation to SBM, all else remaining the same, and that riser unloading is more intense in WBM, with peak liquid and gas outflows surpassing those of SBM manyfold. The results are also used to evaluate the reliability and intended conservative nature of the RGT Worksheet. This work demonstrates through simulation that GIR can be safely handled with existing surface equipment in many rigs. This represents a significant reduction in NPT and minimizes risks. The authors also demonstrate that calculations performed using the simplified methods considered in the RGT Worksheet can give conservative safeguards to offshore operations.