Hydrate Prevention in Subsea Oil Production Dead-Legs

Abstract

Abstract A study was conducted to better understand the hydraulic and thermal behaviour of dead-legs within a stacked spool arrangement designed to produce two well streams from a single manifold. The study included scenarios under different rigid spool orientations and active line flow conditions. The ultimate goal of the study was to optimize inhibitor injection procedures with respect to loss of inhibitor and contamination of the production fluids. A Computational Fluid Dynamics (CFD) technique was employed to simulate the flow and thermal conditions within the flow lines and dead-legs. The results of the simulations were validated with two sets of experiments and in two different scales (sizes). The findings of the study suggest that the thermal and/or hydraulic status of the dead-legs during long shut-in periods pose little risk of hydrate formation. Introduction A dead-leg is defined as an inactive portion of the pipe where the flow is stagnant or has very low velocity. This inactive pipe is normally connected to an active pipe that carries the main flowing stream. Due to the low or stagnant flow created in dead-legs, there is a risk of cool-down and hydrate formation. The stagnant conditions within the dead-legs could also promote corrosion. The formation of hydrates is a well known phenomenon in the oil and gas production and processing systems. Hydrates can form under low temperature and high pressure when water is present in contact with gaseous or live crude oil and light liquid hydrocarbons. The fluids present in the well-stream contain a certain amount of gas which is saturated with vaporized water. Free produced or condensed water can collect in the low-points of the pipeline including pipe segments within dead-legs. Hydrates can be formed as a solid block when the line is left inactive and can potentially plug the pipeline. When hydrate formation happens, the hydrate plug in the pipeline may be either removed by depressurization or injection of inhibitor. By depressurizing one or even two sides of the plug, the hydrate plug starts to melt and can potentially move with high velocity, which could cause severe damage to subsea components. The formation of hydrates in the rigid spools connecting the subsea trees to the main subsea production manifolds could result in complex consequences. The manifold-subsea tree interconnecting spool is connected to the manifold from one side and can be relatively easily depressurized; however the tree side of the spool cannot be depressurized readily and may require costly subsea intervention and an extended loss of production. Addition of hydrate inhibitor will reduce the melting temperature and reduce the thermodynamic potential for hydrate formation. Flushing dead-legs with inhibitors such as methanol can protect them from hydrate formation during shut-in periods.