Hydrate management in deadlegs: Effect of driving force on hydrate deposition

In oil and gas pipeline networks, deadlegs are sections of process pipes equipped for providing maintenance services. As they are gas-filled with stagnant water saturated gas, gas hydrates are often deposited on the pipe wall and grow into a hydrate plug, which causes severe safety risks and flow assurance problems. One strategy for hydrate mitigation is to apply external heating to the pipe wall to lower the driving force for hydrate formation. Here, we studied how hydrate deposition under water saturated gas is affected by the driving force (temperature gradient) with a 1-in. vertical pipe system. Pipe wall temperatures are varied from -10 to 15 degrees C, corresponding to 29 to 4 degrees C subcooling temperatures using a gas mixture of methane/ethane (74.2/25.8 mol%) at 100 bar. Pipe wall temperature is found to affect the temperature gradient in the pipe, and this greatly alters the hydrate kinetics. At high driving forces, most of the water droplets condensed on the pipe wall readily convert into dry hydrate deposits, which are confirmed by dielectric constant responses from a permittivity probe and visual inspection of surface morphologies. As the driving force becomes lowered, some free water remains on the wall, resulting in wet hydrate deposits due to reduced hydrate kinetics. When increasing the pipe wall temperature from -10 to 10 degrees C, the plug time increases from 35 to 500 h. These results demonstrate that control of the driving force is a promising strategy for hydrate management in deadlegs.