Ethylene glycol is often used worldwide in both plants and pipelines to prevent hydrate formation. Worldwide, there are several offshore wet gas pipelines that transport gas through large distances, under hydrate forming conditions with large concentrations of acid gas. Several pipelines use glycol as a means for dehydration. Under certain conditions, extremely severe corrosion rates can be obtained if inhibitors are not used. This paper describes the efforts to develop corrosion inhibitors that prevent severe corrosion in wet gas systems with glycol.
Thermodynamic hydrate inhibitors have often been used to prevent hydrate formation in offshore systems and have also found application in onshore gas processing plants. Ethylene glycol (MEG) and diethylene glycol (DEG) have often been used in subsea multiphase production flow lines. In many subsea production systems, the glycol is regenerated at the separation platform and reused where it is injected at the subsea well manifold. In some studies carbon dioxide corrosion of carbon steel has been observed to be reduced when glycol forms part of the aqueous mixture when compared with systems not containing glycol. In some studies this retardation has been found to experimentally obey a factor given by de Waard. One study found that the anodic reaction decreased strongly with increased glycol concentration. The study was performed in thermostatically controlled glass cells under natural convection conditions. As the velocities generated in the test were from natural convention, the velocities were gentle and not able to disturb protective film formation.
pH stabilization has been used in several gas condensate pipelines where glycol has been used as a hydrate inhibitor. In the study presented by Dugstad, Nyborg and Seiersten, flow loop corrosion studies were performed at a fluid velocity 3 m/s and 6 bar CO2 at temperatures of 20°C and 100°C. Corrosion rates in these experiments reduced with time with the formation of a protective film. Pitting corrosion has been found in glass cell corrosion experiments with glycol solutions with H2S/CO2. Pitting occurred in experiments that had a pH of 6.5 at a temperature of 60°C.
Baker Petrolite has extensive experience in developing corrosion inhibitors for offshore systems that use ethylene glycol as a hydrate inhibitor. In one study a hypothetical mixed production line was modeled to illustrate the conditions at which a off shore system would operate. The study showed that mixture velocities 4.0 and 5.7 m/second could be expected. Corrosion experiments were done in a sparged autoclave (stagnant) and rotoclave apparatus (high shear conditions) at 100°C. High corrosion rates were obtained in both sets of conditions. Pitting corrosion was observed in the stagnant test. The high corrosion rates were inhibited by a chemical that was stable at high temperatures, compatible with ethylene glycol and compatible with system produced fluids and system materials. Further tests under both dynamic and static conditions of lean and rich ethylene glycol and water mixtures were conducted to evaluate pitting corrosion when sodium chloride was added to them. It was found that a corrosion inhibitor was effective in preventing general and pitting corrosion here.
In this work, new results are presented where extremely severe mesa corrosion has developed both in environments containing carbon dioxide with and without hydrogen sulfide. In the tests it was found that an inhibitor could drastically reduce corrosion rates in the test.
