INTRODUCTION
This paper contains the discussion of a case history where corrosion attacks took place in a slightly sour waste water disposal pipeline. The exposure of the waste water to the air had taken place from an open pit, which may have generated elemental sulfur fine particles in the solution. When solids dropped out from the solution, localized attack was detected in the form of underdeposit corrosion. Details are also given on the laboratory testing results and development of a corrosion inhibition program for such an environment.
In many onshore operations, the integrity of a waste water disposal pipeline can be extremely important since any failures of the pipeline may directly result in a shutdown of the entire upstream oil and water separation processes. The main water source to be disposed is generally the produced water after a series of surface facilities, such as, separators, filters, clarifier tanks, cone bottom storage tanks, etc. There are also many waste water sources which are much dirtier in terms of solid content to be transported in the waste line after commingling with the produced water. These waste waters can be from vacuum trucks, drains, scrubbers, soft water cogeneration units and generators and are first dumped into waste pits which could be directly opened to the air. After temporary solids settle out they are pumped into storage tanks and then go through a filtration process whenever the tanks get full. It is then easy to determine that these waste waters are generally full of solids collected from either reservoir or surface facilities. Unfortunately, the filtration process involved can be quite inadequate and ineffective sometimes to get rid of all the suspended solids, particularly the fine particles. The ratio of waste water and the produced water varies from system to system and therefore the quality of the water being sent down to the waste pipeline can differ from month to month, not to mention the variations from the variable field conditions. From a corrosion perspective, it is not hard to concede that the waste water could pose a high risk to the integrity of the carbon steel pipelines in terms of underdeposit corrosion (UDC) due to the presence of a significant amount of solids. The standard reduction potential for oxygen is 1.23 V for equation 1, implicating its strong electron removing capability when compared with other acidic species dissolved in the solution. The detrimental impact of the ingress of oxygen for oil and gas pipeline system had been observed and reported by many operators and researchers1-3. The most effective way to reduce the oxygen impact on the pipeline deterioration is the elimination of the oxygen sources, such as using gas blankets and preventing the leakage. The use of an oxygen scavenger, e.g. bisulfite, is also a typical option in the field. But its effectiveness can be largely limited when the oxygen concentration and location are the uncertain factors, as well as its potential deactivation by other chemicals in the water.
