Protection Against Microbiologically Influenced Corrosion by Effective Treatment and Monitoring During Hydrotest Shut-in

During hydrotesting, a pipeline may be shut-in for a significant period of time.  If unprotected, the pipeline becomes susceptible to corrosion due to bacteria, oxygen, and saline conditions of the hydrotest water typically obtained from surface waters and seawater.  Most notably, indigenous bacterial populations in source water used to fill the pipeline can proliferate in the stagnant shut-in condition and attach to the pipe wall forming biofilms.  These sessile biofilms may contain sulfate reducing bacteria (SRB) and/or acid-producing bacteria (APB) which can contribute to microbiologically influenced corrosion (MIC).  In addition, SRB generate hydrogen sulfide (H₂S) which is hazardous, corrosive, and can form iron sulfide solids.  To protect against these adverse effects, three types of chemicals are generally recommended to be added to the hydrotest water during the pipeline fill:  an oxygen scavenger, a biocide, and a corrosion inhibitor.

Chemical treatment to prevent corrosion invariably deals with balancing risk, damage to the environment and cost. The method of applying chemical is important to ensure protection for extended shut-in times. Monitoring is important to ensure that protection is being provided during shut-in and that risk is minimized after the pipeline is brought into operation. The paper will discuss different methods for discharging the water into the environment as well as procedures to ensure corrosion protection for shut-in times that exceed the original target discharge date.

In support of this discussion, field data is presented from a hydrotest application that was monitored for bacterial growth and biocide residual during shut-in.  This data highlights the importance of protecting the pipeline during shut-in and emphasizes that a quality monitoring program is the key to determining if a chemical program is providing control over the targeted parameters.  A discussion of best practices for MIC-protection during hydrotest is provided.