Pipelines must be properly laid up to prevent corrosion damage during mothballing, or following a hydrostatic test prior to commissioning. Among all the lay-up procedures, wet lay-up using chemically treated water is generally considered the most cost-effective approach. This is particularly true for major subsea pipelines. To implement the procedure, a number of oilfield chemicals, including bactericide, are added to the water. However, at the end of the lay-up period, it has become increasingly difficult to discharge bactericide-treated water because of environmental concerns. A standard procedure for wet lay-up, which can be easily implemented in the field and is also environmentally acceptable, remains to be established. For this reason, a study was conducted to examine the efficacy and stability of different types of bactericides for wet lay-up application. Methods of neutralizing the bactericide residuals remaining in the water at the end of lay-up were investigated using the Microtox assay system. The study was intended to demonstrate that a properly neutralized wet lay-up fluid could be safely discharged into the environment.
This paper reviews the current practices of hydrostatic testing and lay-up for cross-country and subsea pipelines. The paper also presents the study results and recommendations on treatment of wet lay-up fluids.
Among all the alternatives of laying up a pipeline for mothballing, or following a hydrostatic test prior to commissioning, wet lay-up using chemically treated water is considered the most cost-effective approach. The water is usually treated with oxygen scavenger and bactericide to minimize corrosion damage. However, at the end of the lay-up period (which can be a few years), disposal of bactericide-treated water has created environmental concerns because of the potential toxic impact to the receiving environment. Procedures for disposal of such water have yet to be resolved.
Bactericide used for wet lay-up application should provide adequate long-term corrosion protection and is environmentally acceptable as well. However, studies have shown that the most effective bactericides in terms of antimicrobial performance were highly toxic to marine organisms, whereas the least toxic bactericides performed poorly in microbial growth inhibition tests. These results suggest that it is unlikely to find a chemical which can meet both requirements (highly effective antimicrobially and with the lowest aquatic toxicity). A more practical approach to resolve this problem is probably to use physical or chemical methods to detoxify the bactericide-treated water, reducing the toxicity to below detectable limit prior to discharge. This approach should be more economical compared to other options, such as (1) dry lay-up, (2) reducing or eliminating bactericide treatment in the water, thereby taking the risk of corrosion damage during the lay-up period, (3) discharging bactericide-treated water into a lined evaporation pond, (4) rescheduling hydrostatic testing, or (5) using high pH water without bactericide for wet lay-up.
To render a detoxification program cost-effective in field operations, a number of criteria must be considered:
implementability of the procedure
duration of the procedure (completion should occur within a very short time period, such as minutes, not hours or days)
toxicity and biodegradability of the selected "neutralizing agent" (the neutralizing agent should be non-toxic as well as biodegradable to minimize potential environmental impact if over-treatment takes place)
capability to treat a large quantity of water on site