Effect of THPS on Discharge Water Quality: A Lessons Learned Study

Biocides typically have an adverse impact on overboard water. THPS (tetrakishydroxymethyl phosphonium sulfate), one of the most commonly used biocides offshore has a similar effect on produced water. The effect of THPS on seawater used for hydrotesting and bulk storage is seldom studied and rarely documented. The effect of temperature, pH, water depth, dissolved oxygen concentration and various ions in the system is important to note. Once a certain volume of water is treated with any chemical, it is now deemed to be chemically treated seawater which cannot be discharged unless verified using the NOEC (No Observable Effect Concentration) testing method. This experience will provide a detailed understanding of the discharge of chemically treated seawater as well as the interaction of THPS with potential ions in the matrix. Additionally, regular sampling and associated analyses will be presented that demonstrate the degradation and half-life of the THPS molecule in varying temperatures. Periodic sampling of the THPS chemical in the seawater has provided a detailed understanding of the half-life degradation of the chemical. The interaction of the chemical with the cations present in the system and subsequent aversion to the neutralization reaction with hydrogen peroxide has also been studied and presented. Tetrakishydroxymethyl phosphonium sulfate (THPS) is a common biocide utilized to treat water that is used for hydrotesting and bulk storage. Hydrotesting (hydrostatic testing) is conducted to ensure that the system is ready for commissioning and startup. In certain other situations, platform hull tanks are flooded with available water during transport from the location of construction to the location of integration. Treatment of this water used for hydrotesting and bulk storage is typically conducted to mitigate corrosion impacts due to microbially induced corrosion (MIC), oxygen-related corrosion and under-deposit corrosion. The decision to treat this water with biocides such as glutaraldehyde and THPS depends on aspects such as: • Source of water • Depth of extraction • Oxygen concentration • Temperature • Condition of the system to be treated • Nutrient sources and availability • Exposure period Biocides such as gluteraldehyde and THPS mitigate MIC by controlling proliferation of microorganisms such as sulfate reducing bacteria (SRB) and acid producing bacteria (APB) but have a significant impact from an environmental discharge aspect. The following paper describes a case study of a super major's efforts in treating ballast tank bulk storage water, subsequent sampling & testing and discharge.