Abstract Tanks used to store produced water on Floating, Production, Storage and Offloading units (FPSOs) are extremely susceptible for generation of high hydrogen sulfide (H2S) levels due to the activity of sulfate-reducing bacteria (SRB). The FPSOs operated by Petrobras in the Campos Basin, offshore Brazil, all contain slop water tanks, while some also have upstream oil-water separation tankage. Slop water, comprised of produced water, ballast water from oil cargo ships and deck water, contains SRB and their nutrients required for generating H2S. Additionally, solids accumulations at the tank bottoms provide an excellent environment for microbial growth.
A 2002 field trial on an FPSO confirmed the viability of combined batch treatments of anthraquinone and of a THPS blend to control H2S biogeneration better than previous treatment programs. Anthraquinone, a non-toxic SRB inhibitor, and THPS, an oilfield biocide, act synergistically to effectively control the H2S biogeneration in this environment. The combined chemical treatment strategy has now been implemented successfully on six Petrobras FPSOs. Flexibility has been important in developing the treatment programs since operating parameters are different for each FPSO and change due to increased water production rates. Options include the ability to inject the chemicals continuously or batchwise at different locations and to alter the volumes and ratios of chemicals for optimizing control over H2S and corrosion.
This paper describes the individual FPSO water flow and storage systems and discusses the customized chemical treatment programs. Included are field H2S data showing the progression of the programs as they are being continually adjusted to optimize H2S generation and cost effectiveness. Also included are results of laboratory microbial studies showing the synergy of anthraquinone and THPS and of corrosion studies which have impacted the direction of usage of these chemicals.
With over 100 FPSOs operating worldwide, the described treatment program can significantly affect the safety and environmental aspects of processing water containing SRB.
Introduction The use of Floating, Production, Storage, and Offloading (FPSO) and Floating, Storage and Offloading (FSO) units to produce oil and/or process oil and water associated with offshore production has increased to about 106 units currently in operation worldwide. These FPSOs/FSOs are ships containing multiple tanks for separating oil and water, storing oil prior to offloading into tankers, and processing waters. Produced water typically flows into slop tanks where it may also combine with drainage water from decks or ballast water from cargo ships. Slop tanks are in many cases the final separation stage in which residual entrained oil is removed from the water prior to its discharge to the sea, offloading or reinjection. Environmental concerns dictate that the total oil and grease (TOG) is a crucial water quality criterion before discharge, but the level of hydrogen sulfide (H2S) is also critical due to its high toxicity and corrosivity to carbon steel.
Oil-water separators and slop water storage tanks are prime locations for activity of sulfate-reducing bacteria (SRB) and the subsequent generation of high levels of H2S. SRB are particularly abundant in most oilfield waters, including seawater. The slop waters also typically contain all of the nutrients required by the SRB for their growth and dissimilatory respiration, reducing sulfate to sulfide. Environmental conditions in the slop water tanks, especially the presence of sludge and solids deposits at the bottom of the tanks, are quite favorable for these anaerobic bacteria to form biofilms. These solids are also protective to the SRB and make chemical biocide treatments difficult for controlling the bioactivity. Health, safety, and environmental aspects associated with the presence of the toxic gas on offshore structures make it necessary to implement effective SRB and/or H2S control procedures while still maintaining compliant water for discharge.