Summary

The 135 D platform is located in the Campos basin off the coast of Brazil. The platform functions as a central processing facility, receiving degassed liquids for treatment from four remote production platforms. Dry oil is sent through pipelines to Cabiunas, while water with < 20 mg/L total oil and grease (TOG) is discharged overboard. Upon startup, the ability of the systems on 135 D to clean produced water to the required TOG of < 20 mg/L was limited. This paper describes the means by which the fundamental causes of the water-treatment performance problems were identified and resolved. Modifications to the original process design are discussed, and the impact of those modifications on the quality of discharged produced water is presented.

The root causes that limited water-treatment performance on 135 D were identified as a combination of very high variations in arriving-fluid flow rates, unexpected variations in solids concentration, process recycle streams, unexpectedly small oil droplets, and sludge generated by the mixing of excess chemicals with inlet fluids.

On-platform modifications to existing process vessels and to the process itself were required to successfully treat produced water whose character varied significantly from design specifications. Since completing modifications, the platform has routinely discharded produced water with < 20 mg/L TOG at or above plant-design capacity.

Introduction and Background

The volume of oil and the total volume of liquids produced by an offshore South American oil field have been increasing as a result of a successful drilling and development program. The field was developed with three remote production platforms and one central production platform. However, the export pipeline's capacity was not designed to handle the large fluid volumes resulting from the development program.

To alleviate the processing bottleneck, the field operator decided to install a new standalone oil/water-separation facility that could process 27 000 m3/d of degassed liquids with a maximum water production of 8700 m3/d to yield crude with <1% basic sediment and water (BS&W) and water with < 20 mg/L TOG. The crude was to leave the facility by means of a pipeline to the shore terminal, and the clean produced water was to be discharged overboard.

To provide the required separation capacity, the semisubmersible drilling-rig platform, 135 D, was converted into an oil/water-separation facility by installing a new process plant. The conversion included a 10-year life-enhancement upgrade to the rig, along with upgrades to the associated utilities and support systems that were required to treat the crude oil and produced water to be received from the original field platforms. The process-flow diagram for the facility is shown in Fig. 1.

Fluids arrive at 135 D by means of subsea pipelines through three risers. The liquids are heated to 60°C in two steps: first by crossexchangers with the produced water and then by indirectly fired heaters using TEG as the heat-transfer medium. The hot liquids pass through twin degassing drums before entering companion Dual Polarity electrostatic treaters (Coalescers A and B). Crude with < 1% BS&W from the coalescers is sent to the export pipeline. The produced water from the coalescers feeds into a single surge-drum-skimming vessel from which it is pumped through two sets of deoiling hydrocyclones. The water underflow from each hydrocyclone discharges to a dedicated sparger-flotation vessel. The treated water exits the flotation vessels via the oil/produced-water crossexchangers in which water is cooled to below 40°C before being discharged overboard.

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