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.
© 2006. Society of Petroleum Engineers
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History
- Original manuscript received:
2 June 2004
- Revised manuscript received:
12 April 2005
- Manuscript approved:
15 April 2005
- Version of record:
20 February 2006
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