The Beatrice field is located in the Moray Firth approximately 12 miles from the Scottish coastline. The 'A' platform complex was originally provided with two pump-eductor induced gas flotation units for produced water clean-up prior to disposal offshore.
After two years of production it became clear that the units could not achieve their design rating and with produced water rates increasing significantly, dramatic improvements in performance were required.
The paper describes a novel and cost-effective solution to the problem using the principle of dissolved gas flotation. General design and cost considerations relating to produced water clean-up by this technique are also addressed, drawing on the experience of Beatrice and other Britoil operated facilities.
The Beatrice 'A7' platform was installed with two gas eductor flotation units, each specified to handle 35,000 bpd of produced water with a maximum oil inlet concentration of 300 mg/1. The specified oil removal efficiency was 95% giving a theoretical effluent oil concentration of 15 mg/1. The maximum expected produced water flowrate was 20,000 bpd, and the system was designed to operate with one flotation cell on line at any one time.
In practice, efficiencies ranged between 20 to 60% and a single flotation unit was unable to process a water throughput of more than 15,000 bpd with an inlet oil loading greater than 150 mg/1. Under upset conditions, with drilling return contamination, the efficiency of the units deteriorated further, In addition, with five years of production experience behind us, it is now considered that the maximum produced water throughput could reach 80,000 bpd, requiring both units to operate simultaneously and in excess of their original design capacity.
This situation is unfortunately common to many early North Sea platforms and has led to considerable analysis of float cell performance and produced water chemistry. In this particular case a cost-effective solution was sought that would avoid capital-intensive modification or resort to additional equipment, as has proved necessary on a number of other platforms.
In the first instance, a series of offshore tests were initiated to determine the characteristics of the problem. The resulting attempt to make the existing gas eductor system more effective proved unsuccessful, leaving a choice of whether to modify the system or purchase additional equipment.
In practice, a novel and inexpensive modification was proposed which proved to be very successful.
The principal elements of the original Beatrice system are shown diagrammatically in Fig. 1.
The offshore investigation primarily measured oil concentration levels at the outlet of the production separators (two off in total), and at the inlet and outlet of both the gravity separator and the duty flotation unit. The data showed that the flotation units had to deal with very small oil droplets (5 to 40 microns).
From repeated observation of gas bubble generation on both flotation units it was apparent that an excessive concentration of gas existed directly above the downpipe. These downpipes convey the entrained gas carried by recycled produced water into the main influent stream.
