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[daryon]

Hi All,

I'm currently working on a water injection package for offshore Oil & Gas production facility. The system comprises of Sea water (SW) lift pumps and SW treatment (filtration & deaeration). The water injection rate is 60,000 bpd, during early field life this water will be treated SW, but as the wells start to produce with higher water cut the produced water (PW) from the process will also be injected. There is high levels of mercury in the PW and therefore it all must be re-injected.

The SW treatment system is sized for 60,000 bpd which is the required WI rate. The deaerator does not operate efficently at turndown flowrates much lower than the design capacity so during maximum PW prodcution (50,000 bpd) the SW treatment will be run at near full capacity (60,000 bpd)but the majority of the treated SW will be dumped overboard with 10,000 bpd being re-injected with the PW. It is necessary to cool the PW below 50 degC to avoid significant scaling issues when it is mixed with the SW.

During maximum PW production (50,000 bpd) about 90,000 bpd of SW is required to cool the PW below 50 degC. The plan is run SW through the PW cooler and then into the SW treatment system to aviod having to incease the size of the SW lift pumps. Please see attached process schematic of the system.

The flow control valve (FCV) on the deaerator inlet controls the flow through the fine filters. The is also level contol which cascades on to the flow control to adjsut the flowrate set point and control liquid level transients in deaearator. There is another FCV upstream of the fine filters which dumps excess SW used to cool the PW overboard. In order to preferentially inject PW there is another SW dump overboard downstream of the SW booster pump. A Flow transmitter on the main WI pumps suction measures the forward flow and if the flow exceeds the required 60,000 bpd SW is dumped overboard.

I have concerns about the process control scheme, i'm just not sure how stable the control will be. I'm concerned about the dump overboard (upstream of fine filters) starving other SW users. Also the main WI injection pumps are being fed by two booster pumps, one for the PW and one for the SW. Pressure control overide has been added to the flow control to prevent low or high pressure on the WI pump suction.

The WI flowrate can only be adjusted by choke valves located on well head platforms some way from the facility, so the injection rate is basically constant. If the WI rates are changed then the Steam turbine drive speed for the WI pumps will be adjusted to suit.

Does anyone have any thoughts/additional concerns on the control scheme. I appreciate this is a long post and fairly complicated but i'm hoping someone can ease my concerns or suggest improvments to the existing control scheme. I would like to hand this over a process control expert unfortunatley that is not an option. Any thoughts or comments are welcomed. Also I'm very happy to clarify the system if what I have provded is not sufficient. Look forward to your views.

Kind Regards
Daryon

Attached Files

•  Water Injection System.xls   382KB   162 downloads