Case Study: Typical FPSO installation of Argus® Oil in Water monitor from ProAnalysis

The following case describes a typical installation of an Argus® online Oil in Water (OiW) monitor from ProAnalysis, and the benefits the customer gained from replacing a bypass line OiW monitor with ProAnalysis" in-line probe type.

Old bypass solution

Two Oil in Water monitors from a traditional supplier were installed on the outlets from the produced water treatment system onboard the FPSO. The OiW monitors had a bypass line design where a water sample from the process was obtained, run through the monitor and returned to the process.

In order to keep the instruments operative, the units required extensive manual maintenance performed by personnel onboard the FPSO:

Daily service of the units:

• Cleaning of measurement chamber
• Emptying bucket with excess drain water from atmospheric vent, inlet to measurement chamber

Periodic maintenance:

• Disassembling and cleaning the tubing between pump and measurement chamber
• Cleaning and changing the membranes on mechanical pumps (two pumps per instrument; one on outlet from process and one on return to process)
• Changing of UV lamp every 6 months
• On and off problems with the 4-20 mA loops, a frequent problem leading to time-consuming troubleshooting

Despite all the maintenance performed on the OiW monitors, they did not function at a satisfactory level:

• At regular maintenance operations - e.g. back-flushing of hydrocyclones - the units had to be shut down. The temporary high oil concentrations (>200 ppm) from such operations caused contamination of the measurement chamber and cleaning / service work.
• The units were calibrated using a linear calibration curve based on a one-point calibration (20 ppm), resulting in a high level of error readings at higher oil concentrations (>40 ppm). Calibration had to be performed using a special calibration mixture, and could not be based on process medium.
• When the instruments were turned on following shut down, a 24-hour warm-up period was required.

Due to these factors, the OiW monitors were not fulfilling the operator"s needs, and several incidents of high oil discharges to sea forced the operator to consider alternative technologies. Key factors in the search for a new technology were stability, reliable measurements and low-level maintenance.

New in-line probe solution

The FPSO operator has opted to replace the old units with the Argus® technology from ProAnalysis, providing OiW measurement directly in the process by means of an in-line probe and rendering the bypass solution redundant. Ultrasound-based self-cleaning of the probe"s optical window is a key element in order to enable in-line measurements. Two in-line measurement probes were connected to one control unit in a dual-probe solution; one probe was installed at a high-pressure location (14 barg operating pressure), and one at a low-pressure location (atmospheric pressure).

Other characteristics of the solution:

• In-line probes installed directly into the process through ball valves on top of 2" pipe nozzles; a simple and cost-efficient installation.
• Ultrasound-based self-cleaning of the in-line probes (optical interface towards process), keeping the probes clean despite contamination in the process.
• Probes connected to one EExd field enclosure by means of armoured fibre-optical cable.
• Modbus communication to control room, as well as two EExi 4-20 mA displays on site.
• Argus Manager user software, providing full communication from, control with and diagnostics of the instrument and both probes.
• Calibration is set during FAT prior to installation using oil from the field, and is adjusted at start-up and commissioning based on results from lab analysis of manual samples.

Figure 1: Argus® in-line probe (low pressure) and EExd / EExe field enclosure

At present, the FPSO has a system that is working under varying process conditions and without any manual intervention. By using the Argus system, manual maintenance has been reduced from a daily activity to an annual inspection and revision of the system.

Figure 2: Plot from Argus Manager (red curve shows oil concentration at the inlet to the water treatment, blue curve shows the outlet)

The Argus® technology for Oil in Water monitoring from ProAnalysis became commercially available on the international market in 2008, and is gaining a reputation as a technology that:

• Minimises risk by providing reliable OiW measurements under varying process conditions
• Offers the lowest life-cycle cost / cost of ownership
• Provides value for money and high quality
• Enables new applications of OiW monitors at higher pressures throughout the produced water treatment process, offering significant benefits to operators - such as:
  • Optimization of chemical injection ? reduced chemical consumption
  • Condition-based cleaning (back-flushing) of the separators ? reduced level of oil discharged to sea
  • Immediate warning of upset in the produced water treatment systems ? reduced level of oil discharged to sea.
  • Measurement of the efficiency of the produced water treatment system ? reduced level of oil discharged to sea.

Yesterday, there were good reasons for operators to choose a bypass system because an in-line solution was not available. Today, with the Argus® technology, operators can get it right the first time and avoid having to think about future excuses for choosing a bypass solution!