Robust anti-fouling and cleaning technology for optical windows enabling maintenance-free subsea operation of optical instrumentation

1 PROJECT PARTICIPANTS AND MAIN ROLES Christian Michelsen Research - CMR Design and maintain test facilities for contamination and observation of sapphire samples. Assist in the use of the testing facilities. University of Bergen; Nano Group - UiB Development of structured sapphire and evaluation and analysis of all test samples; surface characterization, contact angle measurements, microscopy. Polytec Pre-evaluation and procurement of commercial oleophobic coatings. ProAnalysis Industrial partner, project administration and offshore testing. 2 CLEARVIEW PROJECT BACKGROUND The ClearView projects main objective was the development of anti-fouling technology for optically transparent windows for use with optical instrumentation in subsea environments. The ClearView project aimed at achieving self-cleaning and under water superoleophobic properties for sapphire surfaces. Owing to its excellent mechanical, thermal and optical properties sapphire (crystalline Al2O3) is extensively used to make optical windows for harsh conditions. Various fabrication and characterization techniques have been utilized during this project that are briefly mentioned in the following section. 3 EXPERIMENTAL METHODS 3.1 Atomic force microscopy To visualize the sapphire surface on atomic scale and quantify the surface quality, UiB obtained AFM images of the sapphire surfaces, obtained using a Scientec 5100. All images were recorded in contact mode. 3.2 Contact angle measurement A droplets contact angle to a surface is a common way to evaluate how well the droplets substance sticks to a given surface. To perform contact angle measurements UiB setup a video based optical contact angle measurement system. This was used to measure the contact angle of oils and water on sapphire windows. For oil contact angle measurements in water, an oil droplet of about 3-5?L in volume was gently deposited from the bottom of the system onto the sapphire window surface submerged in water. This method is known as the captive bubble technique and is used for test liquids that have lower density than the surrounding media. 3.3 Surface structuring Surface structuring of sapphire involved use of the cleanroom facilities at UiB. List of various tools utilized in cleanroom for surface modification/structuring is given below. Surface structuring was performed using a combination of optical lithography, surface etching and chemical masks. 3.4 Live testing of sapphire samples 3.4.1 Integrated real-time fouling monitoring system To study the anti-fouling properties of sapphire surfaces, a test set-up was designed at CMR. The setup consists of a polypropylene cylindrical test vessel, of 20-litre capacity and was equipped with window mounts, copper coil, temperature control unit, mixing element and a Ultra-Turrax emulsifying element. The window mounts allowed the test surfaces to be imaged using two CCD cameras placed on the outside of the test vessel. 3.4.2 Optical properties measurement system In order to enable us to quantify the impact on optical performance from thin layers of precipitated scaling, and window surface degradation due to e.g. chemical corrosion or mechanical abrasion, CMR developed a multipurpose optical measurement setup. The system is capable of measuring spectral transmission and surface reflectance. 3.4.3 Offshore testing For the best performing samples in the project we felt that it was important to perform a field test in an industrial environment. ProAnalysis designed and built a specialized holder for multiple sapphire windows, which could be mounted on a normal instrument probe. This allowed us to insert the test samples into an active process pipe and evaluate the performance in a real test environment. 4 CONCLUSIONS The ClearView project has resulted in successful development of novel anti-fouling optical windows based on nanostructured sapphire substrates, showing very promising results for the targeted applications. The technology developed in the project is also highly applicable to a wide range of other optical applications in subsea and/or processing environment.

There is a strong and increasing demand from the petroleum industry for advanced instrumentation providing online process surveillance and condition monitoring for subsea installations. Optical technologies is the solution of choice for such instrumentati on, and has the merit to increase production and cost efficiency, reduce safety risks, avoid production interrupts, and satisfy environmental safeguard regulations. Technology for this purpose is a field of research having both regional and national strat egic importance, being identified as a key focus area by OG21, Norway's official technology strategy for the petroleum sector. However, fouling of optical windows by precipitated salt, biological growth and hydrocarbon deposits are fundamental issues wh ich hinder application of above mentioned optical technologies in subsea environments. There is to our knowledge no existing anti-fouling solution available today which can keep optical windows sufficiently clean and optically transparent over time to ena ble permanent installation of optical instrumentation in subsea environments without the need for frequent maintenance intervention for cleaning or window replacement. The project aims to develop durable, effective anti-fouling technology for optical wi ndow surfaces, enabling maintenance free solutions for permanent subsea installation of optical instrumentation, with focus on solutions for continuous oil-in-water monitoring and optical imaging technology for structural condition monitoring and leakage detection. The proposed innovation combines novel nanostructured anti-fouling and super-hydrophobic surfaces with robust coating materials, monitoring technology and cleaning methods uniquely adapted for two different subsea applications. By this unique approach, the aim is to develop optical windows that are highly resistant to all relevant fouling mechanism such as precipitated salt, biological growth and hydrocarbon deposits.