Biofouling, the unwanted growth of marine organisms, is the principle factor limiting the duration of deployments of submerged equipment. Underwater sensors, cameras, connectors, lights, windows, and other surfaces or structures are susceptible to marine growth that renders them ineffective. In some environments, biofouling is so intense that cleaning or other mitigation strategies are required on a weekly basis.
Manual cleaning, servicing, or replacement of underwater equipment on a regular basis is costly and in some locations impossible. For this reason, the ocean sensing industry has spent considerable effort over many years to develop in-situ biofouling control technologies that allow the sensors or devices to remain functional in the field for extended durations uninhibited by marine growth. The biofouling control technologies resulting from these efforts can be broadly categorized into three groups: 1) volumetric chemical dosing techniques, 2) surface coatings and treatments, and 3) mechanical methods. Their shortcomings led AML to develop a UV-based technology that is non-toxic, non-contact, and suitable for the complex geometries often found on subsea equipment.
As shown above, three instruments were deployed directly adjacent to one another. Two of the instruments were equipped with UV•Xchange and one was left unprotected as a control. Baffles were placed between adjacent sondes to prevent ‘spillover’ UV light from one instrument to the next. Each instrument was equipped with a conductivity sensor, a time of flight sound velocity sensor, and a turbidity sensor. The control sonde was also equipped with a pressure sensor.
Data were recorded from each instrument at a frequency of 1 Hz and a controllable underwater camera was available to visually document the progression of the biofouling.
Visible Results of UV Biofouling Control
As expected, growth on the platform was rapid. Sensor readings from the control sonde demonstrated clear errors due to fouling within a few weeks. Within a few months, the instrument was enveloped in marine growth. The sensors of the UV-protected instruments, in contrast, remained in agreement within their published accuracies. Still images from daily video records were collected and stitched into a time lapse video showing 480 days of progress.
During regular maintenance of the Folger Pinnacle instrument platform in July 2014, AML’s CTD test rig was pulled up along with the other oceanographic equipment. UV2’s conductivity sensor and one LED module was replaced at this time* but no other service, calibration, or cleaning was done. The image below shows effective biofouling prevention in UV1, UV2, whereas heavy fouling is seen on Control CTD.
* In April 2014, an LED module on UV2 failed due to a poor solder connection. As a result, the sensor it had been protecting, the C•Xchange conductivity sensor, began to foul. The appearance of its glass tubes after two months of fouling contrasts starkly with the pristine glass tubes of the C•Xchange on UV1 which were protected for the entire nine months.
“I am quite astounded by the results of this test. I expected the UV antifouling system to be a significant improvement, perhaps doubling the time between required maintenance, but I did not expect it to outperform the TBT protected CTD. We operate our CTDs continuously on the Observatory. The pumped CTDs, protected with TBT, last about 7 or 8 months before the TBT is depleted and we also wear through the pumps. Near hydrophones, where we cannot use pumps, the CTDs last 2 months at best. The UV protected CTD at our most challenging site has lasted over 15 months and shows no loss of accuracy yet.”
Tom Dakin, Sensor Technology Officer at Ocean Networks Canada
The instruments continued to function (foul-free) for 26 months. The camera feed ceased when the platform was heavily damaged by a series of storms rolling through the area. The wave action was intense enough to tear the camera sphere and accompanying equipment off of the observatory platform, at which point the observatory suffered a complete power failure. The image at right was captured prior to the loss of the platform.
The platform was recovered from the pinnacle in April of 2016. The image below shows the test rig post deployment. Of note is that the rig was unpowered between the winter storm event in late 2015 and the platform recovery in April 2016. Again, the UV protected sensor surfaces show the effectiveness of the antifouling system.
Biofouling Control Results: Two Years of Time Series Data
In addition to the visual record of the fouling, sensor readings were monitored and compared over time to assess UV’s effectiveness at eliminating biofouling-induced measurement error. Data from the AML CTD test rig were collected and compared against a reference CTD using a TBT-based biofouling control technique. The graph below shows the conductivity data trends, and a few notable events are highlighted.
- Start of deployment: All sensors working and in agreement with each other within the total error budget
- Event 1: AML Control (lacking any biofouling control) drifts out of specification compared with UV-protected test sensors and TBT-protected reference sensor
- Event 2: LED wiring issue in UV2 results in loss of biofouling protection for conductivity sensor, causing UV2 to drift out of specification compared with UV1 and the TBT protected reference sensor
- Event 3: Platform service (see Figure 5) including replacement of UV2 LED module and fresh Conductivity XchangeTM No other cleaning, calibration, or maintenance.
- Event 4: Ground fault failure of TBT-protected CTD, resulting in long gap in reference data
- Event 5: UV1 and UV2 sensors (AML Conductivity XchangeTM sensors protected by UV•Xchange) are overlapping each other for the entire period where the TBT protected CTD data is missing, up to the next event.
- Event 6: UV1 conductivity drifts out of specification compared with UV2 as the protective LED reaches the end of its service life.
- Event 7: TBT-protected CTD is serviced and replaced. Readings are in line with UV2.
AML’s UV•Xchange is an attractive antifouling technology compared with other techniques currently being used as it is non-contact, non-chemical, and contains no moving parts. This technology demonstration is further confirmation of the technique’s effectiveness at preventing biofouling on sensors in long term in-situ deployments, enabling optimal performance and the expansion of monitoring timelines.
Learn how UV•Xchange can make a difference for you.
Whether it is preventing biofouling-induced drift to keep sensor data accurate, or keeping a camera lens clean to maintain a quality image, our UV antifouling technology optimizes the performance of the instrumentation that it is employed to protect.