Fouling release coatings (FRCs) can become damaged and diminished over exposure. Quantifying adverse effect of scratches on FRCs is crucial for damage control. This study investigated the effect of four pre-defined scratches on the re-fouling of a silicone-based FRC (SiFR) undergoing underwater cleaning utilizing a novel automated underwater cleaning system (AUCS). Moreover, barnacle adhesion and coating detachment formation of scratched SiFR were evaluated. Field testing at the CoaST Maritime Test Centre (CMTC) demonstrated that the scratches varying in depths and widths can significantly affect the biofouling behavior and cleaning efficiency of SiFR surface. For wide scratches (i.e. 3-mm-wide), hard fouling (e.g. barnacles, mussels) was more prone to accumulate, and underwater cleaning was effective in preventing hard fouling but not soft fouling on SiFR surface. Additionally, the re-fouling and cleaning difficulty of hard fouling increased with the depth of wide scratches. For narrow scratches (i.e. <50-μm-wide), SiFR was primarily attached by soft fouling (e.g. biofilm, algae), and underwater cleaning performed positive fouling resistance of algae but not biofilm on SiFR surface. Besides, algae became difficult to remove with the depth of narrow scratches. Notably, biweekly cleaning proved to be highly effective in biofouling control of SiFR with narrow and shallow scratches.
The effectiveness of fouling release coatings (FRCs) may diminish over time in the seawater. Underwater cleaning of FRCs in service is crucial for biofouling control. This study investigated the effects of cleaning parameters (brush moving speed, cleaning force and frequency) on a silicone-based FRC (SiFR) aged for 2 months (A2-SiFR) and 13 months (A13-SiFR), simulating the short-term and long-term FRCs in service. Fouling resistance, re-fouling and cleaning efficiency utilizing a self-designed automated underwater cleaning system (AUCS) were examined. Moreover, coating surface characterization, including visual appearance condition, roughness, water contact angle were evaluated on the testing surfaces. Field testing at the CoaST Maritime Test Centre (CMTC) demonstrated that underwater cleaning exhibited positive effect on the long-term fouling control performance of aged FRCs. Besides, stronger cleaning force, slower moving speed and biweekly cleaning led to better fouling resistance. No significant damages were observed on A2-SiFR surface, while regular spiral and circular scratches appeared on A13-SiFR surface. The damages led to adverse effects on fouling resistance, re-fouling and cleaning efficiency of algae on the cleaned A13-SiFR surface, affecting the coating performance more than cleaning parameters. Despite these damages, the cleaned surfaces still exhibited a higher fouling resistance compared with the ones without cleaning.
Offshore pipelines and structures require regular marine growth removal and inspection to ensure structural integrity. These operations are typically carried out by Remotely Operated Vehicles (ROVs) and demand reliable and accurate feedback signals for operating the ROVs efficiently under harsh offshore conditions. This study investigates and quantifies how sensor delays impact the expected control performance without the need for defining the control parameters. Input-output (IO) controllability analysis of the open-loop system is applied to find the lower bound of the H-infinity peaks of the unspecified optimal closed-loop systems. The performance analyses have shown that near-structure operations, such as pipeline inspection or cleaning, in which small error tolerances are required, have a small threshold for the time delays. The IO controllability analysis indicates that off-structure navigation allow substantial larger time delays. Especially heading is vulnerable to time delay; however, fast-responding sensors usually measure this motion. Lastly, a sensor comparison is presented where available sensors are evaluated for each ROV motion’s respective sensor-induced time delays. It is concluded that even though off-structure navigation have larger time delay tolerance the corresponding sensors also introduce substantially larger time delays.