Anode design and analysis for an undersea cabled observatory with a seawater ground

Abstract

Providing power to the seafloor to operate sensors and equipment reliably for long-term use requires a well thought out power return system as there are numerous obstacles in the harsh undersea environment. Cabled observatories typically employ commercial-off-the-shelf (COTS) telecommunications cables that contain optical fibers and a single power conductor, where the return path utilizes a seawater ground anode at the other end of the cable rather than a second (ground) conductor. Dissolution of the anode substrate, stray current effects, resistance, and hydrogen and chlorine concentrations must all be carefully considered in the design otherwise insurmountable problems can result and the life of the observatory can be shortened. For example, hydrogen can cause an increase in the attenuation of silica-based optical fibers and can degrade the performance of certain electronic components over time. These consequences result in costly cable and equipment replacement. This paper describes the design of an anode recently deployed on an undersea observatory in nearly 2400 meters of seawater, and includes discussions of the mechanical analysis, tipping moment calculations, skid design, rigging and redundancy. The life of the anode is expected to exceed 20 years, which makes this a practical solution for long-term offshore energy applications. In addition to the design, analysis is presented that assisted in optimal selection of the final design parameters. The minimum physical size of the electrodes, anode frame material, distance from the anode to the cable termination and physical positioning (i.e. buried or not buried in seabed sediments) of the anode and cathode are some examples of these parameters. This analysis considers potentials between the anode and cathodes, stray current effects from sea electrodes, chlorine concentrations in the vicinity of the anode, hydrogen concentration in the vicinity of the cathode, and the effects of hydrogen evolution at a platinized titanium cathode. Results from the design and analysis were taken into account in electrical models and simulations. This allowed careful examination of the transmission of DC power through the telecommunications cable and posing (and answering) “what if” questions that might arise to minimize potential problems during operation. Cabled observatories present the inherent challenge that complete system testing is normally impossible as key elements, such as the telecommunications cable, are only integrated at deployment. Even had the cable been available for integration testing, its reactive properties are different deployed than at a test facility. The potential cost implications of such an unexpected interaction post deployment could result in unrecoverable loss. Finally, techniques used to install and operate the anode on the ocean observatory are presented along with actual footage from the deployment. Current and voltage drop measurements taken during deployment were in good agreement with those predicted from our analysis and simulations, which help to validate this design and analysis process. Taking a thorough systems engineering approach from the start of this small but critical piece of the cabled observatory more than justifies the investment when the system performs and operates as expected.