Simulation and validation of the effects of thermally buoyant flow on subsea laser transmission

Point-to-point underwater laser communication systems are being developed to address increasing subsea data needs and bandwidth requirements. Many use cases - such as transmitting data from distributed sensors installed on subsea oilfield infrastructure - feature laser beams passing above heat sources such as heated pipelines. These thermal sources induce natural convection in the surrounding seawater, changing its refractive index. Maintaining point-to-point laser telemetry thus requires characterizing and mitigating the effects of these thermally buoyant flows on beam propagation. A direct numerical simulation was used to model heat and fluid flow for simple two-dimensional geometries representative of conditions where laser communication beams pass over subsea pipelines. Lineouts from the temperature fields and computed refractive index are used to calculate beam deflection for various beam locations and flowline temperatures. These results are used to develop practical guidelines on laser transceiver placement. The simulation output for a subset of test cases is validated experimentally in a laboratory water tank using scaled thermal sources. Laser beam deviation is measured directly and, as intermediate verification, fluid flow and temperature are measured using digital particle image velocimetry, showing good agreement with simulated results.