Working towards single beam acoustic iceberg profiling using active roll control on a Slocum glider

On the Newfoundland Shelf and Labrador Coast icebergs are a major concern for off-shore operations and subsea installations, such as pipelines and wellheads. The shape and in particular the maximum draught of icebergs is of great interest to engineers and scientist, in order to (a) determine if the iceberg poses a threat to the subsea installations and therefore needs to be dealt with and (b) to predict the motion of the iceberg. This paper introduces the concept of using an autonomous underwater glider equipped with an ice-profiling sonar to get an initial estimate of shape and draught of the iceberg. The Iceberg profiling missions proposed in this paper requires the glider to achieve a stable helical gliding motion with a variable range of radii and vertical velocities. We analyzed the experimental data, in particular the roll, pitch and yaw from past field trials. All the data shows small roll angles due to small amounts of error during the trimming of the glider. The roll itself introduces a turning motion which is compensated by feed-back heading controller that uses the vertical rudder to correct the errors in the heading. Using the rudder, a vertical helical motion with variable radii can be achieved. This motion pattern is used for virtual moorings, where a glider spirals up and down through the water column repeatedly collecting data for one spatial location. For the purpose of the ice-profiling mission we are going to explore active roll control in conjunction with the rudder actuator in order to achieve a desired turning rate as well as a desired sonar orientation towards the iceberg target. In order to achieve this we are exploring the relationship between internal mass adjustment and the achievable roll angles using a simplified glider model derived from. The used model is quantified by comparing experimental data from past field trials with simulation results. We are presenting the outcome from numerical simulations where turning rate, roll angle and vertical glide speed are varied as a function of internal mass distribution and rudder angle.