Mitigation of vortex-induced disturbances for the rapidly deployable stable platform

Abstract

The rapidly deployable stable platform (RDSP) is a concept vessel that is being developed for missions, such as at-sea cargo transfer, that will benefit from a stable spar-like platform that can transit under its own power at medium speeds (∼5–10 m/s). The stability and transit qualities of the RDSP extend from its two distinct modes of operation. These operating modes are the horizontal transit mode (HTM), where the RDSP is configured like a trimaran and the vertical operating mode (VOM), where the RDSP is configured like a spar buoy. In VOM, the RDSP will use the thrusters that are located on its spar section to manoeuvre at slow speeds, mitigate disturbances and dynamically position itself. When the RDSP is moving through the water in VOM, vortices will oscillate off of its spar section and produce forces that are primarily perpendicular to the direction of oncoming flow. Mitigating these disturbances is the focus of this work and is done using an adaptive control algorithm. This algorithm includes a disturbance rejection component that is capable of compensating for any unknown disturbance and phase information, as well as a phase locked loop (PLL) that improves the disturbance frequency estimate on-line. Using this technique, the vortex-induced motions are reduced to 18% of their open loop values using control inputs that are obtainable for the proposed RDSP thrusting system.