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.

Direct Adaptive Rejection of Vortex-Induced Disturbances for a Powered Spar Platform

“…A one tenth scale model of a RDSP (11.4 m spar length) has been designed and constructed at Florida Atlantic University (FAU), complete with a sensor suite that is capable of providing all of the states of the RDSP [6]. Preliminary findings for a full scale RDSP with a 10 m diameter reveal vortex-induced oscillations with displacement amplitudes of approximately 1 m [7]. This is significant when dynamically positioning near other vessels for cargo transfer applications.…”

“…This crude estimate of the frequency provides a strong motivation for using the PLL to better-estimate the disturbance frequency in the loop, as proposed in [7].…”

“…where p is the density of sea water, (7) indicates that the relative velocity or acceleration of the water or air with respect to the component that is being referenced, A is the vector of projected areas in the x-and z-directions, V is the submerged volume, Cd (Re) is the vector of coefficients of drag in the x-, y-, and z-directions, C a is the vector of coefficients of added mass in the x-, y-, and z-directions, and f is the combined drag and added mass force vector in the x-, y-, and z-directions. The drag force portion of this equation is a combination of skin friction and residual drag.…”

“…This was previously applied to the RDSP without the inclusion of frequency synthesis for wave and load disturbance rejection in [11][12][13]. The concept of augmenting the algorithm with a PLL in order to reject disturbances at coarsely known frequencies was considered in [7,14], and applied to a simplified 1-DOF RDSP simulation. This paper extends the work of [7] by providing a more detailed description and analysis of the PLL, and applying it to a more realistic and complex simulation of the RDSP that utilizes all 6-DOF (rather than 1-DOF).…”

“…The concept of augmenting the algorithm with a PLL in order to reject disturbances at coarsely known frequencies was considered in [7,14], and applied to a simplified 1-DOF RDSP simulation. This paper extends the work of [7] by providing a more detailed description and analysis of the PLL, and applying it to a more realistic and complex simulation of the RDSP that utilizes all 6-DOF (rather than 1-DOF). Furthermore, multiple sensor and actuator configurations will be considered.…”