Motion control of high-speed hydrofoil vessels using state-space methods

Abstract

Hydrofoil ships cruise at large speeds and are often expected to operate in rough weather conditions. The motion of these ships due to their encounter with ambient waves can become uncomfortable or even dangerous without the use of some form of motion control. The objective of this thesis is to study the active motion control of high-speed hydrofoil vessels. This work is composed of two parts, reflecting the two disciplines applied: hydrodynamics and optimal control theory. In the first part, a two-dimensional computer code is developed for the calculation of forces and the integration of the equations of motion for fully submerged lifting bodies operating near a free surface. A Rankine source boundary element (panel) method is used assuming potential flow around the body. As a result, the motions of a hydrofoil vessel operating at high speed in ambient waves can be estimated in the time domain. In the second part, the application of optimal control theory to motion control of hydrofoil ships is investigated. The code developed in the first part of this work is used as a simulation tool for the assessment of control laws designed using state-space linear-quadratic methods.

(cont.) It is found that a linear-quadratic optimal controller can attenuate the motion response of the vessel advancing in monochromatic or ocean waves, with proper adjustment of the cost matrices that enter the quadratic performance criterion used. Accurate dynamic modeling is crucial in the design of control laws for any system. Vessels that operate on or near the free surface experience hydrodynamic memory effects due to their own motion. Casting the seakeeping equations of motion into a linear, time-invariant state-space model suitable for the design of optimal control laws is challenging since there is no straightforward way of including these memory effects in the model. In this work, the seakeeping equations of motion are cast in a linear state-space form which does not include memory effects, and the motion control simulation results show that this model is satisfactory for the design of hydrofoil vessel control laws.