Asgeir J. Sørensen

DESIGN OF A DYNAMIC POSITIONING SYSTEM USING MODEL-BASED CONTROL

Abstract A dynamic positioning (DP) system includes different control functions for the automatic positioning and guidance of marine vessels by means of thruster and propeller actions. This paper describes the control functions which provide station-keeping and tracking. The DP controller is a model-based control design, where a new modified LQG feedback controller and a model reference feedforward controller are applied. A reference model calculates appropriate reference trajectories. Since it is not desirable, nor even possible, to counteract the wave-frequency movement caused by first-order wave loads, the control action of the propulsion system should be produced by the low-frequency part of the vessel movement caused by current, wind and second-order mean and slowly varying wave loads. A Kalman-filter-based state estimator and a Luenberger observer are used to compute the low-frequency feedback and feedforward control signals. Full-scale experiments with a multipurpose supply ship demonstrate the performance of the proposed controller.

Model-Based Output Feedback Control of Slender-Body Underactuated AUVs: Theory and Experiments

This paper presents the design and experimental results of a novel output feedback controller for slender-body underwater vehicles. The controller is derived using model-based design techniques. Two separate control plant models are employed: a 3-degree-of-freedom (DOF) current-induced vessel model accounting for the current loads acting on the vehicle and a 5-DOF model describing the vehicle dynamics. The main design objective behind this strategy is to incorporate the vehicle dynamics when estimating the current influence on the vehicle. Furthermore, the transit model is based on the notion of constant propeller revolution resulting in a partly linearized model, which subsequently leads to perspicuous and implementable controller and observer structures. The controller is derived using the observer backstepping technique, and the closed loop is proved to be asymptotically stable using Lyapunov and cascaded systems theory. The control objective is to track the desired pitch and heading angle generated by the line-of-sight guidance system while keeping constant forward thrust. Experimental results demonstrate successful performance of the proposed output feedback controller implemented on the Minesniper MkII AUV/ROV.

Design of hybrid controller for dynamic positioning from calm to extreme sea conditions

High-level control of dynamic positioning systems on marine vessels using hybrid controller is developed to extend the operational weather window for marine operations to harsh environments. The types of hybrid controller considered are multi-output PID controllers with position measurements, and multi-output PID and acceleration feedback controllers with position and acceleration measurements. Numerical simulations and experiments in addition to stability proof are conducted to verify the proposed hybrid-controller dynamic positioning system in varying environmental conditions from calm to extreme seas.

Structural issues in the design and operation of marine control systems

Abstract This paper addresses structural issues in the design and operation of marine control systems on ships and offshore installations. In particular, control systems for offshore vessels conducting station keeping and low speed manoeuvring operations are presented. Nevertheless, the methodologies will be valid for other applications and operational conditions. Design issues related to both real-time control and monitoring systems and operational management systems will be addressed. Marine operations with a dynamically positioned drilling rig is presented as an example.

Design of ride control system for surface effect ships using dissipative control

Abstract A ride control system for active damping of heave and pitch accelerations of surface effect ships (SES) is presented. It is demonstrated that distributed effects that are a result of spatially varying pressure in the air cushion result in significant vertical vibrations in low and moderate sea states. In order to achieve high quality human comfort and crew workability it is necessary to damp these vibrations using a control system which accounts for distributed effects owing to spatial pressure variations in the air cushion. To develop such a ride control system a mathematical model describing the motion of the craft in the vertical plane is derived. This mathematical model accounts for accelerations induced by both the dynamic uniform and the spatially varying air cushion pressure. Sensor and actuator placement is discussed, and the stability of the control system is analysed using the theory of passivity. The performance of the ride control system is shown by power spectra of the vertical accelerations and the pressure variations obtained from full-scale experiments with a 35 m SES.

A New Method of Thruster Control in Positioning of Ships Based on Power Control

Abstract Positioning systems include different control functions for automatic positioning and guidance of marine vessels by means of proper action of the thruster and propeller devices. These may be controllable pitch propeller (CPP) with fixed speed, controllable speed with fixed pitch propeller (FPP), or controllable pitch and speed in combination. The positioning controller computes commanded forces in surge and sway and moment in yaw. The thruster allocation algorithm determines the corresponding force and direction each thruster and propeller device must produce in order to fulfil the positioning controller commands. Conventionally, the final pitch or speed set-point signals are determined from stationary propeller force to speed/pitch relations based on data from the propeller manufacturer. However, these relations are strongly influenced by the local water flow around the propeller blades, hull design, operational philosophy, vessel motion, waves and water currents. In this paper, a new method based on torque control and power control of the propeller and thruster devices is introduced. Instead of calculating the propeller speed and pitch set-point signals based on stationary functions, the propeller force to moment and power relations are used. This method gives a significant improvement in the performance and the stability of the electrical power plant network, while the positioning accuracy and bandwidth are improved. Simulations demonstrate the performance of the proposed controller schemes.

Ocean Wave Spectral Estimation Using Vessel Wave Frequency Motions

Wave spectra are estimated from wave frequency motions of a vessel at zero or low advance speed. Minimization of a cost functional that indicates how well the estimated spectrum results in the measured motion spectra was based on sequential quadratic programming and a genetic algorithm. Two procedures have been developed and applied to numerically simulated motions of a 59 m length offshore supply vessel.

Multiple model adaptive wave filtering for dynamic positioning of marine vessels

This paper addresses a filtering problem that arises in the design of dynamic positioning systems for ships and offshore rigs subjected to the influence of sea waves. Its key contribution is twofold: i) it introduces an improved model for filter design, and ii) and it exploits the structure of a multiple model adaptive wave filter that relies on measurements of the vessels position and heading only. Namely, an improvement in the control plant model is proposed that better captures the physics of the problem at hand and a bank of Kalman filters is designed for a finite number of parameter values, each corresponding to a different peak frequency of the assumed wave spectrum model. Tools from multiple model adaptive estimation (MMAE) theory are exploited to blend the information provided by the different observers, yielding position and velocity estimates of the marine vessel. These estimates are then to be used in an appropriately designed feedback control law. Simulations illustrate the efficacy of the MMAE techniques proposed and the improvement in performance that is obtained when compared with other approaches.

Lyapunov-Based Integrator Resetting With Application to Marine Thruster Control

This paper addresses the idea of improving transient behavior by internal state resetting of dynamic controllers, such as controllers with integral action or adaptation. The concept presented here assumes that for a given closed-loop system with a dynamic controller, improved transient performance is achieved when reset of controller states gives negative jumps in the Lyapunov function value. The Lyapunov function constitutes a part of the controller algorithm. By combining this with existing stability theory for switched systems, the stability analysis of the overall system follows directly. The framework assumes that a Lyapunov function is given, and that full state measurement is available for feedback. Moreover, an estimator is needed to give a coarse estimate of the system equilibrium point. An anti-spin feature in local thruster speed control on ships with electric propulsion is in this paper presented as an application for the given framework. Transients arise when the ship operates in extreme seas, where disturbances such as ventilation and in-and-out of water effects may give rise to loss in propeller thrust. A Lyapunov function is used to decide appropriate reset of the integrator state of a standard Pi-controller. The method is illustrated with experimental results.

Positioning of small-waterplane-area marine constructions with roll and pitch damping

Abstract In dynamic positioning systems for floating marine constructions a three-degrees-of-freedom multivariable controller, with feedback from the horizontal-plane positions and velocities in surge, sway and yaw, has been regarded as adequate for the control objective. However, for certain marine constructions with discernible coupling characteristics in the dynamics and kinematics between the horizontal-plane and the vertical-plane, undesirably large roll and pitch oscillations may be induced by the thruster actions applying the conventional horizontal-plane control strategy. In this paper a new multivariable control law accounting for both horizontal and vertical motions is proposed. It is shown that significant roll-pitch damping can be achieved. Moreover, the positioning performance in surge, sway and yaw is also improved because of suppressed roll-and pitch-induced horizontal-plane motions.