Jann Peter Strand

Passive nonlinear observer design for ships using lyapunov methods: full-scale experiments with a supply vessel

Dynamic positioning (DP) and tracking systems for ships are usually designed under the assumption that the kinematic equations can be linearized about a set of predefined constant yaw angles, typically 36 operating points in steps of 10^o, to cover the whole heading envelope. This is necessary when applying linear (Kalman filter)theory and gain scheduling techniques. However, global exponential stability (GES) cannot be guaranteed if linear theory is used. In this paper a nonlinear observer is derived. The observer is proven to be passive and GES. The number of tuning parameters is reduced to a minimum by using passivity theory. This results in a simple and intuitive tuning procedure. The proposed observer includes features like estimation of both the low-frequency position and velocity of the ship from noisy position measurements, bias state estimation (environmental disturbances) and wave filtering. The nonlinear passive observer has been simulated on a computer model of a supply vessel and implemented on full-scale ships with excellent results.

Nonlinear passive weather optimal positioning control (WOPC) system for ships and rigs: experimental results

A new concept for nonlinear positioning control of marine vessels is presented and documented experimentally. The vessel is controlled such that the resulting wind, wave and current force does not produce a yaw moment without measuring the environmental disturbances.

Nonlinear Vectorial Observer Backstepping with Integral Action and Wave Filtering for Ships

Abstract In this paper a nonlinear control system for dynamic positioning and thruster assisted position mooring of ships is proposed. The control system is derived by using the observer backstepping design methodology. A nonlinear observer which provides wave filtering and bias state estimation from position and heading measurements is applied. Integral action is included in both the observer and controller to compensate for slowly-varying (constant) environmental disturbances. Stability is proven by applying Lyapunov stability theory.

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.

Nonlinear Control of Ships: A Locally Optimal Design

Abstract A stabilizing controller for moored and free-floating ships is constructed by backstepping to meet two design objectives: one local and the other global. The local objective is to design an H∞-optimal controller for the linearized plant. The global objective is inverse optimality for the nonlinear system. In the absence of disturbances, the system is rendered globally asymptotically stable and locally exponentially stable. Integral action is obtained by constant parameter adaptation.

Thruster assisted position mooring system for turret-anchored FPSOs

A thruster assisted position mooring system includes different control functions for station keeping and motions damping of the horizontal-plane motions. A new nonlinear passivity-based state observer producing the thruster control actions has been implemented and verified in full-scale tests. Even though sophisticated filtering and control techniques are applied in the high-level control systems, unforeseen load peaks caused by process disturbances acting on the local actuation system may challenge the power plant stability, if no proper precaution is taken. Both high and low level control aspects concerning integrated positioning and electrical propulsion system are addressed.

Positioning of Semi-Submersibles with Roll and Pitch Damping

Abstract Dynamic positioning and thruster assisted position mooring of ships and floating marine constructions include different control functions for automatic positioning in the horizontal plane. A three degrees of freedom multivariable controller with feedback signals from surge, sway and yaw, either of linear or nonlinear type, can be regarded as adequate for the control objective for most surface vessels. However, for certain marine constructions with discernible coupling characteristics in the dynamics between the horizontal plane (surge, sway and yaw) and vertical plane (heave, roll and pitch), undesirably large roll and pitch oscillations may be induced by the thruster actions. Especially for constructions with natural periods in roll and pitch within the bandwidth of the positioning controller, the thruster induced oscillations in roll and pitch may become limitable on the operation. In this paper a new multivariable control law accounting for both horizontal and vertical motions is proposed. Simulations with a semi-submersible demonstrate the effect of the proposed control strategy.

Nonlinear Output Feedback and Locally Optimal Control of Dynamically Positioned Ships: Experimental Results

Abstract In this paper an output feedback controller for dynamically positioned ships is presented. The control law is designed in the framework of locally optimal backstepping design, where locally an H ∞ -optimal controller is found. Globally the system is rendered inverse optimal by construction of the cost function. A nonlinear observer is used in the output feedback backstepping design. Results from experimental tests with a model ship are presented.

Modelling and Control of Riser Angles and Stresses in Dynamic Positioning

Abstract Conventional controller designs for dynamic positioning of ships and floating marine structures have so far been based on the principle on automatic positioning in the horizontal-plane about some desired position and heading co-ordinates. A three degrees of freedom multivariable controller either of linear or nonlinear type, normally with feedback signals from surge, sway and yaw position and velocities, has been regarded as adequate for the control objective. For semi-submersibles feedback from roll and pitch angular rotation velocity may also be included. However, for certain marine operations this control philosophy may not be the most appropriate approach ensuring a safe and cost effective operation. For drilling and work over operations the main positioning objective is to minimise the bending stresses along the riser and the riser angle magnitudes at the well head on the subsea structure and at the top joint as well. A positioning strategy solely based on position and velocity feedback may not be the most optimal solution for these applications. In this paper a new hybrid dynamic positioning controller, that also accounts for riser angle offsets and bending stresses is proposed. It is shown that a significant reduction in riser angle magnitude can be achieved. Simulations with a drilling semi-submersible demonstrate the effect of the proposed control strategy.