Astrid H. Brodtkorb

Increasing the Operation Window for Dynamic Positioned Vessels Using the Concept of Hybrid Control

In order to extend the operational window of marine vessels, the high-level control of the dynamic positioning (DP) system is revised. Major contributions of this paper include the modeling of a hybrid controller for a DP vessel in a varying sea state using the hybrid dynamical systems framework proposed by [1], and establishing global asymptotic stability of the closed-loop hybrid system. Simulations in a sea state varying from calm to extreme are conducted with the hybrid controller, and a single controller with adaptive wave filtering for comparison. The single controller becomes unstable in extreme seas whereas the hybrid controller shows good performance. Switching is based on spectral analysis of the vessel wave frequency motions.Copyright

Sea state estimation using model-scale DP measurements

Complex marine operations are moving further from shore, into deeper waters, and harsher environments. The operating hours of a vessel are weather dependent, and good knowledge of the prevailing weather conditions may ensure cost-efficient and safe operations. This paper considers the estimation of the peak wave frequency of the on-site sea state based on the vessels motion in waves. A sea state can be described by significant wave height, peak wave frequency, wave direction, and often wind speed and direction are added as well. The signal-based algorithm presented in this paper is based on Fourier transforms of the vessel response in heave, roll and pitch. The measurements are used directly to obtain an estimate of the peak frequency of the waves. Experimental results from model-scale offshore ship runs at the Marine Cybernetics Laboratory (MCLab) at NTNU demonstrate the performance of the proposed sea state estimation algorithm.

Sensor-based hybrid observer for dynamic positioning

Observers are important components of dynamic positioning (DP) systems for marine vessels, estimating unmeasured states, filtering noise, filtering out wave induced motions, and predicting states in the case of signal loss. In this paper, a sensor-based hybrid observer concept is investigated. The concept assumes that noisy position measurements are available only occasionally at a non-constant sampling rate. Predictions of position between the samples are provided by integrating acceleration measurements, which are available at a high rate (approximated to be continuous sampling). Estimates with smaller variance are computed by averaging multiple observer copies of position and velocity. The contributions of this paper include the design, stability analysis and simulation of a sensor-based hybrid observer combining noisy acceleration, velocity and position measurements of a vessel in DP using different sampling rates.

Evaluation of shipboard wave estimation techniques through model-scale experiments

The paper continues a study on the wave buoy analogy that uses shipboard measurements to estimate sea states. In the present study, the wave buoy analogy is formulated directly in the time domain and relies only partly on wave-vessel response amplitude operators (RAOs), which is in contrast to all previous works that either are formulated in the frequency domain and/or depend entirely on RAOs. Specifically, the paper evaluates a novel concept for wave estimation based on combined techniques using a wave frequency estimator, not dependent on RAOs, to detect wave frequency and, respectively, nonlinear least squares fitting to estimate wave amplitude and phase. The concept has been previously tested with only numerical simulations but in this study the techniques are applied to model-scale experiments. It is shown that the techniques successfully can be used to estimate the wave parameters of a regular wave train.

Hybrid controller concept for dynamic positioning of marine vessels with experimental results

Abstract The next generation marine control systems will, as a step towards increased autonomy, have more automatic functionality in order to cope with a set of complex operations in unknown, challenging and varying environments while maintaining safety and keeping operational costs low. In this paper a hybrid control strategy for stationkeeping and maneuvering of marine vessels is proposed. The hybrid concept allows a structured way to develop a control system with a bank of controllers and observers improving dynamic positioning (DP) performance in stationary dynamics, changing dynamics including enhancing transient performance, and giving robustness to measurement errors. DP systems are used on marine vessels for automatic stationkeeping and tracking operations solely by use of the thrusters. In this paper a novel method improving the transient response of a vessel in DP is developed. The performance of the hybrid control system, including two observer candidates and one controller candidate, is demonstrated in model-scale experiments and on full-scale field data. The hybrid system has global stability properties.

Time-Varying Model-Based Observer for Marine Surface Vessels in Dynamic Positioning

This paper deals with the problem of transient events in model-based observers for dynamic positioning of marine surface vessels. Traditionally, model-based observers experience a deterioration of performance during transients, and there is a give or take relationship between transient and steady state performance. To remedy this problem, we propose to use time-varying gains for a model-based observer. The gains are aggressive during transients to improve transient performance, and relaxed in steady state to lower the oscillations of the estimates. The proposed observer is analyzed with regard to stability. Its performance is verified in both a high-fidelity simulation model, and on experimental data with the research vessel (R/V) Gunnerus. In addition, a partial closed-loop validation with R/V Gunnerus has been performed.