A time-domain methodology to assess the dynamic positioning performances

This paper presents the procedure for designing a Dynamic Positioning system and evaluating its performance using a dynamic approach. Generally, static analysis is the standard preliminary design method for a Dynamic Position system; however, this has some drawbacks for a realistic representation of the system operating in real conditions since the transient behaviours are neglected. The proposed procedure provides results in a novelty manner, and they are named Time-domain Dynamic Positioning Capability Plots. They were obtained by a model-based design approach using a ship's dynamic simulator. The dynamic simulator is based on a fully actuated testing model. Indeed, the proposed tool has been designed on a tug vessel equipped with two azimuth thrusters and one bow-thruster. In particular, two force allocation logics for the station keeping manoeuvres have been developed; one is an optimised thrust allocation logic, the other is obtained with the primary goal to reduce as much as possible azimuth thrusters rotation. The two different allocations can fulfil both open-loop and closed-loop commands, but only the closed-loop has been discussed. Eventually, the comparison between static capability plots and the capability plots obtained by several dynamic simulations has been provided, presented, and discussed.