This paper is devoted to expand the very promising research undertaken in the authorʹs previous work, basically done on simplified modelling the escort push operation. Now, the other two modes of a tugʹs employment, as stated in the title, are covered. The special focus is again set on the indirect towing in that the towline force is much higher than the thruster force. The ratio of these two forces, referred to as the relative towing force (or amplification ratio) is evaluated together with the hull drift angle and the thruster(-s) angle for a given escort speed. This mutual relationship is known as the tug performance diagram. Although rather generic (container-type) formulas are derived, they are supplied for exemplification purposes with simple, analytically given hull hydrodynamic forces. The aim is also here to provide a basis for further sensitivity analysis of the model and possible improvement/optimisation to the tug design. The obtained charts also could serve as rough and clear guidance for towmasters while escorting. http://www.transnav.eu the International Journal on Marine Navigation and Safety of Sea Transportation Volume 8 Number 3
The International Maritime Organization adopted a strategy to reduce the total annual GHG emissions from international shipping by at least 50% by 2050, compared to 2008 levels. The European Union proposed an even farther reaching transformation: the European Commission adopted a set of proposals to make the EU’s transport policies fit for reducing net greenhouse gas emissions by at least 55% by 2030, compared to 1990 levels. Therefore, all industrial actions in line and consistent with these strategies are essential. One of such activities may be a gradual transition from the most common independent controls of transport ships’ thrusters, propellers, and rudders to an integrated, power optimized, 3 degrees of freedom joystick control. In this paper, the full mission bridge simulator (FMBS) research on potential energy savings and, consequently, a GHG emission reduction, while steering a RoPax twin-screw ferry equipped with bow thrusters by a joystick control, is presented. The task of navigators engaged in the research was to steer the vessel either via classic engine, rudder, and thruster levers or via a joystick while (1) following the predefined straight track, (2) rotating at the turning area, and (3), finally, crabbing (moving sideways) until stopping at the quay fenders. The conclusions are that energy savings of approximately 10% can be expected for berthing manoeuvres controlled by a joystick, compared to independent actuators’ controls. These conclusions have been drawn from a statistical analysis of the ship’s energy consumption during typical manoeuvring phases of 18 berthing operations performed in FMBS.
This paper looks at a typical problem encountered in the process of designing an automatic ship’s course stabilisation system with the use of a relatively new methodology referred to as the Active Disturbance Rejection Control (ADRC). The main advantage of this approach over classic autopilots based on PID algorithms, still in the majority, is that it eliminates the tuning problem and, thus, ensures a much better average performance of the ship in various speed, loading, nautical and weather conditions during a voyage. All of these factors call for different and often dynamically variable autopilot parameters, which are difficult to assess, especially by the ship’s crew or owner. The original result of this article is that the required controller parameters are approximated based on some canonical model structure and analysis of the hydrodynamic properties of a wide class of ships. Another novelty is the use of a fully verified, realistic numerical hydrodynamic model of the ship as a simulation model as well as a basis for deriving a simplified model structure suitable for controller design. The preliminary results obtained indicate good performance of the proposed ADRC autopilot and provide prospects for its successful implementation on a real ship.
The present survey, as part of larger project, is devoted to properties of pure linear models of yaw motion for directionally stable ships, of the first-and second-order, sometimes referred to as the Nomoto models. In rather exhaustive way, it exactly compares and explains both models in that what is being lost in the zigzag behaviour, if the reduction to the simpler, first-order dynamics (K-T model) is attempted with the very famous [Nomoto et al., 1957] approximation: T = T1 The latter three time constants of the second-order model, more physically sound, are strictly dependent on the hydrodynamic coefficients of an essential part of the background full-mission manoeuvring model. The approximation of real ship behaviour in either of the mentioned linearity orders, and the corresponding complex parameters may facilitate designing and evaluating ship steering, and identifying some regions of advanced nonlinear models, where linearisation is valid. As a novel outcome of the conducted investigation, a huge inadequacy of such a first--order model for zigzag simulation is reported. If this procedure is used for determining steering quality indices, those would be of course inadequate, and the process of utilizing them (e.g. autopilot) inefficient.
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