Introduction
Over the last decades ship sizes have increased dramatically for different types of vessels (container carriers, LNG-carriers), while fairways often haven’t increased at the same rate. As a result, bigger ships nowadays sail in areas designed for smaller vessels. Sometimes new infrastructures (locks) make it possible for bigger ships to sail on an existing canal resulting in the same issue: bigger ships sailing in a status quo fairway. The Knowledge Centre Manoeuvring in Shallow and Confined Water (www.shallowwater.be), which is a collaboration between Flanders Hydraulics Research and the Maritime Division of Ghent University, has a lot of experience in evaluating and investigating the possible bottlenecks in such a fairway. This paper presents a general methodology that is used to evaluate sailing in shallow and confined water using manoeuvring simulations.
Systematic investigation
Systematic investigation of ship manoeuvring in shallow and confined water is executed by Flanders Hydraulics Research (FHR) and Ghent University (UGent) through desk studies and simulation techniques. Desk studies take into account the parameter variation of positional, kinematic and/or control variables on isolated force components of the manoeuvring mathematical models. Simulation techniques use mathematical models in 3 to 6 degrees of freedom describing all hydrodynamic and external (due to banks, wind, current) forces on the ship’s hull.
In real time simulations the pilot or skipper controls the engine’s telegraph and rudder angles while in fast time simulations the human factor is excluded by using an autopilot that sets propeller rates and rudder angles based on a predefined decision scheme.
Each of the described methods has advantages and disadvantages. The advantage of real time simulations is the completeness of the simulation technique by introducing the man in the loop in the simulation process based on validated realistic mathematical models. Disadvantages of this tool are the relatively time consuming process for systematic investigation and variability of the human factor during the simulations. An exact repeatability is not possible and the number of different set ups that can be tested is rather low since only about 10 to 20 different simulation runs can be carried out per day.
As the number of parameters can be large during a systematic investigation the disadvantages can be overcome by desk studies or fast time simulations.
Desk study with parameter variation
All parameters are set to a fixed value before the start of the calculation for the desk study. The ship is forced to follow a predefined track, consisting of time functions of velocity, position, heading, propeller rate, rudder deflection and other possible variables. Such track can either be an ideal track (centreline of the fairway) or a track derived from a full scale measurement of a ship.
Due to the forced character of the simulation run, in general there is no equilibrium between the forces and moments acting on the ship and the external forces. During the simulations horizontal forces and moments acting on the ship will be calculated based on the available mathematical manoeuvring models, and are available for further analysis. These forces are for example bank effects, ship – ship interactions , propeller thrust or (increased) resistance. Similarly, vertical motions due to squat can be calculated, too.
Systematic deviations from this track will result in considerations with respect to sensitivity of operational variables. For example, with a set of parallel tracks the impact of the lateral position in the confined fairway on the forces and moments can be investigated.
Fast time simulations
Fast time simulations make use of an autopilot, which is in this context a computer algorithm setting the control variables of a sailing vessel to obtain a realistic track of the vessel in the simulated fairway.
The autopilot which is in use at FHR has been developed for track-keeping in confined channels, specifically focusing on following a desired trajectory with minimum deviations. As a result, the outcome of a fast time simulation not only depends on the ship characteristics and intrinsic behaviour, but also on the control routine settings. As the complete hydrodynamic and external force model has to be solved, the exact position (and heading) of the ship is not known beforehand. As a consequence the settings of the autopilot have an influence on the outcome. Therefore it is of the utmost importance that these settings are handled with care.
Post processing can be used to check certain parameters, for example the rudder demand while taking a bend. As such the easiness or complexity as well as the margin to take the bend can be evaluated without the impact of the human factor.
Conclusion
Both investigation techniques for systematic parameter variation have their merits. In combination with full scale measurements, real time simulations and model tests, the Knowledge Centre Manoeuvring in Shallow and Confined Water is capable of evaluating on an objective and scientific manner the nautical complexities of a fairway.