Maritime University of Szczecin
Akademia Morska w Szczecinie
2014, 37(109) pp. 10–15 2014, 37(109) s. 10–15
ISSN 1733-8670
Ship manoeuvring hydrodynamics in a new inland
shiphandling simulator of SMU – InSim
Jaroslaw Artyszuk, Lucjan Gucma, Maciej Gucma
Maritime University of Szczecin, Faculty of Navigation
70-500 Szczecin, ul. Wały Chrobrego 1–2, e-mail: j.artyszuk@am.szczecin.pl
Key words: inland ship, hydrodynamics, manoeuvring, mathematical model Abstract
The present paper describes the hydrodynamic modelling solutions, applied in the newly developed shiphandling simulator at SMU (Szczecin Maritime University) for the inland navigation – called InSim. The objective is to provide some guidance on the simulator capability and potential while conducting various research and the crew training projects.
Introduction
The manoeuvring simulation models (simula-tors) of sea-going and inland ships, as used in marine traffic engineering, belong to the following two types:
– non-autonomous; – autonomous.
The non-autonomous simulation model is an in-teractive model with a human (operator) input, which mostly works in real time. The simulation with such a model is very sensitive to the operator’s knowledge and skills. Additionally, the obtained results are essentially affected by the technical solu-tions used to simulate the informational (input) environment – e.g. the bridge visual view, and/or the bridge equipment display – and the control (output) environment. The latter is connected with the emulation of steering devices, for instance.
Nowadays, since the simulation almost entirely has been run by the computers, where everything must be mathematically (numerically) modelled and programmed, the advantage of non-autonomous simulation is that we do not have to mathematically model a very complex decision process of a human. On the contrary, the human modelling is absolutely required within the second type of simulation – autonomous one. The key ele-ment of the computer simulation, if applied to physical phenomena of motion, is the adopted
mathematical model (the so-called dynamic model) of the object’s motion. For a floating object, one gets here a hydrodynamic model. The hydrody-namic model of a ship, based on differential equa-tions in a moving frame, operates in a loop, where for given instantaneous environment disturbances and operator’s steering settings the particular forces and moments are calculated. In the next step, through equations integration, the resulting linear and angular velocities, and corresponding dis-placements, are calculated and forwarded to the simulator’s visual system responsible for displaying the ship’s smooth movement.
The functional diagram of such simulator is pre-sented in the figure 1, which also served as the framework for developing hardware and software of the latest, independent InSim simulator of SMU, dedicated to inland navigation. The latter is a new area of interest covered by SMU according to their long-term experience and expansion policy. The layout of InSim bridge is shown in the figure 2.
The major merits behind the computer simula-tion in general, and its applicasimula-tion for evaluasimula-tion of vessel traffic safety in restricted waterways in particular (like InSim), are among others today as follows:
– high degree of conformity with reality;
– mathematical modelling flexibility ensuring fast-built, simple, and adequate models
accord-ing to research objectives, includaccord-ing the stochas-tic models;
– low financial effort in comparison to the physi-cal simulation;
– time-effectiveness of data collection and proc-essing;
– solutions can be tested and objectives met with-out the need to create a real system
non-Fig. 1. The functional diagram of InSim simulator
existing systems can be examined, e.g. a maxi-mum ship for the safe operation of given water-ways;
– virtual extreme loads can be applied on the in-vestigated object, especially those destructive ones.
The present study is devoted to hydrodynamic modelling solutions applied in the InSim simulator. They have direct and indirect effects on reliability of simulation results and potential areas of the simulator’s application.
The software of InSim
All procedures for the ship manoeuvring compu-tation are gathered in a self-contained MS Windows DLL library developed using the C++ language. This basic file “mm_model.dll” together with a ship hydrodynamic database file (of .HDB extension) for each ship model forms a package, referred to as the “SMART DLL”. An additional nautical area file (of .MAP extension), which brings a distribu-tion map of various physical phenomena (wind, current, wave, bank, fender, etc.), can also be loaded. Appropriate programming interfaces to use the dynamic-link library in C++ or Delphi pro-gramming environments are provided. The hydro-dynamic database files can be supplied in encoded or open format, as to allow in the latter case an input from the authorized user by means of any text editor. These ship specific data files comprise all dimensional and nondimensional model parameters, mostly of geometric or hydrodynamic nature, in-cluding among others sophisticated multidimen-sional lookup tables as representing functional rela-tionships. Since the “SMART DLL” is a purely mathematical library, the user is required to provide a graphical interface within his application for ac-quiring steering commands.
The crucial procedure of the DLL library, called “smm_inout (...)”, is basically a single recursive advance of the ship motion state vector (but ex-tended from the usual one as to combine also the dynamics of steering devices). This is done through a numerical integration of the aforementioned ship motion differential equations. Various numerical algorithms for ODE problems are available within the package, but the Euler method is still here suffi-cient and mostly frequently used. If the “smm_inout” procedure is run inside the PC timer controlled loop, then a real-time mode is achieved. Since the procedure is absolutely very fast, even with large sizes of the accompanied ship model HDB files, it is thus well suited also for fast-time (offline) modes.
The SMART library can be easily replaced with another one, preserving the library functions calling interface, since one of the key features of the InSim simulator is open independent architecture, thus allowing an extension of the simulator to meet various, especially future training and/or research the needs. The commercial simulators are mostly prevented from doing even a small modernisation by the user on his own that necessitates placing new simulator upgrading orders to the same manufac-turer.
Mathematical model description of InSim
The differential equations of ship manoeuvring motions in the usual moving frame of reference are as follows:
z z z y z c x z g x g y x z c y m z g y m g x M t m J F v m m v m m t v m m F v m c m v m c m t v m m d d d d d d 66 22 11 11 22 22 11 22 11 (1) where:vx, vy, z – surge, sway and yaw velocity;
t – time;
m – ship’s mass (displacement); m11, m22, m66 – added (virtual) masses;
cm – empirical (viscosity) reduction factor;
Fx, Fy, Mz – external physical excitations as
resul-tant forces and moment;
g, c – superscripts denoting a ship’s ground velocity vector components and the water current velocity vector components. The external excitations are modelled within the modules below (the quoted symbols are also used to distinguish particular components of the resultant forces and moment):
H – hull; P – propeller; R – rudder;
A – wind (aerodynamics);
WV – wave action (of 1st and 2nd order); ICE – ice interaction;
BE – bank effect; SS – ship-to-ship; LTU – lateral thruster unit; FEND – fenders;
ANCH – anchors; TUG – tugs.
Each of these excitation models consists of a dimensional part (mostly being a product of certain physical quantities leading to a proper unit, e.g. N or NM) and a non-dimensional part, essentially constituting a single non-dimensional coefficient, being strictly a constant or a function of multiple parameters. Sometimes, this non-dimensional co-efficient (i.e. the multiple variable function under-neath) is decomposed into a product or a sum of simpler coefficients (functions) that gives much comfort in storing, such relationships in lookup- -tables. Latter can be easily handled if their dimen-sions are limited up to three, in which case we have a three-variable functions. However, the two- -dimensional lookup-tables are the best.
The above lookup-table approach is just fully implemented in the SMART library in that any analytical formulations for hydrodynamic coeffi-cients, e.g. in the form of polynomial expansion, as widely published in the literature, are strongly avoided. The mentioned expansion sometimes serves only as the background (initial guess) for tuning the non-dimensional coefficients in accord-ance with the ship’s performaccord-ance in sea trials.
The both parts of (semi-empirical) model for given excitation, dimensional and non-dimensional one, belong to the art of dynamic modelling. Both required a lot of efforts to arrive at the adequate and flexible solution, where all the essential effects / relationships connected with particular excitation are reflected up to the required level of accuracy. For example, an improper choice of dimensional part may significantly complicate the non-dimen-sional coefficient.
The dynamic model (1) has to be further sup-plemented with the known kinematical relation-ships – differential equations of the first order – for the change of the heading angle and the ship’s origin position.
Other details on model fitting procedures can be found in [1].
Specific data on “Luisa Lynn”
The main particulars of “Luisa Lynn”, the first ship being modelled for InSim simulator, are pre-sented in the table 1.
The abovewater side view and the underwater stern view are shown in the figure 3.
The values of thrust and torque coefficients of the propeller, as functions of the advance coeffi-cient and pitch ratio, were taken from available published model tests on CPPs and preserved in the mathematical model. The same was done with the
Table 1. Basic data of “Luisa Lynn”
Length over all (L) 78.8 m
Breadth extreme 8.0 m
Draught extreme 2.12 m
Trim –
Displacement (fresh water) 1150 t
Main engine power/rpm 265 kW/340 rpm
Gear none
Propeller CPP in half-nozzle, right-handed,
diameter 1.45 m, 3-bladed
Stern rudder underhung, dual-blade,
maximum angle 57 Bow thruster nominal thrust 0.5 t (estimated)
a)
b)
Fig. 3. Selected views of “Luisa Lynn”; a) source: www. marinetraffic.com, b) source: Rentrans Cargo Ltd. (Szczecin, Poland) – the owner’s archive
propeller wake fraction and thrust deduction as functions of forward speed and propeller loading. Finally, the hull resistance coefficient in deep wa-ter, kept, however, constant against the speed, was scaled accordingly (also for other non-zero drift and dimensionless yaw values) to reach the known maximum speed of “Luisa Lynn”, equal to abt. 10.9 km/h. The speed relevant to other settings of the wheelhouse speed control knob is given in the table 2.
Table 2. Estimated speed of “Luisa Lynn”
Speed setting [%] Speed [m/s] Speed [kt] Speed [km/h]
100 3.03 5.89 10.9
80 2.58 5.02 9.3
60 2.26 4.39 8.1
40 1.69 3.29 6.1
-0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 0 100 200 300 400 500 pomiar symul. 0 10 20 30 40 50 60 0 100 200 300 400 500 0 0.5 1 1.5 2 2.5 0 100 200 300 400 500 0 10 20 30 40 50 60 70 0 100 200 300 400 500 time[s] 0 50 100 150 200 250 -150 -100 -50 0 50 vy[m/s] time[s] [] time[s] vx[m/s] time [s] z[/min] yO[m] xO[m] lateral velocity drift angle forward velocity yaw velocity track trial sim.
Fig. 4. Turning circle test (rudder angle 57 PORT, constant throttle) -40 -30 -20 -10 0 10 20 30 40 0 100 200 300 400 0 1 2 3 4 5 6 0 100 200 300 400 pomiar symul. time[s] vx[kt] [] time[s] trial sim. forward velocity heading
Fig. 5. 20/20 zig-zag test (constant throttle)
Concerning the turning and yaw checking abil-ity, determined by means of turning circle and zig-zag tests, the combined trial and simulated charts are presented in figures 4 and 5. The trial data, also as a part of the InSim project, were obtained through measurements at sea using a dedicated hardware and software developed at SMU. The simulated values refer to the model performance after application of a special model fitting proce-dure, mostly connected with calibrating the hull sway force and yaw moment coefficients (as func-tions of drift angle and dimensionless yaw veloci-ty), and the rudder-hull interaction factors as well. The displayed in the figure 4 turning circle trial data, all except the yaw velocity z, have already
included an allowance for the existence of water current at the trial site and refer to the midship posi-tion. The current set and drift, 170 (related to ship’s initial heading) and 0.18 m/s accordingly, were herein established on the principle of getting during the data recalculation process a constant drift angle at the steady phase of turning.
With regard to stopping ability the agreed sea trial program only encompassed coasting stop tests, i.e. with setting the propeller to zero pitch. For a right-handed controllable pitch propeller this leads to the well-known effect of ship’s turning towards the starboard side. This phenomenon is very clear for “Luisa Lynn” too. The figure 6 shows the forward velocity and heading change for the two conducted runs of such stopping. Though of the same steering, they reveal some remarkable differ-ences.
0 20 40 60 80 100 0 100 200 300 400 500 0 1 2 3 4 5 6 0 100 200 300 400 500 pomiar 'ham1' pomiar 'ham2' symul. time[s] vx[kt] [] time[s] trial no. 1 trial no. 2 simul. forward velocity heading
Fig. 6. Coasting stop performance
-180 -150 -120 -90 -60 -30 0 0 40 80 120 160 200 240 -4 -2 0 2 4 6 0 40 80 120 160 200 240 time[s] vx[kt] [] time[s] forward velocity heading distance 168m (2.13 L) time 110s heading 62 to PORT
Fig. 7. Estimated crash stop behaviour (FULL ASTERN)
The reason for such strange behavior can be hardly assessed at this stage of research. It should be added that the water current effect has not been analysed for the coasting stop tests.
The final manoeuvring mathematical model has been essentially fitted against the trial No. 2 in the figure 6 for speed decrease in that the wake frac-tion, thrust deducfrac-tion, and the propeller thrust coef-ficient for zero pitch were tuned in the regions of domain responsible for the coasting stop simula-tion. The yaw effect of the propeller zero pitch has been however assumed in the middle of the both records, since this has rather little influence on the speed decrease rate, as in the case of “Luisa Lynn”.
The plots in the figure 7 demonstrate the estimated performance (essentially based on the adopted model test results of the CPP in concern) during the crash stopping.
Final remarks
The presented ship hydrodynamic model, de-spite the fact that only selected aspects of its have been shown in this paper, is able to interact with any inland waterway infrastructure. The other addi-tional dynamic effects are also included in the model. However, to perform simulation-based safety studies in 3D visual environment provided by InSim, concerning e.g. inland ship-bridge colli-sions and related bridge protection design (see [2] for examples), we need a detailed graphical and mechanical 3D model of inland waterway infra-structure. This is planned in the near future.
Acknowledgements
The work was carried out within the development project of the Polish government: NCBIR – R10002810, entitled Development and construction of an integrated interactive simulator for inland ship navigation and manoeuvring.
References
1. ARTYSZUK J.: Modelling and Simulation in Ship
Manoeu-vring Safety and Effectiveness Issues. Maritime University, Szczecin 2013 (in Polish).
2. GUCMA L.: Risk Management in Ship Collision with Waterway Bridges. Maritime University, Szczecin 2013 (in Polish).
