68
Figure 9 is a picture of the vessel performing an
IMO zig-zag test. As can be seen, the sea in the test
area is calm. Some initial validation studies using the
full-scale tests have been described in section 4. Figure
7 (in section 4) compares turning circle paths and
ways to handle the deviations between measurements
and predictions by either changing the control angel
of the azipull unit or replace the unit by a
conventional rudder/propeller system. It was found
that the applied generic azipull model gave too high
control forces for high angles. Simulations were thus
also run using different azipull angles – the best
turning circle results were obtained using a 25°
control angle to port and 20° to starboard. Also, for
the overshoot angles, there are some differences
between measured and predicted values. Improved
outcomes from VeSim are obtained by tuning the
hydrodynamic coefficients and especially control
system parameters using the deviations between the
initial VeSim predictions and full-scale
measurements.
There are two main reasons for the difference
between sea trial manoeuvres and VeSim predictions.
The first one is due to limitations introduced by the
Hexapod system used by SINTEF Ocean for PMM
tests. For highly manoeuvrable vessels, such as
double-ended ferries with azipull systems, it is not
possible to obtain the high drift angles and yaw rates
that are measured during turning circles and other
tests applying large control unit angles. The second
one comes from the existing model of the hull and
azipull unit interaction in VeSim. CFD is presently
used to study these interactions and new models for
interaction effects will be developed for double-ended
ferry designs.
Figure 9. Zig-zag test seen stern-wise from the bridge of MF
Suløy.
The results of the manoeuvring sea trials have been
used by SINTEF Ocean and HAV Design to tune hull
and propulsion force models for the VeSim simulation
tool. The goal of this work has been to reduce the
differences between sea trial measurements and
VeSim predictions. The reliability of VeSim with
numerical input for new hull forms needs to be
improved so it can be used as one of the design tools
by HAV Design in their work to develop new zero-
/low-emission double-ended ferries with high
manoeuvring performance. Based on tuning
experience, it is concluded that approvements are
needed with respect to:
− Four quadrant asipull model
− Interaction effects between hull and asipull
Two alternative ways of improving VeSim input
for early design phase studies are presently
investigated. One is to extend the present HullVisc
tool to unconventional ship designs by using 3D
potential flow theory. The other one is to generate
input data from a CFD based PMM.
6 SOME COMMENTS ON OPERATIONAL
EXPERIENCE
The IMO standard tests give information for the ship
designer more than for ship captains. Despite double
ended ferries showing a low degree of directional
instability, based on direct and reverse spiral tests,
and qualifying well within IMO’s criteria on zig-zag
tests, there is no guarantee that the ferry is steerable in
critical and typical manoeuvring situations, especially
during the retardation phase. One specific effect that
is purely related to the hull and propulsion units is
the behaviour during retardation. Some of the ferry
captains initially reported unusual effects related to
control of the vessel during retardation manoeuvres.
Analysing the reports, it was concluded that lack of
experience with the actual control system (azipulls
fore and aft) caused some of these effects. The higher
the initial speed when initiating retardation, the more
unstable the vessel will be in the retardation phase.
Such an operational characteristic increases the stress
level for the captains during the final phase of a
voyage. To improve this behaviour, a set of new of
test manoeuvres have been suggested by the captains
to identify the best operational procedure for
operating both of the azipulls during retardation and
docking. The outcome of such tests, and a specific
operational guideline for these part of the operation
under varying environmental conditions, should be
documented in the vessel's Manoeuvring Booklet.
Based on discussions with double-ended ferry
masters, they are asking for better documentation of
low-speed manoeuvring performance, especially for
harsh weather conditions. This should also be part of
the vessel specific Manoeuvring Booklet. In addition
to these tests, which will reveal behaviour during sea
trials, it is even more important to detect
manoeuvring challenges during the design process
and hence modify design to overcome this. Hence, a
methodology for developing a reliable simulation
model that capture the behaviour during retardation
should be specified. The essence is to conduct
systematic CFD studies/ model tests to generate
hydrodynamic coefficient input to the time domain
simulation model so it can be used as an early design
tool. This tool (for instance a simplified version of
VeSim) could then be used by designers to investigate
manoeuvring performance, both IMO standard
manoeuvres and ship specific low speed manoeuvres
requested by captains.