This work continues a
series of the publications devoted to use of visualization tools of modern
CAE-systems for estimating of the ice performance of vessels in special
operating conditions [4].
The CAE-system (Computer Aided Engineering) is the
computer technology modeling and visualizing space time development of the
studied process. At present similar technologies find the application in
different areas of knowledge: mechanics, hydra, gas and thermodynamics,
construction, processing of materials, medicine, nuclear physics,
hydrometeorology, micro and macrocosm, etc. A basis of the CAE-system is the
numerical solver of systems of the differential equations describing behavior
previously sampled (it is normal in a finite element formulation) areas of
space (environment, bodies). At the same time it should be noted that in
addition to the fulfilled numerical methods the convergence of the decision is obtained
with use of the special «artificial» programs, algorithms and procedures.
All set of the sea and river ice technology problems
belongs to the extensive class of mechanics of deformable environments. The
basic scientific novelty of CAE technologies is consideration of problems of
mechanics via modeling of objects interactions unlike modeling of loadings that
is traditional semi-empirical approach considered by the majority of numerical
methods. In problems of assessment and forecasting of ice performances of the
fleet the approach allows to pass to implementation of nonautonomous models of
interaction of the ship hull and its propulsion and steering complex with the
water ice environment. Unlike the classical autonomous solutions [5-8]
considering influence of only quantitatively static isotropic environment on
the vessel, CAE models describe interference of the vessel and environment
taking into account stochasticity of this process in space and time. Also
important advantage of the CAE-analysis it is necessary to acknowledge the
possibility of separation of hydrodynamic and ice loads that it is unavailable
to a model experiment and natural tests.
Essentially significant stage of CAE technologies is
the postprocessing implementing processing of results of modeling by means of
scientific visualization. Post-processor tools in problems of a sea and river
ice technology are three-dimensional graphics with a rich set of options for
the analysis of model (scaling, detailing, tools for measurements, gradient
fields of parameters, levels, isolines, traces, sections, cuts, vectors,
transparency, etc.), animation and also the graphic processor creating
different nodal and element space time functions at a possibility of their
mathematical analysis [12].
A sufficient number of analytical, semi-empirical
and numerical techniques for the forcasting of ice resistance, icebreaking
capability and propulsion ability of vessels so far have been proposed [1-7]. They
give the acceptable level of adequacy in the analysis of ice problems of
operational character. However the applicability of such techniques is fair
only for «trim on an even keel» vessels at their design draft. At the same time,
the trim and a list as factors of case ice resistance and loadings on a
propulsion and steering complex are not considered in principle. It is caused
by the fact that actually transport ship having an ice class should be loaded
on the level of design draft (its strongest strip of «an ice belt»). Therefore
for ice category cargo fleet the removal of sitting of the vessel from the list
of the arguments influencing ice propulsion ability is admissible.
Operation of icebreaking means in comparison with
cargo ships is distinguished by much wider range of the modes and dynamic
working methods. At the same time natural observations of work of the fleet in
ices [8, 9] show relevance of periodic modes of the movement of ice breakers
with the changed sitting that saves relevant a problem of the forecast of ice
maneuverability and propulsion ability for such conditions of their operation.
Scientific and technical research of the author is
connected with estimates of ice performances of vessels for inland and coastal
navigation. Those types of vessels can be classified by ice criteria of belonging
to not Arctic categories. The criteria’s of acceptable safety and efficiency of
ice operations for such fleet can only be satisfied in the conditions of small
ice cakes and brash ice. The formation of such ice environment for the fleet
with low (not Arctic) ice categories is provided with icebreaking means with
multishaft propulsions and steering complexes. This means the vessels for
internal waterways and coastal sea , that is the case of the ice breaker of the
project of 1191 types «Captain Evdokimov».
After preliminary study of ice potential of this
vessel [8], the author did not find any information on quantitative impact
assessments of his sitting on ice propulsion ability. The needs for the
solution of partial problems of icebreaking works are interfered by
restrictions in implementation of natural tests of ice breakers. As an
alternative to model experiment in ice pools for these purposes can not always
be recommended due to a number of its shortcomings those are mainly the
imperfection of model ice (and, especially, by consideration of interaction of
a propulsion and steering complex with ice). The impossibility of separation of
ice and hydrodynamic influence on the ship hull and its propulsion and steering
complex, identifications of the making ice loads (on the hull, propellers,
rudders) is also essential limitation for obtaining representative data in both
above-mentioned cases.
The experience of the author shows that in the
absence of solid natural or empirical data these decisions are admissible to be
received with use of CAE technologies [4, 10, 11, 12]. Below it is illustrated
with results of the next series of CAE ice tests (about application of a
LS-DYNA [12] package) the shallow-draft icebreaker at different options of its
sitting.
The qualitative analysis of CAE simulation of work
of the icebreaker in ice cakes showed that in a real «working» (safe) interval
of retrimming, a list and the draft of the vessel you should not expect notable
interalternative features of ice loads on the interacting structural elements
(the hull, thrusters, rudders). It well explains fig. 1.
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a
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b
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c
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d
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e
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Fig. 1. A condition of «model»
channels in ice cakes at different options of an icebreaker sitting
(a, d – sitting on «even keel» at a
design draft of 2,5 m; b, e – a list of 5 degrees;
c – a list of
5 degrees, a trim by the stern 2 degrees)
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By the form «model» ice channels (the nature of
distribution of ices, their degree of fragmentation and concentration, fig. 1)
the user can judge on a loosely correlation of ship sitting with a condition of
the ice route after passing of the icebreaker. This assumption qualitatively
(Fig. 1d, 1e) and quantitatively (Fig. 2) is also confirmed by the analysis of
modes of vibration of propellers of the vessel.
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a
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b
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c
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d
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(a – the outboard port propeller; b –
the inboard port propeller;
c – the
inboard starboard propeller; d – the outboard starboard propeller)
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In fig. 2 mode of vibration of propellers for three
options of overcoming an ice crossing point by the icebreaker from ice cakes
0,5 m thick and concentration of 9-10 balls are illustrated. At the same time
each option is characterized by individual ship sitting.
The analysis of oscillograms of fig. 2 shows that
the time nature of rotating speed of any propeller (curves A – ship sitting on
«even keel» at a molded draft of 2,5 m; curves B – a vessel list by 5 degrees;
curves C – a list of the vessel of 5 degrees, a trim by the stern 2 degrees)
saves characteristic instability in each of three mentioned icebreaker sitting
options. Also it is necessary to note that in all rated options of CAE testing
predicts almost continuous contact of all propulsion and steering complex of
the vessel with the ice cover which is «flowing round» the hull.
Qualitative interalternative features of the process
of breaking of a compact ice are more obvious in comparison with ice cakes
(Fig. 3). However, anunambiguous identification of the created ice channels on
the basis of an icebreaker sitting is not always available even to the
experienced ice captain or the hydrologist (Fig. 4a, 4b).
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a
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b
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Fig. 3. The nature of breaking of a
compact ice at different options of an icebreaker sitting (bottom view)
(a – sitting
on «even keel» at a design draft of 2,5 m; b – a list of 5 degrees)
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a
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b
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c
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Fig. 4. A qualitative condition of
«model» channels in compact ices at different options of an icebreaker
sitting
(a – sitting on «even keel» at a
design draft of 2,5 m;
b – a trim by
the stern 2 degrees; c – a list of 5 degrees)
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It should be noted that existence of a list causes
stochastic fluctuations of lateral ice hull loads. The last worsens route
stability of the icebreaker [10] at the movement in a continuous ice cover that
can obviously be reflected in a form and a condition of the ice channel (Fig. 4c).
At the same time already at the qualitative level reduction of speed of the
course in relation to sitting option «on an even keel» is noted (Fig. 5, curves
B and A respectively).
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Fig. 5. Time
character of motion speed of the icebreaker in compact ices at different
options of its sitting
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The features in the nature of the movement and
dynamics of the different icebreaker sittings in compact ices illustrated in
fig. 4 and 5. Those features with high degree of probability are accompanied by
interalternative differences in ice loads on a hull and its propulsion and
steering complex.
The quantitative results of CAE estimates of the
resulting ice resistance for several options of the sitting of the icebreaker
in ice cakes are shown in fig. 6.
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Fig. 6.
Comparison of ice resistance of the icebreaker in ice cakes at different
options of its sitting
(A – sitting
on «even keel» at the draft of 1,8 m, smoothed values; B – sitting on «even
keel» at the draft of 2,5 m, smoothed values; C – a trim by the bow 2
degrees, smoothed values; D – list of 5 degrees, smoothed values; Å –
trim by the bow 2 degrees and list of 5 degrees, smoothed values; F – trim by
the stern 2 degrees and list of 5 degrees, smoothed values; G – trim by the
stern of 5 degrees, smoothed values;
H-N –
according to curve A-G results of CAE modeling)
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(Fig. 6, curve A-G) can be inferred by the analysis
of smoothed curves that the level of total ice resistance of the vessel is
individual in each option of its sitting. At the same time one-time differences
in ice loads can accept significant sizes (Fig. 6, it most obviously later ~ 25
s when the vessel completely «goes deep» into the ice field and, except the
hull, begins active contact with ices of thrusters and rudders of the
icebreaker). However in dynamics these distinctions have the sign-variable
character that is a testimonial of the fact that the correlation of an
icebreaker sitting with the level of its total ice resistance is not
significant.
Separation of ice resistance on constructive
components (the hull, rudders, thrusters) stands on hind legs of ice loads,
similar fig. 6 (Fig. 7, designations of curves correspond to fig. 6).
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a
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b
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c
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Fig. 7.
Comparison of components of ice resistance of the icebreaker in ice cakes at
different options of its sitting
(a – the ice
resistance of the hull; b – the ice resistance of propellers; c – the ice
resistance of rudders)
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At the same time it is necessary to pay attention to
sign-variable character of ice resistance of propellers (Fig. 7b). The
considerable stochasticity of simultaneous interaction with the ice environment
of four thrusters gives rather low level (a little more than 5%) of this ice
resistance component in overall balance of longitudinal ice loads on the
vessel.
As a continuous ice cover on this stage of research
is considered, it is possible to state unambiguously only about influence of a
list on the ice resistance of the icebreaker (Fig. 8).
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Fig. 8.
Comparison of ice resistance of the icebreaker in compact ices at different
options of its sitting
(A – sitting
on «even keel» at the draft of 2,5 m, smoothed values; B – a list of 5
degrees, smoothed values; C-D – according to curve A-B results of CAE
modeling)
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At the same time within the safe range of this
parameter we can note the notable gain of total ice resistance of the vessel
(25-30%) in comparison with its sitting on «even keel» (Fig. 8, comparison of
curves A and B).
1. There is a sufficient number of partial problems
of safety in the field of ice navigation. Generally they are caused by
requirements of optimization of vessels maneuvering (including joint) in
different ice conditions during the short-term period of time (ranging from
several minutes to several tens minutes). Expert experience in area has showed
that in such cases the forecast of safe conditions from positions of averaging
of influence of set of ice arguments (as propose traditional solutions) will give
a low forecast success rate. Therefore at impossibility of implementation of
full-scale ice tests of vessels the model experiment (including virtual (CAE
experiment)) remains a reliable alternative of definition of their special ice
performances.
2. Distribution of ices and their degree of
fragmentation in «model» ice channels indicate weak correlation of a ship
sitting and qualitative condition of the ice channel, and functional
communication of icebreaker sitting with the level of total ice resistance is
not significant. At the same time in each option of an icebreaker sitting safe
operating conditions of transport ships of not Arctic ice categories – small
ice cakes and brash ice are provided.
3. For compact ices CAE experiments predict only
influence of a list on the ice resistance of the icebreaker. At the same time
deterioration in its route stability generates tortuosity of the ice channel,
reducing the level of ice safety of the carried-out transport ships of not
Arctic ice categories.
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