Table 13. Ship speeds under different ice, wave, and wind conditions.
Wind direction vs. ship heading
Ice
Wave
Head
Beam
Following
concentration
height
sea
sea
sea
<3 m
(full speed) 1 kn
full speed
full speed
Ice free (full sea)
3 to 5 m
(full speed) 2 kn
full speed
(full speed) + 1 kn
>5 m
(full speed) 6 kn
(full speed) 3 kn
(full speed) 3 kn
<1 m
8 kn
1 to 2 m
7 kn
030%
2 to 3 m
6 kn
(partial sea)
3 to 5 m
5 kn
5 to 7 m
4 kn
>7 m
3 kn
>30% (no sea)
0m
full speed
tion is 30% or greater, insufficient fetch is present
sions with Brigham lead us to believe that our
for a slowing sea to develop, and the base speed
current reduction values are not unrealistic.
established in step 1 is maintained. If ice concen-
Fog. If fog is determined to be present and the
tration is less than 30%, slowing may be necessary.
cargo vessel is not under icebreaker escort, then a
Wind direction is then used to determine whether
slowing factor between 0.5 and 1.0 is chosen at
head seas, following seas, or beam seas are present
random. Ship speed is multiplied by this factor
(Fig. 12), and the ship speed is slowed according
only if it is determined to have a greater effect on
to values given in the table. Larger waves in the
speed than factors attributed to superstructure ic-
presence of a partial ice cover (<30%) is cause for
ing, snowstorms, and darkness. If fog is determined
greater slowing than waves in open water, due to
not to be present or if the cargo ship is under es-
the increased risk of damage from collision with
cort, then no slowing is imposed; i.e., the slowing
ice.
factor is set to 1.0.
Superstructure icing. If topside icing is deter-
Adjust speed for visibility and
mined to be in progress and the cargo vessel is not
maneuverability factors (step 4)
under icebreaker escort, then a slowing factor be-
Ship speed may be slowed by four environ-
tween 0.85 and 1.0 is chosen at random to account
mental variables that lead to degraded visibility
for decreased maneuverability and visibility. Ship
or maneuverability: fog, topside icing, snowstorms,
speed is multiplied by this factor only if it is deter-
and darkness. Fog, icing, and snow are selected by
mined to have a greater effect on speed than fac-
applying the MC algorithm to PDFs that describe
tors attributed to fog, snowstorms, and darkness.
the likelihood that each condition will occur. Dark-
If icing is determined not to be in progress, or if
ness is set by the simulation clock and depends on
the cargo ship is under escort, then the slowing
approximations of day length for the months in
factor is set to 1.0.
which the transits occurs. We assumed night to be
Snowstorms. Decreased visibility is considered
16 hours long in October, and four hours long in
to be the primary effect of falling snow, and ship
April and August. June has 24 hours of daylight.
speed may be decreased if the cargo vessel is not
These factors only come into play if the cargo
under icebreaker escort. If a snowstorm is raging
vessel is not under icebreaker escort. In addition,
and the cargo vessel is not under icebreaker es-
only the factor that has the greatest impact on
cort, then a slowing factor between 0.5 and 1.0 is
speed is actually applied to slow the rate of
chosen at random. Ship speed is multiplied by this
progress. The extent to which the ship speed is
factor only if it is determined to have a greater
slowed by visibility and maneuverability factors
effect on speed than factors attributed to super-
such as these is not well documented in the litera-
structure icing, fog, and darkness. If snow is de-
ture. As a result, the magnitude of our reduction
termined not to be falling or if the cargo ship is
factors is subjective. Values of some factors were
under escort, then the slowing factor is set to 1.0.
adjusted upward during earlier test runs because
Darkness. The simulation clock keeps track of
the extent of slowing seemed too great. Discus-
the time of day, and for those segments traversed
31
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