5.6.1 The Code makes reference to three safety
levels (see table 1 in annex 3) and prescribes the acceptable probability
that each safety level may occur. Level 1 is expected to have a probability
of occurrence of greater than 10-5, i.e. frequent or reasonably probable.
Table 1 in annex 3 reveals that for Safety Level 1 (minor effect)
it only prescribes that horizontal accelerations should not exceed
0.2 g.
5.6.2 In applying these standards it should be
noted that paragraph 4.3.1 of the Code advises that superimposed vertical
accelerations exceeding 1 g at the longitudinal centre of gravity
should be avoided “unless special precautions are taken with
respect to passenger safety”. For vertical accelerations exceeding
1 g then hazards for safe seating of passengers and crew will ensue.
5.6.3 Similarly, table 1 in annex 3 of the Code
stipulates acceptable maximum horizontal accelerations for severe
and extreme operating conditions.
5.6.4 Table 2 in annex 3 of the Code makes it
clear that Safety Level 2 relates to conditions when emergency procedures
are required and passengers may be injured, and Level 3 to conditions
when there is a large reduction in safety margins, and serious injury
to a small number of occupants may occur.
5.6.5 The upper limit of Level 2 corresponds to
the worst intended conditions − see paragraph 3.3.2 of annex 9 of the Code.
Passengers must be seated before the onset of Level 2 in accordance
with the Code provisions in paragraph 4.2.4 and
annex 9, paragraph 3.3.2.
5.6.6 Many forms of high-speed craft may have
safe handling limitations as suggested in paragraph
17.5.4.1 of the Code, for example:
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.1 Amphibious hovercraft may have to avoid certain
speed and drift angle combinations in order that plough-in or skirt
tuck-under and possible capsizing do not occur.
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.2 Many forms of high-speed craft may have to
avoid excessive bow-down trim in order to preserve safe manoeuvring
behaviour, such as avoidance of bow-diving or broaching (see paragraph 17.2.1 of the Code).
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.3 Guidance in this safe handling may be obtained
from appendix 2 and the Revised Guidance to the master for avoiding
dangerous situations in adverse weather and sea conditions (MSC.1/Circ.1228), as appropriate, bearing
in mind that the latter document is largely addressed to conventional
ships.
5.6.7
Chapter 17 of
the Code requires full-scale testing to determine operating limitations
and procedures for operation of the craft within limitations. Annex
9 defines the test procedures needed to develop these operational
limits. In particular section 3 of
annex 9 and table 1 of annex 3 define the horizontal and vertical
acceleration levels which must not be exceeded to ensure passenger
safety. Under normal operation conditions, craft must not exceed Safety
Level 1 (0.2 g in horizontal plane) at maximum operating speed as
per paragraph 3.3 of annex 9 of
the Code. In worst intended conditions, craft should not exceed Safety
Level 2 (0.35 g in horizontal plane). Vertical acceleration measurements
are also required by annex 9, and these limits are driven by structural
limitations for which craft must not exceed the limiting vertical
acceleration at the longitudinal centre of gravity as per paragraph
4.3.1 of the Code and paragraph 5.3.3 of these Guidelines. The above
limits, trial results, and the significant wave height to speed table
inform the process of defining operational limits. It should be noted
that paragraph 17.4 of the Code
requires the trials conducted under annex 9 to include verification
of the effects of failure(s) identified as being critical.
5.6.8 Although paragraph
17.1 of the Code makes provision for use of data from model
tests where appropriate, wherever practicable use of such data should
be confirmed by suitable trials of the craft or an identical craft.
Model tests should be used to evaluate safe limits in situations that
would be hazardous to investigate during sea trials. For these purposes,
model tests should be taken to include mathematical modelling as well
as testing of a physical model.
5.6.9 The references to vertical accelerations
in paragraph 4.3.1 and table 1 of annex 3 of the Code should be interpreted
as referring to the mean of the 1/100th highest accelerations
(not RMS), which should be measured using the criteria of footnote
1 to table 1 of annex 3.