4.1.1 The objective of the tests addressed
in this chapter is the determination of Φ1
(paragraph
3.2.2.3 of the Code). The “angle of roll to windward due to
wave action” Φ1
is defined, according
to weather criterion, as follows:
where Φ1r
is “regular
waves roll-back angle”, that is the resonant roll amplitude
in beam regular waves (heading 90°) having steepness defined in
the following sections. The reduction factor 0.7 takes into account
the actual irregular nature of the sea.
4.1.2 The standard procedure for the
determination of Φ1r
is that by means
of tests in regular waves. The use of alternative procedures is permitted.
Sufficient justification should be provided to the Administration
regarding the selected procedure.
4.1.3 As a reference in selecting the more suitable
procedure it should be noted that:
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.1 The direct measurement of Φ1r
(see
paragraph 4.5) can lead to the need of generating very steep waves
close to the breaking limit if the ship roll period is very short
(see table 4.5.1). Generation of waves with such steepness and sufficient
quality can be sometimes difficult due to breaking close to the wavemaker.
In addition, in carrying out roll tests, care should be taken during
the transient ship behaviour before steady state is reached, because
possible large heeling angles (sometimes eventually leading to capsize)
can occur. It should be underlined that Φ1r
is
the steady state maximum roll angle, for this reason capsize during
initial transient phase of the test does not necessarily lead to not
fulfilment of the criterion. It should be underlined that the methodology
does not allow for corrections for scale effects on roll damping,
and for this reason large models should be preferred when direct measurements
are carried out.
-
.2 The there-step procedure (see paragraph 4.6.1)
is the simplest among the two proposed alternative procedures. This
method was adopted when original weather criterion was developed.
The procedure is sensitive to the quality of execution and analysis
of roll decay tests. The procedure allows to execute tests for the
determination of the effective wave slope coefficient r,
with reasonably small steepnesses, leading to rather simple tests.
The methodology allows, in principle, corrections for scale effects
on roll damping.
-
.3 The parameter identification technique (PIT)
(see paragraph 4.6.2) is a procedure with a large degree of flexibility,
that allows to take into account nonlinearities of both damping and
restoring, and that provides means for allowing frequency dependence
of the "effective wave slope function". The methodology allows, in
principle, corrections for scale effects on roll damping. When used
with only one series of tests for one single wave steepness, the number
of free parameters should be reduced to guarantee robustness of the
methodology. The method can take great advantages (regarding robustness
and accuracy) from the execution of more than one series of tests
at different wave steepness: for this reason the use of at least two
different wave steepnesses is strongly recommended. To guarantee correct
application of the method, a sufficient basic training of personnel
on the theoretical background on which the method is based is needed.