4.1 General principles
4.1.1 A combined consideration of design and operational aspects can effectively be
used to achieve a sufficient safety level. In application, this principle requires
guidance to be provided for the preparation of operational measures, consistent with
the design assessment requirements.
4.1.2 Whereas the principles used in these Guidelines can be applied to consider any
operational problems related to ship behaviour in a seaway, detailed procedures in
these Guidelines cover the following stability failure modes:
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.1 dead ship condition;
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.2 excessive acceleration;
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.3 pure loss of stability;
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.4 parametric rolling; and
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.5 surf-riding/broaching.
4.1.3 These Guidelines consider the operational limitations and operational guidance,
which are defined in 4.3.1. Either operational limitations or operational guidance
can be used for the following four stability failure modes: excessive acceleration,
pure loss of stability, parametric rolling and surf-riding/broaching. For the dead
ship condition failure mode, only operational limitations related to areas or routes
and season (4.3.1.1 and 4.5.1) can be applied. This means that neither operational
limitations related to maximum significant wave height nor operational guidance are
applicable because the ship's main propulsion plant and auxiliaries are inoperable.
This means that the ship is neither able to avoid heavy weather nor control speed
and course.
4.1.4 Operational limitations and operational guidance should provide at least the
same level of safety as that provided by the procedures and standards given by the
Guidelines for vulnerability criteria in chapter 2 or the direct stability
assessment in chapter 3. In particular, the safety level of those loading conditions
that fail design assessment requirements in chapter 2 or chapter 3 should become
sufficient if all combinations of the sailing condition and sea state that are not
recommended by these operational measures are removed from the design assessment.
4.1.5 Whereas the principle in 4.1.4 can be directly used to prepare operational
measures ensuring a required safety level, more detailed procedures were developed
as described in these Guidelines for convenience of ship designers and
Administrations. Using the procedures and standards described herein corresponds to
setting a safety level in accordance with the Guidelines for direct stability
assessment in chapter 3.
4.1.6 Although the application of operational measures can reduce the likelihood of
stability failure to a desired low level, a loading condition for which too many
situations should be avoided to achieve the required safety level should not be
considered as acceptable. Therefore, from practical and regulatory perspectives,
operational measures should not be considered as always sufficient for any loading
condition.
4.1.7 In case operational measures are provided for particular failure mode(s) based
on these Guidelines, they may be applied instead of the relevant provisions in the
guidance provided in MSC.1/Circ.1228.
4.2 Stability failures
4.2.1 The definition of stability failure should be consistent with those used in
either the Guidelines for vulnerability criteria in chapter 2 or the Guidelines for
direct stability assessment in chapter 3.
4.2.2 The provisions given hereunder apply to all ships, except for ships with an
extended low weather deck when considering the dead ship condition failure mode or
the pure loss of stability failure mode.
4.3 Operational measures
4.3.1 These Guidelines consider the following ship specific operational measures:
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.1 Operational limitations which define the limits on a ship's
operation in a considered loading condition, are as follows:
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.1 Operational limitations related to areas or routes and season
permit operation in specific operational areas (either
geographical areas or specific types of operational areas like
sheltered waters) or routes and, if appropriate, the specific
season. For the operational area, route and season, the
environmental conditions are specified by the wave scatter table and
corresponding wind statistics; and
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.2 Operational limitations related to maximum significant wave
height permit operation in conditions up to a maximum
significant wave height. The environmental conditions are specified
by the combination of the wave scatter table related to operational
area or route and season, and corresponding wind statistics. The
wave scatter table limited to a specific significant wave height is
referred to as a limited wave scatter table; and
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.2 Operational guidance which defines the combinations of ship speed
and heading relative to mean wave direction that are not recommended and
that should be avoided in each relevant sea state.
4.3.2 The operational measures specified in 4.3.1 require different amount of
information and planning in their application, as follows:
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.1 operational limitations related to areas or routes and season do not
require weather data during the operation of the ship and thus do not need
any specific information and planning;
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.2 operational limitations related to maximum significant wave height need a
forecast for the significant wave height and the availability of appropriate
routing in a sufficient time before encountering possible storm conditions;
and
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.3 operational guidance requires detailed forecast information about wave
energy spectrum and wind characteristics, together with means for indicating
combinations of ship speed and heading relative to mean wave direction that
should be avoided, which should be available for safe routeing in a
sufficient time before encountering possible storm conditions.
4.3.3 The operational measures specified in 4.3.1 can be combined, e.g. operational
limitations can be applied up to a certain significant wave height and operational
guidance for greater significant wave heights. When operational limitations are
combined with operational guidance, the requirements for operational guidance
apply.
4.4 Acceptance of operational measures
4.4.1 Operational limitations and operational guidance should be accepted by the
Administration according to these Guidelines.
4.4.2 Acceptance of a loading condition for unrestricted operation, limited operation
or operation using onboard operational guidance should be performed following these
Guidelines in combination with the design assessment requirements according to
chapter 2 or chapter 3. A loading condition is considered as:
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.1 acceptable for unrestricted operation, if it satisfies the design
assessment requirements for all five stability failure modes specified in
4.1.2;
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.2 acceptable for limited operation, if it is provided with
operational limitations for one or more stability failure modes specified in
4.1.2 for unrestricted operation and satisfies the design assessment
requirements for the remaining stability failure modes;
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.3 acceptable for operation using onboard operational guidance, if it
is provided with operational guidance for one or more stability failure
modes specified in 4.1.2 for unrestricted operation and is either provided
with operational limitations for unrestricted operation or satisfies the
design assessment requirements for the remaining stability failure modes;
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.4 acceptable for operation in a specified area or on a specified route
during a specified season, if it is provided with operational
limitations for one or more stability failure modes specified in 4.1.2 for
this area or route and season, and satisfies the design assessment
requirements for the remaining stability failure modes;
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.5 acceptable for limited operation in a specified area or on a specified
route during a specified season, if it is provided with operational
limitations for one or more stability failure modes specified in 4.1.2 for a
given significant wave height limit for this area or route and season, and
either has operational limitations without specification of maximum
operational significant wave height for this area or route and season, or
satisfies the design assessment requirements for the remaining stability
failure modes; and
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.6 acceptable for operation using onboard operational guidance in a
specified area or on a specified route during a specified season, if
it is provided with operational guidance for one or more stability failure
modes specified in 4.1.2 for this area or route and season and is either
provided with operational limitations for this area or route and season or
satisfies the design assessment requirements for the remaining stability
failure modes.
4.4.3 Application of the operational limitations related to maximum significant wave
height or operational guidance can reduce the stability failure rate to any low
level. However, if too many sailing conditions in too many sea states should be
avoided for a certain loading condition, such loading condition cannot be considered
as acceptable in practical operation. Therefore:
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.1 a loading condition cannot be considered as acceptable if the ratio of the
total duration of all situations which should be avoided to the total
operational time, is greater than 0.2. In the calculation of this ratio, the
situations that should be avoided include those defined by:
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.2 in the calculation of the ratio in 4.4.3.1, the probabilities of the sea
states are taken according to the full wave scatter table. Wave headings are
assumed uniformly distributed and the ship forward speed is assumed
uniformly distributed between zero and the maximum service speed.
4.4.4 Active means of motion reduction, such as active anti-roll fins and anti-roll
tanks, can significantly reduce roll motions in seaway. Therefore, if such devices
are not considered in the development and application of the operational measures,
the advice to the ship master may be suboptimal or misleading. On the other hand,
the safety of the ship with specific reference to aspects addressed by the present
Guidelines should be ensured also in cases of failure of such devices. Therefore, it
is recommended that the development, application and acceptance of the operational
measures is done both with operating and inactive (or retracted, if retractable)
anti-roll devices.
4.4.5 Operational guidance can indicate some sailing conditions as safe with respect
to roll motion but they may be unattainable due to limits of the propulsion and
steering systems of the ship or undesirable due to other problems, such as excessive
vertical motions or accelerations and slamming. For example, for parametric rolling
in bow waves, roll motions may reduce with increasing forward speed, but high speeds
in bow waves could be either unattainable or could lead to excessive vertical
motions or loads. Neglecting this contradiction can lead to misleading operational
guidance or even put the ship in danger if in some sea state all sailing conditions,
acceptable from the point of view of roll motions, are unattainable or dangerous
because of other reasons.
4.5 Preparation procedures
4.5.1 Operational limitations related to areas or routes and season
4.5.1.1 Operational limitations are prepared following the design assessment
procedures in chapter 2 or chapter 3 with modified environmental conditions assumed
in operation. The modification of the reference environmental conditions is based on
the wave scatter table for a specified area or a specified route during a specified
season and corresponding wind statistics, acceptable to the Administration.
4.5.1.2 The environmental conditions applied in the preparation of the operational
limitations related to specified areas or specified routes during a specified season
should be consistent with the corresponding vulnerability criteria if the
preparation is based on the Guidelines for vulnerability assessment in chapter 2. If
the preparation is based on direct stability assessment these environmental
conditions should be consistent with the Guidelines for direct stability assessment
in chapter 3. Other environmental conditions may be applied, as appropriate.
4.5.1.3 For some Level 1 and Level 2 vulnerability assessment procedures, regular
wave cases should be defined, based on the wave statistics.
4.5.2 Operational limitations related to maximum significant wave height
4.5.2.1 Operational limitations related to maximum significant wave height are
developed using design assessment procedures in chapter 2 or chapter 3 for a
specific environment, which is defined by cutting the wave scatter table for a
specified area or a specified route during a specified season at a specified
significant wave height and by corresponding modification of wind statistics.
4.5.2.2 The environmental conditions applied in the preparation of the operational
limitations related to maximum significant wave height should be consistent with the
corresponding vulnerability criteria, if the preparation is based on the Guidelines
for vulnerability assessment in chapter 2. If the preparation is based on the direct
stability assessment, these conditions should be consistent with the Guidelines for
direct stability assessment in chapter 3. Other environmental conditions may be
applied, as appropriate.
4.5.2.3 For certain Level 1 and Level 2 vulnerability assessment procedures,
definition of the corresponding regular wave cases is required; this is done in the
same way as for operational limitations without specification of maximum operational
significant wave height.
4.5.3 General principles of preparation of operational guidance
4.5.3.1 Operational guidance should indicate all sailing conditions that should be
avoided for each range of sea states in the relevant wave scatter table.
4.5.3.2 Operational guidance should ensure that the considered loading condition
satisfies the design assessment requirements in chapter 2 or chapter 3 after
removing from the design assessment all sailing conditions that should be avoided.
To simplify the preparation and acceptance of operational guidance, three equivalent
approaches, recommended for the preparation of operational guidance, are considered
below in detail. These approaches are based on:
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.1 probabilistic motion criteria and standards (referred to as probabilistic
operational guidance);
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.2 deterministic motion criteria and standards (referred to as deterministic
operational guidance); and
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.3 simplified motion criteria and standards (referred to as simplified
operational guidance).
4.5.3.3 Operational guidance should clearly indicate acceptable and unacceptable
sailing conditions for each relevant sea state and may be presented in the form of a
polar diagram.
4.5.3.4 Other forms different from polar diagrams could be used for displaying
operational guidance, provided that equivalent information is included.
4.5.4 Probabilistic operational guidance
4.5.4.1 This type of operational guidance uses probabilistic criteria, such as the
probability of stability failure during a specified time or the rate of stability
failures, and corresponding probabilistic thresholds to distinguish sailing
conditions which should be avoided.
4.5.4.2 Sailing conditions that should be avoided are those for which:
where r (s-1) is the upper boundary of the 95% confidence interval
of the stability failure rate.
4.5.4.3 Procedures and numerical methods applied for the determination of the failure
rate as referred to in 4.5.4.2 should satisfy the recommendations of the Guidelines
for direct stability assessment in chapter 3.
4.5.4.4 If a certain assumed situation should be avoided, assessment for higher
significant wave heights, with other parameters unchanged, is not required.
Conversely, if a certain assumed situation does not have to be avoided, assessment
for lower significant wave heights, with other parameters unchanged, is not
required.
4.5.5 Deterministic operational guidance
4.5.5.1 Using deterministic criteria, such as maximum roll amplitude in a given
exposure time, represent a simpler but less accurate approach than using
probabilistic criteria. Therefore, in order to provide an equivalent safety level,
the thresholds for deterministic criteria are conservatively selected.
4.5.5.2 Deterministic operational guidance can be prepared using only model tests,
only numerical simulations or their combination. Numerical methods applied in such
simulations should satisfy the recommendations of the Guidelines for direct
stability assessment in chapter 3.
4.5.5.3 Sailing conditions that should be avoided are those for which:
where α = 2 is the scaling factor, x3h is the mean
3-hour maximum roll or lateral acceleration amplitude and xlim
is the corresponding stability failure threshold, as defined in the
Guidelines for direct stability assessment in 3.2.1.
4.5.5.4 To define the mean 3-hour maximum amplitude, the total recommended duration
of a test or simulation is 15 hours at full scale for each considered situation.
4.5.5.5 If a certain assumed situation should be avoided, an assessment for higher
significant wave heights, with other parameters unchanged, is not required.
Conversely, if a certain assumed situation does not have to be avoided, an
assessment for lower significant wave heights, with other parameters unchanged, is
not required.
4.5.6 Simplified operational guidance
4.5.6.1 Whereas probabilistic and deterministic operational guidance provides
accurate and detailed recommendations for the ship forward speed and course in each
sea state, it requires model tests or numerical methods of high accuracy. Therefore,
simpler conservative approaches may be used to develop operational guidance for
acceptable forward speed and course when it is deemed practicable.
4.5.6.2 In principle, any simple conservative estimations for the sailing conditions
that should be avoided in each relevant sea state, can be used if they are shown to
provide a superior safety level compared to the design assessment requirements. In
particular, Level 1 or Level 2 vulnerability criteria of the Guidelines for
vulnerability assessment in chapter 2 can be used. Some examples of recommended
approaches based on Level 1 and Level 2 vulnerability criteria are included below:
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.1 For the excessive acceleration stability failure mode, all
forward speeds should be avoided in all sea states where CS,i
> 10-6, where CS,i is
defined according to 2.3.3.2.1 of the Guidelines for vulnerability
assessment. The transfer function ay(ω)
defined in 2.3.3.2.2 is multiplied by the absolute value of the sine of
the wave heading angle μ and calculated by replacing the wave
frequency ωj with wave encounter frequency
ωej.
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.2 For the pure loss of stability failure mode, nominal ship forward speed of
the ship of 0.752 · L1/2 m/s or greater, should be avoided
in following to beam wave directions in sea states for which
max(C1i,C2i) = 1,
where C1i and C2i are
defined in 2.4.3.3 and 2.4.3.4, respectively, of the Guidelines for
vulnerability assessment.
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.3 For the parametric rolling stability failure mode, forward speed, for
which
CS,i(vs,Hs,Tz),
defined according to 2.5.3.3.1 of the Guidelines for vulnerability
assessment, is equal to 1, should be avoided in all wave directions and all
sea states.
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.4 For the surf-riding/broaching failure mode, either:
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.1 nominal ship speed of 0.94⋅L1/2 (m/s), or
greater, should be avoided when the wavelength, based on mean wave
period, is greater than 80% of the ship length, the significant wave
height is greater than 4% of the ship length L (m) and the
heading angle μ (deg) from the wave direction is less than 45
degrees; or
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.2 alternatively, the critical nominal ship speed provided by the
Level 2 vulnerability criteria (see 2.6.3.4.2) or above should be
avoided in following to beam wave directions in sea states for which
cHT > 0.005, where cHT
is calculated as:
where wij (Hs, Tz) and
C2ij should be calculated based on the
level 2 vulnerability criteria in 2.6.3.2, but with the diffraction
component of the wave force taken into account.
4.6 Application
4.6.1 Operational guidance should be provided as easily accessible and understandable
information in graphical form which clearly indicates unacceptable sailing
conditions for a given sea state, as well as the relevant stability failure modes.
Automatic alert systems can be used for the cases when sailing conditions are close
to or within the areas of unacceptable sailing conditions.
4.6.2 Unacceptable sailing conditions are derived from the pre-defined databases of
probabilistic, deterministic or simplified safety criteria, stored as functions of
the ship forward speed and ship heading with respect to the mean wave direction for
relevant sea states. These sea states are specified by using as input the actual
significant wave height, mean zero-crossing wave period, mean wave direction and
ship course.
4.6.3 The effect of non-parallel wave systems (cross sea) can be reproduced using
these pre-defined databases by combining separate responses to the wind sea and
swell which correspond to the significant wave height, mean zero-crossing wave
period and mean wave direction of each of these wave systems by:
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.1 summing the rate of stability failures for each of these wave systems when
using probabilistic operational guidance;
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.2 summing the maximum responses to each of these wave systems when using
deterministic operational guidance; and
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.3 overlaying the unacceptable sailing conditions for each of these wave
systems when using simplified operational guidance.
The procedure described above is meant to be a practical approximation tool for
addressing cross sea conditions starting from pre-calculations based on simpler
standard sea states. However, such a procedure is an approximate one and sea states
encountered in the ship's operation can be characterized by complex spectra
combining multiple wind sea and swell systems. Therefore, particular caution is
recommended to be exercised during operation when making use of operational guidance
developed according to the described procedure, if the sea state is characterized by
complex combinations of wind sea and swell systems.
4.6.4 The master should ensure that the ship, at any time during the voyage and
considering the available weather forecasts, satisfies the operational limitations
related to maximum significant wave height or operational guidance.