MSC.1/Circ.1461
8 July 2013
GUIDELINES FOR VERIFICATION OF DAMAGE STABILITY REQUIREMENTS FOR
TANKERS
1 The Maritime
Safety Committee, at its ninety-second session (12 to 21 June 2013), having
considered the proposal of the Sub-Committee on Stability and Load Lines and on
Fishing Vessels Safety, at its fifty-fifth session (18 to 22 February 2013),
approved the Guidelines for verification of damage stability requirements
for tankers, as set out in the annex.
2 The Guidelines
consist of two parts, as follows:
.1 part 1: Guidelines for preparation and
approval of tanker damage stability calculations. This part should be applied
to oil tankers, chemical tankers and gas carriers constructed on or after 14
June 2013.
.2 part 2: Guidelines for operation and
demonstration of damage stability compliance. This part should be applied to
all oil tankers, chemical tankers and gas carriers.
3 Member Governments
are invited to bring the annexed Guidelines to the attention of all parties
concerned.
***
ANNEX
GUIDELINES FOR VERIFICATION
OF DAMAGE STABILITY
REQUIREMENTS FOR TANKERS
PART 1
GUIDELINES FOR PREPARATION
AND APPROVAL OF
TANKER DAMAGE STABILITY
CALCULATIONS
Guideline for scope of
damage stability verification on new oil tankers, chemical tankers and gas
carriers1
______________________
1 The application of regulation 27 of the 1988
Load Lines Protocol is explained in appendix 1.
1 APPLICATION
These Guidelines are intended for oil tankers, chemical tankers
and gas carriers constructed on or after 14 June 2013.
2 REFERENCE
2.1 IMO general
instruments
.1 SOLAS chapter II-1, regulations 4.1, 4.2, 5-1
and 19;
.2 Part B, chapter 4 of the International Code
on Intact Stability, 2008 (2008 IS Code), adopted by resolution MSC.267(85), as
amended;
.3 Adoption of amendments to the Protocol of
1988 relating to the International Convention on Load Lines, 1966 (resolution MSC.143(77)),
regulations 27(2), 27(3), 27(11), 27(12) and 27(13)1;
.4 Explanatory notes to SOLAS chapter II-1
subdivision and damage stability regulations (resolution MSC.281(85));
.5 Recommendation on a standard method for
evaluating cross-flooding arrangements (resolution MSC.245(83));
.6 Revised Recommendation on a standard
method for evaluating cross-flooding arrangements (resolution MSC.362(92));
.7 Guidelines on interpretation of the
International Code for the Construction and Equipment of Ships Carrying
Dangerous Chemicals in Bulk (IBC Code) and the International Code for the
Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (IGC Code)
and Guidelines for the uniform application of the survival requirements of the
IBC and IGC Codes (MSC/Circ.406/Rev.1);
.8 Guidelines for damage control plans and
information to the master (MSC.1/Circ.1245); and
.9 Guidelines for the approval of stability
instruments (annex, section 4) (MSC.1/Circ.1229).
2.2 Instrument
applicable to oil tankers
MARPOL Annex I, regulation 28.
2.3 Instruments
applicable to gas carriers
.1 International Code for the Construction and
Equipment of Ships Carrying Liquefied Gases in Bulk (IGC Code), chapter 2,
paragraphs 2.1, 2.4, 2.5, 2.6.2, 2.6.3, 2.7, 2.8 and 2.9; and
.2 Guidelines on Interpretation of the
International Code for the Construction and Equipment of Ships Carrying
Dangerous Chemicals in Bulk (IBC Code) and the International Code for the
Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (IGC Code)
and Guidelines for the Uniform Application of the Survival Requirements of the
IBC and IGC Codes (MSC/Circ.406/Rev.1).
2.4 Instruments
applicable to chemical tankers
.1 International Code for the Construction and
Equipment of Ships Carrying Dangerous Chemicals in Bulk (IBC Code), chapter 2,
paragraphs 2.1, 2.4, 2.5, 2.6.2, 2.7, 2.8 and 2.9; and
.2 Guidelines on Interpretation of the
International Code for the Construction and Equipment of Ships Carrying
Dangerous Chemicals in Bulk (IBC Code) and the International Code for the
Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (IGC Code)
and Guidelines for the Uniform Application of the Survival Requirements of the
IBC and IGC Codes (MSC/Circ.406/Rev.1).
3 GENERAL
3.1 Education and
training
3.1.1 Plan approval of
staff engaged in damage stability verification of new oil tankers, chemical
tankers and gas carriers should have as minimum the following formal
educational background:
.1 a
degree or equivalent from a tertiary institution recognized within the field of
marine engineering or naval architecture; and
.2
competent in the English language commensurate with their work.
3.1.2 Plan approval of
staff engaged in damage stability verification of new oil tankers, chemical
tankers and gas carriers should be trained according to theoretical and
practical modules defined by the Administration or recognized organization (RO)
acting on its behalf, to acquire and develop general knowledge and
understanding applicable to the above-mentioned types of ship and stability
assessment according to the IMO instruments referred to in section 2 above.
3.1.3 Methods of training
may include monitoring, testing, etc. on a regular basis according to the
Administration or RO's system. Evidence of training provided should be
documented.
3.1.4 Updating of
qualification may be done through the following methods:
.1 self-study;
.2 extraordinary seminars in case of significant
changes in the international conventions, codes, etc.; and
.3 special training on specific work, which is
determined by a long absence of practical experience.
3.1.5 Maintenance of
qualification should be verified at annual performance review.
3.2 Scope of stability
verification
3.2.1 The scope of damage
stability verification is determined by the required damage stability standards
(applicable damage stability criteria) and aims at providing the ship's master
with a sufficient number of approved loading conditions to be used for the
loading of the ship. In general, for non-approved loading conditions (by the
Administration or RO acting on its behalf), approved KG/GM limit curve(s) or
approved stability instrument software satisfying the stability requirements
(intact and damage) for the draught range to be covered, should be used to
verify compliance on board.
3.2.2 Within the scope of
the verification determined as per the above, all damage scenarios specified by
the relevant regulations should be determined and assessed, taking into account
the damage stability criteria.
3.2.3 Damage stability
verification and approval requires a review of submitted calculations and
supporting documentation with independent check calculations to confirm that
damage stability calculation results comply with relevant stability criteria.
3.2.4 Examination and
approval of the stability instrument software installed on board (and to be
used for assessing intact and damage stability) should also be carried out. A
stability instrument comprises hardware and software. The accuracy of the
computation results and actual ship data used by the software is to be
verified.
3.3 Assumptions
3.3.1 For all loading
conditions, the initial metacentric height and the righting lever curve should
be corrected for the effect of free surfaces of liquids in tanks.
3.3.2 Superstructures and
deckhouses not regarded as enclosed can be taken into account in stability
calculations up to the angle at which their openings are flooded. Flooding
points (including windows) incapable of weathertight closure are to be included
in any list determined in accordance with paragraph 3.4.2.6. Full compliance
with residual stability criteria must be achieved before any such point becomes
immersed.
3.3.3 When determining the
righting lever (GZ) of the residual stability curve, the constant displacement
(lost buoyancy) method of calculation should be used (see section 6.1).
3.3.4 Conditions of
loading and instructions provided by the submitter for use of the applicable
KG/GM limit curve(s) and variation of loading patterns and representative
cargoes are taken to be representative of how the ship will be operated.
3.4 Documentation to be
submitted for review
3.4.1 Presentation of
documents
The documentation should begin with the following details:
principal dimensions, ship type, designation of intact conditions, designation
of damage conditions and pertinent damaged compartments, KG/GM limit curve(s).
3.4.2 General documents and supporting information
.1 lines plan, plotted or numerically;
.2 hydrostatic data and cross curves of
stability (including drawing of the buoyant hull);
.3 definition of watertight compartments with
moulded volumes, centres of gravity and permeability;
.4 layout plan (watertight integrity plan) for
the watertight compartments with all internal and external opening points including
their connected sub-compartments, and particulars used in measuring the spaces,
such as general arrangement plan and tank plan;
.5 Stability Booklet/Loading Manual including at
least fully loaded homogeneous condition at summer load line draught (departure
and arrival) and other intended operational conditions2;
.6 coordinates of opening points with their
level of tightness (e.g. weathertight, unprotected)3, including
reference to the compartment that the opening is connected to;
.7 watertight door location;
.8 cross- and down-flooding devices and the
calculations thereof according to resolution MSC.245(83) or MSC.362(92), as appropriate, with information
about diameter, valves, pipe lengths and coordinates of inlet/outlet. Cross-
and down-flooding should not be considered for the purpose of achieving
compliance with the stability criteria (see also section 9.2);
.9 pipes in damaged area when the breaching of
these pipes results in progressive flooding (see section 10.1);
.10 damage extents and definition of damage cases;
and
.11 any initial conditions or restrictions which
have been assumed in the derivation of critical KG or GM data, and which must
therefore be met in service.
___________________
2 For the purpose of making a submission of stability information
for approval, the minimum number of loading conditions which should be
submitted for approval is a function of the mode of operation intended for the
ship. MSC/Circ.406/Rev.1
offers guidance in this respect, and identifies the concepts of the
"dedicated service tanker" and "parcel tanker" for the
purpose of undertaking stability approval of ships certified under the IBC and
IGC Codes and the appropriate treatment of ships assigned tropical freeboards.
3 Details of watertight, weathertight and unprotected openings
should be included in the Damage Control Plan and Damage Control Booklet in
accordance with MSC.1/Circ.1245.
The cases and extent of progressive flooding assumed in the damage
stability analysis should be indicated in the Damage Control Booklet and the
Documents for Submission in accordance with the annex to resolution MSC.281(85).
Arrangements to prevent further flooding are to be indicated on the Damage
Control Plan and in the Damage Control Booklet.
3.4.3 Special documents
3.4.3.1 Documentation
.1 Design documentation: damage stability
calculations (including residual stability curves), the arrangements,
configuration and contents of the damaged compartments, and the distribution,
relative densities and the free surface effect of liquids.
.2 Operational documentation: loading and
stability information booklet (stability booklet), Damage Control Plan; and
Damage Control Booklet.
3.4.3.2 Special consideration
For intermediate flooding stages before cross-flooding (see
sections 6.8 and 9.2) or before progressive flooding (see section 6.9), an
appropriate scope of the documentation covering the aforementioned items is
needed in addition. The intermediate stages for cargo outflow and seawater
inflow should be checked. If any stability criteria during intermediate stages
shows more severe values than in the final stage of flooding, these
intermediate stages should also be submitted.
4 OPERATING LIMITS –
DESCRIPTIONS/ASSUMPTIONS
In considering the scope of the verification to be conducted,
consideration of the operating limits is needed.
The following loading options should be permitted:
.1 service loading conditions identical to the
approved loading conditions of the stability booklet (see section 4.2); or
.2 service loading conditions complying with the
approved intact and damage stability limiting curves (where provided) (see
section 4.3); or
.3 service loading conditions which have been
checked with an approved stability instrument with the capability to perform
damage stability calculations (Type 2 or Type 3 of the IS Code and MSC.1/Circ.1229)
either based on KG/GM limit curve(s) or based on direct damage stability
assessment (see section 4.5).
If the above-mentioned proof of compliance is not possible, then
the intended loading conditions should be either prohibited or be submitted for
specific approval to the Administration or RO acting on its behalf. Suitable
instructions to this effect should be included in the stability booklet/loading
manual.
An approved loading condition is one which has been specifically
examined and endorsed by the Administration/RO.
4.1 Specific loading
patterns
4.1.1 Ship-specific design
loading patterns and loading restrictions should be clearly presented in the
stability booklet. The following items should be included:
.1 any required and intended loading conditions
(including the ones corresponding to multiple freeboards when so assigned to
the ship), i.e. symmetrical/unsymmetrical, homogeneous/alternating or ballast/
partial/full;
.2 types (e.g. oil, noxious liquid substances
and LNG) of liquid cargo allowed to be carried;
.3 restrictions to different liquid loads to be
carried simultaneously;
.4 range of permissible densities of liquid
loads to be carried; and
.5 minimum tank filling levels required to
achieve compliance with the applicable stability criteria.
4.1.2 For the verification
of damage stability all loading conditions presented in the stability booklet
except for ballast, light ship and docking conditions are to be examined.
4.2 Range of
permissible loading conditions
In the absence of stability software and KG/GM limit curve(s), in
lieu of approved specific loading conditions, a matrix clearly defining any
allowable ranges of loading parameters (draught, trim, KG, cargo loading pattern
and SG) that the ship is allowed to load whilst remaining in compliance with
the applicable intact and damage stability criteria can be developed for the
stability booklet when a greater degree of flexibility than that afforded by
approved specific loading conditions is needed. If this information is to be
used, it should be in an approved form.
4.3 KG/GM Limit
curve(s) 4
______________________
4 To
avoid difficulties associated with developing suitable KG/GM limit curves and
their restriction on operational capacity, it is recommended that an approved
Type 3 stability software is fitted on board.
4.3.1 Where KG/GM limit
curves are provided, a systematic investigation of damage survival
characteristics should be undertaken by making calculations to obtain the
minimum required GM or maximum allowable KG at a sufficient number of draughts
within the operating range to permit the construction of a series of curves of
"required GM" or "allowable KG" in relation to draught and
cargo tank content in way of the damage. The curves must be sufficiently
comprehensive to cover operational trim requirements.
4.3.2 The verification of
KG/GM limit curves should be conducted without any free surface correction. The
actual loading condition uses the free surface correction (see section 6.5)
when comparing actual and allowable KG values.
4.3.3 It is to be noted
that any change of filling level, draught, trim, or cargo density might have a
major influence to the results of a damage case; therefore the following items
should be considered carefully for the calculation of the KG/GM limit curves:
.1 intact and damage stability criteria
applicable to the ship;
.2 the maximum required damage extent and lesser
extents of damage which provide the most severe damage cases;
.3 draught range of the ship (up to tropical
freeboard if required);
.4 trim range of the ship (see section 6.6);
.5 full and empty cargo tanks;
.6 partially filled cargo tanks (consideration
of increments as necessary);
.7 minimum tank fillings in tonnes if required;
.8 maximum/minimum densities of cargoes; and
.9 ballast tank filling levels as necessary to
achieve compliance.
4.3.4 Damage stability
calculations, on which the KG/GM limit curve(s) is(are) based, should be performed
at the design stage. The KG/GM limit curve(s) drawn out taking stability
criteria (intact and damage) into account should be inserted in the stability
booklet.
4.4 Initial heel
The stability booklet should contain a note for the master to avoid
initial heel greater than 1 degree. A steady heeling angle may have a major
influence on the stability of the ship especially in the case of damage.
4.5 Direct calculation
on board (stability instrument)
4.5.1 Any stability
software installed on board should cover all stability requirements (intact and
damage) applicable to the ship.
4.5.2 The following types
of stability softwares, if approved by an Administration or RO acting on its
behalf (according to the 2008 IS Code and MSC.1/Circ.1229), are applicable for the
calculation of service-loading conditions for tank ships:
.1 Type 2: Checking intact and damage stability
on basis of a KG/GM limit curve(s) or previously approved loading conditions;
and
.2 Type 3: Checking intact and damage stability
by direct application of pre-programmed damage cases for each loading
condition, including capability for calculation of intermediate damage stages.
4.5.3 The software should
be approved by the Administration or RO acting on its behalf. The stability
instrument is not a substitute for the approved stability documentation, but
used as a supplement to facilitate stability calculations.
4.5.4 Sufficient damages,
taking into account lesser damages, and variation of draft, cargo density,
tank-loading patterns and extents of tank filling should be performed to ensure
that for any possible loading condition the most onerous damages have been
examined according to relevant stability criteria.
4.5.5 The methodologies
for determining compliance with relevant stability criteria should be as set
out in these Guidelines.
5 Hull and
compartment modelling tolerances
5.1 Acceptable
tolerances should be in accordance with table 1. Where two values are provided
for the permissible tolerances, the per cent deviation is allowable as long as
it does not exceed the following linear value for the particular hull form
dependent parameter.
5.2 Deviation from
these tolerances should not be accepted unless the Administration or RO acting
on its behalf considers that there is a satisfactory explanation for the
difference and that there will be no adverse effect on the capability of the
ship to comply with the stability criteria.
5.3 No deviation is
generally allowed for input data; however, small differences associated with
calculation rounding or abridged input data are acceptable.
Table 1 (relevant parts of MSC.1/Circ.1229 are reproduced)
|
Hull form dependent |
Tolerances |
|
Displacement
|
2%
|
|
Longitudinal
centre of buoyancy, from AP |
1%/50
cm max |
|
Vertical
centre of buoyancy |
1%/5
cm max |
|
Transverse
centre of buoyancy |
0.5%
of B/5 cm max |
|
Longitudinal
centre of flotation, from AP |
1%/50
cm max |
|
Moment
to trim 1 cm |
2%
|
|
Transverse
metacentric height |
1%/5
cm max |
|
Longitudinal
metacentric height |
1%/50
cm max |
|
Cross
curves of stability |
5
cm |
|
Compartment dependent |
Tolerances |
|
Volume
or deadweight |
2%
|
|
Longitudinal
centre of gravity, from AP |
1%/50
cm max |
|
Vertical
centre of gravity |
1%/5
cm max |
|
Transverse
centre of gravity |
0.5%
of B/5 cm max |
|
Free
surface moment |
2%
|
|
Level
of contents |
2%
|
Deviation in % = [(base value – applicant's value)/base value] x
100
where the "base value" may be taken from the approved stability
information or the computer model.
6 Methodology
6.1 Method of analysis
6.1.1 Independent analysis
uses the "constant displacement"/"lost buoyancy" method.
6.1.2 Within the scope of
damage stability analysis with the deterministic approach, depending on the
subdivision of the ship, the result of applying the standard of damage as
specified in the applicable requirements is the creation of a number of damage
cases, where one or more compartments are open to sea.
6.1.3 The compartment(s),
once damaged, are not considered as contributing to the buoyancy of the ship.
Consequently, a new condition of equilibrium occurs. In order to define the new
equilibrium condition and to assess the stability of the ship after damage the
lost buoyancy/constant displacement method is used.
6.1.4 The new floating
position can be determined by assuming that the damaged displacement is equal
to the intact displacement (constant displacement) minus the weight of liquids
which were contained in the damaged compartments.
6.1.5 Due to the lost
buoyancy of the damaged compartment(s), the remaining intact ship has to
compensate by sinkage, heel and trim until the damaged displacement is reached.
Once the equilibrium has been reached and the final waterline is determined,
the metacentric height (GM), the righting lever curves (GZ) and the centre of
gravity positions (KG), can be calculated in order to verify the stability of
the ship against the applicable requirements.
6.1.6 For the intermediate
stages of flooding and the equalization with compartments cross-connected by
small ducts, i.e. not opened to the sea directly, the added weight method is
used.
6.2 Arguments used in
calculations
The arguments used in the calculation for the verification of
damage stability are the following:
.1 trim: The calculation should be done for the
ship freely trimming;
.2 heel angle at equilibrium: The heel angle at
equilibrium, due to unsymmetrical flooding, should not exceed the maximum
values as indicated in the applicable requirements. Concerning the range of
positive righting levers (GZ), this should be calculated beyond the position of
equilibrium to the extent as so required by the applicable requirements;
.3 free surface of liquid: For the calculation
of the position of the centre of gravity (KG), the metacentric height (GM) and
the righting lever curves (GZ), the effect of the free surfaces of liquids (see
section 6.5) should be taken into account;
.4 immersion of weathertight and unprotected
openings (see sections 6.7 and 10.1)
Unprotected openings:
The positive range of
righting levers is calculated from the angle of equilibrium until the angle of
immersion of the unprotected openings leading to intact spaces;
Weathertight points: see
paragraph 10.1.2;
.5 progressive flooding through internal pipes:
in case of damage of an internal pipe which is connected to an undamaged
compartment, the undamaged compartment should also be flooded, unless
arrangements are fitted (e.g. check valves or valves with remote means of
control), which can prevent further flooding of the undamaged compartments;
.6 permeabilities: care should be taken to apply
the permeabilities as specified in the applicable regulations. Special
attention should be paid in case compartments which are separated by
weathertight boundaries are modeled as one compartment. This simplified method
of modeling the compartments should apply only to compartments belonging to the
same category (same permeability); and
.7 heel angles for the calculation of the GZ
curve: evaluation of damage stability criteria should generally be determined
from data calculated over a range of angles from 0 to 60 degrees. It is
recommended to use an increment not exceeding 5 degrees.
6.3 Adjustments for
cargo run-off
6.3.1 In cases where the
damage involves the cargo hold, it is assumed that cargo is flowing out and
that water ingress starts. During the intermediate stages of flooding it is
considered that both cargo and seawater are existing in the damaged tank (see
section 9.3).
6.3.2 At the final stage
it is assumed that the cargo is completely lost and that the tank is filled
with seawater up to the level of the waterline.
6.3.3 The impact on the
stability of the ship, due to the inflow and outflow of liquid cargo is also
dependent on the following parameters:
.1 the density of the cargo: liquid cargo with
density greater than 0.95 t/m3 should be considered as heavy liquid
cargo. In case of lesser vertical extent of damage, i.e. damage above the tank
top (see appendix 4), the release of heavy liquid cargo might lead to large
angle of heel on the intact side of the ship. Depending on intact draught and
cargo tank filling level, outflow of cargo of lesser density may also cause
heel to the opposite side; and
.2 the permeability of the cargo space, taking
into account that permeabilities smaller than those specified in the applicable
rules can be applied, if justified.
6.4 Handling of
permeabilities
6.4.1 Permeability of a
space means the ratio of the volume within that space, which should be assumed
to be occupied by water to the total volume of that space. The total volume
should be calculated to moulded lines, and no reduction in total volume should
be taken into account due to structural members (i.e. stiffeners, etc.).
Account of structural members is taken in the applicable permeabilities (see
also MSC/Circ.406/Rev.1,
paragraph 3.11).
6.4.2 Depending on the
applicable requirements, the permeabilities assumed for spaces flooded as a
result of damage should be as shown in table 2.
Table
2
|
Spaces
|
Permeabilities
|
|||
|
MARPOL
|
ICLL
1) |
IBC
|
IGC
|
|
|
Appropriated
to stores |
0.6
|
0.95
|
0.6
|
0.6
|
|
Occupied
by accommodation |
0.95
|
0.95
|
0.95
|
0.95
|
|
Occupied
by machinery |
0.85
|
0.85
|
0.85
|
0.85
|
|
Voids
|
0.95
|
0.95
|
0.95
|
0.95
|
|
Intended
for consumable liquids |
0
to 0.95* |
0.95
|
0
to 0.95* |
0
to 0.95* |
|
Intended
for other liquids |
0
to 0.95* |
0.95
|
0
to 0.95* |
0
to 0.95* |
|
* The permeability of partially filled
compartments should be consistent with the amount of liquid carried in the
compartment. 1) Regarding application of ICLL damage stability requirements
refer to appendix 1. |
||||
6.4.3 Whenever damage penetrates a tank containing liquids, it
should be assumed that the contents are completely lost from that compartment
and replaced by seawater up to the level of the final plane of equilibrium.
6.4.4 Other figures for permeability may be used for the damaged
case both during cargo run-off and the final equilibrium condition under the
following provisions:
.1 the detailed calculations and the arguments
used for determining the permeability of the compartment(s) in question, is to
be included in the damage stability booklet;
.2 the water tightness/resistance to water
pressure and the means by which internal fittings/material are secured to the
tank should substantiate the use of such fittings/material in reducing the
permeability of a compartment. Where a ship is fitted with significant
quantities of cargo insulation, the permeabilities of the relevant cargo spaces
and/or the void spaces surrounding such cargo spaces may be calculated by
excluding the volume of insulation material in those spaces from the flooded
volume, provided that the insulating material is shown to comply with the
following conditions:
.1 it is impermeable to water under hydrostatic
pressure at least corresponding to the pressure caused by the assumed flooding;
.2 it will not crush or break up due to
hydrostatic pressure at least corresponding to the pressure caused by the
assumed flooding;
.3 it will not deteriorate or change its
properties over the long term in the environment anticipated in the space it is
installed;
.4 it is highly resistant to the action of
hydrocarbons, where relevant; and
.5 it will be adequately secured so that it will
remain in position if subjected to collision damage and consequent
displacement, distortion of its supporting and retaining structure, repeated
rapid ingress and outflow of seawater and the buoyant forces caused by
immersion following flooding;
.3 the applied permeability should reflect the
general conditions of the ship throughout its service life, rather than
specific loading conditions; and
.4 permeabilities other than those indicated in
table 2 should be considered only in cases, where it is evident that there is a
significant discrepancy between the values shown in the regulations and the
actual values (i.e. due to specific tank structure or insulating material).
6.5 Free surface
calculation (upright, as ship heels and
after cargo run-off)
With respect to the approval of actual loading conditions the
following should be applied:
6.5.1 The free surfaces of
liquids lead to the increase of the centre of gravity (KG) and the reduction of
the metacentric height (GM) and the righting arm (GZ curve) of the ship.
Therefore corrections should be made, taking into account the change of the
centre of gravity of the ship due to the moving of the centre of gravity of the
liquids. Depending on the filling level, free surfaces can exist in tanks with
consumable liquids, seawater ballast and liquid cargo.
6.5.1.1 For consumable liquids account on the free surfaces should
be taken whenever the filling level is equal to or less than 98 per cent:
.1 In calculating the free surface effects in
tanks containing consumable liquids, it should be assumed that for each type of
liquid at least one transverse pair or a single centreline tank has a free
surface and the tank or combination of tanks taken into account should be those
where the effect of free surfaces is the greatest.
.2 Taking into account subparagraph .1, the free
surfaces should correspond to the maximum value attainable between the filling
levels envisaged.
6.5.1.2 During ballasting between departure and arrival condition,
the correction for the free surfaces should correspond to the maximum value
attainable between the filling levels envisaged. This applies also for the
situation where in the departure condition the filling level of a ballast tank
is 0 per cent and in the arrival 100 per cent (or the opposite).
6.5.1.3 For the category of liquids referred to under paragraphs
6.5.1.1 and 6.5.1.2, intermediate loading conditions may be considered as an
alternative, as deemed necessary, covering the stage where the free surfaces
are the greatest. It may be calculated with varying free surface moments (i.e.
actual liquid transfer moments), taking into account actual heel and trim,
depending on the interval angles of the GZ curve. This is a more accurate
method.
6.5.1.4 Except as indicated in regulation 27(11)(v) of the 1988
Load Lines Protocol, for liquid cargo the effect of free surface should be
taken into account for the filling level equal to or smaller than 98 per cent.
If the filling level is fixed actual free surfaces can be applied. The
following two methods can be used for the calculation of the GZ curve, taking
into account the effect of the free surface moments for the intact
compartments:
.1 Calculation with constant effect of free
surfaces, without taking into account the change in heel and trim, for the
interval angles of the GZ curve.
.2 Calculation with varying free surface
moments, actual liquid transfer moments, taking into account actual heel and
trim, depending on the interval angles of the GZ curve (see appendix 2).
6.5.2 For the damaged
compartments, whenever the damage is involving cargo tanks, account should be
taken of the following:
.1 the impact on the stability of the ship due
to the outflow of cargo and ingress of seawater can be verified with the
calculation of the intermediate stages of flooding (see section 9); and
.2 at the final equilibrium the free surface
correction should exclude the free surface moment of the lost cargo.
6.5.3 The free surface
effect should be calculated at an angle of heel of 5° for each individual
compartment or as per paragraph 6.5.1.3.
6.6 Treatment of
operational trim
6.6.1 For the assumed
damage and the resultant damage cases, the damage stability should be assessed
for all anticipated conditions of loading and variations in draught and trim.
6.6.2 Significant trim
values (greater than 1% Lpp) can appear in the aft/fore part of the
ship in the departure and arrival condition. In that case, damage cases
involving the aft/fore part of the ship might be critical for achieving
compliance with the applicable criteria. In order to limit the trim, ballast
water is used during the voyage, as deemed necessary. Under the provision of
paragraphs 6.5.1.2 and 6.5.1.3, for taking account of the free surface effect
during ballasting, if intermediate stages of the voyage are considered, then
the loading conditions representing these stages should be also calculated for
damage stability.
6.7 Down-flooding
points
6.7.1 Down-flooding point
is the lower edge of any opening through which progressive flooding may take
place. Such openings should include air pipes, ventilators and those which are
closed by means of weathertight doors or hatch covers and may exclude those
openings closed by means of watertight manhole covers and flush scuttles, small
watertight cargo tank hatch covers which maintain the high integrity of the
deck, remotely operated watertight sliding doors, and sidescuttles of
non-opening type.
6.7.2 All openings through
which progressive flooding may take place should be defined: both weathertight
and unprotected. As an alternative, it might be accepted to consider only the
most critical openings, which are considered to be the openings with the lowest
vertical position and close to the side shell. Concerning the longitudinal
position it depends on the aft or fore trim of the initial condition and the
trim after damage at equilibrium. Unprotected openings should not be immersed
within the minimum range of righting-lever curve required for the ship. Within
this range, the immersion of any of the openings capable of being closed
weathertight may be permitted.
6.8 Cross-flooding time
6.8.1 Cross-flooding time
should be calculated in accordance with the Recommendation on a standard
method for evaluating cross-flooding arrangements (resolutions MSC.245(83) or MSC.362(92), as
appropriate).
6.8.2 If complete fluid
equalization occurs in 60 s or less, the equalized tank should be assumed
flooded with the tanks initially to be flooded and no further calculations need
to be carried out. Otherwise, the flooding of tanks assumed to be initially
damaged and equalized tank should be carried out in accordance with section
9.2. Only passive open cross-flooding arrangements without valves should be
considered for instantaneous cases.
6.8.3 Where cross-flooding
devices are fitted, the safety of the ship should be demonstrated in all stages
of flooding (see sections 9.2 and 10). Cross-flooding equipment, if installed,
should have the capacity to ensure that the equalization takes place within 10
min.
6.8.4 Tanks and
compartments taking part in such equalization should be fitted with air pipes
or equivalent means of sufficient cross-section to ensure that the flow of
water into the equalization compartments is not delayed.
6.8.5 Spaces which are
linked by ducts of a large cross-sectional area may be considered to be common,
i.e. the flooding of these spaces should be interpreted as instantaneous
flooding with the equalization of duration of less than 60 s.
6.9 Progressive
flooding (internal/external) (see also sections
10.1 and 10.2)
6.9.1 Progressive flooding
is the flooding of compartments situated outside of the assumed extent of
damage. Progressive flooding may extend to compartments, other than those
assumed flooded, through down-flooding points (i.e. unprotected and
weathertight openings), pipes, ducts, tunnels, etc.
6.9.2 The flooding of
compartment(s) due to progressive flooding occurring in a predictable and
sequential manner through a down-flooding point which is submerged below the
damage waterline may be permitted provided all intermediate stages and the
final stage of flooding meet the required stability criteria.
6.9.3 Minor progressive
flooding through the pipes situated within the assumed extent of damage may be
permitted by the Administration, provided the pipes penetrating a watertight
subdivision have a total cross-sectional area of not more than 710 mm2
between any two watertight compartments.
6.9.4 If the opening
(unprotected or fitted with a weathertight means of closure) connects two
spaces, this opening should not be taken into account if the two connected
spaces are flooded or none of these spaces are flooded. If the opening is
connected to the outside, it should not be taken into account only if the
connected compartment is flooded.
7 EXTENTS OF DAMAGE
CONSIDERED
7.1 Maximum extents
The following provisions regarding the maximum extent and the
character of the assumed damage should be applied:
Table
3
|
.1
|
Side
damage: |
MARPOL/IBC/IGC
|
ICLL
(Type A ships) |
|
.1.1
|
Longitudinal
extent: |
1/3
L2/3 or 14.5 m, whichever is less |
Single compartment between adjacent transverse bulkheads as
specified in ICLL paragraph 12(d) 1) |
|
.1.2
|
Transverse
extent: |
B/5 or 11.5 m, whichever is less (measured inboard from the
ship's side at right angles to the centreline at the level of the summer load
line) |
B/5 or 11.5, whichever is the lesser (measured inboard from the
side of the ship perpendicularly to the centreline at the level of the summer
load waterline) 1) |
|
.1.3
|
Vertical
extent: |
upwards without limit (measured from the moulded line of the bottom
shell plating at centreline) |
From
baseline upwards without limit |
|
.2
|
Bottom damage 2): |
MARPOL/IBC/IGC
|
|
|
For 0.3 L from the forward perpendicular of the ship |
Any
other part of the ship |
||
|
.2.1
|
Longitudinal
extent: |
1/3
L2/3 or 14.5 m, whichever is less |
1/3
L2/3 or 5 m, whichever is less |
|
.2.2
|
Transverse
extent: |
B/6
or 10 m, whichever is less |
B/6
or 5 m, whichever is less |
|
.2.3
|
Vertical
extent: |
MARPOL/IBC:
B/15 or 6 m, whichever is less (measured from the moulded line
of the bottom shell plating at centreline) IGC:
B/15 or 2 m, whichever is less (measured from the moulded line
of the bottom shell plating at centreline) |
MARPOL/IBC:
B/15 or 6 m, whichever is less (measured from the moulded line
of the bottom shell plating at centreline) IGC:
B/15 or 2 m, whichever is less (measured from the moulded line
of the bottom shell plating at centreline) |
|
.3
|
Bottom
raking damage 3): |
MARPOL
|
|
|
.3.1
|
Longitudinal
extent: |
in
tankers of 75,000 tonnes deadweight and above: 0.6
L(m) measured from the forward perpendicular of the ship |
|
|
in
tankers of less than 75,000 tonnes deadweight: 0.4
L(m) measured from the forward perpendicular of the ship |
|||
|
.3.2
|
Transverse
extent: |
B/3
anywhere in the bottom |
|
|
.3.3
|
Vertical
extent: |
Breach
of the outer hull |
|
|
1)
|
See
appendix 3. |
||
|
2)
|
Bottom
damage is not required in the ICLL. |
||
|
3)
|
Bottom
raking damage is required only for oil tankers of 20,000 tonnes deadweight
and above. |
||
7.2 Lesser extents
7.2.1 If
any damage of a lesser extent than the maximum damage specified in table 3
would result in a more severe condition, such damage should be considered (see
section 4.5.4).
7.2.2 In
the case of a gas carrier, local side damage anywhere in the cargo area
extending inboard 760 mm measured normal to the hull shell should be
considered, and transverse bulkheads should be assumed damaged when also
required by the applicable subparagraphs of section 2.8.1 of the IGC Code.
7.3 Rationale for reviewing lesser extents including symmetrical
vs. unsymmetrical tank arrangement/geometry – Calculation on weakest side
7.3.1 For
a given loading condition, the following examples of damages of a lesser extent
may result in a more severe situation than that caused by the maximum damage
specified in table 3:
.1 Example of damage on double bottom tanks with
watertight centre girder:
.1 Damage of a lesser extent which could occur
at the bottom plate of the ship, without damaging the centre girder, will lead
to flooding of the double bottom tank on one side of the ship only. This is the
case of unsymmetrical flooding. For the same location, damage of a maximum
extent would cause damage on the centre girder and therefore flooding of the
double bottom tanks on both sides. This is the case of symmetrical flooding
(see appendix 4).
.2 Compared to the symmetrical flooding in the
case of maximum damage extent, unsymmetrical flooding of spaces, caused by
damage of a lesser extent might lead to a more severe situation. Of course, in
case of non-watertight centre girder, the effect of damage of lesser and
maximum extent would be the same.
.2 Example of damage with lesser vertical
extents:
Damage starting from above a
tank top would flood the spaces only above the double bottom (see appendix 4).
This may result in a more onerous residual stability or heeling angle.
7.3.2 Taking
into account the above examples, it is necessary to review damages of lesser
extents considering the symmetrical or unsymmetrical nature of tank
arrangements of the ship and geometry of the ship. The ship's damage stability
is to be ensured, in the most severe or weakest case of damage of lesser
extents.
8 RATIONALE APPLIED FOR LOADING PATTERN EVALUATION
For damage stability calculations of
tank ships the following effects due to different loading methods should be
taken into account in determining the scope of verification and specific cases
of damage to be investigated.
8.1 Homogeneous vs. alternate/partial loading
8.1.1 For
homogeneous loading conditions, the damage to cargo tanks may have a major
effect on residual stability. Outflow of the loaded cargo liquids (and less
inflow of seawater) may reduce the ships' displacement and cause heel to
opposite side of the damage. For alternate loading conditions the residual stability
depends on the damaged cargo tank. Damage to a fully loaded cargo tank might
cause reduction of the initial displacement and heel to the opposite side, but
damage on an empty cargo tank might cause the opposite effect. For the damage
to two adjacent cargo tanks, one filled and one empty, the total effect might
be less severe due to two (partly) neutralizing effects.