1 General
1.1 The purpose of this standard is to provide
procedures and relevant design parameters of limit state design of cargo containment
systems of a novel configuration in accordance with section 4.27 of this Code.
1.2 Limit state design is a systematic approach where
each structural element is evaluated with respect to possible failure modes related
to the design conditions identified in section 4.3.4 of this Code. A limit state can
be defined as a condition beyond which the structure, or part of a structure, no
longer satisfies the requirements.
1.3 The limit states are divided into the three
following categories:
- .1 Ultimate Limit States (ULS), which correspond to the maximum loadcarrying
capacity or, in some cases, to the maximum applicable strain, deformation or
instability in structure resulting from buckling and plastic collapse; under
intact (undamaged) conditions;
- .2 Fatigue Limit States (FLS), which correspond to degradation due to the
effect of cyclic loading; and
- .3 Accident Limit States (ALS), which concern the ability of the structure to
resist accident situations.
1.4 Part A through part D of chapter 4 of this Code
shall be complied with as applicable depending on the cargo containment system
concept.
2 Design format
2.1 The design format in this standard is based on a
Load and Resistance Factor Design format. The fundamental principle of the Load and
Resistance Factor Design format is to verify that design load effects,
Ld
, do not exceed design resistances,
Rd
, for any of the considered failure modes in any scenario:
A design load
Fdk
is obtained by multiplying the characteristic load by a load factor relevant
for the given load category:
A design load effect
Ld (e.g. stresses, strains,
displacements and vibrations) is the most unfavourable combined load effect derived
from the design loads, and may be expressed by:
-
Ld
= q(Fd1
,Fd2
,...,FdN
)
-
where q denotes the functional relationship between
load and load effect determined by structural analyses.
The design resistance
Rd
is determined as follows:
-
-
where:
-
Rk
is the characteristic resistance. In case of materials
covered by chapter 6 of this Code, it may be, but not limited
to, specified minimum yield stress, specified minimum tensile
strength, plastic resistance of cross sections, and ultimate
buckling strength;
-
γR
is the resistance factor, defined as γR
= γm
· γs
;
-
γm
is the partial resistance factor to take account of the
probabilistic distribution of the material properties (material
factor);
-
γs
is the partial resistance factor to take account of the
uncertainties on the capacity of the structure, such as the
quality of the construction, method considered for determination
of the capacity including accuracy of analysis; and
-
γC
is the consequence class factor, which accounts for the
potential results of failure with regard to release of cargo and
possible human injury.
2.2 Cargo containment design shall take into account
potential failure consequences. Consequence classes are defined in table 1, to
specify the consequences of failure when the mode of failure is related to the
Ultimate Limit State, the Fatigue Limit State, or the Accident Limit State.
Table 1: Consequence
classes
| Consequence
class
|
Definition
|
| Low
|
Failure implies minor release
of the cargo.
|
| Medium
|
Failure implies release of the
cargo and potential for human injury.
|
| High
|
Failure implies significant
release of the cargo and high potential for human injury /
fatality.
|
3 Required analyses
3.1 Three dimensional finite element analyses shall be
carried out as an integrated model of the tank and the ship hull, including supports
and keying system as applicable. All the failure modes shall be identified to avoid
unexpected failures. Hydrodynamic analyses shall be carried out to determine the
particular ship accelerations and motions in irregular waves, and the response of
the ship and its cargo containment systems to these forces and motions.
3.2 Buckling strength analyses of cargo tanks subject
to external pressure and other loads causing compressive stresses shall be carried
out in accordance with recognized standards. The method shall adequately account for
the difference in theoretical and actual buckling stress as a result of plate out of
flatness, plate edge misalignment, straightness, ovality and deviation from true
circular form over a specified arc or chord length, as relevant.
3.3 Fatigue and crack propagation analysis shall be
carried out in accordance with paragraph 5.1 of this standard.
4 Ultimate Limit States
4.1 Structural resistance may be established by testing
or by complete analysis taking account of both elastic and plastic material
properties. Safety margins for ultimate strength shall be introduced by partial
factors of safety taking account of the contribution of stochastic nature of loads
and resistance (dynamic loads, pressure loads, gravity loads, material strength, and
buckling capacities).
4.2 Appropriate combinations of permanent loads,
functional loads and environmental loads including sloshing loads shall be
considered in the analysis. At least two load combinations with partial load factors
as given in table 2 shall be used for the assessment of the ultimate limit
states.
Table 2: Partial load
factors
|
Load combination
|
Permanent loads
|
Functional loads
|
Environmental loads
|
| 'a'
|
1.1
|
1.1
|
0.7
|
| 'b'
|
1.0
|
1.0
|
1.3
|
The load factors for permanent and functional loads in load combination
'a' are relevant for the normally well-controlled and/or specified loads
applicable to cargo containment systems such as vapour pressure, cargo weight,
system self-weight, etc. Higher load factors may be relevant for permanent and
functional loads where the inherent variability and/or uncertainties in the
prediction models are higher.
4.3 For sloshing loads, depending on the reliability of
the estimation method, a larger load factor may be required by the Administration or
recognized organization acting on its behalf.
4.4 In cases where structural failure of the cargo
containment system are considered to imply high potential for human injury and
significant release of cargo, the consequence class factor shall be taken as
γC
= 1.2. This value may be reduced if it is justified through risk analysis and
subject to the approval by the Administration or recognized organization acting on
its behalf. The risk analysis shall take account of factors including, but not
limited to, provision of full or partial secondary barrier to protect hull structure
from the leakage and less hazards associated with intended cargo. Conversely, higher
values may be fixed by the Administration or recognized organization acting on its
behalf, for example, for ships carrying more hazardous or higher pressure cargo. The
consequence class factor shall in any case not be less than 1.0.
4.5 The load factors and the resistance factors used
shall be such that the level of safety is equivalent to that of the cargo
containment systems as described in sections 4.21 to 4.26 of this Code. This may be
carried out by calibrating the factors against known successful designs.
4.6 The material factor γm
shall in general reflect the statistical distribution of the mechanical
properties of the material, and needs to be interpreted in combination with the
specified characteristic mechanical properties. For the materials defined in chapter
6 of this Code, the material factor γm
may be taken as:
- 1.1 when the characteristic mechanical properties
specified by the recognized organization typically represents the lower 2.5%
quantile in the statistical distribution of the mechanical properties; or
- 1.0 when the characteristic mechanical properties
specified by the recognized organization represents a sufficiently small
quantile such that the probability of lower mechanical properties than specified
is extremely low and can be neglected.
4.7 The partial resistance factors γsi
shall in general be established based on the uncertainties in the capacity of
the structure considering construction tolerances, quality of construction, the
accuracy of the analysis method applied, etc.
4.7.1 For design against excessive plastic deformation
using the limit state criteria given in paragraph 4.8 of this standard, the partial
resistance factors γsi
shall be taken as follows:
Factors A, B, C and D are defined in section 4.22.3.1 of this Code.
Rm
and Re
are defined in section 4.18.1.3 of this Code.
The partial resistance factors given above are the results of
calibration to conventional type B independent tanks.
4.8
Design against excessive plastic deformation
4.8.1 Stress acceptance criteria given below refer to
elastic stress analyses.
4.8.2 Parts of cargo containment systems where loads
are primarily carried by membrane response in the structure shall satisfy the
following limit state criteria:
-
σm
≤ f
-
σL
≤ 1.5f
-
σb
≤ 1.5F
-
σL
+ σb
≤ 1.5F
-
σm
+ σb
≤ 1.5F
-
σm
+ σb
+ σg
≤ 3.0F
-
σL
+ σb
+ σg
≤ 3.0F
where:
|
σm |
= |
equivalent primary general membrane stress |
|
σL
|
= |
equivalent primary local membrane stress |
|
σb
|
= |
equivalent primary bending stress |
|
σg
|
= |
equivalent secondary stress |
- f =
- F =
With regard to the stresses σm
, σL
, σb
and σg
, see also the definition of stress categories in section 4.28.3 of this Code.
Guidance Note:
The stress summation described above shall be carried out by
summing up each stress component ( σx
, σy
, τxy
), and subsequently the equivalent stress shall be
calculated based on the resulting stress components as shown in
the example below.
|
4.8.3 Parts of cargo containment systems
where loads are primarily carried by bending of girders, stiffeners and plates,
shall satisfy the following limit state criteria:
-
σms
+ σbp
≤ 1.25F (See notes 1, 2)
-
σms
+ σbp
+ σbs
≤ 1.25F (See note 2)
-
σms
+ σbp
+ σbs
+ σbt
+ σg
≤ 3.0F
-
Note 1: The sum of equivalent section membrane stress and
equivalent membrane stress in primary structure (σms +
σbp
) will normally be directly available from three‑dimensional finite
element analyses.
-
Note 2: The coefficient, 1.25, may be modified by the
Administration or recognized organization acting on its behalf
considering the design concept, configuration of the structure, and the
methodology used for calculation of stresses.
-
where:
|
σms
|
= |
equivalent section membrane stress in primary structure |
|
σbp
|
= |
equivalent membrane stress in primary structure and stress in
secondary and tertiary structure caused by bending of primary structure |
|
σbs
|
= |
section bending stress in secondary structure and stress in
tertiary structure caused by bending of secondary structure |
|
σbt
|
= |
section bending stress in tertiary structure |
|
σg
|
= |
equivalent secondary stress |
|
f
|
= |
 |
|
F
|
= |
 |
The stresses σms
, σbp
, σbs
, and σbt
are defined in 4.8.4. For a definition of σg
, see section 4.28.3 of this Code.
|
Guidance Note:
The stress summation described above shall be carried out by
summing up each stress component (σx
, σy
, τxy
), and subsequently the equivalent stress shall be
calculated based on the resulting stress components.
|
Skin plates shall be designed in accordance with the requirements of the
Administration or recognized organization acting on its behalf. When membrane stress
is significant, the effect of the membrane stress on the plate bending capacity
shall be appropriately considered in addition.
4.8.4 Section stress categories
Normal stress is the component of stress normal to the plane of
reference.
Equivalent section membrane stress is the component of the normal stress
that is uniformly distributed and equal to the average value of the stress across
the cross section of the structure under consideration. If this is a simple shell
section, the section membrane stress is identical to the membrane stress defined in
paragraph 4.8.2 of this standard.
Section bending stress is the component of the normal stress that is
linearly distributed over a structural section exposed to bending action, as
illustrated in figure 1.
4.9 The same factors γc
, γm
, γsi
shall be used for design against buckling unless otherwise stated in the
applied recognized buckling standard. In any case the overall level of safety shall
not be less than given by these factors.
5 Fatigue Limit States
5.1 Fatigue design condition as described in section
4.18.2 of this Code shall be complied with as applicable depending on the cargo
containment system concept. Fatigue analysis is required for the cargo containment
system designed under section 4.27 of this Code and this standard.
5.2 The load factors for FLS shall be taken as 1.0 for
all load categories.
5.3 Consequence class factor γc
and resistance factor γR
shall be taken as 1.0.
5.4 Fatigue damage shall be calculated as described in
sections 4.18.2.2 to 4.18.2.5 of this Code. The calculated cumulative fatigue damage
ratio for the cargo containment systems shall be less than or equal to the values
given in table 3.
Table 3: Maximum allowable
cumulative fatigue damage ratio
|
|
Consequence class
|
|
Cw
|
Low
|
Medium
|
High
|
| 1.0
|
0.5
|
0.5*
|
| Note*: Lower value shall be used in
accordance with sections 4.18.2.7 to 4.18.2.9 of this Code,
depending on the detectability of defect or crack, etc.
|
5.5 Lower values may be fixed by the Administration or
recognized organization acting on its behalf, for example for tank structures where
effective detection of defect or crack cannot be assured, and for ships carrying
more hazardous cargo.
5.6 Crack propagation analyses are required in
accordance with sections 4.18.2.6 to 4.18.2.9 of this Code. The analysis shall be
carried out in accordance with methods laid down in a standard recognized by the
Administration or recognized organization acting on its behalf.
6 Accident Limit States
6.1 Accident design condition as described in section
4.18.3 of this Code shall be complied with as applicable, depending on the cargo
containment system concept.
6.2 Load and resistance factors may be relaxed compared
to the ultimate limit state considering that damages and deformations can be
accepted as long as this does not escalate the accident scenario.
6.3 The load factors for ALS shall be taken as 1.0 for
permanent loads, functional loads and environmental loads.
6.4 Loads mentioned in section 4.13.9 (Static heel
loads) and section 4.15 (Collision and Loads due to flooding on ship) of this Code
need not be combined with each other or with environmental loads, as defined in
section 4.14 of this Code.
6.5 Resistance factor γR
shall in general be taken as 1.0.
6.6 Consequence class factors γc
shall in general be taken as defined in paragraph 4.4 of this standard, but may
be relaxed considering the nature of the accident scenario.
6.7 The characteristic resistance Rk
shall in general be taken as for the ultimate limit state, but may be relaxed
considering the nature of the accident scenario.
6.8 Additional relevant accident scenarios shall be
determined based on a risk analysis.
7 Testing
7.1 Cargo containment systems designed according to
this standard shall be tested to the same extent as described in section 4.20.3 of
this Code, as applicable depending on the cargo containment system concept."