4.16
General
4.16.1 The structural design shall ensure that tanks
have an adequate capacity to sustain all relevant loads with an adequate margin of
safety. This shall take into account the possibility of plastic deformation,
buckling, fatigue and loss of liquid and gas tightness.
4.16.2 The structural integrity of cargo containment
systems shall be demonstrated by compliance with 4.21 to 4.26, as appropriate, for
the cargo containment system type.
4.16.3 The structural integrity of cargo containment
system types that are of novel design and differ significantly from those covered by
4.21 to 4.26 shall be demonstrated by compliance with 4.27 to ensure that the
overall level of safety provided in this chapter is maintained.
4.17
Structural analyses
4.17.1
Analysis
4.17.1.1 The design analyses shall be based on accepted
principles of statics, dynamics and strength of materials.
4.17.1.2 Simplified methods or simplified analyses may
be used to calculate the load effects, provided that they are conservative. Model
tests may be used in combination with, or instead of, theoretical calculations. In
cases where theoretical methods are inadequate, model or full-scale tests may be
required.
LR 4.17-01 Where simplified methods or simplified analyses are
proposed, their details are to be agreed with LR before commencement of
application.
4.17.1.3 When determining responses to dynamic loads,
the dynamic effect shall be taken into account where it may affect structural
integrity.
4.17.2
Load scenarios
4.17.2.1 For each location or part of the cargo
containment system to be considered and for each possible mode of failure to be
analysed, all relevant combinations of loads that may act simultaneously shall be
considered.
LR 4.17-02 LR should be consulted for guidance on the relevant
combination of loads to be taken into account in the analysis and this should be
done at as early a stage as possible.
4.17.2.2 The most unfavourable scenarios for all
relevant phases during construction, handling, testing and in service, and
conditions shall be considered.
4.17.3 When the static and dynamic stresses are
calculated separately, and unless other methods of calculation are justified, the
total stresses shall be calculated according to:
where:
-
σx.st
, σy.st
, σz.st
, τxy.st
, τxz.st
and τyz.st
are static stresses; and
-
σx.dyn, σy.dyn, σz.dyn,
τxy.dyn, τxz.dyn
and τyz.dyn
are dynamic stresses,
each shall be determined separately from acceleration components and
hull strain components due to deflection and torsion.
4.18
Design conditions
All relevant failure modes shall be considered in the design for all
relevant load scenarios and design conditions. The design conditions are given in
the earlier part of this chapter, and the load scenarios are covered by 4.17.2.
4.18.1
Ultimate design condition
Structural capacity may be determined by testing, or by analysis, taking
into account both the elastic and plastic material properties, by simplified linear
elastic analysis or by the Code provisions.
4.18.1.1 Plastic deformation and buckling shall be
considered.
LR 4.18-01 Plastic deformation analyses should be conducted in
accordance with an agreed recognised Standard.
4.18.1.2 Analysis shall be based on characteristic load
values as follows:
-
| Permanent
loads:
|
Expected
values
|
| Functional
loads:
|
Specified
values
|
| Environmental
loads:
|
For wave loads:
most probable largest load encountered during
108 wave encounters.
|
4.18.1.3 For the purpose of ultimate strength
assessment, the following material parameters apply:
-
.1.1
Re
= specified minimum yield stress at room temperature
(N/mm2). If the stress-strain curve does not show a defined yield
stress, the 0.2% proof stress applies.
-
.1.2
Rm
= specified minimum tensile strength at room temperature
(N/mm2).
-
For welded connections where under-matched welds, i.e. where the
weld metal has lower tensile strength than the parent metal, are
unavoidable, such as in some aluminium alloys, the respective
Re
and Rm
of the welds, after any applied heat treatment, shall be used. In such
cases, the transverse weld tensile strength shall not be less than the
actual yield strength of the parent metal. If this cannot be achieved,
welded structures made from such materials shall not be incorporated in
cargo containment systems.
-
.2 The above properties shall correspond to the
minimum specified mechanical properties of the material, including the weld
metal in the as-fabricated condition. Subject to special consideration by
the Administration or recognized organization acting on its behalf, account
may be taken of the enhanced yield stress and tensile strength at low
temperature. The temperature on which the material properties are based
shall be shown on the International Certificate of Fitness for the Carriage
of Liquefied Gases in Bulk required in 1.4.
4.18.1.4 The equivalent stress σC
(von Mises, Huber) shall be determined by:
where:
|
σx
|
= |
total normal stress in x-direction; |
|
σy
|
= |
total normal stress in y-direction; |
|
σz
|
= |
total normal stress in z-direction; |
|
τxy
|
= |
total shear stress in x-y plane; |
|
τxz
|
= |
total shear stress in x-z plane; and |
|
τyz
|
= |
total shear stress in y-z plane. |
The above values shall be calculated as described in 4.17.3.
4.18.1.5 Allowable stresses for materials other than
those covered by chapter 6 shall be subject to approval by the Administration or
recognized organization acting on its behalf in each case.
LR 4.18-02 For materials other than those covered by Ch 6,
details of the allowable stresses are to be submitted for consideration.
4.18.1.6 Stresses may be further limited by fatigue
analysis, crack propagation analysis and buckling criteria.
4.18.2
Fatigue design condition
4.18.2.1 The fatigue design condition is the design
condition with respect to accumulated cyclic loading.
4.18.2.2 Where a fatigue analysis is required, the
cumulative effect of the fatigue load shall comply with:
where:
|
ni
|
= |
number of stress cycles at each stress level during the life of
the tank; |
|
Ni
|
= |
number of cycles to fracture for the respective stress level
according to the Wohler (S-N) curve; |
|
nLoading
|
= |
number of loading and unloading cycles during the life of the
tank, not to be less than 1000footnote. Loading and unloading cycles
include a complete pressure and thermal cycle; |
|
NLoading
|
= |
number of cycles to fracture for the fatigue loads due to
loading and unloading; and |
|
Cw
|
= |
maximum allowable cumulative fatigue damage ratio. |
The fatigue damage shall be based on the design life of the tank but not
less than 108 wave encounters.
4.18.2.3 Where required, the cargo containment system
shall be subject to fatigue analysis, considering all fatigue loads and their
appropriate combinations for the expected life of the cargo containment system.
Consideration shall be given to various filling conditions.
4.18.2.4.1 Design S-N curves used in the analysis shall
be applicable to the materials and weldments, construction details, fabrication
procedures and applicable state of the stress envisioned.
4.18.2.4.2 The S-N curves shall be based on a 97.6%
probability of survival corresponding to the mean-minus-two-standard-deviation
curves of relevant experimental data up to final failure. Use of S-N curves derived
in a different way requires adjustments to the acceptable Cw
values specified in 4.18.2.7 to 4.18.2.9.
4.18.2.5 Analysis shall be based on characteristic load
values as follows:
-
| Permanent
loads:
|
Expected
values
|
| Functional
loads:
|
Specified values or
specified history
|
| Environmental
loads:
|
Expected load history,
but not less than 108 cycles
|
If simplified dynamic loading spectra are used for the estimation of the
fatigue life, they shall be specially considered by the Administration or recognized
organization acting on its behalf.
4.18.2.6.1 Where the size of the secondary barrier is
reduced, as is provided for in 4.4.3, fracture mechanics analyses of fatigue crack
growth shall be carried out to determine:
-
.1 crack propagation paths in the
structure;
-
.2 crack growth rate;
-
.3 the time required for a crack to propagate
to cause a leakage from the tank;
-
.4 the size and shape of through thickness
cracks; and
-
.5 the time required for detectable cracks to
reach a critical state.
The fracture mechanics are, in general, based on crack growth data taken
as a mean value plus two standard deviations of the test data.
4.18.2.6.2 In analysing crack propagation, the largest
initial crack not detectable by the inspection method applied shall be assumed,
taking into account the allowable non-destructive testing and visual inspection
criterion, as applicable.
4.18.2.6.3 Crack propagation analysis under the
condition specified in 4.18.2.7: the simplified load distribution and sequence over
a period of 15 days may be used. Such distributions may be obtained as indicated in
figure 4.4. Load distribution and sequence for longer periods, such as in 4.18.2.8
and 4.18.2.9 shall be approved by the Administration or recognized organization
acting on its behalf.
4.18.2.6.4 The arrangements shall comply with 4.18.2.7
to 4.18.2.9, as applicable.
4.18.2.7 For failures that can be reliably detected by
means of leakage detection:
Predicted remaining failure development time, from the point of
detection of leakage till reaching a critical state, shall not be less than 15 days,
unless different requirements apply for ships engaged in particular voyages.
4.18.2.8 For failures that cannot be detected by
leakage but that can be reliably detected at the time of in-service inspections:
Predicted remaining failure development time, from the largest crack not
detectable by in-service inspection methods until reaching a critical state, shall
not be less than three times the inspection interval.
4.18.2.9 In particular locations of the tank, where
effective defect or crack development detection cannot be assured, the following,
more stringent, fatigue acceptance criteria shall be applied as a minimum:
Predicted failure development time, from the assumed initial defect until
reaching a critical state, shall not be less than three times the lifetime of the
tank.
4.18.3
Accident design condition
4.18.3.1 The accident design condition is a design
condition for accidental loads with extremely low probability of occurrence.
4.18.3.2 Analysis shall be based on the characteristic
values as follows:
-
| Permanent
loads:
|
Expected
values
|
| Functional loads:
|
Specified
values
|
| Environmental loads:
|
Specified
values
|
| Accidental loads:
|
Specified
values or expected values
|
4.18.3.3 Loads mentioned in 4.13.9 and 4.15 need not be
combined with each other or with wave-induced loads.