1 General
1.1 The purpose of this standard is to provide procedures and relevant design
parameters of limit state design of fuel containment systems of a novel
configuration in accordance with section 6.4.16.
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 6.4.1.6. A limit state can be defined as a condition beyond which the
structure, or part of a structure, no longer satisfies the regulations.
1.3 The limit states are divided into the three following categories:
- .1 Ultimate Limit States (ULS), which correspond to the maximum load-carrying
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 Section 6.4.1 through to section 6.4.14 shall be complied with as applicable
depending on the fuel 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:
where:
- ɣf is load factor; and
- Fk is the characteristic load as specified in section 6.4.9
through to section 6.4.12.
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:
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 7, it may be, but not limited to, specified minimum yield
stress, specified minimum tensile strength, plastic resistance of cross
sections, and ultimate buckling strength;
- ɣR is the resistance factor, defined as ɣR
= ɣm · ɣs;
- ɣm is the partial resistance factor to take account of the
probabilistic distribution of the material properties (material factor);
- ɣs 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
- ɣC is the consequence class factor, which accounts for the
potential results of failure with regard to release of fuel and possible human
injury.
2.2 Fuel 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 fuel.
|
Medium
|
Failure implies release of the fuel and potential for human
injury.
|
High
|
Failure implies significant release of the fuel 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
fuel containment systems to these forces and motions.
3.2 Buckling strength analyses of fuel 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
fuel containment systems such as vapour pressure, fuel 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.
4.4 In cases where structural failure of the fuel containment system are considered
to imply high potential for human injury and significant release of fuel, the
consequence class factor shall be taken as ɣC = 1.2. This value
may be reduced if it is justified through risk analysis and subject to the approval
by the Administration. 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 fuel.
Conversely, higher values may be fixed by the Administration, for example, for ships
carrying more hazardous or higher pressure fuel. 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 fuel containment systems as described in
sections 6.4.2.1 to 6.4.2.5. This may be carried out by calibrating the factors
against known successful designs.
4.6 The material factor ɣm 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, the material factor ɣm may
be taken as:
- 1.1 when the characteristic mechanical properties specified by the
Administration 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
Administration 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 ɣsi 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 ɣsi shall be taken as follows:
Factors A, B, C and D are defined in 6.4.15.2.3.1. Rm and
Re are defined in 6.4.12.1.1.3.
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 fuel 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
Guidance Note:
|
The stress summation described above
shall be carried out by summing up each stress component (
σx, σy,
σxy), 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 fuel 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 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
- The stresses σms, σbp,
σbs, and σbt are defined in
4.8.4.
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. 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.
Figure 1: Definition of the three categories of section stress (Stresses
σbp and σbs are normal to the
cross section shown.)
4.9 The same factors ɣC, ɣm,
ɣsi 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 6.4.12.2 shall be complied with as
applicable depending on the fuel containment system concept. Fatigue analysis is
required for the fuel containment system designed under 6.4.16 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 ɣC and resistance factor
ɣR shall be taken as 1.0.
5.4 Fatigue damage shall be calculated as described in 6.4.12.2.2 to 6.4.12.2.2.5.
The calculated cumulative fatigue damage ratio for the fuel 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 6.4.12.2.7 to 6.4.12.2.9,
depending on the detectability of defect or crack, etc.
5.5 Lower values may be fixed by the Administration.
5.6 Crack propagation analyses are required in accordance with 6.4.12.2.6 to
6.4.12.2.9. The analysis shall be carried out in accordance with methods laid down
in a standard recognized by the Administration.
6 Accident Limit States
6.1 Accident design condition as described in 6.4.12.3 shall be complied with as
applicable, depending on the fuel 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 6.4.9.3.3.8 and 6.4.9.5 need not be combined with each other
or with environmental loads, as defined in 6.4.9.4.
6.5 Resistance factor ɣR shall in general be taken as 1.0.
6.6 Consequence class factors ɣC 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 Fuel containment systems designed according to this standard shall be tested to
the same extent as described in 16.2, as applicable depending on the fuel
containment system concept.