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DNV-OS-C501 Composite Components
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SECTION 2
Design Philosophy and Design PrinciplesSec.2
A. General
Sec.2
A 100 Objective
Sec.2 A
101 The purpose of this section is to identify and address key issues
which need to be considered for the design, fabrication, and operation
of FRP components and structures. Furthermore, the purpose is to
present the safety philosophy and corresponding design format applied
throughout this Standard.Sec.2
B. Safety philosophy
Sec.2
B 100 General
Sec.2 B
101 An overall safety objective is to be established, planned and
implemented covering all phases from conceptual development until
abandonment of the structure.
Sec.2 B
102 This Standard gives the possibility to design structures or
structural components with different structural safety requirements,
depending on the Safety Class to which the structure or part of
the structure belongs. Safety classes are based on the consequence
of failures related to the Ultimate Limit State (ULS).
Sec.2 B
103 Structural reliability of the structure is ensured by the use
of partial safety factors that are specified in this Standard. Partial
safety factors are calibrated to meet given target structural reliability
levels. Note that gross errors are not accounted for. Gross errors
have to be prevented by a quality system. The quality system shall
set requirements to the organisation of the work, and require minimum
standards of competence for personnel performing the work. Quality
assurance shall be applicable in all phases of the project, like
design, design verification, operation, etc.Sec.2
B 200 Risk assessment
Sec.2 B
201 To the extent it is practically feasible, all work associated
with the design, construction and operation shall ensure that no
single failure is to lead to life-threatening situations for any
persons, or to unacceptable damage to material or to environment.
Sec.2 B
202 A systematic review or analysis shall be carried out at all phases
to identify and evaluate the consequences of single failures and
series of failure in the structure such that necessary remedial
measures may be taken. The extent of such a review is to reflect
the criticality of the structure, the criticality of planned operations,
and previous experience with similar structures or operations.Guidance note:
A methodology for such a systematic review is the Quantitative Risk
Analysis (QRA) which may provide an estimation of the overall risk
to human health and safety, environment and assets and comprises
(i) hazard identification, (ii) assessment of probability of failure
events, (iii) accident development and (iv) consequence and risk
assessment. It should be noted that legislation in some countries
requires risk analysis to be performed, at least at an overall level
to identify critical scenarios, which may jeopardise the safety
and reliability of the structure. Other methodologies for identification
of potential hazards are Failure Mode Effect Analysis (FMEA) and
Hazardous Operations studies (HAZOP).---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.2
B 300 Quality Assurance
Sec.2 B
301 The safety format of this Standard requires that gross errors
(human errors) shall be controlled by requirements to the organisation
of the work, competence of persons performing the work, verification
of the design and Quality Assurance during all relevant phases.Sec.2
C. Design format
Sec.2
C 100 General principles
Sec.2 C
101 The basic approach of the Limit State Design method consists
in recognising the different failure
modes related to each functional
requirement and associating to each mode of failure
a specific limit state beyond
which the structure no longer satisfies the functional requirement.
Different limit states are defined, each limit state being related
to the kind of failure mode and its anticipated consequences.
Sec.2 C
102 The design analysis consists in associating each failure mode
to all the possible failure mechanisms (i.e.
the mechanisms at the material level). A design equation or a failure
criterion is defined for each failure mechanism, and failure becomes
interpreted as synonymous to the design equation no longer being
satisfied.
Sec.2 C
103 The design equations are formulated in the so-called Load
and Resistance Factor Design (LRFD) format, where partial safety factors (load factors
and resistance factors) are applied to the load effects (characteristic
load values) and to the resistance variables (characteristic resistance
values) that enter the design equations.
Sec.2 C
104 The partial safety factors, which are recommended in this
Standard, have been established such that acceptable and consistent
reliability levels are achieved over a wide range of structure configurations
and applications.
Sec.2 C
105 This section discusses the limit states that have been considered
relevant for the design of structures made of FRP materials, presents
the underlying safety considerations for the recommended safety
factors and finally introduces the adopted LRFD format.
Sec.2 C
106 As an alternative to the LRFD format a recognised Structural
Reliability Analysis (SRA) may be applied. The conditions for application
of an SRA are discussed at the end of this section.Sec.2
C 200 Limit states
Sec.2 C
201 The following two limit state categories shall be considered
in the design of the structure:
| — | Ultimate Limit State (ULS) |
| — | Serviceability Limit State (SLS). |
Sec.2 C
202 The Ultimate Limit State shall
be related to modes of failure for which safety is an issue. The
ULS generally corresponds to the maximum load carrying capacity
and is related to structural failure modes. Safety
Classes are defined in accordance with the consequences
of these failure modes on safety, environment and economy. The ULS
is not reversible.
Sec.2 C
203 The Serviceability Limit State should
be related to failure modes for which human risks or environmental
risks are not an issue. The SLS is usually related to failure modes
leading to service interruptions or restrictions. Service Classes are defined in accordance
with the frequency of service interruptions due these modes of failure.
The SLS is usually reversible, i.e. after repair or after modification
of the operating conditions (e.g. interruption of operation, reduction
of pressure or speed) the structure will again be able to meet its
functional requirements in all specified design conditions.Guidance note:
Ultimate Limit States correspond to, for example:| - | loss of static equilibrium
of the structure, or part of the structure, considered as a rigid
body| - | rupture of critical sections of the structure caused
by exceeding the ultimate strength or the ultimate deformation of
the material| - | transformation of the structure into a mechanism (collapse).| - | loss of stability (buckling, etc...) | | | |
Serviceability Limit States corresponds to, for example:| - | deformations which affect the
efficient use or appearance of structural or non-structural elements| - | excessive vibrations producing discomfort or affecting
non-structural elements or equipment| - | local damage (including cracking) which reduces the
durability of the structure or affects the efficiency or appearance
of structural or non-structural elements. | | |
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Sec.2
C 300 Safety classes and Service classes
Sec.2 C
301 Safety classes are
based on the consequences of failure when the mode of failure is
related to the Ultimate Limit State. The operator shall specify
the safety class according to which the structure shall be designed.
Suggestions are given below.
Sec.2 C
302 Safety classes are defined as follows:| — | Low Safety Class, where failure of the structure
implies small risk of human injury and minor environmental, economic
and political consequences. |
| — | Normal Safety Class, where failure of the structure
implies risk of human injury, significant environmental pollution or
significant economic or political consequences. |
| — | High Safety Class, where failure of the structure implies risk
of human injury, significant environmental pollution or very high
economic or political consequences. |
Sec.2 C
303 Service classes are
based on the frequency of service interruptions or restrictions
caused by modes of failure related to the Serviceability Limit
State. These modes of failure imply no risk of human injury and
minor environmental consequences. The operator shall specify the
service class according to which the structure shall be designed.
Suggestions are given below.
Sec.2 C
304 Service classes are defined according to the annual number
of service failures. The Normal and High
Service Classes are defined by the target reliability
levels indicated in Table C1.Sec.2
C 400 Failure types
Sec.2 C
401 Failure types are based
on the degree of pre-warning intrinsic to a given failure mechanism.
A distinction shall be made between catastrophic and progressive
failures, and between failures with or without reserve capacity
during failure. The failure types for
each failure mechanism described in this Standard are specified
according to the following definitions:| — | ductile,
corresponds to ductile failure mechanisms with reserve strength
capacity. In a wider sense, it corresponds to progressive non-linear
failure mechanisms with reserve capacity during failure. |
| — | plastic, corresponds
to ductile failure mechanisms without reserve strength capacity.
In a wider sense, it corresponds to progressive non-linear failure
mechanisms but without reserve capacity during failure. |
| — | brittle, corresponds
to brittle failure mechanisms. In a wider sense, it corresponds
to non-stable failure mechanisms. |
Sec.2 C
402 The different failure types should be used under the following
conditions for materials that show a yield point:| — | failure type ductile may
be used if:
sult > 1.3 syield and eult > 2 eyield |
| — | failure type plastic may
be used if:
sult ³ 1.0 syield and eult > 2 eyield |
| — | in all other cases failure type brittle shall
be used. |
Where sult is
the ultimate strength at a strain eult and
syield is the yield strength at
a strain eyield.
Sec.2
C 500 Selection of partial safety factors
Sec.2 C
501 Partial safety factors depend on the safety class and the failure
type. The partial factors are available for five different levels
and are listed in Section 8.
Sec.2 C
502 The selection of the levels is given in the table C1 for the ultimate
limit state.Sec.2 C
| Table C1 Target reliability
levels for ULS |
| SAFETY CLASS | FAILURE TYPE |
| Ductile/Plastic | Brittle |
| Low | A | B |
| Normal | B | C |
| High | C | D |
Sec.2 C
503 The recommended selection of the levels for the serviceability
limit state is given in the table C2.Sec.2 C
| Table C2 Target reliability
levels for SLS |
| SERVICE
CLASS | SERVICE
FAILURES |
| Normal | A |
| High | B |
Sec.2
C 600 Design by LRFD method
Sec.2 C
601 The Partial Safety Factor format (or Load and Resistance Factor
Design, LRFD) separates the influence of uncertainties and variability
originating from different causes. Partial safety factors are assigned
to variables such as load effect and resistance variables. They
are applied as factors on specified characteristic values of these
load and resistance variables, thereby defining design values of
these variables for use in design calculations, and thereby accounting
for possible unfavourable deviations of the basic variables from
their characteristic values. The characteristic values of the variables
are selected representative values of the variables, usually specified
as specific quantiles in their respective probability distributions,
e.g. an upper-tail quantile for load and a lower-tail quantile for
resistance. The values of the partial safety factors are calibrated,
e.g. by means of a probabilistic analysis, such that the specified
target reliability is achieved whenever the partial safety factors
are used for design. Note that characteristic values and their associated
partial safety factors are closely linked. If the characteristic
values are changed, relative to the ones determined according to
procedures described elsewhere in this document, then the requirements
to the partial safety factors will also change in order to maintain
the intended target reliability level. Guidance note:
The following uncertainties are usually considered:| - | Uncertainties in the loads,
caused by natural variability, which is usually a temporal variability| - | Uncertainties in the material properties, caused by
natural variability, which is usually a spatial variability| - | Uncertainties in the geometrical parameters, caused
by| - | deviations of the geometrical
parameters from their characteristic (normal) value| - | tolerance limits| - | cumulative effects of a simultaneous occurrence of several
geometrical variation | | |
| - | Uncertainties in the applied engineering models| - | uncertainties in the models
for representation of the real structure or structural elements| - | uncertainties in the models for prediction of loads,
owing to simplifications and idealisations made| - | uncertainties in the models for prediction of resistance, owing
to simplifications and idealisations made| - | effect of the sensitivity of the structural system (under-
or over-proportional behaviour) | | | |
| | | |
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Sec.2 C
602 Partial safety factors are applied in design inequalities for
deterministic design as shown by examples in 606.
The partial safety factors are usually or preferably calibrated
to a specified target reliability by means of a probabilistic analysis. Sometimes
the design inequalities include model factors or bias correction
factors as well. Such model or bias correction factors appear in
the inequalities in the same manner as the partial safety factors,
but they are not necessarily to be interpreted as partial safety
factors as they are used to correct for systematic errors rather
than accounting for any variability or uncertainty. Model factors
and bias correction factors are usually calibrated experimentally.
Sec.2 C
603 The following two types of partial safety factors are used in
this standard:| — | Partial
load effect factors, designated in this standard by gF . |
| — | Partial resistance factors,
designated in this standard by gM . |
Sec.2 C
604 In some cases it is useful to work with only one overall safety
factor. The uncertainties in loads and resistance are then accounted
for by one common safety factor denoted gFM.
The following simple relationship between this common safety factor
on the one hand and the partial load and resistance factors on the
other are assumed here corresponding to the general design inequality
quoted in 606:gFM= gF x gM
Sec.2 C
605 The following two types of model factors are used in this Standard:| — | Load
model factors, designated in this Standard by gSd . |
| — | Resistance model factors, designated
in this Standard by gRd . |
Guidance note:
| - | Partial load effect factors gF are applicable to the characteristic
values of the local response of the structure. They account for
uncertainties associated with the variability of the local responses
of the structure (local stresses or strains). The uncertainties
in the local response are linked to the uncertainties on the loads
applied to the structure through the transfer function.
| - | Partial resistance factors gM account for uncertainties associated
with the variability of the strength. | - | Load model factors gSd account
for inaccuracies, idealisations, and biases in the engineering model
used for representation of the real response of the structure, e.g.
simplifications in the transfer function (see section 9). For example, wind characterised
by a defined wind speed will induce wind loads on the structure,
and those loads will induce local stresses and strains in the structure.
The load model factor account for the inaccuracies all the way from
wind speed to local response in the material.| - | Resistance model factors gRd account for differences between true
and predicted resistance values, e.g. differences between test and
in-situ materials properties (size effects), differences associated
with the capability of the manufacturing processes (e.g. deviations
of the geometrical parameters from the characteristic value, tolerance
limits on the geometrical parameters), and differences owing to
temporal degradation processes.| - | Uncertainties or biases in a failure criterion are accounted
for by the resistance model factor.| - | Geometrical uncertainties and tolerances should be included in
the load model factor. | | | | | |
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Sec.2 C
606 A factored design load effect is obtained by multiplying a
characteristic load effect by a load effect factor. A factored design
resistance is obtained by dividing the characteristic resistance
by a resistance factor.
The structural reliability is
considered to be satisfactory if the following design inequalities
are satisfied:General design inequality for the Load Effect and Resistance Factor
Design format:
where,
| gF | partial load effect factor |
| gSd | load model factor |
| Sk | characteristic load effect |
| Rk | characteristic resistance |
| gM | partial resistance factor |
| gRd | resistance model factor. |
Design rule expressed in terms of forces and moments:
where,
| F | code check function (e.g. buckling equation) |
| gF | partial load or load effect factor |
| gSd | load model factor |
| Sk | characteristic load or load effect |
| Rk | characteristic resistance |
| gM | partial resistance factor |
| gRd | resistance model factor. |
Design rule expressed in terms of a local response such as
local strains:
where,
| F | code check function |
| gF | partial load effect factor |
| gSd | load model factor |
| ek | characteristic value of the local response of the structure (strain)
to applied load Sk |
k | characteristic value of strain to failure |
| Rk | characteristic resistance |
| gM | partial resistance factor |
| gRd | resistance model factor. |
Sec.2 C
607 The load model factor shall be applied on the characteristic
local stresses or strains. The resistance model factors apply on
the characteristic resistance of the material used at the location
on the structure where the design rule is to be applied.
Sec.2 C
608 The characteristic values for load effects and resistance variables
are specified as quantiles of their respective probability distributions.
Sec.2 C
609 The characteristic load effect, Sk,
is a value that should rarely be exceeded. For time dependent processes,
it is generally given in terms of return values for occurrence,
e.g., once in a given reference time period (return period). See
section 3 I400 for characteristic loads.
Sec.2 C
610 The characteristic resistance, Rk,
is a value corresponding to a high probability of exceedance, also
accounting for its variation with time when relevant. See section 4 A600 and section 5 A600
for characteristic resistance.
Sec.2 C
611 The partial safety factors are calibrated against the target reliabilities
indicated in Tables C1 and C2. See
also Section 8.
Sec.2 C
612 The partial safety factors defined in this Standard apply to
all failure mechanisms and all safety- and service classes. They
depend on the target reliability, the load distribution type (or
the local response distribution type when applicable) and its associated
coefficient of variation, and on the coefficient of variation associated
with the resistance. When several loads are combined, a combination
factor shall be used with the same set of partial factors. The combination
of several loads is described in section
3 K.
Sec.2 C
613 The load model factors depend on the method used for the structural
analysis. See section 8 C and section 9 L.
Sec.2 C
614 The resistance model factors depend on the uncertainties in
the material strength properties caused by manufacturing, installation
and degradation. See section 8 B.Sec.2
C 700 Structural Reliability Analysis
Sec.2 C
701 As an alternative to design according to the LRFD format specified
and used in this Standard, a recognised Structural Reliability Analysis
(SRA) based design method in compliance with Classification Note
No. 30.6 'Structural Reliability Analysis of Marine Structures' or
ISO 2394 may be applied provided it can be documented that the approach
provides adequate safety for familiar cases as indicated in this Standard.
Sec.2 C
702 The Structural Reliability Analysis is to be performed by suitably
qualified personnel.
Sec.2 C
703 As far as possible, target reliabilities are to be calibrated against
identical or similar designs that are known to have adequate safety.
If this is not feasible, the target reliability is to be based on
the limit state category, the failure type and the Safety or Service
Class as given in Table C3 and Table C4.Sec.2 C
| Table C3 Target annual
failure probabilities PFT |
| | Failure consequence |
| Failure
type | LOW
SAFETY CLASS | NORMAL
SAFETY CLASS | HIGH SAFETY CLASS |
| Ductile failure type (e.g.
as for steel) |
PF = 10-3 |
PF = 10-4 |
PF = 10-5 |
Brittle failure type
(basis
case for composite) |
PF = 10-4 |
PF = 10-5 |
PF = 10-6 |
Sec.2 C
| Table C4 Target reliabilities
in the SLS expressed in terms of annual probability of failure |
| SERVICE
CLASS | SERVICE
FAILURES |
| Normal | 10-3 |
| High | 10-4 |
Sec.2
D. Design approach
Sec.2
D 100 Approaches
Sec.2 D
101 The structure can be designed according to three different
approaches:| — | An analytical approach, i.e.
the stress/strain levels at all relevant parts of the structures
including the interfaces and joints are determined by means of a
stress analyses (e.g. a FEM-analyses, see section
9) and compared with the relevant data on the mechanical
strength. |
| — | Design by component testing only, i.e. full scale or
scaled down samples of the structure or parts of the structure are tested
under relevant conditions (see section
10) such that the characteristic strength of the complete
structure can be determined. |
| — | A combination of an analytical approach and testing,
i.e. the same approach specified in section 10 for updating in combination
with full scale component testing. |
Sec.2 D
102 The structure shall be designed such that none of the failure
mechanisms, identified in the design analysis (see section 3 and 6),
will occur for any of the design cases specified in section 3. The
design against each individual failure mechanism can be checked
with the help of one of the three approaches mentioned in 101.Sec.2
D 200 Analytical approach
Sec.2 D
201 The level of all stress (strain) components in all relevant areas
of the structure, including stress concentrations, shall be determined
according to section 9.
Sec.2 D
202 Failure criteria and safety factors are applied to the load effects,
i.e., the local stresses or strains.
Sec.2 D
203 The analysis provides the link between load and load effect.
If non-linear effects change the mean, distribution type and COV
of the load effect relative to the load itself, the properties of
the load effect shall be used to determine safety factors.
Sec.2 D
204 The partial factors in Section
8 shall be used. Sec.2
D 300 Component testing
Sec.2 D
301 The purpose of this approach is to define the characteristic
strength of the finished and complete structure under relevant load
conditions. If deemed relevant, the resistance may be found by testing
scaled models or parts of the finished structure.
Sec.2 D
302 Details about component testing are given in Section 10 and 7.
Sec.2 D
303 A sufficiently large number of tests shall be carried out in
order to be able to define the characteristic strength of the structure
with a confidence level at least as large as required for the data
used with the analytical approach.
Sec.2 D
304 The failure mode(s), failure mechanism(s) and location(s)
of failure shall be verified during and or after the tests.Sec.2
D 400 Analyses combined with updating
Sec.2 D
401 Analyses of the structure may be complicated and a conservative
bias may have to be introduced in the analyses. The reasons for
such biases may be:| — | Scaling effects. |
| — | Uncertainties in the relevance of the design rules,
e.g. in areas with large stress gradients. |
| — | The analytical models for analysing the stress level
in the structure. |
| — | The effect of the environment on the mechanical properties. |
| — | Etc. |
Sec.2 D
402 In such cases the analyses that have been carried out may be
combined with the procedure for updating given in Section 10C. The purpose of this
approach is to update the predicted resistance of the structure
with the results from a limited number of tests in a manner consistent
with the reliability approach of the standard.
Sec.2 D
403 It is a basic assumption that that all biases are handled
in a conservative way, i.e. that the bias lead to a conservative
prediction of the resistance of the structure.Sec.2
E. Requirements to documentation
Sec.2
E 100 Design Drawings and Tolerances
Sec.2 E
101 Design drawings shall be provided according to general standards.
Sec.2 E
102 Tolerances shall be indicated. Sec.2
E 200 Guidelines for the design report
Sec.2 E
201 The design Report should contain the following as a minimum:| — | Description of the entire structure
and of its components. |
| — | Design input as described in Section 3, including design life,
environmental conditions. |
| — | Relevant design assumptions and conditions including applicable
limitations. |
| — | Description of analysis from design phase, evaluation
of problem areas, highly utilised and critical areas of the structure
and highlighting points that require special attention during subsequent
phases. |
| — | Reference to accepted calculations and other documents verifying
compliance with governing technical requirements for all phases. |
| — | Fabrication procedures giving a concentrated description of
the manufacturing/ fabrication history, reference to specifications,
drawings etc., discussion of problem areas, deviations from specifications
and drawings, of importance for the operational phase identification
of areas deemed to require special attention during normal operation
and maintenance. |
| — | Reference to documentation needed for repair and modification. |
Sec.2 E
202 All failure modes and failure mechanisms shall be clearly
identified and listed in a systematic way, preferably in a
table.
It shall be shown that each combination of identified failure modes
and mechanisms was addressed in the design.
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