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DNV-OS-C102 Structural Design of Offshore Ships
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SECTION 8
Fatigue Capacity Assessment Sec.8
A. Introduction
Sec.8
A 100 General
Sec.8 A
101 This section gives provisions
for assessment of fatigue capacity of the structural details in
the unit. The assessment shall account for all significant loads
contributing to fatigue damage.
Sec.8 A
102 In the assessment of
fatigue life, consideration shall be given to the stress concentration
factors from fabrication imperfections which exceed the values included
in the S-N curves.
Sec.8 A
103 Hull vibration is not
covered by this standard, but should be included if relevant.Sec.8
B. Principles and Methodology
Sec.8
B 100 Assessment principles
Sec.8 B
101 The dynamic response
for all relevant variable loading conditions shall be considered
in the fatigue calculation.
Sec.8 B
102 The
accumulated fatigue damage from both transit and operating conditions
shall be calculated according to the operational characteristics
of the unit. Appropriate fraction of time in each condition and
wave headings shall be considered.
Sec.8 B
103 The fatigue life shall
be calculated considering the combined effects of global and local
structural response.
Sec.8 B
104 The resistance against
fatigue is normally given as S-N curves, i.e. stress range (S) versus
number of cycles to failure (N) based on fatigue tests. Fatigue
failure is defined as when the crack has grown through the thickness.
Sec.8 B
105 Gross scantling may be utilized in the fatigue calculation.Sec.8
B 200 Methods for fatigue capacity
Sec.8 B
201 The fatigue analysis
should be based on S-N data, determined by fatigue testing of the
considered welded detail, and the linear damage hypothesis. When
appropriate, the fatigue analysis may alternatively be based on fracture
mechanics.
Sec.8 B
202 Acceptable analysis methods
for calculation of the accumulated damage are given in DNV Classification Note
30.7 (CN 30.7) and DNV-RP-C203. Analysis methods, e.g. simplified
or spectral methodology, should be specified in the structural design
brief.
Sec.8 B
203 When a wave load analysis is used for a spectral fatigue analysis,
the design basis for transit and operating condition as specified
in Sec.3 Table B1 and B2 shall be
applied.
Sec.8 B
204 For detailed consideration on design loading conditions and
mean stress effect, see unit specific provisions Sec.11 and Sec.12.Sec.8
C. Structural Details and Stress Concentration
Factor (SCF)
Sec.8
C 100 General
Sec.8 C
101 Fatigue sensitive details
in the hull and topside supporting structures shall be documented
to have sufficient fatigue strength. Areas to be checked are given
in unit specific provision Sec.11 and Sec.12.
Sec.8 C
102 Stress concentration
factors of local details may be determined according to CN 30.7.
For details not covered by CN 30.7, or documented in other recognised
publications, detailed FE analysis shall be carried out for determination
of SCFs, according to the procedure given in CN 30.7.Sec.8
D. Design Loads and Calculation of Stress
Ranges
Sec.8
D 100 Wave environment
Sec.8 D
101 The wave date in transit
and operating conditions shall be determined according to unit specific provisions
Sec.11 and Sec.12.Sec.8
D 200 Wind and current
Sec.8 D
201 The effect of wind may
be omitted except for special structures subject to significant
wind exposure, e.g. flare tower, derrick, etc.Sec.8
D 300 Local and global loads
Sec.8 D
301 Typical local load effects
to be considered are:
| — | vortex shedding |
| — | external sea pressure |
| — | tank pressure |
| — | variation of filling level in cargo tanks (low cycle). |
Sec.8 D
302 Typical global loads
to be considered are:| — | wave bending moments and shear
forces |
| — | horizontal and vertical hull deformations/deflections |
| — | wave induced accelerations (inertia loads). |
Sec.8 D
303 The global and local
load effects shall be combined according to the procedures given
in CN 30.7.Sec.8
E. Design Fatigue Factor (DFF)
Sec.8
E 100 General
Sec.8 E
101 DFF is required for different structural elements based on
the consequences of failure and accessibility for in service inspection
and repair.
Sec.8 E
102 The required service life of new units shall
be minimum 20 years assuming that the unit complies with the DNV
requirements for inspection.
Sec.8 E
103 For additional consideration on DFF, see unit specific provisions
Sec.11 and Sec.12.
Sec.8 E
104 Substantial consequences other than pure strength considerations
may require higher design fatigue factors. Such factors should be
specified in the structural design brief.Guidance note:
When defining the appropriate design fatigue factor for a
specific fatigue sensitive detail, consideration shall be given to
the following: Evaluation of likely crack propagation paths (including
direction and growth rate related to the inspection interval), may
indicate the use of a higher design fatigue factor, such that:- Where the likely crack propagation
indicates that a fatigue failure affects another detail with a higher
design fatigue factor.
- Where the likely crack propagation is from a location
satisfying the requirement for a given "Access for inspection
and repair" category to a structural element having another
access categorisation.
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