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DNV-OS-C201 Structural Design of Offshore Units (WSD method)
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Sec.7
A. General
Sec.7
A 100 General
Sec.7 A
101 In this standard, requirements are given in relation to fatigue
analyses based on fatigue tests and fracture mechanics. See DNV-RP-C203
for practical details with respect to fatigue design of offshore
structures. See also Sec.2 B103.
Sec.7 A
102 The aim of fatigue design is to ensure that the structure
has an adequate fatigue life. Calculated fatigue lives should also
form the basis for efficient inspection programmes during fabrication
and the operational life of the structure.
Sec.7 A
103 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 normally defined as when the
crack has grown through the thickness.
Sec.7 A
104 The S-N curves shall in general be based on a 97.6% probability
of survival, corresponding to the mean-minus-two-standard-deviation
curves of relevant experimental data.
Sec.7 A
105 The design fatigue life for the structure components should
be based on the structure service life specified. If a service life
is not specified, 20 years should be used.
Sec.7 A
106 To ensure that the structure will fulfil the intended function,
a fatigue assessment shall be carried out for each individual member,
which is subjected to fatigue loading. Where appropriate, the fatigue
assessment shall be supported by a detailed fatigue analysis. It
shall be noted that any element or member of the structure, every welded
joint and attachment or other form of stress concentration is potentially
a source of fatigue cracking and should be individually considered.
Sec.7 A
107 The analyses shall be performed utilising relevant site specific
environmental data for the area(s) in which the unit will be operated.
The restrictions shall be described in the Operation Manual for
the unit.
Sec.7 A
108 For world wide operation the analyses shall be performed utilising
environmental data (e.g. scatter diagram, spectrum) given in DNV-RP-C205.
The North Atlantic scatter diagram shall be utilised.Sec.7
A 200 Design fatigue factors
Sec.7 A
201 Design fatigue factors (DFF)
shall be applied to reduce the probability for avoiding fatigue
failures.
Sec.7 A
202 The DFFs are dependent on the significance of the structural
components with respect to structural integrity and availability
for inspection and repair.
Sec.7 A
203 DFFs shall be applied to the design fatigue life. The calculated
fatigue life shall be longer than the design fatigue life times
the DFF.
Sec.7 A
204 The design requirement may alternatively be expressed as the
cumulative damage ratio for the number of load cycles of the defined
design fatigue life multiplied with the DFF shall be less or equal
to 1.0.
Sec.7 A
205 The design fatigue factors in Table A1 are valid for units
with low consequence of failure and where it can be demonstrated
that the structure satisfies the requirement to damaged condition
according to the accidental design condition with failure in the
actual element as the defined damage.Sec.7 A
| Table A1 Design fatigue
factors (DFF) |
| DFF | Structural element | 1 | Internal structure, accessible and not welded
directly to the submerged part | | 1 | External structure, accessible for regular
inspection and repair in dry and clean conditions | | 2 | Internal structure, accessible and welded directly
to the submerged part | | 2 | External structure not accessible for inspection
and repair in dry and clean conditions | | 3 | Non-accessible areas, areas not planned to
be accessible for inspection and repair during operation | |
Guidance note:
Units intended to follow normal inspection schedule according
to class requirements, i.e. the 5-yearly inspection interval in
sheltered waters or drydock, may apply a Design Fatigue Factor (DFF)
of 1
For units inspected during operation according to DNV requirements,
the DFF for outer shell should be taken as 1. For units inspected
afloat at a sheltered location, the DFF for areas above 1 m above
lowest inspection waterline should be taken as 1, and below this
line the DFF is 2 for the outer shell. Splash zone is defined as
non-accessible area.
Where the likely crack propagation develops from a location
which is accessible for inspection and repair to a structural element
having no access, such location is itself to be deemed to have the
same categorisation as the most demanding category when considering
the most likely crack path. For example, a weld detail on the inside
(dry space) of a submerged shell plate shall be allocated the same
DFF as that relevant for a similar weld located externally on the
plate.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.7 A
206 The design fatigue factors shall be based on special considerations
where fatigue failure will entail substantial consequences such
as:| — | danger of loss of human life,
i.e. not compliance with the accidental criteria |
| — | significant pollution |
| — | major economical consequences. |
Guidance note:
Evaluation of likely crack propagation paths (including direction
and growth rate related to the inspection interval), may indicate
the use of a different DFF than that which would be selected when
the detail is considered in isolation. For example where the likely
crack propagation indicates that a fatigue failure starting in a
non critical area grows such that there might be a substantial consequence
of failure, such fatigue sensitive location should itself be deemed to
have a substantial consequence of failure.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.7 A
207 Welds beneath positions 150 m below water level should be
assumed inaccessible for in-service inspection.
Sec.7 A
208 Design fatigue factors to be applied for typical structural
details may be found in Sec.11 to
Sec.14.Sec.7
A 300 Methods for fatigue analysis
Sec.7 A
301 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.7 A
302 In fatigue critical areas where the fatigue life estimate
based on simplified methods is below the acceptable limit, a more
accurate investigation or a fracture mechanics analysis shall be
performed.
Sec.7 A
303 For calculations based on fracture mechanics, it should be
documented that the in-service inspections accommodate a sufficient
time interval between time of crack detection and the time of unstable
fracture. See DNV-RP-C203 for more details.
Sec.7 A
304 All significant stress ranges, which contribute to fatigue
damage in the structure, shall be considered. The long term distribution
of stress ranges may be found by deterministic or spectral analysis.
Dynamic effects shall be duly accounted for when establishing the
stress history.
Sec.7 A
305 Local effects, for example due to:| — | slamming |
| — | sloshing |
| — | vortex shedding |
| — | dynamic pressures |
| — | mooring and riser systems. |
shall be included in the fatigue damage assessment when relevant.
Sec.7 A
306 Principal stresses, see DNV-RP-C203, should be applied in
the evaluation of fatigue responses.
Sec.7
A 400 Simplified fatigue analysis
Sec.7 A
401 Simplified fatigue analysis may be undertaken in order to
establish the general acceptability of fatigue resistance, or as
a screening process to identify the most critical details to be
considered in a stochastic fatigue analysis, see 500.
Sec.7 A
402 Simplified fatigue analyses should be undertaken utilising
appropriate conservative design parameters. A two-parameter, Weibull
distribution, see DNV-RP-C203, may be utilised to describe the long-term
stress range distribution:
| n0 | = | total number of stress cycles during the lifetime
of the structure |
| = | extreme stress range (MPa) that is exceeded
once out of n0 stress
cycles. The extreme stress amplitude:
is
thus given by
|
| h | = | shape parameter of the Weibull stress range
distribution |
| = | intercept of the design S-N curve with the
log N axis |
| = | complete gamma function |
| m | = | inverse slope of the S-N curve |
| DFF | = | Design Fatigue Factor. |
Sec.7 A
403 The simplified fatigue evaluation should be based on dynamic
stress from a global analysis, with the stresses scaled to the return
period of the minimum fatigue life of the unit. In such cases, scaling
may be undertaken utilising the appropriate factor found from the
following:
| ni | = | number of stress variations in i years appropriate
to the global analysis |
| = | extreme stress range that is exceeded once
out of ni stress variations. |
Sec.7
A 500 Stochastic fatigue analysis
Sec.7 A
501 Stochastic fatigue analyses shall be based upon recognised
procedures and principles utilising relevant site specific data
or North Atlantic environmental data, see 107 and 108.
Sec.7 A
502 Simplified fatigue analyses may be used as a "screening" process
to identify locations for which a detailed, stochastic fatigue analysis
should be undertaken.
Sec.7 A
503 Fatigue analyses shall include consideration of the directional
probability of the environmental data. Provided it can be satisfactorily
verified, scatter diagram data may be considered as being directionally
specific. Relevant wave spectra and energy spreading shall be utilised
as relevant.
Sec.7 A
504 Structural response shall be determined based upon analyses
of an adequate number of wave directions. Transfer functions shall
be established based upon consideration of a sufficient number of
periods, such that the number, and values of the periods analysed:| — | adequately cover the distribution
of wave energy over the frequency range |
| — | satisfactorily describe transfer functions at, and around,
the wave "cancellation" and "amplifying" periods |
| — | satisfactorily describe transfer functions at, and around,
the relevant natural periods of the unit. |
It should be considered that "cancellation" and "amplifying" periods
may be different for different elements within the structure.
Sec.7 A
505 Stochastic fatigue analyses utilising simplified structural
model representations of the unit (e.g. a space frame model) may
be used to form basis for identifying locations for which a stochastic
fatigue analysis, utilising a detailed model of the structure, should
be undertaken, e.g. at critical intersections.
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