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DNV-OS-C103 Structural Design of Column Stabilised Units (LRFD method)
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SECTION 5
Fatigue Limit States (FLS)Sec.5
A. General
Sec.5
A 100 General
Sec.5 A
101 General requirements for the fatigue limit states are given
in DNV-OS-C101 Sec.6. Guidance concerning fatigue calculations are
given in DNV-RP-C203.
Sec.5 A
102 Units intended to follow normal inspection requirements according
to class requirements, i.e. 5 yearly inspection in sheltered waters
or drydock, may apply a Design Fatigue Factor (DFF) of 1.0.
Sec.5 A
103 Units intended to stay on location for prolonged survey period,
i.e. without planned sheltered water inspection, shall comply with
the requirements given in Appendix A.
Sec.5 A
104 The design fatigue life of the unit shall be minimum 20 years.
Sec.5 A
105 The fatigue capacity of converted units will be considered
on a case-by-case basis, and is a function of the following parameters:
| — | results and findings form surveys
and assessment of critical details |
| — | service history of the unit and estimated remaining
fatigue life. |
Guidance note:
New structural steel on converted units older than 10 years,
may normally be accepted with minimum 15 years documented fatigue
life from the time of conversion.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.5 A
106 Local effects, e.g. due to:| — | slamming |
| — | sloshing |
| — | vortex shedding |
| — | dynamic pressures |
| — | mooring and riser systems. |
shall be included in the fatigue damage assessment when relevant.
Sec.5 A
107 In the assessment of fatigue resistance, relevant consideration
shall be given to the effects of stress concentrations including
those occurring as a result of:
| — | fabrication tolerances, including
due regard to tolerances in way of connections involved in mating
sequences or section joints |
| — | cut-outs |
| — | details at connections of structural sections, e.g.
cut-outs to facilitate construction welding |
| — | attachments. |
Sec.5 A
108 Local detailed finite element analysis of critical connections,
e.g. pontoon and pontoon, pontoon and column, column and deck and
brace connections, should be undertaken in order to identify local
stress distributions, appropriate SCF's, and/or extrapolated
stresses to be utilised in the fatigue evaluation. Dynamic stress
variations through the plate thickness shall be checked and considered
in such evaluations, see DNV-RP-C203, for further details.
Sec.5 A
109 For well known details the local finite element analysis may
be omitted, provided relevant information regarding SCF are available.
Sec.5 A
110 Principal stresses, see DNV-RP-C203 Sec.2.2, should be applied
in the evaluation of fatigue responses.Sec.5
B. Fatigue Analysis
Sec.5
B 100 General
Sec.5 B
101 The basis for determining the acceptability of fatigue resistance,
with respect to wave loads, shall be in accordance with the requirements
given in Appendix B. The required
models and methods are dependent on type of operation, environment
and design type of the unit.Sec.5
B 200 World-wide operation
Sec.5 B
201 For world wide operation the analyses shall be undertaken
utilising environmental data, e.g. scatter diagram, spectrum, given
in DNV-RP-C205. The North Atlantic scatter diagram shall be utilised.Sec.5
B 300 Restricted operation
Sec.5 B
301 The analyses shall be undertaken utilising relevant site specific
environmental data for the area(s) the unit will be operated. The
restrictions shall be described in the operation manual for the
unit. Sec.5
B 400 Simplified fatigue analysis
Sec.5 B
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.5 B
402 Simplified fatigue analyses should be undertaken utilising
appropriate conservative design parameters. A two-parameter, Weibull
distribution, see DNV-RP-C203, Sec.2.14, may be utilised to describe
the long-term stress range distribution. In such cases the Weibull
shape parameter 'h', see 403 for a two-pontoon
semisubmersible unit should have a value of h = 1.1.
Sec.5 B
403 The following formula may be used for simplified fatigue evaluation:
Sec.5 B
404 A simplified fatigue evaluation shall be based on dynamic
stresses from design waves analysed in the global analysis as described
in Sec.4 B. The stresses should
be 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 | = | the number of stress variations in i
years appropriate to the global analysis |
| = | the extreme stress range (MPa) that is
exceeded once out of ni stress
variations. |
Sec.5
B 500 Stochastic fatigue analysis
Sec.5 B
501 Stochastic fatigue analyses shall be based upon recognised
procedures and principles utilising relevant site specific data
or North Atlantic environmental data.
Sec.5 B
502 Simplified fatigue analyses should be used as a "screening" process
to identify locations for which a detailed, stochastic fatigue analysis
should be undertaken.
Sec.5 B
503 Fatigue analyses shall include consideration of the directional
probability of the environmental data. Providing that it can be
satisfactorily checked, scatter diagram data may be considered as
being directionally specific. Scatter diagram for world wide operations
(North Atlantic scatter diagram) is given in DNV-RP-C205. Relevant
wave spectra and energy spreading shall be utilised as relevant.
A Pierson-Moskowitz spectrum and a cos4 spreading
function should be utilised in the evaluation of column-stabilised
units.
Sec.5 B
504 Structural response shall be determined based upon analyses
of an adequate number of wave directions. Transfer functions should
be established based upon consideration of a sufficient number of
periods, such that the number, and values of the periods analysed:| — | adequately cover the wave data |
| — | satisfactorily describe transfer functions at, and around, the
wave "cancellation" and "amplifying" periods
(consideration should be given to take into account that such "cancellation" and "amplifying" periods
may be different for different elements within the structure) |
| — | satisfactorily describe transfer functions at, and around, the
relevant excitation periods of the structure. |
Sec.5 B
505 Stochastic fatigue analyses utilising simplified structural
model representations of the unit, e.g. a space frame model, may
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. See also Appendix B for more details regarding
models and methods.
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