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DNV-OS-C103 Structural Design of Column Stabilised Units (LRFD method)
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SECTION 4
Ultimate Limit States (ULS)Sec.4
A. General
Sec.4
A 100 General
Sec.4 A
101 General requirements in respect to methods of analysis and
capacity checks are given in DNV-OS-C101.
Detailed considerations with respect to analysis methods and models
are given in DNV-RP-C103.
Sec.4 A
102 Both global and local capacity shall be checked with respect
to ULS. The global and local stresses shall be combined in an appropriate
manner.
Sec.4 A
103 Analytical models shall adequately describe the relevant properties
of loads, stiffness, displacement, response, and satisfactorily
account for the local system, effects of time dependency, damping
and inertia.
Sec.4 A
104 Two sets of design load combinations, a) and b) shall be checked.
Partial load factors for ULS checks of column-stabilised units according
to the present standard are given in Table A1.
Sec.4 A
| Table A1 Load factors,
Ultimate Limit States |
| Combination of design loads | Load categories |
Permanent
and variable functional loads, gf,G,Q
| Environmental loads, gf,E | Deformation loads, gf,D |
| a | 1.2 1) | 0.7 | 1.0 |
| b | 1.0 | 1.2 | 1.0 |
- If the load is not well defined, e.g. masses or functional
loads with great uncertainty, possible overfilling of tanks etc.,
the coefficient should be increased to 1.3.
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Sec.4 A
105 The loads shall be combined in the most unfavourable way,
provided that the combination is physically feasible and permitted
according to the load specifications. For permanent and variable
functional loads, a load factor of 1.0 shall be used in load combination
a) where this gives the most unfavourable response.
Sec.4 A
106 The material factor gM for
ULS yield check should be 1.15 for steel structural elements. Material
factors gM for
ULS buckling checks and bolt connections are given in DNV-OS-C101
sec.5. Material factors gM for
ULS weld connections are given in DNV-OS-C101 Sec.9.Sec.4
A 200 Global capacity
Sec.4 A
201 Gross scantlings may be utilised in the calculation of hull
structural strength, provided a corrosion protection system in accordance
DNV-OS-C101, is maintained.
Sec.4 A
202 Ultimate strength capacity check shall be performed for all
structural members contributing to the global and local strength
of the column-stabilised unit. The structures to be checked includes,
but are not limited to, the following:| — | outer skin of pontoons |
| — | longitudinal and transverse bulkheads, girders and decks in
pontoons |
| — | connections between pontoon, columns and bracings |
| — | bracings |
| — | outer skin of columns |
| — | decks, stringers and bulkheads in columns |
| — | main bearing bulkheads, frameworks and decks in the deck
structure |
| — | connection between bracings and the deck structure |
| — | connection between columns and the deck structure |
| — | girders in the deck structure. |
Sec.4
A 300 Transit condition
Sec.4 A
301 The structure shall be analysed for zero forward speed. For
units in transit with high speed, also maximum speed shall be considered
in the load and strength calculations.Guidance note:
Roll and pitch motion at resonance should be somewhat smaller than
calculated by a linear wave theory due to flow of water on top of
the pontoons. This effect may be accounted for provided rational
analysis or tests prove its magnitude.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.4 A
302 Slamming on bracings shall be considered as a possible limiting
criterion for operation in transit. The effect of forward speed
shall be accounted for in the slamming calculations. Sec.4
B. Method of Analysis
Sec.4
B 100 General
Sec.4 B
101 The analysis shall be performed to evaluate the structural
capacity due to global and local effects. Consideration of relevant
analysis methods and procedures are given in DNV-RP-C103, and in Appendix B.
Sec.4 B
102 Model testing shall be performed when significant non-linear
effects cannot be adequately determined by direct calculations.
In such cases, time domain analysis may also be considered as being
necessary. Model tests shall also be performed for new types of
column-stabilised units.
Sec.4 B
103 Where non-linear effects may be considered insignificant,
or where such loads may be satisfactorily accounted for in a linear
analysis, a frequency domain analysis may be adequately and satisfactorily
undertaken. Transfer functions for structural response shall be
established by analysis of an adequate number of wave directions,
with an appropriate radial spacing. A sufficient number of periods
shall be analysed to:| — | adequately cover the site specific
wave conditions |
| — | satisfactorily describe transfer functions at, and around, the
wave "cancellation" and "amplifying" periods |
| — | satisfactorily describe transfer functions at, and around, the
heave resonance period of the unit. |
Sec.4 B
104 Global, wave-frequency, structural responses shall be established
by an appropriate methodology, e.g.:| — | a regular wave analysis |
| — | a "design wave" analysis |
| — | a stochastic analysis. |
Sec.4 B
105 Design waves established based on the 'design wave' method,
see DNV-RP-C103, shall be based on the 90% fractile value
of the extreme response distribution (100 years return period) developed
from contour lines and short term extreme conditions.
Sec.4 B
106 A global structural model shall represent the global stiffness
and should be represented by a large volume, thin-walled three dimensional
finite element model. A thin-walled model should be modelled with
shell or membrane elements sometimes in combination with beam elements.
The structural connections in the model shall be modelled with adequately stiffness
in order to represent the actual stiffness in such a way that the
resulting responses are appropriate to the model being analysed.
The global model usually comprises: | — | pontoon shell, longitudinal
and transverse bulkheads |
| — | column shell, decks, bulkheads and trunk walls |
| — | main bulkheads, frameworks and decks for the deck structure
("secondary" decks which are not taking part in
the global structural capacity should not be modelled) |
| — | bracing and transverse beams. |
Sec.4 B
107 The global analyses should include consideration of the following
load effects as found relevant:| — | built-in stresses due to fabrication
or mating |
| — | environmental loads |
| — | different ballast conditions including operating and
survival |
| — | transit. |
Sec.4 B
108 Wave loads should be analysed by use of sink source model
in combination with a Morison model when relevant. For certain designs
a Morison model may be relevant. Details related to normal practice
for selection of models and methods are given in Appendix B.
Sec.4 B
109 When utilising stochastic analysis for world wide operation
the analyses shall be undertaken utilising North Atlantic scatter
diagram given in DNV-RP-C205.
Sec.4 B
110 For restricted operation 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.4
C. Scantlings and Weld Connections
Sec.4
C 100 General
Sec.4 C
101 Minimum scantlings for plate, stiffeners and girders are given
in DNV-OS-C101 Sec.5.
Sec.4 C
102 The requirements for weld connections are given in DNV-OS-C101
Sec.9.Sec.4
D. Air Gap
Sec.4
D 100 General
Sec.4 D
101 In the ULS condition, positive air gap should in general be
ensured for waves with a 10-2 annual
probability of exeedance. However, local wave impact may be accepted
if it is documented that such loads are adequately accounted for
in the design and that safety to personnel is not significantly impaired.
Sec.4 D
102 Analysis undertaken to check air gap should be calibrated
against relevant model test results when available. Such analysis
should take into account:| — | wave and structure interaction
effects |
| — | wave asymmetry effects |
| — | global rigid body motions (including dynamic effects) |
| — | effects of interacting systems, e.g. mooring and riser
systems |
| — | maximum and minimum draughts. |
Sec.4 D
103 Column "run-up" load effects shall be accounted
for in the design of the structural arrangement in the way of the
column and bottom plate of the deck connection. These "run-up" loads
shall be treated as environmental load component, however, they
should not be considered as occurring simultaneously with other
environmental loads.
Sec.4 D
104 Evaluation of sufficient air gap shall include consideration
of all affected structural items including lifeboat platforms, riser
balconies, overhanging deck modules etc.
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