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DNV-OS-C201 Structural Design of Offshore Units (WSD method)
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SECTION 2
Design PrinciplesSec.2
A. Introduction
Sec.2
A 100 General
Sec.2 A
101 This section describes design principles and design methods
including:
| — | working stress design method |
| — | design assisted by testing |
| — | probability based design. |
Sec.2 A
102 General design considerations regardless of design method
are also given in B.
Sec.2 A
103 This standard is based on the working stress design (WSD)
method also known as the allowable stress method.
Sec.2 A
104 Direct reliability analysis methods are mainly considered
as applicable to special case design problems, to calibrate the
usage factors to be used in the WSD method and for conditions where
limited experience exists.
Sec.2 A
105 As an alternative or as a supplement to analytical methods,
determination of load effects or resistance may in some cases be
based either on testing or on observation of structural performance
of models or full-scale structures.Sec.2
A 200 Aim of the design
Sec.2 A
201 Structures and structural elements shall be designed to:| — | sustain loads liable to occur
during all temporary, operating and damaged conditions if required |
| — | maintain acceptable safety for personnel and environment |
| — | have adequate durability against deterioration during
the design life of the structure. |
Sec.2
B. General Design Considerations
Sec.2
B 100 General
Sec.2 B
101 The design of a structural system, its components and details
should, as far as possible, account for the following principles:| — | resistance against relevant
mechanical, physical and chemical deterioration is achieved |
| — | fabrication and construction comply with relevant, recognised
techniques and practice |
| — | inspection, maintenance and repair are possible. |
Sec.2 B
102 Structures and elements thereof, shall possess ductile resistance
unless the specified purpose requires otherwise.
Sec.2 B
103 Fatigue life improvements with methods such as grinding or
hammer peening of welds should not provide a measurable increase
in the fatigue life at the design stage. The fatigue life should
instead be extended by means of modification of structural details.
Fatigue life improvements due to compression stress level should
not be considered for welded structure, ref. DNV-RP-C203.
Sec.2 B
104 Structural elements may be fabricated according to the requirements
given in DNV-OS-C401.Sec.2
B 200 Overall design
Sec.2 B
201 The overall structural safety shall be evaluated on the basis
of preventive measures against structural failure put into design,
fabrication and in-service inspection as well as the unit's
residual strength against total collapse in the case of structural
failure of vital elements.For vital elements, which are designed according to criteria
given for intact structure, the likelihood and consequence of failure
should be considered as part of the redundancy evaluations. The
consequence of credible accidental events shall be documented according
to Sec.8.
Sec.2 B
202 When determining the overall structural design, particular
care shall be taken such that the solution does not lead to unnecessarily
complicated connections.
Sec.2
B 300 Details design
Sec.2 B
301 In the design phase particular attention should be given to
structural detailing, and requirements for reinforcement in areas
that may be subjected to high local stresses, for example:| — | critical connections |
| — | locations that may be subjected to wave impact |
| — | locations that may be subjected to accidental or operational
damage |
| — | locations where cutouts are made or discontinuities
are present. |
Sec.2 B
302 Structural connections should, in general, be designed with
the aim to minimise stress concentrations and reduce complex stress
flow patterns. Connections should be designed with smooth transitions
and proper alignment of elements. Large cut-outs should be kept
away from flanges and webs of primary girders in regions with high
stresses.
Sec.2 B
303 Transmission of high tensile stresses through the thickness
of plates during welding, block assembly and operation shall be
avoided as far as possible. In cases where transmission of high
tensile stresses through thickness occur, structural material with
proven through thickness properties shall be used. The below sections for
different types of units may give examples where to use plates with
proven through thickness properties.
Sec.2 B
304 Units intended for operations in cold areas shall be so arranged
that water cannot be trapped in local structures or machinery exposed
to the ambient temperature.Sec.2
C. Design Conditions
Sec.2
C 100 Basic conditions
Sec.2 C
101 Different modes of operation or phases during the life of
structure may be governing for the design. The following design
conditions, as defined in Sec.1 C,
should normally be considered:| — | installation condition |
| — | operating conditions(s) |
| — | retrieval condition |
| — | survival condition |
| — | transit condition. |
Guidance note:
For many units the operating condition will be the same as
the survival condition. The retrieval condition is normally applicable
for self-elevating units only.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.2 C
102 Relevant load cases shall be established for the various design
conditions based on the most unfavourable combinations of functional
loads, environmental loads and/or accidental loads, see Sec.3.
Sec.2 C
103 Limiting environmental and operational conditions (design
data) for the different design conditions shall be specified. The
limiting conditions shall be stated in the operation manual.Sec.2
D. Loading Conditions
Sec.2
D 100 General
Sec.2 D
101 Each structural member shall be designed for the most unfavourable
of the loading conditions given in Table D1.
For definitions
and description about the different types of loads see Sec.1 and Sec.3,
respectively. Sec.2 D
| Table D1 Loading conditions |
| Case | Description | a) | functional loads | | b) | maximum combination of environmental loads
and associated functional loads | | c) | accidental loads and associated functional
loads | | d) | annual most probable value of environmental
loads and associated functional loads after credible failures, or
after accidental events | | e) | annual most probable value of environmental
loads and associated functional loads in a heeled condition corresponding
to accidental flooding | |
Sec.2 D
102 For each of the loading conditions in Table D1 and for each
structural element, the combinations of loads, position, and direction
giving the most unfavourable load effect shall be used in the analysis.
Sec.2 D
103 All directions of wind, waves and current relative to the
unit are normally to be assumed equally probable.
Sec.2 D
104 If, however, statistics show clearly that wind, waves and
current of the prescribed probability are different for different
directions, this may be taken into account in the analysis. It is
assumed that orientation of the unit will be under complete control
of the operator.Sec.2
D 200 Load
Sec.2 D
201 The representative values for load components in the different
design conditions shall be based on Sec.3.
Sec.2 D
202 For installation, transit and retrieval the loads may be based
on specified values, which shall be selected dependent on the measurers
taken to achieve the required safety level. The value may be specified
with due attention to the actual location, season of the year, operation
schedule and weather forecast, and consequences of failure.Sec.2
E. Design by the WSD Method
Sec.2
E 100 Permissible stress and usage factors
Sec.2 E
101 In WSD the target component safety level is achieved by comparing
the calculated stress for different loading conditions with maximum
permissible stress defined by multiplication of the characteristic
strength or capacity of the structural member with permissible usage
factors.
Sec.2 E
102 The permissible usage factors are a function of loading condition,
failure mode and importance of strength member.
Sec.2 E
103 The maximum permissible usage factor, hp, is calculated by: hp = bh0
| h0 | = | basic usage factor as given in 200 |
| b | = | coefficient depending on type of structure, failure mode and
reduced slenderness, see Sec.5. |
Sec.2 E
104 Stresses shall be calculated using net scantlings, i.e. with
any corrosion addition deducted.Sec.2
E 200 Basic usage factors
Sec.2 E
201 The basic usage factor for different loading conditions, h0, is given in Table E1.Sec.2 E
| Table E1 Basic usage
factors h0 |
Loading conditions | | a) | b) | c) | d) | e) | h0 | 0.60 1) | 0.80 1) | 1.00 | 1.00 | 1.00 | - For units unmanned during extreme environmental conditions,
the usage factor h0 may
be taken as 0.84 for loading condition b).
| | |
Sec.2 E
202 The basic usage factors account for:| — | possible unfavourable deviations
of the loads |
| — | the reduced probability that various loads acting together
will act simultaneously |
| — | uncertainties in the model and analysis used for determination
of load effects |
| — | possible unfavourable deviations in the resistance of
materials |
| — | possible reduced resistance of the materials in the
structure, as a whole, as compared with the values deduced from
test specimens. |
Sec.2 E
203 If the residual strength of the unit after collapse of a vital
structural member does not satisfy the accidental damage criteria,
the usage factors in Table E1 for the pertinent vital structural
members shall be multiplied by a factor 0.9.Sec.2
F. Design Assisted by Testing
Sec.2
F 100 General
Sec.2 F
101 Design by testing or observation of performance is in general
to be supported by analytical design methods.
Sec.2 F
102 Load effects, structural resistance and resistance against
material degradation may be established by means of testing or observation
of the actual performance of full-scale structures.Sec.2
F 200 Full-scale testing and observation
of performance of existing structures
Sec.2 F
201 Full-scale tests or monitoring on existing structures may
be used to give information on response and load effects to be utilised
in calibration and updating of the safety level of the structure.
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