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DNV-OS-C102 Structural Design of Offshore Ships
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SECTION 6
Strength of Topside StructuresSec.6
A. Introduction
Sec.6
A 100 General
Sec.6 A
101 The requirement in this section is applicable for:
| — | local strength of plate and
stiffener |
| — | simple girders |
| — | calculation of complex girder systems. |
Sec.6 A
102 This section gives provisions for checking of ultimate strength
for typical topside structures such as:| — | drill-floor and substructure |
| — | modules |
| — | gantry structure |
| — | flare tower |
| — | riser balcony |
| — | deck houses which carry loads from risers, mud, brine
etc. |
Guidance note:
For derrick structure, see DNV-OS-E101---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.6 A
103 The topside structures shall be designed to withstand the
relevant loading conditions according to the transit, operating
and survival conditions.
Sec.6 A
104 For the different conditions the topside loads are normally
different, and direct calculations of the accelerations and hull
girder loads may be carried out.
Sec.6 A
105 Topside structures of truss work type of structure as the
primary load-bearing elements and where the plates are not included
in assessment of the global strength, the plates with stiffeners
should normally comply only with the local requirements.
Sec.6 A
106 When the plates with stiffeners are part of the primary load-bearing
structure, both local and global requirements shall be complied
with.
Sec.6 A
107 The deformations due to hull girder bending and stiffness
variations of the supporting structure shall be accounted for in
the structural analyses.
Sec.6 A
108 Deck houses, accommodation or superstructure, which is not
part of the load-bearing structure for typical offshore element
loads, shall comply with the requirements given in unit specific
provisions Sec.11 and Sec.12
Sec.6 A
109 The local requirements to end connections of stiffeners and
design of brackets are given in DNV Rules for Classification of
Ships Pt.3 Ch.1 Sec.3 C.
Sec.6 A
110 For slender structures, e.g. flare tower, the response due
to vortex shedding shall be considered. Ref. DNV-RP-C205.
Sec.6 A
111 Overview of the design principles are given in Table A1.Sec.6 A
Table
A1 Design principles for topside and topside supporting structures | | Design
conditions | Items | Transit | Operation | | Ship
Rules | Direct
calculation | Ship
Rules | Direct
calculation | Local requirements to topside structure
(plates / stiffeners / girders / beams) | Part
B, C and D | | Still water bending moments and shear
forces | Sec.4, limit curves | Sec.4,
limit curves | | Design accelerations | Ship
Rules Pt.3 Ch.1 Sec.4 | Sec.5,
wave load analysis (North Atlantic) | Ship
Rules Pt.3 Ch.1 Sec.4 | Sec.5,
wave load analysis with specified seastate | | Wave bending moments | Ship
Rules Pt.3 Ch.1 Sec.5 | Ship
Rules Pt.3 Ch.1 Sec.5 | | Wind loads | Sec.4 | Sec.4 | | Hull deformation | Part
I and Sec.7 | | Part
I and Sec.7 | | | Green sea | Sec.4, Sec.11 and Sec.12 | | | | | |
Sec.6
A 200 Definition of load point
Sec.6 A
201 Symbols: p = design pressure in kN/m
Sec.6 A
202 The load point for which
the design pressure shall be calculated is defined for various strength
members as follows: - For plates:
midpoint of horizontally stiffened plate field. Half of the
stiffener spacing above the lower support of vertically stiffened
plate field, or at lower edge of plate when the thickness is changed
within the plate field. - For stiffeners:
midpoint of span.
When the pressure is not varied linearly over the span the
design pressure shall be taken as the greater of:
and
pm, pa and pb are
calculated pressure at the midpoint and at each end respectively. - For girders:
midpoint of load area.
Sec.6
B. Local Static Loads
Sec.6
B 100 Local loads on topside structures
Sec.6 B
101 The local static loads
for decks and bulkheads in topside facilities, which are not part
of a tank, are given in Table B1. For areas not specifically mentioned
in Table B1, relevant values in the DNV Rules for Classification
of Ships Pt.3 Ch.1 Sec.9 may be applied.Sec.6 B
| Table B1 Local static
loads |
| Decks | Plates and stiffeners | Girders
(kN/m2) | Global design Evenly
distributed load (kN/m2) | Point load (kN) | | Storage areas in modules 1) | q | 1.5
q | f ´ q | q | Lay down areas 1) | q | 1.5
q | f ´ q | f ´ q | Lifeboat platforms | 9.0 | 9.0 | 9.0 ´ f | - | Area between equipment | 5.0 | 5.0 | 5.0 ´ f | - | Walkways, staircases and platforms, crew
spaces | 4.0 | 4.0 | 4.0 ´ f | - | Walkways and staircases for inspection
only | 3.0 | 3.0 | 3.0 ´ f | - | Minimum values for areas not given above 2) | 2.5 | 2.5 | 2.5 | - - The distributed loads, q, to be evaluated for each case. Lay
down areas should not be designed less than 15 kN/m2. 2) The minimum values shall
be determined considering the weights of the equipment and bulks,
which may be located on the area. The minimum values shall not be
less than 2.5 kN/m2
Notes: | — | wheel loads to be added to distributed
loads where relevant. (Wheel loads can normally be considered acting
on an area of 300 ´ 300 mm.) | | — | point load may be applied on an area 100 ´ 100
mm, and at the most severe position, but not added to wheel loads
or distributed loads | | — | the factor f may be taken as
Where, A is the loaded area in m2. | | — | Global design presents variable functional loads to
be included in the load model for the global analysis. In the capacity
checks, stresses from the global analysis shall be combined with
the effect of local loads, i.e. tank pressures, weight of equipments,
etc. |
| | | | | | | | |
Sec.6
C. Local Requirements to Plates and Stiffeners
Sec.6
C 100 Plates
Sec.6 C
101 The plate thickness shall not to be less than:
| f1 | = | see DNV Rules for Classification of Ships
Pt.3 Ch.1 Sec.2 |
| tk | == | corrosion addition according to the Ship
Rules, Pt.3 Ch.1 Sec.2 Table D1
0 for elements which
are not part of a tank |
Sec.6 C
102 The thickness of plating subjected to lateral pressure shall
not be less than:
| ka | = | correction factor for aspect ratio of
plate field, (1.1 - 0.25 s/l)2
maximum
1.0 for s/l = 0.4
minimum 0.72 for
s/l = 1.0 |
| s | = | stiffener spacing in m |
| l | = | stiffener span in m |
| p | = | local design load in B |
| hP | = | permissible utilisation factors as given
in I |
| fy | = | specified minimum yield stress of the
material in N/mm2 |
Sec.6
C 200 Stiffeners
Sec.6 C
201 The section modulus for longitudinals, beams, frames and other
stiffeners subjected to lateral load shall not be less than:
| l | = | stiffener span in m |
| s | = | stiffener spacing in m |
| p | = | local design load in B |
| km | = | bending moment factor, see DNV Rules
for Classification of Ships Pt.3 Ch.1 Sec.3 Table B1 |
| hP | = | permissible usage factors as given in
I |
| fy | = | specified minimum yield stress of the
material in N/mm2 |
Sec.6 C
202 The requirement in 201 applies to an axis parallel to the
plating. For stiffeners at an oblique angle with the plating, the
required section modulus shall be multiplied by:
| j | = | angle in degrees 1) between the stiffener
web plane and the plane perpendicular to the plating |
- j is to be taken as 90 degrees if the angle is greater
or equal to 75 degrees.
|
Sec.6 C
203 Stiffeners with sniped ends may be accepted where dynamic
stresses are small and vibrations are considered to be of minor
importance, provided that the plate thickness t supported by the
stiffener is not less than:
In such cases the required section modulus in 201 shall be
based on the following parameter values:
km = 8
Guidance note:
For typical sniped end details as described above, a stress
range lower than 30 MPa can be considered as small dynamic stress.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.6
C 300 Green sea loads
Sec.6 C
301 Topside members exposed to green sea loads shall be checked
according to the DNV Rules for Classification of Ships Pt.3 Ch.1
Sec.9 with the design loads given in Sec.11 and Sec.12.Sec.6
D. Local Requirements to Simple Girders
Sec.6
D 100 General
Sec.6 D
101 The requirements in this sub-section give minimum scantlings
to simple girders with respect to yield. When boundary conditions
for individual girders are not predictable due to dependence of
adjacent structures, direct calculations shall be carried out.
Sec.6 D
102 The local requirements to end connections of girders and design
of brackets are given in DNV Rules for Classification of Ships Pt.3
Ch.1 Sec.3 C.
Sec.6 D
103 The requirements for section modulus and web area given in
D500 apply to simple girders supporting stiffeners, or other girders,
exposed to linearly distributed lateral load. It is assumed that
the girder satisfies the basic assumptions of simple beam theory,
and that the supported members are approximately evenly spaced and
similarly supported at both ends. Other loads should be specially
considered based on the same beam-theory.
Sec.6 D
104 The section modulus and web area of the girder shall be taken
in accordance with particulars as given in D500. Structural modelling
in connection with direct stress analysis shall be based on the
same particulars when applicable.
Sec.6 D
105 Dimensions and further references with respect to buckling
capacity are given in sub-section G.Sec.6
D 200 Minimum thickness
Sec.6 D
201 The thickness of web and flange of girders shall not be less
than:| — | for longitudinal girders located lower than 4.0
m above the upper continuous deck of the hull or up to the first
deck in modules or topside deck houses: t = 5 + 0.01
L (mm), maximum 8 mm |
| — | for longitudinal girders at higher locations or transverse
girders: t = 4 + 0.01 L (mm), maximum 7 mm, minimum
5 mm. |
Sec.6
D 300 Effective flange of girders
Sec.6 D
301 The effective flange of girders is determined according to
DNV Rules for Classification of Ships Pt.3 Ch.1 Sec.3 C400.Sec.6
D 400 Effective web of girders
Sec.6 D
401 The effective web of girders is determined according to DNV
Rules for Classification of Ships Pt.3 Ch.1 Sec.3 C500.Sec.6
D 500 Strength requirement of simple girders
Sec.6 D
501 Simple girders subjected to lateral loads and which are not
taking part in the overall strength of the unit, shall comply with
the following:
Sec.6 D
502 Minimum section modulus Section modulus Zg:
Sec.6 D
503 Minimum web area after deduction of cut-outs:
The web area at the middle of the span is not to be less than
0.5 AW
| Sg | = | girder span in m. see DNV Rules for Classification
of Ships Pt.3 Ch.1 Sec.3. |
| b | == | breadth of load area in m (plate flange),
b may be determined as:
0.5 (l1 + l2) where
l1 and l2 are the spans of the supported stiffeners on both sides
of the girder, respectively, or distance between girders |
| p | = | local design load in sub-section B |
| km | = | bending moment factor, see DNV Rules
for Classification of Ships Pt.3 Ch.1 Sec.3 Table B1 |
| kt | = | shear force factor, see DNV Rules for
Classification of Ships Pt.3 Ch.1 Sec.3 Table B1 |
| hP | = | permissible usage factors as given in
sub-section I |
| tp | = | permissible shear stress in N/mm2
0.39 fy for load combination a) in Sec.3 Table C1
0.46 fy for load combination b) in
Sec.3 Table C1 |
| Ns | = | number of supported stiffeners on girder
span |
| Pp | = | average "point load" from
stiffener |
| fy | = | specified minimum yield stress of the
material in N/mm2 |
Sec.6
E. Global Static Loads
Sec.6
E 100 Global static loads in topside structure
Sec.6 E
101 The static loads to be
applied for the global analysis of the topside facilities or in
the still water loading conditions of the unit are in principle
determined by considering the permanent loads and realistic values
for simultaneously acting variable loads.
Sec.6 E
102 The total static load
of a module, excluding tank loads, is determined according to:
Where
| qS | = | static global weight of module (kN) |
| Fs | = | total steel weight of decks (kN) |
| Fe | = | weight of equipment (kN) |
| n | = | total number of heavy equipment (>50kN) |
| K | == | global load reduction factor for the
deck considered to account for simultaneous acting module loads
0.6 |
| Pv | = | evenly distributed design load (kN/m2) for the deck considered,
ref Table B1. |
| m | = | total number of decks |
| A | = | loaded area of deck considered (area
covered by equipment may be excluded) |
Sec.6 E
103 The tank loads within
the module shall be included, if relevant.
Sec.6 E
104 The load used shall include
all equipment over 50 kN plus the sum of all realistic deck loads
accounting for the joint probability of occurrence.Sec.6
F. Global Dynamic Loads
Sec.6
F 100 Global dynamic loads in topside structure
Sec.6 F
101 The global dynamic loads
to be combined with the global static loads are determined by multiplying
the masses with the design accelerations defined in F200.
Sec.6 F
102 Wind force shall be included
for topside structures with large wind area as e.g. modules, flare
tower, derrick structure, etc.Sec.6
F 200 Design accelerations
Sec.6 F
201 The design accelerations
used for design of the topside structures are:| av | = | vertical acceleration |
| at | = | transverse acceleration |
| al | = | longitudinal accelerations |
The sign convention is according to the coordinate system
below:
Sec.6
F 300 Transit conditions
Sec.6 F
301 Design accelerations given in Table F1 may be based on the
DNV Rules for Classification of Ships Pt.3 Ch.1 Sec.4, or alternatively
by direct calculations using the basis given in Sec.3 Table B1 for
the transit condition.
Sec.6 F
302 Load cases shall be generated
for each of the maximum basic responses for the head sea, beam sea
and oblique sea as given in Table F1. For symmetrical structures
about a longitudinal and transverse plane through the centre of
gravity of the topside structure, load combination 4 and 7 may be
omitted.Sec.6 F
| Table F1 Combination
of dynamic responses in transit |
| Heading | Load
case | Maximum response | Combination with fraction of responses | av | at | al | Fw | Head
Sea | 1 | Hull deflection | 0.5 | 0.0 | -r | 1.0 | | 2 | Hull deflection | -0.5 | 0.0 | +r | 1.0 | | Beam
Sea | 3 | at | 1.0 | 1.0 | -c | 1.0 | | 4 | at | 1.0 | -1.0 | -c | 1.0 | | Oblique
Sea | 5 | al | -j | 0.4 | 1.0 | 1.0 | | 6 | at | +m | 1.0 | 0.9 | 1.0 | | 7 | at | +m | -1.0 | 0.9 | 1.0 | - Design accelerations, al,
at, av are based on a 20 year return
period in the North Atlantic
- For wind force, Fw,
see Sec.4.
| | |
Where:
| | | | Values
for L > 200 m | Values
for L < 100 m |
| C | = | -0.003 L + 0.7 | 0.1 | 0.4 |
| J | = | -0.002 L + 0.4 | 0 | 0.2 |
| M | = | -0.004 L + 1.1 | 0.3 | 0.7 |
| R | = | -0.004 L + 1.4 | 0.6 | 1.0 |
| L = Length of unit
(m), shall not be taken higher than 200 nor less than 100 |
Sec.6
F 400 Operating and
Survival condition
Sec.6 F
401 The design accelerations given in Table F2 should be determined
based on the DNV Rules for Classification of Ships Pt.3 Ch.1 Sec.4,
or alternatively by direct calculations using the basis given in Sec.3 Table B1 for the relevant operation
condition.Sec.6 F
| Table F2 Combination
of dynamic responses in operating and survival conditions |
| | Load case | Maximum response | Combination with fraction of responses 1) |
| av | at | al | Fw 2) |
| Operation | 1 | al | -b | -c | 1.0 | 1.0 |
| 2 | at | 0.8 | 1.0 | -e | 1.0 |
| 3 | av | 1.0 | +f | -g | 1.0 |
1) Accelerations need to
be calculated for each relevant design condition.
2)
Other factor may be accepted, if documented |
Where:
| | | | Values
for L > 200 m | Values
for L < 100 m |
| b | = | 0.003 L + 0.2 | 0.8 | 0.5 |
| c | = | -0.002 L + 0.8 | 0.4 | 0.6 |
| e | = | 0.004 L + 0.2 | 1.0 | 0.6 |
| f | = | -0.005 L + 1.3 | 0.3 | 0.8 |
| g | = | 0.004 L + 0.2 | 1.0 | 0.6 |
| L = Length of unit
(m), shall not be taken larger than 200 m nor less than 100 m |
Sec.6
G. Hull Deformation
Sec.6
G 100 General
Sec.6 G
101 Large topside structures
that have significant impact on the total stiffness of the hull
girder interface, e.g. derrick structure and main support stools
for topside process structure, shall be included in longitudinal
hull girder FE model.
Sec.6 G
102 Minor topside modules
may be analysed separately from the hull provided they have less
importance for the total hull girder behaviour. Alternatively the
longitudinal deformation in deck may be estimated by the simplified
formula below: | d | = | longitudinal deformation between sections
1 and 2 |
| M | = | design vertical bending moment at sections
1 and 2 1) |
| Z | = | section modulus at the deck at the interface
with topside structure |
| E | = | Young's modulus of elasticity |
| l1 | = | distance between sections 1 and 2 |
| 1) = | The design bending moment (M) consists of static and dynamic
part and shall be considered for the load combination a), b) and d)
as specified in Sec.3 Table C1. |
Sec.6
H. Complex Girder Systems
Sec.6
H 100 General
Sec.6 H
101 For girders that are parts of a complex 2- or 3-dimensional
structural system, a complete structural analysis shall be carried
out to demonstrate that the stresses are acceptable with respect
to material yield and buckling.
Sec.6 H
102 The method used in the analysis shall be capable of describing
the physical behaviour of the structure when exposed to the local
and global loads.
Sec.6 H
103 For girder systems consisting of slender girders, the assessment
for all load combination in Sec.3 Table C1 may be based on elastic
beam theory. Due attention should be given to:¾ shear area variation, e.g. due to cut-outs
¾ moment of inertia variation
¾ effective flange
¾ local buckling of girder flanges.
Sec.6
H 200 Design loads
Sec.6 H
201 Both local and global loads as defined in sub-section B, E
and F shall be considered. The relevant load combinations given
in Sec.3 Table C1 shall be addressed.
Sec.6 H
202 Green sea loads need not to be considered for the global check
of a girder system.Sec.6
H 300 Impact from surrounding structure
Sec.6 H
301 The impact of structures connected to the part covered by
the capacity model shall be included in the assessment of the girders.Sec.6
I. Acceptance Criteria
Sec.6
I 100 Material yield check
Sec.6 I
101 The maximum permissible usage factor, hp, is calculated by:hP = bh0
| h0 | = | basic usage factor in Sec.3
Table C1 |
| b | = | coefficient depending on type of structure |
Sec.6 I
| Table I1 Usage factor
coefficients |
| Items | Load combination | a) | b) | c) | d) | Basic usage factor, h0 | 0.6 | 0.8 | 1.0 | 1.0 | | Coefficient depending on
type of structure, b | | Local requirements to plates and stiffeners | 1.14 | 1.0 | NA | NA | | Local requirements to section modulus
of girders and stringers | 1.0 | 1.0 | 1.0 | 1.0 | | Global strength of topside load-bearing
elements in general | 1.14 | 1.0 | 1.0 | 1.0 | | Global strength of drill-floor and substructure,
flare | 1.0 | 1.0 | 1.0 | 1.0 | Buckling stability check in general | 1.0 | 1.0 | 1.0 | 1.0 | | | |
Sec.6
I 200 Local liquid tanks
Sec.6 I
201 The local strength requirements
to plates, stiffeners and simple girders in tanks shall comply with
the requirements in DNV Rules for Classification of Ships Pt.3 Ch.1.
The allowable stress for longitudinal members need not be less than
160f1 MPa.For material factor f1,
see Rules for Classification of Ships Pt.3 Ch.1 Sec.2.
Sec.6
J. Buckling Stability
Sec.6
J 100 Bars, beams, columns and frames
Sec.6 J
101 It shall be ensured that there is conformity between the initial
imperfections in the buckling resistance formulas and the tolerances
in the applied fabrication standard.Guidance note:
If buckling resistance is calculated in accordance with Classification
Note 30.1 for bars and frames, the tolerance requirements given
in DNV-OS-C401 should not be exceeded.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.6
J 200 Flat plated structures and stiffened
panels
Sec.6 J
201 The buckling stability of plated structures may be checked
according to DNV-RP-C201.Sec.6
J 300 Tubulars
Sec.6 J
301 Tubular members may be checked according to DNV Classification
Note 30.1 or API RP 2A - WSD. For interaction between local shell
buckling and column buckling, and effect of external pressure, DNV-RP-C202
may be considered.
Sec.6 J
302 Cross sections of tubular member are divided into different
types dependent of their ability to develop plastic hinges and resist
local buckling. Effect of local buckling of slender cross sections
shall be considered.Guidance note:
- Effect of local buckling of
tubular members without external pressure, i.e. subject to axial
force and/or bending moment) given in section 3.8 of DNV-RP-C202
may be used.
- Effect of local buckling of tubular members with external
pressure need not be considered for the following diameter
Dm to thickness t ratio:
where
E = modulus of elasticity and
fy = minimum yield strength.
---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.6 J
303 Tubular members with external pressure, tubular joints and
conical transitions may be checked according to API RP 2A-WSD.Sec.6
J 400 Capacity checks according to other
codes
Sec.6 J
401 Stiffeners and girders may be checked according to provisions
for beams in recognised standards such as AISC-ASD.Guidance note:
The principles and effects of cross section types are included
in the AISC-ASD.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.6
K. Fatigue strength
Sec.6
K 100 General
Sec.6 K
101 Fatigue of topside structures shall be documented according
to the principles and requirements given in Sec.8
Sec.6 K
102 The worst dynamic stress
amplitude using the combinations in Table F1 and Table F2 may be
applied for a simplified fatigue calculation in transit and operation.
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