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DNV-OS-C103 Structural Design of Column Stabilised Units (LRFD method)
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SECTION 3
Design LoadsSec.3
A. Introduction
Sec.3
A 100 General
Sec.3 A
101 The requirements in this section define and specify load components
and load combinations to be considered in the overall strength analysis
as well as design pressures applicable for local design.
Sec.3 A
102 Characteristic loads shall be used as reference loads. Design
loads are, in general, defined in DNV-OS-C101 and described in DNV-RP-C103
and DNV-RP-C205. Guidance concerning load categories relevant for
column-stabilised unit designs are given in this section.Sec.3
B. Definition
Sec.3
B 100 Load point
Sec.3 B
101 The load point for which the design pressure for a plate field
shall be calculated, is defined as midpoint of a horizontally stiffened
plate field, and 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.
Sec.3 B
102 The load point for which the design pressure for a stiffener
shall be calculated, is defined as midpoint of the span. When the
pressure is not varied linearly over the span, the design pressure
shall be taken as the greater of the pressure at the midpoint, and
the average of the pressures calculated at each end of the stiffener.
Sec.3 B
103 The load point for which the design pressure for a girder shall
be calculated, is defined as midpoint of the load area.Sec.3
C. Permanent Loads (G)
Sec.3
C 100 General
Sec.3 C
101 Permanent loads are loads that will not vary in magnitude,
position, or direction during the period considered, and include:
| — | lightweight of the unit, including
mass of permanently installed modules and equipment, such as accommodation,
helideck, drilling and production equipment |
| — | hydrostatic pressures resulting from buoyancy |
| — | pretension in respect to mooring, drilling and production systems,
e.g. mooring lines, risers etc. See DNV-OS-E301. |
Sec.3
D. Variable Functional Loads (Q)
Sec.3
D 100 General
Sec.3 D
101 Variable functional loads are loads that may vary in magnitude,
position and direction during the period under consideration.
Sec.3 D
102 Except where analytical procedures or design specifications
otherwise require, the value of the variable loads utilised in structural
design shall be taken as either the lower or upper design value,
whichever gives the more unfavourable effect. Variable loads on
deck areas for local design are given in DNV-OS-C101, Sec.3 D200.
Sec.3 D
103 Variations in operational mass distributions, including variations
in tank load conditions in pontoons, shall be adequately accounted
for in the structural design.
Sec.3 D
104 Design criteria resulting from operational requirements shall
be fully considered. Examples of such operations may be:| — | drilling, production, workover,
and combinations thereof |
| — | consumable re-supply procedures |
| — | maintenance procedures |
| — | possible mass re-distributions in extreme conditions. |
Sec.3 D
105 Dynamic loads resulting from flow through air pipes during
filling operations shall be adequately considered in the design
of tank structures.Sec.3
D 200 Lifeboat platforms
Sec.3 D
201 Structural strength requirements related to lifeboat platforms
and their supporting structure are given in DNV-OS-C101 Sec.3 D400.Sec.3
D 300 Tank loads
Sec.3 D
301 A minimum design density (r) of 1.025 t/m3 should be considered in the
determination of the required scantlings of tank structures.
Sec.3 D
302 The extent to which it is possible to fill sounding, venting
or loading pipe arrangements shall be fully accounted for in determination
of the maximum design pressure to which a tank may be subjected
to.
Sec.3 D
303 Dynamic pressure heads resulting from filling of such pipes
shall be included in the design pressure head where such load components
are applicable.
Sec.3 D
304 All tanks shall be designed for the following internal design
pressure:
| av | = | maximum vertical acceleration, (m/s2), being the coupled motion
response applicable to the tank in question |
| hop | = | vertical distance (m) from the load point
to the position of maximum filling height. For tanks adjacent to
the sea that are situated below the extreme operational draught
(TE), hop should not be taken less than from the load point to the
static sea level.
Descriptions and requirements related
to different tank arrangements are given in DNV-OS-D101 Ch.2 Sec.3
C300. |
| gf,G,Q | = | partial load factor, for permanent and functional loads
see Sec.4 Table A1 |
| gf,E | = | partial load factor for environmental loads,
see Sec.4 Table A1. |
Sec.3 D
305 For tanks where the air pipe may be filled during filling operations,
the following additional internal design pressure conditions shall
be considered:pd = (r g0 hop + pdyn) gf,G,Q (kN/m2)
| pdyn | = | Pressure (kN/m2)
due to flow through pipes, minimum 25 kN/m2. |
Guidance note:
This internal pressure need not to be combined with extreme environmental
loads. Normally only static global response need to be considered.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.3 D
306 For external plate field boundaries, it is allowed to consider
the external pressure up to the lowest waterline occurring in the
environmental extreme condition, including relative motion of the
unit.Guidance note:
For preliminary design calculations, av may be taken as 0.3 g0 and external pressure for external
plate field boundaries may be taken up to half the pontoon height.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.3 D
307 In cases where the maximum filling height is less than the
height to the top of the air pipe, it shall be ensured that the tank
will not be over-pressured during operation and tank testing conditions.
Sec.3 D
308 Requirements for testing of tank tightness and structural strength
are given in DNV-OS-C401, Ch.2 Sec.4.Sec.3
E. Environmental Loads (E)
Sec.3
E 100 General
Sec.3 E
101 General considerations for environmental loads are given in
DNV-OS-C101 Sec.3 E and Sec.3 F, and DNV-RP-C205.
Sec.3 E
102 Combinations of environmental loads are stated in DNV-OS-C101
Sec.3 Table F1.
Sec.3 E
103 Typical environmental loads to be considered in the structural
design of a column-stabilised unit are:| — | wave loads, including variable
pressure, inertia, wave 'run-up', and slamming
loads |
| — | wind loads |
| — | current loads |
| — | snow and ice loads. |
Sec.3 E
104 The following responses due to environmental loads shall be
considered in the structural design of a column-stabilised unit:| — | dynamic stresses for all limit
states |
| — | rigid body motion, e.g. in respect to air gap and maximum angles
of inclination |
| — | sloshing |
| — | slamming induced vibrations |
| — | vortex induced vibrations,e.g. resulting from wind loads on
structural elements in a flare tower |
| — | environmental loads from mooring and riser system. |
Sec.3 E
105 For column-stabilised units with traditional catenary mooring
systems, earthquake loads may normally be ignored.Sec.3
E 200 Sea pressures
Sec.3 E
201 For load conditions where environmental load effects shall
be considered the pressures resulting from sea loading are to include
consideration of the relative motion of the unit.
Sec.3 E
202 The design sea pressure acting on pontoons and columns of
column-stabilised platforms in operating conditions shall be taken
as:
where
and
| TE | = | extreme operational draught (m) measured
vertically from the moulded baseline to the assigned load waterline |
| Cw | = | reduction factor due to wave particle
motion (Smith effect) Cw = 0.9
unless otherwise documented |
| DD | = | vertical distance in m from the moulded
baseline to the underside of the deck structure
(the
largest relative distance from moulded baseline to the wave crest
may replace DD if this
is proved smaller) |
| zb | = | vertical distance in m from the moulded
baseline to the load point |
| ps | = | permanent sea pressure |
| pe | = | environmental sea pressure. |
Sec.3 E
203 When pressures are acting on both sides of bulkheads, the
load factor shall be applied to the net pressure.
Sec.3 E
204 The Smith effect (Cw =
0.9) shall only be applied for loading conditions including extreme
wave conditions.Sec.3
E 300 Wind loads
Sec.3 E
301 The pressure acting on vertical external bulkheads exposed
to wind shall in general not be taken less than 2.5 kN/m2 for local design.
Sec.3 E
302 Further details regarding wind design loads are given in DNV-RP-C205.Sec.3
E 400 Heavy components
Sec.3 E
401 The forces acting on supporting structures and lashing systems
for rigid units of cargo, equipment or other structural components
should be taken as:
For components exposed to wind, a horizontal force due to
the design gust wind shall be added to PHd.
| av | = | vertical acceleration (m/s2) |
| ah | = | horizontal acceleration (m/s2) |
| M | = | mass of cargo, equipment or other components
(t) |
| PVd | = | vertical design force |
| PHd | = | horizontal design force. |
Sec.3 E
402 Further considerations with respect to environmental loads
are given in DNV-RP-C205.Sec.3
F. Deformation Loads (D)
Sec.3
F 100 General
Sec.3 F
101 Deformation loads are loads caused by inflicted deformations,
such as:| — | temperature loads |
| — | built-in deformations. |
Further details and description of deformation loads are given in
DNV-OS-C101 Sec.3 H.
Sec.3
G. Accidental Loads (A)
Sec.3
G 100 General
Sec.3 G
101 The following ALS events shall be considered in respect to
the structural design of a column-stabilised unit:| — | collision |
| — | dropped objects, e.g. from crane handling |
| — | fire |
| — | explosion |
| — | unintended flooding. |
Sec.3 G
102 Requirements and guidance on accidental loads are given in
DNV-OS-C101 and generic loads are given in DNV-OS-A101.Sec.3
H. Fatigue Loads
Sec.3
H 100 General
Sec.3 H
101 Repetitive loads, which may lead to significant fatigue damage,
shall be evaluated. The following listed sources of fatigue loads
shall, where relevant, be considered:| — | waves (including those loads
caused by slamming and variable (dynamic) pressures). |
| — | wind (especially when vortex induced vibrations may occur) |
| — | currents (especially when vortex induced vibrations
may occur) |
| — | mechanical loading and unloading, e.g. crane loads. |
The effects of both local and global dynamic response shall
be properly accounted for when determining response distributions
related to fatigue loads.
Sec.3 H
102 Further considerations in respect to fatigue loads are given
in DNV-RP-C203 and DNV-RP-C205.
Sec.3
I. Combination of Loads
Sec.3
I 100 General
Sec.3 I
101 Load factors and load combinations for the design limit states
are in general, given in DNV-OS-C101.
Sec.3 I
102 Structural strength shall be evaluated considering all relevant,
realistic load conditions and combinations. Scantlings shall be
determined on the basis of criteria that combine, in a rational
manner, the effects of relevant global and local responses for each
individual structural element.Further guidance on relevant load combinations is given in DNV-RP-C103.
Sec.3 I
103 A sufficient number of load conditions shall be evaluated
to ensure that the characteristic largest (or smallest) response,
for the appropriate return period, has been established.
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