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DNV-OS-C103 Structural Design of Column Stabilised Units (LRFD method)
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SECTION 6
Accidental Limit States (ALS)Sec.6
A. General
Sec.6
A 100 General
Sec.6 A
101 Satisfactory protection against accidental damage shall be
obtained by the following means:
| — | low damage probability |
| — | acceptable damage consequences. |
Sec.6 A
102 The structure's capability to redistribute loads
should be considered when designing the structure. The structural
integrity shall be intact and should be analysed for the following damage
conditions:| — | fracture of braces and major
pillars important for the structural integrity, including their
joints |
| — | fracture of primary girder in the upper hull. |
After damage requiring immediate repair, the unit shall resist functional
and environmental loads corresponding to a return period of one
year.
Sec.6 A
103 Analysis as stated shall satisfy relevant strength criteria given
in this standard and in DNV-OS-C101. The damage consequences of
other accidental events shall be specially considered in each case,
applying an equivalent standard of safety.
Guidance note:
Energy absorption by impact types of accidental events requires the
structure to behave in a ductile manner. Measures to obtain adequate
ductility are:| - | select materials with sufficient
toughness for the actual service temperature and thickness of structural
members| - | make the strength of connections of primary members
to exceed the strength of the member itself| - | provide redundancy in the structure, so that alternate
load redistribution paths may be developed| - | avoid dependency on energy absorption in slender members with
a non-ductile post buckling behaviour| - | avoid pronounced weak sections and abrupt change in strength
or stiffness. | | | | |
---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.6 A
104 The loads and consequential damage due to accidental events
or accidental flooding such as:| — | collision |
| — | dropped objects, e.g. from crane handling |
| — | fire |
| — | explosion |
| — | unintended flooding |
| — | abnormal wave events |
shall not cause loss of floatability, capsizing, pollution
or loss of human life. Requirements for watertight integrity and hydrostatic
stability are given in DNV-OS-C301.
Guidance note:
10-4 waves need
not to be considered as a ALS condition.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.6
B. Collision
Sec.6
B 100 General
Sec.6 B
101 A collision between a supply vessel and a column of a column-stabilised
unit shall be considered for all elements of the unit which may
be exposed to sideway, bow or stern collision. The vertical extent
of the collision zone shall be based on the depth and draught of
the supply vessel and the relative motion between the supply vessel
and the unit.
Sec.6 B
102 A collision will normally only cause local damage of the column.
However, for a unit with slender columns, the global strength of
the unit shall be checked.
Sec.6 B
103 A collision against a brace will normally cause complete failure
of the brace and its connections, e.g. K-joints. These parts shall
be assumed non-effective for check of the residual strength of the
unit after collision.Sec.6
C. Dropped Object
Sec.6
C 100 General
Sec.6 C
101 Critical areas for dropped objects shall be determined on the
basis of the actual movement of potentially dropped objects relative
to the structure of the unit itself. Where a dropped object is a
relevant accidental event, the impact energy shall be established
and the structural consequences of the impact assessed.
Sec.6 C
102 A dropped object on a brace will normally cause complete failure
of the brace or its connections, e.g. K-joints. These parts are
assumed to be non-effective for the check of the residual strength
of the unit after dropped object impact.
Sec.6 C
103 Critical areas for dropped objects shall be determined on the
basis of the actual movement of loads assuming a drop direction
within an angle with the vertical direction:| — | 10º in air, for floating
units |
| — | 5º in air, for bottom supported units |
| — | 15º in water. |
Dropped objects shall be considered for vital structural elements
of the unit within the areas given above.
Sec.6
D. Fire
Sec.6
D 100 General
Sec.6 D
101 The main loadbearing structure that is subjected to a fire shall
not lose the structural capacity. The following fire scenarios shall
be considered:| — | fire inside the unit |
| — | fire on the sea surface. |
Sec.6 D
102 Further requirements concerning accidental limit state events
involving fire is given in DNV-OS-A101.
Sec.6 D
103 Assessment of fire may be omitted provided assumptions made
in DNV-OS-D301 are met.Sec.6
E. Explosion
Sec.6
E 100 General
Sec.6 E
101 In respect to design, considering loads resulting from explosions,
one or a combination of the following design philosophies are relevant:| — | hazardous areas are located
in unconfined (open) locations and that sufficient shielding mechanisms,e.g.
blast walls, are installed |
| — | hazardous areas are located in partially confined locations and
the resulting, relatively small overpressures are accounted for
in the structural design |
| — | hazardous areas are located in enclosed locations and
pressure relief mechanisms are installed, e.g. blast panels designed
to take the resulting overpressure. |
Sec.6 E
102 As far as practicable, structural design accounting for large
plate field rupture resulting from explosion loads should be avoided
due to the uncertainties of the loads and the consequences of the
rupture itself. Sec.6
F. Heeled Condition
Sec.6
F 100 General
Sec.6 F
101 Heeling of the unit after damage flooding, as described in
DNV-OS-C301 shall be accounted for in the assessment of structural
strength. Maximum static allowable heel after accidental flooding
is 17º including wind. Structures that are wet when the
static equilibrium angle is achieved, shall be checked for external
water pressure. Guidance note:
The heeled condition corresponding to accidental flooding
in transit conditions will normally not be governing for the design. ---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.6 F
102 The unit shall be designed for environmental condition corresponding
to 1 year return period after damage, see DNV-OS-C101.Guidance note:
The environmental loads may be disregarded if the material
factor is taken as gM = 1.33.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.6 F
103 Local exceedance of the structural resistance is acceptable
provided redistribution of forces due to yielding, buckling and
fracture is accounted for.
Sec.6 F
104 Wave pressure, slamming forces and green sea shall be accounted
for in all relevant areas. Local damage may be accepted provided
progressive structural collapse and damage of vital equipment is
avoided.
Sec.6 F
105 Position of air-intakes and openings to areas with vital equipment
which need to be available during an emergency situation, e.g. emergency
generators, shall be considered taking into account the wave elevation
in a 1 year storm.
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