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DNV-OS-C102 Structural Design of Offshore Ships
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SECTION 9
Accidental ConditionsSec.9
A. Introduction
Sec.9
A 100 General
Sec.9 A
101 Accidental loads are loads related to abnormal operation or
technical failure. Attention should be given to layout and arrangements
of facilities and equipment in order to minimise the adverse effects
of accidental events.
Sec.9 A
102 Safety assessment shall
be carried out according to the principles given in DNV-OS-A101
for relevant accidental scenarios.
Sec.9 A
103 The overall objective
for design with respect to accidental conditions is that unit's
main safety functions shall not be impaired by accidental events.
Satisfactory protection against accidental damage may be achieved by
two barriers:
| — | reduction of damage probability |
| — | reduction of damage consequences. |
Sec.9 A
104 The design against accidental
loads may be done by direct calculation of the effects imposed by
the loads on the structure, or indirectly, by design of the structure
as tolerable to accidents. Sec.9
B. Design Criteria
Sec.9
B 100 General
Sec.9 B
101 Structures shall be checked
for accidental loads in two steps, according to the loading conditions presented
in Sec.3 Table C1: | — | resistance of the structure
against design accidental loads, i.e. loading condition c) |
| — | post accident resistance of the structure against environmental
loads after accidental damage, i.e. loading conditions d). |
The unit shall be designed for environmental condition corresponding
to 1 year return period after accidental damage.
Sec.9 B
102 Generic values of accidental
loads are given in DNV-OS-A101.
Sec.9 B
103 The different types of
accidental loads require different methods and analyses to assess
the structural resistance. Local exceedance of the structural capacity
is acceptable provided redistribution of forces due to yielding,
buckling and fracture is accounted for.
Sec.9 B
104 The inherent uncertainty
of the frequency and magnitude of the accidental loads, as well
as the approximate nature of the methods for determination of accidental
load effects, shall be recognised. It is therefore essential to
apply sound engineering judgement and pragmatic evaluations in the
design.
Sec.9 B
105 If non-linear, dynamic
FE analysis is applied for design, it shall be verified that all
local failure modes (e.g. strain rate, local buckling, joint overloading,
and joint fracture) are accounted for implicitly by the modelling
adopted, or else subjected to explicit evaluation.
Sec.9
B 200 Dropped objects
Sec.9 B
201 Critical areas for dropped
objects shall be determined on the basis of the actual movement
of potential dropped objects, e.g. crane or other lifting operation
mass, 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.9 B
202 Critical areas for dropped
objects should be determined assuming a minimum drop direction within
an angle of 10° with the vertical direction.
Sec.9 B
203 Setback area shall be
designed to satisfy the dropped object scenario in accordance with
DNV-OS-E101. Guidance note:
The dropped object is calculated according to DNV-RP-C204.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.9
B 300 Fires
Sec.9 B
301 The structure that is
subjected to a fire shall maintain sufficient structural strength
before evacuation has occurred. The following fire scenarios shall
be considered: | — | jet fires |
| — | fire inside or on the hull |
| — | fire on the sea surface. |
Assessment of fire may be omitted provided fire protection
requirements made in DNV-OS-D301 are met.
Sec.9
B 400 Explosions
Sec.9 B
401 One or more of the following
main design philosophies will be relevant: | — | Ensure that hazardous locations
are located in unconfined (open) locations and that sufficient shielding mechanisms
(e.g. blast walls) are installed. |
| — | Locate hazardous areas in partially confined locations
and design utilising the resulting, relatively small overpressure. |
| — | Locate hazardous areas in enclosed locations and install
pressure relief mechanisms (e.g. blast panels) and design for the
resulting overpressure. |
Sec.9 B
402 As far as practicable,
structural design accounting for large plate field rupture resulting
from explosion actions should be avoided due to the uncertainties
of the actions and the consequences of the rupture itself.
Sec.9 B
403 Structural support of
blast walls and the transmission of the blast action into main structural
members shall be evaluated when relevant. Effectiveness of connections
and the possible outcome from blast, such as flying debris, shall
be considered. Sec.9
B 500 Unintended
flooding
Sec.9 B
501 The structural design of the hull against unintended flooding
shall be based on the deepest equilibrium waterline in damaged condition
obtained from damage stability calculations.
Sec.9 B
502 The permissible stresses for local scantling, e.g. plating,
stiffener and girder, in a flooded condition may be taken as 220f1 for normal stresses and 120
f1 for shear stresses
in accordance with DNV Rules for Classification of Ships Pt.3 Ch.1.Sec.9
B 600 Collision
Sec.9 B
601 Collision with a typical supply boat is normally not affecting
the structural integrity as long as the unit complies with stability
requirements from national or international bodies. Collision with
supply boat and accidental flooding are thus not considered in this
standard.Sec.9
B 700 Extreme weather
condition
Sec.9 B
701 Units designed not to disconnect and escape from the extreme
environmental loading condition, as a hurricane or typhoon, shall
be designed using the 100-years return period for the actual loading
as an accidental case, ref. Sec.3 B500. Sec.9
B 800 Loss
of heading control
Sec.9 B
801 For units normally operated with heading control, either by
weather vaning or by thruster assistance, the effects of loss of
the heading control shall be evaluated.
Sec.9 B
802 The loss of heading control condition shall be considered
in the hull, topside and turret structural design.
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