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Sec.7: Fatigue [Table of Contents] Sec.9: Weld Connections

DNV-OS-C201 Structural Design of Offshore Units (WSD method)

[-] Sec.8: Accidental Conditions

SECTION 8
Accidental Conditions

Sec.8
A. General

Sec.8
A 100   General

Sec.8 A
101   Accidental conditions shall in principle be assessed for all units. Safety assessment is carried out according to the principles given in DNV-OS-A101.

Sec.8 A
102   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.8 A
103   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. Examples of the latter are compartmentation of floating units which provides sufficient integrity to survive certain collision scenarios without further calculations.

Sec.8
B. Design Criteria

Sec.8
B 100   General

Sec.8 B
101   Structures shall be checked for accidental loads in two steps, according to the loading conditions presented in Sec.2 Table D1:
—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) and e).

The unit shall be designed for environmental condition corresponding to 1 year return period after accidental damage.

Sec.8 B
102   Typical accidental loads of relevance for mobile offshore units are:
—impact from ship collisions
—impact from dropped objects
—fires
—explosions
—abnormal environmental conditions
—accidental flooding.

Generic values of accidental loads are given in DNV-OS-A101.

Sec.8 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.8 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.8 B
105   If non-linear, dynamic finite element 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.8
B 200   Collision

Sec.8 B
201   Ship collision, e.g. by supply vessels, shall be considered as relevant for the unit operation and transit regions.

Sec.8
B 300   Dropped objects

Sec.8 B
301   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.8 B
302   Critical areas for dropped objects should be determined assuming a minimum drop direction within an angle with the vertical direction:
—5° in air, for bottom supported structures
—10° in air, for floating units
—15° in water.

Dropped objects should be considered for vital structural elements of the unit within the areas given above.

Sec.8
B 400   Fires

Sec.8 B
401   The structure that is subjected to a fire shall maintain sufficient structural before evacuation has occurred. The following fire scenarios shall be considered:
—jet fires
—fire inside or on the hull
—fire on the sea surface.


Sec.8 B
402   Assessment of fire may be omitted provided fire protection requirements made in DNV-OS-D301 are met.

Sec.8
B 500   Explosions

Sec.8 B
501   In respect to design, 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 overpressures
—locate hazardous areas in enclosed locations and install pressure relief mechanisms (e.g. blast panels) and design for the resulting overpressure.


Sec.8 B
502   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.8 B
503   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.8
B 600   Unintended flooding

Sec.8 B
601   Heeling of the unit, during transit condition, after damage flooding as described in DNV-OS-C301 shall be accounted for in the structural strength. Maximum static allowable heel after accidental flooding are specified in Sec.11 to Sec.14 for the different types of units. Structures that are wet when the static equilibrium angle is achieved shall be checked for external water pressure.

Sec.8 B
602   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.8 B
603   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.
Sec.7: Fatigue [Table of Contents] Sec.9: Weld Connections