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L: Fatigue [Table of Contents] App.A: Cross Sectional Types

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

[-] Sec.14: Special Considerations for Deep Draught Floaters (DDF)
[-] M: Accidental Condition

Sec.14
M. Accidental Condition

Sec.14
M 100   General

Sec.14 M
101   The objective of this subsection is to provide supplemental guidance related to design for accidental condition as outlined in Sec.7.

Sec.14 M
102   Units shall be designed to be damage tolerant, i.e. credible accidental damage, or events, should not cause loss of global structural integrity. The capability of the structure to redistribute loads should be considered when designing the structure.

Sec.14
M 200   Fire

Sec.14 M
201   Deck area will be limited for some DDF concepts. Potential fire scenarios shall therefore be carefully considered and taken into account in design and layout planning.

Sec.14
M 300   Explosion

Sec.14 M
301   As for fire, the limiting deck space and protected moon-pool area (potential gas or oil leakage) for some DDF units require that explosions are carefully considered in the design process.

Sec.14 M
302   In respect to design considering loads resulting from explosions one, or a combination of the following main design philosophies are relevant:
  1. Ensure that the probability of explosion is reduced to a level where it is not required to be considered as a relevant design loadcase.
  2. Ensure that hazardous areas are located in unconfined (open) locations and that sufficient shielding mechanisms (e.g. blast walls) are installed.
  3. Locate hazardous areas in partially confined locations and design utilising the resulting, relatively small overpressures.
  4. Locate hazardous areas in enclosed locations and install pressure relief mechanisms (e.g. blast panels) and design for the resulting overpressure.


Sec.14 M
303   As far as practicable, structural design accounting for large plate field rupture resulting from explosion loads should normally be avoided due to the uncertainties of the loads and the consequence of the rupture itself.Structural support of blast walls, and the transmission of the blast load 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.14
M 400   Collision

Sec.14 M
401   Safety assessments shall be the basis for determination of type and size of colliding vessel and impact speed.

Sec.14 M
402   Collision impact shall be considered for all elements of the unit, which may be impacted by sideways, bow or stern collision. The vertical extent of the collision zone shall be based on the depth and draught of attending vessels and the relative motion between the attending vessels and the unit.

Sec.14 M
403   Resistance to unit collisions may be accounted for by indirect means, such as, using redundant framing configurations, collision ring in splash zone and materials with sufficient toughness in affected areas.

Sec.14
M 500   Dropped objects

Sec.14 M
501   Critical areas for dropped objects shall be determined on the basis of the actual movement of potential dropped objects (e.g. crane actions) 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.14 M
502   Generally, dropped object assessment will involve the following considerations:
  1. Assessment of the risk and consequences of dropped objects impacting topside, wellhead, and riser system in moonpool and safety systems and equipment. The assessment shall identify the necessity of any local structural reinforcement or protections to such arrangements.
  2. Assessments of the risk and consequences of dropped objects impacting externally on the hull structure (shell, or bracings) and hull attachments such as strakes, fairleads and pipes. The structural consequences are normally fully accounted for by the requirements for watertight compartmentation and damage stability and the requirement for structural redundancy of slender structural members.

Sec.14
M 600   Unintended flooding

Sec.14 M
601   A procedure describing actions to be taken after relevant unintended flooding shall be prepared. Unintended filling of hard tanks, collision ring and bracings for a DDF will be the most relevant scenarios for the operation phase.

Sec.14 M
602   It must be ensured that counter-filling of tanks and unit uprighting can be performed safely and without delays.

Sec.14 M
603   Structural aspects related to the tilted condition and counter-flooding (if relevant) shall be investigated. This applies to the complete unit including risers and mooring system.

Sec.14 M
604   If the unit can not be brought back to the design draught and verticality by counter-ballasting and redistribution of ballast water, this must be taken into account in design of the unit.

Sec.14
M 700   Abnormal wave events

Sec.14 M
701   Abnormal wave effects are partly related to air-gap and wave exposure to deck or topside structures. Consequences from such wave impacts shall be evaluated and taken into account in design of the relevant structural parts.

Sec.14 M
702   In areas with hurricanes, special considerations have to be made with respect to selection of relevant sea states to be applied in design of the unit.
L: Fatigue [Table of Contents] App.A: Cross Sectional Types