ANNEX M SPECIAL DESIGN PROVISIONS FOR COLUMN STABILIZED UNITS
M.6 ACCIDENTAL LIMIT STATES (ALS)
M.6.1 General
General guidance and requirements concerning accidental events are given in Chapter 9, and Annex A.
Units shall be designed to be damage tolerant, i.e. credible accidental damages, or events, are not to cause loss of global structural integrity, (see M.9.1). The capability of the structure to redistribute actions should be considered when designing the structure.
In the design phase, attention shall be given to layout and arrangements of facilities and equipment in order to minimise the adverse effects of accidental events.
Satisfactory protection against accidental damage may be obtained by a combination of the following principles :
• reduction of the probability of damage to an acceptable level, and,
• reduction of the consequences of damage to an acceptable level.
Structural design in respect to the Accidental damage Limit State (ALS) shall involve a two-stage procedure considering :
• resistance of the structure to a relevant accidental event, and,
• capacity of the structure after an accidental event.
Global structural integrity shall be maintained both during and after an accidental event. Actions occurring at the time of a design accidental event and thereafter shall not cause complete structural collapse, or loss of hydrostatic (or hydrodynamic) stability (See M.8).
Requirements to compartmentation and stability in the damage condition are given in M.8.4. When the upper hull (deckbox) structure becomes buoyant in satisfying requirements to damage stability, /1/, consideration shall be given to the structural response resulting from such actions.
M.6.2 Dropped objects
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.
The impact energy at sea level is normally not to be taken less than 5 MJ for cranes with maximum lifting capacity of more than 30 tonnes, however, reduced impact energy may be acceptable for smaller cranes and special purpose cranes.
The impact energy below sea level is assumed to be equal to the energy at sea level, unless otherwise documented.
Generally, dropped object assessment will involve the following considerations :
• assessment of the risk and consequences of dropped objects impacting topside, wellhead (e.g. on the seafloor), and safety systems and equipment. The assessment shall identify the necessity of any local structural reinforcement or protection to such arrangements.
• assessment of the risk and consequences of dropped objects impacting externally on the hull structure. However the structural consequences are normally fully accounted for by the requirements for watertight compartmentation and damage stability (see M.8) and the requirement for structural redundancy of slender structural members (see M.9.1).
M.6.3 Fire
General guidance and requirements concerning accidental limit state events involving fire are given in Chapter 9 and Annex A.
M.6.4 Explosion
In respect to design considering actions resulting from explosions one, or a combination of the following main design philosophies are relevant :
• Ensure that the probability of explosion is reduced to a level where it is not required to be considered as a relevant design loadcase.
• 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.
As far as practicable, structural design accounting for large plate field rupture resulting from explosion actions should normally be avoided due to the uncertainties of the actions and the consequences of the rupture itself.
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.
M.6.5 Collision
Collision shall be considered as a relevant ALS loadcase for all structural elements of the unit that may be impacted in the event of a collision. The vertical zone of impact shall be based on the depth and draught of the colliding vessel and the relative motion of the vessel and the unit.
In the assessment of the collision condition, the following general considerations with respect to structural strength will normally apply :
• An evaluation shall be undertaken in order to assess the extent of structural damage occurring to the unit at the time of impact.
• The extent of the local damage resulting from the collision should be compared to that damage extent implicit in the NMD regulations covering damage stability of the unit, /1/. Provided that the extent of local damage does not exceed that damage criteria stated in the NMD regulations, the unit shall satisfy the relevant damage stability requirements of the NMD. See also M.8.4. If the extent of the damage exceeds that damage criteria stated in the NMD regulations, /1/, an equivalent level of safety to that implicit in the NMD stability regulations should be
documented.
• NMD damaged condition requirements, /1/, in respect to structural strength of watertight boundaries (including boundaries required for reserve buoyancy) shall be satisfied (see M.6.8).
• Global structural integrity of the unit after the collision shall be evaluated.
• Topside structural arrangements should be designed for the damaged (inclined) condition.
Considerations concerning structural evaluation at, and after, the time of the collision are given below.
Annex M Rev. 1, December 1998 Damage occurring at the time of collision
A structural evaluation shall be performed in order to document the extent of the local damage occurring to the unit at the time of impact.
If a risk analysis shows that the greatest relevant accidental event with regard to collision is a drifting vessel at 2m/s, with a displacement which does not exceed 5000 tonnes, the following kinetic energy occurring at the time of collision may be considered for the structural design ;
• 14 MJ for sideways collision, and,
• 11 MJ for bow or stern collision.
Local damage assessment may be undertaken utilising sophisticated non-linear analytical tools, however, simplified analytical procedures will normally be considered as being sufficient to evaluate the extent of damage occurring under the action of the collision.
Simplified local damage assessment of the collision event normally involves the following considerations :
• The typical geometry of the supply vessel, together with relevant force-deformation curves for side, bow and stern impact, documented in Annex A, may normally be utilised.
• In the local structural strength assessment the side, bow and stern profiles of the supply vessel are progressively 'stepped' into the collision zone of the column stabilised unit, see example shown in Figure M.6-1.
Figure M.6-1 Illustration of the bow profile of a supply vessel being 'stepped' into the structure of a column stabilised unit.
• By considering the local geometry of the supply vessel and the impacted structure, relevant force-deformation curves for the column stabilised unit may be produced.
• For a given action level the area under the force-deformation curves represents the absorption of energy. The distribution and extent of the damage results from the condition of equal collision force acting on the structures, and that the sum of the absorbed energies equals the portion of the impact energy dissipated as strain energy, i.e.
∫
∫ +
=
+ 0Δu u u u
Δs
0 s s s
u
s E P (δ )dδ P (δ )dδ
E (M.6.1)
Where :
u s,P
P he force in the impacting vessel and the impacted unit respectively, and,
u s,δ
δ he deformations in the impacting vessel and the impacted unit respectively.
This procedure is illustrated in Figure M.6-2.
Figure M.6-2 Dissipation of strain energy global structural integrity after collision
Having evaluated the extent of local damage incurred by the relevant collision event (as described above) an assessment of the resulting, global structural capacity (considering environmental actions) shall be undertaken. In such an evaluation the following listed items are relevant :
• In cases where the impact damage is limited to local damage to the column particular consideration should be given to column/deckbox interfaces, and the damaged (impacted) section. (For typical column structures a simplified approach to assess the reduced capacity of the structure in way of the damaged location would be to assume that all the impacted
(deformed) area is ineffective.)
• When counter-flooding is utilised as a means of righting the unit in an accidental event the actions resulting from such counter flooding shall be evaluated.
Annex M Rev. 1, December 1998
• In cases where the impact damage is limited to local damage to a single brace, redundancy requirements given in M.9.1 should adequately cover the required structural evaluation.
• NMD requirements to watertight boundary, structural strength in the damaged condition, see M.6.8, (including inclined angles resulting from requirements to reserve buoyancy) should be satisfied.
• In order to avoid risk assessment considerations in respect to implications of structural failure of topside structures in the inclined condition, (e.g. in respect to the possibility of progressive collapse in the event of structural failure) it is normal practice to consider the structural capacity of topside structural arrangements in the damaged condition.
Capacity exceedances may be accepted for local areas provided that adequate account is given to the redistribution of forces.
Due to the large heel angles in the damaged condition, the in-plane force component of the deckbox mass may be considerable. Normally part of the deck will be submerged and counteract this force.
The most critical condition is therefore generally the heel angle corresponding to a waterplane just below the deckbox corner.
M.6.6 Unintended flooding
Considerations in respect to unintended flooding are generally system and stability design considerations rather than structural design. Requirements with respect to intact and reserve buoyancy conditions are normally considered to adequately cover any structural strength requirements (see M.6.8) in respect to unintended flooding.
M.6.7 Abnormal wave events
Air gap considerations with respect to evaluation of the abnormal wave events are given in M.7.
Should any part of the structure receive wave impact actions in the abnormal wave condition the consequences of such wave impacts shall be evaluated.
M.6.8 Reserve buoyancy
Structural strength of watertight boundaries (both internal and external watertight boundaries, including all stiffeners and girders supporting the plate fields) shall comply with the requirements stated in NMD regulations, /1/.