BIBLIOGRAFÍA

In practice it is not realistic to consider fires and explosions in isolation; it is not only impossible to fully isolate the two phenomena, but to do so risks missing potential high-risk events. This applies particularly where a fire analysis ignores potential damage caused by a preceding explosion or where an explosion occurs during a fire.

The nature of the interaction between explosion and fire will depend upon whether an explosion precedes a fire (the usual base case) or whether it occurs during a fire. The effects of interaction are discussed in the following sections.

For the purposes of this section, explosion shall be assumed to include the effects of projectiles.

4.5.2 Fire response of explosion damaged structures

4.5.2.1 General

There are four categories of explosion damage to structures, three of which may affect subsequent fire endurance.

1. A structure, which has responded to explosion while remaining in the elastic deflection range everywhere and without connection failures. A Category 1 structure can be considered to have been unaffected by explosion when considering its response to fire.

This is the case for structures subject to explosion within the SLB range.

2. A structure, which has responded to an explosion with plastic deformation but without connection failures. A Category 2 structure will be unaffected in its response to fire except in respect of:

o Possible damage to PFP (e.g. due to substrate strains), but noting that there are extremely limited data available on PFP damage following an explosion;

o Loss of straightness of members subject to buckling loads;

o Deformation of supported equipment and pipes;

o Loss of pipe/equipment support.

3. A structure that has responded to explosion loading with or without connection failures (local or global). Category 3 structures will be weakened and behave differently in fire scenarios, compared to undamaged structures, with much reduced fire endurance.

4. A structure, severely damaged by explosion with loss of entire segments of the structure.

Categories 2, 3 and 4 damage relate to structures designed to resist explosion in DLB range.

4.5.2.2 Analytical treatment of explosion damaged structure

For Category 1 damage in an explosion (SLB) it is assumed that there is no weakening of structure with respect to fire endurance hence fire and explosion can be considered independently by different techniques, if required.

In practice it is very difficult to perform fire response analysis of explosion-damaged structure, where the explosion damage may have reduced the reserve structural capacity for dead loads.

The two practical difficulties are:

1. Calculating the reserve structural capacity for distorted and/or weakened structures;

2. Covering a suitable range of explosion damage scenarios.

It is therefore recommended to design the main parts of the structure to survive design events with Category 1 (e.g. 10⁴ years return period) or modest Category 2 damage. In practice this will involve optimising the overall layout of the topsides facilities to minimise explosion pressures.

For damage corresponding to Categories 2, 3 or 4, it would be necessary to apply a structure model that has been fully modified to take account of the explosion damage that has occurred prior to fire.

This is a particularly advanced type of analysis but could be practical where the non-linear software can determine both fire and explosion response. It is probably necessary to account both for geometry changes and the straining that has occurred in strained members and this will affect the material model for those members. For this reason it may not be suitable to use different software for the fire and explosion response and merely use the output geometry from the non-linear explosion software as input to the fire-response software.

4.5.3 Explosion response of structures at elevated temperatures

4.5.3.1 General

As temperature increases, the yield stress and Young’s modulus of metals decrease. This can result in comparatively small temperature rises resulting in a considerable increase in explosion related deflection. This applies particularly where a component is designed to resist explosion through plastic deformation. Explosions during fire can sometimes result from equipment or vessel BLEVEs due to heating in fire or a delayed effect of explosions in one area on an adjacent area, already in flames. This is part of a complex domino situation where a first area is in flames and the explosion in that first area has caused leaks in a second (adjacent) area, and it takes some time before the leaks in the second area ignite and cause the second explosion, causing explosion overpressures in the first area.

Unless, analyses of escalation identify clear limits to potential damage, it is recommended that the fire hazard strategy assumes a “burn down philosophy” and that the fire risk analysis should confirm that the TR is not destroyed with an unacceptable frequency, in which case, a different solution will be required (e.g. a revised layout or separate accommodation jacket).

Other damage can occur due to projectiles caused by vessel BLEVEs and to a lesser extent from pipe failures. Another source of damage may be equipment and structure falling from areas above that has become weakened by fires. The higher the module stack, the more damage a dropped could cause. Where appropriate, this aspect needs to be linked to dropped-object hazard evaluation. On F(P)SOs and converted jack-up type substructures the damage consequence due to impact with the deck might be large and the protection requirements difficult to meet without heavy protection such as thick steel plates or Bi-steel.

4.5.3.2 Analytical treatment of fire-damaged structure

Rigorous numerical analysis for explosion effects on fire-damaged structure is currently not practical in most cases, though the advanced non-linear techniques briefly alluded to in Section 4.5.2.2 might be applicable here.

Coping with the explosion after fire scenarios is principally achieved with a suitable barrier philosophy and distancing (sensitive equipment and structure from hazard). For vessel BLEVEs distancing will not usually be sufficient due to long projectile trajectories. Barriers, (which may be walls or other equipment installed between the location of the BLEVE and vulnerable targets) can reduce projectile trajectories and will be required if projectiles form a significant component of the continuing escalation hazards.

From the practical standpoint, where an explosion during a fire is a significant risk, the temperature of structural steelwork may need to be kept significantly lower than for fire-only loaded steelwork. This is to improve resistance to resist the secondary explosions. This can be accommodated by more extensive application of PFP and this factor should be borne in mind when contemplating reducing the extent of PFP coverage to meet only specific fire scenarios.

An additional consideration is that where there is a significant risk of an explosion during a fire it is necessary to ensure adequate strength and bonding or fixing of passive fire protection materials at high temperatures.

4.6 Safety conflicts