Section
2 Survivability
2.1 General
2.1.1 Survivability
is defined as the probability that a ship can remain operational to
some degree following an attack. The elements of whole ship survivability
are shown in Figure 1.2.1 Whole ship structural survivability. Survivability
is divided into two main aspects:
- Susceptibility, the probability of a threat acquiring, reaching
and detonating on a ship.
- Vulnerability, the probability that a ship will be able to survive
a successful attack and operate at a certain level.
Survivability is normally calculated as the product of susceptibility
and vulnerability. Recoverability is an important aspect as it has
a significant influence on the vulnerability of the overall ship as
a system. It can be defined as a measure of the ability of the ship
to reach a particular level of operation higher than that immediately
following the hit. A variety of levels of operation required following
damage can be defined, see
Vol 1, Pt 4, Ch 1, 2.2 Vulnerability
2.1.2 Generally
there are four basic phases in the classification of naval ships with
respect to survivability:
- Concept phase.
- Assessment phase.
- Build phase.
- Maintenance phase.
2.1.3 The concept
phase is not normally part of classification and is a discussion
between the Owner, Naval Administration, designer and those specialists
able to perform the appropriate calculations. It is used to identify
the potential threats, requirements for the ship structure, machinery
and systems with respect to those threats.
Figure 1.2.1 Whole ship structural survivability
2.1.4 The concept
phase will apply all of the elements shown in Figure 1.2.1 Whole ship structural survivability:
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Identification
of the threat is first and this is usually determined for current
and future threats. Several threats will be identified which affect
the ship in a variety of ways, they may be underwater or above water,
far field, close in, or contact weapons. Typical threat groups are
given in Table 1.3.1 Relationship between
notations.
-
The susceptibility
is the ability of the threat to reach the ship and detonate and is
a function of the capability of the threat, the ship’s signatures
and the ship’s defence systems. Various computer codes and simulations
are available for determining the susceptibility of a hull and the
capability of weapon systems. Consideration should be given to the
degradation of a threat by a ships defence systems.
-
If a threat detonates,
damage may result. The extent and amount of damage is a function of
the vulnerability of the ship, see
Vol 1, Pt 4, Ch 1, 2.2 Vulnerability. A vulnerability assessment may
be used to determine the consequences of the threat detonation. The
consequences are likely to be in the form of fire, flood and physical
damage as shown in Figure 1.2.1 Whole ship structural survivability.
The assessment tools used in the design stage for vulnerability analysis
generally employ simple design formulae which are then verified during
the assessment phase.
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The consequences
of damage can be limited through recoverability, the ability to repair
damage to structure, equipment and systems. This is mainly an operational
matter though it will have an impact on ship design. Damage control
operational procedures will make certain demands on structure and
equipment.
2.1.5 It is
beyond the scope of the Rules to provide further detail on the concept
phase. However on request, LR is able to provide details of suitable
organisations who are able to give specialist advice as necessary.
2.1.6 The assessment phase looks in more detail at the vulnerability of
the ship and uses explicit calculations to assess the capability of the ship based upon
the relevant effects of threats such as blast pressure or fragment size. It is not
necessary to define the actual weapon, just the consequences or effects of threats. A
threat can produce a variety of effects, the manner in which the rules currently address
these effects is detailed in Vol 1, Pt 4, Ch 2 Military Load Specification. In naval ship classification, notations are used to denote that
a calculation for a particular threat has been reviewed. Currently, these are concerned
with structural aspects only, though some aspects of machinery are indirectly addressed
through other notations such as propulsion machinery redundancy, PMR, steering
gear machinery redundancy, SMR and fire safety, FIRE.
2.1.7 The build
phase ensures that the requirements of the assessment phase
are put in place on board the ship. It is identical to the normal
classification requirements for construction, installation and testing
of structure and equipment under LR survey, verifying that the correct
materials, welding fabrication and testing procedures are used.
2.1.8 The maintenance
phase is applied by maintaining a ship in class through life.
It verifies that the original standard to which the ship was built
is maintained and that any new rule requirements are implemented.
It also verifies that modifications to the ship do not compromise
the integrity of the structure, equipment or systems.
2.1.9 It is
the responsibility of the Naval Administration to ensure that each
of the phases is implemented, to define the requirements that are
to be met and advise the Owner on the manner in which particular threats
can be dealt with.
2.2 Vulnerability
2.2.1 The
resistance of a vessel to loadings from military threats can be described
by the term vulnerability which is the probability that once hit by
a specified threat a vessel will lose capability.
2.2.2 This
Chapter deals separately with the effects of a threat on the structure
but when considering the total vulnerability of the ship it will be
necessary to combine all the effects of a weapon detonation to determine
the total damage to the ship as a system. This is normally done very
early in the design stage at a low level of complexity, see
Vol 1, Pt 4, Ch 1, 2.1 General 2.1.4 Several computer codes are
available to determine the consequence of weapon threats on the ship
system.
2.2.3 The
damage to a ship is likely to occur by three mechanisms; fire, flood
and physical damage, see
Figure 1.2.1 Whole ship structural survivability The direct effect of threats on the ship’s crew
is not included in this Section but some features such as shelter
stations and CBRN protection will reduce the risk. Indirect damage
caused by the threat should also be considered and a vulnerability
analysis can be used to site magazines such that they are offered
the maximum protection.
2.2.4 The
methods used to control the spread of fire will have an effect on
structural design, materials, fire insulation, fire divisions and
openings, and must all be considered. One method used to control the
spread of fire is zones and some guidance on the philosophy and structural
requirements are provided in Vol 1, Pt 4, Ch 1, 7 Design guidance for nuclear, biological and chemical defence. However, the precise requirements for fire detection, protection
and extinction are to be provided in the specified fire safety standard(s).
Where this is examined by LR in accordance with Vol 1, Pt 1, Ch 2, 3.10 Other notations 3.10.9 a FIRE notation
may be assigned. Other methods include the use of smoke tight boundaries,
insulation and fire-fighting systems.
2.2.5 The
stability of a vessel and flooding following damage is not covered
in this Chapter. The specified subdivision and stability standard,
should define the extent of damage that the vessel is required to
survive and remain stable. All structure and watertight closing appliances
will need to be assessed to this level. Vol 1, Pt 3, Ch 1, 5 Definitions defines how the minimum extent of watertight subdivision
is to be determined. Vol 1, Pt 3, Ch 4 Closing Arrangements and Outfit contains
details on requirements for closing appliances. For such an extreme
event as flooding, plastic type analysis is appropriate for watertight
structure and this is recognised in the relevant rule requirements.
2.2.6 For
the hull, the effect of the threat can be limited in two ways:
- Ensuring that there is adequate global strength following damage
using a residual strength analysis. Where appropriate a whipping analysis
may also be necessary.
- Ensuring that local structure can contain the threat or limit
the damage. Individual items of structure can be hardened or strengthened
in certain areas to achieve this.
2.2.7 Physical
damage may occur to cables, piping, equipment and machinery, and other
systems. The duplication or protection of these items is dealt with
in Vol 2, Pt 1, Ch 3, 4.9 Military requirements. Where
protection is required, e.g. armour, the impact on the structural
design of the hull is to be considered.
2.2.8 Different
levels of vulnerability can be represented, as illustrated in Figure 1.2.2 Vulnerability contours for a given threat levelEach will have different
acceptance criteria for the hull structure. For an internally detonating
threat, the increasing levels can be visualised as a threat of increasing
magnitude rather than increasing distance.
2.2.9 Level
A is that, closer than which, the hull structure will fail due to
the detonation of a threat. The failure can occur in a number of ways
as detailed in Vol 1, Pt 4, Ch 2 Military Load Specification. At this
level the assessment will normally be performed using plastic criteria
which will result in permanent deformation. The ship may no longer
function effectively but it should remain afloat and not rupture or
fail catastrophically.
2.2.10 Level
B is that, closer than which, the majority of the ship’s machinery
and equipment is damaged such that it will not operate effectively
and the ship can no longer continue to navigate. The hull must therefore
not deform permanently and elastic assessment criteria may be necessary
to determine the global strength of the hull (local deformation may
be sustained).
Figure 1.2.2 Vulnerability contours for a given threat level
2.2.11 Level
C is that, closer than which, the ship’s weapon systems begin
to fail and the vessel is no longer able to operate with full effectiveness.
Normally the global and local criteria would be assessed against elastic
criteria.
2.2.12 Levels
B and C for underwater threats are primarily dealt with by adopting
a suitable shock policy for the ship.
2.2.13 It
is the responsibility of the Owner to specify the levels at which
these should be set. In theory, they could be made to be coincident
but this would provide little reserve within the vessel for recovery
by damage control and repair. Conversely, they should not be set too
far apart as this represents unnecessary armament and strengthening
which is not effectively protecting the equipment and machinery from
attack.
2.2.14 For
an assessment of a threat which also produces effects on machinery,
two structural calculations may have to be performed. One at the equipment
level of failure using failure criteria that result in little or no
deformation (plating only for example) and one approaching a structural
failure level using ultimate strength or plastic collapse criteria.
The requirements in Vol 1, Pt 4, Ch 2 Military Load Specification generally
deal with ultimate strength or plastic criteria, the conventional
rule calculations in Vol 1, Pt 6, Ch 2 Design Tools and Vol 1, Pt 6, Ch 5 Structural Design Factors set elastic failure criteria.
For normal naval ship construction, the hull is usually able to withstand
the threat level at which equipment and systems fail with little or
no permanent deformation, though some check calculations may be necessary
on critical areas.
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