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Slide 1 · LEARN
LEARN
DISCUSS
GROW
SAME OCEAN. DIFFERENT APPROACH.
DETERMINISTIC
VS
PROBABILISTIC
DAMAGE STABILITY
Two methods. One goal.
A safer ship.
UNDERSTAND
THE DIFFERENCE
THAT MATTERS
Slide 2 · 01/04
SOLAS DAMAGE STABILITY
DETERMINISTIC vs
PROBABILISTIC
Two different ways to answer one question:
“Will the ship survive after damage?”
DETERMINISTIC
CAN THE SHIP SURVIVE
A PRESCRIBED DAMAGE?
A specified damage
scenario is assumed,
compartments are flooded,
and the ship must satisfy
prescribed residual
stability criteria.
PROBABILISTIC
WHAT IS THE OVERALL
PROBABILITY THAT
THE SHIP SURVIVES?
Different possible damage
locations and extents are
considered statistically,
with each damage case
contributing according
to its probability and
survival.
KEY DIFFERENCE
Deterministic → Prescribed damage cases
Probabilistic → Probability of damage × probability of survival
The IMO describes the revised SOLAS Chapter II-1 framework
as a harmonised probabilistic approach to subdivision
and damage stability.
Slide 3 · 02/04
SOLAS DAMAGE STABILITY
DETERMINISTIC
APPROACH
ASSUME THE DAMAGE, THEN PROVE SURVIVAL
Under the applicable deterministic standard, the damage extent
and loading assumptions are prescribed by the governing instrument.
TYPICAL PRESCRIBED PARAMETERS
(E.G. TRADITIONAL SOLAS PASSENGER SHIP STANDARD)
DAMAGE LENGTH
TRANSVERSE
PENETRATION
VERTICAL EXTENT
3 m + 3% of L, or 11 m
whichever is less
20% of B
As prescribed by the
applicable damage-
stability regulation.
AFTER FLOODING
The ship is assessed for:
Final equilibrium condition
Heel / list
Residual stability
GZ curve
Progressive flooding
Applicable heeling moments
Prescribed compartment(s) are flooded
and the ship must satisfy the required
residual stability criteria.
IMPORTANT
These numerical assumptions must not be applied universally
to every ship type. The governing Code or Convention
determines the applicable damage scenario.
Slide 4 · 03/04
SOLAS DAMAGE STABILITY
PROBABILISTIC
APPROACH
THINK IN PROBABILITIES
Instead of one prescribed damage case, the probabilistic method evaluates many possible damage cases.
1 DAMAGE PROBABILITY
p_i
Probability that a particular compartment or group of compartments is flooded.
Determined using the prescribed SOLAS probability methodology for damage location and extent.
2 SURVIVAL PROBABILITY
s_i
Probability of survival after flooding the compartment or group of compartments.
Incorporates the residual stability effects, including horizontal subdivision.
3 EACH DAMAGE CASE CONTRIBUTES
A_i = p_i × s_i
The attained index for each damage case is the product of the probability of the damage and the probability of survival.
For each loading condition the partial attained index is obtained by summing over all damage cases:
A = Σ p_i s_i
THREE REFERENCE DRAUGHTS
The assessment is carried out for three loading conditions:
d_s
Deepest subdivision draught
d_p
Partial subdivision draught
d_l
Light service draught
REFERENCE
The definitions and calculation framework come from SOLAS Chapter II-1, Regulations 6 and 7, as amended by Resolution MSC.216(82).
Slide 5 · 04/04
SOLAS DAMAGE STABILITY
THE FORMULA
THAT MATTERS
ATTAINED SUBDIVISION INDEX
The overall attained subdivision index is calculated using three loading conditions:
A = 0.4As + 0.4Ap + 0.2Al
Where:
As = partial index at deepest subdivision draught
Ap = partial index at partial subdivision draught
Al = partial index at light service draught
PARTIAL INDEX CALCULATION
For each loading condition, the partial attained index is obtained by summing over all damage cases:
A = Σ pisi
pi = probability that a compartment or group of compartments is flooded
si = probability of survival after flooding
PASS CONDITION
The design complies when the attained subdivision index meets or exceeds the required subdivision index:
A ≥ R
A ≥ R alone is not sufficient under MSC.216(82). The partial indices must also meet the applicable limits.
PARTIAL INDEX REQUIREMENTS
PASSENGER SHIPS
As, Ap, Al ≥ 0.9R
CARGO SHIPS
As, Ap, Al ≥ 0.5R
REQUIRED SUBDIVISION INDEX (R)
R is calculated according to SOLAS II-1 Regulation 6 and depends on the applicable ship type, length and, for passenger ships, the specified number of persons on board.
KEY TAKEAWAY
Deterministic:
Prescribed damage → Survive it
Probabilistic:
Probability of damage × Probability of survival → Attained A
Drafted by SailR from our daily lessons. AI-drawn illustration. Simplified to explain the idea, not to scale. Last updated 19 Oct 2026.
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