GOAL-BASED NEW SHIP CONSTRUCTION STANDARDS Safety level approach by zpx18157




MARITIME SAFETY COMMITTEE                                                                                  MSC 81/6/14
81st session                                                                                             21 March 2006
Agenda item 6                                                                                       Original: ENGLISH


                                Safety level approach – worked example

                                             Submitted by Germany

    Executive summary:        This document is intended to support the proposed “safety level
                              approach” in goal-based new ship construction standards by providing
                              for a “worked example”
    Action to be taken:       Paragraph 3
    Related documents:        MSC 81/6/1, MSC 81/6/2 and MSC 81/INF.6

1       This document comments on documents MSC 81/6/1, MSC 81/6/2 and MSC 81/INF.6
and is submitted in accordance with the provisions of paragraph 4.10.5 of the Guidelines on the
organization and method of work (MSC/Circ.1099 – MEPC/Circ.405). Especially, this
document refers to documents MSC 81/6/2 and MSC 81/6/8.

2      In an attempt to illustrate the structure of the safety level approach, Germany has
developed the attached worked example.

Action requested from the Committee

3        The Committee is invited to note the information and take action as deemed appropriate.


                          For reasons of economy, this document is printed in a limited number. Delegates are
                          kindly asked to bring their copies to meetings and not to request additional copies.
                                                                                       MSC 81/6/14



           IMO Mission Statement                                                     Tier 0

The mission statement is taken as the basis for the GBS framework:

“The mission of the International Maritime Organization (IMO) is to promote safe, secure,
environmentally friendly and sustainable shipping.”

Goals are specified, the fulfilment of which would ensure that the mission is met.

(From the four aspects of shipping in the mission statement, for the purpose of this example, this
document only deals with safety.)

           Goals                                                                     Tier 1

Regarding safety an overall goal may read:

Overall Goal:

“Ships are to be designed and constructed to be safe, when properly operated and maintained
under the specified operating and environmental conditions, in intact and specified damage
conditions, throughout their life.” (Source: GBS working group)

The term ‘safe’ means that specified, acceptable safety levels are met, regarding the risk to
persons (e.g. crew, passengers), to the ship and to the environment. The widely used principle
for determining criteria for acceptable risks is the ALARP1 principle. The ALARP principle
dictates that risks should be managed to be ‘As Low As Reasonably Practicable’. Both risk levels
and the cost associated with mitigating the risks are considered, and all risk reduction measures
should be implemented, as long as the cost of implementing them is within acceptable limits.

Such Safety levels may be determined e.g. by use of FSA or found by comparison of inherent
safety levels contained in current shipping regulations, or by comparison with other industries.

IMO/flag States may set the target safety levels related to protection of life at sea and the
environment based on analysis of historic data and political requirements.

The goals to be formulated in Tier 1 would contain the target safety levels for human safety and
environmental safety.

        The ALARP principle, As Low As Reasonably Practicable, is defined in IMO FSA Guidelines
        (MSC/Circ.1023 – MEPC/Circ.392).

MSC 81/6/14
Page 2

Goals for human safety had previously been proposed as (MSC 72/16):

“The ship has to be designed, operated and maintained such that the risk of a crew member is
lower than 10-3 fatalities per vessel year.”

“The ship has to be designed, operated and maintained such that the risk of a passenger is
lower than 10-4 fatalities per vessel year.”

“The ship has to be designed, operated and maintained such that the risk of a member of the
public ashore is lower than 10-4 fatalities per year.”

These three examples of goals shall be seen as risk limits defining the intolerable risk area. The
negligible risk limit together with the CAF/CATS still remains to set the ALARP area and allow
for risk mitigation being considered by cost effectiveness assessment.

Hence parameters which must be established by e.g. IMO are the essential parameters in the
safety level approach.

Top-level safety objective           Intolerable              risk Negligible risk CAF/CATS
                                     level                         level
Safety of the ship                           10-a                        10-A        h-value
Safety of the cargo                          10-b                        10 -B
Safety of passengers                         10-c                        10 -C
Protection of the environment                10-d                        10 -D
Safety of third parties                      10-e                        10 -E
Safety of the seafarer (occupational         10-f                        10 -F

           Functional requirements                                               Tier 2

Goals are achieved by a set of functions, e.g.:

In order to achieve the goal that passengers on board a ship are not exposed to a risk exceeding
the safety level defined in Tier 1, the ship has to provide several functions, such as:

-   Manoeuvrability                               -   Fire protection
-   Sea keeping performance                       -   Watertight integrity
-   Stability and floatability                    -   Safety of navigation
-   Emergency protection                          -   Ship structure
-   Habitability                                  -   Propulsion
-   Power generation                              -   Other systems
-   Life-saving appliances

All of these functions may be further broken down into sub-functions.

                                                                                      MSC 81/6/14
                                                                                           Page 3

Setting target failure probabilities

It is obvious, that the loss of a function, or a malfunction, affects the safety of the ship. And the
more often such failures would occur, the more at risk, the less safe, are the ship, crew,
passengers, etc.

Thus, in order to reach the goals with the safety levels defined in Tier 1, for each of the functions
a target failure probability has to be defined. The setting of these values will have to consider the
proportionality between the function failure and its consequences regarding safety.

Such target failure probability may read:

The structural strength of a ship shall not exceed an annual failure probability of 10-x.

At present, the GBS debate addresses structural strength, among other aspects related to ship
construction standards.

Structural strength is a function and within Tier 2 the requirements to be achieved for this
function have to be defined.

Functional requirement for structural strength:

Such requirement may read:

The structural strength shall be such that during the entire service life of the ship:

-   the structure withstands the environmental impacts (e.g. waves, corrosion) (with a
    sufficient reliability),

-   withstands impacts from the designated use of the ship (e.g. cargo loads, handling)
    (without reduction of its structural capability),

IMO/flag States may set the target failure probabilities based on analysis of existing ships and
formal safety assessment. The target failure probabilities must be set such that the target safety
levels from Tier 1 can be met.

             International Rules and Regulations                                    Tier 4

Calibrating detailed rules

Based on functional requirements, quantified with appropriate target failure probabilities,
classification rules may be developed for Tier 4.

Following the above example on structural strength and hull girder capacity:

The development of class rules would have to consider all relevant aspects which have influence
on the hull girder, e.g. environmental loads, ship loading conditions, material strength.

MSC 81/6/14
Page 4

The uncertainties in these aspects, also in calculation models for hull girder strength, can be
calibrated by so-called partial safety factors.

The setting of these safety factors can be based on structural reliability analysis (SRA) of the
loading and capacity for a set of sample ships.

The partial safety factors must be calibrated such that the target failure probability assigned for
hull girder capacity is met.

As a result, the class rules would transfer the functional requirements into
deterministic/prescriptive requirements for a specific component or system. However, they
would be based on a risk-based approach (here: use of SRA), ensuring that the overall risk-based
goals would be met.

           Validation and Verification                                            Tier 3

Verifying compliance

Once classification rules are amended or updated, classification societies have to provide a proof
– the so-called rule commentary – that ships designed based on any new rule with the derived
partial safety factor have a lower or equal failure probability than prescribed in Tier 2.

This commentary can be reviewed to show compliance with the safety levels and the functional
requirements of the higher Tiers.



To top