Risk-Based Approval
4.2 Acceptance of the Alternative Design
4.2.7 Different Acceptance Criterion Philosophies
SOLAS provides the possibility of using alternative designs. Often, however, it is left to “the satisfaction of the Administration”, whether a design is acceptable or not – and various philosophies behind the accept criteria can be applied.
4.2.7.1 Risk Evaluation Criteria at IMO
Risk evaluations criteria normally place the risk in one of three categories; intol- erable/unacceptable, tolerable and negligible/broadly acceptable. Risks that are as- sessed to lie between the boundaries for intolerable and negligible will normally be required to be kept “As Low As Reasonably Practicable” or “ALARP”. Additional criteria for what is reasonably practicable are thus needed and for safety this is often given in terms of the Cost of Averting a Fatality, e.g. CAF.
Similar cost effectiveness criteria for environmental risks related to accidental oil spills are the Cost of Averting a Ton of oil Spilt, e.g., CATS. The currently available risk evaluation criteria are summarized in Chap. 3.
Available risk acceptance criteria are related to safety of human life. Both indi- vidual and societal criteria are documented and their use is promoted by the FSA guidelines.
Risk acceptance criteria related to the protection of the environment beyond the CATS criteria are not yet documented but work is being conducted to develop a societal acceptance criterion related to oil spills.
This issue is described in detail in Chap. 3 of this handbook.
4.2.7.2 Risk Acceptance Criteria for Main Ship Functions
Setting acceptance criteria for ship functions requires that the functions are defined first. Among these functions are structural integrity, watertight integrity, stability, the capabilities for propulsion, power supply, communication, navigation, manoeu- vrability, sea-keeping, emergency control, habitability and cargo handling. Work reported in Chap. 3 addresses acceptance criteria for these functions and concluded that additional work needs to be done to establish acceptance criteria.
It was underlined that for several ship functions and systems, safety is not be the dimensioning issue for acceptance criteria may therefore be irrelevant. Instead, reliability or availability dimensioned by commercial considerations is expected to be the more important requirements for functions such as propulsion availability and capability.
Based on work related to target reliabilities for structures the SAFEDOR report proposed as general procedure to determine acceptance criteria for main ship func- tions and systems the following four steps:
• Develop a risk model that include the function in question–all scenarios that are affected
• Use Cost-Effectiveness criteria
• Derive the requirement (availability, target reliability etc.)
• Use this as target in Risk Based Design.
This issue is described in some detail in other Chap. 3 of the handbook.
4.2.7.3 Cost-Effectiveness
Current decision-making with relation to safety at IMO and classification societies employs cost effectiveness as central decision-making criterion. It is well rooted and used within the ALARP principle. Main advantages are that existing criteria (CAF, CATS) can be used and easy integration into the traditional design framework. How- ever, only design changes can be assessed. Note that this apparent disadvantage is not considered a true disadvantage, since most ship designs are not completely new but are almost always adaptations of existing designs. It is therefore clearly recom- mended to use cost-effectiveness for decision-making in risk-based design.
To find an adequate balance, we need to be able to draw on qualified person- nel, acquainted with the process, the philosophy and the techniques of risk based approval.
This issue is described in detail in other chapters of this book.
4.2.7.4 Risk Balance
In principle, safety equivalency can be used for all functions, systems, sub-systems and components. Based on the same philosophy as for the damage stability require- ments, each function could be treated the same way, imposing similar requirements.
This is illustrated in the figure below, using the symbols from the approved new damage stability regulations (The attained index (A), which is an estimate of the probability of surviving a collision with water ingress, is required to be larger than the R; R is then a risk-based acceptance criterion for damage stability).
This concept may be illustrative as it is intuitive. This is, however, a simplification that will not be generally valid. The condition for using this approach is that the innovative design solutions do only affect one of the functions or systems. In most cases an innovative solution may affect multiple function and accident scenarios. In such cases this description may be too simplistic.
The second route would need explicit safety levels against which to design. This second route probably is necessary for future trade-offs between several aspects of ship design (e.g., damage stability and life saving). Safety levels and acceptance criteria are key elements of the future regulatory framework and the next section discusses these in more detail.
In the short- and medium-term, only partially risk-based designed ships are ex- pected. Thus, prescriptive and probabilistic rules need to be applied together. This coexistence certainly is a challenge for the regulator as elements from one design as- pect (e.g., bulkhead positions related to damage stability) frequently influence other design aspects (e.g., cargo capacity, strength and outflow of fluids, to name a few).
For each element in ship design challenging current rules in isolation, safety equivalency appears to be the best way ahead for the time being. It offers the de- signer freedom for alternative arrangements and/or equipment and offers to the reg- ulator a method for approval which closely follows the SOLAS-II.2/17 approach.
Fig. 4.8 Risk balanced acceptance criteria
The challenge, however, is to define the equivalent design and designing a second ship just for reference purpose is certainly not cost-effective.
The regulatory framework of the future has to facilitate full optimization of safety which is expected to become visible when two or more aspects or functions of ship safety are becoming traded off against each other. It is well known that many ship functions contribute to the overall safety of the vessel. What is not known explicitly is the share each function actually contributes (Fig. 4.8).
A collection of ship functions that contribute to safety are presented in the figure with elements marked in Blue indicating that requirements Rxy are set by IMO and/or class, and with orange-marked elements indicating those requirements which are probably specified by the owner, i.e., owner’s requirements are stricter than IMO requirements. Note that for each function, an individual R will have to be specified together with a procedure to determine A.