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Risk-based rules for crane safety systems

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Abstract

The International Maritime Organisation (IMO) has recommended a method called formal safety assessment (FSA) for future development of rules and regulations. The FSA method has been applied in a pilot research project for development of risk-based rules and functional requirements for systems and components for offshore crane systems. This paper reports some developments in the project. A method for estimating target reliability for the risk-control options (safety functions) by means of the cost/benefit decision criterion has been developed in the project and is presented in this paper. Finally, a structure for risk-based rules is proposed and presented.

Introduction

The development of the DNV Rules for Certification of Lifting Appliances started [4] in the 1970s. At that time, the maritime industry wished a serious and thorough engagement regarding maritime cranes and this led to the development of the first DNV requirements for lifting appliances. The rules have later been revised several times and our requirements are widely copied by other regulators. Up until today, however, the DNV Rules for cranes have had its main focus on the structural strength.

The three decades since the start of the crane rule development have shown a decrease in crane accidents related to structural failure. In the same period, the safety-related systems applied in cranes have become more complex and this development is expected to continue. For the forthcoming years, the system's integrity is assumed to be more and more important for the total safety of cranes.

To cope with this, the crane rule project has initiated development of a new type of functional requirements to crane systems. The project reported here was started in 2004, and it was decided to apply the IMO-Formal Safety Assessment (FSA) method as the basis for development of risk-based rules and requirements for crane systems. One of the objectives in this pilot project was to establish a practical approach for application of the FSA method in development of risk-based rules for systems and components.

This paper first presents the FSA method in general and then how FSA was applied in the project. As a part of the cost–benefit analysis in the FSA step 4, a new method for estimation of target reliability for crane safety functions was required and a new method was developed in the project. This is presented in Section 3. First, assumptions for the barrier cost model and the risk reduction cost model are presented. Then, a manual way of applying the method is described and the cost optimal barrier (cost optimal risk reduction) is identified by applying the FSA decision criterion in the given example. A mathematical expression for estimation of the cost optimal barrier is then developed and presented as an alternative to the manual method for estimation of the target reliability. In Section 4, a proposal for organisation of generic risk-based rules is presented.

Section snippets

About FSA

In the beginning of the rule development project, it was decided to apply the recommended IMO-FSA method. In this section, the FSA method is briefly presented.

Some challenges in the project

The key challenge in the project was to find a method for estimation of the required target reliability of the RCOs and thus defining requirements to e.g. system redundancy. The cost–benefit criterion for estimation of the cost optimal target reliability (probability of failure on demand (PFD)) for the safety functions has been applied. A new estimation method has been developed and is presented in this paper. The method is based on the IMO-FSA cost–benefit decision criterion and concepts

Generic risk-based rules

The FSA method does not give recommendations on how to organise the rule text to be applied by the industry and the certification body. The project has therefore developed a first proposal for organisation of such rules.

The rules prepared in the project are based on the assumption that it is possible to base rules on generic risk contributors and generic RCOs or risk control measures.

Discussion of results

The BIL estimation method is based on generic estimates of several parameters related to future risks, costs and specific operational details. Such estimates may vary for different ships and installations. As technology and cost models for the technology change, this could form the basis for regular updates of the rules or specific deviations from the generic requirements.

The proposed functional requirements in this paper should be seen in relation to requirements established in standards like

Conclusions

The main conclusions from the project are in brief as follows:

  • Application of the IMO-FSA method for risk control of operation of lifting appliances/cranes is feasible.

  • A theoretical method for estimation of the target reliability for the safety functions or barriers (risk control options) has been developed and is applicable.

  • The method has been applied for selected safety functions identified in the hazard identification and risk analysis.

  • The new risk-based rule text has been prepared on the

References (5)

  • IMO MSC/Circ. 1023. Guidelines for formal safety assessment for use in the IMO rule-making...
  • IEC 61508. Functional safety of electrical/electronic/programmable safety related...
There are more references available in the full text version of this article.

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