1. Use similar material requirements for flap components as for conventional rudders
2. Develop procedure for design approval of flap rudders
3. Monitoring/indication of flap rudder angle
4. Measuring of clearances at each bottom survey
5. Regular inspection of pivot link and upper hinge
6. Planned maintenance of bearings/hinges/pivot
7. Develop specific survey guidance for flap rudders
The risk analysis of the flap rudder (FSA Step 2) was based on the generic oil tanker FSA, and used the results in Figure 3 to show that the average annual cost of failures of conventional rudders on oil tankers was $820 per ship year. This was dominated by delays and repair costs, since the costs to people and the environment resulting from groundings consequent on rudder failures were only $20 per ship year. These are based on theoretical predictions using an event tree technique, since no such events had occurred in the period selected for analysis. Based on DNV's failure data, the risks for flap rudders were estimated to be 3 times higher than for conventional rudders, giving a total cost of rudder failures of $2500 per ship year for a tanker with a flap rudder. This is the maximum possible benefit of any additional risk control option, and places a useful upper limit on the cost impact of further rules.
A cost-benefit analysis was carried out for the 7 risk control options for flap rudders (FSA Step 4). The costs included costs to DNV in developing, administering and implementing the proposed rules, and additional costs to the owner in manufacturing, maintaining and inspecting the rudder. The benefits, in terms of reductions to the risks from rudder failures, were estimated mainly by expert judgement based on available failure data.
If only costs of human, environmental and third-party property damage were included, none of the RCOs would have positive net benefits, and all would have ICAFs in excess of $200 million. In other words, none of the RCOs could be justified as part of +1A1 Class. However, if damage costs to the ship were included, 4 out of the 7 RCOs have a positive net benefit, and hence are considered suitable for implementation as part of a voluntary class notation.
These conclusions are now being considered as part of DNV's rule making process. It must be noted that there are major uncertainties in the FSA work, particular in estimating the benefits of the risk control options. In addition, the criteria that in effect limit this type of measure to voluntary class notations are tentative, and not necessarily consistent with other motivations. For these reasons, DNV regard FSA as an input to a decision-making process, rather than the definitive conclusion on it. The rule development process must also consider issues such as market acceptability, data quality and future trends, which at present the FSA cannot model.
4. DISCUSSION
4.1 Lessons Learned
Some lessons from the work to date include:
・ FSA is complex and challenging. Obtaining robust marine risk estimates is difficult, but can be achieved by suitable combinations of historical data, theoretical modelling and expert judgement.
・ Ship rules are inter-dependent and often their purpose is unclear. Quantifying the benefit of existing ship rules is therefore particularly challenging. FSA is more readily applied to new rules.
・ Marine accident data sources are incomplete and sometimes difficult to access. While better data would always help the work, it must be acknowledged that much more could be done with the data sources that are available.
・ Ships are diverse. Even within a generic ship type, there are wide variations in design and operation. Estimation of the risks for average generic ship types is readily achieved using historical data, but the potential variation between individual vessels is largely unknown at present.
