The Naval Architect: February 2017
Passenger ship safety was high on the agenda at the IMO’s Maritime Safety Committee meeting MSC 97 in November as regulators pushed forward with new proposals on damage stability and “survivability” in the event of a collision. For the moment, changes to the rules must wait, with further work on the proposals deemed necessary before the MSC meets again in June next year.
As proposed, the new regulations will apply to all passenger ships contracted from 2020, or with keels laid from 2022, or delivered from 2024, and will supersede previous SOLAS (2009) provisions. The safety proposals, which have involved work by the European Maritime Safety Agency (EMSA), flag state representatives, ship operators, designers and builders, have focused on a ship’s ability not to founder following a collision. The key objective has been to achieve a significant increase in the Required Subdivision Index ‘R’.
“Damage stability is about ships not sinking in the event of a collision or grounding,” explains Ari Huttunen, head of ferry design at Foreship. “The current legislation, embodied in SOLAS 2009, is based on probability and presented as an index – the Required Subdivision Index ‘R’. It must be shown by calculations that a certain ship design exceeds the value of ‘R’”.
In the SOLAS 2009 regulations, the calculation of the ‘R’ depends on the number of persons onboard and the total lifeboat capacity. The formula penalises ships which have fewer lifeboat seats than the total number of persons onboard which, in practice is the case on every passenger ship, as a large proportion of the persons onboard will need to use life rafts. This, says Huttunen, places limits on designers, and especially on ferry design.
“As proposed, the new regulations will raise damage stability requirements on new ferries,” explains Huttunen, “but they will include a different way of calculating ‘R’, so that the number and capacity of lifeboats onboard is no longer a factor in the calculation.”
Huttunen notes a recent surge in demand for passenger ship design consultancy services, as clients seek guidance on what the new regulations will mean from a practical point of view, not least because there are no easy options towards compliance. Huttunen explains that meeting the new regulations is likely to involve trade-offs between various design criteria, with a complex optimisation process necessary to offset gains made on one hand and drawbacks on the other.
Increasing the freeboard of a ferry, for example, should improve her damage stability because she would be able to take in more water before potentially capsizing or sinking. However, damage stability may also be improved by increasing the beam which increases initial stability, i.e. the metacentric height (GM), and reduces the risk of capsize.
However, the adverse result is a shorter roll period. This is uncomfortable for passengers, particularly on upper decks and in heavy seas, and also risks personal injury and damage to cargo, fixtures and fittings. Ferry designers, Huttunen explains, therefore aim for relatively low GMs to improve seakeeping, and an optimisation process is required to find the best compromise between freeboard and GM in the applicable seakeeping range.
A few weeks before the November MSC, Huttunen presented a paper at Interferry’s Manila conference in which he revealed details of recent work completed on a theoretical ferry designed with colleagues at Foreship. The project had been set up specifically to see what design steps were required to ensure that a 1600-POB ferry would comply with the likely SOLAS 2020 regulations.
The ferry, with novel evacuation systems without lifeboats, was designed for 1,500 day-passengers and 100 crew. It had a length of 150m, a breadth of 25m, a depth to the vehicle deck of 9m, a draught of 6m (and therefore a freeboard of 3m), 700 trailer lane-metres, hoistable car decks and an electric dual-fuel propulsion system.
A key objective of the work was to optimise the design variables to achieve the required increase in ‘R’ from 78% to 85% as required by the proposed SOLAS 2020. However, it was also necessary to make sure that the vessel’s “survivability” met extremely demanding water-on-deck requirements as set out in the Stockholm Agreement, 1996. This was where the design team started their optimising.
Foreship’s naval architects confirmed that the Stockholm Agreement water-on-deck regulations were far more demanding than the existing rules set out in SOLAS 2009. The ferry design not only complied with both but was also found to be compliant with the forthcoming SOLAS 2020 rule proposal. This emphasises the significance of the Stockholm Agreement being phased out.
The ship designers also examined ways in which the ferry’s main dimensions could be optimised in relation to other variables including beam, freeboard and subdivision. Based on a draught of 6m, the effect of increasing the beam by 50cm to 25.5m or raising the freeboard by 20cm each reduced the GM requirements by about 10cm.
Next, the team looked at the effect of installing two pairs of partial subdivision bulkheads on the vehicle deck. This resulted in a GM reduction of about 15cm, a significant effect considering the minor impact on tonnage or lightweight. The downside is the reduced extent of the cargo lanes along the ship’s sides.
Overall, Huttunen and his colleagues reached a number of useful conclusions that will be important for ship design when working in compliance with the new SOLAS 2020 regulation. These include:
- Effective subdivision should not affect passenger capacity
- Too much initial stability could cause discomfort onboard, but this can be addressed by optimising various design criteria
Ferries in Europe, particularly those operating in northern waters, are very safe these days. However, further improvements can always be found, Huttunen observes. “For designers, owners and operators, the task at hand is to work within the trade-offs that the new regulations imply and optimise the overall safety and efficiency of passenger vessels.”
