Craig Eason writes in Lloyds List – Tuesday 6 October 2009
NAVAL architects could be creating legally sound ships that are unstable if they focus too much on cargo efficiency in the design process, according to Flensburg Shipyard naval architect Heike Billerbeck.
â€œSome designers only think of cargo capacity and nothing else,â€ Ms Billerbeck said. â€œLegislators should make it a requirement that other criteria such as seakeeping are taken into account more.â€
She cited a design Flensburg made for a ro-ro vessel requested by Seatruck Ferries, a division of the Clipper Group.
Seatruck was the owner of Riverdance , the vessel that ran ashore after a freak wave disabled it in January 2008, and wanted a vessel that was as efficient as possible yet had improved handling in the light of the accident.
Although the UK marine accident investigation board highlighted poor stability and cargo stowage as factors in the 1977-built ro-ro vesselâ€™s demise, the operator has maintained that a freak wave alone led to the vessel developing a list and losing power.
In designing a new vessel for a similar operational criteria, Ms Billerbeck said this could easily be done to meet the requirements laid out in the International Maritime Organizationâ€™s stability rules, IMO resolution A.749, but if too strong a focus on meeting the requirements for cargo capacity was maintained then the eventual vessel could end up with poor seakeeping.
Flensburg was asked to design a vessel for operation in the Irish Sea, with a lane capacity of 2,150 m and service speed of 21 knots produced by two 8 MW main engines. Using these parameters the shipyardâ€™s design team arrived at a four-deck vessel with improved water resistance and fuel performance.
The design met not only the clientâ€™s requirements, but also those of the classification societies and the IMOâ€™s rules on intact and damage stability.
But Ms Billerbeck said that while the design work done on the vessel â€” including scale tests in the Hamburg ship model test basin â€” revealed a statistically correct vessel, it had a tendency to roll up to 30 degrees with wave heights of just over 5 m. In a following sea, the tests revealed a possibility of capsize as a result of the dynamic motion increasing roll.
â€œThere is no absolute universal ship efficiency index as the dynamic effects on a vessel are not taken into account within the IMO stability rules and regulations,â€ she told the German Society for Maritime Technology when it gathered in Hamburg last week to discuss ship efficiency measures.
Ms Billerbeck said the IMO resolution A.749 is based on statistics including mostly vessels under 100 m in length and dating back to the early 20th century.
â€œThis creates a problem and new designs are required for developing further speed,â€ she added.
Therefore ship efficiency needs to look at the whole operation of ships such as ro-ro vessels on tight schedules.
If lashing requirements are reduced due to recorded improved seakeeping then turnaround times in port can be improved, which in turn will allow for lower sailing speeds while still maintaining an allocated timetable.
Seatruck has four new ro-ro vessels on order with Flensburg shipyard.
â€œOperational needs need to be matched against ship efficiency, and seakeeping needs have to be worked into the design right from the start,â€ Ms Billerbeck said.