The world watched the news with dismay when the Korean ferry Sewol capsized and sank in April 2014, leaving more than 300 people dead, mostly secondary school students. Disasters like this and the wrecking of the cruise ship Costa Concordia off the Italian coast in 2012 set scientists pondering at the MARIN research institute. Couldn’t a technology be developed that would buy some time and allow passengers to safely disembark during an emergency?
Ideas abound on how to make ships more stable. In recent years much research has been done on movable hydrofoils that increase stability in high seas, for instance. But an emergency system that can roll into action if a ship starts listing and sinking because of unbalanced cargo or damage doesn't exist yet. With the exception of lifeboats of course.
So the Director of MARIN, Bas Buchner, came up with the idea of Emergency Floatation and Stability Devices (EFSDs) – a system of airbags that would make ships that are flooding more stable and buoyant.
Together with MARIN and engineering company SARC, 4 maritime bachelor students, Cas van Engelen, Bart Ettema, Martijn Witvoet and Feiko van Logchem performed a first quick scan into the feasibility of this type of emergency floatation device. Last week the students presented their prototype and their research to Cora van Nieuwenhuizen, Minister of Infrastructure and Water Management, during a working visit to the Water & Maritime Top Sector.
‘Inflating such large bags in a heartbeat is quite a challenge’
The ship the students worked on is a so-called RoPax ship, a ship, just like the Korean Sewol, which carries passengers and freight, cars mostly. They experimented with a 1:36 scale model in a test basin. Their model is about 5.5 metres long and one metre wide, mimicking a ship that would in reality be more than 200 metres long.
So what do the airbags which they placed on the sides look like? Feiko van Logchem affectionately calls them inflatable bracelets. “During the tests, we used PVC pipes filled with air to mimic the bracelets. They were fixed to the sides of the ship at water level. In reality however, a system would need to be devised with bags made of a strong material such as Kevlar that can be inflated rapidly. Inflating such large bags in a heartbeat is quite a challenge.”
Foul weather conditions
The students found that five EFSDs are sufficient for the typical RoPax ship which is taking water in through the hull to survive sea state 7. Sea state 7 is foul weather conditions with up to nine metre high waves. These EFSDs would have a diameter of five metres and be 15 metres long.
Optical tracking system
The model was equipped with an optical tracking system, which allowed measurements in six degrees of freedom. In addition, three cameras were installed in and around the model to get a better view of both sides of the ship. The camera placed inside monitored the water intake. A wave meter was fixed to the model to measure the relative wave height. The ship was assumed to have no rudder so the waves only approached one side of it.
‘The airbags worked a lot better than we expected’
An initial heeling angle, which could be caused by shifting loads or wind, was simulated to intensify the conditions. Damage to the vessel was then simulated in different sea states and when it caused the ship to capsize, the EFSDs were added. First, all 18 were added. They were then gradually removed step-by-step to find the minimum amount that would prevent the ship from capsizing.
“The airbags worked a lot better than we expected,” says van Logchem. “Still many challenges need to be tackled. One challenge is for the bags to inflate quickly enough. And finding the right location for the EFSDs and attaching them firmly another. They should not be placed beneath the lifeboats as they would hinder the lowering of the lifeboats.”
Another group of bachelor students will follow up on this research.