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Desso – a forgiving design
for enhanced safety
Improved designs and stricter rules have contributed significantly to improved safety on ro-ro/passenger vessels, but extensive damage after i.a. a collision could still lead to severe consequences. The scope of the Desso project has therefore been to keep a severely damaged ro-pax vessel upright and afloat for 24 hours.

Design for survival onboard
In 1999, plans to cover the wreck of Estonia with concrete were abandoned. Instead, the Swedish Government decided to put aside some of the money saved to maritime safety R&D projects. All in all, the state has provided about SEK 50 million (EUR 5.3 million) through Vinnova – the Swedish Governmental Agency for Innovation Systems – and the shipping industry has contributed with the same amount. About EUR one million has been spent on the Desso – Design for Survival Onboard – project that now has been finalised.

In 2003, 12 partners, with SSPA in the lead, was given the task to find a design for a ro-ro passenger vessel aiming at a ship serving as its own lifeboat and also study passenger and crew survival in an emergency situation.
The Desso concept vessel is based on a generic ro-pax design, the EuRoPax 3000, which has been developed by the Italian design office NAOS. The vessel has capacity for 1,500 passengers and 3,000 lane metres for cargo. The EuRoPax 3000 has a high degree of built-in safety features to fulfil floatability rules after sustaining damage.

The design caters for symmetrical flooding, resulting in small heeling angles, which is a requirement for safe evacuation.
The Desso ro-pax is a wider vessel above the waterline than a conventional ro-pax. Enclosed watertight sides casings up to the first accommodation deck gives spare buoyancy without obstruction of the cargo space.

Four compartments
For redundancy the engines have been placed in four separate compartments, where the aft engine rooms also are separated transversely. This arrangement secures propulsion after damage extending along three adjacent compartments or a rip-up penetrating the bottom over 65 per cent of the length of the vessel.
Separate engine rooms also improve fire safety. A number of measures have been taken to restrict spread of fire and smoke and to facilitate a fast and efficient fire fighting response. The entire vessel is protected by a high-pressure water mist systems and by an active smoke control system to keep toxic gases away from corridors. All cabins are equipped with alarm buzzers and mattresses of high fire resistance materials. The cargo spaces are subdivided through fire curtains. The aft port and internal hoistable ramps can be closed remotely to restrict in-flow of air to a fire.

Accident survey as a base
In the initial phase of the project a survey of passenger ship accidents was carried out. In each case an event tree was created and points in the chain of events where a different action or occurrence might have changed the course of events where they were identified. Those points, or “chain-breakers” were categorised and analysed. The study showed that most “chain-breakers” were categorised as management.

According to the Desso project, highly skilled management is one of the primary factors for increasing safety onboard. If a severe accident occurs, the crew must be supported with all the information needed to take the right decisions. The Desso vessel is equipped with a decision support system placed on the bridge. The system calculates stability in a flooding situation and predicts motions and state. It warns of cargo shifting, predicts smoke and fire spread and gives advice on ship handling and on evacuation. The system also has a simulation mode for training.

On a conventional ro-pax, there is just one alarm level, requesting all passengers to gather at the muster stations. The Desso vessel has been designed for a three-step alert ladder. The first step, yellow alert, means that the situation is safe and passengers are allowed to stay in all accommodation areas. At orange alert, all cabins are evacuated and passengers stay in safe areas on the leisure and evacuation decks. At the last step, red alert, all passengers are gathered on the evacuation deck.

This alert ladder will provide the time needed to avoid crowding and to prepare passengers for an orderly evacuation.

Conventional evacuation
The Desso vessel will be evacuated by rather conventional means. Initially the project studied float-free modules and float-free, free-fall lifeboats. These were however considered too costly and too limited. According to Desso, a float-free module can’t be used in a grounding accident and the vessel can’t be abandoned before it sinks. There would also be a higher risk for a large number of passengers if a module fails. Instead, Desso has chosen an evacuation system without lifeboats, but with large life rafts for up to 100 persons. Passengers embarked on the evacuation deck and the raft is launched by crane or lifted by a helicopter.

According to Desso, the main advantage with this design is the time gain before the vessel sinks – the redundancy will make it possible for the ship to return to port or to sheltered waters. How a large number of people can be transferred safely from one ship to another is still to be answered.

//Rolf P Nilsson