A study investigating the use of active ventilation in case of fire in cargo spaces on ships
Mechanical ventilation can be used to change the fire conditions so that heat and fire gases flow in a desired direction. Ventilators are, for example, used in firefighting operations in buildings, but how can ventilation be used in large ro-ro spaces on board ships? In LASH FIRE, a first step of investigation was carried out and states what needs to be known before using mechanical ventilation during a fire in a ro-ro space on board.
In Brandposten #61, an article clarified the effects of ventilation in a roll on/roll off (ro-ro) space on board a ship. In the LASH FIRE project, guidance has been developed to increase the knowledge of opportunities and risks with mechanical ventilation and its use in case of fire. The general guidance is summarised in a matrix in Figure 1.
Presently, when a fire is confirmed in a ro-ro space, the general routine is to shut off mechanical fans and close fire dampers connected to the space. Studies performed in the LASH FIRE project show that keeping the fans on at increased capacity in the early stages of a fire can reduce the amount of smoke inside a closed ro-ro space. This would facilitate manual firefighting but is only applicable for fires close to an exhaust duct. If the fire is located at a distance from ventilation ducts, keeping fans active will thin out smoke downstream, but not enough to facilitate manual firefighting. Switching fans off, as is the practice today, is still the best alternative to reduce the fire intensity but generates worse visibility conditions.
Computer simulations and model-scale tests conducted
The studies included computer simulations (CFD), aided by fire tests, interviews, and in-situ visits. The series of CFD fire simulations focused on investigating the steady-state situation of a 5 MW fire with respect to visibility, temperature, incident radiative heat flux, and smoke layer height. The simulation results were used to determine the distances from the fire where critical values for these parameters were reached (60 °C, 2.5 kW/m², and a visibility of 10 m).
The model for the ro-ro space was based on a drawing provided by Stena Rederi AB. The closed ro-ro space had the following inner dimensions: 162 m long, 25.6 m wide, and 5.8 m high. No fire propagation and no firefighting system were simulated. The results should only be considered if manual firefighting is needed, and if the fire size is at most 5 MW, i.e., at the early stage of a fire.
Figure 1. Guidance matrix showing when the use of mechanical ventilation may be considered in the event of fire in enclosed ro-ro spaces on ships. Important before using the ventilation is to acquire knowledge, understanding and conduct practical training scenarios.
Model-scale (1:8) fire tests were carried out to further investigate mechanical ventilation and study the effects on fire development in a closed model. The model was equipped with four mechanical fans, and different ventilation scenarios were evaluated in 12 fire tests. The fans were controlled by a frequency converter and could achieve capacities of up to 20 air changes per hour. The theoretical volume of the model was 24 m³.
The ventilation design (number of fans, capacities, etc.) is unique to almost all ro-ro spaces, and it is important for the fire team and chief engineers on board to first have knowledge, ship-specific training, and understanding of the capacity of the ship’s ventilation system before using the ventilation in a firefighting strategy.
The guidance includes the following issues to evaluate when considering the use of mechanical ventilation for ro-ro space fires:
- Location of fire: The location of the fire in relation to supply/exhaust fans will affect how fans can be used.
- Surrounding cargo: The risk of fire spread to adjacent cargo with the usage of ventilation needs to be evaluated.
- Available equipment: Reversible fans and temperature sensors can be helpful for a successful fire ventilation strategy, but this has not been evaluated.
- Ventilation set-up: Practise on board if the fan configuration can create the desired airflow in the space before using the strategy in a real situation.
The work is described in the D11.5 report “Elucidation and guidelines for ro-ro space ventilation in case of fire,” which can be downloaded at https://lashfire.eu/.
Anna Karlsson
anna.karlsson@ri.se
For more information, visit www.ri.se/en.
