Full-scale tests of FRP, aluminium and steel partitions at DBI will help create simpler documentation of fire safety for ships with lightweight superstructures.
When the steel ship, Norman Atlantic, caught fire in the Mediterranean in January 2015, wind conditions made it extremely difficult for the rescue helicopters to evacuate the ship. As a consequence, the rescue operation took 24 hours, during which time the passengers had to wait on the sun deck while the interior of the ship was destroyed by the fire. Despite this, the vessels stability and load-bearing capacity were not in immediate danger.
This example underlines the impressive robustness of steel ships in the event of a fire. Nevertheless, the research and development project, Compass (COMposites for PASSenger ships) is working to replace steel with FRP (Fibre-Reinforced Plastic), which, as a rule, has nowhere near as good fire technical properties as steel. On the other hand, FRP structures are considerably lighter than similar steel structures.
Compass is being carried out by DBI in collaboration with DTU Department of Civil Engineering and DTU Mechanical Engineering, while the Danish Maritime Foundation is providing financial support for the project.
Comprehensive series of tests
More recently, Compass has performed a series of major fire tests in DBI’s fire laboratory in order to resolve the dilemma between lightweight structures and fire safety, which will make it easier to design ships with FRP. Thus, in October 2015, six full-scale fire tests were conducted, during which the fire properties of three types of ship partitions (bulkheads), of steel, aluminium and FRP respectively, were tested.
It took over a week to conduct the six full-scale tests and each type of partition was subjected to two fires. Firstly, with a standard fire curve in accordance with ISO 834-1 and, secondly, a calculated fire curve corresponding to a fire in the cafeteria of a ship. The partitions were constructed as realistic ship structures (bulkheads) and were subject to a vertical fire loading in each test. The steel and aluminium partitions were fire-insulated to A-60, the FRP partitions to FRD-60.
Each test continued to the point of physical collapse or until the temperature in the furnace had decreased to a point that was irrelevant to the test.
Ships with lower fuel consumption
But, why perform tests with materials that are less fireproof than steel in the first place? The answer lies in the shipping industry’s fuel consumption and consequent impact on the environment. With lighter materials like FRP, it will be possible to build ships with a lower fuel consumption. Moreover, maintenance costs will be lower as plastic is resistant to corrosion.
However, the challenge is considerable. The International Maritime Organisation (IMO) prioritises safety extremely highly and therefore prescribes steel and other non-inflammable materials in the prescriptive set of rules, SOLAS (Safety of Life at Sea).
FRP is not excluded – however, it requires extraordinary documentation of safety in accordance with a specific section, SOLAS 17, which, in practice, has been almost impossible to establish up to now.
The voluminous data from the fire tests has not yet been fully analysed. But, overall, expectations were confirmed. FRP partitions lose their load-bearing capacity after little more than an hour in a standard fire, aluminium partitions (bulkheads) can hold out for several hours while the steel partition has an “infinite” load-bearing capacity in the event of a fire due to very little utilisation of the steel’s load-bearing capacity.
– The results were as we had expected. The tests underline the fact that it requires achieving adequate levels of safety and robustness when using FRP in ships requires a great deal of forethought. But, on the other hand, the results give rise to no reason whatsoever for dismissing the possibility of using FRP. In all probability, it could be done, as long as the challenges are tackled in the right way, says fire technical consultant Anders Dragsted from DBI.
The challenge for SOLAS ships is – unlike buildings on dry land – that the ship is expected to “be its own lifeboat” and be capable of a “safe return to port”. Therefore, Compass focuses on how you can achieve the same level of safety on board a FRP ship as you would on board a ship with a traditional prescriptive design.
During the tests, DTU took 3D photographs of the surface of the partitions in order to be able to detect fairly small physical deformations. This information can be used, for example, to verify fire simulations of the partitions’ reaction to fire which, in the long term, can make it easier to document safety.
In addition, at DTU a series of smaller tests will be conducted to determine FRP’s strength and structure in the event of a fire.
Conversion of ferries
In specific terms, Compass is seeking to make easier to build or convert large passenger ships with a new lightweight superstructure in which the load-bearing steel is replaced with a structure that contains carbon and/or glass fibre composites.
The project will take as its point of departure a conversion of a the superstructure on a Danish passenger ferry, Prinsesse Benedikte, which sails between Denmark and Germany, to FRP. The aim is to develop an alternative design with at least the same level of safety as a design executed in accordance with the prescriptive regulations described in SOLAS II-2.
Compass is also to develop procedures that document the interplay between the fire properties of the structure and the level of safety achieved in the event of a fire.