Structural Integrity Analysis of Critical Elements of RoPax Ships

Doktorsavhandling, 2008

The potentially costly consequences of a lost ship in the form of fatalities, property, cargo and related industries, as well as pollution of the environment in the form of oil spill, etc., and the public view are the driving forces for developing safer ships. Especially RoRo-ferries, i.e. RoPax, have been in focus during the last few years after a series of accidents claiming a large amount of fatalities. This thesis is on the subject of safety of RoPax-vessels with an emphasis on structural integrity. It comprises three studies: the “event-chain” study, which concerns the safety of RoPax vessels in general, the “bow-door study” and the “collision strength study”. Accidents are seen as a result of a series of events where each one may be trivial, but, when combined, will ultimately end in a disaster. When such a series of events is studied, it is possible to define “chain-breakers”, i.e. points in time and space where a different action or design feature could halt the development. The objective of the event-chain analysis was to identify and analyse chain-breakers from previous accidents. The results were graphics of event-chains with chain-breakers which were used in workshops with experts in two stages. Concepts and ideas regarding fire protection and evacuation, damage stability, surveillance and support systems and management were put forward. Accidents with many casualties due to failure of bow-door systems and several additional incidents are the cause for concern of the safety of bow-door systems. In the current work, the rules of IACS UR S8 were assessed regarding the structural integrity for bow-doors of the “clam door” type. The emphasis was on bow-door supports, transferring load on the doors into the hull, and bow-door arms, carrying the doors during opening and closing. Calculation of the required strength of the supports of an existing RoPax was made according to the IACS S8 rules and compared to a finite element (FE) calculation based on real pressure values and pressure distributions recorded onboard. The results show that the required and the proposed methods of the IACS S8 appear to be approaches that lead to inadequate results. A bow-door arm was also studied in order to analyse whether arms, dimensioned according to a static load, should be permitted to be a part of the locking arrangements of bow-doors. The analysis was carried out with FE-calculations based on field measurements in which it was shown that the analysed bow-door arm could suffer fatigue damage. Hence, bow-door arms should not be permitted to be included in the locking arrangement of bow-doors, unless the structural integrity of the arms regarding dynamic loads has been assessed very carefully. RoRo-vessels, with their large open RoRo-decks and resulting poor damage stability, are sensitive to collision damage. An innovative conceptual deformable inner barrier design is presented with the aim of improving the collision safety in a collision into the side of a ship by facilitation of a deep intrusion before penetration. The design fulfils the design rules by having equivalent strength for operational as well as extreme loads. It also fulfils the stakeholders’ interests regarding the acceptable extra weight and cost. Validation of the conceptual design was made using FE-simulations, based on indata and criteria established in an experimentally verified FE sensitivity analysis, in a comparative study of the original “reference” structure, a costal RoRo-vessel. The FE-simulations show that the amount of absorbed energy can be more than tripled, at the same time as the rules and the stakeholders’ interests are fulfilled.