A casualty which does not qualify as a very serious casualty and which involves: A fire, explosion, grounding, contact, heavy weather damage, ice damage, hull cracking or suspected hull -defect, etc., resulting in : Structural damage rendering the IWT vessel not navigable, such as penetration of the hull underwater, immobilization of main engines, extensive accommodation damage etc. or; Pollution (regardless of quantity) and/or; A breakdown necessitating towage or assistance from the bank.