FLO/FLO Heavy Lift Critical Stability Phases

Abstract

The US Navy has used FLO/FLO heavy lift transport as an alternative to towing for the transport of damaged vessels as well as transport of smaller vessels not suited for ocean transit. There are critical stability considerations that have to be assessed prior to conducting a heavy lift operation, specifically “Draft at Instability” and “Minimum Stability”. During de-ballasting of the heavy lift ship with the lifted vessel on the docking blocks, the reaction of the docking blocks on the lifted vessel is effectively the same as removing weight from the lifted vessel’s keel (similar to what occurs when a floating dry-dock is de-ballasted). This raises the lifted vessel’s centre of gravity thus reducing the lifted vessel’s metacentric height (GM) until the lifted vessel’s GM is zero. During a critical part of the FLO/FLO heavy lift operation the cargo deck of the FLO/FLO heavy lift ship will be completely submerged. During this phase, only the water-plane of the hull structure which extends above the cargo deck will provide stability for the heavy lift ship. The heavy lift ship will pass through a phase of “Minimum Stability”, which should not occur at the same time that the lifted vessel assumes its “Draft at Instability”. The lifted vessel and the heavy lift ship may roll out of phase, causing landing problems, or causing the lifted vessel and/or the FLO/FLO heavy lift ship to become unstable, assume a large list or even capsize. Computer naval architectural software programs HECSALV and POSSE are effective tools in modelling critical stability phases for FLO/FLO heavy lift operations. This paper discusses the critical stability phases of a FLO/FLO heavy lift operation, and the methods and practices to plan for and mitigate the effects of reduced stability at these phases.