Use of Cryogenic Buoyancy Systems for Controlled Removal of Heavy Objects from the Seabed

New methods for marine salvage and decommissioning of structures in the open sea are continually being sought in order to improve control and lower operational costs. This paper investigates the concept design of a lightweight cryogenic marine heavy lift buoyancy system. The approach makes use of a cryogenic system for provision of buoyancy within the ocean environment. The objective is to be able to lift or lower large displacement objects under full remote control. The opening stages of the project work include the development of a system that will operate to a depth of 350m. As part of the design process for such an arrangement, numerical simulation of the complete system has been undertaken in order to develop mechanical, cryogenic and process control systems. The paper considers the overall design concept and associated system development issues. These are illustrated through use of the time accurate simulation of alternative design configurations that confirm their viability. INTRODUCTION Numerous methods of lifting and lowering objects from the seabed have been developed throughout the history of ocean engineering and exploration. Initially, these methods were in practice for the purpose of marine salvage. The first evidence of marine salvage in operation was in 460 B.C., where mention is made of a Greek diver being hired to remove treasure from shipwrecks [1]. Most of the early divers went free diving – using no equipment and staying submerged as long as a breath lasted, this could be as long as 5 or 6 minutes! It was not until the development of a method to supply divers with surface air that diving was really used for salvage operations. From this point on diving became not only a recognized method for salvage, but also a method for deploying and recovering subsea equipment. Since then, many techniques have been used to recover objects from the depths that are much too large for a diver, or even indeed a team of divers, to handle. These include methods such as compressed air being pumped down through lines to fill vessels that had been sealed by divers, building a coffer dam around the object and pumping out the water, and pumping polyurethane foam into hollow objects to provide enough buoyancy to float them free[1]. All these methods have limitations on their scope of operation. Techniques using divers are limited to the maximum depth a diver can descend to without coming to harm, dam building can only be done in shallow water. The methods more commonly used today include cranes and lift bags and an assortment of Remotely Operated Vehicles (ROVs). Cranes are popular for deep water lifts, and can be used to depths of 2000m, but are not without problems. The weight of the cable can end up being more than that of the payload for deeper lifts making the process awkward and costly. There is a limiting sea state due to the motion of the vessel on which the crane is mounted, which leads to operational constraints due to predicted weather windows. Ultimately, it is the excessive cost of hiring and the limited availability of the cranes which are the main problems. For the case of lift bags, control is the limiting factor. The open bottom bags dump excess air from the bottom as they ascend, and the enclosed lift bags only have a limited capacity to dump air through pressure release valves. As such the rate of ascent tends to be controlled somewhat coarsely by adding more weight to the bags, and the slow steady ascent often required