1. Field of the Invention
The invention generally relates to underwater mining systems and in particular to a stabilized hoist rig for lowering, lifting, and supporting a pipe string from a deep ocean mining vessel.
2. Description of the Prior Art
The potential of the ocean for supplying important and basic raw materials is generally recognized. Mining operations for sand, gravel, shell, heavy sands and other materials from continental shelf deposits are presently being performed by dredging techniques. On the ocean floor in deeper waters are vast quantities of mineral reserves. The primary mineral resources presently known are metalliferous deposits of zinc, copper, silver, lead, manganese and phosphate. Exploitation of these minerals is limited primarily by the technology of their recovery or delivery to the surface of the ocean. Among these deposits are mineral concentrations spread over large areas of the ocean floor in the form of nodules. The existance of nodules on the ocean bottom has been known for many years and are believed to be formed over aeons of time due to the precipitation out of the seawater of mineral substances. These nodules are known to consist essentially of iron oxide, manganese oxide, copper, cobalt and nickel, and are generally found in the deep areas of the sea where the floor is relatively hard and flat. The areas in which the nodules are known in sufficient quantities to sustain a profitable mining operation are found generally more than 200 miles off shore and at depths up to 18,000 feet and more.
Among the numerous systems which have been conceived for the recovery of nodules from the ocean floor is the hydraulic system which generally consists of a length of pipe which is suspended from a floating platform or vessel. The system includes a gathering head which is designed to collect and winnow the nodules from the ocean floor sediments and transport them through the pipeline. Means are provided for causing the water inside the pipeline to flow upward with sufficient velocity to suck the nodules into the system and transport them to the surface.
One of the major problems associated with this mining method is the bending stress induced in the pipe string by the pitch and the roll of the support vessel in response to wave movements of the ocean. Another complex problem is that of aligning a hoist rig with the pipe string for pipe stabbing and removal operations during lowering and lifting of the pipe string. A related problem is that of minimizing axial stresses induced by the sudden acceleration and deceleration of the pipe string during lowering and lifting operations.
Nearly vertical vacuum pipe strings, designed to elevate ore nodules from the ocean floor to a transport ship, can become dynamically unstable and fail within certain ranges of the following system parameters: damping of the pipe string; axial tension; ratio of the flow rate to the fundamental pipe string frequency; ratio of the pipe string mass to the contained flowing mass of ore and water mixture; the support vessel motion as it affects pipe tension and end displacement; pipe string inclination angle; and vortex forces caused by the ship's speed and ocean currents. Axial tension in the pipeline is adversely affected by the lifting and lowering operations of the pipe string into the ocean when it becomes necessary to decelerate the pipe string to a stop on the rig floor so that a new length of pipe may be added to or taken from the string. Sudden jarring stops can easily over-stress the pipe string in tension causing premature failure. Such a failure in the pipe string would delay mining operations for an indefinite period of time, and such damage would probably require replacement of the line.
These problems have been minimized in the past by designing special support vessels which do not react significantly to wind and wave action and by limiting the water depth in which these vessels operate. One such design is disclosed in U.S. Pat. No. 3,522,670. However, as the search for ocean mineral deposits advances into deeper waters where increased wave action induces higher roll and pitch reactions in the support vessel, and the length of the pipe string increases to reach abyssal depths, it becomes imperative to minimize the bending action induced in the pipe string and to minimize axial stresses induced into the pipe string by lowering, lifting, and maneuvering operations.