Offshore Berths With Multiple Orientation

Abstract

ABSTRACT Offshore berths of various types for loading and unloading minerals, constructed in recent years or now under construction, are reviewed. These include both breast on, breast off and combination designs and installations that require maximum, none or moderate amounts of warping of the vessels during loading or unloading. A common feature of all these facilities is that the berths have a fixed orientation, selected as the best compromise between variable wind and sea conditions, aimed at attaining the highest over-all berth availability factor. In exposed locations with strong seasonal variations in the direction of winds, currents and swel1s, anyone fixed berth orientation may be unsatisfactory. Designs developed to cope with these conditions are illustrated with two breast-off type berths (one for loading, one for unloading) requiring warping of the vessel and a breast-on type offshore berth for the vessel stationary during loading. It is the conclusion of the authors that (1) designs providing multiple orientation of offshore berths at relatively modest cost are feasible and (2) in certain locations, multiple berth orientation improves berth availability, thereby extending the areas of economic application for this type of facility. INTRODUCTION A recent development in the transportation of bulk materials is the offshore berth for the loading and unloading of are carriers. Offshore loading or unloading terminals often provide the only solution to the problem of accommodating the new generation of giant size vessels. In most locations it is prohibitively expensive to construct a protected harbor of conventional design to handle these new ships, some of which are in the 250,OOO-dwt or larger class. In other locations it would be uneconomical to provide a protected harbor even for smaller ships. The Offshore terminals have to be located on the basis of water depth, tides, currents, waves, winds, subsoil conditions, etc.; in short, all factors affecting the maneuvering of ships and the design of man-made structures in the sea. FIXED ORIENTATION BERTHS The simplest arrangement for offshore ship loading or unloading stations provides a single orientation for a ship which may be tied up to a pier structure Or dolphins or which may be moored to buoys that hold the ship away from the berth structure. The heading of the ship can usually be reversed along the predetermined dock face. In some installations the ship remains in one position during the entire duration of the loading or unloading operations. In other installations the ship is warped; that is, moved forward and backward in order for the dock-mounted loading or unloading equipment to reach every hold of the ship. The decision to load the ship in one position with moderate warping or maximum warping is properly based on an analysis of capital and operating cost, taking into special consideration the time available to load the ships as function of the required throughput, the capital bost of berth structures and loading equipment, the yearly cost of operation and maintenance and other operational considerations.