One type of offshore terminal for mooring a ship includes upper and lower beams extending from the bow of the ship and supporting a turret in rotation about a vertical axis relative to the ship. The lower portion of the turret is anchored to the sea floor, as by heavy chains. An upper bearing arrangement that mounts the turret to the upper beam, includes a thrust bearing that supports the weight of the turret and the load thereon, and that also forms a radial bearing that resists horizontal movement of the turret. A lower bearing arrangement that mounts the turret to the lower beam includes only a radial bearing. The turret is rigid, and the upper and lower bearings must be precisely aligned to avoid large bending stresses on the turret which would result in large radial loads on the bearings that limit their useful life.
In practice, it is found difficult to mount the radial bearings on the upper and lower beams to be precisely aligned. For example, even if the bearings are precisely aligned when the turret is installed on the vessel, misalignment will occur at the site where the terminal is installed when heavy chains are attached to the turret. The heavy weight of the chains causes the upper beam to bend, and in bending its outer ends pivot, which results in misalignment of the bearings. A comprehensive analysis can be made to predict the degree of bending of the upper beam, but in practice such analysis is very difficult to perform accurately. Also, it is difficult to install the turret at a shipyard with the required amount of misalignment and consequent bending load on the turret, which is predicted to be compensated for when the chains are installed. A bearing arrangement which avoided the need to accurately predict the amount of bearing misalignment during final installation and the need to install the bearings under high load with high precision, would be of considerable value.