The transient plane motion of a shuttle tanker connected to a submerged turret loading system in random waves Is studied. The environmental loads considered herein are the first- and second-order wave loads, wind and current loads. The wave loads are calculated by using a panel method within the framework of potential theory. The second-order slowly varying drift forces and moments are expressed in terms of quadratic transfer functions. The transient plane motion caused by headings as well as random trains of incident waves Is predicted by including the time-memory effect. It is shown that the weathervaning takes place quite slowly. The excursion depends strongly on the wave heading and the numerical example indicates that a DP system Is required to assist the submerged turret loading system to enhance the operation limit.
Recently some new loading systems have been developed for possible deployments in an oil field of severe environment. One of promising alternatives is the Submerged Turret loading (STL) system, by which crude oil is to be transferred directly to a shuttle tanker from production risers without any help of storage facilities therebetween. A typical STL system consists of a submerged buoy and a turret mooring. Since the loading system Is completely submerged and It guides the tanker to weathervane, it can be set in operation under heavy weather conditions In hostile seas as like the North Sea. To design the loading system, it is vital to carefully evaluate the motion behavior of a tanker moored to it. As usual the excitation is due to wind, wave and current. The wind and current loads Invoke more and less a quasi-steady horizontal displacement of the ship, while the wave load Is responsible for slow drift motions as well as fast oscillatory motions.
