Diesel oil provision to production and drilling offshore units employing the use of platform supply vessels (PSVs) raises some logistic difficulties regarding time spent in the available terminals by the PSVs during the diesel loading operation in order to access the terminal, follow the queue, load the diesel, and leave the terminal.

An alternative to this approach would be the mooring of a diesel tanker offshore and its use as a Hub, in order to dispatch oil to the PSV fleet. Following this new approach, Petrobras projected and installed some buoys, moored by a single line to a fixed point and capable of receiving available tankers with 16,000 ton of deadweight. It is desirable nowadays to improve the size of these ships, as there are larger ones available in the fleet to perform this job, or else they will be scrapped. The full scale and the numerical models of the behavior of these ships present fishtailing phenomenon, which gets worse when we increase the dimensions of the ships.

In order to model the behavior of these anchored tankers, the computer code Wamit was employed in the frequency domain, and through the use of impulse response functions, we accessed the ships behavior in waves in the time domain. The wind and current actions were represented by a Morison like formulation, using constant coefficients and assuming a force proportional to the relative velocity squared. The mooring lines were represented by a FEM truss model.

Results were obtained for three classes of ship: Petrobras Class 35, 43 and 47, which have 16,000, 38,000 and 40,000ton of deadweight capacity. The class 35 tankers are already being employed as a Diesel Hub for the PSVs. Based on the results presented herein a pilot test using a class 47 tanker is underway, with its position and motions fully monitored during the whole test, in order to compare its behavior with the numerical estimates.

This content is only available via PDF.
You can access this article if you purchase or spend a download.