DOWNTIME ANALYSIS FOR OFFLOADING OPERATION: DP X NON-DP SHUTTLE TANKER

The Brazilian oil company PETROBRAS will install the first FPSO ever in the Gulf of Mexico (GoM). This unit will be installed in a water depth of 2,500m, and in addition to several challenges to develop such a project, one has received special attention: the offloading to be performed by a shuttle tanker in tandem with the FPSO.

In a general offloading analysis, the motions of the shuttle tanker and the hawser tension are evaluated only in the maximum operational environmental conditions. This approach has limitations, the most important being the fact that the worst results do not always happen with the most severe environmental conditions and that it does not provide an indication of the operational downtime.

In this work, an analysis is performed to evaluate the downtime of a shuttle tanker, with and without dynamic positioning (DP) assistance, under the scatter environmental data taken from the GoM METOCEAN technical specification. Due to the large amount of possible environmental combinations of wave, wind and current, a reduced selection of 60 conditions has been chosen based on statistical procedures. The offloading analysis is performed for a turret moored FPSO, connected with two types of shuttle tankers: a non-DP shuttle tanker (ST) or a DP shuttle tanker (DPST). The DPST uses 2 tunnel thrusters in the bow, 1 in the aft and the main propeller, with a total power of 12,500kW. The ST is assisted by one tug in tandem, which applies at least 10t of force at the ST stern.

The calculations are performed with the in-house PETROBRAS software DYNASIM, a fully coupled time domain simulator. In the analysis the position of the moving shuttle tanker is monitored within the green zone, defined as ±45o from the FPSO bow-stern axis, and the mean and maximum hawser tensions, for all defined environmental conditions. The downtime for each loading condition is obtained by the summation of the occurrence probabilities of the environmental conditions under which the ST or DPST results did not stay inside the defined limits of position in the green zone and by the hawser tensions, i.e. the offloading cannot be performed. Machinery failure probability is not considered for the evaluation.

As a result a downtime smaller than 3%, or 11 days per year, was obtained for the ST and smaller than 2.4%, or 9 days per year, for the DPST, with the hawser tension limit exceedence being the main cause.