ANALYSIS OF OIL SPILL RISK IN DP SHUTTLE TANKER DIRECT OFFLOADING OPERATIONS

摘要

DP shuttle tankers performing offloading directly from fixed or geostationary floating offshore installations is addressed in this paper. It is important to ensure that disconnection of offloading hose can be achieved in time given shuttle tanker DP failure and position loss. The accident scenario is the hose fail-to-disconnect while shuttle tanker has an excessive position excursion. The consequence can be oil spill combined with the damage the offloading system. The spill amount can be as much as the crude oil volume in the hose, or over 1000 m3 if isolation and shutdown of oil export pump on the installation are not achieved timely. Various barriers to prevent oil spill have been developed over the past 30 years' history of shuttle tanker offshore loading. However, the direct offloading is a new operational context to the traditional offloading. A quantitative frequency model for oil spill initiated by DP shuttle tanker position loss in direct offloading is presented in this paper. Case study results show that in the base case where only traditional barriers are used, the frequency for large oil spill up to 1000 m3 or more may reach 2.48E-03 per year, given 20 hours offloading cargo transfer time and 52 times offloadings per year. This frequency is not negligible, and risk reduction measures are viewed necessary. Novel safety barriers, i.e. Automatic Shutdown and Release (ASDR), as well as the HPR (Hydroacoustic Position Reference) and BLS (Bow Loading System) weak link mode, are analyzed as sensitivity cases. Results show that the frequency of large oil spill can then be reduced to 3.81E-05 per year, i.e. 1.5% of the base case value, and this is well within 1.0E-04 per year level. Recommendations to minimize oil spill risk during DP shuttle tanker direct offloading operations are proposed in this paper.