Specific product transfer risks from FPSO to Convectional and dedicated Shuttle Tankers
During a tandem offloading operation between the FPSO and tanker, the tanker is stationed astern of the FPSO on approximately the same heading astern of the FPSO. The loading of the tanker is a complex operation conducted jointly by the FPSO and the tanker operational teams.
The Station keeping of offtake tankers can be challenging due to wind, waves and strong currents. Passive or active weathervaning is always a risky operation where tanker misalignments with the FPSO could lead to either tanker collision with the FPSO or tanker breakouts.
During the loading operation, while marine hoses are facilitating transfer of crude oil, the mooring equipment secures the FPSO and the shuttle tankers. This mooring equipment consists of hawser(s), chafe chain(s), quick release mechanism, supporting buoys as well as assistance from tug boats and/or dynamic positioning systems.
In addition to accidental uncontrolled release of oil to the environment, there are significant risks of serious personnel injury and extensive damage to the FPSO and tanker equipment following excessive loading and/or failure of mooring equipment. This excessive loading or failure of mooring equipment could be the consequence of tanker breakouts or excessive and damaging pressure surges.
A tanker breakout is a vessel moving off station and breaking its mooring hawser or similar such incident due to bad weather, strong under water currents, variable tides, tanker mis-manoeuvring, failure of deck equipment, blackout out of dynamic positioning systems.
An extreme and damaging pressure surge can be caused by the inadvertent sudden closure of the butterfly valve (if fitted) during full flow condition or the failure of slamming shut of the discs in the Bow Loading Coupler/North Sea Valve.
Following these incidents, if marine hoses are not fitted with Marine Breakaway Couplings (MBC), the hoses could be permanently compromised or ruptured and significant spill could occur.
Specific risks of FPSO transfer and the consequences of an incident
When the tanker drifts out of control following a tanker breakout then excessive load on the hoses, in the absence of MBCs, will cause rupture and extensive pollution will be inevitable. Several thousands of cubic meters of crude oil could be lost to the sea.
Expensive capital hoses would have to be replaced and the whole string would have to be inspected, tested either onshore or offshore before the FPSO could resume operation.
An example of a successful MBC activation: Nigeria 2015
An FPSO was offloading crude oil to a shuttle tanker when the mooring hawser parted and the shuttle vessel drifted out of control. Extreme tensile loads were applied to the transfer hose system. This tanker breakout was caused by a Soliton (Solitary subsurface wave that can propagate along the boundary between water layers having different densities. Generally occurring in specific regions, solitons are often associated with large but short-term current velocities that can cause severe disruption to subsea operations while showing little indication of their presence on the surface). The MBCs activated successfully – preventing extensive pollution and damage.
Extreme and damaging pressure surge
Extreme pressure surges can be caused by the failure of the butterfly valve or the failure of the bow loading coupler. Following this kind of incident, an excessive and damaging surge could damage hoses, FPSO manifolds and a large spill could occur. The installation of a MBC will prevent the damage caused by this kind of surge and dissipate the energy of the surge.
An example of a damaging pressure surge caused by a bow loading coupler in 2007 in Norway:
During a loading operation, a hydraulic hose failed – leading to the coupler valve on the tanker’s bow loading system to snap shut in 0.5 seconds as opposed to the normal 25-28 seconds. This caused a rapid and extreme buildup of pressure in the hoses of about 115 bars. An MBC was not fitted and the outcome was a ruptured hose at subsea. The oil spill into the sea was 4,400 m3 (27,700 Barrels). The operator was fined USD 4.62 Million. Additional costs which were not divulged included terminal downtime, the cost of replacing the damaged hoses, the cost of the support vessels, the clean-up operation and damage to the company’s reputation.