Ship-to-ship (STS) transfer moves cargo directly between two vessels — typically one larger vessel and a smaller one, moored alongside each other at anchor or a designated STS location — bypassing port facilities entirely.
Why STS Operations Happen
- Reducing draft — transferring some cargo from a very large vessel to a smaller one before entering a port with depth restrictions the larger vessel alone couldn’t meet
- Avoiding port costs and delays — some operations use STS specifically to avoid port fees or congestion
- Sanctions evasion — genuinely, STS transfers have also been used (illegitimately) to obscure cargo origin in sanctions-evasion schemes, which is part of why STS operations receive real regulatory and monitoring attention today
The Real Operational Challenges
Two large vessels operating alongside each other at sea, rather than against a fixed, engineered quay, presents genuine challenges:
- Fendering — large pneumatic fenders are used between the vessels, positioned and monitored throughout given the higher relative motion risk than a fixed berth
- Mooring — a careful mooring arrangement between the two vessels, accounting for weather, current, and the combined motion of both ships
- Continuous communication — both vessels’ bridge teams coordinate throughout, monitoring relative position and any developing issues
OCIMF STS Guidelines
The Oil Companies International Marine Forum (OCIMF) publishes detailed STS operational guidelines, widely followed as the industry standard for planning and conducting these operations safely — covering everything from pre-operation planning to mooring configuration to emergency procedures specific to STS.
Pollution Risk
STS transfer of oil cargo carries genuine spill risk given the operation’s inherent complexity — hose connections, transfer rate control, and monitoring are all real, checked procedures specifically because a spill during STS transfer has real environmental and financial consequences.
Where STS Commonly Occurs
Many STS operations take place at designated STS locations — areas specifically assessed and approved for this type of operation, considering weather, traffic, and environmental sensitivity — rather than arbitrarily chosen open-water locations.
Exam Relevance
STS transfer procedures, OCIMF guidelines, and the real operational and pollution risks involved are core Cargo Work and Ship Knowledge topics in deck officer competency exams, particularly for tanker-focused candidates.