The transfer of cargo between two vessels moored alongside each other at sea or in an anchorage.
Quick facts
Regulation
MARPOL Annex I
In practice
The STS operation requires a detailed pre-transfer plan covering fender type and placement, mooring arrangement, communication procedures, emergency stop signals, and the responsibilities of each vessel's officer in charge. The receiving vessel's chief officer calculates stability at all stages. OCIMF's SIRE inspection programme assesses STS competence and documentation. Lightering — transferring cargo from a large laden vessel to a smaller one to reduce draught before proceeding to a port that cannot accept the larger vessel — is the most common commercial reason for STS operations. The operation carries elevated risk of collision, mooring line failure, and spill, and is therefore subject to heightened watchstanding and environmental vigilance.
Regulatory detail & full definition
Ship-to-Ship (STS) transfer is the process of transferring cargo — most commonly crude oil, petroleum products, liquefied gas, or dry bulk — between two vessels moored alongside each other, either at anchor in a sheltered anchorage or at sea in a designated STS area. MARPOL Annex I regulation 40A requires that vessels engaged in oil STS operations notify the relevant coastal state in advance and follow the procedures in MARPOL's STS guidelines (MEPC.186(59)) and OCIMF's Ship-to-Ship Transfer Guide.
The STS operation requires a detailed pre-transfer plan covering fender type and placement, mooring arrangement, communication procedures, emergency stop signals, and the responsibilities of each vessel's officer in charge. The receiving vessel's chief officer calculates stability at all stages. OCIMF's SIRE inspection programme assesses STS competence and documentation. Lightering — transferring cargo from a large laden vessel to a smaller one to reduce draught before proceeding to a port that cannot accept the larger vessel — is the most common commercial reason for STS operations. The operation carries elevated risk of collision, mooring line failure, and spill, and is therefore subject to heightened watchstanding and environmental vigilance.