STS Operations

Ship-to-ship (STS) cargo transfer is the transfer of cargo — primarily crude oil, petroleum products, or liquefied gas — between two vessels lying alongside one another at sea or at anchor. STS is used for lightering (reducing a deep-draught VLCC or ULCC to allow port entry), spread loading (assembling a full cargo from multiple sources), or emergency cargo transfer. It is one of the higher-risk operations in commercial shipping, combining the hazards of tanker cargo transfer with the dynamics of two large vessels in close proximity.

MARPOL Annex I Chapter 8 has regulated STS operations for oil tankers since 2011. The practical operational standards are set by OCIMF — the Oil Companies International Marine Forum — whose guidance is referenced by flag administrations, port state control, and P&I clubs worldwide.

Regulatory framework

MARPOL Annex I Chapter 8Regulation on STS Operations (in force 1 Jan 2011)
Mandatory for oil tankers of 150 GT and above. Requires an STS Operations Plan approved by the flag administration, advance notification to coastal states, and compliance with OCIMF/ICS guidance.
OCIMF/ICS Ship-to-Ship Transfer Guide (Petroleum), 5th ed.Petroleum STS operational standard
The principal operational reference for oil STS. Covers approach procedures, mooring, hose connection, ESD systems, cargo operations, and disconnection. Widely adopted as the recognised standard by PSC, SIRE inspectors, and P&I clubs.
OCIMF Ship-to-Ship Transfer Guide (Liquefied Gases)LPG / LNG STS guidance
Separate OCIMF guide covering gas carrier STS, including emergency shutdown (ESD) system requirements, vapour return, nitrogen purging, and the additional hazards of cryogenic and pressurised cargo.
Coastal state notification (48 hours typical)Pre-operation notification requirement
Under MARPOL Annex I Regulation 41, vessels must notify the coastal state in advance of an STS operation within its EEZ — typically 48 hours, though some states require 72 hours. Failure to notify is a detainable deficiency.

Operational sequence

1.Pre-arrival planning
STS Operations Plan submitted to coastal state authority. P&I club notified (required by most clubs). Vessel-to-vessel pre-arrival checklists exchanged. POAC (Person in Overall Advisory Control) identified and briefed.
2.Rendezvous
Mother ship (STBL — ship to be lightered) and daughter ship (LSV — lightering service vessel) rendezvous at the designated lightering area. Weather window confirmed.
3.Approach
Parallel approach typical: LSV manoeuvres alongside STBL at minimal closing speed. Closing speed at first contact must not exceed 0.3 knots. Pilot and STS team on station.
4.Fender deployment
Pre-positioned Yokohama pneumatic fenders (or composite fenders for gas) deployed from the smaller vessel. Fender size selected to match vessel freeboard differential and anticipated sea state.
5.Mooring
Ship-to-ship mooring lines passed and made fast. Rope tails used on each line as energy absorbers. Line tension monitored continuously as relative freeboards change with cargo transfer.
6.Hose connection
Cargo hoses rigged with Marine Breakaway Coupling (MBC). LNG/LPG operations use ESD-linked arm connectors. Simultaneous Emergency Shutdown (SESD) drill performed between both vessels before cargo commences.
7.Cargo transfer
Operations conducted on a dedicated ship-to-ship UHF channel. Cargo control officers on both vessels maintain continuous communication. ESD systems armed and tested. Transfer rate agreed between both chief officers.
8.Disconnection
Oil: hoses drained, blown through with inert gas, disconnected. LNG/LPG: hoses/arms purged with nitrogen, cryogenic cooldown lines shut off, arm disconnection sequence followed. Unmooring in reverse order to mooring. Vessels separate at low speed.

Critical fenders

Yokohama pneumatic fenders
The global standard for petroleum STS. Common sizes: 3.3 m × 6.5 m (smaller combinations) and 4.5 m × 9.0 m (VLCC/ULCC). Minimum two fenders deployed; number increases with vessel size differential and sea state above Beaufort 3.
Composite fenders
Used for LNG STS operations where pneumatic fenders may be unsuitable. Must meet cryogenic temperature resistance requirements at LNG temperatures (−163 °C).
Fender allocation
Number and placement calculated using OCIMF methodology based on displacement of the smaller vessel, relative freeboard, and maximum permitted contact energy. Documented in the STS Operations Plan.

Personnel & the POAC

The POAC (Person in Overall Advisory Control) is the individual with overall responsibility for the STS operation, typically a specialist STS Master Mariner supplied by the lightering operator. The POAC boards the STBL from a launch during the approach and coordinates operations between both vessels. For LNG STS operations the team additionally includes dedicated cargo engineers on both ships and a specialist LNG STS supervisor.

Both masters retain ultimate authority over the safety of their respective vessels. The POAC's role is advisory and coordinating, not command-overriding. In practice, any officer on either vessel may call an emergency shutdown if they identify an unsafe condition, regardless of the stage of the operation.

Risk factors

Weather window: STS operations should not commence if wind is forecast to exceed Beaufort 5 or swell above 2 m during the operation. Operations in progress must have agreed abort criteria.
Vessel size differential: Large freeboard difference between STBL and LSV increases mooring line geometry problems and fender loading as cargo is transferred and freeboards change.
Rolling characteristics: A vessel in light condition (LSV at completion) may roll heavily in moderate swell, increasing snap-load risk on mooring lines and hoses.
Anchored vs underway: Anchored STS provides a stable reference but requires a suitable designated anchorage. Underway STS is weather-dependent and requires both vessels to maintain matching course and speed throughout.
Port-state restrictions: Several coastal states restrict or prohibit STS in certain areas of their EEZ, particularly near fishing grounds, marine protected areas, or areas of known current instability.

Insurance

P&I club cover applies to STS operations provided they are declared in advance and conducted in accordance with a recognised industry standard (OCIMF/ICS guides). Most clubs require pre-notification of STS operations involving crude oil or products above a defined volume threshold, and some require pre-approval where the operation involves non-standard arrangements. Ship operators should confirm the specific requirements of their P&I entry before each operation.

Recent incidents

The 2018 collision and oil spill involving the tanker A. Symphony off Hong Kong resulted from an STS lightering operation in adverse conditions. The 2020 ULCC lightering incidents in the eastern Caribbean highlighted the risks of conducting STS in areas with strong uncharted currents. Both cases reinforced industry guidance on weather limits and approach speed controls.