In the maritime industry, cargo transfers are usually linked to busy ports and terminals. But did you know that some of the most important transfers happen on the high seas, and just two ships can accomplish this? This flexible method reduces pressure on ports and improves how things are done. But ship-to-ship (STS) transfers can be tricky. They need to be planned really carefully, there have to be strict safety measures in place.
In this guide, we’ll use our extensive knowledge of the shipping industry and our experience working with lots of shipping clients to go through the details of STS transfers. We’ll cover the STS essential processes, risks, and safety considerations. Whether you’re new to STS or looking to sharpen your knowledge, this guide will help you understand how to carry out these transfers safely and effectively.
What is Ship-to-Ship Transfer?
Ship-to-Ship (STS) Transfer refers to the operation where cargo is transferred between two ships, either while they are stationary or moving slowly side by side.
This method is commonly used for transporting liquids such as crude oil, liquefied gas (LPG or LNG), chemicals, and even bulk cargo, equipment, or personnel. In 2022 alone, more than 460,000 STS operations were carried out worldwide.
STS operations are conducted in a way that allows ships to maintain relative stillness, similar to what is described in physics as “relative motion.”
This approach offers significant flexibility and efficiency in cargo transfers, especially in areas where traditional port facilities may be lacking or too congested.
A Typical dry bulk STS transfer operations, using grab cranesWhy need Ship-to-Ship Cargo Transfer?
STS offers a more efficient cargo transfer solution, with several key advantages depending on the situation:
1. Handling Complex Shipments:
STS is particularly useful for transferring cargo that is difficult to handle in a regular port. Large vessels like tankers and gas carriers may carry bulk cargo that certain ports can’t accommodate. STS enables cargo redistribution to specialized transport vessels, optimizing the flow of goods.
2. Cost and Time Savings:
For both ports and ships, STS operations help reduce congestion and save time and costs. Ships avoid high port berthing fees and minimize the time spent at terminal docks, as cargo can be transferred directly in locations closer to production or consumption points.
3. Overcoming Port Limitations:
As the size of carrier ships increases, certain very large vessels (such as VLCCs, ULCCs, and VLOCs) may not be able to dock at standard ports due to berth size or draft restrictions. STS allows these ships to offload cargo to smaller vessels or barges that can access port facilities.
4. Emergency Situations:
n case of mechanical failure, collision, or grounding, STS transfers can help offload cargo to another ship, reducing the draft or assisting in salvaging the vessel. This not only helps mitigate losses but also ensures timely cargo delivery.
Types of STS Operations
Cargo Transfer
The most common type of STS operation, this involves transferring large quantities of bulk, liquid, or containerized cargo between vessels. It is typically achieved using bulk cranes, grab cranes, and conveyor belts. Offshore cranes with heave compensation are also utilized when necessary to ensure smooth and safe transfers.
Personnel and Crew Transfer
This type of STS operation is used to move people between vessels, such as during crew changes, specialist arrivals, or emergency evacuations. It is typically accomplished with gangways, pilot ladders, man-riding cranes, personal baskets, and small craft or tenders for safe and efficient personnel transport.
Equipment Transfer
Equipment transfer involves moving tools, supplies, spare parts, or small cargo between vessels. This operation is generally carried out using cranes on board the ships and is common in offshore vessels that are equipped with lifting equipment.
STS Transfer of Fuel
This operation refers to the transfer of fuels such as LNG, bunker fuel, and other types of fuel between two vessels. The transfer typically occurs via marine fuel hoses and is commonly carried out on tankers, offshore supply vessels, drillships, and oil rigs.
Offshore Bunkering
Offshore bunkering is a specific type of STS operation where fuel is transferred to a ship for its own consumption, typically using bunker barges or fuel tankers. This method is crucial for supporting long-haul voyages where refueling at port is not feasible.
LNG Ship-to-Ship Transfer
LNG transfer operations share a similar flow to fuel oil transfers but require specialized cryogenic systems. Since LNG must be kept below -160°C, this operation demands careful handling to prevent leaks, which could cause rapid vaporization, fire, or explosion.
Transfer Area
The ship-to-ship (STS) transfer area is a designated location—typically in open sea or sheltered waters—where two vessels safely come alongside to transfer cargo, equipment, or personnel. Selecting the appropriate area is crucial for ensuring safety, efficiency, and regulatory compliance. The location should offer ample sea room, generally at least 10 nautical miles of clear water, with sufficient depth for both vessels and adequate under-keel clearance.
Safe STS transfer areas are typically positioned away from busy shipping lanes, shallow waters, offshore structures, and environmentally sensitive zones. The area should provide shelter from strong winds, waves, and currents, with minimal swell—usually under 3 meters—to reduce vessel movement and collision risks. Good visibility, reliable VHF radio and AIS coverage, and clear access for tugboats or rescue vessels are also essential.
The transfer area may be either anchored or drifting, depending on the operation, but in all cases, it must be approved by the relevant authorities and comply with MARPOL, SOLAS, and local regulations. Careful consideration of sea conditions, traffic density, water depth, and emergency access will ensure a safer, more efficient STS transfer.
STS Transfer at calm and spacious watersSTS Transfer Process
STS operations are carefully planned “at-sea handoffs” involving numerous safety checks. The general process includes the following steps:
1. Planning – The two ships agree on the cargo, fuel, or personnel to be transferred, and discuss the location and safety precautions.
2. Positioning – The ships approach each other side by side at sea or at anchor, often with the assistance of tugboats to maintain stability.
3. Mooring – Heavy ropes are used to secure the ships together to prevent drifting, and fenders are placed between them to absorb impact during the operation.
4. Connecting – Hoses, cranes, or ship gangways are set up depending on whether fuel, cargo, or personnel is being transferred.
5. Transfer – The cargo, fuel, or personnel are moved between ships at a controlled pace, ensuring safety and accuracy throughout the operation.
6. Disconnecting – Once the transfer is completed, hoses or gangways are removed, and the ships are untied.
7. Departure – The ships separate and continue on their respective journeys, completing the operation.
Ship To Ship Transfer Time
An STS operation involves multiple stages, and the total time is the sum of each phase. The duration is typically calculated from the ship’s arrival at the Transfer Area, often referred to as the “berth.”
The overall time for a ship-to-ship (STS) transfer can vary significantly based on several factors, but a typical transfer—especially for large cargo such as crude oil—usually takes between 12 and 36 hours.
STS Equipment
All vessels involved in STS operations must be equipped with complete and well-maintained equipment, and their personnel must be fully trained and proficient in its use. Essential equipment includes:
- Fenders: Primary and secondary fenders, along with cushions for impact protection.
- Mooring Equipment: Winches, mooring lines/wires, and other related tools.
- Cargo Transfer Equipment: Hoses, hose saddles, and marine cranes for efficient cargo handling.
- Safety and Ancillary Equipment: Leak detection systems, alarms, and control devices.
- Other Essential Tools: Communication equipment, oil spill response gear, and firefighting equipment.
Bulk crane for STSSTS Risk Alert
Ship-to-ship (STS) operations involve significant risks, where even minor oversights can lead to serious consequences. Below, we’ve outlined the critical risks to be aware of during an STS operation:
Vessel Contact and Collision Risks
The most common incident in STS operations is the contact or collision between vessels while maneuvering alongside or during departure. Causes include excessive approach speed, incorrect approach angles, poor awareness of weather or tidal conditions, and failure to maintain consistent heading or speed.
Miscommunication between vessels, unexpected yaw of an anchored ship, and equipment failures—such as engine or steering gear malfunctions—also contribute to accidents. In rough sea conditions, vessels may roll together, leading to a loss of control. Incorrect placement of secondary fenders, without considering draft or freeboard changes, may leave vulnerable areas exposed and prone to damage.
Dynamic positioning errors, hydrodynamic interaction effects, and tugboat miscoordination further increase the likelihood of collisions.
Hose Integrity and Handling Risks
Hoses used in STS operations must be specially designed for the cargo being transferred, complying with industry standards such as EN1765, BS1435, and OCIMF guidelines. Using damaged or expired hoses can lead to leaks or ruptures. Improper handling—such as twisting, pinching, dragging over sharp edges, or exceeding the minimum bend radius—weakens hose integrity.
Routine inspections (every 12 months) are essential, and failure to check hose string connections may lead to oil leaks. In LNG transfers, cryogenic temperatures increase hose brittleness, making failures more likely. Preventive measures include using Hose-in-Hose (HiH) systems for secondary containment and pressure/temperature sensors linked to automatic shutdowns.
Flammable vapors pose a fire or explosion risk during oil or gas transfers. IMO regulations require inert gas systems to maintain oxygen levels below 8%, insulating flange kits to prevent electrostatic sparks, and a strict ban on hot work during active transfers.
Environmental Risks
STS operations inherently carry environmental risks, such as oil and chemical spills, which can lead to significant pollution. The consequences of inadequate spill response or lack of contingency planning can worsen the damage. Different cargo types pose unique environmental hazards:
Crude oil: Flammable, forms long-lasting slicks, and harms marine life and coastal economies.
Petroleum products (e.g., diesel, jet fuel): Flammable and toxic but may evaporate faster, posing contamination hazards.
Liquefied gases: Create explosive vapor clouds and pose cryogenic hazards to personnel, though they disperse quickly in water.
Hazardous chemicals: Highly toxic, corrosive, or reactive with water, these chemicals can release dangerous gases, impacting both marine and human health.
Fender and Mooring Equipment Risks
Fenders are crucial in preventing vessel damage during STS operations. Issues arise from insufficient or incorrectly placed fenders, especially around vulnerable hull areas. Primary fenders should be tested every two years, while both primary and secondary fenders should undergo visual inspection before use.
Weak mooring equipment increases risks. Poor-quality lines, worn winches, or improperly maintained fittings can cause line failures and loss of control. Additionally, mooring lines without synthetic fiber tails or high-modulus synthetic ropes are less capable of absorbing shock loads, leading to instability.
Improper mooring procedures, such as over-tightening lines or incorrect anchoring, can destabilize vessels and lead to collisions. Inexperienced crews or inadequate supervision, particularly in poor weather, worsen these risks.
Personnel Safety Risks
STS operations expose crew members to numerous hazards, with accidents often caused by human error, poor communication, inadequate training, and fatigue. Ensuring proper planning, risk assessment, and safety procedures is essential.
Risks include falling overboard during personnel transfer, inadequate use of personal protective equipment (PPE), and transfers conducted too close to cargo handling areas. Onboard accidents—such as slips, trips, falls, or crushing injuries—are common. Mooring line failures can also cause fatal injuries.
Crew members may be exposed to hazardous substances such as oil, chemicals, or vapors during transfers. Fire and explosion risks are particularly high when dealing with flammable products. Additional risks include excessive noise, vibration-related fatigue, heat stress, or hypothermia in extreme weather, and confined spaces with oxygen deficiency or toxic atmospheres.
Regulatory and Planning Risks
Proper planning and adherence to regulations are vital for the safe and efficient execution of STS operations. Failing to comply with international, national, or local rules—such as lacking an approved STS Operations Plan or failing to notify local authorities—can result in legal penalties and delays.
An incomplete or outdated STS plan increases operational risks, as missing procedures, cargo, ballast arrangements, or contingency measures could lead to accidents. The Person in Overall Advisory Control (POAC) must have the necessary qualifications and authority to review and terminate unsafe operations.
Thorough risk assessments must be conducted for each phase—pre-arrival, approach, mooring, cargo transfer, completion, and unmooring—considering vessel compatibility, mooring arrangements, and navigational safety. Deviation from approved plans or industry best practices increases the likelihood of errors.
Failure to properly notify authorities and obtain necessary permits may lead to fines, penalties, or operational delays. Furthermore, inadequate contingency planning for weather changes, equipment failures, or oil spills makes operations vulnerable to unforeseen events.
What is dark STS transfers?
A “dark STS transfer” refers to a covert ship-to-ship operation in which vessels intentionally disable their tracking systems (AIS) to remain undetected. These operations are unauthorized, illegal, and highly risky.
Ships engaged in dark STS transfers are often part of the “shadow fleet,” typically operating under flags of convenience with unclear ownership. These vessels often lack proper insurance and maintenance, and they are frequently used for illicit activities, such as transferring oil or black-market goods.
By hiding their locations and cargo origins, these ships evade sanctions, regulations, and port authorities. This secrecy introduces significant dangers: poorly maintained vessels risk catastrophic oil spills, with no funds available for cleanup, and their invisible operations heighten the likelihood of collisions, endangering surrounding vessels.
Photo of a dark STS operation (Source UN report)Double Banking
While most STS operations involve just two vessels, it is also possible for three ships to operate simultaneously, a practice known as “double banking” or a “three-ship operation.” However, this is not common and is generally discouraged due to the significantly increased safety risks compared to standard two-ship transfers.
In a double banking setup, a larger “mother ship” (such as a large tanker or FSRU) is moored with two smaller “daughter vessels” alongside, typically arranged in a T- or Y-shaped formation. Although this method is occasionally used for specific purposes—like LNG transfers from an FSRU or military replenishment—the close proximity of the vessels makes ship movements and mooring far more complex and hazardous.
The presence of unpredictable water forces, a heightened risk of collision, and the challenges of maneuvering in tight spaces mean that such operations require exceptional planning, expert approval, and specialized expertise, making them rare in commercial shipping.
Double banking operation for three vesselsSTS Requirements and Best Practices
STS operations need to follow the latest transfer guidelines and get approval from the port authorities—yep, there’s a fair bit of paperwork involved. You’ll usually need to submit everything at least 48 hours in advance. The transfer area should have enough water depth, stable sea conditions, and real-time weather monitoring. If wind speeds go over 25 knots, operations should be put on hold, and vessels should stay 3–10 meters apart.
Before starting, the type of cargo, associated hazards, and safety procedures should be clearly explained to all crew members. A reliable communication channel must be established between vessels, with backup systems in place. The ship’s Master and the appointed STS coordinator or superintendent are generally responsible for overall control.
During the maneuver, vessels must maintain precise positioning and stability, assisted by tugboats if necessary. The delivering vessel should approach at less than 0.5 knots and align parallel within 30–50 meters.
All STS equipment—such as fenders, hoses, and mooring gear—must be certified by classification societies, regularly tested, inspected, and properly maintained. An Emergency Response Plan should be in place, with spill control equipment (booms, skimmers) staged in advance and fire extinguishers readily available. Emergency response readiness should not exceed 30 minutes.
When transferring fuel, hoses must be flushed with inert gas or water after disconnection to clear residual cargo.
Ship To Ship Transfer Checklist
A Ship-to-Ship (STS) transfer checklist is a standardized and essential document that ensures the safety and efficiency of every STS operation. Studies show that nearly 80% of STS incidents are linked to checklist non-compliance, highlighting its critical importance.
The checklist outlines all necessary procedures, checks, and precautions to be followed before, during, and after the transfer of cargo between two vessels. A comprehensive STS checklist is typically divided into sections, such as:
- Pre-Arrival and Pre-Berthing Checks
- Berthing and Mooring Checks
- Cargo Transfer Checks
- Post-Transfer and Unberthing Checks
- Emergency Procedures
It should also include contact information for emergency services and authorities. The Person in Overall Advisory Control (POAC) and the Master of each vessel are responsible for supervising the checklist, while senior crew members complete the relevant sections. Two copies are made: the original remains onboard as an internal record, and a duplicate is submitted to the head office for inspection.
Coming Ashore During STS Operations
You might wonder what happens when two ships are conducting ship-to-ship (STS) cargo transfer operations offshore, and personnel need to carry cargo samples or documents ashore.
In these situations, tender boats are commonly used. These small, versatile boats facilitate the safe transfer of personnel from the larger vessels to the shore. Tender boats are typically enclosed, providing protection for the crew and make sure they don’t get wet during the transfer.
However, it’s important to ensure favorable weather conditions and calm seas for safe operations. Additionally, crew members involved in small-boat embarkation and disembarkation should be properly trained to handle these operations safely.
Summary
That concludes our guide. To summarize, a successful STS operation depends on seamless cooperation between a professional STS service provider, reliable equipment, a qualified Mooring Master, and the vessels involved. With adequate preparation, the process is straightforward and efficient.
OUCO offers a comprehensive range of ship-to-ship (STS) transfer solutions, including specialized equipment and expertise for safe and efficient cargo transfers. Our product line includes offshore cranes, man-riding cranes, active heave-compensation winches, ship gangways, bulk grab buckets, and bulk handling equipment. These ensure secure personnel and cargo transfers, along with efficient bulk material handling. Explore our full product range, watch informative videos, and feel free to leave us a message if you’re interested in learning more.
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