Similar-size VLCC STS is not unsafe by default, but it is less forgiving than it looks.
At anchorage, the real risk begins when two hulls stop moving as one controlled system.
We were preparing for a ship-to-ship (STS) transfer at a busy South-East Asian anchorage when the company shared a past STS incident for safety learning. It involved other vessels, but the operation was close enough to make me read carefully.
The investigation report described two VLCCs alongside when the weather deteriorated. Within 20 minutes, mooring lines parted, the bow opened, the hose string came under strain and parted, cargo was stopped, and emergency response followed.
But something in that explanation did not fully settle the matter for me. Tankers do not simply open out because the wind has increased to 30 or 35 knots. Mooring lines do not part one after another without a load path. A hose string does not come under strain unless the relative position of the two ships has already changed. The report recorded what happened, but the operational question remained how did two large tankers, already made fast alongside, lose their geometry so quickly?
Later onboard, I discussed the case with the same Mooring Master, or POAC. That discussion moved the event from a weather explanation to ship movement.
The Same-Length Problem
Similar-size VLCC STS alters the fundamentals of the mooring layout.
When a VLCC transfers to a smaller tanker, the difference in length gives useful geometric leverage. The smaller vessel’s head and stern lines can be led at more effective angles forward and aft, giving better longitudinal and transverse restraint. The two-ship relationship is also easier to read visually from the bridge wing.
With two VLCCs of similar length, the mooring arrangement has far less leverage. Conventional head and stern lines cannot work in the same way as they do when a smaller tanker is alongside a larger one. Because the parallel body profiles closely match, more of the physical control shifts to breast lines, springs, and carefully planned head and stern arrangements.
The Falkonera judgement remains an important legal and technical precedent because it puts this operational reality on record. In VLCC-to-VLCC STS, same-length hulls are not an exception. The arrangement is fundamentally different from a conventional mother-and-daughter vessel setup. This does not mean VLCC-to-VLCC STS is unsafe by default, but it simply has less natural forgiveness.
The Anchorage Trap
At anchorage, STS can look stable because the approach itself is controlled. One vessel is lying at anchor. The manoeuvring vessel comes up slowly with tug assistance, checks the drift, matches the heading, reduces the closing speed, and settles alongside on the primary fenders. From the bridge wing, it can feel similar to bringing a ship alongside a berth.
But an anchored VLCC still yaws, sheers, and ranges on her cable. The cable restrains the ship, but it also becomes the pivot around which the two-ship system can turn. When a squall passes or the tidal stream shifts, the anchored vessel may rotate around that restraint faster than the vessel alongside can follow.
Beyond the obvious change of freeboard, the deeper hazard is the response reversal that takes place during the transfer. At the beginning of the operation, the discharging vessel at anchor is loaded, deeper, heavier, and has more underwater grip. By the later stages, that same vessel has become lighter, higher-sided, and more exposed to wind. The receiving vessel alongside has become deeper, heavier, and slower to follow.
As the freeboard difference changes, the breast-line lead also changes. A line intended to restrain lateral separation may begin carrying more vertical component and less horizontal holding power. By the later stage, the line can still look tight, but it may no longer be giving the same sideways control.
The anchor cable is now restraining the vessel most sensitive to wind, while the heavier vessel alongside may not rotate at the same rate. The ships may still share similar length, but they no longer share the same response. Hour thirty is not hour one.
Freeboard and Response Reversal During STS
The hulls remain similar in length, but draft, windage and mooring lead geometry change as cargo transfers.
The Active Gap
The space between two parallel VLCCs alongside is not neutral.
Above the waterline, one hull can alter the wind reaching the other. Fujiwara and NMRI’s work on side-by-side offloading shows how shielding and interaction can change wind forces between adjacent hulls.
Below the waterline, the gap also matters. Side-by-side hydrodynamic studies, including Li’s work on multi-body resonance and shielding, show that adjacent hulls can develop relative motion, uneven fender compression, and higher mooring loads.
In the case reviewed, one ship had more windage and less draft. The other had more draft and more inertia. The gap amplified the imbalance.
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The Failure Path
The original incident sharing recorded the visible sequence, weather deteriorated, mooring lines parted, the bow opened, and the hose string failed. The operational reconstruction came later, after my discussion with the POAC.
By the later stage of the transfer, the anchored discharging vessel had become lighter and more exposed to wind. The receiving vessel was deeper, heavier, and slower to follow. When the environmental forces shifted, the two ships no longer responded at the same rate. That difference mattered most forward. As the anchored vessel swung back, the receiving vessel’s bow could not follow with the same movement. The forward mooring group became the main restraint resisting separation.
Winch brake holding capacity is part of the same load path. If the brake does not render at the intended load, the line can become a fixed restraint and part before the rest of the mooring group shares the force.
If the forward lines are not carrying equal tension, the load does not spread. The first fully loaded line parts, the next inherits the shock, and progressive failure follows. Once the bow opens, the hose is pulled outside the geometry planned for cargo transfer.
CHIRP Maritime explains why this is difficult: vessels alongside can have different motion periods, creating snatch loading and making load balance difficult. It also stresses that lines within a working group should be of the same size, construction, strength, and length.
An unevenly tensioned line is a delayed shock load waiting for movement.
Moorings as an Active Control System
In a similar-size transfer, moorings are the active control system holding a shifting, 600,000-tonne combined mass together.
Every winch, BHC, chock, fairlead, and line lead directly affects how safely that load is distributed. If one part of the mooring group stops sharing load, the system weakens.
Skuld’s STS safety guidance provides a practical framework for this. It highlights that proper fendering, robust hose support, strict adherence to minimum bend radii, and constant tracking of relative freeboard changes are essential. It also highlights the critical importance of enclosed fairleads and correctly deployed rope tails so that line direction and elasticity remain effective as the vessels move dynamically.
During pre-STS training, the deck team must be briefed to read the mooring group as a load-sharing system, not just report line condition.
Indicators Beyond the Anemometer
A wind speed limit is necessary, but it is not enough. The same gust can either press the vessels together or open a high-freeboard bow, depending on direction, cable lead, and vessel response.
The 2025 OCIMF, CDI, ICS, and SIGTTO STS Guide remains the current industry benchmark, but the final judgement at sea is still physical.
Are the two vessels still moving as a single, controlled system? If that answer becomes uncertain, the operation has already changed.
DeepDraft View
Similar-size VLCC STS should not be rejected on vessel dimensions alone. It has clear commercial value, an operational track record, and it can be carried out safely. But it must never be treated as a standard STS operation simply scaled up.
It is a specialized interface with reduced tolerance for geometric error. One ship lightens while the other loads. One is held by an anchor cable while the other lags behind its movement. Wind and water interaction in the gap can amplify the imbalance. Mooring groups must continue sharing load, and the hoses remain safe only while the relative position of the two ships remains controlled.
Failure starts when the two VLCCs stop moving as one.
Weather monitoring is only the first layer. In anchorages known for sudden squalls, the operation must assume that weather may arrive faster than support can be arranged. If the warning-to-wind window is only fifteen or twenty minutes, a tug still being organised is already late.
Tug standby must be part of the STS plan and JPO, not a last-minute commercial discussion. It must also have a defined role before the risk develops, whether checking the anchored vessel’s swing, helping the receiving vessel’s bow follow, or taking another position judged appropriate by the POAC.
I have seen how easily this becomes a cost debate placed on the Master at the worst possible time. That is not where the decision belongs. If tug support is required to preserve alignment, it must be available while the risk exists, and the cost must be built into the operation. This is where company support, POAC judgement, and STS provider quality matter. The same interface risk exists in crew transfer.A good POAC does more than bring the vessels together. The value is in setting practical limits, reading the developing geometry, and advising when to slow, stop, stabilise, or separate.
The real test of command is preserving cohesion in the final alignment of two moving ships until the last line is safely let go.
Media Section
Sources Reviewed
- OCIMF, CDI, ICS and SIGTTO, Ship to Ship Transfer Guide for Petroleum, Chemicals and Liquefied Gases, 2nd Edition, 2025.
- IMO, MARPOL Annex I, Chapter 8, Prevention of Pollution During Transfer of Oil Cargo Between Oil Tankers at Sea.
- Skuld, Ship to Ship Transfer Safety.
- CHIRP Maritime, Ship to Ship Mooring Incident.
- Falkonera Shipping Company v Arcadia Energy Pte Ltd, Court of Appeal judgment.
- Fujiwara / National Maritime Research Institute, Wind Effect Estimation in Side by Side Offloading Operation.
- Li, B., Multi-body Hydrodynamic Resonance and Shielding Effect of Vessels Parallel and Nonparallel Side-by-Side, Ocean Engineering, 2020.
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