Why STS failures escalate faster than crews expect
Estimated read time: 70–85 minutes
Audience: Cadet → AB → Junior Officer → Chief Mate
Introduction – When two ships become one unstable system
Ship-to-ship operations are often described as controlled, planned, and procedural. Checklists are extensive, communications are formal, and equipment is purpose-built. This creates the impression that STS operations are inherently safer than improvised cargo or bunker transfers.
In reality, STS operations are among the most unforgiving deck evolutions at sea.
The reason is simple: once two vessels are alongside, their motions, loads, and failures are coupled. A problem on one ship immediately becomes a problem on the other. There is no isolation, no pause button, and no easy escape once energy starts transferring between hulls.
What STS really is from a deck perspective
From the bridge, STS is about headings, speeds, and clearances.
On deck, STS is about contact forces, stored energy, and elastic systems under constant adjustment.
Fenders compress and rebound. Mooring lines stretch and relax. Hoses carry pressurised cargo while being pulled in multiple directions. None of these systems are static. They are continuously responding to swell, wind shifts, and human corrections.
The most dangerous moment is not the initial coming-alongside. It is the long middle period when everything appears settled — and crews relax their vigilance.
Fender systems: not cushions, but energy stores
STS fenders are often spoken about as buffers. This language is misleading. Pneumatic and foam fenders do not simply absorb impact; they store energy and release it back into the system. As vessels surge apart and come together, fenders cycle through compression and rebound.
If a fender shifts, loses position, or fails structurally, the stored energy is released asymmetrically. That imbalance transfers instantly into mooring lines and hull contact points. Loads spike without warning, often beyond what crews believe is possible under “normal” conditions.
Mooring geometry between moving hulls
STS moorings are under constant adjustment. Lines that were balanced minutes ago may become heavily loaded as vessels yaw or surge. Unlike berth moorings, there is no fixed reference point. Both ends are moving.
This is why snap-back risk during STS is often underestimated. The line may appear calm until a sudden relative movement tightens it violently. Crews standing in what felt like a safe position moments earlier can find themselves directly in a snap-back path.
🔻 Real-World Failure: STS Collision and Oil Spill off Johor, Malaysia (2017)
In January 2017, two tankers conducting an STS operation off Johor, Malaysia collided during transfer preparations. The incident resulted in a significant oil spill and structural damage to both vessels.
Investigations identified failures in relative position control, communication, and load anticipation. Environmental forces were present but not extreme. The operation had been planned and briefed. What failed was the assumption that conditions would remain stable once alongside.
Deck crews reported rapid changes in line tension and fender loading immediately before contact. Those changes occurred faster than corrective actions could be taken. Once hull-to-hull contact became uncontrolled, escalation was immediate.
The lesson from Johor is clear:
STS operations do not degrade slowly.
They fail abruptly when coupled systems fall out of balance.
Why STS accidents feel “sudden”
STS incidents are often described as sudden because the system hides stress well. Elastic components smooth out small changes — until they can’t. When capacity is exceeded, the transition from control to failure happens in seconds.
This is why experienced STS teams focus less on checklists and more on continuous observation of line behaviour, fender compression, and relative motion.
Knowledge to Carry Forward
STS operations are not two ships working side by side. They are a single, unstable mechanical system made of steel, rope, rubber, and fluid. Safety depends on recognising early signs of imbalance and acting before stored energy is released unevenly. Once that release begins, no procedure can stop escalation.
Competent deck officers treat STS as temporary controlled instability, not steady-state operations.
Tags
On Deck, STS Operations, Ship to Ship Transfer, Fenders, Mooring Lines, Stored Energy, Oil Transfer, Deck Safety, Human Factors, Failure Modes