What STS is, and the three reasons it happens
A ship-to-ship transfer moves cargo directly between two vessels moored alongside each other, either at anchor or underway, without either touching a berth. It is a routine commercial operation in the oil, chemical, gas and dry bulk trades, not an exceptional one.
Transfers happen for three distinct reasons, and confusing them causes most of the planning errors:
| Reason | What it is | Typical driver |
|---|---|---|
| Lightering | Discharging part cargo to reduce draft | The receiving port cannot take the vessel loaded |
| Cargo transfer | Moving a full or partial parcel between vessels | Trading decision, parcel splitting, or a change of destination |
| Bunkering | Transferring fuel rather than cargo | Supply at anchorage where no berth is available |
Lightering is draft-driven and usually happens close to the destination. Cargo transfer is commercially driven and can happen anywhere the parties agree. They carry different risk profiles and different documentation.
The two configurations
Every transfer is either at anchor or underway, and the choice governs almost everything else about the operation.
| At anchor | Underway | |
|---|---|---|
| Where | Designated anchorage or transfer area | Open water on a steady heading |
| Manoeuvring | One vessel anchored, the other comes alongside | Both making way; constant-heading approach |
| Weather tolerance | Lower — swell on the beam is the limit | Higher — heading can be chosen to suit the sea |
| Cost | Lower | Higher; more time and more crew |
| Typical use | Most commercial transfers | Where no suitable sheltered anchorage exists |
Underway transfers look more demanding and are, but they give the masters control over the relative heading to wind and swell, which is why they persist in exposed areas where an anchored transfer would be unworkable.
Who is in charge
This is the question that causes the most confusion, and it has a clear answer: the masters retain command of their own vessels throughout. Nobody displaces them.
What sits alongside that is the Person in Overall Advisory Control, usually written POAC. The POAC is a specialist — often supplied by the STS service provider — who advises on the approach, the mooring, the transfer and the unmooring. The word that matters is advisory. The POAC coordinates; the masters decide.
- Masters — retain command and remain responsible for their own vessel at all times.
- POAC — advises and coordinates the operation between the two vessels.
- Service provider — supplies fenders, hoses, mooring assistance and often the POAC.
- Charterers — usually appoint and pay for the service provider under the charter party.
- Support craft — mooring launches and standby vessels, positioned before the approach begins.
The pre-transfer conference between both masters and the POAC is where the plan is agreed. If either master is not satisfied, the operation does not begin — and that authority is not negotiable, whatever commercial pressure exists.
The equipment that does the work
STS equipment is unglamorous and absolutely determinative of whether the operation is safe.
| Equipment | Function | What goes wrong |
|---|---|---|
| Primary fenders | Large pneumatic fenders separating the hulls | Undersized for the vessel pair; damaged or under-inflated |
| Secondary fenders | Protect at the ends of the parallel body | Omitted to save cost |
| Cargo hoses | Carry the transfer between manifolds | Age, certification lapsed, incorrect rating for the product |
| Mooring lines | Hold the vessels in position | Mixed materials with different elasticity in the same pattern |
| Messenger and support craft | Run lines and stand by | Not on scene before the approach starts |
Fender sizing is the single most common technical failing. Fenders must suit the actual pair of vessels — their freeboard difference, parallel body length and displacement — not a generic specification. A fender adequate for two similar vessels can be badly wrong where a large tanker works with a much smaller one.
How a transfer is sequenced
The sequence is standard, and each stage has a stop point where the operation can be halted without consequence.
- Planning and compatibility check — manifold heights, hose ratings, mooring arrangements, cargo compatibility, weather window.
- Pre-transfer conference — both masters and the POAC agree the plan, the abort criteria and the communications.
- Fender deployment — on the designated vessel, checked and positioned before any approach.
- Approach and mooring — the manoeuvring vessel comes alongside the constant-heading or anchored vessel.
- Hose connection and pre-transfer checks — the checklist is completed jointly and signed.
- Transfer — at an agreed rate, with continuous monitoring of moorings, fenders and the interface.
- Hose drain and disconnect — before any unmooring.
- Unmooring and departure — as briefed, with support craft still on scene.
The abort criteria agreed at the conference are the most valuable part of the plan. They are set when everyone is calm and the sea is behind schedule pressure, which is exactly when a decision to stop is easiest to make correctly.
Where transfers go wrong
Incidents cluster around a short list of causes, and almost none of them are exotic.
| Failure | Usual root cause |
|---|---|
| Contact damage during approach | Approach speed and angle; misjudged relative motion |
| Fender failure | Undersized or damaged fenders for this vessel pair |
| Hose rupture or spill | Age and certification, or transfer rate exceeding the agreed limit |
| Mooring line failure | Mixed line materials, or ranging in rising swell |
| Weather deterioration mid-transfer | No agreed abort criteria, or reluctance to invoke them |
Weather is the recurring theme. A transfer begun in a marginal forecast is the situation where commercial pressure and safety judgement pull hardest against each other, and where a written abort criterion earns its keep.
The commercial and insurance dimension
STS operations sit awkwardly across charter parties and insurance, and the awkwardness is worth resolving before the vessels meet rather than after.
- Charter party permission. Many forms require STS to be expressly permitted; some restrict it by area or require owner approval of the service provider.
- Who pays. Service provider costs, fenders, hoses and support craft are usually charterers' account, but this must be stated.
- Time. Whether STS time counts as laytime, and how waiting for a suitable weather window is treated, is a frequent dispute.
- P&I cover. Clubs generally expect operations to follow recognised industry guidance; departures from it can affect a claim.
- Hull and machinery. Contact damage between the two vessels is a foreseeable exposure and should be understood before the operation.
For the operational record, the useful discipline is the same as any port call: every event with a time, a source and a context. STS disputes are decided on the transfer log, the pre-transfer checklist and the weather record — see demurrage and laytime for how that evidence base is used.
Frequently asked questions
What is a ship-to-ship transfer?
An operation moving cargo or fuel directly between two vessels moored alongside each other, at anchor or underway, without either using a berth. It is routine in the oil, chemical, gas and dry bulk trades.
What is the difference between STS and lightering?
Lightering is a type of STS done specifically to reduce a vessel's draft so it can enter a port that cannot take it fully loaded. Other STS transfers are commercially driven — splitting a parcel or changing destination — and are not about draft.
Who is in charge during an STS operation?
Each master retains command of their own vessel. A Person in Overall Advisory Control, or POAC, coordinates the operation between the two vessels, but the role is advisory — the masters decide, and either can stop the operation.
What equipment is needed for STS?
Primary and secondary pneumatic fenders sized for the specific vessel pair, certified cargo hoses rated for the product, mooring lines of consistent material, and support craft on scene before the approach begins. Fender sizing is the most common technical failure.
What are the main risks in an STS transfer?
Contact damage during the approach, fender failure, hose rupture, mooring line failure and deteriorating weather mid-transfer. Almost all of them trace back to planning: wrong equipment for the vessel pair, or no agreed criteria for stopping.