A dolphin is a freestanding mooring or berthing structure, separate from the main quay or wharf, used to secure vessels or absorb their berthing impact.
Mooring Dolphins vs Breasting Dolphins
The two main types serve different purposes. Mooring dolphins provide additional points to make fast mooring lines beyond what the main berth face offers — useful when a vessel is longer than the quay itself, or when extra restraint is needed against wind and current. Breasting dolphins are positioned to take the vessel’s weight as it comes alongside, absorbing berthing impact through fendering, and keeping the hull at a safe standoff distance from fixed structures.
Where Dolphins Are Used
Dolphins are especially common at oil and gas terminals, where a jetty is often built specifically for loading/discharging rather than general cargo handling — the vessel moors against a line of dolphins (breasting dolphins near midships, mooring dolphins fore and aft) rather than a continuous quay wall. This layout also keeps a firebreak-style separation between the vessel and the terminal’s fixed piping and infrastructure, a meaningful safety consideration when handling flammable cargo.
Construction
A dolphin is typically built as a cluster of steel or concrete piles driven into the seabed, capped with a platform, and fitted with mooring bollards or fenders depending on its role. Because it stands independent of the shore, it can be positioned exactly where berthing geometry requires, rather than being constrained to a fixed quay line.
Why Officers Need to Know the Distinction
Berthing plans at dolphin terminals require deck officers to understand which dolphin serves which function before arrival — approaching a breasting dolphin at the wrong angle, or missing a mooring dolphin’s line-handling sequence, complicates what’s already a more exposed and less forgiving berthing operation than alongside a continuous quay.
Exam Relevance
Dolphin structures, their role at terminal berths, and mooring/breasting terminology are covered under port operations and mooring topics in deck officer competency exams.