Dolphins are part of OPD in-port service and are built to extend a berth (a berthing dolphin) or providing a point to moor to (a mooring dolphin). The mooring dolphins are often used to "cushion" ship impacts.

The structures typically consist of a number of piles driven into the seabed or riverbed and connected above the water level to provide a platform or fixing point. The piles can be wood, steel or reinforced concrete. Smaller dolphins can have the piles drawn together with bolts, but larger dolphins are typically fixed using a reinforced concrete capping or a structural steel frame.