A graving dock is a dry dock excavated from land directly adjoining a seaway. The excavation is lined with concrete, and closed on the side facing the sea by a gate. This allows a vessel to be floated in, the gates closed, and the water pumped free.
- Gates are typically floating panels, brought into place & flooded (may also be flaps attached to seabed). Hydrostatic pressure holds the gate in place against lips on the side of the drydock and the sill on the bottom.
- There is some leakage around the gate, so behind the sill is a trough which leads to a pump well.
- The floor of the dry dock slopes aft to the pump well also, for rainwater or washdowns.
- The pump well is connected to the pumphouse, allowing the small flow of water past the gate to be continuously pumped out.
- Pumphouses are often on both sides, with the dock operator station in one of them.
- The pumphouse also controls sluice valves or similar, for controlled flooding of the graving dock.
- Traditionally, the walls were stepped to allow for shorings. Modern vessels tend to be blockier & supported by bilge blocks, so starting in the 1940's, graving docks walls have become more vertical, with less steps.
- The floor is of thick concrete, allowing for heavy and/or unbalanced loads.
- At ground level, a concrete apron reinforces the top edge & supports equipment. Cranes, particularly gantry cranes, may be fitted to railroads embedded in the apron.
- Docks attempt to minimize flood time of their drydock, to keep garbage & fish levels low
- Heavier vessels require less pumping time, as they displace more water
- Location is extremely important, the depth of the dock limits ship size: maximum vessel draught is directly related to the sill height, block heights, & tide levels.
- Graving refers to the cleaning of hull, i.e., scraping or burning of marine growth (fouling).
- Once built, it is extremely expensive to redesign (unlike floating dry docks).