Graving Dock

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Graving Dock

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.

Contents

Construction

  • 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.

Notes

  • 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).

Incidences

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