page_title: Air lock diving-bell plant
section_title: Operations
hierarchical_section_title: Air lock diving-bell plant / Operations
caption_attribution_description: English: Mobile Air Lock Diving Bell, cross section drawing, built for the Admiralty in 1902
context_page_description: Gibraltar Harbour's air lock diving-bell plant, or caisson diving bell barge, was a purpose-built barge for the laying, examination and repair of moorings for battleships. It was designed by Siebe Gorman & Company of Lambeth and Forrestt & Co. Ltd of Wivenhoe in Essex, who built and supplied it in 1902 for the British Admiralty.
  Local conditions at Gibraltar dictated the need for such a craft. The heavy harbour moorings have three chains extending out radially along the seabed from a central ring, each terminating in a large anchor. Most harbours have a soft seabed, and it is usual to lay down moorings by settling anchors in the mud, clay or sand, but this could not be done in Gibraltar harbour where the seabed is hard rock.
  For Gibraltar the seabed needed to be dug out sufficiently for an anchor point to hold. The barge would be towed over the work site, moored in place with anchors, and the bell would be lowered to the seabed. Air pumped into the bell would displace the water, and workers accessed the bell through the airlock in the central access shaft.
context_section_description: The Plant's task was to bury each of three anchors for a mooring deep into the seabed.
  A mooring vessel (in the 1960s, HMS Moorland) would first lay out the mooring components in position. 
  Good weather was required for operation of the bell, and the barge would be moored with six or eight anchors to keep it in place. The barge had no means of propulsion, and was towed into place over each mooring anchor by tugs which also laid out the barge's anchors. Once anchored, the barge was capable of making fine adjustments to its position by using the winches to haul on the anchors.
  The access tube on the bell was sealed at the top, making it airtight, and the bell lowered through a moon pool to the seabed over the position where the anchor was to be bedded. A compressor was used to equalize the air pressure inside the bell and access tube with the water pressure at the bottom opening of the bell, this would displace the water in the bell and leave an air-filled space above the level of the rim in which workers could operate. The working chamber of the bell was bottomless, allowing workmen to stand and work directly on the seabed within an air filled space.
  Men would climb down into the entry tube and through an airlock and then go down a vertical ladder into the working chamber. Once the anchor's position was marked, a winch (mounted on the deck's lifting frame, its wires passing through the chamber) would lift the anchor and the barge would move, placing the anchor to one side. The barge would then return to its original position and four men in the chamber would commence digging a hole big enough for placement of the anchor. 
  Working in dank, dingy, pressurized conditions, the men would skim off a thin layer of mud and attack the rock, first with pickaxes; as the hole deepened, one man would hold a crowbar for another to swing a sledgehammer. The rock, chipped out into boulders, would then be passed up by hands from the hole. Digging the hole would take two or three days until it was shoulder-high and deep enough for the anchor.
  Once the anchors were embedded in the holes, the mooring tender vessel would take the place of the barge to complete the mooring system, using her two 10-ton winches for the job in hand. The completed moorings would then be left in place, with buoys for markers, as permanent installations.
caption: A cross section of the mobile air lock diving bell
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