Caisson Elevation View
Caisson Elevation View
Figure 3-69. Drydock caisson general arrangement and seal location.
Figure 3-70. Plan view of drydock seat and caisson seal interface.
220.127.116.11 Stop Logs. Some drydock configurations flood the drydock through flooding sluice gates and tunnels. These tunnels provide a path for water into the drydock during pumping/flooding operations. The underwater intake to the flooding tunnel is usually located just outboard of the caisson assembly. A steel plate called a "stop log" is normally installed across the intake opening and provides a secondary seal for the sluice gates inside the flooding tunnel. It is sometimes necessary for divers to inspect the stop logs and correct excessive leakage past the stop log seals prior to maintenance on valves and pumps located inside the flooding tunnel.
18.104.22.168 Drydock Sill. The drydock sill is located at the outboard edge of the drydock concrete structure at the seafloor/ drydock interface. The drydock structure usually extends 5 to 10 feet above the seafloor at this point. Mud/silt buildup is a common problem in this area and must be removed if it extends above the drydock sill level. Otherwise, interference at the seal area and/or difficulty in positioning the caisson may result.
22.214.171.124 Docking Blocks. Docking blocks are used to support the keels and hulls of ships, barges, and vessels during overhaul operations. They are positioned and fixed on the drydock floor prior to ship docking and flooding operations. However, during ship docking operations, docking blocks may be inadvertendy moved out of position. If this condition is suspected, divers
<J! VL/Cn cuurtco are used to verify docking block position and in some cases remove (float) the blocks for subsequent reinstallation after dewater-ing the drydock.
3.10.2 Deterioration of the Graving Drydock Structure
In general, the concrete (or stone masonry) portion of the drydock is subject to deterioration through cracking, spalling, swelling, and disintegration, not unlike other concrete or stone masonry structures (refer to Section 3.6, Concrete Structures and Section 3.8, Stone Masonry Structures.) However, during drydocking operations various conditions exist where additional damage to the structure may occur -namely, during ship and caisson positioning operations. Contact between the ship and drydock structure may occur as a ship is moved into the drydock. Likewise, inadvertent contact may occur during positioning of the caisson. In either of these cases, damage to the drydock structure may occur as a result of impact.
The caisson, whether a concrete or steel structure, is also subject to damage as a result of drydock operations. Deterioration of the caisson may occur as a result of caisson positioning operations (impact and/or abrasion damage) or from long term exposure to seawater (corrosion, biofouling, marine organisms, and/or impact).
For a steel caisson, corrosion of the metal structure is always a concern. Although the caisson is painted and "zincs" replaced at regular intervals during overhaul periods (usually every 5 to 6 years), corrosion may be accelerated in areas where the steel has been exposed (scraped, impacted, etc.)
Damage to a concrete caisson may show up as cracking, loss of material, swelling of the concrete, chemical deterioration, and/or exposure and corrosion of the steel substructure. These conditions can be a result of impact, abrasion, wave action, poor construction methods/techniques, and long-term exposure to a seawater environment.
The cross section of a DM-29 wood caisson seal is shown in Figure 3-71. The wood portion of the seal transfers the load (due to hydrostatic pressure) from the caisson to the drydock seat and provides the primary seal. The small rubber gasket provides a secondary seal. Several physical and environmental conditions exist that can cause either accelerated or eventual deterioration of the wood:
• Marine borers
• Insect infestation
Of these, splintering and marine borers are the most prevelant.
Similar to the caisson structure, the seal assembly is also subject to impact and/or abrasion damage during caisson positioning operations. Wood deterioration can be confirmed by visual inspection and is evidenced by wood splintering and cracking where impact has occurred.
Several marine organisms attack and infest wood structures exposed to seawater. The types of organisms and their probability and rate of occurrence are based on several environmental conditions:
• Water temperature
• Sunlight exposure (variations/cycles)
• Wood preservative (when used)
In some areas, the speed of deterioration is surprisingly fast, particularly in a warm air/ water environment. A summary of the most common organisms and their associated effect on wood structures is provided in Section 3.6.2, Deterioration of Timber Structures.
The rubber component of the DM-29 wood seal and the newer all-rubber seal configuration (Figure 3-72) are not normally attacked by marine organisms, and are thus considered immune to this problem.
As with the drydock structure and caisson assembly, deterioration of the drydock seat due to impact and exposure to seawa-ter may occur. The condition of the seat and its resistance to deterioration will vary based on the amount and severity of impact events, seawater exposure, and composition of seat surface (steel, concrete, or stone).
In cases where an inner and outer seat exist and where the caisson has been positioned at the inner seat, the outer seat can become fouled with marine growth. This condition must be rectified before the caisson can be positioned at the outer seat. A similar situation occurs before superflood-ing. The inward facing seat is normally ex posed to seawater and marine fouling and must be inspected and cleaned prior to superflooding the drydock to ensure an adequate seal.
Underwater inspection of the graving drydock structure, caisson assembly, and associated components will most likely be accomplished at incremental component levels and not in a fixed, sequential order, as inspection requirements and frequency vary with each drydock component. A typical underwater inspection of a drydock facility will involve inspection of the drydock structure, caisson assembly, caisson seal, drydock seat, stop logs, and drydock sill area.
Procedures for underwater inspection of steel, concrete, timber, and stone masonry waterfront structures are discussed in Sections 3.4.3, 3.6.3, 3.7.3, and 3.8.3, respectively. In general, these inspection procedures are applicable to graving drydocks, caisson assemblies, and associated components.
Equipment and tool requirements for inspection of waterfront steel, concrete, timber, and stone masonry structures are discussed in Sections 3.4.5, 3.6.4, 3.7.4, and 3.8.4, respectively. In general, these requirements are applicable to graving drydocks, caisson assemblies, and associated components.
Wood fitted and bedded to caisson structure
Mounting stud, nut, and washer
Mounting stud, nut, and washer
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