Removing Of Buoy With Crane Procedure

Underwater Construction

Figure 5-16. Mooring leg chain stopper.

This procedure should be repeated until the chain remaining in the water equals the water depth plus freeboard, plus a nominal 20 feet. During inhaul, the chain should be waterblasted to remove mud and debris and then carefully laid on deck to aid in inspection and reinstallation.

The crane should be used to recover the remaining bights of chain as well as the anchor.

• The load from the winch should be transferred to the crane and the inhaul wire removed.

• The crane should be raised to the maximum height and the chain maneuvered into the chain stopper, as illustrated in Figure 5-16.

• The stopper gate should be closed and the load transferred to the stopper.

Figure 5-16. Mooring leg chain stopper.

• After waterblasting the chain, it should be placed on the deck.

• The crane hook should be resecured to a link inboard of the stopper, the load taken on the crane, and the chain released from the stopper.

• The remaining chain should be hauled in and the anchor placed on deck after cleaning thoroughly.

Recovered mooring legs should be thoroughly inspected for wear and corrosion. Any components showing greater than 30 percent loss of material must be replaced. All data are to be recorded, including jewelry size and specifications and length. The anchors should be sandblasted to remove encrustation and scale. Each mooring leg should be reinstalled immediately after recovery and refurbishment. Reinstallation should be performed in accordance with installation procedures described in NAV-FAC DM-26, Harbor and Coastal Facilities, and NAVFAC MO-124, Mooring Maintenance.

5.4.2 Total Replacement

It may be necessary to replace a fleet mooring because the deterioration is so widespread that other repair methods would be uneconomical. Riser moorings are recovered by removing the buoy and then removing one leg at a time:

• The mooring buoy is lifted into the air to expose the ground ring.

• After the ground ring and legs are exposed, one leg is laid across the corner bitt to steady the chain.

• A buoy and line are attached to the leg, and the first A-link below the detachable link is then severed by cutting with a torch.

• A second leg is then picked up and separated in the same manner.

• The last leg is secured to a corner bitt, and the leg is separated by removing the detachable link to which the ground leg is attached.

• Again, a buoy and line are attached to the leg.

• The mooring buoy is then washed off with a stream of seawater and brought on deck.

Each of the legs and anchors are retrieved, raised by a crane, washed down, and stored neatly on deck. A new fleet mooring is then installed in accordance with NAVFAC DM-26 and NAVFAC MO-124 installation procedures.

While the above procedure appears to be relatively simple and straightforward as described, it is actually a difficult and dangerous operation. The weights and forces involved are extremely high and any miscalculation or mistake could have disastrous consequences. It is recommended that total replacement be undertaken only by personnel experienced in the procedures.

5.4.3 Cathodic Protection

Cathodic protection is a highly desirable maintenance technique for fleet moorings. A system consists of:

• Sacrificial anodes to provide the necessary electrical current.

• Wire ropes for imparting electrical continuity throughout the ground tackle

• Fittings necessary for joining the components of the system to the mooring and to each other.

There are three types of sacrificial anodes typically used in fleet mooring ca-thodic protection systems:

• Special-D anodes

Buoy anodes are mounted directly on the buoy in a location that is not subjected to impact by moored vessels. They have a nominal length of 36 inches, a nominal cross-sectional area of 16 in.2, and a nominal weight of 150 pounds.

Link anodes are cast on special chain links that are joined direcdy onto the ground tackle system before the mooring is laid. The anode castings have a nominal length of 24 inches, a nominal cross-sectional area of 80 in.2, and a nominal weight of 500 pounds.

Special-D anodes are the same size as link anodes. An insulated copper wire is passed through a 3/4-inch-diameter steel pipe core cast inside the anode, bared and soldered at each pipe end, and potted with epoxy to reduce the stress on the connection. These wires extend about 30 feet from the anode, where they are joined in place to wire ropes. The wire ropes are periodically woven back and forth through the ground tackle and joined to it using metal hose clamps to provide the necessary electrical continuity. Wire ropes are typically 5/8- to 3/4-inch-diameter galvanized steel wire.

Buoy anodes are supported by a pair of brackets welded direcdy to the buoy shell that have threaded fittings for simple installation/replacement of anodes. Link anodes are joined to the ground tackle by a standard joining link. Special-D anodes can be secured by clamps to the wire ropes which, in turn, are joined to the ground tackle by common metal hose clamps.

Buoy anodes usually are installed ashore during buoy overhauls, but are easily replaced on-site by lifting the buoy out of the water and removing and replacing the nuts from bolts or threaded studs.

Link anodes are usually installed ashore when the ground tackle is being overhauled. These anodes are located where shots of chain are joined together.

Special-D anodes typically are installed by divers. The anode leads must be sufficiently long and located such that the anode will not be displaced by movement of the ground legs from mooring forces. Wire ropes can either be installed after the ground tackle has been arranged on the deck of the installing vessel or in place by divers. The wire rope is loosely woven back and forth through about every eighth link and then clamped to every eighth link. The coating should be scraped away from the steel at this location to ensure a good contact surface.

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