Note

Inexperienced operators should not operate lift bags.

WARNING All lift bags and rigging shall be thoroughly inspected before use. Failure could result in serious injury to the diver.

Figure 2-31. Method for tying pumpkin float to cables.

2.5.4 Surface Lift

For large lifting operations, there are many devices that are controlled and operated from the deck of a surface vessel and powered by pneumatic, hydraulic, and electric means. Powered lifting devices available through the OCEI include:

• Cable reel stands

• Utility winches

• Puller-tensioner winches

• Mooring winches

These devices can be mounted on an ocean construction support platform, such as:

• Modular Construction Platform

The lifting cable may be rigged over an end of a vessel (such as in a pipe-pulling operation) or over some type of A-frame rigging system.

Wire ropes have been conventionally used as lift lines in the past. An optional replacement line is a Kevlar (aramid) line. A wide variety of rope sizes and constructions are available in the commercial market. For a given strength, Kevlar line has the same diameter as the wire rope. Therefore, it is compatible with the existing winch and sheaves. Kevlar lines do not surge or snap back like nylon or polyester lines. The Kevlar line can be easily terminated by an eye splice like a wire rope. The cost of a Kevlar line is much higher than a wire rope, however, the higher cost may be justified by the ease in handling both on deck and underwater by the divers.

Lifting ring Maneuvering

Iris diaphragm strap dump valve V

Maneuvering strap

Lifting eye

Maneuvering strap

Lift bridle

Grommet, bolt, bushing

Lanyard handle

Maneuvering strap

Lift bridle

Grommet, bolt, bushing

Lanyard handle

Lift sling Load transfer line

Bridle

Load lifting bridle

Connecting sling

Safety shackle

Iris diaphragm dump valve

Lifting eye

Iris diaphragm dump valve

Control zipper

Lift sling Load transfer line

Bridle

Safety shackle

Dump valve lanyard

Control zipper

Dump valve lanyard

Figure 2-32

Diver lift system - bags available in three sizes: up to 500, 1,200, and 3,000 pounds.

Since surface-powered lifting devices are normally used when heavy weights or large tension forces are involved, an in-line, end-of-line, or running line dynamometer is used. This allows support personnel to monitor the tension forces in the lift lines and to watch for impending danger due to overload. Particular care must be exercised when heavy objects break the water surface as loads increase when the buoyancy effect of the water is lost. UCT personnel using surface-controlled lifting devices should be aware of the hazards involved and should have a thorough working knowledge of rigging procedures. All pulling/lifting cables should be run through fair-lead sheaves at appropriate intervals to prevent a line from moving beyond the control of the operators.

the effect of ship heave by paying out additional lift line as the ship heaves up and by taking in the line when the ship dips down. An air compressor or a set of compressed air cylinders is required to charge the unit initially. The unit can also be used during cable laying or pipe pulling operations to maintain steady line tension.

2.5.5 Underwater Running Rigging System

Underwater running lines are sometimes required to move bottom sitting weights with winches located either on a surface platform or on shore. These lines are often used with fairlead or multiple sheave blocks to direct the proper tension force to the weight. The conventional running rigging system consists of either a 3/4-inch-diameter 6 x 37 wire rope, or a 1-inch-diameter polyester rope. The wire rope system can handle a load up to 10,000 pounds whereas the polyester line can hoist a maximum load of 5,000 pounds. A 3/4-inch-diameter braided aramid fiber (Kevlar)

WARNING Personnel should stand clear of heaving lines when lifting is in progress.

For heavy lift operations in relatively high sea states, provision should be made to compensate for ship heave motion, which is detrimental to lifting safety. A ram tensioner ship heave motion compensator is available through OCEI. It is capable of supporting up to 25,000 pounds. The unit consists of a pneumatic/hydraulic cylinder and a set of sheaves. This compensator unit should be placed between the deck winch and the outboarding sheave (Figure 2-33). It acts like a giant shock absorber and compensates for

Ship Motion Compensator

Ship Motion Compensator

"A" Frame

"A" Frame

Payload

Payload

Figure 2-33 Fantail rigging for ship motion compensator.

line with a braided polyester outer jacket is available commercially to replace the stiff, heavy, and greasy wire rope as the primary running rigging system. The cost of the ara-mid rope is much higher than the wire rope. Lightweight blocks consisting of nylon sheaves are also commercially available for underwater weight handling. Although such lightweight hardware is costly, it does reduce the diver's strenuous activities underwater, increases work efficiency, and enhances the safety of the operation.

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