Nodules

In 1972 Kennecott Explorations Inc. conducted a three months cruise to the central Pacific as part of a study of deep-ocean manganese nodules. One of the objectives of the voyage was to collect at least 200 tons of nodules for extractive metallurgy research and various engineering tests. It was determined to cut off production at a maximum of 200 tons, should we reach that amount before the end of the 38 days. We were able to reach that goal; 200 tons of nodules were collected, representing an average return of 2340 lb from 171 dredge runs. Once equipment and techniques were refined, normal returns consistently exceeded 2 tons per haul.

Up to this time, Kennecott was sampled extensively with the use of small free-fall devices using wire rope and winch equipment. These devices have never offered a viable method for collecting nodules in bulk, and we sought to develop a dredging capability which gave some assurance of meeting future requirements with efficiency and economy. After testing a series of increasingly complex dredge at sea, it was decided to utilize two of the simpler configurations, tor modification provided the basic design for the used.

The dredge system consisted of a main traction with storage reel, A-frame, dredge, hydraulically operated unloading rack, nodule receiving bin, nodule storage bins and instrument and laboratory vans.

Thirty-thousand feet of 7/8 in. wire rope are used for the dredging wire. This is led from the dredge tow- point up through a 36 in. head block on the A-frame, then down through a fairlead block and tenslometer to the traction winch, where it is reived in a figure eight through two 54 in. bull wheels which provide primary and secondary traction. The wire then runs forward across the top of the storage bins to take-up reel, which provides additional traction.

A second wire rope is used for an in-haul line and leads aft from the small hydraulic winch mounted on the nodule receiving bin. The winch, an unloading rack, a screw conveyer and two auxiliary tuggers are operated by a separate electro-hydraulic unit installed within the splash shield immediately aft of the main winch. This unit requires electrical power from the host ship, but can be configured as a self- contained unit if necessary.

Deployment of the dredge is straight forward, the unit being restrained by the in-haul line as it is pulled to the extern by the dredging rope. The main purpose of the in-haul is to counteract the upward component of the main wire’s force keeping the dredge on deck until the moment of launch. Once the dredge arrives at deck edge, the in-haul can be slacked and removed; the dredge is then slipped into the water by main wire alone.

For recovery, the in-haul is hooked forward through the rails and onto the loading rack, where it is chained in place, using pre-cut chain stoppers. The unloading rack is then elevated and the load dumped into the receiving bin, where it is transported through the screw-conveyor to the storage bins. The storage is compartmentalized to reduce any free-surface effect and distribution of the nodules to the 1 various compartments is accomplished through a portable pivoting service resembling a coal chute.

Fig. 16. Dredge operating on Bonanza Creek

Dredge Specifications

The dredge itself is 7 ft wide and 11 ft 9 in. long. In height, it tapers from 3 ft 6 in. at the front to 3 ft at the rear; the cross-section reducing from a maximum of 24.5 sq ft to a minimum 21 sq ft. Dredge capacity is a normal 255 cu ft. Exclusive of the chain clump and towing attachments, the dredge weighs approximately 2800 lb in air.

The forward assembly is constructed of 3/8 in, HRS plate, A vertical brace, fabricated from 1/2 x 3 in. HHS, contains the towing attachment points. A 6 in, cutting blade is installed on the bottom of the drudge at an angle of 20° down from the horizontal. Two twin points are provided, one at 3 in. and one at 6 in. up from the bottom of the unit.

The main longitudinal strength member's of the dredge basket are constructed of 1/2 in. diameter 3x19 wire rope. These stringers are separated by circumferential stiffeners made 3/0 x 3-in. HRS. The bag is constructed of cyclone conveyor-belt material, wired to the strength members. A 500-lb chain clump is attached to the rear of the unit by a 6-ft I, providing stabilizing ballast and tending to keep the flexible bag straight when under tension on the bottom.