Design Improvements in Box Dredges To Enhance the Collection of Manganese Nodules

Abstract

ABSTRACT Design of three box dredges along with the technique and modifications effected to enable bulk sampling of manganese nodules is presented. Incorporation of a sled, angled cutting edges, retention plates and increased size of the dredge resulted in significant increase of the dredge haul. Consequent field data is included. Design of a simple feeding concept to enhance the concentration of nodules at the dredge mouth is briefly dealt with for future application. INTRODUCTION It was from a routine dredge haul in the Atlantic Ocean that manganese nodules were first discovered in 18731. This century old technique of drag-line dredging continues to be universally used within the Oceanographic community for collecting manganese nodules and consolidated substrate materials. The analysis from the samples collected from dredge hauls have been the subjects of numerous publications. The universal use of the bucket or box dredge can be attributed to its dual ability to function both as a reconnaissance survey tool and as a bulk sampler. Other favorable characteristics are its simple design, low cost, the flexibility it allows to operate in almost any water depth and adaptability to almost any research vessel having the necessary deck gear like a winch, wire rope and an 'A' frame. Generally considered to be an archaic and blind sampling method, often they are the only tools available for bulk sampling of seafloor substrate2. Compared to its wide spread use, sparse literature relating to improvement in dredge designs for collecting manganese nodules and crust seem to exist3–8. The purpose of this paper is to present the evolution of designs and refinements carried out in the dredges to enable bulk sampling of manganese nodules and encrustations during surveys in the Central Indian Basin (CIB). HISTORY AND BACKGROUND For India, the National Institute of Oceanography's Research Vessel R.V. Gaveshani collected the first nodules on 26th January 1981 from the Indian Ocean. Extensive exploration in the CIB lead to the recognition of India as a "Pioneer Investor" in sea bed mineral exploration in April 1982. The first dredge was designed during this period for collecting manganese nodules. The 10mm diameter cable on board R.V. Gaveshani having a breaking load of 5.4 tons restricted the development of a larger dredge. The period August '82 to March '88 saw accelerated activity in the Central Indian Basin. Three chartered vessels M.V. Skandi Surveyor, M.V. Farnella, M.V. G.A Reay in addition to the Indian research vessel ORV Sagar Kanya were deployed during different periods for the project 'Surveys for Polymetallic Nodules'. The requirement of tons of manganese nodules for development of metallurgical process resulted in the design and operation of various types of dredges. DESIGN CONSIDERATIONS Ship size and deck machinery apart, the diameter and strength of the wire rope chosen for dredging directly influences the design of a dredge.