In Japan, marine aggregates have conventionally been dredged by centrifugal suction pumps installed on-board vessels. Due to dredging operations in shallower areas being prohibited for environmental reasons, for more than ten years, aggregates have been mined at depths of over 30 meters. Because of the inconvenience of using conventional on-board suction pumps at these depths, submersible pump systems were developed in Japan specifically for deeper applications. Recently, dredging at depths of 50 to 60 meters is commonly carried out and the use of a submersible electric motor-driven pump has enabled operations to be carried out at depths of up to 100 meters.
Due to environmental concerns and limited reserves, it is becoming more and more difficult to mine sand from traditional onshore resources. This situation is causing concrete and aggregate suppliers to look for other sources of this sand commodity. The answer may be found in the mining of marine aggregate. Marine sand is suitable for use in the construction industry, and is an economically sound alternative to mining aggregate on land. The demand for this resource can be expected to increase as corporations increasingly realize the benefit of procuring this commodity offshore. In Japan, the mining of marine aggregate began in the 1950s, and annual output, now standing at approximately 80 million tons, is increasing every year.
(1) Changes in dredging facilities The dredging vessels are of the same structure as route dredgers, but are provided with on-board screening facilities for aggregate selection, and onshore discharge facilities for aggregate unloading. Concerning the dredging method, stationary dredging is dominant in Japan whereby the vessel is permitted to dredge marine aggregates within a restricted licensed area while anchored.
