Development and Possible Applications of Mebo200 for Geotechnical Investigations for the Underwater Mining

Abstract

Abstract Deepsea mining sites are usually large areas of polymetallic nodules or seafloor massive sulfide (SMS) deposits. Prior to underwater mining campaigns, successful exploration programs have to be taken into account in order to include selection of target areas by means of seafloor and sub-seafloor surveys of the rocks and sediments. Coring of the submarine sediments plays a major role for evaluating of the marine sediments and they are useful for future mining operations. The main geology which hosts the polymetallic nodules is pelagic clay. On the seafloor and specifically at the mudline, this kind of soils has very low undrained shear strength up to 20 kPa. In order to properly design the nodule harvester, which will be in contact with the sea floor, physical properties of the seabed and shallow subsurface (i.e. the upper 100–200m)are needed to compute bearing capacity. Underwater soil cores can be used to evaluate soil type, cohesion, unit weight of soil, modulus of elasticity. Seafloor massive sulfide (SMS) deposits are very different from the polymetallic nodules. In this case, cores are needed in order to assess possible anomalies. Measured uniaxial compressive strength (UCS) values strengths in the SMS zone can be very high. Drill cores are required for assessing the mineral content at these deposits. The MeBo200 is a sea bed drill rig presently developed by the MARUM Center for Marine Environmental Sciences at the University of Bremen and the department of Maritime Technologies of BAUER Maschinen GmbH. It is well suited for core drilling in formations of comparable strength and therefore it is suitable preparatory geotechnical exploration. MeBo200 is the second generation of MeBo. From 2008 to 2012 MeBo was employed in 9 expeditions. 66 deployments took place in up to 2,050 m water depth. 1,445 m were cored in different type of geologies, concentrating more on the quality of the cores. These include crystalline rocks from the Porcupine Bank west of Ireland, as well as very soft homogenous silty clays in the Gela Basin with very low undrained shear strength values. The recovery rate was good for hard rocks and cohesive soft sediments. Taking into account the case histories of MeBo it is possible to state that for underwater mining applications the new MeBo200 can be utilized for geological and geotechnical drilling campaigns.