Abstract

The proposed paper describes the design and operation of a remote sea floor, hybrid rotary-vibracore drill for shallow sampling in the more difficult, densely packed, clay, sand, and gravel deposits. Weighing about 2500 lbs., the system was designed for remote operation from work boats as small as 60 feet. The system is capable of taking a 2 inch core, 16 feet in length.

The basic design and operation of the rotary-vibracore drill involves the employment of a dual barrel system whereby the inner-tube functions as the vibracore barrel and the outertube serves as the rotary barrel. The vibracore barrel, driven by a pair of hydraulic rotary vibrators rigidly attached to the barrel, functions as a conventional vibracore, complete with liner and core catcher (as required). The outer rotary barrel is driven by a high torque hydraulic motor via a simple gear train at about 30 rpm. A specially designed bit is attached to the rotary barrel and provides the desired rotary induced, shear effect. This in turn reduces side wall friction on the vibracore barrel and enhances penetration and core recovery. Water is introduced to the annular space between the two barrels via a water swivel mounted between the vibrator and rotary drive sections. This serves to lubricate the barrels, purge sand, and provide water to the rotary bit, at the point of exit.

Other design features include a hydraulic cylinder, sheave, and cable arrangement for remote drill system/core barrel assembly pull-out and pull-down. The cylinder hydraulics are also designed so as to bypass on remote command to permit controlled gravity down feed while coring. Also, a fathometer transducer is shock-mounted on the rotary frame (minimal vibration) to provide depth of penetration and penetration rate information.

The rotary-vibracore drill is hydraulically powered and electrically actuated. A hydraulic pressure and return hose is deployed from a power supply on the support vessel to a drill frame mounted, remotely controlled distribution valve system. The control valve system container is oil filled and pressure compensated. The electrically actuated control valves are operated via a simple deck-mounted panel, however the system may also be computer controlled.

The rotary-vibracore drill was recently tested from the 60 foot work boat, R/V Kit Jones. A core of approximately 14.5 feet was recovered from a section including about 8 feet of loose, silty sand overlying a very dense, desiccated clay. Previous tests at the site with a conventional hydraulic powered vibracore system have yielded only about 1 to 1.5 feet penetration in the dense clay.

The successfully tested design provides a compact, low cost, shallow coring system for inshore sampling of the more difficult, densely packed sands, gravels, and clays that typically refuse the conventional vibracore. The system can be operated remotely from low cost work boats of opportunity. The means of rotary cutting the core and introducing the remote pulldown/ pull-out feature insures greater percentage core recovery and the ability to deploy and recover in rougher sea conditions leading to overall savings in time and cost.

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