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Victoria University Antarctic Research Expedition Science and Logistics Reports 1993-94: VUWAE 38

Equipment Concept and Design

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Equipment Concept and Design

Background

The vibracorer was developed to recover a complete and undisturbed sample of the sea floor for glacial and climate history studies. This was difficult for the sea floor around Antarctica because sampling both soft mud in the upper metre or two and compact diamictite beneath, a mixture of mud, sand and stones deposited by the last expansion of the Antarctic ice sheet, can only be satisfactorily achieved with a vibrating tube. Existing vibracorers either were limited to water depths of 200 m (but most Antarctic sea floor is deeper, to 1000 m), or they were too large and heavy to move and operate from the fast ice, where at least part of the science goal could be met and where VUW had considerable operational experience. VUW decided then to build its own corer, which would also have sensors for recording tilt and orientation of the core, important for studying changes in past sedimentation and magnetic field directions.

The corer was successfully checked out in a series of tests in early October 1992, culminating with the recovery of a 3.7 m core from Petone Wharf on the northern margin of Wellington Harbour, and was first deployed in Antarctica a few weeks later. The first two Antarctic deployments were made in 700 m of water in Granite Harbour, outlet for the well-studied Mackay Glacier 150 km northwest of Scott Base. On both occasions the corer tilted on the soft sea floor mud as the core barrel was driven in, the barrel bending as retrieval was attempted. On its return to Wellington the corer was redesigned with larger feet and a lower centre of gravity, and successfully tested in the middle of Wellington Harbour (which is muddier than Granite Harbour). It should be noted that the mechanical and electrical systems in the corer have consistently performed well from the time of the recovery of the first core from Petone Wharf.

Logistic operation and constraints

The first part of the programme, to core from the fast ice at about 10 sites in Granite Harbour, would not only provide near shore scientific data but also a safe and stable platform from which to gain experience with the corer before ship deployment, with the considerable time pressures involved, in subsequent seasons. The party moved equipment and personnel with a D5 bulldozer, 2 five tonne cargo sledges, a sledge mounted accommodation/ laboratory wannigan, a steel sledge and a RN75 Nodwell vehicle with HIAB crane for lifting and drilling ice holes. This mode of operation has been used successfully for some years, and allows about 3 weeks field time after getting to Antarctica, testing equipment at Scott Base, travelling to the site and returning by around December 6, when summer warming cracks the sea ice.

The style of operation constrained the concept and design of the corer, putting a premium on lightness to reduce weight for the cargo sledges and for air cargo to and from Antarctica. Also because the equipment was developmental and had potential uses in back in New Zealand it was not considered feasible to send it to Antarctica the previous season by ship so it had to be air transported at the beginning of the season.

For operation from the fast ice up to 2.5 m thick the vibracorer needed to fit down a reasonably small hole, say 1.5 m in diameter. Also it was designed to operate from batteries to avoid an expensive and heavy power umbilical and large generator (15-20 KVA?) at the surface. It weighed 1.6-1.8 tonne in air and 1.2-1.5 tonne in water, though the exact weight is not known because of modifications to the legs in early 1993.

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The winch was designed to have a line pull of 2-2.5 tonne and again be kept as light as possible. It was powered from the Power Take Off of the hydraulic system on the Nodwell tracked vehicle. The line chosen was a polyester 20 mm diameter double braid rope with a breaking load of 7.5 tonne driven by a capstan onto a separately driven take up drum. A thimble eye splice at the end reduced the breaking load to 90% of the rated value, down to 6.75 tonne. With the corer weighing a maximum in water of 1.5 tonne the safety factor is 4.5. Polyester rope is low stretch and has the best resistance to UV, freezing temperatures and abrasion of the commonly used synthetic fibres.