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SCAR Report No 16,

Appendix 7

APPROACHES TO MARINE SHALLOW DRILLING ON THE ANTARCTIC SHELF
Yngve Kristoffersen,
University of Bergen, Norway.

Shelf sediments which document past glacial advances are often represented by relatively coarse material compacted by the iceload and therefore difficult to penetrate by conventional sediment sampling devices lowered from a vessel. Penetration more than a meter or so in overconsolidated material can only be obtained by drilling. On the positive side is that glacial erosion of inclined strata on a prograding shelf often expose truncated sediment sequences at the sea floor and provide access to a geological record which range from the young sediments at the shelf edge to succesively older strata towards land or the front of an ice shelf. Important information can therefore be obtained by shallow drilling (say less than 100 m sub-bottom).

Goal:

To obtain undisturbed sediment cores +50 m long in water depths up to 1000 m for geological ground truth and interpretation of past glacial environments.
Approaches:

 

- Drilling from landfast ice
- Ice strengthened dedicated drilling vessels;
- Mobile systems adapted to existing Antarctic research and support vessels;

Drilling from landfast sea ice platforms has successfully been carried out in the Ross Sea with great scientific reward during the MSSTS, CIROS projects and currently at Cape Roberts as international cooperative efforts. Holes have been drilled down to over 700 m below the seabed with good recovery using conventional offshore drilling technology. The sediment cores document the earliest glaciation in the Ross Embayment during the Oligocene. Choice of drill sites are however, constrained by the presence of suitable fast-ice. Unexpected changes in the ice situation from one year to another represent a challenge, and relocation on short notice is difficult.

Very few ice-strengthened geotechnical drilling vessels are commercially available, and rates are more than $ 50k/day. Mobile systems based on standard offshore drilling technology using heavy API pipe also require a substantial size of the rig and relatively large mobilisation costs. They require a moon pool of a size that is not available in the current Antarctic fleet of research vessels, and therefore a dedicated vessel.

Small mobile systems which can be lowered to the seafloor and remotely operated from a research vessel have been built and successfully used in the deep ocean environment. However, cores are limited to 3 m length. Engineering proposals have existed for more advanced versions which can take longer cores by adding pipe and exchanging core barrels by remote control, but no reports of prototype development have been recorded.

Developments:

Objectives of ANTOSTRAT as well as Nansen Arctic Drilling in the Arctic Ocean have spurred development of an alternative approach using light weight mining technology for shallow drilling from Antarctic research vessels.

A concept using mining exploration equipment adapted for marine shallow drilling from a research vessel was first attempted during the Nordic Antarctic Expedition 1995/96 on the Queen Maud Land Shelf. Initial tests had been carried out in Norwegian fjords and offshore Helsinki by University of Bergen in cooperation with Geo Drilling A/S, of Namsos, Norway. The site was in 212 m water depth on the west side of Kvitkuven Ice Rise in a bay where the Finnish research vessel "Aranda" could be tied to fast ice. The rig was mounted on the stern of the vessel. The riser and drill string performed very well, but penetration of the glacial till was hampered by frequent loss of circulation. The drill bit reached 16 m below the seafloor before the site had to be abandoned because of a change in the ice situation. Total sediment recovery was 18% and included drilling through three basalt boulders.

A second trial was made from "Polar Queen" the following year with a different type of rig mounted over the side of the vessel. The riser and drill string was protected by a guard below the waterline. Drilling were attempted at three sites in water depths ranging from 200 m to 286 m with the ship docked in fast ice as well as under dynamic positioning in open water with drifting sea ice in up to 30 knots wind. Problems with repeated loss of circulation gave a penetration of 6-8 m with recovery varying from 0% to 70%.

These drill sites were located stratigraphically below and above a distinct seismic reflector which is apparent in reflection profiles across the shelf over a distance of several thousand kilometers along the Eastern Weddell Sea continental margin. This reflection event represent an unconformity and mark the onset of vigorous shelf progradation. Microfossils in the shallow drill cores have the potential of providing the first age reference points for Late Neogene and younger glacial advances of the East Antartic Ice Sheet in Queen Maud Land.
The most challenging attempt was carried out in August 1996 from the Swedish icebreaker "Oden" in the Arctic Ocean about 200 km from the North Pole. The vessel was able to keep position while breaking 2-4 m thick drifting sea ice of 8-9/10 cover. The riser was lowered through a moon pool and landed on the Lomonosov Ridge in 970 m water depth. As the drill string was run in the riser an obstruction was encountered at 250 m depth and eventually the attempt had to be aborted.

The experience from two Antarctic and one Arctic season have led to design of a new rig which will meet the specific needs for being mobile and adaptable to a wide range of polar research vessels. The rig comes as a regular 20 foot container sized unit which is positioned with one end extending 2 m over the side or stern of the vessel, or over the well if a moon pool is available. The derrick is mounted in port, and the drill unit and heave compensator guide folds out from the top of the container. Additional containers are used for storage, workshop, mud mixing and a 245 kW power plant. Sea trials are scheduled for the fall 1998.

Valuable experience has been gained in operating under a wide range of ice conditions, with and without a moon pool. Drilling from a ship is safe and can be done with a minimum impact on the environment. This approach has the potential of providing a cost effective tool for geological sampling on the Antarctic continental shelf and make shallow drilling an integral part of the marine research activity conducted by a number of nations around Antarctica in the quest for the climate history of the continent.