U.S. Antarctic Program Shallow Drilling Project (SHALDRIL) Cruises Completed Using DOSECC Tools
S. Wellner University of Houston
L. Holloway ConocoPhillips
SHALDRIL (for SHALlow DRILling) is a project of the United States Antarctic Program aimed at developing and demonstrating a system to obtain drill core from the Antarctic continental margin. The concept behind SHALDRIL is to drill through the stiff glacial overburden that covers nearly all of the Antarctic continental shelf and has historically inhibited piston coring, in order to sample older deposits where they come close to the seafloor. In addition to being able to sample older lithified material, the system developed must also have the ability to sample soft sediment. SHALDRIL is designed to operate between the continental slope, where traditional drill ships typically work, and the fast-ice zone, where ANDRILL is best suited. The success of the SHALDRIL method is dependent upon having a mobile drilling platform, capable of operating in ice-covered waters, and a drilling system that can retrieve high-quality core during an extremely short operating window.
Figure 1- Nathanial B. Palmer with drill rig installed, SHALDRIL project, Antarctica
In 1994, a workshop was convened at Rice University in Houston, TX. Fourteen scientists attended the workshop, which focused on new data from the continental shelf offshore of Seymour Island (Anderson et al., 1992; Sloan et al., 1995). There was strong consensus from the group that shallow drilling would yield key information about this critical region, the “Last Refugium of Antarctica.” The workshop participants also discussed drilling technology. In the final analysis, it was determined that the technology for SHALDRIL was not yet ready. A SHALDRIL committee was formed to monitor technical advances. The result of this work is several reports outlining different methods and techniques that might be utilized. In 2000, the SHALDRIL steering committee learned about improved drilling systems, including diamond coring, capable of drilling in water depths of several hundred meters and to core a few hundred meters beneath the seafloor. In 2003 a contract was signed with Seacore Limited (UK) to design and build a vessel specific drilling rig and to conduct the drilling operations for SHALDRIL. The rig was mounted over a small moonpool that was installed through the starboard deck of the RV/IB Nathaniel B. Palmer (Fig. 1).
SHALDRIL II was conducted in the northwestern Weddell Sea and the primary drilling targets were in the northern portion of the James Ross Basin, which contains one of the most complete Neogene successions anywhere in Antarctica (Anderson, 1999). Previous seismic investigations have revealed a virtually continuous succession of seaward-dipping strata on the continental shelf. The succession spans the late Eocene through Pleistocene.
Throughout both cruises the Seacore rig operated without difficulty and only minor adjustments were made to it between the cruises. The sampling tools for SHALDRIL I worked extremely well when taking push samples through soft Holocene sediments but were not so successful at getting through the stiff Pleistocene till. Thus, a change was made for the 2006 cruise to develop a modified set of sampling tools supplied to Seacore through DOSECC.
DOSECC Bottom Hole Assembly
The primary set of drilling hardware includes a suite of DOSECC tools that were designed to work within a common bottom hole assembly (BHA). The DOSECC tools were originally designed to operate with a drill bit with an internal diameter (ID) of 3.345″. However, for SHALDRIL, the bit throat was modified to accommodate a larger 3.85” ID. The larger throat would have allowed the Seacore piggy-back coring hardware (PBCH) to be deployed through the common BHA bit if material was encountered where high-speed diamond coring was required. This option was not deployed during the 2006 campaign. Since the time at any drill site was severely limited due to drifting ice, it proved important to have multiple tools fitting the same BHA in order to reduce tripping times to change the BHA.
Two coring assemblies and one non-coring tool were developed. These include an extended-nose spring-loaded corer and another DOSECC assembly known as the Alien corer. The extended nose corer is typically used for sediments after push or piston sampling is exhausted. The alien corer is similar to the extended nose corer but is designed to sample harder material and uses a triple core barrel inner tube assembly that is rotated in conjunction with the larger outer BHA. Due to increasing the bit and tools to accept the larger throat size in the primary BHA bit, all tools were then able to sample the same core size. The alien corer proved to be the tool used during the majority of the SHALDRIL II cruise. Sample recovery was high using this system. In addition, drilling through the subglacial and glacial-marine diamictons that form the overburden above most of the targeted strata was just as fast with the alien corer as with the non-coring tool (i.e., center bit installed in place of a coring assembly).
Figure 2- Alien drill bit with glacial diamicton collected during SHALDRIL
Drill-in Bottom Hole Assembly
As noted above, the first hardware option allowed up to three sampling systems to be deployed through the same BHA. Others tools are available but were not part of the suite of tools purchased by Seacore for SHALDRIL II. Should hard rock be encountered at a very shallow depth where the common BHA bit cannot be advanced easily, a different operating system can be deployed. This hardware, which was originally developed at ODP and slightly modified for SHALDRIL, uses a robust five roller-cone bit and a one-cone center bit latched into its throat. The center bit is removable via wire line once the BHA has been drilled to depth to allow a clear passage for the PBCH to be initiated. This hardware was not deployed during SHALDRIL II.
During SHALDRIL II, the ice conditions along the existing seismic lines did not allow any of the proposed sites to be occupied in their exact locations. The ability to continue drilling into sections of the desired age was dependent on collection of new seismic data during the cruise. In effect, areas of open water were identified, then seismic data was collected and correlated, and then sites were selected and drilled before changes in ice conditions. Without this ability to follow the water, chances are none of the objectives would have been met.
While none of the planned sites were drilled, nearly all of the coring objectives were met by drilling in alternate locations. A total of five holes reached the targeted older material yielding samples from the Eocene, Oligocene, Miocene, and Pliocene, although only a few meters were recovered from any one hole due to being driven away by ice. Also during the SHALDRIL II cruise, a long Holocene core was recovered from the Firth of Tay that will complement other Holocene cores of the area, including that from Maxwell Bay collected during the 2005 SHALDRIL cruise. The only SHALDRIL drilling objective that was not met was to gain a continuous sample through a thick (80-100 m) grounding zone wedge deposit. This was simply not possible due constant problem of drifting ice. However, based on the samples obtained from the overburden at other sites, there is every reason to believe this would have been technically possible.
The sea ice conditions encountered during SHALDRIL II were the worst-case scenario, thick multiyear ice drifting at rates that were unpredicted. The fact that SHALDRIL is so mobile enabled us to improvise and overcome severe sea-ice conditions and exploit alternate sites, proving the efficacy of the “drill and run” strategy. The drilling and sampling system is capable of penetrating up to 20 meters of glacial overburden and sampling older strata within 24 hours time. Core recovery in partially lithified sedimentary material is quite good (greater than 80%). The fact that success was found in more ice, and thus much shorter windows for drilling, than predicted will allow future SHALDRIL plans to be made for a much broader set of conditions. We hope that this is the beginning of a long program of drilling from icebreakers in the Antarctic.
We thank the crew of the NBP and the dedicated staffs at Raytheon Polar Services Company and Seacore for helping to make SHALDRIL possible. We are supported by NSF Office of Polar Programs grant ANT-0125526 to John Anderson (Rice University), in collaboration with P. Manley (Middlebury College), S. Wise (Florida State University), and J. Zachos (University of California Santa Cruz.)
Anderson, J.B., S.S. Shipp, and F.P. Siringan (1992), Preliminary seismic stratigraphy of the northwestern Weddell Sea Continental Shelf, in Y. Yoshida, K. Kaminuma, and K. Shiraishi (Eds.), Recent Progress in Antarctic Earth Science, Terra Scientific Publishing, Tokyo, 603-612.
Sloan, B.J., L.A. Lawver, and J.B. Anderson (1995), Seismic stratigraphy of the Palmer Basin, in A.K. Cooper, P.F. Barker, and G. Brancolini (Eds.), Geology and Seismic Stratigraphy of the Antarctic Margin, American Geophysical Union, Antarctic Research Series, 68, 235-260.