ANDRILL: ANtarctic geological DRILLing
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Coulman High Project
Drilling & Operations
"Plumb-Bob" Method of Drilling from a Fast Moving Ice Shelf
The existing ANDRILL Sea Riser and Drill Rig was designed and proven for a riser deployment of 1000 m (ice and water thickness). The primary new engineering challenge for the CHP will be to deploy and operate for ~50 days and accommodate bending in the pipe produced by the ~100 m northward movement of the ice shelf. Water column currents and the tidally driven rise and fall of the ice shelf must also be accommodated.
Stress Engineering Services (SES) in Houston, Texas, performed an analysis of various options for drilling from the moving ice shelf (Supplementary Documents [Fast Ice Shelf Riser Analysis Report]). SES riser modeling and the subsequent review by the ANDRILL Engineering Task Force (AETF) identified the “downstream spud-in” (“plumb bob”) method to meet these challenges. This method deploys a cable and weight attached to the riser to pull the riser downstream of the rig, where it can spud into the seafloor ahead of the direction of ice movement (see slides on the following pages).
Riser modeling has also identified the high bend angles in the riser and drill strings that occur at the sea floor in the early and latter part of the 50-day window. The curvatures can be mitigated with non-standard higher fatigue strength drill strings and by varying lengths of core barrels with greater clearances to pass inside the high curvature zones. The downstream spud-in method with bespoke drill strings and barrels will achieve the goal of 50 days of drilling while connected to the seafloor, with an expected penetration of up to 1000 m.
A key piece of equipment required for this project is a small-diameter ROV to attach the plumb bob to the riser and pull it downstream. A shallow-water (~300 m) version of the Submersible Capable of sub-Ice Navigation and Imaging (SCINI), developed at Moss Landing Marine Laboratories, was used during the CH site surveys in 2010-2011 to make observations and conduct experiments below the Ross Ice Shelf. The CHP needs a Deep-SCINI ROV that can operate in up to 1000 m of water and conduct observations and simple tasks requiring a gripper.
Funds to construct the Deep-SCINI are being sought by the US-ANDRILL Science Management Office at the University of Nebraska-Lincoln, who (1) participated in a September 2011 proposal to NASA, with colleagues at the University of Texas at Austin and elsewhere, that contains a request for partial funding of a prototype Deep-SCINI development, and (2) submitted a January 2012 instrument development proposal to the NSF Major Research Instrumentation Program to request funds for Deep-SCINI. If these requests are unsuccessful, the ANDRILL CH Project Plan and Budget identifies funding to develop the Deep-SCINI system (Section I-5(b), Pyne and Falconer, 2010).
The front of the RIS evolves by steady advance at rates of mostly 0.6–1.0 km/yr, with major calving events spaced at intervals of many decades. The advance rate at the site in November 2010 was 2.0 m/day. East of Ross Island (170°–177°E), there is a record of at least one calving event between the 1842 Ross Erebus and Terror expeditions and the 1902 Scott Discovery expedition, but no major calving after 1902 until the C-16 and C-19 calving events in 2000 and 2002, which occurred on fissures that had been present for decades before the calving. MODIS satellite images show several large fissures in the RIS that are potential sites of major iceberg calving in future decades, but none appear likely to affect the Coulman High drill site within the next decade.
Deployment of the Riser for Spud-in and Drilling during the CHP
SES riser modeling and the subsequent review by the ANDRILL Engineering Task Force (AETF) identified the “downstream spud-in” (“plumb bob”) method to meet the challenges of drilling from a fast moving ice shelf. This method deploys a cable and weight attached to the riser to pull the riser downstream of the rig, where it can spud into the seafloor ahead of the direction of ice movement, as explained below (also see slides on the following pages).
Pre-spud-in preparation illustration
(1) Pre-spud-in preparations
  • Three hot water drill holes are made through the ice shelf at CH, where the ice is approximately 250-270 m thick, using a hot water drill system;
  • A slim ROV (Deep-SCINI) is used to transfer a tag line between the drill hole where the riser will be deployed and the winch hole, which is located about 150 to 200 meters north of the main drill hole in the flow direction of the ice shelf. The tag line is connected to a “plumb bob” weight attached to the drill riser;
  • The tag line is progressively replaced with heavier lines until the main “cable” from the winch to the riser is in place;
  • The ROV continues to observe and monitor the deployment of riser and the impact of the plumb bob weight.
Spud-in (Day 0)
(2) Spud-in (Day 0) 
  • The winch cable is tensioned to pull the “plumb bob” mass (5 tons) away from the riser, thereby deflecting the riser and sea floor assembly at least 50 m in the direction of ice flow, to allow the riser to be landed on the sea floor at the preferred spot;  
  • The sea floor assembly mass of 5 tons ensures that the entry of the riser into the sea floor is nearly vertical. The “plumb bob” mass enables controlled deviation of the riser and eliminates the need to set a sea floor anchor. Contact with the sea floor is likely to be timed when currents can assist the deflection process. The riser is likely to be washed and possibly drilled into the sea floor to a depth of 10-20 m before cementing it in, to anchor and stabilize the riser; 
  • Modeling confirms that the greatest stress and curvature in the riser occurs near the sea floor and at the base of the ice shelf. In both areas the riser is strengthened with a stress joint (additional 7-5/8 inch casing sleeves) however these two areas now become the points which have the greatest effect on the rotation of the internal drill strings and operation of the coring tools.
(3) Day 12-15
  • The completion of coring with the first coring string [4.5 inch (“PHD”)] is expected at a depth ideally between 250 mbsf and 300 mbsf, approximately 12-15 days after spud-in;
  • The 4.5” drill string is then cemented in the hole and made ready for continued coring with the next smaller diameter drill string that is lowered down and nested inside the previous string and the riser pipe to guide core barrels and other tools to the bottom of the hole;
  • The ROV is available 24/7 for a variety of observational missions and for other light tasks as required
Drill img
(4) Day 29-32
  • The completion of coring with the second coring string [3.5 inch (“HQ”)] is expected ideally between 700 mbsf and 800 mbsf, approximately 29-32 days after the initial spud-in; this 3.5” drill string would also be cemented in the hole and used as casing for the next smaller diameter drill string to be deployed and nested inside of it to continue coring to total depth;
  • The ROV is available 24/7 for a variety of observational missions and for other light tasks as required.
Drill img
(5) Day 49
  • The completion of coring with the third drill string [2.875 inch (“BQ)”] is expected at 1000 mbsf, approximately 49 days after the initial spud-in. Additional time, if any, could be devoted to down hole logging as the drill strings are recovered to the surface to terminate the drilling in this hole;
  • The ROV is available 24/7 for a variety of observational missions and for other light tasks as required.
drill img
(6) Recovery of Riser
  • Following the completion of coring and any down hole logging that is conducted during the recovery of the drill strings, the riser is recovered back through the water column to the ice shelf surface;
  • The ROV is available 24/7 for a variety of observational missions and for other light tasks as required.
Operational challenges will result from the movement of the ice shelf northward over the sea floor at ~2 m/day. Deflection of the riser cannot be more than 8% of the water layer thickness of 630 m (below ice base). The riser will be pulled north ahead of the rig using a weight lowered through a second hole in the ice shelf, to spud-in and begin drilling while the rig drifts northward over the hole. (2) Spud-in. The winch cable is tensioned to pull a plumb bob mass (5 tonne) from the riser deflecting it and the sea floor assembly in the direction of ice flow by at least 50 m. Contact with the seafloor should be timed when currents can assist in the deflection. The riser is then washed or drilled into the seafloor to a depth of 10–20 m and cemented in place. (5) The final piece of tapered drill string will allow drilling to a depth of 1000 meters below seafloor (mbsf) before day 49. At this time, any downhole logging operations must be completed and the cemented casings cut and recovered to disconnect the riser from the seafloor.